ZnO Nanorod Photoanode Research Articles

Fabrication of ZnO Scaffolded CdS Nanostructured Photoanodes with Enhanced Photoelectrochemical Water Splitting Activity under Visible Light.

CdS, characterized by its comparatively narrow energy band gap (∼2.4 eV), is an appropriate material for prospective use as a photoanode in photoelectrochemical water splitting. Regrettably, it encounters several obstacles for practical and large-scale applications, including issues such as bulk carrier recombination and diminished conductivity. Here, we have tried to address these challenges by fabricating a novel photoelectrode (ZnO/CdS) composed of one-dimensional ZnO nanorods (NRs) decorated with two-dimensional CdS nanosheets (NSs). A facile two-step chemical method comprising electrodeposition along with chemical bath deposition is employed to synthesize the ZnO NRs, CdS NSs, and ZnO/CdS nanostructures. The prepared nanostructures have been investigated by UV-visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy. The fabricated ZnO/CdS nanostructures have shown enhanced photoelectrochemical properties due to the improvement of the semiconductor junction surface area and thereby enhanced visible light absorption. The incorporation of CdS NSs has been further found to promote the rate of the charge separation and transfer process. Subsequently, the fabricated ZnO/CdS photoelectrodes achieved a photocurrent conversion efficiency 3 times higher than that of a planar ZnO NR photoanode and showed excellent performance under visible light irradiation. The highest applied bias photon-to-current conversion efficiency (% ABPE) of about ∼0.63% has been obtained for the sample with thicker CdS NSs on ZnO NRs with a photocurrent density of ∼1.87 mA/cm2 under AM 1.5 G illumination. The newly synthesized nanostructures further demonstrate that the full photovoltaic capacity of nanomaterials is yet to be exhausted.

Gradient doping of Cu(I) and Cu(II) in ZnO nanorod photoanode by electrochemical deposition for enhanced photocurrent generation

This study describes the synthesis and characterization of Cu-doped ZnO nanorods (NRs) by an electrochemical method in the presence of two different Cu precursor (Cu+2 and Cu+) in order to improve photocurrent generation. Analyses of the resulting materials by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis and electrochemical photocurrent (ECP) spectroscopy confirm the formation of well-aligned ZnO Würtzite nanostructures in the form of hexagonal rods. For both doping source with a concentration of up to 0.5%, the following changes were observed: a distortion of the ZnO morphology, an increase in transmittance to 96% for ZnO doped with Cu+2, and a reduction of the energy gap from 3.36 eV to 3.06 and 3.02 eV for ZnO doped with Cu+2 and Cu+, respectively. From photoelectrochemical tests, the photo-current density was improved up to 0.05 mA cm-2 in the presence of Cu doping, which is twelve times superior to that of undoped ZnO nanorods, which means that the incorporation of Cu+2 or Cu + significantly improves the separation efficiency of photogenerated electron-hole pairs. These results can be considered promising for optoelectronic and photocatalysis applications.

Efficient Photocatalytic Hydrogen Evolution over TiO2-X Mesoporous Spheres-ZnO Nanorods Heterojunction.

Photocatalytic water splitting into hydrogen is regarded as one of the key solutions to the deterioration of the global environment and energy. Due to the significantly reduced grain boundaries, ZnO nanorods facilitate a fast electron transfer through their smooth tunnels and are well suited as a photocatalyst. However, the photocatalytic hydrogen evolution performance of pristine ZnO nanorods is still low due to the high recombination rate of photogenerated electron-hole pairs and the less light absorption. Here, a novel structure about black ZnO nanorods (NRs)/TiO2-X mesoporous spheres (MSs) heterojunction has been prepared and the photocatalytic hydrogen evolution performance has been explored. The photocatalytic activity test results showed that ZnO NRs/TiO2-X MSs exhibited higher catalytic activity than ZnO NRs for hydrogen production. Compared to the pure ZnO NRs photoanode, the photocurrent of ZnO NRs/TiO2-X MSs heterojunction photoanode could reach 0.41 mA/cm2 in view of the expanding spectral response region and effective inhibition of e−/h+ recombination at the same condition. Using a relatively integrated experimental investigation and mechanism analysis, we scrutinized that after being treated with NaBH4, TiO2 MSs introduce oxygen vacancies expanding the photocatalytic activity of pure TiO2, and improving conductivity and charge transport capabilities through coating on ZnO NRs. More importantly, the results provide a promising approach in the NRs/MSs composite structure serving as photoanodes for photocatalytic hydrogen production.

Porous Zn1-xCdxS nanosheets/ZnO nanorod heterojunction photoanode via self-templated and cadmium ions exchanged conversion of ZnS(HDA)0.5 nanosheets/ZnO nanorod

Herein, we synthesized porous Zn1-xCdxS nanosheets (PNS)/ZnO nanorod (NR) heterojunction photoanode via self-templated conversion using successively hydrothermal and Cd2+ ion exchange methods. Moreover, after conversion of ZnO to inorganic–organic hybrid ZnS-1,6-hexanediamine (HDA)0.5nanosheets/ZnO NR material, Cd2+ ion exchange was conducted. It was confirmed from X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM) analyses that the inorganic–organic hybrid ZnS(HDA)0.5 NS was transformed into Zn1-xCdxS PNS/ZnO NR heterojunction photoanode via the replacement of Zn2+ by Cd2+ ion. Zn1-xCdxS PNS/ZnO NR-160C3H heterojunction photoanode synthesized at 160 °C for 3 h showed the highest photocurrent density of 4.10 mA cm−2 (vs. RHE) under 1.5 G illumination, which was 7.9 times higher than that of bare ZnO NR photoanode. The porous nanostructured morphology and larger surface area of Zn1-xCdxS PNS/ZnO NR heterojunction photoanode fabricated by Cd2+ ion exchange result in efficient light absorption and effective charge transfer pathway. The photoluminescence (PL) and time-resolved photoluminescence (TRPL) results show the shorter lifetime (37 ns) and reduced recombination in Zn1-xCdxS PNS/ZnO NR-160C3H heterojunction photoanode. During PEC analysis, the possible charge transfer mechanism in Zn1-xCdxS PNS/ZnO NR heterojunction photoanode was proposed. Surface passivated Zn1-xCdxS PNS/ZnO NR-160C3H photoanode shows improved photostability and exhibited 3.8 times higher H2 evolution (161 μmol) than the Zn1-xCdxS PNS/ZnO NR-160C3H (42 μmol) photoelectrode at 0.9 V vs. RHE.

Layered Double Hydroxide onto Perovskite Oxide-Decorated ZnO Nanorods for Modulation of Carrier Transfer Behavior in Photoelectrochemical Water Oxidation.

Despite the fact that perovskite oxides with high photoelectrochemical (PEC) stability have gained widespread concern in the field of photo(electro)catalytic water splitting, the potential as a photoelectrode has not yet fully exploited. Herein, perovskite oxide-decorated ZnO nanorod photoanode improves the vital issue that photoproduced electron-hole pairs are apt to be quenched, in which type II band alignment between perovskite oxide and ZnO plays a crucial role in extracting carriers. Further, coupling with layered double hydroxide (LDH) onto the heterostructure not only tunes surface injection behavior of charge carriers by facilitating the interface reaction dynamics but also suppresses ZnO self-corrosion for extended durability. As a result, the optimized CoAl-LDH/LaFeO3/ZnO nanorod photoanode yields a much enhancive effect for the PEC property in terms of photocurrent density (2.46 mA cm-2 at 1.23 V vs reversible hydrogen electrode under AM 1.5G), onset potential, and stability. This work signifies a feasible design to combine promising perovskite oxides with the traditional photoelectrode system for achieving efficient water splitting.

UV-assisted water splitting of stable Cl-doped ZnO nanorod photoanodes grown via facile sol-gel hydrothermal technique for enhanced solar energy harvesting applications

Vertically aligned pristine ZnO and Cl-doped ZnO nanorod arrays were grown by a simple, cost-effective sol-gel and hydrothermal method. These nanorods (NRs) were fabricated to demonstrate their potential as highly efficient photoelectrodes to be used in photoelectrochemical water splitting applications. XRD measurements indicate that all the fabricated NRs have preferably grown along c-axis (0 0 2) direction. FESEM images confirmed hexagonal shaped NRs grown along (0 0 2) direction. The light harvesting efficiency of pristine ZnO NRs was enhanced with increase in doping concentration which is due to rise in absorbance as verified by UV–Vis absorption spectroscopy. When used as photoanodes in PEC water splitting under UV illumination, these NR arrays exhibits an enhanced photocurrent density of 2.16 mA cm−2 at 1.2 V vs. Ag/AgCl which is much higher than undoped ZnO NRs (0.103 mA cm−2). Cl_Z3 photoanode exhibits a stable photocurrent density even after continuous illumination of 12 h. The significantly improved photoresponse behavior and high photostability with suitable bandgap and high light harvesting efficiency of these photoanodes provides valuable platforms for efficient photoelectrochemical water splitting applications.

Activating ZnO nanorods photoanodes in visible light by CdS surface sensitiser

Thin films of c-axis aligned uniform ZnO nanorods (NRs) were fabricated on to fluorine-doped tin oxide-coated soda lime glass substrate by a two-step chemical route. Thereafter ZnO NRs/CdS core shell structures were successfully synthesised by depositing CdS layer on top of vertically aligned ZnO NRs using less hazardous nanocrystal layer deposition technique. The presence of CdS in ZnO NRs/CdS core shell structures was confirmed by energy dispersive X-ray analysis. Examination of structure and morphology of the fabricated films by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) revealed that both films have one-dimensional hexagonal wurtzite structure. Optical properties evaluated from ultraviolet-visible and photoluminescence spectra demonstrated better photo response of ZnO NRs/CdS core shell structure with respect to bare ZnO NR structure. Optical to chemical conversion efficiency of ZnO NRs/CdS photoanode was found to be similar to 1.75 times higher than bare ZnO NRs photoanode in photo electrochemical water splitting under visible light.

Bamboo shoots shaped FeVO4 passivated ZnO nanorods photoanode for improved charge separation/transfer process towards efficient solar water splitting

Among many novel and promising semiconductors, the triclinic iron vanadate (FeVO4) has been rarely applied in the field of solar water splitting due to its poor charge carrier separation ability. In this work, we prepared FeVO4-passivated ZnO heterojunction nanorods (FVO/ZnO NRs) photoanodes by simple spin-coating and calcination method for the first time. The optimal photoanode exibihits stable and effective PEC water oxidation performance, compared to pristine ZnO, affording 130 mV negative shift in onset potential and 2.4 folds augment in photocurrent density. Particularly, X-ray photoelectron spectroscopy and Mott-Schottky test reveal the establishment of heterojunction, which subdue photo-induced carriers bulk recombination and improve the charge separation efficiency. Open-circuit photovoltage and electrochemical impedance spectroscopy were recorded to further clarify that the FeVO4-passivated layer eliminates the surface trapping state, thus increasing the photovoltage and promoting the interface charge transfer kinetics. These demonstrations are expected to provide new thread for the rational application of new photoanode candidates and to optimize PEC performance.

Facile synthesis of novel ZnO/Cd0.5Zn0.5S photoanode for dye-sensitized solar cell

Present study reports facile wet chemical synthesis of novel ZnO/Cd0.5Zn0.5S core–shell photoanode by employing ZnO nanorods as core and Cd0.5Zn0.5S nanoparticles as shell for application in dye sensitized solar cell. The growth mechanism of core–shell photoanode exhibits that Cd0.5Zn0.5S nanoparticles are evenly distributed on the ZnO nanorods. However, ZnO nanorods coalesce in larger size during Cd0.5Zn0.5S nanoparticles shell formation via Oswald ripening. The optimized efficiency for ZnO/Cd0.5Zn0.5S core–shell photoanode was found to be 2.94%, which is about 2.5 fold increase as compared to bare ZnO nanorods photoanode.

Enhanced charge transfer activity in Au nanoparticles decorated ZnO nanorods photoanode

Charge-transfer and photoactivity at an interface of a photoactive materials, such as ZnO, is critically influence the performance of dye-sensitized solar cell (DSSCs). The performance of DSSCs utilizing Au decorated ZnO nanorod photoanode with Au nanoparticles grown at seven different growth times from 0.12 up to 6 min were studied. It was found that the charge transfer characteristic of DSSCs devices utilizing Au decorated ZnO nanorods increased with the increasing of Au nanoparticles size and density and optimum at Au nanoparticles size of ca. 15 nm and density of approximately 4–10 nanoparticles/nanorods. At this condition, the charge transfer resistance as low as 7.4 Ω was recorded. This is equivalent to power conversion efficiency of 1.11%. The charge transfer resistance gradually declined when the growth time further elongated. The efficient charge transfer can be associated to the excellent catalytic properties of Au nanoparticles on ZnO nanorods. This also improves its carrier transport in the device as judged from significantly low dark current properties. The improvement of light absorption can also be considered to improve the performance, attributed to the surface plasmon resonance (SPR) effect of the gold nanoparticles. The preparation of Au nanoparticles decorated ZnO nanorods and the mechanism of charge transfer characteristics will be discussed.

Gradient doping of copper in ZnO nanorod photoanode by electrodeposition for enhanced charge separation in photoelectrochemical water splitting

New and improved electrochemical synthetic approaches have been developed to improve the photoelectrochemical performance of ZnO nanorods by homogenous and gradient doping of copper. The ZnO, Cu-doped ZnO, ZnO Cu-doped ZnO homojunction and also Cu gradient doped ZnO photoanodes were electro-synthesized and characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and UV–visible absorption spectroscopy. A comparative study was done between samples and Cu gradient doped ZnO photoanodes exhibit significantly enhanced photocurrent density in photoelectrochemical cell applications as compared to other photoanodes. By comparing the results, it is found out that the homogeneous Cu-incorporated ZnO photoanodes show an enhanced photoelectrochemical response, but not as well as Cu gradient doped ZnO photoanodes. The photoelectrochemical performance improvement by adjusting the copper concentration profile is attributed to an extra charge separation effect. The linear sweep voltammetry, electrochemical impedance spectroscopy and Mott-Schottky analysis were used to confirm the improved charge separation.

One-step facile preparation of ZnO nanorods as high-performance photoanodes for photoelectrochemical cathodic protection

In this work, ZnO nanorod photoanodes were fabricated by electrodeposition on a conductive indium tin oxide glass substrate, enabling photoelectrochemical cathodic protection (CP) of both stainless and carbon steels in a chloride solution. The surface morphology, composition, crystalline structure and optical absorption property of the prepared ZnO photoanodes were characterized by scanning electron microscopy, energy dispersive X-ray spectrum, X-ray diffraction and UV–Visible diffuse reflection absorption spectrometry, respectively. Electrochemical and photoelectrochemical properties of the photoanodes were measured. Results demonstrate that the ZnO nanorod photoanodes are able to achieve sufficient cathodic polarization to 304 stainless steel and X52 carbon steel in 3.5 wt.% NaCl solution under light illumination. Compared to previously reported ZnO photoanodes, the mass activity of the prepared ZnO nanorod photoanodes in this work is improved nearly 10 times and 3–6 times when coupled with stainless steel and carbon steel, respectively. The unique surface nanostructure of the generated ZnO nanorods contributes to the high photoelectrochemical activity of the photoanodes. Particularly, the ZnO nanorod photoanode prepared at the depositing potential of −0.9 V (SCE) possesses the higher photoelectrochemical activity of 19.5 μA/mg than those fabricated at more negative potentials. Under identical illuminating conditions, the ZnO nanorod photoanode can cathodically polarize the stainless steel much more substantially than carbon steel, causing negative potential shifts of 306.6 mV and 39.1 mV, respectively.

ZnO/Nb2O5 core/shell nanorod array photoanode for dye-sensitized solar cells

In this paper, ZnO/Nb2O5 core/shell nanorod arrays were synthesized and used as photoanodes for dye-sensitized solar cells (DSSCs). We first synthesized ZnO nanorod array on fluorine-doped tin oxide (FTO) glasses by a hydrothermal method, and then ZnO/Nb2O5 core/shell nanorod array was directly obtained via solvothermal reaction in NbCl5 solution. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images revealed that the ZnO nanorods were uniformly wrapped by Nb2O5 shell layers with a thickness of 30–40 nm. Photovoltaic characterization showed that the device based on ZnO/Nb2O5 core/shell nanorod photoanode exhibited an improved efficiency of 1.995%, which was much higher than the efficiency of 0.856% for the DSSC based on bare ZnO nanorod photoanode. This proved that the photovoltaic performance of ZnO nanorods could be improved by wrapping with Nb2O5 shells.

Cover Feature: Investigating the Role of Oxygen Vacancies and Lattice Strain Defects on the Enhanced Photoelectrochemical Property of Alkali Metal (Li, Na, and K) Doped ZnO Nanorod Photoanodes (ChemElectroChem 8/2018)

The Cover Feature shows the charge carrier generation, recombination, and transportation in alkali-metal-doped ZnO nanorods photoanode under the illumination of visible solar light. More information be found in the Article by K. Karmakar et al. on page 1147 in Issue 8, 2018 (DOI: 10.1002/celc.201800097).

Investigating the Role of Oxygen Vacancies and Lattice Strain Defects on the Enhanced Photoelectrochemical Property of Alkali Metal (Li, Na, and K) Doped ZnO Nanorod Photoanodes

AbstractThis work demonstrates the significance of defect engineering in tuning the visible‐light‐driven photoelectrochemical property of alkali metal (Li, Na, and K) doped ZnO nanorods. The large concentration of oxygen vacancies introduced into the sub‐bandgap, because of alkali metal doping, serve as the light‐absorbing donor sites and also photoelectron recombination centers, resulting in the enhanced photocurrent and hole separation in the valance band. The lattice strain developed in the nanorods, owing to doping, contributes to the easy electron transportation and mobility. Defect engineering also tunes the electronic structure of photoanodes, resulting in bandgap modification and band edge engineering, boosting charge‐carrier migration and reduced electron−hole pair recombination for enhanced oxygen evolution.

Stable and Enhanced Visible-Light Water Electrolysis Using C, N, and S Surface Functionalized ZnO Nanorod Photoanodes: Engineering the Absorption and Electronic Structure

Highly oriented arrays of C, N, and S surface functionalized/surface doped ZnO Nanorods (NRs) were fabricated by a simple chemical bath deposition followed by wet chemical method. Surface functionalization significantly improved both the photocurrent and photostability of the ZnO NRs electrodes under visible-light irradiation along with the reduction in onset potential for water oxidation. The C-ZnO, N-ZnO, and S-ZnO NRs photoanodes exhibited 6.5, 5.5, and 3 times increase in photoelectrochemical water oxidation efficiency, respectively, as compared to that measured for pure ZnO NRs under visible-light illumination (10 mW·cm–2, wavelength >420 nm, 0.5 M Na2SO4). Surface engineered ZnO NRs also exhibited enhanced visible light harvesting efficiency and significantly quenched electron–hole recombination leading to greatly enhanced carrier separation. Surface doping remarkably tuned the electronic structure of the photoanodes changing the band position and band bending at electrode–electrolyte interface lead...

Layer-by-Layer Electrodeposited Reduced Graphene Oxide-Copper Nanopolyhedra Films as Efficient Platinum-Free Counter Electrodes in High Efficiency Dye-Sensitized Solar Cells

Efficient and low-cost platinum-free counter electrodes consisting of reduced graphene oxide (rGO) and copper nanopolyhedra (Cu) were fabricated on indium tin oxide (ITO) glass using a layer-by-layer electrochemical deposition. The structure and morphology of the as-fabricated counter electrodes were confirmed by X-ray diffraction, field-emission electron microscopy, and Raman spectroscopy. rGO-Cu-modified ITO electrodes, with different rGO loadings, were applied as catalytic counter electrodes in dye-sensitized solar cells (DSSCs) with an electrodeposited ZnO nanorod array as the photoanode. The current density−voltage curves and transient photocurrent measurements illustrate that the power conversion efficiency of the ZnO nanorod photoanode is significantly improved by the formation of rGO on the counter electrode. On the basis of electrochemical impedance spectroscopy, it is determined that the increased electrocatalytic activity of the rGO-Cu counter electrode is due to an enhanced conductivity of the counter electrodes. The maximum conversion efficiency is obtained for the rGO-Cu counter electrode with 0.5 mg ml−1 of rGO and has one and one half the efficiency achieved using a counter electrode modified only with Cu nanopolyhedra.

Amorphous NiO electrocatalyst overcoated ZnO nanorod photoanodes for enhanced photoelectrochemical performance

This research demonstrated that introducing amorphous NiO electrocatalysts onto the surface of ZnO photoanodes can effectively facilitate their PEC performance.

Enhanced photovoltaic performance of QDSSCs via modifying ZnO photoanode with a 3-PPA self-assembled monolayer

A SAMs layer of 3-PPA with a phosphonic acid headgroup and carboxylic acid tailgroups has been used to modify the surface of ZnO nanorods photoanode in CdS quantum dots sensitized solar cells. Its effects on the photovoltic performance have been investigated in detail by adjusting the concentration and deposition time of 3-PPA. Especially, we utilize ultraviolet photoelectron spectroscopy to characterize the variation of band alignment after introducing 3-PPA layer in the solar cells. The results reveal that the 3-PPA not only suppress the electron-hole recombination process due to its passivation on the surface defects, but forms an energy barrier to efficiently retard the back transfer of electrons, which finally results in the enhancement of conversion efficiency of solar cells.

Nickel Hydroxide Decorated Hydrogenated Zinc Oxide Nanorod Arrays with Enhanced Photoelectrochemical Performance

Photoelectrochemical water splitting over transition metal oxide photoanodes has attracted increasing attention as environmentally friendly and cost-effective method to produce hydrogen. In this work, we demonstrate that hydrogenation and nickel hydroxide coating can significantly improve the photoelectrochemical properties of ZnO nanorod photoanode. The nickel hydroxide decorated hydrogenated ZnO (Ni(OH)2/H-ZnO) nanorods exhibit substantially higher photoelectrochemical performance than the pristine ZnO and H-ZnO nanorods, which achieve a remarkable photocurrent density of ∼910μAcm−2 at 0.4V (vs. Ag/AgCl) under white light irradiation. Moreover, the Ni(OH)2/H-ZnO nanorods has a good stability without any photocurrent decay after 3h irradiation.