Photoelectrochemical (PEC) water splitting powered by solar radiation offers a promising way to produce hydrogen in an environment friendly manner. However, its efficiency is still limited by slow charge separation and low light absorption in standard metal oxides like TiO2. Engineering semiconductor heterojunctions has emerged as an effective strategy to overcome these limitations by broadening spectral absorption, optimizing band alignment, and enhancing interfacial charge transfer. This work reports the fabrication of SnS2/TiO2 and ZnS/TiO2 nanotube (NT) heterojunctions, achieved via a scalable chemical vapor deposition (CVD) and magnetron sputtering approach. Structural analyses using X-ray diffraction and Raman spectroscopy confirmed the crystalline integrity of the SnS2 and ZnS overlayers. Field-emission scanning electron microscopy characterization revealed consistent decoration of sulfide nanoflakes on the TiO2 NT framework. X-ray photoelectron spectroscopy elucidated the chemical composition and electronic states of the heterojunction. High-resolution transmission electron microscopy reveals the heterojunctions' high crystallinity and well-defined interfaces. The PEC measurements in 0.5 M Na2SO4 electrolyte under simulated sunlight having intensity of 100 mW/cm2, revealed enhanced photocurrent densities for both SnS2/TiO2 (∼136 μA/cm2) and ZnS/TiO2 (∼256 μA/cm2) heterojunctions relative to bare TiO2 NT (∼70 μA/cm2), with ZnS/TiO2 exhibiting the improvement of nearly 4-fold at 1.4 V vs RHE. Along with the increased density of catalytically active surface sites provided by the sulfide layers, the enhancement is attributed to the formation of type-II band alignment at the heterojunction, which enables effective photogenerated charge separation and transportation. These findings underscore the potential of CVD-derived metal sulfide/TiO2 heterojunctions as cost-effective, large-area photoanodes for next-generation solar water oxidation technologies.

Green hydrogen (H2) generation via photoelectrochemical (PEC) water splitting has emerged as a sustainable and carbon–neutral route for producing clean fuel, simultaneously providing a direct pathway for harnessing solar energy beyond conventional fossil resources. Among potential photoanodes, titanium dioxide (TiO2) remains attractive owing to its excellent chemical robustness and photostability; however, its intrinsically wide band gap severely restricts visible light absorption and thereby limits PEC efficiency. To circumvent this drawback, rational construction of heterostructures by coupling TiO2 with narrow-bandgap semiconductors has gained considerable attention as an effective strategy to broaden the absorption window and facilitate interfacial charge separation. In the present work, we demonstrate the fabrication of hierarchical TiO2 nanotubes (TNTs) functionalized with ZnSe and PtSe2 layers to synergistically enhance PEC activity. Comprehensive structural and surface analyses, including transmission electron microscopy and X-ray photoelectron spectroscopy, unequivocally confirm the successful formation of PtSe2/ZnSe/TNTs hybrid architectures. The hybrid photoanode delivers an enhanced photocurrent density of 430 µA/cm2 in 0.5 M Na2SO4, nearly six times higher than bare TNTs, attributed to efficient charge separation via a dual type-II heterojunction. The fabricated photoanodes also exhibits stable PEC performance for 5 h under continuous light illumination. These results underscore the efficacy of the PtSe2/ZnSe/TNT hybrid architecture as a robust platform for advancing solar-driven water splitting, highlighting its potential for integration into next-generation solar fuel conversion systems.

Engineering efficient heterojunction interfaces remains a promising route to unlock enhanced photoelectrochemical (PEC) water splitting systems. In this work, we report the strategic integration of layered tantalum diselenide (TaSe2) with narrow band gap Bi2Se3 and wide band gap ZnSe to construct novel heterojunctions. A polycrystalline TaSe2 film was directly grown on flexible Ta metal foil using chemical vapor deposition, which served as a robust and conductive scaffold. X-ray diffraction and X-ray photoelectron spectroscopy analyses confirmed the successful formation of the desired heterojunctions and elucidated their crystalline, chemical, and electronic states. PEC studies demonstrated a substantial enhancement in photocurrent density upon heterojunction formation. The ZnSe/TaSe2 heterojunction exhibited a markedly enhanced photocurrent density of similar to 252.3 mu A/cm(2) at 1.4 V vs. RHE, representing nearly six-fold improvement relative to bare TaSe2 (similar to 42.7 mu A/cm(2)) and similar to 1.8-fold enhancement compared to the Bi2Se3/TaSe2 counterpart (similar to 145.3 mu A/cm(2)). This improvement is attributed to synergistic interfacial interactions at the ZnSe/TaSe2 junction, including effective charge separation and increased density of active sites. The superior conductivity of TaSe2 further facilitates rapid carrier transport and minimizes recombination losses. This study highlights the electrochemical versatility of TaSe2-based heterojunctions and establishes a rational design framework for constructing efficient photoelectrodes for solar fuel generation on scalable and flexible substrates.

The present study investigates the integration of charge density wave material TiSe2 pyramids grown on Ti metal foil with wide bandgap ZnSe film for sunlight-driven water-splitting. X-ray diffraction and X-ray photoelectron spectroscopy analysis verified the excellent crystallinity and successful fabrication of the ZnSe/TiSe2 pyramids heterostructure. The photoelectrochemical measurements were performed in 0.5 M Na2SO4 electrolyte under AM = 1.5 G solar condition, demonstrating ZnSe/TiSe2 exhibited photocurrent density of 175.2 µA/cm2 at 0.8 V vs Ag/AgCl. The enhanced photocurrent density is attributed to the excellent electrical conductivity between ZnSe and TiSe2, increased catalytic sites, and rapid charge carrier separation and migration due to favorable band alignment between ZnSe and TiSe2. These results underscore the potential of TiSe2-based heterostructure in optimizing solar-to-hydrogen conversion and offer insights into band engineering for improving PEC performance.

The effects of 65 keV Si ion beam implantation on hematite (alpha-Fe2O3) film are reported. Grazing incidence X-ray diffraction (GI-XRD) patterns confirm a higher degree of displacement of atoms at the film surface and stimulated disordered lattice structure. The film implanted with higher Si fluence (1 x 10-16 ions cm-2) shows a higher degree of surface amorphization compared to the film implanted with lower Si fluence (1 x 10-15 ions cm-2). The results of GI-XRD and Raman spectra indicate the oxidization of implanted Si ions to form SiO2 at the surface structure. The analysis of Rutherford backscattering (RBS) spectra suggests that the implanted Si ions penetrated deep from the surface and deposited inside the films. The RBS spectra indicate the formation of nonstoichiometric oxides of (Fe/Si)O at the interface of the film and the substrate. The optical absorbance spectra show fluence-independent nature in the UV region, whereas the absorbance varies with fluence in the visible region. The presence of multiple bandgaps (2.41-2.43 and 4.69-4.77 eV) indicates multiple layers/interfaces within the films. In turn, varied refractive indices (1.95-2.57) indicate the material's potential in optoelectronic devices and optical filters.

Gallium nitride (GaN) nanostructures are highly promising for photoelectrochemical (PEC) water splitting due to their excellent electron mobility, chemical stability, and large surface area. However, the wide bandgap ( 3.4 eV) of GaN limits its ability to absorb a broad spectrum of solar radiation, restricting its PEC performance. To address this limitation, MoS2/GaN nanorods (NRs) heterostructures for enhanced PEC applications were fabricated on thin tungsten foil using a combination of atmospheric pressure chemical vapor deposition (CVD) and laser molecular beam epitaxy (LMBE). The Raman spectroscopy and X-ray diffraction revealed the hexagonal phase of GaN and MoS2. X-ray photoelectron spectroscopy examined the electronic states of the GaN and MoS2. PEC measurements revealed that the MoS2-decorated GaN NRs exhibited a photocurrent density of approximately172 μA/cm2, nearly 2.5-fold compared to bare GaN NRs ( 70 μA/cm2). The increased photocurrent density is ascribed to the Type II band alignment between MoS2 and GaN, which promotes effective charge separation, the decrease in charge transfer resistance, and the increase in active sites. The findings of this work underscore that the CVD and LMBE technique fabricated MoS2/GaN heterostructures on W metal foil substrate can provide the vital strategy to raise the PEC efficiency toward solar water splitting.

This study investigates the dark and ultraviolet (UV) light-illuminated cold field emission (CFE) characteristics of self-assembled GaN nanocolumn network (NCN) synthesized over flexible titanium (Ti) metal foil substrate using a laser molecular beam epitaxy technique. UV-light illumination has led to an increase in CFE current, causing a decrease in the turn-on field from 2.3 to 1 V/mu m at a current density of 10 mu A/cm2 under 31.32 mW/cm2 of light power. Further, a 6.2-fold enhancement of the field-emission current density, that is from 136 to 844 mu A/cm2, is obtained at an electric field of 9 V/mu m. The results reveal the potential of GaN NCN as light-triggered field-emission switches in flexible nano optoelectronics and pulsed electron stream applications.

Group 10 transition metal dichalcogenide PtSe2 has drawn considerable attention toward the search for an efficient visible light photocatalyst due to its strong light–matter interaction nature. Here we report PtSe2/TiO2 heterostructure for solar energy conversion through photoelectrochemical (PEC) water splitting. The PtSe2/TiO2 heterostructure was prepared by sputtered Pt film followed by the chemical vapor deposition method. Raman spectroscopy and X-ray diffraction analysis confirmed the growth of hexagonal phase PtSe2 and anatase TiO2 corresponding to their distinguished peak characteristics. The field-emission scanning electron microscopy and transmission electron microscopy studies reveal that TiO2 nanotubes were decorated with PtSe2 nanoflakes forming PtSe2/TiO2 heterostructure. The chemical composition of the prepared samples was studied by using the X-ray photoelectron spectroscopy and indicated the sole presence of PtSe2 and TiO2 compounds. The PEC measurements were taken in 0.5 M Na2SO4 electrolyte solution under simulated sunlight (AM = 1.5 G). The maximum photocurrent density of 234.7 µA/cm2 at 1.4 V versus RHE was recorded for PtSe2/TiO2 heterostructure, which is 1.7 times higher than that of Pt/TiO2. The improved PEC performance of PtSe2/TiO2 as a result of efficient solar absorption over a wider spectrum and effective charge separation produced by type-II band alignment between PtSe2 and TiO2.

The present work shows the improved humidity sensing characteristics of TiO2 nanoparticles in the form of a nanocomposite with multiwalled carbon nanotubes (MWCNTs) prepared by a hydration-dehydration method. The structural and morphological characterizations of TiO2-MWCNTs confirm the nanocomposite formation without any other impurities and with an improved surface area. The pure TiO2 and nanocomposite films are deposited on IDE coated flexible poly-ethylene terephthalate (PET) substrates by a drop casting method. The nanocomposite shows improved sensitivity (1246.2 MΩ/%RH) and an ultrafast response/recovery time (2 s/1 s) with a minimal hysteresis of 0.27%RH. Further, the flexible nanocomposite sensor is tested for human healthcare applications including respiratory monitoring, apnea like situations, and skin moisture detection. The sensor can distinguish different breath patterns like normal, fast, deep and apnea like situations significantly. Skin moisture detection can also be performed using the nanocomposite sensor in a non-invasive manner. Overall, this study represents an environmentally friendly, easy to fabricate, flexible TiO2-MWCNT nanocomposite based improved humidity sensor for application in human healthcare and wearable devices.

Optoelectronic properties of GaN are underexplored on good lattice matching SrTiO3 (STO) due to STO's instability at the high GaN growth temperatures (800-1100 degrees C) required by traditional techniques. Here, GaN is grown on STO (100) at lower temperatures (500, 600, and 700 degrees C) using the laser-assisted molecular beam epitaxy (LMBE) technique, and their morphological, crystalline, optical, and photodetection properties are analyzed. Further, heterojunction of Bi2Se3 thin film (bandgap of 0.3 eV) is formed on the highest photo-responsive LMBE-GaN/STO to fabricate a self-powered broadband photodetector. The fabricated self-powered heterojunction photodetectors device exhibits a high responsivity of 2.93 x 10(2) mAW(-1) in ultraviolet region and a notable responsivity of 2.3 and 12 mAW(-1) in visible and near-infrared spectral regions, respectively. In addition, photoresponse properties of fabricated devices on bare LMBE-GaN and its heterojunction are compared under UV light illumination. The photoresponsivity of heterojunction in UV region is estimated to be 3.05 x 10(4) mAW(-1), which is enhanced by 100% compared to bare LMBE-GaN. Combining the unique optoelectronic properties of GaN and rigidity of STO, epitaxy of GaN on STO enables construction of robust photodetector devices. Further, Bi2Se3-functionalized GaN can provide self-sufficient and high-quality futuristic optoelectronics devices.

In the present paper, we studied SnO2 nanostructured thin film-based resistive type humidity sensor fabricated by spin coating method on alumina substrate and showed its application in real time respiratory monitoring. Experiments show the growth of crystalline SnO2 nanoparticles with a tetragonal phase, with an average particle size of-15 nm and a high surface roughness of the order of 50-60 nm. The gold-interdigitated electrodes were patterned on the SnO2 film surface by DC sputtering technique to study the sensing response of the film. The sensor shows a high sensitivity of 1.34 k & omega;/%RH, a minimal hysteresis of 0.94%, and good repeatability. The developed sensor is tested for monitoring human breath patterns in different physical conditions, and apnea like situations, which suggests its potential for clinical applications. Overall, the present work proposes an envi-ronmental friendly, easy-to-synthesize, highly stable, and simple facile SnO2 based deployable humidity sensor to diagnose human health patterns in a non-contact manner.

We have grown a few layers MoS2 and MoSe2 on Si (100) substrates using a chemical vapor deposition (CVD) technique at different approaches and growth temperatures. Structural, surface morphology and electronic properties were investigated by Raman spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy (XPS). The MoS2 thin films were grown using three-zone CVD system by employing the MoO3 and sulfur powder precursors. We have obtained characteristics E12g and A1g Raman modes of MoS2 peaks and the separation between these two modes revealed the formation of a 2-monolayer of MoS2 on Si (100) substrate. The triangular morphology of MoS2 layer with a thickness of ∼1.4 nm i.e. nearly 2-monolayer was deduced by atomic force microscopy. Similarly, a few layers of MoSe2 were grown on Si (100) by selenization of Mo-coated Si (100) and 2H-MoSe2 characteristics Raman peaks were obtained. The XPS analysis revealed the formation of pure MoS2 and MoSe2 thin films on Si (100) substrates. The two-dimensional materials on Si (100) can be used for broadband photodetector applications.

We have demonstrated a comparative study of NO2 gas sensing behavior of reactive sputtered growth WO3−x nanocrystalline thin films and its functionalization with Ag nanoparticles (Ag−WO3−x) on Si/SiO2 substrates. X-ray diffraction and transmission electron microscope characterizations demonstrate the formation of polycrystalline monoclinic phase of porous WO3-x thin film. X-ray photoelectron spectroscopy experiments reveal that W6+ charge state has higher concentration compared with that of W4+ and W5+. The Ag−WO3−x films exhibit a sensitivity of about 70

Thin films of Bi2Se3 were deposited on Si (100) substrates using RF magnetron sputtering at a deposition temperature of 633 K. The surface morphology of Bi2Se3 films revealed the variation of grain sizes with thickness, while elemental analysis showed the stoichiometric Bi2Se3 films. The high-intensity x-ray diffraction peaks along (0003), (0006) and (00015) planes showed the dominant orientation of Bi2Se3 along the c-axis on Si (100). Three prominent Raman characteristic peaks, E2g, A11g, and A21g modes, were observed which revealed a pure hexagonal phase. X-ray photoelectron spectroscopy analysis showed the growth of high-purity Bi2Se3 films. The temperature-dependent electrical conductivity measurements showed the increase in conductivity with temperature due to the semiconducting nature. Seebeck coefficient of these films falls in the range of −153 to −236 µV/K near room temperature. These findings pave the way for economically preparing large-area and high-performance Bi2Se3 thin films for future thermoelectric devices.

The Bi2Se3 thin films of different thicknesses (100, 200 and 400 nm) were deposited on mica substrates by magnetron sputtering technique and their structural and optical properties have been studied. The X-ray diffraction analysis revealed the crystalline nature of Bi2Se3 films with preferred orientation along the c -axis. The presence of three well-pronounced Raman modes i.e. A(1)(1g) , E-2g, and A(21g) confirms the formation of pure Bi2Se3 compound on mica substrates. The field emission scanning electron microscopy and elemental analysis measurements showed the growth of Bi2Se3 film with varying grain sizes with thickness while maintaining the stoichiometric ratio of 2:3 of Bi and Se. Surface roughness was analysed using atomic force microscopy and it revealed that the roughness increases from 2.16 to 28.3 nm when the film thickness increases from 100 to 400 nm. The optical bandgap analysed with broadband absorption spectroscopy showed a decrease in bandgap from 1.3 to 1.08 eV with increasing film thickness. The sputtered Bi2Se3 films on mica possess good crystalline and structural quality with tunable bandgap that can be used for futuristic optoelectronic applications. Copyright (C) 2022 Elsevier Ltd. All rights reserved.

Thin films of Bi2Se3 were deposited on various substrates such as sapphire (0001), quartz, and GaN/sapphire (0001) using magnetron sputtering. The crystalline quality of deposited thin films was studied by high-resolution X-ray diffraction and {0003n} diffraction confirms that the films are oriented along the c-axis. Raman spectroscopy measurements revealed the pure rhombohedral phase of Bi2Se3 thin films. These films possess truncated hexagonal morphology, and elemental analysis showed stoichiometric Bi2Se3. The chemical and electronic states of the Bi2Se3 thin films were validated by X-ray photoelectron spectroscopy which revealed the formation of Bi2Se3 compound. The optical properties of the Bi2Se3 thin films were studied using broadband absorption spectroscopy and the optical bandgap of Bi2Se3 thin films (~ 150 nm) was found in the range of 1.40–1.48 eV. The Bi2Se3 thin film deposited using magnetron sputtering with good structural and optical quality will pave the way for the development of futuristic devices.

We have grown epitaxial GaN nanowalls network (NWN) on AlN buffered pre-nitridated sapphire (11–20) substrate using AlN buffer by laser-assisted molecular beam epitaxy (LMBE) technique at 700 °C. The scanning electron microscopy measurement revealed the formation of honeycomb GaN NWN having a wall width ~ 15–30 nm and pore sizes of 150–250 nm. High-resolution X-ray diffraction analysis disclosed the highly c-axis-oriented growth of hexagonal GaN structure. X-ray photoemission spectroscopy study reveals the formation of AlN buffer on pre-nitridated sapphire. A low value of tensile biaxial stress (~ 0.12 GPa) shows that GaN NWN grown on sapphire (11–20) is nearly stress free. Further, we have fabricated a metal–semiconductor–metal ultra-violet (UV) photodetector (PD) on GaN NWN and studied photo-response behaviour of UV PD device. We have obtained the photoresponsivity of 0.46 A/W at 3 V with detectivity of ~ 2.18 × 107 Jones and noise equivalent power (NEP) of ~ 3.6 × 10–10 W/Hz1/2 for the fabricated GaN NWN PD.

We have studied the crystalline, optical and photo-response properties of epitaxial GaN films grown on bare- and pre-nitridated a-sapphire substrates with different thicknesses of low temperature (LT) GaN buffer layer using laser-assisted molecular beam epitaxy (LMBE). It is found that the GaN grows epitaxially on a-sapphire along the c-axis and has low biaxial stress of ~ 0.3 GPa. Room-temperature photoluminescence spectroscopy disclosed the near band edge emission peak at ~ 3.4 eV with a line width of 100 ~ 110 meV for GaN grown on thin LT-GaN buffer, whereas an additional prominent blue luminescence emission is obtained for GaN film grown on thicker LT-GaN buffer. Metal–semiconductor-metal based ultra-violet (UV) photodetectors (PD) fabricated on GaN film grown on pre-nitridated sapphire with thin LT-GaN buffer showed a responsivity of ~ 1.35 A/W at 3 V applied bias. It is also observed that the PDs with thin LT-GaN buffer have nearly 5 times higher responsivity with a faster response than that with thick LT-GaN buffer. We found that the photoresponse characteristics of GaN-based UV PDs on a-sapphire are critically dependent on crystalline and optical defects.

We have grown various epitaxial GaN nanostructures on sapphire (11-20) substrates by tuning the buffer layer growth conditions in laser molecular beam epitaxy (LMBE) process. The pre-nitridation and buffer layer GaN growth at low temperature (LT) on sapphire (11-20) critically affect the surface morphology and structural properties. Granular GaN thin film (similar to 160 nm) was grown on pre-nitridated sapphire whereas nano-column (NC)-GaN was obtained on LT-GaN buffer layer on bare sapphire having a height of similar to 370 nm at the growth temperature of 700 degrees C. Nano-porous (NP)-GaN was obtained with pore sizes in the range of 70 similar to 110 nm having vertical height of similar to 560 nm under similar growth conditions on LT-GaN buffered pre-nitridated sapphire. In-situ reflection high energy electron diffraction, high-resolution x-ray diffraction and Raman spectroscopy measurements indicated the epitaxial growth of c-axis oriented, wurtzite crystalline GaN nanostructures on sapphire (11 similar to 20) substrate with nearly negligible biaxial stress (0.03-0.23 GPa). Further, metal-semiconductor-metal (MSM) ultra-violet (UV) photodetectors were fabricated on epitaxial GaN nanostructures. The photo responsivity studies revealed that the NP-GaN MSM device has a photoresponse of similar to 358 mA/W at an applied bias of 1V. The photo-responsivity of NP-GaN MSM device is higher than that of GaN film (similar to 36 mA/W) and NC-GaN (similar to 7 mA/W) which revealed the importance of shape and size of GaN nanostructures on the responsivity of UV-photodetector devices. These results demonstrate the capability of LMBE technique to grow different GaN nanostructures on sapphire (11-20) substrate by tuning buffer layer conditions for their application as UV-photodetectors.