Active facets on titanium(III)-doped TiO2: an effective strategy to improve the visible-light photocatalytic activity

Hierarchical TiO2 spheres decorated with Au nanoparticles for visible light hydrogen production

Hierarchical TiO₂ spheres composed of nanosheets are successfully synthesized via a simple solvothermal route.

Copper(i) cysteine complexes: efficient earth-abundant oxidation co-catalysts for visible light-driven photocatalytic H2 production

Inspired by nature, a Cu(i)–cysteine complex serving as an oxidation co-catalyst to consume photo-induced holes has been developed for highly efficient visible light-driven photocatalytic H₂ production.

Factors influencing the photocatalytic activity of rutile TiO2 nanorods with different aspect ratios for dye degradation and Cr(vi) photoreduction

The aspect ratio of rutile TiO₂ nanorods is a key factor influencing photocatalytic MB degradation. The conduction band potential plays significant roles in the photocatalytic reduction of Cr(vi).

Theoretical studies of heteroatom-doping in TiO2 to enhance the electron injection in dye-sensitized solar cells

Density functional calculations have been explored to analyze the structural, electronic and charge transfer properties of a doped TiO₂ substrate and catechol–TiO₂ interfaces for dye-sensitized solar cells.

Steering charge kinetics in photocatalysis: intersection of materials syntheses, characterization techniques and theoretical simulations

This review outlines the recent progress on probing and steering charge kinetics toward designing highly efficient photocatalysts.

One-step synthesis of a hierarchical Bi2S3 nanoflower\In2S3 nanosheet composite with efficient visible-light photocatalytic activity

Hierarchical Bi₂S₃ nanoflower\In₂S₃ nanosheet composites were prepared and showed excellent visible-light photocatalytic activity.

Increasing the visible light absorption of graphitic carbon nitride (melon) photocatalysts by homogeneous self-modification with nitrogen vacancies

A novel reduced melon photocatalyst with a bandgap of 2.03 eV developed here has a widened visible light absorption range and suppressed radiative recombination of photo-excited charge carriers due to the homogeneous self-modification with nitrogen vacancies. As a consequence, the reduced melon shows a much superior photocatalytic activity compared to the pristine melon in generating •OH radicals and degrading the organic pollutant Rhodamine B.

Triple-layered nanostructured WO₃ photoanodes with enhanced photocurrent generation and superior stability for photoelectrochemical solar energy conversion

Unique nanorods/nanoparticles/nanoflakes (NRs/NPs/NFs) WO3 triple-layers are grown on a metallic W foil by a simple one-step anodization method. The triple-layered structure is formed through a self-organization process, the film thickness (up to 3 μm) being controlled by the anodization time. A first layer made of an array of WO3 densely-packed vertically-aligned NRs (1.2-1.4 μm in height) grow atop the tungsten foil, followed by a second layer of small NPs (50-80 nm) and finally a third layer made of rectangular NFs (200-300 nm). When irradiated by white light in a photoelectrochemical cell these WO3 triple-layers generate a photocurrent as high as 0.9 mA cm(-2) at 1.2 V/RHE. Moreover, we show that the stability of the triple-layered WO3 photoanodes can be considerably enhanced by adding an ultrathin (10 nm) TiO2 protective overlayer.

Reduced TiO2 rutile nanorods with well-defined facets and their visible-light photocatalytic activity

Stable reduced TiO2 rutile nanorods with well-defined facets were prepared by a solvothermal route in the presence of Zn powder. The oxygen vacancy in the TiO2 nanorods, which can be tuned by the amount of Zn, results in a narrow band gap and visible-light photocatalytic activity.

A facile and versatile method for preparation of colored TiO2 with enhanced solar-driven photocatalytic activity

Colored TiO2 has attracted enormous attention due to its visible light absorption and excellent photocatalytic activity. In this report, we develop a simple and facile solid-state chemical reduction approach for a large-scale production of colored TiO2 at mild temperature (300-350 °C). The obtained sample possesses a crystalline core/amorphous shell structure (TiO2@TiO2-x). The oxygen vacancy results in the formation of a disordered TiO2-x shell on the surface of TiO2 nanocrystals. XPS and theoretical calculation results indicate that valence band tail and vacancy band below the conduction band minimum appear for the TiO2-x, which implies that the TiO2@TiO2-x nanocrystal has a narrow band gap and therefore leads to a broad visible light absorption. Oxygen vacancy in a proper concentration promotes the charge separation of photogenerated carriers, which improves the photocatalytic activity of TiO2@TiO2-x nanocrystals. This facile and general method could be potentially used for large scale production of colored TiO2 with remarkable enhancement in the visible light absorption and solar-driven H2 production.

Fluorine-doped porous single-crystal rutile TiO2 nanorods for enhancing photoelectrochemical water splitting

Fluorine-doped hierarchical porous single-crystal rutile TiO(2) nanorods have been synthesized through a silica template method, in which F(-) ions acts as both n-type dopants and capping agents to make the isotropic growth of the nanorods. The combination of high crystallinity, abundant surface reactive sites, large porosity, and improved electronic conductivity leads to an excellent photoelectrochemical activity. The photoanode made of F-doped porous single crystals displays a remarkably enhanced solar-to-hydrogen conversion efficiency (≈0.35 % at -0.33 V vs. Ag/AgCl) under 100 mW cm(-2) of AM=1.5 solar simulator illumination that is ten times of the pristine solid TiO(2) single crystals.

Molecule-based water-oxidation catalysts (WOCs): cluster-size-dependent dye-sensitized polyoxometalates for visible-light-driven O2 evolution

From atomic level to understand the cluster-size-dependant behavior of dye-sensitized photocatalysts is very important and helpful to design new photocatalytic materials. Although the relationship between the photocatalytic behaviors and particles' size/shape has been widely investigated by theoretical scientists, the experimental evidences are much less. In this manuscript, we successfully synthesized three new ruthenium dye-sensitized polyoxometalates (POM-n, n relate to different size clusters) with different-sized POM clusters. Under visible-light illumination, all three complexes show the stable O2 evolution with the efficient order POM-3 > POM-2 > POM-1. This cluster-size-dependent catalytic behavior could be explained by the different numbers of M = Ot (terminal oxygen) bonds in each individual cluster because it is well-known that Mo = Ot groups are the catalytically active sites for photooxidation reaction. The proposed mechanism of water oxidation for the dye-sensitized POMs is radical reaction process. This research could open up new perspectives for developing new POM-based WOCs.

High detectivity solar-blind high-temperature deep-ultraviolet photodetector based on multi-layered (l00) facet-oriented β-Ga₂O₃ nanobelts

Fabrication of a high-temperature deep-ultraviolet photodetector working in the solar-blind spectrum range (190-280 nm) is a challenge due to the degradation in the dark current and photoresponse properties. Herein, β-Ga2O3 multi-layered nanobelts with (l00) facet-oriented were synthesized, and were demonstrated for the first time to possess excellent mechanical, electrical properties and stability at a high temperature inside a TEM studies. As-fabricated DUV solar-blind photodetectors using (l00) facet-oriented β-Ga2O3 multi-layered nanobelts demonstrated enhanced photodetective performances, that is, high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability, importantly, at a temperature as high as 433 K, which are comparable to other reported semiconducting nanomaterial photodetectors. In particular, the characteristics of the photoresponsivity of the β-Ga2O3 nanobelt devices include a high photoexcited current (>21 nA), an ultralow dark current (below the detection limit of 10(-14) A), a fast time response (<0.3 s), a high R(λ) (≈851 A/W), and a high EQE (~4.2 × 10(3)). The present fabricated facet-oriented β-Ga2O3 multi-layered nanobelt based devices will find practical applications in photodetectors or optical switches for high-temperature environment.

Facet-dependent photocatalytic properties of TiO(2) -based composites for energy conversion and environmental remediation

Titanium dioxide (TiO2 ) is one of the most widely investigated metal oxides because of its extraordinary surface, electronic, and photocatalytic properties. However, the large band gap of TiO2 and the considerable recombination of photogenerated electron-hole pairs limit its photocatalytic efficiency. Therefore, research attention is being increasingly directed towards engineering the surface structure of TiO2 on the atomic level (namely morphological control of {001} facets on the micro- and nanoscale) to fine-tune its physicochemical properties; this could ultimately lead to the optimization of selectivity and reactivity. This Review encompasses the fundamental principles to enhance the photocatalytic activity by using highly reactive {001}-faceted TiO2 -based composites. The current progress of such composites, with particular emphasis on the photodegradation of pollutants and photocatalytic water splitting for hydrogen generation, is also discussed. The progresses made are thoroughly examined for achieving remarkable photocatalytic performances, with additional insights with regard to charge transfer. Finally, a summary and some perspectives on the challenges and new research directions for future exploitation in this emerging frontier are provided, which hopefully would allow for harnessing the outstanding structural and electronic properties of {001} facets for various energy- and environmental-related applications.

Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization

Titanium dioxide (TiO2) is one of the most widely investigated metal oxides due to its extraordinary surface, electronic and catalytic properties. However, the large band gap of TiO2 and massive recombination of photogenerated electron-hole pairs limit its photocatalytic and photovoltaic efficiency. Therefore, increasing research attention is now being directed towards engineering the surface structure of TiO2 at the most fundamental and atomic level namely morphological control of {001} facets in the range of microscale and nanoscale to fine-tune its physicochemical properties, which could ultimately lead to the optimization of its selectivity and reactivity. The synthesis of {001}-faceted TiO2 is currently one of the most active interdisciplinary research areas and demonstrations of catalytic enhancement are abundant. Modifications such as metal and non-metal doping have also been extensively studied to extend its band gap to the visible light region. This steady progress has demonstrated that TiO2-based composites with {001} facets are playing and will continue to play an indispensable role in the environmental remediation and in the search for clean and renewable energy technologies. This review encompasses the state-of-the-art research activities and latest advancements in the design of highly reactive {001} facet-dominated TiO2via various strategies, including hydrothermal/solvothermal, high temperature gas phase reactions and non-hydrolytic alcoholysis methods. The stabilization of {001} facets using fluorine-containing species and fluorine-free capping agents is also critically discussed in this review. To overcome the large band gap of TiO2 and rapid recombination of photogenerated charge carriers, modifications are carried out to manipulate its electronic band structure, including transition metal doping, noble metal doping, non-metal doping and incorporating graphene as a two-dimensional (2D) catalyst support. The advancements made in these aspects are thoroughly examined, with additional insights related to the charge transfer events for each strategy of the modified-TiO2 composites. Finally, we offer a summary and some invigorating perspectives on the major challenges and new research directions for future exploitation in this emerging frontier, which we hope will advance us to rationally harness the outstanding structural and electronic properties of {001} facets for various environmental and energy-related applications.

Microwave-assisted self-doping of TiO2 photonic crystals for efficient photoelectrochemical water splitting

In this article, we report that the combination of microwave heating and ethylene glycol, a mild reducing agent, can induce Ti(3+) self-doping in TiO2. A hierarchical TiO2 nanotube array with the top layer serving as TiO2 photonic crystals (TiO2 NTPCs) was selected as the base photoelectrode. The self-doped TiO2 NTPCs demonstrated a 10-fold increase in visible-light photocurrent density compared to the nondoped one, and the optimized saturation photocurrent density under simulated AM 1.5G illumination was identified to be 2.5 mA cm(-2) at 1.23 V versus reversible hydrogen electrode, which is comparable to the highest values ever reported for TiO2-based photoelectrodes. The significant enhancement of photoelectrochemical performance can be ascribed to the rational coupling of morphological and electronic features of the self-doped TiO2 NTPCs: (1) the periodically morphological structure of the photonic crystal layer traps broadband visible light, (2) the electronic interband state induced from self-doping of Ti(3+) can be excited in the visible-light region, and (3) the captured light by the photonic crystal layer is absorbed by the self-doped interbands.

A p-type Ti(iv)-based metal–organic framework with visible-light photo-response

Photoelectrochemical studies on a new Ti(iv)-based porous metal–organic framework (NTU-9, bandgap 1.72 eV) indicated that NTU-9 is a p-type semiconductor with visible-light-driven photoactivity.

Investigation of the facet-dependent performance of α-Fe2O3 nanocrystals for heavy metal determination by stripping voltammetry

The electrochemical performances of α-Fe₂O₃ nanocubes, nanoplates, and nanorods modified GCEs toward Pb²⁺ detection strongly depend on their exposed facets.

Self-modification of titanium dioxide materials by Ti3+ and/or oxygen vacancies: new insights into defect chemistry of metal oxides

In this review, we highlight the recent research efforts towards understanding the defect chemistry of titanium dioxide. Particular attention is paid to the synthesis of self-modified TiO₂ materials with Ti³⁺/oxygen vacancies and the favorable effects of these defects on the properties and applications of the obtained materials.

Preparation of NiS/ZnIn2S4 as a superior photocatalyst for hydrogen evolution under visible light irradiation

In this study, NiS/ZnIn2S4 nanocomposites were successfully prepared via a facile two-step hydrothermal process. The as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Their photocatalytic performance for hydrogen evolution under visible light irradiation was also investigated. It was found that the photocatalytic hydrogen evolution activity over hexagonal ZnIn2S4 can be significantly increased by loading NiS as a co-catalyst. The formation of a good junction between ZnIn2S4 and NiS via the two step hydrothermal processes is beneficial for the directional migration of the photo-excited electrons from ZnIn2S4 to NiS. The highest photocatalytic hydrogen evolution rate (104.7 μmol/h), which is even higher than that over Pt/ZnIn2S4 nanocomposite (77.8 μmol/h), was observed over an optimum NiS loading amount of 0.5 wt %. This work demonstrates a high potential of the developing of environmental friendly, cheap noble-metal-free co-catalyst for semiconductor-based photocatalytic hydrogen evolution.

Exciton-free, nonsensitized degradation of 2-naphthol by facet-dependent BiOCl under visible light: novel evidence of surface-state photocatalysis

Photoreactivity for photodegradation of 2-NAP on BiOCl nanosheets with dominant exposed (010) and (001) facets is studied under visible light via an exciton-free and nonsensitized mechanism. This phenomenon cannot be explained by semiconductor theory or self-sensitized (as those involve dyes) mechanisms. The photocatalytic activities are mainly owing to the formation of the surface state, which is confirmed to be the surface complex Bi-O-C10H7. This surface complex is characterized with ultraviolet-visible diffuse reflectance spectra, Fourier transformed infrared spectroscopy, Raman scattering, X-ray photoelectron spectroscopy, and photoelectrochemical measurement. The optical absorptivity of BiOCl is shifted from the ultraviolet light toward the visible light via a charge-transfer-complex pathway. Charge transfer after the excitation of visible light induces efficient visible photocatalytic activities. The results show that single-crystalline BiOCl nanosheets exposing (010) facets exhibit higher photoactivity due to more surface complex and more terminal bismuth atoms on the surface of BiOCl (010). Our current work is expected to offer new insight into photocatalytic theory for better understandings to photocatalytic reactions and for rational design and synthesis of photocatalyst with high activity.

Facile fabrication of highly efficient g-C3N4/Ag2O heterostructured photocatalysts with enhanced visible-light photocatalytic activity

Highly efficient visible-light-driven g-C3N4/Ag2O heterostructured photocatalysts were prepared by a simple liquid phase synthesis method at room temperature. The composition, structure, morphology, and optical absorption properties of the as-prepared g-C3N4/Ag2O composites were characterized by XRD, FTIR, XPS, TEM, and UV-vis DRS, respectively. We found interestingly that the photogenerated charge carriers separations of the as-prepared g-C3N4/Ag2O composites were closely related to the mass ratio of g-C3N4 and Ag2O. When the mass ratio of g-C3N4 and Ag2O reached 1:4, the as-prepared composite exhibited the highest photocatalytic activity, which was almost 11 and 1.2 times as high as that of individual g-C3N4 and Ag2O, respectively. The enhancement of photocatalytic activity could be attributed to the synergetic effects between g-C3N4 and Ag2O as well as the improved dispersibility and the decreased particle size of Ag2O. Moreover, the as-prepared composites showed excellent stability toward the photodegradation of methyl orange (MO). Finally, a possible photocatalytic and charge separation mechanism was proposed.

Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light

Graphitic carbon nitride nanosheets are extracted, produced via simple liquid-phase exfoliation of a layered bulk material, g-C3N4. The resulting nanosheets, having ≈2 nm thickness and N/C atomic ratio of 1.31, show an optical bandgap of 2.65 eV. The carbon nitride nanosheets are demonstrated to exhibit excellent photocatalytic activity for hydrogen evolution under visible light.

Photocatalytic activity of hydrogenated TiO2

Photocatalysis is a promising advanced water treatment technology, and recently the possibility of using hydrogenation to improve the photocatalytic efficiency of titanium dioxide has generated much research interest. Herein we report that the use of high-temperature hydrogenation to prepare black TiO2 primarily results in the formation of bulk defects in the material without affecting its electronic band structure. The hydrogenated TiO2 exhibited significantly worse photocatalytic activity under simulated sunlight compared to the unhydrogenated control, and thus we propose that high-temperature hydrogenation can be counterproductive to improving the photocatalytic activity of TiO2, because of its propensity to form bulk vacancy defects.

Facile synthesis of thermal- and photostable titania with paramagnetic oxygen vacancies for visible-light photocatalysis

A novel dopant-free TiO(2) photocatalyst (V(o)(.)-TiO(2)), which is self-modified by a large number of paramagnetic (single-electron-trapped) oxygen vacancies, was prepared by calcining a mixture of a porous amorphous TiO(2) precursor, imidazole, and hydrochloric acid at elevated temperature (450 °C) in air. Control experiments demonstrate that the porous TiO(2) precursor, imidazole, and hydrochloric acid are all necessary for the formation of V(o)(.)-TiO(2). Although the synthesis of V(o)(.)-TiO(2) originates from such a multicomponent system, this synthetic approach is facile, controllable, and reproducible. X-ray diffraction, XPS, and EPR spectroscopy reveal that the V(o)(.)-TiO(2) material with a high crystallinity embodies a mass of paramagnetic oxygen vacancies, and is free of other dopant species such as nitrogen and carbon. UV/Vis diffuse-reflectance spectroscopy and photoelectrochemical measurement demonstrate that V(o)(.)-TiO(2) is a stable visible-light-responsive material with photogenerated charge separation efficiency higher than N-TiO(2) and P25 under visible-light irradiation. The V(o)(.)-TiO(2) material exhibits not only satisfactory thermal- and photostability, but also superior photocatalytic activity for H(2) evolution (115 μmol h(-1) g(-1)) from water with methanol as sacrificial reagent under visible light (λ>400 nm) irradiation. Furthermore, the effects of reaction temperature, ratio of starting materials (imidazole:TiO(2) precursor) and calcination time on the photocatalytic activity and the microstructure of V(o)(.)-TiO(2) were elucidated.