ARTICLE pubs.acs.org/JPCC
Water Photooxidation by TiSi2TiO2 Nanotubes Subarna Banerjee, Susanta K. Mohapatra, and Mano Misra* Chemical and Materials Engineering, Mail Stop 388, University of Nevada, Reno, Nevada 89557, United States
bS Supporting Information ABSTRACT: A composite photocatalyst comprised of self-assembled titania (TiO2) nanotubes (NTs) coupled with titanium disilicide (TiSi2) nanoparticles has been synthesized, characterized, and tested for photoelectrochemical hydrogen generation. The TiO2 NT array is prepared by a sonoelectrochemical anodization method using ethylene glycol, ammonium fluoride, and disodium salt of ethylene diaminetetraacetic acid (Na2[H2EDTA]). TiSi2 nanoparticles are produced from commercial bulk particles by a multiple ball milling followed by an ultrasonication process. The coupled catalyst is prepared by impregnating TiSi2 nanoparticles into the TiO2 nanotubes by multiple soaking processes, which leads to a noble structure with TiSi2 nanorods inside the TiO2 nanotubes. The heterostructural composite photoanode exhibited an enhanced photocurrent density of 3.49 mA/cm2 at 0.2 VAg/Ag/Cl compared to TiO2 nanotubes alone (0.9 mA/cm2) and can be considered as a potential possible candidate for the water splitting reaction using visible light. Pure TiSi2 particles coated on ITO are also evaluated for the water splitting reaction, and it is found that TiSi2 possesses good visible light activity (around 55% of the total activity). The combined light absorption (both UV and visible regions) of TiSi2TiO2 NT material as evident from experimental results and the high charge transport properties of a self-assembled one-dimensional nanotube array make the composite potentially promising.
1. INTRODUCTION In recent years, there has been a growing interest in finding sustainable alternative energy sources because of the increasing cost of fossil fuels and the drastic effects of global climate change.14 It is envisioned that hydrogen (H2) has the potential to supplement and ultimately replace fossil fuels. Solar hydrogen production from water is one of the promising green technologies available today. Generally, this process utilizes a semiconductor material to transform solar energy (which is ∼1 kW/(h m2) on the earth's surface) for the production of H2 from water.5 For this purpose, titanium dioxide (TiO2) is widely studied as a semiconductor for hydrogen production due to its high corrosion resistance in aqueous solution and proper band edge positions to split water. However, owing to its relatively large band gap (3.2 eV), TiO2 absorbs light in the ultraviolet (UV) region and hence has a small energy conversion efficiency of solar energy.610 Similar to TiO2, the band gap of zinc oxide (ZnO) (∼3.3 eV) is also too large to effectively use visible light, though it has been used for solar-assisted splitting of water.1113 Hematite (Fe2O3) is theoretically an ideal candidate for photoelectrochemical applications as it is economical, abundant, and chemical and photocorrosion stable. Its low band gap (around 2 eV) is also very suitable for such applications. Unfortunately, the photoefficiency remained at very low values, mostly due to the high level of electronhole recombination in the solid and at grain boundaries.14 CdS is becoming popular in hydrogen evolution but tests are still going on regarding its sacrificial role, though there are reports which suggest differently.15 Thus, there is a real need for a material r 2011 American Chemical Society
which absorbs in the visible light of the solar spectrum, is stable in water, and at the same time is economical. Demuth and co-workers proposed titanium disilicide (TiSi2) as a new semiconductor catalyst for the photocatalytic splitting of water.16 For a semiconductor, TiSi2 has very unusual optoelectronic properties that are ideal for use in solar technology. In addition, this material absorbs light over a wide range of the solar spectrum, is easily obtained, and is inexpensive. The lightabsorption characteristics of TiSi2 are ideal for solar applications: broad-band absorbance measurements show a band gap range from 3.4 eV (ca. 360 nm) to 1.5 eV (ca. 800 nm) for TiSi2.16 This behavior is atypical of semiconductors since these materials usually exhibit small band gap spreads. There have been reports on the optoelectronic and thermoelectric properties of semiconducting silicides.17 Other silicides which have been used for the production of hydrogen from water are β-iron silicide (β-FeSi2)13 and sodium silicide (NaSi).18 Yamaguchi and co-workers have reported the semiconducting properties of β-FeSi2 that are essential for photovoltaic applications.18 The paper discusses some of the properties of semiconducting β-FeSi2 essential for photovoltaic applications and reports highly oriented β-FeSi2 film of 100 nm in thickness deposited on silicon (Si) substrates and the synthesis of single crystals, both p-type and n-type, of β-FeSi2. Lefenfeld and Dye have reported the production of hydrogen from water Received: July 23, 2010 Revised: May 24, 2011 Published: May 25, 2011 12643
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The Journal of Physical Chemistry C by reacting water with NaSi, though this method does not involve photocatalytic splitting of water.19 TiSi2 is an excellent electronic material as it is one of the most conductive silicides (resistivity ∼ 10 μΩ cm)20,21 and is a crucial material in the ultra-large-scale integration semiconductor industry because of its thermodynamic stability and low sheet and contact resistance in gate and source/drain areas which allow fast operation.22,23 Recently, Wang and co-workers have reported on the spontaneous growth of highly conductive two-dimensional single-crystalline TiSi2 nanonets.24 Even though photocatalytic properties of TiSi2 to split water are reported,16 its photoelectrochemical properties are not well investigated. Recently, Wang and co-workers reported a noble TiSi2TiO2 coreshell photocatalyst.25 They prepared the catalyst by filling TiSi2 nanoparticles into the randomly oriented TiO2 nanotubes prepared by the atomic layer deposition method. This new catalyst exhibited good photoelectrochemical properties to split water. In this work, we have explored the possibility of a selfassembled TiO2 nanotube array electrode filled with TiSi2 nanoparticles. The catalyst is investigated for the water splitting reaction using a photoelectrochemical process. The photoelectrochemical process separates the hydrogen and oxygen in two separate compartments, which is a safer and cheaper process than the photocatalytic one. The motivation behind the use of selfassembled TiO2 nanotubes (NTs) are due to their fast charge transport properties and fabrication opportunity, as well as a cost-effective scale-up process (without changing the photoefficiency of the material).2636 The synthesis, characterization, and photoelectrocatalytic properties of this heterojunction photocatalyst are investigated here.
2. MATERIALS AND METHODS 2.1. Materials. Ethylene glycol (Fischer, 99.5%), ammonium fluoride (NH4F, Spectrum, 98%), titanium(IV) chloride (TiCl4, Alfa Aesar, 99.0%), titanium foil (Ti, Alfa Aesar, 0.127 mm thickness, 99.7% purity on metal basis), disodium ethylenediaminetetraacetate (Na2[H2EDTA]) (Fisher Scientific, 99.6%), titanium disilicide (TiSi2, 99.5% metal basis, Alfa Aesar, 325 mesh powder), indium tin oxide coated glass (ITO, Aldrich, 3060 Ω/sq surface resistivity), and P25 titania (TiO2, Degussa) nanoparticles (NPs) are used as received. 2.2. Synthesis of Self-Assembled TiO2 Nanotube Array. Synthesis of TiO2 NTs is carried out on a titanium (Ti) foil in organic media (5% water in ethylene glycol þ 0.5 wt % NH4F þ 0.25 wt % Na2[H2EDTA, pH 6.46.5) using the electrochemical anodization method at a constant temperature of 15 °C. The anodization is carried out for 30 min at an applied potential of 80 Vdc. Ti foil acts as anode and platinum (Pt) acts as cathode. The detailed procedure to synthesize a TiO2 nanotube array has been reported previously.37 The anodized samples are properly washed with distilled water to remove the occluded ions and dried in an air oven to take out the TiO2 layer. This film is etched with aqueous hydrofluoric acid (HF) (5%) from the back side whereby the barrier layer is dissolved. A membrane is obtained by this process with both ends open. The use of Na2[H2EDTA] and high voltage is to make the diameter of the nanotubes bigger than the conventional process so that TiSi2 particles can be loaded into the TiO2 nanotubes. 2.3. Synthesis of TiSi2/TiO2 NT Photocatalyst. The aspurchased large particles are converted to nanoparticles by multistep ball milling followed by ultrasonication in methanol.
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These ball milled and ultrasonicated TiSi2 particles are impregnated into the TiO2 NT surface with the help of 1-octanol. The impregnated material is then annealed under nitrogen (N2) atmosphere in a chemical vapor deposition furnace (CVD furnace, FirstNano) at 500 °C for 6 h to crystallize the TiO2 nanotube array as well as to remove the organics. The prepared TiSi2TiO2 material is then coated on Ti foil using TiCl4 solution followed by annealing at 500 °C for 3 h under N2. This also helped in sintering of the TiSi2 nanoparticles inside the TiO2 nanotubes to form a nanorod array. This process is found to be very simple to make a stable composite electrode of TiSi2 and TiO2 nanotube array (TiSi2/TiO2 NTs). For a comparison, the pure TiO2 NTs are also annealed under N2 atmosphere at 500 °C for 6 h to convert the amorphous TiO2 NTs to crystalline ones. The flow rate of N2 is kept at 200 sccm (standard cubic centimeters per minute) throughout the annealing process. The photoactivities of TiO2 NTs and TiSi2/TiO2 NTs are compared with that of commercial TiO2 NPs (P25, Degussa). A dip-coating method is adopted to coat the P25 NPs on a Ti foil. For this purpose, the P25 NPs (30 mg) are dispersed in a solution containing ethylene glycol (30 mL), ethanol (15 mL), and polyvinylpyrrolidone (15 mg). The Ti foil after P25 coating is dried in an air oven (120 °C) overnight, followed by annealing in an oxygen atmosphere for 3 h at 500 °C in a CVD furnace. This process removes the organics from the thin layer of P25 and makes a uniform film on the Ti foil. The P25/Ti catalyst is characterized by field emission scanning electron microscopy (FESEM), which is available in the Supporting Information (Figure S1). 2.4. Synthesis of TiSi2 on ITO (TiSi2/ITO) and TiSi2 on Ti (TiSi2/Ti). A dip-coating method is adopted to coat the TiSi2 nanoparticles (NPs) on ITO coated glass plate and Ti metal foil. The ball milled and ultrasonicated TiSi2 particles are coated on the ITO and Ti metal support using the dip-coating process. For this purpose, the TiSi2 NPs (30 mg) are dispersed in a solution containing ethylene glycol (30 mL), ethanol (5 mL), and polyvinylpyrrolidone (15 mg). The ITO plate and Ti metal foil (1 cm2) after TiSi2 coating are dried in an air oven (120 °C) overnight, followed by annealing in an oxygen atmosphere at 350 °C for 2 h (ITO plate) and 500 °C for 2 h (Ti foil) in a CVD furnace. This process removes the organics from the thin layer of TiSi2 and makes a uniform film on the ITO plate. For a fair comparison, we tried to keep the thickness of all the materials close to 20 μm. 2.5. Characterization. A field emission scanning electron microscope (FESEM; Hitachi, S-4700) is used to analyze the nanotubenanoparticle composite formation and morphology. Diffuse reflectance ultraviolet and visible (DRUVvis) spectra of the samples are measured from the optical absorption spectra using a UVvis spectrophotometer (UV-2401 PC, Shimadzu). Fine BaSO4 powder is used as a standard for the baseline, and the spectra are recorded in the range 200800 nm. Glancing angle X-ray diffraction (GXRD) is done using a Philips-12045 B/3 diffractometer. The target used in the diffractometer is copper (Cu, λ = 1.54 Å), and the scan rate is 1.2 deg/min. A scanning transmission electron microscope (STEM; Phillips CM 300) equipped with ESVision software is used for mapping and crystal distribution of the samples. A small amount of sample is placed on a carbon coated Cu grid and subjected to high resolution transmission electron microscopy (HRTEM), STEM, and fast Fourier transformation (FFT) measurements. STEM mapping of TiSi2/TiO2 NTs is given in the Supporting Information. 12644
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Figure 1. Schematic showing the formation of TiSi2/TiO2 NT composite structure.
2.5. Photoelectrochemical (PEC) Tests. Experiments on H2 generation from water are carried out using a PEC process in a glass cell with photoanodes (TiSi2/ITO and TiSi2/TiO2 NTs) and cathode (Pt foil) compartments. The compartments are connected by a fine porous glass frit. Ag/AgCl electrode is used as the reference electrode. The cell is provided with a 60 mm diameter quartz window for light incidence. The electrolyte used is an aqueous solution of 1 (M) KOH. A computer-controlled potentiostat (SI 1286) is used to control the potential and record the photocurrent generated. A 300-W solar simulator (69911, Newport-Oriel Instruments) is used as the light source. An AM 1.5 filter is used to obtain one sun intensity, which is illuminated on the photoanode (87 mW/cm2, thermopile detector from Newport-Oriel is used for the measurements). The samples are anodically polarized at a scan rate of 5 mV/s under illumination, and the photocurrent is recorded. All the experiments are carried out under ambient conditions.
3. RESULTS AND DISCUSSION 3.1. Design of Photocatalyst. We have tried to put TiSi2 nanoparticles into the anodized disk (TiO2 nanotube/Ti). In this case, we found that most of the nanoparticles stay on top of the nanotubes and form a film type of structure rather than going inside the nanotubes. This might be due to the air filling the nanotubes which hinders the adsorption of solvent containing TiSi2 nanoparticles. On the other hand, when the nanotubes are open at both ends, the solvent is able to go through the nanotube channels by pushing out the air present inside them.37 The other advantage of this architecture is better photocurrent: the TiO2 NT membrane is free of the barrier layer lying between the NTs and the underlying Ti substrate. The barrier layer can enhance the recombination loss by hindering the electron transfer process to the metal electrode (cathode). This gives less photocurrent which in turn makes the photoanode less photoefficient.38,39 Figure 1 shows the schematic of the formation of TiSi2/TiO2 NT composite structure.
Figure 2. Schematic view to show the effectiveness of the design of TiSi2TiO2 composite to harvest sunlight. e1 = photoelectrons generated from the TiSi2 nanoparticles; e2 = photoelectrons generated from the TiO2 nanotubes; e = (e1 þ e2) = overall photoelectrons generated from the system; h = (h1 þ h2) = overall holes transferred to the interface to react with the electrolyte.
Figure 2 shows the schematic view as to how effective the design of the TiSi2TiO2 composite is to harvest sunlight. The overall photoconversion efficiency of a multiband gap semiconductor is dependent on the position of its conduction and valence bands as well as their geometric arrangement. For efficient interparticle electron transfer between the semiconductors TiSi2 and TiO2, the conduction band of TiO2 must be more anodic than the corresponding band of the sensitizer, i.e., TiSi2. These thermodynamic conditions also favor the phenomenon of electron injections, and thus TiSi2and TiO2 match very well (Figure 2).16 When the system is under UVvis irradiation, both semiconductors (TiSi2 and TiO2) are excited; electrons are injected from TiSi2 to TiO2 as in the case of visible illumination. In addition to this, more photoelectrons are generated from TiO2 NTs by harvesting UV photons. In this case, a high concentration of 12645
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Figure 5. (A) TEM image and (B) HRTEM image and FFT pattern of a single annealed TiSi2 particle. It shows that TiSi2 particle is highly crystalline in nature and crystallizes in orthorhombic structure.
Figure 3. FESEM images of TiO2 NTs prepared in organic medium (5% water in ethylene glycol þ 0.5 wt % NH4F, 0.5 wt % Na2[EDTA], pH 6.8) at 70 Vdc for 30 min. (A) Cross-sectional view, (B) cross-sectional view showing close-up view of the nanotubes, (C) top view, and (D) bottom view of the TiO2 NTs. The internal tube diameter is observed around 122 nm, and the length of the tubes is observed around 22 μm.
Figure 6. FESEM image of TiSi2 particles sintered into the TiO2 NT array. Inset shows a TEM image of TiSi2TiO2 nanotubes. Figure 4. FESEM images of (A) as-received TiSi2 particles (
