Photocatalytic degradation of organic dyes under UV–Visible light using capped ZnS nanoparticles

Purification techniques like ozonization, chlorination and filtration have their own limitations of corresponding energy sources and harmful waste generation. However, heterogeneous photo catalysis is used for producing oxidative agent (hydroxyl radical) which has been used as an environmentally harmonious decontamination process. Such safe and low energy consumable photo catalytic system is required for purification of polluted water. Degradation of dyes is a standard method to check the photocatalytic activity of any type of photo catalyst. In this paper thioglycerol capped and uncapped ZnS nanoparticles are studied in detail for their photocatalytic activity and generation of electron hole pairs. Bromophenol blue, crystal violet and reactive red dyes were successfully photo reduced using ZnS nanoparticles after 3.0 h of irradiation. Since the photocatalytic activity depends on the generation of electron hole pairs and the existence of different phases, we have tried to correlate the optical and morphological studies with these results to understand the phenomenon of photocatalytic activity at nanoscale. Though the Ultra violet irradiation can efficiently degrade the dyes, naturally abundant solar radiation is also very effective in the mineralization of dyes. Hence, it may be a viable technique for the safe disposal of textile wastewater into the water streams.     

Photoelectrochemical hydrogen generation with nanostructured CdS/Ti–Ni–O composite photoanode

Composite photocatalysts have aroused great interest due to combination of favorable electronic and optical properties. Herein, novel CdS/Ti–Ni–O composite photoanodes were constructed through anodic fabrication of nanostructured Ni-doped TiO₂ (Ti–Ni–O) oxide films and CdS deposition by successive ionic layer adsorption and reaction (SILAR). The morphology and composition evolution, optical properties and photoelectrochemical (PEC) performance of the photoanodes were investigated. The composite nanofilms mainly consisted of micropores and nanotubes. The CdS/Ti–Ni–O composite photoanode demonstrated remarkable PEC hydrogen generation properties with a high photocurrent density (6.72 mA·cm^?2 at 0 V vs Ag/AgCl) which was 18.2 times to that of the bare Ti–Ni–O photoanode. The synergy of Ni-doping and CdS-coupling on the enhancement of PEC performance offers useful ideas to the exploitation of effective photocatalysts and contributes to the development of solar-driven PEC hydrogen generation.     

Lanthanide-organic-frameworks modified ZnIn2S4 for boosting hydrogen generation under UV–Vis and visible light

Novel Ln-MOF with microrods shape were successfully combined with ZnIn₂S₄ (ZIS) microsphere and used for photocatalytic hydrogen generation under UV–Vis and visible light. The Ln-MOFs/ZIS system comprises lanthanide-carboxylate coordination networks (Tm and Gd as metal ions, and 1,3,5-benzenetricarboxylic acid (BTC) as the organic linker) deposited on ZnIn₂S₄ microspheres. Effect of the amount of ((Tm,Gd)-BTC) (1, 5, 10 wt%) on the optical properties and photocatalytic hydrogen evolution performance was investigated. ZIS microsphere shows the marigold flower-like morphology and hexagonal polytopic crystal form. Our results proved that the combination of ZIS microsphere, Ln-MOF and Pt nanoparticles (NPs) caused significant enhancement in hydrogen generation. Amount of formed hydrogen was raised from 196.3 to 7782.1 μmol g^?1 for pristine ZIS and ZIS decorated with 1% (Tm, Gd)-BTC/Pt under UV–Vis light, respectively.     

Co-catalyst free direct Z–scheme photocatalytic system with simultaneous hydrogen evolution and degradation of organic pollutants

Photocatalytic hydrogen evolution coupled with simultaneous pollutant degradation is a promising but challenging strategy for both wastewater treatment and renewable energy production. In this work, a series of direct Z-scheme heterostructural catalysts, denoted as CeO₂/Cu–I-bpy, were designed and constructed by in-situ synthesizing CeO₂ nanoparticles on the surface of a coordinate polymer, named Cu–I-bpy. The simultaneous photocatalytic evolution of hydrogen and photo-degradation of organic pollutants was successfully achieved with this hybrid catalyst, even without any co-catalyst. The hybrid catalyst exhibited a 6.9 folds higher hydrogen evolution efficiency than the corresponding individual photo-reduction catalyst (Cu–I-bpy here), and 2.5 times degradation rate of the corresponding individual photo-oxidation catalyst (CeO₂ here), after optimizing the CeO₂ loading. After extensively characterized via XRD, XPS, Raman spectroscopy, UV–Vis absorption spectroscopy, SEM, TEM, BET, photo-electrochemical method and photoluminescence, it was found that the hybrid catalysts show higher charge separation and transfer efficiency than corresponding individual CeO₂ and Cu–I-bpy. Further mechanism investigation via active species trapping, EPR and electron flow experiments indicates a direct Z-scheme charge transfer mechanism in the CeO₂/Cu–I-bpy, which not only boosts the photogenerated electron-hole separation efficiency, but also retains the high reduction and oxidation ability the corresponding individual catalyst for hydrogen evolution and pollutant degradation, respectively. This work demonstrates the possibilities of turning waste water into energy resources using carefully designed photocatalytic systems.     

Promoting hydrogen production by loading PdO and Pt on N–TiO2 under visible light

Nitrogen/titanium dioxide (N/TiO₂) visible light photocatalysts were prepared using the sol–gel method. The catalysts were characterized using transmission electron microscopy, reflective UV–visible spectroscopy, specific surface area measurements, and X-ray diffraction. The prepared catalysts were used to generate hydrogen gas through the water-splitting reaction under visible light (wavelengths greater than 400 nm). Various N/Ti addition ratios were tested, and the hydrogen generation rates were compared to determine the optimal ratio. The maximal hydrogen production rate (approximately 55 μmol h⁻¹ g⁻¹) was attained when the N/Ti ratio of N–TiO₂ was 10. When PdO and Pt were loaded onto the N–TiO₂ catalyst, the hydrogen generation rates increased to 544 and 772 μmol h⁻¹ g⁻¹, respectively. The highest hydrogen production rate (2460 μmol h⁻¹ g⁻¹) was obtained when bimetallic 0.05 wt% PdO-0.10 wt% Pt/N–TiO₂ was used. After three times use the hydrogen yield of the catalyst was maintained as 83%. A possible mechanism of water splitting catalyzed by this visible light photocatalyst is proposed.     

Rational design of ZnO–CuO–Au S-scheme heterojunctions for photocatalytic hydrogen production under visible light

An integration of S-scheme heterojunction catalyst with surface plasmon resonance effect is the prime focus of current research activites in the field of visible light driven photocatalytic hydrogen (H₂) evolution. Herein, a sol-gel route is used to design a heterojunction of ZnO–CuO–Au. The effect of process parameters, including irradiation time, catalyst dose, and sacrificial reagents on the hydrogen evolution is studied. The S-scheme ZnO–CuO–Au heterojunction catalyst demonstrated high surface area, better optical absorption response in the visible part of light spectrum, and improved separation and transportion of charge carriers as verified by DRS, PL, and photoelectrochemical studies. The maximum H₂ evolution rateof ZnO–CuO–Au reaches 4655 μmolh⁻¹g⁻¹, which is 5 and 3.2 times higher than ZnO–CuO and Au–ZnO catalysts, respectively. A possible reason of this increase in H₂ evolution rate is inhibited recombination of charge carriers because of the S-scheme design to increase electrons with strong reduction potential and prolong lifetime, Au serves as an SPR source and conductive channel to swift the transfer of electrons and high density of active sites. This work offers innovative insight into designing plasmonic metals-modified S-scheme systems for solar fuel production.     

CdO–CdS nanocomposites with enhanced photocatalytic activity for hydrogen generation from water

Nanocomposites of CdO–CdS have been prepared in ethylene glycol water mixture followed by heating at 300 °C. TEM and XRD studies confirmed the atomic scale mixing of CdO and CdS nanoparticles, leading to the formation of CdSO₃ phase at the interfacial region between CdO and CdS. Photocatalytic studies for hydrogen generation from water show an enhanced activity for CdO–CdS composites compared to individual components namely CdO or CdS nanoparticles. Based on optical absorption, surface area measurements, steady state and time resolved fluorescence studies, it is established that, enhanced absorption in the visible region, higher surface area and increase in lifetime of the charge carriers are responsible for the observed increase in hydrogen yield from water when composite sample was used as the photocatalyst compared to individual components. The composite sample when combined with Pt as co-catalyst exhibit a large increase in the photocatalytic activity.     

Photocatalytic system with water soluble nickel complex of S,S′-bis(2-pyridylmethyl)-1,2-thioethane over CdS nanorods for hydrogen evolution from water under visible light

In this paper, we report a new nickel complex, [(bpte)NiCl₂] (bpte = S,S′-bis(2-pyridylmethyl)-1,2-thioethane) that can serve as a catalyst both for electrochemical and photochemical driven hydrogen production from water. As an electrocatalyst, [(bpte)NiCl₂] can electrocatalyze hydrogen generation from a neutral buffer with a turnover frequency (TOF) of 555.78 mol of hydrogen per mole of catalyst per hour (mol H₂/mol catalyst/h) at an overpotential (OP) of 837.6 mV. Together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H₂A) as a sacrificial electron donor, the nickel complex also can photocatalyze hydrogen evolution in heterogeneous environments and can work for 107 h. Under an optimal condition, the photocatalytic system can afford 24900 mol of H₂ per mole of catalyst during 83 h irradiation, with a TOF of 300H₂ per catalyst per hour. The average value of apparent quantum yield (AQY) is ～24% at 420 nm.     

2D-2D heterostructured CdS–CoP photocatalysts for efficient H2 evolution under visible light irradiation

Visible-light photocatalytic water splitting for hydrogen evolution has attracted tremendous attention in past decades, but it still suffers from low solar-hydrogen conversion efficiency. In this paper, two-dimensional (2D) CdS was synthesized by the hydrothermal method, and 2D CoP nanosheets were synthesized by successive hydrothermal, oxidation and phosphodation process. Then 2D-2D CdS–CoP photocatalysts were formed by ultrasonically dispersing the mixed solution of CdS and CoP, and their heterostructure was confirmed by transmission electron microscopy, X-ray photoelectron spectroscopy, fluorescence spectrometer and so on. CdS–CoP with 2% CoP loading amounts exhibits a maximum photocatalytic performance of 56.3 mmolg⁻¹h⁻¹ under visible light irradiation, which is 11.3 times as high as bare CdS. The enhanced photocatalytic performance of CdS–CoP should be due to the following two points: (1) high catalytic activity of CoP; (2) highly efficient separation and transfer of electron-hole pairs photogenerated in CdS due to the synthesized 2D-2D heterostructure.     

Photocatalytic overall water splitting under visible light over an In–Ni–Ta–O–N solid solution without an additional cocatalyst

A sort of mixed nitrogen oxides, with a general formula In–Ni–Ta–O–N, has been synthesized with a solid state reaction method and used as the catalyst for visible light (λ > 400 nm) driven overall water splitting without the extra loading of a cocatalyst. Hydrogen and oxygen are evolved with a high rate in an ideal ratio of 2 to 1. The non-nitrided mixed oxide and individual oxide or nitride do not show any activity under the same reaction conditions. The elemental ratio plays a key role and the composition with the best activity is found as In:Ni:Ta:O:N = 0.9:0.1:1:3.21:0.774, with which the sample has an absorption edge at 550 nm.     

Noble-metal-free Z-Scheme MoS2–CdS/WO3–MnO2 nanocomposites for photocatalytic overall water splitting under visible light

To fabricate an efficient two-component Z-scheme system for visible light induced overall water splitting, CdS/WO₃ nanocomposites, with cubic CdS nanoparticles grown on the surface of hexagonal WO₃ nanorods, were prepared via a facile precipitation of Cd²⁺ with S²⁻ in the presence of pre-obtained hexagonal WO₃ nanorods. MnO₂ and MoS₂, the co-catalysts for O₂ and H₂ generation respectively, were selectively deposited on WO₃ and CdS in the CdS/WO₃ nanocomposites. The resultant MoS₂–CdS/WO₃–MnO₂ composites show photocatalytic activity for overall water splitting under visible light, with an optimized performance observed over 2.0%MoS₂-0.2 CdS/WO₃-1.0%MnO₂. The visible light induced overall water splitting over MoS₂–CdS/WO₃–MnO₂ nanocomposites can be attributed to the presence of a Z-scheme charge transfer pathway in the CdS/WO₃ nanocomposites, ie, the transfer of the photo-generated electrons from the CB of WO₃ to the VB of CdS to recombine with the photo-generated holes through an efficient interface between cubic CdS and hexagonal WO₃. The left photo-generated holes in VB of WO₃ and the photo-generated electrons in CB of CdS therefore can accomplish the water oxidation and water reduction simultaneously, with the assistance of the surface deposited cocatalysts (MnO₂ and MoS₂). This work demonstrated the great potential of fabricating the two-component direct Z-scheme photocatalytic systems for overall water splitting from two semiconductors with a staggered band structure.     

ZnO–Bi2O3/graphene oxide photocatalyst with high photocatalytic performance under visible light

Abstract

Photocatalytic nanomaterials are attracting more and more attention because of their potential for solving environmental problems. ZnO, as one of the most promising photocatalysts, can only be excited by ultraviolet (UV) or near UV radiation. The objective of the study is to describe an efficient visible light driven ZnO based photocatalyst. In this regard, we communicate the preliminary research on the synthesis, characterisation and photocatalytic properties of ZnO–Bi₂O₃/graphene oxide (GO) composite materials. It was found that the photodegradation of methylene blue in the presence of ZnO–Bi₂O₃/GO reached 99·62% after irradiation with visible light for 2 h. The presence of GO enhances the stability of ZnO–Bi₂O₃ and reduces the recombination of charge carriers. ZnO–Bi₂O₃/GO also shows high photocatalytic activity for the degradation of acid blue, acid yellow, reactive red, acid red, reactive yellow and reactive blue under visible light irradiation. The novel aspect is the combination of GO and Bi₂O₃ doped ZnO. The use of GO enhances the efficiency of photocatalysis, and Bi₂O₃ doping ZnO excites the absorption of visible light. The impact of the research concerns the study of ZnO–Bi₂O₃/GO, which can be used as a promising photocatalyst for the treatment of textile wastewater.     

Fabrication of CdS/β-SiC/TiO2 tri-composites that exploit hole- and electron-transfer processes for photocatalytic hydrogen production under visible light

In this work, CdS/SiC/TiO₂ tri-composite photocatalysts that exploit electron- and hole-transfer processes were fabricated using an easy two-step method in the liquid phase. The photocatalyst with a 1:1:1 M ratio of CdS/SiC/TiO₂ exhibited a rate of hydrogen evolution from an aqueous solution of sodium sulfite and sodium sulfide under visible light of 137 μmol h^?1 g^?1, which is 9.5 times that of pure CdS. β-SiC can act as a sink for the photogenerated holes because the valence band level of β-SiC is higher than the corresponding bands in CdS and TiO₂. In addition, the level of the conduction band of TiO₂ is lower than those of CdS and β-SiC, so TiO₂ can act as the acceptor of the photogenerated electrons. Our results demonstrate that hole transfer and absorption in the visible light region lead to an effective hydrogen-production scheme.     

Ag–AgBr/g-C3N4/ZIF-8 prepared with ionic liquid as template for highly efficient photocatalytic hydrogen evolution under visible light

In this study, a ternary photocatalyst named Ag–AgBr/g-C₃N₄/ZIF-8 (A/g/Z) was prepared by ionic liquid assisted in-situ growth method. The structure and composition of samples are studied by means of XRD, SEM, XPS, TEM, EIS, etc. The AgBr prepared by ionic liquid assisted method has good dispersion, and the protonated carbon nitride has better specific surface area and morphology structure. The structure of the composites is optimized by modifying g-C₃N₄ with MOFs and noble metals, and the existence of Ag⁺ can play a bridging role, so as to form multi-path electronic transmission route. The high efficiency of electron transfer greatly improved the efficiency of hydrogen evolution. It is worth noting that the surface plasmon resonance (SPR) effect produced by silver ions on the surface of g-C₃N₄ can improve the absorption of visible light. The A (1.1)/g/Z (6.5) exhibit high hydrogen evolution efficiency (2058 μmol g^?1 h^?1, which is 49 times than g-C₃N₄) at the absence of Pt co-catalyst. Furthermore, the transient photocurrent density of the composites is much higher than that of g-C₃N₄ and AgBr, the semicircle radius of EIS is also less than both. Through the five cycle experiment, the photocatalytic efficiency of the composite material remained above 91%.     

Constructing atomic Co1–N4 sites in 2D polymeric carbon nitride for boosting photocatalytic hydrogen harvesting under visible light

Two?dimensional polymeric carbon nitride (2DPCN) is a versatile support for constructing single atom catalysts (SACs). Herein, we demonstrated a facile strategy for constructing atomic Co sites on 2DPCN. Atomic dispersion of Co was confirmed by the aberration?corrected high angle annular dark field scanning transmission electron microscopy, while the extended X?ray absorption fine structure spectroscopy analysis precisely corroborated the existence of atomic Co¹–N₄ non square planar sites on 2DPCN support. Remarkably, Co¹/2DPCN catalyst yielded H₂ at a rate of 28.3mmol_H2 h^?1 g_Co^?1, which is approximately 4.2 times higher than that of CoNPs/2DPCN (6.8mmol_H2 h^?1g _Co^?1) counterpart. Notably, with 0.75 wt % Pt as a co?catalyst, H₂ evolution activity for Co¹/2DPCN and CoNPs/2DPCN reached as high as 138.6 mmol_H2 h^?1 g_metal^?1 and 22.4mmol_H2 h^?1 g_metal^?1, respectively, which could be attributed to the synergistic effect of atomic Co¹–N₄ sites and Pt. Both DFT calculations and spectroscopic results revealed that as constructed atomic Co¹–N₄ sites, reduced band gap energy, improved light harvesting ability and decreased energy barrier for H₂ evolution.     

Hydrogen generation from splitting water with Al–Bi(OH)3 composite promoted by NaCl

In this paper, a novel Al–Bi(OH)₃ system hydrogen-generating material is investigated. Hydrolysis experiments show that the hydrolysis properties of the Al–10 wt% Bi(OH)₃ composite are significantly improved by doping with sodium chloride, and the Al–10 wt% Bi(OH)₃–5 wt% NaCl composite has a low activation energy (10.4 kJ mol⁻¹). With the further optimization of milling time, the hydrogen yield of Al–10 wt% Bi(OH)₃–5 wt% NaCl composite reaches 1000 mL g⁻¹ in 1 min. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive spectroscopy and thermogravimetric analysis are applied to characterize the composite and explore the hydrolysis mechanism. The characterization results show that the activation of aluminum mainly comes from three factors: (1) The formation of alumina during ball milling plays an important role in preventing the agglomeration between Al–Bi, Al–Al and Bi–Bi; (2) Bismuth generated during ball milling can form micro-galvanic cell with aluminum to promote the corrosion of aluminum; (3) Sodium chloride as a grinding aid contributes to crush aluminum powder, and chloride ions facilitate the corrosion of aluminum in the hydrolysis process. In addition, the drying method and initial water temperature have a great influence on by-products. The composite is expected to be used in mobile emergency fuel cell due to its rapid hydrogen production capacity.     

One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light

The development of efficient and stable noble-metal-free photocatalysts is crucial for hydrogen evolution from water splitting as clean energy. This study reports uniform CdS/NiS spherical nanoparticles through a simple one-pot hydrothermal method with the aid of KOH. The prepared CdS/NiS composites show superior photocatalytic activities toward the water splitting under visible light. A suitable amount of KOH in the synthesis benefits to form CdS/NiS photocatalysts with the improved activity. The CdS/NiS composite including 10 mol% metal percentage of Ni exhibits the highest photocatalytic activity. The high hydrogen evolution rate of 24.37 mmol h^?1 g^?1 is achieved over the CdS/NiS composite photocatalyst. The CdS/NiS photocatalyst has good photocatalytic stability in the recycling uses. The present CdS/NiS as a noble-metal-free photocatalyst provides the superior visible-light driven catalytic activity for hydrogen evolution.     

Synthesis and characterization of composite visible light active photocatalysts MoS2–g-C3N4 with enhanced hydrogen evolution activity

Molybdenum disulfide (MoS₂) and graphitic carbon nitride (g-C₃N₄) composite photocatalysts were prepared via a facile impregnation method. The physical and photophysical properties of the MoS₂–g-C₃N₄ composite photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microcopy (HRTEM), ultraviolet–visible diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. The photoelectrochemical (PEC) measurements were tested via several on–off cycles under visible light irradiation. The photocatalytic hydrogen evolution experiments indicate that the MoS₂ co-catalysts can efficiently promote the separation of photogenerated charge carriers in g-C₃N₄, and consequently enhance the H₂ evolution activity. The 0.5wt% MoS₂–g-C₃N₄ sample shows the highest catalytic activity, and the corresponding H₂ evolution rate is 23.10 μmol h⁻¹, which is enhanced by 11.3 times compared to the unmodified g-C₃N₄. A possible photocatalytic mechanism of MoS₂ co-catalysts on the improvement of visible light photocatalytic performance of g-C₃N₄ is proposed and supported by PL and PEC results.     

Photocatalytic hydrogen evolution under visible light irradiation by the polyoxometalate α-[AlSiW11(H2O)O39] -Eosin Y system

It is important to construct a stable and efficient dye sensitization system for visible-light photocatalytic hydrogen evolution. Eosin Y (EY)-sensitized α-[AlSiW₁₁(H₂O)O₃₉]⁵⁻ (AlSiW₁₁) (an Al³⁺ substituted Keggin polyoxometalate (POM)) for the hydrogen evolution under visible light irradiation (λ > 420 nm) has been carried out in the presence of triethanolamine as electron donor and Pt as co-catalyst. EY can coordinate with AlSiW₁₁. The coordination association between AlSiW₁₁ and EY is beneficial to the charge transfer from EY to AlSiW₁₁ and to stability of EY. The system displays efficient and stable photocatalytic hydrogen evolution. The average apparent quantum efficiency and turnover number of EY during 20 h irradiation (λ > 420 nm) are 10.3% and 473, respectively. The highest quantum efficiency amounts to 28.0% under 520 nm monochromatic light irradiation. The present study highlights linking between dye and POM molecule as a way to develop new visible-light stable photocatalyst or system.