Valence Band Level Research Articles

Highly efficient Z-scheme structured visible-light photocatalyst constructed by selective doping of Ag@AgBr and Co3O4 separately on {010} and {110} facets of BiVO4: Pre-separation channel and hole-sink effects

To improve the visible-light photocatalytic activity and photo-carriers separation of BiVO4, highly efficient Z-scheme structured visible-light photocatalyst Ag@AgBr/BiVO4/Co3O4 was constructed by selective doping of Ag@AgBr and Co3O4 on {010} and {110} facets of BiVO4. Due to the different energy levels of conduction band (CB) and valence band (VB) between {010} facet and {110} facet, BiVO4 could act as the ‘pre-separation channel’ to achieve spatial charge separation. Such pre-separation channel effect improved photo-electrons transfer from BiVO4{010} facets to electron mediator (Ag°) for subsequent recombination with holes of AgBr. Thus, the photo-induced electrons of AgBr could be accumulated on CB for reduction of O2 to O2−. On the other hand, holes of BiVO4 accumulated on {110} facets were captured by deriving-hole-type co-catalysts Co3O4 for organics oxidation. In addition, Ag° not only served as the electron mediator of Z-scheme but also improve the visible-light utilization due to SPR-effect. Being attributed to the synergistic effect of pre-separation channel and deriving-hole-type co-catalysts, Z-scheme structured Ag@AgBr/BiVO4/Co3O4(0.15 wt%) achieved high photodegradation rate of organics. Even after 9th cycles, Ag@AgBr/BiVO4/Co3O4(0.15 wt%) could still photodegraded more than 90% of OTC within 24 min. DRS, PL and photoelectrochemical analyses also confirmed the improvement of visible-light utilization and charge separation of Ag@AgBr/BiVO4/Co3O4. The radical trapping experiments and EPR demonstrated that both superoxide radical (O2−) and holes (h+) were main active species for organics photodegradation. In summary, this work not only constructed a highly efficient Z-scheme structured visible-light photocatalyst, but also provided a new method to diminish the photo-carriers recombination by pre-separation channel and hole-sink effects.

Band Tunable CdSe Quantum Dot-Doped Metals for Quantum Dot-Sensitized Solar Cell Application

Quantum dots are drawing great attention as a material for the next-generation solar cells because of the high absorption coefficient, tunable band gap, and multiple exciton generation effect. In search of the viable way to enhance the power conversion efficiency of quantum dot-sensitized solar cells, we have succeeded in preparing the quantum dot solar cells with high efficiency based on CdSe:X (Mn2+ or Cu2+) nanocrystal by successive ionic layer absorption and reaction. The morphological observation and crystalline structure of photoanode were characterized by field-emission scanning electron microscopy, X-ray diffraction, and the EDX spectra. In addition, the electrochemical performance of photoelectrode was studied by the electrochemical impedance spectra. As a result, we have succeeded in designing QDSSCs with a high efficiency of 4.3%. Moreover, the optical properties, the direct optical energy gap, and both the conduction band and the valence band levels of the compositional CdSe:X were estimated by the theory of Tauc and discussed details. This theory is useful for us to understand the alignment energy structure of the compositions in electrodes, in particular, the conduction band and valence band levels of CdSe:X nanoparticles.

Influence of heavy metal sorption pathway on the structure of biogenic birnessite: Insight from the band structure and photostability

Birnessite, primarily formed by biooxidation, is a common semiconducting Mn oxide in nature and a major controller of heavy metals cycling processes. In turn, the heavy metal sorption pathway alters its structural chemistry and thus the electronic structure. We synthesize three kinds of birnessite, namely biogenic (bio-birnessite), Cu coprecipitated (co-birnessite) and Cu adsorbed birnessite (ad-birnessite), to investigate the influence of Cu(II) occupancy on the structure, semiconducting property and photostability of birnessite. XRD show co-birnessite holds the same hexagonal symmetry as bio-birnessite, whereas ad-birnessite transforms to triclinic symmetry as reflected by the splitting reflections at 2.44 and 1.42 Å. The adsorption process is accompanied by a more intense releasing of Mn(II) (156.88 μM/L) from a bio-birnessite analog δ-MnO2 in light than in dark (19.55 μM/L), suggesting the photoreductive releasing of Mn(II) promotes structure transformation. Conformably, both the Mn average oxidation state (AOS) (3.32) and mole ratio of Cu/Mn (0.08) in ad-birnessite is lower than those in co-birnessite (AOS: 3.47, Cu/Mn ratio: 0.17). EXAFS demonstrate different Cu complexing features in ad- and co-birnessite that the ratio of Cu incorporated (INC) into vacancies is 1.16 and 0.45 for ad- and co-birnessite, respectively. UV–vis diffuse reflection spectra (DRS) and photoelectron spectrometer (PS) exhibit the band gap (Eg) and valence band (VB) of bio-birnessite are 2.05 and −5.58 eV, while there is 0.1 and 0.05 eV decrease of Eg, and 0.08 and 0.16 eV lowering of VB in co- and ad-birnessite, respectively. The reduced Eg guarantee them to generate photoelectron-hole pairs under mild indoor visible light, and the lower energy level of VB in ad-birnessite makes its VB holes more reactive to accept electrons from electron donors. Further density functional theory (DFT) calculations focus on interpreting the fine structures brought by different Cu sorption pathways on the electronic structure of birnessite. A Mn vacancy in a 2 × 2 × 1 hexagonal birnessite cell significantly reduces Eg from 1.60 to 0.28 eV by hybridizing Mn 3d and O 2p states in VB. The doped Cu reduces Eg mainly through incorporating Cu 3d orbital in VB. The INC Cu in hexagonal birnessite contributes more to reduce Eg (1.60–0.34 eV) than TCS Cu (1.60–1.20 eV), while in triclinic birnessite cell, TCS Cu causes more obvious reduction of Eg (1.68–0.28 eV) than INC Cu (1.68–1.55 eV). Thus, we conclude the vacancy imposes more effect on the electronic structure of hexagonal co-birnessite than doped Cu in TCS or INC sites; and the band structure of co-birnessite is more sensitive to INC Cu, in contrast to TCS Cu affecting more in ad-birnessite. Overall, the crystal structure differs according to Cu fixation pathway, which imposes a comprehensive effect on band structure and photostability contributed by vacancies, Cu occupancy sites and structural symmetry. Cu coprecipitating with birnessite is suggested as a preferred method in practical clean-up of metals such as Cu, due to the higher adsorption capacity for Cu, better stability of TCS Cu and better photostability influenced by Cu.

(111) Facets-Oriented Au-Decorated Carbon Nitride Nanoplatelets for Visible-Light-Driven Overall Water Splitting.

Development of a simple and stable photocatalyst for overall water splitting is a promising avenue for solar energy conversion. Here, carbon nitride (CN) nanosheet panels decorated with in situ-formed (111) facets-oriented Au nanoparticles (AuNPs) have been prepared by vapor-deposition polymerization followed by an easy immersion technique. Benefiting from the enhanced visible light absorption, the surface plasmon resonance effect of AuNPs, rapid transportation and separation of charge carriers, as well as better-aligned valence band levels, the as-obtained photocatalyst shows effective overall water splitting with stoichiometric H2 and O2 evolution even without any sacrificial agent, distinct from the half-reaction of Pt-decorated CN. This work opens up a brand-new route for facet self-selective growth of metal on two-dimensional conjugated carbon nitride materials, which has been demonstrated to be effective for artificial photosynthesis applications.

Improvement of organic solar cell performance via the incorporation of a MgO cathode interlayer fabricated by a reaction of thermally deposited Mg with MoO3

The use of magnesium oxide (MgO), which was formed by a reaction of thermally deposited magnesium with molybdenum trioxide, as a cathode interlayer material in vacuum-processed, planar pn-heterojunction organic solar cells (OSCs) using copper phthalocyanine and C70 and bulk-heterojunction (BHJ) OSCs using tris[(5-phenylthiophen-2-yl)phenyl]amine and C70 reduced the series resistance, thus improving the fill factor and power conversion efficiency (PCE). On the other hand, the use of the MgO cathode interlayer in solution-processed BHJ OSCs using poly(3-hexylthiophene) and [6,6]-phenyl-C71-butyric acid methyl ester improved the open-circuit voltage, thus improving the PCE. It is speculated that the improved performance is attributed to the high hole blocking ability of the MgO layer due to the deep-lying valence band level of MgO.

Invalidity of Band-Gap Engineering Concept for Bi3+ Heterovalent Doping in CsPbBr3 Halide Perovskite.

Heterovalent CsPbBr3 doping with Bi results in a significant red shift of the optical absorption of both single-crystal and powdered samples. The results of low-temperature (3.6 K) photoluminescence studies of perovskite single crystals indicate that the position of the excitonic luminescence peak remains unaffected by Bi doping that, in turn, infers that the band gap of Bi-doped perovskite is not changed as well. The position and state density distribution of the valence band and Fermi level of single-crystal perovskites were determined by another direct method of ultraviolet photoelectron spectroscopy. The obtained results show that Bi3+ doping causes no changes in the valence band structure but an increase in the Fermi level by 0.6 eV. The summary of the obtained results directly demonstrates that the concept of the band-gap engineering in Bi3+-doped CsPbBr3 halide perovskite is not valid.

Enhanced Visible-Light-Driven Hydrogen Production of Carbon Nitride by Band Structure Tuning

In this study, the band gap of carbon nitride has been decreased to 2.21 eV by merely optimizing the synthesis temperature to achieve a much higher utilization of visible light. To investigate the effect of synthesis temperature on the band gap structure and hydrogen production, a series of carbon nitrides were synthesized by directly heating melamine between 500 and 650 °C in Ar. The band gap structures of the samples prepared were studied based on characterization analysis in combination with the results of theoretical calculations, which revealed that their variation was caused by nitrogen vacancies. Higher synthesis temperatures cause the loss of −NH2 groups, and the nitrogen vacancies make the valence band level significantly more negative, thus allowing a remarkably enhanced H2 production rate under visible light (λ > 480 nm). This work explains and demonstrates the reduction of the carbon nitride band gap by structural means and presents a simple method of developing high-performance visible light ...

Magnesium-Doped MAPbI3 Perovskite Layers for Enhanced Photovoltaic Performance in Humid Air Atmosphere.

Despite the high efficiency of MAPbI3 perovskite solar cells, the long term stability and degradation in humid atmosphere are issues that still needed to be addressed. In this work, magnesium iodide (MgI2) was first successfully used as a dopant into MAPbI3 perovskite prepared in humid air atmosphere. Mg doping decreased the valence band level, which was determined from photoelectron yield spectroscopy. Compared to the pristine MAPbI3 perovskite film, the 1.0% Mg-doped perovskite film showed increased crystal grain size and formation of pinhole-free perovskite film. Performance of the solar cell was increased from 14.2% of the doping-free solar cell to 17.8% of 1.0% Mg-doped device. Moreover, 90% of the original power conversion efficiency was still retained after storage in 30-40% relative humidity for 600 h.

Constructive effects of the interfacial properties: A strategy to design hole transport materials for high performance perovskite solar cells

Properties of three organic hole transport materials (HTMs), FDT, PEH-2, and SNE (See Scheme 1) are investigated by combination of molecular dynamics and first principle calculations. Besides the isolated molecules, the interfacial properties of HTMs adsorbed on CH3NH3PbI3 (110) surface are firstly theoretically investigated in detail to evaluate the HTM performance. The overall performance of isolated HTMs is evaluated in terms of frontier molecular orbital, absorption spectrum, and hole mobility. After HTMs are adsorbed on CH3NH3PbI3 (110) surface, the significantly reduced band gap of SNE can improve the light harvesting ability of perovskite solar cells (PSCs). Moreover, the energy difference between HOMO energy level of HTM and valence band of CH3NH3PbI3 (110) surface for SNE is the smallest in three studied molecules indicating the largest Voc of PSCs. Although HTM-CH3NH3PbI3 system is closer to the real environment than isolated HTMs, the adsorbed HTM-CH3NH3PbI3 is rarely considered because of its expensive computational cost and structural complexity. We expected that this work would be helpful to deeply understand the property of HTMs applied in PSCs.

Optical properties and dynamic process in Cd1−xMnxSe quantum dots sensitized solar cells

Overall, the quantum dots sensitized solar cells were successfully prepared with the enhanced short current density from 7.7 mA/cm2 to 19 mA/cm2 based on Mn2+ ions doped on CdSe nanocrystal by successive ionic layer absorption and reaction. There were rapid effects of Mn2+ ions on optical, physical, chemical, and photovoltaic properties of the quantum dots sensitized solar cells. Consequently, the highest efficiency of the quantum dots sensitized solar cells increased dramatically from 1.7% for pure CdSe nanocrystal to 3.8% for Mn2+ ions doped on pure CdSe nanocrystal. Actually, the Mn2+ dopant rise in the conduction band of pure CdSe nanocrystal, reduces recombination, enhances the efficiency of high harvesting, and improves the charge transfer and collection. In addition, the optical properties, the direct optical energy gap, and both conduction band and valence band levels of the compositional CdS, Cd1−xMnxSe were estimated by Tauc model and discussed in details. This Tauc model is useful for us to understand the alignment energy structure of the compositions in electrodes, in particular, the conduction band and valence band levels of CdS, Cd1−xMnxSe nanoparticles. As a result, a rise or a drop of the conduction band and valence band levels when Mn2+ content was changed. Correspondingly, the Electrochemical Impedance Spectra were carried to determine dynamic resistances in QDSSCs.

Understanding the energy level matching relationships between semiconductor photocatalysts and organic pollutants for effective photocatalytic degradations

In this work, representative semiconductors CdS, V2O5, WO3 and P25 (Degussa TiO2 consisting of 80 wt% anatase and 20 wt% rutile) were chosen as the photocatalysts, and the common phenol along with its derivatives (p-nitrophenol, p-chlorophenol and hydroquinone) were designated as the probe pollutants. The energy levels of the frontier molecular orbitals (EHOMO and ELUMO) of organics, and the valence band (VB) and conduction band (CB) energy levels (EVB and ECB) of photocatalysts were obtained through experiments and calculations. By correlating the photocatalytic activities of photocatalysts for the degradations of the organic pollutants under the irradiation of a 300 W xenon lamp with their above-mentioned energy levels, some new findings can be acquired. To be specific, the more positive EHOMO of an organic pollutant leads to its easier photodegradation by photocatalysts. Meanwhile, the more negative VB position for a photocatalyst results in the higher photocatalytic activities for the degradations of organic pollutants. Therefore, the matching correlation between the EHOMO of an organic pollutant and the EVB of a photocatalyst is of great importance to the efficiency of photocatalytic degradation.

Role of random magnetic anisotropy on the valence, magnetocaloric and resistivity properties in a hexagonal Sm2Ni0.87Si2.87 compound

In this work, we report the effect of random magnetic anisotropy (RMA) on the valence, magnetocaloric and resistivity properties in a glassy intermetallic material Sm2Ni0.87Si2.87. On the basis of detailed studies on the valence band and core level electronic structure, we have established that both the Sm3+ and Sm2+ ions are present in the system, suggesting the compound to be of mixed valence in nature. The significant observation of positive magnetic entropy change in zero-field cooled measurement has been argued due to the presence of RMA that develops due to local electronic environmental variations between the rare-earth ions in the system. The quantum interference effect caused by the elastic electron–electron interaction is responsible for the resistivity upturn at low-temperature for this disordered metallic conductor.

Optimization of Source/Drain Schottky Barrier Height for be SOI Mosfet

New devices architectures have been proposed to enable the evolution of semiconductors technology. One possibility is the reconfigurable transistors [1], i.e., transistor that can act as an n-type or as a p-type, which could enable circuits performing more than one function [2]. A new reconfigurable planar device was proposed/fabricated at Laboratorio de Sistemas Integraveis (LSI) from University of Sao Paulo (USP) using SOI substrate [3]. The named BE (Back Enhanced) SOI MOSFET (Patent BR 102015020974-6, 2015) is a kind of undopped junctionless SOI transistor and it shows the benefit of a simpler fabrication process compared to non-planar similar devices. In order to enable the good operation of the BE SOI for both types of transistors, the source/drain Schottky contact electrode perform an essential role. This work investigates the influence of the source/drain electrode metal workfunction on the drain current levels of the n-type and p-type BE SOI MOSFET in order to optimize its behavior. Figure 1 shows a schematic profile of the device. The work principle of the BE SOI MOSFET is based on different back bias (VGB) in each case. For a positive enough VGB, an electron channel is formed at the back interface that connects the source, channel and drain. In this case it is working like the n-type BE SOI MOSFET. This current path can be interrupted by the front gate (VGF). The electrostatic coupling due to VGB and VGF can deplete the channel, thus ceasing the drain current. Analogously, for a negative enough VGB, a hole channel is formed at the back interface and the p-type BE SOI MOSFET is obtained. The drain current characteristics of this transistor were explored in previous work [4]. Schottky source and drain contacts are required in this device. When the current flows, the charges tunnel through the Schottky barriers as indicated in figure 2. Consequently, the Schottky Barrier Height (SBH) influences at the current level. Higher SBH reduces the amount of tunneling charges. Figures 3 and 4 shows the simulated drain current in the BE SOI MOSFET as a function of VGF for metal workfunctions (ΦM) varying between 4.1V and 5.1V. The simulations performed in numerical Synopsys Sentaurus TCAD [5] considered only the dependence of the SBH on the metal workfunction and were calibrated using experimental data. The |VGB| value used is 25V and the drain to source voltage |VDS| is 100mV. The SBH for electrons directly increases with ΦM, therefore the simulated drain current in the n-type BE SOI (figure 3) decreases as ΦM rises. For lower values of ΦM (near conduction band level of the silicon), it was observed higher currents. In these situations the SBH is small, thus most of the electron current is by thermionic emission over the barrier and the contact exhibit an ohmic behavior. An analogous phenomenon is observed in the p-type device. The SBH for holes directly decreases with ΦM, hence the simulated drain current in the p-type BE SOI (figure 4) increases as ΦM reduces. In the same manner, the hole current for higher values of ΦM (near valence band level) is mostly by thermionic emission. In order to evaluate similar current levels for the n-type and p-type transistors, an optimal ΦM value must be selected. Figure 5 shows the comparison of the simulations of both types of transistors for an overdrive voltage VGT=VGF-VT=1V, where VT is the threshold voltage. It is possible to observe that the ratio of the maximum current levels is compatible with the charge mobility difference between electrons and holes. The inset graph shows the details of the region where the current of the n-type and p-type transistors intersect (at ΦM=4.57V). This workfunction value corresponds the BE SOI MOSFET with similar current levels for both types. The parameter that most affects the current level in the n-type and p-type BE SOI MOSFET is the metal workfunction of the drain and source Schottky contacts. Other optimizations can be performed to aim higher currents such the use of UTBB SOI substrate or alternative materials in order to enhance the performance of this device. This work evaluated the difference of the current levels in both types of transistors. Simulations results showed that ΦM=4.57V enables similar SBH, and therefore, similar electron and hole current levels. The simplicity of fabrication and the reconfigurability aspect makes the BE SOI MOSFET an interesting candidate for future applications. Figure 1

Fluoride ion-promoted hydrothermal synthesis of oxygenated g-C3N4 with high photocatalytic activity

Fluoride ion, as a very nucleophilic anion, was introduced into the hydrothermal treatment of g-C3N4 for the synthesis of oxygenated g-C3N4. The existence of F- promotes the hydrothermal oxygenation process by enhancing the hydrolysis and condensation progress of g-C3N4 to form the oxygenated g-C3N4 (CNFs). Plenty of the oxygen-containing groups like CO, CO and NO were formed during the hydrothermal treatment of g-C3N4, accompanying with the stepwise fragmentization of g-C3N4 skeleton, which help to lower the valence band level, enhance the electron-hole separation efficiency under irradiation, and lower the impedance of g-C3N4. Specifically, CNF-6 shows the maximal photoresponse and the highest photocatalytic activity for the degradation of AO7 under visible irradiation.

Influence of O2 plasma treatment on NiOx layer in perovskite solar cells

We fabricated perovskite solar cells (PSCs) with an inverted p–i–n planar structure using a NiOx film as a hole-transporting layer. Since the surface of the NiOx film fabricated by sputtering is hydrophobic, O2 plasma treatment under various conditions was performed to improve its wettability. Water contact angles after the treatment under both normal and weak conditions on the NiOx film reached approximately 15°. After the treatment, the valence band level of the NiOx film was deeper by about 0.15 eV. The maximum efficiency of the NiOx-based device under the optimized O2 plasma condition reached 12.3%.

Visible light-driven photocatalytically active g-C3N4 material for enhanced generation of H2O2

Reduced g-C3N4 material was prepared by a thermal treatment of g-C3N4 in presence of NaBH4 under N2 atmosphere. The prepared catalyst material was characterized by using elemental analyzer, FTIR and XPS and the analysis showed that the reduction treatment created nitrogen vacancies followed by a formation of functional group CN owing to a break-up reaction in the pyridine nitride of a s-triazine-C3N4. The findings of UV–vis DRS and DFT calculation revealed that the formed functional group CN results in a narrowed energy band gap owing to positive shift in the conduction band as well as valence band. The downshift observed in the valence band level made the catalyst material with a feature of visible light-driven water oxidation capacity, that was confirmed by the electron and hole sacrifice and OH trapping-EPR techniques. The intermediate energy level within the band gap of g-C3N4 originated from the vacancies caused an extended absorption, especially to the visible region. The analysis of PL emission spectrum confirmed that the reduction treatment could facilitate the spatial separation of photo-excited electron and hole, and enhance the charge transfer as well. RDE studies showed that the selective production of H2O2 by two-electron reduction of O2 was a predominant reaction step using the reduced g-C3N4. The reduced g-C3N4 prepared at 370 °C exhibited an efficient visible light driven catalytic performance on H2O2 production (170 μmol/L h−1) from pure H2O and O2 at ambient atmosphere in the absence of organic electron donors. The solar-to-H2O2 chemical conversion efficiency and apparent quantum yield approached to ∼0.26%, ∼4.3%, respectively. In addition, the experimental results obtained on recycling of the prepared g-C3N4 evidenced the photocatalytic stability of the material.

First-principles calculation of geometric, electronic structures and optical properties of Lindqvist-type polyoxometalates functionalized carbon nitride

The geometric and electronic structures of nano-composites based on Lindqvist-type polyoxometalates (POMs) and monolayer graphitic carbon nitride (g-C3N4) were investigated by first-principles method. We investigated the most stable adsorption site on the g-C3N4 surface and the electronic structures of Lindqvist-type POMs functionalized g-C3N4, including band structures and density of states (DOS). Based on the conduction band (CB) and valence band (VB) levels of Lindqvist-type POMs, H2W6O19, H2Mo6O19 and H2WnMo6−nO19 (n = 1, 2, 3, 4, 5) are suitable to generate composites with g-C3N4, which are expected for electron and hole transfer between POMs and g-C3N4. The results indicate that the band gap of g-C3N4 composing with Lindqvist POMs obviously decreases comparing to pristine g-C3N4 and the charge transfer is from g-C3N4 to POMs which could enhance the photocatalytic ability. Moreover, the absorption strength of W6O19/g-C3N4 obviously increases in visible light comparing with g-C3N4.

Ab-initio study of (Ga,Cr)N and (Ga,Mn)N DMSs: under hydrostatic pressure

The influence of hydrostatic pressure between 0–100 GPa on structural, electronic and magnetic properties of CrxGa1−xN and MnxGa1−xN (x = 0.25) diluted magnetic semiconductors has been studied. The calculations have been performed using DFT as implemented in code SIESTA. LDA + U as exchange-correlation (XC) potential have been used to study the parameters. Under external pressure, shifting in both valence band and conduction band energy levels from their actual positions has been observed, which lead to modification of electronic properties. Also, N0α, s-d exchange constant and p-d exchange constants, N0β have been calculated at different pressures. Both the compounds show half metallic nature at studied pressure range.

Silicon Oxidation by NAPPES: From Dangling Bonds to Oxygen Islands to 2D SiOx Layer to the Onset of Bulk SiO2 Formation

Valence band and core level photoelectron spectral measurements at near-ambient pressures (NAP; up to 0.5 mbar) were made in the presence of molecular oxygen to explore the various oxidation stages of silicon surfaces. Dangling bonds feature observed on clean Si-surfaces in the valence band at ultrahigh vacuum decreases dramatically due to oxygen adsorption between ambient temperature and up to 400 K at 0.1 mbar of O2 pressure. The adsorption of oxygen on dangling bonds appears to be localized as islands; this reflects in the surface heterogeneous character and also responsible for the broadening in the oxygen gas phase vibrational features. This is further supported by an increase in the work function and can be correlated to the presence of Hofer (molecular) precursor. When the temperature was increased to 500 K, molecular precursor species dissociates to form – Si═O species. This is fully supported by the change in the Si work function as well as from the observation of oxidized Si-species from Si 2p c...

Balancing Catalytic Activity and Interface Energetics of Electrocatalyst-Coated Photoanodes for Photoelectrochemical Water Splitting.

For photoelectrochemical (PEC) water splitting, the interface interactions among semiconductors, electrocatalysts, and electrolytes affect the charge separation and catalysis in turn. Here, through the changing of the bath temperature, Co-based oxygen evolution catalysts (OEC) with different crystallinities were electrochemically deposited on Ti-doped Fe2O3 (Ti-Fe2O3) photoanodes. We found: (1) the OEC with low crystallinity is highly ion-permeable, decreasing the interactions between OEC and photoanode due to the intimate interaction between semiconductor and electrolyte; (2) the OEC with high crystallinity is nearly ion-impermeable, is beneficial to form a constant buried junction with semiconductor, and exhibits the low OEC catalytic activity; and (3) the OEC with moderate crystallinity is partially electrolyte-screened, thus contributing to the formation of ideal band bending underneath surface of semiconductor for charge separation and the highly electrocatalytic activity of OEC for lowering over-potentials of water oxidation. Our results demonstrate that to balance the water oxidation activity of OEC and OEC-semiconductor interface energetics is crucial for highly efficient solar energy conversion; in particular, the energy transducer is a semiconductor with a shallow or moderate valence-band level.