Improved Photoelectrochemical Water Splitting Performance of Cu2O

Article pubs.acs.org/JPCC

Improved Photoelectrochemical Water Splitting Performance of Cu2O/SrTiO3 Heterojunction Photoelectrode Dipika Sharma,† Sumant Upadhyay,† Vibha R. Satsangi,‡ Rohit Shrivastav,† Umesh V. Waghmare,§ and Sahab Dass*,† †

Department of Chemistry, Dayalbagh Educational Institute, Agra 282 110, India Department of Physics & Computer Sciences, Dayalbagh Educational Institute, Agra 282 110, India § Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India ‡

ABSTRACT: Nanostructured thin films of Cu2O modified by overlayering SrTiO3 with varying thickness have been studied for the first time as photoelectrode in photoelectrochemical (PEC) water splitting. Effective mass calculations for electrons and holes in bulk SrTiO3 and Cu2O using DFT firstprinciples have also been attempted to explain the enhanced charge separation at Cu2O/SrTiO3 interface. All samples were characterized using XRD, SEM, and UV−vis spectrometry. The influence of surface modification of Cu2O with varying thickness of SrTiO3 on PEC performance has been investigated. Photocurrent density for Cu2O/SrTiO3 heterojunction with overall thickness of 343 nm at 0.8 V/SCE was found to be 2.52 mA cm−2 which is 25 times higher than that of pristine Cu2O (0.10 mA cm−2 at 0.8 V/SCE). Theoretical studies showed that the electrons in SrTiO3 had large effective masses as compared to electrons in Cu2O at conduction band minima indicating weak mobility of photogenerated electrons in SrTiO3 and strong mobility in Cu2O leading to improved separation of charge carriers resulting in the enhancement of photocurrent densities at the Cu2O/SrTiO3 heterojunction.

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perovskite cubic structure with good flatband potential and photovoltage (Voc) indicating its ability to photolyze the water.9−13 On the other hand SrTiO3 is a wide band gap material, and absorbs in the UV region which limits its efficiency toward photoelectrochemical splitting of water. It is expected that heterojunction of SrTiO3 with a small band gap material such as Cu2O,14 CdS,15 and α-Fe2O316 may overcome its limitation of absorption in UV region. In the present study modifications were attempted on Cu2O with SrTiO3 (both possess cubic crystal structure with lattice constant of about 4.26 and 3.905 Å, respectively), by making the ITO/Cu2O/ SrTiO3 heterojunction thin films with varying thickness of SrTiO3. Prepared Cu2O/SrTiO3 heterojunction photoelectrodes were also characterized for their structural, electrical, and optical properties. Effective mass calculations for charge carriers using first-principles density functional theory (DFT) were also carried out for the first time to understand the charge separation mechanism at Cu2O/SrTiO3 interface.

INTRODUCTION Photoelectrochemical (PEC) method is considered to be a safe and highly promising method for hydrogen generation due to several reasons: (a) PEC technology is based on solar energy, which is a perpetual source of energy, and uses water which is a renewable energy resource, (b) PEC technology may be used on both large and small scales, and (c) PEC technology is relatively uncomplicated.1 Various methodological attempts have been carried out toward enhancing the efficiency of PEC water splitting including heterojunction systems,2 doping,3 dye sensitization,4 swift heavy ion irradiation,5 etc. Use of a heterojunction semiconductor system (as photoelectrode) is a promising strategy attempted to improve the efficiency of PEC cell. A heterojunction of two semiconductor materials, one with wide band gap and another with low band gap, having a staggered band-edge alignment (i.e., type-II band alignment) in which the valence band (VB) of one semiconductor is positioned (energetically) between the valence band (VB) and conduction band (CB) of another semiconductor, and its CB is positioned above the VB and CB of other semiconductor, can improve separation of photogenerated charge carriers leading to enhanced photoactivity of photoelectrodes. The small band gap semiconductor is also responsible for visible light absorption and sensitizes the wide band gap material through electron or hole injection.6 Among the various metal oxides, strontium titanate (SrTiO3), on account of its favorable properties, becomes a favorable candidate to be used as photoanode in the PEC splitting of water.7,8 SrTiO3 has © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Cu2O Thin Films. Nanostructured thin films of Cu2O were deposited on the conducting glass substrate (ITO) using simple spray-pyrolysis (Holmarc, India) method. For this spray precursor, copper(II) acetate monoReceived: July 15, 2014 Revised: October 9, 2014 Published: October 9, 2014 25320

dx.doi.org/10.1021/jp507039n | J. Phys. Chem. C 2014, 118, 25320−25329

The Journal of Physical Chemistry C

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hydrate [Cu(CH3COO)2·H2O (Aldrich 98%)] and dextrose (Fisher 99.5%) dissolved in water were used as starting compounds. In addition 20 vol % of 2-propanol [(CH3)2CHOH (Qualigens 99.5%)] was added to the abovedescribed aqueous solution. Precursor solution was sprayed at a pressure of 2 kg cm−2 through a pneumatic nebulizer with a nozzle diameter of 0.1 mm for 30 s with the time interval of 10 s, onto ITO glass substrate, and kept on substrate heater at 280 °C temperature. One third part of ITO substrate was initially covered with aluminum foil to establish the electrical contact to convert them into electrodes.17 2.2. Preparation of Heterojunction Cu2O/SrTiO3 Thin Films. Heterojunction Cu2O/SrTiO3 thin films were obtained by over layering the SrTiO3 thin films with various thicknesses using a sol−gel spin coating method. The precursor solutions comprised strontium acetate [Sr(CH3COO)2, Aldrich 99% ] and titanium isopropoxide [Ti(OC2H4CH3)4, Aldrich 97%]; detailed methodology for SrTiO3 thin films has been reported elsewhere.18 To optimize the photoelectrochemical response of the Cu2O/SrTiO3 heterojunction, thin films (1, 2, and 5 layers) were tried for the deposition of SrTiO3 thin films. A schematic diagram of the Cu2O/SrTiO3 heterojunction photoelectrode at the irradiated side is shown in Figure 1. Descriptions about

was placed 90° on the half edge of the sample holder for taking cross-sectional SEM image. Surface morphology of Cu2O/ SrTiO3 heterojunction thin film was obtained using scanning electron microscope (Hitachi S3700, Japan). The band gap energy was determined using absorption data of thin films recorded by UV−vis spectrophotometer (Shimadzu, UV-2450, Japan). The thickness of all samples was measured by using αstep profilometer (tencor Alpha Step D-120). XPS measurements were performed using an Mg Kα source (Omicron Nanotechnology). All the binding energy was referenced to the adventitious C 1s peak at 284.8 eV. 3.1. Photoelectrochemical Study. PEC performance measurements for samples were carried out in aqueous electrolyte of 0.1 M NaOH (pH 13). Pristine Cu2O, SrTiO3, and Cu2O/SrTiO3 heterojunction thin films were used as working electrode, saturated calomel (PAR, Model K0077) as a reference electrode (SCE), and platinum gauze as a counter electrode. Current−voltage characteristics were recorded at a scan rate of 20 mV/s using scanning potentiostat (PAR, Model: VersaStat II), under darkness and by illuminating the photoelectrode with visible light (150 W xenon arc lamp, Bentham, output intensity 150 mW/cm2), which was first passed through a water jacket to prevent IR radiation. All the photoelectrodes were scanned between −1.0 and +1.0 V/SCE. The photocurrent density was extracted from I−V curves, by taking the difference between the current observed under illumination and under darkness per square centimeter exposed area of the working electrode. IPCE measurements were also carried out in the same three electrode PEC cell under illumination using electrochemical workstation (Zahner, PP211, CIMPS-pcs, Germany) with wideband white LED tunable light source (output intensity = 1.02 W/m−2) and a separate monochromator covering 430−720 nm. The incident photon to electron conversion efficiency (IPCE) values of all samples were calculated using the following expression19

Figure 1. Schematic diagram of the heterojunction photoelectrode at the irradiated side.

films thicknesses with other details of all the samples prepared in this study are summarized in Table 1. All the films were converted into photoelectrodes using copper wire, silver paste, and epoxy (Hysol, Singapore) for its use as photoelectrode in PEC cell.

IPCE =

I is the measured photocurrent density, λ is the wavelength of the incident light, and Jlight is the measured irradiance at the measurement wavelength. The resistivity of all the samples was calculated from the slope of current−voltage characteristic curves under dark conditions. 3.2. Mott−Schottky Analysis. To obtain the Mott− Schottky curves, the capacitance (C) at the semiconductor/ electrolyte junction with an ac signal frequency of 1 kHz was measured using a LCR meter (Agilent Technology, model 4263 B, Singapore) at varying electrode potentials in the same threeelectrode configuration under dark conditions. Linear variations of 1/C2 with applied potential (Vapp) were plotted to get Mott− Schottky (MS) curves. From the intercept of these Mott− Schottky curves, flatband potentials were calculated for all the samples using the following relation:20

Table 1. Description for All the Thin Film Samples sr no.

Cu2O film thickness (nm)

1 2 3 4 5

183 183 183 183

SrTiO3 film thickness (nm)

overall thickness (nm)

acronym

160 160 352 495

167 160 343 535 678

A B C D E

1240I λJLight

3. CHARACTERIZATIONS The phases and crystalline structure of Pristine Cu2O, SrTiO3, and Cu2O/SrTiO3 heterojunction thin films were characterized using X-ray diffraction (XRD) with a Bruker AXS, Germany, Xray diffractometer (model D8 Advance), having Cu Ka (λ = 1.5418 Å) radiation in the scanning angle range (20−60°) 2θ. The cross sectional and ingredient of heterojunction thin films was examined using field emission scanning electron microscope (FE-SEM) (INCA Penta FET X3, TESCAN, Inter University Accelerator Centre, New Delhi). For the interface cross-sectional SEM image of heterojunction thin film, the sample was cut into half to make an edge. After that, the sample

⎛ 2 ⎞⎛ 1 kT ⎞ =⎜ ⎟ ⎟⎜Vapp − Vfb − 2 q ⎠ C ⎝ qε0εsN ⎠⎝

Here, ε0 is the permittivity of the vacuum, N is the donor density, Vapp is the applied potential, Vfb is the flatband potential, εs is the dielectric constant of the semiconductor, q is the electronic charge, and kT/q is the temperature dependent term. 25321



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Figure 2. (a) Electronic band structure and (b) DOS of Cu2O.

Figure 3. (a) DOS and (b) electronic band structure of SrTiO3.

3.3. Computational Details. The band structures and density of states (DOS) of SrTiO3 and Cu2O were calculated using the Quantum Espresso package21 within the framework of density functional theory using the generalized gradient approximation (GGA).22 Ultrasoft pseudopotentials23 were used in the description of the electron−ion interactions. A Monkhorst-pack mesh24 of 4 × 4 × 4 k points was used in sampling the integrals over the Brillouin zone, and a much finer mesh was used for accurate determination of the density of the electronic states. Kohn−Sham wave functions were represented in a plane-wave basis with cutoff energy of 27 Ry, and the charge density was represented with a plane-wave basis with a cutoff of 162 Ry. Structural optimization was carried out using Hellman−Feynman forces to minimize the total energy. Reported electronic structures have been calculated at the optimized structures.

calculation of electron and holes, calculated for Cu2O and SrTiO3, are shown in Figure 2a,b and Figure 3a,b, respectively. It is clear that the conduction band minima and valence band maxima of Cu2O are mainly constructed by O-2p orbitals and Cu-3d orbitals, respectively, and lie at same point (Γ) indicating its direct band gap behavior.25 In contrast, the valence band top of SrTiO3 is mainly constructed by O-2p orbitals. The conduction band bottom of SrTiO3 consists of Ti-3d orbitals. Energy band structure of SrTiO3 shows the conduction band minima and valence band maxima lying at different points indicating it to be an indirect band gap semiconductor.26 4.1. Effective Mass Calculation. The effective mass of electron me at the conduction band minima and hole mh at valence band maxima for SrTiO3 and Cu2O were evaluated by fitting the conduction and valence bands to a parabola according to E = ℏ2k2/2mem0, where m0 denotes the electron rest mass. For Cu2O and SrTiO3, electron effective mass was calculated at CBM and hole effective mass at VBM near Γ point in Γ-R direction.27,28 For SrTiO3 hole effective mass near M point in M-X direction is also calculated as VBM lies at M point (Figure 3b) as shown in Table 2. From the calculated values of effective masses, it is clear that elctrons in Cu2O have low effective mass as compared to the electrons in SrTiO3. In contrast, the effective mass of hole is lower for SrTiO3 than in Cu2O; hence, the mobility of electrons is higher in Cu2O while mobility of holes is higher in SrTiO3, respectively. Furthermore,

4. RESULTS AND DISCUSSION To explain the enhancement in the separation of charge carriers or reduction in recombination rate at the interface of Cu2O/ SrTiO3 heterojunction, which may be one of the reasons for high photocurrent densities and IPCE for heterojunction samples, the effective masses of charge carriers were calculated theoretically using DFT based calculations. The lower the effective mass is, the greater the mobility of charge carriers will be. Electronic band structure and DOS for effective mass 25322



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Table 2. Effective Mass Componentsa CBM at Γ point Γ-R (111) direction VBM at Γ point Γ-R (111) direction VBM at M point M-X (110) direction a

Cu2O

SrTiO3

0.676 3.80

1.23 0.982 1.12

In unit of m0.

band-offset calculations have also been carried out to determine the possible mechanism of enhanced charge separation within Cu2O/SrTiO3 heterojunction interface. Band Offset Calculations for Cu2O/SrTiO3 Interface. Model of the Cu2O/SrTiO3 Interface. The Cu2O/SrTiO3 interface is modeled with a slab consisting of a Cu2O (001) layer of four unit cell thickness interfacing with a SrTiO3 (001) layer of 4 unit cell thickness (see Figure 4). This atomic arrangement is Figure 5. Macroscopically average electrostatic potential where the averaging steps along the Z-direction of the Cu2O/SrTiO3 supercell.

valence band (VB) of Cu2O is positioned (energetically) between the valence band (VB) and conduction band (CB) of SrTiO3, and the CB of Cu2O is positioned above the VB and CB of SrTiO3, which favors the charge carrier separation or photoelectrochemical activity of Cu2O/SrTiO3 heterojunction (see Figure 5). Hence, theoretical results reveal that overlayering of SrTiO3 on Cu2O could improve mobility of charge carriers resulting in the enhanced separation of photogenerated charge carriers in Cu2O/SrTiO3 heterojunction as shown in Figure 11.31 4.2. Crystal Phase Analysis. XRD analysis was used to identify the crystal structure of the samples. Figure 6 shows the

Figure 4. Supercell model considered in the present work.

weakly polar along the longitudinal c-axis, and we use a thick layer of vacuum to minimize the effects of polarity and periodic images associated with the supercell. Band Offset. The band offset (BO) is typically split into two terms:29,30

BO = ΔEv, c + ΔV

Figure 6. X -ray diffraction pattern for (A) pristine Cu2O, (B) pristine SrTiO3, and (C−E) Cu2O/SrTiO3 heterojunction thin film samples with overall thickness of 343, 535, and 678 nm deposited on conducting glass substrate, ITO (Sn/In2O3), respectively.

(1)

The first contribution comes from the difference in valence (or conduction) band energies derived from the band structure of bulk materials on the two sides of the interface. Knowing the value of ΔEv, we obtain ΔEc by adding the experimental band gaps because DFT-based calculations of band gaps are wellknown to give underestimation. The second term, ΔV, is the step-like jump in the average of the electrostatic potential across the heterojunction, and is obtained using planar and macroscopic averages of the electrostatic potential (see Figure 5). From band offset calculations we conclude that the Cu2O/ SrTiO3 heterojunction interface after contact is type-II or having staggered band-edges alignment meaning that the

X-ray diffraction (XRD) patterns of pristine Cu2O, SrTiO3, and heterojunction Cu2O/SrTiO3 thin films. The XRD pattern of heterojunction thin film exhibited diffraction peaks at 2θ = 29.16, 36.53, 42.43 which can be indexed to (110), (111), (200) planes, respectively, of the cubic cuprous oxide phase (JCPDS file no. 065-3288). The lattice constant of Cu2O obtained is a = 4.26 Å, which is in agreement with the value a = 4.26 Å obtained by Mao et al.,32 and peaks at 2θ = 32.45, 46.52, and 57.85°, which can be indexed to (110), (200), and (211) plane, respectively, of the cubic strontium titanate phase 25323



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(JCPDS file no. 073-0661). The lattice constant of SrTiO3 obtained from the present data is a = 3.905 Å, which is in agreement with results reported by Solanki et al.18 No other peaks were observed in addition to the In2O3 peaks, which were from the ITO substrates implying that the products are highly pure and no mixed oxide has been formed in case of heterojunction. The crystallite size for the Cu2O and SrTiO3 thin film was calculated to be 32 and 29 nm, respectively, from fwhm of the most intense diffraction peak using Debye− Scherrer’s equation.33 4.3. UV−Vis Absorption Spectrum. Figure 7 shows the UV−vis absorption spectrum for (A) pristine Cu2O, (B)

pristine SrTiO3, and (C−E) Cu2O/SrTiO3 heterojunction thin films with varying thickness of SrTiO3 in the wavelength range 300−800 nm. Pristine Cu2O, SrTiO3 shows absorption edge in visible 530 nm and UV 375 nm light, respectively. For a heterojunction, samples of Cu2O/SrTiO3 thin films showed blue shift from 515 to 478 nm in absorption edge with increase in the thickness of SrTiO3 which clearly indicate the influence of SrTiO3 on the absorption of Cu2O thin films. The absorption intensity of the heterojunction thin films in the UV region increases slightly with the increase in thickness of SrTiO3, which can be explained by the Lambert−Beer law34 according to which film thickness (d) is inversely proportional to the transmitted light intensity (I), resulting in greater absorption of UV light with increased film thickness of SrTiO3. 4.3. Cross-Sectional Surface Morphological and X-ray Energy Dispersive Spectroscopy Analysis. Figure 8A shows the cross-sectional SEM image for sample C which clearly indicates the successful deposition of 155 nm thick SrTiO3 thin film over the Cu2O thin film with thickness of 190 nm. Calculated values of the thickness are in good agreement with the values calculated using surface profilometer. The elemental composition for Cu2O/SrTiO3 thin film (sample C) at the interface was ascertained from the EDAX analysis as shown in Figure 8B with atomic percentage of the Sr, Ti, Cu, and O as 8.23%, 12.58%, 36.45%, and 42.74%, respectively. SEM image for heterojunction sample C shown in Figure 8C clearly shows the granular porous deposition of SrTiO3 on uniformly deposited Cu2O thin film with particle size in the ranges 25−28 and 35−40 nm, respectively, which is also clearly indicated in cross-sectional SEM image.

Figure 7. UV−vis absorption spectra for (A) pristine Cu2O, (B) pristine SrTiO3, and (C−E) heterojunction Cu2O/SrTiO3 thin film samples with overall thickness of 343, 535, and 678 nm, respectively.

Figure 8. Cross-sectional SEM (A), energy dispersive X-ray image (B), and SEM image (C) for Cu2O/SrTiO3 heterojunction sample C. 25324



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Figure 9. XPS spectra of (A) full-spectrum scan, (B) Cu 2p, (C) Sr 3d, (D) O 1s, and (E) Ti 2p for Cu2O/SrTiO3 heterojunction (sample C).

4.5. Photoelectrochemical Measurements. The photocurrent density versus applied potential curves for all samples was measured in 0.1 M NaOH electrolyte. The dark currents obtained for all of the samples were similar. Under illumination, the Cu2O/SrTiO3 photoelectrodes exhibited a p-type photocurrent which increases with an increase in the cathodic bias. Photocurrent density exhibited by heterojunction Cu2O/ SrTiO3 (sample C) was consistently much higher than that of the pristine Cu2O film, which is expected due to the synergy of improved solar absorption and better charge transportation in the heterojunction of Cu2O and SrTiO3. The photocurrent responses of the Cu2O/SrTiO3 heterojunction were assessed with respect to the thickness of SrTiO3 thin films in the given electrolyte (Figure 10). Heterojunction sample with SrTiO3 thickness of 160 nm and overall thickness of 343 nm (sample C) exhibited the highest photocurrent density of 2.52 mA cm−2 at 0.8 V/SCE. Increment in photocurrent density for Cu2O/ SrTiO3 heterojunction can be explained with a theoretical

4.4. XPS Spectra. The surface chemical compositions of the Cu2O/SrTiO3 heterojunction samples were further analyzed using XPS study, and spectra are illustrated in Figure 9 A. The XPS survey scan spectra showed the presence of Sr, Ti, O, and Cu in the film. The XPS peak for C 1s at 284.8 eV is ascribed to adventitious carbon from the XPS instrument.35 The XPS spectrum for Cu 2p in Figure 9B shows two peaks at 932.5 and 952.7 eV that are assigned to Cu 2p3/2 and 2p1/2, respectively, which is a characteristic of Cu2+ in Cu2O and peak at 538.7 eV is for O 1s region shown in Figure 9C. In the spectrum of Sr 3d (Figure 9D), two peaks at 132.7 and 135.8 eV are attributed to 3d5/2 and 3d3/2, respectively. As shown in Figure 9E, the peak at 458.6 is observed in the spectrum of Ti 2p, which corresponds to the 2p3/2 states of Ti4+. The XPS results further confirmed the coexistence of Cu2O and SrTiO3 in the Cu2O/SrTiO3 heterojunction thin film,36,37 which agrees well with the XRD results. 25325



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Cu2O/SrTiO3 heterojunction thin film samples were found to be stable under applied potentials for long illumination time and without any loss of obtained photocurrent which implies enhanced stability due to overlayering stable SrTiO3 thin film onto Cu2O thin film (samples C−E). To further elucidate the possible causes of marked increase in the photoresponse of the heterojunction samples, the resistivity of the samples was calculated as presented in Table 3. Also, the maximum value of flatband potential (Table 3) exhibited by sample C supports maximum photocurrent density exhibited by this sample. 4.6. Mott−Schottky Study. The curves of 1/C2 vs. voltage (V/SCE) are shown in Figure 12, and the slope of these curves gives the value of flat band potential which plays an important role in deciding performance of photoelectrode in PEC water splitting. The positive shift in the Vfb of relative to Cu2O implies a decrease in band bending and thus facilitates electron transfer.40 Calculated values of flatband potentials for all the samples have been given in Table 3. It was found that value of Vfb increases from 0.15 V/SCE for pristine Cu2O to 0.91 V/ SCE for Cu2O/SrTiO3 heterojunction (sample C). The highest value of the flatband potential obtained for sample ‘C’ also supports the best photocurrent density exhibited by this sample. 4.7. Efficiency Calculation. The overall applied bias to incident photon conversion efficiency of a PEC, for the water splitting reaction, can be determined by the following expression.41

Figure 10. Photocurrent density vs applied potential curve for (A) pristine Cu2O, (B) SrTiO3, and (C−E) heterojunction Cu2O/SrTiO3 thin film samples with overall thickness of 343, 535, and 678 nm, respectively, under visible light illumination in 0.1 M NaOH.

expression that photocurrent is proportional to the amount of electrons produced by absorption of photons by semiconductor and their rate of transfer toward the counter electrode. Initial increase in the film thickness of SrTiO3 (with one coating film thickness of 160 nm) may allow the penetration of the light (photons) to Cu2O, and cumulative harvesting of photons upon irradiation with visible light may give rise to a larger number of photoelectrons. Further increase of the SrTiO3 layer on the nanostructured Cu2O thin films is likely to reduce the contact area between the sample and the electrolyte, leading to increased charge recombination in the SrTiO3. Hence, with a more than critical amount of SrTiO3, the photocurrent density of the heterojunction will decrease.38 Second, in the case of the heterojunction, electric fields built at the interface also drive the separation of photogenerated charge carriers SrTiO3 (n-type) and Cu2O (p-type) with band position of Cu2O and SrTiO3 favorable for the separation of charge carriers (Figure 11).39

η(%) =

0 [Jp (Erev − Eapp)]

I0

× 100

Here, Jp is the photocurrent density in mA/cm2, and E0rev is the standard reversible potential for water splitting (1.23 V), Eapp = Emeas − Eoc, where Emeas is the electrode potential (V/SCE) of the working electrode corresponding to the measured Jp, and Eoc is the open circuit potential (V/SCE) of the same working electrode under the same conditions of illumination and electrolyte. At I0 = 150 mW cm−2, efficiency of 1.44% at 0.8 V/ SCE was obtained for sample C shown in Figure 13. Beyond the optimum thickness of SrTiO3 (i.e., in sample C), applied bias photon-to-current efficiency decreases with the further increase in thickness of SrTiO3 which may be due to increased electron−hole pair recombination or due to increase in distance traveled by the charge carriers resulting in loss of charge carrier during their movement across the film and at the surface.42 To verify the photocurrent at 0.8 V/SCE the incident photon to current conversion efficiency (IPCE) was obtained in wavelength range 440−730 nm (Figure 14). Cu 2 O/SrTiO 3 heterojunction (sample C) showed the highest IPCE of over 15% at 480 nm, which gradually decreased to zero at 720 nm. This behavior is the same as that of the Cu2O photoelectrode. The large increase in IPCE indicates that the heterojunction is better in harvesting incident photons to induce water splitting. IPCE values for other pristine and heterojunction samples have been given in Table 3. Similar results are also reported by Jinzhan Su et al., for WO3/BiVO4 heterojunction.43 Hydrogen gas generated at the working electrode was collected using visible light source of irradiance 150 mW/cm2 in an inverted test tube and measured by the water displacement method at 0.8 V/SCE for the best photoresponsive heterojunction sample. The amount of measured hydrogen production has been plotted in Figure 15 showing maximum rate of production of hydrogen as 2.4 mL h−1 cm−2. A linear behavior of hydrogen

Figure 11. Energy band diagram of Cu2O and SrTiO3 before and after formation of p−n junction. 25326



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Table 3. Photochemical Performance of (A) Pristine Cu2O, (B) SrTiO3, and Heterojunction Cu2O/SrTiO3 (C, D, E) Thin Film Samples sample identification

open-circuit photovoltage (Voc, V/SCE)

resistivity (×105 Ω cm)

photocurrent density at 0.8 V/SCE (mA cm−2)

flatband potential (Vfb)

ABPE efficiency at 0.8 V/SCE

A B C D E

0.26 0.27 0.43 0.31 0.29

3.8 4.2 1.6 2.8 3.5

0.10 0.036 2.52 0.76 0.33

0.15 −0.36 0.91 0.67 0.58

0.046 0.013 1.44 0.37 0.15

Figure 14. IPCE performance of (A) pristine Cu2O, (B) SrTiO3, and (C−E) heterojunction Cu2O/SrTiO3 thin film samples under visible light illumination in 0.1 M NaOH electrolytic solution. Figure 12. Mott−Schottky curve for all samples.

Figure 15. Rate of hydrogen collection for sample C under visible light source of irradiance 150 mW cm−2 at the position of sample.

SrTiO3 heterojunction and possible transfer mechanism of photogenerated charge carriers are proposed, as illustrated in Figure 11. The band diagram after contact is hypothetical, by taking the band bending into account. A similar energy band diagram for the Bi2O3/BiVO4 heterojunction has also been reported earlier.48 Before contact, the Fermi levels of two semiconductors (SrTiO3 and Cu2O) are well-aligned. After the formation of heterojunction or after contact, the sample is immersed in the electrolyte, and is irradiated with light, photoexcited electrons from Cu2O conduction band transfer to the conduction band of SrTiO3 while holes move from SrTiO3 to Cu2O, preventing the photocorrosion of Cu2O via the influence of the electric field formed at the p−n junction of the Cu2O/SrTiO3 heterojunction.40 The overall process of generation and separation of electron−hole pairs and transfer of holes to the electrolyte increases due to the presence of the

Figure 13. Applied bias photon-to-current efficiency (ABPE) versus sample thickness curve for pristine SrTiO3 and Cu2O/SrTiO3 heterojunction thin films.

generation with time confirms the stability of the photoelectrode.44 4.8. Charge Separation Mechanism. On the basis of the band gaps and the band edge positions of SrTiO3 and Cu2O before coming in contact with each other calculated using the equation EVB = X − Ee + 0.5Eg and ECB = EVB − Eg,45−47 where Ee is energy of free electron on the hydrogen scale, Eg is the band gap and X is the absolute electronegativity of the semiconductor (geometric mean of electronegativity of constituents atoms), an energy band diagram of Cu2O/ 25327



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additional SrTiO3 layer. This effect leads to the enhancement of photocurrent densities and IPCE values.31

5. CONCLUSIONS In the present study, spray pyrolytically deposited p-Cu2O thin films modified with overlyering of n-SrTiO3 were used as photoelectrode in photoelectrochemical water splitting. DFT based calculations attempted for effective mass for electron and holes in order to explain charge carriers separation at Cu2O/ SrTiO3 interface agree well with experiment and provide a qualitative picture of the charge separation mechanism responsible for the improved photoelectrochemical performance of Cu2O/SrTiO3. Maximum conversion efficiency of 1.4% was exhibited by Cu2O/SrTiO3 heterojunction photoelectrode with overall thickness 343 nm as compared to CuO/SrTiO3 heterojunction system reported by Surbhi et al.49 Heterojunction samples were also stable under applied potentials for long illumination time without any significant loss indicating high stability and photocorrosion resistivity of modified Cu2O (samples C−E). The improved photoresponse of this photoelectrode may be attributed to the enhanced transfer of photogenerated charge carriers at the interface and reduction in resistance.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-9219695960. Fax: +91-562-2801226. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors gratefully acknowledge financial support received from the Department of Science & Technology, New Delhi, India, vide project no. SR/NM/NS-147/2010. We are also thankful to Dr. Saif Khan IUAC, New Delhi, India for Cross-SEM analysis of samples.

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REFERENCES

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Improved Photoelectrochemical Water Splitting Performance of Cu2O

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