Self-powered highly enhanced broad wavelength (UV to visible) photoresponse of ZnO@ZnO1−xSx@ZnS core–shell heterostructures

Study on interface engineering and chemical bonding of the ReS2@ZnO heterointerface for efficient charge transfer and nonlinear optical conversion efficiency

Rhenium sulfide (ReS2) has emerged as a promising two-dimensional material, demonstrating broad-spectrum visible absorption properties that make it highly relevant for diverse optoelectronic applications. Manipulating and optimizing the pathway of photogenerated carriers play a pivotal role in enhancing the efficiency of charge separation and transfer in novel semiconductor composites. This study focuses on the strategic construction of a semiconductor heterostructure by synthesizing ZnO on vacancy-containing ReS2 (VRe-ReS2) through chemical bonding processes. The ingeniously engineered built-in electric field within the heterostructure effectively suppresses the recombination of photogenerated electron-hole pairs. A direct and well-established interfacial connection between VRe-ReS2 and ZnO is achieved through a robust Zn-S bond. This distinctive bond configuration leads to enhanced nonlinear optical conversion efficiency, attributed to shortened carrier migration distances and accelerated charge transfer rates. Furthermore, theoretical calculations unveil the superior chemical interactions between Re vacancies and sulfide moieties, facilitating the formation of Zn-S bonds. The photoluminescence (PL) intensity is increased by the formation of VRe-ReS2 and ZnO heterostructure and the PL quantum yield of VRe-ReS2 is improved. The intricate impact of the Zn-S bond on the nonlinear absorption behavior of the VRe-ReS2@ZnO heterostructure is systematically investigated using femtosecond Z-scan techniques. The charge transfer from ZnO to ReS2 defect levels induces a transition from saturable absorption to reverse saturable absorption in the VRe-ReS2@ZnO heterostructure. Transient absorption measurements further confirm the presence of the Zn-S bond between the interfaces, as evidenced by the prolonged relaxation time (τ3) in the VRe-ReS2@ZnO heterostructure. This study offers valuable insights into the rational construction of heterojunctions through tailored interfacial bonding and surface/interface charge transfer pathways. These endeavors facilitate the modulation of electron transfer dynamics, ultimately yielding superior nonlinear optical conversion efficiency and effective charge regulation in optoelectronic functional materials.

Understanding of mobility limiting factors in solution grown Al doped ZnO thin film and its low temperature remedy

The solution grown doped ZnO based transparent electrode has shown great potential in future generation flexible and smart devices due to its abundance in earth, low cost, simple and low temperature synthesis process. But solution grown doped ZnO possesses one major drawback, its mobility decreases rapidly with an increase in doping concentration. To eliminate this issue, the understanding of factors that limiting mobility is a prerequisite. But till date, there are very limited resources with detailed understanding of mobility limiting factors in solution grown TCO. Here in this report, with the morphological, optical and electrical investigations, the mobility limiting factor comes out to be surface related property and assigned to be the defects related to surface adsorbed oxygen and oxygen species at the surface. Furthermore, we have modified the surface to remove the surface adsorbed oxygen species by a low temperature (70 °C) simple solution process. Surface modified sample shows more than two orders of improvement in resistivity without any significant change in the transparency in visible range.

Defect-Enriched ZnO/ZnS Heterostructures Derived from Hydrozincite Intermediates for Hydrogen Evolution under Visible Light

Introducing defect engineering into ZnO/ZnS heterojunction photocatalysts is an effective method to simultaneously improve their visible light performance and photocatalytic efficiency. Herein, a defect-enriched ZnO/ZnS heterostructure photocatalyst was synthesized through a hydrozincite [Zn₅ (OH)₆ (CO₃ )₂ ] intermediate-deriving reaction. The mechanism analysis showed that there were interstitial Zn and Zn vacancies in the hydrozincite-derived ZnO, while S vacancies and interstitial S and Zn vacancies were formed in ZnS components after calcination. These specific defect states endowed visible light response ability to both ZnO and ZnS components in the ZnO/ZnS photocatalysts. Under visible light irradiation, the photocatalytic hydrogen evolution rate of ZnO/ZnS reached 11.68 mmol h^-1  g^-1 , and under simulated sunlight irradiation, the best photocatalytic hydrogen evolution rate could reach 27.94 mmol h^-1  g^-1 , which was much higher than most previous reports. The analysis of energy band structure and photodeposition showed that the photocatalytic reduction sites were mainly on ZnS, and the photocatalytic reaction mainly followed the typical Z-type mechanism. This work presents a simple and low-cost method for the preparation of defects-enriched ZnO/ZnS-based photocatalytic materials with high photocatalytic activity and stability.

Photocatalytic Degradation of Rhodamine B Dye and Hydrogen Evolution by Hydrothermally Synthesized NaBH4—Spiked ZnS Nanostructures

Metal sulphides, including zinc sulphide (ZnS), are semiconductor photocatalysts that have been investigated for the photocatalytic degradation of organic pollutants as well as their activity during the hydrogen evolution reaction and water splitting. However, devising ZnS photocatalysts with a high overall quantum efficiency has been a challenge due to the rapid recombination rates of charge carriers. Various strategies, including the control of size and morphology of ZnS nanoparticles, have been proposed to overcome these drawbacks. In this work, ZnS samples with different morphologies were prepared from zinc and sulphur powders via a facile hydrothermal method by varying the amount of sodium borohydride used as a reducing agent. The structural properties of the ZnS nanoparticles were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques. All-electron hybrid density functional theory calculations were employed to elucidate the effect of sulphur and zinc vacancies occurring in the bulk as well as (220) surface on the overall electronic properties and absorption of ZnS. Considerable differences in the defect level positions were observed between the bulk and surface of ZnS while the adsorption of NaBH₄ was found to be highly favourable but without any significant effect on the band gap of ZnS. The photocatalytic activity of ZnS was evaluated for the degradation of rhodamine B dye under UV irradiation and hydrogen generation from water. The ZnS nanoparticles photo-catalytically degraded Rhodamine B dye effectively, with the sample containing 0.01 mol NaBH₄ being the most efficient. The samples also showed activity for hydrogen evolution, but with less H₂ produced compared to when untreated samples of ZnS were used. These findings suggest that ZnS nanoparticles are effective photocatalysts for the degradation of rhodamine B dyes as well as the hydrogen evolution, but rapid recombination of charge carriers remains a factor that needs future optimization.

Defect-Assisted Broad-Band Photosensitivity with High Responsivity in Au/Self-Seeded TiO2 NR/Au-Based Back-to-Back Schottky Junctions

TiO₂ nanorods (NRs) have generated much interest for both fundamental understanding of defect formation and technological applications in energy harvesting, optoelectronics, and catalysis. Herein, we have grown TiO₂ NR films on glass substrates using a self-seeded approach and annealed them in H₂ ambient to modify their surface defects. It has been shown that broad-band photosensing properties of Au/self-seeded TiO₂ NR/Au-based two back-to-back Schottky junctions (SJs) for a broad wavelength of light are much superior as compared to those of the pristine and the control samples. Photoresponsivity values for the H₂-annealed sample are 0.42, 0.71, 0.07, and 0.08 A/W for detecting, respectively, 350, 400, 470, and 570 nm lights. Very low dark current and high photocurrent lead to a gain value as high as 1.85 × 10⁴ for 400 nm light. Unprecedentedly modified NR-based SJs show excellent photoresponsivity for detecting as low as 25, 36, 48, and 28 μW/cm² power densities of 350, 400, 470, and 570 nm lights, respectively. It is found that Ti³⁺ defects play a key role in an efficient photoelectron transfer from TiO₂ to Au. Our work, for the first time, highlights the simplicity and reveals the rationale behind the excellent properties of Au/self-seeded TiO₂ NR film/Au back-to-back SJs.

Fabrication and characterization of distinctive ZnO/ZnS core-shell structures on silicon substrates via a hydrothermal method

A distinctive novel ZnO/ZnS core-shell structure on silicon was reported in this study. Compared with previous studies, ZnO nanorods encapsulated by 5 nm ZnS nanograins were observed using a scanning electron microscope. Furthermore, strong (111) cubic ZnS crystalline structures were confirmed using high resolution transmission electron microscopy, selected area diffraction, and X-ray diffraction. The optical properties changed and the antibacterial behaviors were suppressed as the ZnS shells were attached onto the ZnO nanorods. Moreover, the results also indicate that the hydrophobicity could be enhanced as more ZnS nanograins were wrapped onto the ZnO nanorods. The ZnO/ZnS core-shell structures in this research show promise for use in future optoelectronic and biomedical applications.