Top-Down Fabrication of III-V Semiconductor Nanopillars for Photoelectrochemical Water Splitting Applications

Abstract

Research into hydrogen generation via photoelectrochemical (PEC) water splitting is ever growing owing to its potential to generate clean and portable form of energy. In PEC water splitting, a semiconductor absorbs sunlight and splits water into hydrogen and oxygen to produce hydrogen fuel.1, 2 However, the lack of a single robust semiconductor material with a narrow bandgap that straddles both water redox potentials is the main barrier to develop practical PEC water splitting systems. III-V semiconductors can make ideal materials for PEC water splitting due to their outstanding optoelectronic properties including tunable band gaps to cover the entire solar spectrum, band edges straddling water redox potentials, high absorption coefficients and high crystalline quality. In addition, III-V photoelectrodes based on one-dimensional nanostructures are shown to greatly enhance the PEC performance from improved light absorption, increased semiconductor/electrolyte interface area and reduced carrier diffusion length over their planar counterparts. In this talk, I will introduce cost-effective and scalable fabrication of random semiconductor nanopillars (NPs) with controlled dimensions developed in our group using ICP etching and self-assembled random mask techniques for PEC water splitting (as illustrated in Figure 1). I will also present our results on the investigation of GaN, InGaN quantum wells and InP nanopillars (NPs), fabricated using top-down method, as photoelectrodes for PEC water splitting application. The band gap tunability with varying In content and high chemical stability makes InxGa1-xN a superior candidate for photoelectrodes. The PEC performance of random GaN NPs photoanodes was tested in 1 M NaOH under simulated one sun illumination. NPs generated the highest ever reported photocurrent density for GaN photoanodes due to exceptional absorption by the NPs, and increased depletion layer area and semiconductor/electrolyte interface area. Moreover, the PEC performance of NPs was found to be strongly influenced by carrier concentration and NP dimensions.3 We further engineered the band gap of GaN NPs by incorporating InGaN/GaN multiple quantum wells (MQWs) to further improve the PEC performance of GaN NPs. Photoluminescence and diffuse reflectance measurements confirmed that the introduction of InGaN/GaN MQWs into GaN extended the optical absorption by the NPs into the visible part of the solar spectrum, thereby contributing to the substantial improvement in photocurrent density for InGaN/GaN MQW NPs.4 Furthermore, GaN NP photoanodes were found to exhibit improved photostability after being decorated with co-catalysts such as Co3O4. The accumulation of photogenerated charge carriers at semiconductor surface trigger the self-oxidation of GaN photoanode during water oxidation reaction and results in photocorossion of GaN photoanodes. Deposition of co-catalyst significantly reduced the self-oxidation of GaN photoanode by rapidly extracting the photogenerated holes out of semiconductor that participate in water oxidation reaction.5 We also developed InP NPs using the random mask technique via top-down approach, removed plasma surface damage using wet treatment in sulphur dissolved oleylamine (S-OA) solution and investigated the PEC performance of InP NP photocathodes. We achieved stable and excellent PEC performance for NP photocathodes without any oxide protection layer after wet treatment in S-OA. The NPs exhibited saturation photocurrent density of ~34 mA/cm2, which is close to theoretical limit and power-saved cathodic efficiency of over 5%. The saturation photocurrent density and power-saved cathodic efficiency of NPs improved by 60% and 33%, respectively, compared to their counterpart planar photocathodes. We also carried out the time-resolved photoluminescence, optical and impedance spectroscopy characterisations to elucidate the PEC performance of InP photocathodes. References Fujishima, K. Honda, “Electrochemical Photolysis of Water at a Semiconductor Electrode”, Nature 1972, 238, 37–38.Walter, E. Warren, J. McKone, S. Boettcher, Q. Mi, E. Santori and N. S. Lewis, “Solar Water Splitting Cells”, Chemical Society Reviews 2010, 110, 6446–6473.Parvathala Reddy Narangari, Siva Krishna Karuturi, Mykhaylo Lysevych, Hark Hoe Tan, and Chennupati Jagadish, “Improved Photoelectrochemical Performance of GaN Nanopillar Photoanodes” Nanotechnology 2017, 28, 154001.Joshua Butson, Parvathala Reddy Narangari, Siva Krishna Karuturi, Rowena Yew, Mykhaylo Lysevych, Hark Hoe Tan, and Chennupati Jagadish, “Photoelectrochemical Studies of InGaN/GaN MQW Photoanodes”, Nanotechnology 2018, 29 045403.Guanyu Liu and Siva Krishna Karuturi, Alexandr N. Simonov, Monika Fekete, Hongjun Chen, Noushin Nasiri, Nhien H. Le, N Parvathala Reddy, Mykhaylo Lysevych, Thomas R. Gengenbach, Adrian Lowe, Hark Hoe Tan, Chennupati Jagadish, Leone Spicciac and Antonio Tricoli, “Robust Sub-Monolayers of Co3O4 Nano-Islands: a Highly Transparent Morphology for Efficient Water Oxidation Catalysis”, Advanced Energy Materials 2016, 6, 1600697. Figure 1