Study the Structural and Morphological Properties of (Cu /Ag) doped TiO2 Nanoparticle Prepared via Solvothermal Method

INTRODUCTION
Nanotechnology has opened new ways to modify material properties in recent decades to satisfy certain scientific and technological demands [1]. Because of their remarkable photocatalytic characteristics, (TiO2) nanoparticles have emerged as a center of attention of scientific investigation among these materials [2]. TiO2 has attracted an enormous amount of interest lately because of its non-toxic properties, high chemical stability [3], high optical transmission and enhanced photo-catalytic performance [4], good durability, high dielectric constant and high refractive index [5,6]. TiO2 has also been regarded as a promising material due to its electrical properties [7] [8] and it can be utilized in extensive range of applications, such as solar cells [9], gas sensors [10] and photo-catalysis systems [11]. Due to its wide band gap (3.2 eV), TiO2 has limited optoelectronic applications in the visible region of the spectrum [12]. 
Recently, metal doping has been applied to improve the physical and chemical properties of TiO2 nano-fibers and nanoparticles (NPs) [13]. The electronic structural and optical properties of TiO2 particles are modified by doping metal and nonmetal ions to enhance the efficiency of interfacial charge transfer, slow down the recombination rate of the electron-hole pairs, and enhance their response [14]. Additionally, metal doping might affect the crystal structure of titania, which would improve the application fields significantly [13].
Doping TiO2 with metals such as silver (Ag) and copper (Cu) is applied to improve its photo-catalytic performance, particularly under visible light, and to enhance its antimicrobial properties [15, 16]. Ag ion is used to improve the photo-catalytic activity of TiO2 through surface plasmon resonance (SPR), which assists in the absorption of visible light, thus extending TiO2 photo-catalytic performance beyond its usual ultraviolet (UV) absorption range. This leads to a more effective generation of electron-hole pairs for photo-catalytic [17, 18]. On the other hand, Cu modulates the electronic structure of TiO2, allowing to enhance light absorption in the visible spectrum and to reduce electron-hole recombination [19]. Furthermore, Cu doping produces oxygen vacancies that improve photo-catalytic efficiency [20]. Moreover, both Ag and Cu doping contribute to TiO2 antimicrobial properties, making it useful for environmental and biomedical applications such as water purification and self-cleaning surfaces [21] [22]. Various methods such as chemical deposition [23], sol–gel [24], hydrothermal [25], microwave [26], and solvothermal synthesis [27] have been used to synthesis pure and doped TiO2 NPs with metals.
In this work, (copper and silver)-doped titania (Cu/Ag-TiO2) NPs have been synthesized by solvothermal method at annealing temperature 500 °C. On the other hand, the influence of doping on the structural and morphological characteristics of the modified titania has been investigated.

MATERIALS AND METHODS
Chemicals
Titanium (IV) tetra-isopropoxide (TTIP, 97 % Aldrich Company), Copper (II) nitrate trihydrate (Cu(NO3)2.3H2O), 99 %, Silver Nitrate (AgNO3), Ethanol (C2H5OH, 99.5 %) and deionized water (DI) were used in synthesis of (Cu/Ag) doped TiO2 NPs.

Synthesis of (Cu/Ag) doped TiO2 nanoparticles
To prepare TiO2 NPs, two distinct solutions were prepared: solution A and solution B. Solution A was formed by mixing 5 ml of (TTIP) with 25 ml of ethanol. To ensure homogeneity, the solution was vigorously stirred for 15 min at room temperature. Meanwhile, solution B was prepared by mixing 25ml of ethanol and 5ml of distilled water. To synthesis (Cu/Ag) doped TiO2 NPs, this step was followed by the preparation of solution C. Hence the aqueous solutions of specific amounts from (1.5, 1, 1%) wt. of Cu(NO3)2.3H2O and (1, 1.5, 1%) wt. of (AgNO3) were both added drop-wise to solution B. Next, solution C was added drop-wise to solution A with continuous vigorous stirring for 1 hour. The final solution was then placed in an oven at 80 °C for 30 min. Then, the precipitate was centrifuged and washed 4 to 5 times with distilled water and ethanol in order to remove the impurities from the solution at 4000 rpm for 10 min. The resulting precipitate was dried under vacuum conditions in an oven at 100 °C for 1 hour. Finally, the generated white powder samples were annealed at 500 °C for 2 hour. In this work, the prepared samples of pure and doped TiO2 NPs with (1.5% Cu:1% Ag), (1% Cu:1.5% Ag) and (1% Cu:1% Ag) will be indicated as T0, T1, T2 and T3, respectively. Fig. 1 illustrates the preparation steps of the un-doped and (Cu/Ag) doped TiO2 NPs via solvothermal method.

Characterizations
Powder (XRD) patterns of the synthesized samples were recorded in the range 2θ◦ of 10–80°, operating at 40 kV of voltage and 40 mA of current. The XRD source was a Cu anode with a Cu-Kα1 (λ =1.54060 Å) radiation. The Crystallite size and (D) was calculated using the Scherer equation [28]:

where K denotes the shape factor (= 0.9), λ is the incident X-ray wavelength, β represents the full width at half maximum (FWHM) and θ is the diffraction angle at the peak maximum of the determined plane. 
To determine the lattice parameters (a, b, and c) of the samples, the following formulas (pertaining to the tetragonal crystal system) were used [29]:

In this equation, d represents the inter-planar spacing, and (h, k, l) refer to the Miller indices. The lattice constants, a and c, were determined from the lattice spacing of the anatase peaks observed at the (101) and (200) planes. This calculation was based on the Bragg equation for a tetragonal lattice structure, where the a and b values are equal, but the c value differs. From the lattice constants (a and c), the unit cell volume (V) of the samples can be calculated from the following relation [29]:

Morphological characterization was conducted using filed emission scanning electron microscopy (FESEM, MIRA3TESCAN, 15keV). The samples were placed on a conductive carbon tape and subsequently coated with a thin layer of gold in a vacuum environment. The size distributions of the examined nanostructures, was obtained from filed emission scanning electron microscopy (FESEM) images, using Image J software. A fluorescence spectrometer (Hitachi F4500, Tokyo, Japan) was utilized to measure the luminescence characteristics of the synthesized composite materials. FTIR analysis of TiO2 and Cu-Ag-TiO2 NPs was conducted using the Attenuated Total Reflectance (ATR) mode with the IR Tracer100 Shimadzu as an accessory. Prior to the analysis, a background scan was performed. The spectra were recorded within the range of 4000 to 400 cm-1, utilizing a resolution of 8 cm-1, and averaged over 256 scans. 

RESULTS AND DISCUSSION
X-Ray Diffraction (XRD) analysis
The structural characteristics and nanoparticle’s phase of the synthesized samples were studied using XRD. The XRD patterns of pure and doped TiO2 (T0, T1, T2 and T3) NPs were shown in Fig. 2. The existence of the anatase phase was confirmed by the peaks at 25.47°, 37.04°, 37.91°, 38.63°, 48.17°, 54.08°, 55.19°, 62.80°, 68.78°, 70.36° and 75.10° corresponding to planes (101), (103), (004), (112), (200), (105), (211), (204), (116), (220), and (215), respectively, according to the reference JCPDS data (01-073-1764). 
Although there were missing peaks related to Ag and Cu, nevertheless the main diffraction peaks shifted toward the lower 2θ value and broaden with increasing doping ion concentration which may be due to lattice strain present in the samples. The ionic radius of Cu2+ (0.72 Å) is slightly greater than that of Ti4+ (0.68 Å). Thus, it is possible to substitute small amounts of Ti4+ instead of Cu2+ in the structure, which could be accompanied by a weak lattice expansion, generated by the small difference between its ionic radii [30] [31]. However, it is improbable that Ag⁺ could replace Ti4+ in the TiO2 matrix because the Ag+ ion radius (1.26 Å) is larger than that of Ti4+ (0.68 Å), which would generate a significant lattice expansion. Therefore, it is less probable to find Ag+ within the TiO2 crystal lattice. Consequently, silver is presumed to be present as Ag NPs on the materials surface [32]. The absence of distinct Ag and Cu peaks in the XRD analysis after doping with TiO2 suggests that silver and copper are either present in an amorphous state, highly dispersed within the TiO2 matrix, or incorporated into the TiO2 lattice as substitutional or interstitial species. 
The results showed that the average crystallite size reduced as a result of the TiO2 NPs doping, as shown in Table 1. The introduction of dopant ions into the TiO2 lattice was responsible for this decrease in crystallite size because it caused lattice strain, which inhibited crystal growth, leading to smaller crystallites. 

Fourier transform infrared (FTIR) analysis
FTIR is the most suitable method for examining changes in structures, bond vibrations, and interactions within the material. Fig. 3 shows the FT-IR spectra of the different wt. % of Cu-Ag doped Titania NPs compared to pure TiO2 over the wavenumber range of 400 cm-1 to 4000 cm-1. It is demonstrated that the synthesized nanoparticles exhibit multiple absorption bands, which show distinct variations in their intensities. The FT-IR spectra indicated that the absorption characteristics of all synthesized nanoparticles exhibit a similar pattern, with no distinct new absorption bands. The large peak in the region from 2200 cm-1 to 2400 cm-1 explains the C– O bands [33]. The strong peak at the region from 1000 cm-1 to 1500 cm-1 was corresponds to O–H bond stretching vibrations group [34]. The peak in the range 500 cm-1 to 900 cm-1 was attributed to the stretching vibration of the Ti-O bond [32].

Field emission scanning electron microscope (FESEM) and energy dispersive Xray-spectroscopy (EDX) analysis  
The morphology analysis of the Cu/Ag-co-doped TiO2 NPs was done by the FESEM technique. The FESEM micrographs of pure and Cu/Ag-doped TiO2 NPs annealed at 500 °C were illustrated in Fig. 4. FESEM micrographs indicated that a non-uniform distribution of particles, consisting of either individual particles or clusters of particles. Typically, all Cu/Ag-doped TiO2 NPs have smaller particle sizes compared to pure TiO2. The image of FESEM displayed spherical particle shapes that were agglomerated in some extensions. These agglomerations were the result of reduced electrostatic repulsion due to the doping that changes the surface charge, which forced the particles to stick together. The average particle size was calculated from FESEM images utilizing Image J. The results revealed that the particle size decreased by doping. The average particle size of sample T0 was observed to be 23.83 nm, but the particle sizes of samples T1, T2 and T3 were 16.48 nm, 16.11 nm and 18.57 nm, respectively [35] [32].
 EDX was used to investigate the microscopic composition of the Cu, Ag co-doped TiO2 NPs. The EDX results of both un-doped and doped TiO2 NPs are illustrated in Fig. 5. The EDX measurement qualitatively confirms the presence of titanium and oxygen in all synthesized nanoparticles. The doped NP Samples exhibited peaks of copper at 1 keV and silver at 3 keV. Consequently, it may be inferred that doping TiO2 has been effectively accomplished. Atomic and weight percentages of elements were shown in Table 2.
To verify the deposition of Cu and Ag co-doped TiO2 NPs, elemental mapping was performed and is shown in Fig. 6. Fig. 6a illustrates the mapping analysis of TiO2, confirming the existence of titanium and oxygen elements without any contaminants. Figs. 5b, c, and d provide the mapping that indicates the existence and uniform distribution of Cu, Ag, Ti, and O.
Obviously, the EDX and mapping images show the presences of Cu and Ag, which means that their concentration or crystallite size may be too small for XRD to pick up, or they may be found as nanoscale clusters that don’t produce typical diffraction peaks.

Photoluminescence (PL) spectroscopy analysis
Photoluminescence (PL) spectroscopy is an effective method for investigating the electronic structure of nanomaterial, the transfer behavior of photo-excited electron-hole pairs in semiconductors, and the recombination rate. Fig. 7 illustrates the PL spectra of pure and Cu/Ag-doped TiO2 nanoparticles, excited at a wavelength of 320 nm. The emission peaks of pure TiO2 closely resemble those of Cu/Ag-doped TiO2; however, the photoluminescence intensity of Cu/Ag-doped TiO2 decreases compared to that of pure TiO2. The UV emission peaks at 380 and 385 nm are regarded as the band edge emission of the host TiO2, attributed to self-trapped excitations localized within TiO6 octahedra. [36]. The emission peak in the range of 630–650 nm can be attributed to generated electron-hole pairs. These charge carriers may get trapped in defect states, such as oxygen vacancies, within the TiO2 lattice. The recombination of these trapped carriers can result in visible light emission, including wavelengths around 630 nm [37]. Moreover, TiO2 nanoparticles often exhibit surface states due to their high surface-to-volume ratio. These surface states can trap electrons or holes, and their recombination can lead to emissions in the visible spectrum [38]. In compounds containing copper, it is observed that with increasing weight percentage of copper, the emission intensity decreases and as a result, the recombination decreases [39]. At the same time, in compounds where Ag was doped to TiO2, it was observed that the emission intensity, i.e. the rate of electron-hole rearrangement, increased with increasing weight percentage.

CONCLUSION
Titanium dioxide nanoparticles doped with different concentration of Cu and Ag, were synthesized using the solvothermal method, and annealed at 500 °C for 2 hours. The structural morphological and optical properties were investigated by XRD, FTIR, FESEM and PL. XRD analysis revealed that a shift of the main diffraction peak toward lower 2θ value and decreasing in crystallite size upon Ag and Cu doping compared to the un-doped sample, indicating lattice strain and structural distortion. The FESEM images showed that the distinctive spherical structure of the obtained nanoparticles with aggregation of tiny crystals. Future research on titanium dioxide doped with Cu and Ag holds significant promise for enhancing the materials properties and broadening its applications in photocatalysis, energy storage and environmental remediation.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.