15900
J. Phys. Chem. C 2008, 112, 15900–15907
Electrodeposition of r-Fe2O3 Doped with Mo or Cr as Photoanodes for Photocatalytic Water Splitting Alan Kleiman-Shwarsctein,† Yong-Sheng Hu,‡ Arnold J. Forman,§ Galen D. Stucky,†,§ and Eric W. McFarland*,‡ Materials Department, UniVersity of California Santa Barbara, California 93106, Department of Chemical Engineering UniVersity of California Santa Barbara, California 93106, and Department of Chemistry and Biochemistry, UniVersity of California Santa Barbara, California 93106 ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: June 22, 2008
Electrochemical methods for the codeposition of Cr or Mo with R-Fe2O3 (hematite) have been developed to produce doped iron oxide films with varying compositions of Cr and Mo which are active photoanodes for photoelectrochemical (PEC) water decomposition (“water splitting”). The films were characterized by scanning electron microscopy, X-ray diffraction, UV-vis optical spectroscopy, and X-ray photoelectron spectroscopy to determine the effect of the dopants on the hematite structure and PEC performance. Upon doping, the microstructures of the films varied; however, no preferred crystallographic orientation or dopant phase segregation was observed. The best performing samples were 5% Cr and 15% Mo doped which had Incident Photon Conversion Efficiencies (IPCE’s) at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4V vs Ag/AgCl. These IPCE values were 2.2× and 4× higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. The increase in performance is attributed to an improvement in the charge transport properties within the films and not due to significant changes in the electrocatalytic rates due to dopants residing at the surface. The onset potential for photocurrent in all the samples was approximately -350 mV vs Ag/AgCl. The optical absorption spectra of the films showed bandgaps of approximately 2.0-2.1 eV for all samples regardless of doping. 1. Introduction The first report of photoelectrochemical (PEC) decomposition of water into hydrogen and oxygen (i.e., “water splitting”) in 1972 by Fujishima1 and Honda using ultraviolet illumination inspired many subsequent researchers to work toward efficiently producing H2 from water, sunlight, and a photocatalyst. If an industrially scalable cost-effective photocatalytic system could be found to transform solar energy into chemical potential energy, it would have a significant effect on how society’s energy use impacts the environment. To be “cost-effective”, a PEC system must balance energy conversion efficiency with capital and operating costs such that the price of producing PEC hydrogen is lower than alternatives such as photovoltaic driven electrolyzers or biomass reforming. High efficiency PEC material systems have been proposed and demonstrated;2-4 unfortunately, the costs of these optimized heterostructures would far exceed any commercially acceptable value for large volume, low value, and fuel production. Costeffective PEC material systems will utilize low-cost materials in low-cost reactor configurations. Abundant and inexpensive metal oxides such as TiO2,1,5-7 ZnO,8-10 and WO311-15 have a wide bandgap and are therefore limited in overall terrestrial solar energy efficiency to less than ∼4%. Thus, their low material cost would be overshadowed by the expense of structures * To whom correspondence should be addressed. E-mail: mcfar@ engr.ucsb.edu. † Materials Department, University of California Santa Barbara. ‡ Department of Chemical Engineering University of California Santa Barbara. § Department of Chemistry & Biochemistry, University of California Santa Barbara.
covering enormous areas for the solar exposure required to make substantial quantities of PEC derived fuels. Compared to other abundant metal oxides, R-Fe2O3 (hematite) has the advantage that its bandgap is ∼2 eV allowing for absorption of roughly 40% of the solar spectrum.16 The maximum theoretical energy efficiency of hematite under AM 1.5 illumination is approximately 13% when water splitting (Eo ) 1.23 V) is the reaction.17 The maximum efficiency is calculated as, [(1.23V × photocurrent)/ (power in AM 1.5G spectra)], where the photocurrent is defined as the maximum current generated if all photons of energy g to the bandgap are collected external of the semiconductor with 100% quantum efficiency (QE). Further, it has the necessary valance band position for efficient oxygen evolution. It is stable against photocorrosion in most alkaline electrolytes, and as an inexpensive, abundant, and nontoxic material, it could be deployed in a large scale. Despite the seemingly favorable characteristics of hematite, several factors have limited its performance. The maximum experimental solar-to-chemical efficiency reported in the literature is less than 3% under AM 1.5 illumination. The low efficiency is thought to be a result of several factors: (1) low electron mobility18,19 (∼0.1 cm2 V-1 s-1) which limits the electron collection efficiency,20 (2) high electron-hole recombination rates,16 (3) inhibited water electro-oxidation rates,16,21 (4) inhibited hydrogen evolution22 due to the conduction band energetics (∼0.2 V vs NHE), and (5) anisotropic conductivity, which is 4 orders of magnitude higher within the (001) plane than parallel to [001].19,23-25 Doping of hematite with transition metals has been studied extensively to improve its PEC properties. Dopants such as Ti,26-28 Si,28-32 Ge,30 Mg,33 Zn,34-37 Ta,38 Nb22,39 and ternary
10.1021/jp803775j CCC: $40.75 2008 American Chemical Society Published on Web 09/13/2008
Electrodeposition of R-Fe2O3 Doped with Mo or Cr mixtures with Al-Ti40 have been investigated. Iron oxide photoelectrodes have been synthesized by a number of diverse methods including: single crystal synthesis,41,42 ceramic processing,38 sol-gel,43,44 spray pyrolysis,35,36,40,45,46 physical vapor deposition,47 chemical vapor deposition,20,32,48,49 aqueous selfassembly of nanostrutures,50,51 and most recently by electrochemical deposition.52-55 Most of the doped samples in previous work have shown little or no improvement in the PEC properties of iron oxide with the exception of Ti and Si. Titanium doping was shown to increase the conductivity of polycrystalline samples by at least 4 orders of magnitude27 and later shown to increase the photocurrent.40 The enhancement of photocurrent from silicon doping is ascribed to an improvement of the crystallinity and preferential orientation of the [110] axis normal to the substrate.32 The influence of Group VI dopants such as Cr and Mo has not been widely studied. In one report, Cr was shown to have a negative effect on the photoelectrochemical performance of hematite, which they attribute to Cr3+ stabilizing Fe3+ and decreasing the redox mediated conductivity of the hematite.56 Recently, we have shown modest improvements by platinum55 doping using electrochemical methods. In spite of the limited success to date, there remain sound reasons to expect that use of dopants to reduce the intrinsic material limitations may be successful. In this work, we report recent results addressing the following questions: (1) Can iron hydroxide be codeposited with Cr or Mo by electrochemical deposition techniques to yield doped R-Fe2O3 after calcination? (2) Can the stoichiometry of the dopant in the electrodeposited film be controlled? (3) Will the photoactivity of the iron oxide be improved by controlling the electrodeposition conditions and the concentrations of the Cr and Mo in the film? (4) Are changes in the performance attributable to surface electrocatalysis or modification in the bulk electronic properties of the material? 2. Experimental Section Electrodeposition of Thin Films. Electrodeposition was carried out using a standard three-electrode configuration. A graphite rod counter electrode, a Ag/AgCl reference electrode (saturated by KCl), and a Pt/Ti working electrode (1.5 µm Pt /500 nm Ti deposited on unpolished quartz by electron beam evaporation) were used. Fluorine-doped tin oxide (FTO, TEC 15, Pilkington glass) substrates were used to electrodeposit samples for X-ray Diffraction (XRD) characterization since the background signal of the FTO substrates generated smaller, less intrusive substrate diffraction peaks and resulted in higher quality hematite XRD spectra as compared to the Quartz Ti/Pt substrates. The electrodeposition method is a modification of the method by Schrebler et al.52 The electrodeposition solution consisted of 5 mM FeCl3 + 5 mM KF + 0.1 M KCl + 1 M H2O2. Fluoride was added to decrease the reduction potential of Fe3+ to Fe2+ and the KCl was used as a supporting electrolyte. There are two functions of H2O2: one is to produce OH- during the reduction process so that the pH in the immediate vicinity of the working electrode increases, aiding the electrodeposition of iron hydroxides; the other is to stabilize the dopant-precursor in aqueous solution. Either CrCl3 or MoCl5 was used as Cr and Mo precursors in the electrolyte for preparing the doped samples. The molar ratios in the electrolyte (Cr/(Cr + Fe)) and (Mo/ (Mo + Fe)) ranged from 0% to 20% for both dopants as a means of varying the dopant concentration in the final samples. All dopant percentages reported herein are those of the electrodeposition solution and not the actual dopant concentration in the electrodeposited film, unless otherwise indicated. The relation-
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15901 ship between the electrolyte dopant concentration and the film composition was established by XPS (see below). Cyclic voltammograms were performed using a potentiostat (EG&G 273A) controlled by custom software written in LabView (National Instruments). The applied voltage was scanned between -0.49 and 0.41V vs Ag/AgCl at a rate of 0.2 V/sec. For the electrodes grown on Pt/Ti coated quartz wafers, 5 cycles were enough to deposit an appreciable amount of iron hydroxide, giving samples ∼0.7 ( 0.2 µm thick. For the FTO substrates, 50 cycles where required to create a uniform film. After electrodeposition, the films were washed with DI water 3× to remove any residual electrolyte followed by calcination in air at 700 °C for the Pt/Ti substrate and 600 °C for the FTO substrate with a heating rate of 2 °C/min and a dwell time of 4 h for all samples. Sample Characterization. X-ray diffraction (XRD) was conducted on samples deposited on FTO rather than on the platinum electrodes using a powder diffractometer (Phillips PANalytical X’PERT, using Cu KR radiation with a CNRS position sensitive detector). The XRD measurements were obtained on FTO substrates because they are less crystalline than the Pt/Ti substrates and thus generated smaller, less intrusive, substrate diffraction peaks resulting in higher quality hematite XRD spectra. Scanning electron microscopy (SEM) was performed on the Pt substrate samples with acceleration voltage of 5 keV and a working distance of ∼5 mm from the tip to the sample plane (FEI Co. XL 40; UHR mode). X-ray photoelectron spectroscopy (XPS) was utilized for compositional analysis to characterize the Fe, Cr and Mo valance states and compositions in the electrodeposited films. XPS was performed with a monochromated Al KR source (1486.6 eV) for excitation (Kratos Axis Ultra with an eight-channel detector). The base pressure was 5 × 10-10 torr, and the spectra were calibrated to C1s at 285.0 eV and processed with Casa XPS. The Cr/(Cr + Fe) and Mo/(Mo + Fe) ratios were determined from the survey scan from the Fe2p, Mo3p, and Cr2p photoelectron emission after fitting a Shirley background and normalizing with Scofield sensitivity factors. Ion etching was conducted at an argon partial pressure of 10-7 torr while applying 4 kV at an ion beam current of 10 mA/cm2, resulting in a ∼20 nm/min etching rate. The thickness of the samples was obtained with a Dektak 5 (Veeco Instruments Inc.) surface profilometer. UV-vis spectroscopy was conducted on a Shimadzu UV3600 UV-NIR Spectrometer with an integrating sphere using a bare FTO substrate as reference for samples deposited on FTO. Photoelectrochemical Performance. The photoelectrochemical properties of the samples were measured using a highthroughput photoelectrochemical setup described elsewhere.8,9 In brief, the experimental system consists of a robotic probe head containing a Pt counter electrode, Ag/AgCl reference electrode, gas diffuser, and an optical fiber bundle which is isolated from the electrolyte via a flat wall borosilicate tube. The probe is contacted to the sample (working electrode) and a seal made with an O-ring exposing a geometric area of 0.24 cm2 to the electrolyte and the illumination source. Before and during the experiments, the solution within the probe head was continuously degassed with N2 (to decrease Oxygen Reduction reaction at the counter electrode). The electrolyte consisted of a 1 M solution of NaOH in high purity water (Milli-Q, Millipore Corp.; 18.2 MΩ) at pH 13.8. A 1000 W ozone free xenon lamp (Newport Corporation #6271), was used for illumination. The lamp was filtered through a water filter, a monochromator (Newport Corporation CS 260 1/4m) and focused to the optical fiber bundle. For wavelength-dependent photocurrent measure-
15902 J. Phys. Chem. C, Vol. 112, No. 40, 2008
Kleiman-Shwarsctein et al.
Figure 1. SEM of electrodeposited samples (a) undoped sample, (b) 15% Cr in solution, (c) 10% Cr in solution, (d) 2% Mo in solution, (e) 5% Mo in solution, and (f) 15% Mo in solution.
ments a diffraction grating was used to give a ∼20 nm bandpass (fwhm) from 360-680 nm together with cutoff filters to eliminate second-harmonics. For visible light photoelectrochemical measurements, the diffraction grating was switched to a mirror in the monochromator and a 400 nm cutoff filter was used to remove the UV from the lamp source. The power density of the collimated beam of white light was controlled by adjusting the mirror within the monochromator. The light intensity exiting the fiber optic bundle was measured with a radiometer (Thorlabs, S120UV/S121B detectors) with power densities from 200 - 800 mW/cm2 for white light experiments and 20-40 mW/cm2 for monochromatic light. The incident photon to electron conversion efficiency (IPCE) of the samples was calculated as follows:
IPCE(%) )
(1240)(iphotcurrentµA ⁄ cm2) (λnm)(jphotonsµW ⁄ cm2)
× 100%
The photocurrent from the experiments was measured with a multimeter (HP 3440A), while in a three-electrode configuration, the potential was applied with the potentiostat and the current was measured by the multimeter. IV curves were measured at 100 mV/sec with a potential range from -0.5 to 0.8 V. The high-throughput8 screening system is computer controlled by custom software written in LabView (National Instruments). 3. Results and Discussion The microstructure of the R-Fe2O3 after electrodeposition and calcination are shown in Figure 1. The morphology of the undoped films is significantly different from that of the doped samples. The control films deposited without dopant, Figure 1a, had the smallest particles, with size continually increasing through 5% and 10% Cr doping, Figure 1b and c. The 2% and 5% Mo doped samples, Figure 1d and 1e, show relatively small particles compared to those in the 15% Mo samples, Figure 1f. The changes in morphology of the samples are a combined effect of changes induced by the electrodeposition conditions and those due to different rates of sintering during the high temperature calcination. SEM images of the samples before calcination (data not shown) show that the undoped and Cr doped samples all
have nearly identical morphologies after electrodeposition, while the Mo doped samples have larger electrodeposited particle sizes and a denser structure. XRD spectra from the electrodeposited samples are shown in Figure 2. The spectra are normalized to the peak at 37.68° of the FTO substrate which corresponds to the cassiterite (200) peak as indexed by reference pattern JCPDS 21-1250 (pattern not shown), while the hematite (R-Fe2O3, space group: R3c (167), a ) 0.5035, b ) 0.5035, c ) 1.3748 nm) reference pattern used to index the peaks is JCPDS 33-0664. The XRD patterns of the undoped, Cr or Mo doped samples in Figure 2, show that the undoped and doped samples have the same crystal structure and that only R-Fe2O3 is present. However, the diffraction patterns of the samples differ from that of the reference pattern in the relative intensity of the peaks. The most intense reflection for all samples is the (104), but the relative intensities for the (012) and (110) peaks, as compared to the reference pattern, are much stronger. At Mo > 10% the (116) and (214) peaks are not crystallized. Although there are small variations in peak intensities, the XRD data does not support the hypothesis that the Cr or Mo doped films grow with a preferential orientation under these conditions. In previous work, Gra¨tzel,32 et al. explained an observed increase in doped iron oxide PEC performance as due, in part, to the enhanced electron transport along the (110) basal plane due to the anisotropic conductivity of iron oxide.25 Under our growth conditions, we do not attribute the improvement of the PEC properties of the Cr and Mo doped iron oxide films (below) to preferential growth or the anisotropic conductivity of iron oxide. One alternative explanation is the possibility of conductivity enhancement by introduction of dopant atoms which are not limited by spin forbidden electron transport. Determination of the crystallite size using the Scherrer equation for the (104) peak shows a size of 38, 42, and 43 nm for the undoped, 5% Cr, and 15% Mo doped samples respectively; variations of 1-6 nm were observed with different % of Cr and Mo dopants. The crystallite size obtained from XRD for any of the samples is smaller than the size of the particles observed in SEM. This is in agreement with the interpretation that small particles with various crystallographic orientations
Electrodeposition of R-Fe2O3 Doped with Mo or Cr
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15903
Figure 2. XRD of Samples (a) Cr doped samples and (b) Mo doped samples.
Figure 3. UV-vis spectra of pure hematite and selected doped samples.
sinter together more easily with increasing concentrations of dopants in the film. The UV-vis spectra of the undoped and doped samples on FTO substrates obtained in transmission mode are shown in Figure 3. The absorption peaks at ∼311, ∼400, and ∼530 nm observed in all samples are consistent with transitions seen in iron oxide previously by Marusak,57 et al. and attributed to 1t1uV f 2t2gV, 3egv f 3egV, 6A1 f 4E ligand field transitions for the high energy absorption as well as interlevel transitions for the lower energy peaks. The optical bandgap of the samples was calculated from a Tauc Plot (Figure 4), and the best fit was found for the direct bandgap transition rather than an indirect bandgap with values of 2.1 eV for the undoped and 2.0 eV for both the Cr and Mo doped samples. This observation matches with that of Schrebler,52 whose samples were also deposited by electrodeposition, while Glasscock28 and others58 assigned the bandgap transition as indirect. Small differences in the observed optical densities of the samples may be due to variations in the thickness as well as morphologically dependent scattering of the deposited films. Nonetheless, the small changes in the absorption properties of the undoped and doped samples do not account for the increased PEC performance (below). Glasscock,28 et al. have observed similar effects whereby the addition of Si and Ti had no effect on the optical absorption properties of the films compared to the undoped hematite material. Figure 5a shows the XPS of the Fe 2p region for the undoped electrodeposited film and selected compositions of Cr and Mo.
The Fe 2p spectra of all of the samples are quite similar and attributed completely to Fe3+, while no Fe2+ shakeup satellite peaks at 715.5 eV59 were observed in any of the samples.59 Data fitting of the Fe 2p3/2 for all of the samples by 4 Fe3+ multiplet peaks as proposed by Grosvenor, et al.60 shows values for binding energies of 710 ( 0.2, 710.6 ( 0.1, 711.4 ( 0.1, and 712.2 ( 0.1 eV and fwhm of 0.8 ( 0.2, 1.1 ( 0.2, 1.3 ( 0.2, and 3.1 ( 0.3 eV for each of the multiplet peaks (peak 1 to 4 in Figure 5b), respectively (see Table S1 in Supporting Information). This data is in agreement with that observed by Grosvenor for R-Fe2O3 although the fwhm for the last peak is much wider than that found by Grosvenor. We speculate that the broadening of the high-binding energy peak may be related to the decrease of the crystal field energy of Fe3+ atoms located at the surface in comparison to those in the bulk.60,61 The high resolution Mo 3d XP spectra are shown in Figure 5c. The Mo 3d5/2 peak is centered for all samples at 232.8 ( 0.1 eV with a very narrow fwhm of 0.9 eV corresponding to Mo6+.62,63 No Mo5+ was found in any sample by XPS. The high resolution Cr 2p XPS spectra are shown in Figure 5d for samples ranging from 2% to 20% Cr in the electrolyte. The Cr 2p spectra was fit to Cr6+ at 580 eV,64 Cr4+ at 576.5 eV, and Cr3+ at 577.4 eV,64,65 while no Cr0 at 574.4 eV or Cr2+ at 575.4 eV were observed for any of the doped samples. The valance state distributions of the chromium from the peak fittings are shown in Table 1. (Calculated values for Mo and Cr peaks can be found in Supporting Information Tables S2 and S3.) It should be noted that Mo is stable exclusively in the +6 state as a dopant while the oxidation state distribution of Cr changes as a function of % Cr in the sample. As is the case of the Mo shown here, isovalence is also exhibited by Pt (as Pt4+) when electrochemically doped into hematite.55 From the survey scan the Fe 2p, Cr 2p, and Mo 3d regions were integrated to quantify the surface dopant concentrations for the electrodeposited films. From Figure 6a, it is seen that the concentration of Cr in the surface of the sample can be controlled by adjusting the amount of Cr in the electrodeposition solution. For the Mo-doped samples, the Mo concentration on the surface of the sample is higher than that of the electrolyte; the percent of dopant in solution has less of an effect on the surface concentration with the composition of the electrolyte in the 5-15% atomic percent range.
15904 J. Phys. Chem. C, Vol. 112, No. 40, 2008
Figure 4. Tauc plots for (a) an undoped sample, (b) Cr doped sample, and (c) Mo doped sample.
The dopant depth profiles for the films are shown in Figure 6b. These XPS measurements were obtained following sequential Ar+ ion etching of the 5% Cr and 15% Mo films. The etch rate for Cr in pure Cr2O3 is about 10% higher than that of Fe in Fe2O3 as determined by Viefhaus;66 however, no corrections were made for difference in etch rates of Cr and Mo vs Fe. Therefore, the etch rates for the doped samples are estimated to be ∼20 nm/min, independent of the species and constant along the thickness of the sample. On the basis of this, the 5% Cr sample has an enriched surface layer (∼6 nm thick), and becomes approximately constant, at 3.4%, after ∼10 nm. Small variations are due to the signal-to-noise ratio increasing as the sputtering time is increased. This Cr surface enrichment is contrary to the results of Bernasik, et al.67 who observed that chromium impurities in hematite segregate away from the surface after annealing at 900 °C leaving a depleted surface layer. It was expected that the surface would be enriched with
Kleiman-Shwarsctein et al. Cr ions since there is a positive mismatch between the atomic radii of Fe3+ and Cr3+ (64 pm vs 69 pm); however, Cr4+ (55 pm) would have a smaller radius and, therefore, it may not segregate to the surface such as was observed for the smaller Al3+ (50 pm) by Bernasik et al.67 The surface of the 15% Mo doped samples are highly enriched, Figure 6b. The surface has approximately 15.6% Mo, falls to 5.6% at ∼7 nm and remains relatively uniform after 40 nm. The atomic radius of Mo6+ (73 pm) is larger than that of Fe3+ (64 pm) and again this size mismatch would suggest an increase in the concentration of Mo in the surface layer which is consistent with the experimental results obtained. The photoelectrochemical performance of the samples was characterized by cyclic voltammetry for the undoped and doped R-Fe2O3 samples. Figure 7a shows that the photocurrent for the undoped sample at 0.0 V vs Ag/AgCl is higher than that of the doped samples; however, the photocurrent at this applied potential is very low (see Figure 7 insets). The hematite films are n-type photoanodes and a positive applied bias will increase the photocurrent generation as the Fermi level moves to assist charge separation and facilitate water splitting. At an applied potential of 0.2 V, the photocurrent of the 5% Cr doped samples is approximately the same as that of the undoped sample, and the 10% Cr doped samples is ∼0.8× that of the undoped sample. However, the performance of the samples is increased at higher applied bias and at 0.4 V the increase in photocurrent is 1.9 and 1.5× for the 5% Cr and 10% Cr, respectively. To explain the observed improvement in PEC for the Crdoped films at applied potentials greater than 0.2 V, we propose the following: the electronic properties of the bulk film are modified by stable Cr4+ which increases the conductivity compared to the undoped film. This mechanism is consistent with our XPS results for the samples with 2% and 5% Cr which showed a majority of Cr4+. The substitution of a Fe3+ by Cr4+ would provide an extra 3d electron (n-doping) to enhance electronic conductivity. The substitution of a Cr3+ would be expected to decrease the conductivity based on the calculations of Velev68 who used density functional theory (DFT) to clarify that the remaining three filled 3d states (t2g) fall below the Fermi energy near the top of the O-p complex. Merchant56 et al. reported that Fe2-xCrxO3 samples were not photoactive and had a resistivity at room temperature of 106 Ω · cm; the lack of photoactivity was attributed to a substitution of the Fe3+ by a Cr3+ (3d6) which did not introduce any Fe2+ states. For samples doped with molybdenum, Figure 7b, the photocurrent at 0.0 V vs Ag/AgCl shows an improvement upon doping of 1.6× for the 2% and 10% and 2.2× for the 15% Mo. At an applied potential of 0.2 V vs Ag/AgCl the photocurrent increases 2.2, 2.4, and 3× for the 2%, 10%, and 15% Mo doped samples, respectively, while at an applied potential of 0.4 V the increase is 2, 2.1, and 2.7× that of the undoped sample. As seen in the XPS data, the Mo6+ dopant atoms segregate to the surface leaving a gradient in concentration with respect to the bulk of the sample. Furthermore, the Mo6+ would be in a 4d0 configuration where it could permit an electron from the adjacent Fe atom to move freely into the empty band. We speculate that the Mo atoms in the bulk of the sample increased the conductivity of the sample which results in an increase in the observed PEC performance of the samples. The photocurrent onset potentials were obtained from the chopped IV curves (see Supporting Information Figure S1) and found to be approximately -350 mV vs Ag/AgCl for all samples, doped or undoped. The onset potential is assumed to be approximately equal to the flatband potential.8,21 The lack
Electrodeposition of R-Fe2O3 Doped with Mo or Cr
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15905
Figure 5. XPS: (a) Fe 2p spectra for pure, Cr and Mo doped samples; (b) Fit of Fe 2p for the 2% Cr doped sample; (c) Mo 3d; and (d) Cr 2p; spectra of doped samples.
TABLE 1: Distribution of Dopant Oxidation States As a Function of Concentration electrolyte speciation Mo 2-15% Cr 2% Cr 5% Cr 10% Cr 20%
oxidation state distributions (%) +3
+4
14 12 20 19
62 66 14 24
+6 100 24 22 66 57
of change in the flatband potential would suggest that the conduction band edge of the Fe2O3 has not been changed by the addition of dopants. Similar effects have been observed by Jaramillo, et al.8 on doped ZnO. Figure 8 shows the action spectra for the best performing Cr (5%) and Mo (15%) doped samples compared to the undoped control sample. Significant performance gains were observed upon doping throughout the illumination wavelengths. For the 5% Cr-doped films, the performance at 0.2 V is similar to the undoped sample; however, at 400 nm and 0.4 V, the performance is 2.2× higher than that of the undoped control, all of which is consistent with the IV-curves. At 480 nm, the IPCE at an applied bias of 0.4 V is 2.5% which corresponds to a 2.2-fold improvement over the undoped hematite. For the 15% Mo-doped films, the IPCE at 400 nm and 0.2 and 0.4 V is 8% and 12%, respectively. This corresponds to a generous 4-fold increase in IPCE over the undoped sample for the 0.2 and 0.4 V applied bias. For the same films at 480 nm, the IPCE at 0.4 and 0.2 V is 4.3 and 3.5 times higher, respectively. The large enhancement of the IPCE at low wavelengths for doped and undoped hematite thin films has been observed previously28,32,69 and explained as an increased efficiency for charge separation of the more energetic electron/hole pairs. The high energy photons are absorbed in the outermost layers of hematite and, therefore, the photogenerated holes have a shorter
diffusion path to reach the surface where they will participate in oxidative chemistry. A positive applied potential will increase the collection efficiency of the electrons and the IPCE will be increased as shown in Figure 8. Furthermore, the applied bias will facilitate hydrogen production at the Pt counter electrode by shifting its potential negative to overcome the approximately 0.2 V difference between the hematite flatband potential and the hydrogen redox level. The improved PEC performance is not likely related to an increase of the absorption of the doped samples since the absorption spectra of the different samples show little change, Figure 3. Differences in the electrocatalytic properties of the different doped surfaces were also ruled out as major contributors to the improvement in the PEC performance by examination of the IV-curves of the samples in the dark where no shifts in the oxygen evolution reactions (OER) were observed, and the measurement of IPCE of the undoped sample at a high applied potential (0.6 V vs Ag/AgCl, Figure S2 Supporting Information). Under high bias (0.6 V vs Ag/AgCl) the oxygen and hydrogen evolution reaction rates will not be rate limiting, and the IPCE of the undoped sample remained lower than the doped samples at 0.2 and 0.4 V bias. We observed that the IPCE for the undoped sample reached 3.4% at 400 nm which is lower than that of the Mo doped sample at 0.2 V vs Ag/AgCl and the Cr doped sample at 0.4 V vs Ag/AgCl. We would expect nearly identical performance of the doped and undoped samples at high applied potentials if the improvements were related to surface catalysis. The electrocatalytic effect of Cr as Cr2O3 is also unlikely because the enhancement of the photocurrent (Figure 6a) at 10% Cr occurs at a higher applied bias in the IV data than that of the 5% Cr. The loading of Cr2O3 (identified as Cr6+ by XPS) would indicate that there is more electrocatalyst on the surface at 10% Cr than that of the 5% Cr sample (see Table 1) in which case the performance of the 10% Cr sample should
15906 J. Phys. Chem. C, Vol. 112, No. 40, 2008
Kleiman-Shwarsctein et al.
Figure 6. (a) Atomic % of dopant for the electrodeposited samples as measured by XPS and (b) Thin film dopant concentration for 5% Cr and 15% Mo samples as a function of depth below surface obtained from Ar+ etching.
Figure 7. IV Curves of (a) Cr doping and (b) Mo doping at 410 mW/cm2 illumination. Inset shows magnification of origin.
the dopant in the sample can be controlled. Although we found no evidence of phase segregation between the host and dopants, there was surface-layer dopant enrichment, particularly in the Mo doped samples. The photoactivity of the iron oxide was improved by codeposition with Mo or Cr. The best performing samples were 5% Cr and 15% Mo doped, which had IPCEs at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4V vs Ag/AgCl. These IPCE values were 2.2 and 4× higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. No evidence was found that the improved performance is due to the electrocatalytic effects of the dopant at the surface of the hematite thin film. The major effect of the Mo and Cr dopants is to improve the charge transport properties of the hematite such that a greater fraction of the photon generated electron/hole pairs are available for surface redox chemistry.
Figure 8. IPCE of the electrodeposited films with and without doping at different applied potentials in N2 degassed 1 M NaOH.
be the same or higher than that of the 5% Cr sample if this was due to an electrocatalytic effectswhich is not observed in the Cr samples. Conclusions We have shown that Cr or Mo can be electrochemically codeposited with iron hydroxide to yield doped iron oxide films. By modifying the electrolyte composition, the concentration of
Acknowledgment. Major funding was provided by the Hydrogen Program of the Department of Energy though Award Number DE-FG36-05GO15040 and DE-FG36-03GO13062. Partial support was provided by the MRSEC Program of the National Science Foundation under Award No. DMR05-20415. Fabrication of the substrates was done at the UCSB Nanofabrication Facility, a part of the NSF-funded NNIN Network. The authors would like to thank Professor Thomas F. Jaramillo, Mr. Daniel Hazen, Ms. Wei Tang, Dr. Tom Mates and Dr. Mark Cornish for technical assistance. Supporting Information Available: Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.
Electrodeposition of R-Fe2O3 Doped with Mo or Cr References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Khaselev, O.; Turner, J. A. Science 1998, 280, 425. (3) Khaselev, O.; Bansal, A.; Turner, J. A. Int. J. Hydrogen Energy 2001, 26, 127. (4) Licht, S.; Wang, B.; Mukerji, S.; Soga, T.; Umeno, M.; Tributsch, H. Int. J. Hydrogen Energy 2001, 26, 653. (5) Kim, D. H.; Anderson, M. A. EnViron. Sci. Technol. 1994, 28, 479. (6) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735. (7) Fujishima, A.; Inoue, T.; Honda, K. J. Am. Chem. Soc. 1979, 101, 5582. (8) Jaramillo, T. F.; Baeck, S. H.; Kleiman-Shwarsctein, A.; Choi, K. S.; Stucky, G. D.; McFarland, E. W. J. Comb. Chem. 2005, 7, 264. (9) Jaramillo, T. F.; Baeck, S. H.; Kleiman-Shwarsctein, A.; McFarland, E. W. Macromol. Rapid Commun. 2004, 25, 297. (10) Hotchandani, S.; Kamat, P. V. J. Electrochem. Soc. 1992, 139, 1630. (11) Baeck, S. H.; Choi, K. S.; Jaramillo, T. F.; Stucky, G. D.; McFarland, E. W. AdV. Mater. 2003, 15, 1269. (12) Baeck, S. H.; Jaramillo, T.; Stucky, G. D.; McFarland, E. W. Nano Lett. 2002, 2, 831. (13) Su, L. Y.; Zhang, L. G.; Fang, J. H.; Xu, M. H.; Lu, Z. H. Sol. Energy Mater. Sol. Cells 1999, 58, 133. (14) Gondal, M. A.; Hameed, A.; Yamani, Z. H. Energy Sources 2005, 27, 1151. (15) Marsen, B.; Miller, E. L.; Paluselli, D.; Rocheleau, R. E. Int. J. Hydrogen Energy 2007, 32, 3110. (16) Ahmed, S. M.; Leduc, J.; Haller, S. F. J. Phys. Chem. 1988, 92, 6655. (17) Murphy, A. B.; Barnes, P. R. F.; Randeniya, L. K.; Plumb, I. C.; Grey, I. E.; Horne, M. D.; Glasscock, J. A. Int. J. Hydrogen Energy 2006, 31, 1999. (18) Itoh, K.; Bockris, J. O. J. Electrochem. Soc. 1984, 131, 1266. (19) Rosso, K. M.; Smith, D. M. A.; Dupuis, M. J. Chem. Phys. 2003, 118, 6455. (20) Quinn, R. K.; Nasby, R. D.; Baughman, R. J. Mater. Res. Bull. 1976, 11, 1011. (21) Dareedwards, M. P.; Goodenough, J. B.; Hamnett, A.; Trevellick, P. R. J. Chem. Soc.-Faraday Trans. I 1983, 79, 2027. (22) Sanchez, C.; Sieber, K. D.; Somorjai, G. A. J. Electroanal. Chem. 1988, 252, 269. (23) Benjelloun, D.; Bonnet, J. P.; Doumerc, J. P.; Launay, J. C.; Onillon, M.; Hagenmuller, P. Mater. Chem. Phys. 1984, 10, 503. (24) Morin, F. J. Phys. ReV. 1954, 93, 1195. (25) Iordanova, N.; Dupuis, M.; Rosso, K. M. J. Chem. Phys. 2005, 122. (26) Uekawa, N.; Watanabe, M.; Kaneko, K.; Mizukami, F. J. Chem. Soc.-Faraday Trans. 1995, 91, 2161. (27) Morin, F. J. Phys. ReV. 1951, 83, 1005. (28) Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N. J. Phys. Chem. C 2007, 111, 16477. (29) Kennedy, J. H.; Shinar, R.; Ziegler, J. P. J. Electrochem. Soc. 1980, 127, 2307. (30) Kennedy, J. H.; Anderman, M.; Shinar, R. J. Electrochem. Soc. 1981, 128, 2371. (31) Cesar, I.; Kay, A.; Martinez, J. A. G.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 4582. (32) Kay, A.; Cesar, I.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 15714. (33) Leygraf, C.; Hendewerk, M.; Somorjal, G. A. J. Phys. Chem. 1982, 86, 4484. (34) Ingler, W. B.; Khan, S. U. M. Electrochem. Solid State Lett. 2006, 9, G144. (35) Ingler, W. B.; Baltrus, J. P.; Khan, S. U. M. J. Am. Chem. Soc. 2004, 126, 10238. (36) Ingler, W. B.; Khan, S. U. M. Thin Solid Films 2004, 461, 301.
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15907 (37) Fan, Z. Y.; Wen, X. G.; Yang, S. H.; Lu, J. G. Appl. Phys. Lett. 2005, 87. (38) Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E.; Stepanyan, G. M.; Turner, J. A.; Khaselev, O. Int. J. Hydrogen Energy 2002, 27, 33. (39) Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E.; Stepanyan, G. M.; Khachaturyan, E. A.; Wang, H.; Turner, J. A. Solar Energy 2006, 80, 1098. (40) Sartoretti, C. J.; Alexander, B. D.; Solarska, R.; Rutkowska, W. A.; Augustynski, J.; Cerny, R. J. Phys. Chem. B 2005, 109, 13685. (41) Wu, D. H.; Zhang, Z. X.; Xu, L. F.; Shen, L. L.; Wang, X. Chin. J. Inorg. Chem. 2004, 20, 31. (42) Iwasaki, M.; Hara, M.; Ito, S. J. Mater. Sci. 2000, 35, 943. (43) Cui, H. T.; Zayat, M.; Levy, D. J. Sol-Gel Sci. Technol. 2005, 35, 175. (44) Gash, A. E.; Tillotson, T. M.; Satcher, J. H.; Poco, J. F.; Hrubesh, L. W.; Simpson, R. L. Chem. Mater. 2001, 13, 999. (45) Kumari, S.; Singh, A. P.; Tripathi, C.; Chauhan, D.; Dass, S.; Shrivastav, R.; Gupta, V.; Sreenivas, K.; Satsangi, V. R. Int. J. Photoenergy 2007, 87467. (46) Ingler, W. B.; Khan, S. U. M. Int. J. Hydrogen Energy 2005, 30, 821. (47) Nagahama, J.; Yumoto, H. Surf. Coat. Technol. 2003, 169, 658. (48) Hardee, K. L.; Bard, A. J. J. Electrochem. Soc. 1976, 123, 1024. (49) Mathur, S.; Sivakov, V.; Shen, H.; Barth, S.; Cavelius, C.; Nilsson, A.; Kuhn, P. Thin Solid Films 2006, 502, 88. (50) Vayssieres, L.; Guo, J. H.; Nordgren, J. J. Nanosci. Nanotechnol. 2001, 1, 385. (51) Beermann, N.; Vayssieres, L.; Lindquist, S. E.; Hagfeldt, A. J. Electrochem. Soc. 2000, 147, 2456. (52) Schrebler, R.; Bello, K.; Vera, F.; Cury, P.; Munoz, E.; del Rio, R.; Meier, H. G.; Cordova, R.; Dalchiele, E. A. Electrochem. Solid State Lett. 2006, 9, C110. (53) Zotti, G.; Schiavon, G.; Zecchin, S.; Casellato, U. J. Electrochem. Soc. 1998, 145, 385. (54) Schrebler, R.; Llewelyn, C.; Vera, F.; Cury, P.; Munoz, E.; del Rio, R.; Meier, H. G.; Cordova, R.; Dalchiele, E. A. Electrochem. Solid State Lett. 2007, 10, D95. (55) Hu, Y.-S.; Kleiman-Shwarsctein, A.; Forman, A.; Park, J.-N.; McFarland, E. W. Chem. Mater. 2008, 20, 3803. (56) Merchant, P.; Collins, R.; Kershaw, R.; Dwight, K.; Wold, A. J. Solid State Chem. 1979, 27, 307. (57) Marusak, L. A.; Messier, R.; White, W. B. J. Phys. Chem. Solids 1980, 41, 981. (58) Watanabe, A.; Kozuka, H. J. Phys. Chem. B 2003, 107, 12713. (59) Gota, S.; Guiot, E.; Henriot, M.; Gautier-Soyer, M. Phys. ReV. B 1999, 60, 14387. (60) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Surf. Interface Anal. 2004, 36, 1564. (61) Droubay, T.; Chambers, S. A. Phys. ReV. B 2001, 64. (62) Anwar, M.; Hogarth, C. A.; Bulpett, R. J. Mater. Sci. 1989, 24, 3087. (63) Chowdari, B. V. R.; Tan, K. L.; Chia, W. T.; Gopalakrishnan, R. J. Non-Cryst. Solids 1990, 119, 95. (64) Brow, R. K. J. Am. Ceram. Soc. 1987, 70, C129. (65) Ikemoto, I.; Ishii, K.; Kinoshita, S.; Kuroda, H.; Alariofranco, M. A.; Thomas, J. M. J. Solid State Chem. 1976, 17, 425. (66) Viefhaus, H.; Hennesen, K.; Lucas, M.; Mu¨ller-Lorenz, E. M.; Grabke, H. J. Surf. Interface Anal. 1994, 21, 665. (67) Bernasik, A.; Janowski, J.; Hirschwald, W.; Stolze, F.; Nowotny, J. J. Mater. Sci. 1991, 26, 2527. (68) Velev, J.; Bandyopadhyay, A.; Butler, W. H.; Sarker, S. Phys. ReV. B 2005, 71. (69) Hida, Y.; Kozuka, H. Thin Solid Films 2005, 476, 264.
JP803775J
