Cost Effective Solvothermal Method to Synthesize Zn-Doped TiO 2 Nanomaterials for Photovoltaic and Photocatalytic Degradation Applications

: Titanium dioxide (TiO 2 ) is a commonly used wide bandgap semiconductor material for energy and environmental applications. Although it is a promising candidate for photovoltaic and photocatalytic applications, its overall performance is still limited due to low mobility of porous TiO 2 and its limited spectral response. This limitation can be overcome by several ways, one of which is doping that could be used to improve the light harvesting properties of TiO 2 by tuning its bandgap. TiO 2 doped with elements, such as alkali-earth metals, transition metals, rare-earth elements, and nonmetals, were found to improve its performance in the photovoltaic and photocatalytic applications. Among the doped TiO 2 nanomaterials, transition metal doped TiO 2 nanomaterials perform efﬁciently by suppressing the relaxation and recombination of charge carriers and improving the absorption of light in the visible region. This work reports the possibility of enhancing the performance of TiO 2 towards Dye Sensitised Solar Cells (DSSCs) and photocatalytic degradation of methylene blue (MB) by employing Zn doping on TiO 2 nanomaterials. Zn doping was carried out by varying the mole percentage of Zn on TiO 2 by a facile solvothermal method and the synthesized nanomaterials were characterised. The XRD (X-Ray Diffraction) studies conﬁrmed the presence of anatase phase of TiO 2 in the synthesized nanomaterials, unaffected by Zn doping. The UV-Visible spectrum of Zn-doped TiO 2 showed a red shift which could be attributed to the reduced bandgap resulted by Zn doping. Signiﬁcant enhancement in Power Conversion Efﬁciency (PCE) was observed with 1.0 mol% Zn-doped TiO 2 based DSSC, which was 35% greater than that of the control device. In addition, it showed complete degradation of MB within 3 h of light illumination and rate constant of 1.5466 × 10 − 4 s − 1 resembling zeroth order reaction. These improvements are attributed to the reduced bandgap energy and the reduced charge recombination by Zn doping on TiO 2 . maxima (FWHM), and is calculated from the predominant anatase (101) plane. The estimated crystalline sizes were about 12, 10, 9, 8, and 7 nm for undoped, 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO 2 nanomaterials, respectively. These results demonstrate that Zn doping reduces the crystalline size of nanomaterials. Huang et al. supported this observation by reporting that Zn–O–Ti bonds are formed during the doping of Zn on TiO 2 , which inhibit the growth of crystal grains of TiO 2 and subsequently reduce the size of particles [18]. The lattice strain of the nanomaterials was calculated using Williamson–Hall (W–H) plot method.


Introduction
The global population is increasing rapidly, and the limited supply of nonrenewable energy leads to rising energy demands, especially in developing countries. This risks the depletion of cheap fossil energy sources and an increase in environmental pollution as well as climate change [1]. Hence, researchers around the world were constantly inventing solutions to expand the energy sources and reduce greenhouse gas emissions. This took renewable energy sources into the spotlight [2]. Renewable energy sources such as solar, wind, geothermal, hydropower, biofuel, and biomass are considered to be cleaner and more environmentally friendly sources of power. However, energy production from these alternative energy sources is yet restricted because of high production costs and poor energy conversion efficiency. Nanotechnology is increasingly playing an active role in overcoming the above limitations [3,4].
Titanium dioxide (TiO 2 ) was widely used as a golden standard nanomaterial for sustainable energy generation and the removal of environmental pollutants [5,6] due to its efficient photocatalytic activity, low cost, nontoxic nature and high stability [7]. Although TiO 2 exhibits the desired performance in ultraviolet light, its overall performance is still limited because of low mobility of porous TiO 2 [8] and its limited spectral response wide bandgap (3.0-3.2 eV) that cannot make use of visible light [9]. Several strategies were investigated and reported to overcome this limitation, which include preparation of nanocomposites [10][11][12], doping/codoping [5,13,14], and synthesis of particles with different nanostructures [15,16], such as nanorods, nanowires, nanotubes, etc. Among these strategies, doping/co-doping displays major impacts on the band structure and trap states of TiO 2, and hence, alters its properties such as conduction band energy, charge transport, recombination, and collection significantly [17]. Even though nonmetals, alkaliearth metals, and rare-earth elements are being used as dopants, transition metal doped TiO 2 nanomaterials are notable candidates for photovoltaic and photocatalytic degradation applications as they improve absorption in the visible region and suppress relaxation and recombination of charge carriers [13].
Zinc (Zn) is one of the promising n-type transition metals; it improves the photocurrent of Dye Sensitized Solar Cell (DSSC) when incorporated with TiO 2 [18]. It also enhances the photocatalytic activity in visible region [19] by reducing the bandgap of TiO 2 [20,21]. Zhu et al. demonstrated the photovoltaic analysis of hydrothermally synthesized Zndoped TiO 2 photoanode, and their findings reveal that incorporation of Zn 2+ ions into an anatase lattice of TiO 2 elevates the edge of conduction band (CB) of the photoanode and the Fermi level is shifted towards the CB edge, which contributes to the improvement in open-circuit voltage (V OC ), and thus, enhanced photovoltaic performance [22]. Huang et al. reported that the device fabricated with Zn-doped TiO 2 nanomaterials, synthesized by agarose gel method, shows better photovoltaic performance due to the improved short circuit current density (J SC ) [18]. Zn-doped TiO 2 is widely used in photocatalysis as well. Zhao et al. analyzed the photocatalytic degradation of Rhodamine B dye using Zn-doped TiO 2 , synthesized by hydrogen-oxygen diffusion flame method, where Zn doping creates appropriate energetic position between ZnO and the excited state of dye molecule, which enhances the electron injection into the conduction band of TiO 2 by capturing electrons, and subsequently promotes the formation of reactive oxygen species which enhances the degradation process [20]. Chen et al. demonstrated the degradation of Methyl orange by Zn-doped TiO 2 synthesized by stearic acid gel method, and higher photocatalytic activity was achieved due to greater BET surface area by Zn doping [23]. Zn-doped TiO 2 was synthesized using facile sol-gel reflux method by Tariq et al., and the synthesized nanomaterials were used to degrade methylene blue (MB) and methyl orange dyes. Here, Zn acted as a reductant to facilitate the Ti 3+ formation and a stabilizer for the oxygen vacancies [21]. Hence, Zn effectively modified the properties of TiO 2 and improved the degradation of dye molecules.
The doped TiO 2 nanomaterials can be synthesized by different methods including hydrothermal synthesis [22], solvothermal synthesis [24], spray pyrolysis [25], microwave synthesis [26], and spin coating [27]. However, hydrothermal and solvothermal methods are preferable, as high crystalline nanomaterials are formed in these syntheses which leads to uniform particle distribution [28]. Although several studies reported on photovoltaic and photocatalytic activities of Zn-doped TiO 2 , the present study reports a simple solvothermal synthesis of Zn-doped TiO 2 nanomaterials utilizing cost effective reaction bottles instead of autoclave followed by characterization of the same, and investigates their efficiency on photovoltaic and photocatalytic degradation applications.

Results and Discussion
The Zn-doped and undoped TiO 2 nanomaterials, synthesized by a simple solvothermal method, were structurally and optically characterized utilizing XRD, EDX (Energy Dispersive X-ray), and UV-Visible spectroscopies, and their photovoltaic and photocatalytic degradation activity were studied.

X-ray Diffraction (XRD) Spectroscopy
The diffraction pattern and crystal structure of the synthesized, undoped, and 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO 2 nanomaterials were examined by XRD, as illustrated in Figure 1. For XRD analysis, the powder sample was placed on a microscopic glass plate and pressed by another glass plate to obtain thin layer of the sample.

Results and Discussion
The Zn-doped and undoped TiO2 nanomaterials, synthesized by a simple solvothermal method, were structurally and optically characterized utilizing XRD, EDX (Energy Dispersive X-ray), and UV-Visible spectroscopies, and their photovoltaic and photocatalytic degradation activity were studied.

X-Ray Diffraction (XRD) Spectroscopy
The diffraction pattern and crystal structure of the synthesized, undoped, and 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO2 nanomaterials were examined by XRD, as illustrated in Figure 1. For XRD analysis, the powder sample was placed on a microscopic glass plate and pressed by another glass plate to obtain thin layer of the sample.  (224), respectively. The observed peaks, corresponding to only anatase phase of TiO2, for all undoped and Zn-doped TiO2 nanomaterials indicate that incorporation of Zn on TiO2 did not influence phase transformation. This may be attributed to the similar ionic radii of Ti 4+ (0.74 Å) and Zn 2+ (0.605 Å) ions [29] which allow Zn 2+ ions to be easily accommodated on to the TiO2 lattice without altering their crystal structure. The average crystalline size of the synthesized nanomaterials was estimated by Debye-Scherrer equation using the predominant anatase (101) plane.

=
(1) Where, d is the average crystalline size of the particles, k is the dimensionless shape factor which has a typical value of about 0.89, λ is the wavelength of the X-ray beam (0.5406 nm), θ is the Bragg angle, and β is the full width at half maxima (FWHM), and is calculated from the predominant anatase (101) plane. The estimated crystalline sizes were about 12, 10, 9, 8, and 7 nm for undoped, 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO2 nanomaterials, respectively. These results demonstrate that Zn doping reduces the crystalline size of nanomaterials. Huang et al. supported this observation by reporting that Zn-O-Ti bonds are formed during the doping of Zn on TiO2, which inhibit the growth of crystal grains of TiO2 and subsequently reduce the size of particles [18]. The lattice strain of the nanomaterials was calculated using Williamson-Hall (W-H) plot method. The average crystalline size of the synthesized nanomaterials was estimated by Debye-Scherrer equation using the predominant anatase (101) plane.
where, d is the average crystalline size of the particles, k is the dimensionless shape factor which has a typical value of about 0.89, λ is the wavelength of the X-ray beam (0.5406 nm), θ is the Bragg angle, and β is the full width at half maxima (FWHM), and is calculated from the predominant anatase (101) plane. The estimated crystalline sizes were about 12, 10, 9, 8, and 7 nm for undoped, 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO 2 nanomaterials, respectively. These results demonstrate that Zn doping reduces the crystalline size of nanomaterials. Huang et al. supported this observation by reporting that Zn-O-Ti bonds are formed during the doping of Zn on TiO 2 , which inhibit the growth of crystal grains of TiO 2 and subsequently reduce the size of particles [18]. The lattice strain of the nanomaterials was calculated using Williamson-Hall (W-H) plot method.
where, λ is the wavelength of X-ray radiation, β is FWHM, and θ is the Bragg angle of the diffraction peaks, d is the crystallite size with lattice strain, and ε is the effective value of the lattice strain. βCosθ is plotted against 4Sinθ, and after linear fitting, the slope gives the value of lattice strain [30,31]. The values of the lattice strain are 7.53 × 10 −5 , 1.16 × 10 −4 , 5.38 × 10 −4 , −0.00499, and −0.00296 for the undoped, and 0.5, 1.0, 1.5, and 2.0 for the mol% Zn-doped TiO 2 nanomaterials, respectively. Positive lattice strain value indicates that the system is under tensile strain, and negative value indicates that the system is under compressive strain [32]. Lattice strain of the 1.0 and 0.5 mol% of Zn-doping is higher than undoped TiO 2 due to the reduced crystalline size, and the values are negative for the 1.5 and 2.0 mol% of Zn doping [32,33]. The formation of ZnO in the higher Zn doping may cause distortion, which might be the reason for the negative value. Further, the absence of peaks for Zn in the XRD of Zn-doped TiO 2 nanomaterials may be attributed to the very small quantity of Zn doped on TiO 2 and homogeneous dispersion of Zn on TiO 2 ; the same was reported by Yanqi et al. [34] and Feng et al. [22].

Energy Dispersive X-ray (EDX) Spectroscopy
The existence of Zn in the Zn-doped TiO 2 nanomaterials was confirmed by EDX analysis, as shown in Figure 2. The summary of EDX results, presented in Table 1, confirms that the amount of Zn doped in TiO 2 rises with the increase in Zn dopant used for the synthesis of Zn-doped TiO 2 nanomaterials, and the distribution of Zn dopant in TiO 2 is found to be uniform.
Where, λ is the wavelength of X-ray radiation, β is FWHM, and θ is the Bragg an of the diffraction peaks, is the crystallite size with lattice strain, and ε is the effect value of the lattice strain. βCosθ is plotted against 4Sinθ, and after linear fitting, the sl gives the value of lattice strain [30,31]. The values of the lattice strain are 7.53 × 10 −5 , 1.1 10 −4 , 5.38 × 10 −4 , −0.00499, and −0.00296 for the undoped, and 0.5, 1.0, 1.5, and 2.0 for mol% Zn-doped TiO2 nanomaterials, respectively. Positive lattice strain value indica that the system is under tensile strain, and negative value indicates that the system under compressive strain [32]. Lattice strain of the 1.0 and 0.5 mol% of Zn-doping higher than undoped TiO2 due to the reduced crystalline size, and the values are negat for the 1.5 and 2.0 mol% of Zn doping [32,33]. The formation of ZnO in the higher doping may cause distortion, which might be the reason for the negative value. Furth the absence of peaks for Zn in the XRD of Zn-doped TiO2 nanomaterials may be tributed to the very small quantity of Zn doped on TiO2 and homogeneous dispersion Zn on TiO2; the same was reported by Yanqi et al. [34] and Feng et al. [22].

Energy Dispersive X-ray (EDX) Spectroscopy
The existence of Zn in the Zn-doped TiO2 nanomaterials was confirmed by E analysis, as shown in Figure 2. The summary of EDX results, presented in Table 1, c firms that the amount of Zn doped in TiO2 rises with the increase in Zn dopant used the synthesis of Zn-doped TiO2 nanomaterials, and the distribution of Zn dopant in T is found to be uniform.

UV-Visible Spectroscopy
To study the optical properties of the Zn-doped and undoped TiO 2 , the powder samples were coated on the microscopic glass plate by doctor-blade method and analyzed by UV-visible spectroscopy, as shown in Figure 3. Zn doping in TiO 2 led to a red-shift in the UV-visible light absorption of TiO 2 and the shift moved towards longer wavelength when the Zn content increased in the Zn-doped TiO 2 nanomaterials.

UV-Visible Spectroscopy
To study the optical properties of the Zn-doped and undoped TiO2, the powder samples were coated on the microscopic glass plate by doctor-blade method and analyzed by UV-visible spectroscopy, as shown in Figure 3. Zn doping in TiO2 led to a red-shift in the UV-visible light absorption of TiO2 and the shift moved towards longer wavelength when the Zn content increased in the Zn-doped TiO2 nanomaterials. The Tauc's formula, given below, was used to estimate the bandgap of the films from UV-visible Spectra: where is the absorbance coefficient, ℎ is the photon energy, A is a constant, is the bandgap energy, and n is the exponential constant index which depends on the nature of transition (n = ½ and 2 for indirectly and directly allowed transitions, respectively). The estimated bandgap values obtained from Tauc-plot are 3.3, 3.1, 3.0, 2.9, and 2.7 eV for the undoped, 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO2, respectively. The above bandgap values, which reveal Zn-doping in TiO2 reduces the bandgap of TiO2, are in good agreement with the theoretical values reported by the Wang et al. [35] and experimental trends reported by Arunachalam et al. [36] and Aware et al. [37]. It is noteworthy that observations contradictory to the present study were also reported in literature [22].

Photovoltaic Measurement
The photovoltaic performance of the fabricated DSSCs with undoped and Zn-doped The Tauc's formula, given below, was used to estimate the bandgap of the films from UV-visible Spectra: where α is the absorbance coefficient, hν is the photon energy, A is a constant, E g is the bandgap energy, and n is the exponential constant index which depends on the nature of transition (n = 1 2 and 2 for indirectly and directly allowed transitions, respectively). The estimated bandgap values obtained from Tauc-plot are 3.3, 3.1, 3.0, 2.9, and 2.7 eV for the undoped, 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO 2 , respectively. The above bandgap values, which reveal Zn-doping in TiO 2 reduces the bandgap of TiO 2 , are in good agreement with the theoretical values reported by the Wang et al. [35] and experimental trends reported by Arunachalam et al. [36] and Aware et al. [37]. It is noteworthy that observations contradictory to the present study were also reported in literature [22].  Figure 4, and the related photovoltaic parameters are summarized in Table 2.  The results reveal improved JSC, open circuit voltage (VOC), and fill factor (FF) values for DSSCs fabricated with 0.5 and 1.0 mol% Zn-doped TiO2 compared to that of the control device. The FF value increased up to 0.69 for the device made with 1.0 mol% Zn-doped TiO2, followed by a reduction to 0.66 for the one made with 2.0 mol% Zn dopant. The VOC sequentially increased from 0.67 to 0.71 V with increase in Zn content in the Zn-doped TiO2. Nevertheless, the performance of the device, η, was mainly dominated by its JSC value (as illustrated in Figure 4 (b)). The observed highest JSC value of 11.64 mAcm −2 corresponds to the device with 1.0 mol% Zn-doped TiO2 photoanode. Zn is a n-type dopant, and hence, Zn doping increases the electron density in the conduction band of TiO2, and thereby improves charge transport in the device [38]. A similar trend was observed in the present study up to 1.0 mol% of Zn content, but the charge transport was reduced with 1.5 and 2.0 mol% of dopant concentrations, maybe due to the recombination of electron-hole pairs. Jianping et al. reported on the formation of a shallow trapping centre of photo-generated carriers at low amounts of Zn dopant, which results in efficient separation of electron-hole pairs and long carrier lifetime and subsequent reduction in carrier recombination followed by rise in photocurrent. However, at higher amounts of Zn dopant, ZnO clusters onto the surface of TiO2 by forming surface states.  The results reveal improved J SC , open circuit voltage (V OC ), and fill factor (FF) values for DSSCs fabricated with 0.5 and 1.0 mol% Zn-doped TiO 2 compared to that of the control device. The FF value increased up to 0.69 for the device made with 1.0 mol% Zn-doped TiO 2 , followed by a reduction to 0.66 for the one made with 2.0 mol% Zn dopant. The V OC sequentially increased from 0.67 to 0.71 V with increase in Zn content in the Zn-doped TiO 2 . Nevertheless, the performance of the device, η, was mainly dominated by its J SC value (as illustrated in Figure 4 (b)). The observed highest J SC value of 11.64 mAcm −2 corresponds to the device with 1.0 mol% Zn-doped TiO 2 photoanode. Zn is a n-type dopant, and hence, Zn doping increases the electron density in the conduction band of TiO 2, and thereby improves charge transport in the device [38]. A similar trend was observed in the present study up to 1.0 mol% of Zn content, but the charge transport was reduced with 1.5 and 2.0 mol% of dopant concentrations, maybe due to the recombination of electron-hole pairs. Jianping et al. reported on the formation of a shallow trapping centre of photo-generated carriers at low amounts of Zn dopant, which results in efficient separation of electron-hole pairs and long carrier lifetime and subsequent reduction in carrier recombination followed by rise in photocurrent. However, at higher amounts of Zn dopant, ZnO clusters onto the surface of TiO 2 by forming surface states. Hence, ZnO acts as a defect site and captures the electrons, which results in an increase in the recombination rate and causes a decrease in J SC [38]. The present study exhibited an improved PCE of 5.67% for 1.0 mol% Zn-doped TiO 2 photoanode-based device, which was 35% higher than that of the control device fabricated with undoped TiO 2 . This improvement in PCE is mainly attributed to the enhanced J SC value; this observation was further confirmed by the EIS measurements.

Electrochemical Impedance Spectroscopy (EIS)
The EIS measurements were performed on the fabricated DSSCs by two electrode method at frequencies from 10 −2 to 10 6 Hz in the dark, with a bias applied voltage of 10 mV. Figure 5 shows the Nyquist plot of the electrochemical impedance spectra of DSSCs fabricated with undoped and Zn-doped TiO 2 photoanodes.
Catalysts 2021, 11, x FOR PEER REVIEW 7 of 14 mV. Figure 5 shows the Nyquist plot of the electrochemical impedance spectra of DSSCs fabricated with undoped and Zn-doped TiO2 photoanodes. As shown in the Figure 5, these Nyquist plots are related to the charge recombination resistance (R2) across the TiO2/electrolyte interface, with a partial contribution from electron transport and accumulation in TiO2 photoanode [39]. R2 value of the undoped and Zn-doped TiO2 photoanodes-based DSSCs can be estimated from the diameter of the respective semicircles [13]; a high R2 value indicates low charge recombination rate [13,40]. In the present study, the R2 value of undoped TiO2 is lower than those of 0.5 and 1.0 mol% Zn-doped TiO2 based DSSCs and higher than that of 1.5 and 2.0 mol% Zn-doped TiO2 based device. Hence, the lowest charge recombination rate is attributed to the 1.0 mol% Zn-doped TiO2 based device. These results are in good agreement with the JSC values and the corresponding η values of the fabricated devices.

Photocatalytic Degradation Measurement
Photocatalytic degradation of MB using Zn-doped and undoped TiO2 nanomaterials was performed under direct sunlight. Prior to the light exposure, the reaction suspension was allowed to establish adsorption-desorption equilibrium by stirring the suspension in dark for 30 min. Subsequently, the suspension was exposed to direct sunlight for a stipulated period (noon daytime) during which the MB retained in the suspension was measured at regular time intervals. The concentration of MB decreased with time, and the absorption peak corresponding to MB (λ = 663 nm) was found to disappear after 2.5 h. The concentration vs. time plots obtained for different Zn-doped TiO2 (0.5, 1.0, 1.5, and 2.0 mol%) and undoped TiO2 samples are depicted in Figure 6. As shown in the Figure 5, these Nyquist plots are related to the charge recombination resistance (R 2 ) across the TiO 2 /electrolyte interface, with a partial contribution from electron transport and accumulation in TiO 2 photoanode [39]. R 2 value of the undoped and Zn-doped TiO 2 photoanodes-based DSSCs can be estimated from the diameter of the respective semicircles [13]; a high R 2 value indicates low charge recombination rate [13,40]. In the present study, the R 2 value of undoped TiO 2 is lower than those of 0.5 and 1.0 mol% Zn-doped TiO 2 based DSSCs and higher than that of 1.5 and 2.0 mol% Zn-doped TiO 2 based device. Hence, the lowest charge recombination rate is attributed to the 1.0 mol% Zn-doped TiO 2 based device. These results are in good agreement with the J SC values and the corresponding η values of the fabricated devices.

Photocatalytic Degradation Measurement
Photocatalytic degradation of MB using Zn-doped and undoped TiO 2 nanomaterials was performed under direct sunlight. Prior to the light exposure, the reaction suspension was allowed to establish adsorption-desorption equilibrium by stirring the suspension in dark for 30 min. Subsequently, the suspension was exposed to direct sunlight for a stipulated period (noon daytime) during which the MB retained in the suspension was measured at regular time intervals. The concentration of MB decreased with time, and the absorption peak corresponding to MB (λ = 663 nm) was found to disappear after 2.5 h. The concentration vs. time plots obtained for different Zn-doped TiO 2 (0.5, 1.0, 1.5, and 2.0 mol%) and undoped TiO 2 samples are depicted in Figure 6.
The MB dye was used as probing molecule to explore the photocatalytic degradation property of the synthesized nanomaterials. For the overall reaction, the zeroth order kinetic model with the optimum rate constant of 1.5466 ± 0.0873 × 10 −4 s −1 for TiO 2 doped 1.0% Zn doping (Table 3). There was a minimum 41% of increase in the activity while Zn was doped in 0.5% over bare TiO 2 . Moreover, the dark adsorption was carried out for 30 min to let the adsorption-desorption equilibrium be achieved, and during this process, the dye molecules are merely transferred from liquid stage to the solid state. With the absorbance vs. wavelength plot (as illustrated in Figure S1a) of dark adsorption studies, once the adsorption-desorption equilibrium is achieved, there is not much change in the concentration of dye with time. However, the degradation occurs under the light exposure in the presence of catalyst (as illustrated in Figure S1b), and these results validate the contribution of catalyst in the dye degradation process. Further, the study without catalyst confirms that there is no self-degradation of dyes (as illustrated in Figure S1c). In summary, the catalysts used in this study are very effective against dye degradation in the presence of sunlight. Further, recycling study was carried out with 1% Zn-doped TiO 2 nanomaterial to examine its reusability. After the photocatalytic experiment, the catalyst was separated from the reaction mixture by centrifugation and used for 2nd cycle by following the same experimental conditions. The MB dye was used as probing molecule to explore the photocatalytic degradation property of the synthesized nanomaterials. For the overall reaction, the zeroth order kinetic model with the optimum rate constant of 1.5466 ± 0.0873 × 10 −4 s −1 for TiO2 doped 1.0% Zn doping (Table 3). There was a minimum 41% of increase in the activity while Zn was doped in 0.5% over bare TiO2. Moreover, the dark adsorption was carried out for 30 min to let the adsorption-desorption equilibrium be achieved, and during this process, the dye molecules are merely transferred from liquid stage to the solid state. With the absorbance vs. wavelength plot (as illustrated in Figure S1a) of dark adsorption studies, once the adsorption-desorption equilibrium is achieved, there is not much change in the concentration of dye with time. However, the degradation occurs under the light exposure in the presence of catalyst (as illustrated in Figure S1b), and these results validate the contribution of catalyst in the dye degradation process. Further, the study without catalyst confirms that there is no self-degradation of dyes (as illustrated in Figure S1c). In summary, the catalysts used in this study are very effective against dye degradation in the presence of sunlight. Further, recycling study was carried out with 1% Zn-doped TiO2 nanomaterial to examine its reusability. After the photocatalytic experiment, the catalyst was separated from the reaction mixture by centrifugation and used for 2nd cycle by following the same experimental conditions.  As shown in Figure 7, the photocatalyst did not exhibit any significant loss in the 2nd cycle of degradation of MB under the same conditions. The calculated degradation percentage were found to be 99% and 82% for 1st and 2nd use, respectively. The results confirm that the synthesized Zn-doped TiO 2 photocatalyst shows good stability and sus-tainability. Moreover, the rate of the photocatalytic degradation results obtained in this study is compared with that of the literature and summarized in the As shown in Figure 7, the photocatalyst did not exhibit any significant loss in the 2nd cycle of degradation of MB under the same conditions. The calculated degradation percentage were found to be 99% and 82% for 1st and 2nd use, respectively. The results confirm that the synthesized Zn-doped TiO2 photocatalyst shows good stability and sustainability. Moreover, the rate of the photocatalytic degradation results obtained in this study is compared with that of the literature and summarized in the Table 4.    Although the experimental conditions and other factors vary, based on the summary of the table, our proposed synthesised nanomaterials effectively degrade the MB under the sunlight compared to that of the other reports. Therefore, this cost effective solvothermal method is preferable for the synthesis of nanomaterials for energy and environmental applications.

Synthesis of Zn-Doped and Undoped TiO 2 Nanomaterials
A 3 mL mixture of TTIP and ethanol in 1:16 volume ratio was vigorously stirred in a 50 mL reaction bottle. Appropriate amounts of Zn(CH 3 COO) 2 .2H 2 O were then added separately to the above solution to prepare different photocatalysts with 0.5, 1.0, 1.5, and 2.0 mol% Zn, and the resultant mixtures were stirred well for one hour. Subsequently, a solution containing ethanol and deionized water in 1:1 volume ratio was added separately to the above mixtures. A white color precipitate was attained after addition of ethanolwater mixture. Then, the products were sintered at 90 • C for 6 h, dried at 100 • C to get powder samples, and subsequently calcinated at 500 • C for 2 h to obtain 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO 2 nanomaterials [46]. The undoped TiO 2 was synthesized by adopting the same procedure without employing Zn(CH 3 COO) 2 .2H 2 O.

Device Fabrication
The Fluorine doped Tin Oxide (FTO) coated glass substrates (Sigma-Aldrich surface resistivity 7.5 Ω/cm 2 ) were cleaned in ultrasonic bath for 10 min with soap water, distilled water and ethanol, successively. The synthesized undoped and Zn-doped TiO 2 nanomaterials were coated separately on cleaned FTO substrates by doctor-blade method. Then, the resultant films were calcinated at 500 • C for 30 min and soaked in 0.3 mM solution of N719 dye and prepared in a mixture of acetonitrile and tert-butyl alcohol [46,47] for 12 h. Afterwards, the dye coated films were rinsed in acetonitrile and subsequently dried. The platinum (Pt) coated glass substrate [48] was assembled with each dye-coated photoanode as counter electrode. Finally, a small amount of I − /I − 3 electrolyte was injected in between the dye coated photoanode and Pt counter electrode to complete the fabrication of DSSC.

Photocatalytic Degradation
Photocatalytic degradation of MB solution was performed using undoped and 0.5, 1.0, 1.5, and 2.0 mol% Zn-doped TiO 2 nanomaterials. As reported in our previous study [49], 25.0 mg of the photocatalyst was suspended in 50 mL of MB solution (initial concentration was 10 ppm) under direct sunlight (intensity of~100 mWcm −2 ). Prior to irradiation, the MB solution with photocatalyst was stirred in the dark for 30 min to ensure the establishment of adsorption or desorption equilibrium. Periodically, 3 mL of suspension was withdrawn and centrifuged, and its absorbance was obtained using UV-visible spectrophotometer (JENWAY 6800, OSA, UK).

Characterization
The structural properties of the synthesized nanomaterials were studied by the Xray diffraction method (PANalytical-AERIS, Almelo, The Netherland) using scan range (2θ) between 20-90 • with step size of 0.02 • and scan speed of 1 • /min. The optical absorbance spectra were recorded using Shimadzu 1800 Scanning Double Beam UV-visible spectrophotometer. The elemental composition of the synthesized nanomaterials was analyzed by the energy-dispersive X-ray spectroscopy technique. The photovoltaic performance of the cells was studied using Keithley-2400 source measurement unit (SMU) under simulated irradiation by 150 W Xe lamp of intensity 100 mWcm −2 with AM 1.5 filter (Peccell-PEC-L12, Kanagawa, Japan), and the effective area of the devices is 0.25 cm 2 . Elec-trochemical impedance spectroscopy (EIS) measurements were carried out on the DSSCs using Metrohm Autolab potentiostat/galvanostat (PGSTAT 128N, Utrecht, Netherlands) with a frequency response analyzer (FRA 32M).

Conclusions
In the present study, undoped and Zn-doped TiO 2 nanomaterials were synthesized by a simple solvothermal method and characterized. The XRD study revealed the existence of anatase phase without any phase transformation in all the synthesized nanomaterials. The UV-visible spectra of Zn-doped TiO 2 nanomaterials demonstrated red shift compared to that of the spectrum of undoped TiO 2 . The EDX spectroscopy confirmed the inclusion of Zn element in the Zn-doped TiO 2 nanomaterials. The photovoltaic performances of DSSCs fabricated with the synthesized undoped and Zn-doped nanomaterials were analyzed; 1.0 mol% Zn-doped TiO 2 based device exhibited PCE of 5.67%, which was greater than 35% enhancement compared to that of the control device due to increased J SC . Moreover, 1.0 mol% Zn-doped TiO 2 displayed efficient photocatalytic property in the degradation of MB under direct sun light. The improved performance revealed by the 1.0 mol% Zn-doped TiO 2 nanomaterials in both photovoltaic and photocatalytic applications could be attributed to its reduced charge recombination rate and enhanced visible light harvesting ability.