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

Facile Green Synthesis of WO₃·H₂O Nanoplates and WO₃ Nanowires with Enhanced Photoelectrochemical Performance

The synthesis of nanostructured materials with controlled shape without using a capping agent and/or a hazardous chemical is one of the major existing challenges. Herein, we report a facile precipitation method to synthesize stacked orthorhombic tungsten trioxide hydrate (WO₃·H₂O) nanoplates by simply mixing WCl₆ (0.025 M) in ethanol at room temperature for 1 h. On subsequent solvothermal treatment of WO₃·H₂O nanoplates at 200 °C in ethanol, formation of monoclinic tungsten trioxide (WO₃) nanowires of <20 nm diameter is demonstrated. The morphology evolution of WO₃ nanowires from WO₃·H₂O nanoplates and change in growth direction through dissolution and recrystallization process is further confirmed by varying the solvothermal duration and temperature. The as-synthesized WO₃·H₂O nanoplates and WO₃ nanowires are used as photoanodes for the hydrogen generation through photoelectrochemical (PEC) water splitting in a neutral pH. The photocurrent density of WO₃ nanowires is found ∼21 times higher than that of WO₃·H₂O nanoplates at 1.0 V vs saturated calomel electrode (SCE) and also higher than the reported WO₃ nanostructures. The superior PEC performance of WO₃ nanowires is justified on the basis of its (200) oriented one-dimensional morphology, large surface area, and small interfacial charge transfer resistance

Effect of Etching on Electron–Hole Recombination in Sr-Doped NaTaO₃ Photocatalysts

Sodium tantalate (NaTaO₃) photocatalysts doped with Sr²⁺ produce core–shell-structured NaTaO₃–SrSr_1/3Ta_2/3O₃ solid solutions able to split water efficiently, when prepared via the solid-state method. In this study, the photocatalysts were chemically etched to examine the different roles of the core and shell with respect to the recombination of electrons and holes. Under excitation by Hg–Xe lamp irradiation, the steady-state population of electrons in the core–shell-structured photocatalyst with a bulk Sr concentration of 5 mol % increased by 130 times relative to that of the undoped photocatalyst. During etching for the first 10 min, the shell detached from the top of the core, and the electron population in the uncovered core further increased by 40%. This population enhancement indicates that electrons are excited in the core and recombined in the shell. Etching up to 480 min resulted in the reduction of the electron population. To interpret the population reduction in this stage of etching, a Sr concentration gradient that controls the electron population in the core is proposed

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

Bimetallic Au@M (M = Ag, Pd, Fe, and Cu) Nanoarchitectures Mediated by 1,4-Phenylene Diisocyanide Functionalization

Hybridization with gold has attracted a lot of attention in many application areas such as energy, nanomedicine, and catalysts. Here, we demonstrate electrochemical hybridization of two different metals by using bare and 1,4-phenylene diisocyanide (PDI) functionalized gold nanoislands (GNIs) supported on a Si substrate. As pristine GNIs are not tightly locked on the Si surface, bimetallic Au@M (M = Ag, Pd, Fe, and Cu) core–shell type nanostructures are produced by an electric-field-induced clustering of GNIs and metal deposition. On the other hand, upon functionalization of GNIs by PDI, 3D island growth on the functionalized GNI template is observed as PDI acts as a protector against the electric-field-induced clustering. Depth-profiling X-ray photoelectron spectroscopy reveals no discernible difference in the interfacial electronic structures of hybrid metals prepared by using pristine and PDI-functionalized GNI templates. This work demonstrates a new approach to produce a secured template and to manipulate growth of hybrid nanoparticles on this template supported on a Si substrate by using electrodeposition and organic functionalization

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

Synthesis and Photophysical Properties of an Eu(II)-Complex/PS Blend: Role of Ag Nanoparticles in Surface-Enhanced Luminescence

A novel Eu­(II) complex with 2-ethylhexyl hydrogen 2-ethylhexyl phosphonate (EHHEHP or PC88A) was synthesized and blended with polystyrene polymer (PS). Both an independent complex and the Eu­(II)/PS blend excited by near-UV light produced blue luminescence, arising from the 5d→ 4f transitions of Eu­(II). Time-dependent density functional theory (TD-DFT) calculations on electronic structures of the complex molecule indicated that the absorbing and emitting center was associated with the ²A­(d_<i>z</i>²) state under the C₂ crystal field. We also synthesized silver nanoparticles (Ag NPs) with an average particle size of 4.48 nm (σ = 0.91 nm) using EHHEHP as a stabilizer. The effects of Ag NPs as a colloidal suspension and an interfacial layer on the luminescence intensity of the blend were investigated as functions of the concentration of Ag NPs and the thickness of the Ag NP layer, respectively. The critical concentration of the colloidal Ag NPs and the critical thickness of the interfacial Ag NP layer were ∼355 ppm and ∼0.16 μm, respectively. Under critical conditions, the Ag NPs increased the luminescence intensity by 4.4 times as a colloidal suspension in CH₂Cl₂ and 2.2 times as an interfacial-layer state

Highly Active Tungsten Oxide Nanoplate Electrocatalysts for the Hydrogen Evolution Reaction in Acidic and Near Neutral Electrolytes

An efficient, cost-effective, and earth-abundant catalyst that could drive the production of hydrogen from water without or with little external energy is the ultimate goal toward hydrogen economy. Herein, nanoplates of tungsten oxide and its hydrates (WO₃·H₂O) as promising electrocatalysts for the hydrogen evolution reaction (HER) are reported. The square-shaped and stacked WO₃·H₂O nanoplates are synthesized at room temperature under air in ethanol only, making it as a promising green synthesis strategy. The repeated electrochemical cyclic voltammetry cycles modified the surface of WO₃·H₂O nanoplates to WO₃ as confirmed by X-ray photoelectron and Auger spectroscopy, which leads to an improved HER activity. Hydrogen evolution is further achieved from distilled water (pH 5.67) producing 1 mA cm^–2 at an overpotential of 15 mV versus the reversible hydrogen electrode. Moreover, WO₃·H₂O and WO₃ nanoplates demonstrate excellent durability in acidic and neutral media, which is highly desirable for practical application. Improved hydrogen evolution by WO₃(200) when compared to that by Pt(111) is further substantiated by the density functional theory calculations

ZnO-TiO₂ Core–Shell Nanowires: A Sustainable Photoanode for Enhanced Photoelectrochemical Water Splitting

We present the synthesis of a unique vertically aligned ZnO-TiO₂ core–shell nanowires (NWs) heterostructure on an Si-wafer using a chemical vapor deposition method. The structural study shows the well-developed ZnO-TiO₂ core–shell NWs heterostructure. This unique ZnO-TiO₂ core–shell NWs heterostructure displays a photocurrent density of 1.23 mA cm^–2, which is 2.41 times higher than pristine ZnO NWs. A cathodic shift in the flat band potential and a lower onset potential of a ZnO-TiO₂ core–shell NWs heterostructure over ZnO NWs indicates more favorable properties for photoelectrochemical water splitting with a photoconversion efficiencey of 0.53%. A higher photocurrent density/photoconversion efficiency is due to the effective addition of photogenerated electron–hole separation originating from the ZnO NWs core and the conformal covering of a amorphous TiO₂ passivation shell. Therefore, these results suggest that the vertically aligned one-dimentional ZnO-TiO₂ core–shell NWs heterostructure is a promising photoanode for solar energy conversion devices

Crystal Phase and Size-Controlled Synthesis of Tungsten Trioxide Hydrate Nanoplates at Room Temperature: Enhanced Cr(VI) Photoreduction and Methylene Blue Adsorption Properties

Controlling the crystal phase of a material using solution-based method is a challenging task and has significant consequence to the material’s properties. Herein we report the phase and size-controlled synthesis of tungsten oxide hydrates at room temperature via a simple precipitation method. In the absence and presence of oxalic acid, orthorhombic WO₃·H₂O and monoclinic WO₃·2H₂O nanoplates of size in the range of 200–600 (thickness <50 nm) and 40–200 nm (thickness <20 nm) were respectively synthesized. Oxalic acid is found to play the central role in the phase transition due to its chelating nature that facilitates bonding of oxalate ions to tungsten cations leading to formation of WO₃·2H₂O. Upon annealing at 400 °C for 2 h under air, both WO₃·H₂O and WO₃·2H₂O nanoplates were converted to monoclinic WO₃ nanoplates. These nanoplates were demonstrated to be highly efficient for the photocatalytic detoxification of toxic Cr­(VI) in the acidic pH under the visible light irradiation. The best Cr­(VI) reduction performance was obtained with WO₃·2H₂O nanoplates due to its smaller band gap and larger effective surface area. In addition, a lower pH value is found to facilitates the Cr­(VI) reduction. Furthermore, highly concentrated methylene blue was efficiently removed (>95%) by adsorption on the nanoplates within a minute, suggesting the importance and potential of a material that can be synthesized at room temperature