TiO2-ZnO Binary Oxide Systems: Comprehensive Characterization and Tests of Photocatalytic Activity

A series of TiO2-ZnO binary oxide systems with various molar ratios of TiO2 and ZnO were prepared using a sol-gel method. The influence of the molar ratio and temperature of calcination on the particle sizes, morphology, crystalline structure, surface composition, porous structure parameters, and thermal stability of the final hybrids was investigated. Additionally, to confirm the presence of characteristic surface groups of the material, Fourier transform infrared spectroscopy was applied. It was found that the crystalline structure, porous structure parameters, and thermal stability were determined by the molar ratio of TiO2 to ZnO and the calcination process for the most part. A key element of the study was an evaluation of the photocatalytic activity of the TiO2-ZnO hybrids with respect to the decomposition of C.I. Basic Blue 9, C.I. Basic Red 1, and C.I. Basic Violet 10 dyes. It was found that the TiO2-ZnO material obtained with a molar ratio of TiO2:ZnO = 9:1 and calcined at 600 °C demonstrates high photocatalytic activity in the degradation of the three organic dyes when compared with pristine TiO2. Moreover, an attempt was made to describe equilibrium aspects by applying the Langmuir-Hinsherlwood equation.


Introduction
Photocatalysis is an effective process for creating minerals out of pollutants in the air and water such as simple inorganic compounds in the presence of a catalyst [1]. The most common and widely described heterogeneous photocatalysts are transition metal oxides and semiconductors such as TiO 2 , ZnO, SnO 2 , and CeO 2 [2][3][4][5]. Titanium dioxide is the most active of the compounds that have been tested. It is relatively cheap, photochemically stable, non-toxic, easily UV-activated, and insoluble in most reaction environments [6,7]. However, its application is limited because of its narrow photocatalytic region (α < 400 nm) and its ability to absorb only a small fraction (5%) of incident solar irradiation, which results from its relatively large band gap (anatase,~3.2 eV) [8]. Many recent studies have focused on modifying the morphology and crystalline structure of TiO 2 to improve its photocatalytic activity. Modification may be performed by adding transition metal ions (such as Cr, Zr, Mn, and Mo) [9][10][11][12], preparing a reduced form of TiO 2−x , sensitization using dyes [13,14], doping with non-metals (such

Preparation of TiO 2 -ZnO Oxide Systems Using the Sol-Gel Method
The synthesis of TiO 2 -ZnO oxide hybrids with TiO 2 :ZnO molar ratios of 9:1, 5:2, and 1:3 was performed by the sol-gel method. First, a reactor equipped with a T25 Basic type high-speed stirrer (IKA Werke GmbH, Staufen im Breisgau, Germany) was filled with a mixture containing an appropriate amount of TTIP in IPA. The resulting mixture was stirred at 1000 rpm. Afterward, an appropriate amount of 15% zinc acetate solution (the precursor of ZnO was dissolved in a mixture of IPA:H 2 O at a volume ratio of 1:3) was introduced at a constant rate of 5 cm 3 /min using an ISM833A peristaltic pump (ISMATEC, Wertheim, Germany). The synthesis was performed at room temperature. The reaction system was additionally stirred for 10 min. After this time, the promoter of hydrolysis (a mixture of ammonia and deionized water at a volume ratio of 1:3) was added at a constant rate of 1 cm 3 /min. The colloidal suspension was mixed for 1 h and the resulting alcogel was dried at 120 • C for 24 h. To remove impurities, the white precipitate was washed several times with deionized water. Lastly, the powder was dried at 80 • C for 3 h and additionally calcined at 600 • C for 2 h (Nabertherm P320 Controller, Lilienthal, Germany). The methodology of the synthesis of TiO 2 -ZnO oxide materials is presented in Figure 1. (IKA Werke GmbH, Staufen im Breisgau, Germany) was filled with a mixture containing an appropriate amount of TTIP in IPA. The resulting mixture was stirred at 1000 rpm. Afterward, an appropriate amount of 15% zinc acetate solution (the precursor of ZnO was dissolved in a mixture of IPA:H2O at a volume ratio of 1:3) was introduced at a constant rate of 5 cm 3 /min using an ISM833A peristaltic pump (ISMATEC, Wertheim, Germany). The synthesis was performed at room temperature. The reaction system was additionally stirred for 10 min. After this time, the promoter of hydrolysis (a mixture of ammonia and deionized water at a volume ratio of 1:3) was added at a constant rate of 1 cm 3 /min. The colloidal suspension was mixed for 1 h and the resulting alcogel was dried at 120 °C for 24 h. To remove impurities, the white precipitate was washed several times with deionized water. Lastly, the powder was dried at 80 °C for 3 h and additionally calcined at 600 °C for 2 h (Nabertherm P320 Controller, Lilienthal, Germany). The methodology of the synthesis of TiO2-ZnO oxide materials is presented in Figure 1.

Analysis of Materials
The particle sizes of the synthesized materials were measured by the non-invasive backscattering method (NIBS) using a Zetasizer Nano ZS (Malvern Instruments Ltd., Worcester, UK) instrument and enabling measurements in the diameter range 0.6-6000 nm. Each sample was prepared by dispersing 0.01 g of the tested product in 25 cm 3 of propan-2-ol. The resulting system was sonicated for 15 min and then placed in a cuvette and analyzed.
The surface microstructure and morphology of the TiO2-ZnO binary oxide systems were examined on the basis of SEM images recorded from an EVO40 scanning electron microscope (Zeiss, Jena, Germany). Before testing, samples were coated with gold (Au) for 5 s using a Balzers PV205P (Oerlikon Balzers Coating SA,. Brügg, Switzerland) coater.
The crystalline structure of the synthesized binary oxide materials was analyzed by the X-ray diffraction method (XRD) using a D8 Advance diffractometer (Bruker, Karlsruhe, Germany) operating with Cu Kα radiation (α = 1.5418 Å), Ni filtered. The patterns were obtained in a stepscanning mode (Δ2θ = 0.05°) over an angular range of 10° to 80°.
High resolution transmission electron microscopy (HRTEM) images as well as dark field scanning TEM (DF STEM) selected area TEM diffractograms and EDS elemental maps were recorded by means of JEOL ARM 200F microscope (JEOL, Peabody, MA, USA) operating at an accelerating voltage of 200 kV. In order to prepare specimens, particular powders were dispersed in alcohol and then a few drops of such solution were placed on copper grids coated with carbon and formvar.

Analysis of Materials
The particle sizes of the synthesized materials were measured by the non-invasive backscattering method (NIBS) using a Zetasizer Nano ZS (Malvern Instruments Ltd., Worcester, UK) instrument and enabling measurements in the diameter range 0.6-6000 nm. Each sample was prepared by dispersing 0.01 g of the tested product in 25 cm 3 of propan-2-ol. The resulting system was sonicated for 15 min and then placed in a cuvette and analyzed.
The surface microstructure and morphology of the TiO 2 -ZnO binary oxide systems were examined on the basis of SEM images recorded from an EVO40 scanning electron microscope (Zeiss, Jena, Germany). Before testing, samples were coated with gold (Au) for 5 s using a Balzers PV205P (Oerlikon Balzers Coating SA,. Brügg, Switzerland) coater.
The crystalline structure of the synthesized binary oxide materials was analyzed by the X-ray diffraction method (XRD) using a D8 Advance diffractometer (Bruker, Karlsruhe, Germany) operating with Cu Kα radiation (α = 1.5418 Å), Ni filtered. The patterns were obtained in a step-scanning mode (∆2θ = 0.05 • ) over an angular range of 10 • to 80 • .
High resolution transmission electron microscopy (HRTEM) images as well as dark field scanning TEM (DF STEM) selected area TEM diffractograms and EDS elemental maps were recorded by means of JEOL ARM 200F microscope (JEOL, Peabody, MA, USA) operating at an accelerating voltage of 200 kV. In order to prepare specimens, particular powders were dispersed in alcohol and then a few drops of such solution were placed on copper grids coated with carbon and formvar.
The surface composition of TiO 2 -ZnO oxide hybrids (content of TiO 2 and ZnO) was analyzed by using energy dispersive X-ray spectroscopy (EDS) using a Princeton Gamma-Tech unit equipped with a prism digital spectrometer (Princeton Gamma-Tech, Princeton, NJ, USA). Representative parts of each sample (500 µm 2 ) were analyzed to determine their actual surface composition.
The parameters of the porous structure of the obtained oxide powders were measured using a physisorption analyzer (ASAP 2020, Micromeritics Instrument Co., Norcross, GA, USA) operating based on a low-temperature adsorption of nitrogen. Before measurement, all materials were degassed at 120 • C for 4 h. Surface area was determined by the multipoint BET method using adsorption data in a relative pressure (p/p 0 ) range of 0.05-0.30. The desorption isotherm was used to determine the pore size distribution based on the Barrett, Joyner, Halenda (BJH) model.
Characteristic functional groups present on the surface of the obtained materials were identified using Fourier transform infrared spectroscopy (FTIR). The measurements were performed using a Vertex 70 spectrophotometer (Bruker, Karlsruhe, Germany). Samples were prepared by mixing with KBr and pressing into small tablets. FTIR spectra were obtained in the transmission mode between 4000 cm −1 and 400 cm −1 .
A thermogravimetric analyzer (Jupiter STA 449F3, Netzsch, Selb, Germany) was used to investigate the thermal stability of the synthesized materials. Measurements were carried out under nitrogen flow (10 cm 3 /min) at a heating rate of 10 • C/min over a temperature range of 30 • C to 1000 • C with an initial sample weight of approximately 5 mg.

Photocatalytic Tests
The photocatalytic activity of the obtained TiO 2 -ZnO binary oxide systems was evaluated based on the decomposition of C.I. Basic Blue 9 (MB), C.I. Basic Red 1 (R6G), and C.I. Basic Violet 10 (RhB) dyes (see , Table 1) in an initial concentration of 5 mg/dm 3 . The surface composition of TiO2-ZnO oxide hybrids (content of TiO2 and ZnO) was analyzed by using energy dispersive X-ray spectroscopy (EDS) using a Princeton Gamma-Tech unit equipped with a prism digital spectrometer (Princeton Gamma-Tech, Princeton, NJ, USA). Representative parts of each sample (500 µm 2 ) were analyzed to determine their actual surface composition.
The parameters of the porous structure of the obtained oxide powders were measured using a physisorption analyzer (ASAP 2020, Micromeritics Instrument Co., Norcross, GA, USA) operating based on a low-temperature adsorption of nitrogen. Before measurement, all materials were degassed at 120 °C for 4 h. Surface area was determined by the multipoint BET method using adsorption data in a relative pressure (p/p0) range of 0.05-0.30. The desorption isotherm was used to determine the pore size distribution based on the Barrett, Joyner, Halenda (BJH) model.
Characteristic functional groups present on the surface of the obtained materials were identified using Fourier transform infrared spectroscopy (FTIR). The measurements were performed using a Vertex 70 spectrophotometer (Bruker, Karlsruhe, Germany). Samples were prepared by mixing with KBr and pressing into small tablets. FTIR spectra were obtained in the transmission mode between 4000 cm −1 and 400 cm −1 .
A thermogravimetric analyzer (Jupiter STA 449F3, Netzsch, Selb, Germany) was used to investigate the thermal stability of the synthesized materials. Measurements were carried out under nitrogen flow (10 cm 3 /min) at a heating rate of 10 °C/min over a temperature range of 30 °C to 1000 °C with an initial sample weight of approximately 5 mg.

Photocatalytic Tests
The photocatalytic activity of the obtained TiO2-ZnO binary oxide systems was evaluated based on the decomposition of C.I. Basic Blue 9 (MB), C.I. Basic Red 1 (R6G), and C.I. Basic Violet 10 (RhB) dyes (see , Table 1) in an initial concentration of 5 mg/dm 3 . Photocatalysis was carried out in a laboratory reactor of UV-RS2 type (Heraeus, Hanau, Germany) equipped with a 150 W medium-pressure mercury lamp as a UV light source surrounded by a water-cooling quartz jacket. First, an appropriate amount of photocatalyst (TiO2-ZnO binary oxide material) was added to a glass tube reactor containing 100 cm 3 of the model organic impurity. The suspension was stirred using an R05 IKAMAG magnetic stirrer (IKA Werke GmbH, Staufen im Breisgau, Germany) for 30 min in darkness to determine the adsorption/desorption equilibrium. After this time, the radiation was turned on to initiate the photocatalytic reaction. The process was carried out for a maximum of 150 min. In the next step, the irradiated mixtures were collected from the reactor at regular intervals and centrifuged to separate the photocatalyst. The concentration of C.I. Basic Blue 9, C.I. Basic Red 1, or C.I. Basic Violet 10 (after adsorption and UV irradiation) was analyzed using a UV-Vis spectrophotometer (V-750, Jasco, Oklahoma City, OK, USA) at a wavelength of 664 nm (for MB), 526 nm (for R6G), or 553 nm (for RhB) using water as a reference. The photocatalytic activity of the TiO2-ZnO binary oxide systems was determined by calculating the yield of dye degradation (W) using the formula below. The surface composition of TiO2-ZnO oxide hybrids (content of TiO2 and ZnO) was analyzed by using energy dispersive X-ray spectroscopy (EDS) using a Princeton Gamma-Tech unit equipped with a prism digital spectrometer (Princeton Gamma-Tech, Princeton, NJ, USA). Representative parts of each sample (500 µm 2 ) were analyzed to determine their actual surface composition.
The parameters of the porous structure of the obtained oxide powders were measured using a physisorption analyzer (ASAP 2020, Micromeritics Instrument Co., Norcross, GA, USA) operating based on a low-temperature adsorption of nitrogen. Before measurement, all materials were degassed at 120 °C for 4 h. Surface area was determined by the multipoint BET method using adsorption data in a relative pressure (p/p0) range of 0.05-0.30. The desorption isotherm was used to determine the pore size distribution based on the Barrett, Joyner, Halenda (BJH) model.
Characteristic functional groups present on the surface of the obtained materials were identified using Fourier transform infrared spectroscopy (FTIR). The measurements were performed using a Vertex 70 spectrophotometer (Bruker, Karlsruhe, Germany). Samples were prepared by mixing with KBr and pressing into small tablets. FTIR spectra were obtained in the transmission mode between 4000 cm −1 and 400 cm −1 .
A thermogravimetric analyzer (Jupiter STA 449F3, Netzsch, Selb, Germany) was used to investigate the thermal stability of the synthesized materials. Measurements were carried out under nitrogen flow (10 cm 3 /min) at a heating rate of 10 °C/min over a temperature range of 30 °C to 1000 °C with an initial sample weight of approximately 5 mg.

Photocatalytic Tests
The photocatalytic activity of the obtained TiO2-ZnO binary oxide systems was evaluated based on the decomposition of C.I. Basic Blue 9 (MB), C.I. Basic Red 1 (R6G), and C.I. Basic Violet 10 (RhB) dyes (see, Table 1) in an initial concentration of 5 mg/dm 3 . Photocatalysis was carried out in a laboratory reactor of UV-RS2 type (Heraeus, Hanau, Germany) equipped with a 150 W medium-pressure mercury lamp as a UV light source surrounded by a water-cooling quartz jacket. First, an appropriate amount of photocatalyst (TiO2-ZnO binary oxide material) was added to a glass tube reactor containing 100 cm 3 of the model organic impurity. The suspension was stirred using an R05 IKAMAG magnetic stirrer (IKA Werke GmbH, Staufen im Breisgau, Germany) for 30 min in darkness to determine the adsorption/desorption equilibrium. After this time, the radiation was turned on to initiate the photocatalytic reaction. The process was carried out for a maximum of 150 min. In the next step, the irradiated mixtures were collected from the reactor at regular intervals and centrifuged to separate the photocatalyst. The concentration of C.I. Basic Blue 9, C.I. Basic Red 1, or C.I. Basic Violet 10 (after adsorption and UV irradiation) was analyzed using a UV-Vis spectrophotometer (V-750, Jasco, Oklahoma City, OK, USA) at a wavelength of 664 nm (for MB), 526 nm (for R6G), or 553 nm (for RhB) using water as a reference. The photocatalytic activity of the TiO2-ZnO binary oxide systems was determined by calculating the yield of dye degradation (W) using the formula below. The surface composition of TiO2-ZnO oxide hybrids (content of TiO2 and ZnO) was analyzed by using energy dispersive X-ray spectroscopy (EDS) using a Princeton Gamma-Tech unit equipped with a prism digital spectrometer (Princeton Gamma-Tech, Princeton, NJ, USA). Representative parts of each sample (500 µm 2 ) were analyzed to determine their actual surface composition.
The parameters of the porous structure of the obtained oxide powders were measured using a physisorption analyzer (ASAP 2020, Micromeritics Instrument Co., Norcross, GA, USA) operating based on a low-temperature adsorption of nitrogen. Before measurement, all materials were degassed at 120 °C for 4 h. Surface area was determined by the multipoint BET method using adsorption data in a relative pressure (p/p0) range of 0.05-0.30. The desorption isotherm was used to determine the pore size distribution based on the Barrett, Joyner, Halenda (BJH) model.
Characteristic functional groups present on the surface of the obtained materials were identified using Fourier transform infrared spectroscopy (FTIR). The measurements were performed using a Vertex 70 spectrophotometer (Bruker, Karlsruhe, Germany). Samples were prepared by mixing with KBr and pressing into small tablets. FTIR spectra were obtained in the transmission mode between 4000 cm −1 and 400 cm −1 .
A thermogravimetric analyzer (Jupiter STA 449F3, Netzsch, Selb, Germany) was used to investigate the thermal stability of the synthesized materials. Measurements were carried out under nitrogen flow (10 cm 3 /min) at a heating rate of 10 °C/min over a temperature range of 30 °C to 1000 °C with an initial sample weight of approximately 5 mg.

Photocatalytic Tests
The photocatalytic activity of the obtained TiO2-ZnO binary oxide systems was evaluated based on the decomposition of C.I. Basic Blue 9 (MB), C.I. Basic Red 1 (R6G), and C.I. Basic Violet 10 (RhB) dyes (see, Table 1) in an initial concentration of 5 mg/dm 3 . Photocatalysis was carried out in a laboratory reactor of UV-RS2 type (Heraeus, Hanau, Germany) equipped with a 150 W medium-pressure mercury lamp as a UV light source surrounded by a water-cooling quartz jacket. First, an appropriate amount of photocatalyst (TiO2-ZnO binary oxide material) was added to a glass tube reactor containing 100 cm 3 of the model organic impurity. The suspension was stirred using an R05 IKAMAG magnetic stirrer (IKA Werke GmbH, Staufen im Breisgau, Germany) for 30 min in darkness to determine the adsorption/desorption equilibrium. After this time, the radiation was turned on to initiate the photocatalytic reaction. The process was carried out for a maximum of 150 min. In the next step, the irradiated mixtures were collected from the reactor at regular intervals and centrifuged to separate the photocatalyst. The concentration of C.I. Basic Blue 9, C.I. Basic Red 1, or C.I. Basic Violet 10 (after adsorption and UV irradiation) was analyzed using a UV-Vis spectrophotometer (V-750, Jasco, Oklahoma City, OK, USA) at a wavelength of 664 nm (for MB), 526 nm (for R6G), or 553 nm (for RhB) using water as a reference. The photocatalytic activity of the TiO2-ZnO binary oxide systems was determined by calculating the yield of dye degradation (W) using the formula below. Photocatalysis was carried out in a laboratory reactor of UV-RS2 type (Heraeus, Hanau, Germany) equipped with a 150 W medium-pressure mercury lamp as a UV light source surrounded by a water-cooling quartz jacket. First, an appropriate amount of photocatalyst (TiO 2 -ZnO binary oxide material) was added to a glass tube reactor containing 100 cm 3 of the model organic impurity. The suspension was stirred using an R05 IKAMAG magnetic stirrer (IKA Werke GmbH, Staufen im Breisgau, Germany) for 30 min in darkness to determine the adsorption/desorption equilibrium. After this time, the radiation was turned on to initiate the photocatalytic reaction. The process was carried out for a maximum of 150 min. In the next step, the irradiated mixtures were collected from the reactor at regular intervals and centrifuged to separate the photocatalyst. The concentration of C.I. Basic Blue 9, C.I. Basic Red 1, or C.I. Basic Violet 10 (after adsorption and UV irradiation) was analyzed using a UV-Vis spectrophotometer (V-750, Jasco, Oklahoma City, OK, USA) at a wavelength of 664 nm (for MB), 526 nm (for R6G), or 553 nm (for RhB) using water as a reference. The photocatalytic activity of the TiO 2 -ZnO binary oxide systems was determined by calculating the yield of dye degradation (W) using the formula below.
where C 0 and C t are the concentrations of the dye prior to and after irradiation, respectively.

Kinetic Study
Kinetic energy of the photocatalytic decomposition of selected organic dyes was described based on the Langmuir-Hinsherlwood equation [40] assuming that pollutant decomposition is of a pseudo-first-order reaction nature. The equation presents dependence between the dye concentration in the aqueous vs. time of UV irradiation.
Assuming that the degradation process of the dye is of pseudo-first-order reaction nature, the constant reaction rate can be determined as the slope of the linear regression.
where k is the degradation rate of organic dye, min −1 , K is the equilibrium constant of adsorption of the dye on the surface of the catalyst, C 0 , C t are concentrations of the dye compound in aqueous solution before irradiation (t = 0) and after define time t. Estimation of constant reaction rate k enables determination of the half-life time of the model organic pollutant.

Dispersive and Morphological Characteristics
The results of dispersive analysis (see Table 2) show that both synthetic TiO 2 and ZnO (without thermal treatment) have monomodal particle size distributions. The TiO 2 and ZnO samples (denoted Ti and Zn) contain particles in the diameter ranges of 531-1720 nm and 220-615 nm, respectively. Dispersive analysis of the synthetic TiO 2 -ZnO oxide systems showed that the molar ratio of the precursors significantly affects the particle sizes of the resulting materials. Samples obtained with TiO 2 :ZnO molar ratios of 9:1; 5:2, and 1:3 denoted as Ti9Zn1, Ti5Zn2, and Ti1Zn3, which contain particles in the ranges 459 nm to 1110 nm, 459 nm to 1480 nm, and 396 nm to 825 nm, respectively. The results show that products with smaller particles were obtained when a higher content of ZnO was used.
It was also confirmed that increasing the temperature of calcination leads to the production of products with larger particles as a result of sintering and agglomerate formation. This situation was observed for all of the oxide materials except samples Ti5Zn2_600 and Ti1Zn3_600. All calcined TiO 2 -ZnO oxide systems exhibit monomodal particle size distributions. Synthetic oxide systems obtained with different TiO 2 :ZnO molar ratios (samples Ti9Zn1_600, Ti5Zn2_600, and Ti1Zn3_600) contain particles in the diameter ranges 531 nm to 1280 nm, 459 nm to 955 nm, and 255 nm to 615 nm, respectively. Among the calcined samples, those obtained with the highest molar contribution of ZnO were composed of the smallest particles.
The SEM microphotographs of TiO 2 and ZnO samples (uncalcined and calcined at 600 • C, Figure 2a,b,i,) show the presence of particles of almost spherical shape with high homogeneity. Moreover, the SEM micrographs for all analyzed oxide samples confirm the presence of particles, which exhibit a high tendency towards agglomeration. SEM observations of the synthesized TiO 2 -ZnO oxide systems (see Figure 2c-h) show that the molar ratio of the precursors does not have any significant effect on the morphology of the resulting systems. The SEM microphotographs of the studied samples confirm the presence of particles with precisely designed diameters, which corresponds to those indicated in the particle size distributions. Wang et al. [41] who synthesized a TiO 2 -ZnO oxide system through a sol-gel method using ammonia as a catalyst obtained results analogous to those reported here. Tsai et al. [42] noted that the TiO 2 -ZnO oxide system contains particles of a spherical shape, which show a high tendency towards agglomeration. Similarly, Ullah et al. [43] demonstrated that a TiO 2 -ZnO oxide system synthesized via a sol-gel method using dimethylaminoethanol was composed of particles of spherical shape with a high tendency to agglomerate. confirm the presence of particles with precisely designed diameters, which corresponds to those indicated in the particle size distributions. Wang et al. [41] who synthesized a TiO2-ZnO oxide system through a sol-gel method using ammonia as a catalyst obtained results analogous to those reported here. Tsai et al. [42] noted that the TiO2-ZnO oxide system contains particles of a spherical shape, which show a high tendency towards agglomeration. Similarly, Ullah et al. [43] demonstrated that a TiO2-ZnO oxide system synthesized via a sol-gel method using dimethylaminoethanol was composed of particles of spherical shape with a high tendency to agglomerate.

Structural Characteristics
The XRD pattern of titanium dioxide calcined at 600 • C (see Figure 3a) shows a strong diffraction peak at 2θ = 25.2, which corresponds to the anatase structure (JCPDS (Joint Committee on Powder Diffraction Standards), No.

Structural Characteristics
The XRD pattern of titanium dioxide calcined at 600 °C (see Figure 3a) shows a strong diffraction peak at 2θ = 25.2, which corresponds to the anatase structure (JCPDS (Joint Committee on   The XRD patterns of the synthetic, un-calcined TiO2-ZnO oxide hybrids (see Figure 4a) do not show diffraction peaks of the TiO2 and ZnO phases. The obtained samples are amorphous. It has been reported that the combination of titania with zinc oxide leads to inhibition of the phase formation of the ZnO crystalline structure. These obtained results suggest that some Zn 2+ cations can incorporate into the titania network [44], which follows from the fact that the ionic radii of Zn 2+ (ca. 60 pm) and Ti 4+ (ca. 60.5 pm) are similar [45]. The XRD pattern of sample Ti9Zn1_600 (obtained with a TiO2:ZnO molar ratio of 9:1 and calcined at 600 °C) confirms the formation of a crystalline material containing both titania and zinc oxide phases (see Figure 4b). Our results are in agreement with those of Stroyanova, Shalaby, and Moriadi [21,46,47]. Anatase was observed to be the dominant phase in sample Ti9Zn1_600. Characteristic diffraction peaks found at 25 Figure 4c) with increasing content of ZnO, the characteristic peaks of anatase, and rutile TiO2 gradually The XRD patterns of the synthetic, un-calcined TiO 2 -ZnO oxide hybrids (see Figure 4a) do not show diffraction peaks of the TiO 2 and ZnO phases. The obtained samples are amorphous. It has been reported that the combination of titania with zinc oxide leads to inhibition of the phase formation of the ZnO crystalline structure. These obtained results suggest that some Zn 2+ cations can incorporate into the titania network [44], which follows from the fact that the ionic radii of Zn 2+ (ca. 60 pm) and Ti 4+ (ca. 60.5 pm) are similar [45]. The XRD pattern of sample Ti9Zn1_600 (obtained with a TiO 2 :ZnO molar ratio of 9:1 and calcined at 600 • C) confirms the formation of a crystalline material containing both titania and zinc oxide phases (see Figure 4b). Our results are in agreement with those of Stroyanova, Shalaby, and Moriadi [21,46,47]. Anatase was observed to be the dominant phase in sample Ti9Zn1_600. Characteristic diffraction peaks found at 25 Figure 4c) with increasing content of ZnO, the characteristic peaks of anatase, and rutile TiO 2 gradually decreased [48]. Moreover, characteristic diffraction peaks found at 36.25 • , 56.6 • , 62.86 • , and 67.96 • are strictly related to the ZnO phase. In Ti9Zn1_600 and Ti5Zn2_600 samples crystallization of photoactive ZnTiO 3 , which is the result of reaction between titania and zinc oxide was observed. For the analyzed materials, the intensity of the ZnTiO 3 peaks also increased. The XRD pattern of the TiO 2 -ZnO oxide system obtained with a TiO 2 :ZnO molar ratio of 1:3 and calcined at 600 • C (sample Ti1Zn3_600, Figure 4d)  decreased [48]. Moreover, characteristic diffraction peaks found at 36.25°, 56.6°, 62.86°, and 67.96° are strictly related to the ZnO phase. In Ti9Zn1_600 and Ti5Zn2_600 samples crystallization of photoactive ZnTiO3, which is the result of reaction between titania and zinc oxide was observed. For the analyzed materials, the intensity of the ZnTiO3 peaks also increased. The XRD pattern of the TiO2-ZnO oxide system obtained with a TiO2:ZnO molar ratio of 1:3 and calcined at 600 °C (sample Ti1Zn3_600, Figure 4d)   HRTEM measurements confirmed that all prepared materials exhibit highly crystalline forms (see Figure 5a-c).
In order to confirm the crystalline structure of studied samples in the selected area, TEM diffraction experiments were conducted. The obtained results clearly confirmed high crystallinity of investigated samples and proved that crystallinity of TiO2-ZnO is quite complex. However, the TEM diffractograms of the same high resolution as XRD results show well-distinguishing diffraction rings, which corresponds to data from XRD. The results of SATEM diffraction are presented in Figure 5d-f. The diffractograms were analyzed using CHT Diffraction Analysis [52]. The most distinctive rings of diffraction were indexed and compared to XRD data. EDS (energy dispersive spectroscopy) experiments were carried out to verify the distribution of materials' components (elements) within the samples. The results (see Figure 6) indicate that distribution of Zn is not uniform. HRTEM measurements confirmed that all prepared materials exhibit highly crystalline forms (see Figure 5a-c).
In order to confirm the crystalline structure of studied samples in the selected area, TEM diffraction experiments were conducted. The obtained results clearly confirmed high crystallinity of investigated samples and proved that crystallinity of TiO 2 -ZnO is quite complex. However, the TEM diffractograms of the same high resolution as XRD results show well-distinguishing diffraction rings, which corresponds to data from XRD. The results of SATEM diffraction are presented in Figure 5d-f. The diffractograms were analyzed using CHT Diffraction Analysis [52]. The most distinctive rings of diffraction were indexed and compared to XRD data. EDS (energy dispersive spectroscopy) experiments were carried out to verify the distribution of materials' components (elements) within the samples. The results (see Figure 6) indicate that distribution of Zn is not uniform.   Figure 7 presents the percentage content of titanium oxide and zinc oxide in the analyzed oxide systems. The results confirmed the efficiency of the sol-gel route of synthesis. Moreover, energy dispersive X-ray microanalysis showed that changing the molar ratio of the initial precursors affects the content of the corresponding oxides in the structure of the final materials. As was expected, the highest quantity of titania (84.0%) was observed in sample Ti9Zn1_600 (obtained with the molar ratio TiO2:ZnO = 9:1) and the highest quantity of zinc oxide (50.6%) in sample Ti1Zn3_600. It was   Figure 7 presents the percentage content of titanium oxide and zinc oxide in the analyzed oxide systems. The results confirmed the efficiency of the sol-gel route of synthesis. Moreover, energy dispersive X-ray microanalysis showed that changing the molar ratio of the initial precursors affects the content of the corresponding oxides in the structure of the final materials. As was expected, the highest quantity of titania (84.0%) was observed in sample Ti9Zn1_600 (obtained with the molar ratio TiO2:ZnO = 9:1) and the highest quantity of zinc oxide (50.6%) in sample Ti1Zn3_600. It was  Figure 7 presents the percentage content of titanium oxide and zinc oxide in the analyzed oxide systems. The results confirmed the efficiency of the sol-gel route of synthesis. Moreover, energy dispersive X-ray microanalysis showed that changing the molar ratio of the initial precursors affects the content of the corresponding oxides in the structure of the final materials. As was expected, the highest quantity of titania (84.0%) was observed in sample Ti9Zn1_600 (obtained with the molar ratio TiO 2 :ZnO = 9:1) and the highest quantity of zinc oxide (50.6%) in sample Ti1Zn3_600. It was concluded that the sol-gel method makes it possible to obtain materials with strictly defined properties whose composition is mainly determined by the molar ratio of the initial precursors. concluded that the sol-gel method makes it possible to obtain materials with strictly defined properties whose composition is mainly determined by the molar ratio of the initial precursors.

Porous Structure Parameters
The surface area of any material is the most important factor for influencing its catalytic activity. The results of textural characteristics of the obtained materials are summarized in Table 3. Samples of TiO2-ZnO oxide systems that were not subjected to thermal treatment exhibit a relatively high surface area. The highest value, ABET = 494.7 m 2 /g, was observed for sample Ti9Zn1, which may be directly related to the dispersive nature of the analyzed material. This sample contained particles with smaller diameters than those of pure TiO2, which is directly linked to the porous structure parameters of the products of synthesis. Slightly poorer porous structure parameters were observed for pure TiO2 and sample Ti5Zn2 (with a molar ratio of TiO2:ZnO = 5:2), which had surface areas (ABET) of 488.6 m 2 /g and 475.8 m 2 /g. Moreover, an increase in the molar contribution of zinc oxide in the final product caused a significant decrease in the specific surface area, which was measured at 97.0 m 2 /g for sample Ti1Zn3 (with a molar ratio of TiO2:ZnO = 1:3) and 27.2 m 2 /g for ZnO. Our observations align with those of Prasannalakshmi and Shanmugam [51]. The samples that had undergone calcination exhibited a large decrease in the surface area. The highest surface area (7.6 m 2 /g) for these TiO2-ZnO oxide materials was recorded for sample Ti9Zn1_600. Thermal treatment also led to a significant decrease in the pore volume and a slight increase in the

Porous Structure Parameters
The surface area of any material is the most important factor for influencing its catalytic activity. The results of textural characteristics of the obtained materials are summarized in Table 3. Samples of TiO 2 -ZnO oxide systems that were not subjected to thermal treatment exhibit a relatively high surface area. The highest value, A BET = 494.7 m 2 /g, was observed for sample Ti9Zn1, which may be directly related to the dispersive nature of the analyzed material. This sample contained particles with smaller diameters than those of pure TiO 2 , which is directly linked to the porous structure parameters of the products of synthesis. Slightly poorer porous structure parameters were observed for pure TiO 2 and sample Ti5Zn2 (with a molar ratio of TiO 2 :ZnO = 5:2), which had surface areas (A BET ) of 488.6 m 2 /g and 475.8 m 2 /g. Moreover, an increase in the molar contribution of zinc oxide in the final product caused a significant decrease in the specific surface area, which was measured at 97.0 m 2 /g for sample Ti1Zn3 (with a molar ratio of TiO 2 :ZnO = 1:3) and 27.2 m 2 /g for ZnO. Our observations align with those of Prasannalakshmi and Shanmugam [51]. The samples that had undergone calcination exhibited a large decrease in the surface area. The highest surface area (7.6 m 2 /g) for these TiO 2 -ZnO oxide materials was recorded for sample Ti9Zn1_600. Thermal treatment also led to a significant decrease in the pore volume and a slight increase in the pore diameters of the obtained materials. The calculated values also imply that the surface area decreases with increased ZnO content.
pore diameters of the obtained materials. The calculated values also imply that the surface area decreases with increased ZnO content.

FTIR Analysis
The FTIR spectra of TiO2-ZnO binary oxide materials (see Figure 8) show absorption peaks at 550 cm −1 and 650 cm −1 ascribed to symmetric stretching vibrations of ≡Ti−O−Ti≡ and the vibration mode of −Zn−O−Ti≡ groups [47,50,53]. The band at 1400 cm −1 indicates stretching vibrations of C-O bonds [54]. Moreover, the FTIR spectra of the synthetic TiO2-ZnO oxide systems contain absorption peaks at 3440 cm −1 and 1630 cm −1 , which is attributed to physically adsorbed water (-OH) and N-H stretching vibrations [55][56][57]. The FTIR spectra of titanium dioxide (uncalcined and calcined) show three characteristic bands at 550 cm −1 , 1400 cm −1 , and 3400 cm −1 , which is associated respectively with stretching vibrations of ≡Ti−O, C-O, and -OH bonds. Analysis of the FTIR spectra of zinc oxide reveals a peak characteristic for zinc oxide (Zn-O) at 500 cm −1 . The broad absorption peak appearing at 700 cm −1 to 1100 cm −1 is characteristic for non-reacted products such as CH3COO − and NH4 + . Moreover, the peak at approximately 3400 cm −1 is ascribed to stretching vibrations of O-H bonds, which are indirectly related to water physically adsorbed on the surface. The FTIR results for synthetic TiO2-ZnO oxide hybrids showed absorption peaks for ≡Ti−O−Ti≡ bonds at 550 cm −1 , Zn−Ti−O bonds at 650 cm −1 , and -OH groups at 3400 cm −1 . Analysis of the FTIR spectra for samples Ti9Zn1, Ti5Zn2, and Ti1Zn3 reveals significant changes in the intensities of the relevant bands, which depend on the molar ratio of the precursors. Moreover, for TiO2-ZnO oxide systems calcined at 600 °C (Ti9Zn1_600, Ti5Zn2_600, Ti1Zn3_600), a decrease in the intensity of the -OH peak at 3400 cm −1 was observed. The spectra show that the intensity of the absorption bands around 650 cm −1 , which correspond to ≡Ti−O−Ti≡ bonds, increases with a growing molar ratio of the TiO2 precursor. It was also observed that the peaks at 1630 cm −1 for O-H bending vibrations at 1400 cm −1 for C-O stretching vibrations decrease when the calcination temperature is increased.

Thermal Analysis
Analysis of the thermograms of samples Ti, Ti9Zn1, Ti5Zn2, and Zn (see Figure 9a) indicates a one-step degradation process. The degradation step in the temperature range 30 °C to 380 °C is associated with a significant decrease in mass by about 19%, 18.5%, 17.0%, and 2.5% for samples Ti, Ti5Zn2, Ti9Zn1, and Zn, respectively. The mass loss is mainly related to the local elimination of water bonded with the surface of the materials. When the temperature is above 380 °C, the samples stabilize and their mass remains almost unchanged. For sample Ti1Zn3, three mass losses were observed on the TGA curves. The first sample in the range of 30 °C to 300 °C corresponds to the loss of free water The FTIR spectra of titanium dioxide (uncalcined and calcined) show three characteristic bands at 550 cm −1 , 1400 cm −1 , and 3400 cm −1 , which is associated respectively with stretching vibrations of ≡Ti−O, C-O, and -OH bonds. Analysis of the FTIR spectra of zinc oxide reveals a peak characteristic for zinc oxide (Zn-O) at 500 cm −1 . The broad absorption peak appearing at 700 cm −1 to 1100 cm −1 is characteristic for non-reacted products such as CH 3 COO − and NH 4 + . Moreover, the peak at approximately 3400 cm −1 is ascribed to stretching vibrations of O-H bonds, which are indirectly related to water physically adsorbed on the surface. The FTIR results for synthetic TiO 2 -ZnO oxide hybrids showed absorption peaks for ≡Ti−O−Ti≡ bonds at 550 cm −1 , Zn−Ti−O bonds at 650 cm −1 , and -OH groups at 3400 cm −1 . Analysis of the FTIR spectra for samples Ti9Zn1, Ti5Zn2, and Ti1Zn3 reveals significant changes in the intensities of the relevant bands, which depend on the molar ratio of the precursors. Moreover, for TiO 2 -ZnO oxide systems calcined at 600 • C (Ti9Zn1_600, Ti5Zn2_600, Ti1Zn3_600), a decrease in the intensity of the -OH peak at 3400 cm −1 was observed. The spectra show that the intensity of the absorption bands around 650 cm −1 , which correspond to ≡Ti−O−Ti≡ bonds, increases with a growing molar ratio of the TiO 2 precursor. It was also observed that the peaks at 1630 cm −1 for O-H bending vibrations at 1400 cm −1 for C-O stretching vibrations decrease when the calcination temperature is increased.

Thermal Analysis
Analysis of the thermograms of samples Ti, Ti9Zn1, Ti5Zn2, and Zn (see Figure 9a) indicates a one-step degradation process. The degradation step in the temperature range 30 • C to 380 • C is associated with a significant decrease in mass by about 19%, 18.5%, 17.0%, and 2.5% for samples Ti, Ti5Zn2, Ti9Zn1, and Zn, respectively. The mass loss is mainly related to the local elimination of water bonded with the surface of the materials. When the temperature is above 380 • C, the samples stabilize and their mass remains almost unchanged. For sample Ti1Zn3, three mass losses were observed on the TGA curves. The first sample in the range of 30 • C to 300 • C corresponds to the loss of free water and amounts to about 7.5%. In the range 300 • C to 470 • C, there is a second mass loss of about 5%, which is related to the thermal decomposition of the ZnO precursor. The total mass loss for sample Ti1Zn3 was 14.0%. and amounts to about 7.5%. In the range 300°C to 470 °C, there is a second mass loss of about 5%, which is related to the thermal decomposition of the ZnO precursor. The total mass loss for sample Ti1Zn3 was 14.0%. The thermograms of samples Ti_600 and Ti9Zn1_600 (see Figure 9b) show a minor mass loss in the temperature range of 30 °C to 350 °C by about 0.6% and 0.2%, respectively. This is related to the presence of a small amount of moisture in the systems [58,59]. In the range of 350 °C to 1000 °C, there is a second mass loss of about 0.7% and 0.4% for samples Ti_600 and Ti9Zn1_600, respectively. A slightly different thermogravimetric curve was observed for samples Ti5Zn2_600, Ti1Zn3_600, and Zn_600. In all three cases, the first degradation step in the temperature range of 30 °C to 300 °C with a mass loss of about 0.1% (for samples Ti5Zn2_600 and Ti1Zn3_600) and 0.2% (for sample Zn_600) is related to the local elimination of water bonded with the surface of the products. The next mass loss of about 0.2%, 0.4%, and 0.5% for samples Ti1Zn3_600, Ti5Zn2_600, and Zn_600, respectively, in the temperature range of 300 °C to 800 °C is related to the thermal decomposition of unreacted zinc acetate [60]. The third degradation step is probably associated with the phase transformation as a result of the applied high temperatures. The total mass loss for samples Ti5Zn2_600, Ti1Zn3_600, and Zn_600 is slightly above 0.8%, 0.3%, and 1.1%, respectively.
Wang et al. [61] who obtained nanoparticles of TiO2-ZnO via a sol-gel process observed three steps of mass loss, which are associated with the evaporation of water, the dehydroxylation of precursors, and the polymorphic transformation of TiO2. The results presented above indicate that the obtained TiO2-ZnO oxide systems have greater thermal stability than materials obtained in other studies [41,61]. In addition, titanium dioxide and zinc oxide following thermal treatment have similar thermal stability to what was reported in the literature [58][59][60][61].

Photocatalytic Activity
Titanium dioxide is known as an effective photocatalyst for the photo-oxidation of different kinds of hazardous organic pollutants in waste water [15]. Zinc oxide is another attractive semiconductor oxide with similar photocatalytic properties [28]. For this reason, a key element of the present research was an evaluation of the photocatalytic activity of the obtained TiO2-ZnO binary oxide systems. The evaluation was based on the decomposition of MB, R6G, and RhB dyes under UV irradiation. Titanium dioxide and samples obtained with TiO2:ZnO molar ratios of 9:1 and 5:2, additionally calcined at 600 °C, were subjected to photocatalytic tests. The results are presented in Figure 10. The thermograms of samples Ti_600 and Ti9Zn1_600 (see Figure 9b) show a minor mass loss in the temperature range of 30 • C to 350 • C by about 0.6% and 0.2%, respectively. This is related to the presence of a small amount of moisture in the systems [58,59]. In the range of 350 • C to 1000 • C, there is a second mass loss of about 0.7% and 0.4% for samples Ti_600 and Ti9Zn1_600, respectively. A slightly different thermogravimetric curve was observed for samples Ti5Zn2_600, Ti1Zn3_600, and Zn_600. In all three cases, the first degradation step in the temperature range of 30 • C to 300 • C with a mass loss of about 0.1% (for samples Ti5Zn2_600 and Ti1Zn3_600) and 0.2% (for sample Zn_600) is related to the local elimination of water bonded with the surface of the products. The next mass loss of about 0.2%, 0.4%, and 0.5% for samples Ti1Zn3_600, Ti5Zn2_600, and Zn_600, respectively, in the temperature range of 300 • C to 800 • C is related to the thermal decomposition of unreacted zinc acetate [60]. The third degradation step is probably associated with the phase transformation as a result of the applied high temperatures. The total mass loss for samples Ti5Zn2_600, Ti1Zn3_600, and Zn_600 is slightly above 0.8%, 0.3%, and 1.1%, respectively.
Wang et al. [61] who obtained nanoparticles of TiO 2 -ZnO via a sol-gel process observed three steps of mass loss, which are associated with the evaporation of water, the dehydroxylation of precursors, and the polymorphic transformation of TiO 2 . The results presented above indicate that the obtained TiO 2 -ZnO oxide systems have greater thermal stability than materials obtained in other studies [41,61]. In addition, titanium dioxide and zinc oxide following thermal treatment have similar thermal stability to what was reported in the literature [58][59][60][61].

Photocatalytic Activity
Titanium dioxide is known as an effective photocatalyst for the photo-oxidation of different kinds of hazardous organic pollutants in waste water [15]. Zinc oxide is another attractive semiconductor oxide with similar photocatalytic properties [28]. For this reason, a key element of the present research was an evaluation of the photocatalytic activity of the obtained TiO 2 -ZnO binary oxide systems. The evaluation was based on the decomposition of MB, R6G, and RhB dyes under UV irradiation. Titanium dioxide and samples obtained with TiO 2 :ZnO molar ratios of 9:1 and 5:2, additionally calcined at 600 • C, were subjected to photocatalytic tests. The results are presented in Figure 10. The first stage of photocatalytic testing involved evaluating the photocatalytic activity of TiO2-ZnO oxide systems in the removal of C.I. Basic Blue 9 (see Figure 10a). The TiO2-ZnO sample obtained with a molar ratio of TiO2:ZnO = 9:1 exhibited significantly better photocatalytic activity than pure titanium. Applying the Ti9Zn1_600 photocatalyst, the degree of decomposition of MB dye was 97.2% after 60 min of UV irradiation. The efficiency of C.I. Basic Blue 9 photodegradation in the presence of samples Ti_600 and Ti5Zn2_600 was 89.3% and 81.6% (after 120 min), respectively. Additionally, the photodecomposition of this organic dye increased with increasing irradiation time.
The decolorization of R6G under UV irradiation showed that sample Ti9Zn1_600 had good photo-oxidation activity (the efficiency of its degradation of C.I. Basic Red 1 was 93.6% after 60 min), which is shown in Figure 10b. Samples Ti_600 and Ti5Zn2_600 showed lower photocatalytic activity in the decomposition of C.I. Basic Red 1. The degradation efficiency was 87.2% (after 120 min) in the presence of sample Ti_600 and slightly lower (59.1%) in the case of photocatalysis using the sample Ti5Zn2_600.
Lastly, the photocatalytic experiments showed that a combination of titania with zinc oxide in a molar ratio of 9:1 exhibited significantly better photocatalytic activity than samples Ti_600 and Ti5Zn2_600 in the degradation of RhB (see Figure 10c). After 60 min of UV irradiation applying the Ti9Zn1_600 photocatalyst, the degree of decomposition of C.I. Basic Violet 10 reached 93.4%. The efficiency of degradation of RB dye in the presence of samples Ti_600 and Ti5Zn2_600 was 87.7% and 71.1%, respectively.
Our results imply that the photocatalytic activity of the synthesized samples depends not only on their BET surface area or crystallinity but can rather be attributed to dispersion and surface morphology. Moreover, based on research reports regarding heterogeneous photocatalysis [31,62,63], we propose a probable mechanism (see Figure 11) and reactions of the photodegradation of organic dyes using TiO2-ZnO oxide materials. The first stage of photocatalytic testing involved evaluating the photocatalytic activity of TiO 2 -ZnO oxide systems in the removal of C.I. Basic Blue 9 (see Figure 10a). The TiO 2 -ZnO sample obtained with a molar ratio of TiO 2 :ZnO = 9:1 exhibited significantly better photocatalytic activity than pure titanium. Applying the Ti9Zn1_600 photocatalyst, the degree of decomposition of MB dye was 97.2% after 60 min of UV irradiation. The efficiency of C.I. Basic Blue 9 photodegradation in the presence of samples Ti_600 and Ti5Zn2_600 was 89.3% and 81.6% (after 120 min), respectively. Additionally, the photodecomposition of this organic dye increased with increasing irradiation time.
The decolorization of R6G under UV irradiation showed that sample Ti9Zn1_600 had good photo-oxidation activity (the efficiency of its degradation of C.I. Basic Red 1 was 93.6% after 60 min), which is shown in Figure 10b. Samples Ti_600 and Ti5Zn2_600 showed lower photocatalytic activity in the decomposition of C.I. Basic Red 1. The degradation efficiency was 87.2% (after 120 min) in the presence of sample Ti_600 and slightly lower (59.1%) in the case of photocatalysis using the sample Ti5Zn2_600.
Lastly, the photocatalytic experiments showed that a combination of titania with zinc oxide in a molar ratio of 9:1 exhibited significantly better photocatalytic activity than samples Ti_600 and Ti5Zn2_600 in the degradation of RhB (see Figure 10c). After 60 min of UV irradiation applying the Ti9Zn1_600 photocatalyst, the degree of decomposition of C.I. Basic Violet 10 reached 93.4%. The efficiency of degradation of RB dye in the presence of samples Ti_600 and Ti5Zn2_600 was 87.7% and 71.1%, respectively.
Our results imply that the photocatalytic activity of the synthesized samples depends not only on their BET surface area or crystallinity but can rather be attributed to dispersion and surface morphology. Moreover, based on research reports regarding heterogeneous photocatalysis [31,62,63], we propose a probable mechanism (see Figure 11) and reactions of the photodegradation of organic dyes using TiO 2 -ZnO oxide materials.
h + + dye → oxidation products (8) e − + dye → reduction products (12) Prasannalakshmi and Shanmugam [51] reported that TiO 2 -ZnO oxide hybrids obtained using a sol-gel method produce almost complete degradation of C.I. Basic Blue 9 within 25 min of irradiation. Pérez-González et al. [45] obtained (TiO 2 ) 1−x -(ZnO) x thin films, with x = 0.00, 0.25, 0.50, 0.75, and 1.00, by the sol-gel process, which were deposited on glass. The synthesized films were evaluated for their ability to degrade MB. The authors found that the photocatalytic performance was improved by decreasing the value of x with the TiO 2 thin films displaying the highest response. Araújo et al. [64] produced TiO 2 -ZnO hierarchical hetero nanostructures following a two-step procedure in which the hydrothermal growth of nanorods took place on the surface of decorated electrospun fibers. The resulting material was applied as a photocatalyst in the photodegradation of Rhodamine B. Photocatalytic tests showed the TiO 2 -ZnO composite to have good photocatalytic activity. Agrawal et al. [65] obtained hierarchically nanostructured hollow spheres composed of ZnO-TiO 2 mixed oxides as a potential candidate for photocatalytic application. Pei and Leung [62] prepared TiO 2 -ZnO nanofibers from a nozzle-less electrospinning solution system. The authors evaluated the photocatalytic activities of different TiO 2 -ZnO composites in the photodegradation of Rhodamine B (RhB) under irradiation with 420 nm visible light. ZnO/TiO 2 hybrid nanofibers were prepared via electrospinning by Chen et al. [63]. Based on the photodegradation of RhB, it was shown that the synthesized products exhibited high degradation efficiency. The ZnO/TiO 2 (1 wt %) nanofibers degraded 90% of the dye in about 15 min.
Prasannalakshmi and Shanmugam [51] reported that TiO2-ZnO oxide hybrids obtained using a sol-gel method produce almost complete degradation of C.I. Basic Blue 9 within 25 min of irradiation. Pérez-González et al. [45] obtained (TiO2)1−x-(ZnO)x thin films, with x = 0.00, 0.25, 0.50, 0.75, and 1.00, by the sol-gel process, which were deposited on glass. The synthesized films were evaluated for their ability to degrade MB. The authors found that the photocatalytic performance was improved by decreasing the value of x with the TiO2 thin films displaying the highest response. Araújo et al. [64] produced TiO2-ZnO hierarchical hetero nanostructures following a two-step procedure in which the hydrothermal growth of nanorods took place on the surface of decorated electrospun fibers. The resulting material was applied as a photocatalyst in the photodegradation of Rhodamine B. Photocatalytic tests showed the TiO2-ZnO composite to have good photocatalytic activity. Agrawal et al. [65] obtained hierarchically nanostructured hollow spheres composed of ZnO-TiO2 mixed oxides as a potential candidate for photocatalytic application. Pei and Leung [62] prepared TiO2-ZnO nanofibers from a nozzle-less electrospinning solution system. The authors evaluated the photocatalytic activities of different TiO2-ZnO composites in the photodegradation of Rhodamine B (RhB) under irradiation with 420 nm visible light. ZnO/TiO2 hybrid nanofibers were prepared via electrospinning by Chen et al. [63]. Based on the photodegradation of RhB, it was shown that the synthesized products exhibited high degradation efficiency. The ZnO/TiO2 (1 wt %) nanofibers degraded 90% of the dye in about 15 min.   Table 4 presents results from the literature concerning the efficiency of decomposition of C.I. Basic Blue 9, C.I. Basic Red 1, and C.I. Basic Violet 10 dyes when different photocatalysts were used.
Analysis of the kinetics of photochemical decomposition of organic dyes shows significant differences in the rate of degradation of the analyzed impurities in the presence of catalysts (see Table 5). Regardless of the type of organic dye, the highest values of the degradation reaction rate k (0.0596 min −1 -C.I. Basic Blue 9, 0.0459 min −1 -C.I. Basic Red 1 and 0.0453 min −1 -C.I. Basic Violet 10) were recorded when the Ti9Zn1_600 oxide system was used as a photocatalyst. Furthermore, in the presence of Ti9Zn1_600 material, the highest values of the half-life time (t 1/2 = 11.632 min-C.I. Basic Blue 9, 15.086 min-C.I. Basic Red 1 and 15.301 min-C.I. Basic Violet 10) of tested organic dyes were noted.

Conclusions
The proposed methodology of synthesis of the TiO 2 -ZnO binary oxide materials using the sol-gel method proved to be very effective.
We studied how the TiO 2 :ZnO molar ratio and calcination temperature affects the physicochemical and photocatalytic properties of synthetic TiO 2 -ZnO oxide hybrids. It was found that the particle sizes, crystalline phase, surface area, pore structures, and photocatalytic activity of the TiO 2 -ZnO oxide systems are strongly dependent on the amount of zinc oxide in the product as well as on the calcination temperature.
The results of XRD analysis show that the quantity of zinc oxide in the product and the calcination temperature have significant effects on crystallizing the resulting materials. The porous structure parameters of the TiO 2 -ZnO oxide systems decreased with an increasing quantity of zinc oxide and temperature of calcination.
The TiO 2 -ZnO oxide hybrid obtained in a molar ratio of TiO 2 :ZnO = 9:1 and calcined at 600 • C (sample T9Zn1_600) showed the highest photocatalytic activity. This is attributed to the fact that this sample is composed with titanium, zinc oxide, and ZnTiO 3 phases as well as with anatase as the dominant phase. Moreover, analysis of the kinetics of the photocatalytic process performed based on the Langmuir-Hinshelwood equation confirmed that degradation of the model organic dyes occurred most intensely in the presence of the Ti9Zn1_600 catalyst.