Seed Layer-Assisted Chemical Bath Deposition of Cu2O Nanoparticles on ITO-Coated Glass Substrates with Tunable Morphology, Crystallinity, and Optical Properties

A seed layer-assisted chemical bath deposition method performed at low temperature has been developed to grow uniform and high-quality crystal cuprous oxide (Cu2O) nanoparticles on transparent conductive/glass substrates. The annealing process by continuous beam (CW) of CO2 laser was used prior to growing the Cu2O nanoparticles. In this study, the controlled synthesis of Cu2O films was investigated by controlling the growth temperatures at 55 °C, 60 °C, 65 °C, and 70 °C, respectively. The modified seeding substrate reflect enhanced structural properties with laser annealing temperature of 450 ℃. In addition, Cu2O nanoparticles with flower-like stricter show a greater density containing a smaller particle with 75 nm average dimension and flower particle size was about 85 nm. Results suggest an effective synthesis route for developing high-quality Cu2O nanoparticles for optical and electronic applications.


Introduction
Cu 2 O is a semiconducting material with a cubic crystal structure; it is inexpensive and has low toxicity with relatively low direct band gap of about 2.0-2.6 eV [1, 2]. Furthermore, it has superior photoelectronic properties, is abundantly available, and can be created using simple preparation procedures [3,4]. Furthermore, Cu 2 O has a unique cuprite structure through a body-centered cubic packing of copper atoms, with oxygen atoms occupying the tetrahedral sites [5,6]. In recent years, Cu 2 O has attracted great interest due to the encouraging results related to its use in several applications and technological fields. In general, Cu 2 O and CuO are semiconductor p-type materials and are thus potentially useful for fabricating junction devices, such as p-n junction solar cells and diodes [7][8][9]. In addition, they can be used as heterogeneous catalysts for solid state gas sensor heterocontacts [10][11][12], microwave dielectric materials [13], and several environmental processes [14]. Furthermore, CuO has been employed in various applications, such as power sources [15], photovoltaic (PV) devices [16], and antibacterial activity [17], and have been used as electrode materials in lithium batteries [18][19][20]. The synthesized methods of Cu 2 O films can be mainly divided into two groups, namely, physical and chemical methods, including chemical preparation methods, such as electro-deposition [21], sol-gel method [22], spray pyrolysis [23], pulsed laser deposition [24,25], and chemical bath deposition (CBD) [26]. CBD is one of the traditional methods of chemical or physical deposition, and the Cu 2 O prepared using the CBD method has shown unique and excellent properties compared with products of other synthesis methods, such as those operating at low temperature and using low-cost equipment, high deposition rate, and high quality of the deposited films [21][22][23]27], as well as those that control of growth parameters and show excellent adhesion to substrates by depositing with large-area films [26,28].
Size control is one of the most effective requirements in a wide range of applications. It requires controlling the size and geometry of nanoparticles, which in turn, lead to the control of optical and electrical properties. Excellent physical properties have been obtained using nanometer-scale solid particles, which have been engineered at molecular or atomic size leads, to construct superior and novel properties that are not otherwise obtainable using conventional bulk materials. These nano-sized particles act similar to a complete unit in relation to properties. All materials have a critical value of size below which their physical properties change drastically [29][30][31][32]. A study on functional materials, such as monometals and towered trimetals, has shown an improvement in physical properties, especially when we talk about nanodimensions [33].
At the same time, metal-organic frameworks continue to attract great research attention. Due to their tunable properties, in terms of molecular arrangement, chemical nature, size, and shape of cavities, they have very important and potential applications in diverse fields, such as biomedicine, gas storage and separation, heterogeneous catalysis, and others. Most of the important metal-organic frameworks studied in the literature are built from divalent transition metal cations such as Cu 2+ , Zn 2+ , Co 2+ , Ni 2+ , and others leading to thousands of open framework architectures with specific porosity [34]. Therefore, the growth and tailoring of Cu 2 O or CuO to produce surface densities, shapes, and specific sizes have been studied due to their various new and futur applications, such as photo electrochemical Co 2 reduction, optical photodetectors, chemical sensors, lithium ion batteries, photoluminescence, and No 2 gas sensors [35][36][37][38].
At present, many important studies have been conducted using the CBD deposition method to prepare Cu 2 O nanoparticles under multiple mechanisms, growth conditions, and parameters [39]. For example, Saadaldin et al. [40] fabricated CuO thin films at different substrate annealing temperatures of 200 °C, 300 °C, and 400 °C in the air by CBD deposition method at 70 °C; the properties of the prepared CuO films were found to be related to annealing temperature. Sultana et al. [41] reported the production of CuO thin films with different thicknesses (60-178 nm) on silicon (n-Si) substrates by using the CBD method at 85 °C. They reported the effects of adjusted growth time on the optical and structural properties, chemical composition, and structural quality of CuO films. In addition, the CuO thin films at 110 nm thicknesses showed the best crystal quality, dielectric constant, refractive index, and optical properties [41]. Reyes et al. [42] reported uniform and crystalline Cu 2 O thin films on corning glass substrates coated with a CuxS seed layer by the CBD method. They investigated the effects of pH and growth temperature (≤70 °C) on the electrical, morphological, optical and structural properties of Cu 2 O thin films. Other studies have reported that the increase of growth temperature can lead to an increased stimulation of the morphologies' formation and thickness, which in turn, reduces the band gap and enhances transmittance [43][44][45].
Meanwhile, indium-doped tin oxide (ITO), which has been used as a seed layer, has been reported to have a critical role in determining the quality of the growth process of the nano dimension semiconductor films with good crystal quality [43][44][45][46]. The relationship between the seed layer and nanostructure films must be studied due to the properties of the nanostructure, which depend mainly on the properties of seeds, such as roughness, crystalline density morphology, crystalline, and grain size [47][48][49][50]. Therefore, the synthesis of a high-quality seed layer can be achieved by using the most attractive method of synthesizing Cu 2 O films, which has been shown to have the ability to promote the development of novel devices and potential applications [51][52][53][54].
In line with the abovementioned findings, many recent studies have investigated the influence of the seed layerassisted chemical bath deposition (SCBD) method on the growth of Cu 2 O and CuO nanostructures. Zhu et al. [55] reported on CuO thin films on CuO seed layers and ITOcoated glass substrate through the SCBD method. Forat et al. [56] reported Cu 2 O flowers grown on ITO seed layercoated glass substrate using SCBD at 70 °C; the ITO seeds with a thickness of 75 nm were prepared on glass substrates using magnetron sputtering-radio frequency (RF). The results exhibited that the Cu 2 O film had good crystallinity, a cubic structure, and was uniformly formed on the seed/ glass substrates. Muiva et al. [57] reported on the growth of one-dimensional (1D) CuO nanostructure films through the SCBD method on CuO seed layers deposited by two methods: chemical spray pyrolysis (CSP) and coated float glass substrates using the successive ionic layer adsorption (SILAR) technique. The CSP/CBD film yielded large grain size and was less strained than the SILAR/CBD film. The average grain sizes were 35.8 nm and 11.9 nm for the CuO nanostructures deposited on the CSP and SILAR CSP seed crystals, respectively. The seed layer can be influenced by adjusting its properties using multiple mechanisms and growth conditions and parameters, thickness, quality, and heat treatment process. The laser annealing temperature of the seed layer is considered one of the critical conditions for controlling the properties of nanostructure films, and reducing or removing the damage of semi-conductor surfaces has garnered great research attention among researchers in 1977 [58]. High-power laser beam quickly heats the surface regions of semiconductor films to a high temperature or dissolves them above melting temperatures. The CW laser or pulse leads to directed energy procedures characterized within a short time by energy dropping on the surface of the nanoparticle semiconductor. The surface of the nanoparticles undergoing rapid heating and cooling might lead to an extremely homogeneous form of film surface. Laser annealing using continuous or pulsed wave depends on various conditions that lead to the melting surface of semiconductor films as a result of the absorption of light energy, which can be converted to heating. This heat energy is rapidly transmitted to the electronic structure and then to phonons with less than of 1 Psec [59,60].
To the best of our knowledge, this work marks the first time in which Cu 2 O nanoparticle growth using the SCBD method is presented, along with the effects of CW CO 2 laser as a thermal energy source to anneal the seed layer and control the morphological, optical, and structural properties of the materials.

Preparation and Annealing Process of Seeds
The square pieces of glass slides used as substrate (10 mm × 10 mm) were cleaned ultrasonically with both alcohol and acetone prior to the seed layer [28]. ITO thin film as seed was prepared by RF magnetron with thickness of approximately 75 ± 5 nm. The thickness of the seed layer and Cu 2 O films were measured using Filmetrics F20. The heat treatment of the seed layer was carried out by laser annealing temperature using continuous wave laser of 10.6 μm at 450 ℃ in the air with power of 25 W [61, 62].

Growth of Cu 2 O Nanoparticles
The Cu 2 O nanoparticles were grown on seeded ITO substrate SCBD method. Then, a post-heat treatment was done on the seed layer using CO 2 laser. Copper chloride solution was obtained by dissolving (1.705 g) copper (II) chloride dehydrated in (100 mL) DDI water and then stirring for 1 h. The molarity of the obtained solution was 0.18 M. The solution was made to boil at different growth temperatures of 55 °C, 60 °C, 65 °C, and 70 °C [37].

Characterizations
The structural characterization of Cu 2 O nanomaterials was studied and analyzed using a diffractometer (PANalytical X'Pert PRO diffractometer engaged with 1.5 Å Cu-Kα X-ray sources). Optical results were obtained by employing the PL system of Jobin Yvon HR 800 UV (Edison, NJ, USA) type, using 325 nm, 20 mW He-Cd laser as the excitation source. The morphological properties of seed layers and the products of the Cu 2 O nanostructures were determined using a FESEM of FEI/Nova Nano SEM450, with energy dispersive X-ray spectroscopy (EDX). The atomic force microscopy using Burker Dimension Edge was employed in this study to probe the surface roughness of the seed layer. Using Shimadzu UV-Vis 1800 (Japan) double-beam spectrophotometer, the optical transmission and optical energy gab were tested. The energy gap was also estimated using Tauc relation wherein the extrapolation of the straight line between (αhν) 2 and incident photon energy was taken, thus representing the energy gap. The following schematic diagram presents the experimental flow chart (Fig. 1). Figure 2a and c show the FESEM images of sputtered seeds with and without laser treatment, respectively. In Fig. 2a, the granular structures and polycrystalline cannot be viewed due to the nature of the surface morphology. Figure 2c indicates the increase in crystallized and polycrystalline structures using laser treatment, as reported in other works [63,64]. The surface morphology of seeds that changed with and without laser treatment at 450 ℃ were examined using AFM. Figure 2b and d indicated that the roughness of seeds increased using the laser annealing temperature as a result of a growth in the seeds' grain size [65]. The RMS of the as-grown seeds was 1.01 nm. Figure 3a and b present the X-ray diffraction (XRD) patterns of seeds without annealing and with laser annealing of 450 ℃, respectively. As shown in Fig. 3, the seed patterns without annealing did not reveal a peak, thereby indicating an amorphous structure [60,66]. At the same time, the XRD patterns of ITO seeds at different laser annealing temperatures showed diffraction patterns with different major peaks of (211), (222), (400), (440), and (622), according to ICSD Card No. 050849. This condition causes a variation of the seed building toward the polycrystalline structures, where a crystalline material with a single grain is called a single crystal, whereas a crystalline material consisting of many grains of different orientations is called polycrystalline [67].

Growth Characteristics of Cu 2 O
To study the effects of the growth temperature on the properties of Cu 2 O nanostructures on sputtered ITO seeds/glass substrate via the SCBD method, we conducted a series of experiments using various growth temperature, while other factors remained constant. The optical, structural, and morphological properties of Cu 2 O nanoparticles at different growth temperatures on seeds via SCBD were determined using UV-Vis, XRD, and FESEM.

Morphological Investigating
Various Cu 2 O nanostructure morphologies on seeds coated with ITO glass substrates via SCBD method under different growth temperatures (55 °C, 60 °C, 65 °C, and 70 °C) were identified by FESEM. The results are shown in Fig. 4.
As shown in Fig. 4, the effects of growth temperature on the size and shape of the Cu 2 O nanoparticles were detected. The Cu 2 O on ITO seeds at a growth temperature of 55 °C appeared to have a high-density and large-area evolution of Cu 2 O films resembling cubes (~ 75 nm in width and length), as revealed in Fig. 4(a 1 , a 2 ). The transformation in the shape and size of Cu 2 O film at growth temperature of 60 °C showed a spherical cluster, which included a smaller particle (around 85 nm in length and width) and with high density, thus providing a closer look at the typical shape of flowers, as revealed in Fig. 4(b 1 , b 2 ). As presented in Fig. 4(c 1 , c 2 ), the Cu 2 O film developed at a growth temperature of 65 ℃ showed the agglomerations of flower morphology. The flower-shaped Cu 2 O nanoparticles revealed greater density containing a smaller particles with length and width of approximately 75 nm. The packing density of the flower-shaped Cu 2 O nanoparticles increased when the growth temperature rose to 70 °C, as revealed in Fig. 4(d 1 , d 2 ). The surface of the substrate was densely covered with the flower-shaped Cu 2 O more systematically, consistently, and with high-density (flower particle sizes were within 85 nm width and length) similar behaviour employing other material could be shown in other work [68,69].

Crystal Building Analysis
The X-ray patterns of the Cu 2 O nanoparticles on seeds at different temperatures of 55 ℃, 60 ℃, 65 ℃, and 70 ℃, are revealed in Fig. 6. All detected peaks indicated that the Cu 2 O nanoparticles with different growth temperatures were successfully grown on all films corresponding with the Cu 2 O source of JCPDS-05/0667 (cubic crystal). That the intensities of the preferred orientation (111) increases along with increasing temperature of growth products suggests that the overall crystallization rate of Cu 2 O nanoparticles increases   Fig. 4 High (a 2 -d 2 ) and low (a 1 -d 1 ) magnification FESEM image of Cu 2 O on ITO seeds/ glass substrate via SCBD method at different growth temperature of 55 ℃, 60 ℃, 65 ℃, and 70 ℃ with growth temperature. Furthermore, the maximum intensity of the (111) direction has been achieved at a growth of 70 ℃, thus indicating the highest crystallization of this film [70]. The absence of diffraction patterns for the seeds and Cu metal also indicated that the Cu 2 O nanoparticles films were grown at a high density, in accordance with the findings reported by Forat et al. [29] and Kumar et al. [71,72].
The optical transmission of the Cu 2 O films was obtained by using a Cary System 500 (Varian) double beam UV-Vis-NIR spectrophotometer. The obtained results are shown in Fig. 7, as can be seen, the optical transmission (T) rate of samples (visible region) increases with the growth temperature. The highest value of T of Cu 2 O films was obtained at a temperature of 70 ℃. This is due to the enhancement crystallization and higher formation [73]. To calculate the direct optical band gap (Eg) of the Cu 2 O nanoparticles estimated by Tauc plots (Fig. 8), the following formula is used: hv = A ( hv-E g ) 1/2 , where ν is the Fig. 5 (a-c) EDs results of the Cu 2 O on seeds/glass substrate via SCBD method at different growth temperature (55-70 °C) respectively photon frequency, h is Planck's constant, A is a constant, and α is the absorption coefficient [61]. By studying the relation between (αhν) m as a function of incident photon energy in the visible and near-IR region, we are able to find the value of the band gap by taking the extrapolation of the obtained straight line from the relation. The value of m depends on the type of band gap, where m = 1/2 for indirect band gaps and 2 is for the direct band gaps.
The band gap energies decreased from 2.107 eV to 2.282 eV for Cu 2 O films at a temperature of 70 ℃. These values of the Eg match with the reported values [74,75].  [76], and the Moss-Bustein effect. The inverse relation between the energy gap and particle is true for the quantum size particle due to the quantum confinement effect, and this occurs when particle radius is very close to the Bohr radius (1-10 nm) that is beyond the quantum size effect.
Furthermore, particle size affects the band gap value of the material, but only if the particle size is in the quantum confinement regime. This means that the band gap value is affected if the size of the particle is comparable, equal, or smaller than the Bohr exciton radius of the material (whose particle size is under consideration). When the size is small, the discretization of energy levels occurs due to the due to quantum confinement. The valance band levels move downwards while the conduction band levels move upwards, thus resulting in the increase of the band gap. In our case, the particle size is beyond 500 nm, and such phenomenon does not exist.
The dependence of absorption on material composition and structure has been reported in other works [77,78]. In thin film technology the grain size may depend on film thickness, but it is difficult to predict the direction. The grain size has no direct relation with the thin film thickness. Second, the grain size mainly depends on the film thickness; in other words, the chemical composition of the thin film is responsible for the grain size distribution. Thus, if the film composition does not vary with the film thickness, a linear relation is observed between the grain size and the film thickness. It may increase or decrease, but the band gap never changes in this case. Meanwhile, if the composition changes with thickness, it can directly affect the material (thin film) band gap. Finally, due to the scattering, the grain size of the thin film affects the average transmission of the film, but this does not affect the energy band gap until the composition remains unchanged [78].

Conclusion
In conclusion, the growth of the Cu 2 O nanoparticles was successfully conducted by the SCBD method using glass substrates at low temperature. This case was established using sputtered ITO seeds and subsequent heat treatment using continuous CO 2 laser beam, with laser annealing temperature of 450 ℃ yielding enhanced structural properties. Furthermore, Cu 2 O particles with flower-like morphology show elevated density with smaller particles (75 nm) and minimum particle size of about 85 nm.
The findings of this study suggests a synthesis route for developing high-quality Cu 2 O nanoparticles with uniform dimension and size for optical and electronic applications. Moreover, the particles have high crystallinity and high transmission. This approach, which has lower substrate cost and uses low-growth temperature, can be used in various flexible nano-devices.