Lingxia Zheng

Hierarchical MoS2 Nanosheet@TiO2 Nanotube Array Composites with Enhanced Photocatalytic and Photocurrent Performances.

A novel type of hierarchical nanocomposites consisted of MoS2 nanosheet coating on the self-ordered TiO2 nanotube arrays is successfully prepared by a facile combination of anodization and hydrothermal methods. The MoS2 nanosheets are uniformly decorated on the tube top surface and the intertubular voids with film appearance changing from brown to black color. Anatase TiO2 nanotube arrays (NTAs) with clean top surfaces and the appropriate amount of MoS2 precursors are key to the growth of perfect compositing TiO2 @MoS2 hybrids with significantly enhanced photocatalytic activity and photocurrent response. These results reveal that the strategy provides a flexible and straightforward route for design and preparation nanocomposites based on functional semiconducting nanostructures with 1D self-ordered TiO2 NTAs, promising for new opportunities in energy/environment applications, including photocatalysts and other photovoltaic devices.

Novel UV–Visible Photodetector in Photovoltaic Mode with Fast Response and Ultrahigh Photosensitivity Employing Se/TiO2 Nanotubes Heterojunction

A feasible strategy for hybrid photodetector by integrating an array of self-ordered TiO2 nanotubes (NTs) and selenium is demonstrated to break the compromise between the responsivity and response speed. Novel heterojunction between the TiO2 NTs and Se in combination with the surface trap states at TiO2 help regulate the electron transport and facilitate the separation of photogenerated electron-hole pairs under photovoltaic mode (at zero bias), leading to a high responsivity of ≈100 mA W-1 at 620 nm light illumination and the ultrashort rise/decay time (1.4/7.8 ms). The implanting of intrinsic p-type Se into TiO2 NTs broadens the detection range to UV-visible (280-700 nm) with a large detectivity of over 1012 Jones and a high linear dynamic range of over 80 dB. In addition, a maximum photocurrent of ≈107 A is achieved at 450 nm light illumination and an ultrahigh photosensitivity (on/off ratio up to 104 ) under zero bias upon UV and visible light illumination is readily achieved. The concept of employing novel heterojunction geometry holds great potential to pave a new way to realize high performance and energy-efficient optoelectronic devices for practical applications.

Shell-thickness dependent electron transfer and relaxation in type-II core–shell CdS/TiO2 structures with optimized photoelectrochemical performance

Core–shell CdS/TiO2 structures are promising for solar-to-fuel conversion applications because their ideal type-II band alignment helps effective charge transfer to form the CdS+/TiO2− system. A better understanding of the charge carrier dynamics is critical to provide guiding principles for designing photoelectrochemical (PEC) devices. Hence, TiO2 shell-thickness dependent charge carrier dynamics and competition between electron relaxation in CdS (e.g. recombination and trapping) and electron transfer from CdS to TiO2 were investigated using ultrafast transient absorption (TA) spectroscopy. The results indicate that the CdS/TiO2 nanocomposite with a molar ratio of 2 : 1 exhibits the highest electron transfer rate constant of ET = 2.71 × 1010 s−1, along with an electron relaxation rate of CdS/TiO2 = 3.43 × 1010 s−1, resulting in an electron transfer quantum efficiency of QET = 79%, which also corresponds to the best PEC hydrogen generation in the CdS/TiO2 core–shell composites. However, the electron transfer rate decreases with increasing thickness of the TiO2 shell consisting of aggregated nanoparticles. One possible explanation is that the CdS and TiO2 form relatively larger, separate particles, or less conforming small particles, with poor interfaces with increasing TiO2, thereby reducing electron transfer from CdS to TiO2, which is supported by SEM, and TEM data and consistent with PEC results. The thickness and morphology dependence of electron transfer and relaxation provides new insight into the charge carrier dynamics in such composite structures, which is important for optimizing the efficiency of PEC for solar fuel generation applications.

Uniform carbon-coated CdS core–shell nanostructures: synthesis, ultrafast charge carrier dynamics, and photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using solar energy has received widespread attention, and strong performance photocatalysts are highly desired. In this work, uniform carbon-coated CdS nanostructures have been fabricated using ascorbic acid as the carbon source by a facile hydrothermal method and characterized using transmission electron microscopy (TEM). The thickness of the carbon layer can be well controlled by the amount of ascorbic acid added during the reaction. Compared to pristine CdS, carbon-coated CdS nanostructures exhibit stronger light absorption and more efficient electron transfer as determined by absorption and photoluminescence (PL) spectroscopy. Ultrafast charge carrier dynamics in the composite CdS/C structures were studied using femtosecond transient absorption (TA) spectroscopy, which revealed direct evidence of effective charge transfer from CdS to the carbon layer. In addition, the CdS/C composites were employed as photoanodes for PEC hydrogen generation, which showed significant improvement in photoactivity over pristine CdS nanospheres. The photocurrent density (−1.0 V vs. Ag/AgCl) of one of the composite structures, CdS/7-C, exhibited ∼20 times enhancement compared with that of pristine CdS. The enhanced PEC property can be attributed to increased light scattering and consequently the light harvesting throughout the whole spectral wavelength, and the effective electron transfer from CdS to the carbon layer. Such carbon-coated semiconductor composites based on a simple and low-cost synthesis method should be useful in PEC as well as other applications such as photovoltaics, detectors and sensors.

Wavelength-Tunable Electroluminescent Light Sources from Individual Ga-Doped ZnO Microwires

Electrically driven wavelength-tunable light emission from biased individual Ga-doped ZnO microwires (ZnO:Ga MWs) is demonstrated. Single crystalline ZnO:Ga MWs with different Ga-doping concentrations have been synthesized using a one-step chemical vapor deposition method. Strong electrically driven light emission from individual ZnO:Ga MW based devices is realized with tunable colors, and the emission region is localized toward the center of the wires. Increasing Ga-doping concentration in the MWs can lead to the redshift of electroluminescent emissions in the visible range. Interestingly, owing to the lack of rectification characteristics, relevant electrical measurement results show that the alternating current-driven light emission functions excellently on the ZnO:Ga MWs. Consequently, individual ZnO:Ga MWs, which can be analogous to incandescent sources, offer unique possibilities for future electroluminescence light sources. This typical multicolor emitter can be used to rival and complement other conventional semiconductor devices in displays and lighting.

Novel Structure for High Performance UV Photodetector Based on BiOCl/ZnO Hybrid Film

A novel type of high performance ultraviolet (UV) photodetector (PD) based on a ZnO film has been prepared by incorporating a BiOCl nanostructure into the film. The responsivity of the BiOCl/ZnO hybrid film PD in UV region can reach 182.87 mA W-1 , which is about 2.72 and 6.87 times for that of TiO2 /ZnO hybrid film PD and pure ZnO film PD. The rise/decay time of BiOCl/ZnO hybrid film PD is 25.83/11.25 s, which is much shorter than that of TiO2 /ZnO hybrid film PD (51.94/26.05 s) and pure ZnO film PD (69.34/>120 s). The BiOCl nanostructure can inject photogenerated electrons into the ZnO film under UV light illumination, leading to the increase of photocurrent, and forms barriers to block the straight transmission of electrons between electrodes, resulting in the decrease of decay time. The results of control experiment show that the transfer path of photogenerated electrons formed by p-n junction will be cut off after depositing gold nanoparticles on the film surface, which means this hybrid film is a unique and novel structure to improve the optoelectronic performance of photodetectors. This novel BiOCl/ZnO hybrid structure paves new route for the development of film PDs based on ZnO film.

Efficiency enhancement of TiO2 self-powered UV photodetectors using a transparent Ag nanowire electrode

A novel efficient way to realize high performance TiO2 self-powered UV photodetectors (PDs) with enhanced responsivity is proposed in this work. By constructing asymmetric electrodes consisting of a commercial mixed silver paste and two different forms of Ag nanostructures (nanofilms and nanowires) on the active layer of TiO2, respectively. Both PD devices exhibited attractive photovoltaic characteristics in a self-powered mode (at 0 V bias). Interestingly, the responsivity of the TiO2 UV PDs is found to be enhanced by 9 times at the wavelength of 350 nm without any power supply when the configuration of the Ag nanostructure electrode was changed from nanofilms to transparent nanowires. The enhanced photoelectric conversion efficiency is mainly attributed to a modified Schottky barrier through reducing hole traps and a higher light absorptivity through using a transparent electrode according to the experimental results and physical models based on the energy band theory. In addition, TiO2 UV PDs with a Ag nanowire electrode also exhibited excellent photoelectric properties in terms of a high linear dynamic range of 103.7 dB, a fast response speed (rise time of 2 ms and fall time of 41 ms), and excellent stability. The design solutions proposed here may provide additional opportunities for further development of Schottky junction-based self-powered UV PDs with high efficiencies.