Facile fabrication of silicon nanowires as photocathode for visible-light induced photoelectrochemical water splitting

We report the fabrication of one dimensional Silicon nanowires (Si NWs) using p-Si (100) substrate through facile two step metal assisted chemical etching (MACE) approach. The evolution of structural and optical properties of Si NWs by etching Si substrate was studied as a function of hydrogen peroxide (H₂O₂), a strong oxidation agent. The length of the NWs increased linearly with the H₂O₂ concentrations and reached maximum of 51 μm for etching of 60 min. The merits of metal free Si NWs as photocathode in the photoelectrochemical (PEC) neutral water splitting under the visible light was investigated. The performance of the photocathode highly depends on the morphology of Si nanostructure. A high density and well separated Si NWs fabricated by 0.6 M of H₂O₂ results in maximum photocurrent density of 6 mA cm^?2 with applied bias photocurrent conversion (ABPE) efficiency of 1.1% under visible light illumination.     

Photocatalytic hydrogen generation by splitting of water from electrospun hybrid nanostructures

MWCNT-TiO₂ hybrid nanostructures are prepared using sol–gel and electrospinning followed by post annealing of as-spun nanofibers at 450 °C per 1 h in air. These hybrid nanostructures composed of MWCNTs varied from 0 to 20% (w/w) and are characterized by SEM, TEM, XRD, and FT-IR analysis. MWCNT-TiO₂ hybrid structures are utilized in commercially available Methylene blue (MB) dye degradation and found that 2% of MWCNT exhibit superior kinetic constant 6.379 × 10⁻³ min⁻¹ extracted. In addition, we demonstrate that the doping of MWCTs within TiO₂ leads to a significant enhancement of the UV–vis light assisted photocatalytic activity is optimized in comparison with higher (5, 10 and 20%) compositions. UV–vis assisted photocatalytic hydrogen is evolved by photoelectrolytic splitting of water by using MWCNT-TiO₂ hybrid nanostructures as electrode.     

Solar water splitting for hydrogen production with monolithic reactors

The present work proposes the exploitation of solar energy for the dissociation of water and production of hydrogen via an integrated thermo-chemical reactor/receiver system. The basic idea is the use of multi-channelled honeycomb ceramic supports coated with active redox reagent powders, in a configuration similar to that encountered in automobile exhaust catalytic aftertreatment.Iron-oxide-based redox materials were synthesized, capable to operate under a complete redox cycle: they could take oxygen from water producing pure hydrogen at reasonably low temperatures (800 °C) and could be regenerated at temperatures below 1300 °C. Ceramic honeycombs capable of achieving temperatures in that range when heated by concentrated solar radiation were manufactured and incorporated in a dedicated solar receiver/reactor. The operating conditions of the solar reactor were optimised to achieve adjustable, uniform temperatures up to 1300 °C throughout the honeycomb, making thus feasible the operation of the complete cycle by a single solar energy converter.     

Surface modification of the p-type Cu2ZnSnS4 photocathode with n-type zinc oxide nanorods for photo-driven salt water splitting

The sulfurization of co-sputtering Cu–Zn–Sn metal precursors was employed to prepare the quaternary copper-zinc-tin-sulphide (Cu₂ZnSnS₄, CZTS) photocathodes on substrates. Influence of [Zn]/[Sn] ratios in CZTS photocathodes on their phases, morphologies, and the efficiencies of photo-driven salt-water splitting was examined. Pristine p-type CZTS photocathodes showed the highest photo-driven performance of 0.61 mA cm^?2 in an electrolyte containing 1 M sodium chloride with the external bias kept at ?1.0 V vs. Ag/AgCl. An n-type zinc oxide (ZnO) nanorod arrays layer was then coated on the CZTS photocathode to improve its photo-driven salt-water splitting performance. The CZTS/ZnO photoelectrode had the best photo-driven performance of 1.87 mA cm^?2 in the 1 M NaCl solution under illumination with the external bias set at ?1.0 V vs. Ag/AgCl. From results of electrochemical impedance spectra measurements for the samples in the electrolyte, the CZTS/ZnO sample had good photo-driven salt-water splitting performance due to its lowest charge transfer resistance and p-n junction formed at the sample. Intensity modulated photocurrent spectroscopy and electrochemical impedance spectra results of samples indicated that the surface states at the CZTS/ZnO interface were the recombination centers with the electrons from the CZTS sample and holes from the ZnO and therefore improved its photo-driven salt-water splitting performance.     

A model of photon-induced self-driven electrochemical cell for water splitting to hydrogen

An equation for cell current in a self-driven photon-induced electrochemical cell, having both electrodes as semiconducting photoelectrodes, has been derived and applied to water splitting to hydrogen. The cell current and the cell potential depend on various semiconductor properties and the properties of the ions in solution. The computed dependence of cell current and potential for specific combinations of electrodes, e.g. nSrTiO₃/p-GaP and nTiO₂/p-GaP, show the same trends as the experimental observation. Further calculations suggest that it should be possible to attain an efficiency of conversion of light up to 18% for water splitting to hydrogen with p-InP (Pt-electrocatalyst)/n-Si (electrocatalyst) and up to 17% with p-Si (Pt)/n-InP(c) using appropriate electrocatalyst on the nSi and on the nInP electrodes.     

Cu2O as an emerging photocathode for solar water splitting - A status review

Recently, cuprous oxide (Cu₂O) based photocathodes have gained research attention for hydrogen (H₂) production through photoelectrochemical (PEC) water splitting reactions due to marginally lower synthesis cost and low energy intensity fabrication processes. Unique properties of Cu₂O, such as tunable bandgap, appropriate band edge potentials with water redox levels and non-toxic nature makes it beneficial for PEC applications. Cuprite is mainly studied under visible light to facilitate enhanced H₂ gas production upon illumination. However, notoriously photocorrosion degrades the PEC performance and restricts the photoactivity of Cu₂O. Moreover, because of the redox potentials lies within the band gap of Cu₂O; self-photocorrosion or self-oxidation upon illumination is unavoidable. Improvement in the Cu₂O photocathodes was achieved by finding elegant solutions such as forming thin heterojunction layers by atomic layer deposition (ALD) as well other methods, co-catalyst deposition, tuning crystal facets and surface modifications with different synthetic methods. In this review, we discuss the improvements in Cu₂O photocathodes achieved over the years for enhanced H₂ production with recently studied photocathodes.     

Zn-doped hematite modified by graphene-like WS2: A p-type semiconductor hybrid photocathode for water splitting to produce hydrogen

A p-type Zn-doped hematite (α-Fe₂O₃(Zn)) in spindle-shape with an acceptor density of ca. 4.21 × 10¹⁸ cm^?3 were synthesized by a facile hydrothermal method. After α-Fe₂O₃(Zn) was modified with graphene-like WS₂ (α-Fe₂O₃(Zn)/WS₂), the photoelectrochemical performances of the composite can be further enhanced. A photocell composed of the p-type α-Fe₂O₃(Zn)/WS₂ nanocomposite as photocathode and n-type α-Fe₂O₃ as photoanode was assembled to estimate the photocatalytic activity of α-Fe₂O₃(Zn)/WS₂. The amount of the hydrogen and oxygen produced from this tandem cell with the optimal electrodes under 2 h simulated solar light irradiation is 12.5 μmol and 4.3 μmol, respectively.     

Graphene and silicene nanodomains in a ultra-thin SiC layer for water splitting and hydrogen storage. A first principle study

First-principles calculations within the density functional theory (DFT) have been addressed to investigate the energetic stability, electronic and optical properties of graphene and silicene nanodomains in a SiC single layer (h-SiC). We observe that graphene domains form a planar structure and give rise to an occupied and an empty electronic levels inside the h-SiC band gap, leading the h-SiC to present a strong optical absorption peak in the visible region. On the other hand, when a silicene nanodomain is present the system is no longer planar and present a corrugated structure similar to the silicene structure. The silicene nanodomain introduce three empty electronic levels within the band gap, leading the h-SiC with optical absorption in the visible region. These results show that a graphene nanodomain in h-SiC is appropriate for optical devices, while silicene nanodomains form almost sp³ quantum dots. This finding suggest that the graphene and silicene nanodomains in a SiC single layer increase the possibility to use h-SiC to produce new electronic and optical devices as well for energy storage by hydrogen adsorption. In fact, we study the H₂ and O₂ adsorption on the pristine system and on the nanodomains, we observe that the presence of the nanodomais increase the binding energies of the adsorbed molecules.     

Nanostructured materials for the visible-light driven hydrogen evolution by water splitting: A review

The conversion of abundantly available photonic energy into useful chemical energy is considered to be a greener protocol for addressing the energy shortage. Recently, since most of the emphasis has been centralized on the semiconductor-based photocatalysis; the designing and fabrication of the novel semiconductor photocatalytic material is happening at a blistering rate. Recently, the nanostructured materials have attracted ever-growing research attention as photocatalytic material for hydrogen generation reaction by dissociation of water. Such photocatalytic nanomaterials are known to exhibit superior activity than their corresponding bulk counter-parts because of the improved interfacial charge separation and the broad surface area providing sufficient active sites. However, the improvement in the efficiency and selectivity towards hydrogen production reaction under solar or visible light radiation always remains a challenging assignment. In the present review, the segregation of the so far reported nanostructured photocatalysts into different categories, based on their dimensionality such as 0-D, 1-D and 2-D materials, is implemented. Furthermore, their synthetic route and the photocatalytic hydrogen evolving efficiencies are explored and briefly summarized. Moreover, the methodology of development of nanocomposite materials leading to the construction of heterojunctions including Type-I, Type-II, Type-III, Z-Scheme and S-Scheme system is also discussed. In addition, an in-depth investigation on the charge carrier's generation, separation and their transportation is also reviewed. Finally, the future perspectives regarding the designing of an efficient, stable and economic photoactive nano-architecture material for the efficient hydrogen production via photocatalytic dissociation of water are also pointed.     

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe₃O₄/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe_(1−y)O at 575 °C.     

Spectral beam splitting technology for increased conversion efficiency in solar concentrating systems: a review

Solar concentrating systems that employ one or more quantum receivers may realize improved energy utilization and higher electric conversion efficiency by incorporating spectral beam splitting technology. Such techniques were investigated in thermophotovoltaic conversion, introduced in the early 1960s, and in concentrating PV devices using cells of different band-gap materials, proposed as early as 1955. One major application was found in systems combining quantum and thermal receivers. This article presents a review of the various solar hybrid beam splitting systems proposed in the literature and the different spectrum splitting strategies employed.     

Cu2O nanowires based p-n homojunction photocathode for improved current density and hydrogen generation through solar-water splitting

Hydrogen generation through solar-water splitting is expected to address the global energy crisis by providing a source for a safer and sustainable alternative fuel. Herein, we report a facile synthesis of Cu₂O nanowires and show that the magnetic field could influence the nanowires’ distribution and alignment. Orientation of nanowires was observed to become more inclined towards the magnetic field lines as the values of full-width at half maximum decreased from 140° to 46.2° with the increase in the field strength. Crystallographic, morphological, optoelectronic, and photoelectrochemical properties of the constructed p-n homojunction were analyzed by using different characterization techniques. A high built-in potential of +0.93 V vs. RHE was observed for a 50 nm layer of n-Cu₂O over p-Cu₂O nanowires that resulted in a significantly high photocurrent density of −7.42 mA/cm². The stability in the photoelectrochemical medium was maintained for 14 h, generating 20 mmol/cm² of H₂.     

ZnBi38O60/Bi2O3 photocathode for hydrogen production from water splitting

Hydrogen production from water splitting into photoelectrochemical cells is a promising alternative for reducing the use of fossil fuels. Here, we synthesize by spray pyrolysis a porous ZnBi₃₈O₆₀/γ-Bi₂O₃ film with a surface area of 744 m² g⁻¹ for use as a photocathode in water-splitting cells. The film of ZnBi₃₈O₆₀ with 3 wt% Bi₂O₃ has 2.3 eV bandgap energy and a conduction band energy of −2.14 V vs. RHE at pH 6.99, which is thermodynamically suitable for reducing H⁺ to H₂. Under illumination, the film produces a current density of −1.55 mA cm⁻² at 0 V vs. RHE with an onset potential of 0.84 V vs. RHE. HC-STH efficiency is 0.09% at 0.17 V vs. RHE and IPCE at 0 V vs. RHE is 3.8% at 480 nm. Under continuous operation, the ZnBi₃₈O₆₀/γ-Bi₂O₃ film shows a stable photocurrent of −0.4 mA cm⁻² at 0 V vs. RHE for 1800 s with 100% Faradaic efficiency.     

Cadmium sulfide Co-catalyst reveals the crystallinity impact of nickel oxide photocathode in photoelectrochemical water splitting

Nickel oxide (NiO) with p-type semiconducting behaviour was prepared via a direct anodisation of nickel (Ni) foam followed by calcination treatment. This method offers a direct photoelectrode synthesis without the intermediate step using a pre-synthesised NiO powder. NiO photocathodes with modulated crystallinity were prepared under elevated calcination temperatures. The beneficial effect of having higher crystallinity in generating higher cathodic photocurrent became obvious in the aid of cadmium sulfide (CdS) deposition. It was found that CdS can promote the excited charge transportation of NiO towards water reduction, thus revealing the effect of NiO crystallinity modulation. The role of CdS as co-catalyst rather than a photosensitiser can be useful in the future design of photoelectrodes.     

Electrodeposited Ni-Co-Sn alloy as a highly efficient electrocatalyst for water splitting

Water splitting to produce hydrogen and oxygen is considered as a feasible solution to solve the current energy crisis. It is highly desirable to develop inexpensive and efficient electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this paper, nanostructured Ni-Co-Sn alloys were electrodeposited on copper foil and the excellent electrocatalytic performances for both HER and OER in alkaline media were achieved. The optimized Ni-Co-Sn electrode shows a low onset overpotential of −18 mV and a small Tafel slope of 63 mV/dec for the HER, comparable to many state-of-the-art non-precious metal HER catalysts. For the OER, it produces an overpotential of 270 mV (1.50 V vs. RHE) at current density of 10 mA/cm², which is better than that of the commercial Ir/C catalyst. In addition to high electrocatalytic activities, it exhibits good stability for both HER and OER. This is the first report that Ni-Co-Sn is served as a cost-effective and highly efficient bifunctional catalyst for water splitting and it will be of great practical value.     

Nanoporous NiFeMoP alloy as a bifunctional catalyst for overall water splitting

High performance bifunctional catalysts for water splitting are very promising. Transition metal phosphides as catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have attracted considerable attention due to their high performance. However, the catalyst with excellent properties still remains a significant challenge. Herein, the nanoporous NiFeMoP(np-NiFeMoP) ribbon was prepared by quenching and dealloying method. It was found that np-NiFeMoP showed excellent HER and OER performance in 1 M KOH. The overpotential of OER is as low as 197 mV at a current density of 20 mA·cm⁻². When the current density is 10 mA·cm⁻², the overpotential of HER is 223 mV. Moreover, np-NiFeMoP only needs a cell voltage of 1.41 V when current density is 10 mA·cm⁻² for water splitting. Our current work may provide some new insights on rationally constructing nanoporous structure with rich active sites to boost the catalytic performances for overall water splitting.     

Pulsed sonoelectrodeposition of leaf-like Bi2WO6 film with exposed [001] facet as a photocathode with a remarkable high photovoltage in water splitting

Bi₂WO₆ is one of the promising triplet bismuth compound that has a layered structure with a high photocatalytic activity for photo-electrochemical (PEC) water splitting systems. Here, Bi₂WO₆ synthesizes by the sonoelectrochemical method in pulse mode of ultrasound. Unexpectedly, synthetic samples show photocathode rather than photoanode activity in PEC systems. Applying of this method creates a leaf-like morphology with exposed [001] crystal facet and controllable amount of surface defects. The co-existence of oxygen and metal vacancies play a significant role in suppressing charge recombination and enhancing charge transporting in photoelectrodes. The creation of high surface vacancies leads to change the conduction and valence band positions and cause hydrogen evolution by Bi₂WO₆ photoelectrodes. Another surprising result for the synthesized film by pulse mode is the creation of high photovoltage about 1.25 V that has a remarkable effect in suppressing charge recombination rate and proposed driving force for water reduction. Furthermore, the onset potential of the photoelectrodes improves and records high efficiencies (ABPE = 2.46% at −0.8 V and IPCE≈28% at 450 nm). The obtaining results introduces the sonoelectrochemical method as a promising method for the synthesis of highly efficient photoelectrodes.     

Epitaxial n- and p-type Emitters for High Efficiency Solar Cell Concepts

Reducing the module prices by increasing the efficiency of solar cells is one of the major challenges in today＇s photovoltaic research. The emitter formation by epitaxial growth offers a cost-efficient and faster alternative to the standard furnace diffusion process. The efficiency potential of epitaxial emitters 〉 22% has already been proven using a single wafer, low pressure, chemical vapour deposition tool. The purpose of this work is to show the potential of epitaxially grown emitters by APCVD （atmospheric pressure chemical vapour deposition） compared to diffused emitters. The APCVD formation of epitaxial emitters at 1,050 ~C can be realised as high throughput inline process and only takes 1-2 min, whereas the diffusion process using POCI3 takes up to 60 min. Simulations show an increase in voltage of AVoc = ＋10 mV and a reduction in saturation current ,1o of 30% for the epitaxial emitter. The lifetime experiments of solar cells with epitaxial emitter exhibit a diffusion length Leff〉 750μm and an emitter saturation current of Joe 〈 50 fA/cm2 on a planar 10 Ω2cm p-type FZ wafer. Another important aim of this work is to evaluate the limitations of epitaxial emitters due to high thermal budget, interface recombination and the change of reflective properties on textured wafers due to the deposition process. Solar cell efficiencies up to 18.4% on p-type and 20.0% on n-type wafers presented in this paper underline that the emitter epitaxy by APCVD is a competitive process for the emitter formation.     

Role of heterocyclic dye (Azur A) as a photosensitizer in photogalvanic cell for solar energy conversion and storage: NaLS–ascorbic acid system

Photogalvanic effect was studied in a photogalvanic cell containing NaLS–ascorbic acid and Azur A as a surfactant, reductant and photosensitizer, respectively. The photopotential and photocurrent generated by this system were 770 mV and 160 μA, respectively. The effect of different parameters like pH, diffusion length, electrode area, light intensity, temperature, etc. on the electrical output of the cell were observed, current–voltage characteristics of the cell have also been studied and mechanism has been proposed for the generation of photocurrent in photogalvanic cell. The observed conversion efficiency and storage capacity for this system were 0.5461 and 110.0 min, respectively.     

Carbon-coated cobalt molybdenum oxide as a high-performance electrocatalyst for hydrogen evolution reaction

Synthesis of high-performance and cost-effective catalysts towards the hydrogen evolution reaction (HER) is critical in developing electrochemical water-splitting as a viable energy conversion technique. For non-precious metal Co- and Ni-based catalysts, hydroxides were found to form on the surface of the catalysts under alkaline environments and benefit the catalytic performance, whereas there is limited systematic study on the explicit influence of hydroxides on the electrocatalytic mechanism and performance of these catalysts. Herein, we report a close correlation observed between the amount of the surface hydroxides formed and the resulting electrocatalytic performance of a Co-Mo-O nanocatalyst through careful comprehensive structural and property characterizations. We found that an appropriate amount of hydroxide can be moderated by simply coating the catalyst surface with carbon shells to optimize the catalytic properties. As a result, a carbon-coated Co-Mo-O nanocatalyst was successfully developed and is among the best reported non-precious HER catalysts with a superior electrocatalytic activity and outstanding durability for the HER under alkaline environment. First-principles calculations were further conducted to probe the nature of the active sites and the role of hydroxides in the Co-Mo-O@C/NF catalyst towards the HER.