Electronic structure, photovoltage, and photocatalytic hydrogen evolution with p-CuBi₂O₄ nanocrystals

Abstract

a As a visible light active p-type semiconductor, CuBi2O4 is of interest as a photocatalyst for the generation of hydrogen fuel from water. Here we present the first photovoltage and photocatalytic measurements on this material and DFT results on its band structure. Single crystalline CuBi2O4 nanoparticles (25.7 ± 4.7 nm) were synthesized from bismuth and cupric nitrate in water under hydrothermal conditions. Powder X-ray diffraction (XRD) confirms the CuBi2O4 structure type and UV-Vis spectroscopy observes a 1.75 eV optical band gap. Surface photovoltage (SPV) measurements on CuBi2O4 nanoparticle films on fluorine doped tin oxide yield 0.225 V positive photovoltage at >1.75 eV photon energy confirming holes as majority carriers. The photovoltage is reversible and limited by light absorption. When dispersed in 0.075 M aqueous potassium iodide solution, the CuBi2O4 particles support photochemical hydrogen evolution of up to 16 µmol h-1 under ultraviolet but not under visible light. Based on electrochemical scans, CuBi2O4 is unstable towards reduction at -0.2 V, but pH-dependent photocurrents of 6.45 A cm -2 with an onset potential of +0.75 V vs. NHE can be obtained with 0.01 M Na2S2O8 as a sacrificial electron acceptor. The photoelectrochemical properties of CuBi2O4 can be explained on the basis of the band structure of the material. DFT calculations show the valence and conduction band edges to arise primarily from the combination of O 2p and Cu 3d orbitals, respectively, with additional contributions from Cu 3d and Bi 6s orbitals just below the Fermi level. Trapping of photoelectrons in the Cu 3d band is the cause for reductive photocorrosion of the material, while the p-type conductivity arises from copper vacancy states near the VB edge. These findings provide an improved understanding of the photophysical properties of p-CuBi2O4 and its limitations as a proton reduction photocatalyst.A) The DFT+U electronic band structures of (left) pristine CuBi2O4 and (right) VCu -CuBi2O4. The Fermi level (EF) is set at 0 eV. The presence of VCu leads to partial occupation of the bands around the VB edge. This gives rise to p-type behavior in CuBi2O4. B) The DFT+U projected density of states of VCu -CuBi2O4.

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting

“…[100,112] CuBi 2 O 4 is ap -type semiconductor with E G = 1.6-1.8 eV,w hich corresponds to am aximum theoretical j ph of 20 mA cm À2 .I ts crystal structure is formed by stacks of [CuO 4 ] that are connected to Bi (Figure 5c). [100] After demonstrating the high photoactivity of CuBi 2 O 4 in 2007, various synthetic methodsh ave been explored, such as electrodeposition, [114][115][116] low-temperature crystal growth, [117] hydrothermal reactions, [118] ands pray pyrolysis. [5] Although the hole mobility at 10 À3 cm 2 V À1 s À1 is relatively low,t he flat-band potential, approximately equal to ÀE VS /Q 0 ,i sa bout 1.26-1.29 V versus RHE, which is quite positivec ompared with that of other p-type p-PEs.…”

Efficient Solar‐Assisted O2 Reduction Using a Cofacial Iron Porphyrin Dimer Catalyst Integrated into a p‐CuBi2O4 Photocathode

“…Recently, CuBi 2 O 4 immobilized on FTO electrode has been emerged as a promising p‐type photocathode for energy storing and conversion processes due to its convenient band gap energy ( E g =1.5–1.8 eV) for absorbing a substantial portion of visible energy, its suitable conduction band minimum (CBM) and valence band maximum (VBM) energies, and its construction from earth abundant materials. Indeed, the CBM and VBM of CuBi 2 O 4 are located at more negative and more positive potentials than the thermodynamic water reduction and oxidation potentials, respectively, enabling solar‐driven water reduction . ORR is thermodynamically much easier than water reduction and using FTO/CuBi 2 O 4 photocathode modified with a suitable O 2 reduction catalyst, as the current Fe 2 TPFPP‐TMP is a promising approach to achieve solar‐assisted ORR.…”

“…[7] We were then interested in integratingt he Fe 2 TPFPP-TMP into as uitable p-type photocathode and investigate its solarassisted ORR. Recently,C uBi 2 O 4 immobilized on FTO electrode has been emerged as ap romisingp -typep hotocathode for energy storing and conversion processes due to its convenient band gap energy (E g = 1.5-1.8 eV) for absorbing as ubstantial portion of visible energy, [41][42][43][44][45] its suitable conduction band minimum (CBM) and valence band maximum (VBM)e nergies, and its construction from earth abundant materials. Indeed, the CBM and VBM of CuBi 2 O 4 are located at more negative and more positive potentials than the thermodynamicw ater reduction and oxidation potentials, respectively,e nabling solardriven water reduction.…”

Recent Advances in Bismuth‐Based Nanomaterials for Photoelectrochemical Water Splitting

“…Hahn et al reportt he synthesis of CuBi 2 O 4 thin films by electrochemical deposition for the hydrogen evolution reaction. [106] The narrow band gap and the p-type behavior originate from Cu vacancies in the lattice.T he photocorrosion arises from trapping of electrons near the conduction band edge of the Cu dband. CuBi 2 O 4 can be synthesized by various methods, including hydrothermal synthesis, solid-state technique,e lectrodeposition, spray pyrolysis, and flux method, for photocatalytic applications.…”

“…[103] Despite the small band gap and suitable band-edgep osition of CuBi 2 O 4 ,H 2 evolution is relativelyl ow (44-55 mAcm À2 at À0.2 Vv s. RHE). [106] Berglund et al have performed adetailed study to understandt he absorption properties, chargetransport kinetics, and stability of the CuBi 2 O 4 photocathode. [104] Wang et al have synthesized the hierarchical architecture, hedgehog-like nanostructure of CuBi 2 O 4 ,w hich shows ntype semiconducting behavior.…”