Characterization of Commercial TiO2 P90 Modified with ZnO by the Impregnation Method

,is article is devoted to TiO2/ZnO nanocomposites’ creation by modifying with the commercial TiO2/P90 product using the impregnation method and identifying the effect of the ZnO modifier on its adsorption, structural, photocatalytic, and electrical properties. ,e synthesized TiO2/ZnO nanocomposites were characterized by XRD, XRF, XPS, and low-temperature nitrogen adsorption-desorptionmethods. As a result, nanostructured TiO2/ZnO composites with the ZnO content of 2, 5, 10, and 15%were obtained. It was shown that the phase composition of TiO2/P90 does not change during the nanocomposite synthesis. XPS studies of TiO2/ZnO nanocomposites indicated the presence of Ti, Zn, O, and OH states on their surface, which is associated with TiO2, ZnO, and hydroxide ions. ,e nitrogen adsorption-desorption method showed that the commercial TiO2/P90 sample is nonporous, and all TiO2/ZnO nanocomposites are characterized by almost the same homogeneous mesoporous structure. Experimentally established sorption and photocatalytic properties depend on the specific surface area and electrostatic interaction with dyes. ,e effect of the ZnO modifier on I-V characteristics of the TiO2/P90 sample was revealed. ,e obtained experimental data showed that the TiO2/P90 sample contains one type of current carriers, and TiO2/2ZnO and TiO2/5ZnO nanocomposites are characterized by two types of current carriers.


Introduction
Semiconductor nanomaterials based on metal oxides have recently attracted more and more attention due to their interesting properties. ey can be successfully used in applied ecology, in particular, as catalysts, photocatalysts, and other materials where their unique electrical properties are required [1][2][3][4].
One of the well-known semiconductor metal oxide nanomaterials is titanium (IV) oxide (TiO 2 ), as evidenced by more than 20,000 publications on TiO 2 in ScienceDirect over the past 5 years. TiO 2 forms several structural modifications, the main of which are tetragonal (rutile and anatase) and orthorhombic (brookite). Among them, anatase and rutile modifications are the most widely recognized in the scientific community as a promising nanomaterial for environmental applications [5].
Heterogeneous photocatalysis is the most popular application of TiO 2 . It is well known that the ultimate photoefficiency of each photocatalyst strongly depends on its physicochemical properties such as specific surface area, particle size, and phase composition, as well as on the synthesis method and precursors' type used to obtain it [6][7][8][9][10][11]. Another potential TiO 2 application is sensory, where it can be successfully used as a sensitive chemoresistive layer of gas sensors [12][13][14]. e use of TiO 2 in photovoltaics is also promising and practically significant [15,16]. ese are only a few examples of titanium (IV) oxide applications, based on prominent characteristics of TiO 2 such as structural, adsorption, (photo) catalytic, and electrical properties, which, depending on the synthesis conditions, can differ significantly [17][18][19].
ZnO crystallizes in three crystalline structures-cubic rock salt, cubic zinc blende, and hexagonal wurtzite-among which only the latter is thermodynamically stable under normal conditions. ZnO is characterized by unique electrical and optical properties that allow its successful environmental application, namely, as a photocatalyst, which has long competed with the most popular TiO 2 , and as a sensitive layer of gas sensors [33,34].
It is known that both titanium (IV) oxide and zinc oxide are n-type broadband semiconductors with a bandgap of 3.2 eV (anatase) and 3.37 eV, respectively. ey have similar physicochemical properties and characteristics such as nontoxicity, high photoactivity, and low cost [35][36][37]. However, high photoactivity of both TiO 2 and ZnO is limited by absorption of wavelengths not exceeding 385 nm and high recombination rate of "electron-hole" pairs [37]. It is reported that a significant improvement in optical and photochemical properties can be achieved by creating nanostructures with both TiO 2 and ZnO due to better charge separation in them [35][36][37][38]. Besides, it is noted that the obtainment of nanocomposites of different TiO 2 /ZnO morphology results in nanocomposite structures capable to absorb wavelengths in the visible light region [38]. us, the similarity of TiO 2 and ZnO properties can make their combined use in the form of nanocomposites a successful solution to the problems of low concentration of charged carriers, high recombination rate of "electron-hole" pairs, light absorption difficulty in the visible region, etc.
Various researchers have shown that, in order to create effective nanocomposite photocatalysts, it is necessary to vary synthesis methods, particle morphology, surface chemistry, degree of crystallinity/amorphousness, mass ratios of components in the TiO 2 -ZnO system, etc. [39][40][41]. Increased photostability, accompanied by improved catalytic activity, is one of the factors justifying the creation of nanocomposites of mixed (binary) oxides, which are considered as promising photocatalysts for environmental applications. Such materials can also be used in lithium batteries, supercapacitors, and fuel cells [42]. e interaction of TiO 2 and ZnO in nanocomposite structures is expected to improve physical and chemical properties (such as electrical conductivity and charge carrier concentration) compared to individual phases of pure oxides [43]. us, creation of binary structures, namely, TiO 2 -ZnO, will promote the formation of new nanocomposite materials with new and/or improved characteristics. Although several studies on the creation and investigation of TiO 2 -ZnO nanocomposites have been presented in the literature to date, it is still unclear how ZnO and its content affect photocatalytic and electrical properties. erefore, additional research on the effect of ZnO on the TiO 2 characteristics will provide a better understanding of the change in its properties to purposefully alter them.
To obtain such binary nanocomposites and study their photocatalytic and electrical properties, the use of commercial TiO 2 samples is of particular interest, which are widely available and already well studied in the literature [44,45]. Among commercial photocatalysts, TiO 2 P25 (Evonik, Germany) is widely used as a reference of its thorough studies [46][47][48]. However, in our opinion, the commercial product of TiO 2 P90 from the same company may be a more interesting material for creating adsorbents, (photo) catalysts, gas sensor sensitive layers, and other functional materials due to the larger specific surface area [44]. P90 is characterized by almost 2 times higher specific surface area (90-110 m 2 /g) compared to P25 (50-70 m 2 /g). e average size of P90 particles is smaller (∼30 nm) than in the P25 sample (∼50 nm), and the content of the anatase phase in the P90 sample is slightly higher (91-92%) than that in P25 TiO 2 (87-88%). erefore, the current work is aimed at the modification of the commercial product TiO 2 P90 with zinc oxide by a simple impregnation method to obtain TiO 2 -ZnO nanocomposites and to evaluate the effect of the ZnO modifier on the adsorption, structural, photocatalytic, and electrical properties of TiO 2 P90.

Synthesis.
e synthesis of nanocomposites was performed by the impregnation method [49]. 10 mL of zinc nitrate solution was added dropwise under stirring to 1 g of TiO 2 . Concentration of zinc nitrate solution was calculated so that it corresponded to 2, 5, 10, and 15% of ZnO in the final nanocomposite. e resulting suspensions were first left for 12 hours, then dried at 100°C for 60 minutes, and calcined at 500°C for 60 minutes. As a result, samples labeled TiO 2 /2ZnO, TiO 2 /5ZnO, TiO 2 /10ZnO, and TiO 2 /15ZnO were obtained, respectively.
Volt-ampere characteristics (I-V characteristics) of the studied samples were measured in the quasi-stationary mode using an automated tester 14TKC-100 both in the dark and under illumination 0.03 AM0. For this, the powders were placed between two clear glass plates with ITO electrodes and a resistance of 30 Ohm/□, and the distance between the plates was 2 μm. Figure 1 presents diffraction patterns of samples TiO 2 /P90, TiO 2 /2ZnO, TiO 2 / 5ZnO, TiO 2 /10ZnO, and TiO 2 /15ZnO, respectively.

XRD and Chemical Composition.
According to the presented diffractograms, the phases of anatase and rutile are observed for the TiO 2 /P90 sample; for TiO 2 /ZnO nanocomposites in all cases, there are phases of anatase, rutile, and wurtzite (ZnO). Table 1 shows the phase composition (automatically calculated by the PDXL software package), ZnO content (determined by the X-ray fluorescence method), and the crystallite size (calculated by the Scherrer equation). e presented phase composition for the TiO 2 /P90 sample and TiO 2 /ZnO nanocomposites indicates that the modification and synthesis temperature (500°C) do not change the phase composition of TiO 2 /P90. X-ray phase and X-ray fluorescence analysis methods (Table 1) confirm the theoretically calculated content of zinc oxide in nanocomposites. e ZnO content is 2, 5, 10, and 15%. e average crystallite size for all samples is in the range of 5.1-6.9 nm for TiO 2 and 3.4-6.0 nm for ZnO. us, it can be stated that TiO 2 /ZnO nanocomposites with the theoretically calculated ZnO content were obtained without changing the phase composition of TiO 2 /P90.

XPS.
Surface states of TiO 2 /ZnO nanocomposites were evaluated by X-ray photoelectron spectroscopy, resulting in XPS spectra, which are shown in Figure 2. e survey spectra of TiO 2 /ZnO nanocomposites (Figure 2(a)) indicate the presence of Ti, Zn, O, and C (hydrocarbons from the XPS device). Native XPS spectra of Ti 2p, Zn 2p, and O 1s for TiO 2 /ZnO nanocomposites are shown in Figures 2(b), 2(c), and 2(d), respectively. e XPS data report showed the presence of Ti 4+ for titanium, Zn 2+ for zinc, and O 2− for oxygen on the surface of nanocomposites. e spectrum for Ti 2p (Figure 2(b)) contains two peaks: the first one at about 458.5 eV refers to the binding energy of Ti 2p3/2, and the second one at about 464 eV corresponds to the binding energy of Ti 2p1/2 [30]. For all nanocomposites, the XPS Zn 2p spectra (Figure 2(c)) also show two peaks approximately at 1021.1 eV and 1044 eV, which are characteristics of Zn 2p3/2 and Zn 2p1/2, respectively [50][51][52]. e absence of other Zn 2p peaks indicates that zinc ions form ZnO and that they are not part of the TiO 2 lattice. e XPS O 1s spectra (Figure 2(d)) for all nanocomposites are also identical and are characterized by peaks at 529.2 eV and 531.7 eV, which correspond to oxygen bound in the crystal lattice of metal oxides and hydroxyl oxygen (OH) on the sample surface, respectively [53].
us, the study of TiO 2 /ZnO nanocomposites by XPS analysis revealed the surface element compositions: Ti 4+ for titanium, Zn 2+ for zinc, and O 2for oxygen. In addition, the presence of hydroxide ions (OH) on their surface was found. Figure 3 displays nitrogen adsorption-desorption isotherms for all samples. As can be seen from the isotherms, the TiO 2 /P90 sample and TiO 2 /ZnO nanocomposites have different porous structures.

Structural and Adsorption Characteristics.
us, according to the IUPAC classification [54], the TiO 2 /P90 sample has type III adsorption isotherm, which is a characteristic of nonporous or macroporous materials. e isotherms of all samples modified with zinc oxide have a slightly different form; the isotherm of type III is transformed into type V, namely, to V (a).
is indicates a transition from a nonporous structure to a mesoporous one, which developed as a result of the modification.
Analyzing the resulting hysteresis loops in TiO 2 /ZnO nanocomposites, it can be noted that they belong to type H2 (b), although being very similar to type H1 [54]. e reason for this might be that nanocomposites have open ink-bottle pores that are homogeneous and have approximately the same neck size and pore width. e pore size distribution for all samples is shown in Figure 4, which confirms the presence of a nonporous structure for TiO 2 /P90 and the appearance of mesopores of almost the same size for TiO 2 /ZnO nanocomposites. Specific surface areas determined by the Brunauer-Emmett-Teller (BET) equation decrease from 113 m 2 /g to 60 m 2 /g with the increase in the ZnO content (Table 2). is indicates the aggregation of titanium (IV) oxide particles during modification and heat treatment. Obtained by the Barrett-Joyner-Halenda (BJH) method, the average pore diameter for all samples correlates with the obtained isotherm types and porous structures and is in the range of 6.8-10.8 nm.

Adsorption and Photocatalytic
Properties. Adsorption-photocatalytic properties were investigated by the discoloration degree of aqueous dye solutions (methylene blue at 664 nm and Congo red at 505 nm), which was determined by the following equation (%) [31]:

Journal of Chemistry
where A 0 is the optical density of the initial dye solution and A 1 is the optical density of the solution after adsorption/ photocatalysis. e results of adsorption-photocatalytic studies are presented in Table 3.
It can be seen that both TiO 2 /P90 sample and TiO 2 / ZnO nanocomposites have low sorption activity towards the cationic dye MB (0-7%) and high sorption activity towards the anionic dye CR (82-96%). It is probably associated with the negative charge of their surface [30] and the corresponding electrostatic interaction between the surface of nanocomposites and dyes. In this case, as a result of the sorption interaction of the studied samples with MB, there is low dye removal efficiency on the TiO 2 / 5ZnO sample, and there is low removal efficiency of CR on TiO 2 /2ZnO. Photocatalytic removal of MB from the aqueous solution is significantly different from the sorption experiments. It is characterized by a greater discoloration degree, which is 19-91%, and decreases with the increase in the zinc oxide content.
is fact correlates well with the change in the specific surface area. Photocatalytic discoloration of CR is in accordance with the sorption removal of this dye on TiO 2 / P90 and TiO 2 /ZnO nanocomposites and is slightly higher, which indicates, first of all, strong electrostatic interaction of the studied samples with the anionic dye.

Electrical
Characteristics. I-V characteristics measured for three samples of TiO 2 /P90, TiO 2 /2ZnO, and TiO 2 /5ZnO are shown in Figures 5-7, respectively. ese two nanocomposites were chosen for the study because of the similarity of their properties with TiO 2 /10ZnO and TiO 2 / 15ZnO samples in terms of adsorption and structural characteristics and their better sorption-photocatalytic properties.
Additionally, dimensionless sensitivity (α) was calculated for the studied samples by determining the slope of the curves in the double logarithmic scale (log-log scale) according to the equation α � dl gI/dlg V. us, analyzing the I-V characteristics, we can see that all samples have both common features and differences. General features include the following phenomena. At high applied voltages, both in the dark and under the illumination, I-V characteristics almost merge, which indicates that injection processes mainly take place at the grain boundaries. Conductivity at high voltage is almost linear, which is also further confirmed by the value of dimensionless sensitivity α � 1. Besides, photosensitivity in the low-voltage range was detected for all samples. In general, I-V curves of all samples in the region up to 1 V are predominantly nonlinear, where the superlinear sections alternate with the sublinear sections several times. e differences found in the samples are interesting. us, individual features of pure titanium (IV) oxide TiO 2 / P90 show the presence of the region with α � 0.75 slope in the dark in the low-voltage region. When illuminated, I-V characteristics become sublinear with α � 0.5, which is a characteristic of rectifying I-V characteristic. For the I-V curve obtained in the dark, there are regions with α � 2.0 that is a characteristic of monomolecular recombination when one type of current carriers dominates. When illuminated, a region with α � 1.5 appears on the I-V curve, which is a      characteristic of bimolecular recombination, when the concentrations of the main and nonbasic current carriers are close to each other. at is, illumination facilitates the injection of nonbasic current carriers from the anode contact. e ratio of currents in the light and in the dark is 6. e TiO 2 /2ZnO sample has slightly different features. us, the addition of ZnO in the amount of 2% increases the current almost by one order of magnitude in the dark, and I-V characteristics are also characterized by a region with α � 0.75. I-V characteristics of the TiO 2 /2ZnO sample under the illumination have a photovoltaic region, and both I-V characteristics have areas with α � 1.5, which is typical of bimolecular recombination, so the addition of ZnO promotes the injection of nonbasic current carriers.

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Further increase of the ZnO content to 5% in pure titanium (IV) oxide significantly increases, up to five orders, the current in the dark compared to TiO 2 /2ZnO. I-V characteristics for this sample are linear with α � 1 in the entire applied voltage range. I-V characteristics of the TiO 2 /5ZnO sample under illumination are also characterized by α � 1.5, which is inherent in bimolecular recombination.
us, it can be stated that the addition of ZnO to TiO 2 changes both quantitative and qualitative parameters of I-V characteristics of the TiO 2 /P90 sample. e photovoltaic effect in the presence of 2% ZnO and the inversion of the light current in case of 5% ZnO concentration are unusual. e found regularities indicate the prospects of using the TiO 2 /P90 sample in structures for which there must be one type of current carriers, for example, photoresistors and sensors based on the effect of the main current carrier injection. TiO 2 /2ZnO and TiO 2 /5ZnO can be used in  Journal of Chemistry structures for which two types of current carriers are required, for example, in the converters of solar energy into electricity.

Conclusions
TiO 2 /ZnO nanocomposites were obtained by impregnating commercial TiO 2 /P90 manufactured by Evonik in order to determine the effect of the ZnO modifier on the adsorption, structural, photocatalytic, and electrical properties of the nanocomposites. e obtained nanocomposites were characterized by XRD, XRF, XPS, and low-temperature nitrogen adsorption-desorption method, and their sorption, photocatalytic, and electrical properties were investigated. It was found that modification does not change the phase composition of TiO 2 /P90, and the ZnO content determined by diffraction and X-ray fluorescence methods corresponds to the theoretically calculated one. In all cases, nanostructured TiO 2 /ZnO composites were obtained (crystallite sizes in the range of 3.4-6.9 nm). e study of the surface chemical states of TiO 2 /ZnO nanocomposites by XPS analysis revealed the degree of oxidation on their surface: Ti 4+ for titanium, Zn 2+ for zinc, and O 2− for oxygen. Besides, the presence of hydroxide ions (OH) on their surface was found.
Studies of the adsorption and structural characteristics of TiO 2 /ZnO nanocomposites have shown that the commercial TiO 2 /P90 sample is nonporous. Its modification with zinc oxide leads to the aggregation of TiO 2 particles and the development of mesoporosity. As a result, the specific surface area of TiO 2 /P90 decreases (from 113 m 2 /g to 60 m 2 / g) with the following dependence: the higher the ZnO content, the smaller the specific surface area. e results of sorption-photocatalytic experiments point at better adsorption properties towards the anionic dye and better photocatalytic activity towards the cationic dye that decreases with the increase in the ZnO content, which correlates well with the data on the specific surface area. e study of electrical properties indicates a significant effect of ZnO on the quantitative and qualitative I-V characteristics of the TiO 2 /P90 sample. e established regularities allow stating the prospects of using the TiO 2 /P90 sample in structures, for which one type of current carriers is required (for example, photoresistors and sensors). TiO 2 /2ZnO and TiO 2 /5ZnO nanocomposites can be effectively used in structures, for which two types of current carriers are required (for example, for converters of solar energy into electricity).

Data Availability
All the data generated and/or analyzed during this study are included within this published article.