The paper presents the results of characterization and study of adsorption-photocatalytic properties of commercial and synthesized-by-hydrothermal method TiO2 and TiO2-SnO2 nanocomposites. Hydrothermal synthesis of TiO2-based nanocomposites was performed in two ways: single-stage and two-stage methods. Characterization was carried out by XRD, X-ray fluorescence method, XPS, EPR, PL, and low-temperature adsorption-desorption of nitrogen, which showed that TiO2-SnO2 nanostructured composites were obtained with tin(IV) oxide content of 10 wt.% and had acidic surface and different porous structures. Besides, modification of a commercial sample with tin(IV) oxide led to a slight decrease in the specific surface area, while modification of a synthesized-by-hydrothermal method TiO2 sample led to an increase. It was found that sorption properties of the obtained nanocomposites and pure TiO2 are better towards anionic dyes. Photocatalytic activity, on the contrary, is higher towards cationic dyes, which is consistent with additional studies on the destruction of these dyes. It was established that in terms of photocatalytic activity, TiO2-SnO2 nanocomposites are more promising than solid solutions, and modification of TiO2 with tin(IV) oxide, in general, leads to improvement of its photocatalytic activity.

1. Introduction

Advanced Oxidation Processes (AOPs) used to remove organic pollutants from wastewater can be fairly considered to be green technologies for environmental restoration, which attributes them to safe and sustainable water treatment technologies [1, 2]. These technologies include: ozonation, ultrasound, microwaves, γ-irradiation, Fenton-like processes, homogeneous and heterogeneous photocatalysis. Among these processes, heterogeneous photocatalysis seems, in our opinion, to be the most promising due to advantages such as mild conditions and short process duration and no use of chemical reagents. Besides, photocatalysts remain chemically unchanged during and after the photocatalytic process and can be reused several times. A promising photocatalyst for the oxidation of organic compounds in aqueous media is nanodispersed titanium(IV) oxide (TiO2) due to its high surface area, biological and chemical stability, low cost, low toxicity, and high photocatalytic activity [35].

Titanium(IV) oxide has been one of the most studied nanomaterials in recent decades due to its high potential for use in the energy sector and for environmental protection. Also, there are many other areas of TiO2 application: disinfectants and antibacterial agents, self-cleaning surfaces, food and pharmaceutical additives, pigments, etc. [68].

TiO2 exists in nature in three modifications: rutile (tetragonal crystal lattice), anatase (tetragonal crystal lattice), and brookite (rhombic crystal lattice). In all cases, crystal structures consist of TiO6 octahedra [9, 10]. Figure 1 shows crystal lattices and structures of TiO2 modifications.

It is known [12] that TiO2 in the anatase form shows higher photocatalytic activity compared to rutile and brookite. This is explained as follows: the band gap for anatase is approximately 3.2 eV, while for rutile ~3.0 eV, indicating better light absorption by rutile than anatase. But, according to the data on photoconductivity [13], the lifetime of the electron-hole pair is longer in anatase than in rutile, and therefore, there are more charge carriers in anatase.

Despite all the abovementioned advantages of TiO2 as a photocatalyst, its commercial application is still limited because of its low photoactivity under visible light and fast recombination of photoexcited electrons and holes leading to poor efficiency. To enhance efficiency and usage of titanium(IV) oxide in a wide range of technological conditions, namely, under solar radiation, TiO2 is doped and modified and nanocomposites are created on its basis [14, 15]. It should also be noted that improvement of TiO2 photocatalytic activity can be also achieved by varying its morphology, combining different crystal modifications of TiO2, and increasing its specific surface area [9, 16].

To increase TiO2 efficiency under visible light, metal doping is widely used. Photoactivity of metal-doped TiO2 photocatalysts largely depends on the nature of the doping ion, its level in the structure of titanium(IV) oxide, doping method, modification of TiO2 being doped, etc. TiO2 doping with metals is considered to result in overlapping of titanium 3d orbitals with d-levels of metals, which makes such TiO2 photoactive in the visible region [14]. It was found [17] that doping of TiO2 nanoparticles with Li, Na, Mg, Fe, and Co ions expands the range of photocatalytic reaction to visible light. In the sample doped with sodium, titanium exists as Ti4+ and Ti3+. Conversion established between them prevented recombination of electrons (e) and holes (h+).

It has been shown that doping with metal ions promotes formation of crystalline TiO2 phases that can generate electrons (e) and holes (h+) to a greater extent. Doping TiO2 with nonmetals such as C, B, I, F, S, and N leads to its even greater photocatalytic activity in the visible region compared to metal doping [18]. This effect is associated with impurity states near the edge of the valence band, and as they do not act as charge carriers, their role as recombination centers is minimized. It was found that TiO2 nanoparticles, doped with nitrogen and carbon, show higher photocatalytic activity when irradiated with visible light compared to TiO2 doped with other nonmetallic dopants. The modification method is also widely studied to increase photoactivity of pure TiO2. As TiO2 modifiers, most attention is paid to nanoparticles of noble metals, such as Ag, Pt, Pd, Rh, and Au [19]. In this case, nanoparticles of the noble metal act as a transfer in the transport of photogenerated electrons in TiO2 particles. Photocatalytic activity increases in this case because the recombination rate of charge carriers decreases.

Undoubtedly, creation of nanocomposites is a promising method to increase photocatalytic activity of photocatalysts [2022]. TiO2-based nanocomposites are created to increase efficiency of photon utilization in them under ultraviolet and visible radiation. Metal oxides such as SnO2, ZnO, WO3, and Fe2O3 are used for this purpose. Among these metal oxides, SnO2 plays an essential role in nanocomposite structures with TiO2 due to the production of more hydroxyl radicals in such a composite compared to others [23]. Titanium(IV) oxide and tin(IV) oxide have similar ionic radii of the cations (0.605 Å for Ti4+ and 0.69 Å for Sn4+) and have similar structural (tetragonal structure of rutile type) and electronic properties [24]. Band gaps of SnO2 and TiO2 (anatase) and TiO2 (rutile) are 3.6 eV and 3.2 eV and 3.0 eV, respectively, while the conduction band of tin(IV) oxide is approximately 0.5 V more positive than one of the titanium(IV) oxide conduction bands [11]. Therefore, creation of such composites is very promising due to possibility of efficient charge separation between crystalline phases of oxides that increases the lifetime of charge carriers and, therefore, reduces the probability of their recombination. As a result, electrons are localized in the conduction band of tin(IV) oxide, while holes are located in the valence band of titanium(IV) oxide. Thus, simultaneous combination of two different semiconductors and two phases with different energy levels can significantly increase mobility of the charge carriers, preventing their recombination and thus improving photoactivity of such a composite photocatalyst [23, 25].

Currently, there are many studies on the creation of TiO2-SnO2 nanocomposites to increase photoactivity. In papers [2628], TiO2-SnO2 nanocomposites were synthesized, which indicate that, depending on their synthesis method and precursor type, powders with a wide range of properties could be obtained. At the same time, it is possible to obtain both metal oxides TiO2-SnO2 and solid solutions TixSn1-xO2. In particular, the authors of article [26] obtained both nanocomposites TiO2-SnO2 and their solid solutions by sol-gel method. The latter is quite possible due to the structural analogy of TiO2 with SnO2, and in study [26], it occurs when molar content of Sn in composites is less than 15 wt.%. Other researchers [27] also synthesized TiO2-SnO2 nanoparticles by sol-gel method from titanium(IV) n-butoxide and tin(II) ethylhexanoate precursors. They found that at low concentrations of the tin precursor, TiO2 particles formed solid solutions when being doped, while TiO2-SnO2 nanocomposites were obtained at high concentrations of the tin precursor. This fact confirms the results of the previously considered study. In paper [28], synthesis of TiO2-SnO2 nanoparticles was carried out by hydrolysis under hydrothermal conditions from anhydrous titanium and tin chlorides. Formation of solid solutions in the whole molar ratio was shown. Such different results also indicate a significant influence of the precursor type used to create TiO2-SnO2 systems. Even in the synthesis of pure TiO2 from different precursors, it is possible to obtain both monophase powders and nanocomposite particles consisting of different TiO2 phases [29]. All this indicates that, despite the considerable amount of work devoted to the synthesis of TiO2-SnO2 nanocomposites, there are still more questions than answers about the physicochemical characteristics of the resulting binary systems and the impact on these characteristics of precursor types, methods, used ratios, etc. In addition, an increase in TiO2 photocatalytic activity is observed both in the case of TiO2-SnO2 nanocomposite synthesis and in obtaining solid solutions. However, according to the literature data, higher photocatalytic activity is associated with TiO2-SnO2 nanocomposite structures, rather than their solid solutions [27].

For obtaining TiO2-SnO2 nanocomposites in the form of both powders and films, various methods are used, such as sol-gel [3033], hydrothermal synthesis [3436], chemical vapor deposition [37], spray and laser pyrolysis [3841], coprecipitation [42, 43], and green [44]. Each of these methods has its advantages and practical application when a certain nanocomposite structure is needed. For obtaining powdered TiO2-SnO2 nanocomposites, the hydrothermal synthesis method seems to be the most promising due to relative simplicity, crystallization at lower temperatures than, for example, in the case of sol-gel method or precipitation method, possibility of obtaining homogeneous nanocomposite particles of different structures, etc.

In our previous study [24], TiO2-SnO2 systems were synthesized and characterized, obtained by both the hydrolytic and hydrothermal methods at low tin content in composites (up to 15 wt.%) using the tin(IV) chloride precursor. In that paper, it was shown that in case of using precursors such as titanium(IV) isopropoxide and tin(IV) chloride (content in the composite 10 wt.%) in both synthesis methods, mainly solid solutions are formed, which indicates easy incorporation of Sn4+ ions into the crystal lattice of titanium(IV) oxide of rutile phase. For further research, hydrothermal synthesis has been chosen as a more promising synthesis method, because the powders obtained by this method were characterized by larger surface areas and developed mesoporosity and better photocatalytic properties towards dyes of different nature compared to the hydrolytic method. The literature data show greater prospects of composite nanostructures with low tin content as photocatalysts. Therefore, it is of considerable interest to obtain TiO2-SnO2 nanocomposite structures with a small content of tin in them.

According to [45, 46], obtaining nanocomposites with low tin concentrations is possible when using the tin(II) chloride precursor. Therefore, in this work, in contrast to [24], synthesis of TiO2-SnO2 nanocomposites was performed using the tin(II) chloride precursor by hydrothermal method. Besides, TiO2-SnO2 nanocomposites in this research were obtained by single-stage and two-stage methods. For comparison, the TiO2-SnO2 nanocomposite based on a commercial TiO2 sample also was synthesized. Synthesis was performed in such a way that it was possible to compare photocatalytic properties of the obtained nanocomposites with previously obtained and studied solid solutions in paper [24]. Thus, the aim of this work was to carry out synthesis of TiO2-SnO2 nanocomposites and their characterization, comparison of sorption-photocatalytic properties of the obtained nanocomposites with pure TiO2 and with similarly obtained solid solutions in article [24], and study of dye degradation of different nature by synthesized nanocomposite photocatalysts.

2. Materials and Methods

2.1. Materials

Reagents of analytical grade were used in the research: titanium(IV) isopropoxide 98+% (C12H28O4Ti, Acros Organics, China); tin(II) chloride (, Merck KGaA, Germany); isopropyl alcohol (С3Н7ОН, Ukraine); nitric acid (НNO3, 65%, Merck KGaA, Germany); methylene blue dye (С16H18ClN3S, Carlo Erba Reagents, France); Congo red dye (C32H22N6Na2O6S2, Carlo Erba Reagents, France); and titanium(IV) oxide (TiO2, Aeroxide® TiO2P25, Evonik, Germany).

2.2. Nanocomposite Synthesis

Nanocomposites were synthesized by hydrothermal method in two ways: single-stage and two-stage methods.

Single-stage synthesis of titanium(IV) oxide and tin(IV) oxide nanocomposites was performed as follows: 7.5 mL of isopropyl alcohol and 2.5 mL of distilled water were mixed in a Teflon reactor. To the resulting mixture, 4x prediluted nitric acid was added dropwise to pH 1.5; then, 5 mL of a titanium(IV) isopropoxide solution was slowly added under constant stirring. After that, 0.2 g of tin(II) chloride was added and stirred vigorously for 20 minutes, and then pH was measured, which was 2.6. Next, the Teflon reactor was placed in a steel autoclave and hydrothermal treatment was performed at 453 K for 12 hours. Then, the resulting suspension was cooled, centrifuged (5000 rpm, MPW-310 centrifuge, Poland), and washed until pH 6. The washed powders were dried for 12 hours at 353 K and then grounded.

Two-stage synthesis of TiO2-SnO2 nanocomposites was carried out as follows: first, titanium(IV) oxide was synthesized by hydrothermal method from titanium(IV) isopropoxide; then, in the presence of titanium(IV) oxide, tin(IV) oxide was synthesized. In the first stage, synthesis was carried out in the same way as in the single-stage method except from introduction of the tin(II) chloride precursor into the reaction medium [24]. Obtained in this way, pure TiO2 was dried for 12 hours at a temperature of 353 K and grounded. Next, the second stage of nanocomposite synthesis was performed. 7.5 mL of isopropyl alcohol and 2.5 mL of distilled water were mixed in a Teflon reactor, and 1.32 g of the synthesized composites in the first stage TiO2 (or a commercial sample) was added under stirring. After formation of a homogeneous suspension, 0.2 g of SnCl2 was added under vigorous stirring. The solution thus obtained had pH of 2.5. The Teflon reactor was placed in a steel autoclave, and hydrothermal treatment was performed at 453 K for 12 hours. Then, the suspension was cooled, centrifuged (5000 rpm, MPW-310 centrifuge, Poland), and washed until pH 6. The washed powders were dried for 12 hours at 353 K and grounded.

Thus, three composites were synthesized and labeled as follows: Р90TiO2-SnO2, s1TiO2-SnO2, and s2TiO2-SnO2 (description is given in Table 1). Additionally, description of two other TiO2 samples (commercial and laboratory-synthesized) is given, which were studied in this work for comparison.

2.3. Characterization of ТіО2 Samples and TiO2-SnO2 Composites

Chemical composition of the synthesized nanocomposites was determined by X-ray fluorescence analysis using an EXPERT 3L INAM analyzer (Ukraine). The measurement range of mass fractions (concentrations) of elements was from 0.005% to 100%.

A study of the phase composition of the samples was performed on an X-ray diffractometer Rigaku Ultima IV (Japan) with CuKα radiation (40 kW, 30 mA), calculation of which was performed automatically using standard cards: No. 00-021-1276 (rutile), No. 00-021-1272 (anatase), and No. 00-041-1445 (cassiterite). The average crystallite size was calculated by Scherrer’s formula.

Nitrogen adsorption-desorption isotherms were obtained on a Quantachrome® Nova 4200e analyzer (USA). The mass of adsorbed and desorbed nitrogen was determined by a built-in algorithm based on the Langmuir or Brunauer-Emmett-Teller (BET) isotherm equation. Porous structure of the samples was determined by the Barrett-Joyner-Halenda (BJH) method.

X-ray photoelectron spectroscopy (XPS) spectra were obtained using a Kratos AXIS 165 spectrometer with Al mono Kalfa X-ray. EPR spectra were recorded at room temperature (298 K) on a RADIOPAN SE/X 2547 spectrometer. A fluorescence spectrophotometer (PerkinElmer, LS55) was used to record photoluminescence (PL) spectra. The electrons of the test samples were excited at the wavelength of 230 nm (5395 eV). For surface acidity determination, 1% suspensions of the samples were prepared, and pH of the obtained suspensions was measured for 2 hours using a Portlab 102 pH meter (Russia) according to the method described in [47].

2.4. Sorption and Photocatalytic Properties

Sorption and photocatalytic properties were evaluated by the discoloration degree of dyes of different nature (methylene blue and Congo red), as well as by the degree of their destruction.

Sorption properties of the samples were studied as follows: 0.05 g of a photocatalyst sample was added to 50 mL of a dye solution (methylene blue (MB), C16H18ClN3S or Congo red (CR), C32H22N6Na2O6S2) and stirred in an ultrasonic bath (40 kHz, UZM-004-1, Ukraine). Then, the resulting suspension was stirred for another 20 minutes on a magnetic stirrer, and the spent catalyst was separated from the solution by centrifugation. Concentration of the dye in the solutions before and after sorption was determined using spectrophotometer UV-5800PC (Shanghai Metash Instruments, China).

A study of the photocatalytic properties of the samples was performed similarly, except that UV irradiation of the suspension (368 nm) was additionally used during stirring on a magnetic stirrer for 20 minutes.

The discoloration degree (%) of a dye solution was determined by relative change in optical density of the dye solutions (for methylene blue dye, it was measured at a wavelength of 664 nm; for Congo red dye, the wavelength was 505 nm):where is the optical density of the initial dye solution and is the optical density of the solution after experiment.

Destruction of dyes was studied by absorption of CO2 released in the photocatalytic process by alkali, followed by its recalculation into organic carbon content. The process of dye destruction was carried out on an experimental installation of original design, the main elements of which were a quartz flask and a gas absorption flask, tightly connected. Dye solution and catalyst were placed in a quartz flask and stirred using sonication for 5 minutes. Next, the quartz flask was connected to the gas absorption flask, into which 25 mL of sodium hydroxide solution (concentration 0.1 mol∙L-1) had been poured, and then vacuum was created in it. After this, photocatalytic destruction was performed by stirring under ultraviolet radiation (368 nm). CO2 gas released as a result of the photocatalytic process bubbled into the gas absorption flask and was absorbed by alkali solution, the change in concentration of which was determined by titration with hydrochloric acid (concentration 0.1 mol∙L-1).

3. Results and Discussion

The following transformations took place in the process of nanocomposite synthesis:

Synthesis of titanium(IV) oxide was carried out according to reaction (2), and formation of tin(IV) oxide by both single-stage and two-stage hydrothermal syntheses occurred according to reaction (3).

3.1. XRD

Figure 2 shows X-ray patterns of all obtained samples that were automatically analyzed. Standard cards were used for identification: anatase, JCPDS 01-070-7348; rutile, JCPDS 01-070-7347; and cassiterite, JCPDS 00-041-1445.

As a result of the analysis, it has been established (Table 2) that a commercial sample of TiO2P25 consists of approximately 25% rutile and 75% anatase that corresponds to the literature data [44], while hydrothermally synthesized sample sTiO2 has pure anatase modification. Phase composition analysis of nanocomposites Р25TiO2-SnO2, s1TiO2-SnO2, and s2TiO2-SnO2 (Table 2) shows that nanocomposites have similar phase composition of pure TiO2 and additional phase of cassiterite, the content of which is 10-11% that corresponds to the theoretical calculation.

Analysis of structural characteristics of nanocomposites and TiO2 samples (Table 2) shows that parameters , , and of crystal lattices for all phases are quite close to the theoretical values ( nm and  nm for anatase;  nm and  nm for rutile; and  nm and  nm for cassiterite). Therefore, significant distortions of crystal lattices are not observed.

Crystallite sizes of all samples are in the range of 4.0-20.1 nm. Thus, all samples are nanocrystalline. P25TiO2-SnO2 nanocomposite, based on the commercial sample AEROXIDE® TiO2P25, has the largest crystallite size that is probably because of the aggregation of initial particles. Nanocomposites based on synthesized TiO2 have smaller crystallite sizes, and for s1TiO2-SnO2 nanocomposite obtained by single-stage synthesis, crystallite sizes are smaller than those for pure TiO2 and s2TiO2-SnO2 nanocomposites synthesized in two stages. Thus, single-stage synthesis allows obtaining TiO2-SnO2 nanocomposites with smaller crystallite size. The two-stage method in case of using the commercial TiO2 sample promotes its aggregation but does not change the crystallite size of the hydrothermally synthesized pure TiO2.

3.2. Chemical Composition

Chemical analysis of the samples confirmed 10 wt.% content of cassiterite. The results of chemical analysis are presented in Table 3. Thus, based on XRD results and taking into account chemical composition of TiO2-SnO2 nanocomposites, it can be stated that in all cases, nanocomposites were obtained with cassiterite phase content of 10 wt.%.

3.3. ХPS, EPR, and PL

Chemical composition and available surface chemical states of TiO2-SnO2 nanocomposites were studied by XPS method. Obtained survey spectra for nanocomposites and synthesized TiO2 are shown in Figure 3 and indicate the presence of Ti, Sn, O, and C (hydrocarbons from the XPS device). An XPS data report showed the presence of the following surface states: Ti3+ and Ti4+ for titanium and Sn4+ for tin. Besides, tin concentration in the P25TiO2-SnO2 and s2TiO2-SnO2 composites is higher (6.1-6.5%) than that in the s1TiO2-SnO2 (3.9%) sample. That is due to the fact that P25TiO2-SnO2 and s2TiO2-SnO2 nanocomposites were obtained by two-stage synthesis, and in this case, tin was deposited on the surface. s1TiO2-SnO2 nanocomposite was synthesized via single-stage synthesis, so tin was distributed in the whole material volume.

In addition, Supplementary Materials contains native XPS spectra of Ti 2p, Sn 3d, and O 1s for TiO2-SnO2 nanocomposites (Figures S1 , S2, and S3, respectively). According to the spectra of Ti 2p (Figure S1), 2 peaks are observed: the first is about 464 eV, which corresponds to the binding energy of Ti 2p 1/2, and the second is about 459 eV, which refers to the binding energy of Ti 2p 3/2 [21]. In the XPS spectra of Sn 3d for all nanocomposites (Figure S2), two peaks were also present that correspond to the binding energy at approximately 495 eV and 486 eV and are characteristic for Sn 3d 3/2 and Sn 3d 5/2, respectively [4850]. XPS spectra of O 1s (Figure S3) for P25TiO2-SnO2 and s2TiO2-SnO2 composites are characterized by peaks at 530 eV, 530.5 eV, and 535.5 eV that correspond to oxygen bound in TiO2 and SnO2 and in the form of surface hydroxyl oxygen (OH), respectively [5153]. s1TiO2-SnO2 nanocomposite is characterized by similar peaks except the one corresponding to hydroxyl oxygen.

Also, EPR spectra (Figure S4) of TiO2P25, sTiO2, Р25TiO2-SnO2, s1TiO2-SnO2, and s2TiO2-SnO2 are shown in Supplementary Materials. The EPR spectra obtained at room temperature demonstrate weak signals, which is most likely associated with the short lifetime of electrons and holes under these conditions.

Figure 4 shows photoluminescence spectra of the samples. As can be seen from Figure 4, all samples demonstrate an emission at the wavelength of 405-406 nm (~3.08 eV), the intensity of which is approximately the same. However, peak intensity is the highest for the samples TiO2P25 and s2TiO2-SnO2 and the lowest for the P25TiO2-SnO2 sample. This indicates that among all samples, the TiO2P25 and s2TiO2-SnO2 samples have the highest charge recombination rate, while the P25TiO2-SnO2 sample has the lowest [54].

3.4. Structural-Adsorption Characteristics

Figure 5 presents the isotherms of nitrogen adsorption-desorption and pore size distribution for all tested samples. As can be seen from the presented isotherms, all samples have different porous structures. Thus, according to the IUPAC classification, the P25TiO2 sample has isotherm type III, which is typical of nonporous or macroporous materials. The isotherm of the commercial sample P25TiO2 after modification with tin(IV) oxide has a slightly different form; namely, isotherm type III has transformed into type V. This indicates transition of a nonporous or macroporous structure to a mesoporous-microporous structure that has developed as a result of the modification. It can also be seen that synthesized samples of sTiO2, s1TiO2-SnO2, and s2TiO2-SnO2 are characterized by isotherms of type IV, which are characteristic for mesoporous materials [55].

Nature of hysteresis loops for samples Р25TiO2-SnO2, sTiO2, s1TiO2-SnO2, and s2TiO2-SnO2 is different. Therefore, they have different pore structures. According to the IUPAC classification, the hysteresis loop of the P25TiO2-SnO2 sample belongs to the H3 type, which is characteristic of lamellar structures with the presence of macropores [55].

Thus, as a result of modification of commercial TiO2 surface, small mesoporosity developed. At the same time, the specific surface area decreased slightly from 109 m2/g for P25TiO2 to 99 m2/g for P25TiO2-SnO2 (Table 4), which may indicate slight aggregation of titanium(IV) oxide particles as a result of modification. The synthesized sTiO2 sample has a hysteresis loop of type H4 [55], which indicates mesoporous-microporous structure of this sample. The structure of s2TiO2-SnO2 nanocomposite is close to the structure of sTiO2, which is quite understandable since it was obtained by modification of sTiO2 with tin(IV) oxide, which led to a small change in porous structure of titanium(IV) oxide. As the result of modification, the surface area increased slightly from 172 m2/g for sTiO2 to 223 m2/g for s2TiO2-SnO2. Different methods of obtaining pure TiO2 explain this unequal effect on the specific surface area due to modification. Probably, hydrothermal conditions under which modification took place caused aggregation of the commercial sample particles, in contrast to the synthesized sTiO2 under the same conditions. The hysteresis loop of the s1TiO2-SnO2 sample belongs to the H2 type (), which is characteristic of complex porous structures that have pores with a large neck diameter [55].

Obtained structural characteristics (average pore diameter and total pore volume) for all samples correlate well with the types of isotherms and porous structures.

3.5. Surface Acidity

Figure 6 illustrates the change in pH of all test sample suspensions according to the method presented in [43]. The shape of curves indicates that all samples have Lewis acid centers, and based on the slope of the curves, the largest number of the centers belong to s1TiO2-SnO2 and s2TiO2-SnO2 nanocomposites. Of particular interest is s1TiO2-SnO2 nanocomposite; change in pH of its suspensions was established very quickly and did not change over time. This behavior indicates the presence of mainly Lewis centers and is confirmed by its XPS spectra, in which there are no surface OH groups present.

After 2 hours (when equilibrium was achieved), the isoionic point (pHiip) value was 4.15 for P25TiO2, 5.78 for sTiO2, 3.53 for P25TiO2-SnO2, 2.84 for s1TiO2-SnO2, and 3.33 for s2TiO2-SnO2. pHiip values indicate acidic nature of the surface for all samples. In this case, sTiO2 is characterized by the lowest acidity, while s1TiO2-SnO2 nanocomposite by the highest. Comparing pHiip of pure TiO2 samples and TiO2-based composites, it can be seen that modification with SnO2 in all cases increases acidity. It should be noted that despite the lower acidity of the sTiO2 sample in comparison with P25TiO2, modification of their surface with tin(IV) oxide leads to different results. Acidity of the nanocomposites based on sTiO2 is much higher than in the case of a composite based on a commercial sample. This is probably due to the modification in acidic media and larger specific surface area of sTiO2 compared to P25TiO2 that resulted in greater sorption of H+ ions by the sTiO2 surface from the reaction solution during synthesis.

3.6. Adsorption-Photocatalytic Properties

Adsorption and photocatalytic activity of all samples was studied using the most widely used model dye solutions of different nature: methylene blue (cationic dye) and Congo red (anionic dye) [56, 57].

Results of the adsorption and photocatalytic study on methylene blue are shown in Figure 7. Diagram data show that methylene blue is almost not adsorbed on the samples. This is because of the acidity of all samples resulting in total positive charge of their surface that eventually caused low cationic dye adsorption.

Photocatalytic activity (Figure 7) is higher, and the dye removal degree in photocatalysis ranges from 50% to 90%. The best results were shown by a modified commercial sample P25TiO2-SnO2 that indicates a positive effect of modification. A similar situation is observed in case of modified and unmodified samples: modification leads to an increase in photocatalytic degradation of methylene blue from 21% for sTiO2 to 53% for s1TiO2-SnO2 and 56% for s2TiO2-SnO2.

Figure 8 illustrates adsorption-photocatalytic properties of all test samples regarding Congo red. Adsorption properties of almost all samples are much better towards anionic dye, and adsorption efficiency ranges from 10% to 78%. Photocatalytic extraction of Congo red is slightly greater than adsorption and, in general, does not indicate significant differences for modified and unmodified samples.

Supplementary Materials contains absorption spectra of initial dye solutions (S5) and solutions after photocatalytic extraction of cationic (S6) and anionic (S7) dyes. In general, they indicate a decrease in dye concentration after the photocatalytic process. However, in some cases, dye concentration did not change or even increase, or peaks of initial dye solutions were observed in the ultraviolet region, for which the degradation degree was not calculated. In addition, when Congo red was treated using the s1TiO2-SnO2 and s2TiO2-SnO2 samples, the main absorption peak of the anionic dye was shifted, caused by high acidity of these samples.

In addition, kinetic dependences were obtained regarding degradation of the most widely used dye in photocatalytic studies—methylene blue. Figure 9 shows photocatalytic removal of MB utilizing each photocatalyst sample over time. As seen from Figure 9(a), the P25TiO2-SnO2 sample is the most efficient to remove MB from its aqueous solution. This is consistent with the results shown in Figure 7 and is explained by the lower recombination rate according to PL spectra.

Figure 9(b) shows linearized kinetic curves, determination coefficients (, Table 5) of which indicate that reaction kinetics for all samples is adequately described by the Langmuir-Hinshelwood model, being in full agreement with the literature data [5860]. The corresponding apparent rate constants (, Table 5) confirm the previously obtained results regarding the highest photocatalytic activity of the P25TiO2-SnO2 sample (apparent rate constant of which is 1.75 times higher than that of unmodified TiO2P25).

Thus, subsequent studies on the stability and reusability of the obtained photocatalysts were carried out utilizing the P25TiO2-SnO2 sample.

Figure 10 shows results of the reusability tests of the Р25TiO2-SnO2 sample in photocatalytic extraction of MB. As can be seen from the diagram, a high degree of dye removal (95-82%) by the chosen photocatalyst is observed during five cycles, which slightly decreases by the end of the fifth cycle. The obtained results indicate good stability and reusability of the tested photocatalyst.

3.7. Dye Destruction and Analysis of Photocatalytic Studies

Additionally, it was decided to study destruction of methylene blue and Congo red (by the amount of CO2 released) in the photocatalytic process for its deeper understanding. Table 6 shows destruction results and, for comparison, photocatalytic extraction data (obtained by spectrophotometric method). The photocatalytic extraction degree of dyes by the TiO2-SnO2 sample obtained hydrothermally but from different precursors (tin(IV) chloride [24], which resulted in a TiO2 solid solution formation) is also mentioned in Table 6.

As can be seen from Table 6, dye degradation under UV radiation (368 nm) occurs even without photocatalysts and is approximately 18% for methylene blue and 5% for Congo red. Destruction of methylene blue with photocatalysts is greater and is even higher in case of the photocatalytic process utilizing modified samples of TiO2, both commercial and synthesized. In general, the destruction degree is lower than the photocatalytic extraction degree that indicates partial extraction of methylene blue either due to sorption or due to decomposition into smaller structural units. A similar pattern is observed for Congo red. At the same time, the degradation degree of Congo red is lower than that of methylene blue and much lower than its photocatalytic extraction degree. Therefore, anionic dye extraction occurs more due to adsorption interactions or due to its decomposition into smaller structural units. The lower destruction degree of Congo red compared to methylene blue can be explained by the fact that more energy is required for the complete destruction of a large Congo red molecule. It should be added that the destruction degree of methylene blue by the P25TiO2-SnO2 nanocomposite is much higher (4.8 times) than by the commercial P25TiO2 sample. Therefore, the positive effect of modification with tin(IV) oxide was also observed in case of anionic dye. Thus, results of dye photocatalytic extraction are fully consistent with the results of destruction and indicate a general positive effect of modification with tin(IV) oxide leading to increased photocatalytic activity of TiO2.

Comparison of the results of dye photocatalytic extraction obtained in this research with the photocatalytic activity of the TiO2-SnO2 sample obtained in [24] (Table 6) shows better prospects of TiO2-SnO2 nanocomposites obtained using the tin(II) chloride precursor as evidenced by their higher photocatalytic activity towards methylene blue. Comparison of the obtained results with the results reported by other researchers, for example, in [58, 59], in which 60–75% of the dyes are removed within 20 minutes of the photocatalytic process, also indicates that development of TiO2-SnO2 nanocomposites is highly promising.

4. Conclusions

In the paper, TiO2-SnO2 nanocomposites were obtained by hydrothermal synthesis using the single-stage and two-stage methods. In addition, TiO2-SnO2 nanocomposite based on the commercial sample AEROXIDE® TiO2P25 was synthesized. They were characterized by XRD, X-ray fluorescence method, XPS, EPR, PL, and low-temperature nitrogen adsorption-desorption method. The effect of TiO2 modification with tin(IV) oxide on sorption-photocatalytic properties of obtained nanocomposites was studied.

It was found that in all cases, nanostructured TiO2-SnO2 composites were obtained (crystallite sizes range from 4.0 nm to 20.1 nm), which contain 10 wt.% of the SnO2 phase. A study of the surface chemical states of TiO2-SnO2 nanocomposites revealed the presence of OH ions on the surface of nanocomposites obtained in two stages and their absence in the nanocomposite synthesized in one stage. The absence of hydroxide ions is confirmed by the results of a total acidity study of nanocomposite surface, according to which all samples have acidic nature of their surface.

Adsorption-structural studies have shown that the commercial sample of TiO2 is nonporous, and its modification under hydrothermal conditions leads to a slight aggregation of TiO2 particles and development of minor mesoporosity. TiO2 synthesis in hydrothermal conditions, as well as creation of nanocomposites based on it, leads to obtaining mesoporous powders with a pore diameter of 2.6-3.8 nm and specific surface area in the range of 172-223 m2/g but with different pore structures.

Sorption-photocatalytic properties of TiO2-SnO2 nanocomposites and pure TiO2 powders indicate higher adsorption efficiency towards anionic dye (consistent with acidity) and higher photocatalytic activity towards cationic dye. Moreover, studies on the destruction of both dyes are consistent with photocatalytic experiments, and comparison of photocatalytic properties of TiO2-SnO2 nanocomposites with a previously synthesized solid solution indicates greater prospects of nanocomposites. Therefore, it can be noted that in general, TiO2 modification by tin(IV) oxide leads to a photocatalytic activity increase.

Data Availability

No data were used to support this study.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Supplementary Materials

S1: XPS spectra (Ti 2p) of TiO2-SnO2 nanocomposites. S2: XPS spectra (Sn 3d) of TiO2-SnO2 nanocomposites S3: XPS spectra (O 1s) of TiO2-SnO2 nanocomposites. S4: EPR spectra of (a) TiO2P25, (b) sTiO2, (c) Р25TiO2-SnO2, (d) s1TiO2-SnO2, (e) s2TiO2-SnO2. S5: Absorption spectra of initial dyes.S6: Absorption spectra after photocatalytic removal of Methylene blue.S7: Absorption spectra after photocatalytic removal of Congo red. (Supplementary materials)