Surface Of ZnO Nanowires Research Articles

Construction of ZnO–SnO2 n-n junction for dual-sensing of nitrogen dioxide and ethanol

As inspired by the recent reports that many unique properties can be achieved by using two dissimilar components to form heterostructures, especially for materials in nano-size. In this letter, one-dimensional ZnO–SnO2 heterostructures were constructed by uniformly growing SnO2 nanoparticles on the surfaces of ZnO nanowires. Compared with pure ZnO nanowires, ZnO–SnO2 heterostructures exhibited significant enhancement in response and selectivity to NO2 and ethanol. The excellent response/recovery characteristics and stability of ZnO–SnO2 heterostructures demonstrate that the heterostructure construction is promising for dual sensing of NO2 and ethanol using one sensor. The possible enhancement sensing mechanisms were discussed based on the depletion control and potential barrier control models.

Chemisorbed CO2 molecules on ZnO nanowires (100 nm) surface leading towards enhanced piezoelectric voltage

Absract Here, we have studied the behavior of ZnO based piezoelectric mechanical transducer in the presence of CO2. We have analyzed reduction-oxidation mechanism of CO2 gas on ZnO nanowire surface and studied its behavior for the enhancement piezoelectric output voltage. Rise in piezoelectric potential for various CO2 exposures time intervals from 30 min to 3 h has been observed. Chemiresistive behavior of CO2 modified ZnO nanowires, is being explained by the generation of high piezoelectric voltage. ZnO nanowires based nanogenerator has been exposed to 30 min, 60 min, 90 min, 120 min and 3 h in a sealed chamber at 20 ppm, corresponding output piezoelectric voltages of 503.9 mV, 1.15 mV, 1.559 V and 1.795 V have been recorded respectively by using Picoscope 5204. It has also been observed that output voltage value has been saturated after 2 h of CO2 exposure. ZnO nanowire based VING (vertically integrated nanowire generator) structure is being used for the study under minute external force of 100 nN. Growth orientation of ZnO nanowires has been ensured along c-axis perpendicular to substrate.

ZnO nanowires/YAG:Ce functional heterostructure coatings with tunable optical properties

Luminescent ZnO nanowires (NWs)/YAG:Ce heterostructure coatings have been elaborated by impregnating cheap hydrothermally-grown ZnO NWs array with ground commercial YAG:Ce nanoparticles (NPs). TEM and photoluminescence quantum yield measurements have been used to make YAG:Ce NPs suitable for soaking within the ZnO NWs. Structural, morphological and optical studies of functional nanocomposite coatings have evidenced an optimal amount of YAG:Ce allowing the visible photoluminescence signal of ZnO NWs nearly to triple whereas slight red shift accompanied by an enlargement on the long wavelength side has been observed for YAG:Ce emission. These phenomena have been explained through the existence of structural defects on ZnO NWs surface and emission mechanisms implying either scattering effects or the passivation of NWs surface by YAG:Ce NPs. Furthermore, an energy diagram involving different structural defects of ZnO NWs as well as Ce3+ ions energy levels has been built from excitation and emission spectra and from decay curves recorded monitoring the orange-red emission of ZnO. The elaborated heterostructure functional coatings have proved to exhibit tunable optical properties in terms of spectral distribution playing on the excitation source. Hence, they appear as a smart way for developing LEDs-based devices with flexible photometric parameters for a broader range of optical applications.

Enhancing ZnO nanowire gas sensors using Au/Fe2O3 hybrid nanoparticle decoration

Heterojunctions are an important strategy for designing high performance electrical sensor materials and related devices. Herein, a new type of metal-semiconductor hybrid nanoparticle has been successfully used to remarkably sensitize the surface of ZnO nanowires for detecting NO2 with high responses over a broad temperature window ranging from room temperature to 600 °C. These hybrid nanoparticles are comprised of iron oxide nanowires with well dispersed single crystalline Au nanoparticles. The hybrid nanoparticle decorated ZnO nanowires have achieved a giant response, as high as 74 500 toward NO2 gas, about 42 times that of Au decorated ZnO nanowire sensors. This dramatic enhancement may be attributed to the efficient charge transfer across the Au–Fe2O3 Schottky and Fe2O3–ZnO semiconductor heterojunction interfaces. Due to the incorporation of thermally-stable Fe2O3 nanoparticles as the support of Au nanoparticles, the working temperature of nanowire sensors was successfully extended to higher temperatures, with an increase of 200 °C, from 400 °C to 600 °C. Such a combination of semiconductor heterojunction and semiconductor-metal Schottky contact presents a new strategy for designing high performance electrical sensors with high sensitivity, stability, selectivity, and wide operation temperature window, which are potentially suitable for advanced energy systems such as automotive engines and power plants.

High Performance UV-LED Activated Sensor Development By Modulating the Morphology and Composition

Metal Oxide Semiconductor (MOS) gas sensors have been studied for monitoring indoor/outdoor air pollutants. A drawback of conventional chemical-resistive gas sensing is the high operating temperature (up to 500°C) that is required for surface reactions. The high temperature operation, however, causes drift problems and makes sensors unsafe for environments containing flammable gases.Ultraviolet light-emitting diode (UV-LED) is a new technological field that has recently seen rapid improvements and novel possible applications. These include the development of UV-LED based gas sensors. UV photons emitted from UV-LED could activate sensors to allow detection of chemicals at ambient temperature. UV irradiation can also improve the sensor performance in harsh environments, where SO2 or NOx gases are present, through desorbing surface contaminants. The state-of-the-art ultraviolet light emitting diode (UV-LED) that emit radiation at a power of a few milli-watts is a promising UV source for this application.UV-activated sensors have a variety of advantages, compared to the traditional chemi-resistive metal oxide semiconductor (MOS) sensors, such as higher stability, lower preparation time, and the ability to safely detect flammable gases. The advances in ultra-violet light emitting diode (UV-LED) technology and its utilization in the gas sensing industry may offer a breakthrough in UV-based sensors, providing several important progresses such as broader application, smaller size, and higher efficiency of the developed sensors.Among the various metal oxides that have been studied for gas sensing applications, ZnO nano-materials have shown promisebecause of high electron conductivity features. ZnO has strong luminescent properties, broad UV absorption, and high stability. Thephoto-activity of ZnO can be improved by synthesizing one-dimensional (1D) nanostructuresand by functionalizing with catalytic metal nanoparticles. This combination lowers the requiredactivationenergy and reduces the recombination rate of photo-generated charge carriers.We will present the fabrication of a sensitive 1D nanostructure ZnO gas sensor decorated with Pt nanoparticles to detect low concentrations of toxic gases at room temperature under UV-LED irradiation (Figure 1). ZnO nanoparticles and nanowires decorated with platinum nanoparticles in different loading concentrations were prepared to detect low concentrations of NO2 under UV-LED irradiation. Solution precipitation, self-assembly crystallization, and photo-deposition techniques were used to synthesize the sensing material. The synthesized sensors at different stages of development were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analyses.To obtain a certain visual evidence of Pt nanoparticleson the surface of ZnO nanowires, High-resolution transmission electron microscopy (HRTEM) and STEM imaging were employed. The presence of Pt nanoparticles was further confirmed through high-angle annular dark-field (HAADF) STEM analysis.Although the sensing response of the pristine ZnO nanoparticles was relatively high (0.4) for NO2 detection, the sensing response improved significantly using ZnO nanowires in the identical photo-activation settings. The decoration ofthe surface of the ZnO nanowires with Pt nanoparticles further enhanced the sensing performance, whereas the 0.1wt% Pt-decorated ZnO nanowire sensor exhibited a response of more than one order of magnitude higher (1.5) than the response generated by the ZnO nanoparticles. This improved performance is attributed to the role of Pt active sites in promoting NO2 adsorption on the ZnO nanowires’ surface as well as enhancing the layer electron utilization.We will present the results and discuss how UV-LED has the potential to significantly impact sensor development and portable gas monitoring systems by enabling novel sensor design concepts. Figure 1

Pentagram-Shaped Ag@Pt Core-Shell Nanostructures as High-Performance Catalysts for Formaldehyde Detection.

High-performance HCHO sensors are of great importance in various application fields such as indoor air quality assessments. Herein, bimetallic Ag-Pt nanoparticles are synthesized as high-performance catalysts for ZnO-based gas sensors. Spherical aberration (Cs)-corrected transmission electron microscopy images with atomic resolution clearly indicate that the prepared nanoparticles exhibit a novel Ag@Pt core-shell nanostructure with a pentagram shape. For high-performance HCHO sensor construction, integrated micro-electrodes are first fabricated with the microelectromechanical system (MEMS) technology. Then, the hydrothermal route is used to self-assemble well-aligned ZnO nanowire arrays onto the sensing microregion. After that, the pentagram-shaped Ag@Pt nanoparticles are loaded onto the surface of ZnO nanowires with the inkjet printing technique to form MEMS sensors with Ag@Pt@ZnO as the sensing material. The thoroughly sensing experiments indicate that the Ag@Pt nanoparticles exhibit satisfied catalytic activation to HCHO molecules. The experimental observed detection limit of our sensor to HCHO reaches the parts per billion level. To elucidate the HCHO-sensing mechanism, the online mass spectrum (online MS) is utilized to analyze the components of exhaust gas stream of HCHO flowing through the Ag@Pt@ZnO material. The online MS indicates that with the Ag@Pt catalyst, HCHO molecules are partially oxidized to HCOOH molecules at low temperatures and are completely oxidized to CO2 molecules at high temperatures.

Enhanced NO2 sensing performance of ZnO nanowires functionalized with ultra-fine In2O3 nanoparticles

Structure and surface properties of the sensing materials have been recognized as the primary consideration for fabricating metal oxide semiconductor gas sensors. In this study, one-dimensional ZnO nanowires with large length-to-diameter ratio were firstly synthesized by a facile hydrothermal method. Then, different contents of ultra-fine In2O3 nanoparticles were directly grown on their surface. The structural characterization confirmed that the In2O3 nanoparticles were only 3–5 nm in diameter and well dispersed on the surface of ZnO nanowires. The gas sensing results showed that the ultra-fine In2O3 nanoparticles gave rise to a significant improvement in NO2 response at their optimal operating temperature of 150 °C. The highest response of 54.6 to 1 ppm NO2 was obtained for the sensor based on In2O3/ZnO composites with the In/Zn molar ratio of 5 %, which was about 8 times higher than that of pure ZnO nanowires. Importantly, a high response of 18.4 to a relatively low NO2 concentration of 250 ppb was also observed for In2O3-functionalized ZnO nanowires. The enhanced NO2 sensing mechanisms of ZnO nanowires functionalized with ultra-fine In2O3 nanoparticles were discussed.

A synergistic combination of zinc oxide nanowires array with dual-functional zeolitic imidazolate framework-8 for hybrid nanomaterials-based gas sensors

The complementary hybridization of nanomaterials enables to realize newly synergistic properties, which is a central concept in next-generation widespread applications. In particular, it is well-established that the hybridization of ZnO nanostructure with zeolitic imidazolate framework-8 (ZIF-8) allows the improvement of gas selectivity owing to molecular sieving capability caused by inherently size-defined porous structures of the ZIF-8. Here, we focused on a different viewpoint related with the preconcentration effect of ZIF-8 hybridized with ZnO nanowires (NWs) array on NO2, NH3, and H2 gas response. Hydrothermally-grown, structurally optimized ZnO NWs array are rationally employed for the nucleation sites of ZIF-8 nanocrystals as well as gas sensing channel. The chemical conversion of the surface of ZnO NWs to ZIF-8 was systematically implemented for the formation of core-shell hybrid structure, where morphological features of the ZIF-8 nanocrystals hybridized with the ZnO NWs array were optimized by simply adjusting synthetic conditions. The thickness of encapsulated ZIF-8 could be effectively manipulated by altering the concentration of 2-methylimidazole (HmIM), which strongly affects to determining gas response of ZIF-8@ZnO NWs-based gas sensors because of a variation in gas-loading capabilities related with the preconcentration effect. For the gas sensor based on ZIF-8@ZnO NWs synthesized using 8 mM HmIM solution, the gas response of NO2, NH3, and H2 gases were improved compared to those of the pristine ZnO NWs-based gas sensor. In addition, we consolidated the molecular sieving effect of the ZIF-8 by measuring the gas response of CH4 and C3H8 for suggesting the dual functionality of the hybrid system.

TiO2/ZnO heterostructure nanowire based NO2 sensor

A highly selective NO2 sensor based on TiO2/ZnO heterostructure nanowires (NWs) has been demonstrated successfully. The sensor film is realised by modifying the ZnO NWs surface with Ti (~30 nm) and subsequent annealing at 350 °C. XPS studies indicated that the Ti is present in the form of TiO2 on the NW surface. Low annealing temperature of 350 °C is attributed to the presence of a mixed phase containing both rutile and anatase phase, forming TiO2–ZnO heterostructure film. The heterostructure film exhibited a sensor response of ~7 towards 5 ppm of NO2 at an operating temperature of 200 °C. Moreover, NO2 detection was achieved with faster response kinetics wherein the response and recovery times of 65 and 98 s, respectively were obtained. Interestingly the response parameters were found to be independent of the NO2 concentration i.e., no significant variation in response/recovery times were observed for different gas concentrations. Additionally, a stable response for the measurement period of over 70 days was also obtained. A sensing mechanism has been established based on the outcomes of sensing characteristics, photoluminescence, X-ray photoelectron spectroscopy and Kelvin probe studies. Our results indicated that the TiO2/ZnO NW heterostructure film is highly sensitive and selective towards NO2.

Redox-Inactive CO2 Determines Atmospheric Stability of Electrical Properties of ZnO Nanowire Devices through a Room-Temperature Surface Reaction.

Emerging interactive electronics for the Internet of Things era inherently require the long-term stability of semiconductor devices exposed to air. Nanostructured metal oxides are promising options for such atmospherically stable semiconductor devices owing to their inherent stability in air. Among various oxide nanostructures, ZnO nanowires have been the most intensively studied for electrical and optical device applications. Here, we demonstrate a strategy for achieving the atmospheric electrical stability of ZnO nanowire devices. Although the chemically active oxygen and water in air are strong candidates for affecting the electrical stability of nanoscale metal oxides, we found that the ppm-level redox-inactive CO2 in air critically determines the atmospheric electrical stability of hydrothermally grown single-crystalline ZnO nanowires. A series of analyses using atmosphere-controlled electrical characterization of single nanowire devices, Fourier transform infrared spectroscopy, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy consistently revealed that atmospheric CO2 reacts substantially with the ZnO nanowire surfaces, even at room temperature, to form an electrically insulative zinc carbonate thin layer. The formation of this layer essentially limits the atmospheric electrical stability of the ZnO nanowire devices. Based on this surface carbonation mechanism, we propose a strategy to suppress the detrimental surface reaction, which is based on (1) reducing the density of surface hydroxyl groups and (2) improving the nanowire crystallinity by thermal pretreatment. This approach improves the atmospheric electrical stability to at least 40 days in air.

Improving Photovoltaic Performance of ZnO Nanowires Based Colloidal Quantum Dot Solar Cells via SnO2 Passivation Strategy

Colloidal quantum dot solar cells (CQDSCs) based on one-dimensional metal oxide nanowires (NWs) as electron transport layer (ETL) have attracted much attention due to its larger ETL/colloidal quantum dots (CQDs) contact area and longer electron transport length than other structure CQDSCs, such as planar CQDSCs. However, it is known that defect states in NWs would increase the recombination rate because of the high surface area of NWs. Here, the defect species on the ZnO NWs surface which resulted in the surface recombination and SnO2 passivation effects were investigated. Comparing with the solar cells using pristine ZnO NWs, the CQDSCs which based on SnO2 passivated ZnO NWs electrode exhibited a beneficial band alignment to charge separation, and the interfacial recombination at ZnO/CQDs interface was reduced, eventually resulted in a 40% improvement of power conversion efficiency (PCE). Overall, these findings indicate that surface passivation and the reduction of deep level defect in ETLs could contribute to improve the PCE of CQDSCs.

Surface doping of ZnO nanowires with Bi: Density-functional supercell calculations of defect energetics

Defect calculations using the density and hybrid functionals in combination with the supercell approach are employed to characterize the electrical properties of a number of ZnO nanowires of various thicknesses doped with Bi atoms occupying surface sites. The variation of the differences between the total energies of charged and neutral supercells with the supercell size is studied, which led the authors to devise an extrapolation procedure to obtain reliable defect energetics in the dilute defect limit. The calculated defect formation energies indicate that although the substitution of Bi into Zn or O sites can take place spontaneously under suitable thermodynamic conditions, the substitution into Zn sites is generally more likely. The defect (charge-state) transition energies are computed and parameterized as a function of the nanowire thickness. It is revealed that the substitution of Bi into O (Zn) sites on the surface of ZnO nanowires yields deep acceptor (shallow donor) levels (except for extremely thin nanowires). It is therefore concluded that the incorporation of Bi into the surface of ZnO nanowires results in $n$-type doping.

Synthesis of ZnO nanowires/Au nanoparticles hybrid by a facile one-pot method and their enhanced NO2 sensing properties

ZnO nanowires (ZNWs) and ZnO nanowires/Au nanoparticles hybrid (Au-ZNWs) with various Au concentrations were synthesized by a facile one-pot hydrothermal method and characterized by XRD, SEM, TEM, XPS, and FTIR. The structural characterization results exhibited that Au nanoparticles were self-assembled onto the surface of ZNWs and the c-axis growth of ZNWs is suppressed by the addition of HAuCl4 in the synthesis of Au-ZNWs hybrid. Gas sensing properties demonstrated the favorite sensing performance could be achieved for 1 mol% Au-ZNWs compared to pure ZNWs and Au-ZNWs with other Au concentrations. The maximum response of 1 mol% Au-ZNWs to 1 ppm NO2 was 31.4 at 150 °C, which was nearly 4 times higher than 8.2 of pure ZNWs. And the shortest response and recovery times could also be achieved by 1 mol% Au-ZNWs in a wide range of operating temperature. Au-ZNWs with various Au concentrations showed better selectivity to NO2 than pure ZNWs. The mechanism of enhanced NO2 sensing performance of Au-ZNWs was investigated by the combination of electronic and chemical sensitizations via Au nanoparticles functionalization.

NO2 sensing properties of one-pot-synthesized ZnO nanowires with Pd functionalization

The application of NO2 sensor based on metal oxide semiconductor is limited due to high operating temperature and poor selectivity. In this work, a low-temperature and high-performance NO2 sensor based on Pd-functionalized ZnO nanowires (Pd-ZNWs) prepared by a facile one-pot hydrothermal method was reported. The Pd was formed and self-assembled onto the surface of ZnO nanowires (ZNWs) during this one-pot hydrothermal process. Microstructure characterization of Pd-ZNWs indicated that the obtained ZNWs with diameter of 100–250 nm and length of 2–10 μm had a single crystal hexagonal structure, and Pd/PdO nanoparticles of approximately 2–5 nm in diameter were distributed on their surface. Gas sensing measurement showed that Pd-ZNWs exhibited higher response, lower optimal operating temperature, and faster response/recovery speeds towards NO2 than those of pure ZNWs. The maximum responses of 1 mol%, 2 mol%, and 5 mol% Pd-ZNWs towards 1 ppm NO2 were 13.5, 9.4, and 9.4, respectively, which were obtained at a low operating temperature of 100 °C and 30% RH. Pd-ZNWs also showed a significant improvement in sensing selectivity towards NO2. At a high RH condition of 60%, the sensors based on pure and Pd-ZNWs still exhibited noticeable responses, fast response/recovery speeds, and good long-term stability to NO2 gas. The sensing mechanism of Pd-ZNWs towards NO2 was discussed by the combination of electronic and chemical sensitization.

Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles

WO3 nanoparticle-decorated ZnO nanowires are synthesized by the hydrothermal and vapor-liquid solid (VLS) methods, respectively. WO3 nanoparticles are decorated on the surface of ZnO nanowires to fabricate the highly sensitive hydrogen gas sensor. This structured sensor presents a sensing response that is 4.3 times higher than its counterpart for 2000 ppm hydrogen gas. The variation in depletion layer width of the ZnO nanowires increases as the WO3 nanoparticles are decorated. This improves the sensing response of the WO3 nanoparticle-decorated ZnO nanowire sensor compared with its counterpart. In this research, a synthetic method of WO3 nanoparticle-decorated ZnO nanowires is described, and its physical and chemical properties are analyzed. Moreover, the sensing responses are measured, and the sensing mechanism is discussed in this literature.

Spray-assisted vapor deposition of CdS nanoparticles on ZnO nanowires with enhanced photocatalytic performance

ZnO nanowires were synthesized by a hydrothermal method, and CdS nanoparticles were deposited on ZnO nanowires by spray-assisted vapor deposition. The SEM image of ZnO nanowires showed their diameter of ∼100 nm. The TEM image of ZnO/CdS composite showed that the CdS nanoparticles were decorated on the surface of ZnO nanowires. The crystallization of CdS, ZnO and ZnO/CdS was determined by XRD. The photocatalytic performance of CdS, ZnO and ZnO/CdS was investigated by degrading Rhodamine B (RhB) under visible light irradiation. The results showed that the ZnO/CdS composite showed a higher catalytic performance than single phase CdS or ZnO under the same reaction condition. The stability of ZnO/CdS composite was investigated by running RhB degradation reaction in 10 cycles, which showed that ZnO/CdS composite had a high stability over photocorrosion. The mechanism for the enhanced photocatalytic performance in ZnO/CdS composite was also explained.

Synthesis, photoluminescence and ultrafast nonlinear optical properties of copper nanolayers/particles uniformly decorated ZnO nanowires

Copper (Cu) nanolayers/particles decorated zinc oxide (Cu-ZnO) nanowires have been successfully synthesized by two-step chemical vapour deposition to prepare ZnO nanowires and magnetron sputtering method to deposit Cu nanolayers/particles. The field emission scanning electron microscopy images indicated the Cu nanolayers/particles are clearly visualized on surface of ZnO nanowires. The impact of sputtering time and sputtering power was analyzed. The results show that the atomic percent of Cu was gradually increased with the sputtering time and sputtering power was increased. The photoluminescence results demonstrated two major emission peaks for the Cu-ZnO nanowires, and the visible emission peaks of Cu-ZnO nanowires stronger than that of unmodified ZnO nanowires. Moreover, the results of open-aperture Z-scan measurements that all samples show the two-photon absorption behavior, and the two-photon absorption coefficients of Cu-ZnO is higher than that of unmodified ZnO nanowires. The results of closed-aperture Z-scan results demonstrated that all samples show self-focusing optical nonlinearity. These studies promise their potential applications in multifunctional optical devices.

(Invited) Quantum Dot/Wide-Bandgap Semiconductor Hetero-Junction Interface Engineering for Efficient Infrared Solar Cells

Fulfilling the potential of solar power is essential toward the realization of a low-carbon society. In doing so, ultra-high efficiency solar cells play a crucial role, and several different concepts for ultra-high efficiency have been proposed: e. g. multiple exciton generation, hot-carrier extraction, intermediate band, multi-junction (M-J). Among these concepts, M-J solar cells based on III-V semiconductors are the only solar cells that have exceeded the single-junction limit (approximately 30% under one-sun illumination). However, the solar cells rely heavily on high cost solar cell technology, which makes a wide implementation of the solar cells difficult. We have been exploiting the potential of solution-processed M-J solar cells. In constructing M-J solar cells, OPVs, and perovskite solar cells can be used for the top and/or middle cells of M-J solar cells. However, there are few materials to choose from for the bottom cells that can operate in the short-wave infrared. Therefore, the development of low cost and efficient short-wave infrared solar cells is required. One of the promising candidates for use as the middle and/or bottom cells is lead chalcogenide colloidal quantum dots (CQDs) because the bandgap of bulk PbS is located in the infrared region (3.1 μm) and can be readily tuned by controlling quantum dot synthesis conditions. CQDs are compatible with low-temperature solution-based technologies. Although we see recent progress in solar cell performance of PbS CQD-based solar cells, there are many problems to be solved. The active layer thickness of the solar cells is needed to be thick enough to absorb solar energy in a wide range of the solar spectrum, particularly in the near-infrared and short-wave infrared region. The typical exciton diffusion length of the solar cells is approximately 200 nm in PbS QD/ZnO planar solar cells (Fig. 1 insert (a)). We then focused on PbS CQDs and ZnO nanowire (NW) to construct heterojunction structures to achieve simultaneous enhancement in carrier transport and light harvesting efficiency ((Fig. 1 insert (b)). Unlike PbS QD/ZnO planar structures, PbS QD/ZnO NW hybrid structures forming bulk-heterojuntion allow almost all the photo-generated carriers to reach the PbS QD/ZnO NW interface even when the active layer gets thicker than the carrier diffusion lengths [J. Phys. Chem. Lett., 4, 2455 (2013)]. Our recent study revealed that NW-type solar cells give an effective carrier diffusion length of over 1 μm [J. Phys. Chem. C, 119, 27265 (2015)]. The charge recombination at the interface is a major issue that degrades solar cell performance. Therefore, passivation of the surface of ZnO NWs is an effective way to suppress the recombination process at the interface. In fact, polyethylenimine treatment was successfully performed to decrease the intensity of the PL from defect states of ZnO NWs, which results in increasing the Voc of the NW-type solar cells by approximately 10%, compared to the untreated solar cells. Narrowing the bandgap is necessary to efficiently utilize the sun light. We synthesized different-sized PbS CQDs showing absorption bands in the wavelength region ranging from the visible to short-wave infrared (from 300 nm to 2000 nm). The external quantum efficiency (EQE) spectra of the ZnO NW type solar cells give an EQE peak originating from the first exciton absorption. From the EQE spectra, we confirmed that the solar cells were able to convert photon energy to electricity from 300 nm to 2000 nm (Fig. 1) [ACS Energy Lett., 2, 2110 (2017)]. The solar cells using PbS CQDs having the first exciton absorption peak locating at approximately 1510 nm produced an EQE of 30% at the peak, which is the highest value ever reported on the solution-processed solar cells. These features of PbS QD/ZnO NW solar cells show the high potential for the subcells of solution-processed M-J solar cells. The solar cells that can operate in the infrared region have other interesting features such as low Voc loss as well as relatively high Voc, which is as high as that of Ge solar cells. PbS QD/ZnO NW solar cells have been confirmed to have long-term stability in the air and under illumination [Phys. Stat. Sol. rrl, 8 (12), 961 (2014)]. 5-year air stability was also verified [Sci. Technol. Adv. Mater., 19, 336 (2018)]. Figure 1

Highly active and stable nanobiocatalyst based on in-situ cross-linking of phospholipase D for the synthesis of phosphatidylserine

Phospholipase D (PLD) was effectively immobilized on a ZnO nanowires/macroporous SiO2 composite support through an in-situ cross-linking method. An anionic and long-chained bi-epoxy cross-linker was used by adsorbing on the surface of ZnO nanowires through static interaction before cross-linking. Under the fine control of in-situ cross-linking the immobilized PLD has loading amount as high as 113.7 mg/gsupport, possessing high specific activity from 13,987 to 16,142 U/gprotein in all the range of loading amount. The immobilized PLD showed high activity and stability in catalyzing the conversion of phosphatidylcholine (PC) to phosphatidylserine (PS). The reaction conditions such as loading amount of PLD, substrate molar ratio, temperature, solution pH, and reaction time were optimized for the finding of best synthetic process. Under optimized conditions and the PS yield reached 94.8% within 40 min at 50 °C. The immobilized PLD exhibited not only better thermostability and resistance to pH inactivation than free PLD but also the greatly improved storage stability and reusability. It was found that 81.5% of initial activity retained after incubation at 4 °C for 60 days and that 80.4% of PS yield retained after 13 cycling reuses.

Enhanced surface-related ultraviolet–visible photoresponse in carbothermal ZnO nanowires by intertwined single-walled carbon nanotubes

An intertwined single-walled carbon nanotubes (SWCNTs)/carbothermal ZnO nanowires (ZNWs) composite is synthesized using a facile ultrasonic stirring technology, which is deposited on Au-interdigitated electrodes for ultraviolet–visible (UV–Vis) photoresponse enhancement. It is found that the SWCNT attached on ZNW surface optimizes the surface-related chemical processes and provides effective carrier transport strategies, responsible for the improvement in photo-responsivity and photo-sensing speed. The UV–Vis absorption of ZNWs has been improved after compositing with the high-conductive SWCNTs, contributing to the significantly improved photo-conductance. The photodetector based on SWCNT/ZNW composites (CZWs) with an optimal SWCNT content of 0.1 wt% shows a 300% improvement in photocurrent density, a 200% improvement in responsivity, and a 400% improvement in on/off current ratio over the pure ZNWs-based device under a UV irradiation of 3.26 mW cm−2 and a bias voltage of 1.0 V. This synthetic method could be scaled up for large-scale fabrication of CZWs on substrate at low cost for practical photodetecting applications.