Synergy of Low-Energy {101} and High-Energy {001} TiO₂ Crystal Facets for Enhanced Photocatalysis

Controlled crystal growth determines the shape, size, and exposed facets of a crystal, which usually has different surface physicochemical properties. Herein we report the size and facet control synthesis of anatase TiO₂ nanocrystals (NCs). The exposed facets are found to play a crucial role in the photocatalytic activity of TiO₂ NCs. This is due to the known preferential flow of photogenerated carriers to the specific facets. Although, in recent years, the main focus has been on increasing the surface area of high-energy exposed facets such as {001} and {100} to improve the photocatalytic activity, here we demonstrate that the presence of both the high-energy {001} oxidative and low-energy {101} reductive facets in an optimum ratio is necessary to reduce the charge recombination and thereby enhance photocatalytic activity of TiO₂ NCs

Nitrogen Doped Reduced Graphene Oxide Based Pt–TiO₂ Nanocomposites for Enhanced Hydrogen Evolution

Electrochemical hydrogen production from water is an attractive clean energy generation process that has enormous potential for sustainable development. However, noble metal catalysts are most commonly used for such electrochemical hydrogen evolution making the process cost ineffective. Thereby design of hybrid catalysts with minimal use of noble metals using a suitable support material is a prime requirement for the electrolysis of water. Herein, we demonstrate the superior hydrogen evolution reaction (HER) activity of the platinum nanoparticles (Pt NPs) supported on faceted titanium dioxide (TiO₂) nanocrystals (Pt–TiO₂) and nitrogen doped reduced graphene oxide (N-rGO) based TiO₂ nanocomposite (Pt–TiO₂–N-rGO). The ternary Pt–TiO₂–N-rGO nanocomposite exhibits a superior HER activity with a small Tafel slope (∼32 mV·dec^–1), exchange current density (∼0.22 mA·cm^–2), and excellent mass activity (∼3116 mA·mg_pt^–1) at 300 mV overpotential. These values are better/higher than that of several support materials investigated so far. The excellent HER activity of the ternary Pt–TiO₂–N-rGO nanocomposite is ascribed to the presence of Ti­(III) states and enhanced charge transportation properties of N-rGO. The present study is a step toward reliable electrochemical hydrogen production using faceted TiO₂ nanocrystals as support material

Engineered Electronic States of Transition Metal Doped TiO₂ Nanocrystals for Low Overpotential Oxygen Evolution Reaction

Electrochemical oxygen evolution reaction (OER) involves high overpotential at the oxygen evolving electrode and thereby suffers significant energy loss in the proton exchange membrane water electrolyzer. To reduce the OER overpotential, precious ruthenium and iridium oxides are most commonly used as anode electrocatalyst. Here we report marked reduction in overpotential for the OER using transition metal (TM) doped TiO₂ nanocrystals (NCs). This reduction in overpotential is attributed to d-orbitals splitting of the doped TMs in the TM-doped TiO₂ NCs and their interactions with the oxyradicals (intermediates of OER) facilitating the OER. The d-orbital spitting of TMs in TM-doped TiO₂ NCs is evident from the change in original pearl white color of undoped TiO₂ NCs and UV–vis absorption spectra

Green Synthesis of Anatase TiO₂ Nanocrystals with Diverse Shapes and their Exposed Facets-Dependent Photoredox Activity

The exposed facets of a crystal are known to be one of the key factors to its physical, chemical and electronic properties. Herein, we demonstrate the role of amines on the controlled synthesis of TiO₂ nanocrystals (NCs) with diverse shapes and different exposed facets. The chemical, physical and electronic properties of the as-synthesized TiO₂ NCs were evaluated and their photoredox activity was tested. It was found that the intrinsic photoredox activity of TiO₂ NCs can be enhanced by controlling the chemical environment of the surface, i.e.; through morphology evolution. In particular, the rod shape TiO₂ NCs with ∼25% of {101} and ∼75% of {100}/{010} exposed facets show 3.7 and 3.1 times higher photocatalytic activity than that of commercial Degussa P25 TiO₂ toward the degradation of methyl orange and methylene blue, respectively. The higher activity of the rod shape TiO₂ NCs is ascribed to the facetsphilic nature of the photogenerated carriers within the NCs. The photocatalytic activity of TiO₂ NCs are found to be in the order of {101}+{100}/{010} (nanorods) > {101}+{001}+{100}/{010} (nanocuboids and nanocapsules) > {101} (nanoellipsoids) > {001} (nanosheets) providing the direct evidence of exposed facets-depended photocatalytic activity

Exploration of Diverse Interactions of l‑Methionine in Aqueous Ionic Liquid Solutions: Insights from Experimental and Theoretical Studies

Here, we have investigated some physicochemical parameters to understand the molecular interactions by means of density (ρ) measurement, measurement of viscosity (η), refractive index(nD) measurement, and conductance and surface tension measurements between two significant aqueous ionic liquid solutions: benzyl trimethyl ammonium chloride (BTMAC) and benzyl triethyl ammonium chloride (BTEAC) in an aqueous l-methionine (amino acid) solution. The apparent molar volume (Φv), coefficient of viscosity (B), and molar refraction (RM) have been used to analyze the molecular interaction behavior associated in the solution at various concentrations and various temperatures. With the help of some important equations such as the Masson equation, the Jones–Doles equation, and the Lorentz–Lorenz equation, very significant parameters, namely, limiting apparent molar volumes (Φv0), coefficient of viscosity (B), and limiting molar refraction (RM0), respectively, are obtained. These parameters along with specific conductance (κ) and surface tension (σ) are very much helpful to reveal the solute–solvent interactions by varying the concentration of solute molecules and temperature in the solution. Analyses of Δμ10#, Δμ20#, TΔS20#, ΔH20#, and thermodynamic data provide us valuable information about the interactions. We note that l-Met in 0.005 molality BTEAC ionic liquid at 308.15 K shows maximum solute–solvent interaction, while l-Met in 0.001 molality BTMAC aqueous solution of ionic liquid at 298.15 K shows the minimum one. Spectroscopic techniques such as Fourier transform infrared (FTIR), 1H-NMR, and UV–vis also provide supportive information about the interactions between the ionic liquid and l-methionine in aqueous medium. Furthermore, adsorption energy, reduced density gradient (RDG), and molecular electrostatic potential (MESP) maps obtained by the application of density functional theory (DFT) have been used to determine the type of interactions, which are concordant with the experimental observations

Charge Separation in TiO₂/BDD Heterojunction Thin Film for Enhanced Photoelectrochemical Performance

Semiconductor photocatalysis driven by electron/hole has begun a new era in the field of solar energy conversion and storage. Here we report the fabrication and optimization of TiO₂/BDD p-n heterojunction photoelectrode using p-type boron doped diamond (BDD) and n-type TiO₂ which shows enhanced photoelectrochemical activity. A p-type BDD was first deposited on Si substrate by microwave plasma chemical vapor deposition (MPCVD) method and then n-type TiO₂ was sputter coated on top of BDD grains for different durations. The microstructural studies reveal a uniform disposition of anatase TiO₂ and its thickness can be tuned by varying the sputtering time. The formation of p-n heterojunction was confirmed through I–V measurement. A remarkable rectification property of 63773 at 5 V with very small leakage current indicates achieving a superior, uniform and precise p–n junction at TiO₂ sputtering time of 90 min. This suitably formed p-n heterojunction electrode is found to show 1.6 fold higher photoelectrochemical activity than bare n-type TiO₂ electrode at an applied potential of +1.5 V vs SHE. The enhanced photoelectrochemical performance of this TiO₂/BDD electrode is ascribed to the injection of hole from p-type BDD to n-type TiO₂, which increases carrier separation and thereby enhances the photoelectrochemical performance