Tailoring morphological characteristics of zinc oxide using a one-step hydrothermal method for photoelectrochemical water splitting application

In the present paper describe the zinc oxide (ZnO) with various morphologies have been synthesized using the one-step hydrothermal method, in which the growth of ZnO nanostructures are significantly tailored by adjusting the pH level between 9 and 12 using 0.1 M Sodium hydroxide (NaOH). Significant results reveal the morphological properties of ZnO nanostructures varied with different pH values with the formation of ZnO nanostructures have different morphological such as a baton, star, flower, and rod-like structures. The present results show the rod-like structure of ZnO nanostructures exhibits the highest photocurrent density of 746.61 μAcm⁻² (at 1.23 V vs RHE) under simulated solar AM 1.5G illumination in Potassium hydroxide (KOH) medium, also the other morphologies. The dependent of the photoelectrochemical (PEC) water splitting properties on the different morphological of ZnO nanostructures are studied. Achieving the morphological evolution mechanism has become one of the method to obtain the production of the hydrogen growth regime used for solar energy conversion and their applied storage potentials. The application of the ZnO nanostructures for PEC water splitting was proposed with the adoption of screen-printed carbon electrodes (SPCEs). These are to quantify the best degree of the highest photocurrent density with one-step tailoring with an ideal modeling system to enhance PEC water splitting performances. Thus, the screen-printed carbon electrodes (SPEs) has been used as an alternative method for fabrication and photoelectrodes testings.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Photocatalytic hydrogen production performance of 1-D ZnO nanostructures: Role of structural properties

Synthesis of zinc oxide (ZnO) nanowires (NWs) grown via vapor-liquid-solid (VLS) process using Gold (Au) as a catalyst metal on aluminum-doped zinc oxide (AZO) seed layer is reported in the present work. During the growth procedure, the nucleation process helps us to obtain ZnO nanowires with Au on the tip, confirming the VLS growth mechanism. Different morphologies were obtained after the variation in the growth parameters in the VLS process, and further, their role in the photocatalytic performance was studied. Changes in the structural properties of nanowires allowed us to modify the aspect ratio and surface area of the nanostructures. X-ray diffraction (XRD) showed that the principal orientation of the nanowires was (002) in the present case. Scanning electron microscopy (SEM) showed the structural properties of 1-D nanostructures (nanowires), and statistical analysis revealed that the average diameter in the present case was found to be varied from 57 to 85 nm. Scanning transmission electron microscopy (STEM) technique revealed the different elements present on the surface of ZnO NWs. Further, the compositional profile of nanostructures was cross-verified using Energy dispersive Spectroscopy (EDS). Photoluminescence (PL) and UV Visible studies were employed to study the optical properties of nanowires. UV–Vis measurements showed the role of different structural properties of nanowires on the absorption spectra, especially in the visible region. The ZnO nanowires were tested as photocatalysts for hydrogen production from water splitting reaction, and it was found in particular nanowires with random orientation with optimal diameter distribution show the stable and highest photocatalytic performance.     

Seed-layer-free deposition of well-oriented ZnO nanorods thin films by SILAR and their photoelectrochemical studies

Morphological forms of ZnO nanostructures play a vital role in deciding properties such as high internal surface area, efficient light scattering and harvesting, lowest charge transfer resistance, etc. which are important for photoelectrochemical (PEC) performance. Herein successful deposition of well oriented ZnO nanorods thin films over fluorine doped tin oxide (FTO) coated glass substrate is achieved by using simple, soft solution and scalable method known as successive ionic layer adsorption and reaction (SILAR). For the first time a compact ZnO layer over large area is deposited in one step synthesis approach, without any assistance of seed layer, by using hydrazine hydrate as a source of hydroxyl ions. The plausible growth mechanism of the morphological variation (alignment and orientation) happening with increasing SILAR cycles and its consequence on PEC performance are discussed in detail. All ZnO thin films show wurtzite crystal structure, however variations in their texture coefficients were found with SILAR cycles, which turns out to be a major aspect for PEC application. Anodic shift was observed in flat band potential values with increment in number of SILAR cycles. The ZnO thin films deposited for 120 cycles showed preferential orientation along (0002) plane and showed better PEC performance with photocurrent of 0.19 mA/cm² (1 V) and maximum photo conversion efficiency of 0.084% at 0.45 V. On the other hand, film deposited for 60 (photocurrent of 0.11 mA/cm² (1 V); efficiency of 0.055%) and 180 cycles (photocurrent of 0.15 mA/cm² (1 V); efficiency of 0.063%) demonstrated inferior PEC performance.     

Co incorporated Ni3S2 hierarchical nano/micro cactus for electrochemical water splitting

Hierarchical nano/micro Ni₃S₂ cactus like structures were fabricated using cost effective and eco-friendly hydrothermal technique. Both pure and Co:Ni₃S₂ composite nanostructures were grown on nickel foam under different synthesis temperatures. The synthesized nanostructures showed a wide variation of hierarchical morphology with variation of synthesis temperatures. The obtained nanostructures were investigated for electrochemical water splitting performance. The morphologies of the samples were found to acquire higher aspect ratio with increment in synthesis temperature. This morphological tuning led to remarkable enhancement in hydrogen evolution performance with very low overpotential 206 mV at 100 mA cm⁻² in acidic medium. Owing to this superior morphology tuned water splitting performance, this low dimensional Ni₃S₂ system can be recognized as potential leader in electrochemical energy harvesting sector.     

Mixed-metal organic framework-coated ZnO nanowires array for efficient photoelectrochemical water oxidation

Designing of high-performance photoanodes is essential for efficient solar energy conversion in photoelectrochemical (PEC) water splitting. Herein, we report an effective approach to synthesize three dimensional (3D) mixed-metal organic framework-coated ZnO nanowires array (ZnNi MOF@ZnO) for the effective PEC performance. The ZnO nanowires act as photon absorber as well as rapid charge transporter; whilst the ZnNi MOF provides the active sites for PEC process by lowering the energy barrier of water oxidation and suppressing electron-hole recombination. The 3D nanostructure of ZnNi MOF@ZnO nanowires array provides intimate interfacial contact through covalent interactions between the ZnNi MOF and ZnO nanowires which facilitates the rapid charge transfer during photocatalytic oxygen evolution reactions. As a result, the ZnNi MOF@ZnO nanowires array exhibited excellent photoelectrochemical water oxidation with very low onset potential (0.31 V vs. RHE) and high photocurrent density (1.40 mA/cm²) as compared to the Zn MOF @ZnO and ZnO nanowires array. This facile strategy provides a promising direction towards high performance photoanode design for adequate solar energy conversion.     

ZnO@Ni foam photoelectrode modified with heteroatom doped graphitic carbon for enhanced photoelectrochemical water splitting under solar light

In this study, an electrocatalyticaly inactive ZnO@Ni foam photoelectrode was modified with heteroatom doped graphitic carbon to achieve enhanced photoelectrochemical (PEC) water splitting performance. The O, S and N doped graphitic carbon was simultaneously deposited with ZnO on Ni foam substrate under hydrothermal deposition. One dimensional ZnO nanorods with flower-like graphitic carbon on their surface were obtained on the Ni foam substrate, which was directly used as photoelectrode to derive photoelectrochemical water splitting under solar light irradiation. The pristine ZnO@NF exhibit unattractive PEC performance evidenced by the high overpotential required for the oxygen evolution reaction (OER) couple of water splitting reaction (398 mV vs. RHE). The carbon modified ZnO–C@NF photoelectrode lowers the overpotential required to 317 mV. This enhancement was attributed to the carbon modification which serves as both active site and photoelectron reservoir; facilitating the sluggish kinetics of OER couple reaction and promoting separation of photogenerated charge carriers.     

Structural properties and enhanced photoelectrochemical performance of ZnO films decorated with Cu2O nanocubes

Combination of ZnO and Cu₂O semiconductors is remarkable for efficient photovoltaic cells and enhanced photoelectrochemical (PEC) performance due to the high electronic energy band alignment of these materials and their controllable electronic structure at the interface. This study reports on a systematic analysis of the effects of Cu₂O nanocube doping on the structural properties and PEC performance of ZnO films. ZnO samples doped with Cu₂O were prepared by a practical electrochemical method. Characterization of the materials was performed by XRD, Raman, FTIR spectroscopy and electrochemical techniques. The XRD, Raman, FTIR spectroscopy analyses indicated a single phase of ZnO for the lower Cu₂O deposition time, while a secondary phase of Cu₂O evolved for the 5 min deposition time. This study showed that ZnO doped with Cu₂O grown for 3 min had the best PEC performance. ZnO/Cu₂O photoelectrodes are recommended as an attractive, competitive and alternative candidate for advanced PEC sensing and this may be for the extended field of water splitting into oxygen and hydrogen under sunlight.     

Enhanced photoelectrocatalytic hydrogen production performance of porous MoS2/PPy/ZnO film under visible light irradiation

Photoelectrochemical (PEC) water splitting provides a “green” approach for hydrogen production. However, the design and fabrication of high-efficient catalysts are the bottleneck for PEC water splitting owing to the involved thermodynamic and kinetic challenges. Herein, we report a new strategy for constructing a porous MoS₂/PPy/ZnO thin film photocatalyst with large specific surface area and excellent conductivity to achieve photoelectrochemical water splitting under visible light irradiation. Porous PPy/ZnO was synthesized via template-assisted electrodeposition, and MoS₂ was further electrodeposited to construct porous MoS₂/PPy/ZnO thin film photocatalyst. The hydrogen evolution rate of MoS₂/PPy/ZnO exhibits about 3.5-fold increase to 40.22 μmol cm⁻² h⁻¹ under visible light irradiation. The enhancement for photoelectrochemical hydrogen production is not only ascribed to enlarged specific surface area of the porous structure, but also attributed to the synergistic effects of MoS₂ and porous PPy/ZnO, which could dramatically improve its visible light absorption capacity and enhance the separation and transfer of photogenerated charges. Thus, more abundant photogenerated electrons and holes participate in photoelectrochemical process, which significantly enhances its photoelectrochemical hydrogen production performance.     

Defects-free single-crystalline zinc oxide nanostructures for efficient photoelectrochemical solar hydrogen generation

The article reports a novel and highly efficient methodology for the development of surface defects-free zinc oxide (ZnO) nanostructures, which are highly useful for various optoelectronic and electronic devices. Using this approach, we have developed high-quality ZnO nanostructures with comparable physical and chemical properties to high-temperature grown ones. Initially, ZnO nanostructures were developed by low-temperature chemical bath deposition, and the surface defects passivated structures were obtained by atomic layer deposition of homo-molecular clusters, i.e., Zn and O atomic layers. The surface passivated ZnO nanostructures exhibited excellent chemical stoichiometry between their constituents with enhanced crystalline quality. These nanostructures also showed improved light transmittance in the wavelengths range of 450–1000 nm, and light emission in the ultraviolet region. Further, the surface passivated nanostructures exhibited remarkable device performance as photoanodes with a greatly improved photocurrent density, more than 3 times, and reduced cathodic current of 6.17 × 10⁻⁷ A@-0.4 V. Significantly, the light-to-dark current ratio of the PEC devices fabricated with passivated ZnO nanostructures is found to be 1761.     

PEC electrode of ZnO nanorods sensitized by CdS with different size and its photoelectric properties

The gradient PEC electrodes of ZnO nanorods sensitized by CdS with different size were fabricated via successive ionic layer adsorption and reaction (SILAR) method and applied in photochemical water splitting. The concentrations of reaction solution and SILAR cycles were investigated in the synthetic process and the working mechanism of the gradient PEC electrode was suggested. The results showed that the hydrogen generation efficiency of 4.88% was achieved for the ZnO/CdS gradient PEC electrode constructed by decreasing of the CdS quantum dots size on ZnO nanorods due to the improved absorption and appropriate energy gap structure, which was confirmed by enhanced absorption spectrum. The expected products have potential application in photoelectrochemical water splitting.     

Nanomaterials for photoelectrochemical water splitting – review

Photoelectrochemical (PEC) water splitting using nanomaterials is one of the promising techniques to generate hydrogen in an easier, cheaper and sustainable way. By modifying a photocatalyst with a suitable band width material can improve the overall solar-to-hydrogen (STH) energy conversion efficiency. Nanomaterials can tune their band width by controlling its size and morphology. In many studies, the importance of nanostructured materials, their morphological and crystalline effects in water splitting is highlighted. Charge separation and transportation is the major concern in PEC water splitting. Nanomaterials are having high surface to volume ratio which facilitates charge separation and suppress electron-hole pair recombination. This review focuses on the recent developments in water splitting techniques using PEC based nanomaterials as well as different strategies to improve hydrogen evolution.     

Hierarchically self-assembled ZnO architectures: Establishing light trapping networks for effective photoelectrochemical water splitting

Here we develop photoanodes based on hierarchical zinc oxide (ZnO) nanostructures such as vertically aligned nanorods (NR), nanorods interconnected by thin nanosheets (NR@TN) and nanorods interconnected by dense nanosheets (NR@DN). The morphological variations were successfully controlled by secondary growth time and the plausible formation mechanisms of these hierarchical ZnO architectures were explained based on the experiment analysis. Under simulated light illumination (AM 1.5, 100 mW cm^?2), NR@TN produced a photocurrent density of 0.62 mA/cm² at 1.23 V vs. reversible hydrogen electrode (vs. RHE). Importantly, 35% enrichment in photoconversion efficiency was observed for NR@TN at much lower bias potential (0.77 V vs. RHE) compared with NR (0.135%) and NR@DN (0.13% at 0.82 V vs. RHE). Key to the improved performance is believed to be synergetic effects of excellent light-trapping characteristics and the large surface-to-volume ratios due to the nanosheet structures. The nanorod connected with thin nanosheet structures improved the efficiency by means of improved charge transfer across the nanostructure/electrolyte interfaces, and efficient charge transport within the material. We believe that the hierarchical ZnO structures can be used in conjunction with doping and/or sensitization to promote the photoelectrochemical (PEC) performance. Further, the ZnO nanorod interconnected with nanosheets morphology presented in this article is extendable to other metal oxide semiconductors to establish a universal protocol for the development of high performance photoanodes in the field of PEC water splitting.     

Enhanced solar-driven water splitting of 1D core-shell Si/TiO2/ZnO nanopillars

In this work, 1D core-shell Si/metal oxide nanopillar (NP) photoanodes were synthesized for enhanced solar-driven water splitting processes. The core-shell structures were fabricated by atomic layer deposition of different metal oxides (TiO₂ and ZnO) onto Si NP, which were synthesized by metal-assisted chemical etching and nanosphere lithography. In order to characterize produced photoanodes various experimental techniques (SEM/TEM, XRD, Transmittance, Reflectance, Raman spectroscopy) were applied. Photoelectrochemical (PEC) water oxidation of produced photoanodes was studied. It was shown that composition of n-Si/TiO₂/ZnO NP exhibited enhanced photocurrents due to barrier effects. The enhanced PEC properties of core-shell Si/TiO₂/ZnO NP are caused by efficient charge separation of photogenerated electron-hole pairs in the TiO₂/ZnO shell and effective holes transfer to the shell-electrolyte interface. The superior photoelectrochemical performance of a photoanode based on core-shell Si/TiO₂/ZnO NP has been confirmed through electrochemical impedance spectroscopy and voltamperometric measurements under electrode irradiation. 1D core-shell Si/TiO₂/ZnO NP offer a new approach for preparing stable and highly efficient photoanodes for PEC water-splitting process.     

Sensitization of vertically grown ZnO 2D thin sheets by MoSx for efficient charge separation process towards photoelectrochemical water splitting reaction

Development of advanced materials for photoelectrochemical (PEC) water splitting has become an essential issue for efficient, green, and economical hydrogen production. In this context, vertically grown thin sheets of ZnO is developed, which can function as an efficient photoanode in PEC water splitting reaction. Further, the PEC activity of ZnO is enriched by decorating a newly developed co-catalyst, which is amorphous MoS_x through efficient charge transportation. MoS_x nanostructure is decorated on the surface of ZnO nanosheet via electrodeposition technique. MoS_x adorned ZnO shows enhanced activity towards photoanodic PEC water splitting compared to bare ZnO. ZnO@MoS_x can generate photocurrent density nearly three times higher compared to bare ZnO at an applied potential of ‘0.5998’ V vs. RHE. Sensitization of MoS_x on ZnO surface results in an enhancement in carrier density; ZnO@MoS_x shows nearly 7.4-times higher carrier density compared to bare ZnO. Maximum photoconversion efficiency, 0.934% is achieved in the case of ZnO@MoS_x. The determined band alignment of ZnO and MoS_x indicate the formation of type-II heterostructure which allow facile charge carrier separation. Efficient charge separation is also confirmed with the help of PL spectroscopy. It further restricts the electron-hole recombination in ZnO, leading to enhanced PEC activity. ZnO@MoS_x thin sheets are very stable even up to 1000 s under chopped illumination condition.     

Electrochemical synthesize and characterization of ZnO/ZnS nanostructures for hydrogen production

In present study, zinc oxide/zinc sulfide (ZnO/ZnS) nanostructures were fabricated by anodization of Zn. The morphological, structural and compositional properties of ZnO/ZnS were studied by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). From FESEM results, well-defined and smooth spherical-like ZnO/ZnS nanostructures were obtained with tube-like ZnO nanostructures underneath the compact layer. XRD patterns revealed that achieved materials offer high crystallinity hexagonal ZnO as dominant with hexagonal close-packed polycrystalline Zn. Moreover, measure of water contact angle was realized to learn about wetting property of the electrodes. Electrochemical impedance spectroscopy (EIS) was recorded to examine electrocatalytic performance of electrodes against hydrogen evolution reaction (HER) in 1 M KOH solution. The values of energy consumption and energy efficiency were calculated as 571.9 kJ mol⁻¹ and 49.5% at current density of 50 mA cm⁻² for the HER on 40-ZnO/ZnS/Zn electrode at 25°C.     

Bias-free visible light-driven photoelectrochemical water splitting of type II ZnO/CuS core/shell heterojunction nanotube arrays

Solar-driven water splitting of semiconductor photoelectrodes via photoelectrochemical (PEC) cell has been regarded as the most promising approach to mitigate the energy crisis and environmental issues in the future. In this work, CuS nanoparticles (NPs) are deposited on ZnO nanotube arrays (ZnO/CuS NTAs) via successive ion layer absorption and reaction method for PEC water splitting under visible light irradiation without applying bias. The excellent light harvesting capacity of CuS NPs from visible to near infrared region not only expands the light harvesting of ZnO NTAs into near infrared region, but also substantially boosts light absorption ranging from 300 to 800 nm. Moreover, CuS NPs coupled on ZnO NTAs can establish a type-II band alignment between ZnO and CuS. Consequently, the ZnO/CuS NTAs photoanode exhibits the significantly boosted PEC water splitting performance under visible light illumination (λ > 420 nm) without applying bias. The photocurrent density of the ZnO/CuS NTAs photoanode is 21.2 μA/cm², which is increased by 9 times compared to that of the pure ZnO NTAs photoanode. The enhancement in PEC water splitting performance for ZnO/CuS NTAs is attributed to (i) the cooperative actions of ZnO and CuS; (ii) significant enhancement in light absorption from the visible to near infrared region achieved by CuS NPs and (iii) efficient charge carrier separation achieved by type-II band alignment.     

Electrochemically etched triangular pore arrays on GaP and their photoelectrochemical properties from water oxidation

Large-scale, triangular pore arrays on GaP were successfully prepared via a simple and high-efficient approach of electrochemical etching under high field. The obtained ordered porous GaP exhibited high performance in the photoelectrochemical (PEC) properties compared with bulk GaP. The photocurrent of the porous GaP exceeded one order of magnitude higher than that of bulk material under 0.1 V compared to the reversible hydrogen electrode (RHE), which indicated the porous structure could enhance photoresponse and facilitate the separation of photo-induced carrier charges and their collection. The structure of triangular pore arrays cooperated with its depth determined the PEC performance of GaP. The optimal etching depth was obtained via testing the PEC performance. The hydrogen production from bulk GaP and its porous structure material were also tested from water splitting. Upon the porous structure, significantly enhanced hydrogen production has also been observed, which indicated that the porous GaP should have important potential in photocatalytic water splitting.     

Impact in rational synthesis Co-ZIF templates for derived the structural Co@NC catalysis for high efficient hydrogen and oxygen evolution reactions

Developing highly active and stable non-noble metal bifunctional electrocatalysts are urgently demanded in overall water splitting. Herein, tunable precursor ratio synthesis of cobalt-based ZIFs as a template derived active cobalt embedded N-doped carbon (Co@NC) catalyst. The rational synthesis of ZIF templates significantly impacts the complex nanostructure and properties of the catalyst (Co@NC). Consequently, the different nanostructures on Co@NC exhibit significance for the electrocatalyst of hydrogen and oxygen evolution reactions. The optimized Co@NC-20 provides excellent electrocatalytic activity with the lowest overpotential of 172 and 301 mV for HER and OER, respectively, at the current density of 10 mA cm^?2. The bifunctional Co@NC-20 reveals a potential for overall water splitting as low as 1.68 V of 10 mA cm^?2. After continuously working for 24h, the exceptional stability activity maintains 75% of the catalytic performance on Co@NC-20. The beneficial character in the synergistic effects between high-active Co species with well-protection of the metal core by carbon shell promotes their excellent performance. This study provides an essential reference for the rational design of ZIF templates for electrocatalysts with more complex structures in the future.