Zn-doped SnO2 nanocrystals were successfully synthesized by a simple hydrothermal method. It is found that Zn doping into SnO2 can induce a negative shift in the flat-band potential (VFB) and increase the isoelectric point. As a result, dye-sensitized solar cells (DSCs) based on Zn-doped SnO2 nanocrystal photoanodes exhibit longer electron lifetimes and higher dye loading compared to undoped SnO2 based DSCs. The overall power conversion efficiency (η) of the optimized Zn-doped SnO2 based DSC reaches 4.18% and increases to 7.70% after the TiCl4 treatment. More importantly, a remarkable η of 8.23% is achieved for DSCs based on a high-quality double-layer SnO2 photoanode with the TiCl4 treatment, to the best of our knowledge, which is so far the best reported efficiency for DSCs based on SnO2 photoanodes.

A novel three-dimensional (3D) nanoarchitecture consisting of hybrid graphene nanosheets (GNs)/graphene foam (GF) was fabricated on the FTO conducting substrate as a high efficient counter electrode (CE) for dye sensitized solar cells (DSSCs). The GNs with various sized such as large-sized heat-reduced graphene nanosheets (H-GNs) and small-sized laser-reduced graphene quantum dots (L-GQDs) were synthesized and used as catalytic materials incorporated into a 3D GF network, respectively. In this design, the aggregations and restacking of GNs were efficiently reduced, which is beneficial for increasing the amount of the active defective sites at the edges of graphene to the electrolyte solution. Especially, L-GQDs with smaller dimension less than 100 nm have more active defective sites at edges, providing superiority over the large-sized H-GNs in terms of electrocatalytic activity. Meanwhile, the GF network with high conductivity provides fast electron transport channels for charge injection between the GNs and FTO. The DSSC with this hybrid CE exhibited energy conversion efficiency (η) of 7.70% with an open circuit voltage (VOC), short circuit photocurrent density (JSC) and fill factor (FF) of 760 mV, 15.21 mA cm−2, and 72.0%, respectively, which is comparable to that of the conventional Pt CE (7.68%).

Graphene nanosheets@ZnO nanorods as a three-dimensional high efficient counter electrode for dye sensitized solar cells is described, highlighting the ZnO nanorods as a 3D framework nanostructure for preventing the aggregations of GNs. It is demonstrated that, when the nanohybrids are used as CE materials for DSSCs, compared to their pristine GNs, the GNs@ZnO nanorod hybrids without aggregations of GNs have a significant improvement in catalytic performance toward the reduction of triiodide, which were reflected in their electrochemical properties such as high current density, narrow peak-to-peak separation (EPP) and low charge transfer resistance (RCT). The enhancement of electrochemical performance can be attributed to more active defective sites at the edges of GNs and the reduction of the restacking of GNs due to the 3D ZnO nanorod nanostructure. The DSSC with this hybrid CE exhibited energy conversion efficiency (η) of 8.12% with an open circuit voltage (VOC), short circuit photocurrent density (JSC) and fill factor (FF) of 765 mV, 21.7 mA cm−2, and 67.1%, respectively, which is comparable to that of the conventional Pt CE (8.82%).

A novel self-powered UV photodetector (UVPD) based on the photoelectrochemical cell (PECC) has been constructed using the TiO2 coated SnO2 mesoporous spheres (SnO2-MS@TiO2). This self-powered UVPD displays a higher photocurrent density compared to the UVPD with the pure SnO2-MS. By means of external quantum efficiency (EQE), UV-vis absorption, and electrochemical impedance measurements, we scrutinize the intrinsic role of the TiO2 coating layer on the photocurrent enhancement. Under UV irradiation, this UVPD exhibits a high on/off ratio of 11519, a fast rise time of 0.007 s and decay time of 0.006 s, together with the excellent visible-blind characteristic and linear optical signal response. The self-powered photodetector is a promising candidate for application in high-sensitivity and high-speed UVPDs.

The effect of air and oxygen annealing on the structural and the optical properties of hydrothermally synthesized ZnO nanorods was investigated. After hydrothermal synthesis, the resulting ZnO nanorods were annealed in air and under an oxygen atmosphere at 370°C for 1 h. X-ray diffraction results revealed that the oxygen-annealed nanorods possessed high crystallinity with a hexagonal-wurtzite crystal structure in the (002) plane. Evaluation of strain showed a tensile lattice strain of 0.426% resulting from oxygen annealing. The photoluminescence measurements showed that the relative intensity ratio of the near-band-edge emission (NBE) to the green emission (INBE/IGE) increased from ~2.6 for the as-grown ZnO nanorods to ~68.7 when the nanorods were annealed under oxygen. After annealing, a red shift of ~30 and ~44 meV in the NBE was observed for the nanorods that were annealed in air and under oxygen, respectively. This shift is attributed to the interaction between the neutral acceptors and the adsorbed oxygen atoms.

The anatase and rutile phases existing in commercially available P25 powder were separated according to their different particle size and chemical affinity. Firstly, electrical double layer was established on TiO2 surface to achieve a highly dispersed TiO2 colloid due to electrostatic repulsive forces. Additionally, strong electrolyte HCl was added to neutralize the weak electrical double layer of rutile particles, and makes them precipitated from TiO2 colloid priorly by centrifugal force. As the amount of HCl increased from 0 to 2 ml, the anatase percent increased from 83.2 to 100%, the diameter decreased from 30.0 to 19.7 nm while the specific surface increased from 52.9 to 76.3 m2/g. Further, the conversion efficiency of dye-sensitized solar cells fabricated from the separated powder increased from 8.52 to 10.1% under 100 mW cm−2 AM 1.5 illumination.

A new chemical technique for preparing screen-printing paste from commercially-available P-25 powder was proposed to fabricate TiO2 electrodes for dye-sensitized solar cells (DSSCs). The TiO2 powder and acetic acid were dispersed in ethanol, following that the reactants solutions kept at 80 °C for 12 h, then dried in oven for 6 h. This process made the TiO2 particle positively charged and highly dispersed in water. Additionally flocculating reaction using hydrochloric acid was done to change the dispersed TiO2 colloid into viscous paste, which omitted the process of adding polymers as thickener, and kept the purity and porosity of electrodes. The mesoporous transparent TiO2 films about 14 μm thickness without cracking and peeling-off were fabricated on conducting glass substrates. A high conversion efficiency of 8.07% was obtained under 100 mW cm−2 AM 1.5 illumination.Graphical abstractHighlights► A chemical technique was proposed to make P-25 powder highly dispersed in water. ► Flocculating reaction was taken to fabricate screen-printing TiO2 paste for DSSCs. ► The transparent porous films about 14 μm were fabricated on FTO glasses. ► An impressive power conversion efficiency of 8.07% was exhibited.

Zn-doped SnO2 nanocrystals were successfully synthesized by a simple hydrothermal method. It is found that Zn doping into SnO2 can induce a negative shift in the flat-band potential (VFB) and increase the isoelectric point. As a result, dye-sensitized solar cells (DSCs) based on Zn-doped SnO2 nanocrystal photoanodes exhibit longer electron lifetimes and higher dye loading compared to undoped SnO2 based DSCs. The overall power conversion efficiency (η) of the optimized Zn-doped SnO2 based DSC reaches 4.18% and increases to 7.70% after the TiCl4 treatment. More importantly, a remarkable η of 8.23% is achieved for DSCs based on a high-quality double-layer SnO2 photoanode with the TiCl4 treatment, to the best of our knowledge, which is so far the best reported efficiency for DSCs based on SnO2 photoanodes.