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Development of flexible, low cost, highly sensitive, and large-area compliance pressure sensors that can detect human physiological signals have become one of the vital component units for wearable electronics. Herein, a highly flexible and stretchable carbon black-porous polydimethylsiloxane-based pressure sensor has been successfully fabricated using a simple and effective fabrication technique with a sacrificial power template method. The pore size on the performance of pressure sensor is systematically investigated using KCl, NaCl, and sugar as powder templates to produce the porous PDMS. Notably, the KCl-templated PDMS pressure sensor exhibits a maximum pressure sensitivity (1.07 kPa−1 in 0–2 kPa), which is significantly higher than that of the NaCl-templated PDMS pressure sensor (0.86 kPa−1 in 0–2.5 kPa) and the sugar-templated PDMS pressure sensor (0.40 kPa−1 in 0–2.8 kPa). The KCl-templated PDMS pressure sensor has other advantages as well, including a fast response time of − 16 ms, a wide sensing range of 0–11,700 Pa, a low detection limit of − 2 mN, and a good reproducibility for over 3000 cycles. With these advantages, it is demonstrated that the pressure sensor can monitor minute mechanical events and human physiological signals, such as weak airflow blown, swallowing motion, finger touch, and pronunciation. Therefore, this new pressure sensor, which exhibit excellent comprehensive performance, will have wide application prospects in health monitoring and wearable devices.

Three trinuclear silver nanoclusters, [Ag3(dppm)3(tBuCC)](CF3SO3)2 (1), [Ag3(dppm)3(CO3)(CH3OH)](CF3SO3)·H2O (2) and [Ag3(dppm)3(Cl)2](CF3SO3)·3DMF (3) (dppm = 1,2-bis-(diphenylphosphino)methane) were obtained and characterized by a series of techniques including single-crystal X-ray diffraction (SCXRD), Fourier transform infrared spectrometry (FT-IR), powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). In the three compounds, their main body skeletons are made up of similar Ag3 cores shaped in a silver triangle. All the three compounds consist of three silver atoms, three dppm bis-(phosphine) ligands, which are further stabilized by CF3SO3− anions. While the other auxiliary ligands, tBuCC−, CO32− and Cl− are separately attached to similar Ag3 cores, making them structurally different. Moreover, the luminescence properties, photocurrent responses and electrochemical properties of compounds 1–3 have been investigated. The results indicate that different structures have a very important influence on their properties. The structure–activity relationship of silver cluster materials provides positive guidance for further design and synthesis of functional silver cluster materials.

We report the fabrication of P-doped α-Fe2O3 nanorod/BiVO4 heterojunction photoelectrode for photoelectrochemical (PEC) water splitting. The heterojunction photoelectrode is fabricated by combining methods of hydrothermal deposition, wet impregnation and metal organic decomposition. The structure and PEC activity of the photoelectrode are systemically studied. This heterojunction photoelectrode exhibits a surprising photocurrent density of 1.17 mA/cm2 at 0.8 V vs. Ag/AgCl, which is approximately 3.55 times higher than that of the pristine α-Fe2O3. The photoconversion efficiency is 9.58 times higher than α-Fe2O3 and the onset potential has an obvious negative shift of 450 mV compared to the α-Fe2O3 for the heterojunction photoelectrode. The significant enhancement of PEC water splitting performance is attributed to the heterojunction structure and the synergistic effects of P-doping, which contributes to expediting electron and hole transfer and reducing carriers recombination. This novel design may provide an efficient way to developing efficient heterojunction photoelectrodes for practical PEC application.

We report the fabrication of a novel WO3 nanorod/graphene/BiV1−xMoxO4 heterojunction photoelectrode for photoelectrochemical (PEC) water splitting. The heterojunction arrays were fabricated via a hydrothermal deposition of WO3 nanorods on FTO glass, with subsequent depositing of graphene nanosheets and BiV1−xMoxO4 by spin-coating method, respectively. The structure of materials and PEC water splitting properties of the photoelectrodes were systemically investigated. The heterojunction exhibits an enhanced photocurrent density (2.27 mA/cm2 at 0.9 V vs. Ag/AgCl), which leads to a significant improvement in photoconversion efficiency (1.00% at about 0.7 V vs. Ag/AgCl). This remarkable PEC performance is mainly due to the constructive effect of the graphene (RGO) in expediting electron transfer and reducing charge recombination to realize enhanced use ratio of photo-generated carriers for the water splitting reaction. This result demonstrates the superiorities of the new graphene-mediated composites photoelectrode and provides a promising route for high-performance photoelectrochemical systems.

A high-yield silver alkynyl assembly [Ag8(CCtBu)5(CF3COO)3(CH3CN)]n (1) constructed from [AgCCtBu]n ligand, CF3COOAg and CH3CN auxiliary ligands with a one-dimensional infinite chain structure has been obtained in one pot. Compound 1 has been well-defined and characterized. The photocurrent properties and the temperature-sensitive luminescent properties of 1 have been investigated.

Large-area Si/ZnO core/shell nanowire (NW) arrays have been fabricated by metal-assisted chemical etching and sol-gel processes. We demonstrated the dependence of photocurrent density on the annealing temperature of the Si/ZnO NW arrays. The core/shell NW arrays annealed at 900 °C enhanced the photocurrent by 2.4 times compared to the samples annealed at 500 °C due to the core/shell array annealed at higher temperature have lower defect density. We also observed that n-Si/n-ZnO NW arrays exhibited a larger photocurrent than p-Si/n-ZnO NW arrays due to n/n junctions enhance the charge separation and minimize recombination.

Large-area highly ordered P-type silicon nanohole (SiNH) arrays have been fabricated by the combination of metal assisted electroless etching and nanospheres lithography technique, and systematic investigations of photoelectrochemical hydrogen evolution behavior of SiNH arrays were also reported in this study. The SiNH arrays exhibited a maximum limiting photocurrent density of 28 mA cm−2, higher than that of SiNW arrays (24 mA cm−2) and the planar Si (21.5 mA cm−2). The improved performance was attributed to SiNH arrays providing a patterned and ordered nanoholes structure as a result of enhancement of the light harvesting as well as charge transportation and collection efficiency. Furthermore, we have demonstrated that the photoelectrode consisting of longer SiNH yielded a higher limiting current. However, when the length of SiNH was increased further, the limiting current dramatically reduced, which is due to an increased interface recombination and scattering resulting from the increased surface area of SiNH begin to play a dominant role. The relationship between the diameter of nanoholes and the photoconversion efficiency of SiNH arrays was also discussed. By implementing this ordered SiNH arrays into electrode design, one may be able to advantageously impact PEC and photovoltaic device performance.

An ultraviolet photodetector based on p-NiO/n-ZnO heterojunction was fabricated using thermal oxidation and hydrothermal growth processes. The properties of the UV photoresponse under various strains were investigated. The results showed that the magnitude and the sign of the strain (tension/compression) have a great influence on the decay time by coupling piezoelectric and surface oxygen adsorption/desorption effects. These results demonstrate an effective approach for designing and fabricating strain modulated fast reset UV photodetectors.

In this research work, a p-NiO/n-ZnO heterostructure was fabricated using thermal oxidation and hydrothermal growth processes. The p-NiO films were oxidized at different temperatures. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and UV–visible spectral analysis were used to characterize the p-NiO/n-ZnO heterostructure. The results indicated that the NiO films oxidized at higher temperature have wider optical band gap and lower defect density. In particular, by comparing the photoresponse properties of the UV photodetectors oxidized at different temperatures we suggest that the oxidation temperatures have a great influence on the photoresponse time. The defect density of NiO film decreases with increasing oxidation temperature. And the defect density affects the photoresponse characteristics that the decay time decreases with the decreasing of defect density as the NiO oxidation temperature increases. This work could serve as a valuable guideline for designing and improving the p-NiO/n-ZnO UV photodetectors in a low-cost and large-scale way.

A p-CuO/n-ZnO heterostructure was synthesized on an indium tin oxide coated flexible substrate (polyethylene terephthalate) by a simple and low-cost hydrothermal method. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction were used to characterize the as-synthesized heterostructure. The results indicated that flower-like CuO nanostructure conformably formed on the ZnO nanorods. Furthermore, a nanogenerator based on the p-CuO/n-ZnO heterostructure was fabricated for harvesting energy from environment, and the output performance of the device was investigated. A larger output current of ~100 nA was obtained in comparison with ZnO based nanogenerator, which could be ascribed to the formation of p–n heterojunctions in the CuO/ZnO composites, reducing the screening effect of excessive electrons. This strategy may provide a highly promising platform for realizing a high-performance flexible piezoelectric energy harvester by new composite piezoelectric material.

Enhancing the output power of a nanogenerator is essential in applications as a sustainable power source for wireless sensors and personal electronic devices. As one possible solution, NiO–ZnO heterojunction is used for piezoelectric performance enhancement via both suppressing the screening effect in the ZnO film and forming a reliable p–n junction at the interfaces instead of the Schottky barriers. From pristine ZnO nanogenerators, alternating current (AC)-type output signal was observed, while from NiO–ZnO nanogenerators, direct current (DC)-type output signal was surprisingly obtained under the same vertical compressive force. By forming a NiO–ZnO heterostructure, the DC output voltage was up to 430 mV and the maximum output current density was of 40 nA cm −2 , which are approximately 21-fold higher voltage and 13 times larger current density by comparison to the pristine ZnO nanogenerators. The results provide a simple, low-cost and effective approach for enhancing the piezoelectric performance.

In this study, a nanogenerator based on Cu2O–ZnO p–n junction has been fabricated on Cu wire substrates for harvesting mechanical energy from the environment. The flexible nanogenerator is composed of a Cu substrate, a Cu2O layer, ZnO nanorods and an outer Au-coated paper electrode; the Cu2O layer was obtained by oxidizing Cu wires directly and the ZnO nanorods were grown on the Cu2O layer using a low-temperature hydrothermal method. The existence of the Cu2O–ZnO p–n junction makes a contribution towards reducing the number of excess electrons in the ZnO, which facilitates in improving the output signal and also overcomes short circuits. An Au-coated paper electrode can involve more nanorods in the power generation process. The DC output voltage was up to 42 mV and the maximum output current density was 400 nA, which are approximately a 13-fold higher voltage and a one order of magnitude larger current in comparison to devices without a Cu2O layer, respectively. This study may provide important insight into a facile fabrication method for low-cost and high-performance energy harvesting devices.

Piezoelectric nanogenerators have been intensively developed in terms of their materials and applications; however, only modest structural progress has been made due to limitations in the growth mechanisms of nano-materials. In this work, a piezoelectric nanogenerator based on ZnO micro-thornyballs (ZMTBs) was introduced. ZMTBs were synthesized by chemical vapor deposition method without the presence of any seed layers or substrates. Electrical characterization was subsequently performed to reveal the characteristics of the contacts formed between the ZnO micro-thornyballs and the copper electrode. The electric output ability of the ZNTTs nanogenerators has been studied in reference to the experiment and the numerically calculation.

In this work the piezoelectric effect of one-dimensional ZnO gear-shaped microwires (GMWs) is studied by the finite element method. By calculating the piezopotential of substrate-bound gear-shaped ZnO microwires oriented along the c-axis with gear groove depth of 0–5 μm, we demonstrate that the GMW has more advantage for piezoelectric nanogenerators than the common hexagonal microwire (HMW). To understand the anisotropic electricity generation, the calculated piezopotential distribution in the bent ZnO GMW and HMW has been analyzed under an external force with the different directions. Moreover, the calculation also indicates that the effect of geometric size on piezopotential for the GMW is similar to the HMW. Our results will be a guidance for the design of high-efficiency energy harvesting devices.

A simple and effective method of synthesizing nanorods (NRs) and the ability to control the size and aspect ratio of them are crucial for fabricating nanodevices. In this paper, we present a systematic study of the growth of ZnO NRs on common paper substrates using a hydrothermal approach by adjusting the growth conditions. By a slight variation of the solution concentration and the growth time, significant changes in morphology and size (aspect ratio) of the obtained ZnO NRs have been controlled. Moreover, the piezoelectric power generation from ZnO-paper nanogenerators grown with different precursor concentration and growth time are also investigated. It is found that the morphology and aspect ratio of NRs have significant influence on the piezoelectric behavior. This type of flexible piezoelectric nanogenerator will have potential applications in implantable biosensors and wearable self-powered electronic devices.

Large-area highly ordered silicon nanohole (SiNH) arrays on Si substrate have been fabricated by the combination of nanosphere lithography and metal assisted electroless etching. The diameter, length of nanoholes, and the center-to-center distance of adjacent nanoholes, can be accurately controlled by nanosphere lithography and metal assisted electroless etching conditions. The sub-wavelength structure of SiNH arrays had excellent antireflection property with a low reflectance of 3.5% within the wavelength range of 300–1000 nm. Compared to the planar Si, the SiNH samples exhibited a higher photoelectrochemical hydrogen generation performance. The improved performance was attributed to SiNH arrays providing an effective light-trapping and a higher semiconductor/electrolyte interface areas which reduce the overpotential required for photoelectrochemical hydrogen reaction. Furthermore, decorating the SiNH arrays with platinum nanoparticles (PtNPs) yielded a significantly high photovoltage of 0.12 V. The photoconversion efficiency of Pt-decorated SiNH (Pt/SiNH) arrays was 22% under the illumination of 100 mW/cm2, higher than that of SiNH arrays (15.5%) and the planar Si (8.1%).

One-dimensional (1D) ZnO microwires were successfully synthesized by chemical vapor deposition and their structural and morphological properties were analyzed by X-ray diffraction and scanning electron microscopy, demonstrating that the microwires were single crystalline with perfect hexagonal structure and smooth surface. Using these 1D microstructures, we fabricated a novel ZnO-based ethanol gas sensor. Operating at room temperature, the sensor was found to have good sensing characteristics. The reliability and stability of the sensor could be improved by connecting multiple 1-wire devices (1-WD) in parallel into a multi-wires device. In interior natural lighting environment and under 3 V bias, the response and recovery time of the 1-WD to 200 ppm ethanol gas were

Wafer-scale high density aligned p-type silicon nanowire (SiNW) arrays decorated with discrete platinum nanoparticles (PtNPs) have been fabricated by metal assisted electroless etching followed by an electroless platinum deposition process, and systematic investigations of photoelectrochemical behavior of Pt/SiNW were also reported in this study. Coating of PtNPs on SiNW sidewalls yielded a more positive onset potential (Vos), which enhances the photoelectrochemical hydrogen generation performance of the photoelectrodes, though excessive PtNPs deposition leads to a decreased photocurrent. Additionally, we have demonstrated that the photoelectrode consisting of longer SiNWs yielded a higher limiting current. However, when the length of SiNWs was increased further to >4 μm, the limiting current dramatically reduced, which is presumably because an increased interface recombination and scattering resulting from the increased surface area of SiNWs begin to play a dominant role. The results demonstrate Pt/SiNW to be a promising hybrid system for photoelectrochemical water splitting, and device performance may be further improved via optimal conditions of PtNPs deposition time and SiNWs length.

Si/ZnO core/shell nanowire (NW) arrays were fabricated using atomic layer deposition of ZnO shell on n-Si NW arrays prepared by metal assisted electroless etching method. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were utilized to characterize the core/shell structures. Water splitting performance of the core/shell structures was preliminarily studied. The Si/ZnO core/shell NW arrays yielded significantly higher photocurrent density than the planar Si/ZnO structure due to their low reflectance and high surface area. The photoelectrochemical efficiency was found to be 0.035 and 0.002 % for 10 μm-long Si/ZnO NW array and planar Si/ZnO sample, respectively. These results suggested that core/shell structure is superior to planar heterojunction for PEC electrode design. We demonstrated the dependence of photocurrent density on the length of the core/shell array, and analyzed the reasons why longer NW arrays could produce higher photocurrent density. The relationship between the thickness of ZnO shell and the photoconversion efficiency of Si/ZnO NW arrays was also discussed. By applying the core/shell structure in electrode design, one may be able to improve the photoelectrochemical efficiency and photovoltaic device performance.

We prepared Ag doped zinc oxide (ZnO:Ag) films using pulsed laser deposition method and studied the Raman spectra of the unannealed and annealed films. The results indicate that the ZnO:Ag films could still retain their wurtzite structure; the A 1 (LO) mode resulting from oxygen vacancies and zinc interstitials was observed to be very strong in the Raman spectra. A local vibrational mode at 493 cm −1 induced by Ag doping could also be clearly observed in the Raman spectra, which may be used as an indication of Ag incorporation into the ZnO lattice.

ZnO microwires have been synthesized by a simple chemical vapour deposition method. The morphology and optical properties were investigated by scanning electron microscopy, photoluminescence, and Raman spectrum, respectively. The strong ultraviolet emission with the weak deep-level emission observed in the photoluminescence spectrum indicates that the ZnO microwires have good optical properties with limited structural defects. In the Raman spectrum, the intense E 2 modes indicate that the ZnO microwires have perfect wurtzite structure. Additionally, the red shift of the Raman peaks can be attributed to the lattice distortion and piezoelectric effect of the nanostructures.

There are numerous sources of mechanical energy in our environment, such as ultrasonic waves, body movement, and irregular air flow/vibration. Here, we present a simple, cost-effective approach for fabricating a flexible nanogenerator and apply it to harvest energy from environmental mechanical vibrations. The nanogenerator was based on ZnO nanorods grown on common paper substrate using a low-temperature hydrothermal method. Piezoelectric currents were measured by attaching the nanogenerator on the surface of a cantilever and a wind-up drum, respectively. At the same time, the vibrations of the cantilever and wind-up drum could also be characterized by the corresponding output signals. This is a practical and versatile technology with the potential for converting a variety of environment energy into electric energy, and also with the application for pre-warning of emergency, such as earthquake and burgling.

A self-powering nanosystem that harvests its operating energy from the environment is an attractive proposition for sensing and personal electronics. In this paper, we present a simple, low-cost approach to convert low-frequency mechanical energy into electric power using piezoelectric ZnO nanorods grown on common Cu wires. This energy conversion device has ultrahigh flexibility and piezoelectric sensitivity and can produce an output current of about 50 nA, which can last for almost 5 seconds. Furthermore, the I–V curves of the device under different strains are also examined, and the I–V characteristic is highly sensitive to strain. This energy-harvesting technology provides a simple and cost-effective platform to capture low-frequency mechanical energy, such as body movements, for practical applications.

A simple two-step hydrothermal process was proposed for enhancing the performance of the nanogenerator on flexible and wearable terylene-fabric substrate. With this method, a significant enhancement in output voltage of the nanogenerator from ∼10 mV to 7 V was achieved, comparing with the one by conventional one-step process. In addition, another advantage with the devices synthesized by two-step hydrothermal process was that their output voltages are only sensitive to strain rather than strain rate. The devices with a high output voltage have the ability to power common electric devices and will have important applications in flexible electronics and wearable devices.

Nanogenerators capable of harvesting energy from environmental mechanical energy are attractive for many applications. In this paper, we present a simple, low-cost approach to convert low-frequency mechanical energy into electric power using piezoelectric ZnO nanorods grown on a common paper substrate. This energy conversion device has ultrahigh flexibility and piezoelectric sensitivity and can produce an output voltage of up to 10 mV and an output current of about 10 nA. It is demonstrated that the device's electric output behavior can be optionally changed between four types of mode simply by controlling the straining rate. Furthermore, it is also shown that the electric output can be enhanced by scaling the size of the device. This energy-harvesting technology provides a simple and cost-effective platform to capture low-frequency mechanical energy, such as body movements, for practical applications.

High-quality Schottky devices based on ZnO microwires (MWs) by using a simple and effective method are reported. Cu-contained silver paste (SPC) was used as the contact electrodes of one end of ZnO MW-based metal-semiconductor-metal structures instead of the conventional method, in which customised silver paste was used as the contact electrodes of both ends of the structure. The experimental results reveal that SPC used in this study can increase the opportunity for obtaining the device with rectifying behaviour. The best-produced device exhibited a very high rectifying ratio of 105 at ±1 V. In addition, the rectifying I–V characteristics were greatly improved after annealing treatment because of the elimination of the bubbles in the conductive pastes. Moreover, the influence of contact area on the electrical properties was also investigated. It was shown that the contact area between ZnO MW and the electrode also played an important role in determining the rectifying performance.

Ag-doped ZnO (ZnO:Ag) films are prepared on c-plane sapphire substrates by pulsed laser deposition at different substrate temperatures. The effect of substrate temperature on the ZnO:Ag film is studied in detail by EDX, XRD and Raman spectroscopy. The results reveal that raising the substrate temperature is beneficial for incorporating Ag into ZnO:Ag films in the range of our experimental temperatures and a number of Ag atoms incorporation into ZnO:Ag films may cause the (002) peak positions of the XRD spectra shift to a lower angle direction, but hardly affect the c-axis orientation of the films. The (002) peak shift ought to be due to the increase of lattice constant in the c-axis direction caused by the partial substitution of Zn2+ ions by Ag+ ions. In addition, a local vibrational mode (LVM) at 492 cm−1 induced by doping Ag occurred in the Raman spectra of all the ZnO:Ag films and its peak position hardly shifted with increasing substrate temperature. It means that the LVM can act as an indication of Ag incorporation into ZnO:Ag film.