CuNb3O8: A p-Type Semiconducting Metal Oxide Photoelectrode

May 23, 2012 - Thus, a new p-type semiconductor is discovered for potential applications in solar energy conversion. ... (6, 7) While many n-type semi...
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CuNb3O8: A p-Type Semiconducting Metal Oxide Photoelectrode Upendra A. Joshi and Paul A. Maggard* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: A new p-type CuNb3O8 polycrystalline photoelectrode was investigated and was determined to have indirect and direct bandgap sizes of 1.26 and 1.47 eV, respectively. The p-type polycrystalline film could be prepared on fluorine-doped tin oxide glass and yielded a cathodic photocurrent under visible-light irradiation (λ > 420 nm) with incident photon-to-current efficiencies of up to ∼6−7% and concomitant hydrogen evolution. A Mott−Schottky analysis yielded a flat band potential of +0.35 V versus RHE (pH = 6.3) and a calculated p-type dopant concentration of ∼7.2 × 1015 cm−3. The conduction band energies are found to be negative enough for the reduction of water under visible light irradiation. A hole mobility of ∼145 cm2/V·s was obtained from J(I)−V2 measurements using the Mott−Gurney relation, which is ∼50% higher than that typically found for p-type Cu2O. DFT-based electronic structure calculations were used to probe the atomic and structural origins of the band gap transitions and carrier mobility. Thus, a new p-type semiconductor is discovered for potential applications in solar energy conversion. SECTION: Energy Conversion and Storage; Energy and Charge Transport first photolectrochemical characterization of the p-type CuNb3O8 semiconductor with a much smaller bandgap size. The CuNb 3 O 8 powder was prepared by heating a stoichiometric mixture of Cu2O and Nb2O5 in an evacuated fused silica tube at 750 °C for 24 h. The product purity was characterized by powder X-ray diffraction and by UV−visible diffuse reflectance spectroscopy (DRS). Polycrystalline films were prepared on FTO (fluorine-doped tin oxide; TEC-7 from Pilkington Glass Inc.) using ethanol as the dispersant and annealed at 450 °C for 3 h under vacuum. All electrochemical experiments were carried out in a Teflon cell using a threeelectrode system, with the CuNb3O8 film as the working electrode, Pt as the counter electrode, and a standard calomel reference electrode (sat. KCl). Argon gas was bubbled through the electrolyte solution (0.5 M Na2SO4) for 30 min before the measurement and purged continuously throughout. The films were backside-irradiated using a high-pressure Xe lamp and visible-light cutoff and band-pass filters (λ ≥ 420 nm). Further details are given in the Supporting Information and in a previous report.31 A linear-sweep cyclic voltammogram of the polycrystalline CuNb3O8 film electrode, under chopped visible-light irradiation, is shown in Figure 1A. The photocathodic current increases with the applied bias voltage (+0.44 to −0.35 V plotted versus RHE), which is representative of typical p-type semiconductor behavior. The electrode film achieves a cathodic photocurrent density of −0.40 mA/cm2 at a −0.35 V applied bias versus RHE, after subtraction of the dark current. The onset potential of the photocurrent is estimated at ∼0.30 V

T

he direct conversion of sunlight to chemical fuels with the use of a stable semiconducting photoelectrode is currently a key challenge in the field of solar energy photoelectrochemistry (PEC). Although n-type TiO2 is one of the most well studied photoelectrodes employed for use in PEC cells, its large band gap of 3.2 eV and its relatively low-energy conduction band both remain important limitations.1 The reduction of water or carbon dioxide to produce hydrogen or methane/ carbon-oxygenates, respectively, can be driven on the surfaces of a p-type semiconductor (i.e., a photocathode) with suitable band positions and bandgap sizes.2−5 A major drawback of current p-type III/V and II/VI semiconductors is their photocorrosion in aqueous solutions under prolonged irradiation.6,7 While many n-type semiconducting photoanodes have been extensively studied,8−17 very few metal-oxide p-type semiconducting photocathodes are currently known, such as Cu2O,18−21 metal-doped Fe2O3,22,23 CaFe2O4,24,25 and Rhdoped SrTiO3.26 While p-type Cu2O is currently extensively studied in PEC applications, its tendency to photocorrode remains a significant drawback. Recently, some successful efforts have been made to stabilize Cu2O using surface coatings of Al2O3, ZnO, and TiO2.27 The preparation of stable p-type metal oxide semiconductors with suitable bandgap sizes and band energies (Eg) is thus of critical importance for solar-driven fuel production. In this research area, our efforts have focused on the investigation of new p-type semiconductors that are comprised of both early and late transition metals. These combinations yield valence and conduction bands comprised of the filled Cu(I)-based d10 orbitals and empty Nb(V)/Ta(V)-based d0 orbitals.28−30 Recently, we reported the first example of such a p-type semiconductor in CuNbO3 (Eg ≈ 2.0 eV) that exhibited stability against photocorrosion.31 In this study, we present the © 2012 American Chemical Society

Received: April 19, 2012 Accepted: May 23, 2012 Published: May 23, 2012 1577

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Figure 1. (A) Linear-sweep cyclic voltammogram of a working CuNb3O8 electrode under chopped visible light irradiation (λ > 420 nm) in an aqueous 0.5 M Na2SO4 (pH = 6.3) electrolyte solution. (B) Plot of Mott−Schottky data for the same electrode.

Figure 2. For CuNb3O8, (A) a plot of J(I)−V2, (B) a Tauc plot of the indirect bandgap size of 1.26 eV, (C) chronoamperometry measurement at −0.156 V versus RHE under visible-light irradiation, and (D) XRD patterns before and after the chronoamperometry experiment.

versus RHE (at pH 6.3). This onset potential gives a rough first estimate of the flat band potential, which is located near the valence band edge of a p-type semiconductor. However, errors arising from interfacial charge-transfer limitations, for example, can occur that need to be considered. A more accurate determination of the flat band potential was conducted via a Mott−Schottky analysis using electrochemical impedance methods at 5 mV AC and 1 and 12 kHz frequencies, shown in Figure 1B. The negative slope of the Mott−Schottky data is the expected behavior for a p-type semiconductor. The X-axis

intercept (V0 = 0.32 V) can be used to determine the flat band potential from the equation V0 = Vfb − kT/e, where k is the Boltzmann constant, T is the temperature, and e is the electron charge. This calculation yields a flat band potential (Vfb) of +0.35 V versus RHE for both frequencies (1 and 12 kHz). The slopes of the Mott−Schottky data are nearly always frequencydependent, as found here, thus giving rise to some error in the subsequent calculation. As the polycrystalline CuNb 3O8 photoelectrode has been prepared via solid-state methods, this type of nonideal behavior is expected. At a frequency of 12 1578

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Figure 3. (A) Photoelectrochemical hydrogen evolution (black line) and oxygen evolution (red line) over a CuNb3O8 photocathode (and Pt counter electrode) under visible light irradiation at −0.156 V applied bias potential versus RHE (pH 6.3). The blue line represents a Faradaic efficiency of 100% for hydrogen production. (B) Overlaid IPCE measurement and UV−Vis DRS of CuNb3O8 versus wavelength.

representative time-dependent photocathodic current of a CuNb3O8 film electrode shows some stability over at least a few hours. Fluctuations in the photocurrent have been observed due to mechanical instability (peeling of particles from the film) or rearrangement of particles on the surface. An X-ray diffraction (XRD) analysis was performed after the chronoamperometry measurements, shown in Figure 2D, with no apparent changes in the peak intensities or positions, and no impurities were detectable. The UV−vis DRS of the CuNb3O8 films also do not show any significant changes. For a p-type semiconductor, the valence band energy (Ev) can be calculated from the flat band potential (Vfb) with the equation, Ev = −Vfb + kT ln(NA/NV), where NA is the dopant concentration and NV is the effective density of states (typically ∼1019) at the valence band edge.36 The conduction band edge is obtained by adding the optical bandgap size to the valence band energy. Hence, the valence band potential is calculated from the Mott−Schottky flat band potential to occur at ∼+0.55 V versus RHE at pH = 6.3. A similar value of ∼+0.50 V is calculated from the less-accurate value of the photocurrent onset potential. The conduction band position is determined to therefore occur at ∼−0.71 and ∼−0.76 eV versus RHE from the Mott−Schottky and photocurrent onset potential, respectively. Thus, the conduction band of the CuNb3O8 film is sufficiently more negative than the reduction potential of water and excited electrons can be used for the reduction of water to hydrogen under visible-light irradiation. However, the valence band position is not more positive than the oxidation potential of water (EO2/H2O is ∼0.85 V at pH = 6.3). However, a dual band gap system would be suitable that combines p-type CuNb3O8 with a suitable n-type semiconductor having a lowerenergy valence band. The photoelectrochemical reduction of water to hydrogen using the CuNb3O8 film under visible light irradiation was measured at an applied potential of −0.156 V versus RHE, as shown in Figure 3A. The dotted line represents the ideal Faradaic efficiency for hydrogen production. This shows a measured Faradaic efficiency of 62%. However, gas bubbles were also observed to be adhered to the CuNb3O8 photoelectrode as well as on the Pt counter electrode. This served to reduce the amount of measured hydrogen to lower than that expected, among other sources of possible gas leakage.37−39 Also, almost a stoichiometric amount of oxygen was detected, as would be generated at the counter electrode. Similar results

kHz, an acceptor density (NA) from the slope of the Mott− Schottky plot was calculated to be ∼7.2 × 10 15 cm−3 (calculation details are provided in the Supporting Information). This is a similar, but somewhat low, dopant density as that known for other p-type semiconductors, such as that found for p-type Cu2O.32,33 The mobility of the majority charge carriers, that is, holes, in p-type CuNb3O8 was measured from current−voltage data taken on a pressed pellet using Pt ohmic contacts from atomiclayer deposition. Details are provided in the Supporting Information. The data shown in Figure 2A exhibit an ohmic region at ∼1.5 V using the Mott−Gurney equation, J(I) = (9/8)(ε0εrμ(V2/ L3)),34 where J(I) is the current density, ε0 is the permittivity of free space, εr is the dielectric constant, L is the pellet thickness, and μ is the carrier mobility. This gives a hole mobility of ∼145 cm2/V·s, which compares favorably to the hole mobility in ptype Cu2O of ∼80−100 cm2/V·s.33 The hole mobility and carrier density yield a value for the electrical conductivity (σp) for CuNb3O8 of ∼0.16 S cm−1, from σp = NA·μ·e, where e is the electron charge. Further increases in the electrical conductivity should be possible with higher dopant densities. Diffuse reflectance UV−vis measurements were taken on the CuNb3O8 powder, and the size and type of the band gap transition were obtained from Tauc plots of (αhν)n versus (hν),35 where hν is the photon energy, α is the absorption coefficient, and n = 1/2 or 2 for indirect and direct band gap transitions, respectively. Shown in Figure 2B, the plot of (αhν)0.5 versus hν yields a lowest-energy band gap transition of ∼1.26 eV, while the lowest-energy direct band gap transition is at a higher transition of ∼1.47 eV (see the Supporting Information). Thus, the direct band gap transition (λabs < 843 nm) is suitable to absorb nearly the entire visible-light range of sunlight. The structural and atomic origins of the band gap transition were probed by DFT-based electronic structure calculations that are described below. The stability of the CuNb3O8 film against photocorrosion was investigated by chronoamperometry measurements under visible-light irradiation at an applied bias of −0.156 V versus RHE. Argon gas was bubbled continuously throughout the experiment in order to avoid ambiguity about the reduction of dissolved oxygen at the electrode. Shown in Figure 2C, a 1579

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band edge by an additional ∼0.5 eV. These show that the nextlowest valence band states consist of filled copper d10 orbitals (weak σπ-to-pπ interactions to O 2p orbitals), and the nexthighest conduction band states consist of d orbital contributions from all niobium atoms. The calculated band structure is shown in Figure 5 and confirms the indirect nature of the band gap transition. However, the relative flatness of the bands also yields a direct band gap transition only a little higher in energy.

were obtained under zero bias condition; however, the total amount of hydrogen was less. Control experiments without light at an applied bias of −0.156 V versus RHE did not yield any detectable hydrogen or oxygen. The incident photon-to-current efficiency (IPCE) action spectrum versus wavelength is shown in Figure 3B, plotted together with the UV−visible DRS data. The black color of the CuNb3O8 film is consistent with the low-energy bandgap size of 1.26 eV. At the highest visible light energies, an IPCE efficiency of ∼4% was attained, which rose steeply to 6−7% at higher photon energies. The IPCE action spectrum is blue-shifted by ∼300 nm relative to the band gap absorption edge. Its origin is currently unknown. The IPCE spectrum shows at what photon energies (i.e., using monochromatic light) the excited electrons are produced in the working electrode that can be used to drive the electrochemical reaction for water reduction to hydrogen at its surfaces. However, the onset value in the IPCE can often be blue-shifted compared to the optical bandgap size obtained from UV−visible spectroscopy. The IPCE onset can also depend on other electrode properties, such as its conductivity, trapping sites, and overpotential. These reactions are occurring on the bare CuNb3O8 surfaces and thus likely require a larger driving force to overcome the significant kinetic barriers to H2 formation. Further research is underway to test this via the deposition of surface cocatalysts that can serve to lower these kinetic barriers. To probe the origin of the photon-driven band gap excitations in CuNb3O8, electronic structure calculations were performed on the geometry-optimized structure based on density functional theory within the CASTEP program package.40 Shown in Figure 4A are electron density plots of

Figure 5. Calculated band structure diagram for CuNb3O8, with conduction and valence bands colored light and dark blue, respectively.

In conclusion, a new p-type polycrystalline CuNb3O8 film shows a small bandgap size, and suitable conduction band position, for the visible-light-driven reduction of water to hydrogen. Overall IPCE and Faradic efficiencies of up to ∼6−7 and ∼62% were achieved, respectively. The film exhibits a high mobility of carriers of ∼145 cm2 /V·s, and a dopant concentration of ∼7.2 × 1015 cm−3 (at 12 kHz). Thus, a promising new p-type metal oxide semiconductor is found that can find application in photoelectrode-mediated solar fuel production. Further research efforts are aimed at increasing its overall efficiency and stability, such as by increasing the dopant concentration, adjusting particle sizes and surfaces, as well as densification of the photoelectrode film to improve hole mobility within the film.

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ASSOCIATED CONTENT

* Supporting Information S

Figure 4. (A) Electron density plot of valence (dark blue) and conduction band (light blue) states within 0.5 eV of the band edge. (B) The same plot for valence and conduction band states within 0.5− 1.0 eV of the band edge. O, Cu, and Nb atoms are red, blue, and light blue, respectively.

Detailed experimental procedure, calculations, and UV−vis DRS before and after PEC measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

the highest valence band states and the lowest conduction band states, that is, within 0.5 eV of each of the band edges. The valence band edge consists primarily of Cu(I) d10 orbitals mixed with O 2p orbitals (σ* interactions), whereas the conduction band edge consists mainly of one type of Nb atom d0 orbitals (nonbonding). Thus, the lowest-energy band gap transition is a metal-to-metal charge transfer between Cu(I) and Nb(V). Under irradiation, the electrons in the CuNb3O8 film are excited into Nb-based crystal orbitals, which can lead to increased stability against photocorrosion as compared to ptype Cu2O. Shown in Figure 4B are electron density plots of the valence and conduction band states further away from the

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Research Corporation for Science Advancement (P.M. is a Scialog awardee) and from the U.S. Department of Energy (DE-FG0207ER15914). The authors also thank Dr. Jon-Paul Maria and David Harris of the Materials Science and Engineering for assistance with the collection of J(I)−V data. 1580

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