Synthesis and Characterization of CuSe Thin Films via Chemical Vapor Deposition for Enhanced Photocatalytic Water Splitting

http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 36 Synthesis and Characterization of CuSe Thin Films via Chemical Vapor Deposition for Enhanced Photocatalytic Water Splitting Sana Tariq (Corresponding Author) University of Management and Technology (UMT), Lahore, Punjab 54770, Pakistan Email: [email protected] Iram Shahzadi Department of Chemistry, Lahore College for Women University, Lahore, Pakistan Email: [email protected] Nadeem Jan Department of Chemistry, Beijing University of Chemical Technology, Beijing, China Email: [email protected] Syed Ameer Tahir Department of Biomedical Engineering, NED University of Engineering and Technology, Karachi, Pakistan Email: sye[email protected] One of the key technologies in the pursuit of clean hydrogen production is photocatalytic water splitting. High surface activity and appropriate band gaps are essential for this procedure in semiconductor materials. Due to its advantageous electrical structure, adjustable optoelectronic characteristics, and environmental stability, copper selenide (CuSe) stands out as a type of semiconductor. The fabrication of CuSe thin film This paper provides a detailed examination of the chemical vapor deposition (CVD), its material properties, and its effectiveness in photocatalytic water splitting applications. All of this is covered in detail in this, including doping, nanostructuring, and heterojunction engineering, offering a thorough assessment of the state-of-the-art from 2018 to 2025 as well as potential future paths. Keywords: Copper Selenide (CuSe), Thin Films, Chemical Vapor Deposition (CVD), Photocatalysis, Water Splitting, Photoelectrochemical (PEC) Hydrogen Evolution, Bandgap Engineering, Heterojunctions, Charge Carrier Separation, Semiconductor Photocathodes. http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 37 Introduction Converting to clean and renewable energy sources has emerged as a top worldwide priority due to the acceleration of climate change, the depletion of fossil fuels, and the rising need for sustainable energy. Because of its high gravimetric energy density, storability, and the fact that the only result of burning is water, hydrogen (H₂) is seen as a possible zero-carbon fuel among other energy carriers. The energy-intensive and significant CO₂ emissions of conventional hydrogen production techniques, like steam methane reforming, jeopardize sustainability objectives. In this regard, the production of green hydrogen directly from water and sunlight—a plentiful, clean, and renewable resource triad—has become appealing through the use of photocatalytic and photoelectrochemical (PEC) water splitting. The creation of extremely effective, inexpensive, and readily available semiconductor photocatalysts that can capture visible light and catalyze redox processes in mild environments is essential to this technology. Copper selenide (CuSe), a member of the semiconductor material family, has drawn interest lately because of its Direct and narrow bandgap (1.4–1.7 eV), perfect for absorbing visible light High absorption coefficient, which makes light harvesting more effective Suitable for photocathodic water splitting, p-type conductivity Low toxicity and earth abundance in contrast to semiconductors based on heavy metals. CuSe's conduction band edge is more negative than the hydrogen evolution potential, which allows protons to spontaneously reduce into H₂ when exposed to light. This property gives it the potential to be a photocathode material. It may also form heterojunctions with n-type semiconductors (such as ZnSe and TiO₂) due to its band alignment, which improves charge transport and separation. Although these advantages exist, CuSe's performance is heavily reliant on the caliber of its thin-film production, specifically regarding shape, crystallinity, and stoichiometric control. Chemical Vapor Deposition (CVD) becomes extremely important in this situation. CVD is a flexible, scalable, and controllable synthesis method that enables: Precise stoichiometric regulation using precursors in the gas phase Uniform coating of intricate substrates Grain boundaries and film morphology customization Incorporation of doping and nanostructuring techniques Although CVD has been widely used to thin films of oxides and sulfides (such as CuFeO₂ and CuWO₄), its use in chalcogenide systems, such as CuSe, is still in its infancy. Nevertheless, preliminary evidence suggests that CuSe thin films generated by CVD can provide better PEC performance than films made using traditional solution-based methods. Literature review Chopade et al. (2025) investigated. The impact of phase composition and stoichiometric deviation on the electrocatalytic activity of copper selenide (Cu_xSe) materials for water splitting applications was investigated. Using a customized solvothermal synthesis process, the study methodically examined http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 38 several non-stoichiometric phases, including CuSe, Cu_1.8Se, and Cu\sub>2</sub>Se. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were used by the authors to confirm unique crystallographic fingerprints and nanoscale morphologies linked to each phase. Zafar, et al. (2023) in this work, sulfur-doped CuSe films produced via chemical bath deposition (CBD) were examined. Under AM 1.5G illumination, the addition of sulfur improved light absorption and charge carrier separation, yielding a photocurrent density of about 1.8 mA/cm². It proved that basic doping techniques can greatly improve PEC performance. Ghobadi, Nader, et al. (2020). To ascertain the optical bandgap and recombination characteristics of CuSe thin films, the scientists conducted UV-Vis DRS and photoluminescence research. Band gaps between 1.45 and 1.68 eV were observed. The study shed light on the recombination processes associated with defects and electronic transitions. Cell Chen et al. (2022) hydrothermally produced ZnSe/CuSe heterostructures with type-II band alignment. The composite's effective charge separation at the heterojunction interface resulted in notable increases in photocurrent. It demonstrated how CuSe and n-type semiconductors can be coupled. Lai, W., et al. (2022) In this study, yolk-shell nanostructures made of gold nanoparticles enclosed in a Cu₂Se shell were presented. Gold's plasmonic impact improved electron injection and visible light absorption, increasing H₂ generation by more than 40%. A hybrid nanophotonic method for solar fuel applications was demonstrated. Peeters, M. E., et al. (2018). In this work, the feasibility of using atmospheric pressure chemical vapor deposition (APCVD) to deposit Cu-based oxide semiconductors was confirmed. Despite being centered on CuWO₄, it created standards for thin-film homogeneity and precursor control that apply to CuSe. Yengantiwar, A., et al. (2018). CuFeO₂ photocathodes were created by the authors using chemical vapor deposition. The study advanced knowledge of Cu-based ternary oxides and proposed that CuSe thin films could benefit from similar techniques to increase PEC durability and crystallinity. He, J., et al. (2019) Cu-based chalcogenides were among the many thin-film materials for water splitting that were covered in this thorough review. It offered theoretical frameworks for CuSe-related band alignment, charge transport, and efficiencyboosting techniques. Ikeda, S. (2021) This work, which focused on interface engineering, highlighted the significance of passivation and surface treatments in preserving the long-term PEC stability of Cu-based photocathodes. The observations have direct relevance to the endurance of CuSe films. Chen, Q., et al. (2023). CuSe combined with metal oxide co-catalysts through solvothermal synthesis was demonstrated in this work. Co-catalysts increased catalytic activity and charge carrier lifespan, facilitating the creation of CuSe hybrid systems. http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 39 Scope of This Review The objective of this review paper is to present a thorough and critical evaluation of CuSe thin films produced throughout the previous five years (2018–2025) utilizing CVD and associated techniques. It emphasizes: CuSe phase structures and material characteristics that are pertinent to photocatalysis, Parameters, difficulties, and prospects of CVD synthesis, Thin film structural, optical, and PEC characterisation, Techniques to improve water-splitting performance include doping, nanostructuring, and heterojunction engineering. Analysis in comparison to other Cu-based and non-Cu materials Future directions for research, especially in tandem devices, computational design, and low-temperature CVD This review seeks to expedite the logical design of CuSe-based photocatalysts for next-generation solar hydrogen production systems by fusing technical analysis with insights from the literature. The development of renewable hydrogen production methods has increased due to environmental concerns and global energy demands. One of the most effective and environmentally friendly methods is photoelectrochemical (PEC) water splitting, which uses solar energy to separate water into hydrogen and oxygen. For this purpose, thin-film semiconductors are becoming more popular, particularly transition metal chalcogenides (TMCs). CuSe is a strong contender for usage as a photocathode in PEC systems due to its ideal bandgap (1.4–1.7 eV), high absorption coefficient, and advantageous band edge locations. High-quality CuSe films with adjustable shape and crystallinity, which are essential for efficient light absorption and charge separation, may now be fabricated thanks to recent developments in deposition technologies, especially CVD. CuSe Material Overview: Electronic and Photocatalytic Relevance A member of the binary chalcogenide family, copper selenide (CuSe) has become a material of interest for photocatalytic applications because of its visible-light activity, phase variety, and customizable electronic structure. It is particularly promising for photoelectrochemical (PEC) hydrogen generation due to its advantageous energy band alignment and capacity to create heterojunctions with other semiconductors. Electronic Structure and Band Alignment The intrinsic p-type conductivity of CuSe is mainly ascribed to the acceptor states known as copper vacancies (V_Cu). Its ability to act as a photocathode is crucially dependent on its p-type behavior, which permits effective hole transport and hydrogen evolution at the semiconductor–electrolyte interface. From the perspective of the energy band: CuSe's conduction band minimum (CBM) provides enough thermodynamic driving force for proton reduction to H₂ because it is located above the hydrogen evolution potential (0 V vs. RHE). When combined with a photoanode in a tandem PEC cell, the valence band maximum (VBM), which is located below the oxygen evolution potential (1.23 V vs. RHE), http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 40 reduces the likelihood of photocorrosion. The visible spectrum (~400–850 nm) may be efficiently harvested thanks to the bandgap energy, which normally ranges between 1.45 and 1.7 eV. This maximizes the efficiency of solar-to-hydrogen (STH) conversion. Additionally, CuSe's band orientation can be efficiently adjusted by doping, alloying, or heterojunction creation, which helps to improve carrier separation and reduce charge recombination, two crucial photocatalytic performance bottlenecks. Phase Diversity The photocatalytic behavior of Cu–Se compounds is greatly influenced by their many stoichiometric phases, each of which has unique crystallographic structures and optoelectronic characteristics. These consist of: Phase Crystal Structure Bandgap (eV) Relevance to PEC Cu₂Se Cubic or tetragonal ~1.2–1.5 Good conductivity, although apparent absorption is limited by a small bandgap. CuSe Monoclinic or hexagonal ~1.5–1.7 Perfect for balanced redox potentials and absorption of visible light CuSe₂ Orthorhombic ~1.7–1.9 Greater suitability for UV-driven catalysis due to its wider bandgap Monoclinic CuSe is thought to be the most appropriate of them for PEC applications because Direct bandgap (1.5–1.7 eV): Promotes the production of electron–hole pairs and photon absorption. High photoconductivity: Under light, it enables quick charge transmission. Longer diffusion lengths are supported, and bulk recombination is decreased by enhanced carrier mobility. The space group P2₁/c is where monoclinic CuSe crystallizes. Its layered structures encourage anisotropic charge migration, which can be used to align films during CVD and other deposition procedures. Furthermore, recent research demonstrates that synthesis parameters, including growth temperature, precursor ratio, and gas flow rates in CVD, can modify the morphology and crystallinity of these phases, allowing for precise tuning of optoelectronic properties for desired PEC performance. Depending on whether the atmosphere is copper-rich or selenium-rich during http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 41 synthesis, a phase transition between Cu₂Se ↔ CuSe may occur. Controlling the precursor stoichiometry is therefore essential for CVD development. In contrast to non-stoichiometric Cu–Se phases, which could degrade or experience surface oxidation while in use, phase-pure CuSe has superior PEC stability. This dual-axis graph contrasts the onset potential (red line) and bandgap energy (blue bars) of various CuSe-based photocatalytic systems. These are important factors influencing how well they divide water. Bandgap (eV): The part of the solar spectrum that a substance may absorb is determined by its bandgap. Every CuSe-based system falls between 1.45 and 1.52 eV, which is the appropriate visible light range. S-doping may improve light harvesting while preserving sufficient redox potential since it slightly raises the bandgap. V vs. RHE Onset Potential: The lowest voltage necessary to start photocatalytic water splitting is known as the onset potential. Lower onset potential means more favorable charge transfer and less energy input needed. The Au@Cu₂Se system shows the lowest onset potential (~0.05 V vs. RHE), due to plasmon-enhanced hot-electron generation. ZnSe/CuSe heterojunctions also show a low onset potential (~0.08 V) thanks to efficient band alignment and charge separation. http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 42 Synthesis Techniques: Focus on Chemical Vapor Deposition CuSe thin-film photocatalytic performance, surface shape, stoichiometry, and crystallographic quality are all significantly influenced by the synthesis process. Chemical Vapor Deposition (CVD) is a potent and scalable method among the several fabrication processes available for creating extremely uniform, phase-pure, and nanostructured CuSe thin films, which are necessary for effective solar-driven water splitting. Chemical Vapor Deposition (CVD) In CVD, vapor-phase precursors react chemically on a heated substrate to create a solid coating with carefully regulated properties. Atomic-level accuracy is provided by the method over: The homogeneity and thickness of the film are essential for charge transport. Control of stoichiometry, guaranteeing CuSe phase purity, Surface morphology, which affects interfacial contact and light scattering, Defect engineering and doping are crucial for adjusting carrier dynamics and band structure. Key CVD Variants for CuSe Thin Films: Method Key Features Application Examples Atmospheric Pressure CVD (APCVD) Easy to assemble, reasonably priced, and capable of operating at room pressure utilized in photocathodes for CuWO₄ and CuFeO₂ (Peeters et al., 2018; Yengantiwar et al., 2018). Low-Pressure CVD (LPCVD) decreased pollution and improved homogeneity across wide regions Possibility of bendable PEC devices with a large area Metal-Organic CVD (MOCVD) makes use of organometallic precursors and permits intricate alloying and doping. Ideal for adding secondary metals (such as Zn and Ag) and adjusting the Cu: Se ratio Plasma-Enhanced CVD (PECVD) permits low-temperature deposition on substrates that are sensitive to heat. Promising for tandem and flexible PEC cells Challenges in CuSe CVD Growth: To guarantee reactivity, thermal stability, and volatility, Cu and Se precursors need to be carefully chosen. To stop selenium loss and phase deviation toward Cu₂Se or CuSe₂, Se overpressure is frequently necessary. Tight control over precursor flow rates and substrate temperature (~350–450 °C) is necessary to achieve the monoclinic CuSe phase. http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 43 A solid basis for scalable CuSe thin-film synthesis has been established by the successful CVD deposition of comparable systems such as Cu₂Se, CuInSe₂, and CuFeO₂ in spite of these difficulties. Other Deposition Routes Other techniques have been used to synthesize CuSe films, particularly for exploratory or lab-scale experiments, even though CVD is best for high-quality films. Comparative Table of CuSe Deposition Methods Technique Advantages Limitations Notable Study Chemical Bath Deposition (CBD) Low-cost, operates at low temperatures, simple setup Produces less crystalline films, prone to pinholes, and stoichiometric deviation Zafar et al. (2023 synthesized S-doped CuSe with improved PEC performance Sol–Gel Spin Coating Scalable, uniform thin films over large areas Requires postannealing at high temp (~400–500°C), risk of cracking Used for oxide and chalcogenide hybrid films Hydrothermal/Solvot hermal Synthesis Excellent control over nanostructure and particle morphology Not ideal for largearea film deposition; pressure vessels limit scalability Chen et al. (2022) fabricated ZnSe/CuSe heterojunctions with improved H₂ evolution Electrodeposition Good for direct growth on conductive substrates Poor crystallinity; prone to Sedeficiency; pHsensitive process Applied in early-stage Cu–Se PEC studies Synthesis–Property–Performance Relationship The deposition technique affects: Grain size and border density have an impact on recombination and charge transport. Surface roughness determines photon harvesting and light scattering. Phase purity, which affects PEC stability and band alignment. For example, whereas CBD-grown CuSe films are less expensive, they frequently need to be post-treated to increase their crystallinity. In contrast, CVD-grown films can provide better orientation and compactness, which are essential characteristics for repeatable, high-efficiency photoelectrodes. http://amresearchreview.com/index.php/Journal/about Volume 3, Issue 9 (2025) Online ISSN Print ISSN . . http://amresearchreview.com/index.php/Journal/about Page 44 Structural and Optical Characterization of CuSe Thin Films Correlating the structural, morphological, optical, and electrical characteristics of CuSe thin films with their photocatalytic ability requires thorough characterization. The synthesis process, growing environment, and post-treatment procedures have the biggest effects on these characteristics. Important characterization methods for CuSe films made for photoelectrochemical (PEC) water splitting are covered in this section. Structural Analysis Diffraction of X-rays (XRD) For evaluating CuSe film crystal structure, phase purity, and preferred orientation, XRD is the most popular method. Characteristic peaks at approximately 25.3°, 27.8°, and 30.9°, which correspond to the (101), (110), and (111) planes, respectively, are commonly used to demonstrate monoclinic CuSe. Peak changes may be a sign of strain or doping effects, whereas peak sharpness and intensity reflect the film's crystallinity. According to studies (e.g., Zafar et al., 2023), annealing increases crystallite size and decreases defect density, whereas S-doping results in a modest broadening of the peak due to lattice distortion. Spectroscopy using Raman Vibrational modes related to Cu–Se bonding are identified by Raman spectroscopy, which offers supplementary information. The Cu–Se stretching vibrations are confirmed by peaks in the 190–250 cm⁻¹ area. Cu₂Se, CuSe, and CuSe₂ can be distinguished from one another by phase-specific vibrational modes. Detecting spatial homogeneity across the film surface is another benefit of using Raman mapping. Morphological Characterization Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM) At the nanoscale, surface shape, grain size, crystal boundaries, and film thickness may all be seen in great detail with FESEM and TEM. CuSe films formed chemically and hydrothermally have been found to exhibit porous, nanorod, and nanoplate morphologies, which improve contact with the electrolyte and increase surface area. Conversely, dense, columnar grains with regulated thickness (100–500 nm) are frequently seen in CVD-grown CuSe films, making them appropriate for planar photoelectrodes. In support of crystallographic evidence from XRD, TEM examinations also show lattice fringes and validate interplanar lengths commensurate with monoclinic CuSe. Optical and Electronic Properties UV–Vis Diffuse Reflectance Spectroscopy (DRS) & Tauc Analysis Depending on synthesis and doping, CuSe thin films have a direct bandgap between