Optimal engineering of CdS/PbS co-sensitized TiO2 nanotube arrays for enhanced photoelectrochemical performance

TiO₂ nanotube arrays (NTAs) are decorated with CdS/PbS nano-sensitizers by successive ionic layer adsorption and reaction (SILAR) method. The uniform growth of the CdS and PbS nanoparticles on the surface and inner side of TiO₂ Nanotube Arrays (NTAs) has been confirmed by Transmission Electron microscopy measurements. The impact of the CdS and PbS semiconductor quantum dots (SQDs) on the photoelectrochemical performance (PEC) of TiO₂ NTAs was systematically investigated, and the optimal decoration of the CdS and PbS SQDs on the TiO₂ NTAs was obtained. CdS/PbS co-sensitized TiO₂ NTA photoanode films show excellent response to visible light (with absorption extended to 825 nm) and enhanced PEC performance. The best performing device showed an enhanced photocurrent density under the 0.62V vs SCE up to 8.2 mA/cm², and high photoconversion efficiency up to 5.35%, which is 16.7 times higher than the pure TiO₂ NTAs. The enhanced PEC performance of TiO₂ NTAs is attributed to the co-sensitization, heterojunction formation and electron “pool” effect imparted on the NTAs by the coupling of CdS and PbS SQDs.     

MoS2 co-catalyst sensitized 3D TiO2/CdS photoanodes with enhanced photoelectrochemical performances

Regulating morphology and constructing heterojunctions to enhance the light absorption and boost the separation of electrons and holes are common and effective means to boost the photoelectrochemical (PEC) performances of TiO₂ photoanodes. In this study, TiO₂ nanoflowers (NFs)/CdS quantum dots (QDs)/MoS₂ nanosheets (NSs) hybrids with two type II band alignments were synthesized by facile hydrothermal, successive ionic layer adsorption and reaction, and dipping methods, respectively. The effects of different amount of MoS₂ co-catalysts on CdS decorated TiO₂ photoanodes were investigated. TiO₂ NFs/CdS QDs/MoS₂ NSs hybrids showed dramatically enhanced PEC performance, especially under visible light illumination. The photocurrent density of TiO₂ NFs/CdS QDs/MoS₂-50 was more than 10 times higher than that of TiO₂ NFs/CdS QDs. This innovative work sheds light on efficiently improving the light absorption by forming heterojunctions and accelerating the electron and hole transfer via specific band engineering design.     

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.     

Enhancement of photocatalytic and photoelectrochemical properties of BiOI nanosheets and silver quantum dots co-modified TiO2 nanorod arrays

In this study, TiO₂ nanorod arrays (TNR), Ag quantum dots (QDs) sensitized with TNR TiO₂/Ag, bismuth oxyhalide (BiOI) nanosheets, and Ag QDs co-modified with TNR and TiO₂/BiOI/Ag (TBA) were prepared by a stepwise process. The morphological, structural, compositional, optical, photocatalytic (PC), and photoelectrochemical (PEC) properties of the samples were investigated. The TBA-2 sample exhibited the highest photocurrent density (281.8 μA/cm²) and photodegradation efficiency (93.3%), with values 9.7 times and 2.25 times higher than those for TNR, respectively. The improvement in sample performance can be attributed to the formation of a heterojunction between BiOI and TiO₂, thereby enhancing the absorption of visible light and improving the charge separation efficiency; Ag QDs limit interfacial electron-hole pair recombination. The experimental results show that TBA can effectively promote light-induced carrier transport and visible light absorption, while inhibiting the recombination rate of the electron-hole pairs, PEC, and PC.     

Iron-doped TiO2 nanotubes with high photocatalytic activity under visible light synthesized by an ultrasonic-assisted sol-hydrothermal method

A series of iron-doped anatase TiO₂ nanotubes (Fe/TiO₂ NTs) catalysts with iron concentrations ranging from 0.88 to 7.00 wt% were prepared by an ultrasonic-assisted sol-hydrothermal process. The structures and the properties of the fabricated Fe/TiO₂ NTs were characterized in detail and photocatalytic activity was examined using a reactive brilliant red X-3B aqueous solution as pollutant under visible light. The lengths of the NTs were determined to range from 20 nm to 100 nm. The incorporation of the iron ions (Fe³⁺) into the TiO₂ nanotubes shifted the photon absorbing zone from the ultraviolet (UV) to the visible wavelengths, reducing the band gap energy from 3.2 to 2.75 eV. The photocatalytic activity of the Fe/TiO₂ NTs was 2–4 times higher than the values measured for the pure TiO₂ nanotubes.     

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.     

Degradation of dye wastewater in a thin-film photoelectrocatalytic (PEC) reactor with slant-placed TiO2/Ti anode

A thin-film photoelectrocatalytic (PEC) reactor with slant-placed TiO₂/Ti anode was developed and successfully applied to degrade Rhodamine B (RhB) and textile effluent. Using a 5–150 mg L⁻¹ RhB solution as the model system, thin-film PEC removed total color and TOC by 99–28% and 78–15%, respectively, in 1 h, which is much higher than 82–7% and 60% to zero by conventional PEC. The enhanced treatment efficiency achieved by thin-film PEC process was attributed to the significantly reduced path length of irradiation light source. The wastewater was kept circulating during the experiments to timely refresh the aqueous film on the TiO₂/Ti anode and promote the mass transfer of the target pollutants and the degradation products in the bulk solution. The thin-film PEC reactor can degrade both simulated and real dye wastewater efficiently under UV light irradiation. Results suggested that thin-film PEC was particularly superior for treating a high concentration solution. The thin-film PEC reactor was also applied to treat RhB solution efficiently under solar light irradiation. The recycle experiments demonstrated excellent stability and reliability of the slant-placed TiO₂/Ti anode. This study proposed a simple and effective method to design PEC reactor applicable for industrial dye wastewater treatment.     

Hydrothermal deposition of AgCl/AgBr co-sensitizers to modify TiO2 NTs for enhanced photocatalytic performance

AgCl/AgBr co-sensitizers were prepared on TiO₂ nanotube arrays (TiO₂ NTs/AgCl/AgBr) by the hydrothermal method. The composition, morphology, optical absorption, photoelectric and photoelectrocatalytic (PEC) performances of TiO₂ NTs/AgCl/AgBr were influenced by the concentration ratio of KCl/KBr. The investigation results revealed that the TiO₂ NTs/AgCl/AgBr photocatalyst exhibited dramatically strong visible light absorption and outstanding photoelectrochemical capacity. The photoelectrode produced high visible light surface photovoltage (-0.43 V), transient photocurrent (0.47 mA/cm²) and carrier concentration (4.39 × 10²⁰ cm^-3). The sample also showed high PEC activity in the organic dye and Cr(VI) removal, and the photocatalytic mechanism and charge carrier transfer path were described based on PEC results. The investigation would offer prospective insight to the sensitization of TiO₂ NTs, which would result in extensive attraction in the preparation and application of semiconductor materials as photoelectrodes and photocatalysts with superior photoelectrochemical performances.     

Carbon-based TiO2-x heterostructure nanocomposites for enhanced photocatalytic degradation of dye molecules

Application of brown titanium dioxide (TiO_2-x) and its modified composite forms in the photocatalytic decomposition of organic pollutants in the environment is a promising way to provide solutions for environmental redemption. Herein, we report the synthesis of effective and stable TiO_2-x nanoparticles with g-C₃N₄, RGO, and multiwalled carbon nanotubes (CNTs) using a simple hydrothermal method. Among all the as-synthesized samples, excellent photocatalytic degradation activity was observed for RGO-TiO_2-x nanocomposite with high rate constants of 0.075 min^?1_, 0.083 min^?1 and 0.093 min^?1 for methylene blue, rhodamine-B, and rosebengal dyes under UV–Visible light irradiation, respectively. The altered bandgap (1.8 eV) and the large surface area of RGO-TiO_2-x nanocomposite impacts on both absorption of visible light and efficiency of photogenerated charge electron (e^?)/hole (h⁺) pair separation. This resulted in enhanced photocatalytic property of carbon-based TiO_2-x nanocomposites. A systematic study on the influence of different carbon nanostructures on the photocatalytic activity of brown TiO_2-x is carried out.     

Titania ultrafiltration membrane: Preparation,characterization and photocatalytic activity

A mesoporous photocatalytic titania (TiO₂) membrane on alumina support is successfully fabricated via the sol–gel processing method. Several techniques such as dynamic light scattering, X-ray diffraction (XRD), TGA, N₂-sorption, and SEM are utilized to investigate the optimized processing parameters and their influence on the final properties of the developed membrane. The prepared titania sol containing organic additives (HPC and PVA) has an average particle size of 55.6 nm with a narrow distribution. The resulting TiO₂ membrane with thickness of 1 μm exhibits homogeneity with no cracks or pinholes. It also maintains small pore size (4.7 nm), large specific surface area (75 m²/g), and small crystallite size (8.3 nm).The permeability and photocatalytic properties of the titania membrane were measured. The permeability coefficient of the fabricated membrane is 30.09 cm³ min^?1 bar^?1 cm^?2. These measurements indicate an optimum processing condition for the preparation of the membrane. The prepared titania membrane has a great potential in developing high efficient water treatment and reuse systems because of its multifunctional capability such as decomposition of organic pollutants and physical separation of contaminants.     

Fabrication of TiO2 nanotubes by using electrodeposited ZnO nanorod template and their application to hybrid solar cells

Vertically aligned TiO₂ nanotubes have been fabricated on the indium-doped tin oxide (ITO) by a simple and versatile technique using the electrochemically deposited ZnO nanorods, oriented along the c-axis, as a template in the spin-on based sol-gel reaction of a Ti precursor. The diameter, length, and shape of TiO₂ nanotubes were controlled by changing the initial ZnO nanorod template and the spin conditions during sol-gel process of a Ti precursor. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were used to confirm the successful formation of TiO₂ nanotubes and characterize their structure and morphology. Furthermore, as an application of the TiO₂ nanotubes, hybrid solar cells based on TiO₂ and poly[2-methoxy,5-(2′-ethyl-hexyloxy)1,4-phenylenevinylene] (MEH-PPV) were successfully fabricated.     

Absorption boost of TiO2 nanotubes by doping with N and sensitization with CdS quantum dots

A process of obtaining N-doped TiO₂ nanotubes sensitized by CdS nanoparticles is presented, including detailed characterizations performed along the synthesis. Transparent TiO₂ films consisting of nanotubes, 2.5 µm long and of ~60 nm inner diameter, were obtained after anodization of a titanium film deposited onto FTO glass substrate. N-doping was achieved by annealing of TiO₂ film in ammonia. X-ray Photoelectron Spectroscopy measurements showed that nitrogen was substitutionally incorporated in the TiO₂ matrix, with the N:Ti concentration ratio of 1:100. The doping changed the optical properties of the material in such a way that the absorption edge was shifted from 380 nm to 507 nm, as observed from diffuse reflectance spectra. The influence of the microwave (MW) irradiation on the synthesized CdS quantum dots and their optical properties was investigated. It was shown that the diameter of CdS nanoparticles was increased due to releasing of S^2- ions from dimethyl sulfoxide (DMSO) as a consequence of the MW treatment. The (N)TiO₂ films were then used as substrates for matrix assisted pulsed laser deposition of the CdS quantum dots with DMSO as a matrix. The laser parameters for the deposition were optimized in order to preserve the nanotubular structure open, the latter being an important feature of this type of photoanode. The structure obtained under optimized conditions has an additional absorption edge shift, reaching 603 nm.     

Controllable preparation of Ti/TiO2-NTs/PbO2–CNTs–MnO2 layered composite materials with excellent electrocatalytic activity for the OER in acidic media

High oxygen evolution overpotential and low corrosion resistance are the main challenges for oxygen evolution materials in acidic media. In this study, a novel composite material, Ti/TiO₂-NTs/PbO₂–CNTs–MnO₂, with high oxygen evolution electrocatalytic activity was successfully prepared. First, TiO₂ nanotubes (TiO₂-NTs) were synthesized in situ on a Ti sheet via anodization and used as an intermediate layer. Subsequently, the adhesion and conductivity of the TiO₂-NTs layer were increased through additional anodization, annealing, and electrochemical reduction. Finally, PbO₂ was electrodeposited with a constant current in a lead acetate medium and doped with carbon nanotubes (CNTs) and MnO₂. The surface morphology, phase composition, and electrochemical performance of the composite materials were investigated. Notably, in an acidic electrolyte (150 g/L H₂SO₄), Ti/TiO₂-NTs/PbO₂–CNTs–MnO₂ exhibited good stability (30 h) and a low oxygen evolution overpotential of 410 mV at 50 mA/cm², which is almost equivalent to that of precious metals (RuO₂ and IrO₂) and 499 mV lower than that of the industrial Pb–0.76 wt% Ag alloy. The outstanding performance is mainly attributed to the high aspect ratio of the TiO₂-NT structure, synergistic effects of the active particles, and inherently good electrochemical properties of the active particles. Therefore, this study provides a new synthetic route for oxygen evolution materials in acidic media.     

Z-scheme g-C3N4 nanosheet photocatalyst decorated with mesoporous CdS for the photoreduction of carbon dioxide

Herein, novel mesoporous CdS nanoparticle (NP)-incorporated porous g-C₃N₄ nanosheets with large surface areas and varying CdS NP percentages were constructed for the first time. The synergistic effect of mesoporous CdS NPs and porous g-C₃N₄ nanosheets indicated effective charge carrier separation and promoted CO₂ photoreduction to form CH₃OH upon illumination. The highest yield of CH₃OH over 3% CdS-g-C₃N₄ heterostructures was determined to be approximately 1735 μmol g^?1, which was 3.8- and 5.50 times greater than those of mesoporous CdS NPs and pristine g-C₃N₄ nanosheets, respectively. In addition, the mesoporous 3%CdS-g-C₃N₄ heterostructure showed an outstandingly enhanced CO₂ photoreduction rate of 192.7 μmol g^?1 h^?1, which was estimated to be ~4.1 and 5.9- times better than CdS (47.1 μmol g^?1 h^?1) and pristine g-C₃N₄ (32.6 μmol g^?1 h^?1), respectively. The photoreduction performance was retained at approximately 94.7% after five cycles of illumination for 45 h. The remarkable synthesized mesoporous CdS-g-C₃N₄ heterostructure played an essential role, with its narrow bandgap and high surface area enabling improved photoinduced carrier separation and a widened range of light absorption. A plausible mechanism for CO₂ photoreduction by the mesoporous CdS-g-C₃N₄ heterostructure was proposed and verified by photoelectrochemical and photoluminescence measurements.     

Synthesis and electrochemical properties of TiO2/nanodiamond nanocomposite

Undoped nanodiamond (ND) powders were coated with TiO₂ through two steps: firstly Ti lay was deposited by cycled vacuum-feeding chemical vapour deposition from gaseous TiCl₄/H₂, and secondly an oxidation treatment was carried out in air. The structure and the morphology of the TiO₂/ND composite were characterized by Raman spectrum and transmission electron microscopy. The electrochemical properties of the TiO₂/ND powder electrode in a solution containing [Fe(CN)₆]^3?/4? or NO₂^? were investigated. The results showed that a continuous coating consisting of Ti nanoparticles covered on ND particle after the deposition. The following oxidation at 500 °C resulted in the formation of anatase phase TiO₂ nanoparticles of about 10 nm. The electrochemical results confirmed that the TiO₂/ND powder electrode exhibited higher electrochemical activity than the pristine ND electrode, especially higher catalytic ability toward the oxidation of nitrite anions. Moreover, the TiO₂/ND powder electrode presented fast response towards nitrite oxidation with a detection limit of 0.55 µM and a linear range of 0.05 to 1.0 mM.     

Effect of CdS contents on H<Subscript>2</Subscript> production from Pt-(CdS/TiO<Subscript>2</Subscript>) film-typed photocatalysts

Pt-(CdS/TiO₂) film-typed photocatalysts are prepared with a doctor-blade method followed by a chemical bath deposition (CBD) process, and the films are characterized by UV-vis spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy. The film-typed structure is composed of photocatalysts and Pt metal part on a FTO substrate without additional electric device, so it is relatively simpler than the conventional photoelectrochemical cell. CdS quantum dots are introduced as a sensitizer for visible light response. Amounts of CdS quantum dots on TiO₂ surface are increased with increasing CBD cycles, but they start to aggregate after certain CdS concentration due to oversaturation phenomenon. This high CdS content induces high electron losses, and therefore it reduces amounts of hydrogen production. As a result, there is a saturation point of CdS content at Cd/Ti ratio of 0.197, and the amounts of evolved hydrogen are 5.407 μmol/cm²·h at this photocatalyst formulation.     

LaCoO3-modified RuO2–TiO2/Ti electrode as an efficient electrocatalyst for oxygen evolution reaction

A RuO₂–TiO₂/Ti electrode modified with LaCoO₃ was successfully fabricated by thermal decomposition and its electrochemical properties were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and stability tests. In contrast with an unmodified RuO₂–TiO₂/Ti electrode, the LaCoO₃–RuO₂–TiO₂/Ti electrode displays a more uniform layer with smaller microparticles. This electrode also displays higher OER performance with lower overpotential (289 mV vs. 348 mV) at a current density of 10 mA cm⁻² and lower Tafel slope (87 mV dec⁻¹ vs. 104 mV dec⁻¹) than the unmodified RuO₂–TiO₂/Ti electrode. The modified LaCoO₃–RuO₂–TiO₂/Ti electrode possesses larger current density, higher specific voltammetric charge, and lower charge transfer resistance (R_ct) than the unmodified RuO₂–TiO₂/Ti electrode does in a KOH solution, according to CV and EIS studies. The LaCoO₃–RuO₂–TiO₂/Ti electrode is very stable. The results show that the modified LaCoO₃–RuO₂–TiO₂/Ti electrode presents higher electrocatalytic activity and good stability for OER.

Graphic abstract

     

Enhanced charge separation of CuS and CdS quantum-dot-cosensitized porous TiO2-based photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using high-performance catalysts shows considerable promise in generating environment-friendly hydrogen energy. Its practical applications, however, suffer from several shortcomings, such as low photocurrent density, large onset-voltage value, and poor durability. In this study, CuS and CdS quantum-dot-cosensitized porous TiO₂-based PEC catalysts (CuS-CT) have been successfully synthesized via in situ sulfuration of CuO and CdO coexisting inside a porous TiO₂ monolith by a hydrothermal method. Compared to porous TiO₂, CuS-sensitized porous TiO₂ (CuS-TiO₂), and CdS-sensitized porous TiO₂ (CdS-TiO₂) in terms of PEC performance, the CuS-CT photoanode exhibited a significantly high anodic photocurrent for water splitting under simulated sunlight radiation. The photocurrent produced by the optimized sample of 7% CuS-5% CdS-TiO₂ (7% CuS-CT) was nearly 2.7 times higher than that of pure porous TiO₂ at 1.0 V versus a reversible hydrogen electrode (RHE). Porous TiO₂ possesses large surface areas that can drive fast electrolyte transport and afford more surface reaction active sites. On the other hand, CuS and CdS quantum dots not only broaden the visible light absorption range, but also improve photoinduced electron-hole separation efficiency. The co-sensitized multi-nanostructures photoanodes lead to a remarkable and promising application in PEC water splitting reactions.     

Preparation of CdS/TiO2 nanotube arrays and the enhanced photocatalytic property

In this paper, a series of CdS/TiO₂ NTs have been synthesized by SILAR method. The as-prepared CdS/TiO₂ NTs have been analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS), and ultraviolet–visible (UV–vis). And their photocatalytic activities have been investigated on the degradation of methylene blue under simulated solar light irradiation. XRD results indicate that TiO₂ NTs were anatase phase, CdS nanoparticles were hexagonal phase. FESEM results indicate that low deposition concentration can keep the nanotubular structures. UV–vis results indicate that CdS can be used to improve the absorbing capability of TiO₂ NTs for visible light, and the content of CdS affects the band gap. Photocatalytic results indicate that CdS nanoparticles are conducive to improve the photocatalytic efficiency of TiO₂ NTs, and the highest degradation rate can reach 93.8%. And the photocatalytic mechanism of CdS/TiO₂ NTs to methylene blue is also described.     

Fabrication of a novel ZnO–CoO/rGO nanocomposite for nonenzymatic detection of glucose and hydrogen peroxide

Herein, a novel nanocomposite of binary ZnO–CoO nanoparticles loaded on the graphene nanosheets (ZnO–CoO/rGO) has been successfully constructed via a facile, economical and two–step process. The obtained ZnO–CoO/rGO hybrids with high electrical conductivity and abundant active sites, could be modified on a glassy carbon electrode to detect glucose and H₂O₂ multi–functionally. The fabricated biosensor exhibits wide linear range for glucose (10 μM to 11.205 mM) and H₂O₂ (25 μM to 11.1 mM), and their corresponding sensitivity are 168.7 μA mM^?1 cm^?2 and 183.3 μA mM^?1 cm^?2. The limits of detection are 1.3 μM and 0.44 μM for the oxidation of glucose and the reduction of H₂O₂, respectively. Furthermore, remarkable selectivity, long–term stability and outstanding reproducibility of the non–enzyme biosensor prove that ZnO–CoO/rGO hybrids are the promising candidate in practical applications.