MoO3, TiO2, and MoTiO5 based oxide semiconductor for photovoltaic applications

Topographic essential synthesis of nanomaterials by adjusting easy preparatory factors is an effective way to improve a variety of nanostructured materials. The SILAR technique is used to evaluate the manufacturing samples of MoO3, TiO2, and MoTiO5 nanostructures. These nanostructures of MoO3, TiO2, and MoTiO5 are used as electrode materials in photovoltaic systems. The link between photoelectrochemical characteristics and MoO3, TiO2, and MoTiO5 nanostructures is studied in depth. The photoelectrochemical characteristics of MoO3, TiO2, and MoTiO5 nanostructures are discovered to be highly dependent. At a 5mV/s scan rate, the photocurrent of MoO3, TiO2, and MoTiO5 electrodes surged fast when sunlight was turned on, reaching values of 1.03 mA cm−2, 1.68 mA cm−2, and 14.20 mA cm−2, respectively. As soon as the solar illumination was turned off, the photocurrent value dropped to zero. Photocurrent transitions showed a quick, homogeneous photocurrent counterpart; this suggested that charge transfer in these ingredients is speedy and possibly related to the crystal buildings of MoO3, TiO2, and MoTiO5. MoTiO5 nano-belt and nano-disc thin films have typical uses in the photoelectrochemical sector because they have the best photoresponse and stability.

Until this point in time, different strategies have been created to magnify nanoparticles of IV-VI semiconducting materials. To prepare IV-VI semiconductors, atomic layer epitaxy (ALE) [21], chemical bath deposition (CBD) [22], chemical vapor deposition (CVD) [23], and consecutive ionic layer adsorption and reaction (SILAR) [24] have been traditionally used. SILAR depends on the dip of the substrate into independently positioned cations and anions followed by washing after every response, it makes possible to give a heterogeneous response among the solid stage and the solvated ions in the solution. In order to form thin films which are uniform, compact and crystalline, it is ideal to use the SILAR method. Moreover, SILAR does not need a target or vacuum, the deposition rate and the film thickness can be easily controlled over a long range by varying the number of deposition cycles.
In this study, a different new method for synthesis of MoO 3 , TiO 2 , and MoTiO 5 nano-structures based on SILAR to indium tin oxide (ITO) electrodes is presented. MoO 3 /TiO 2 /MoTiO 5 nano-structures obtained were recognized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, energy dispersive spectroscopy (EDS), X-ray diffraction, and photo-current analysis. Improvement in photocurrent density between MoO 3 , TiO 2, and MoTiO 5 electrodes and electrochemical impedance spectroscopy (EIS) to determine the charge transfer resistance at the interface studies have been done. It is indicated by the test results that, by setting the spool time, control of the morphology and size of MoO 3 , TiO 2 , MoTiO 5 nano-structures can be possible. Good photovoltaic properties are shown by the nano-structured MoO 3 , TiO 2 , MoTiO 5 photo-electrodes formed and they can be utilized in applications of solar energy conversion. Using optical-quality enlarged nano-structures with a low defect concentration which has a stable and repeatable photo-current through many cycles is the general test strategy of this study as can be given in Figure 1.

Materials and methods
MoO 3 , TiO 2 , and MoTiO 5 nanofilms were formed by reaction SILAR method and sequential ionic layer adsorption. All of the electrolyte solutions used in this study were prepared using deionized water (i.e.>18MΩ) from a Milli-Ωsystem. In this method, 25 mL of 0.05 M Titanium III chloride (TiCl 3 ) and 25 mL of 0.05 M of sodium molybdate (MoNa 2 O 4 ) as cationic and 25 mL of 0.02 M of sodium hydroxide (NaOH) as an ionic pioneer were expended. Four beaker SILAR contrivance was used to prepare MoO 3 and TiO 2 thin films with different nanostructures, where cationic and ionic precursors were formed from two rinse steps. The cleaned ITO substrate was submersed for 20 s in a cationic solution (MoNa 2 O 4 ) for the adsorption of molybdenum ions onto the substrate and in the anionic solution (NaOH) for 20 s to form MoO 3 materials. One might as well say, the well-cleaned ITO substrate was submersed in a cationic solution (TiCl 3 ) for 20 s for the adsorption of the titanium ions on the substrate and anionic solution (NaOH) for 20 s to form TiO 2 materials. A six-beaker SILAR system was expended to arrange MoTiO 5 nanostructured thin films, where cationic and ionic solutions were individuated by a rinse step. The cleaned ITO substrate was submersed in a cationic solution (MoNa 2 O 4 ) for 20 s for the adsorption of molybdenum ions on the substrate and a cationic solution (TiCl 3 ) for 20 s for the adsorption of the titanium ions on the substrate. The ITO substrate was washed in deionized water for 10 s to remove the loosely bound molybdenum, titanium, and hydroxide ions. Washing the substrate using deionized water for 10 s again distinguish surplus or nonreaction ions. In this manner, a SILAR cycle of accumulating each of MoO 3 , TiO 2 , MoTiO 5 is finished. This type of 80 SILAR period was reiterated to obtain the optimum thickness of MoO 3 , TiO 2 , and MoTiO 5 thin films.

Analytical methods
An electrochemical workstation (attached to a three-electrode cell, BAS 100B/W) was exercised for photoelectrochemical tests (Chrono-Amperometry measurements). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were performed with Gamry (600+) potentiostat systems connected to a three-electrode cell. ITO-coated quartz (10 Ω cm −2 ) was used as the working electrode for the electrochemical measurements and the photoelectrochemical measurements. The counter and reference electrodes that were used included a Pt wire and Ag/AgCl (saturated KCl), respectively. The photocatalytic performance of MoO 3 , TiO 2, and MoTiO 5 electrodes was determined by performing cyclic voltammetry studies for the one-electron reduction of reversible Fe(CN) 6 3− /Fe(CN) 6 4− redox system in a solution containing 10.0 mM K 3 Fe(CN) 6 and 0.1 M KCl. X-ray diffraction plots of the deposited films were registered with a Rigaku powder X-ray diffraction meter with a CuK X-ray source (λ = 1.5406 Ả). Morphological workout and identification of the elemental composition (Mo/O), (Ti/O), and (Mo/Ti/O) of MoO 3 , TiO 2 , MoTiO 5 nanostructures were performed by an EDS united with a scanning electron microscope (ZEISS system). X-ray photoelectron spectroscopy (XPS, Spect-Flex spectrometer) measurements of metal oxide samples were obtained by using a standard Al X-ray source. Atomic force microscopy images of electrodeposits were acquired in ambient conditions, with a Hitachi 5100N instrument. Ultraviolet-visible (UV-Vis) spectroscopy measurements were obtained from a Shimadzu UV-3600 Plus spectrophotometer. Photoelectrochemical quantifications of MoO 3 , TiO 2 , and MoTiO 5 nanostructures were made at room temperature using 0.1 M KCl solution. The photocurrent intensity was enrolled under illumination by AM 1.5 G (1 sun, 100 mW/cm 2 ) exploiting a solar simulator (SolarLight-16 S).  (Figures 2c-2f) show MoTiO 5 , the combined structures of nanobelts and nano-discs. With the increase of photo-absorption efficiency on MoTiO 5 nanostructured thin films, which have impressive photosensitivity, light absorption, reflection and scattering can be increased significantly. The SEM image of MoTiO 5 can be seen in Figure 2(f) and the results indicate that the MoTiO 5 nanostructures [25] were designed perfectly after the SILAR procedure. Well connection between MoO 3 nanoparticles in nano-belts shape and TiO 2 nanoparticles in nano-disc shape can be observed. Additionally, to verify the elements' composition, the EDX analysis of the MoTiO 5 was performed and the results are given in Figure 2 To examine the crystal growth of the as-designed samples, a systematic XRD study was performed and Figure 3 shows the results. The diffraction peaks with reference to the (101), (111), (2 0 0), (2 1 1), (2 0 4), and (301) crystal orientations with the lattice constants a = 3.755 Å and c = 9.5114 Å confirm the tetragonal anatase phases of the TiO 2 nanoparticles in compliance with the JCPDS file 21-1272 [26]. Regarding the unalloyed MoO 3 , the XRD planes displayed at 2θ = 12.8°, 25.8°, 38.9°, 46.1°, and 49.3°correspond to the (020), (040), (021), (061), and (002) orientations of orthorhombic construction MoO 3 with the JCPDS Card no. 05-0508 and the lattice literals a 5 3.96 Å and c 5 3.7 Å [27]. It was shown in the sample that nano-belts average thickness is approximately 50 nm. The MoTiO 5 sample exhibited diffraction peaks at 28.3°, 47.1°, and 49.9°, which MoTiO 5 had phases with peaks at (101), (301), and (311), respectively. From the MoTiO 5 curve [28], it can be well observed that the diffraction peaks of MoTiO 5 can be successfully prepared by this process. Figure 4(a) indicates UV-visible absorption spectra of arrant TiO 2 , MoO 3, and MoTiO 5 . In Figure 4a, it is clearly seen that the absorption intensity in MoTiO 5 is higher at about 300 nm compared to TiO 2 and MoO 3 . The band gap values of the films were calculated by plotting (Ahν) 2 vs. (hν) and extrapolating the linear portion of the graph to the energy axis ( Figure  4b) [29]. The plots of (Ahν) 2 versus (hν) are illustrated in Figure 4(b), the band gaps of the synthesized MoO 3 , TiO 2, and MoTiO 5 thin films were found to be 3.0 eV, 3.2 eV, and 3.6 eV, respectively. As can be seen, among the developed materials, MoTiO 5 is one of the semiconductor metal oxides with a wide band gap of 3.6 eV.

Results
XPS study was performed to indicate the presence of Mo/Ti/O bond in the nano-composites and Figure 5 shows the results. It is shown in Figure 5(a) that it is shown in the large-scan XPS spectrum that the overborne C. Ti, O, and Mo elements are formed through patterns among these elements and the C element is from the XPS instrument itself, and no other elements are appointed. The high-definition XPS spectrum of O 1s and the location of the binding energy of 530.3 eV are shown in Figure 5 Figure 6 shows the transient photocurrent of MoO 3 , TiO 2, and MoTiO 5 nanostructured electrodes produced under chopped one-sunlight illumination. Chrono-Amperometry measurements were carried out in 0.1 M KCl electrolyte under the irradiance of 100 mW/cm 2 from SolarLight-16 S for Pt counter and Ag/AgCl reference electrodes. The photocurrent counterpart was measured in 8 s on-off cycles at the short-circuit potential in 0.10 M KCl electrolyte solution without compromising reactive or co-catalysts. When exposed to solar illumination, photocurrents of MoO 3 , TiO 2, and MoTiO 5 electrodes rise to 1.03 mA cm -2 , 1.68 mA cm -2 , and 14.89 mA cm -2 , respectively. When there is no solar illumination, the photocurrent value drops to zero. Figure 6 depicts the quick and monotonic photocurrent in these transients and the rapid change transport mechanism and its connection to single crystalline MoTiO 5 . Furthermore, the photocurrents produced by the MoTiO 5 nanostructures are constant and reproducible across several cycles, showing that the electrode is photocorrosion-free. The chronoamperometry study present in Figure 6, also supports the results obtained from CV and EIS measurements. Moreover, it is also beneficial for their photocatalytic performance that the peerless structure of MoTiO 5 would make the light storage better [31,32].
Electrochemical performances of MoO 3 , TiO 2 , and MoTiO 5 materials were investigated by using CV and EIS techniques. CVs obtained in an electrochemical solution containing 0.1 M KNO 3 and 10 mM Fe(CN) 6 3-/ Fe(CN) 6 4for MoO 3 , TiO 2 and MoTiO 5 prepared in metal oxide materials are shown in Figure 7a. While the electrochemical activity of MoO 3 , and TiO 2 materials was quite low, the electrochemical activity of MoTiO 5 improved as seen in Figure 7a. Nyquist graphs obtained for TiO, MoO 3, and MoTiO 5 composite films prepared on ITO electrode in 0.1 M KCl solution containing 10 mM Fe(CN) 6 3− / Fe(CN) 6 4− are shown in Figure 7b. Nyquist graphs are fitted according to the electrical circuit given in Figure 7b. Here, the faradaic electron transfer resistance (Rp) corresponds to the diameter of the formed semicircle. The solution resistance (R w ) is the intersection point of the Real Z' axis of the graph [33]. The stationary phase element (CPE) is the capacitance of the double layer [34]. Accordingly, electron transfer resistances for MoO 3 , TiO 2, and MoTiO 5 composite films were determined as 862 Ω, 179 Ω, and 10 Ω, respectively. When the electron transfer values obtained were evaluated, it was determined that the electron transfer resistance was quite low since the electrochemical activity of the MoTiO 5 composite film was the highest. Thus, the impedance study carried out also supported the photocurrent studies.

Discussion
In conclusion, the structure of MoO 3 , TiO 2 , and MoTiO 5 nanostructures was effectively altered using the SILAR approach by adjusting basic preparation conditions. Experiments also indicated that altering the deposition time may change the size of MoTiO 5 nanostructures. MoTiO 5 nanostructures, photocurrent measurements demonstrated a decreased fault concentration and higher optical quality. The photocurrent produced by the MoTiO 5 nanostructures is steady and reproducible over many cycles, showing that the electrode is photocorrosion-free. The characterization methods and photoelectrochemical studies on these metal oxides have shown that the method applied in the synthesis of these materials affects the crystal structure and grade of the materials at the same time, the nanosize of the synthesized material has a direct effect on the electronic properties and performances of these materials.