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Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts† Jing Qiu,a Guangtong Zeng,b Prathamesh Pavaskar,c Zhen Lic and Stephen B. Cronin*bc Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for high efficiency photocatalytic water splitting. However, the electrochemical instability of most III–V semiconductors severely limits their applicability in photocatalysis. In this work, we passivate p-type GaP with a thin layer of n-type TiO2 using atomic layer deposition. The TiO2 passivation layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO2 passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn-region. Plasmonic Au nanoparticles deposited on top of the TiO2-passivated GaP further increases the photoconversion efficiency through local field enhancement. These two enhancement mechanisms are separated by systematically varying the thickness of the TiO2 layer. Because of the tradeoff between the quickly decaying plasmonic fields and the formation of the pn-charge separation region, an optimum performance is achieved for a TiO2 thickness of 0.5 nm. Finite difference time domain (FDTD) simulations of the electric field profiles in this

Received 5th November 2013, Accepted 18th December 2013

photocatalytic heterostructure corroborate these results. The effects of plasmonic enhancement are

DOI: 10.1039/c3cp54674h

structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au. This general approach of

distinguished from the natural catalytic properties of Au by evaluating similar photocatalytic TiO2/GaP passivating narrower band gap semiconductors enables a wider range of materials to be considered for

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plasmon-enhanced photocatalysis for high efficiency water splitting.

In photocatalysis, the energy of photons can be utilized to drive many important chemical reactions, including H2 production,1 CO2 reduction,2–4 and water purification.5,6 Unfortunately, the efficiencies of most photocatalytic processes are far too low for practical large scale applications. The direct conversion of solar-to-chemical energy has several advantages over solar-toelectric energy conversion, most notably, the ability to store large amounts of energy (BGW) in chemical bonds that can later be released without producing harmful byproducts. As the cost of direct solar-to-electrical energy becomes competitive with fossil fuels, there will be a need to store large amounts of the solar energy for use during nights, cloudy days, and winter months. Therefore, our energy infrastructure has much to gain by finding more efficient ways to enhance these photocatalytic processes. a

Department of Materials Science, University of Southern California, Los Angeles, California 90089, USA b Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, USA. E-mail: [email protected] c Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp54674h

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Over the past few years, several research groups have demonstrated a new method for improving the efficiency of photocatalytic processes by exploiting the strong plasmon resonance of small metal nanoparticles.7–12 While these studies have clearly shown proof-of-principle of this enhancement mechanism, the overall photoconversion efficiencies are still very low.8 Torimoto et al. also demonstrated enhanced photocatalytic water splitting by depositing CdS nanoparticles on SiO2-coated Au nanoparticles.13 However, these plasmonicphotocatalytic complexes demonstrated enhancement factors less than 2. Liu et al. and Ingram et al. observed enhanced photocatalytic water splitting under visible illumination by depositing plasmonic metal nanoparticles on top of anatase TiO2.13–18 While these previous works reported enhancement factors of approximately 10-fold with the incorporation of plasmonic nanoparticles, the overall photoconversion efficiencies were still quite low in the visible wavelength range because of TiO2’s large band gap (Eg = 3.2 eV). As a result, these proof-ofprinciple studies relied on short-lived, sub-band gap defect states for optical absorption in the visible wavelength range. GaP (Eg = 2.25 eV) has a substantially smaller band gap than TiO2, can absorb more than 18% of the solar spectrum, and is better matched to the plasmon resonance energy of

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Au nanoparticles. While GaP absorbs a more extended range of the solar spectrum than TiO2, its main advantage as a photocatalyst is its relatively high conduction band energy, which exceeds that of TiO2 by more than 1 eV. However, GaP is more expensive than TiO2 and is not oxidatively stable over a wide range of pH. As a result, a vast majority of previous photocatalytic studies have been carried out on TiO2. In the work presented here, we investigate the photocatalytic stability of TiO2-passivated GaP using atomic force microscopy (AFM), photoelectrochemistry, and optical microscopy. The photocatalytic efficiency is studied systematically as a function of TiO2 layer thickness. Further catalytic enhancement is explored with the addition of plasmonic gold nanoparticles, and is studied as a function of TiO2 layer thickness. The effects of plasmonic enhancement are distinguished from the natural catalytic properties of Au by evaluating similar photocatalyticTiO2/GaP structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au.

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Zn doped p-type (100) oriented GaP with a dopant concentration of 2  1018 cm3 was used as a photocathode for water splitting. Atomic layer deposition (ALD) of TiO2 was performed at 250 1C on the p-GaP wafers using TiCl4 as the titanium source and water vapor as the oxygen source. The carrier gas during the deposition was argon with a flow rate of 20 sccm. The rate of deposition was about 0.4 Å per cycle. A 500 nm thick aluminum film was evaporated on the back of the p-GaP to form an Ohmic contact. We then evaporated a gold film with a nominal thickness of 5 nm on the top surface of the TiO2. This thin gold film is known to form island-like growth that is strongly plasmonic and serves as a good substrate for surface enhanced Raman spectroscopy (SERS)19–21 and photocatalytic enhancement.14,22 The spectral response of these films has been studied in our previous work.24 Samples were prepared with and without the plasmonic gold nanoparticles, as shown in the schematic diagrams of Fig. 1a and 2a. The aluminum contact was then connected to the external circuitry with a

Fig. 1 (a) Schematic diagram of sample geometry and (b) photocatalytic current–potential curves measured for GaP photocatalysts with various thicknesses of TiO2 under 1 W cm2 532 nm illumination in a 0.5 M Na2SO4 pH = 7 solution. (c) Relative decrease of the overpotential required to initiate this reaction and (d) calculated built-in voltage plotted as a function of TiO2 thickness. Energy band diagrams for (e) thin and (f) thick TiO2 layers. (g) Photocatalytic current plotted as a function of potential for thicker TiO2 layers.

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Fig. 2 (a) Schematic diagram of sample geometry and (b) photocurrent plotted as a function of voltage for GaP photocatalysts with various thicknesses of TiO2 with Au nanoparticles under 1 W cm2 532 nm illumination in a 0.5 M Na2SO4 solution.

copper wire and coated with epoxy cement to insulate it from the electrolytic solution, as shown in the figure. The photocatalytic reaction rates of two sets of samples were measured in 0.5 M Na2SO4 and 0.5 M H2SO4 solutions using a three-terminal potentiostat with the prepared samples, a Ag/AgCl electrode, and a graphite electrode functioning as the working, reference, and counter electrodes, respectively. Finite difference time domain (FDTD) simulations were carried out using a cell of size 1000 nm  800 nm  500 nm. A grid spacing of 2 Å is used in the volume of 500 nm  500 nm  40 nm around the film and 10 nm elsewhere. A 0.002 fs temporal grid is used with a total of 100 000 time steps. In the simulation, the samples are irradiated with a planewave source with a Gaussian pulse containing a spectrum of wavelengths ranging from 300 nm to 800 nm. Perfectly matched layers (PML) boundary conditions are used with 25 layers. The dielectric function of Au is based on the optical constants given by Palik and Ghosh.28 Fig. 1b shows the photocurrent–voltage curves for GaP passivated with various thicknesses of TiO2 measured in a pH = 7 solution of 0.5 M Na2SO4 under 532 nm illumination. Bare GaP (blue curve) has an onset of photocurrent at a potential of approximately 0.66 V. For TiO2 passivated GaP, we see a clear shift in the overpotential that scales linearly with the thickness of the TiO2, as shown in Fig. 1c. For 10 nm TiO2 (pink curve), this overpotential/onset potential is shifted by approximately 0.46 V. This shift is attributed to the formation of a pn-junction, since the TiO2 is n-type doped due to oxygen vacancies.23 While TiO2 does not absorb light at 532 nm, the pn-junction formed with the GaP enables separation of photogenerated charge in the actively absorbing GaP. Fig. 1d shows the built-in potential for the junction calculated using the WD2 q Na Nd ðNa ea þ Nd ed Þ , with a doping relation Vbi ¼ 2e0 ea ed ðNa þ Nd Þ2 concentration of Na = 5  1018 cm3. Here, WD is the depletion width of the GaP–TiO2 junction, which is a function of the TiO2 thickness layer. This calculation shows a similar trend to the experimentally observed shift in the overpotential. Typically, p-type GaP acts as a photocathode due to the direction of band bending at its interface with water, promoting photoelectron flow to the water. For n-type semiconductors, the bending is usually upward, resulting in photohole flow

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to the water. It is, therefore, somewhat surprising that when n-type TiO2 up to 10 nm thick is coated on top of p-GaP, the electrode still functions as a photocathode. This is most likely due to the finite thickness of the TiO2 layer, which is fully depleted and does not provide enough surface charge to invert the material from p- to n-type, as illustrated in Fig. 1e. When the TiO2 thickness is further increased, enough donor impurities to bend the band upward, hindering the electron flow to the water, as shown in Fig. 1f. This result was corroborated experimentally, when the thickness of TiO2 is increased beyond 15 nm, we observe no photocurrent, as shown in Fig. 1g. Fig. 2 shows the photocatalytic I–V characteristics for TiO2 passivated GaP with 5 nm gold nanoparticles. Au nanoparticles deposited on bare GaP without TiO2 passivation (purple curve) improve the photocatalytic reaction rate by a slight shift of the I–V curve to a lower overpotential with respect to bare GaP by approximately 0.2 V. Depositing Au nanoparticles on top of TiO2 passivated GaP results in a further improvement in the I–V characteristics. For 0.5 nm thick TiO2, however, we observe the largest enhancement in the photocatalytic reaction rate. For this dataset, we observe both a downshift of the overpotential (by approximately 0.58 V) and an increase in the photocurrent (i.e., increased photoinduced charge) due to plasmonic field enhancement. At V = 0.7 V, the photocurrent is enhanced by a factor of 4 with respect to bare GaP, after accounting for the shift in the overpotential. The photocatalytic properties of samples with 0.5 nm TiO2 are substantially better than those with 1 nm TiO2. This anomalous behavior of 0.5 nm thick TiO2 was observed in several other samples consistently, and is the result of a tradeoff between pn-junction formation and coupling of the localized plasmonic fields of the Au nanoparticles to the actively absorbing GaP layer, as discussed below. In order to separate the effects of plasmonic enhancement from the natural catalytic properties of Au, we evaluated similar photocatalytic TiO2/GaP structures with Pt instead of Au, which is a catalytic, non-plasmonic metal. Fig. S1 in the ESI† document shows the photocurrent versus potential plots taken for GaP photocatalysts with Pt nanoparticles under 532 nm illumination. No significant photocurrent is observed for overpotentials below 1 V indicating that the enhancement observed with Au nanoparticles originates from the plasmon resonance.

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Fig. 3 Electric field distributions calculated using the finite difference time domain method (a) in the plane of the Au nanoparticles and (c, d) in the perpendicular direction across the GaP/TiO2/Au/electrolyte interface. (b) Calculated electric field enhancement factor plotted as a function of TiO2 thickness using eqn (1).

Fig. 3 shows the results of a finite difference time domain (FDTD) simulation performed on a 5 nm gold nanoparticle (nanoisland) film. The electric field distribution in the plane of the gold island film is shown in Fig. 3a. Here, localized hot spots can be seen in regions between nearly touching nanoparticles–nanoislands separated by approximately 2–3 nm. This phenomenon has been studied in detail previously.14,22,25,26 Fig. 3c shows the electric field distribution in the perpendicular direction, across the GaP/TiO2/Au/electrolyte interface at one of the hot spots between two nearly touching Au nanoparticles with a TiO2 thickness of 0.5 nm. Here, the local electric field intensity can reach 1000 the incident electric field. These plasmonic nanoparticles couple light very effectively from the far field to the near field at the GaP/TiO2 pn-junction. This is advantageous for photocatalysis for two reasons. First, there is an increased electron–hole pair generation rate in close proximity to the electrolyte interface and charge separating region, enabling a larger fraction of the photoinduced charge to diffuse to the catalytic surface and contribute to catalysis. We can calculate this fraction by integrating E2 over the volume of the catalyst from the GaP surface to one minority carrier diffusion length (100 nm) below this surface, as described by eqn (1). Ð0

EF ¼

Ð 2 100nm dz jEj dxdy Ð0 Ð 2 100nm dz jE0 j dxdy

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(1)

In the denominator, the electric field intensity without the TiO2 and Au nanoparticles, E0, is integrated over the same volume as in the numerator. The second enhancement mechanism arises from the increased light intensity at the pn-junction, which   nkT IL produces a larger open circuit voltage. Voc ¼ ln þ1 , q I0 and this further reduces the overpotential. Here, the plasmonenhanced electric fields create a larger photocurrent IL, which in turn produces a larger Voc. In addition to electric field

Fig. 4 Two terminal photocurrent density (absolute value) plotted as a function of the applied overpotential for various GaP/TiO2/Au nanoparticle photocatalysts measured in a pH = 0, 0.5 M H2SO4 solution under 1 W cm2 532 nm illumination.

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enhancement, these plasmonic nanoparticles may produce hot electrons that can drive catalytic processes at a lower applied overpotential.9 Fig. 3d shows the electric field distribution for a

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GaP/TiO2/Au nanoparticle structure with a 3 nm thick TiO2 film. Here, the localized plasmonic fields do not extend into the actively absorbing GaP layer. Fig. 3b shows the integrated

Fig. 5 (a) Time dependence of the photocurrent density of bare GaP illuminated with 1 W cm2 532 nm light in a 0.5 M Na2SO4 solution at an applied overpotential of 0.7 V. (b) Optical microscope image, (c) atomic force microscope image, and (d) surface topography of the GaP surface after the 5 hour reaction.

Fig. 6 (a) Time dependence of the photocurrent density of TiO2 passivated GaP illuminated with 1 W cm2 532 nm light in a 0.5 M Na2SO4 solution at an applied overpotential of 0.7 V. (b) Optical microscope image, (c) atomic force microscope image, and (d) surface topography of the GaP/TiO2 surface after the 12 hour reaction.

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electric field enhancement factor calculated using eqn (1) plotted as a function of TiO2 thickness. In this figure, a maximum enhancement of 42% can be seen at 0.5 nm TiO2 thickness. The sharp drop off of the enhancement factor as the TiO2 thickness increases is a direct result of the highly localized nature of the plasmon-enhanced fields. The 42% electric field enhancement factor is significantly smaller than those obtained in our previous studies of plasmon-enhanced TiO2 (without GaP).14 This is a result of GaP’s relatively long minority carrier diffusion length (100 nm), which results in a larger integrated volume in eqn (1). Fig. 4 shows the two terminal photocurrent densities plotted as a function of the applied overpotential for various GaP/TiO2/Au nanoparticle photocatalysts, measured with a Pt counter electrode in a pH = 0, 0.5 M H2SO4 solution illuminated at 532 nm. A large reduction in the overpotential from approximately 0.8 to 0.2 V can be seen for the GaP/TiO2/Au structure with 0.5 nm TiO2 thickness. Again, the plasmonenhanced photocatalyst with 0.5 nm TiO2 is substantially better than those with thicker TiO2 films and TiO2 without gold nanoparticles. Perhaps the most important aspect of these TiO2 passivated GaP photocatalysts is their photochemical stability in the electrolytic solution. Recently, Chen et al. reported 2–10 nm TiO2 can protect Si photoanode for water oxidation from corrosion.27 Fig. 5 and 6 show the time dependence of the photocatalytic current and surface roughness of GaP without and with TiO2 passivation, respectively. In Fig. 5a, the photocurrent density is plotted as a function of time for bare GaP illuminated at 532 nm in a 0.5 M Na2SO4 solution with an applied overpotential of 0.7 V for 5 hours. An exponential decay can be seen with a time constant of 0.45 hours indicating significant corrosion of the surface. An abrupt drop in the photocurrent occurs at 4.8 hours when the device failed. Fig. 5b and c show optical microscope and atomic force microscope images of the GaP surface after 5 hours of illumination. Fig. 5d shows a plot of the surface topography obtained along the dashed white line indicated in Fig. 5c, showing an RMS roughness of 143 nm. In contrast, the photocurrent density of TiO2 passivated GaP is stable for 12 hours, as plotted in Fig. 6a. The optical microscope image (Fig. 6b) and atomic force microscope image (Fig. 6c) exhibit no evidence of surface corrosion or damage after 12 hours, with an RMS surface roughness of 1.0 nm (Fig. 6d). In conclusion, plasmon-enhanced photocatalytic water splitting is observed on TiO2 passivated GaP. The TiO2 passivation layer prevents corrosion of the GaP surface, making it stable in pH = 0 solution. In addition to preventing corrosion, the TiO2 passivation layer provides enhancement in the photoconversion efficiency through the formation of a charge separating pn-region, which decreases carrier recombination and lowers the overpotential required to initiate this reaction. Plasmonic Au nanoparticles deposited on top of TiO2 passivated GaP further improve the photocatalytic process through plasmonic field enhancement. These two enhancement mechanisms result in an optimum thickness of the TiO2 layer of 0.5 nm.

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Electromagnetic simulations performed using the finite difference time domain (FDTD) method indicate that a 0.5 nm film of TiO2 enables significant coupling of the localized plasmonic fields of the Au nanoparticles with the actively absorbing GaP layer. This general approach of passivating narrower band gap semiconductors with TiO2 will enable more efficient photocatalysts to be developed.

Acknowledgements This research was supported by ONR Award No. N00014-12-10570 (J.Q.) and NSF Award No. CBET-0854118 (G.Z.).

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Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts.

Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for high efficiency photocatalytic water splitting...
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