Annealed Polycrystalline TiO₂ Interlayer of the n-Si/TiO₂/Ni Photoanode for Efficient Photoelectrochemical Water Splitting

Abstract

High photovoltage generation from a photoelectrode is important for efficient solar-driven water splitting. Here, we report a thermal treatment process that greatly enhances photovoltage generation from an n-Si/TiO 2 /Ni photoanode. By selectively annealing the TiO 2 interlayer, the photoanode generates a high photovoltage of 570 mV, which is very competitive as compared with photovoltages produced using other similar metal−insulator−semiconductor structures with earth-abundant metal catalysts. Different annealing conditions and junction layer thicknesses were systematically investigated. It is found that the optimal annealing temperature occurs between 500 and 600 °C. Within this temperature range, the deposited amorphous Ti is converted into polycrystalline anatase phase TiO 2 . The optimal annealing time scales linearly with TiO 2 thickness and inversely with annealing temperature. The large photovoltage generation is attributed to the reduced defect states and improved junction barrier height by the annealed TiO 2 interlayer. This study demonstrates that thermal annealing offers an attractive approach to modify the TiO 2 interlayer material's properties for photovoltage optimization.

Design Principles for Efficient and Stable Water Splitting Photoelectrocatalysts

“…The presence of these interfacial defect states will show up as a nonideality in the models discussed above. The common approaches to address these defects include annealing or chemical treatments aimed at removing the defect states. ,, In addition to interfacial defects, nonidealities can result from image force lowering, barrier height inhomogeneity, recombination in the semiconductor depletion region, and field emission . While these nonidealities may be negligible for some MIS systems, including the ones discussed above, removing these sources of nonidealities would require alternative approaches for different systems.…”

“…In these systems, a stabilizing insulator layer is placed between a semiconductor and a metal-based electrocatalyst forming so-called metal–insulator–semiconductor (MIS) device architectures. , One of the first MIS water splitting systems used a TiO 2 insulator layer to stabilize Si . Since this work, many protective insulators have been utilized in Si-based MIS photocatalysts, including Al 2 O 3 , − HfO 2 , − SiO 2 , − SrTiO 3 , TiO 2 , ,− and ZrO 2 . The widespread deployment of these insulators was enabled by atomic layer deposition (ALD), which is a layer-by-layer growth technique to deposit conformal and pinhole-free layers of materials with sub-nanometer precision. ,,,− …”

Low-cost, scalable, highly stable Si-based metal-insulator-semiconductor photoanodes for water oxidation fabricated using thin-film reactions and electrodeposition

“…In PEC cells, semiconductors play a key role in absorbing photons from the light source to create mobile charge carriers. Various semiconductor materials have been studied for the high performance PEC cells, including metal oxides [1][2][3][4] , nitrides 5,6 , Si [7][8][9][10][11][12][13] , -compound semiconductor materials [14][15][16] , and others 17,18 . Among these, Si-based photoelectrodes have attracted substantial interest due to silicon's moderate bandgap (1.12 eV), high charge mobility and diffusion lengths, and well established technological infrastructure 19 .…”

Scalable, highly stable Si-based metal-insulator-semiconductor photoanodes for water oxidation fabricated using thin-film reactions and electrodeposition

“…In PEC cells, semiconductors play a key role in absorbing photons from the light source to create mobile charge carriers. Various semiconductor materials have been studied for the high-performance PEC cells, including metal oxides [1][2][3][4] , nitrides 5,6 , Si [7][8][9][10][11][12][13][14][15][16][17] , III-V compound semiconductor materials [18][19][20] , and others 21,22 . Among these, Si-based photoelectrodes have attracted substantial interest due to silicon's moderate bandgap (1.12 eV), high charge mobility and diffusion lengths, and well-established technological infrastructure 23 .…”

“…An efficient metal catalyst at the surface improves the reaction kinetics of photoanodes, reducing the overpotential for OER 17,[24][25][26] . Corrosion of Si in aqueous electrolytes is suppressed in MIS photoelectrode structures by protecting the Si surface using thin layers of insulators such as TiO 2 13,27,28 , NiO X 9,11,12 , SrTiO 3 8 , SiN X 29 , and SiO X 9-11, [14][15][16] .…”