Enhanced visible-light-active photocatalytic performance using CdS nanorods decorated with colloidal SnO2 quantum dots: Optimization of core–shell nanostructure

Interface engineered cascade-type electronic structure of 2D/0D/2D CdS-CdCO3/SnO2 quantum dots/g-C3N4 nanocomposite for boosting solar-driven photocatalysis

A rational design of heterojunctions with high-quality contacts is essential for efficiently separating photogenerated charge carries and boosting the solar-driven harvesting capability. Herein, we fabricated a novel heterojunction of SnO2 quantum dots-anchored CdS-CdCO3 with g-C3N4 nanosheets as a superior photocatalyst. SnO2 quantum dots (SQDs) with positively charged surfaces were tightly anchored on the negatively charged surface of CdS nanosheets (NSs). The resulting CdS@SnO2 was finally decorated with g-C3N4 NSs, and a new crystalline phase of CdS-CdCO3 was formed during the hydrothermal decoration process, g-C3N4 decorated CdS-CdCO3@SnO2 (CdS-CdCO3@SnO2@g-C3N4). The as-synthesized photocatalysts were evaluated for the degradation of methyl orange dye under solar light conditions. The CdS-CdCO3@SnO2@g-C3N4 exhibited 7.7-fold and 2.3-fold enhancements in photocatalytic activities in comparison to those of the bare CdS and CdS@SnO2 NSs, respectively. The optimal performance of CdS-CdCO3@SnO2@g-C3N4 is primarily attributed to the cascade-type conduction band alignments between 2D/0D/2D heterojunctions, which can harvest maximum solar light and effectively separate photoexcited charge carriers. This work provides a new inspiration for the rational design of 2D/0D/2D heterojunction photocatalyst for green energy generation and environmental remediation applications.

CdS Nanoparticles Supported by Cobalt@Carbon-Derived MOFs for the Improved Adsorption and Photodegradation of Ciprofloxacin

The design and synthesis of efficient photocatalysts that promote the degradation of organic pollutants in water have attracted extensive attention in recent years. In this work, CdS nanoparticles are grown in situ on Co@C derived from metal-organic frameworks. The resulting hierarchical CdS/Co@C nanostructures are evaluated in terms of their adsorption and photocatalytic ciprofloxacin degradation efficiency under visible-light irradiation. The results show that, apart from offering a large surface area (55.69 m²·g^-1), the prepared material can effectively suppress the self-agglomeration of CdS and enhance the absorption of visible light. The CdS/Co@C-7 composite containing 7% wt Co@C has the highest photodegradation rate, and its activity is approximately 4.4 times greater than that of CdS alone. Moreover, this composite exhibits remarkable stability after three successive cycles of photocatalysis. The enhanced photocatalytic performance is largely ascribed to the rapid separation of electron-hole pairs and the effective electron transfer between CdS and Co@C, which is confirmed via electrochemical experiments and photoluminescence spectra. The active substance capture experiment and the electron spin resonance technique show that h⁺ is the main active entity implicated in the degradation of CIP, and accordingly, a possible mechanism of CIP photocatalytic degradation over CdS/Co@C is proposed. In general, this work presents a new perspective on designing novel photocatalysts that promote the degradation of organic pollutants in water.

ZnWO4 nanorod-colloidal SnO2 quantum dots core@shell heterostructures: Efficient solar-light-driven photocatalytic degradation of tetracycline

Zinc tungsten oxide (ZW) and colloidal SnO₂ quantum dots (CS) were synthesized individually by hydrothermal and wet chemical methods. ZW-CS core@shell nanorods were prepared using a sonochemical method for the enhanced photocatalytic activity of tetracycline (TC) degradation. ZW-CS core@shell nanorods were systematically characterized by structural, morphological mapping and optical techniques. All characterization techniques were synchronized to confirm the construction of core@shell nanorods. Optical absorption studies indicate an increased light-capturing efficiency along with a reduced bandgap from 3.56 to 3.23 eV, which is further supported by photoluminescence. Mapping analysis from SEM and HR-TEM evidence the presence of elements as well as a core@shell nanostructure. The optimized sample of ZW-CS 1.0 shows improved photocatalytic degradation of TC under stimulated solar light. The TC degradation efficiency by ZW-CS 1.0 core@shell nanorods was about 97% within 2 h. The formation of core@shell nanorod structure might be the reason for the better photocatalytic tetracycline degradation performance.

Carbon dots decorated cadmium sulphide heterojunction-nanospheres for the enhanced visible light driven photocatalytic dye degradation and hydrogen generation

Carbon dots (C-dots) developed from beetroot is used for the rational design of cadmium sulphide based heterojunction photocatalysts (C-dots@CdS) using hydrothermal technique. The crystal structure, phase, morphology and optical characteristics of the synthesised materials are determined using X-ray diffraction (XRD), High resolution transmission electron microscopy (HR-TEM), Field emission scanning electron microscopy (FESEM), Energy dispersive X-ray analysis (EDAX), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, UV-Visible diffuse reflectance spectroscopy (UV-DRS), photoluminescence spectroscopy (PL spectroscopy), BET adsorption, X-ray photoelectron spectroscopy (XPS) and electrochemical studies. Using C-dots@CdS catalytic system, a superior photocatalytic activity relative to the undecorated CdS is observed. Among the C-dots@CdS samples, the CdS loaded with 6 wt% of C-dots exhibited enhanced hydrogen evolution rate compared with other samples considered for the study. CdS nanospheres modified with C-dots (6 wt%) resulted in the photocatalytic hydrogen evolution rate of 1582 µmolg^-1 against 849 µmolg^-1 evolution rate obtained for CdS nanospheres within 3 h. In spite of being 0D/0D type nano-heteroarchitecture, C-dots@CdS system obtained an apparent quantum yield of 6.37 % for the catalytic dosage of 20 mg under the irradiation of visible light. CdS in the C-dots@CdS system serves as the light harvester while C-dots with discernible edges can maintain the continuous supply of photo-excited charge carriers and hence can reduce the charge-carrier recombination. Further, the photodegradation of crystal violet dye using the optimised dosage of C-dots@CdS-6 exhibited an efficiency of 97.3 % in 120 min of visible light irradiation under neutral conditions. The detailed kinetic study reveals that the mechanism of photodegradation of crystal violet dye using C-dots@CdS system can be described using pseudo-second-order kinetics. The presence of oxygen rich hydrophilic surface functionalities of C-dots, the formation of near-surface heterojunction and the suitable band structure of C-dots@CdS system leading to the optimum charge carrier separation kinetics can be attributed to the enhanced photocatalytic performance. This work offers a promising strategy to develop bio-derived C-dots based heterojunction photocatalyst to address the burgeoning energy and environmental demands.

Heterostructured Nanoscale Photocatalysts via Colloidal Chemistry for Pollutant Degradation

With the further acceleration in the industrialization process, organic pollutants and gas pollution in the environment have posed severe threats to human health. It has been a global challenge regarding achieving an efficient solution to pollutant degradation. In such a context, photocatalysts have attracted researchers’ attention for their simplicity, efficiency, cleanliness and low cost. However, the single photocatalyst is facing a research bottleneck owing to its narrow light absorption spectrum and high photocarrier recombination rate. Given that heterojunctions can achieve efficient separation of photogenerated carriers in space, constructing heterostructured photocatalysts has become the most perspective method to improve the performance of photocatalysts. Furthermore, nanoparticles prepared through colloidal chemistry have the characteristics of high dispersion, stability and adsorption, further enhancing the degradation efficiency of heterostructured photocatalysts. This article reviews the primary methods for preparing heterostructured photocatalysts through colloidal chemistry, classifies the heterojunction types by transport routes of photogenerated carriers and summarizes the recent progress of heterostructured photocatalysts in pollutant degradation. To implement environmental remediation, it is crucial to explore economical and efficient photocatalysts for practical applications. It is hoped that this review will stimulate further exploration of colloidal heterostructured photocatalysts for pollutant degradation.

In situ growth of 1D/2D CdS-Bi2MoO6 core shell heterostructures for synergistic enhancement of photocatalytic performance under visible light

Stability of the photocatalyst, maximum solar energy harvesting and effective photogenerated charge carrier separation are yet demanding key features of the photocatalysis for pollutant abetment and photo-electrochemical applications. Herein, we report the in situ solvothermal synthesis of CdS-Bi₂MoO₆ core-shell heterostructures (CdS-Bi₂MoO₆ CSHs) for the photocatalytic elimination of methyl orange (MO) under visible light. The as-synthesized CdS-Bi₂MoO₆ CSHs exhibited highest photocatalytic performance of 98.5%, which is approximately 10 and 4 folds higher than pristine Bi₂MoO₆ nanosheets (NSs) and CdS nanorods (NRs), respectively. This significantly enhanced photocatalytic performance is attributed to the core-shell heterostructure that improves the visible-light harvesting ability, facilitates efficient separation and transfer of the photogenerated charge carriers, as well as synergistic band alignment of both CdS NRs and Bi₂MoO₆ NSs. The CdS-Bi₂MoO₆ CSHs also showed efficient photocatalytic performance toward methylene blue (MB) as colored dye and tetracycline hydrochloride (TCH) as a colorless emerging contaminant. Additionally, the outcomes of transient photocurrent, electrochemical impedance, and photoluminescence study further corroborate that the construction of core-shell heterostructures with tight contact, leading to effective charge carrier separation. The hole (h⁺) and superoxide radical anion (^•O₂^-) were determined to be the predominant active species accountable for the MO dye degradation. Furthermore, the CdS-Bi₂MoO₆ CSHs exhibited a satisfactory recycling efficiency over five cycles (reduced by approximately 6%), owing to the protective Bi₂MoO₆ NSs shell over the CdS NRs core, demonstrating their applicability in wastewater purification and photo-electrochemical applications.

Recent progress in transition metal oxide/sulfide quantum dots-based nanocomposites for the removal of toxic organic pollutants

Water is an essential solvent that is extremely necessary for the survival of life. Water pollution due to the increased utilization of water for various processes, including domestic and industrial activities, poses a special threat that contaminates both surface and ground water. In recent years, advanced oxidation processes (AOPs) have been applied to deal with wastewater problems, which is a green method used to oxidize organic contaminants with strong oxidative radical species. Among the AOPs, photocatalytic technology is one of the most promising strategies for wastewater cleaning, which fulfills the aims of environmentally friendly and sustainable development. Owing to their unique electronic, optical, and structural properties, nanoscale semiconductors have received substantial interest as materials for AOPs, particularly inspired by their superb quantum confinement effects and large surface-area-to-volume ratio, which are essential for catalytic reaction kinetics. Recent advancements have revealed that semiconductor nanocrystals, known as quantum dots (QDs), are newly emerging zero-dimensional (0-D) nanomaterials, which have garnered much attention owing to their special physiochemical characteristics such as high conductivity, thermo-chemical and opto-mechanical stability, high adsorption coefficients, and, most importantly, their admirable recyclability. In this review, we provide a clear understanding of the importance of semiconductor QD-based nanocomposites in the degradation of organic pollutants, in addition to the mechanism involved in the reaction process. Following this, the enhancement of different materials, such as metal oxides and metal sulfide QD-based nanocomposites, is discussed in the context of combating environmental pollution.

Enhanced solar-light-driven photocatalytic and photoelectrochemical properties of zinc tungsten oxide nanorods anchored on bismuth tungsten oxide nanoflakes

At present, sustainable water supply and energy generation are the most important challenges faced by humankind globally. Thus, it is crucial to progress ecological techniques for sustainable removal of organic pollutants from wastewater and generation of hydrogen as an alternative to fossil fuels. In this study, zinc tungsten oxide (ZnWO₄) nanorods, bismuth tungsten oxide (Bi₂WO₆) nanoflakes, and Bi₂WO₆/ZnWO₄ (BO-ZO) nanocomposites were prepared via a simple hydrothermal approach. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, diffuse reflectance spectroscopy, and electrochemical analyses were conducted to confirm the formation of the BO-ZO heterostructure. The structural and morphological analyses revealed that the ZnWO₄ nanorods were moderately dispersed on the Bi₂WO₆ nanoflakes. The bandgap tuning of BO-ZO nanocomposite confirmed the establishment of the heterostructure with band bending properties. The BO-ZO nanocomposite could degrade 99.52% of methylene blue (MB) within 60 min upon solar-light illumination. The photoelectrochemical (PEC) measurement results showed that the BO-ZO nanocomposite showed low charge-transfer resistance and high photocurrent response with good stability. The BO-ZO photoanode showed a low charge-transfer resistance of 35.33 Ω and high photocurrent density of 0.1779 mA/cm² in comparison with Ag/AgCl in a 0.1 M Na₂SO₃ electrolyte under solar-light illumination. The MB photocatalytic degradation and PEC water oxidation mechanisms of the nanocomposite were investigated.

ZnO nanosheets-decorated Bi2WO6 nanolayers as efficient photocatalysts for the removal of toxic environmental pollutants and photoelectrochemical solar water oxidation

Herein we report the fabrication of novel Bi₂WO₆/ZnO heterostructured hybrids for organic contaminant degradation from wastewater and photoelectrochemical (PEC) water splitting upon solar illumination. The Bi₂WO₆/ZnO photocatalysts were synthesized using a simple and eco-friendly hydrothermal process without the support of any surfactants. From the photocatalytic experiments, heterostructured Bi₂WO₆/ZnO nanohybrid catalysts exhibited considerably better photocatalytic performance for rhodamine B (RhB) degradation under solar illumination. The BWZ-20 nanocomposite demonstrated superior photodegradation of RhB dye up to 99% in about 50 min. Furthermore, BWZ-20 photoelectrode showeda lower charge-transfer resistance than other samples prepared, suggesting its suitability for PEC water splitting. The photocurrent densities of Bi₂WO₆/ZnO photoelectrodes were evaluated under the solar irradiation. The BWZ-20 photoelectrode exhibited a significant photocurrent density (0.45 × 10^-3A/cm²) at +0.3 V vs. Ag/AgCl, which was~1036-times higher than that of pure Bi₂WO₆, and ~4.8-times greater than the pure ZnO. Such improved photocatalytic and PEC activities are mainly attributed to the formation of an interface between ZnO and Bi₂WO₆, superior light absorption ability, low charge-transfer resistance, remarkable production of charge carriers, easy migration of charges, and suppression of the recombination of photogenerated charge carriers.

In Situ Construction of a MgSn(OH)6 Perovskite/SnO2 Type-II Heterojunction: A Highly Efficient Photocatalyst towards Photodegradation of Tetracycline

Using solar energy to remove antibiotics from aqueous environments via photocatalysis is highly desirable. In this work, a novel type-II heterojunction photocatalyst, MgSn(OH)₆/SnO_2, was successfully prepared via a facile one-pot in situ hydrothermal method at 220 °C for 24 h. The obtained heterojunctions were characterized via powder X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, and ultraviolet-visible diffuse reflectance spectroscopy. The photocatalytic performance was evaluated for photodegradation of tetracycline solution under ultraviolet irradiation. The initial concentration of tetracycline solution was set to be 20 mg/L. The prepared heterojunctions exhibited superior photocatalytic activity compared with the parent MgSn(OH)₆ and SnO₂ compounds. Among them, the obtained MgSn(OH)₆/SnO₂ heterojunction with MgCl₂·6H₂O:SnCl₄·5H₂O = 4:5.2 (mmol) displayed the highest photocatalytic performance and the photodegradation efficiency conversion of 91% could be reached after 60 min under ultraviolet irradiation. The prepared heterojunction maintained its performance after four successive cycles of use. Active species trapping experiments demonstrated that holes were the dominant active species. Hydroxyl radicals and superoxide ions had minor effects on the photocatalytic oxidation of tetracycline. Photoelectrochemical measurements were used to investigate the photocatalytic mechanism. The enhancement of photocatalytic activity could be assigned to the formation of a type-II junction photocatalytic system, which was beneficial for efficient transfer and separation of photogenerated electrons and holes. This research provides an in situ growth strategy for the design of highly efficient photocatalysts for environmental restoration.