Ni-Doped ZnO nanofibers have been synthesized by a simple, facile electrospinning method. The structure and morphology of the products were characterized by SEM, HRTEM and XRD analysis. X-ray photoelectron spectroscopy (XPS) confirmed that Ni2+ ions exist in the ZnO nanofiber structures. The photoelectric gas-sensing tests reveal that the response was significantly enhanced by Ni doping, and the 0.7% molar ratio Ni-doped sample (NZ0.7) exhibits the highest response of 532.7% to 100 ppm HCHO at room temperature. Photoluminescence (PL) measurements show that there are more donors in Ni-doped ZnO nanofibers by the introduction of Ni2+ ions. Furthermore, the surface photovoltage (SPV) and transient photovoltage (TPV) tests indicated that Ni doping can effectively enhance the donor density, which could facilitate charge separation and transport in the semiconductor, and promote photogenerated holes to move toward the irradiated surface of the samples. These results contribute to the photocatalytic oxidation of HCHO so as to achieve higher gas-sensing properties for Ni-doped ZnO nanofibers.

ZnO nanofibers, nanoplates, nanoflowers have been successfully synthesized by simple electrospinning and hydrothermal routes, respectively. The characterization of three samples was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV–vis diffuse reflectance spectrum (UV–vis DRS). The effects of ZnO nanostructure morphology on the gas-sensing property of HCHO have been investigated. The gas sensor based on ZnO nanofibers showed a high sensitivity, reversible response and good selectivity towards HCHO under the irradiation of 365 nm UV-light at room temperature. These desirable sensing features can be attributed to the special network structure with larger specific surface areas and its one-dimensional arrangement of ZnO nanocrystallites with a larger proportion of the depletion layer, which results in more obvious adjustment of the depletion layer width. The results confirm that ZnO nanofibers obtained by the electrospinning are attractive chemical sensing materials.

Three dimensional (3D) center-hollow ZnO architectures assembled by nanoparticles have been successfully fabricated on a large scale via a template-free method using an oil bath. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller specific surface area, surface photocurrent and UV-Vis diffuse reflectance spectroscopy. The photoelectric gas-sensing results demonstrated that the 3D porous, center-hollow ZnO structures exhibited excellent sensitivity and good selectivity to formaldehyde under 365 nm light irradiation at room temperature. The gas response to 1 ppm formaldehyde can reach 70%, which is superior to the results reported in the literature, indicating that the 3D center-hollow ZnO architectures are ideal candidate materials for photoelectric gas sensors. The underlying mechanisms responsible for the high sensitivity and selectivity to formaldehyde are discussed, which provide a new pathway for designing novel VOC sensors. Moreover, the facile method presented in this paper has the advantage of low-cost and high-yield, which is suitable for the practical production processes.

The composite Bi2O3/ZnO photocatalyst, a novel p–n type heterojunction with different molar ratios of Bi to Zn, has been fabricated using a hydrothermal method. The XRD, XPS, UV-vis DRS, SEM, HRTEM studies were used to characterize the as-obtained products. The photocatalytic activities of the p-Bi2O3/n-ZnO heterojunctions were investigated for their efficiency on the degradation of alizarin red (AR) dye under visible light irradiation (λ > 420 nm), and the results showed that the composites possessed remarkable photocatalytic activities, which were conducive to their separation, recycling, and reuse. The photogenerated charge-transfer properties were tested by surface photocurrent (SPC) and surface photovoltage (SPV) experiments and results show that the interfacial electric field located between Bi2O3 and ZnO played an important role in promoting the separation of photogenerated electron and hole pairs. These results are helpful to design and construct high efficiency heterogeneous semiconductor photocatalysts.

Hybrid Pt–Co:ZnO nanostructure photocatalysts were prepared via a facile two-step synthetic strategy. SPS and TPV investigations demonstrate the existence of the synergetic effect between Pt and Co dopants. Such synergetic effect could make use of visible photons as well as facilitates the separation of photogenerated charges to prevent recombination, effectively prolongating the charges lifetime to participate photocatalytic reaction. The synergetic effect exist in Pt–Co:ZnO inducing as high as 7.7-fold in photovoltaic response and 10-fold in the photo–activity for hybrids compared to Co:ZnO.Keywords: hybrid nanostucture; photocatalytic activity; photovoltaic properties; Pt−Co:ZnO; synergetic effect;

Ag–ZnO nanorods with enhancement in UV light photoelectric gas-sensing response properties were synthesized by a facile two-step process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution electron microscope (HRTEM) and UV–vis diffuse reflectance spectroscopy (UV–vis DRS). The results revealed that Ag nanoparticles have decorated the surface of ZnO nanorods successfully. Formaldehyde (HCHO) sensing properties of sensors based on the Ag–ZnO nanorods were also investigated under 370 nm light irradiation at room temperature with the assistance of surface photocurrent technique. It was found that the as-prepared Ag–ZnO nanorods showed excellent gas-sensing property that the gas response to 5 and 40 ppm formaldehyde can reach to 9.4% and 119.8% under 370 nm light irradiation at room temperature, respectively. To better understand the reason for the enhanced gas-sensing properties of Ag–ZnO, the separation and transfer behavior of the photogenerated charges were characterized by means of surface photovoltage and phase spectra. The results indicated that an electronic interaction between Ag and ZnO formed. Ag, as electron acceptor, effectively promoted the charge separation and transfer ability and, at the same time, inhibited the recombination of photogenerated charge carriers in ZnO, thus the Ag–ZnO nanorods had a high gas-sensing response to HCHO at room temperature.

A visible-light-active ZnO photocatalyst system in the presence of manganese ions (Mn/ZnO) was prepared via a simple and rapid approach. XRD, XPS, Raman scattering and UV-Vis DRS confirmed the manganese exists in multivalent forms (Mn3+/Mn2+) in the ZnO lattice, furthermore, ZnO light absorption is extended to the visible region. The photocatalytic activities of the catalysts were evaluated by measuring the photodegrading efficiency of 2,4-dichlorophenol (DCP) under visible light irradiation. With an optimal molar ratio of 5% in Mn/ZnO the highest rate photodegradation was achieved under the experimental conditions. We have characterized the separation and transfer behavior of the photogenerated charges in the visible region by means of surface photovoltage (SPV), surface photocurrent (SPC) and transient photovoltage (TPV) techniques. Based on the comprehensive investigation of the photovoltaic properties of Mn/ZnO photocatalyst, we illustrate the behavior of photogenerated charges have distinct effects on the photocatalytic activity. It is demonstrated that the incorporation of multivalent Mn in ZnO promoted the separation of photogenerated charges, inhibited the recombination of photogenerated carriers, and thus prolonged the charges lifetime to participate in the photocatalytic reaction, resulting in highly efficient photocatalytic activity, which is attributed to the formation of a strong electronic interaction between the multivalent Mn and ZnO.

In2O3-sensitized flowerlike ZnO with visible light photoelectric response properties were synthesized by a facile two-step process, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX, HRTEM and UV-vis diffuse reflectance spectroscopy. The results revealed that In2O3 nanoparticles have grown and eventually coalesced on the surface of the flowerlike ZnO successfully, and the samples exhibited significant response to visible light. The photoelectric gas-sensing of the In2O3-sensitized ZnO was also studied to formaldehyde (HCHO) under 460 nm light irradiation at room temperature with the help of surface photocurrent technique. It was found that ZnO sensitized with In2O3 could enhance the gas response to HCHO under the visible light illumination. This may be due to the fact that the composite structure of In2O3–ZnO extends the photo absorbing range to visible light area, inhibits the recombination of photo-generated electrons and holes, and thus increases the utilization of photo-generated carriers in photoelectric gas detection, resulting in the higher sensing response in some extent. The gas response to 5 ppm and 100 ppm formaldehyde can reach to 19% and 419% under visible light irradiation at room temperature, respectively. These results should be valuable for designing a new type of visible-light assisted gas sensor at room temperature.

ZnO nanostructures materials with two-dimensional (2D) plate-like and three-dimensional (3D) flower-like morphologies have been synthesized via facile hydrothermal process. Scanning electron microscopy (SEM), high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD) have been used to characterize the samples. We have compared the surface photoelectric properties of flower-like ZnO with that of plate-like ZnO by means of surface photovoltage (SPV), field-induced surface photovoltage (FISPV), transient photovoltage (TPV) and surface photocurrent (SPC) techniques. Based on the photovoltage responses, the influence of the morphology on transfer characteristics of photogenerated charge carriers was discussed. The synthesized flower-like ZnO exhibited more efficient separation and transfer properties of photogenerated charge carriers than that of plate-like ZnO in the UV region due to the special flower-like hierarchical nanostructures. We have also evaluated the photocatalytic activity of the ZnO samples by photodegradation efficiency of RhB. The experiments demonstrated that RhB in aqueous solution was more efficiently photodegraded using flower-like ZnO than plate-like ZnO. The results also show the consistency between the carrier dynamics of photogenerated electron–hole pairs and photocatalytic activity.Highlights► Flower-like ZnO possesses higher specific surface area compared to plate-like ZnO. ► Flower-like ZnO exhibits an efficient separation of photogenerated charge carriers. ► Flower-like ZnO shows better activities in the UV region than the plate-like ZnO. ► The SPV response of plate-like ZnO appeared in the visible region by field-induced. ► The correlation among the morphology, surface photovoltage property and photocatalytic activity is studied in this work.

In this work, Fe-doped flowerlike ZnO powders with various doping contents were successfully fabricated by a hydrothermal method. The results of X-ray diffraction and UV–vis DRS spectra revealed that the Fe ions have been successfully doped into the crystal lattice of the ZnO host structure, and the optical absorption response of Fe-doped ZnO was extended into the visible region for the incorporation of Fe ions. The room-temperature photoelectric gas sensing of formaldehyde (HCHO) based on the Fe-doped ZnO was also studied under 532 nm light irradiation provided by a green laser pointer. It was found that the as-prepared Fe-doped ZnO samples showed excellent sensitivity, in which the gas response to 5 and 100 ppm formaldehyde can reach to 22% and 287% under 532 nm light irradiation at room temperature, respectively. The sensing mechanism of the obvious visible-light-induced photoelectric gas sensing was discussed with the help of surface photovoltage measurement. Our results demonstrated that visible light irradiation was a promising approach to achieving a large response for gas sensors at room temperature. This work will pave a way for the development of a low-cost practical gas sensor.

In this paper, a series of BiOI/ZnO photocatalysts containing various BiOI contents were prepared by a facile two-step synthetic method. The structure and crystal phase, morphology, surface element analysis, optical property of as-prepared samples are measured by X-ray diffraction (XRD), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and UV–Vis diffuse reflectance spectrometry (DRS). BiOI/ZnO photocatalytic activities of the prepared photocatalysts were evaluated by photocatalytic degradation of phenol under simulated light irradiation. The phenol degradation rate reached 99.9% within 2 h under simulated solar light irradiation. The probable photocatalytic mechanism of composites photocatalysts is discussed by active species trapping experiments, the surface photovoltage (SPV), the transient photovoltage (TPV) and photoluminescence (PL) measurements. The results manifest that the superior photocatalytic activity of BiOI/ZnO composites is derived from the strong internal electric field between BiOI and ZnO, which is beneficial for the effective separation and transfer of photogenerated charges in ZnO. Moreover, the loading of BiOI on the surface of ZnO inhibited the recombination of photogenerated charge carriers in ZnO, resulting in excellent photocatalytic activity. On the contrary, the effect of an extension of the light absorption range induced by the introduction of BiOI on the phenol degradation activity is not significant.Figure optionsDownload full-size imageDownload high-quality image (54 K)Download as PowerPoint slide

In2O3-sensitized flowerlike ZnO with visible light photoelectric response properties were synthesized by a facile two-step process, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX, HRTEM and UV-vis diffuse reflectance spectroscopy. The results revealed that In2O3 nanoparticles have grown and eventually coalesced on the surface of the flowerlike ZnO successfully, and the samples exhibited significant response to visible light. The photoelectric gas-sensing of the In2O3-sensitized ZnO was also studied to formaldehyde (HCHO) under 460 nm light irradiation at room temperature with the help of surface photocurrent technique. It was found that ZnO sensitized with In2O3 could enhance the gas response to HCHO under the visible light illumination. This may be due to the fact that the composite structure of In2O3–ZnO extends the photo absorbing range to visible light area, inhibits the recombination of photo-generated electrons and holes, and thus increases the utilization of photo-generated carriers in photoelectric gas detection, resulting in the higher sensing response in some extent. The gas response to 5 ppm and 100 ppm formaldehyde can reach to 19% and 419% under visible light irradiation at room temperature, respectively. These results should be valuable for designing a new type of visible-light assisted gas sensor at room temperature.

Three dimensional (3D) center-hollow ZnO architectures assembled by nanoparticles have been successfully fabricated on a large scale via a template-free method using an oil bath. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller specific surface area, surface photocurrent and UV-Vis diffuse reflectance spectroscopy. The photoelectric gas-sensing results demonstrated that the 3D porous, center-hollow ZnO structures exhibited excellent sensitivity and good selectivity to formaldehyde under 365 nm light irradiation at room temperature. The gas response to 1 ppm formaldehyde can reach 70%, which is superior to the results reported in the literature, indicating that the 3D center-hollow ZnO architectures are ideal candidate materials for photoelectric gas sensors. The underlying mechanisms responsible for the high sensitivity and selectivity to formaldehyde are discussed, which provide a new pathway for designing novel VOC sensors. Moreover, the facile method presented in this paper has the advantage of low-cost and high-yield, which is suitable for the practical production processes.