A highly efficient and noble metal-free photocatalytic system using NixB/CdS as photocatalyst for visible light H2 production from aqueous solution

Photocatalytic H₂ production from water is one of the most attractive issues for the conversion of solar energy into chemical energy. In this study, the Ni_xB/CdS photocatalyst was firstly used for photocatalytic H₂-evolution reaction and showed efficient visible-light photocatalytic activity and good stability for H₂-evolution from aqueous solution. The optimal Ni_xB loading content was determined to be 0.8 wt.%, and the corresponding H₂-production rate reached up to 4.8 mmol·h^− 1·g^− 1 after a 10 h visible light irradiation. It is proposed that the loading of Ni_xB on the surface of CdS could effectively increase the separation of photo-generated electrons and holes and greatly enhance the photocatalytic activity.     

Electric-assisted strategy enhances the photocatalytic performance of mixed-dimensional CdS/MoS2 photocatalysts

As an effective semiconductor catalyst, cadmium sulfide (CdS) is used in the field of split water hydrogen evolution because of its suitable band gap (~2.4 eV), good photocatalysis activity. However, the rapid recombination of photogenerated electron–hole pairs limits the application of CdS in the field of catalytic hydrogen evolution. Here, we synthesize a CdS/MoS₂ mixed-dimensional heterojunction by a simple hydrothermal method. In this process, CdS nanoparticles were supported on MoS₂ nanosheets, where MoS₂ acts as a loading platform and co-catalyst to improve the photocatalytic performance of CdS. A series of characterizations confirmed that CdS nanoparticles with a size of approximately 130 nm were uniformly grown on the surface of MoS₂ nanosheets. Photoelectrochemical (PEC) tests show that this CdS/MoS₂ mixed-dimensional heterojunction has enhanced photocatalysis hydrogen evolution activity, which is due to the CdS/MoS₂ mixed-dimensional heterojunction can promote the separation of photogenerated electron–hole pairs and positive synergy of MoS₂ nanosheets as a co-catalyst. In addition, a new electrically assisted method is used to enhance the photocatalytic activity, and the photocatalytic performance of the CdS/MoS₂ mixed-dimensional heterojunction is improved by nearly four times when a voltage of 0.6 V is applied. This phenomenon is attributed to the fact that providing an appropriate voltage can further promote the separation of photogenerated electron–hole pairs and rapid carrier mobility, thereby effectively improve the photocatalytic activity of the photocatalyst. This work provides a good guiding significance for the design and construction of high-performance hydrogen evolution catalysts.     

Constructing magnetically recoverable CdS/CaFe2O4 P–N heterojunctions for enhanced photoelectrochemical properties towards H2 evolution reaction

A novel CdS/CaFe₂O₄ (CS/CFO) heterogeneous p-n junction was created by thermal deposition of CaFe₂O₄ nanoparticles on CdS rods. The CS/CFO hetero-structured photocatalysts exhibited increasingly efficient visible light harvesting compared to the bare CdS. The CS/CFO composites also presented higher photocurrent and slower decay of photoluminescence, suggesting a better separation of the photo-generated electrons and holes. The photocatalytic H₂ evolution quantity on the optimized CS/CFO composite from water in the presence of ethanol was up to 2200 μmol after 3-h visible light illumination, which is more than twice that of the pristine CdS. The chemical interaction between CdS and CaFe₂O₄ was confirmed by the shifts in the XPS peaks, which made it possible for the charge carriers to transfer across the p-n junction interface. This research highlights the importance of forming an interfacial p-n heterojunction between two semiconductors for efficient charge separation and improved photocatalytic performance.     

Hydroxy Acid-Assisted Synthesis of Highly Dispersed Ni-NiS on CdS as Effective Photocatalyst for Hydrogen Evolution

To achieve the well-dispersed Ni–NiS dual-cocatalysts anchored CdS, the samples have been successfully constructed by a cheap and convenient method of hydroxy acid assisted hydrothermal method. Based on the coordination and reduction effects of hydroxy acids, Ni²⁺ can be facilely transformed into the high dispersed dual-function sites of Ni⁰ electrons trap and NiS holes reservoir. The highly dispersed Ni–NiS dual-cocatalysts not only provide more dual-function active sites but also present distinctly enhanced visible light absorption, effectively separated electron hole pairs and quickly migrated charge carriers. The optimized Ni–NiS/CdS–CA presented an excellent photocatalytic H₂ generation rate of 57.88 mmol·h^?1·g^?1, which is about 15.35 times higher than that of NiS/CdS. Moreover, the stability can be distinctly increased by modulating the surface cover of Ni–NiS with a suitable Ni/(Ni?+?Cd) atomic ratio. This work would provide a unique strategy to design the high effective photocatalysts with high dispersed bi-function dual cocatalysts.

Graphic Abstract

The well-dispersed Ni-NiS dual-cocatalysts anchored CdS in situ have been successfully constructed via the coordination and reduction effects of hydroxy acid assisted hydrothermal method. Ni-NiS/CdS-CA not only presents dual-function active sites but also exhibits distinctly enhanced visible light absorption, effectively separated electron hole pairs and quickly migrated charge carriers, resulting in a remarkable enhancement in photocatalytic H2 evolution activity.

     

Rational design of MoS2/g-C3N4/ZnIn2S4 hierarchical heterostructures with efficient charge transfer for significantly enhanced photocatalytic H2 production

The rational design of hierarchical heterojunction photocatalysts with efficient spatial charge separation remains an intense challenge in hydrogen generation from photocatalytic water splitting. Herein, a noble-metal-free MoS₂/g-C₃N₄/ZnIn₂S₄ ternary heterostructure with a hierarchical flower-like architecture was developed by in situ growth of 3D flower-like ZnIn₂S₄ nanospheres on 2D MoS₂ and 2D g-C₃N₄ nanosheets. Benefiting from the favorable 2D-2D-3D hierarchical heterojunction structure, the resultant MoS₂/g-C₃N₄/ZnIn₂S₄ nanocomposite loaded with 3 wt% g-C₃N₄ and 1.5 wt% MoS₂ displayed the optimal hydrogen evolution activity (6291 μmol g^?1 h^?1), which was a 6.96-fold and 2.54-fold enhancement compared to bare ZnIn₂S₄ and binary g-C₃N₄/ZnIn₂S₄, respectively. Structural characterizations reveal that the significantly boosted photoactivity is closely associated with the multichannel charge transfer among ZnIn₂S₄, MoS₂, and g-C₃N₄ components with suitable band-edge alignments in the composites, where the photogenerated electrons migrate from g-C₃N₄ to ZnIn₂S₄ and MoS₂ through the intimate heterojunction interfaces, thus enabling efficient electron-hole separation and high photoactivity for hydrogen evolution. In addition, the introduction of MoS₂ nanosheets highly benefits the improved light-harvesting capacity and the reduced H₂-evolution overpotential, further promoting the photocatalytic H₂-evolution performance. Moreover, the MoS₂/g-C₃N₄/ZnIn₂S₄ ternary heterostructure possesses prominent stability during the photoreaction process owing to the migration of photoinduced holes from ZnIn₂S₄ to g-C₃N₄, which is deemed to be central to practical applications in solar hydrogen production.     

Constructing 0D/2D Z-Scheme Heterojunction of CdS/g-C3N4 with Enhanced Photocatalytic Activity for H2 Evolution

0D/2D Pt-C₃N₄/CdS heterojunction photocatalyst were fabricated with CdS quantum dots interspersed on g-C₃N₄ nanosheets via successive ionic layer absorption process. The obtained Pt-C₃N₄/CdS Z-scheme heterojunction with Pt cocatalyst deposited on g-C₃N₄ nanosheets exhibited H₂ production rate of 35.3 mmol g^?1 h^?1, which is 3.1 times higher than that of Pt-CdS/C₃N₄. The enhanced photocatalytic activity are attributed to the Z-scheme charge carrier transfer mechanism with stronger redox ability. The photocatalytic mechanism of the CdS/g-C₃N₄ composite is investigated and demonstrated in this work. It may provide unique insights to design 0D/2D Z-scheme heterojunction photocatalyst systems using a facile method for highly efficient H₂ production.

Graphic Abstract

Schematic illustration of charge transfer modulated by the metal cocatalyst selective deposition on heterojunction-type II (a) and direct Z-Scheme mechanisms (b) over the C₃N₄/CdS heterostructure composites under visible light irradiation.

     

Improved H2-generation performance of Pt/CdS photocatalyst by a dual-function TiO2 mediator for effective electron transfer and hole blocking

Surface modification with noble metal cocatalysts was proved to be a useful route for boosting photocatalytic efficiency of various photocatalysts. Nevertheless, considering the random dispersion of metallic cocatalysts on the photocatalyst surface, the noble metal-loaded photocatalyst generally shows a limited enhancement of its activity because the noble metals can also work as the recombination sites of photoinduced charges. In this paper, TiO₂ as a dual-function mediator (for effective electron transport and hole block) is successfully introduced into the interface of Pt and CdS to form PtTiO₂/CdS photocatalyst, with an aim of suppressing the high recombination rate of electron-hole pairs on the Pt active sites. Under visible light, all the prepared PtTiO₂/CdS displayed distinctly enhanced photocatalytic hydrogen-generation performance and the PtTiO₂/CdS(8%) attains the highest photocatalytic H₂-production rate (294.2?μmol/h), a value significantly higher than that of Pt/CdS about 3.2 time. A dual-function TiO₂-mediated mechanism was put forward to account for the superior hydrogen production of PtTiO₂/CdS photocatalyst, namely, the TiO₂ layer in the PtTiO₂/CdS not only works as electron-transport layers to effectively transfer photogenerated electrons to promote the H₂-production reaction on Pt cocatalysts, but also acts as hole-block layer to prevent the possible recombination of photogenerated charges on the Pt active sites, resulting in a distinct improvement of final H₂-generation activity.     

Multi-layered mesh-like MoS2 hierarchical nanostructure fabricated on Ti foil: An efficient noble metal-free photocatalyst for visible-light-driven H2 evolution from water

In the present work, multi-layered mesh-like MoS₂ hierarchical nanostructure was fabricated on a Ti foil in a hydrothermal process. Meanwhile, photocatalytic H₂ evolution from water over the as-prepared MoS₂ hierarchical nanostructure was investigated under visible irradiation. The results indicate that the as-prepared MoS₂ hierarchical nanostructure consists of the vertically grown few-layers MoS₂ nanosheets. And this three-dimensional mesh-like MoS₂ hierarchical nanostructure possesses high photocatalytic activity for visible-light-driven H₂ evolution from water. A rate of H₂ evolution of approximately 240 μmol g^− 1 h^− 1 was achieved under optimal conditions. Furthermore, the photocatalytic mechanism was preliminarily discussed.     

High-performance piezocatalytic hydrogen evolution by (Bi0.5Na0.5)TiO3 cubes decorated with cocatalysts

Piezocatalysis has great potential in the field of new energy. Developing robust piezocatalyst with high hydrogen evolution reaction (HER) performance is of great significance but still challenging. Here, the hydrothermal-synthesized (Bi_0.5Na_0.5)TiO₃ (BNT) cubes are demonstrated to the excellent piezocatalytic HER performance. Based on the strong piezoelectric property, the BNT cubes allow an efficient water splitting by the action of ultrasonic vibration, with a H₂ yield of 0.38 mmol g⁻¹ h⁻¹ and a H₂O₂ yield of 0.25 mmol g⁻¹ h⁻¹. To further improve the piezoelectric HER efficiency, Ag and CoO_x nanoparticles are employed as the cocatalysts through photochemical decoration. Consequently, the HER performance of BNT cubes is significantly enhanced. These cocatalysts provide the unique channels for charge transfer, thus increase the charge separation efficiency, thereby enhancing the piezocatalytic HER performance. This work not only demonstrates the eco-friendly BNT is a promising and superior piezocatalyst for new-energy applications, but also provides a rational strategy to enhance the piezocatalytic performance.     

Cd12Ge17B8O58: A bulk borate material capable of photocatalytic H2 evolution from pure water

It is quite interesting to develop photocatalytic applications among borate materials, in addition to their applications in optics. The diversity in structural chemistry may provide suitable electronic structures, possessing low e⁻-h⁺ recombination rate. With this rationale, polycrystalline Cd₁₂Ge₁₇B₈O₅₈ was unraveled as a new efficient photocatalyst for H₂ evolution in pure water. The calculated electronic structure suggests different effective masses of e⁻ and h⁺, facilitating the anisotropic migration of charges. Various cocatalysts (Ag, Au, Pd, Pt, CuO_x, RuO_x, NiO_x) were applied to significantly enhance the activity, and the optimal H₂ evolution rate in pure water achieved 163 μmol/h/g.     

Sonochemical preparation and photocatalytic properties of CdS QDs /Bi2WO6 3D heterojunction

Hierarchical CdS quantum dots (QDs)/ Bi₂WO₆ three-dimensional (3D) heterojunction photocatalyst was successfully synthesized by a facile green ultrasonic method for the first time. Photocatalytic activities under visible light irradiation were tested by the degradation of Rhodamine B (RhB) and tetracycline hydrochloride (TC), and the reduction of Cr(VI) in aqueous solution. As compared to pure CdS and Bi₂WO₆, CdS QDs/ Bi₂WO₆ heterojunctions manifested a significantly enhanced photocatalytic activity for these treatments. When the effect of the mass ratio of CdS QDs to Bi₂WO₆ was investigated, 3% CdS QDs/ Bi₂WO₆ heterojunction showed the highest photocatalytic efficiency: the efficiency for RhB degradation was 94.5% for 30?min and this value was about 6 times and 1.5 times higher than those of pure Bi₂WO₆ and CdS QDs. This enhancement was majorly accredited to the synergetic effect between Bi₂WO₆ and CdS QDs, which included intimate contact and matched band gap potentials between 0D CdS QDs and 3D Bi₂WO₆, which contributed to the efficient electron-hole separation and fast transfer of charge carriers between CdS QDs and Bi₂WO₆. A possible Z-scheme photocatalytic mechanism was proposed, in which the sample was provided with the efficient charge transfer pathway and was endowed with excellent oxidation and reduction ability.     

P-doped MoS2/Ni2P/Ti3C2Tx heterostructures for efficient hydrogen evolution reaction in alkaline media

By combining the advantages of doping to change the electronic structure of molybdenum disulfide (MoS₂), transition metal phosphides, and MXene, we proposed the idea of designing and preparing a new type of composite material, P-doped MoS₂/Ni₂P/Ti₃C₂T_x heterostructures (denoted as P@MNTC), to serve as the hydrogen evolution reaction (HER) catalyst of electrochemical water splitting. The as-prepared P@MNTC heterostructures show a significant HER activity with an overpotential of 120 mV at 10 mA cm^–2 in alkaline electrolyte, with decreasing 105 and 125 mV compared with those of MoS₂ and MXene, respectively. The density functional theory indicates that the P doping and synergy effect of Ti₃C₂T_x can enhance the activation of MoS₂ and thus promote dissociation and absorption of H₂O during HER process. This strategy provides a promising way to develop high-efficiency MoS₂- and Ti₃C₂T_x-based composite catalysts for alkaline HER.     

Hydrothermal synthesis of CdS/CoWO4 heterojunctions with enhanced visible light properties toward organic pollutants degradation

Effective solar energy harvesting and charge carrier separation are two key factors of the photocatalysis system. In this work, the heterojunction photocatalyst of CdS/CoWO₄ was fabricated by a facile hydrothermal method. Compared with the pristine CdS and CoWO₄, the CdS/CoWO₄ heterojunction photocatalyst showed enhanced photocatalytic activity for the methylene blue (MB) degradation under visible light irradiation. Particularly, the sample with molar ratio of CdS:CoWO₄ (sample C2) controlled at 3:5 showed the highest MB degradation ratio (83%) in 1 h among all samples, which is about 3 times over the pure CdS and 8 times over pure CoWO₄, respectively. The greatly enhanced photocatalytic activity (3–8 times) of CdS/CoWO₄ is due to the efficient separation of electron-hole pairs by the heterojunction structure and strong visible light absorption of CdS. This work provides a new insight into the application of tungstate-based heterojunction photocatalysts in environmental remediation.     

Heterostructure-based 3D-CdS/TiO2 nanotubes/Ti: Photoelectrochemical performances and interface simulation investigation

Heterojunction can effectively improve the charge separation efficiency and facilitate electron transfer, producing a strong photoelectric signal. By using 3D-TiO₂ nanotubes/Ti foil as support, CdS–TiO₂ heterojunction electrodes with different CdS proportions were fabricated as photoelectrochemical (PEC) biosensor to respond the visible irradiation and improve the PEC performance of TiO₂ nanotubes. Density functional theory (DFT) simulation was conducted to clarify the PEC process of CdS–TiO₂NTs and revealed the important role of CdS in enhancing electron–hole separation on TiO₂ nanotubes. Owing to the 3D tubular structure of the support, 2 mM CdS–TiO₂ nanotubes/Ti PEC electrode exhibited low detection limit of 0.27 μM and good sensitivity of 328.87 μA mM^?1 cm^?2 for glucose in the range of 2–9 μM under visible illumination. The fabricated CdS–TiO₂ nanotubes/Ti biosensor also showed high selectivity and good stability, which indicated a new candidate for biosensors.     

Efficient H2 production on CoSx/Zn0.3Cd0.7S heterojunction by selective oxidation of p-chlorobenzyl alcohol in organic phase

Developing high-efficient photocatalysts for H₂ production is still the focus of current research. A series of CoS_x/Zn_0.3Cd_0.7S (CoS_x/ZCS) samples with different mass ratios were successfully synthesized for H₂ production by the oxidation of p-chlorobenzyl alcohol (Cl-PhCH₂OH). Herein, Zn_0.3Cd_0.7S (ZCS) solid-solution nanoparticles are distributed on the surface of CoS_x polyhedron, and CoS_x is obtained from ZIF-67, composed of Co_1−xS and CoS₂. The photocatalytic H₂ production was performed by Cl-PhCH₂OH oxidation in N,N-Dimethylformamide (DMF) solution under visible light irradiation, and 5% CoS_x/ZCS sample showed the best photocatalytic activity, with the H₂ generated rate high to 2.80 mmol g⁻¹ h⁻¹. Meanwhile, the selectivity of Cl-PhCH₂OH oxidation to p-chlorobenzaldehyde (Cl-PhCHO) is 97.6%, and the rate is 2.95 mmol g⁻¹ h⁻¹. Moreover, the reasons for the excellent photocatalytic performance of CoS_x/ZCS samples were deeply analyzed by experimental and theoretical results, such as trapping agents, photoelectrochemistry test, electron paramagnetic resonance (EPR), specific surface area, and density functional theory (DFT) calculation. This work provides a new theoretical guidance and experimental experience for the application of H₂ application and design of novel photocatalysts.     

Novel Ag2S/ZnS/carbon nanofiber ternary nanocomposite for highly efficient photocatalytic hydrogen production

Novel Ag₂S/ZnS/carbon nanofiber (CNF) ternary nanocomposite with high photocatalytic H₂ production performance was synthesized by combination of an in-situ solid-state process and a cation-exchange reaction, using organic–inorganic layered zinc hydroxide nanofibers as precursor. Moreover, the loading amount of Ag₂S nanocrystals can be readily regulated by changing the AgNO₃ concentration, and the optimized H₂ production rate was 224.9 μmol h^− 1, significantly higher than that of the reported ZnS-based composite photocatalysts. The synergistic effect of CNF and Ag₂S as water reduction and oxidation cocatalyst, respectively, can greatly suppress the charge recombination thus resulting in high photocatalytic H₂ production activity.     

Nanoflower-like CdS and SnS2 loaded TiO2 nanotube arrays for photocatalytic wastewater treatment and hydrogen production

In this work, nanoflower-like CdS/SnS₂/TiO₂ NTs ternary heterojunction photocatalysts were synthesized by a hydrothermal method, the relationship between the morphology, microscopic morphology, crystallinity, elemental presence state and hydrogen production performance of the ternary photocatalysts were investigated by SEM, TEM, XRD and XPS, respectively. The photocatalytic performance, electrochemical property and hydrogen production capacity of CdS/SnS₂/TiO₂ NTs were compared with pure TiO₂ NTs, CdS/TiO₂ NTs and SnS₂/TiO₂ NTs. After 2 h of photocatalytic reaction, the removal efficiency of MB wastewater reached 100%, and the photocatalytic efficiencies toward RhB and Cr(VI) removal reached 86.08% and 80.93% after 3 h, respectively. The electron spin resonance (ESR) technique certified the active radical groups that played a role in the catalytic process and further investigated the possible photocatalytic mechanism. Hydrogen production per unit time achieved 97.14 μmol h^?1 cm^?2, this work provides the new technique to achieve solar energy conversion for hydrogen generation.     

A high H2 evolution rate under visible light of a CdS/TiO2@NiS catalyst due to a directional electron transfer between the phases

The photocatalytic activity of CdS can be greatly improved by co-modification of NiS and TiO_2 materials; furthermore the order of connection affects much. A directional electron transfer route via CdS → TiO_2→ NiS is found crucial to the enhancement of ternary catalyst, where TiO_2 acts as an electron reservoir and Ni S works as an effective cocatalyst. Cd S/TiO_2@Ni S with Ni S loaded on TiO_2 has an activity of H_2 evolution 2.5 times higher than NiS@Cd S/TiO_2 with Ni S pre-loaded on Cd S. Faster e-/h+separation rates is obtained of Cd S/TiO_2@Ni S under visible light than under extra UV light irradiation, which in turn demonstrates the importance of directional electron transfer route.     

Oriented rutile TiO2 nanorod arrays for efficient quantum dot-sensitized solar cells with extremely high open-circuit voltage

TiO₂ nanoparticles are typically employed to construct the porous films for quantum dot-sensitized solar cells (QDSCs). However, undesirable interface charge recombination at grain boundaries would hinder the efficient electron transport to the conducting substrate, giving rise to the decline of open-circuit voltage (V_oc). In this work, vertically aligned architectures of oriented one-dimensional (1D) TiO₂ nanorod arrays hydrothermally grown on substrates pave a way in designing highly efficient QDSCs with efficient radial-directional charge transport. SEM, TEM, XRD, and Raman spectroscopy were employed to characterize the as-prepared TiO₂ nanorods, showing the rutile phase with single-crystalline structure. The homogeneous deposition of CdS/CdSe QDs on the surface of TiO₂ nanorods has been achieved by in-situ grown strategies (i.e., successive ionic layer absorption and reaction, and chemical bath deposition). An extremely high V_oc value up to 0.77 V has been achieved for CdS/CdSe QDSCs based on the well-ordered 1D nanorod arrays. To the best of our knowledge, it is the highest V_oc reported for TiO₂-based QDSCs. Dependencies of photovoltaic performance, optical absorption, and interfacial charge behavior on the length of nanorods were systematically investigated. A 1.7 μm nanorod-array photoelectrode-based QDSC delivers a remarkable power conversion efficiency up to 3.57% under simulated AM 1.5 100 mW cm⁻² illumination, attributed to the balance of competition between the increase of QD loading and suppression of interfacial recombination. This work highlights the combination of QDs with high absorption coefficient 1D architectures possessing efficient charge transport for constructing high efficiency solar cells.