Self-Assembly of Metal and Metal Oxide Nanoparticles and Nanowires

Article pubs.acs.org/cm

Self-Assembly of Metal and Metal Oxide Nanoparticles and Nanowires into a Macroscopic Ternary Aerogel Monolith with Tailored Photocatalytic Properties Florian J. Heiligtag,† Wei Cheng,† Vagner R. de Mendonça,‡ Martin J. Süess,†,§ Kathrin Hametner,⊥ Detlef Günther,⊥ Caue Ribeiro,∥ and Markus Niederberger*,† †

Laboratory of Multifunctional Materials, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, CH-8093 Zurich, Switzerland ‡ Department of Chemistry, Federal University of São Carlos, Rod. Washington Luís, km 235, São Carlos, Sao Paulo CEP 13560-970, Brazil § Scientific Center for Optical and Electron Microscopy (SCOPEM), Auguste-Piccard-Hof 1, CH-8093 Zurich, Switzerland ⊥ Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093 Zurich, Switzerland ∥ Embrapa Instrumentation, Rua XV de Novembro, 1452, Caixa Postal 741, São Carlos, Sao Paulo CEP 13560-970, Brazil S Supporting Information *

ABSTRACT: Self-assembly processes represent the most powerful strategy to produce complex materials with unique structural and compositional sophistication. Here we present such a self-assembly route to a three-component aerogel from preformed nanoparticle building blocks. Starting with a mixture of gold and anatase nanoparticles and tungsten oxide nanowires, controlled cogelation resulted in the formation of a macroscopic aerogel monolith with high specific surface area and porosity, remarkable transparency, and excellent crystallinity. The modular approach enables us to finetune the composition of the aerogels, and thus their properties, by choosing the appropriate building blocks and their relative concentration ratios. As an illustrative example, we show the targeted tailoring of the photocatalytic activity: the gold nanoparticles and the tungsten oxide nanowires both add their specific beneficial effects to the anatase aerogel matrix, leading to a superior performance of the three-component system.

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INTRODUCTION The design of materials with new and improved properties in terms of energy generation, energy storage, and energy conversion is one of the big challenges in materials science and technology. Although solar cells,1 and battery materials2 address generation and storage, respectively, the field of (photo)catalysis holds the most promising research to advance energy conversion.3,4 In photocatalysis research, like in other areas of materials chemistry, two general strategies can be followed on the way to develop materials with optimized performance: (i) the discovery of completely new materials, or (ii) the combination of well-known materials with additive or synergistic properties and innovative morphologies. Anatase nanoparticles are one of the most prominent examples of nanosized photocatalysts, and many reviews have been published about this subject in the last years.4−8 Pursuing strategy (ii), anatase is an excellent example for the possibilities to improve the performance of a known materialfor example, by band gap engineering (to optimize the absorption of light), electron−hole pair stabilization (to prolong the lifetime of the active species and thus the catalyst’s efficiency), and by © 2014 American Chemical Society

increasing the accessibility of the catalyst’s surface and its affinity for target molecules (to ensure a constant supply of target molecules). The chemist’s toolbox to challenge these problems is rich and includes doping for band gap narrowing and electron−hole pair stabilization,9 or decoration of the catalyst with metals or other semiconductors for electron−hole pair stabilization by electron extraction.10,11 The combination of different materials also offers possibilities for adjusting the adsorption−desorption equilibrium between the target molecules and the catalyst by a change of surface polarity.12 To ensure a good supply of target molecules, the accessibility of the surface of the photocatalyst represents another important parameter. Although nanoparticles typically show superior performance, because of their high surface-to-volume ratio, their tendency to agglomerate can dramatically reduce the accessible surface area. The problem can be solved by using a nanostructured bulk material with high porosity, in which the Received: June 6, 2014 Revised: September 2, 2014 Published: September 23, 2014 5576

dx.doi.org/10.1021/cm502063f | Chem. Mater. 2014, 26, 5576−5584

Chemistry of Materials

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aerogels with a monolithic body. To preserve the fine pore structure of the multicomponent gels, critical point drying with liquid CO2 was performed. The photocatalytic properties of the aerogels were characterized by their capability of ·OH radical formation and methylene blue dye degradation under UV light.

nanoparticles are connected with each other in a threedimensional open network, thus preventing agglomeration, but still offering high surface area and excellent accessibility of the active surfaces. Aerogels are materials that perfectly fulfill these requirements.13 The general synthesis approach involves aqueous sol−gel routes.14 However, these methods suffer from drawbacks that are particularly detrimental if the aerogels are designed for applications in photocatalysis. One major problem is the amorphous nature of the as-synthesized aerogels. Any type of postsynthetic thermal treatment to induce crystallization is associated with a loss of surface area and porosity,11 thus reducing the main benefit of aerogels. The second issue is related to the limited compositional complexity accessible by aqueous sol−gel chemistry. The vast majority of work is dedicated to silica-based aerogels15 and thus does not provide a large range of functionalities. In fact, it is a great challenge to synthesize binary or ternary transition-metal-oxidebased aerogels or composite aerogels consisting of different types of materials via sol−gel routes. Even if the chemical reactivity of the different precursors is carefully adjusted,14 it is not possible to prepare aerogels consisting of metal and metal oxide nanoparticles with different sizes and shapes combined within one body. All these drawbacks of the sol−gel route can be avoided by the use of preformed particles as building blocks.16 The first examples using this strategy were reported for CdS gels and aerogels,17 where the controlled destabilization of highly concentrated nanoparticle dispersions lead to a gel and finally an aerogel. The crystalline phase of the resulting aerogel can be predefined by the nanoparticles used as building blocks and remains unchanged during processing. Most importantly, features related to the nanosize like the luminescence properties of quantum dots could be maintained.18 Meanwhile, the family of nanoparticle based aerogels has grown considerably, including PbS,19 ZnS,19 or CdSe,20 BaTiO3,21 Au,22 Ag,22 and Pt,22 as well as two-component materials such as CdS-Au24 and CdS-Ag,24 Although in recent examples, the gold nanoparticles were grown on the aerogel matrix formed by the CdS nanoparticles, in other cases, the second component was added separately before the gelation was induced, for example, in CdTe-Au,25 TiO2−Au,26 TiO2− SiO2,27 Au−Ag,22 and Pt−Ag.22 A very recent work used uniformly distributed, metallic building blocks to synthesize even a four-component Au−Ag−Pt−Pd aerogel.28 It is obvious that the extension of such a cogelation approach beyond components of a single material class and beyond isotropic building blocks would open up fascinating possibilities for the creation of aerogels with unprecedented architectural complexity. Here we report an important step in this direction: the preparation of a ternary aerogel consisting of gold nanoparticles and tungsten oxide nanowires embedded in an anatase TiO2 matrix. Each of these three components specifically contributes to the performance of the aerogel as a photocatalyst. The anatase framework offers a high surface area with a large open porosity and defined crystallinity in the photocatalytically most active modification. The tungsten oxide nanowires provide enhanced adsorption of the target molecules,12 and the gold nanoparticles increase the lifetime of the electron−hole pairs produced during the photocatalytic process.29 After optimization of the cogelation process (i.e., controlled destabilization of the stable nanoparticle dispersions), the process can achieve not only high homogeneity in the distribution of all building blocks in the material but also high transparency of gels and

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RESULTS AND DISCUSSION The process of anatase gelation is a significantly improved adaptation of a previously published protocol,26 involving a controlled destabilization of trizma (2-amino-2-(hydroxymethyl)-1,3-propanediol)-functionalized anatase nanoparticles in water. When these dispersions are slightly heated, desorption of trizma molecules from the {001} facets on opposite sides of the anatase nanoparticles leads to a destabilization of these facets, inducing oriented attachment along [001].30 Whereas under dilute conditions the oriented attachment results in the formation of individual pearl-necklace-like structures up to several micrometers in length,30,31 in higher-concentration dispersions, the oriented attachment is less perfect, leading first to branching and cross-linking of the linear assemblies and then to the solidification of the dispersion into a gel.26 The addition of gold nanoparticles as a second and especially tungsten oxide nanowires as a third component not only enhances the photocatalytic performance of the aerogels but also illustrates that nanoparticles with different morphological motives (like needles and wires) can be incorporated in the anatase aerogel network. In principle, the three nanobuilding blocks can be mixed in different ratios and over a wide concentration range, as long as the corresponding dispersions can be prepared in sufficient quality. Of course, the amount of titania relative to the other components has to be high enough to provide the rigid framework structure. Collapse of the open pore structure is prevented by a supercritical drying process with CO2, resulting in translucent, monolithic aerogels with macroscopic dimensions (Figure 1).

Figure 1. Digital photograph of a translucent monolith of a ternary anatase−tungsten oxide−gold composite aerogel.

Such a high degree of translucency indicates a high level of structural homogeneity, which is only achievable if the initial nanoparticle dispersion of all three components is of high quality. One of the big advantages of a particle-based aerogel is its defined crystallinity. The high crystallinity of the nanoscale building blocks, which is a characteristic of the nonaqueous 5577



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pronounced diffraction peak is the (010) reflection, indicating that the nanowires grow along the [010] direction.33 Independent of the number of components, the XRD patterns of all aerogels only reveal the anatase phase simply due to the fact that the concentrations of the other two components is below the detection limit, which can be estimated to be at about 5%, because the nanosize of our building blocks leads to severe peak broadening and makes it especially difficult to distinguish different phases in the XRD pattern. To confirm the presence of the tungsten oxide nanowires and the gold nanoparticles in the aerogel, inorganic elemental analysis on the aerogels by laser ablation coupled to inductively coupled plasma mass spectrometry (LA-ICP-MS) was performed. The operation conditions are given in Table S1 in the Supporting Information, and the results in weight fractions are summarized in Table 1. Molar fractions can be found in Table S2 in the Supporting Information. All ratios are calculated with respect to TiO2 as internal standard, which is set to 100 wt % for the samples without tungsten oxide and to 95.4 wt % for the samples with tungsten oxide to account for the high amount of tungsten oxide of the total weight. For the calculation of the weight ratios, a formula of WO3 for the tungsten oxide nanowires was used. The weight ratio of WO3 in the binary (TiO2−WOx) and in the ternary (TiO2−WOx−Au) aerogel matrices was 4.6 ± 0.15% and 7.2 ± 0.39%, respectively. The small standard deviation of the values points to a homogeneous distribution of the tungsten oxide nanowires in the macroscopic samples. The content of gold in the TiO2−Au and TiO2−WOx−Au aerogels is 880 ± 40 ppm and 1130 ± 20 ppm, respectively. Also here, the standard deviation, reflecting the local distribution of gold within the samples, is small. The observation of homogeneously distributed gold nanoparticles and tungsten oxide nanowires in the aerogel underlines again the good quality of the initial dispersions. Interesting to mention is the relatively high chlorine content of more than 0.2%, which is a residue of TiCl4 used as precursor for the anatase nanoparticles. The morphology of the final aerogels was evaluated by scanning electron microscopy (SEM). An overview SEM image at low magnification shows a homogeneous architecture of the TiO2−WOx−Au aerogel sample over a large area (Figure 3a). At higher magnification, a continuous porous network is visible with a pore size ranging from around 50 to a few hundreds of nanometers (Figure 3b). The very fine and branched network of the anatase nanoparticles becomes obvious when a very thin part on the edge of the aerogel structure is monitored (Figure 3c). The other building blocks are not distinguishable in these SEM images due to their low concentrations and, in the case of tungsten oxide nanowires, due to their morphology similar to the anatase network. To further study the nanostructure of the aerogels and their nanoparticle building blocks, transmission electron microscopy

sol−gel route used for their synthesis, remains unaffected during the gelation and processing of the nanoparticles into the gel and aerogel. The crystalline phase of the titania and the tungsten oxide building blocks as well as the one of the resulting aerogels was characterized by powder X-ray diffraction (XRD). The corresponding patterns are shown in Figure 2. The

Figure 2. XRD patterns of the tungsten oxide nanowires, trizmafunctionalized anatase nanoparticles, the pure titania aerogel, and the ternary titania−tungsten oxide−gold composite aerogel.

as-synthesized trizma-functionalized titania nanoparticles can exclusively be assigned to the anatase phase (ICDD file card no. 1-70-6826). The crystallite size was determined to be 2.8 nm by the Scherrer equation using the (101) reflection. The XRD pattern of the pure anatase aerogel is basically the same with only two minor differences. The crystallite size in [101] direction slightly increases to 3.3 nm, and also the crystallite size in [001] direction is more elongated in comparison to the as-synthesized nanopowder due to the oriented attachment along this direction. However, the latter effect cannot be quantified by the Scherrer equation, because the (004) reflection is superimposed by the (103) and (112) reflections. Moreover, the effect is not as pronounced as in the case of a perfect oriented attachment. Penn and Banfield, for example, in their seminal work on coarsening of anatase nanparticles, observed that under hydrothermal conditions, there is a preferred growth along [001], which resulted in the sharpening of the corresponding reflections in the XRD pattern.32 In our case, the nanoparticle assembly occurs at much lower temperatures, so that it can be expected that most of the crystallites do not fully fuse together and also the large degree of orientational misalignment prevents the development of more intense reflections along the c-axis.30 This is particularly true for higher-order reflections like the (004) peak. The assynthesized tungsten oxide nanowires can be assigned to W18O49 (ICDD file card no. 36-0101). The only well-

Table 1. Results of the Inorganic Elemental Analysis by LA-ICP-MS

TiO2 aerogel TiO2−Au aerogel TiO2−WOx aerogel TiO2−WOx-Au aerogel a

TiO2

WO3

Au

Cl

wt %

wt %

ppm

ppm

100 100 95.4 95.4

0.012 ± 0.014 0.0040 ± 0.0006 4.42 ± 0.15 7.26 ± 0.39

270 ± 120 880 ± 40 see belowa 1130 ± 20

2260 2600 2820 2560

± ± ± ±

210 200 390 120

Value not measured. 5578



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confirms the presence of gold (Figure 4d). Besides gold, high amounts of copper from the TEM grid and small amounts of titanium from the aerogel matrix were measured. Individual, single tungsten oxide nanowires are not detectable, because they have a diameter of just 1 nm, as shown in a HRTEM image of a pure tungsten oxide sample as reference (Figure 5a). However, bundles of tungsten oxide nanowires are visible in a TEM image of the aerogel (Figure 5b). A high-resolution TEM image clearly shows the crystalline tungsten oxide nanowires embedded in the crystalline anatase matrix (Figure 5c). The high crystallinity of all building blocks is confirmed by fast Fourier transform (FFT) of the areas marked with colored squares in Figure 5a,c. The matrix (blue square) can be assigned to the anatase phase and the tungsten oxide nanowires in Figure 5a, and in Figure 5c, both can be assigned to W18O49 in agreement with XRD data. For the determination of the specific surface area, pore-size distribution, and porosity, nitrogen sorption experiments were carried out. The specific surface area calculated by the multipoint Brunauer−Emmett−Teller (BET) method ranges from 509 m2/g for the pure TiO2 aerogel to 473 m2/g for the three-component aerogel. These values are larger than those previously published for nanoparticle-based aerogels prepared from trizma-functionalized anatase nanoparticles (302−405 m2/g),26 due to optimization of the synthesis protocol. Initially we prepared our titania nanoparticle dispersions from dried powders, and now we use the wet precipitates, which considerably improves the quality of the dispersions. The specific surface area of the aerogels is also significantly larger than that of the as-synthesized dried trizma-functionalized anatase powder (252 m2/g). The reduced surface area of the assynthesized powder is clearly a result of random agglomeration, which is avoided in the aerogels. The samples were screened for micropores according to the t-plot method, but no microporosity was found. The Barrett−Joyner−Halenda (BJH) poresize distribution calculations carried out on the desorption branch of the nitrogen sorption isotherm produced questionable results, because repeated adsorption−desorption cycles resulted in a shift of the isotherm in all prepared samples (Figure 6a). The adsorption branch of the isotherm shows a very steep rise during the first run of the measurement without a plateau region. This observation shows that it is highly probable that there are pores large enough to be outside the measurement range, in this case larger than 220 nm.34 These pores cannot be found in the second and third measurement run, and these runs show a pronounced plateau region. Although equilibration time is a known problem for the measurement of aerogels,35,36 here we do not expect this to be the source of shifting isotherms, because a change in equilibration time did not result in a significant change of this effect. The effect of changes in the isotherm can be better explained by a breakdown of the pore structure during the desorption cycle of the measurement, caused by capillary forces induced by the condensation and evaporation of N2 in the pores.36,37 This is also reflected by the pore-size distribution graphs according to the BJH (Figure 6b) or the density functional theory (DFT) method (Figure 6c), which show small amounts of residual porosity larger than 30 nm after the first run and no such porosity after the second run. Also a reduction of the BET specific surface area can be observed during the measurements (Figure 6d), additionally supported by the observation of a reduction in translucency of the sample after every measurement run. This collapse of the pore

Figure 3. SEM images of an anatase−tungsten oxide−gold composite aerogel in (a) low and (b) high magnification. (c) SEM image from the edge of such a ternary composite aerogel.

(TEM) was used. An image of a TiO2−WOx−Au aerogel at intermediate magnification shows the fine-structure of the titania network (Figure 4a). Linear sections of 50−60 nm in length are connected with each other by junctions. Z-contrast reveals the gold nanoparticles as bright dots in the anatase network when high-angle annular dark field-scanning TEM (HAADF-STEM) mode is used (Figure 4b, red arrows). The tungsten oxide nanowires are not distinguishable in these images. A high-resolution TEM (HRTEM) image of a 10 nm gold nanoparticle shows high crystallinity, visible by the welldeveloped lattice fringes (Figure 4c). Energy-dispersive X-ray spectroscopy (EDX) of the area with the gold nanoparticle 5579



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Figure 4. (a) TEM image of a TiO2−WOx−Au aerogel showing linear assemblies of anatase nanoparticles branched to a network. (b) HAADFSTEM image showing gold nanoparticles as bright spots (red arrows) in the anatase matrix in the aerogel TiO2−WOx−Au. (c) HRTEM image of a gold nanoparticle. (d) EDX of this gold nanoparticle. The Cu signature in (d) stems from the copper grid.

structure caused by the measurement makes it difficult to determine the actual pore-size distribution. The fact that the small forces of N2 desorption are easily able to change the sample can be explained by the nature of the sample. Our nanoparticle-based aerogels are even more fragile than aerogels synthesized by sol−gel chemistry. The building blocks stick together only very weakly by an often imperfectly oriented attachment. For this reason, it is not possible to perform mercury porosimetry, which would destroy the fragile structure of the aerogel. Finally, we can only estimate the pore-size distribution to have major contributions of pores larger than around 75 nm. The onset point of the adsorption branch of the first run of the measurement being at a relative pressure of 0.977 corresponds to pores of 74 nm. The total pore volume is supposed to be larger than the measured 4.7 cm3/g, which means that the porosity is expected to be larger than 95%. The steep rise of the isotherm at high relative pressures and the lack of a plateau region in the first run suggest the existence of further pores outside the measurement range of 220 nm. The porosity can be adjusted through the concentration of the nanoparticles in the initial dispersion. However, at higher nanoparticle concentrations, the stability of the nanoparticle dispersion becomes an issue, and the final aerogels lose their optical translucency. To elaborate the benefits of the multicomponent nature of our aerogels, the photocatalytic performance of the composites was evaluated by the capability of ·OH radical formation, as well as by the photocatalytic decomposition of methylene blue

(MB) as a positively charged model system, both under UV light. The decrease of the MB concentration over time is shown in Figure 7a. Although all the samples showed remarkable photoactivity, the measurement with the standard TiO2 material P25 showed the best result, and the aerogel samples containing WOx performed slightly better compared to the other aerogels (Figure 7a). WOx is known to increase the interaction between the dye and the photocatalyst. Li et al.12 studied the Zeta potential of a TiO2−WOx composite, and they found a shift of the isoelectric point (IEP) to lower pH values from ∼6.3 to ∼5.0 for TiO2−WOx compared to pure TiO2. The pH of our MB solution, measured by a pH meter, is around 6.5, and therefore, we can assume that the pure TiO2 aerogel is not able to have a good interaction with the positively charged MB dye. This assumption is supported by the fact that the trizma-functionalized titania used for our aerogels was shown to have an IEP between 7 and 8,27 which is higher than the IEP for pure TiO2 as reported by Li et al.12 Based on Li’s work, the composites containing WOx should provide a more negative surface at the pH of the MB solution, favoring the interaction between the positively charged dye and the photocatalyst. As a matter of fact, we found that aerogels containing WOx were able to adsorb more dye molecules than those without WOx. This observation was based on adsorption measurements, where the catalysts were equilibrated in a dye solution in the dark, similar to the photocatalysis tests, and after equilibration, the free dye concentration was determined by UV−vis measurements. Taking the average MB adsorption of 5580



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Figure 5. (a) HRTEM image of a single tungsten oxide nanowire in a pure tungsten oxide sample. (b) Overview TEM image of a bundle of tungsten oxide nanowires embedded in the anatase matrix in a binary aerogel. (c) HRTEM image of the W18O49 nanowire bundle. (d−f) FFT from images (a) and (c) proving the phase of the structures.

Figure 6. (a) N2 sorption isotherms measured three consecutive times using a TiO2 aerogel as example. (b) BJH pore-size distribution calculated from the desorption branches of the isotherms of (a). (c) Pore-size distribution by DFT calculations of the isotherms. (d) Development of the BET specific surface area during the three runs of the measurement calculated from (a).

are still, at least partly, attached to the surface of the corresponding nanoparticle building blocks. Accordingly, TGA measurements (data not shown) reveal that the aerogels contain about 7% adsorbed water and about 15% adsorbed organics. However, it seems as if they do not significantly affect the photocatalytic properties. The photocatalytic performance does not change significantly during the photocatalytic reaction

all aerogels containing tungsten oxide in comparison to the average MB adsorption of all aerogels without tungsten oxide, the incorporation of tungsten oxide doubles the adsorption of MB after an equilibration time of 120 min (Figure 7b). The dye adsorption of P25 is lower, due to a lower specific surface area, which is typically around 50 m2/g. It is important to mention that all the organic stabilizers (trizma, octylamine, and citrate) 5581



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Figure 7. Overview of the photocatalytic experiments. (a) Photodegradation of MB in the presence of the different aerogels and P25. (b) MB concentration after equilibration of aerogels in MB solution showing strong adsorption for tungsten oxide containing aerogels. (c) Spectra related to 2-hydroxyterephtalic acid formation after different times of UV irradiation. (d) Plot showing the induced fluorescence intensity at 425 nm against UV irradiation time.

Table 2 presents the values of both kMB and kOH for the various aerogels and the dependence of kMB on kOH is plotted in

due to possible decomposition of the organics on the surface, even if the photocatalyst is recovered after the reaction and reused. Due to the fragility of the aerogel monoliths, the organic compounds cannot be simply removed by calcination without pulverizing the macroscopic body and without considerable crystal growth. Plots of the logarithm of the relative concentration of MB in the solution ln([MB]/[MB]0) versus irradiation time (not shown) are approximated to be linear, as expected for a pseudofirst-order kinetics,38 and therefore, the slope corresponds to the rate constant kMB. This rate constant can be determined for each sample and used as a measure of the samples’ photoactivity. The concentration of a blank MB solution without any catalyst hardly changed under irradiation conditions, hence ruling out a significant contribution of MB photolysis (Figure 7a). To further investigate the photocatalytic properties of the samples, we performed an analysis of ·OH radical formation under UV radiation using terephtalic acid as a radical trap.39 The 2-hydroxyterephtalic acid, product of the reaction between ·OH radical and terephtalic acid, is detected by its strong fluorescence at 425 nm when excited at 315 nm (Figure 7c). The intensity of fluorescence is proportional to the ·OH concentration. Figure 7d summarizes the obtained results. The slope of the line in Figure 7d, kOH, is proportional to the rate of ·OH radical formation, and therefore, this value was used to evaluate the composites’ photocatalytic efficiency.

Table 2. Values of kMB and kOH and Their Standard Deviation δ for the Different Aerogels samples TiO2 TiO2−WOx TiO2−Au TiO2−WOx−Au

kMB ± δ [103 min−1] 7.2 8.5 7.2 8.7

± ± ± ±

0.4 1.4 0.3 0.6

kOH ± δ [min−1] 1.37 1.26 1.80 1.78

± ± ± ±

0.23 0.15 0.04 0.03

Figure 8. The two samples with Au nanoparticles show better performances in ·OH radical generation, clearly showing the effect of Au nanoparticles in the composites, probably by increasing the lifetime of the photogenerated charges.29 Within the limits of the measurement, WOx does not affect the efficiency of TiO2 and TiO2−Au composite in the formation of ·OH radicals, but rather, it does affect the degradation rate for MB. The increase in kMB is directly related to the addition of WOx in the composite (blue arrow), while the increase in kOH is, in turn, related to the presence of Au nanoparticles in the aerogel (green arrow). The three-component aerogel was able to maintain both enhancements related to each component, showing thus improved properties compared to the two component composites or to pure TiO2, as highlighted by the red arrow in Figure 8. Consequently, not every component contributes to every photocatalytic reaction. Instead, the 5582



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ACKNOWLEDGMENTS

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REFERENCES

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We thank the Scientific Center for Optical and Electron Microscopy (SCOPEM) of ETH for access to TEM and SEM facilities. We thank Damian Renggli for his work in the lab, Dr. Alessandro Lauria for his assistance with the sample preparation for the LA-ICP-MS measurements, Marta J. I. Airaghi Leccardi for providing the photograph of the aerogel, and Niklaus Kränzlin for performing the SEM measurements. F.J.H. and M.N. are grateful for the financial support from ETH Zurich (ETH-07 09-2). W.C. acknowledges a fellowship from the China Scholarship Council. V.R.M. thanks CNPq for financial support. Figure 8. Dependence of the rate constant of ·OH radical formation (kOH) on the rate constant for MB photodegradation (kMB).

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different nanoparticles are able to catalyze different reactions, making these aerogels a truly multifunctional photocatalyst. Nevertheless, the scope of these tests was not necessarily the development of a superior photocatalyst but to present a proofof-concept that such a modular approach makes it possible to combine different types of nanoparticles homogeneously in one material. Moreover, during the photocatalytic tests in water, the monolithic body falls apart into pieces. Therefore, to take full advantage of the monolithic structure, in a next step, the mechanical stability of the aerogels has to be improved, or they have to be stabilized within a mold. On the other hand, it can be expected that the aerogels are stable enough for gas-phase reactions.

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CONCLUSIONS We showed that the nanoparticle-based fabrication of aerogels gives access not only to monolithic aerogels with high surface area, good porosity, defined crystallinity, and outstanding transparency, but the method also allows us to acquire tailormade multifunctional materials with high compositional and morphological complexity. On the basis of a three-component example consisting of anatase and gold nanoparticles as well as tungsten oxide nanowires, we show how such a combination of different building blocks can be used to systematically tune the chemical properties like the photocatalytic activity. The beneficial contributions of all three components make this material superior to the two-component systems. Such a modular approach represents a versatile tool to predesign the aerogels and their properties by a careful selection of the concentration, crystal phase, and shape of the respective building blocks.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details about nanoparticle syntheses, aerogel processing, and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5583



Article

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