Photocatalytic generation of hydrogen over mesoporous CdS nanoparticle: Effect of particle size, noble metal and support

Kinetic research and optimized particle size for high-performance of CdS NP photocatalyst

Metal semiconductors are important materials for photocatalytic technology, and their morphology and size have the great influence on photocatalytic performance. Therefore, the detailed exploration of the size effect is significant for the photocatalytic reaction. Herein, CdS nanoparticles (NPs) with different particle sizes were prepared for photodegradation of methylene blue, and the effects of particle size on photodegradation were studied using CdS NPs as a catalyst. The rate-determining step of photodegradation is determined theoretically by deriving the kinetic order of the photodegradation reaction, and the relationships of size and performance are explored. The results show that the particle size of the CdS NP catalyst has an obvious influence on the photodegradation rate and the rate constant, and the size effects show a "volcano" trend, implying that the catalyst activity is best when the particle size of the catalyst is moderate. The size influences on the adsorption/desorption, light utilization, and carrier efficiency of the catalyst originate from the influence of size on the surface properties and energy band. This study provides a new understanding on the effect of catalyst size on its properties, and this finding of optimum sizes of catalyst possessing better activity is valuable, which has a significant guidance for developing high-performance catalytic materials.

Conjugated Polymer Polypyrrole Nanostructures: Synthesis and Photocatalytic Applications

Conjugated polymers (CPs) have been recently widely investigated for their properties and their applications in different fields including photocatalysis. Among the family of CPs, polypyrrole (PPy) has been the most extensively studied owing to its good environmental stability, high electrical conductivity, superior redox properties and easy synthesis. Besides, nanostructured polypyrrole-based nanomaterials are a type of active organic materials for photocatalysis, which is one of their emerging applications. Nanostructuration of polypyrrole can reduce the electron-hole recombination because of short charge transfer distances and reactant adsorption, and product desorption can be enhanced owing to the high surface area offered by nanostructures. This review summarizes synthesis of different nanostructures based on π-conjugated polymer polypyrrole and the latest developments for photocatalytic applications, including degradation of organic pollutants and hydrogen generation.

Biotemplated CdS Nano-Aggregate Networks for Highly Effective Visible-Light Photocatalytic Hydrogen Production

In the last few decades, many new synthesis techniques have been developed in order to obtain an effective visible-light responsive photocatalyst for hydrogen production by water splitting. Among these new approaches, the biotemplated synthesis method has aroused much attention because of its unique advantages in preparing materials with special morphology and structure. In this work, Hydrilla verticillata (L. f.) Royle was used as a biotemplate to synthesize a CdS photocatalyst. The as-synthesized sample had the microstructure of nano-scaled aggregate networks and its activity for photocatalytic hydrogen production was six times higher than that of CdS synthesized without a template in an Na₂S-Na₂SO₃ sacrificial system. The use of Pt and PdS as cocatalysts further improved the hydrogen production rate to 14.86 mmol/g·h under visible-light (λ ≥ 420 nm) irradiation, so the hydrogen production can be directly observed by the naked eye. The results of characterization showed that the as-synthesized CdS photocatalyst has a high specific surface area and narrow band gap, which is favorable for light absorption and photocatalytic reaction. This work provides a new way to search for efficient visible-light catalysts and confirms the uniqueness of a biotemplated synthesis method in obtaining specially structured materials.

Design of noble metal-free CoTiO3/Zn0.5Cd0.5S heterostructure photocatalyst for selective synthesis offurfuraldehyde combined withH2production

The development of photocatalytic systems composed of earth-abundant metal-based catalysts for efficient production of clean fuel, H₂ as well as value-added chemicals is of significant importance towards sustainable generation of energy resources. Consequently, herein we report rational construction of Z-scheme CoTiO₃/xZn_0.5Cd_0.5S (x = 5 (S1), 10 (S2), 15 (S3) and 20 wt% (S4)) heterostructures featuring suitable band structure for efficient photocatalytic reduction of protons of water to H₂ combined with selective oxidation of furfuryl alcohol (biomass derivative) to a value-added product, furfuraldehyde. Electron microscopy analysis of heterostructure S2 revealed that Zn_0.5Cd_0.5S nanoparticles are decorated over the surface of CoTiO₃ microrods. The photocatalytic studies showed higher catalytic performance by S2, for selective oxidation of furfuryl alcohol to furfuraldehyde with 95% yield coupled with a H₂ generation rate of 1929 μmol g^-1h^-1 which is about 4-fold higher than that of pristine Zn_0.5Cd_0.5S. The enhanced catalytic performance of heterostructure S2 has been ascribed to synergistic interaction aided by the Z-scheme heterojunction formation between CoTiO₃ and Zn_0.5Cd_0.5S. Overall, this work demonstrates the application of noble metal-free photocatalyst for simultaneous production of H₂ and value-added chemical under mild and environment-friendly conditions.

NiO-TiO2 p-n Heterojunction for Solar Hydrogen Generation

Photocatalytic water splitting for hydrogen production has been widely recognized as a promising strategy for relieving the pressure from energy crisis and environmental pollution. However, current efficiency for photocatalytic hydrogen generation has been limited due to a low separation of photogenerated electrons and holes. p-n heterojunction with a built-in electric field emerges as an efficient strategy for photocatalyst design to boost hydrogen evolution activities due to a spontaneous charge separation. In this work, we investigated the effect of different preparation methods on photocatalytic hydrogen production over NiO-TiO2 composites. The results demonstrated that a uniform distribution of NiO on a surface of TiO2 with an intimate interfacial interaction was formed by a sol-gel method, while direct calcination tended to form aggregation of NiO, thus leading to an uneven p-n heterojunction structure within a photocatalyst. NiO-TiO2 composites fabricated by different methods showed enhanced hydrogen production (23.5 ± 1.2, 20.4 ± 1.0 and 8.8 ± 0.7 mmolh−1g−1 for S1-20%, S2-20% and S3-10%, respectively) as compared with pristine TiO2 (6.6 ± 0.7 mmolh−1g−1) and NiO (2.1 ± 0.2 mmolh−1g−1). The current work demonstrates a good example to improve photocatalytic hydrogen production by finely designing p-n heterojunction photocatalysts.

Biomedical Waste Management by Using Nanophotocatalysts: The Need for New Options

Biomedical waste management is getting significant consideration among treatment technologies, since insufficient management can cause danger to medicinal service specialists, patients, and their environmental conditions. The improvement of waste administration protocols, plans, and policies are surveyed, despite setting up training programs on legitimate waste administration for all healthcare service staff. Most biomedical waste substances do not degrade in the environment, and may also not be thoroughly removed through treatment processes. Therefore, the long-lasting persistence of biomedical waste can effectively have adverse impact on wildlife and human beings, as well. Hence, photocatalysis is gaining increasing attention for eradication of pollutants and for improving the safety and clearness of the environment due to its great potential as a green and eco-friendly process. In this regard, nanostructured photocatalysts, in contrast to their regular counterparts, exhibit significant attributes such as non-toxicity, low cost and higher absorption efficiency in a wider range of the solar spectrum, making them the best candidate to employ for photodegradation. Due to these unique properties of nanophotocatalysts for biomedical waste management, we aim to critically evaluate various aspects of these materials in the present review and highlight their importance in healthcare service settings.

Wide Band Gap Chalcogenide Semiconductors

Wide band gap semiconductors are essential for today's electronic devices and energy applications because of their high optical transparency, controllable carrier concentration, and tunable electrical conductivity. The most intensively investigated wide band gap semiconductors are transparent conductive oxides (TCOs), such as tin-doped indium oxide (ITO) and amorphous In-Ga-Zn-O (IGZO), used in displays and solar cells, carbides (e.g., SiC) and nitrides (e.g., GaN) used in power electronics, and emerging halides (e.g., γ-CuI) and 2D electronic materials (e.g., graphene) used in various optoelectronic devices. Compared to these prominent materials families, chalcogen-based (Ch = S, Se, Te) wide band gap semiconductors are less heavily investigated but stand out because of their propensity for p-type doping, high mobilities, high valence band positions (i.e., low ionization potentials), and broad applications in electronic devices such as CdTe solar cells. This manuscript provides a review of wide band gap chalcogenide semiconductors. First, we outline general materials design parameters of high performing transparent semiconductors, as well as the theoretical and experimental underpinnings of the corresponding research methods. We proceed to summarize progress in wide band gap (E_G > 2 eV) chalcogenide materials-namely, II-VI MCh binaries, CuMCh₂ chalcopyrites, Cu₃MCh₄ sulvanites, mixed-anion layered CuMCh(O,F), and 2D materials-and discuss computational predictions of potential new candidates in this family, highlighting their optical and electrical properties. We finally review applications-for example, photovoltaic and photoelectrochemical solar cells, transistors, and light emitting diodes-that employ wide band gap chalcogenides as either an active or passive layer. By examining, categorizing, and discussing prospective directions in wide band gap chalcogenides, this Review aims to inspire continued research on this emerging class of transparent semiconductors and thereby enable future innovations for optoelectronic devices.

Recent Progress in Energy-Driven Water Splitting

Hydrogen is readily obtained from renewable and non-renewable resources via water splitting by using thermal, electrical, photonic and biochemical energy. The major hydrogen production is generated from thermal energy through steam reforming/gasification of fossil fuel. As the commonly used non-renewable resources will be depleted in the long run, there is great demand to utilize renewable energy resources for hydrogen production. Most of the renewable resources may be used to produce electricity for driving water splitting while challenges remain to improve cost-effectiveness. As the most abundant energy resource, the direct conversion of solar energy to hydrogen is considered the most sustainable energy production method without causing pollutions to the environment. In overall, this review briefly summarizes thermolytic, electrolytic, photolytic and biolytic water splitting. It highlights photonic and electrical driven water splitting together with photovoltaic-integrated solar-driven water electrolysis.

CdS Nanowires Decorated with Ultrathin MoS2 Nanosheets as an Efficient Photocatalyst for Hydrogen Evolution

CdS nanowires decorated with ultrathin MoS2 nanosheets were synthesized for the first time by ultrasonic exfoliation by using dimethylformamide as the dispersing agent. An excellent hydrogen evolution rate of 1914 μmol  h(-1) (20 mg catalyst) under visible-light irradiation (λ ≥ 400 nm, ≈ 154 mW cm(-1) ) and an apparent quantum yield of 46.9% at λ=420 nm were achieved over the MoS2 /CdS composite. The presence of ultrathin MoS2 nanosheets (rich in active edge sites) on the CdS surface promotes the separation of photogenerated charge carriers and facilitates the surface processes of photocatalytic hydrogen evolution.

Rational design of semiconductor-based photocatalysts for advanced photocatalytic hydrogen production: the case of cadmium chalcogenides

This review summarizes the recent advances in developing CdX (X = S, Se, Te)-based photocatalyst systems for photocatalytic hydrogen production from water.

Screening and characterization for the optimization of CdS-based photocatalysts

The photocatalyst with a precursor composition In_0.2–Cd_0.8S showed the highest photocurrent under light illumination.

Optimization of the experimental conditions of hydrogen production by the Pt–(CdS/ZnS) system under visible light illumination

The photocatalytic activity of a Pt–(CdS/ZnS) system towards hydrogen generation from water and in presence of a sacrificial organic is studied.

Exceptional activity of sub-nm Pt clusters on CdS for photocatalytic hydrogen production: a combined experimental and first-principles study

Here we report the first ever observation of outstanding photocatalytic hydrogen production activity of sub-nm Pt cluster modified CdS nanocatalysts.

Enhanced photocatalytic degradation of tetracycline antibiotics by reduced graphene oxide–CdS/ZnS heterostructure photocatalysts

Ternary RGO–CdS/ZnS heterostructures with enhanced visible-light photocatalytic activity have been prepared by a facile hydrothermal method.

Photocatalytic Hydrogen Production from Water Splitting on CdS and Cd0.8Zn0.2S Nanorods: Influence of Ni(OH)2 Modification

The composite photocatalysts of Ni (OH)2modified Cd1-xZnxS (x=0, 0.2) nanorods were synthesized via a simple deposition-precipitation method using nanorods as support and Ni (NO3)2as nickel hydroxide precursor. The structures, morphologies and optical properties of the catalysts were characterized by X-ray diffraction, scanning electron microscopy, high resolution transmission electron spectroscopy and UV-visible absorption spectroscopy. The photoactivities of the catalysts were examined toward hydrogen production by water splitting under simulated solar irradiation. Results show that the catalyst of Ni (OH)2-Cd0.8Zn0.2S nanorods exhibits a significantly enhanced H2-production activity in compared to the Ni (OH)2-CdS nanorods. The reason for the different effects of Ni (OH)2modification on the photoactivities of the two catalysts was discussed, and the possible mechanism related to the photocatalytic process was proposed.

CdS nanoparticles sensitization of Al-doped ZnO nanorod array thin film with hydrogen treatment as an ITO/FTO-free photoanode for solar water splitting

Aluminum-doped zinc oxide (AZO) nanorod array thin film with hydrogen treatment possesses the functions of transparent conducting oxide thin film and 1-D nanostructured semiconductor simultaneously. To enhance the absorption in the visible light region, it is sensitized by cadmium sulfide (CdS) nanoparticles which efficiently increase the absorption around 460 nm. The CdS nanoparticles-sensitized AZO nanorod array thin film with hydrogen treatment exhibits significantly improved photoelectrochemical property. After further heat treatment, a maximum short current density of 5.03 mA cm-2 is obtained under illumination. They not only are much higher than those without CdS nanoparticles sensitization and those without Al-doping and/or hydrogen treatment, but also comparable and even slightly superior to some earlier works for the CdS-sensitized zinc oxide nanorod array thin films with indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) as substrates. This demonstrated successfully that the AZO nanorod array thin film with hydrogen treatment is quite suitable as an ITO/FTO-free photoanode and has great potentials in solar water splitting after sensitization by quantum dots capable of visible light absorption.

Photocatalytic activities of Cd-doped ZnWO4 nanorods prepared by a hydrothermal process

The ZnWO(4) nanorods doped with cadmium ions have been successfully synthesized by a hydrothermal crystallization process. The products were characterized in detail by multiform techniques: X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results of the photocatalytic degradation of rhodamine B (RhB) in aqueous solution showed that cadmium ions doping greatly improved the photocatalytic efficiency of ZnWO(4) nanorods. The Cd-doped ZnWO(4) nanorods with atomic ratio of Cd to Zn being 0.06 had the best activity in photo-degradation of RhB in aqueous solution under UV light irradiation, when the nanorods have prepared at pH 8.

Synthesis and the effect of calcination temperature on the physical-chemical properties and photocatalytic activities of Ni,La codoped SrTiO3

A series of highly vis-light active Ni,La-codoped SrTiO(3) photocatalysts were successfully synthesized with sol-gel process. The characterization results show that the calcination temperature has a strong influence on the physical-chemical properties of as-synthesized photocatalysts. The surface area and porosity, even the initial adsorption rate for malachite green (MG), decreased with increasing calcination temperature. To evaluate the photocatalytic activities, the photodegradation of a water contaminant (MG) was carried out under visible light irradiation. The as-synthesized photocatalysts exhibited a high vis-light activity, and a 100% degradation of MG was observed for the Ni,La-SrTiO(3)-x catalysts calcined at low temperature under visible light irradiation for 1h, during which only 7% and 15% of MG was degraded for self-degrade and commercially available photocatalyst Degussa P25, respectively. The high vis-light activity is a result of the best combination of many properties, such as the intensive visible light response, the large surface area and pore volume and the high initial adsorption rate for substrate.

Nanostructured Photocatalysts and Their Applications in the Photocatalytic Transformation of Lignocellulosic Biomass: An Overview

Heterogeneous photocatalysis offer many possibilities for finding appropiate environmentally friendly solutions for many of the the problems affecting our society (i.e., energy issues). Researchers are still looking for novel routes to prepare solid photocatalysts able to transform solar into chemical energy more efficiently. In many developing countries, biomass is a major energy source, but currently such countries lack of the technology to sustainably obtain chemicals and/or fuels from it. The Roadmap for Biomass Technologies, authored by 26 leading experts from academia, industry, and government agencies, has predicted a gradual shift back to a carbohydrate-based economy. Biomass and biofuels appear to hold the key to satisfy the basic needs of our societies for the sustainable production of liquid fuels and high value-added chemicals without compromising the scenario of future generations. In this review, we aim to discuss various design routes for nanostructured photocatalytic solid materials in view of their applications in the selective transformation of lignocellulosic biomass to high value-added chemicals.