Cascaded electron transition in CuWO4/CdS/CDs heterostructure accelerating charge separation towards enhanced photocatalytic activity

Enhanced interfacial effect via acid theory with boosted photocatalytic performance of nitenpyram removal

Surface reaction is a prominent aspect that affects the efficiency of photocatalysis. In this work, acid theory was employed to facilitate the reaction dynamics and enhance the interfacial effect between photocatalysts and target molecules. The photocatalytic removal efficiency of NTP was 66 % for bare CdS in 50 min with apparent rate constants of 0.023 compare to 96 % with apparent rate constants of 0.065 for 5% Ce-CdS. The introduced Ce atom as bifunctional active site reduces the energy barrier of O2 adsorption, strengthens the interfacial effect and accelerates the electrons transfer, which could facilitate surface reaction process and boost the photocatalytic performance.

Hydrothermal Synthesized CoS2 as Efficient Co-Catalyst to Improve the Interfacial Charge Transfer Efficiency in BiVO4

The bare surface of BiVO4 photoanode usually suffers from extremely low interfacial charge transfer efficiency which leads to a significantly suppressed photoelectrochemical water splitting performance. Various strategies, including surface modification and the loading of co-catalysts, facilitate the interface charge transfer process in BiVO4. In this study, we demonstrate that CoS2 synthesized from the hydrothermal method can be used as a high-efficient co-catalyst to sufficiently improve the interface charge transfer efficiency in BiVO4. The photoelectrochemical water splitting performance of BiVO4 was significantly improved after CoS2 surface modification. The BiVO4/CoS2 photoanode achieved an excellent photocurrent density of 5.2 mA/cm2 at 1.23 V versus RHE under AM 1.5 G illumination, corresponding to a 3.7 times enhancement in photocurrent compared with bare BiVO4. The onset potential of the BiVO4/CoS2 photoanode was also negatively shifted by 210 mV. The followed systematic combined optical and electrochemical characterization results reveal that the interfacial charge transfer efficiency of BiVO4 was largely improved from less than 20% to more than 70% due tor CoS2 surface modification. The further surface carrier dynamics study performed using an intensity modulated photocurrent spectroscopy displayed a 6–10 times suppression in surface recombination rate constants for CoS2 modified BiVO4, which suggests that the key reason for the improved interfacial charge transfer efficiency possibly originates from the passivated surface states due to the coating of CoS2.

One Step Synthesis of Oxygen Defective Bi@Ba2TiO4/BaBi4Ti4O15 Microsheet with Efficient Photocatalytic Activity for NO Removal

Photocatalysis is an effective technology for NO removal even at low concentrations in the ambient atmosphere. However, the low efficiency of this advanced process and the tendency of producing toxic byproducts hinder the practical application of photocatalysis. To overcome these problems, the Bi@Ba2TiO4/BaBi4Ti4O15 photocatalytic composites were successfully prepared by a one-step hydrothermal method. The as-synthesized photocatalysts exhibited an efficient photocatalytic performance and generated low amounts of toxic byproducts. X-ray diffraction studies show that Bi3+ is successfully reduced on the surface of Ba2TiO4/BaBi4Ti4O15 (BT/BBT). After L-Ascorbic acid (AA) modification, the photocatalytic NO removal efficiency of Bi@Ba2TiO4/BaBi4Ti4O15 is increased from 25.55% to 67.88%, while the production of the toxic byproduct NO2 is reduced by 92.02%, where the initial concentration of NO is diluted to ca. 800 ppb by the gas stream and the flow rate is controlled at 301.98 mL·min−1 in a 150 mL cylindrical reactor. Furthermore, ambient humidity has little effect on the photocatalytic performance of theBi@Ba2TiO4/BaBi4Ti4O15, and the photocatalyst exhibits excellent reusability after repeated cleaning with deionized water. The improved photocatalytic effect is attributed to the addition of AA in BT/BBT being able to reduce Bi3+ ions to form Bi nanoparticles giving surface plasmon effect (SPR) and generate oxygen vacancies (OVs) at the same time, thereby improving the separation efficiency of photogenerated carriers, enhancing the light absorption, and increasing the specific surface areas. The present work could provide new insights into the design of high-performance photocatalysts and their potential applications in air purification, especially for NO removal.

Regulating directional transfer of electrons on polymeric g-C3N5 for highly efficient photocatalytic H2O2 production

Graphite carbon nitride (g-C₃N₅) has been widely used in various photocatalytic reactions due to its higher thermodynamic stability and better electronic properties compared to g-C₃N₄. However, it is still challenging to endow g-C₃N₅ with high performance on photocatalytic hydrogen peroxide (H₂O₂) production. Herein, potassium and iodine are co-doped into g-C₃N₅ (g-C₃N₅-K, I) for photocatalytic production of H₂O₂ with high efficiency. As expected, the photocatalytic H₂O₂ production rate over the g-C₃N₅-K, I (2933.4 μM h^-1) reaches to 84.22 times as that of g-C₃N₅. The excellent photocatalytic H₂O₂ production activity is mainly ascribed to the co-doping of K and I, which significantly improves the capacity of oxygen (O₂) adsorption, selectivity of two-electrons oxygen reduction reaction (2e^- ORR) and separation efficiency of charge carriers. The density functional theory (DFT) calculations reveal that O₂ molecules are more conducive to being adsorbed on g-C₃N₅-K, I. Besides, the result of excited states further indicates that photo-generated electrons can be directionally driven to the adsorbed O₂ molecules, which are effectively activated to form H₂O₂. The findings will contribute to new insights in designing and synthesizing g-C₃N₅ based photocatalysts for the H₂O₂ production.

Carbon nitride-based Z-scheme heterojunctions for solar-driven advanced oxidation processes

Solar-driven advanced oxidation processes (AOPs) via direct photodegradation or indirect photocatalytic activation of typical oxidants, such as hydrogen peroxide (H₂O₂), peroxymonosulfate (PMS), and peroxydisulfate (PDS), have been deemed to be an efficient technology for wastewater remediation. Artificial Z-scheme structured materials represent a promising class of photocatalysts due to their spatially separated charge carriers and strong redox abilities. Herein, we summarize the development of metal-free graphitic carbon nitride (g-C₃N₄, CN)-based direct and indirect Z-scheme photocatalysts for solar-driven AOPs in removing organic pollutants from water. In the work, the classification of AOPs, definition and validation of Z-schemes are summarized firstly. The innovative engineering strategies (e.g., morphology and dimensionality control, element doping, defect engineering, cocatalyst loading, and tandem Z-scheme construction) over CN-based direct Z-scheme structure are then examined. Rational design of indirect CN-based Z-scheme systems using different charge mediators, such as solid conductive materials and soluble ion pairs, is further discussed. Through examining the relationship between the Z-scheme structure and activity (charge transfer and separation, light absorption, and reaction kinetics), we aim to provide more insights into the construction strategies and structure modification on CN-based Z-schemes towards improving their catalytic performances in AOPs. Lastly, limitations, challenges, and perspectives on future development in this emerging field are proposed.

Understanding the mechanism of interfacial interaction enhancing photodegradation rate of pollutants at molecular level: Intermolecular π-π interactions favor electrons delivery

Uncovering the interaction between photocatalyst and reaction substrate as well as subsequent electron transfer process is critical to achieve high-performance photodegradation of pollutants. Herein, based on the reduced density gradient (RDG) method, we visualize the simulation of the π-π interactions between photocatalyst (g-C₃N₄) and pollutant molecule (flumequine, FLU). Results revealed that π-π interactions between g-C₃N₄ and FLU favor electrons delivery, resulting in enhanced charge separation efficiency and direct hole oxidation of FLU. Moreover, it is found that the charge transfer rate is determined by the valence band (VB) level of g-C₃N₄ and E_HOMO of FLU, of which the deeper VB position of g-C₃N₄ favors faster charge transfer, leading to further enhancement in photocatalytic degradation rate of FLU. Additionally, the possible degradation pathways of FLU were proposed by theoretical calculation and the determined intermediates. Our work afforded a new insight into pollutants degradation and the rational design of highly efficient photocatalysts.

Nonmetal doped carbon nitride nanosheet as photocatalyst for degradation of 4, 5-dichloroguaiacol

Metal-free photocatalysts for high efficient photocatalytic degradation of pollutants have attracted growing concern in recent years. Herein, relying on density functional theory (DFT) calculations, boron and phosphorus doped C₂N layers were explored for the potential of utilization as photocatalysts for 4, 5-dichloroguaiacol (4, 5-DCG) removal. Our computations revealed that the adsorption energy of 4, 5-DCG on B@N-doped C₂N layers were 26.56 kcal mol^-1, and the ΔG^≠ of initial reactions of 4, 5-DCG with OH were also reduced onto the B@N-doped C₂N substrates. The band gap of B@N-doped C₂N was 2.27 eV. The obtained results showed that the doping of boron atom into C₂N layer narrows bandgap, and retains well catalytic performance and adsorption properties. Hence, B@N-doped C₂N layer is a promising photocatalyst for organic pollutants removal. Possible degradation pathways of 4, 5-DCG and aquatic toxicity assessment during degradation were also carried out. Products with higher toxicity would be formed and the transformation products were still toxic to three nutrient levels of aquatic organisms (green algae, fish, and daphnia).

Revealing the stability of CuWO4/g-C3N4 nanocomposite for photocatalytic tetracycline degradation from the aqueous environment and DFT analysis

Graphitic carbon nitride (g-C₃N₄) is an emerging metal-free photocatalyst, however, engineering the photocatalytic efficiency for the effective degradation of hazardous molecules is still challenging. An unstable and low bandgap CuWO₄ was composited with g-C₃N₄ to achieve synergistic benefits of tuning the visible light responsiveness and stability of CuWO₄. CuWO₄/g-C₃N₄ nanocomposite exhibited a relatively high visible light absorption region and the bandgap was modified from 2.77 to 2.53 eV evidenced via UV-DRS. Moreover, the fast electron transfer rate was observed with CuWO₄/g-C₃N₄ nanocomposite as confirmed using PL and photocurrent studies. XRD, FT-IR, and HR-TEM analyses signified the formation of CuWO₄/g-C₃N₄ nanocomposite. CuWO₄/g-C₃N₄ nanocomposite showed enhanced photocatalytic degradation of Tetracycline (TC) about ∼7.4 fold greater than pristine g-C₃N₄ in 120 min. Notably, the OH^• and ^•O₂^- radicals played a most significant role in photocatalytic TC degradation. Furthermore, the energy band structure, density of state, and Bader charge analyses of these molecules were performed.

Enhanced photocatalytic benzyl alcohol oxidation over Bi4Ti3O12 ultrathin nanosheets

Ultrathin Bi₄Ti₃O₁₂ nanosheets (NS) with the thickness about 3.9 nm were successfully synthesized by a hydrothermal method and were used as a photocatalyst for the oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). The photocatalytic performance of NS is about 8 times higher than that of bulk Bi₄Ti₃O₁₂. In-situ FTIR of pyridine adsorption and NH₃-TPD reveal that NS has more surface Lewis acid sites (Ti⁴⁺) for the adsorption and activation of BA. The photogenerated electrons (e^-) and holes (h⁺) of NS can be fully used to produce the superoxide radicals and carbon-centered radicals, respectively. The monolayer nanosheet structure of NS not only greatly promotes the separation of photogenerated carriers, but also achieves the efficient activation of BA molecules via the CO⋯Ti coordination. This work successfully reveals the surface/interface interactions between the surface active sites of a photocatalyst and the reactive molecules via using ultrathin nanosheet as a molecular platform.

Amorphous CoSx decorated Cd0.5Zn0.5S with a bulk-twinned homojunction for efficient photocatalytic hydrogen evolution

Amorphous CoSx was combined with Twinned-Cd0.5Zn0.5S to construct homo-heterojunction for efficient photocatalytic H2 evolution. This work provides new ideas for constructing noble metal-free photocatalyst with homo-heterojunction.

Effective enhancement of electron migration and photocatalytic performance of nitrogen-rich carbon nitride by constructing fungal carbon dot/molybdenum disulfide cocatalytic system

To find a cocatalyst that can replace noble metals, fungal carbon dot (CD) modified molybdenum disulfide (MoS₂) cocatalyst system was designed. The composites were prepared by hydrothermal and calcination methods with different ratios of CDs, MoS₂ and nitrogen-rich carbon nitride (p-C₃N₅). p-C₃N₅ has excellent electronic properties, and MoS₂ modified by CDs (D-MoS₂) can significantly enhance the photocatalytic performance of p-C₃N₅ by improving the photogenerated electron migration efficiency. The experiments showed that the developed CDs/MoS₂/C₃N₅ composites exhibited excellent performance in both photocatalytic hydrogen (H₂) evolution and methylene blue (MB) degradation, with CMSCN5 (D-MoS₂ with 5% mass fraction) showing the best photocatalytic activity. The corresponding H₂ evolution rate of CMSCN5 was 444 μmol g^-1h^-1 and 1.45 times higher than that of unmodified p-C₃N₅, by 120 min, the removal rate of MB was up to 93.51%. The 5 cycle tests showed that CMSCN5 had great stability. The high charge mobility and high density of H₂ evolution active sites of MoS₂ nanosheets, together with the electron storage and transfer properties of CDs can obviously improve electron migration and reduce the photogenerated carrier recombination on the p-C₃N₅ surface. The design and preparation of such composites offer broad prospects for the development of photocatalytic systems with noble metal-free cocatalysts.

Probing the role of surface acid sites on the photocatalytic degradation of tetracycline hydrochloride over cerium doped CdS via experiments and theoretical calculations

Surface acid site regulation of photocatalysts is a promising strategy to improve their performance. Herein, surface acid sites of cadmium sulfide were rationally regulated by cerium doping, which resulted in significantly increased photocatalytic activity for tetracycline hydrochloride (TC-HCl) degradation. The generated Brønsted acid sites were verified to favor the adsorption of organic molecules because of their strong affinity. Meanwhile, Lewis acid sites acted as the active sites for C-C bond cleavage via a nucleophilic substitution process, which was testified by the Fukui function and electrostatic potential. Besides, Ce³⁺ doping suppressed the recombination of electron-hole pairs, which also boosted the performance of TC-HCl degradation. Moreover, the degradation pathway of TC-HCl was deduced based on theoretical calculations and HPLC-MS results. The toxicity of pollutants and intermediates was also evaluated. This work provided new insight into the rational design and preparation of highly efficient photocatalysts for environmental purification.

Multipath elimination of bisphenol A over bifunctional polymeric carbon nitride/biochar hybrids in the presence of persulfate and visible light

Polymeric carbon nitride (PCN) has become a star material either in photocatalysis or in persulfate (PS) activation. In this work, we synthesized bifunctional biochar (BC)-doped PCN through a facile one-pot thermal treatment process. The PCN/BC hybrid (CNBC) with an optimized proportion could not only activate PS directly, but also possessed improved optical properties. Amorphous BC domains generated from the carbonization of external corncob provided attachments for the in-situ growth of PCN and upgraded its catalytic ability including electron transport property, visible light (VIS) utilization, and oxidation power. Mechanism studies demonstrated that in the CNBC/PS system without VIS, a nonradical electron transfer route was responsible for the degradation of bisphenol A (BPA), while in the CNBC/PS/VIS system, radical/nonradical mixing mechanisms including mediated electron transfer, radical oxidation, and hole oxidation were unveiled. Degradation pathways of BPA were deduced including direct oxidation at the aromatic ring, β-scission of isopropyl, and ring cleavage. Most of the intermediates were less toxic than BPA as assessed by the ECOSAR software. The CNBC/PS/VIS system showed satisfactory resistance to environmental interferences except for HCO₃^-. This work provides a simple but effective strategy for the synthesis of PCN-based bifunctional catalysts and deepens mechanistic insights into hybrid advanced oxidation technologies.

Lattice-Matched CoP/CoS2 Heterostructure Cocatalyst to Boost Photocatalytic H2 Generation

Transition-metal phosphides and sulfides are considered as promising cocatalysts for the photocatalytic hydrogen evolution reaction (HER), and the cocatalytic effect can be improved by directed heterostructure engineering. In this study, a novel lattice-matched CoP/CoS₂ heterostructure having a nanosheet morphology was developed as an HER cocatalyst and integrated in situ onto graphitic carbon nitride (g-C₃N₄) nanosheets via a successive phosphorization and vulcanization route. First-principles density functional theory calculations evidenced that the construction of the lattice-matched CoP/CoS₂ heterostructure resulted in the redistribution of interface electrons, enhanced metallic characteristics, and improved H* adsorption. As a result of these effects, the CoP/CoS₂ heterostructure cocatalyst formed a 2D/2D Schottky junction with the g-C₃N₄ nanosheets, thus promoting photoelectron transfer to CoP/CoS₂ and realizing fast charge-carrier separation and good HER activity. As expected, the CoP/CoS₂ heterostructure exhibited excellent cocatalytic activity, and the optimal loading of the cocatalyst on g-C₃N₄ enhanced its HER activity to 3.78 mmol g^-1 h^-1. This work furnishes a new perspective for the development of highly active noble-metal-free cocatalysts via heterostructure engineering for water splitting applications.

One-Dimensional CdS/Carbon/Au Plasmonic Nanoarray Photoanodes via In Situ Reduction-Graphitization Approach toward Efficient Solar Hydrogen Evolution

Photoelectrochemical (PEC) hydrogen evolution has been acknowledged as a promising "green" technique to convert solar energy into clean chemical fuel. Photoanodes play a key role in determining the performance of PEC systems, spurring numerous efforts to develop advanced materials as well as structures to improve the photoconversion efficiency. In this work, we report the rational design of a plasmonic hierarchical nanorod array, composed of oriented one-dimensional (1D) CdS nanorods decorated with a uniformly wrapped graphite-like carbon (C_PDA) layer and Au nanoparticles (Au NPs), as highly efficient photoanode materials. An interfacial in situ reduction-graphitization method has been conducted to prepare the CdS/C_PDA/Au nanoarchitecture, where polydopamine (PDA) coating was used as a C source and a reductant. The CdS/C_PDA/Au nanoarray photoanode demonstrates superior photoconversion efficiency with a photocurrent density of 8.74 mA/cm² and an IPCE value (480 nm) of 30.2% (at 1.23 V vs RHE), under simulated sunlight irradiation, which are 12.7 and 13.5 times higher than pristine CdS. The significant enhancement of PEC performance is mainly benefited from the increase of the entire quantum yield and efficiency due to the formation of a Schottky rectifier, localized surface plasmon resonance (LSPR)-enhanced light absorption, and promoted hot-electron injection from interlayered graphene-like carbon. More importantly, thanks to the inhibited charge carrier recombination process and transferred oxidation reaction sites, the fabricated CdS/C_PDA/Au photoelectrode exhibits lengthened electron lifetimes and better photostability, illustrating its wonderful potential for future PEC application.

Surface dual redox cycles of Mn(III)/Mn(IV) and Cu(I)/Cu(II) for heterogeneous peroxymonosulfate activation to degrade diclofenac: Performance, mechanism and toxicity assessment

Advanced oxidation processes (AOPs) based on heterogeneous catalytic activated peroxymonosulfate (PMS) have been becoming alternatives to conventional wastewater treatment technologies to directly degrade chemical contaminants. To build dual/multi redox cycles of different metal ions may be an effective means for better PMS activation. Herein, this study designed Mn₃O₄/CuBi₂O₄ with dual redox cycles of Mn(III)/Mn(IV) and Cu(I)/Cu(II) to activate PMS for efficiently decomposing and mineralizing diclofenac sodium (DCF). Under optimal reaction conditions, DCF (50 mg/L) was degraded totally within 10 min, and TOC removal rate reached up to 74.3%. The possible mechanism of PMS activation by Mn₃O₄/CuBi₂O₄ was proposed, wherein dual redox cycles of Mn(III)/Mn(IV) and Cu(I)/Cu(II) on Mn₃O₄/CuBi₂O₄ effectively facilitated PMS activation to generate ·O₂^-, ¹O₂, SO₄·^- and ·OH, which was responsible for DCF degradation. Moreover, combined with degraded products detected by high resolution liquid chromatography coupled to mass spectrometry and corresponding toxic assessment results, the possible degradation pathways of DCF were proposed and the relative toxicity of degraded products was evaluated. This work may be useful for developing stronger heterogeneous activators of PMS to construct more efficient AOPs for purifying wastewater.

Single-step solvothermal synthesis of highly uniform CdxZn1−xS nanospheres for improved visible light photocatalytic hydrogen generation

A single step synthesis of a solid solution of Cd_xZn_1−xS is demonstrated with optimum Cd and Zn percentage for enhanced photocatalytic hydrogen generation under visible light.

A uniformly decorated and photostable polydopamine-organic semiconductor to boost the photoelectrochemical water splitting performance of CdS photoanodes

Photoelectrochemical (PEC) water splitting to produce renewable H2 fuel by storage of solar energy has attracted increasing attention as it could reduce carbon footprint and solve the global consumption growth. Herein, a photostable polymer polydopamine (PDA) was introduced to enhance the PEC performance by forming a uniform inorganic-organic hybrid heterostructure with CdS. The organic semiconductor PDA not only forms a strong coordinate bond to facilitate the transfer of electrons, but also acts as a passivation layer, contributing to improve the stability of the photoelectrode. A photocurrent density of 1.08 mA cm-2 was achieved for CdS/1PDA, which was about 2.4 times that of bare CdS at 0.28 V vs. RHE, and CdS/1PDA featured a reasonable photocurrent stability compared with bare CdS. The Co-Pi co-catalyst, as a hole acceptor, further prohibited charge recombination and promoted the water oxidation kinetics. The photocurrent density of CdS/1PDA/5Co-Pi was up to 2.68 mA cm-2 (0.28 V vs. RHE), which was 5.7 and 2.5 times higher than that of bare CdS and CdS/1PD, respectively. The strategy provides a beneficial insight to design an inorganic-organic uniform heterostructure for the enhancement in PEC performance.

Highly value-added utilization of H2S in Na2SO3 solution over Ca-CdS nanocrystal photocatalysts

Alkaline-earth metal Ca²⁺ modified CdS nanocrystals have been designed for the first time for highly efficient H₂ evolution from hydrogen sulfide (H₂S) with Na₂SO₃ as a favourable reaction medium. The advantage of Na₂SO₃ was revealed by an electrochemical test, and the conversion of Na₂SO₃ during the reaction was carefully studied. Particularly, most of Na₂SO₃ was converted into Na₂S₂O₃. Highly value-added utilization of waste H₂S is therefore achieved via photocatalysis.