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Composite Titanium Dioxide Nanomaterials Michael Dahl,† Yiding Liu,†,‡ and Yadong Yin*,†,‡ †
Department of Chemistry and ‡Materials Science and Engineering Program, University of California at Riverside, Riverside, California 92521, United States 1. INTRODUCTION 1.1. Scope
Titanium dioxide (titania, TiO2) has been fervently researched over the past few decades due to its potential applications across many different areas. Thanks to its bulk properties, including high refractive index and ultraviolet (UV) light absorption, TiO2 has seen considerable use as a white pigment in paint, food coloring, and personal care products and as a UV absorber in sunscreens.1−7 These applications utilize TiO2 across a wide range of sizes from hundreds of nanometers to several micrometers. Although these applications account for the majority of global TiO2 consumption, its utilization in nanoscale research has primarily focused on its near semiconductor electronic properties. Beginning with the initial discovery of the production of hydrogen from a TiO2 anode under UV irradiation,8 much work has been done, which has expanded use of TiO2 to numerous new applications. These applications range from photovoltaic cells9 to photocatalysts for hydrogen production and environmental remediation10 as well as photoelectrochemical sensors.11 Many other niche uses have also been studied, particularly in the medical and biological fields, where TiO2-based nanomaterials have been investigated for in vivo imaging,12,13 cancer therapy,10,14 and protein separation/purification15−24 and as bactericides.25−28 Although applications as a pigment/UV absorber are typically possible using pure TiO2, it has become clear that this is less feasible for applications utilizing photoelectrochemical properties as well as a number of biological applications. Thus, much research has been dedicated to the construction of nanoscale TiO 2 composite materials. Various excellent reviews have looked at TiO2 and its applications to each of the aforementioned fields of study over the years and give a significant amount of background for each TiO2 material.9,10,26,29−41 However, many of these reviews look at either a multitude of materials (i.e., not limited to TiO2 composites) and/or a specific application of TiO2 materials with a narrow focus. Thus, the intent of this review will be to discuss the synthesis of TiO2 composite nanomaterials and their applications across a wide range of fields. To clarify the focus of this review, we will define a composite as a combination of one or more materials (metal, metal oxide, metal sulfide, etc.) with TiO2. These combinations may appear in many forms such as layered or core−shell structures and can be produced by various methods including chemical synthesis, solution- or gas-phase deposition, and templated fabrication.
CONTENTS 1. Introduction 1.1. Scope 1.2. Advantages of TiO2 Composites 2. Metal and Metal Oxide−TiO2 Composites 2.1. Group I and II Metals 2.2. Early Transition Metals 2.3. Middle Transition Metals 2.3.1. Vanadium, Niobium, and Tantalum 2.3.2. Chromium, Molybdenum, and Tungsten 2.3.3. Manganese and Rhenium 2.4. Late 3d Transition Metals 2.4.1. Iron and Cobalt 2.4.2. Nickel, Copper, and Zinc 2.5. p-Block Metals 2.6. Lanthanides 3. Noble Metal and Metal Oxide−TiO2 Composites 3.1. Ruthenium and Rhodium 3.2. Palladium 3.3. Platinum 3.4. Silver 3.5. Gold 4. Nonoxide Semiconductor−TiO2 Composites 4.1. Metal Pnictogenides 4.2. Metal Chalcogenides 4.2.1. Cadmium Chalcogenides 4.2.2. Other Single-Metal Chalcogenides 4.2.3. Semiconductor Alloy Chalcogenides 5. Carbon−TiO2 Composites 5.1. Carbon Nanotubes 5.2. Graphene and Graphene Oxide 5.3. Other Carbon 6. Templated Composites 6.1. Yolk−Shell and Core−Shell 6.2. Ordered Mesoporous Silica and Zeolites 6.3. Anodized Aluminum Oxide (AAO) 7. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References
© 2014 American Chemical Society
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Special Issue: 2014 Titanium Dioxide Nanomaterials Received: November 3, 2013 Published: July 11, 2014 9853
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Figure 1. (a) General model of photocatalysis on TiO2. Reactions occur in three steps: (i) absorption of photons greater than the band gap energy to produce an electron−hole pair; (ii) separation of charges and migration to the surface; (iii) redox reactions with adsorbed reactants. (b) TiO2 composite structure exhibiting a heterojunction and charge trapping on TiO2 and the second component.
detailed. It is also necessary to note that a number of composites consist of multiple materials which span different categories. In these cases the composite will be discussed in the section relevant to the specific component that is yielding the greatest benefit or is the most novel addition.
We do concede that many of the composite materials discussed herein have shown little improvement in performance toward their applications and, as such, have been superseded by superior materials; however, in the interest of being comprehensive, they will be covered as well. Further, we acknowledge that the utilization of TiO2 for the photocatalytic degradation of dyes is only a model system to test photocatalytic activity; however, especially with how ubiquitous it is in the literature, this technique has become a useful tool for determining and comparing the gross improvements of various composite materials. It is frequently included within this review to make such comparisons, although we disregard the likelihood of its extension to a commercial product. Since the study of TiO2 has become such a widespread, multidisciplinary field, a significant amount of research has been dedicated to it over the last several decades. Consequently, a brief search of the literature in the SciFinder Scholar database for “TiO2” returns over 250 000 results. This can be narrowed to just over 11 000 results when using “TiO2 composite”, although many composite materials that are not specifically labeled as such are left out. Thus, it becomes apparent that in order to best review the state of TiO2 composite materials, criteria for separating types of composites must be determined. Here, we will first organize composite materials by reviewing specific metal composites based on their positions in the Periodic Table. Then, we will switch to categorization using specific material types. The initial sections of the review will look at metal and metal oxide−TiO2 composites. Metals that are similar chemically or utilized for similar applications will be grouped together in order to make simpler comparisons. It is worth noting that while there are numerous composites with different types of metals and metal oxides there are still some elements which are seldom used outside of doping, and as such, a number of metals will not be considered. Following this we will discuss nonoxide semiconductor−TiO2 composites, which have been the subjects of significant research in the fields of photocatalysis and photoelectrochemical cells. Carbon−TiO2 composites, which have been heavily researched due to the recent increase in the production of carbon nanomaterials such as nanotubes and graphene, will then be discussed. The subsequent section will discuss templated composites, which have seen an increase in study recently. The synthesis and utilization of complex TiO2 nanostructures using templates such as polystyrene, silica, zeolites, and aluminum oxide will be
1.2. Advantages of TiO2 Composites
The photoactivity of TiO2 has been shown to be dependent on several key properties: crystal phase, surface area, exposed crystal facets, uncoordinated surface sites, defects in the lattice, and degree of crystallinity. Morphology control of TiO2 via synthesis of composite materials has allowed for the improvement and fine tuning of many of these properties. Additionally, TiO2 composite structures can create and tune other properties such as mid-band-gap electronic states which can alter charge migration or produce a red shift in the absorption spectrum. Further, formation of heterojunctions between TiO2 and other materials can yield visible light absorption by the added material with charge separation facilitated by the TiO2. The two main polymorphs of TiO2 which show the highest photoactivity are the anatase and rutile phases, which have typically reported band-gap values of 3.2 and 3.0 eV, respectively. Although the band gap of rutile is narrower, the anatase phase is typically considered more favorable as it has a higher reduction potential and a slower rate of recombination of electron−hole pairs.30,32 Unfortunately, its wide band gap dictates that it will primarily absorb UV radiation. Here the utility of mixed phase TiO2 must be discussed. Optimization and control of the phase transition and its applications have been covered in detail elsewhere;42−47 however, commercial Degussa P25, or simply P25 for short, is one of the most commonly used mixed phase TiO2 composites. This mixed phase material allows for utilization of visible light wavelengths through excitation of the rutile phase while also containing benefits of anatase TiO2, such as a decreased recombination rate of charge carriers. The initial mechanism for this enhancement was ambiguous; however, utilizing electron paramagnetic resonance (EPR), Hurum et al. studied the fate of photogenerated charge carriers in order to shed light on the mechanism.48,49 They found that electrons which were photogenerated by the rutile component were transferred to a previously proposed electron trap site in the anatase lattice which lies 0.8 eV below the anatase conduction band.50 Further, this is situated lower than the rutile conduction band. A decrease in electron−hole recombination is achieved since the 9854
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such, most of the metals below will be discussed as their metal oxides. Further, the most common oxide will be preferentially discussed as other oxides are likely to be unstable and thus do not form stable composites.
photogenerated holes remain within the rutile component and the electrons are spatially separated into the anatase component. It was later determined that the photogenerated holes are preferentially trapped on the surfaces, whereas electrons become trapped within the lattice. As such surface electron trap sites increase the recombination rates, which indicates that a composite with a second material which is either a hole or an electron sink can further increase catalytic efficiency. Beyond the use of mixed phase TiO2, composites with nonTiO2 materials are a very promising means to extend the usefulness of anatase into the visible wavelengths. Alternatively, for composites consisting of the rutile phase, inclusion of higher work function materials can yield slower charge carrier recombination. Other methods to decrease charge carrier recombination include increasing crystallinity, which can be done by high-temperature calcination, addition of dopants, or specific synthetic protocols. Additionally, defects, which can serve as charge carrier traps and reduce the recombination of photogenerated electron−hole pairs, can also be either induced or stabilized by formation of a composite. Composites which can help to tune the grain size have also been shown, such as metal oxide sol−gel precursors which can form composites with TiO2 and inhibit crystallinity. Figure 1a shows a general model for photocatalysis on anatase TiO2 in which light is absorbed to produce an electron−hole pair, charges migrate to the surface, and redox reactions occur. This ideal case assumes low charge recombination and easy charge migration to the surfaces. Charge separation can be enhanced by creation of features such as surface defects where electrons and holes can be trapped to prevent recombination or, as discussed earlier, in P25 where a natural electron trap exists below the anatase conduction band. Figure 1b shows an improved case utilizing a TiO2 composite heterojunction. In this case, a structure with a narrower band gap can utilize visible light to produce an electron−hole pair. Assuming a favorable band offset, the electron can migrate to the TiO2, while the hole is trapped in the second material. Redox reactions are now free to occur at the separate surfaces since the likelihood of charge recombination has been diminished. A heterojunction composite structure can then be rationally designed in order to produce a favorable band offset and band positions in order to develop a catalyst for the needs of specific reactions, such as water splitting. In addition to the improvement of photocatalysis, composite structures can yield other benefits. Such advantages include the ability to tune the surface properties, i.e., acidity/basicity or open coordination sites, of the resultant materials, which is of importance to the adsorption of molecules, a critical factor relevant to catalysis, separation, and further modification. Numerous mixed metal oxide/TiO2 composites are beneficial for stabilization of thermal catalysts where reactions such as high-termperature NOx reduction are improved. Composites such as core−shell materials are also beneficial toward the stabilization of nanoparticles against phenomena such as sintering or aggregation. Further, composites can be of great use to create highly porous materials, hollow shells, or hierarchical structures by templating methods.
2.1. Group I and II Metals
Due to their high oxidation potentials and instability in aqueous solutions, composites of the Group I and II elements are mainly based on their oxides. Additionally, many of these elements are also frequently used for formation of metal−titanate materials, which exceeds the scope of this review and will not be specifically discussed. The large majority of the Group I and II metal oxide−TiO2 composites were initially studied as part of more complex alumino−silicate and alumino−titanate glasses and ceramics for a variety of applications.51−53 These composites are not strictly TiO2 composites, however, as they are formed from melts of metal oxide mixtures and typically result in titanate products. Lithium oxide specifically has little use in composites besides as a component in glasses which, once melted, typically form lithium titanate materials.54,55 Oxides of sodium, rubidium, and cesium have also seen a considerable portion of their usage as components in glasses.56,57 Interestingly, potassium oxide, as K2O, has seen interest as a component in composites for catalytic processes. In a system containing a mixed composite of TiO2−ZrO2, it was shown that a mixture of K2O and CO2 can be beneficial in increasing both the reactivity and the selectivity of the dehydrogenation of ethylbenzene to styrene.58 Conversion of ethylbenzene was increased from 60.59% to 71.95% with addition of 3 wt % K2O, and the selectivity was increased from 97.04% to 99.63%. It is worth noting that one previous study of the addition of only K2O to the TiO2−ZrO2 composite for this reaction reported decreased activity due to K2O neutralizing the acidic sites on the TiO2−ZrO2 composite,59 although it was shown that this neutralization did have the added benefit of increasing the selectivity by suppressing the dealkylation products.60 The basicity of K2O has also been shown to influence other reactions as well. In one study, by again neutralizing the acidic sites in a V2O5(WO3)/TiO2 composite, selective reduction of NO by NH3 was poisoned to nearly zero conversion with 1 wt % K2O.61 On the other hand, catalytic applications that can utilize the basicity of K2O have also been studied with positive results. Recently, Salinas et al. demonstrated the transesterification of commercial canola oil over a K2O/TiO2 catalyst for production of biodiesel.62 The results show that a 20 wt % K2O/TiO2 catalyst can achieve total conversion to methyl esters in refluxing methanol after 7 h of reaction time. This compares favorably to other catalysts which utilize potassium in the forms of KOH, KNO3, or KI supported on alumina, which have conversions of approximately 90% over reaction times ranging from 8 to 10 h for similar commercial oils.63−65 A follow up study on this system showed that upon calcination above 600 °C the composite formed an even more active titanate catalyst which yielded total conversion in under 3 h.66 In addition to K2O/TiO2 composites, MgO/TiO2 composites have also seen use in biodiesel production with similarly active catalysts being developed.67,68 After the work by Tanabe et al. in 1978, which detailed the catalytic activity of composites of MgO/TiO2 based on acidity/basicity,69 little research had been conducted until the past decade. Aside from the production of biodiesel, recent work has been established
2. METAL AND METAL OXIDE−TIO2 COMPOSITES Most metals are either too chemically reactive, especially on the nanoscale, not having properties that can be taken advantage of readily, or unnecessary with regard to composites with TiO2. As 9855
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to a 0.35 Mg/Ti molar ratio; above this ratio the blockage of hole escape by MgO on TiO2 is thought to be excessive, and thus, the performance decreased. This recombination was previously studied by Bandara et al., whereby they monitored the degradation of various organic molecules under UV irradiation with TiO2 loaded with different amounts of MgO.73 These catalysts consisted of clusters of MgO on the surface of TiO2, and the optimal catalyst was determined to have a composition of 3 wt %. The MgO clusters act as coordinately unsaturated defect sites where anion vacancies can efficiently separate and trap electrons with the resulting holes trapped in the TiO2, thus preventing recombination and allowing for optimal work to be done by the system. This charge separation was later confirmed by DFT simulation of MgO clusters on a rutile TiO2 (110) surface. 74 The combination of these findings has inspired new research into these MgO/TiO2 composite structures as inexpensive materials with improved catalytic performances.75,76 Whereas MgO has seen considerable research in composites with TiO2 in recent years, the remaining Group II metals have been little used, which is likely due to the abundance of research on the more useful and interesting systems with strontium and barium titanates. Some research has been conducted utilizing CaO in a composite with TiO2 where a basic oxide is needed for catalytic applications,77 akin to the use of potassium oxide mentioned above. However, mixtures of CaO and TiO 2 have seen more research as ceramic materials,78,79 wherein again the titanate product is most commonly formed.80 TiO2@SrO core@shell nanowires have recently been prepared and their photocatalytic properties tested for dye degradation.81 The composite was prepared by dip coating TiO2 nanowires in an aqueous Sr(NO3)2 solution for different times and then calcining the products in air to 450 °C. The SrO coating, however, shows no well-defined SrO peaks by X-ray diffraction and appears to be amorphous under the given conditions, although higher temperature calcination may form a strontium titanate phase rather than SrO.82 It is worth noting that compared to the pure TiO2 nanowires, there was still an improvement in the rate of dye degradation when the shell thickness was optimized. Barium oxide has seen recent usage as part of TiO2 and/or Al2O3 composites for NOx storage/reduction applications. Initial results out of the Toyota Motor Corp. laboratories showed the ability of NOx to adsorb onto the basic barium oxide surface to form Ba(NO3)2, which stored the NOx as a nitrate for reduction at a later step.83 Adsorption of NO2 on BaO/TiO2 composites was improved greatly when utilizing TiO2 as a support as compared to Al2O3, which was attributed to the weaker surface Lewis acid sites of TiO2 binding NOx as well, compared to limited binding by the more acidic Al2O3.84,85 Andonova et al. additionally found that Ti-containing domains had a strong affinity toward Bacontaining domains, to the point that upon calcination at 600 °C barium titanates and barium−titanium−aluminates were formed as shown in Figure 3. They further determined that the strong affinity between Ti and Ba domains can be beneficial for inhibiting formation of BaSO4, which poisons the NOx uptake efficiency.85−87
utilizing MgO/TiO2 composites for photocatalysis and photoelectrochemical cells. One example of such work done by Taguchi et al. utilized a thin layer of MgO over TiO2 for stabilization of solid-state dye-sensitized solar cells (DSSCs).70 Magnesium methoxide was deposited onto a TiO2/F-doped SnO2 (FTO) cell in an ethanolic solution and then sintered to form a thin (∼0.25 nm) MgO layer. The cell was completed with Ru 535 dye, CuI as a hole scavenger, and an Au-coated FTO slide. The role of MgO was to stabilize the cell and act as a physical barrier between TiO2 and CuI to prevent charge recombination and, most importantly, to prevent back electron transfer between TiO2 and the dye molecules. Consequently, both the open-circuit voltage (Voc) and the short-circuit current density (Jsc) improved from 430 to 510 mV and 8.38 to 8.74 mA/cm2, respectively, and the efficiency increased from 2.13% to 2.90%. Additionally, the stability over time for both parameters improved markedly with the MgO coating as shown in Figure 2. The increase in Jsc was unexpected as the
Figure 2. Stability of Voc (open square), Jsc (open triangle), fill factor (cross), and efficiency (filled circle) of solid-state DSSCs (a) without MgO layer and (b) with MgO layer under continuous illumination. Adapted from ref 70 with permission from The Royal Society of Chemistry.
MgO layer may reduce the efficiency of electrons from excited dyes being injected into TiO2; however, an unforeseen benefit of the MgO layer was an increase in dye adsorption of ∼20% as compared to the TiO2 surface alone. Later work by Jung et al. also showed similar increases when dye-sensitized solar cells (DSSCs) were fabricated from TiO2 particles (Degussa, P25) which were predecorated with MgO.71 They also found an optimal weight percentage to be 0.6 wt % MgO, which yielded an improvement from 3.1% to 4.5% in their as-constructed cells. Construction of DSSCs for hydrogen production was also observed to show a minor increase in H2 production when a MgO/TiO2 composite cell was constructed utilizing an overcoat of MgO on a TiO2:ITO cell.72 It was found that under bias voltage the decrease in charge recombination was beneficial for MgO loading amounts of up
2.2. Early Transition Metals
Although oxides of scandium have not been commonly utilized in composites with TiO2, other early transition metal oxides have seen considerable use. Here we will focus on the utilization of yttrium, zirconium, and hafnium oxide−TiO2 9856
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Figure 3. Illustration of the effect of calcination and BaO deposition on an Al2O3 substrate. Reprinted with permission from ref 85. Copyright 2009 American Chemical Society.
composites, where lanthanum will be reserved for the lanthanides section later. Yttrium oxide has been most notably utilized in composites with TiO2 for enhancement of upconversion particles. Lü et al. showed that Y2O3 doped with Tm3+ and Yb3+ (Y2O3:Tm3+,Yb3+), when coated with a TiO2 layer, showed an upconversion emission intensity enhancement of up to 5.4 times that of the uncoated sample.88 These core@shell structures were prepared by a modified Stöber process to coat the cores with TiO2 using tetrabutyl titanate in place of a silica precursor. The reaction was allowed to proceed for various times in order to tune the shell thickness. The TiO2 layer showed an amorphous structure by X-ray powder diffraction, since the structure was not calcined post synthesis and drying. The enhancement of the emission intensity was attributed to the TiO2 layer, which provides a ligand field for surface lanthanide dopant ions to convert from a “dormant” state to an active one. However, it is noted that in composites where the TiO2 shells were too thick, absorption of the incident light and reabsorption of the emitted light increased to the point that the enhancement factor would begin to decrease. Although optimal samples were denoted by optimum coating time, statistical analysis for an optimized thickness coating was not performed. In addition to the upconversion enhancement, composites of lanthanide-doped Y2O3 and TiO2 have also been utilized for photocatalysis and DSSCs. Li et al. prepared a composite of Y2O3:Tm3+,Yb3+/TiO2 by hydrolyzing TiCl4 in an ethanolic solution containing the upconversion particles, followed by calcination at 500 °C.89 The optimal upconversion particle was found to have a Yb3+ concentration of 2 mol % with a fixed Tm3+concentration of 0.5 mol %. They found that when compared to pure TiO2 prepared using the same methods there was an increase in the degradation rate of methyl orange (MO) by solar irradiation. Pure TiO2 degraded approximately 40% of the MO in 150 min, whereas the composite achieved 100% degradation. The photocatalytic enhancement was attributed to infrared radiation in the solar spectrum being upconverted to UV light, which could then be utilized by the TiO2 portion of the composite. Recently, Y2O3:Er3+ nanorods were utilized to improve the efficiency of a TiO2:FTO DSSC.90 The cells were constructed by depositing a 12 μm thick layer of a TiO2 colloid onto FTO, then mixing the Y2O3:Er3+ nanorods with a separate TiO2 colloid solution and adding a 4 μm thick layer of the mixture on top, followed by sintering at 450 °C. On the basis of several trials, the optimum weight percent of the upconversion particles was found to be 5 wt % compared to the overall TiO2
amount. At this concentration, the cell efficiency was found to increase from 5.84% in the pure TiO2 cell to 7.00%, with corresponding increases in Voc, Jsc, and the fill factor (FF). Although the increase in efficiency is modest, the benefit of converting infrared light into more usable wavelengths to increase DSSC performance is notable. Zirconium oxide has seen considerable usage in TiO2 composite materials. However, it is most frequently incorporated as part of a mixed oxide ceramic system or as a mixed oxide catalytic support system, although some more recent usage has focused on the interaction between ZrO2 and TiO2 for photocatalytic applications. The frequent use as a support can be attributed to optimal surface acidity of the ZrO2/TiO2 composite, 91 which can be fine tuned based on the concentration of ZrO2 within the composite as well as the calcination temperature and the synthetic conditions.92,93 The acid−base tunability of these supports can be utilized for the catalytic, or photocatalytic, oxidation of organic molecules;94−97 dehydrogenation reactions;59,98 and NOx reduction,99,100 among other reactions, which have been reviewed previously.101,102 Also of interest recently is the utilization of ZrO2/ TiO2 composites for photocatalysis, which is tested by dye degradation or water splitting.103−106 Addition of ZrO2 to TiO2 has been shown to retard or even prevent formation of rutile phase within TiO2 composites, which corresponds to a decrease in the growth of TiO2 grains in the microstructure and an increase in the surface area,106−109 although the robust nature of ZrO2 makes it difficult to remove from the TiO2 composite unlike the case of SiO2. Chen et al. recently utilized this increase in surface area to optimize ZrO2/TiO2 composite microspheres for the adsorption of heavy metal oxides.110 They were able to reach a maximum surface area of 413 m2/g in a 30:70 Zr:Ti molar ratio composite, which compares favorably to the pure phase surface areas of 108 and 104 m2/g for TiO2 and ZrO2, respectively. One recent study for the use of this system in a photocatalyst was done by Li et al., who synthesized a series of composites containing ZrO2, CeO2, and TiO2.105 The composites were prepared via the evaporation-induced self-assembly (EISA) method utilizing Pluronic P123 as a surfactant, Ce(NO3)3, ZrOCl2·8H2O, and titanium n-butoxide in an ethanolic solution followed by calcination at 500 °C. The ZrO2-containing composite consistently showed increases in surface area, pore diameter, and pore volume when compared to samples without ZrO2, indicating the influence of the ZrO2 on the microstructure of the composite. The resistance to sintering provided 9857
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by the ZrO2 allows for stabilization of long-range, ordered pores throughout the microstructure. The photocatalytic activity was further tested via the visible light degradation of Rhodamine B (RhB). The mesoporous ZrO2−CeO2−TiO2 composite showed the highest photocatalytic activity when compared to the other catalysts including P25, as shown in Figure 4.
materials. One study by Triyoso et al. showed that in an optimum synthesis by atomic layer deposition the mixed oxide deposition is favorable when compared to alternating layers of HfO2 and TiO2.112 The mixed oxides yield κ values of 38 at a 10:1 HfO2/TiO2 ratio and 28 at a 1:2 ratio, as compared to κ values between 21 and 25 with layered structures. Other recent studies have also shown improvement of the relative permittivity of mixed oxide systems as compared to pure HfO2, with better charge mobility and reasonable charge leakage.113−116 2.3. Middle Transition Metals
2.3.1. Vanadium, Niobium, and Tantalum. Much like how ZrO2 has been utilized in composites with TiO2, vanadium oxides in the form of V2O5 have seen frequent usage when combined with TiO2 as a support material for reactions such as the catalytic reduction of NOx117−119 and oxidation of organic molecules.120,121 Other applications of V2O5/TiO2 composites which have seen some study are for use in gas sensors122,123 and photocatalysis.124−126 One recent study by Yang et al. found that nanotube arrays consisting of a V2O5/TiO2 composite have beneficial properties in supercapacitor applications.127 The nanotube arrays, which are shown in Figure 5a−f, were fabricated through anodization of Ti:V alloy plates (Ti:V ratio ranging from 0.2 to 18 atom %) in an electrolyte consisting of ethylene glycol and HF. It had been shown previously that V2O5 demonstrated significant Li+intercalation properties such as a high capacity and rate, which is improved in the presence of TiO2.128 Pure phase V2O5 nanotubes could not be grown due to instability under typical anodization electrolytes, so TiO2 nanotubes provided an optimal and a stable structure to utilize for the composite. As shown in Figure 5g, the cyclic voltammograms show the highest current densities for nanotubes fabricated from the 18 atom % vanadium alloy. Upon calcination, the nanotubes showed improved performance until formation of the rutile phase, which is considered to reduce the conductivity.129 Ultimately, the optimized nanotubes showed specific capacitance values up to 220 F/g with an energy density of 19.56 Wh/kg. Additionally, other oxides of vanadium, most notably vanadium(IV) oxide, VO2, have been used in composites with TiO2, although much of this research is limited due to the inherent instability of the lower oxides. However, because of the considerable amount of research regarding the thermochromic transition of VO2,130−132 composites with TiO2 have been investigated to enhance both the chemical stability of VO2 and the transmittance, with some good results to date,133−136 including a slight decrease in the transition temperature, though it still remains too high for practical use. Both niobium and tantalum oxides have not been frequently researched for use in composites with TiO2. Niobium(V) oxide Nb2O5 has seen some applications in photocatalysis for dye degradation,137,138 selective photooxidation of organic molecules,139,140 and DSSCs.141−144 Nb2O5/TiO2 composites for DSSC applications focus mostly on the use of Nb2O5 as a blocking layer between the conducting electrode and TiO2 to reduce the recombination rate of the charge carriers, which has been recently shown to improve efficiency in DSSCs.145,146 Incorporation of Nb2O5 has been observed to improve the cell efficiency by between 22% and 35%, depending on the structure of the composite and the thickness of the Nb2O5 layer on the TiO2, where thicknesses on the order of a few nanometers are optimal. In addition to its use as a blocking
Figure 4. Photocatalytic adsorption, degradation, and reduction of total organic carbon percentages (a) and photodegradation kinetic curves (b) of RhB on P25, TiO2, CeO2−TiO2, mesoporous CeO2− TiO2, and mesoporous ZrO2−CeO2−TiO2. Reprinted from ref 105, with permission from Elsevier. Copyright 2013.
Hafnium oxide (HfO2 )−TiO 2 composites have seen extensive research recently as potential materials for applications as gate dielectrics due to the high relative permittivity (dielectric constant, κ) of HfO2 and TiO2 compared to the standard gate dielectric, SiO2. The increase of the permittivity of the gate dielectric is of considerable importance since it allows for a higher capacitance with a smaller or equal thickness as compared to SiO2. TiO2 alone should be an ideal material based on its permittivity measured in the range of κ = 80−110 as compared to SiO2, in which κ = 3.9. Unfortunately, due the presence of stable Ti3+ within a TiO2 structure, oxygen vacancies at these reduced sites act as carrier traps and highleakage paths.111 Thus, HfO2 (κ = 25) is seen as a better alternative, especially when improved κ values with reduced current leakage can be found in HfO2/TiO2 composites. Additionally, composites of HfO2/TiO2 should also improve the charge mobility, which is one other drawback to pure HfO2 9858
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a dopant in TiO2 materials. Further, Ta2O5 has also been studied as a single catalyst system for photocatalytic water splitting.153−155 2.3.2. Chromium, Molybdenum, and Tungsten. Although chromium has been recognized as one of the optimal non-noble metal dopants in TiO2 materials for enhanced visible light photoactivity,156,157 research on composites with TiO2 and chromium oxides has been demonstrated. Composites utilizing TiO2 as a support material for active chromium oxides have been studied for NOx reduction158−160 as well as photocatalytic reactions161−163 and magnetic applications.164−166 Since the discovery of large negative magnetoresistance in CrO2, it has been researched in depth; however, one significant downside of CrO2 is its poor thermodynamic stability; as such, composites with TiO2 have been studied to improve this feature. Research done to this point has shown that composites maintain much of the necessary properties of CrO2, although its stability has yet to be significantly enhanced. Oxides of both molybdenum and tungsten have seen use for applications within TiO2 composite materials. The promise in photocatalysis has drawn a considerable amount of attention to these systems, especially in the case of tungsten(VI) oxide.167−171 On the other hand, molybdenum oxides, such as MoO3, have seen less research, and in comparison to WO3/ TiO2 composites, the photocatalytic activity is commonly lower.170 As such, the focus here will be on tungsten oxides, which have been frequently utilized in composites with TiO2 for photocatalytic applications. Early studies by Do et al. and later Kwon et al. demonstrated an enhanced photocatalytic activity of WO3/TiO2 composites for oxidation of 1,4dichlorobenzene (DCB).172,173 Both of these studies utilized P25 as the TiO2 component and used an incipient wetness technique to impregnate the P25 with the tungsten oxide precursor, followed by calcinations to produce the oxide phase. It was found that the optimal WO3 concentration was 3 mol %, which led to degradation rates up to 2.5 times higher than P25 under UV irradiation. Similar studies using thin transparent films showed a similar activity enhancement in other photocatalytic degradation experiments as well.170 The improved photocatalytic activity has been attributed to both favorable band gap positions for efficient charge separation and formation of reduced W5+ species, which assist charge separation and act as reduction sites.172,174,175 Photocatalytic degradation experiments have also been extended to utilize these composite structures under visible light irradiation as well.174,176−179 The narrower band gap of WO3 (∼2.8 eV) allows for absorption of visible light photons by the composite, and accordingly, the rate of photocatalytic degradation increases markedly as compared to pure TiO2.174,176 In one study, Li et al. prepared TiO2 colloids by hydrolysis of titanium n-butoxide, followed by in situ hydrolysis of ammonium tungstate and at different concentrations.174 Samples were then calcined at 700 °C, and their photocatalytic activity was measured through the degradation of methylene blue under visible light irradiation. As shown in Figure 6a, the 3 mol % WOx/TiO2 composite showed optimal photocatalytic activity. Although the XRD pattern in Figure 6b shows no discernible tungsten oxide phase, the XPS spectrum, Figure 6c, shows a majority of tungsten appearing as W6+, which corresponds to the WO3 phase. An interesting additional result shown in this study is the effect of WO3 on retardation of the anatase to rutile phase transition. As seen in the XRD pattern of the pure TiO2, rutile phase is clearly present as the main phase, whereas in the WOx/
Figure 5. Cross-section SEM images of ordered nanotube arrays (ONTs) on different substrates: (a) pure Ti, (b) Ti:0.2 V alloy, (c) Ti:3 V alloy, and (d) Ti:18 V alloy by anodization; (e and f) highmagnification cross-section and top-view SEM images of the Ti:18 V alloy ONTs. (g) Cyclic voltammograms of Ti and Ti-V oxide nanotubes, performed over a voltage window between 0 and 0.8 V in 0.1 M HClO4 electrolyte with a scan rate of 50 mV s−1. Adapted from ref 127 with permission from The Royal Society of Chemistry.
layer, Nb has also been incorporated into TiO2 thin films and nanotubes. Although this Nb inclusion is typically as a dopant, incorporation of Nb can yield a noticeable red shift in the absorption as well as a preference for rutile phase formation upon calcination.147,148 Unfortunately, this incorporation also yields an increase in recombination centers which is not ideal and diminishes the photocatalytic properties.149 Synthesis of Ti−Nb composite nanotubes from anodization of a Ti−Nb alloy has been shown to allow for significant control on the length and diameter of composite nanotubes. These nanotubes can be synthesized with lengths between 0.5 and 8 μm and diameters of 30−120 nm by changing the potential applied. Importantly, upon calcination to 650 °C, the tubular morphology is well maintained.150 Tantalum(V) oxide, Ta2O5, composites with TiO2 have also seen some research for photocatalysis,151,152 although it is more commonly seen as 9859
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structure, followed by growth of another shell layer on the outside. After the samples were dried, they were calcined at 600 °C, and it was found that the crystallinity of TiO2 in the composite decreased with increasing WO3 content. 2.3.3. Manganese and Rhenium. TiO2 composites containing both manganese and rhenium oxides are frequently used for supported catalytic reactions, such as NOx reduction for manganese oxide183−185 and the water−gas shift reaction for metallic rhenium/rhenium oxide.186−189 More recently, however, MnO2/TiO2 composites have been developed for electrochemical applications, specifically for supercapacitors190 and Li+ batteries.191,192 MnO2 is utilized due to its high theoretical capacity193,194 (∼1230 mAh/g), and development of composites with TiO2 allows for improved device construction on a well-developed support material. Although the breadth of research to date using MnO2/TiO2 composites for electrochemical applications is limited, these materials show great promise for future growth as research on advanced energy storage technology progresses. As a cocatalyst with Pt, Re metal supported on TiO2 has shown significant enhancement in the performance of a low-temperature water−gas shift reaction. It has been demonstrated that the activity of Re is facilitated by formation of oxides. These sites can be reduced by CO to form CO2 and then reoxidized by H2O, forming H2.188 Utilization of rhenium or rhenium oxides on TiO2 has been rather limited beyond these reactions; however, due to its catalytic enhancement, it is an avenue that can still see some improvement when utilizing rational catalytic design. 2.4. Late 3d Transition Metals
2.4.1. Iron and Cobalt. Due to the instability of metallic iron on the nanoscale, it has not been commonly utilized for composites with titania. Although there have been some niche applications utilizing iron oxide/TiO2 composites,195−198 a significant amount of research over the past decade has focused on utilizing the magnetic properties of iron oxide within a composite. Composites of iron oxide@TiO2 have been extensively implemented to enable the magnetic recoverability of TiO2 from reaction media.199−206 Recently, these structures have seen considerable interest based on the significant morphological control that can be employed. Lou et al. demonstrated a direct coating of TiO2 onto α-Fe2O3 by hydrolysis of TiF4 in an ethanolic solution.201 The core@shell composites could then be converted to magnetic Fe3O4@TiO2 by reduction under H2 flow at 300 °C. The TiO2 shell within the structure showed good crystallinity, with the morphology retained after crystallization. Further, if so desired, the TiO2 composite could be made completely hollow by dissolution of the α-Fe2O3 in dilute HCl. Xuan et al. utilized poly(styreneacrylic acid) (PSA) as a template onto which they adsorbed Fe3+ ions, which were chemically converted to Fe3O4 in a solution of NH3·H2O and Na2SO3.202 The composites were then coated with TiO2 by hydrolysis of titanium(IV) n-butoxide in an ethanolic solution, and the original template was removed by dissolution in THF. The as-prepared Fe 3 O 4 @TiO 2 composites were then used for photocatalytic degradation of RhB under UV irradiation and showed good photoactivity as well as recoverability and cyclability. Later Ye et al. established a hierarchical structure where superparamagnetic Fe3O4 cores (Figure 7a) were first coated by a layer of SiO2 by a modified Stöber process (Figure 7b). This was followed by a coating with TiO2 using titanium(IV) nbutoxide in an ethanolic solution to obtain an Fe3O4@SiO2@
Figure 6. (a) Total organic carbon (TOC) removal during MB degradation. (b) XRD pattern showing evolution of crystal phase with increasing tungsten oxide content. (c) XPS fitting spectrum of 3% WOx−TiO2 powder showing W 4f: arrows indicate the presence of WO3, W6+ 4f spectrum, WxOy a mixed spectrum of W5+ and W6+, and WO2 W4+ 4f. Adapted from ref 174, with permission from Elsevier. Copyright 2001.
TiO2 composite samples only anatase phase is present. This phenomenon has been seen in other tungsten oxide/TiO2 composites,180 and moreover, other studies have seen not just an inhibition of the phase transition but a decrease in crystallinity with increasing WO3 content.181 This decrease in crystallinity was also observed in a series of core@shell TiO2@ WO3 microspheres. In this study, Yang et al. synthesized core@ shell structures by coating hydrothermally synthesized yolk@ shell TiO2 particles using an incipient wetness impregnation method.182 As the concentration of tungsten precursor increased, WO3 first filled in the cavity within the yolk@shell 9860
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as opposed to carboxylic acids especially, can selectively bind to the metal oxide surface and be removed from a mixture. One of the first of these studies by Chen et al. produced Fe3O4@TiO2 composites by preparing Fe3O4 particles by precipitation of Fe2+ and Fe3+ with an ammonia solution, followed by adding a thin layer of SiO2 and finally TiO2 from deposition of titanium(IV) n-butoxide in ethanol.15 Similar composites with good enrichment capabilities have also been synthesized.17,21 Separately, one composite of γ-Fe2O3/TiO2 synthesized by Lu et al. was a nanocrystal cluster system comprising TiO2 and γFe2O3 nanocrystals, which were combined in a “micelle” solution to form clusters.20 The clusters were then coated by SiO2, calcined, and finally etched by NaOH to obtain a dispersible, magnetically recoverable γ-Fe2O3/TiO2 composite, which is shown in Figure 8a. The XRD pattern seen in Figure 8b indicates crystal phases of both anatase TiO2 and γ-Fe2O3, and the elemental composition is further confirmed by EDX as shown in Figure 8d. Due to their high surface area, these particles had good enrichment capability, which can be seen in the MALDI-TOF mass spectrum of the enriched β-casein sample in Figure 8c. Contrary to iron oxides, less research has been conducted on cobalt oxide/TiO2 composites, although recently there has been interest in cobalt oxide/TiO2 composites for photocatalysis. CoO is a semiconductor with a band gap of ∼2.4 eV, and Co3O4 has a band gap of ∼2.19 eV. Recently, Zhang et al. synthesized TiO2 nanotubes by anodization of TiO2 foil, followed by cathodic deposition of Co(NO3)3 and calcination to 450 °C to form a CoO/TiO2 composite.207 Addition of CoO resulted in considerable improvement for the photocatalytic degradation of methyl orange (MO) under UV irradiation. The enhanced photocatalysis was attributed to the reduced recombination of photogenerated electrons with holes due to the p−n junction of the CoO/TiO2 composite. A similar method was utilized by Dai et al., although Co(NO3)2 was deposited by hydrolysis in a NaOH solution to form Co(OH)2 nanoparticles on the TiO2 nanotubes. The composite was then decomposed at 220 °C to form a Co3O4/TiO2 composite.208 Wang et al. also synthesized a Co3O4/TiO2 composite, although this was obtained via coprecipitation of TiCl4 and Co(acac)3 in benzyl alcohol, followed by calcination at 400 °C.209 Samples were then tested for UV photocatalytic hydrogen production from water with either methanol or ethanol as a sacrificial reagent. It was found that the ideal composition was 2.9 atom % Co, which resulted in a H2 production rate of 2.17 mmol/g h with 10% methanol by volume. This represents a significant improvement when compared to P25, which had a H2 production rate of 0.02 mmol/g h. Recently, more attention has been given to the fabrication of Co3O4/TiO2 composite structures for Li+-battery applications.209−211 In one such study Luo et al. synthesized TiO2 nanobelts by a hydrothermal reaction between Ti foil and NaOH, followed by ion exchange with HCl and calcination at 450 °C.211 The nanobelts were then introduced into another hydrothermal reaction with Co(NO3)2, urea, and NH4F at 120 °C for 5 h, followed by washing and calcination at 400 °C. The TiO2@Co3O4 composites were then coated with graphene oxide to complete the synthesis and showed capacities of ∼431, ∼345, and ∼204 mA h/g at 100, 200, and 800 mA/g, respectively. 2.4.2. Nickel, Copper, and Zinc. Both nickel and copper oxides have been extensively studied in composites with TiO2.
Figure 7. (Top) Scheme of as-synthesized composite structure. (Bottom) TEM images of (a) Fe3O4, (b) Fe3O4/SiO2, (c) Fe3O4/ SiO2/TiO2, and (d) calcined Fe3O4/SiO2/TiO2 particles. Insets in c and d are magnified images of portions of the composite particles showing the morphological change in the TiO2 shell due to calcination. Adapted with permission from ref 203. Copyright 2010 John Wiley and Sons.
TiO2 composite as shown in Figure 7c.203 Finally, the composite was calcined at 500 °C to crystallize the TiO2 shell. As shown in Figure 7d, the as-calcined structures are uniform and have well-defined morphologies with anatase crystalline TiO2 on the surface. The composites also showed good photocatalytic activity for degradation of RhB under UV irradiation which exceeded that of P25. Further, the magnetic recoverability and good cyclability allowed for a catalyst which showed little drop in efficiency over 18 cycles. The synthetic control shown for these systems indicates that other features could be added as desired, such as noble metal nanoparticles for further enhanced photocatalytic activity. A separate application for magnetic iron oxide/TiO 2 composites that has shown promise in recent years is magnetically recoverable TiO2 for phosphopeptide enrichment.15,17,19−24 The field of metal oxide affinity chromatography (MOAC) has seen an increase in research partially due to magnetically recoverable composites, which allow for easy separation of phosphopeptides from a complex mixture.18 MOAC relies on the affinity of phosphorylated peptides for metals in metal oxides. The Lewis base phosphate group can bind in a bidentate fashion to the Lewis acid metal oxides, and when the pH and solvent are optimized, the phosphate group, 9861
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Figure 8. (a) TEM images of γ-Fe2O3/TiO2 multiple-component clusters (red circles are used to highlight γ-Fe2O3 nanocrystals) and corresponding primary γ-Fe2O3 nanocrystals (inset). (b) XRD pattern of composite sample ((*) anatase-phase TiO2, (#) maghemite-phase γ-Fe2O3). (c) MALDI mass spectra of the tryptic digest of β-casein (100 μL, 10−8 M) after enrichment. Insets are photos of the composite sample dispersed in water before and after exposure to external magnetic fields. (d) EDX analysis of the as-synthesized composite sample. Reprinted with permission from ref 20. Copyright 2010 American Chemical Society.
for pseudocapacitive properties,232 and Wang et al. utilized NiO/TiO2 nanowire arrays to fabricate stable (to 600 cycles) composites with a high areal capacity.233 While these studies can still benefit from additional optimization, they have shown promise for potential pseudocapacitor applications. Copper oxides have been heavily researched as composites with TiO2 recently due to their inherent p-type configuration. The band gaps of CuO and Cu2O are ∼1.4 and ∼2.2 eV, respectively, which makes both materials promising for research in the conversion of solar radiation.35,234,235 Most composites featuring copper oxides have shown a considerable dependence on the copper oxide loading percent in order to create optimal catalysts. The photocatalytic degradation of organic molecules over Cu2O/TiO2 composites has been of particular interest, especially to extend the activity of TiO2 to visible wavelengths. In addition to visible light excitation, formation of a type II heterojunction between the n-type TiO2 and p-type Cu2O leads to improved charge separation where photoexcited electrons are passed from Cu2O to TiO2 and holes remain localized in Cu2O. Initial studies utilizing composites of Cu2O/TiO2 showed that deposition of Cu2O onto P25 enhanced both the UV and the visible light degradation of dyes as compared to bare P25.236−239 Recently, other architectures have been considered as well, including Cu2O deposited on TiO2 nanowires, Cu2O on TiO2 nanosheets, and Cu2O@TiO2 core@shell structures, all of which showed improved organic molecule degradation under visible light when compared to pure TiO2.240−242 Chu et al. prepared Cu2O@TiO2 core−shell heterojunction composites by precipitation of Cu2O from CuCl, followed by direct coating with titanium(IV) nbutoxide.241 Samples were subsequently calcined at 400 °C
Nickel supported on TiO2 has been of interest for decades for a wide range of catalytic reactions such as hydrogenation,212−214 reforming,215 as well as others.216−218 More recently, however, nickel oxide and hydroxide composites have been utilized in p− n junction nickel oxide or hydroxide/TiO2 catalysts for photocatalytic organic molecule degradation,219−223 hydrogen production,224−226 and DSSCs.227−230 In one recent report by Lin et al., TiO2 nanobelts were produced by hydrothermal synthesis using P25 and NaOH, followed by treatment with HCl to form protonated H2Ti3O7 nanobelts.222 The nanobelts could subsequently be coarsened by hydrothermal treatment in H2SO4. NiO was deposited on the nanobelts by wet impregnation with Ni(NO3)2, followed by calcination at 600 °C. The products of each step can be seen in the SEM images in Figure 9a−d, which clearly show both the coarsened nanobelts (Figure 9c) and the NiO deposited on the nanobelts (Figure 9d). The activity of the NiO/TiO2 composite nanobelts was then tested by both UV and visible light degradation of methyl orange (MO), as shown in Figure 9e and 9f. It is apparent that addition NiO, as well as the coarsening of the nanobelts, greatly improves the photocatalytic activity. Yu et al. showed impressive results when making a simple mixture of Ni(OH)2 deposited on P25.224 At an optimized 0.5 wt %, the evolution of H2 under UV irradiation was 3.056 mmol/h g in a 25 vol % aqueous methanol solution. DSSCs fabricated from deposition of NiO onto TiO2 have been studied recently, and utilization of NiO produced up to a 16% increase in cell efficiency when compared to cells without NiO.229 Additional studies on nickel/TiO2 composites have been done with applications in batteries.231−233 Kim et al. recently synthesized NiO/TiO2 nanotube arrays which were measured 9862
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Figure 10. (a and b) TEM images of Cu2O/TiO2 nanosheets, (c) degradation, and (d) kinetic curves of phenol degradation catalyzed by pure TiO2 nanosheets and Cu2O/TiO2 nanosheets under visible light. Adapted from ref 242 with permission from The Royal Society of Chemistry.
production.245−249 One study by Lalitha et al. showed significant H2 production from Cu2O-decorated P25 using a 5 vol % glycerol:water solution while under simulated solar irradiation.247 These composite structures have exhibited the significant advantage of Cu2O/TiO2 composites, although they still suffer the inherent problem of the thermodynamic instability of copper(I) oxide. Copper(II) oxide (CuO), which is the more thermodynamically stable oxide of copper under typical conditions, has also been utilized in composites with TiO2 as a photocatalyst and for production of hydrogen.235,250−252 A promising recent use was reported by Mor et al., who developed a p−n-junction diode from TiO2 and CuO/TiO2 nanorod arrays for photoelectrochemical water splitting,235 as shown in the scheme in Figure 11a. TiO2 and CuO/TiO2 nanotubes were synthesized from anodization of Ti and Ti−Cu films on FTO, respectively. Nanotube arrays are shown in Figure 11b and 11c, which consisted of ∼74% Cu−26% Ti and showed lengths of ∼1000 nm, pore diameters of ∼65 nm, and wall thicknesses of ∼35 nm. The length could be decreased by increasing the Ti content, resulting in a sample of 60% Cu−40% Ti with similar pore sizes and wall thicknesses but a length of only ∼850 nm. The CuO/TiO2 nanotube arrays could then be calcined to improve the crystallinity. When the diode was placed under AM1.5 illumination incident on the TiO2 side, the TiO2 absorbed the UV irradiation and allowed visible light to penetrate to the CuO/TiO2 side, where it was likewise absorbed. Since the majority of UV light was absorbed by the pure phase TiO2, photocorrosion of CuO/TiO2 was minimal after 5 h of operation in 0.1 M Na2HPO4. The overall conversion efficiency of a water:methanol:diethylamine (5:5:2) solution was found to be 0.48% and might be improved with further optimization. In addition to copper oxides, metallic copper has also seen some utilization for catalysis.253−257 Since metallic copper has similar properties to silver and gold, it can be photodeposited onto a substrate and utilized as a cheaper alternative. These composites to this point have seen use for CO oxidation, H2 production, and dye degradation and as bactericides. However, as the activity of gold catalysts is typically much higher, use of metallic copper has not been widespread as of yet.
Figure 9. SEM images of (a) TiO2 nanobelts (NBs), (b) NiO−NP/ TiO2 NBs, (c) surface-coarsened TiO2 NBs, and (d) NiO−NP/TiO2 coarsened NBs. Degradation of MO by (e) UV and (f) visible light where catalysts are (a) pristine TiO2 NBs, (b) NiO−NP/TiO2 NBs, (c) surface-coarsened TiO2 NBs, (d) NiO−NP/TiO2 coarsened NBs, (e) pure NiO nanoparticles, (f) and P-25. Adapted from ref 222 with permission from The Royal Society of Chemistry.
under N2 protection to avoid oxidation of the copper(I). The samples showed a significant improvement over the as-prepared pure TiO2 and P25 for photocatalytic degradation of 4nitrophenol under visible light irradiation. It is important to note that the photocatalytic activity of the composite is much greater than pure TiO2, despite the fact that adsorbed 4nitrophenol will sensitize TiO2 and improve visible light photocatalytic activity.243 It is unclear what effect this may have played on the composite structure; however, it is presumed to be a minor contributor as compared to the advantages of the heterojunction created by the composite. A separate report on the synthesis of core@shell Cu2O@TiO2 composites showed improved morphological control as well. Su et al. demonstrated that Cu2O octahedra, which were individually coated with thin layers of TiO2, could show improved surface voltage, although photocatalytic experiments were not carried out.244 One study by Liu et al. utilized Cu2O loaded on TiO2 nanosheets with exposed (001) facets.242 Figure 10a and 10b contains TEM images of ∼2 nm Cu2O loaded on the TiO2 nanosheets with a ∼3 atom % Cu loading. Figure 10c and 10d shows the degradation profile and rate constant plot for visible light degradation of phenol with the composite structures. It is evident that Cu2O/TiO2 composites with an optimal Cu loading show an increase in activity of about 12 times compared to the undecorated nanosheets and about 3 times greater than nitrogen-doped nanosheets (N−TiO2 NS). In addition to the degradation of organic compounds, Cu2O/TiO2 composites have also been demonstrated for photocatalytic hydrogen 9863
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Figure 11. (a) Illustration of photoelectrochemical diode for water splitting comprised of n-type TiO2 and p-type Cu−Ti−O nanotube array films, (b) lateral and (c) top view FESEM images of a Cu−Ti−O nanotube array sample. Adapted with permission from ref 235. Copyright 2008 American Chemical Society.
Figure 12. TEM images of (a) PS/ZnO−TiO2 composite particles and (b) ZnO−TiO2 hollow spheres. Electron energy loss (c) “Zn” and (d) “Ti” element mapping images of ZnO−TiO2 hollow spheres. Reprinted with permission from ref 259. Copyright 2009 American Chemical Society.
The definition of zinc as a transition metal is arbitrary here, as it is more accurately described as a “post” transition metal based on its available oxidation states having filled d orbitals; however, it can still be categorized as a 3d-block metal.258 Nonetheless, ZnO/TiO2 composites have been utilized for photocatalysis,259−264 hydrogen production,265,266 and construction of solid state DSSCs.267−273 Agrawal et al. developed a hollow ZnO@TiO2 void@shell@shell structure with good morphology control and enhanced photocatalytic activity.259 The microspheres, as shown in Figure 12a and 12b, were made by coating polystyrene beads with ZnO, followed by a subsequent coating through hydrolysis of titanium(IV) ethoxide. The microspheres were then rendered hollow by calcination in air at 550 °C. XRD shows a majority anatase phase with some rutile phase and minor ZnO peaks, indicating good crystallinity after calcination. The composite structures showed a significant improvement for degradation of RhB under UV irradiation as compared to hollow ZnO or TiO2 alone. Since both oxides have similar band gaps (∼3.37 for ZnO and 3.2 for TiO2) and absorb in the UV portion of the electromagnetic spectrum, the improved photocatalytic activity of ZnO/TiO2 composites is attributed to more efficient charge separation due to the favorable band positions of ZnO and TiO2. When ZnO is irradiated to produce photogenerated charge carriers, the electrons remain in the ZnO while the holes transfer to TiO2. This charge separation can improve the photocatalytic efficiency by reducing the recombination rate of the charge carriers.261 In addition to improved photocatalytic performance, ZnO/ TiO2 composites have been prepared for use as DSSCs. One early study by Wang et al. showed that additional coprecipitation of ZnO and TiO2 onto a TiO2-coated FTO substrate resulted in nearly a 27% improvement in efficiency when the cell was constructed.267 This was attributed to improved electron transport by ZnO as well as a decrease in
dark current generation. Since ZnO has a greater electron mobility than TiO2, Park et al. constructed DSSCs from ZnO particulates and coated them with a TiO2 layer, followed by calcination at 400 °C.269 The TiO2 overlayer led to an increase in cell efficiency from 5.2% to 6.3% or an increase of over 20%, which was later increased further to 30% upon optimization of the calcination conditions.270 2.5. p-Block Metals
Many oxides of the p-block metals have been utilized in composites with TiO2 for a variety of applications. Of these metal oxides, alumina, or Al2O3, is likely the one most frequently integrated with TiO2 due to the resultant properties when both are combined as a catalyst support, e.g., tunable acidity/basicity. Since there are few other applications which benefit from the combination of TiO2 and Al2O3, these composite supports will not be further reviewed. Additionally, those elements which are colloquially referred to as “metalloids” (Si, Ge, As, Sb, and Te) will also not be specifically discussed here. Gallium oxide has been scarcely researched for TiO2 composites,74,274−278 much of which utilizes TiO2 as a support. One recent application demonstrated by Chandiran et al. was the deposition of Ga2O3 atop a TiO2 layer within a DSSC to act as a tunneling layer; the result was a reduction of the charge carrier recombination rate by nearly 2 orders of magnitude.278 After four Ga2O3 atomic layer deposition cycles, open circuit voltages increased from 692 mV to an impressive 1100 mV, with corresponding increases in Jsc from 3.6 to 5.1 mA/cm2 and cell efficiency from 1.4% to 4.0%. This recent development will likely pave the way for additional research on Ga2O3/TiO2 composites for DSSC applications. As with gallium oxide, indium oxide−TiO2 composites have not been considerably researched. Most of the composites that have been developed have been used for photocatalytic 9864
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degradation of organic molecules.279−284 Addition of In2O3, which has a band gap of ∼2.5 eV, allows for visible light absorption of In2O3/TiO2 composites. Further, since the conduction band of In2O3 (−0.63 V versus NHE) is offset higher than that of TiO2 (−0.4 V versus NHE), an efficient heterostructure can be formed which supports efficient separation of photogenerated charges.282,283 These beneficial attributes ensure that research on In2O3/TiO2 composites is likely to increase in the future. Tin(IV) oxide, SnO2, has seen considerable usage in composites with TiO 2 for photocatalysis, 2 8 5 − 2 9 2 DSSCs,293−296 and battery applications.297−302 In one recent study by Xu et al., N-doped TiO2 was synthesized, followed by addition of SnO2 of different compositions and calcination at 400 °C.291 The SnO2/N−TiO2 composites showed superior performance for degradation of RhB under visible light irradiation as shown in Figure 13a. Interestingly, the N−TiO2
capacity. The large theoretical specific capacity for SnO2 (∼790 mAh/g) as compared to TiO2 (∼170 mAh/g), combined with the better structural and chemical stability of TiO2, suggests a promising candidate electrode material.303 Jeun et al. synthesized hollow SnO2@TiO2 void@shell@shell nanotubes which show Li+ capacities higher than TiO2, with superior stability to pure SnO2.302 As shown in the scheme in Figure 14a, the composite was prepared via plasma-enhanced atomic
Figure 14. (a) Schematic diagram of the fabrication procedures and schematic illustration of Li-ion insertion/extraction in SnO2@TiO2 double-shell nanotubes. (b and c) Cyclability of SnO2 and SnO2@ TiO2 nanotube electrodes at 800 and 1500 mA/g, respectively. Adapted from ref 302 with permission from The Royal Society of Chemistry.
Figure 13. (a) Rates of RhB degradation mediated by TiO2, SnO2, nitrogen-doped TiO2, TiO2/SnO2 with different Ti/Sn mole ratios, and nitrogen-doped TiO2/SnO2 with different mole ratios under visible light (λ > 400 nm) irradiation. (b) Schematic of the anticipated charge migration and separation on N−TiO2/SnO2 composite photocatalysts under visible light irradiation. Adapted with permission from ref 291. Copyright 2012 American Chemical Society.
layer deposition of SnO2 and then TiO2 on polyacrylonitrile electrospun nanofibers. The composites were then calcined at 700 °C to burn off the organic core and leave a hollow cavity, which provided space for expansion of the composite upon Li+ insertion. Figure 14b and 14c shows the stability of the discharge capacity of the composite structures. After the initial first cycle capacity loss and 50 subsequent cycles, the reversible capacities of the SnO2@TiO2 composites were 300 and 200 mA h/g at current densities of 800 and 1500 mA/g, respectively. This compares favorably to the case of pure SnO2 nanotubes,
samples showed better performance under UV irradiation. A proposed mechanism is shown in Figure 13b. It was asserted that RhB dye sensitization of the N−TiO2 catalyst, followed by improved charge separation and migration on the composite structure, was responsible for the improved photocatalytic activity in comparison to the N-doped TiO2 alone. SnO2/TiO2 composites have also been used as electrode materials for a lithium ion battery anode to enhance the 9865
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photoactivity of a Rh/TiO2 composite to enhance the reduction of CO2 to CO in the reverse water−gas shift reaction (reverse of eq 1).341,342 The reaction conditions, especially the Rh loading and metallic nature, were controlled to ensure that the photocatalytic reduction was at its highest when Rh appeared in a metallic state. Further, the loading amount of Rh was optimized in order to prevent reduction of CO to CH4 in a reverse of eq 2, which occurred at high Rh loading. The Rh/TiO2 composite helps facilitate reduction through formation of formate ions from the reaction of CO2 and H2 which occurs on the partially reduced Rh surface. It is worth noting, however, that long irradiation time will decrease catalytic performance as reduction of Rh to metallic states will increase dissociation of H2 and thus favor production of CH4 over CO. Ru and Rh/TiO2 composites have also been applied to the photocatalytic degradation of organic molecules.343−347 These composites showed little improvement on their own when compared to TiO 2 ; however, with addition of cocatalysts347 or dye sensitization,345 the photocatalytic efficiency increased markedly.
which showed a similar initial capacity but continuously decreased as the cycling continued. Bismuth oxide, Bi2O3, composites with TiO2 have seen little research. However, some recent work has been done for applications in the photocatalytic degradation of organic molecules.237,304−307 Bi2O3/TiO2 composites have shown improved photocatalytic degradation rates relative to pure TiO2, especially when under visible light irradiation. 2.6. Lanthanides
Typically, metals from the lanthanide series are used as dopants for TiO2 nanomaterials, and as such most research on TiO2− lanthanide composites is limited to a few applications. TiO2 composites with La2O3 have been investigated the most as supports for catalysis in reactions such as NOx reduction.308−310 There are also some reports covering TiO2 composites with Er2O3,311,312 Nd2O3,313 and Eu2O3;314,315 however, the most frequently utilized oxide from the lanthanide series is cerium oxide, CeO2. Recently, CeO2/TiO2 composites have attracted significant attention as a means to improve both the UV and the visible photocatalytic degradation of organic molecules.316−322 CeO2/TiO2 composites have been shown to have a higher porosity and surface area compared to pure TiO2 samples, which has proven to be beneficial in photocatalytic degradation.318,322 Further TiO2 has been shown to promote and stabilize the Ce3+ oxidation state, leading to enhancement of chemical activity. These Ce3+ centers and corresponding oxygen vacancies within the lattice of mixed CeOx/TiO2 composites act as hole acceptors which reduce recombination of charge carriers.316,323,324 Additionally, CeOx/TiO2 composites have shown the ability to enhance the activity of metal nanoparticles (Cu, Au, Pt) for the water−gas shift reaction.325,326 It is believed that the Ce3+ ions with improved stability in the composites effectively bind and dissociate water on the oxide surface. In the presence of a metal nanoparticle, OH bound to the oxide and CO bound to the metal form a HO−CO intermediate to H2 and CO2 gas products. In this case inclusion of each component is necessary in order to best improve the reactivity.
CH4(g) + H 2O(g) → CO(g) + 3H 2(g) 3.2. Palladium
Much like the later noble metal nanoparticles, palladium nanoparticle composites with TiO2 have seen an extensive amount of research in recent years. Utilization of Pd/TiO2 composites has provided improved materials for methanol reforming,348−351 hydrogenation,352,353 photocatalysis,354−359 and H2 production.360−364 Methanol reforming has shown considerable potential as a source of hydrogen from hydrocarbons, especially when compared to the energy-intensive syngas method for hydrogen production. Al-Mazroai et al. showed that Pd deposited on P25 via an incipient wetness method showed considerable activity for methanol reformation under visible light irradiation.350 This room-temperature synthesis is much less energy intensive than syngas formation, which occurs at temperatures up to 1000 °C. The enhancement of the photocatalytic activity of TiO2 through addition of Pd nanoparticles is evident by the numerous reports detailing improved dye degradation capabilities of the composites. One of the simplest methods for preparing Pd/TiO2 composites is photodeposition of Pd nanoparticles on P25. Iliev et al. utilized this method to synthesize a Pd/P25 composite with 0.5 and 1.0 atom % Pd concentration.354 These composites showed improvement over the blank P25 samples for degradation of xylenol orange under UV irradiation. The extension of Pd/TiO2 composites for degradation under visible light irradiation has been carried out as well. Mohapatra et al. synthesized TiO2 nanowires from anodization of Ti foil, followed by functionalization with PdCl2 via an incipient wetness method and subsequent calcination under H2/Ar atmosphere to crystallize the TiO2 and convert the Pd salt to metallic Pd.355 The as-synthesized composite with an optimized 1.25 wt % Pd showed considerable photocatalytic improvement compared to the bare TiO2 nanotubes. In addition to dye degradation, photocatalytic H2 production has also been carried on Pd/TiO2 composites. Early reports from Fujishima and Honda demonstrated that Pd on P25 can split water under UV irradiation, with activity near that of platinum.365 Since this report, considerable effort has been devoted to improving the activity and extending these
3. NOBLE METAL AND METAL OXIDE−TIO2 COMPOSITES 3.1. Ruthenium and Rhodium
In early research on TiO2 photocatalysis, ruthenium(IV) oxide (RuO2) was heavily investigated as a cocatalyst for generation of O2 during photolysis of water.327−332 More recently, composites consisting of either ruthenium or rhodium with TiO2 have seen some more classic catalytic uses to date with little recent research on the topic of photocatalysis. One classic type of catalytic reaction is the use of TiO2 as a support for Ruor Rh-mediated hydrogenation.333,334 Another such reaction which has seen research utilizing either Ru or Rh/TiO2 composites is the water−gas shift reaction.335,336 In these composites, TiO2 acts as a support for the active metal in the conversion of CO to CO2 and H2 as shown in eq 1. CO(g) + H 2O(g) → CO2(g) + H 2(g)
(2)
(1)
TiO2 also acts as a support for Ru and Rh in the catalytic oxidation of methanol for formation of syngas by the reaction shown in eq 2.337−340 These applications of Ru and Rh/TiO2 composites have been well developed; however, they are also of less use when compared to other catalysts utilizing either Pt as the active metal or CeO2 as the support. One study utilized the 9866
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To maximize the efficiency of the Pd/TiO2 catalysts, considerable effort has been made to rationally design and optimize the morphology of the composite. Recent attention has been especially given to the synthesis of Pd@TiO2 core@ shell catalysts.366−368 In a recent study from Zhang et al., Pd@ TiO2 core@shell particles were synthesized by first reducing the palladium precursor salt to form nanoparticles, followed by addition of a TiF4 solution and hydrothermal treatment at 180 °C for 48 h.367 SEM and TEM images of the resulting products are shown in Figure 16a and 16b, respectively, which clearly show the core@shell structure. The same procedure was utilized for synthesis of Au@TiO2 and Pt@TiO2. The resulting core@shell particles showed a high degree of crystallinity, as evidence by the XRD pattern in Figure 16c. Further, the Pd@ TiO2 composites showed the highest activity for visible light degradation of RhB as shown in Figure 16d. This result shows
composites to visible light activity. Sayed et al. recently prepared a nitrogen-doped visible light active Pd/TiO 2 composite.362 This composite showed improved H2 production by incorporation of N doping as compared to a sample with only Pd deposition. Ye et al. prepared TiO2 nanotube arrays by three-step anodization of Ti foil followed by calcination and hydrothermal reduction of Pd nanoparticles onto the crystalline TiO2 nanotubes in the presence of polyvinylpyrrolidone (PVP) and NaI.364 The size of the loading could be tuned via controlling both the PVP concentration and the hydrothermal reaction time. Figure 15a and 15b shows the SEM image of the as-
Figure 15. (a) Top and (b) cross-sectional SEM images of TiO2 nanotube arrays (TNTAs) obtained from electrochemical anodization. SEM images of Pd QDs deposited on TNTAs: (c) top view at low and high (inset) magnifications, and (d) cross-sectional view (high magnification of a broken tube is shown in the inset). (e) TEM image of TiO2 nanotubes with Pd QDs deposited showing that they were uniformly dispersed on the nanotube. (f) Amount of hydrogen generated by TiO2 nanotubes and Pd@TNTAs nanocomposites as photoanodes and Pt foil and Pd@TNTA nanocomposites as cathodes. Pd% = 2.15 wt%. PE and CE indicate photoanode electrode and cathode electrode, respectively. Adapted with permission from ref 364. Copyright 2012 American Chemical Society.
prepared nanotube array, and Figure 15c and 15d shows the array after hydrothermal Pd deposition. Figure 15e shows a TEM image of a nanotube with Pd nanoparticles clearly deposited. Production of H2 was carried out through fabrication of a three-electrode photoelectrochemical cell (PEC). H2 production is shown in Figure 15f, where the optimal catalyst utilized a Pd loading of 2.15 wt %, and the optimal PEC was constructed with the Pd/TiO2 composite as both the photoanode and the cathode.
Figure 16. SEM (a) and TEM (b) of the as-prepared Pd@TiO2 core@ shell composite. (c) XRD patterns of the as-prepared M@TiO2 (M = Au, Pd, Pt) core−shell nanocomposites; peaks indicated by dashed lines, and symbols are ascribed to the anatase phase of TiO2 and the corresponding noble metal. (d) Performances of M@TiO2 (M = Au, Pd, Pt) core−shell nanocomposites and commercial TiO2 P25 for photocatalytic degradation of RhB under irradiation of visible light (λ > 400 nm). Adapted with permission from ref 367. Copyright 2011 American Chemical Society. 9867
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reforming and thus hydrogen production.380,381 As the standard reduction potentials of methanol and ethanol, 0.13 and 0.224 V, respectively, are lower than the standard reduction potential of oxygen to water, 1.23 V, use of alcohols is more favorable for the hydrogen-yielding oxidation as well as prevention of back reactions. This has resulted in a large number of Pt/TiO2 hydrogen production catalysts utilizing alcohols as the main reactant (reforming)382−387 or as the sacrificial reagent in an aqueous solution.388−394 Further, selection of alcohols has been extended to higher alcohols such as isopropanol and polyols such as glycerol in order to increase the number of moles of hydrogen produced in a reaction system. In addition to the extensive research done on hydrogen production from Pt/TiO2 composites, oxidation/degradation of organic pollutants has seen a significant amount of research as well, especially since it can more easily demonstrate the photoactivity of the catalyst.395−402 The improvement of photocatalytic activity with Pt incorporation is typically ascribed to formation of a Schottky barrier at the metal−TiO2 interface. This occurs because the work function of Pt (∼5.36−5.63 eV)403 is greater than that of TiO2 (∼4.6−4.7 eV);403,404 so, the electrons transfer to the Pt and the holes are localized within the TiO2, thus effectively separating the charge carriers to improve the photocatalytic efficiency. To enhance the activity of Pt/TiO2 composites, steps have been taken to optimize the interaction between the Pt and the TiO2. In one study by Kandiel et al. they were able to effectively tune the surface area and crystallinity of the TiO2 within their prepared Pt/TiO2 composite and demonstrate that although a high surface area is beneficial an increase in crystallinity is preferential.405 The increase in photocatalytic activity is attributed to the decrease in defect sites, which act as centers for charge recombination, when the crystallinity increases. Yu et al. recently showed the improvement of photocatalytic water splitting when using Pt on TiO2 nanosheets with exposed (001) facets as the catalysts.406 The nanosheets, as shown in Figure 17a and 17b, were prepared via a hydrothermal method with titanium(IV) n-butoxide and hydrofluoric acid and had previously showed improved photocatalytic activity by testing dye degradation as compared to P25.407 The HRTEM image in Figure 17b clearly indicates the lattice spacing parallel to the top and bottom facets is ∼0.235 nm, corresponding to the (001) plane of anatase TiO2. Pt nanoparticles were then photodeposited onto the nanosheets, as shown in Figure 17c and 17d. When Pt nanoparticles were incorporated, photocatalytic hydrogen production from an aqueous ethanol solution increased as compared to both the nonplatinized sheets with exposed (001) facets and the platinized nonfaceted particles. Also of note, the fluoridated surface increased the activity compared to the nonfluoridated one as well. This was attributed to the ability of fluoride surface ions to trap electrons, which would slow recombination of the charge carriers and then transfer them to the Pt surface. It is evident from these studies that further optimization of TiO2 can greatly benefit the photocatalytic activity of Pt/TiO2 composites in the future. One further phenomenon that has been found in Pt/TiO2 composites is a red shift of the absorption spectra. Although not frequently reported, it has nonetheless been apparent in a number of studies.397,408−410 This red shift has been attributed to unreduced Pt species, as Pt(OH)2 or PtO2, on the surface of the TiO2 producing lower energy levels on the surface and
both the utility of Pd/TiO2 composites and the potential as core@shell catalysts. 3.3. Platinum
Since the initial reports of hydrogen evolution from water by Honda and Fujishima, platinum has been intertwined with TiO2, even as a just a simple electrode. Kiwi and Grätzel first demonstrated the promotion of hydrogen evolution from water utilizing poly(vinyl alcohol)-stabilized Pt colloids.369 Later Kiwi et al. demonstrated the visible light photolysis of water to produce H2 and O2 on a composite consisting of Pt/RuO2/ TiO2,327 which was followed by subsequent studies from the same group.328−331 From these studies as well as others at the time, photocatalytic production of H2 by Pt/TiO2 composites has consistently shown an increasing amount of research.365,370,371 Additionally, other researchers were utilizing Pt/TiO2 composites as well for other photocatalytic reactions. Kraeutler and Bard showed that a Pt/TiO2 composite can catalyze the decomposition of acetic acid selectively to methane under UV illumination.372 Further work from Bard showed significant development of Pt/TiO2 for a number of reactions such as decarboxylation, radical-induced synthesis of amino acids, and decomposition of organic molecules.373−375 These studies, among others, cemented the role of Pt in TiO2 composites. Recently, with the advances in nanoscale synthesis and controllable/tunable properties of nanomaterials, interest in Pt/TiO2 composites has only increased. Synthetic controls have allowed for optimization of parameters such as morphology, crystal phase, crystallinity, porosity, and surface area, each of which can alter the photoactivity of a composite. Other optimization of the photocatalytic system, such as use and type of sacrificial agents, has seen a considerable amount of research as well. Although Pt/TiO2 composites have been frequently utilized for production of hydrogen from water, there is a significant drawback that must be considered. As reported by Sato et al., a Pt/TiO2 composite (as well as Pd/TiO2 and Rh/TiO2) not only catalyzes production of H2 and O2 from water but also catalyzes the back reaction as well.376 As such, effective water splitting was not possible. Sayama and Arakawa later showed that addition of an aqueous solution of sodium carbonate to a reaction system containing Pt/TiO2 effectively suppresses the back reaction and therefore promotes formation of both H2 and O2.377,378 The role of the carbonate was proposed to be both a preferential adsorbate as compared to O2 and a hole (h+) scavenger. The carbonate species as hole traps could then form peroxocabonates which could irreversibly decompose to CO2 and O2. This production of O2 does not need to be on the catalyst surface, and as such, it is less likely to quickly react with H2 to form water. Iodide anions have also been found to be advantageous for production of hydrogen and oxygen.379 It was found that addition of iodide can bind to the Pt surface and form an iodine layer which can also suppress recombination of H2 and O2 at the Pt surface. It must be noted though that addition of too much of either anion will reduce the beneficial aspects as too much will bind to the Pt surface and leave too few reactive sites for hydrogen reduction. More recently, however, the focus of reaction systems has been on hydrogen production rather than oxygen production, and thus, production of hydrogen has been supplemented by addition of alcohols such as methanol or as a sacrificial hole acceptor. Early work by Kawai and Sakata showed the efficacy of a Pt/ TiO2 composite for photocatalytic methanol and ethanol 9868
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layer. It was shown that when the SiO2 layer was thinner the photocatalytic degradation of methylene blue increased, even without any TiO2 contact. In another study, Standridge et al. demonstrated the effect of the distance between silver nanoparticles and a dye on the photocurrent of a DSSC.417 The system was composed of Ag nanoparticles coated with varying thicknesses of TiO2 separating the particles from the dye layer. As shown in Figure 18b, the thinnest TiO2 coatings
Figure 17. (a) TEM and (b) HRTEM of prepared TiO2 nanosheets. (c) TEM and (d) HRTEM of nanosheets clearly showing Pt deposits. Reprinted with permission from ref 406. Copyright 2010 American Chemical Society.
resulting in visible light absorption. This may be useful for some visible light photocatalysis; however, it implies that the stability of these catalysts is likely limited since further photoexcitation of the TiO2 will complete the reduction of the Pt species to metallic platinum and thus likely eliminate the red shift produced. It is further possible that a small amount of doping may occur, as this can lead to a red shift in the absorption as well by producing sub energy levels between the valence band and the conduction band of TiO2.411 Production of a red shift via this mechanism will be much more stable and allow for improved visible light activity in Pt/TiO2 composite structures.
Figure 18. IPCEs and overall cell trends. (a) IPCEs for cells with 125 cycles of amorphous TiO2. (b) IPCEs for cells with silver NPs and dye. Arrow indicates increasing TiO2 thickness. Top four spectra and bottom four spectra correspond to amorphous and anatase TiO2, respectively. (c) Efficiencies of the cells. Dark blue, orange, and green symbols correspond to anatase samples. Light blue, red, and yellow symbols correspond to amorphous samples. (d) Calculated plasmonenhancement factor as a function of TiO2 thickness. Reprinted with permission from ref 417. Copyright 2009 American Chemical Society.
yielded the greatest enhancements in the incident photon conversion efficiencies (IPCEs) due to the larger electromagnetic field imparted on the dye by the LSPR of the silver nanoparticles. This in turn led to greater cell efficiencies as shown in Figure 18c. In order to better examine the effects of the metal−TiO2 interaction in a Ag/TiO2 composite, core@shell Ag@TiO2 structures have been synthesized. Initial core@shell composites were produced for drug delivery systems by Liz-Marzán et al.,440,441 followed by others;420,442−445 however, core@shell Ag@TiO2 composites made specifically for studying the interaction were first synthesized and studied by Hirakawa and Kamat.446,447 Hirakawa and Kamat showed a reversible charging and discharging of the Ag core by photogenerated electrons from the TiO2 shell. When the composite was irradiated, electrons moved to the Ag core where they were trapped as the holes generated in the TiO2 were scavenged by ethanol. As electrons were captured by the Ag core, the surface plasmon resonance band would subsequently blue shift up to 30 nm, as shown in Figure 19a. Irradiation of the system would then be discontinued, and once an electron acceptor was added to the system, the electrons would be discharged by the Ag core and the SPR band would red shift nearly back to its initial position. This serves to indicate the propensity of a metal, based on the work function in relation to TiO2, to capture photogenerated electrons from TiO2 rather than to inject them via LSPR.
3.4. Silver
Due to its low cost, as compared to other noble metals, intense localized surface plasmon resonance (LSPR), and easy shape control, silver nanomaterials have been utilized in composites with TiO2 to a significant extent. Silver has been utilized widely with TiO2 in order to make composites for photocatalytic organic molecule degradation,199,412−415 DSSCs,416−423 photoactive bactericides,424−430 photochromic materials,431−437 and other applications. Although silver cannot recombine H+ atoms for hydrogen production, based on its slightly larger work function, ∼4.7 and 4.6 eV for Ag and TiO2, respectively,413,438 it still has the capability of attracting photogenerated electrons from TiO2 and thus improving charge separation. As such, many initial reports showed the improved UV photocatalytic degradation of organic pollutants by Ag/TiO2 composites as compared to bare TiO2. Although the exact nature of the LSPR effect on enhanced photocatalytic activity is not entirely understood, it is clear that it improves the photocatalytic activity of TiO2 and generation of photoelectrochemical current. In one study, Awazu et al. demonstrated that direct contact between Ag and TiO2 is not necessary for photocatalytic enhancement, suggesting that the increase in the electromagnetic field by the Ag LSPR was the cause.439 In their study the silver nanoparticles were embedded in SiO2 layers with different thicknesses, followed by a TiO2 9869
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are comparable to or better than Pt/TiO2 composites for hydrogen production when under visible light irradiation because of the LSPR effect of gold in visible wavelengths. In addition to applications for organic molecule degradation and hydrogen production, Au/TiO2 composites have been heavily used for CO oxidation, which is of environmental importance due to its release from fossil fuel burning in internal combustion engines. Au/TiO2 composites have been shown to effectively convert CO to CO2, even at temperatures below 0 °C.449,450 Additionally, a variety of factors can affect the activity of Au/TiO2 composites for CO oxidation, most notably the size of the Au particles, but also crystallinity of TiO2 and preparation method of the composite.451−454 Deactivation of the composite due to sintering of Au particles is particularly important because for practical applications such as catalytic converters the composite would be subject to high temperatures (>750 °C). To prevent this deactivation, one recent study by Lee et al. used a yolk@shell Au@TiO2 catalyst which can effectively prevent sintering of Au nanoparticles by placing a physical TiO2 barrier.455 In this report, they synthesized a Au@TiO2 composite consisting of a gold nanoparticle (∼15 nm) within an otherwise hollow TiO2 shell. The composite was made by coating Au nanoparticles first with a SiO2 layer and then coating the composite with a TiO2 layer. SiO2 could then be removed from the composite by dissolution with NaOH as shown in Figure 20a. The composite could then be calcined and crystallize the TiO2 with no change to the Au nanoparticle as shown in Figure 20b. This result was compared to calcination of an Au/TiO2−P25 composite where it is clearly evidenced that the Au nanoparticles sinter substantially (Figure 20d). By comparison to the as-prepared Au/TiO2−P25 composite, the yolk@shell composites had similar CO oxidation activity, as shown in Figure 20e. Although the catalyst was not significantly improved, there is still room for improvement by both decreasing the size of the Au nanoparticle and improving the crystallinity of the TiO2 shell. Production of hydrogen from Au/TiO2 composites has seen a considerable amount of research, although for most reactions the activity is commonly lower than that of both Pd/TiO2 and Pt/TiO2.350,438,456,457 Regardless, Au/TiO2 composites have been applied to reactions for hydrogen production from water splitting with a sacrificial agent,360,388,393,458−460 steam and alcohol reforming,461−466 and the water−gas shift reaction.253,363,467−469 Sastre et al. recently detailed a systematic study in which they compared various noble metal catalysts of the same loading on P25 for their photoactivity in hydrogen production from the water−gas shift reaction under simulated solar light.363 As shown in Table 1, the Au/TiO2 catalyst showed the highest CO conversion percent as well as the highest H2 production when compared to other noble metal catalysts and Au supported on CeO2. Additionally, the catalyst showed little syngas reactivity as evidenced by the relatively minor formation of CH4. Au/TiO2 composites have also been shown to be highly efficient for photooxidation of organic molecules.255,399,470−477 More importantly, many of these catalysts have utilized visible light irradiation to induce the photoactivity. In one recent study by Zhang et al. a core−shell hierarchical SiO2@Au@TiO2 composite was developed that had high photocatalytic activity for degradation of RhB.474 The composite was synthesized starting with a SiO2 core, to which Au nanoparticles were chemically attached via interactions with an amine-modified SiO2 surface. The composite was then coated with TiO2 and
Figure 19. (A) Absorption spectra of Ag@TiO2 colloids in ethanol. (a) Before UV irradiation, (b, c, and d) after exposure to UV light (λ > 300 nm) for 10, 30, and 60 s, and (e) after exposure of the UVirradiated suspension to air. Transmission electron micrograph is shown in the inset. (B) Shift in the plasmon absorption peak during UV excitation followed by its exposure to air. Bottom schematic illustrates the mechanism of the photoinduced charge separation and charging of the metal core in Ag@TiO2. Adapted with permission from ref 446. Copyright 2004 American Chemical Society.
Since the antibacterial effect via the oligodynamic effect of colloidal silver has been common knowledge scientifically for over a century, finding a method to enhance its activity while mitigating the leaching of silver is beneficial. Due to this, Ag/ TiO2 composites have also seen a significant amount of research in the past decade. Ag/TiO2 composites have been shown to be very effective in eliminating bacteria and preventing growth of bacteria such as Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) in Ag concentrations less than 10 μg/mL. Remaining under this concentration is of utmost importance as it is the point at which silver becomes toxic in human cells.448 One report by Zhang et al. showed that when utilizing small Ag nanoparticles in a 3.9 wt % Ag/TiO2 composite E. coli growth could be suppressed by 99.9% when the concentration of Ag was as low as 1.6 μg/mL. These composites have shown utility for low-cost, highly active antibacterial applications. 3.5. Gold
As compared to both platinum and silver, gold has advantageous characteristics of each. Like silver, gold has a tunable LSPR which can be utilized to improve photocatalytic activity, but gold also has a more robust chemical stability akin to platinum that silver does not have. Further, like platinum, gold has a high work function (∼5.1−5.3 eV).403,438 On the basis of this, Au/TiO2 composites are used for many of the same applications as Pt and Ag, with some improvements depending on the specific case. In general, Au/TiO2 composites have better photoactivity than Ag/TiO2 composites because the higher work function of gold as compared to silver allows for better charge separation from TiO2. Au/TiO2 composites also 9870
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Table 1. Photocatalytic Activity for Hydrogen Evolution and Product Distribution after 22 h Irradiation with Concentrated Sunlighta
photocatalyst TiO2 Pd/TiO2 (1%) Pt/TiO2 (1%) Au/TiO2 (1%) Ag/TiO2 (1%) CeO2 Au/CeO2 (0.2%) Au/CeO2 (0.5%)
CO conversion (%)
H2 production (μmol/g catalyst)
CO2 production (μmol/g catalyst)
CH4 production (μmol/g catalyst)
3 24
264 4178
649 4585
4 1
25
4279
4763
64
71
10 506
13 447
7
10
1650
1823
8
4 8
350 1380
607 1526
5 15
28
4880
4762
2
a
Adapted from ref 363 with permission from The Royal Society of Chemistry.
composite completely degraded the dye in 40 min, whereas in the same time commercial P25 had only degraded ∼38% of the dye. Further, the optimal catalyst showed a significant decrease in the amount of gold necessary compared to many other catalysts as the optimal loading amount was only 0.10 wt %. The catalyst also showed improvement by nitrogen doping of the TiO2 shell, which was supposedly doped by decomposition of the amine layer when the composite was calcined.
4. NONOXIDE SEMICONDUCTOR−TIO2 COMPOSITES Nonoxide semiconductors (e.g., CdS, CdSe, GaP) have seen an increase in their utilization in composites with TiO2 after development of quantum dot-sensitized solar cells (QDSSCs). In addition to QDSSCs, other applications centered on photocatalysis have led to a significant interest of these nonoxide semiconductors in composites with TiO2. Controlling the size of quantum dots allows for the ability to tune the energetics of the composite system which allows for easier fabrication of composites with desired properties.
Figure 20. (a) TEM image of as-prepared Au@TiO2 yolk@shell structure. (b) TEM of calcined Au@TiO2 yolk@shell structure. (c) TEM image of Au/P25 sample as prepared. (d) TEM image of Au/ P25 sample after calcination. (e) Time dependence of the carbon monoxide coverage on gold (ΘCO, circles) and the carbon dioxide partial pressure (PCO2, squares) during room-temperature oxidation of CO with O2 on gold/titania catalysts. The two left-hand panels correspond to our Au@TiO2 yolk@shell catalyst and the two righthand panels to a reference Au/TiO2−P25 sample. First and third panels were obtained by first introducing 200 Torr of CO into the cell and then adding 200 Torr of O2; in the second and fourth panels the sequence was reversed. The coverage of CO on Au and the gas-phase CO2 pressure were estimated from the integrated intensities of the DRIFT signals in the 2090−2145 and 2300−2400 cm−1 regions, respectively. Turnover frequencies (TOF), in units of molecules of CO2 produced per surface Au atom per second, are also reported. Adapted from ref 455 with permission, Copyright 2011 John Wiley and Sons.
4.1. Metal Pnictogenides
Semiconductors containing pnictogenides (N, P, As, and Sb anions) have seen limited usage toward composites with TiO2 to this point. Early reports of photoelectrochemical cells for water splitting showed the ability of composites constructed of TiO2 anodes and GaP cathodes to form stable self-driven cells.478−480 However, the overall efficiencies of these cells (∼0.01%) were still too low for practical use. Other metal pnictogenides have seen some utilization in composites with TiO2, typically with applications to solar cells. In one study, Ren et al. coated GaAs nanowires with amorphous TiO2 using sol−gel methods and found nearly a 20% increase in their cell efficiency.481 Other pnictogenides using indium as the precursor have seen usage in composites with TiO2 as well.482−486 In one report by Nedeljković et al., InP nanocrystals were grown directly onto TiO2 nanorods with the focus on developing composites with good contact for later applications where charge transfer is important (i.e., solar cells and photocatalysis). Synthesis consisted of using hydrothermally prepared Na2Ti3O7 nanotubes, which are subsequently treated with acid to obtain H2Ti3O7 and calcined to
calcined at 500 °C as shown in the scheme in Figure 21a. Figure 21c shows the energy-dispersive X-ray (EDX) elemental mapping confirming the “sandwich” structure where the Au nanoparticles are located in between the SiO2 core and the TiO2 layer. This composite showed impressive photocatalytic activity of the composite structure for degradation of RhB dye under direct solar irradiation as shown in Figure 21d. The 9871
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Figure 21. (a) Schematic illustration of the fabrication process of the sandwich-structured SiO2/Au/TiO2 photocatalyst. (b) Typical SEM image of the composite photocatalyst. (c) Elemental mapping of a single particle with the distribution of individual elements shown in the bottom row. (d) Photodegradation of RhB under direct sunlight illumination. Adapted from ref 474 with permission, Copyright 2012 John Wiley and Sons.
Figure 22. Charge injection of excited CdSe quantum dot into TiO2 nanoparticle. The scheme on the right shows the tuning of energy levels (and hence the charge injection) by size control. Reprinted with permission from ref 504. Copyright 2008 American Chemical Society.
CdSe and vice versa has led to even greater utilization of these materials. 492−494 Additionally, although tellurium is an increasingly rare element, it has seen a considerable amount of research in composites with TiO2 as well. The significant synthetic controls over size, morphology, and composition of cadmium chalcogenide nanocrystals495−501 has led to a substantial amount of research and review articles being published on these composites. In a typical scheme, traditional dye sensitization is replaced by narrow band-gap semiconductor nanoparticles such as CdSe, as shown in Figure 22. This sensitization can be further tuned in terms of excitation wavelength by controlling the properties of the semiconducting particles, e.g., shape and composition. Many such composites have been reported over recent years with varying architectures and methods, many of which are very promising.502−511 One recent report of interest from the Kamat group was the development of a “solar paint” which consisted of CdS, CdSe, and TiO2 particles dispersed in a mixed tert-butyl alcohol/water solvent to form a paste.510 This could then be “painted” onto conducting electrodes followed by completion of a cell with the counter electrode and electrolyte to produce a simple solar cell. Although the highest cell efficiency reported was 1.08% with an open circuit voltage of 585 mV and short circuit current of 3.1 mA/cm2, this technique has the advantage of facile cell
produce TiO2 rods. These nanorods could then be dispersed in 9-octadecene and toluene, where InP nanocrystals could be grown by oxidation of indium nanoparticles with P(SiMe3)3. Other composites containing metal pnictogenides such as InAs and InN have been applied toward fabrication of solar cells with TiO2 as well, but overall research on these composites has been limited by the more common usage of metal chalcogenides. 4.2. Metal Chalcogenides
As opposed to metal pnictogenides, metal chalcogenides (S, Se, and Te anions) have seen a considerable amount of research in regard to composites with TiO2. Since the most common composition of quantum dots is of metal chalcogenides (CdS, CdSe, etc.), quantum dot-sensitized solar cells most commonly utilize one of these in their composites with TiO2. Additionally, metal chalcogenides have been further utilized in composites with TiO2 for improving the photocatalytic activity of TiO2. Here we will look at some of the key recent developments regarding metal chalcogenides and TiO2 composite materials. 4.2.1. Cadmium Chalcogenides. The discovery that cadmium chalcogenides (Cds, CdSe, CdTe) can act as sensitizers for TiO2 has led to these materials becoming the prototypical quantum dot−TiO2 composites over the past few decades.487−491 Additionally, fabrication of core@shell CdS@ 9872
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construction and easy to scale methods. Improvement of this technique in the future may allow for a much larger commercial application of QDSSCs for next-generation energy production. In addition to the development of solar cells, cadmium chalcogenides have also been utilized in composites with TiO2 for photocatalysis. These composites typically are applied to either photocatalytic hydrogen production from water512−516 or photocatalytic oxidation of organic molecules517−519 since they can extend the usable wavelengths of light into the visible region. Although these materials have seen some success, significant drawbacks occur such as leaching of the cadmium ions or in the case of hydrogen production needing sacrificial agents such as Na2S or Na2SO3. Due to this, noble metal and metal oxide TiO2 composites still show greater promise for cleaner systems for photocatalysis. 4.2.2. Other Single-Metal Chalcogenides. Although the most common metal chalcogenide semiconductor materials for research with TiO2 have been cadmium compounds, other materials have seen some utilization as well in order to mitigate the utilization of cadmium which is highly toxic. In recent years, lead sulfide (PbS) has been utilized as a potential composite with TiO2.520−525 PbS is beneficial since it has a narrow band gap (∼0.41 eV) and a large Bohr exciton radius. PbS can also improve the photoactivity of TiO2 due to its multiple exciton generation.521 As such these composites have seen some use for both photocatalytic and photovoltaic applications and will likely see more in the future due to the low toxicity of the compound stemming from its low solubility. Another metal chalcogenide that has seen some recent research in composites with TiO2 is tin(II) sulfide, SnS.526−529 This material is promising as both tin and sulfur are abundant and have low toxicities compared to other metal chalcogenides. Fabrication of devices to this point has been minimal; however, there is significant room to expand this in the future. 4.2.3. Semiconductor Alloy Chalcogenides. Alloys of metals within semiconductor nanoparticles have become an area of research with an increasing amount of interest. Much of this is due to the decrease in the necessary amount of either rare or toxic metals and replacement of them with metals such as zinc or copper. Alloys such as copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) have begun to be utilized as composite materials with TiO2.530−535 Another alloy, copper indium sulfide (CuInS2), has seen more significant research in composites with TiO2 to date for applications in both photovoltaics536−541 and photocatalysis.542−544 Much like other semiconductors, they have shown great promise to date; however, as they have not been studied to the extent of systems such as CdS or CdSe, optimization is still needed to yield results which are to the same efficiency as the better known systems. Recently, both Santra et al. and Chang et al. established systems where a CuInS2/TiO2 composite for photovoltaic applications has been further sensitized using CdS.540,541 Addition of CdS into these composites has been able to increase cell efficiencies significantly; however, they still fall under the efficiencies of many pure semiconductor/TiO2 composites and will need further optimization in the future to become feasible.
been some early reports of the observation of CNTs,545 their isolation by Iijima546 accelerated their utilization over the past two decades. On the basis of the work function of CNTs, ∼5 eV,547 it is apparent that they can act as electron acceptors to more effectively separate photogenerated charges on TiO2. Initial reports for the synthesis of CNT/TiO2 composites were made by Vincent et al., who demonstrated the embedding of multiwall carbon nanotubes (MWCNTs) in an amorphous TiO2 matrix.548 Hernadi et al. later showed a homogeneous direct coating of an amorphous TiO2 layer on MWCNTs via direct, solvent-free impregnation of the nanotubes in liquid titanium(IV) ethoxide.549 Jitianu et al. then demonstrated a homogeneous coating via both hydrothermal and sol−gel coatings, followed by crystallization by annealing to 300 °C.550,551 These sol−gel coatings deposit a thin homogeneous TiO2 layer onto the MWCNTs. These coatings could also be mediated with cetyltrimethylammonium bromide (CTAB); however, the crystallinity was decreased upon calcination as compared to those prepared without surfactant. These coating methods provided the foundation for later utilization of these composites, and although some CNT/TiO2 composites have been utilized for photocatalytic hydrogen production,552−555 they are more frequently utilized in composites for organic molecule degradation.556−561 Proposed mechanisms for the CNT-mediated enhancement of photocatalysis on TiO2 composite structures have been investigated and typically fall under one of three categories:562 (1) CNTs as electron sinks, as mentioned above;563 (2) excitation of the CNT itself followed by charge injection into TiO2;564 (3) introduction of carbon impurities in TiO2 forming Ti−O−C bonds leading to creation of energy states within the band gap of TiO2 to facilitate visible light absorption.565 Any of these proposed mechanisms can be correct depending on the method of preparation of the CNT/ TiO2 composite structure. In addition to improved photocatalytic activity, the composites have the additional benefit of enhanced adsorption of dye molecules onto the CNT surface, which can enhance the rate of degradation as well. Further, these composites can be improved by addition of other cocatalysts such as noble metals, which can further improve charge separation.359,566,567 In addition to applications for photocatalysis, CNT/TiO2 composites have been utilized for applications with DSSCs.568−572 Typical CNT/TiO2 composites for DSSC applications indicate that ∼0.10 wt % CNT is optimal, with cell efficiencies that increase between 19% and 50%. The increase in cell efficiencies has been attributed to the improved electron transport mediated by the CNTs. Further utilization of CNT/TiO2 composites has been done for applications to Li+ battery materials.573−577 One report by Cao et al. utilized well-controlled TiO2 coatings on MWCNTs from hydrolysis of titanium(IV) butoxide to form well-defined “coaxial nanocables”, shown in the TEM images in Figure 23a and 23b.574 These composites could achieve a specific capacity of 244 mA h g−1 at a current density of 3000 mA g−1, as shown in Figure 23c. This capacity was retained when cycled over 100 times under a current density of 1000 mA g−1 and within a potential window of 0.01−3 V (Figure 23d).
5. CARBON−TIO2 COMPOSITES
Graphene, which has many extraordinary properties such as high electron mobility and surface area, has been investigated heavily with respect to composites with TiO2 over the past decade. Similar to the case of carbon nanotubes, the higher work function of graphene (∼4.9−5.2 eV)578,579 with respect to
5.2. Graphene and Graphene Oxide
5.1. Carbon Nanotubes
The emerging prominence of carbon nanotubes (CNTs) has led to their use across many applications. Although there had 9873
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on the surface of amine-functionalized amorphous TiO2 microspheres.598
Figure 23. (a) TEM images of TiO2-precursor-coated CNTs. (b) HRTEM images of CNT@TiO2 nanocables. (c) Comparison of the rate performance of CNT@TiO2, TiO2-free CNT, and CNT-free TiO2 sample between voltage limits of 0.01−3 V. Specific storage capacities in CNT core and TiO2 sheath are also shown. Shaded areas represent the capacity contribution from TiO2 or CNT in the CNT@TiO2 nanocables. (d) Cycling performance of CNT@TiO2 nanocables under a current density of 1000 mA g−1 between voltage limits of 0.01−3 V. (Inset) Corresponding Coulombic efficiency profiles. Adapted with permission from ref 574. Copyright 2010 American Chemical Society.
TiO2 allows for an increase in charge separation by electron injection into the graphene sheets. Chemical utilization of graphene has been established through reduction of graphene oxide (graphitic oxide, GO) sheets, which are commonly synthesized by the Hummers method580 or a modification thereof.581 Reduction is done by a number of means such as by hydrothermal/solvothermal treatment582 or, more frequently, in situ with TiO2 deposition. One recent study by Zhu et al. simultaneously formed the graphene/TiO2 composite and reduced the GO sheets by utilizing TiCl3 as the reductant for GO and precursor for TiO2.583 Concurrent reduction of GO and crystallization of TiO2 is one of the optimal methods as it provides good contact between the two materials with wellcrystallized morphology. It must be noted that reduction is not always carried out in prior reports, and many claimed graphene/TiO2 composites are in reality graphene oxide/ TiO2. These graphene/TiO2 composites have seen applications in the construction of DSSCs,584−586 Li+ battery applications,587−592 and, most prominently, enhanced photocatalysis.593−602 One simple composite reported by Zhang et al. consisted of graphene oxide mixed with P25. The components were mixed and held at 120 °C for 3 h in water−ethanol mixture within a Teflon-sealed autoclave to achieve both reduction of the GO and deposition on P25.593 This P25−graphene (P25−GR) composite showed improved photoactivity for degradation of methylene blue under both UV and visible light irradiation, as compared to bare P25, and a slight improvement when compared to a P25−carbon nanotube composite as shown in Figure 24. In a study by Lee et al., graphene oxide was wrapped
Figure 24. Photodegradation of methylene blue under (a) UV light and (b) visible light (λ > 400 nm) over (1) P25, (2) P25−CNTs, and (3) P25−GR photocatalysts. Reprinted with permission from ref 593. Copyright 2010 American Chemical Society.
These composites were then hydrothermally treated to reduce the GO, followed by calcination under argon at 400 °C. Figure 25a−d shows the synthetic scheme with corresponding SEM images of the composite. Figure 25b shows the positively charged, amine-functionalized TiO2 microspheres, and Figure 25c shows the composite structure after the negatively charged graphene oxide is wrapped on the positively charged TiO2. Figure 25d then shows the composite after hydrothermal treatment. At this stage the graphene oxide has been reduced to graphene and the TiO2 has crystallized to anatase phase. Samples showed improved photocatalytic activity compared to a two-step method where GO was coated on precalcined anatase microspheres and then subsequently reduced, indicating that the order of preparation was critical. Degradation of MB by visible light is shown in Figure 25e, showing the improved photocatalytic activity of the composite as compared to P25, bare anatase particles, and the two-step method. An extension of this method was later shown by Zhang et al.,602 where they wrapped GO on a core@shell SiO2@amine− TiO2 composite, followed by reduction and etching of the SiO2 core to form a hollow TiO2/GO composite structure. With the simplified synthesis of graphene only recently established it is 9874
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carbon coating on P25 from hydrothermal deposition by glucose, followed by graphitization of the carbon by calcination at 800 °C under nitrogen.612 The study showed that there was an optimal thickness of the carbon layer for enhancement of the photocatalytic degradation of formaldehyde. It was proposed then that a thin carbon coating of only a few nanometers can be beneficial for charge separation. However, when the coating is too thick, absorbance and scattering of photons by the carbon layer led to less light reaching the TiO2 surface, resulting in a decrease of the photocatalytic activity.
6. TEMPLATED COMPOSITES In order to have a well-defined and easy to control morphology, templates such as SiO2 and polystyrene have been utilized with techniques such as incipient wetness impregnation and sol−gel coatings. These composite structures can yield TiO2 with tailored properties such as morphology, high surface area, and well-controlled crystallinity. In this section, we will focus on templated composite structures which yield pure TiO2 upon removal of the template, regardless of if the template was removed or not. 6.1. Yolk−Shell and Core−Shell
Template-mediated syntheses have been utilized quite regularly in recent years to create hollow TiO2 shells with well-controlled properties. Synthesis of these materials typically follows several sequential steps: (i) preparation of the core material template; (ii) deposition of the shell materials on the surface of the templates, usually via a sol−gel process; (iii) removal of the core materials. A number of core@shell and yolk@shell materials have been mentioned to this point utilizing a variety of core materials such as Ag, Au, SiO2, iron oxides, and glucose; however, they were synthesized with a focus on the synergistic effects of the composite. Here we will focus on methods that yield pure TiO2 as the final product or can yield this with a simple removal of the template. One template material that has been utilized readily over the past decade is polystyrene (PS) beads.618−629 Zhong et al. utilized commercially purchased PS beads which were first assembled into a crystalline array, followed by infiltration with the sol−gel precursor, titanium(IV) isopropoxide.619 The composite could then be immersed in toluene to dissolve the polystyrene template, followed by sonication to free the hollow amorphous TiO2 spheres from the substrate. Shiho and Kawahashi synthesized similar core@shell particles and subsequently removed the core PS bead by calcination under air.618 It was found that calcination to 600 °C for 3 h could completely remove the PS core and crystallize the remaining hollow TiO2 shell to anatase phase. Calcination to 900 °C would crystallize the TiO2 to rutile; however, the shell morphology would be completely degraded. Using similar templated composites, Syoufian et al. later showed that an optimal amount of TiO2 precursor allowed for the highest anatase crystallinity, which in turn yielded the best photocatalytic activity for degradation of MB.625 In addition to PS beads, TiO2 has also been deposited on other carbonaceous templates which can easily be burned away to reveal a hollow TiO2 structure. Most prominently, glucose can be hydrothermally treated to form colloidal carbon spheres.630 Ao et al. showed that these carbon spheres could then be coated by TiO2 via a sol−gel process, followed by calcination to remove the carbon core.631 Doping of the TiO2 shell for improved photocatalytic activity was also demonstrated by addition of
Figure 25. (A) Schematic illustration of the synthesis steps for graphene-wrapped anatase TiO2 NPs. (B) SEM images of bare, amorphous TiO2 NPs prepared by sol−gel method. (C) SEM images of GO-wrapped amorphous TiO2 NPs. (D) SEM images of graphenewrapped anatase TiO2 NPs. All scale bars are 200 nm. (B) Photodegradation of methylene blue (MB) under visible light (λ > 420 nm) by (a) P25, (b) bare anatase TiO2 NPs, (c) graphene−TiO2 NPs (two-step hydrothermal), and (d) graphene−TiO2 NPs. The weight ratio of graphene to TiO2 in the graphene−TiO2 hybrid materials was 0.02:1. Adapted from ref 598 with permission, Copyright 2012 John Wiley and Sons.
expected that further research on composites of graphene and TiO2 is only to increase in the coming years. 5.3. Other Carbon
In addition to the above forms of carbon, other types have seen usage in combination with TiO2 as well. One such material is activated carbon (AC), which has been used extensively to promote the photocatalytic degradation of organic molecules.603−608 The high surface area allows for a greater adsorption of the molecules which can then be more readily degraded by the TiO2 component. Other carbonized materials such as polymers, glucose, and sucrose have seen use as either a template for TiO2 coating or as a coating material on top of TiO2. Many of these composites are then utilized for applications in photocatalysis609−613 or Li+ battery applications.614−617 One report by Zhang et al. demonstrated a thin 9875
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dopant sources during the sol−gel coating.632−634 Although good control of the coating of TiO2 onto PS beads and glucose has been demonstrated, it has fallen out of favor as compared to other templates such as silica. Coating of TiO2 onto silica templates has been widely demonstrated over the past two decades with varying degrees of control.635−639 One significant drawback for many of these early composites is that after crystallization at high temperature and removal of the SiO2 the TiO2 shell would collapse. This is likely due to the large degree of structural reorganization that occurs upon high-temperature crystallization of the amorphous shell. Once the SiO2 core is removed, the support for these crystals is gone and the shell falls apart. Interestingly, if a spacer of different composition, such as PS or SnO2, is placed between the SiO2 core and the TiO2 layer the shell morphology can be retained after calcination and removal of either the spacer or the SiO2 core.640,641 More recently, however, research on these SiO2@TiO2 core@shell composites has led to synthetic strategies which yield stable, hollow TiO2 shells. In a recent report by Joo et al., hollow crystalline TiO2 with well-defined crystallinity and morphology was synthesized by a SiO2@TiO2@SiO2 core@shell@shell composite.642 The composite was synthesized by sequential sol−gel coatings of TiO2 and SiO2 on a colloidal SiO2 core. The composite was then crystallized by high-temperature calcination, and the SiO2 core and outer shell materials were removed by dissolution with NaOH. The as-obtained hollow TiO2 consisted of small anatase grains (∼5 nm) and displayed a high surface area (∼300 m2/g) and porosity. Although a significant improvement in morphology was shown, the photocatalytic activity was still minimal due to the small grains yielding more sites for recombination of photogenerated charge carriers. It was determined that the silicate oligomers could penetrate into the amorphous TiO2 matrix and inhibit significant crystallization, thus resulting in small anatase grains.643 As such, a subsequent report detailed a “partial etching” method where the SiO2 at the SiO2−TiO2 interface was selectively dissolved followed by recalcination and then complete removal of the SiO2.644 It was determined that the Ti−O−Si bonds are less organized and weaker than the Ti−O−Ti and Si−O−Si bonds such that they are preferentially etched. The scheme and corresponding TEM images in Figure 26 show the evolution of the structure by the reported procedure. This method could significantly increase the crystallinity as shown in Figure 27a−d, where the grain size increased from ∼4 nm (Figure 27a) in the original sample to >14 nm (Figure 27d), as determined by the Scherrer equation from the XRD pattern (Figure 27e). The hollow shells still showed a highly porous nature with a high specific surface area (∼300 m2/g). The increase in crystallinity yielded hollow TiO2 anatase shells that had comparable photocatalytic activity to P25 for UV degradation of RhB. Other methods to both increase the crystallinity and simplify synthetic procedures have also been reported utilizing SiO2@TiO2 composites. Shen et al. recently reported highly crystalline hollow TiO2 shells starting from a SiO2@TiO2 composite.645 After synthesis, the composite was calcined at a desired temperature and the silica was removed by etching with NaOH in a Teflon-lined autoclave at 80 °C for 4 h. The resulting TiO2 maintained its well-defined hollow shell morphology with large crystal grains. These hollow structures have shown well-controlled crystallinity and porosity that have good photocatalytic activity and allow for facile transport of reactants into the core of the
Figure 26. (Left) Schematic illustration of the partial etching and recalcination procedure for fabrication of mesoporous TiO2 shell nanostructures. (Right) Corresponding TEM images of the SiO2@ TiO2@SiO2 particles after sequential treatments: (a) original calcination, (b) partial etching, (c) recalcination, and (d) final etching. After removing all silica, the products are mesoporous TiO2 shells. Adapted from ref 644 with permission from The Royal Society of Chemistry.
structure.646,647 Further, they can be functionalized even further with other shell coatings besides SiO2. One such structure was previously mentioned where SiO2@TiO2 was coated with graphene oxide followed by reduction to graphene and dissolution of the SiO2 core to form a hollow TiO2@graphene composite.602 In a separate report by Zhang et al., a similar structure was synthesized but a final layer of SiO2 was coated on top of the entire composite. However, the final morphology was not as well maintained as other published methods. 6.2. Ordered Mesoporous Silica and Zeolites
Ordered templates based on silica such as MCM-41 and SBA15 and zeolites have seen some usage as “hard” templates for synthesis of mesoporous TiO2 materials. These templates allow for synthesis of highly porous TiO2 networks which can be used 9876
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Figure 28. Four main types of guest assembling inside ordered mesoporous silica (OMS) nanotubular mesopores: (I) ensemble of nanoparticles with diameter less than that of OMS mesopores; (II) isolated nanoparticles of dimension comparable with OMS mesopores diameter; (III) guest phase layer; (IV) combination of guest nanoparticles inside the OMS mesopores with large guest particles at the outer surface of OMS crystals. Reprinted from ref 663 with permission from Elsevier. Copyright 2005.
synthesis and proliferation of ordered mesoporous silica structures like MCM-41 and SBA-15 the use of zeolites decreased. In one recent study Yue et al. showed that by changing the type of TiO2 precursor added to a SBA-15 template they could control the phase and structure of the resulting mesoporous TiO2.661 Titanium(IV) isopropoxide was hydrolyzed and then dissolved by different acids to yield Ti(NO3)4 or TiCl4. The precursors were then loaded into SBA-15, followed by calcination and template removal via etching with NaOH. It was found that the nitrate precursor would form a interconnected rutile network upon calcination (Figure 29a, 29c, and 29e), whereas the chloride precursor would form a particulate network consisting of anatase grains (Figure 29b, 29d, and 29f). These interesting properties allow for the tailored synthesis of TiO2 from mesoporous silica templates which are hard to form otherwise. 6.3. Anodized Aluminum Oxide (AAO)
Since the discovery of porous alumina templates made by anodization of aluminum films, composites with these templates have been widely researched. The ease of deposition of precursors into the alumina pores allows for controlled height and composition of the deposited nanowires. The general scheme for synthesis of these nanowires is shown in Figure 30, where an aluminum film is anodized to form a porous network. This anodization can be repeated to control the pore width and height to better enhance the tuning of the later deposited material. The pores are then filled by deposition of the precursor material, e.g., titanium alkoxides, and the alumina/aluminum can be selectively dissolved to yield free nanorods/nanowires. Alternatively, commercial alumina templates with larger pore widths (200−250 nm in diameter) are available as filters. These templates can then be utilized in the same fashion for deposition and subsequent template removal. These templating methods have made the controllable synthesis of highly crystalline TiO2 nanowires simple and reproducible.670−679 Single-crystal TiO2 nanowires have been reported by Zhang et al.673 and Miao et al.674 utilizing electrochemical oxidative hydrolysis and sol−gel deposition of TiO2 precursors, respectively. This templating method has shown great utility for creation of uniform nanowire arrays which can be used for DSSCs or photocatalysis.676−679
Figure 27. (a) TEM image of mesoporous TiO2 shells after calcining SiO2@TiO2@SiO2 particles and NaOH etching (sample corresponds to “etched” in XRD). (b−d) TEM images of hollow mesoporous TiO2 shells prepared by calcination followed by partial etching and recalcination at (b) 700 °C for 4 h (PE-700), (c) 800 °C for 4 h (PE-800), and (d) 800 °C for 16 h (PE-800−16h). (e) XRD patterns of a−d; PE-600 is not pictured. Adapted from ref 644 with permission from The Royal Society of Chemistry.
for applications in photocatalytic degradation of organic molecules,648−657 H2 production,658−660 and Li+ battery applications.661,662 Further, easy removal of the template can be done by chemical etching with NaOH or HF. Figure 28 shows the typical host−guest interactions between the template and the TiO2 precursor, with cases I−III being most common depending on the concentration of precursor and type of template and case IV not typically seen when using impregnation methods.663 Composites with zeolites were initially utilized more frequently due to their abundance and ease of purchase.664−669 These composite structures were shown to enhance photocatalytic activity of reactions such as photooxidation of alcohols and water splitting. Later, with the
7. SUMMARY AND OUTLOOK Synthesis of TiO2 composites materials is a field that has seen significant growth over the past few decades and will likely increase in the coming years. Enhancements to the photocatalytic activity of TiO2 through the use of composite materials 9877
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Figure 29. (a and c) TEM images of mesoporous TiO2 from nitrate precursor calcined at (a) 200 and (c) 300 °C. (b and d) TEM images of mesoporous TiO2 from chloride precursor calcined at (b) 200 and (d) 300 °C. (e and f) HRTEM images of mesoporous TiO2 from (e) nitrate and (f) chloride precursors calcined at 300 °C. Reprinted from ref 661 with permission, Copyright 2009 John Wiley and Sons.
Figure 30. General scheme for formation of nanowire-type materials from an anodized aluminum template.
addition to these metals, carbon materials may allow for cheap composites with enhanced activity as well. With the decreasing cost in the synthesis of carbon materials such as graphene or with synthetic techniques such as carbonization of glucose, composites of carbonaceous materials with TiO2 have become much cheaper and easier to produce. As such they are likely to see a considerable amount of research in the coming years as an alternative composite material with significant advantages such as good photocatalytic activity and high adsorption capacities. Templated composites show a significant number of advantages that are not always seen with the less ordered composite materials. Although many template syntheses are labor and energy intensive, much recent progress has been made in this field in order to mitigate this. Yolk@shell composites hold great promise for high-temperature catalytic applications where sintering of nanoparticles degrades catalyst performance. Additionally, some of these structures have possible applications as nanoreactors or bifunctional catalysts where the inner core and outer shell are functionalized differently. AAO templated synthesis of nanowires has interesting benefits such as making composite nanowires of altering compositions, which may be useful for future applications. Overall, composite TiO2 materials can be synthesized in such a way that they can take advantage of many properties of different materials and be applied to a significant number of
has great potential for developing commercial catalysts for hydrogen production, environmental remediation, production of fine chemicals, as well as other applications. Further composites with TiO2 show promise in applications for improved solar cells and lithium ion batteries. With the increasing need for clean and sustainable energy sources, TiO2 composite materials may hold the key to developing solutions to many of these problems. Composites with materials such as noble metals have been heavily investigated and hold much promise, though market prices for these metals may slow progress in their commercialization. However, optimization of the physicochemical properties of these composites has shown a significant reduction in the necessary quantity of noble metals, which will be of importance to reactions where noble metals remain necessary. These noble metal/TiO2 composites have shown a substantial potential for applications in hydrogen production and environmental remediation, two significant challenges society faces going forward. More interestingly, however, composites with cheaper metals and metal oxides as well as carbon materials hold significant promise as they become further developed in the coming years. These composites with metals such as copper or nickel and the oxides of each have the potential to be developed to the point that they may be utilized in place of noble metals for future applications in order to reduce costs without a significant decrease in activity. In 9878
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energy- and environment-related applications. Synthetic strategies such as the design of core@shell Cu2O@TiO2 particles by controlled hydrolysis and condensation of TiO2 precursor onto the Cu2O core can yield heterojunctions which can more effectively separate photogenerated charge carriers as well as increase the visible light response. Many of these structures can be tuned by controlling the composition in order to decrease the charge carrier recombination rate or adjust the band gap of the composite. This is the case with materials such as CNT/ TiO2 composites where the CNTs can act as electron sinks or create energy states between the TiO2 bands, depending on the synthesis method. Many properties of these composites are only now being explored and controlled, and as the synthetic approaches mature even further, it is likely that fabrication of devices with tailored properties using TiO2 composites will be made easier, faster, and cheaper.
Riverside, under the supervision of Yadong Yin. His research interests include the synthesis and self-assembly of complex colloidal nanostructures for energy applications.
AUTHOR INFORMATION Yadong Yin received his Ph.D. degree in Materials Science and Engineering from the University of Washington in 2002, then worked as a postdoctoral fellow at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, and became a staff scientist at LBNL in 2005. In 2006 he joined the faculty at the Department of Chemistry at the University of California, Riverside. His research interests include the synthesis and application of nanostructured materials, self-assembly processes, and colloidal and interface chemistry.
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies
ACKNOWLEDGMENTS We are thankful for the financial support provided by the U.S. Department of Energy (DE-FG02-09ER16096). REFERENCES (1) Weir, A.; Westerhoff, P.; Fabricius, L.; Hristovski, K.; von Goetz, N. Environ. Sci. Technol. 2012, 46, 2242. (2) Pfaff, G.; Reynders, P. Chem. Rev. 1999, 99, 1963. (3) Meacock, G.; Taylor, K. D. A.; Knowles, M. J.; Himonides, A. J. Sci. Food Agric. 1997, 73, 221. (4) Phillips, L. G.; Barbano, D. M. J. Dairy Sci. 1997, 80, 2726. (5) Robichaud, C. O.; Uyar, A. E.; Darby, M. R.; Zucker, L. G.; Wiesner, M. R. Environ. Sci. Technol. 2009, 43, 4227. (6) Jaroenworaluck, A.; Sunsaneeyametha, W.; Kosachan, N.; Stevens, R. Surf. Interface Anal. 2006, 38, 473. (7) Morison, W. L. N. Engl. J. Med. 2004, 350, 1111. (8) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (9) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (10) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (11) Yang, L.; Luo, S.; Cai, Q.; Yao, S. Chin. Sci. Bull. 2010, 55, 331. (12) Endres, P. J.; Paunesku, T.; Vogt, S.; Meade, T. J.; Woloschak, G. E. J. Am. Chem. Soc. 2007, 129, 15760. (13) Smith, L.; Kuncic, Z.; Ostrikov, K.; Kumar, S. J. Nanomaterials 2012, 2012, 1. (14) Yamaguchi, S.; Kobayashi, H.; Narita, T.; Kanehira, K.; Sonezaki, S.; Kubota, Y.; Terasaka, S.; Iwasaki, Y. Photochem. Photobiol. 2010, 86, 964. (15) Chen, C.-T.; Chen, Y.-C. Anal. Chem. 2005, 77, 5912. (16) Jones, B. J.; Vergne, M. J.; Bunk, D. M.; Locascio, L. E.; Hayes, M. A. Anal. Chem. 2007, 79, 1327. (17) Li, Y.; Xu, X.; Qi, D.; Deng, C.; Yang, P.; Zhang, X. J. Proteome. Res. 2008, 7, 2526. (18) Leitner, A. TrAC, Trends Anal. Chem. 2010, 29, 177. (19) Lu, Z.; Ye, M.; Li, N.; Zhong, W.; Yin, Y. Angew. Chem. 2010, 122, 1906.
Michael Dahl received his B.S. in Chemistry at California State University, Fullerton, in 2009. He is currently a Ph.D. student in Chemistry at the University of California, Riverside, under the supervision of Yadong Yin. His research interests include synthesis and utilization of nanostructured materials for applications in catalysis and energy storage.
Yiding Liu received his B.S. degree in Materials Physics at the University of Science and Technology of China in 2010. He is currently pursuing his Ph.D. degree at the University of California, 9879
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