Photoelectrochemical Investigation and Electronic Structure of a p

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Photoelectrochemical Investigation and Electronic Structure of a p-Type CuNbO3 Photocathode Upendra A. Joshi, Andriy M. Palasyuk, and Paul A. Maggard* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States

bS Supporting Information ABSTRACT:

A new p-type CuNbO3 photoelectrode was prepared on fluorine-doped tin oxide (FTO) glass and characterized by X-ray diffraction (XRD), UV
vis spectroscopy, and photoelectrochemical techniques. Solid-state syntheses yielded a red-colored CuNbO3 phase (space group: C2/m (No. 12), Z = 8, a = 9.525(1) Å, b = 8.459(2) Å, c = 6.793(1) Å, β = 90.9(2)
) with a measured optical bandgap size of ∼2.0 eV. Phase-pure samples could be deposited and annealed on FTO slides at 400
C under vacuum. Photoelectrochemical measurements showed the onset of a photocathodic current driven under visible-light irradiation and reaching incident-photon-tocurrent efficiencies exceeding ∼5%. The p-type CuNbO3 film also exhibits a stable photocurrent and notable resistance to photocorrosion, as shown by X-ray diffraction. Electronic structure calculations based on density functional theory reveal the visiblelight absorption originates from a nearly direct bandgap transition owing primarily to copper-to-niobium (d10-to-d0) excitations. A promising new p-type semiconductor is thus revealed of potentially broad use in solar-energy conversion.

1. INTRODUCTION The direct conversion of solar energy to chemical fuels using semiconductor photoelectrodes has been a topic of intense research for several decades.1,2 The technological realization of the solar-driven photoelectrochemical (PEC) production of hydrogen or hydrocarbons would provide the capability to switch to renewable and domestically-produced energy carriers. The reduction of either water or carbon dioxide to generate hydrogen or hydrocarbon fuels respectively can be driven on p-type (photocathodic) electrodes with suitable band energies. The highest-efficiency p-type semiconductors to date are based on crystalline III
V semiconductors with outstanding optoelectronic properties but limited corrosion resistance.3 Metal-oxide semiconductors, by contrast, can exhibit superior corrosion resistance but are typically difficult to dope as p-type with a high mobility of carriers. Whereas a plethora of research has been directed at highly efficient n-type semiconductor photoanodes,4 a much smaller number of p-type metal-oxide semiconductors is currently known, such as cuprous oxide (Cu2O),5 metal-doped Fe2O3,6
8 and CaFe2O4.9,10 Current results thus necessitate the r 2011 American Chemical Society

utilization of alternative approaches to identify promising p-type semiconductors for use in solar-driven fuels production. Recently, our research efforts have sought to investigate the reduction of the bandgap sizes of early transition-metal oxides via the incorporation of transition metals with d10 electron configurations, that is, specifically Cuþ and Agþ. For example, a number of new Agþ/Re7þ and Cuþ/Re7þ mixed oxide-organic solids reveal tunable and significantly reduced optical bandgap sizes in the range of ∼3.5
2.0 eV within many structurally-flexible networks.11,12 An extension of these investigations into the photcatalytically-relevant niobates and tantalates has recently been shown to yield a promising class of new semiconductors based on mixed Cuþ/Ta5þ and Cuþ/ Nb5þ metal oxides.13
15 For the UV-photocatalyst Na2Ta4O11,16 for example, the isoelectronic substitution of Cuþ for Naþ results in the site-differentiated solid-solution (Na1
xCux)2Ta4O11 with an accompanying modification of the bandgap transition and size from ∼4.0 eV to ∼2.65 eV.14 However, their photoelectrochemical Received: May 18, 2011 Published: May 27, 2011 13534

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The Journal of Physical Chemistry C properties for application as new p-type semiconductors have never previously been investigated. This study presents the first results, to our knowledge, of a new p-type CuNbO3 film electrode and a detailed investigation of its underlying electronic structure based on density functional theory calculations.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Preparation of high-purity CuNbO3 powder was performed by finely grinding a stoichiometric mixture of Nb2O5 (99.9% Aldrich) and Cu2O (99.9% Alfa Aesar) under argon atmosphere in a glovebox, followed by reaction in a sealed fused-silica tube at 950
C for 4 days. The final product was obtained in the form of a homogeneous cherry-red powder. To prepare films of CuNbO3, fluorine-doped tin oxide glass (TEC-7 and TEC-15 from Pilkington Glass Inc.) slides were sonicated in deionized water to remove water-soluble impurities from the surface for 30 min. After that, they were sonicated in ethanol for 30 min and acetone for 30 min to remove any organic impurities from the surface. This process was carried out three times. All glass slides were then dried under flowing N2 to remove residual acetone and stored to avoid contamination. A 1 cm2 area was masked off with scotch tape on the glass side of each FTO slide. The CuNbO3 powder was ground in a mortar and pestle using ethanol as a dispersant and then spread over the glass slide using the doctor-blade technique. The film was dried at room temperature, the tape was removed, and then each film was calcined at 400
C for 3 h under vacuum. The freshly-prepared films were either used directly for photoelectochemical measurements, or were mildly oxidized by heating in air to ∼250
C for 3 h. Electrical contacts were made to the FTO slide using copper tape that was coated with a conducting adhesive. 2.2. Characterization. Powder X-ray diffraction (XRD) data of all samples were collected on an INEL diffractometer using Cu KR1 (λ = 1.54056 Å) radiation from a sealed-tube X-ray generator using a curved position sensitive detector (CPS120). The experimental powder XRD pattern of CuNbO3 matched well with the theoretical one generated from the single-crystal XRD study.17 All XRD peaks were indexed and the lattice parameters were refined using the LATCON program to a = 9.525(1), b = 8.459(2), c = 6.793(1) Å, β = 90.9(2)
.18 The UV
vis diffuse reflectance spectra were collected on a Shimadzu UV
vis-NIR Spectrometer UV-3600 with the use of an integrating sphere. The sample was mixed with barium sulfate and pressed onto a holder and placed along the external window of the integrating sphere. A second sample of pressed barium sulfate powder was prepared as a reference and the data were plotted as the remission function F(R¥) = (1
R¥)2/(2R¥), where R is diffuse reflectance based on the Kubelka
Monk theory of diffuse reflectance.19 Field-emission scanning electron microscopy (FE-SEM) analyses were performed on JEOL SEM 6400 (Peabody, MA). All films were coated with Pt to enhance the conductivity before SEM analysis. A 45
sample holder was employed to obtain SEM images for measurements of the film thickness. 2.3. Photoelectrochemical Measurements. Photoelectrochemical measurements were carried out using a three-electrode system in a Teflon cell, with CuNbO3 films as working electrodes, Pt foil as the counter electrode, and a standard calomel reference electrode (sat. KCl). The film was immersed in a 0.5 M Na2SO4 electrolyte solution that was purged with argon gas for 30 min prior and continuing throughout the measurements. The pH of the electrolyte was adjusted using dilute NaOH and H2SO4 solutions. An electrochemical analyzer (Princeton

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Applied Research, PARSTAT 2263) with PowerSuite software was employed to measure photocurrent and to carry out chronoamperometry experiments. A 400 W Xe arc lamp (Oriel-Newport) equipped with an ultraviolet cutoff filter (λ g 420 nm) was utilized as the visible-light source. All photoelectrochemical experiments were carried out by irradiation of the back side of the films. Incident-photon-to-current conversion efficiency (IPCE) measurements were carried out using bandpass filters of four different wavelengths (351.95, 451.48, 522.49, and 602.75 nm). The number of photons of monochromatic light reaching the electrode surface was measured at each wavelength using a Si photodiode. The monochromatic IPCE is defined as follows.   mA 1239:8  photocurrent density cm2    100 IPCEð%Þ ¼ mW wavelength½nm  photon flux cm2 2.4. Electronic Structure Calculations. Band-structure calculations of CuNbO3 were carried out with the use of the planewave DFT package CASTEP.20 According to the ultrasoft core potentials scheme,21 the following configurations were used in the valence atomic configuration of CuNbO3: 3d104s1 for copper, 4d45s1 for niobium, and 2s22p4 for oxygen.

3. RESULTS AND DISCUSSION Cyclic voltammetry measurements of the CuNbO3 films on FTO glass were carried out under chopped visible-light irradiation from a 400 W Xe lamp (λ g 420 nm). Shown in part A of Figure 1 is the measured current
potential curve obtained under an applied bias voltage scanned from þ0.2 to
0.6 eV versus a standard calomel electrode. A significant photocathodic current is clearly observed to indicate that CuNbO3 is a p-type semiconductor. The electrode achieves a cathodic photocurrent density of
0.10 mA/cm2 at
0.6 V applied bias versus SCE (after subtracting dark current). Conversely, no photoanodic current is observed. The on-set potential of the photocathodic current was observed at ∼þ0.10 V versus a standard calomel electrode (SCE), which corresponds to þ0.35 V versus the normal hydrogen electrode (NHE, at pH 6.3). Prior to the oxidation treatment of the CuNbO3 film, a more negative onset potential is observed of
0.11 V versus SCE as well as higher dark current (Figure S3 of the Supporting Information). This onset potential can give a rough measure of the flat-band potential and which for a p-type semiconductor occurs nearer to the valence band edge with increasing dopant density. An estimate of the conduction band edge is obtained by adding to it the measured optical bandgap size. Shown in part B of Figure 1, the UV
vis diffuse-reflectance spectrum of the CuNbO3 film is plotted together with the incident-photon-to-current conversion efficiency (IPCE) spectrum. Full details of the IPCE calculations are given in the Supporting Information. The optical bandgap edge was found to be at ∼635 nm, and which matches well with the onset of the photocathodic current that rises initially at ∼600 nm. At the lowest visible-light energies, IPCE efficiencies of ∼1
2% are attained and which rise steeply to >5% at even higher incident photon energies. The photocurrent measurements confirm that the visible-light absorption leads to the generation and separation of electron
hole pairs and transfer 13535

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Figure 1. (a) Current
potential curve in an aqueous 0.5 M Na2SO4 solution (pH 6.3) under chopped visible-light irradiation (λ g 420 nm) for a CuNbO3 electrode. Insert shows a photograph of the CuNbO3 film on FTO glass. (b) (i) Incident-photon-to-current conversion efficiency spectrum of CuNbO3 obtained by applying
0.4 V vs SCE to the working electrode and (ii) the UV
vis spectrum of CuNbO3.

Figure 2. Results of the chronoamperometry experiment for the CuNbO3 film electrode at
0.6 V applied versus SCE using a 0.5 M Na2SO4 electrolyte under full arc irradiation. Left: shorter period of time, right: longer period of time.

of electrons to the electrolyte and holes to the back contact. As the IPCE efficiency is lowest near the band-edge energies, this suggests that the band-edge electrons are much less efficiently migrating to and transferring across the electrode interfaces. To investigate the photostability of the CuNbO3 film, the time-dependent photocurrent response was measured, as shown in Figure 2, and which was performed at a
0.6 V applied potential (versus SCE) under full-arc irradiation. To avoid the competitive electrochemical reduction of dissolved oxygen, the electrolyte solution was continuously bubbled with argon gas. At an applied bias potential of
0.6 V (vs SCE), a cathodic dark current was observed, which was subtracted from the photocurrent response for the subsequent IPCE calculations. While irradiated, the photocurrent remained constant, and then it decreased rapidly back to the dark current value when the light was switched off. A strong dependence of the photocurrent on the pH value of the electrolyte solution was also observed. The photocurrent reached a maximum at a pH of ∼6.31 before then decreasing for either higher or lower pH values. This pH was selected for all subsequent measurements. The strong dependence of photocurrent on the pH of electrolyte indicates that the charge transfer kinetics are facilitated more efficiently at pH 6
7. The time-dependent photocurrent data plotted in Figure 2 (right side) show that the CuNbO3 electrode exhibits a relatively stable photocurrent over the course of a few hours, indicating that limited to no photocorrosion takes place on the surface of the CuNbO3 film. It is well-known that the stability and

Figure 3. Powder X-ray diffraction patterns from (a) that calculated/ simulated from the CuNbO3 structure, (b) the experimental pattern from the as-synthesized CuNbO3 film, and (c) the experimental pattern from the CuNbO3 film after the photoelectrochemical measurements were performed.

photoresponse of a metal-oxide film is highly dependent upon its preparation conditions.22 A higher dark current observed before the mild oxidation treatment is likely due to the cathodic corrosion of the film (Figure S3 of the Supporting Information). After the mild oxidation treatment (250
C for 3 h), the dark current is reduced, and that indicates an improved stability. 13536

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Figure 4. Cross-sectional SEM images of the CuNbO3 film on FTO glass both before (a and b) and after (c and d) the photoelectrochemical measurements. The average measured CuNbO3 film thicknesses of ∼53
55 μm are labeled by the horizontal red lines.

Typically, p-type semiconductors such as Cu2O can be susceptible to cathodic photodecomposition in aqueous solution under illumination, as the minority charge carriers are electrons that migrate to the surface.23 In the case of Cu2O, its redox potential for surface decomposition is well below the conduction band and therefore causes the reduction of Cu2O to Cu metal at very low surface electron concentrations.24 In contrast, CuNbO3 is more resistant to photocorrosion owing to the electrons being excited to the lowerenergy Nb5þ (d0) orbitals rather than the Cu-based orbitals (Electronic Structure Calculations below). To further investigate the effect of irradiation on the CuNbO3 film electrode, both powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were carried out. Shown in Figure 3 are the powder XRD patterns based on a simulated theoretical pattern for the CuNbO3 structure, as well as the experimental powder XRD patterns of the CuNbO3 film both before and after several hours of photocurrent measurements. Each peak in the XRD pattern could be indexed and fitted to that of the known structure, with the refined lattice constants given in the Experimental Section above, and with no detectable traces of Cu metal or other impurities. Cross-sectional SEM images of the CuNbO3 film prepared on the FTO glass, both before and after the photoelectrochemical measurements, are shown in Figure 4 with the film thicknesses labeled with horizontal red lines. The overall uniformity of the film is confirmed and the average thickness of the film is shown to be ∼53
55 μm. Shown in parts c and d of Figure 4, the SEM images of the CuNbO3 film after photoelectrochemical measurements show that it has maintained its uniformity and approximate film thickness. There was no detectable surface Cu metal and no evidence for dissolution or degradation of the film resulting from photocorrosion. These results further confirmed the stability of the prepared CuNbO3 film.

Figure 5. On the left, the calculated electronic densities-of-states (DOS) of CuNbO3 using CASTEP, including the total DOS (black), and the projected orbital contributions from Cu d-orbitals (blue), Nb d-orbitals (red), and O p-orbitals (green). On the right, the band structure k-space diagram for a narrower energy range falling within a few eV above and below the Fermi level (Ef).

To probe the origin of the photon-driven bandgap excitations in CuNbO3, electronic-structure calculations were performed on the geometry-optimized structure based on density functional theory within the CASTEP program package.25 The calculated total and partial electronic densities-of-states and bandgap size are shown in Figure 5 (left). The conduction band of CuNbO3 is comprised of empty Nb-based d-orbitals (red line) that are increasingly mixed at higher energies with empty O-based p-orbitals (green line). Conversely, the uppermost energy of the valence band is comprised of filled Cu-based d-orbitals (blue line) mixed to a small extent with the filled O-based p-orbitals at lower energies. Thus, the lowest-energy bandgap excitation is 13537

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Figure 7. On the left, a polyhedral view of the structure of CuNbO3 with the unit cell outlined; on the right, a polyhedral view of a single symmetry-unique layer of the structure; red polyhedra = NbO6, yellow atoms = oxygen, blue atoms = copper. Figure 6. Combined plot of the electron densities that comprise the lowest-energy conduction band states (red electron density, concentrated on Nb) and the highest-energy valence band states (blue electron density, concentrated on Cu).

found to be a metal-to-metal charge transfer (MMCT) between Cu (d10) to Nb (d0)-based crystal orbitals. By comparison, the alkali-metal niobates are found to exhibit bandgap sizes ∼1.5 eV higher in energy, for example NaNbO3 at 3.4 eV,26 owing to a primarily oxygen-to-niobium transition. Thus, the isoelectronic substitution of Cuþ for Naþ has resulted in the insertion of a higher-energy valence band based on the 3d10 orbitals of the former. Shown in part B of Figure 5 is a plot of the band-structure diagram (i.e., energy versus k, or momentum space) for crystal orbitals that fall within a few electron volts of the bandgap transition, as defined in its primitive space group setting. The lowest-energy bandgap is nearly direct, with the calculated lowestenergy direct transition at the k = M vector being only ∼0.04 eV higher in energy than the lowest-energy indirect transitions stemming from k = M f L or M f Z. This situation arises because of the flatness of the lowest-energy conduction-band crystal orbital, located at ∼2 eV in Figure 5 (right). The flatness of this particular band leads to a higher effective mass and lower charge mobility of the excited electrons. This arises because the effective mass is inversely proportional to the rate of change of the band energy in k-space (i.e., m* R (∂E)2/∂2k).27 Thus, relative to the optical bandgap edge, the small blue-shift in obtaining a high photocurrent and high IPCE is explained as a result of the additional photon energy necessary to excite into the more disperse and higher-energy conduction bands beginning at ∼2.3 eV in Figure 5. Electron-density plots of the highest-energy valence band and lowest-energy conduction band are overlaid in Figure 6, shown as blue- and red-colored electron density, respectively. These show that the uppermost valence band states reside in dz2-type Cubased orbitals, as expected for its linear coordination geometry. However, the lowest-energy and flat conduction-band states are concentrated in Nb d(π) orbitals within the plane of the layer. A polyhedral view of the CuNbO3 structure, and the niobate layer in which the conduction-band crystal orbitals reside, are shown together in Figure 7. The corrugated niobate layer is comprised of squares of four vertex-shared NbO6 octahedra and that bridge to neighboring squares via edges. Within the plane of this layer, the Nb d(π)in-plane orbitals find no interaction with the O ligands as a function of the k-space vector. Typically, extended chains of vertexor edge-bridged MO6 octahedra should lead to a high band dispersion

for the d(π) orbitals in that reciprocal direction of k-space owing to increasing π* interactions to the oxygen p-orbitals.28 However, the niobate layer in CuNbO3 is highly corrugated and contains NbO6 octahedra that alternate between vertex shared and edge shared in the plane and leads to the disruption of possible π* interactions in any direction of k-space for this in-plane d-orbital, as found in its lowermost conduction band in Figure 5. The slightly higher-energy conduction bands consist of the out-ofplane d(π)-orbitals with significant π* interactions as a function of k-vector and thus a greater band dispersion and charge mobility. Compared to the more intensely explored p-type Cu2O photocathodes, the nature of the bandgap excitations that drive the photocurrent response in CuNbO3 offers a promising alternate approach. These results demonstrate the first successful preparation of a new type of p-type oxide semiconductor that can be prepared in the copper(I)
niobate system. Further synthetic and photoelectrochemical investigations are underway that suggest further promising progress and more detailed results will be forthcoming.

’ ASSOCIATED CONTENT

bS

Supporting Information. Digital photograph of experimental setup and details of the calculations of IPCE measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; Tel.: 1-919-515-3616.

’ ACKNOWLEDGMENT The authors acknowledge support of this research from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (DE-FG02-07ER15914). ’ REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 315, 737. (2) (a) Khaselev, O.; Turner, J. A. Science 1998, 280, 425. (b) Bard, A. J. Science 1980, 207, 139. (3) (a) Nozik, A. J. J. Cryst. Growth 1977, 39, 200. (b) Bak., T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Int. J. Hydrogen Energy 2002, 27, 991. 13538

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