Semiconductor Photocatalysis Research Articles

Enhanced photocatalytic activity of two dimensional SrTiO3 nano structures for dye degradation

The combination of the properties of nanostructures, photocatalytic semiconductors, and plasmonic materials can significantly contribute to improving the performance of the entire photocatalysis process. We have produced a two-dimensional nanostructure which was then coated with gold as a plasmonic material followed by SrTiO3 as semiconductor material to produce a unique catalyst. The result was the production of the first catalyst of its kind made in this way, which offered notable results. The degradation ability of the sample was examined with different degrees of pH of rhodamine B dye, where we achieved the highest degradation, 16%, at pH10.

Carbon nitride for photocatalytic water splitting to produce hydrogen and hydrogen peroxide

Semiconductor photocatalysis provides a sustainable way to solve the problems of energy crisis and environmental pollution. As a typical visible-light-response organic semiconductor photocatalyst, however, bulk carbon nitride suffers from poor photoreactivity due to the quick recombination of electron–hole pairs, low specific surface area, and weak light absorption property, which severely hampers its practical applications. This review summarizes the recent progresses of carbon nitride based photocatalysis for water splitting to produce hydrogen gas (H2) and hydrogen peroxide (H2O2). The strategies to improve the photoreactivity of carbon nitride from the aspects of the charge separation and light absorption are highlighted, which include cocatalysts modification, morphological control, crystallinity enhancement, element doping, semiconductor coupling, and surface functionalization. At last, the remaining challenges are analyzed, and the future developments of carbon nitride for photocatalytic water splitting were prospected.

Review on the Synthesis Method of Nanocomposites and Approach to Making Semiconductors Visible Light Active

Many studies have been concentrated on the degradation of toxic organic compounds in waste water via photocatalysis of various semiconductors. It has attracted great attention in modern science because of its potential in solving many current environmental problems such as air and water pollution. The common photocatalysts are primarily nanocomposite metal oxides, are known to be good photocatalysts for the degradation of several environmental contaminants due to their high photosensitivity, stability and non-toxic nature. There are different approaches for the synthesis of nanomaterials: Top down and bottom up approaches. Top- down approach is best approach and refers to successive cutting of a bulk material to get nano sized particles. These applications have their interest in controlling particle size, particle shape, size distribution, particle composition and degree of particle agglomeration. Some nanoparticles have large band-gap which leads to high recombination rate of photogenerated electron–hole pairs limit their utilization for photocatalytic applications. More recently, significant efforts have also been made to develop new or modified semiconductor photocatalysts that are capable of using visible-light (λ = 400–700 nm) including semiconductor coupling, metal ion doping, nonmetallic element doping, and sensitization with organic dyes. Coupling of two semiconductor nanoparticles with different band gap widths has been demonstrated in many studies as one of the most effective ways to reduce the recombination of electron–hole pairs and consequently, achieving a higher photocatalytic activity. Moreover, the ternary nanocomposites have high visible light photocatalytic activity and organic dyes can be decomposed efficiently, implying the higher photocatalytic activity of the ternary nanocomposites.

Designing Nanoengineered Photocatalysts for Hydrogen Generation by Water Splitting and Conversion of Carbon Dioxide to Clean Fuels.

Semiconductor photocatalysis has received tremendous attention in the past decade as it has shown great promise in the context of clean energy harvesting for environmental remediation. Sunlight is an inexhaustible source of energy available to us throughout the year, although it is rather dilutely dispersed. Semiconductor based photocatalysis presents one of the best ways to harness this source of energy to carry out chemical reactions of interest that require external energy input. Photocatalytic hydrogen generation by splitting of water, CO2 mitigation, and CO2 conversion to green fuel have therefore become the highly desirable clean and sustainable processes for a better tomorrow. Although numerous efforts have been made and continue to be expended to search and develop new classes of photocatalyst materials in recent years, several significant challenges still remain to be resolved before photocatalysis can reach its commercial potential. Therefore, major attention is required towards improving the efficiencies of the existing photocatalysts by further manipulating them and parallelly employing newer strategies for novel photocatalyst designs. This personal account aims to provide a broad overview of the field primarily invoking examples of our own research contributions in the field, which include photocatalytic hydrogen generation and CO2 reduction to value added chemicals. This account reviews the state-of-the-art research activities and scientific possibilities which a functional material can offer if its properties are put to best use through goal-oriented design by combining with other compatible materials. We have addressed fundamental principles of photocatalysis, different kind of functional photocatalysts, critical issues associated with them and various strategies to overcome the related hurdles. It is our hope that this current personal account will provide a platform for young researchers to address the bottleneck issues in the field of photocatalysis and photocatalysts with a sense of clarity, and to find innovative solutions to resolve them by a prudent choice of materials, synthesis protocols, and approaches to boost the photocatalysis output. We emphasize that a targeted or goal-directed photocatalyst nanoengineering as perhaps the only way to realize an early success in this multiparametric domain.

W5+–W5+ Pair Induced LSPR of W18O49 to Sensitize ZnIn2S4 for Full‐Spectrum Solar‐Light‐Driven Photocatalytic Hydrogen Evolution

AbstractThe localized surface plasmon resonances (LSPR) effect makes W18O49 an effective visible and near‐infrared (NIR) light antenna to realize full‐spectrum solar‐light driven photocatalysis, yet the precise origin remains elusive. Here, the LSPR originates from the localized electron confinement around lattice W5+–W5+ pairs in the unique structure of W18O49 by density‐functional theory calculation, which gives W18O49 a broad absorption ranging from visible to NIR region, independent of the particle shape and size is confirmed. This unique periodic LSPR simplifies the design of W18O49‐sensitized photocatalytic composite into enhancing the light absorbance of W18O49 and screening photocatalytic semiconductors with suitable energy band potentials. To this end, hierarchical‐structure W18O49 microflowers with high absorbance have been coated with ZnIn2S4 nanosheets to achieve cocatalyst‐free photocatalytic composite, which presents an outstanding H2 production rate of 902.57 µmol within 3 h under simulated solar‐light. Comprehensive characterizations, including ultrafast transient absorption spectroscopy, prove the injection of hot electrons from W18O49 to ZnIn2S4 and the increase of long‐lived active electrons. This work clarifies the LSPR origin of oxygen‐deficient semiconductors and paves the way for the search of broad‐spectrum active photocatalyst.

Chemical fabrication, structural characterization and photocatalytic water splitting application of Sr-doped SnO2 nanoparticles

Semiconductor photocatalysis has gained considerable attention in recent years due to their enabling nature to convert solar energy into fuels of renewable hydrocarbon. However, many of them suffer from some drawbacks like the inability to visible light irradiation and wide band gaps. Herein, we have synthesized monophasic strontium (Sr) doped SnO2 nanoparticles by a cost-effective and environmental friendly hydrothermal method. As-synthesized nanoparticles showed rutile crystalline structure with irregular and rough cubical shape and no other elemental impurities. Sr-doped SnO2 nanoparticles show a constant decrease in bandgap with increasing dopant concentration, which is estimated for excellent photocatalytic activity. The photocatalytic water splitting of as-prepared Sr-doped SnO2 nanoparticles for H2 generation shows a large influence of the increasing dopant concentration related to the narrowing bandgap on H2 generation rate. Hence, the tunable bandgap with adjusted dopant concentration indicates that band gap tuning through doping for produced nanostructures may open up a new opportunities for photocatalytic and other optoelectronic applications.

Different coupling level induced mode shift of plasmon resonance in Au/ZnO heterojunction

Plasmon resonance properties of Au nanoparticle embedded in ZnO thin film are simulated by COMSOL Multiphysics software in Wave Optics Module. The absorption spectra, surface electric field distribution and the dielectric constant of heterojunction structures with semi and fully embedded Au nanoparticles are investigated. For the fully embedded situation, the local field enhancement caused by resonance is several times more than that of the semi-embedding case. We deduce the parameter A theoretically described size and interface effects in the dielectric constant equation to explain the coupling effect. The results confirm that ε′(ω)/ε″(ω) can be determined by function A, which illustrates that surface plasmon resonance (SPR) mode is transformed into local surface plasmon resonance (LSPR) mode. The resonance hot-spots are gradually formed at the surface of Au nanoparticle, which can be confirmed by calculating the increasingly penetration depth and reach its maximum value at thickness of 150 nm, indicating that the formation of LSPR can be verified. The exploration on the resonance reaction mechanism determined by the function A will promote the development of theoretical system and further application in semiconductor photoluminescence, photocatalysis and surface-enhanced Raman scattering devices.

Black titanium dioxide nanomaterials for photocatalytic removal of pollutants: A review

• Black TiO 2 nanomaterials exhibited superior performance in environmental pollutant removal. • Black TiO 2 synthesis, its basic properties, environmental contaminant removing mechanism were analyzed. • Resent development of black TiO 2 on photocatalytic removal of different kinds of environmental pollutants were summarized. • The stability and recoverability of black TiO 2 were firstly discussed. Semiconductor photocatalysis is one of the most widely used environment-friendly technologies for removing various contaminants. As a well-developed photocatalyst, titanium dioxide (TiO 2 ) still has limits in its wide bandgap and rapid recombination rate of photogenerated charge carriers. Recently, black TiO 2 appears as a strong candidate in the improvement of sunlight harvesting, because of its excellent absorption capacity and utilization of solar radiation. Despite extensive applications in both environmental and energy fields, the use of black TiO 2 as a photocatalyst in pollutant removal is ambiguous. The primary objective of the review is to comprehensively evaluate the applications of black TiO 2 in photocatalytic removal of contaminants, including conventional organic contaminants, emerging contaminants, microbes, and heavy metals. The basic properties, photocatalytic mechanism, and synthesis of black TiO 2 have been summarized and analyzed. Moreover, the stability and recoverability of black TiO 2 have also been discussed. Finally, the perspectives of the application of black TiO 2 in pollutant removal have been further discussed.

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution.

Metal-organic frameworks (MOFs) have been intensively studied as a class of semiconductor-like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL-125-NH2 , is integrated with the metal oxides (MoO3 and V2 O5 ) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built-in electric field of MIL-125-NH2 , boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H2 production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor-like nature, which would greatly promote MOF photocatalysis.

Charge Separation by Creating Band Bending in Metal–Organic Frameworks for Improved Photocatalytic Hydrogen Evolution

AbstractMetal–organic frameworks (MOFs) have been intensively studied as a class of semiconductor‐like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL‐125‐NH2, is integrated with the metal oxides (MoO3and V2O5) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built‐in electric field of MIL‐125‐NH2, boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H2production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor‐like nature, which would greatly promote MOF photocatalysis.

Layered photocatalytic nanomaterials for environmental applications

The increasing pollution and human demand for a cleaner environment have made achieving the environmental sustainability a current research focus. As a “green” technology, semiconductor photocatalysis is of great significance to the environmental purification. Benefiting from the unique anisotropic crystal structure and electronic properties, layered photocatalytic nanomaterials show great potential for efficient photocatalytic environmental treatment. This review comprehensively summarizes the recent progress on layered photocatalytic nanomaterials for oxidation or reduction of pollutants in water and air along with the basic understanding of related mechanisms and developments in this field. First, the existing diversified layered photocatalysts are classified, and their different synthesis and modification strategies are discussed in detail to provide a comprehensive view of the material design that affects their photocatalytic performance. Subsequently, the extensive applications of the above-mentioned layered photocatalytic nanomaterials in environmental fields are systematically summarized, including photooxidation of water and air pollutants, and photoreduction of heavy metal pollutants, NO 3 − , BrO 3 − and CO 2 . Finally, based on the current research achievements in layered photocatalysts for environmental remediation, the future development direction and challenges are proposed. Layered photocatalytic nanomaterials show great potential for efficient photocatalytic environmental remediation. The recent progress of metal-containing and metal-free layered photocatalytic nanomaterials for the oxidation or reduction of pollutants in water and air are highlighted, providing guidelines for designing photocatalysts with layered structure.

Oxygen vacancy-rich 2D GO/BiOCl composite materials for enhanced photocatalytic performance and semiconductor energy band theory research

Bismuth-based materials are extensively studied for photocatalytic applications because of their unique crystal structure. Herein, we reported a binary graphene oxide (GO)/BiOCl composite material prepared by a hydrothermal method and analyzed its photodegradation mechanism through the semiconductor energy band theory. The degradation rate of the GO/BiOCl composite towards Rhodamine B could reach 93.6% within 8 min, and its performance exceeded that of most photocatalysts. The influencing factors for improving the photocatalytic activity are as follows: (1) abundant oxygen vacancies generated on the tight recombination interface; (2) a 2D-2D electron transfer channel between GO and BiOCl; and (3) GO acting as a load to provide more reaction sites for BiOCl nanosheets. This work provides a simple solution and theoretical explanation for the rapid degradation of pollutants, and has broad application prospects.

Adsorption-photocatalysis processes: The performance and mechanism of a bifunctional covalent organic framework for removing uranium ions from water

Photocatalytic reaction could become an effective way to solve the energy crisis and environmental restoration and governance. Herein, we reported a covalent organic framework (COF-TpBpy) was connected by polyhydroxy-containing unit (Tp) and bipyridine unit (Bpy), which was utilized as photocatalytic semiconductor for photocatalytic detoxification of U(VI). Firstly, TpBpy was comprehensively characterized by physical chemistry (PXRD, SEM, TEM, FT-IR, BET, EA) and photoelectrochemistry (Vis-DRS, M-S plots, photocurrent-time, EIS, PL). Hereafter, the adsorption capacity of TpBpy for U(VI) was investigated by batch adsorption experiment, reaching 455 mg/g within 60 min. The adsorption experimental data conformed to the second-order kinetics, suggesting the chemisorption or surface complexation mechanism. Following that, TpBpy exhibited the narrower optical band gap (Eg = 2.2 V, ECB = -0.95 and EVB = 1.25 V vs. NHE) due to its extended π-conjugation system and electron-rich units of the skeleton (N-14.56%, O-27.73%). After visible light irradiation for 420 min, the photoreduction removal rate of U(VI) by TpBpy was about 55.4%. ESR spectra corroborated that O2– radicals and photoelectrons were the main active species involved in the photoreduction process. XPS analysis revealed the formation of NU bond and UO bond, whether in the process of adsorption or photoreduction. In short, the structure of TpBpy possessed strong coordination ability with U(VI) ions, and promoted the transfer of electrons from electron-rich groups to U(VI), thereby reducing to U(IV). As a multifunctional material, TpBpy possess the advantage in eliminating nuclear waste streams.

Intrinsic Mechanisms of Morphological Engineering and Carbon Doping for Improved Photocatalysis of 2D/2D Carbon Nitride Van Der Waals Heterojunction

Van der Waals (VDW) heterojunctions in a 2D/2D contact provide the highest area for the separation and transfer of charge carriers. In this work, a top‐down strategy with a gas erosion process was employed to fabricate a 2D/2D carbon nitride VDW heterojunction in carbon nitride (g‐C3N4) with carbon‐rich carbon nitride. The created 2D semiconducting channel in the VDW structure exhibits enhanced electric field exposure and radiation absorption, which facilitates the separation of the charge carriers and their mobility. Consequently, compared with bulk g‐C3N4 and its nanosheets, the photocatalytic performance of the fabricated carbon nitride VDW heterojunction in the water splitting reaction to hydrogen is improved by 8.6 and 3.3 times, respectively, while maintaining satisfactory photo‐stability. Mechanistically, the finite element method (FEM) was employed to evaluate and clarify the contributions of the formation of VDW heterojunction to enhanced photocatalysis, in agreement quantitatively with experimental ones. This study provides a new and effective strategy for the modification and more insights to performance improvement on polymeric semiconductors in photocatalysis and energy conversion.

An ultra-high performance for reduction of 4-nitroaniline with optimized noble-metal-free Bi4NbO8Cl nanosheets under visible light irradiation

The reduction conversion of 4-nitroaniline (4-NA) to p-phenylenediamine (PPD) via semiconductor photocatalysis has been considered as one of the most promising technologies to satisfy the requirement of modern industrial development. In this work, Bi4NbO8Cl materials with different calcination temperatures were successfully obtained by a facile synthesis method. Attractively, Bi4NbO8Cl nanosheet was put forward to catalyze 4-nitroaniline into p-phenylenediamine (PPD), which presented an ultra-high performance for the reduction of 4-nitroaniline under visible light irradiation. The optimal Bi4NbO8Cl nanosheets without an additive of noble metal cocatalyst can achieve conversion of 4-nitroaniline into p-phenylenediamine in 5 min. Furthermore, the crystal structure, optical properties, microscopic morphology, element chemical states, and photoelectrochemical properties of these photocatalysts were systematically investigated by various physical and chemical characterizations. Thereinto, Bi4NbO8Cl material displays a uniform morphology of nanosheets, and the size of nanosheets becomes larger and larger with the increase of calcination temperature. Simultaneously, prominent durability and reproducibility were accomplished via the above-mentioned photocatalyst. In addition, the CO2− radicals acting as primary active species can be confirmed by some controlled experiments and active species trapping experiments. This work provides a high-efficiency photocatalyst for the hydrogenation of 4-nitroaniline into p-phenylenediamine (PPD), and a possible reaction mechanism was uncovered and proposed.

Boosting the Photocatalytic H2 Evolution and Benzylamine Oxidation using 2D/1D g-C3N4/TiO2 Nanoheterojunction.

The present research aims at the elevation of solar-to-chemical energy conversion with extortionate performance and sustainability. The nanostructured materials are revolutionizing the water splitting technology into decoupled hydrogen with simultaneous value-added organic chemical production. Yet, the bottleneck in semiconductor photocatalysis is rapid charge recombination and sluggish reaction kinetics. Herein, we demonstrate an efficient and non-noble metal-based catalyst for successful redox reaction with a theoretical modeling through density functional theory (DFT) study. Implementing this robust approach on 2D/1D ultrathin g-C3N4 nanosheets and TiO2 nanowires heterojunction, we achieved H2 production of 5.1 mmol g-1 h-1 with apparent quantum efficiency of 7.8% under visible light illumination and 93% of benzylamine conversion to N-benzylidene benzylamine in situ. The interface of 2D g-C3N4 nanosheets and 1D nanowires provide ample active sites and extends the visible light absorption with requisite band edge position for the separation of photoinduced charge carriers with superior stability. The electronic properties, band structure, and stability of the heterojunction are further investigated via DFT calculations which corroborate the experimental results and in good agreement for the enhanced activity of the heterojunction.

S-Scheme BiOCl/MoSe2 Heterostructure with Enhanced Photocatalytic Activity for Dyes and Antibiotics Degradation under Sunlight Irradiation.

Semiconductor photocatalysis is considered to be a promising technique to completely eliminate the organic pollutants in wastewater. Recently, S-scheme heterojunction photocatalysts have received much attention due to their high solar efficiency, superior transfer efficiency of charge carriers, and strong redox ability. Herein, we fabricated an S-scheme heterostructure BiOCl/MoSe2 by loading MoSe2 nanosheets on the surface of BiOCl microcrystals, using a solvothermal method. The microstructures, light absorption, and photoelectrochemical performances of the samples were characterized by the means of SEM, TEM, XRD, transient photocurrents, electrochemical impedance, and photoluminescence (PL) spectra. The photocatalytic activities of BiOCl, MoSe2, and the BiOCl/MoSe2 samples with different MoSe2 contents were evaluated by the degradation of methyl orange (MO) and antibiotic sulfadiazine (SD) under simulated sunlight irradiation. It was found that BiOCl/MoSe2 displayed an evidently enhanced photocatalytic activity compared to single BiOCl and MoSe2, and 30 wt.% was an optimal loading amount for obtaining the highest photocatalytic activity. On the basis of radical trapping experiments and energy level analyses, it was deduced that BiOCl/MoSe2 follows an S-scheme charge transfer pathway and •O2−, •OH, and h+ all take part in the degradation of organic pollutants.

Electrospun Semiconductor‐Based Nano‐Heterostructures for Photocatalytic Energy Conversion and Environmental Remediation: Opportunities and Challenges

Harvesting solar energy to drive the semiconductor photocatalysis offers a promising tactic to address ever‐growing challenges of both energy shortage and environmental pollution. Design and synthesis of nano‐heterostructure photocatalysts with controllable components and morphologies are the key factors for achieving highly efficient photocatalytic processes. One‐dimensional (1D) semiconductor nanofibers produced by electrospinning possess a large ratio of length to diameter, high ratio of surface to volume, small grain sizes, and high porosity, which are ideally suited for photocatalytic reactions from the viewpoint of structure advantage. After the secondary treatment of these nanofibers through the solvothermal, gas reduction, in situ doping, or assembly methods, the multi‐component nanofibers with hierarchical nano‐heterostructures can be obtained to further enhance their light absorption and charge carrier separation during the photocatalytic processes. In recent years, the electrospun semiconductor‐based nano‐heterostructures have become a “hot topic” in the fields of photocatalytic energy conversion and environmental remediation. This review article summarizes the recent progress in electrospinning synthesis of various kinds of high‐performance semiconductor‐based nano‐heterostructure photocatalysts for H2 production, CO2 reduction, and decomposition of pollutants. The future perspectives of these materials are also discussed.

Surfactant-assisted tuning of K2V3O8 nanorods for robust charge dynamics in semiconductor photocatalysis

This work presents a deep investigation into the structural and optoelectronic properties of K2V3O8 nanorods for testing their photocatalytic potentials. K2V3O8 nanorods have been synthesized by a simple surfactant-assisted hydrothermal method, with the surfactant SDS being incorporated to tune the morphology of the nanorods. Morphological analyses have revealed a profound effect of SDS in tailoring the morphology of the nanorods, due to a systematic layer by layer assembly of K2V3O8 nuclei along the surfactant chains. Various structural and optoelectronic analyses have been performed to establish the enhanced charge migration and charge separation efficiency along the smooth nanorod surfaces. The spectroscopic analyses have been theoretically studied by generating electronic structures via DFT calculations. The tuned nanorods demonstrated excellent photocatalytic activity, achieving 93% degradation of complex pharmaceutical levofloxacin and a robust photocurrent density of 0.7 mA/cm2 at 1.23 V vs RHE. Thus, this work establishes K2V3O8 nanorods as a novel material for photocatalysis and also explores their structure–property relationships for achieving optimal photocatalytic activity.

Sulfur Vacancy-Mediated Electron–Hole Separation at MoS2/CdS Heterojunctions for Boosting Photocatalytic N2 Reduction

Defect engineering is considered as an efficient method for improving the photocatalytic activity of semiconductor photocatalysis because defects can not only serve as trapping centers for electrons and holes but also work as active sites for reaction. Herein, we synthesized a series of MoS2/CdS heterojunctions with abundant sulfur vacancies and used for photocatalytic N2 reduction. The sulfur vacancies at MoS2/CdS heterojunctions were confirmed by UV–vis diffuse-reflectance spectroscopy (UV-DRS) and electron paramagnetic resonance. The charge transfer behaviors of the photocatalysts were characterized by X-ray photoelectron spectroscopy, transient photocurrent, steady and transient photoluminescence spectroscopy, and so forth. Under visible light irradiation for 4 h, the production of NH3 at 3% MoS2/CdS heterojunctions (249.7 mg L–1 g–1) was about 5.4 and 3.9 times higher than that of pure MoS2 (45.9 mg L–1 g–1) and pristine CdS (64.5 mg L–1 g–1), respectively. The reaction mechanisms and pathways were further studied by in situ diffuse reflection infrared Fourier transform spectroscopy.