p-CuInS2 /n-Polymer Semiconductor Heterojunction for Photoelectrochemical Hydrogen Evolution

Enhancing Photocathodic Performances of Particulate-CuGaS2-Based Photoelectrodes via Conjugation with Conductive Organic Polymers for Efficient Solar-Driven Hydrogen Production and CO2 Reduction

Modification with conductive organic polymers consisting of a thiophane- or pyrrole-based backbone improved the cathodic photocurrent of a particulate-CuGaS2-based photoelectrode under simulated solar light. Among these polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) was the most effective in the improvements, providing a photocurrent 670 times as high as that of the bare photocathode. An incident-photon-to-current efficiency (IPCE) for water reduction to form H2 under monochromatic light irradiation (450 nm at 0 V vs RHE) was ca. 11%. The most important point is that modification of the conductive organic polymers does not involve any vacuum processes. This importance lies in the use of an electrochemically oxidative polymerization, not in a physical process such as vapor deposition of metal conductors. This is expected to be advantageous in the large-scale application of photocathodes consisting of particulate photocatalyst materials toward industrial solar-hydrogen production using photoelectrochemical-cell-based devices. Artificial photosynthesis of water splitting and CO2 reduction under simulated solar light was demonstrated by combining the PEDOT-modified CuGaS2 photocathode with a CoOx-loaded BiVO4 photoanode. Furthermore, how the cathodic photocurrent of the particulate-CuGaS2-based photocathode was drastically improved by the modification was clarified based on various characterizations and control experiments as follows: (1) selectively filling cavities between the particulate CuGaS2 photocatalysts and a conductive substrate (FTO; fluorine-doped tin oxide) with the polymers and (2) using a large driving force for carrier transportation governed by the polymers' redox potentials adjusted by functional groups.

Synthesis and hybridization of CuInS2 nanocrystals for emerging applications

Copper indium sulfide (CuInS2) is a ternary A(I)B(III)X(VI)2-type semiconductor featuring a direct bandgap with a high absorption coefficient. In attempts to explore their practical applications, nanoscale CuInS2 has been synthesized with crystal sizes down to the quantum confinement regime. The merits of CuInS2 nanocrystals (NCs) include wide emission tunability, a large Stokes shift, long decay time, and eco-friendliness, making them promising candidates in photoelectronics and photovoltaics. Over the past two decades, advances in wet-chemistry synthesis have achieved rational control over cation-anion reactivity during the preparation of colloidal CuInS2 NCs and post-synthesis cation exchange. The precise nano-synthesis coupled with a series of hybridization strategies has given birth to a library of CuInS2 NCs with highly customizable photophysical properties. This review article focuses on the recent development of CuInS2 NCs enabled by advanced synthetic and hybridization techniques. We show that the state-of-the-art CuInS2 NCs play significant roles in optoelectronic and biomedical applications.

Recent Advances in CuInS2-Based Photocathodes for Photoelectrochemical H2 Evolution

Photoelectrochemical (PEC) H₂ production from water using solar energy is an ideal and environmentally friendly process. CuInS₂ is a p-type semiconductor that offers many advantages for PEC H₂ production. Therefore, this review summarizes studies on CuInS₂-based PEC cells designed for H₂ production. The theoretical background of PEC H₂ evolution and properties of the CuInS₂ semiconductor are initially explored. Subsequently, certain important strategies that have been executed to improve the activity and charge-separation characteristics of CuInS₂ photoelectrodes are examined; these include CuInS₂ synthesis methods, nanostructure development, heterojunction construction, and cocatalyst design. This review helps enhance the understanding of state-of-the-art CuInS₂-based photocathodes to enable the development of superior equivalents for efficient PEC H₂ production.

Optimization of PEC and photocathodic protection performance of TiO2/CuInS2heterojunction photoanodes

The establishment of heterojunction is a powerful strategy to enhance the photoresponse performance of photoanode. Here, TiO₂/CuInS₂(T/CIS) composites were prepared via a two-step hydrothermal method, and their morphologies were controlled by adjusting the reaction time. The absorption spectra show that CuInS₂can significantly improve the absorption of visible light. The T/CIS2 (2 h reaction time) photoanode exhibited the most outstanding photoelectrochemical (PEC) performance, with a photocurrent density of 168% that of the pure TiO₂photoanode. Under simulated sunlight, this photoanode can supply a protective photocurrent of 0.49 mA cm^-2and a protective voltage of 0.36 V to stainless steel (304ss), which are about 4 and 2 times those of the TiO₂sample. The enhancement in the photocathodic protection performance is attributed to enlarged visible light absorbance and higher charge separation rate. This study demonstrates that the TiO₂/CuInS₂photoanode is a promising candidate for application in photoinduced cathodic protection of metallic materials.

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

CuInS2 Photocathodes with Atomic Gradation-Controlled (Ta,Mo)x(O,S)y Passivation Layers for Efficient Photoelectrochemical H2 Production

An atomic gradient passivation layer, (Ta,Mo)_x(O,S)_y, is designed to improve the charge transportation and photoelectrochemical activity of CuInS₂-based photoelectrodes. We found that Mo spontaneously diffused to the a-TaO_x layer during e-beam evaporation. This result indicates that the gradient profile of MoO_x/TaO_x is formed in the sublayer of (Ta,Mo)_x(O,S)_y. To understand the atomic-gradation effects of the (Ta,Mo)_x(O,S)_y passive layer, the composition and (photo)electrochemical properties have been characterized in detail. When this atomic gradient-passive layer is applied to CuInS₂-based photocathodes, promising photocurrent and onset potential are seen without using Pt cocatalysts. This is one of the highest activities among reported CuInS₂ photocathodes, which are not combined with noble metal cocatalysts. Excellent photoelectrochemical activity of the photoelectrode can be mainly achieved by (1) the electron transient time improved due to the conductive Mo-incorporated TaO_x layer and (2) the boosted electrocatalytic activity by Mo_x(O,S)_y formation.