Structure and Properties of Water on the Anatase TiO₂(101) Surface: From Single-Molecule Adsorption to Interface Formation

The interaction of water with titanium dioxide surfaces has a vital role in many energy- and environment-related applications, such as dye-sensitized solar cell, photocatalytic or photoelectrochemical hydrogen production, and environmental purification. Structure and properties of water on the anatase TiO₂(101) surface have been studied by using a combination of density functional theory and force field molecular dynamics. Owing to the amphotericity of this surface and the competition between water–water and water–substrate interactions, the structure and properties of water on the anatase TiO₂(101) surface exhibited some peculiar and complicated features. The overall evolutionary process of interface formation has been obtained by investigating the coverage-dependent adsorption configuration and energy of water. The competition between water–water and water–substrate interaction results in the existence of a stable bilayer of water (Θ ≥ 2 ML) and an ice-like structure of water at higher coverage (Θ ≥ 3 ML). Both static and dynamic calculation results have showed that a highly ordered structure occurs in the first few water molecule layers, and this order decreases as one moves toward the bulk region. The electric fields across the interface and in the electric double layer were estimated to be about 10 and 2 eV, respectively. This study may provide new insight into the static and dynamic properties of the water–TiO₂ interface and elucidate the reactions that occur on the TiO₂ surface

Back Electron Transfer at TiO₂ Nanotube Photoanodes in the Presence of a H₂O₂ Hole Scavenger

Adding charge scavengers, which usually are more unstable than water, is an effective method to quantify the quantum efficiency loss of photoelectrode during the charge separation, transfer, and injection processes of the water splitting reaction. Here, we detected, on TiO₂ nanotube photoanodes after using hydrogen peroxide (H₂O₂) as a hole scavenger, a nearly 40% saturated photocurrent decrease in alkaline electrolyte and a negligible saturated photocurrent difference in acid electrolyte. We found that the photoelectrons were trapped in the surface states of TiO₂ with nearly the same storage capacity of electrons in a wide range of pH values from 1.0 to 13.6. However, kinetics of a back reaction, H₂O₂ reduction by the photoelectrons trapped in surface states, is about 10 times higher for that in alkaline electrolyte than in acid electrolyte. As a result, the pH-dependent kinetic difference in H₂O₂ reduction induced the negative effects on the saturated photocurrent. Our results offer a new insight into understanding the effects of back electron transfer on electrochemical behaviors of surface states and charge scavengers

Trivalent Nickel-Catalyzing Electroconversion of Alcohols to Carboxylic Acids

The comprehension of activity and selectivity origins of the electrooxidation of organics is a crucial knot for the development of a highly efficient energy conversion system that can produce value-added chemicals on both the anode and cathode. Here, we find that the potential-retaining trivalent nickel in NiOOH (Fermi level, −7.4 eV) is capable of selectively oxidizing various primary alcohols to carboxylic acids through a nucleophilic attack and nonredox electron transfer process. This nonredox trivalent nickel is highly efficient in oxidizing primary alcohols (methanol, ethanol, propanol, butanol, and benzyl alcohol) that are equipped with the appropriate highest occupied molecular orbital (HOMO) levels (−7.1 to −6.5 eV vs vacuum level) and the negative dual local softness values (Δsk, −0.50 to −0.19) of nucleophilic atoms in nucleophilic hydroxyl functional groups. However, the carboxylic acid products exhibit a deeper HOMO level (<−7.4 eV) or a positive Δsk, suggesting that they are highly stable and weakly nucleophilic on NiOOH. The combination (HOMO, Δsk) is useful in explaining the activity and selectivity origins of electrochemically oxidizing alcohols to carboxylic acid. Our findings are valuable in creating efficient energy conversions to generate value-added chemicals on dual electrodes

Solution-Chemical Route to Generalized Synthesis of Metal Germanate Nanowires with Room-Temperature, Light-Driven Hydrogenation Activity of CO₂ into Renewable Hydrocarbon Fuels

A facile solution-chemical route was developed for the generalized preparation of a family of highly uniform metal germanate nanowires on a large scale. This route is based on the use of hydrazine monohydrate/H₂O as a mixed solvent under solvothermal conditions. Hydrazine has multiple effects on the generation of the nanowires: as an alkali solvent, a coordination agent, and crystal anisotropic growth director. Different-percentage cobalt-doped Cd₂Ge₂O₆ nanowires were also successfully obtained through the addition of Co­(OAc)₂·4H₂O to the initial reaction mixture for future investigation of the magnetic properties of these nanowires. The considerably negative conduction band level of the Cd₂Ge₂O₆ nanowire offers a high driving force for photogenerated electron transfer to CO₂ under UV–vis illumination, which facilitates CO₂ photocatalytic reduction to a renewable hydrocarbon fuel in the presence of water vapor at room temperature

Theoretical Insight into Charge-Recombination Center in Ta₃N₅ Photocatalyst: Interstitial Hydrogen

Ideal Ta₃N₅ is a promising candidate photocatalyst for solar water splitting. In a common synthetic route, both oxygen and hydrogen impurities are inevitably formed during the nitridation of TaO_<i>x</i> precursor by ammonia. The introduced hydrogen impurities would bond with oxygen in the form of hydroxyl groups, resulting in additional bands bracketing band edges. This configuration adds Ta₃N₅ electron–hole recombination centers, leading to a high onset potential. Hydrogen impurities would also introduce hydrogen bonds which aggravate charge recombination by additional charge transport paths from anions to hydroxyl recombination centers. In addition, hydride ions of hydroxyl groups may be activated into protons at high bias and may relay hole transport in Ta₃N₅, endowing the material with high saturated photocurrent. In summary, hydrogen impurities would aggravate the onset potential of Ta₃N₅ in the way of high electron–hole recombination. More broadly, hydrogen impurities may be common in (oxy)­nitrides and other covalent materials; they may add to the photocatalysts’ high onset potential via electron localizations and might introduce high charge recombination for covalent semiconductors

Controllable Conformation Transfer of Conjugated Polymer toward High Photoelectrical Performance: The Role of Solvent in Induced-Crystallization Route

In the present work, polyimides with different conformation (dendritic, spherulitic, and laminar) were synthesized in different solvents. The strong solvent-dependent conformation variation is found intimately related to the specific interactions between the polymer chain and solvent molecules that have a primary driving force for the chain diffusion and rearrangement. Moreover, the photoelectrical properties of polyimide were sensitively influenced by the crystal structure and polymer morphology. In laminar conformation with strong intermolecular π–π interactions, the light absorption as well as the mobility and separation of photoinduced carriers were greatly improved compared with similar values of dendritic and spherulitic ones. For photocatalytic hydrogen evolution from water splitting, polyimide with laminar conformation exhibits 16 times enhancement in activity than the dendritic. This work provides further insight into the intrinsic interacting mechanism of solvent-induced crystallization of conjugated polymer and paves an innovative way for synthesis of efficient polymer semiconductor photocatalysts

Ultrathin, Single-Crystal WO₃ Nanosheets by Two-Dimensional Oriented Attachment toward Enhanced Photocatalystic Reduction of CO₂ into Hydrocarbon Fuels under Visible Light

An ultrathin, single-crystal WO₃ nanosheet of ∼4–5 nm in thickness, corresponding to six repeating unit cells of monoclinic WO₃ along the <i>c</i> axis, was synthesized with laterally oriented attachment of tiny WO₃ nanocrystals formed using a solid–liquid phase arc discharge route in an aqueous solution. Size-quantization effects in this ultrathin nanostructure alter the WO₃ band gap to enable the nanosheet to exhibit enhanced performance for photocatalytic reduction of CO₂ in the presence of water in hydrocarbon fuels that do not exist in its bulk form

Oxygen-Impurity-Induced Direct–Indirect Band Gap in Perovskite SrTaO₂N

Oxynitride semiconductors are considered to be promising candidates for solar water splitting. In this work, we show that oxygen-rich SrTaO₂N has a band gap with direct–indirect character through twin valence-band maximums (VBMs), resulting in good photoelectronic responses. Compared with the direct band gap of ideal SrTaO₂N, the additional indirect VBM of the oxygen-rich solid solution was found to be due to strontium–oxygen hybridization, using orbital projections based on hybrid/GW density functional theory (DFT). This twin-VBM character was validated by strontium K-edge absorption through extended X-ray absorption fine structure (EXAFS) analysis. The twin-VBM character of the band structure can enhance the photoelectronic response and hole transport. Our findings provide a viable strategy for enhancing the solar water splitting performance of oxynitrides

Formation of Hierarchical Structure Composed of (Co/Ni)Mn-LDH Nanosheets on MWCNT Backbones for Efficient Electrocatalytic Water Oxidation

Active, stable, and cost-effective electrocatalysts are attractive alternatives to the noble metal oxides that have been used in water splitting. The direct nucleation and growth of electrochemically active LDH materials on chemically modified MWCNTs exhibit considerable electrocatalytic activity toward oxygen evolution from water oxidation. CoMn-based and NiMn-based hybrids were synthesized using a facile chemical bath deposition method and the as-synthesized materials exhibited three-dimensional hierarchical configurations with tunable Co/Mn and Ni/Mn ratio. Benefiting from enhanced electrical conductivity with MWCNT backbones and LDH lamellar structure, the Co₅Mn-LDH/MWCNT and Ni₅Mn-LDH/MWCNT could generated a current density of 10 mA cm^–2 at overpotentials of ∼300 and ∼350 mV, respectively, in 1 M KOH. In addition, the materials also exhibited outstanding long-term electrocatalytic stability

Study on the Ambient Temperature as an Important but Easily Neglected Factor in the Process of Preparing Photovoltaic All-Inorganic CsPbIBr₂ Perovskite Film by the Elegant Solvent-Controlled Growth Strategy

All-inorganic CsPbIBr2 perovskite has received extensive attention in the field of solar cells due to its good wet and thermal stability as well as a moderate band gap. In the preparation of CsPbIBr2 film by one-step spin-coating method, the amount of dimethyl sulfoxide solvent remaining in the precursor film has a great influence on the process of film growth. Therefore, it is necessary to ensure that an appropriate amount of solvent exists in the precursor film before annealing. Herein, we adopted the solvent-controlled growth (SCG) strategy, that is, standing by the precursor films in the nitrogen glovebox for a period of time before annealing, to make sure that excess solvent can be evaporated from the precursor film. In this work, we found that the ambient temperature is an important but easily neglected factor in the process of preparing CsPbIBr2 film by the SCG strategy. When the ambient temperature is 20 °C, SCG treatment is required to obtain a flat and dense CsPbIBr2 film. However, SCG treatment is not essential at 30 °C. The ambient temperature has an impact on the evaporation rate of the solvent in the precursor film, and thus affects the effect of the SCG strategy. This work highlights that, when preparing CsPbIBr2 film by a one-step spin-coating method, in order to obtain a high-quality CsPbIBr2 film, the influence of ambient temperature on solvent-controlled growth strategy should be considered