Journal of Nanomaterials
Volume 2020 (2020), Article ID 8349480, 13 pages
https://doi.org/10.1155/2020/8349480

Research Article

Enhanced Photocatalytic Activity of TiO₂/SnO₂ Binary Nanocomposites

Tetiana A. Dontsova, Anastasiya S. Kutuzova, Kateryna O. Bila, Svitlana O. Kyrii, Iryna V. Kosogina, and Daria O. Nechyporuk

Department of Inorganic Substances, Water Purification and General Chemical Technology, Chemical Technology Faculty, National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 03056, Prosp. Peremohy 37, Kyiv, Ukraine

Correspondence should be addressed to Tetiana A. Dontsova; dontsova@xtf.kpi.ua

Received 22 May 2020; Revised 31 July 2020; Accepted 1 August 2020; Published 18 August 2020

Academic Editor: Yasuhiko Hayashi

Copyright © 2020 Tetiana A. Dontsova et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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 [3–5].

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. [6–8].

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.

(a)

(b)

(c)

Figure 1: Crystal lattices and structures of TiO₂ modifications: (a) rutile, (b) anatase, and (c) brookite [10, 11].

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 [20–22]. 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 [26–28], 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 [30–33], hydrothermal synthesis [34–36], chemical vapor deposition [37], spray and laser pyrolysis [38–41], 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.

Table 1: ТіО₂ samples and TiO₂-based composites.

P25TiO₂
Commercial sample AEROXIDE® TiO₂P25

sTiO₂
TiO₂ synthesized according to the method described in [21]

1Р25TiO₂-SnO₂
Commercial AEROXIDE® TiO₂P25 modified with tin oxide

s1TiO₂-SnO₂
TiO₂-SnO₂ composite synthesized by single-stage hydrothermal synthesis

s2TiO₂-SnO₂
TiO₂-SnO₂ composite synthesized by two-stage hydrothermal synthesis

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.

Figure 2: Diffraction patterns of the samples: 1: TiO₂P25; 2: sTiO₂; 3: Р25TiO₂-SnO₂; 4: s1TiO₂-SnO₂; and 5: s2TiO₂-SnO₂.

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.

Table 2: XRD analysis.

Sample
Phase
(nm)
(nm)
(nm)
Crystallite size (nm)
Mass content (%)

P25TiO₂
Anatase
0.378
0.378
0.950
4.4
75

Rutile
0.459
0.459
0.296
4.0
25

sTiO₂
Anatase
0.378
0.378
0.947
7.0
100

Р25TiO₂-SnO₂
Anatase
0.379
0.379
0.950
15.2
69

Rutile
0.459
0.459
0.296
20.1
21

SnO₂
0.475
0.475
0.319
11.6
10

s1TiO₂-SnO₂
Anatase
0.379
0.379
0.945
5.7
89

SnO₂
0.497
0.497
0.497
5.2
11

s2TiO₂-SnO₂
Anatase
0.379
0.379
0.949
7.0
89

SnO₂
0.483
0.483
0.321
6.8
11

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.%.

Table 3: Chemical composition of TiO₂-SnO₂ composites.

Р25TiO₂-SnO₂
s1TiO₂-SnO₂
s2TiO₂-SnO₂

Element
Mass fraction (%)

22Ti
90.5
88.1
89.5

50Sn
9.5
11.9
10.5

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.

Figure 3: XPS spectra of TiO₂-SnO₂ nanocomposites and TiO₂ synthesized by hydrothermal method.

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 [48–50]. 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 [51–53]. 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].

Figure 4: PL spectra: 1: TiO₂P25; 2: sTiO₂; 3: Р25TiO₂-SnO₂; 4: s1TiO₂-SnO₂; and 5: s2TiO₂-SnO₂.

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].

(a)

(b)

(c)

(d)

(e)

Figure 5: Nitrogen adsorption-desorption isotherms and pore size distribution: (a) TiO₂P25, (b) sTiO₂, (c) Р25TiO₂-SnO₂, (d) s1TiO₂-SnO₂, and (e) s2TiO₂-SnO₂.

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].

Table 4: Structural-adsorption characteristics of nanocomposites.

Sample
Specific surface area (m²/g)
Average pore diameter (nm)
Total pore volume (cm³/g)

P25TiO₂
109
—
—

sTiO₂
172
3.8
0.32

Р25TiO₂-SnO₂
99
1.5
0.25

s1TiO₂-SnO₂
223
2.6
0.28

s2TiO₂-SnO₂
192
3.7
0.31

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.

Figure 6: Total acidity of sample surface: 1: TiO₂P25; 2: sTiO₂; 3: Р25TiO₂-SnO₂; 4: s1TiO₂-SnO₂; and 5: s2TiO₂-SnO₂.

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.

Figure 7: Adsorption and photocatalytic studies using methylene blue (dye concentration 10 mg/L, reaction time 20 min): 1: TiO₂P25; 2: sTiO₂; 3: Р25TiO₂-SnO₂; 4: s1TiO₂-SnO₂; and 5: s2TiO₂-SnO₂.

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.

Figure 8: Adsorption and photocatalytic studies using Congo red (dye concentration 40 mg/L, reaction time 20 min): 1: TiO₂P25; 2: sTiO₂; 3: Р25TiO₂-SnO₂; 4: s1TiO₂-SnO₂; and 5: s2TiO₂-SnO₂.

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.

(a)

(b)

Figure 9: Change in the concentration of MB during the photocatalytic process: (a) kinetic dependences and (b) linearized curves .

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 [58–60]. 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).

Table 5: Apparent rate constants () and coefficients of determination () of linearized kinetic curves.

Sample
(min⁻¹)

P25TiO₂
0.0624
0.976

sTiO₂
0.0572
0.965

Р25TiO₂-SnO₂
0.1089
0.967

s1TiO₂-SnO₂
0.0074
0.973

s2TiO₂-SnO₂
0.0079
0.951

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.

Figure 10: Degree of photocatalytic removal of MB in reusability tests with the P25TiO₂-SnO₂ sample (dye concentration 10 mg/L, reaction time 20 min).

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.

Table 6: Comparative table of photocatalysis and destruction under UV radiation (368 nm).

Sample
Photocatalysis (%)
Destruction (%)

Methylene blue
Congo red
Methylene blue
Congo red

No sample, only UV radiation
—
—
18
5

P25TiO₂
65
61
36
10

sTiO₂
21
80
25
10

Р25TiO₂-SnO₂
92
63
52
48

s1TiO₂-SnO₂
53
74
52
10

s2TiO₂-SnO₂
56
78
44
9

TiO₂-SnO₂ obtained in [24]
31
75
—
—

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)

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