Oxygen Vacancies In ZnO Research Articles

High-efficient separation of photoinduced charge carriers in CoFe2O4-ZnO magnetic heterostructures mediated by oxygen vacancies

In this work, magnetic heterostructures were obtained by the in-situ growth of various ZnO amounts on the preformed CoFe2O4 nanoparticles. The X-ray diffraction characterization shows the presence of both ZnO and CoFe2O4 crystalline phases, and the crystallite size is about 13–14 nm for both components. X-Ray Photoelectron Spectroscopy (XPS) analysis revealed a CoFe2O4 inversion degree of 0.7. The composite samples demonstrated an extended absorption edge into the visible range. The sample’s magnetic properties were investigated by vibrating-sample magnetometer (VSM) and correlated with Electron Paramagnetic Resonance (EPR) results. The magnetic behavior was attributed to the combination of CoFe2O4 and ferromagnetic ZnO. The emergence of a ferromagnetic phase within ZnO was elucidated to originate from charge/spin transfer of spin-up polarized states from the CoFe2O4 Fermi level into empty oxygen vacancies of ZnO. High efficiency in Rhodamine B (RhB) degradation under visible light was observed in the prepared composite materials, particularly with an optimal CoFe2O4-ZnO ratio of 1:5, achieving an 81 % degradation rate within 6 h. The proposed photodegradation mechanism, based on various spectroscopic and magnetic investigations, highlights the role of spin transfer and oxygen vacancies in facilitating reactive oxygen species (ROS) generation for pollutant degradation.

Enhanced photocatalytic activity of green synthesized zinc oxide nanoparticles using low-cost plant extracts

Developing stable and highly efficient metal oxide photocatalysts remains a significant challenge in managing organic pollutants. In this study, zinc oxide nanoparticles (ZnO NPs) were successfully synthesized using various plant extracts, pomegranate (P.M), beetroot roots (B.S), and seder, along with a chemical process. The produced ZnO NPs were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–Vis), Field Emission Scanning Electron Microscope (FESEM), High-Resolution Transmission Electron Microscopy (HRTEM), and Surface Area. For all prepared samples, the results indicated that the composition of the plant extract affects several characteristics of the produced particles, such as their photocatalytic properties, energy bandgap (Eg), particle size, and the ratio of the two intensity (0 0 2) and (1 0 0) crystalline planes. The particle size of the produced NPs varies between 20 and 30 nm. To examine NPs' photocatalytic activity in the presence of UV light, Methyl Orange (MO) was utilized. The Eg of ZnO synthesized by the chemical method was 3.16 e. V, whereas it was 2.84, 2.63, and 2.59 for P.M, Seder, and B.S extracts, respectively. The most effective ZnO NPs, synthesized using Beetroots, exhibited a degradation efficiency of 87 ± 0.5% with a kinetic rate constant of 0.007 min−1. The ratio of the two intensity (0 0 2) and (1 0 0) crystalline planes was also examined to determine a specific orientation in (0 0 2) that is linked to the production of oxygen vacancies in ZnO, which enhances their photocatalytic efficiency. Furthermore, the increase in photocatalytic effectiveness can be attributed to the improved light absorption by the inter-band gap states and effective charge transfer.

Effective Photogeneration of Singlet Oxygen and High Photocatalytic and Antibacterial Activities of Porous Mn-Doped ZnO-ZrO<sub>2</sub> Nanocomposites

Disperse porous Mn-doped ZnO-ZrO<sub>2</sub> nanocomposites were prepared using the facile polymer-salt method. The effect of Mn content on the crystal structure, composite morphologies, their ability to photogenate the singlet oxygen, luminescence properties, and bactericidal activities were studied. The crystal structure and morphology of these materials were investigated using XRD and SEM analysis. It was found that obtained nanocomposites consist of small (~9 nm) hexagonal ZnO and fine ZrO<sub>2</sub> crystals and the embedding of Mn ions expands the crystal cells of ZnO crystals. Photoluminescence spectra indicate the presence of different structural defects (interstitial Zn ions and oxygen vacancies in ZnO and oxygen vacancies in ZrO<sub>2</sub> crystals). Mn-doped ZnO-ZrO<sub>2</sub> nanocomposites can photogenerate singlet oxygen under visible (λ = 405 nm) irradiation. The increased power density of the exciting blue (λ = 405 nm) light significantly enhances the singlet oxygen photogeneration by prepared composites. The dependence of the intensity of singlet oxygen photogeneration by composites on the power density of exciting radiation (at its variation in the range 0.8 ÷ 1.6 W/cm<sup>2</sup>) is close to linear. Mn-doped ZnO-ZrO<sub>2</sub> composites demonstrate superior antibacterial activity against the gram-positive bacteria <em>Staphylococcus aureus ATCC 209P</em>. It was found that highly dispersed porous Mn-doped ZnO-ZrO<sub>2</sub> nanocomposites are promising for practical environmental and medical applications.

Impact of oxygen vacancies on enhanced NO2 gas sensing performance of lithium doped Co:ZnO thin films

This article explored the influence of lithium on cobalt-doped ZnO thin films fabricated via the sol–gel spin coating technique for NO2 gas sensing applications. The abundance of oxygen vacancies in ZnO can be proven by scanning electron microscopy, four-probe Hall measurements, photoluminescence spectroscopy, UV-visible spectroscopy, and x-ray photoelectron spectroscopy. The presence of lithium plays a crucial role in generating more oxygen vacancies in Co-doped ZnO was discussed. Among the fabricated samples, (Li-Co) co-doped ZnO exhibits better sensitivity (2940.17%), selectivity, repeatability, and stability (after 90 days) toward 75 ppm NO2 gas.

Modulating oxygen vacancy of ZnO for visible-light driven photocatalytic H2 evolution via facile gas treatment approach

Photocatalytic hydrogen production through water splitting is one of the most promising approaches for sustainable and renewable energy source. In recent years, research on defect rich metal oxide semiconductors has increased tremendously. N-type semiconductor ZnO exhibits a wide direct band gap of 3.3eV, limiting the optical absorption at UV range, hindering the photocatalytic performance of pristine ZnO. Furthermore, short charge carriers life time can also decrease the efficiency for hydrogen evolution. In this report, crystal defect engineering via oxygen vacancy tailoring was conducted, narrowing the bandgap of ZnO, while creating interstitial defects that reduces the recombination rate of electrons and holes. ZnO was synthesized via hydrothermal method using zinc acetate dihydrate and sodium hydroxide as precursors before subjecting for conditional annealing for 30, 60, 90 and 120 min for 200 °C, 300 °C, 400 °C and 500 °C. A series of characterization techniques were used to authenticate the effects of oxygen vacancy in ZnO. Experimental studies revealed that ZnO annealed at 400 °C for 120 min exhibit the highest amount of hydrogen produced through photochemical reactions. Therefore, the degree of oxygen vacancies in ZnO semiconductor can be regulated through diligent control of annealing parameters. Specifically, finding the optimum annealing temperature and duration were the focus in this study that relates to its photocatalytic hydrogen evolution rate.

Revealing the Dynamics of Oxygen Vacancy in ZnO1–x/Cu during Robust Methanol Synthesis from CO2

Revealing the Dynamics of Oxygen Vacancy in ZnO_1–<i>x</i>/Cu during Robust Methanol Synthesis from CO₂

Effect of UV-O3 treatment on the performance of UV detector based on zinc oxide/molybdenum disulfide nanosheet heterostructures

UV photodetectors based on ZnO nanostructures, using a metal-semiconductor-metal (MSM) configuration, are popular due to their simple fabrication. However, they experience high dark currents and slow photoresponse speeds. This is due to a high recombination rate for photogenerated electrons and holes as well as low charge carrier mobility attributable to ZnO's inherent defects. In this study, MoS2 nanosheets was employed to cover the surface of ZnO nanorods, creating a unique architecture that focuses the impact of MoS2 solely on light absorption and charge transfer on the ZnO surface. Subsequently, a 15-min UV-O3 exposure is applied to the ZnO/MoS2 structure. The ZnO/MoS2UVo heterostructure exhibits remarkable results, including a six-fold increase in sensitivity compared to pure ZnO, Additionally, the Detectivity increases by 265 %, while the Responsivity and Quantum Efficiency rise by up to 102 %. These enhancements are attributed to the reduction of dark current coming from the reduction of oxygen vacancies as sulfur edge atoms from MoS2 fill these vacancies. Furthermore, additional dark current reduction post UV-O3 treatment is due to both the reduced ZnO oxygen vacancies and the formation of a thin MoO3 passivation layer on the ZnO NR surface. Despite these achievements, the study acknowledges the presence of multiple junctions between ZnO, MoS2, and MoO3, which still pose challenges for efficient charge transfer to the electrodes.

Effect of oxygen vacancy modification of ZnO on photocatalytic degradation of methyl orange: A kinetic study

This study investigated a novel kinetic model of the adsorption and photocatalytic degradation of methyl orange, an anionic dye, using a commercial ZnO and reduced commercial ZnO photocatalysts. The results were compared and discussed with our previous study on methylene blue, a cationic dye, providing new insights into the interaction of the catalyst with molecules with different charges. The kinetic model was applied to describe adsorption in the dark, followed by photocatalytic degradation under simulated sunlight. In addition, the predicted pKa values of methyl orange were estimated using the kinetic model, which were found to be 4.40 and 9.34. The effect of ZnO oxygen vacancies (created through hydrogen reduction at 500 °C) on the adsorption and photocatalytic degradation of methyl orange was investigated. The existence and effects of oxygen vacancies on ZnO were analyzed using TPR, PL, XPS, XRD, FE-SEM, EDS, BET, Tauc plot, Mott-Schottky, and cyclic Voltammetry. The appearance of oxygen vacancies was clearly observed by the characterization results, while the morphology of the nanoparticles was found to be unchanged. Additionally, the band gap of ZnO was decreased from 3.22 to 3.07 eV, and the maximum valence band of the catalyst was increased from 2.43 to 2.67 eV due to the reduction pretreatment. The findings suggest that the photocatalytic degradation rate of the reduced ZnO substantially increased. However, the adsorption removal of the anionic dye declined, which could be attributed to the more negative zeta potential of ZnO after hydrogen reduction. Finally, the quantum yield of photocatalytic process was calculated and compared with the results of methylene blue.

Strategic enhancement of oxygen defects in ZnO from ZnS for water splitting to generate green electricity by hydroelectric cell

The green energy produced by the material device Hydroelectric Cell possesses a huge potential to transform the energy generation by water splitting. It is an efficient, clean and green electricity generating device accomplishing the ambitious Net Zero target. The present study reports a facile and unique process to obtain highly defective and nanoporous ZnO for water splitting at room temperature by strategic heat treatment of ZnS and lithium substituted ZnS. The creation of defects enhanced microstrain and dislocation density in ZnO has been confirmed by X-ray diffraction analysis. Surface and lattice defects presence have been distinctly evidenced by photoluminescence (PL) spectroscopy. The oxygen vacancy in ZnO and Li-ZnO has been also confirmed by the g value of 2.06 and 2.09 calculated from electron paramagnetic resonance (EPR) measurements. As soon as water is sprinkled on the Hydroelectric cell, the surface defects chemidissociate water into H3O+ and OH− ions. Hydronium ions trapped in nanopores create a high electric potential that sustains water splitting. Due to redox reactions at electrodes of Hydroelectric cell, voltage and electric current is generated. The fabricated hydroelectric cells (4 cm2 area) of pristine ZnO, ZnS converted ZnO, and Li-ZnO exhibited short circuit current, ISC 5.6 mA, 15 mA and 53 mA respectively by instant water splitting. Innovative method of oxygen defects creation in ZnO by heat treatment of bulk ZnS is an environment friendly, novel and simple non-photocatalytic technique to generate green electricity by water splitting at room temperature.

Synergistic enhancement of Ce–ZnO/g-C3N4 photocatalytic performance using N,O-bis-vacancy induction and S-scheme heterojunctions

The photocatalytic decontamination of organic pollutants is an environmentally friendly and efficient water treatment method. The introduction of vacancies, heterojunctions, and doping can enhance the performance of photocatalysts for such applications. In this study, N,O double-vacancy Ce–ZnO/g-C3N4 S-scheme heterojunction photocatalysts were prepared. It is found that the electron separation and transfer efficiencies are considerably improved by the presence of nitrogen vacancies in graphitic carbon nitride (g-C3N4) and oxygen vacancies in ZnO, in addition to the S-scheme heterojunction created between the two materials. In the degradation of methylene blue and ciprofloxacin, the degradation rates exhibited by Ce–ZnO/g-C3N4 are significantly higher than those of the g-C3N4/ZnO system. This remarkable photocatalytic performance of Ce–ZnO/g-C3N4 may result from three key factors: (i) Ce doping, which produces impurity energy levels; (ii) the generation of electron capture centres by the N and O vacancies; and (iii) the synergistic effect of the S-type heterojunction formed at the interface between the two materials. This study therefore provides new approaches for the use of doping, vacancies, and heterojunctions to induce the efficient degradation of organic pollutants.

Melamine-Assisted Thermal Activation Method for Vacancy-Rich ZnO: Calcination Effects on Microstructure and Photocatalytic Properties.

Defect engineering is considered an effective method to adjust the photocatalytic properties of materials. In this work, we synthesized the vacancy-rich ZnO rods with (100) planes via the melamine-assisted thermal activation method. A high concentration of oxygen vacancies was successfully introduced into non-polar oriented ZnO rods by calcination. The effect of oxygen vacancy on the photocatalytic properties of non-polar-oriented ZnO rods was investigated. Raman and XPS spectra revealed the formation of oxygen vacancies in the ZnO. The results showed that the growth habit and defects in ZnO can be controlled by changing the ratio of ZnO to melamine. The higher ratio of ZnO to melamine led to more amounts of (100) planes and oxygen vacancies in ZnO, and it reached the highest when the ratio was 1.2:1. When the ratio was 1.2:1, ZnO exhibited a high methyl orange degradation rate (95.8%). The differences in oxygen vacancy concentration and non-polar planes were responsible for the improvement in photocatalytic performance. ZnO exhibited good stability and regeneration capacity. After recycling four times, the degradation rate was still at 92%. Using the same method, vacancy-rich α-Fe2O3 was obtained. This work could offer a new and simple strategy for designing a photocatalyst with oxygen vacancies.

Extraordinary photooxidation properties of ZnO nanosheets with surface oxygen vacancies of low density

Surface defect engineering strategies are critical for catalytic activity, especially in the photodegradation of organic pollutants. However, the exact function of the oxygen vacancies for the photooxidation performance of ZnO is unclear. Oxygen vacancies might act as electron donors to encourage carrier separation and then boost catalytic efficiency; besides, excessive oxygen vacancies might act as recombination centers for photogenerated electron–hole, which impairs photocatalytic performance. Therefore, the specific effect of the amount of oxygen vacancies in ZnO needs to be investigated. We developed a facile method, which is inspired by the aforementioned strategy, to adjust the amount of oxygen vacancies in ZnO nanosheets for exploring its photocatalytic activity. ZnO nanosheets with oxygen vacancies of low density exhibit a greatly enhanced photooxidation ability and can rapidly and stably degrade high concentration (4 × 10−4 mol/L) of phenol compared with nearly inactivity of the ZnO nanosheets with plentiful oxygen vacancies. This mechanism of the improved photocatalytic activity is proposed to be mainly due to the significant reduction in oxygen vacancies as carrier recombination centers and the increase in photogenerated electron–hole mobility. This research presents a new idea for the surface defect engineering design of regulated photocatalysts to enhance photocatalytic activity.

Effect of Sulfur Contents in NiZnS Composite Photocatalysts on Solar Water Splitting

Solar energy is attracting much attention as an eco-friendly source for future energy needs. Herein, NiZnS photocatalysts were synthesized with a hydrothermal method at various sulfur contents. The ZnS material is widely used as a photocatalyst because of its high stability, low toxicity, and excellent charge separation characteristics. Nickel is considered a co-component in the ZnS base to improve hydrogen evolution efficiency, because nickel sulfide has a narrow band gap. Field emission scanning electron microscopy analysis was used to observe particle size and shape. As the sulfur ratio increased, the particle size increased, and relatively uniform particle sizes were obtained at the 2:2 molar ratio of NiZn:S. X-ray diffractometer analysis showed the formation of ZnO crystals at low sulfur contents in the NiZnS photocatalysts. Among the various NiZnS compositions, the NiZn:S ratio of 2:2 resulted in the highest hydrogen production rate (1541.5 μmol/g/h) with stable reproducibility. UV-vis spectroscopy was used to analyze light absorbance, and the band gap changed with different sulfur contents due to the oxygen vacancies in ZnO, as identified by X-ray photoelectron spectroscope. High amounts of thiourea used to introduce the sulfur increased the particle sizes and blocked sunlight coming to NiZnS surfaces, thereby degrading photocatalytic performance. Therefore, changing the sulfur content when fabricating the NiZnS composite photocatalysts affected the crystalline structures and band characteristics of the materials, and it finally resulted in improved light absorption, charge separation, and the hydrogen production rate of the photocatalysts.

Oxygen vacancy engineering of zinc oxide for boosting piezo-electrocatalytic hydrogen evolution

Vibration-driven piezo-electrocatalytic hydrogen evolution reaction has drawn great interest in the past years as it provides an effective approach to harvest the dispersed and extensive mechanical energy. However, its wide applications remain a grand challenge due to the unsatisfied catalytic activity of currently available piezo-electrocatalysts. Here, the defect engineering is applied in the hydrothermally-synthesized ZnO by heat treatment to fabricate ZnO catalysts with various concentrations of oxygen vacancy. Under the stimulation of ultrasonic vibration, ZnO with optimal oxygen vacancy concentration shows a piezo-electrocatalytic H2 yield of 3.8 mmol·h−1·g−1. The controlled experiments of piezo-current response and electrochemical impedance spectroscopy disclose that the piezo-generated carriers’ separation and transfer are greatly enhanced in ZnO after introducing oxygen vacancies. Density functional theory calculations uncover that the oxygen vacancies of ZnO play an important role in facilitating H desorption. This work delivers an efficient strategy to achieve the simultaneous improvement of carrier dynamics and surface reaction and ultimately promote piezo-electrocatalytic H2 evolution.

Highly sensitive and lower detection-limit NO[formula omitted] gas sensor based on Rh-doped ZnO nanofibers prepared by electrospinning

As a kind of toxic gas, NO2 can cause damages to the environment and threaten the health of human beings. To develop highly sensitive NO2 gas sensor is necessary. Here, the Rh-doped ZnO nanofibers were prepared by electrospinning. The Rietveld refinements results of XRD patterns have proved the successful doping of Rh into the crystal lattice of ZnO. The XPS spectra reveals the existence of large amounts of surface oxygen vacancy defects, which are beneficial to the enhancement of the sensitivity. The morphology and crystal lattice of the nanofibers were investigated by SEM and HRTEM, respectively. The highly sensitive NO2 gas sensor based on 1% Rh-doped ZnO nanofibers, with the experimental detection limit of 50 ppb (or 18.6 ppb, theoretical signal-to-noise ratio ¿ 3) was fabricated. At the optimal operating temperature of 150 °C, its response values (Rg/Ra) are 1.04 and 36.17, with the corresponding response time of 45 s and 32 s, towards 50 ppb and 10 ppm NO2, respectively. The sensor also exhibits good selectivity, repeatability and linearity between the relative humidity and the response values. The doping of Rh introduces large amounts of surface oxygen vacancy defects. The defects are more favored adsorption sites for NO2. The reasons for the ultrasensitivity of the sensor are as followings: (1) The binding energy between NO2 and ZnO with surface oxygen vacancy defects is about 1.5 times stronger than that for adsorption on the stoichiometric surface; (2) The surface oxygen vacancies of ZnO can have a reaction with NO2 to produce NO inducing the charge transfer; (3) The catalytic activities of Rh towards NO2 and H2O.

A comparative study of the ZnO Fibers-based photodetectors on n-Si and p-Si

The ZnO fibers (Fs)/p-Si (labeled D1) and ZnO Fs/n-Si (labeled D2) photodetector heterojunctions were fabricated and both devices gave a clear rectifying I–V characteristic with a high rectifying ratio, in the dark. At zero bias, D1 showed self-driven characteristics, while D2 had not and D1 was found to be more stable than D2 in time-dependent measurements. Optoelectronics results revealed that D1 had high sensitivity to both visible and excellent stability after 20 days. At zero bias, the ON/OFF ratio of the D1 photodetector was as high as 33 650 and in the dark, a rectification ratio of 67 400 within ±2 V was obtained for the D1 device. The maximum responsivities of the devices was ∼0.8 mA W−1, and their detectivity was ∼109 Jones. Furthermore, the ZnO Fs/p-Si (labeled D3) and ZnO Fs/n-Si (labeled D3) photodetectors yielded excellent response to 365 nm and 395 nm UV light (10 mW cm−2). Responsivity, detectivity (D), and external quantum efficiency values reached as high as 5.28 A W−1, 1.02 × 1013 Jones, and %16.6, respectively under 365 nm UV light. The excellent responses of the photodetectors to visible and UV light were attributed to the oxygen vacancies in ZnO and the formation of electron–hole pairs by the light effect and their separation by the electric field in the device formed between ZnO and Si.

Effect of inert ambient annealing on structural and defect characteristics of coaxial N-CNTs@ZnO nanotubes coated by atomic layer deposition

The structure and intrinsic defects of ZnO determine the photogeneration of electron-hole pairs. The control of these characteristics allows for improving the activity of photocatalytic nanostructures. The intrinsic defects of ZnO atomic layer deposited around carbon nanotubes, coaxial N-CNT@ZnO, and its evolution after annealing in a nitrogen atmosphere are studied. TEM showed that the coaxial N-CNT@ZnO structure is preserved up to 500 °C; some damage is seen at 600 and 700 °C. Raman and XPS reveal a low surface defect concentration for as-grown ZnO, increasing to a maximum for the 600 °C annealed sample. Cathodoluminescence shows a broad low-intensity green emission associated with oxygen vacancies of ZnO. The surface defects improved the photocatalytic degradation of amaranth dye, however, damage to the coaxial structure lowers the catalytic activity.

Laser engineering of ITO/ZnO/ITO structures for photodetector applications

ZnO nanomaterials have received much attention due to their suitability for applications such as gas sensors, UV detectors, and solar collectors. However, the functionality of ZnO in optical applications is often limited by its wide bandgap (3.15 eV) which restricts the response to shorter wavelengths. In view of this limitation, there has been much interest in tuning the optical properties of ZnO through defect engineering. In this work, we show that processing ZnO thin films with nanosecond (ns) laser irradiation is a simple and effective way to introduce interband defects lowering the bandgap and increasing the sensitivity of ITO/ZnO/ITO photodetector structures at longer wavelengths. In particular, we show that the concentration of oxygen vacancies in ZnO is proportional to laser fluence below 700 mJ/cm2, but that an increase in laser fluence above this value results in thermal heating that anneals the film and lowers the relative abundance of oxygen vacancies. On the other hand, the surface morphology of ZnO does not change significantly even though the fluence reaches 700 mJ/cm2. To illustrate the way in which laser processing can be utilized to improve the optical properties of ZnO films in photodetector applications, we have fabricated transparent ITO/ZnO/ITO stacked structures and measured their response at various optical wavelengths. We find that processing with ns laser radiation is effective in enhancing the responsivity and detectivity of these devices at blue (460 nm) and UV (390 nm) wavelengths. The response of the photodetector is also increased at green wavelengths (570 nm) and red wavelengths (620 nm) when processed with laser fluences in the 480–600 mJ/cm2 range.

Photo-response of Fe-doped ZnO electron transport interlayer on Poly(3-hexylthiophene-2,5-diyl): Phenyl-C61-butyric acid methyl ester based thin film device for photodiode application

An electron transport material - Fe doped ZnO layer with atomic ratio Zn: Fe = 90:10 was prepared for enhancing the photocurrent between an optically active poly(3-hexylthiophene-2,5-diyl): phenyl-C61-butyric acid methyl ester film and an indium tin oxide film used as an anode. The crystallite size of ZnO decreases with increasing Fe doping as estimated from major X-ray diffraction (100), (002), (101), and (110) peaks using the Scherrer formula. ZnO particle size reduction on Fe doping was also observed by field emission scanning electron microscopy. The photoluminescence study on Fe-doped ZnO shows interstitial oxygen defects band at ∼2.28 eV and the bound exciton caused by oxygen vacancy in ZnO. The X-ray photoelectron spectra show shift of the Zn2p3/2, and Zn2p1/2 peaks to lower binding energy due to Fe3+ doping in the ZnO. Time-correlated single-photon counting experiment reveals faster decay of lifetime of Fe-doped ZnO than undoped material at excitation wavelength of 650 nm due to higher electron transport rate through the interface according to Langevin recombination formula. The photoresponse property of the films was measured under solar light illumination (10 mW/cm2), and the results show that Fe-doped ZnO can be a better than undoped ZnO based multilayer device for photodiode application.

Crystal-facet and microstructure engineering in ZnO for photocatalytic NO oxidation

Photocatalysis is believed to be an important way of reducing NO pollutant in air and the facet engineering of semiconducting oxides could enhance the efficiency of the photocatalysis. ZnO nanoparticles with different exposed crystalline facets were successfully synthesized using a hydrothermal method and their photocatalytic degradation towards NO was investigated. The crystals from ZnCl2 precursor were hexagonal mesoporous ones with exposed (0002) facet, while those from zinc acetate were in the form of flakes or wheat ears with enhanced exposure of (101(−)1) facet. Calcination in air imparted an enhanced the textural coefficient of the orientated facets as well as the oxygen defects. The nanocrystals with enhanced (0002) facet and lower flat-band energy did better in photoelectrochemical water-oxidation than those with exposed (101(−)1) facet that showed superior photocatalytic activity (approaching 76.7 ± 0.6% under 365 nm photons) for NO oxidation. According to theoretical calculations, (101(−)1) facet with O termination showed much higher affinity to NO molecules than other configurations, and the oxygen vacancy in ZnO played an minor role in the photocatalytic oxidation of NO. A high quantum efficiency approaching 97.5 ± 1.4% under 275 nm photons was obtained for the ZnO crystals from zinc acetate with mixed (0002) and (101(−)1) facets. This research explores the special characteristics of ZnO with different exposed facets and is important for the future design of highly efficient photocatalyst for hazardous material removal.