Simple but Effective Way To Enhance Photoelectrochemical Solar-Water-Splitting Performance of ZnO Nanorod Arrays: Charge-Trapping Zn(OH)2 Annihilation and Oxygen Vacancy Generation by Vacuum Annealing

Multicomponent hydroxides supported Cu/Cu2O nanoparticles for high efficient photocatalytic ammonia synthesis

Environmentally friendly photocatalytic N₂ fixation process has attracted considerable attention. Developing efficient photocatalysts with high electron-hole separation rates and gas adsorption capacities remains quite challenging. Herein, a facile fabrication strategy of Cu-Cu₂O and multicomponent hydroxide S-scheme heterojunctions with carbon dot charge mediators is reported. The rational heterostructurebrings excellent N₂ absorption ability and high photoinduced electron/hole separation efficiency, and the ammonia produced yield reach above 210 µmol·g_cal^-1·h^-1 during the nitrogen photofixation process. More superoxide and hydroxyl radicals are generated simultaneously in the as-prepared samples under light illumination. This work offers a reasonable construction method to further develop suitable photocatalysts for ammonia synthesis.

Selective Oxidation of Biomass Molecules via ZnO Nanoparticles Modified Using Charge Mismatch of the Doped Co ions

A charge mismatch between transition-metal-ion dopants and metal oxide nanoparticles (MO NPs) within an engineered complex engenders a significant number of oxygen vacancies (V_O) on the surface of the MO NP construct. To elucidate in-depth the mechanism of this tendency, Co ions with different charge states (Co³⁺ and Co²⁺) were doped into ZnO NPs, and their atomic structural changes were correlated with their photocatalytic efficiency. We ascertained that the increase of the Zn-O bond distances was distinctly affected by Co³⁺-ion doping, and, subsequently, the number of V_O was noticeably increased. We further investigated the mechanistic pathways of the photocatalytic oxidation of 2,5-hydroxymethylfurfural (HMF), which have been widely investigated as biomass derivatives because of their potential use as precursors for the synthesis of sustainable alternatives to petrochemical substances. To identify the reaction products in each oxidation step, selective oxidation products obtained from HMF in the presence of pristine ZnO NPs, Co³⁺- and Co²⁺-ion-doped ZnO NPs were evaluated. We confirmed that Co³⁺-ion-doped ZnO NPs can efficiently and selectively oxidize HMF with a good conversion rate (∼40%) by converting HMF to 2,5-furandicarboxylic acid (FDCA). The present study demonstrates the feasibility of improving the production efficiency of FDCA (an alternative energy material) by using enhanced photocatalytic MO NPs with the help of the charge mismatch between MO and metal-ion dopants.

Some Distinct Attributes of ZnO Nanorods Arrays: Effects of Varying Hydrothermal Growth Time

This study investigates the growth time effect on the structural, morphological, optical, and photoelectrochemical characteristics of highly oriented ZnO nanorod arrays (ZNRAs). The nanorod arrays were grown on ITO substrates using the unified sol-gel spin coating and hydrothermal techniques. ZnO nanoparticles (ZNPs) were synthesized using the sol-gel spin coating method. In contrast, the hydrothermal method was used to grow the ZnO nanorods. The hydrothermal growth time investigated was between 4 and 12 h. The synthesized ZNRAs were used as the photoanode electrodes to investigate their photoelectrochemical (PEC) electrode potency. The as-prepared ZNRAs were characterized using various analytical tools to determine their structures, morphologies, optical, and photoelectrochemical traits. EDX spectra showed the presence of uncontaminated ZnO chemical composition, and FTIR spectra displayed the various functional groups in the samples. A rod-shaped ZnO nanocrystallite with mean lengths and diameters of 300-500 nm and 40-90 nm, respectively, is depicted. HRTEM images indicated the nucleation and growth of ZNRAs with a lattice fringe spacing of 0.26 nm and a growth lattice planer orientation of [002]. The optimum ZNRAs (grown at 8 h) as photoelectrode achieved a photoconversion efficiency of 0.46% and photocurrent density of 0.63 mA/cm², that was 17 times higher than the one shown by ZNPs with Ag/AgCl as the reference electrode. Both values were higher than those reported in the literature, indicating the prospect of these ZNRAs for photoelectrode applications.

Verifying the relationships of defect site and enhanced photocatalytic properties of modified ZrO2 nanoparticles evaluated by in-situ spectroscopy and STEM-EELS

Base treatment and metal doping were evaluated as means of enhancing the photocatalytic activity of ZrO₂ nanoparticles (NPs) via the generation of oxygen vacancies (O_vS), and the sites responsible for this enhancement were identified and characterized by spectroscopic and microscopic techniques. We confirmed that O_vS produced by base treatment engaged in photocatalytic activity for organic pollutant degradation, whereas surface defects introduced by Cr-ion doping engaged in oxidative catalysis of molecules. Moreover, we verified that base-treated ZrO₂ NPs outperformed their Cr-ion doped counterparts as photocatalysts using in situ X-ray photoelectron spectroscopy and scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS). Thus, our study provides valuable information on the origin of the enhanced photocatalytic activity of modified ZrO₂ NPs and demonstrates the practicality of in situ spectroscopy and STEM-EELS for the evaluation of highly efficient metal oxide photocatalysts.

Structural, photocatalytic and photoelectrochemical properties of porous ZnO nanosheets prepared by air cold plasma

Porous ZnO nanosheets with different thickness were prepared on zinc substrate by air cold plasma for photocatalytic degradation and photoelectrochemical water splitting. The ZnO nanosheets consisted of nanocrystallines with high-density oxygen-related defects characterized by the strong red luminescence. The UV absorption tended to be saturated as the thickness increased, and the saturation occurred at a thickness of about 2.3μm. Under UV irradiation (365 nm), the 2.3μm thick sample with higher content of oxygen vacancies and oxygen interstitials showed the highest photocatalytic activity (and higher than P25 TiO₂) in degradation of gaseous ethyl acetate. Due to the excellent UV-vis absorption ability and the effective transfer of photogenerated carriers, the ZnO nanosheets with thickness of 3.3μm showed a photocurrent density as high as 0.22 mA cm^-2at -0.28 V (versus Ag/AgCl) under AM 1.5 G 100 mW cm^-2.

The enhanced water splitting activity of a ZnO-based photoanode by modification with self-doped lanthanum ferrite

The difficult separation and transfer of photoexcited charge carriers in composite photoelectrodes is a decisive factor limiting the efficiencies of semiconductor-based photoelectrochemical water splitting systems. Herein, to further enhance the photoelectrochemical properties of ZnO-based photoanodes, we constructed composite ZnO nanoarray photoanodes with Fe-self-doped lanthanum ferrite (denoted as La1-xFe1+xO3/ZnO NRs), which had the effect of killing two birds with one stone. This improvement strategy differs from the previously popular multi-step modification process, and integrates the dual benefits of a heterojunction and cocatalyst using the same material, the doped LaFeO3, which bypasses the shortcomings of multi-step charge transfer. Gratifyingly, benefitting from the suitable energy bands and excellent electrocatalytic oxygen evolution activity of La0.9Fe1.1O3, the photoanode exhibits outstanding bulk charge separation and surface charge utilization efficiencies, as well as achieving a photocurrent density that is over three times higher than that of pristine ZnO NRs, with a small onset potential (0.33 V vs. RHE). This electrode modification concept provides guidance for the development of other highly active photoelectrodes.

Coffee ground derived biochar embedded Ov-NiCoO2 nanoparticles for efficiently catalyzing a boron‑hydrogen bond break

The catalytic boron‑hydrogen bond break is usually regarded as an important reaction both in the area of environment treatment and hydrogen energy, attracting increasing attention in the past decades. Due to the limitation of conventional noble metal-based catalyst, cost-effective transition metal-based catalysts with high activity have been recently developed to become the promising candidates. Herein, the coffee ground waste was utilized as the biochar substrate loaded with ultrafine NiCoO₂ nanoparticles. The abundant function groups on the biochar substrate efficiently adsorbed the metal ions and confined the crystal growth spatially, making the NiCoO₂ nanoparticles highly dispersed on the surface. Moreover, the oxygen vacancies were further created in the catalysts by a vacuum-calcination strategy to boost their catalytic activity towards boron‑hydrogen bond break both in the systems of 4-nitrophenol reduction by NaBH₄ and hydrogen release from NH₃BH₃. The results indicated that the moderate presence of oxygen vacancies could effectively accelerate the boron‑hydrogen bond break and the catalytic activity performed a satisfied stability during several recycles. The theoretical calculation method was adopted to analysis and discuss the mechanism within this process. This design strategy on active catalysts not only offered a novel solution of biowaste resource reuse but also demonstrated the significant role of oxygen vacancies in energy and environmental catalysis.

One-step synthesis of novel Cu2ZnNiO3 complex oxide nanowires with tuned band gap for photoelectrochemical water splitting

Although alloying and nanostructuring offer a great opportunity for enhancing photoelectrochemical behavior and band gap tuning, these methods have not been investigated extensively. This article reports the synthesis of Cu2ZnNiO3 complex oxide nanowires (∼200 nm) grown on German silver alloy via a one-step optimized hydrothermal route and their utilization to split water photoelectrochemically. Surface characterizations were used to elucidate the formation mechanism of the Cu2ZnNiO3 complex oxide nanowires. The nanowires exhibited an exceptional visible light absorption extending from 400 to 1400 nm wavelengths with a tuned band gap of ∼2.88 eV calculated from the corresponding Tauc plot. In tests to split water photoelectrochemically, the nanowires generated a significant photocurrent of up to −2.5 mA cm−2 at −0.8 V versus Ag/AgCl and exhibited an exceptional photostability which exceeded 2 h under light-off conditions with no photocurrent decay. Band edge positions related to water redox potentials were estimated via Mott–Schottky and diffuse reflectance spectroscopy analysis with the density of charge carriers reaching as high as 5.15 × 1018 cm−3. Moreover, the nanowires generated ∼1100 µmol of H2 in 5 h. These photoelectrochemical results are much higher than the reported values for similar structures of copper oxide, zinc oxide and nickel oxide separately under the same conditions, which can be attributed to the advantages of Cu, Zn and Ni oxides (such as visible light absorption, photostability, and efficient charge carrier generation and transport) being combined in one single material. These promising results make German silver a robust material toward photoelectrochemical water splitting.

Enhancing the quasi-theoretical photocurrent density of ZnO nanorods via a lukewarm hydrothermal method

A ∼10-μm-long one-dimensional (1D) ZnO nanorod array (NRA) vertically oriented on a fluorine-doped tin oxide (FTO) coated glass substrate is successfully fabricated via a lukewarm hydrothermal method. The reflection of light from the rough surface of this ultralong ZnO NRA, resulting from the variation in the characteristic length of individual ZnO NRs in a tapered geometry, is largely suppressed. This in turn favors the ZnO NRA as a photoelectrode effectively harnessing UV-light for solar water splitting, as evidently manifested in the quasi-theoretical photocurrent density that reached ∼0.9 mA cm^-2 at 1V_Ag/AgCl. A further contribution to such an outstanding performance stems from additional photocurrent generation by the ZnO NRA upon visible light illumination. This is attributed to a variety of native defects and the surface hydroxyl groups present in the ZnO NRA, giving rise to the mid-gap states that mediate the associated electronic transitions. Moreover, those lattice imperfections further boost the carrier concentration of the ZnO NRA to facilitate the carrier transport which in turn enhances the photoelectrochemical activity.

Increased Active Sites on Irregular Morphological α-Fe2O3 Nanorods for Enhanced Photoelectrochemical Performance

Uniform rectangular α-Fe₂O₃ nanorods (R-Fe₂O₃) and irregular α-Fe₂O₃ nanorods (D-Fe₂O₃) with a random size vertically aligned on fluorine-doped tin oxide were prepared with a facile one-step hydrothermal procedure. X-ray diffraction (XRD) measurements and Raman spectra confirm that the obtained samples are α-Fe₂O₃, and XRD patterns show that D-Fe₂O₃ has two extra (012) and (104) planes of hematite in addition to the identical peaks to R-Fe₂O₃. The carrier density of the D-Fe₂O₃ sample is four times larger than that of R-Fe₂O₃. Finally, the D-Fe₂O₃ photoelectrode exhibited a better photoelectrochemical (PEC) performance under visible illumination than that of R-Fe₂O₃, achieving the photocurrent density of 0.15 mA cm^-2 at 1.23 V versus reversible hydrogen electrode. In addition, incident photo-to-current conversion efficiency of D-Fe₂O₃ is nearly three times larger than that of R-Fe₂O₃. Hence, the improved PEC performance of D-Fe₂O₃ can be ascribed to higher carrier density resulting from the amount of oxygen vacancies and more activated exposed surface facets.

Investigation of Strain Effects on Photoelectrochemical Performance of Flexible ZnO Electrodes

In this report, the growth of zinc oxide (ZnO) nanocrystals with various morphologies, nanoflower, nanosheet, and nanorod, on flexible stainless steel (SS) foils to be utilized as photoanodes in photoelectrochemical (PEC) solar cells has been presented. It has been aimed to provide flexibility and adaptability for the next generation systems with the incorporation of SS foils as electrode into PEC cells. Therefore, physical deformation tests have been applied to the prepared ZnO thin film photoanodes. These thin films have been thoroughly characterized before and after straining for better understanding the relationship between the morphology, straining effect and photoelectrochemical efficiency. We observed a notable increase in the maximum incident photon-to-current efficiency (IPCE) and durability of all ZnO photoelectrodes after straining process. The increase in IPCE values by 1.5 and 2.5 folds at 370 nm has been observed for nanoflower and nanorod morphologies, respectively after being strained. The maximum IPCE of 69% has been calculated for the ZnO nanorod structures after straining. Bending of the SS electrodes resulted in the more oriented nanorod arrays compared to its flat counterpart, which improved both the light absorption and also the photo-conversion efficiency drastically. The finite-difference time-domain simulations have also been carried out to examine the optical properties of flat and bent ZnO electrodes. Finally, it has been concluded that SS photoanodes bearing ZnO semiconducting material with nanoflower and nanorod morphologies are very promising candidates for the solar hydrogen generator systems in terms of efficiency, durability, flexibility, and lightness in weight.

In Situ Formation of WO3-Based Heterojunction Photoanodes with Abundant Oxygen Vacancies via a Novel Microbattery Method

Non-stoichiometric ratio semiconductor materials have exhibited excellent performance in energy conversion and storage fields. However, the hydrogen treatment method that is commonly used to introduce oxygen vacancies is expensive and dangerous. In this paper, a novel microbattery method using Zn powder and Fe powder as reductant has been developed to synthesize the oxygen vacancy modified WO_{3- x} films and oxygen-deficient heterojunction films (ZnWO_{4- x}/WO_{3- x} and Fe₂O_{3- x}/WO_{3- x}) at room temperature. The as-prepared WO_{3- x} and ZnWO_{4- x}/WO_{3- x} heterojunction films exhibit improved photoelectrochemical performance. It is worth noting that this microbattery method can quickly introduce oxygen vacancies into semiconductor materials, including powders and films at room temperature.

Synergistic doping effects of a ZnO:N/BiVO4:Mo bunched nanorod array photoanode for enhancing charge transfer and carrier density in photoelectrochemical systems

One-dimensional heterojunction nanorods are highly attractive as photoanodes for developing efficient photoelectrochemical (PEC) systems for the effective photogeneration of charge carriers and transport. ZnO/BiVO4 nanorod arrays (NRAs) are excellent candidates if their charge transferring and recombination issues can be improved. In the current work, we have studied the synergistic doping effects of N-doped ZnO/Mo-doped BiVO4 NRAs for enhancing the photoanode activity in PEC devices. The N-doping of ZnO NRs enhances the charge carrier density ∼3-fold over undoped ZnO NRs through increased oxygen vacancies induced by N dopants. The Mo dopants in BiVO4 improve the mobility of photogenerated charge carriers and contribute to reducing charge recombination. The synergistic doping effects of both ZnO and BiVO4 could increase the charge transfer rate constant (kct) of the ZnO:N/BiVO4:Mo heterojunction by ∼40% and decrease the charge transfer resistance ∼1.9-fold compared to those of undoped ZnO/BiVO4, which were confirmed by time resolved photoluminescence (PL) and electrochemical impedance (EIS) analyses. Our optimally fabricated ZnO:N/BiVO4:Mo NRA photoanode could achieve an excellent photocurrent of 3.62 mA cm-2 without the application of any co-catalysts. This work presents a useful strategy for designing efficient heterojunction photoanodes in PEC systems.

Enhancing Durability and Photoelectrochemical Performance of the Earth Abundant Ni-Mo/TiO2 /CdS/CIGS Photocathode under Various pH Conditions

Cu(In,Ga)(S,Se)₂ (CIGS) is a promising photocathode material owing to its high absorption coefficient, adjustable band gap, and suitable band edge for the hydrogen evolution reaction (HER). However, most CIGS photocathodes have suffered from instability in applications that require a wide range of pH conditions and have utilized noble metal HER catalysts to achieve a high performance. Thus, improving the durability of the CIGS photocathode under various pH conditions and developing a cost-effective non-noble metal catalyst are critical issues in the photoelectrochemical (PEC) application of this promising photocathode material. Here, we catalyze the CIGS photocathode with Ni-Mo as a non-noble metal to enhance the PEC efficiency, and we employ atomically grown TiO₂ to passivate the CdS/CIGS surface and improve the stability under a wide range of pH conditions. Our Ni-Mo alloy exhibits the best HER catalytic activity among reported earth-abundant HER catalysts in both acidic and alkaline solutions. The Ni-Mo/CdS/CIGS photocathode yields an onset potential of 0.5 V (vs. RHE) and a short-circuit photocurrent density as high as 15-25 mA cm^-2 under various pH conditions ranging from 0.4 to 14, which is highly comparable to that of Pt/CdS/CIGS. Furthermore, the passivation of CdS/CIGS with a thin TiO₂ layer, obtained by atomic layer deposition, effectively prevents the photocorrosion of CdS and the dissolution of the Mo back contact, which are the main causes of the degradation of the photocathode. The optimized Ni-Mo/TiO₂ /CdS/CIGS photocathode produces a stable photocurrent density at 0 V_RHE for 100 minutes except under strong alkaline conditions. The current work presents a very useful method to improve the efficiency and durability of the CIGS photocathodes with an earth-abundant metal catalyst, which completely replaces Pt.

Dual-Axial Gradient Doping (Zr and Sn) on Hematite for Promoting Charge Separation in Photoelectrochemical Water Splitting

One of the crucial challenges to enhance the photoelectrochemical water-splitting performance of hematite (α-Fe₂ O₃ ) is to resolve its very fast charge recombination in bulk. Herein, we describe the design and fabrication of dual-axial gradient-doping on 1D Fe₂ O₃ nanorod arrays with Zr doping for x-axial and Sn doping for y-axial directions to promote the charge separation. This dual-axial gradient-doping structure fulfills the requirements of a greater electron-carrier concentration for increasing conductivity as well as a higher charge-separation efficiency across the dual-axial direction of Fe₂ O₃ nanorods, ultimately showing an excellent photocurrent density of 1.64 mA cm^-2 at 1.23 V vs. RHE, which is 26.3 times more than that of the bare Fe₂ O₃ . Furthermore, the remarkably improved photocurrent density, when comparing the uniform Zr-doped Fe₂ O₃ nanorod arrays (1.0 mA cm^-2 at 1.23 V vs. RHE) with dual-axial gradient-doped (Zr and Sn) Fe₂ O₃ nanorod arrays, highlights the additional charge-separation effect resulting from gradient codoping of Zr and Sn. Hence, this promising design may provide guidelines for dual-axial gradient doping into photoelectrodes to realize efficient PEC water splitting.

Efficient Photocatalytic Hydrogen Evolution via Band Alignment Tailoring: Controllable Transition from Type-I to Type-II

Considering the sizable band gap and wide spectrum response of tin disulfide (SnS₂ ), ultrathin SnS₂ nanosheets are utilized as solar-driven photocatalyst for water splitting. Designing a heterostructure based on SnS₂ is believed to boost their catalytic performance. Unfortunately, it has been quite challenging to explore a material with suitable band alignment using SnS₂ nanomaterials for photocatalytic hydrogen generation. Herein, a new strategy is used to systematically tailor the band alignment in SnS₂ based heterostructure to realize efficient H₂ production under sunlight. A Type-I to Type-II band alignment transition is demonstrated via introducing an interlayer of Ce₂ S₃ , a potential photocatalyst for H₂ evolution, between SnS₂ and CeO₂ . Subsequently, this heterostructure demonstrates tunability in light absorption, charge transfer kinetics, and material stability. The optimized heterostructure (SnS₂ -Ce₂ S₃ -CeO₂ ) exhibits an incredibly strong light absorption ranging from deep UV to infrared light. Significantly, it also shows superior hydrogen generation with the rate of 240 µmol g^-1 h^-1 under the illumination of simulated sunlight with a very good stability.

Decreased Charge Transport Barrier and Recombination of Organic Solar Cells by Constructing Interfacial Nanojunction with Annealing-Free ZnO and Al Layers

To overcome drawbacks of the electron transport layer, such as complex surface defects and unmatched energy levels, we successfully employed a smart semiconductor-metal interfacial nanojunciton in organic solar cells by evaporating an ultrathin Al interlayer onto annealing-free ZnO electron transport layer, resulting in a high fill factor of 73.68% and power conversion efficiency of 9.81%. The construction of ZnO-Al nanojunction could effectively fill the surface defects of ZnO and reduce its work function because of the electron transfer from Al to ZnO by Fermi level equilibrium. The filling of surface defects decreased the interfacial carrier recombination in midgap trap states. The reduced surface work function of ZnO-Al remodulated the interfacial characteristics between ZnO and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM), decreasing or even eliminating the interfacial barrier against the electron transport, which is beneficial to improve the electron extraction capacity. The filled surface defects and reduced interfacial barrier were realistically observed by photoluminescence measurements of ZnO film and the performance of electron injection devices, respectively. This work provides a simple and effective method to simultaneously solve the problems of surface defects and unmatched energy level for the annealing-free ZnO or other metal oxide semiconductors, paving a way for the future popularization in photovoltaic devices.

Highly efficient UV-sensing properties of Sb-doped ZnO nanorod arrays synthesized by a facile, single-step hydrothermal reaction

The synthesis of electrical and optical property-modulated, low-dimensional metal oxide semiconductors has been adopted for the development of nanodevices.

Enhanced photoelectrochemical water oxidation performance of a hematite photoanode by decorating with Au–Pt core–shell nanoparticles

The charge transfer process of the AuPt/α-Fe₂O₃ composite photoanode for photoelectrochemical water oxidation.