Plasmonic Photocatalyst for H2 Evolution in

Dec 14, 2010 - Jiun-Jen Chen,† Jeffrey C. S. Wu,*,† Pin Chieh Wu,‡ and Din Ping Tsai‡ ... consume either e- or h+2-4 or modifying photocatalys...
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Plasmonic Photocatalyst for H2 Evolution in Photocatalytic Water Splitting Jiun-Jen Chen,† Jeffrey C. S. Wu,*,† Pin Chieh Wu,‡ and Din Ping Tsai‡ Department of Chemical Engineering and Department of Physics, National Taiwan UniVersity, Taipei, Taiwan 10617 ReceiVed: August 6, 2010; ReVised Manuscript ReceiVed: NoVember 27, 2010

The effect of surface plasmon resonance (SPR) on the photocatalytic water splitting was studied by employing the photocatalyst, Au/TiO2, to produce renewable solar hydrogen. It is well-known that metal particles on TiO2 can behave as electron traps, retarding the recombination of electron-hole pairs, thereby improving reaction activity. However, the electron trap is not the only mechanism responsible for the photoreaction enhancement. Our experiment on methylene blue photodegradation over Au particles proved that the SPR phenomenon was also involved in the photoreaction enhancement. Furthermore, the photocatalytic water splitting was performed on Au/TiO2 prepared by the photodeposition method. The production of hydrogen was significantly increased because Au particles not only acted as electron traps as well as active sites but also played an important role in the SPR enhancement. The intensified electric field at the interface between the Au particle and the subdomain on TiO2 was illustrated by finite element method (FEM) electromagnetic simulation. 1. Introduction Due to global environmental problems and energy issues, scientists have paid a great deal of attention to the utilization of solar energy for the production of hydrogen from water using photocatalysts. Water splitting has been studied for a long time since the discovery of the Honda-Fujishima effect,1 which involves a TiO2 semiconductor electrode. Water splitting on semiconductors is initiated by the absorption of a photon with energy equal to, or greater than, the semiconductor bandgap. This promotes electrons from the valence band (VB) to the conduction band (CB), with the consequent formation of electron-hole pairs. The produced electrons and holes, respectively, induce the reduction of the H+ ion and the oxidation of H2O, both absorbing on the semiconductor surface. The overall photocatalytic water splitting reaction is thus formulated as in eq 1. Catalyst/hν

2H2O 98 2H2 + O2

(1)

One of the major problems of photoreaction is its low activity due to the high recombination rate of photogenerated electronhole pairs, setting a limit to the efficiency of light energy conversion. In recent years, several research groups have made efforts to increase the photoactivity of semiconductor metal oxides, for example, adding sacrificial agents to efficiently consume either e- or h+2-4 or modifying photocatalysts by noble metal loading to favor the separation of charge carriers.5-9 Methanol and other organic species are commonly used as sacrificial agents in photocatalytic water splitting. They can capture photogenerated valence-band holes more efficiently than water, making conduction band electrons readily available for hydrogen production from water. Although the quantum ef* Corresponding author. Phone: 886-223631994. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Department of Physics.

ficiency of such a method may be increased with the expense of sacrificial agents, it is not pure water splitting considered from the viewpoint of solar energy conversion. The loading of noble metal is usually used to enhance the activity of photoreaction. The presence of noble metal particles on the surface of the photocatalyst increases the electron-hole pair separation because photogenerated electrons can be captured by the noble metal. Noble metal particles can serve as electron traps. Under light irradiation, the electrons are transferred from the TiO2 conduction band to the metal, and the holes accumulated in the TiO2 valence band. Hence, photogenerated electrons and holes are efficiently separated. However, the active sites are blocked, resulting in an activity decrease when too much noble metal is loaded.8 In addition, the noble metal clusters at a higher concentration may work as a recombination center. The recombination rate between electrons and holes increases exponentially with the increase in loading concentration because the average distance between trapping sites decreases by increasing the number of the clusters confined within a particle.5 Noble metal particles, such as gold and silver, are interesting catalytic nanomaterials because the peculiar activities are strongly related to their size, shape, and surface charge. The optical properties of gold nanoparticles are dominated by their surface plasmon resonance (SPR), defined as the collective motions of the conduction electrons induced by light irradiation.10,11 This is associated with a considerable enhancement of the electric near-field. The resonance wavelength strongly depends on the size and shape of the nanoparticles, the interparticle distance, and the dielectric property of the surrounding medium.12,13 As shown in Figure 1, electrons from the valence band are excited to the conduction band in TiO2 by UV light irradiation. The electrons then migrate to the gold particle on TiO2. The SPR effect induced by appropriate visible light irradiation can boost the energy intensity of trapped electrons resulting in the photocatalytic activity enhancement.14 To investigate the sole SPR phenomenon of nanogold particles, the photodegradation of methylene blue (MB) aqueous solution was conducted over Au particles deposited on a quartz

10.1021/jp1074048  2011 American Chemical Society Published on Web 12/14/2010

Plasmonic Photocatalyst for H2 Evolution

Figure 1. Schematic illustration of Au-loaded TiO2 for water splitting by the SPR effect.

plate. The combination effect of SPR and photocatalysis was studied using nanogold particles on TiO2. A numerical simulation was used to discern the SPR phenomenon of the Au/TiO2 system in aqueous solution. The simulation used a frequencydomain three-dimensional finite element method to solve Maxwell’s equations of electric field distributions. In the end, the photocatalytic water splitting was performed to illustrate the SPR-enhanced photoactivity. 2. Experimental Section 2.1. Preparation of Au on a Quartz Plate and Au/TiO2. A gold dispersion was prepared according to the sodium citrate reduced method.15 Distilled water was added to 2 mL of HAuCl4 solution containing 50 mg of gold and made up to 500 mL. When the solution was boiling, 50 mL of 1% sodium citrate solution was added under vigorous stirring. After 30 min of continuous boiling, the solution was allowed to cool. This method produces a stable, deep-red suspension of gold particles. A pipet was used to take 8 mL of nanogold suspension solution. Then the solution was spread on a clean quartz plate. To increase the adhesion of nanogold particles on the quartz plate, the resulting sample was then heated from room temperature to 100 °C. Next, the heating temperature was increased from 100 to 500 °C within 30 min and kept at 500 °C for 60 min.16,17 The photodeposition of nanogold particles on TiO2 was carried out by the method suggested in the literature.18,19 Degussa P25, i.e., TiO2, was precalcined in air at 773 K for 4 h. The calcined TiO2 powder was then added into a beaker containing the appropriate amount of 0.002 M HAuCl4 solution, and the solution pH was adjusted to 5.5 by dropwise addition of 0.2 N NaCO3. The suspension solution was irradiated with the 100 W high-pressure mercury lamp operated at 8 W cm-2 for 1 h with vigorous stirring. The color of solution became clear, indicating the completion of Au photodeposition. Then the suspension was filtrated and washed several times with distilled water, until no Cl- was detected. The solid was dried under vacuum at room temperature for 16 h. 2.2. Characterization. The Au loading on TiO2 by photodeposition was estimated from the residual Au concentration in the precursor solution measured by atomic absorption spectroscopy (GBC 906AA). The light absorption of photocatalysts was characterized by reflective diffusive UV-vis spectroscopy (Varian, Cary 100). Field-emission scanning electron microscopy (FE-SEM) was carried out on a Hitachi model S-800 instrument. The X-ray photoelectron spectroscopy (XPS) was carried out to determine the chemical composition of the as-prepared Au/TiO2 particles and the chemical status of

J. Phys. Chem. C, Vol. 115, No. 1, 2011 211 various species. The XPS was carried out on a Thermo Theta Probe instrument. The photocatalyst was pressed into a pellet and stuck to the sample holder using a carbon tape. Carbon (1s, 284.5 eV) was used as an internal standard for binding energy calibration. Transmission electron microscopy (TEM) of the photocatalysts was carried out on a Hitachi model H-7100 instrument. The particle size distribution (PSD) of gold nanoparticles was measured by the Particle Size and Zeta Potential Analyzer (Malvern, Nano-ZS). 2.3. Photocatalytic Activity Measurement. 2.3.1. Photodegradation of Methylene Blue. The methylene blue (MB) aqueous solution of 2.4 × 10-5 M was photodegraded in a glass reactor at 25 °C. The Au-deposited quartz plate (5 cm × 5 cm × 1 mm) immersed in the solution was irradiated by the Xe lamp (λ > 400 nm). The Varian, Cary 100, reflective diffusive UV-vis spectroscope was used to measure the concentrations of the MB aqueous solution based on the absorption peak of 664.3 nm during the photodegradation. 2.3.2. Photocatalytic Splitting of Water. The photocatalytic splitting of water to generate hydrogen and oxygen was carried out using the system shown in Figure 2. TiO2 with 3 wt % of Au was prepared for the water splitting experiment. In a typical reaction, 0.2 g of photocatalyst was added to 140 mL of deionized water in the Pyrex reactor. Before photocatalytic reaction, the reactor was heated at 50 °C and evacuated for 30 min with continuous stirring to remove dissolved air in the water. Next, the reactor was purged with high-purity argon gas and evacuated again. This Ar purge/evacuation process was repeated five times, and then the residual air content was checked by GC. The reactor was irradiated using a Xe lamp (λ > 400 nm) after the residual air in the reactor was confirmed to be negligible. The light intensity in front of the reactor was measured using a Lumen meter (Goldilux, GRP-1 70234). The intensity of incident visible light (λ > 400 nm) was 1.68 W/cm2, which was projected onto the reactor-side surface of 96 cm2. The UV source was a 254 nm UV lamp and was inserted into the center of the reactor. Cooling water was circulated inside the reactor to maintain the reaction temperature at 25 °C. The reaction was carried out for 7 h, and the reaction products were analyzed by GC using an online sampling loop (1 mL) at intervals of 1 h. The GC (China Chromatography 2000 GC) system was equipped with a 3.5 m Molecular Sieve 5A column and a thermal conductivity detector, with Ar flowing at 20 mL/ min as the carrier gas. Blank reactions were performed without photocatalyst in the presence of light and with photocatalyst in the dark. In both cases, no production of hydrogen was observed. 2.4. Simulation of Surface Plasmon Resonance. Threedimensional FEM electromagnetic simulation (COMSOL Multiphysics) was used to simulate the electromagnetic field distribution of a 3 nm nanogold particle on a 100 nm × 100 nm × 50 nm TiO2. A normal incident light with linear polarization transverse to the plane of incidence (i.e., TM polarization) radiated from the top of the gold nanoparticle. TM (short for transverse magnetic) polarized light means the electromagnetic wave in which the magnetic field vector is everywhere perpendicular to the plane of incidence (the plane of incidence which includes the normal to the surface and the incident wave vector). In our numerical computation and simulation, refractive indexes of 1.335, 2.592, and 3.300 were used for water, gold, and TiO2, respectively, and the light wavelength of 562.8 nm was employed. The size of the gold nanoparticle is quite small as compared with that of TiO2. We choose the periodic boundary condition for the simulation of many gold nanoparticles attached to the TiO2.

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Figure 2. Apparatus for photocatalytic water splitting. A, Ar for purging; B, ultrahigh-purity Ar; C, on/off valve; D, sampling loop; E, six-way valve; F, on/off valve; G, GC; H, magnetic stirrer; I, UV; J, water lock; K, Xe lamp; PI, pressure gauge; and R, Pyrex reactor.

Figure 3. (a) TEM micrograph of nanogold dispersion and (b) particle size distribution.

3. Results and Discussion 3.1. Photocatalyst Characterization. Figure 3(a) shows the TEM of gold nanoparticles prepared by the sodium citrate reduced method. The gold nanoparticles dispersed well in liquid solution. From the TEM micrograph, the average size of gold

nanoparticles was approximately 20 nm, which is consistent with the PSD result shown in Figure 3(b). Figure 4 shows the TEM micrographs of pure TiO2 (P25) and Au/TiO2 prepared by the photodeposition method. The near spherical Au particles were observed as dark spots having obvious contrast with the TiO2 support as shown in Figure 4(b). The mean size of Au was near 3 nm, which is smaller than that of the particles produced by the sodium citrate reduced method. The morphology and size of Au particles on the quartz plate are shown in Figure 5. The Au particles aggregated slightly after calcination, leading to an increased size to about 50 nm. The plasmon absorption arises from the collective oscillations of the free conduction band electrons that are induced by the incident electromagnetic radiation in Au0 particles. Figure 6 shows the UV-vis spectrum of gold particles on the quartz plate, indicating an absorption band between the wavelength of 500 and 600 nm. The maximum absorption is at 533.4 nm.

Figure 4. TEM micrograph of (a) TiO2 (P25) and (b) 3.0 wt % Au on TiO2.

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Figure 7. UV-vis spectrum of TiO2 (P25) and photodeposited Au/ TiO2. Figure 5. SEM micrograph of Au particles on a quartz plate.

Figure 6. UV-vis spectrum of Au particles dispersed on the quartz plate.

According to the relevant references,12,13 the absorption band is ascribed to the plasmonic resonance of metallic Au particles. The size of the nanogold particles would effect the position of the maximum absorption peak of SPR. Figure 7 shows the UV-vis spectrum of TiO2 (P25) and photodeposited Au/TiO2. For the TiO2 (P25), the absorption edge was around 400 nm, which is between the absorption edge of anatase (387 nm) and rutile (418 nm). This suggests that the TiO2 consists of both anatase and rutile phases.20 Since the size of Au particles produced by the photodeposition method was smaller than that of the sodium citrate reduced method, the SPR absorption peaks were found at somewhat different positions. In the case of Au/TiO2, the absorption band was shifted to near 562.8 nm. Figure 8 shows the XPS spectra of 3 wt % Au/TiO2. As shown in Figure 8(a), the spin-orbit components (2p3/2 and 2p1/2) of Ti 2p are well deconvoluted by two peaks at approximately 459.1 and 464.7 eV, corresponding to Ti4+ in a tetragonal structure. Meanwhile, the O 1s XPS spectrum (Figure 8(b)) shows a narrow peak with slight asymmetric distribution and a binding energy of 530.4 eV. This peak was attributed to the Ti-O in TiO2. The double peaks for nanogold particles were centered at 83.5 and 87.2 eV as shown in Figure 8(c). According to the

relevant literature,21-23 the doublet peaks located at 83.3 and 87.2 eV for nanogold particles can be assigned to the characteristic doublets of Au0 loaded on TiO2, suggesting that only elemental Au is formed on the TiO2 surface. 3.2. MB Degradation. Figure 9 shows the result of the degradation of MB solution. The factors affecting MB removal can be categorized into three kinds.24-27 The first is the adsorption of MB on gold particles, and the second is the light effect alone, under which MB would be photodegraded. The third is the photocatalytic degradation of MB in the solution. Prior to the photocatalytic reaction, blank experiments were conducted. To verify the adsorption effect and the light effect alone on the degradation of MB, respectively, reaction with the liquid solution containing nanogold particles in the dark and the reaction with liquid solution under light irradiation without any nanogold particles were performed. Figure 9 shows that after 6 h the adsorption effect contributed 4% in MB reduction, while 14% was observed for the light effect alone. However, when a Xe lamp containing visible light was used as the light source, 31.5% in MB degradation was found for nanogold particles after 6 h of irradiation. Since the Au particle is not a photocatalyst, this degradation is not caused by photoinduced electrons generated under light irradiation. The electromagnetic field of incident light couples with the oscillations of conduction electrons in gold particles, resulting in strong-field enhancement of the local electromagnetic fields near the surface of gold nanoparticles. Such enhanced local field strength can be much higher than the applied electromagnetic field (i.e., incident light). Thus, it is suggested that when the wavelength of incident light (533.4 nm) matches with the SPR band of Au the resonance of the electrons was induced, as if forming an extra electron magnetic field to enhance the light effect, thus increasing the degradation rate of MB. In another experiment, a 250-450 nm filter was attached to the light source so that only light within this wavelength range may pass through. The SPR effect was faded out by blocking away the SPR absorption band of Au (i.e., 533.4 nm). Only 19% in MB degradation resulted after 6 h of irradiation, which is nearly equal to the summation of MB degradation efficiency in both blank tests (i.e., 4% adsorption + 14% light effect alone). Therefore, the extra ∼13% increase in MB degradation was attributed to the SPR on Au particles. 3.3. Photocatalytic Splitting of Water. The yields of H2 and O2 evolution from the photocatalytic water-splitting reaction

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Figure 9. Results of MB photodegradation.

TABLE 1: Photocatalytic Water Splitting to Form H2 and O2 over 3.0 wt % Au/TiO2 after 7 h of Irradiationa yield (µmol/g-cat) catalysts and conditions

H2

O2

Au/TiO2 irradiated by UV and visible light Au/TiO2 irradiated by UV Au/TiO2 irradiated by visible light TiO2 irradiated by UV

53.75 35.04 ND 0.79

25.87 17.52 ND 0.38

a ND: not detected. UV: wavelength ) 254 nm, intensity ) 30 mW cm-2. Visible: wavelength > 400 nm, intensity ) 1.68 W cm-2.

Figure 8. XPS spectra of 3.0 wt % Au/TiO2. (a) Ti 2p, (b) O 1s, and (c) Au 4f.

by Au/TiO2 are listed in Table 1. The product ratio of H2 and O2 was about 2:1, which fitted the stoichiometric ratio of water splitting. The H2 evolution from pure water using TiO2 and Au/ TiO2 photocatalysts prepared by the photodeposition method is shown in Figure 10 (O2 evolution not shown). It can be seen that the activity of water splitting was improved by the loading of Au on TiO2. After 7 h of UV irradiation on TiO2, only a small amount of H2 was produced, which was about 0.79 µmol/ g-cat. Under the same conditions, Au/TiO2 greatly enhanced the production of H2, which was about 35.04 µmol/g-cat. This improvement was due to the presence of nanogold particles that played the role of the electron sinks, retarding the recombination

Figure 10. Photocatalytic activity for water splitting (photocatalyst 0.2 g of 3.0 wt % Au/TiO2). O2 evolution is not shown.

of electron-hole pairs. Moreover, nanogold particles also offered active sites resulting in the substantial increase of H2 yield. Figure 10 also shows the result of H2 yield for Au/TiO2 under both UV and visible light irradiation after 7 h. As expected, the additional visible light further boosted the H2 yield of Au/ TiO2 to 53.75 µmol/g-cat. Such a result was due to the SPR effect generated by nanogold particles under the irradiation of appropriate visible light. In summary, under the irradiation of both UV and visible light, nanogold particles not only played the role of electron sinks but also offered active sites as well as the SPR enhancement to significantly increase the production of H2. Since TiO2 is a UV responsive photocatalyst, sufficient electron-hole pairs will be generated under UV irradiation to

Plasmonic Photocatalyst for H2 Evolution

Figure 11. Schemes of Au on TiO2 simulation. (a) TiO2 only, (b) a full spherical gold particle on TiO2, and (c) a half spherical gold particle on TiO2.

give H2. On the other hand, under the irradiation of visible light alone, no H2 production was observed on Au/TiO2. This implies that water splitting with only the SPR effect from nanogold particles cannot be successfully performed. Therefore, for the water splitting over TiO2 loaded with nanogold particles, the ability to offer electron sinks as well as active sites should be the major merit of Au, while the effect of SPR is less significant but important. Although the real phenomenon of SPR on Au/TiO2 is complicated, it can be elucidated by simulation. The numerical calculation of electromagnetic field intensity of the Au particle on TiO2 could be illustrated by the three-dimensional FEM simulation. As shown in Figure 11, there are three cases in our computations, namely, TiO2, with a water interface, a 3 nm full spherical gold nanoparticle on the TiO2, and a 3 nm half spherical gold nanoparticle on the TiO2. Simulation results shown in Figure 12 clearly demonstrate the occurrence of electromagnetic field enhancement in the vicinity of gold nanoparticle in close proximity to TiO2 under the normal illumination of a linear TM polarized light. The maximum intensity of the electric field in the second and third case is 4.19 and 1.88 times higher than that in the first case, which has a water-TiO2 interface only. This indicates that near-field intensity enhancement of surface plasmon is present to effectively promote the photocatalytic process.

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Figure 12. FEM electromagnetic intensity simulation of (a) TiO2 only, (b) a full spherical gold particle on TiO2, and (c) a half spherical gold particle on TiO2. The diameter of the gold nanoparticle is 3 nm, and the size of TiO2 is 100 nm × 100 nm × 50 nm. A wavelength of 562.8 nm and TM polarized light is incident from the top. The definition of electromagnetic intensity is |E| ) (|Ex|2 + |Ey|2 + |Ez|2)(1/2), and the subscripts denote the component of total electric field E.

in sunlight for solar energy harvest, especially for wide bandgap materials, such as TiO2. Thus, both photon energies of UV and visible light can be absorbed and converted to chemical energy, i.e., hydrogen via water splitting. From the result of methylene blue degradation, the influence of the excitation wavelength related to the plasmon band absorption was observed in accordance with electromagnetic theory. The degradation efficiency increased when the incident wavelength matched with the surface plasmon resonance absorption band of nanogold particles. The photocatalytic water splitting is one of the best direct routes to generate renewable hydrogen from sunlight. Our results in water splitting clearly indicated that the role of electron sinks is not the only mechanism responsible for the activity enhancement by incorporating Au particles. The SPR phenomenon of Au that functions to provide extra electromagnetic field was found to be important as well for the enhancement of H2 production in photocatalytic water splitting. Acknowledgment. The authors would like to acknowledge the National Science Council of Taiwan for financial support of this research under project no. 98-2120-M-002-004.

4. Conclusion The surface plasmon resonance on the metal-loaded photocatalysts can be an important way to utilize the full spectrum

References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38.

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