Au-TiO<sub>2</sub> Nanocomposites and Efficient Photocatalytic Hydrogen Production under UV-Visible and Visible Light Illuminations: A Comparison of Different Crystalline Forms of TiO<sub>2</sub>

Jose, Deepa; Sorensen, Christopher M.; Rayalu, Sadhana S.; Shrestha, Khadga M.; Klabunde, Kenneth J.

doi:https://doi.org/10.1155/2013/685614

International Journal of Photoenergy

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Abstract Introduction Materials and Methods Results Conclusions Acknowledgments Supplementary Materials References Copyright Related Articles

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Progress and Perspectives in Visible-Light-Driven Photocatalysis

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Volume 2013 | Article ID 685614 | https://doi.org/10.1155/2013/685614

Au-TiO₂ Nanocomposites and Efficient Photocatalytic Hydrogen Production under UV-Visible and Visible Light Illuminations: A Comparison of Different Crystalline Forms of TiO₂

Deepa Jose,¹Christopher M. Sorensen,²Sadhana S. Rayalu,^1,3Khadga M. Shrestha,¹and Kenneth J. Klabunde¹

Academic Editor: Elias Stathatos

Received14 Jan 2013

Revised05 Mar 2013

Accepted11 Mar 2013

Published04 Apr 2013

Abstract

nanocomposites were prepared by the solvated metal atom dispersion (SMAD) method, and the as-prepared samples were characterized by diffuse reflectance UV-visible spectroscopy, powder XRD, BET surface analysis measurements, and transmission electron microscopy bright field imaging. The particle size of the embedded Au nanoparticles ranged from 1 to 10 nm. These Au/TiO₂ nanocomposites were used for photocatalytic hydrogen production in the presence of a sacrificial electron donor like ethanol or methanol under UV-visible and visible light illumination. These nanocomposites showed very good photocatalytic activity toward hydrogen production under UV-visible conditions, whereas under visible light illumination, there was considerably less hydrogen produced. Au/P25 gave a hydrogen evolution rate of 1600 μmol/h in the presence of ethanol (5 volume %) under UV-visible illumination. In the case of Au/TiO₂ prepared by the SMAD method, the presence of Au nanoparticles serves two purposes: as an electron sink gathering electrons from the conduction band (CB) of TiO₂ and as a reactive site for water/ethanol reduction to generate hydrogen gas. We also observed hydrogen production by water splitting in the absence of a sacrificial electron donor using Au/TiO₂ nanocomposites under UV-visible illumination.

1. Introduction

Ever since the first report of water splitting using a TiO₂ photoanode by Fujishima and Honda, researchers all over the world have been trying to develop efficient solar energy harvesting semiconductor materials for water splitting and thereby producing clean hydrogen energy [1]. Several strategies like band gap engineering of semiconductor materials, cocatalyst loading, two-step photoexcitation (Z scheme), and so forth have been developed to achieve efficient overall water splitting to produce hydrogen and oxygen [2–5]. Recently, people have been trying to exploit the UV and visible absorption of metal nanoparticles like Au, Ag, and Cu by embedding them on a semiconductor material [6, 7]. Among the various semiconductor catalysts, TiO₂ is one of the most promising due to its high chemical stability, nontoxicity, and low cost [8]. The only limitation of TiO₂ as a photocatalyst is its poor efficiency in the visible region of the solar spectrum due to its wide band gap (3.2 eV for anatase). However, TiO₂ can show some visible photocatalytic activity due to the presence of some rutile form of TiO₂ and/or by doping it with metal or nonmetallic elements [9–11] or by deposition of noble metal nanoparticles on TiO₂ [12–15]. Metal nanoparticle loaded TiO₂ showed enhanced photocatalytic activity under UV or visible light illumination. The improved UV activity of these photocatalysts is attributed to the better charge separation by electron transfer from the conduction band (CB) of TiO₂ to Au nanoparticles [16–18], whereas the observed visible activity is explained as photoexcitation of Au nanoparticles (due to surface plasmon resonance effect) and charge separation by the transfer of photoexcited electrons from Au to the CB of TiO₂ [16, 19–25]. There are some other reports explaining the enhanced visible activity of Au/TiO₂ due to local electric field enhancement near the TiO₂ surface by the Au surface plasmon resonance [26–29].

There are several methods reported in the literature for the synthesis of TiO₂ supported gold nanoparticles such as adsorption of preformed Au colloids [30], photodeposition [31, 32], deposition precipitation [33], impregnation [33], and chemical reduction [34]. We used a different approach for the synthesis of these Au loaded TiO₂ nanocomposites: the solvated metal atom dispersion (SMAD) method. The SMAD technique has been used extensively in our laboratories for the gram scale synthesis of nanoparticles of metals like Au, Ag, Cu, metal chalcogenides, [35–38]. The advantages of this method over other routes reported in the literature are that no byproducts are formed, reproducibility and scalability. We used different types of TiO₂: anatase (UV active), rutile (visible active), and P25 (a mixture of 75% anatase and 25% rutile) for Au loading. These Au (~1 wt%) loaded TiO₂ nanocomposites, prepared by the SMAD method were used as photocatalysts toward photocatalytic hydrogen production, in the presence or absence of a sacrificial electron donor ethanol (5 volume%) under UV-visible and visible conditions. This paper will discuss our findings regarding the role of Au nanoparticles and the role of different crystalline forms of TiO₂.

2. Materials and Methods

2.1. Materials

The different phases of TiO₂ used were anatase (Alfa Aesar, 99.9% and anatase prepared by aerogel method in our laboratories), rutile (Sigma Aldrich, 99.99%), and P25 (Degussa). Sodium sulfite (98.3%) was purchased from Fischer Scientific. Butanone (Sigma Aldrich, 99.7%) was distilled and freeze pump thaw processed for five cycles prior to use in SMAD experiments. Ethanol (200 Proof) from Decon Labs was used for photochemical reactions.

2.2. Instrumentation

Diffuse reflectance UV-visible spectra (DRS UV) were recorded using a Cary 500 scan UV-Vis-NIR spectrophotometer operating in air at room temperature over the range from 200 to 800 nm. Brunauer-Emmet-Teller (BET) measurements of surface area and pore size distribution of TiO₂ and Au/TiO₂ nanocomposites were determined using a Quantachrome NOVA 1200 N₂ gas adsorption/desorption analyzer at liquid nitrogen temperature. Powder XRD analysis of these samples was carried out using a Scintag-XDS-2000 spectrometer with Cu Kα radiation with applied voltage of 40 kV and current of 40 mA. TEM bright filed images were taken using a Phillips CM100 electron microscope operating at 100 kV. Bulk elemental analysis was carried out using Perkin Elmer Optima 4300DV spectrometer (ICP-OES) at Galbraith laboratories Inc.

2.3. Preparation of Au/TiO₂ Nanocomposites

Au loaded TiO₂ nanocomposites were prepared by the SMAD method. The details of the SMAD technique are given elsewhere [39]. In a typical experiment, the crucible was loaded with Au shot (~60 mg) and TiO₂ (P25, anatase commercial, anatase aerogel, or rutile) (~2960 mg) was placed in the bottom of the reactor. Liquid nitrogen cooling and vacuum were applied to the reactor and the crucible was heated resistively under vacuum in such a manner that there was cocondensation of Au atoms with solvent molecules (butanone) on the walls of the reactor. The reactor was brought up to room temperature under Ar atmosphere once the metal evaporation was complete. Upon matrix (Au-butanone) melt down, the Au-butanone colloid comes into contact with TiO₂ at the bottom of the reactor. The Au-butanone/TiO₂ mixture was stirred vigorously under Ar atmosphere for 2 h. The color of the slurry changed from dark blue to purple during the stirring process. Butanone was removed from the mixture by applying vacuum, and the dried powder was used for further studies. In addition to the vacuum drying, Au/TiO₂ samples for control experiments were calcined in air at 200°C for 2 h (heating rate, 5°C/min) to make sure that there was no butanone remaining. The catalysts prepared by the SMAD method are shown in Figure S1 (see Supplementary Material available online at doi: http://dx.doi.org/10.1155/2013/685614), and their color varied from lavender to purple.

2.4. Photocatalytic Water Splitting Experiments Using a 450 W Mercury Lamp

The photocatalytic water splitting experiments were carried out in a glass enclosed reaction chamber with a quartz inner radiation reaction vessel. The glass chamber was connected to a gas circulation evacuation and water cooling system. In a typical experiment, 255 mg catalyst (as prepared Au/TiO₂ or Au/TiO₂ calcined at 200°C for 2 h), 322 mL distilled water, and 18 mL ethanol (or 18 mL methanol or 20 mM sodium sulfite) as sacrificial electron donor was taken in the glass reactor with a magnetic stir bar. The reaction mixture was evacuated and filled with Ar five times to remove all the dissolved gases. This was followed by irradiation using a 450 W high pressure Hg lamp via a quartz tube. Water at 20°C was circulated continuously through the outer walls of the reactor and the quartz vessel to make sure that the temperature of the reaction mixture did not exceed 35°C [40]. The activity of these catalysts for hydrogen production was investigated during the first 5 h irradiation period, using a fresh catalyst each time. 2 M NaNO₂ solution was used as a filter to cut off UV radiation in our visible studies (UV-visible absorption spectrum of 2 M NaNO₂ is given in the supporting information). H₂ production was monitored using an online GC system (GOW-MAC 580 model) employing an All Tech molecular 80/100 sieve 5 A column with Ar as the carrier gas and a thermal conductivity detector.

3. Results and Discussions

3.1. Au/TiO₂ Nanocomposites Characterization

Au nanoparticles (~1 wt%) loaded on to a variety of anatase, rutile, and a mixture of anatase and rutile (P25) TiO₂ were prepared by the SMAD method. The as-prepared samples were characterized by diffuse reflectance UV-visible spectroscopy, BET surface area analysis, transmission electron microscopy, and powder XRD. The diffuse reflectance spectra of Au/TiO₂ nanocomposites showed 2 sets of peaks (Figure 1): band gap transition band of TiO₂ with a maximum in the UV region and surface plasmon resonance band in the visible region (Au/anatase commercial with a maxima at 526 nm, Au/anatase aerogel with a maxima at 535 nm, Au/rutile with a maxima at 525 nm, and Au/P25 with a maxima at 560 nm). The position and shape of the surface plasmon resonance band depends on particle size, shape, and dielectric constant of the medium [41]. The red shift of the surface plasmon band maxima in these nanocomposites can be attributed to the refractive index of the TiO₂ matrix. The band gap values of the respective phases of TiO₂ remained unchanged even after Au loading as per the DRS UV data (anatase = 3.2 eV, rutile = 3.0 eV, and P25 = 3.1 eV).

(a)

(b)

The presence of loaded Au nanoparticles on the TiO₂ matrix was confirmed by TEM bright field imaging. The TEM bright field images of different phases of TiO₂ before and after Au loading are given in Figures 2 and 3. Commercial anatase has micron sized particles (0.1-0.2 μm, Figure 2(a)) with a surface area 13 m²/g, whereas anatase prepared by the aerogel method consists of small nanometer sized particles (10–15 nm, Figure 2(b)) with a high surface area of 102 m²/g (Figure S1 and Table S1). TEM bright field images of these anatase particles showed embedded Au nanoparticles of size ranging from 1 to 9 nm (Figures 2(c) and 2(d)) with an average particle diameter of 2.4 nm and 4.3 nm (Figure 4) for aerogel and commercial anatase, respectively. The surface areas of these Au/anatase nanocomposites were less than the respective TiO₂ matrix used (Table S1). The rutile phase of TiO₂ used in our studies showed micron sized particles (0.2–0.6 μm, Figure 3(c)). The surface area of rutile was found to be very low (Table S1). The TEM bright field image (Figure 3(d)) showed embedded Au nanoparticles on the semiconductor with a size ranging from 3 to 8 nm with an average particle diameter of 4.7 nm (Figure 4). In the case of P25, the particles size ranged from 15 to 20 nm (Figure 3(a)) with a surface area of about 48 m²/g (Table S1). TEM bright field images exhibited Au nanoparticles of size ranging from 3 to 10 nm (Figure 3(b)) with an average particle diameter of 6.7 nm (Figure 4). The powder XRD pattern (Figure S3) of these Au/TiO₂ nanocomposites did not show any Au reflections due to the low concentration (Table S2) of Au in these samples.

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

A conclusion from these characterization studies is that the final gold particle size depends a great deal on the surface area of the TiO₂ support. Therefore, as the Au-butanone colloid encountered the cold TiO₂, gold nanoparticle growth is limited/controlled by the TiO₂ surface available.

3.2. Photocatalytic Activity of Au/TiO₂ Nanocomposites under UV-Visible Conditions

All Au/TiO₂ photocatalysts prepared by our SMAD method showed UV-visible activity to generate hydrogen in the presence of a sacrificial electron donor, irrespective of the TiO₂ phase present in the system (Table 1). Among the different catalysts used, Au/P25 showed the highest activity with a hydrogen production rate of 1600 μmol/h in the presence of ethanol. In addition to ethanol as a sacrificial agent, we used methanol and sodium sulfite as an electron scavenger in Au/P25 system. The rate of hydrogen evolution was lower compared to ethanol when methanol was used as the sacrificial agent with a photocatalytic hydrogen production of 816 μmol/h. The photocatalytic hydrogen production was very low when 20 mM sodium sulfite solution was used as a sacrificial agent (20 μmol/h). These results clearly indicate that during sacrificial electron donor mediated hydrogen production, there is a considerable contribution toward hydrogen generation from the sacrificial agent in addition to the hydrogen generation.

Au/TiO₂ photocatalysts were found to be much more active compared to their samples without gold. The higher activity could be attributed to the better charge separation achieved in these catalysts where Au particles are good electron scavengers as well as catalyst surfaces for hydrogen formation. The subsequent holes in photoexcited TiO₂ are quenched by the sacrificial electron donor ethanol. The Fermi level of Au nanoparticles ( = +0.45 V versus NHE at pH 7 for bulk Au) is more positive than the bottom of the conduction band of TiO₂ ( = −0.5 V versus NHE at pH 7) which favors the electron transfer from photoexcited TiO₂ to Au. However, the reduction potential of water is −0.41 V (NHE at pH 7), and the transferred electrons cannot reduce water until the Fermi level of Au is raised to negative potentials. There are several reports in the literature regarding metal nanoparticle storing electrons in its Fermi level and shifting the Fermi level to more negative potentials when they come into contact with a photoexcited semiconductor nanoparticle like TiO₂ [17, 18, 42–46]. This kind of electron transfer from semiconductor to metal continues until Fermi level equilibration takes place. Choi et al. showed that under UV irradiation, Au@TiO₂ core-shell nanoparticles exhibit a blue shift in the surface plasmon resonance band maxima due to the transfer of electrons from TiO₂ to Au core and the Fermi level shift to negative potentials [47]. Au cannot undergo this kind of the Fermi level shift without taking electrons from a semiconductor [47].

3.3. Photocatalytic Activity of TiO₂ and Au/TiO₂ Nanocomposites under Visible Conditions

We used 2 M NaNO₂ solution as a filter to cut off the UV radiations under our visible light illumination photocatalytic experiments. From the UV-visible spectrum of NaNO₂ (Figure S4) it is clear that this chemical filter cuts off radiation only up to 400 nm. Since rutile does absorb in the 400–410 nm range, some charge transfer from TiO₂ to Au could take place. As expected, the rutile phase of TiO₂ is visible light active, whereas the anatase phase is visible light inactive due to its slightly large band gap. Au loading improved the visible light activity of TiO₂ nanoparticles containing the rutile phase. Thus, the enhanced visible activity of these rutile-containing photocatalysts could be due to better charge separation in the presence of Au nanoparticles by transferring electrons from the CB of TiO₂ to the Au Fermi level [16–18].

In addition to the charge transfer from TiO₂ to Au, there are several reports where Au imparts visible activity to wide band gap visible light inactive semiconductor nanoparticles like TiO₂, SrTiO₃, and CeO₂ [19–25, 48–51]. García and coworkers prepared visible active Au loaded P25 or CeO₂ nanocomposites for hydrogen or oxygen production by water splitting in the presence of a sacrificial electron donor (methanol or EDTA) or acceptor [16, 48]. According to these reports, Au nanoparticles absorb visible light by the surface plasmon resonance effect; charge separation at the Au nanoparticle takes place by the transfer of excited electrons from Au to the CB of TiO₂; subsequent holes are quenched by the sacrificial electron donor. Kimura et al. showed that the rutile form of TiO₂ more favors this kind of interfacial electron transfer from Au to TiO₂ [19]. Essentially, in these reported systems, Au nanoparticle acts as a sensitizer as in the case of dye-sensitized solar cells [49].

However, this kind of electron transfer from Au to the CB of TiO₂ is not energetically favored [26]. On the other hand, Tian and Tatsuma showed that this kind of electron transfer is possible due to the formation of a Schottky barrier at Au/TiO₂ junctions [20, 50]. In the case of Pt_(05wt%)/SrTiO₃, the Pt metal decreases the band gap energy of SrTiO₃ by the formation of a Schottky barrier at the Pt/SrTiO₃ interface [51]. Thus, there is still controversy regarding electron flow to or from metal particles with TiO₂.

To get a better overall understanding about how Au nanoparticles impart visible activity to TiO₂ semiconductor, we examined the visible activity of Au/anatase nanocomposites. Two different types of anatase were used: a commercial one with micron sized particles and anatase prepared by an aerogel method with nanometer sized particles. Regardless of the anatase used for Au loading, we did not see any visible light activity for the first five hours in the presence of water ethanol mixture. However, there was a very small amount of hydrogen evolution sixth-hour onwards. The mechanism of formation of small amount of hydrogen after five hours of visible light illumination is not clear to us, which could be either due to the reduction of water or due to the thermal decomposition of ethanol, aided by small nanoparticles undergoing nonradiative decay forming localized hot spots [52, 53].

It has also been reported that Au nanoparticles can also impart visible activity to a plasmonic-metal/semiconductor nanocomposite by surface plasmon mediated local electric field enhancement [26–29]. Interaction of a semiconductor nanoparticle with this kind of localized electric field could allow the formation of electron/hole pairs in the near surface region which can migrate to the surface without undergoing electron/hole pair recombination. For example, electromagnetic simulations showed that the increased photocatalytic water splitting under visible illumination of anodic TiO₂ with Au nanoparticles is due to the local electric field enhancement near the TiO₂ surface instead of electron transfer from Au to the CB of TiO₂ [26]. Awazu et al. called this kind of surface plasmon-induced localized electric field enhancement and improved photocatalytic activity as “plasmonic photocatalysis” [54]. Christopher et al. explained the enhanced visible activity of Ag/TiO₂ as due to the higher concentration of charge carriers in the semiconductor by plasmon mediated radiative transfer of energy from Ag nanoparticles to TiO₂, no electron flow from Ag to TiO₂ was observed [55]. Recently, Seh et al. showed enhanced visible light activity for the Janus shaped anatase Au-TiO₂ nanocomposites as due to the strong localization of plasmonic near fields close to the Au-TiO₂ interface and the coupling of plasmonic near fields to optical transitions involving localized electronic states in amorphous TiO₂ [29]. In our case, this kind of localized electric field created by Au nanoparticles could not generate electron/hole pairs in the near surface region of the semiconductor as the surface plasmon absorbance of Au nanoparticle does not overlap with the TiO₂ absorption spectrum. Linic and Ingram observed enhanced visible activity with Ag loaded N-TiO₂, whereas Au loaded N-TiO₂ did not show any significant enhancement in the photocurrent due to the red shift in the position of surface plasmon absorption band of Au nanoparticles compared to that of Ag [56].

3.4. Au Nanoparticles in Au/TiO₂ Nanocomposites Act as an Electron Sink for Better Charge Separation and Total Water Splitting

Our results indicate that there is a transfer of electrons from TiO₂ to Au and Au nanoparticle acts as a sink for photogenerated electrons. If this is true, Au nanoparticles could aid charge separation without using a sacrificial electron donor. We strengthened this hypothesis by doing photocatalytic water splitting under UV-visible conditions using Au/P25 nanocomposites in the absence of a sacrificial electron donor. There was an evolution of hydrogen (Figure 5) which again supports the proposed mechanism of charge separation achieved by Au nanoparticles (acts as a sink for photogenerated electrons and the Fermi level equilibration) and water molecules (quench the holes in the valence band of TiO₂). In order to prove that the hydrogen evolution is not caused by some organic impurities like butanone incorporated into the Au/P25 photocatalyst during sample preparation, we used calcined Au/P25 (at 200°C for 2 h). This sample showed photocatalytic activity for several cycles (Figure 5). We also did control experiments, wherein we used Degussa P25 and tried photocatalytic water splitting in the absence of ethanol; there was no hydrogen evolution. These results imply a mechanism shown in Scheme 1.

The rate of hydrogen evolution was low for Au/P25 in the absence of ethanol. P25 produced no hydrogen in the absence of ethanol; however, in the presence of ethanol, there was hydrogen evolution. This implies two things: (1) charge separation in pure TiO₂ takes place only in the presence of a sacrificial electron donor; (2) charge separation takes place in TiO₂ nanoparticles which are in good interfacial contact with Au nanoparticles even in the absence of a sacrificial electron donor. In the absence of ethanol, initially, charge separation takes place only on those TiO₂ particles which are in good interfacial contact with Au nanoparticles by transferring photoelectrons from the CB of TiO₂ to the Au Fermi level; holes are being quenched by water molecules to produce H₂O₂ or O₂. Thus, charge separation takes place in the case of photoexcited TiO₂ nanoparticles which are in good interfacial contact with TiO₂, and the Au/TiO₂ Fermi level equilibration takes place. Subsequent water reduction and hydrogen evolution are observed. The amount of hydrogen evolved in the absence of ethanol is low compared to the hydrogen evolved in the presence of ethanol. This could be due to the low concentration (1 wt%) of Au (which causes the charge separation) in these catalysts. We could not see any oxygen evolution during these experiments. This could be due to several reasons: (1) the reaction mixture was deaerated before photoillumination, and evolved small amount of oxygen could dissolve in water due to the high solubility of oxygen in water compared to hydrogen [57], (2) evolved oxygen can accept photoexcited electron from TiO₂ to form [58, 59]. The rate of the hydrogen evolution decreases with time as the evolved oxygen acts as a better electron acceptor to form superoxide.

4. Conclusions

These Au/TiO₂ photocatalysts were found to be resilent photocatalysts for the generation of hydrogen under UV-visible conditions. The photocatalytic hydrogen evolution rate was found to be 1600 μmol/h which is higher than Au/TiO₂ nanocomposites prepared by the photodeposition method (in our hands). Comparisons of different TiO₂ crystalline forms with different surface areas suggest that, in our case, the best explanation of any visible light activity is that the rutile form can absorb some energetic visible light, and then electrons are scavenged by the Au nanoparticles. This is the same mechanism as with more energetic UV light, where the process is much more efficient.

When ethanol scavenger is present, these photocatalysts are long lived. In the absence of ethanol, catalyst degradation takes place slowly (over days); this process of degradation is not understood, and hydrogen generation is much slower.

Further work comparing different metal nanoparticles is underway.

Conflict of Interests

The authors declare no financial conflict of interests.

Acknowledgments

The authors are grateful to the Department of Energy (Basic Energy Sciences, DE-SC 0005159) for funding. They thank Dr. Dan Boyle for assistance with TEM measurements.

Supplementary Materials

Supplementary Material provided includes N₂ adsorption/desorption isotherms of all the samples; surface areas, pore sizes, and pore volumes; amounts of gold loaded on the various samples; powder XRD patterns; and the UV-visible spectrum of the UV filter material NaNO₂.

Supplementary Materials

References

A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature, vol. 238, no. 5358, pp. 37–38, 1972.
View at: Publisher Site | Google Scholar
K. Maeda, “Photocatalytic water splitting using semiconductor particles: history and recent developments,” Journal of Photochemistry and Photobiology C, vol. 12, no. 4, pp. 237–268, 2011.
View at: Publisher Site | Google Scholar
K. Maeda and K. Domen, “Photocatalytic water splitting: recent progress and future challenges,” Journal of Physical Chemistry Letters, vol. 1, no. 18, pp. 2655–2661, 2010.
View at: Publisher Site | Google Scholar
R. Abe, “Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation,” Journal of Photochemistry and Photobiology C, vol. 11, no. 4, pp. 179–209, 2010.
View at: Publisher Site | Google Scholar
A. Kubacka, M. Fernández-García, and G. Colón, “Advanced nanoarchitectures for solar photocatalytic applications,” Chemical Society Reviews, vol. 112, no. 3, pp. 1555–1614, 2012.
View at: Publisher Site | Google Scholar
S. Linic, P. Christopher, and D. B. Ingram, “Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy,” Nature Materials, vol. 10, pp. 911–921, 2011.
View at: Publisher Site | Google Scholar
S. C. Warren and E. Thimsen, “Plasmonic solar water splitting,” Energy Environmental Science, vol. 5, pp. 5133–5146, 2012.
View at: Publisher Site | Google Scholar
O. Carp, C. L. Huisman, and A. Reller, “Photoinduced reactivity of titanium dioxide,” Progress in Solid State Chemistry, vol. 32, no. 1-2, pp. 33–177, 2004.
View at: Publisher Site | Google Scholar
V. Brezová, A. Blažková, L. Karpinský et al., “Phenol decomposition using Mⁿ⁺/TiO₂ photocatalysts supported by the sol-gel technique on glass fibres,” Journal of Photochemistry and Photobiology A, vol. 109, no. 2, pp. 177–183, 1997.
View at: Publisher Site | Google Scholar
X. Wang, M. Blackford, K. Prince, and R. A. Caruso, “Preparation of boron-doped porous titania networks containing gold nanoparticles with enhanced visible-light photocatalytic activity,” ACS Applied Materials and Interfaces, vol. 4, no. 1, pp. 476–482, 2012.
View at: Publisher Site | Google Scholar
D. B. Hamal and K. J. Klabunde, “Synthesis, characterization, and visible light activity of new nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO₂,” Journal of Colloid and Interface Science, vol. 311, no. 2, pp. 514–522, 2007.
View at: Publisher Site | Google Scholar
A. Primo, A. Corma, and H. García, “Titania supported gold nanoparticles as photocatalyst,” Physical Chemistry Chemical Physics, vol. 13, no. 3, pp. 886–910, 2011.
View at: Publisher Site | Google Scholar
X. Wang and R. A. Caruso, “Enhancing photocatalytic activity of titania materials by using porous structures and the addition of gold nanoparticles,” Journal of Materials Chemistry, vol. 21, no. 1, pp. 20–28, 2011.
View at: Publisher Site | Google Scholar
H. Tada, T. Kiyonaga, and S. I. Naya, “Rational design and applications of highly efficient reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium(iv) dioxide,” Chemical Society Reviews, vol. 38, no. 7, pp. 1849–1858, 2009.
View at: Publisher Site | Google Scholar
V. Subramanian, E. Wolf, and P. V. Kamat, “Semiconductor-metal composite nanostructures. To what extent do metal nanoparticles improve the photocatalytic activity of TiO₂ films?” Journal of Physical Chemistry B, vol. 105, no. 46, pp. 11439–11446, 2001.
View at: Publisher Site | Google Scholar
C. G. Silva, R. Juá rez, T. Marino, R. Molinari, and H. García, “Influence of excitation wavelength (UV or visible light) on the photocatalytic activity of titania containing gold nanoparticles for the generation of hydrogen or oxygen from water,” Journal of the American Chemical Society, vol. 133, no. 3, pp. 595–602, 2011.
View at: Publisher Site | Google Scholar
V. Subramanian, E. E. Wolf, and P. V. Kamat, “Catalysis with TiO₂/Gold nanocomposites. Effect of metal particle size on the fermi level equilibration,” Journal of the American Chemical Society, vol. 126, no. 15, pp. 4943–4950, 2004.
View at: Publisher Site | Google Scholar
M. Jakob, H. Levanon, and P. V. Kamat, “Charge distribution between UV-irradiated TiO₂ and gold nanoparticles: determination of shift in the Fermi level,” Nano Letters, vol. 3, no. 3, pp. 353–358, 2003.
View at: Publisher Site | Google Scholar
K. Kimura, S. I. Naya, Y. Jin-nouchi, and H. Tada, “TiO₂ crystal form-dependence of the Au/TiO₂ plasmon photocatalyst's activity,” Journal of Physical Chemistry C, vol. 116, no. 12, pp. 7111–7117, 2012.
View at: Publisher Site | Google Scholar
Y. Tian and T. Tatsuma, “Mechanisms and applications of plasmon-induced charge separation at TiO₂ films loaded with gold nanoparticles,” Journal of the American Chemical Society, vol. 127, no. 20, pp. 7632–7637, 2005.
View at: Publisher Site | Google Scholar
D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, and T. Hirai, “Gold nanoparticles located at the interface of anatase/rutile TiO₂ particles as active plasmonic photocatalysts for aerobic oxidation,” Journal of American Chemical Society, vol. 134, no. 14, pp. 6309–6315, 2012.
View at: Publisher Site | Google Scholar
A. Furube, L. Du, K. Hara, R. Katoh, and M. Tachiya, “Ultrafast plasmon-induced electron transfer from gold nanodots into TiO₂ nanoparticles,” Journal of the American Chemical Society, vol. 129, no. 48, pp. 14852–14853, 2007.
View at: Publisher Site | Google Scholar
Y. Nishijima, K. Ueno, Y. Kotake, K. Murakoshi, H. Inoue, and H. Misawa, “Near-infrared plasmon-assisted water oxidation,” Journal of Physical Chemistry Letters, vol. 3, no. 10, pp. 1248–1252, 2012.
View at: Publisher Site | Google Scholar
Y. Nishijima, K. Ueno, Y. Yokota, K. Murakoshi, and H. Misawa, “Plasmon-assisted photocurrent generation from visible to near-infrared wavelength using a Au-nanorods/TiO₂ electrode,” Journal of Physical Chemistry Letters, vol. 1, no. 13, pp. 2031–2036, 2010.
View at: Publisher Site | Google Scholar
E. Kowalska, O. O. P. Mahaney, R. Abe, and B. Ohtani, “Visible-light-induced photocatalysis through surface plasmon excitation of gold on titania surfaces,” Physical Chemistry Chemical Physics, vol. 12, no. 10, pp. 2344–2355, 2010.
View at: Publisher Site | Google Scholar
Z. Liu, W. Hou, P. Pavaskar, M. Aykol, and S. B. Cronin, “Plasmon resonant enhancement of photocatalytic water splitting under visible illumination,” Nano Letters, vol. 11, pp. 1111–1116, 2011.
View at: Google Scholar
H. Wang, T. You, W. Shi, J. Li, and L. Guo, “Au/TiO₂/Au as a plasmonic coupling photocatalyst,” Journal of Physical Chemistry C, vol. 116, no. 10, pp. 6490–6494, 2012.
View at: Publisher Site | Google Scholar
Y. Lu, H. Yu, S. Chen, X. Quan, and H. Zhao, “Integrating plasmonic nanoparticles with TiO₂ photonic crystal for enhancement of visible-light-driven photocatalysis,” Energy Environmental Science, vol. 46, pp. 1724–1730, 2012.
View at: Google Scholar
Z. W. Seh, S. Liu, M. Low et al., “Janus Au-TiO₂ photocatalysts with strong localization of plasmonic near-fields for efficient visible-light hydrogen generation,” Advanced Materials, vol. 24, no. 17, pp. 2310–2314, 2012.
View at: Publisher Site | Google Scholar
J. Li and H. C. Zeng, “Preparation of monodisperse Au/TiO₂ nanocatalysts via self-assembly,” Chemistry of Materials, vol. 18, no. 18, pp. 4270–4277, 2006.
View at: Publisher Site | Google Scholar
T. Soejima, H. Tada, T. Kawahara, and S. Ito, “Formation of au nanoclusters on TiO₂ surfaces by a two-step method consisting of Au(III)-complex chemisorption and its photoreduction,” Langmuir, vol. 18, no. 11, pp. 4191–4194, 2002.
View at: Publisher Site | Google Scholar
C. Yogi, K. Kojima, T. Takai, and N. Wada, “Photocatalytic degradation of methylene blue by Au-deposited TiO 2 film under UV irradiation,” Journal of Materials Science, vol. 44, no. 3, pp. 821–827, 2009.
View at: Publisher Site | Google Scholar
W.-C. Li, M. Comotti, and F. Schüth, “Highly reproducible syntheses of active Au/TiO₂ catalysts for CO oxidation by deposition-precipitation or impregnation,” Journal of Catalysis, vol. 237, no. 1, pp. 190–196, 2006.
View at: Publisher Site | Google Scholar
M. C. Hidalgo, M. Maicu, J. A. Navío, and G. Colón, “Effect of sulfate pretreatment on gold-modified TiO₂ for photocatalytic applications,” Journal of Physical Chemistry C, vol. 113, no. 29, pp. 12840–12847, 2009.
View at: Publisher Site | Google Scholar
S. Stoeva, K. J. Klabunde, C. M. Sorensen, and I. Dragieva, “Gram-scale synthesis of monodisperse gold colloids by the solvated metal atom dispersion method and digestive ripening and their organization into two- and three-dimensional structures,” Journal of the American Chemical Society, vol. 124, no. 10, pp. 2305–2311, 2002.
View at: Publisher Site | Google Scholar
A. B. Smetana, K. J. Klabunde, and C. M. Sorensen, “Synthesis of spherical silver nanoparticles by digestive ripening, stabilization with various agents, and their 3-D and 2-D superlattice formation,” Journal of Colloid and Interface Science, vol. 284, no. 2, pp. 521–526, 2005.
View at: Publisher Site | Google Scholar
A. A. Ponce and K. J. Klabunde, “Chemical and catalytic activity of copper nanoparticles prepared via metal vapor synthesis,” Journal of Molecular Catalysis A, vol. 225, no. 1, pp. 1–6, 2005.
View at: Publisher Site | Google Scholar
S. Cingarapu, Z. Yang, C. M. Sorensen, and K. J. Klabunde, “Synthesis of CdSe quantum dots by evaporation of bulk CdSe using SMAD and digestive ripening processes,” Chemistry of Materials, vol. 21, no. 7, pp. 1248–1252, 2009.
View at: Publisher Site | Google Scholar
K. J. Klabunde, P. L. Timms, P. S. Skell, and S. Ittel, “Introduction to metal vapor synthesis,” Inorganic Synthesis, vol. 19, pp. 59–86, 1979.
View at: Publisher Site | Google Scholar
Y. Kuo, C. D. Frye, M. Ikenberry, and K. J. Klabunde, “Titanium-indium oxy(nitride) with and without RuO₂ loading as photocatalysts for hydrogen production under visible light from water,” Catalysis Today, vol. 199, pp. 15–23, 2013.
View at: Publisher Site | Google Scholar
K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment,” Journal of Physical Chemistry B, vol. 107, no. 3, pp. 668–677, 2003.
View at: Publisher Site | Google Scholar
P. V. Kamat, “Manipulation of charge transfer across semiconductor interface. A criterion that cannot be ignored in photocatalyst design,” Journal of Physical Chemistry Letters, vol. 3, no. 5, pp. 663–672, 2012.
View at: Publisher Site | Google Scholar
A. Takai and P. V. Kamat, “Capture, store and discharge. Shuttling photogenerated electrons across TiO₂-silver interface,” ACS Nano, vol. 5, no. 9, pp. 7369–7376, 2011.
View at: Publisher Site | Google Scholar
V. Subramanian, E. E. Wolf, and P. V. Kamat, “Green emission to probe photoinduced charging events in ZnO-Au nanoparticles. Charge distribution and Fermi-level equilibration,” Journal of Physical Chemistry B, vol. 107, no. 30, pp. 7479–7485, 2003.
View at: Google Scholar
T. Hirakawa and P. V. Kamat, “Photoinduced electron storage and surface plasmon modulation in Ag TiO 2 clusters,” Langmuir, vol. 20, no. 14, pp. 5645–5647, 2004.
View at: Publisher Site | Google Scholar
A. Wood, M. Giersig, and P. Mulvaney, “Fermi level equilibration in quantum dot-metal nanojunctions,” Journal of Physical Chemistry B, vol. 105, no. 37, pp. 8810–8815, 2001.
View at: Publisher Site | Google Scholar
H. Choi, W. T. Chen, and P. V. Kamat, “Know thy metal neighbor. Plasmonic versus electron charging effects of gold nanoparticles in dye sensitized solar cells,” ACS Nano, vol. 6, no. 5, pp. 4418–4427, 2012.
View at: Publisher Site | Google Scholar
A. Primo, T. Marino, A. Corma, R. Molinari, and H. García, “Efficient visible-light photocatalytic water splitting by minute amounts of gold supported on nanoparticulate CeO₂ obtained by a biopolymer templating method,” Journal of the American Chemical Society, vol. 133, no. 18, pp. 6930–6933, 2011.
View at: Publisher Site | Google Scholar
B. O'Regan and M. Grätzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO₂ films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991.
View at: Google Scholar
J. Tang, M. White, G. D. Stucky, and E. W. McFarland, “Electrochemical fabrication of large-area Au/TiO₂ junctions,” Electrochemistry Communications, vol. 5, no. 6, pp. 497–501, 2003.
View at: Publisher Site | Google Scholar
T. Puangpetch, T. Sreethawong, S. Yoshikawa, and S. Chavadej, “Hydrogen production from photocatalytic water splitting over mesoporous-assembled SrTiO3 nanocrystal-based photocatalysts,” Journal of Molecular Catalysis A, vol. 312, no. 1-2, pp. 97–106, 2009.
View at: Publisher Site | Google Scholar
C.-W. Yen and M. A. El-Sayed, “Plasmonic field effect on the hexacyanoferrate (III)-thiosulfate electron transfer catalytic reaction on gold nanoparticles: electromagnetic or thermal?” Journal of Physical Chemistry C, vol. 113, no. 45, pp. 19585–19590, 2009.
View at: Publisher Site | Google Scholar
J. R. Adleman, D. A. Boyd, D. G. Goodwin, and D. Psaltis, “Heterogenous catalysis mediated by plasmon heating,” Nano Letters, vol. 9, no. 12, pp. 4417–4423, 2009.
View at: Publisher Site | Google Scholar
K. Awazu, M. Fujimaki, C. Rockstuhl et al., “A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide,” Journal of the American Chemical Society, vol. 130, no. 5, pp. 1676–1680, 2008.
View at: Publisher Site | Google Scholar
P. Christopher, D. B. Ingram, and S. Linic, “Enhancing photochemical activity of semiconductor nanoparticles with optically active Ag nanostructures: photochemistry mediated by Ag surface plasmons,” Journal of Physical Chemistry C, vol. 114, no. 19, pp. 9173–9177, 2010.
View at: Publisher Site | Google Scholar
D. B. Ingram and S. Linic, “Water splitting on composite plasmonic-metal/semiconductor photoelectrodes: evidence for selective plasmon-induced formation of charge carriers near the semiconductor surface,” Journal of the American Chemical Society, vol. 133, no. 14, pp. 5202–5205, 2011.
View at: Publisher Site | Google Scholar
http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html.
J. Kiwi and M. Grätzel, “Optimization of conditions for photochemical water cleavage. Aqueous Pt/TiO₂ (anatase) dispersions under ultraviolet light,” Journal of Physical Chemistry, vol. 88, no. 7, pp. 1302–1307, 1984.
View at: Google Scholar
M. V. Rao, K. Rajeshwar, V. R. Pal Verneker, and J. DuBow, “Photosynthetic production of H₂ and H₂O₂ on semiconducting oxide grains in aqueous solutions,” Journal of Physical Chemistry, vol. 84, no. 15, pp. 1987–1991, 1980.
View at: Google Scholar

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Copyright © 2013 Deepa Jose et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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International Journal of Photoenergy

Progress and Perspectives in Visible-Light-Driven Photocatalysis

Au-TiO2 Nanocomposites and Efficient Photocatalytic Hydrogen Production under UV-Visible and Visible Light Illuminations: A Comparison of Different Crystalline Forms of TiO2

Abstract

1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Instrumentation

2.3. Preparation of Au/TiO2 Nanocomposites

2.4. Photocatalytic Water Splitting Experiments Using a 450 W Mercury Lamp

3. Results and Discussions

3.1. Au/TiO2 Nanocomposites Characterization

3.2. Photocatalytic Activity of Au/TiO2 Nanocomposites under UV-Visible Conditions

3.3. Photocatalytic Activity of TiO2 and Au/TiO2 Nanocomposites under Visible Conditions

3.4. Au Nanoparticles in Au/TiO2 Nanocomposites Act as an Electron Sink for Better Charge Separation and Total Water Splitting

4. Conclusions

Conflict of Interests

Acknowledgments

Supplementary Materials

References

Copyright

Au-TiO₂ Nanocomposites and Efficient Photocatalytic Hydrogen Production under UV-Visible and Visible Light Illuminations: A Comparison of Different Crystalline Forms of TiO₂

2.3. Preparation of Au/TiO₂ Nanocomposites

3.1. Au/TiO₂ Nanocomposites Characterization

3.2. Photocatalytic Activity of Au/TiO₂ Nanocomposites under UV-Visible Conditions

3.3. Photocatalytic Activity of TiO₂ and Au/TiO₂ Nanocomposites under Visible Conditions

3.4. Au Nanoparticles in Au/TiO₂ Nanocomposites Act as an Electron Sink for Better Charge Separation and Total Water Splitting