Enhanced Photocatalytic Water Splitting by Plasmonic TiO2âFe2O3

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Enhanced Photocatalytic Water Splitting by Plasmonic TiO-FeO Cocatalyst Under Visible Light Irradiation 2

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Wei-Hsuan Hung, Tzu-Ming Chien, and Chuan-Ming Tseng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5033965 • Publication Date (Web): 27 May 2014 Downloaded from http://pubs.acs.org on June 3, 2014

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The Journal of Physical Chemistry

Enhanced Photocatalytic Water Splitting by Plasmonic TiO2-Fe2O3 Cocatalyst under Visible Light Irradiation Wei-Hsuan Hung1*, Tzu-Ming Chien1, and Chuan-Ming Tseng2 1

Feng Chia University, Taichung 407, Taiwan ＊ E-mail: [email protected] 2 Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

Abstract In this study, we introduce a plasmonic TiO2-Fe2O3 cocatalyst photoelectrode to improve the water-splitting process. The absorption of incident photons and the separation rate of photo-generated electron-hole pairs are enhanced due to the broadband absorption and strong electric field of the composite formed from these two metal oxide semiconductors and plasmonic silver nanoparticles (Ag NPs). Plasmonic TiO2-Fe2O3 cocatalyst photoelectrodes were fabricated using a precipitation and solution processing method. Under visible light irradiation, a photocurrent that is 20 times higher than that of pure Fe2O3 was observed using an optimized ratio of the plasmonic TiO2-Fe2O3/Ag cocatalyst. The mechanism for this enhancement in the plasmonic cocatalyst system was investigated using different structural configurations of the photoelectrode. Both the crystallinity and absorption band edge of the TiO2-Fe2O3 cocatalyst were characterized using X-ray diffraction (XRD) and ultraviolet-visible absorption spectroscopy (UV-Vis). Furthermore, the spatial distribution of the photocurrent was investigated using this plasmonic cocatalyst system.

Keywords: silver nanoparticles, charge separation, solar energy conversion, plasmon resonance

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Figure 1(a) Schematic of the plasmonic TiO2-Fe2O3 cocatalyst photoelectrode and water-splitting process (b) Energy band diagram of metal oxide semiconductors and the redox potentials of water Alternative and renewable energy sources have been important areas of research over the past few decades. Many areas of research have focused on new energy sources with low carbon emissions and minimal environmental impacts, including solar energy

1-2

, geothermal heat 3, wind, tides 4, and various forms of biomass

5-6

.

Solar energy is the most investigated source of renewable energy because it has the largest energy resource base of any natural process. Efficiently harvesting solar energy has become a topic of intense study and interest in the energy field. The conversion of solar energy into hydrogen via photocatalytic water splitting is the most direct method for achieving the goal of clean and renewable energy. Titanium dioxide (TiO2) is a common semiconductor photoelectrode used in the water-splitting process; the low cost, simple preparation, non-toxicity and chemical stability of TiO2 make it an attractive photoelectrode material. However, due to its large band gap (3.2 eV), TiO2 does not absorb photons in the visible region of the electromagnetic spectrum, which significantly reduces its solar energy conversion efficiency. To improve its solar energy conversion efficiency, many methods, such as doping with nitrogen or heavy metals, have been used in an attempt to extend the absorption edge of TiO2 into the visible wavelength region, which would create additional energy states within the band gap of TiO2 for the absorption of longer wavelengths. Many of these approaches have the drawback of increasing crystal deformations or introducing defects that act as exciton recombination centers.

7-14

Iron oxide (Fe2O3), an n-type semiconductor, is

another typical photocatalyst utilized in water splitting.

15-18

The band gap of iron

oxide (2.3 eV) is smaller than that of TiO2 and is capable of absorbing visible light photons. Similar to TiO2, iron oxide is inexpensive to produce, is non-toxic and is a 2


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stable material for photocatalytic reactions.19-23 Nevertheless, the two main disadvantages are low conductivity and a short exciton diffusion length (2-20 nm), which significantly lower the photocatalytic efficiency of iron oxide

24-27

. The short

exciton diffusion length of iron oxide constrains the effective absorption of photons to within 20 nm of the iron oxide surface, thus limiting hydrogen production. To increase photocatalytic hydrogen production from the water-splitting process, we introduce a novel plasmonic TiO2-Fe2O3 cocatalyst photoelectrode in this work. The concept of the plasmonic TiO2-Fe2O3 cocatalyst photoelectrode is illustrated in Figure 1. The absorption of incident photons by the plasmonic TiO2-Fe2O3 cocatalyst photoelectrode produces electron–hole pairs (excitons). Next, the holes oxidize water at the TiO2-Fe2O3 cocatalyst surface to form oxygen, and the electrons migrate to the counter electrode to reduce the water, thereby forming hydrogen. Our plasmonic cocatalyst photoelectrode improves the water-splitting process through three strategies. First, wide-range photon absorption can be achieved in the TiO2-Fe2O3 cocatalyst system; ultraviolet and visible photons in the solar spectrum can be effectively absorbed by TiO2 and Fe2O3 due to the appropriate energy band gaps of these two metal oxide semiconductors. Second, strong inherent electric fields at the heterogeneous interfaces of TiO2-Fe2O3 (heterojunctions) suppress the recombination of photo-generated electron-hole pairs.28 Third, with the integration of Ag NPs into the TiO2-Fe2O3 matrix, the absorption of incident light and the rate of exciton separation can be further increased due to near-field effects and light scattering from the plasmonic NPs.29-30 Figure 1b shows an energy band diagram of metal oxide semiconductors and the redox potentials of water. The defect state in the TiO2 results from the presence of impurities from the fabrication process, which causes the absorption of lower energy photons by TiO2. A significantly enhanced water-splitting photocurrent is achieved in the plasmonic TiO2-Fe2O3 photoelectrode in this work. Different plasmonic TiO2-Fe2O3 structure configurations are employed to understand the mechanism for the improvement in the photocurrent in the water-splitting process. Spatially resolved photocurrent maps are measured to investigate the potential of producing large-area cocatalyst photoelectrodes.

Experimental The iron oxide (Fe2O3) was prepared from FeCl2‧4H2O (0.199 g, 1.0 mmol) and FeCl3‧6H2O (0.54 g, 2.0 mmol). Sodium hydroxide solution (0.16 M), a precipitating agent, was slowly added to the precursor solution. After stirring the solution at 80 ℃ for 30 minutes, the color of the solution changed from yellow to black due to the 3



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formation of Fe3O4 (Reaction: Fe2+ + 2Fe3+ + 8 OH-→ Fe(OH)2 + 2Fe(OH)3→ Fe3O4 + 4 H2O). Black iron oxide powder (Fe3O4) was obtained through filtration and post-bake steps. However, Fe3O4 possesses a much lower photocatalytic activity than Fe2O3.31 Therefore, the Fe3O4 phase was converted to the Fe2O3 phase by dissolution in pure alcohol. Finally, the iron oxide (Fe2O3) membrane was fabricated using a spin-coating method followed by sintering at 450°C for 3 hours. Titanium dioxide (TiO2) was produced using the standard sol-gel process 32-34 with tetraethyl titanate as the precursor in our study. TiO2-Fe2O3 cocatalyst samples were synthesized by adding Fe2O3 powder to the TiO2 sol-gel solution to form the cocatalytic colloidal solution. The crystallinity of the TiO2-Fe2O3 cocatalyst system was analyzed by X-ray diffraction (XRD). The plasmonic layer of Ag NPs was prepared by direct chemical reduction on the surfaces of indium tin oxide (ITO) and the TiO2-Fe2O3 cocatalyst while immersing the photoelectrodes in the silver acetylacetonate precursor for 16 hours.35-36 The sizes, shapes and morphologies of these silver NPs were examined using field-emission scanning electron microscopy (FE-SEM), and the FE-SEM images are provided in Figure S1 in the supporting information. Electrical measurements were conducted using five different photoelectrodes, Fe2O3, TiO2, TiO2-Fe2O3, Ag/TiO2-Fe2O3 (top plasmon-enhanced mode) and TiO2-Fe2O3/Ag (bottom plasmon-enhanced mode), to investigate the improvement in the water-splitting photocurrent under 532 nm laser irradiation. In addition, the crystal structure of the TiO2-Fe2O3 cocatalyst was investigated using high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). A field-emission scanning electron microscope and a UV-Visible spectrometer (UV-Vis) were employed to characterize the morphology and plasmon resonance wavelength of the developed photoelectrodes.

Results and Discussion

Figure 2 (a-b) Low-magnification TEM image, high-resolution TEM image and electron diffraction pattern of the TiO2-Fe2O3 cocatalyst (c) SEM image and corresponding EDX elemental mappings of Fe, Ti, and O in TiO2-Fe2O3 (d) SEM 4


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cross-sectional view of TiO2-Fe2O3/Ag and (e) XRD patterns of different electrodes and silver NPs. The configuration of the TiO2-Fe2O3 cocatalyst was determined from the low-magnification transmission electron microscopy (TEM) image presented in Figure 2a, which showed alternating nanocrystalline networks of TiO2 and Fe2O3. The high-magnification TEM image presented in Figure 2b reveals a set of lattice fringes with a d-spacing of 3.5 Å, which corresponds to the (101) plane of the anatase phase of TiO2. Similarly, the phase of iron oxide was also identified by the lattice fringes with d-spacings of 2.9 Å and 4.7 Å, which correspond to the (111) and (220) planes of γ-Fe2O3. The average grain sizes of TiO2 and Fe2O3 were also determined to be approximately 7-10 nm. Furthermore, the SAED pattern presented in Figure 2b indicates that a polycrystalline structure was present in the cocatalyst system. The crystal structures of these photoelectrodes were also analyzed by X-ray diffraction (XRD), as shown in Figure 2e. The XRD patterns of the TiO2-Fe2O3 mixed electrode only show the main peak of anatase, and the intensity of the peak is lower than that of pure TiO2. This result might be attributed to structural deformation and the presence of defects in the TiO2. Some of the defects were the result of the substitution of Ti4+ with Fe3+, which slightly decreases the crystallinity of TiO2. The XRD pattern of the TiO2-Fe2O3/Ag structure is similar to the pattern observed for TiO2-Fe2O3; however, an additional peak at 37.93° was observed due to the presence of Ag nanoparticles. The energy-dispersive X-ray spectroscopy (EDX) mapping shown in Figure 2c provides more detailed information about the element distribution within the TiO2-Fe2O3 cocatalyst and further confirms the high uniformity of the TiO2-Fe2O3 cocatalyst. Figure 2d presents a cross-sectional view of the TiO2-Fe2O3/Ag sample, which indicates that the thickness of the TiO2-Fe2O3/Ag was approximately 200 nm.

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Figure 3 Absorption spectra of different photoelectrode configurations Figure 3 shows the absorption spectra of different photoelectrodes. TiO2 shows a typical absorption band edge at 390 nm. With the integration of Fe2O3, the TiO2 absorption band edge is extended to 550 nm due to the smaller band gap of iron oxide. Both the Ag/TiO2-Fe2O3 and TiO2-Fe2O3/Ag structures show higher photon absorption in the visible light region than does the TiO2-Fe2O3 structure alone. The highest visible light absorption is obtained with the Ag/TiO2-Fe2O3 structure due to the strong surface plasmon resonance of the Ag NPs on top of the TiO2-Fe2O3 surface. Further discussion of the relationship between the absorption spectrum and enhanced photocurrent will be presented below.

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Figure 4 (a) Photocurrent responses with different mixing ratios of TiO2-Fe2O3 (b) Photocurrent as a function of TiO2-Fe2O3 composition and (c) Illustration of the charge separation in different TiO2-Fe2O3 compositions. Samples with varying TiO2-Fe2O3 compositions were tested to obtain the optimized TiO2-Fe2O3 mixing ratio before integration with the plasmonic silver layer. Figure 4a shows the photocurrent response of TiO2-Fe2O3 with different mixing ratios under laser irradiation at 300 mW and 532 nm. By analyzing the photocurrent responses for the different compositions, we obtained a similar asymmetric parabolic profile for both 300 mW and 600 mW sources, as shown in Figure 4b. The photocurrent increases with an increase in the ratio of iron oxide because of the improvement in visible photon absorption. The highest photocurrent response occurs at a ratio of 57.12 mol%. The decrease in the photocurrent beyond the optimized ratio of iron oxide (57.12 mol%) is due to a more pronounced short exciton diffusion length effect resulting from the substantial increase of iron oxide in the TiO2-Fe2O3 matrix. Notably, most of the TiO2-Fe2O3 compositions show higher photocurrents than Fe2O3 alone, which indicates that the creation of heterogeneous interfaces between TiO2 and Fe2O3 enhances the exciton separation rate, thereby compensating for the short 7



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exciton diffusion length effect in pure iron oxide. The enhancement of exciton separation in the TiO2-Fe2O3 photoelectrode is shown in Figure 4c, which also indicates that with the proper creation of heterogeneous interfaces of TiO2 and Fe2O3, exciton recombination can be effectively suppressed. Photocurrents that are 4 times and 21 times higher than those of pure iron oxide and TiO2, respectively, were achieved by the optimizing the ratio of the TiO2-Fe2O3 cocatalyst. After determining the optimal ratio of TiO2-Fe2O3, the next step was to integrate a plasmonic silver layer into the TiO2-Fe2O3 electrode to form the desired plasmonic cocatalyst system.

Figure 5(a) The photocurrent responses of different photoelectrodes (b) Power dependence study of TiO2-Fe2O3/Ag and (c) Photocurrent response comparison with TiO2-Fe2O3 photoelectrode under irradiation with different laser powers. Figure 5a shows the photocurrent responses of different photoelectrode structural configurations under irradiation with a 532 nm green laser with a power of 600 mW. Even with the two-fold greater laser power input, the TiO2 photocurrent remains negligible

under

532

nm

irradiation. Similarly,

TiO2-Fe2O3 exhibits

higher

photocurrent than TiO2 and Fe2O3 alone due to the enhanced charge separation rate at the TiO2-Fe2O3 heterogeneous interface. To improve the water-splitting performance, silver nanoparticle layers were integrated into the TiO2-Fe2O3 cocatalyst system to increase the absorption of incident light. Two different plasmonic structural configurations were investigated in this study: Ag/TiO2-Fe2O3 (top plasmon-enhanced mode) and TiO2-Fe2O3/Ag (bottom plasmon-enhanced mode). The water-splitting photocurrent was successfully improved in both configurations compared to the TiO2-Fe2O3 only photoelectrode. The highest photocurrent was found with the geometry in which the plasmonic silver nanoparticles were located underneath the TiO2-Fe2O3 (TiO2-Fe2O3/Ag, bottom plasmon-enhanced mode). This geometry yields 8


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a photocurrent that is 2 times higher than that for silver nanoparticles on the top of TiO2-Fe2O3. This result is not consistent with what was observed with UV-Vis absorption, which showed that the strongest light absorption occurs when the plasmonic silver layer is loaded on the top of TiO2-Fe2O3. However, this discrepancy can be explained by the shading effect generated from the Ag/TiO2-Fe2O3 configuration: the larger coverage of silver nanoparticles reduces the number of available reaction sites (on the TiO2-Fe2O3 surface) accessible to the reactants. Therefore, TiO2-Fe2O3/Ag may a better approach to utilize plasmonic enhancement in this cocatalyst system without interfering with active reaction sites. All of the photocurrent responses in Figure 5a were measured under a small applied voltage of 0.02 V, and additional photocurrent responses at higher applied voltages are provided in Figure S2 in the supporting information. Furthermore, prolonged photocurrent measurements were performed to examine the stability of the TiO2-Fe2O3/Ag photoelectrode, which are provided in the supporting information (Figure S3). Figure 5b shows the results of the power dependence study of TiO2-Fe2O3/Ag, which demonstrates a linear relation between the laser power and water-splitting photocurrent in the range from 100 mW to 600 mW. The threshold laser power was also determined from the intersection with the x axis, which indicates a minimum laser power of 90 mW. Improving the interface surface of the plasmonic silver layer with TiO2-Fe2O3 could reduce this threshold. For further comparison, the results from the power dependence study of the TiO2-Fe2O3 photoelectrode have also been included, which show a similar minimum laser power but with a smaller slope for the photocurrent increase shown in Figure 5c. To further elucidate the role of the silver nanoparticles, we also replaced Ag with Pt to confirm that the photocurrent enhancement is due to plasmonic effects; these results are provided in Figure S4 in the supporting information. Finally, the spatial distribution of the photocurrent was examined using a step design structure in the photoelectrode. The concept behind step design is that rather than covering the full area, the plasmonic silver layer only covers half of the photoelectrode underneath the TiO2-Fe2O3 layer. In Figure 6a, an optical image of the step design photoelectrode shows the clear boundary line at the middle of photoelectrode along with the grid used to measure the photocurrents. Spatially resolved mapping photocurrents were measured across both plasmon-enhanced and non-plasmon-enhanced regions with a 300 mW laser beam by rastering the laser focus through every intersection of the grid. A stair-like spatial photocurrent response was observed from this step design photoelectrode, as shown in Figure 6b. The top of the stair exhibits an enhanced photocurrent in an area of 3 mm x 8 mm due to the 9



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existence of the plasmonic silver layer. The photocurrent distribution is not perfectly uniform in the entire enhanced area, which can be attributed to the arbitrary shape of plasmonic Ag NPs. However, this entire plasmon-enhanced area still shows more than four times the enhanced photocurrent with respect to the bottom of the stair (TiO2-Fe2O3 only). This difference demonstrates the potential application of the plasmonic cocatalyst photoelectrode for improving solar water splitting over large areas.

Figure 6 (a) Optical image of the step design photoelectrode and (b) Spatially resolved

mapping

photocurrents

across

both

non-plasmon-enhanced regions

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plasmon-enhanced

and

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Conclusion In this study, plasmonic TiO2-Fe2O3 cocatalyst photoelectrodes were successfully fabricated using a completely solution-based process. Using the optimized ratio of TiO2-Fe2O3 cocatalyst, photocurrent enhancements of 4 and 21 times were achieved with respect to those of pure iron oxide and TiO2, respectively, under visible light irradiation due to the appropriate creation of heterogeneous interfaces between TiO2 and Fe2O3. With the integration of a plasmonic Ag NP layer, a 5-fold improvement in the water-splitting photocurrent was observed compared to TiO2-Fe2O3 alone under irradiation with a 532 nm green laser. This additional photocurrent enhancement is attributed to the strong near-field and light scattering effects from the plasmonic Ag NPs. Spatially resolved mapping of the photocurrent was also performed across both the plasmon-enhanced and non-plasmon-enhanced regions, which demonstrates a potential application for the plasmonic cocatalyst photoelectrode in improving solar water splitting over large areas.

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ASSOCIATED CONTENT

Supporting Information SEM images of Ag nanoparticles (Figure S1); The photocurrent response of TiO2-Fe2O3/Ag at 1 V (Figure S2); Prolonged Photocurrent measurement of TiO2-Fe2O3/Ag photoelectrode (Figure S3); Comparisons of photocurrent response of Pt/TiO2-Fe2O3 and TiO2-Fe2O3/Pt at 0.02 V (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Phone: +886-4-24517250-5309 *Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Science Council Foundation (NSC 101-2218-E-035 -007 -MY3).

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Enhanced Photocatalytic Water Splitting by Plasmonic TiO2âFe2O3

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