•Eosin Y and carbon dots are used as co-sensitizers of TiO2 NT photoanode.•Carbon dots can increase the adsorption amount of Eosin Y on TiO2 NT surface.•Co-sensitization sequence has little effect on adsorption amount of Eosin Y.•A favorable charge transfer model dominates in carbon dots/Eosin Y/TiO2 NT.•Co-sensitized TiO2 owns better light adsorption and hydrogen production rate.According to a stepwise adsorption approach, TiO2 NT photoanode was co-sensitized with Eosin Y and carbon dots for photoelectrochemical water splitting. The adsorption amount of Eosin Y in co-sensitized samples increases due to the presence of carbon dots regardless of co-sensitization sequence. The absorbance of Eosin Y/carbon dots/TiO2 NT in visible light region is stronger than that of carbon dots/Eosin Y/TiO2 NT, while carbon dots/Eosin Y/TiO2 NT exhibits higher photocurrent density and hydrogen production rate. Based on the shortest electron transport time, the electron diffusion coefficient in carbon dots/Eosin Y/TiO2 NT electrode is up to 14.94 × 10−19cm2s−1. A favorable energy levels alignment dominates in carbon dots/Eosin Y/TiO2 NT, which can further account for enhancement in charge transfer.

•Better carbon dots is prepared by electrochemical ablation of graphite rods at 40 V for 5 h.•Carbon dots/TiO2 nanotube arrays exhibit the higher photoelectrochemical properties.•More carbon dots can be better combined with TiO2 nanotube arrays than TiO2 nanoparticles.•The role of carbon dots depends on the up-converted PL behavior and alkaline electrolyte.Carbon dots with various functional groups can be employed as the potential sensitizer. In this study, carbon dots are obtained by electrochemical ablation of graphite rods in alkaline electrolyte. The better preparation condition is the applied potential of 40 V and the ablation time of 5 h. TiO2 nanotube arrays and TiO2 nanoparticles photoelectrodes are sensitized by the as-prepared carbon dots through using impregnation method. Carbon dots/TiO2 nanotube arrays electrodes exhibit greater photoelectrochemical hydrogen production activities than carbon dots/TiO2 nanoparticles electrodes. It is because more carbon dots can be well combined with TiO2 nanotube arrays. Based on the IPCE values in visible light region, the role of carbon dots on TiO2 nanotube arrays electrode depends on the up-converted PL behaviors from their surface states and the alkaline electrolyte. The results provide insight into carbon dots that serve as sensitizer of TiO2 photoelectrode in water splitting system of alkaline solution.

Molecular dynamics simulations have been conducted to study the interaction between anatase TiO2(001), (100), and (101) surfaces and water at room temperature. The dynamic interfacial structure and properties of water on anatase TiO2 surfaces are obtained by analyzing the water density, the diffusion coefficient of water, the surface charge distribution, electric fields and the electrostatic potential distribution. The simulation results have revealed that a highly-ordered water layer structure can be formed near to the anatase TiO2 surface and have also given the Helmholtz layer width and potential drop at the water–TiO2 interface. By correlating the Helmholtz layer with the depletion layer, the depletion layer widths of three surfaces (001), (100), and (101) have been calculated as 474 Å, 237 Å and 99 Å, respectively. The resulting order of the photoelectrochemical activity of the anatase TiO2 surfaces is (001) > (100) > (101), which is consistent with the experimental results. This study may provide a useful correlation of the depletion layer with the Helmholtz layer based on simulations results for the prediction of the behavior and the control of photon-energy conversion devices.

•TiO2 NPs were deposited by fs-PLD at room temperature under different chamber pressure.•The morphology of the sample obtained in 1 mbar shows the sparser nanoagglomerates.•TiO2 NPs films with pure rutile phase exhibited the higher photocurrent densities.•The optimal deposition conditions for better PEC properties of TiO2 NPs films were deduced.TiO2 nanoparticles (NPs) were deposited on Ti foil by fs-PLD at room temperature under different chamber pressure from high vacuum (∼10−6 mbar) to oxygen pressure (0.1 mbar, 1 mbar). The present work focuses on investigating the effects of deposition conditions on crystalline phase, morphology and photoelectrochemical properties of the resulting TiO2 NPs-assembled films electrode. The results show that the morphology of the sample obtained in 1 mbar shows the sparser nanoagglomerates than that of TiO2 NPs deposited under vacuum or in 0.1 mbar. Moreover, TiO2 NPs films with pure rutile phase and the thickness of less than 400 nm exhibited the higher photocurrent densities and the greater charge carrier densities. Combined with the analysis of the photovoltage, it can be deduced that the optimal deposition condition for better PEC performance of TiO2 NPs films produced by fs-PLD is the higher chamber pressure (1 mbar) and longer deposition time (3 h).

•Ternary carbonates with different components ratio were modified by adding NaOH or NaNO3.•NaOH has greater effects on reducing the melting point of mixed carbonates for heat storage.•Six samples have lower initial crystallization point than that of corresponding ternary salts.•Specific heat of optimized samples is fitted with the quadratic polynomial of temperature well.•Thermal stability of optimized samples is proven to be good by repeatability of heating curves.In order to meet the requirements of high temperature solar thermal power generation better, the melting point of mixed carbonates should be further reduced. Starting with ternary carbonates, we modified their composition ratios and added NaNO3 or NaOH as additives. Their new melting points, initial crystallization points, decomposition temperatures, specific heats and thermal stabilities were analyzed. While the melting points of the mixed nitrate/carbonate salts were maintained at 394.7 °C, the melting points of the mixed hydroxide/carbonate salts were reduced to 323.2 °C. Also the initial crystallization temperature of the mixed hydroxide/carbonate salts was dependent on their composition. Among the hydroxide-mixed molten salts, there are six samples with lower initial crystallization temperature. The decomposition temperature of the selected six samples was nearly 800 °C. Their specific heats were fitted with the quadratic polynomial of the temperature and their thermal stability was shown by the excellent repeatability of five heating curves. Only the very first heating curves showed that the initial measured value of a melting point was higher than all the later values of the molten salts due to starting with the powder. Furthermore, possible issues that arise with the addition of NaOH are discussed.

The present work investigates the photoelectrochemical behavior of nanotubular N/C-TiO2 electrode for hydrogen production. Via the sonoelectrochemical anodization process of 1 h, N-containing TiO2 based nanotube arrays(N-TNT) with the length of about 650 nm were fabricated in fluoride aqueous solution added 0.25 M NH4NO3; C-containing TiO2 based nanotube arrays(C-TNT) with the length of about 2 μm were prepared in fluoride ethylene glycol solution. In virtue of the longer tubes with the larger surface areas, C-TNT can harvest more light and produce more photoactive sites than N-TNT, which also made the charge transfer resistance in C-TNT larger than that in N-TNT. Considered the more negative flat band potential of C-TNT, C-TNT has the smaller energy barrier and the better photoelectrochemical activity. It may be attributed to the appropriate defect concentration gradient owing to the modification of C element. Under UV–vis light (320–780 nm) irradiation, the average hydrogen generation rate of C-TNT was 282 μL h−1 cm−2. The surface properties and near-surface properties of the resultant electrode were synthetically analyzed by using UV–vis diffuse reflectance spectra(DRS), field emission scanning electron microscopy (FESEM), I-t curves, and electrochemical impedance spectroscopy (EIS) techniques.Highlights► N-containing TiO2 nanotube arrays were anodized in fluoride aqueous solution. ► C-containing TiO2 nanotube arrays were anodized in ethylene glycol solution. ► Flat band potential of C-TNT is more negative than that of N-TNT. ► The appropriate defect concentration gradient may be created by doping C element. ► C-TNT exhibited the better surface properties and near-surface properties.

TiO2 nanotube arrays were fabricated by sonoelectrochemical anodic oxidation and calcined in nitrogen, air, or 5% hydrogen/nitrogen which was denoted as TNT-A, TNT-N, and TNT-H, respectively. All annealed TiO2 nanotube arrays samples exhibited similar surface morphology. With UV illumination (365 ± 15 nm), the photocurrent density of the TNT-A, TNT-N and TNT-H was about 0.27 mA/cm2, 0.45 mA/cm2 and 0.60 mA/cm2, respectively. The trapped electron at the Ti4+ center of TiO2 nanotube arrays shows absorption at around 500–700 nm. From the XPS measurement, it was found that annealing in 5% hydrogen/nitrogen helped the sample obtain a greater defect density. Because of the reduction of Ti4+ and the formation of oxygen vacancies, the charge transfer resistance appeared in this order: TNT-A > TNT-N > TNT-H. Thus TNT-H harvested the greatest charge carrier density of 9.86 × 1020 cm−3, TNT-N and TNT-A obtained a charge carrier density of 1.38 × 1020 cm−3 and 1.06 × 1020 cm−3, respectively. Accordingly, the hydrogen production rate by water splitting over TNT-A, TNT-N and TNT-H (320–780 nm irradiation, 3 h) was about 120 μL/h cm2, 159 μL/h cm2 and 231 μL/h cm2, respectively.

The photocatalyst TiO2 with a wormhole-like mesoporous structure and narrow pore distribution were successfully synthesized using triethanolamine as template. The complex oxides InVO4 with a relatively high surface area, a visible light active hydrogen-evolving photocatalyst, were obtained by means of the templating hydrothermal method with a variety of surfactants. InVO4 with wormhole-like structure were fabricated by templating of cetyltrimethylammonium bromide. By comparing with the non-mesoporous samples, the mesoporous photocatalysts exhibited higher photocatlytic hydrogen evolution activity for water splitting in the absence and in the presence of oxalic acid.

Molecular dynamics simulations have been conducted to study the interaction between anatase TiO2(001), (100), and (101) surfaces and water at room temperature. The dynamic interfacial structure and properties of water on anatase TiO2 surfaces are obtained by analyzing the water density, the diffusion coefficient of water, the surface charge distribution, electric fields and the electrostatic potential distribution. The simulation results have revealed that a highly-ordered water layer structure can be formed near to the anatase TiO2 surface and have also given the Helmholtz layer width and potential drop at the water–TiO2 interface. By correlating the Helmholtz layer with the depletion layer, the depletion layer widths of three surfaces (001), (100), and (101) have been calculated as 474 Å, 237 Å and 99 Å, respectively. The resulting order of the photoelectrochemical activity of the anatase TiO2 surfaces is (001) > (100) > (101), which is consistent with the experimental results. This study may provide a useful correlation of the depletion layer with the Helmholtz layer based on simulations results for the prediction of the behavior and the control of photon-energy conversion devices.