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Article

Enhanced Photocatalytic Performances of SnS2/TiO2 Composites via a Charge Separation Following Z-Scheme at the SnS2/TiO2{101} Facets

by
Nkenku Carl
1,
Muhammad Fiaz
2,
Hyun-Seok Oh
1 and
Yu-Kwon Kim
1,2,*
1
Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea
2
Department of Chemistry, Ajou University, Suwon 16499, Republic of Korea
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(7), 442; https://doi.org/10.3390/catal14070442
Submission received: 6 June 2024 / Revised: 3 July 2024 / Accepted: 8 July 2024 / Published: 10 July 2024
(This article belongs to the Special Issue Recent Advances in Environment and Energy Catalysis)
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Abstract

:
The formation of heterojunctions for efficient charge separation has been practiced for the preparation of efficient semiconductor-based photocatalysts for applications such as hydrogen production and environmental remediation. In this study, we synthesized a composite structure with a heterojunction between SnS2 and TiO2 through a microwave-assisted hydrothermal process, in which SnS2 nanoparticles grew on nanocrystalline TiO2 nanosheets preferentially at the exposed {101} facets. Appropriate exposure of the {001} and {101} facets of the TiO2 nanosheet in the composite with a preferential growth of SnS2 nanoparticles at the {101} facets was the origin of the charge separation following a direct Z-scheme mechanism to result in enhanced photocatalytic performances in photodegradation of organic dyes such as methylene blue (MB) and rhodamine B (RhB) compared to that of SnS2 and TiO2 alone. A plot of photodegradation rates vs. SnS2 ratios in the composites gave an overall volcano-shaped curve with a maximum at the SnS2 ratio of about 33% at which small SnS2 nanoparticles were populated at the {101} facets of the TiO2 nanosheets with a high surface area (118.2 m2g−1). Our results suggest the microwave-assisted hydrothermal process can be a good synthetic approach for composite-based photocatalysts with a preferential heterojunction structure.

Graphical Abstract

1. Introduction

The design of efficient photocatalytic materials can pave the way not only to utilize energy directly from the Sun as a means to meet the growing world’s energy demand but also to eliminate harmful organic compounds for environmental remediation through photocatalytic degradation via direct light-induced chemical reactions mediated by photogenerated electrons and holes [1,2]. Organic dyes can be good examples of harmful compounds that can be soluble in water and remain for a long time to affect our ecosystems. Organic dyes are commonly used in various areas of our daily life, such as papers, printing, and textiles, and usually have a high degree of chemical stability [1]. To eliminate such stable but harmful compounds, the use of photocatalysts can ultimately be an environmentally friendly way of accelerating degradation reactions.
Among various photoactive materials, TiO2 is considered a preferred photocatalyst due to its high reduction potential with a sufficiently large band gap, in addition to its nontoxicity, low cost, and high stability [3]. Furthermore, studies based on shape-controlled synthesis of TiO2 have revealed many interesting aspects of the origin of its photoactivity. For example, the photoactivity of anatase TiO2 has been shown to vary depending on the exposed facets, such as {101} and {001} [4]. The ratio of exposed facets has been effectively controlled to observe high photoactivities attributed to {001} [5,6,7] and {101} facets [8], respectively, despite the advantageous properties of TiO2, the wide band gap of about 3.2 eV limits light absorption to the UV region (wavelength below 400 nm) and its photocatalytic activity as well.
The photoactivity of TiO2 can be further enhanced by making use of both the preferential charge separation of TiO2 and the enhanced absorption by a visible-light-active photocatalyst through a proper heterojunction between them. Forming a heterojunction of the photocatalyst in the preferred facets of TiO2 can effectively suppress the recombination of photogenerated e−–h+ pairs, which can extend the lifetime of the hot carriers on the surface to improve surface reaction efficiencies [9]. Combining TiO2 with narrow bandgap semiconductors by forming heterojunctions is a successful approach to extend not only the light absorption range but also the charge carrier lifetime via charge separation at the heterojunctions [10]. As a result, enhanced photoactivity has been reported for various types of TiO2-based composites such as MoS2/TiO2 [11], SnO/TiO2 [12], TiO2/g-C3N4 [10] and CdS/TiO2 [13].
SnS2 is a layered transition metal dichalcogenide (TMD) with a narrow band gap (2.18–2.44 eV) that can act as a charge transfer medium as well as active sites for catalytic reactions [14] and is gaining increasing attention as a photocatalyst for solar-driven reactions and degradation of organic pollutants. This is due to its cost-effectiveness, non-toxicity, and unique physicochemical characteristics [15]. However, SnS2 is also limited by its high recombination rate, which can be mitigated by forming heterojunctions. These heterojunctions enhance the charge separation of photo-excited electron–hole pairs [16].
In this study, we synthesized composites of SnS2/TiO2 with the intention of exploring the photodegradation performances of the composites in relation to properties such as charge separation at heterojunctions and enhanced visible light absorption due to SnS2. As a composite preparation method, we employed a microwave-assisted hydrothermal process, which was found to be a facile hydrothermal method of preparing SnS2/TiO2 composites with coupled 2D heterojunctions. The synthesized composites were found to show enhanced photocatalytic activity in the photodegradation of both methylene blue (MB) and rhodamine B (RhB) in an aqueous solution, largely due to enhanced charge separation at the heterojunction via the Z-scheme.

2. Results and Discussion

2.1. Characteristics of SnS2/TiO2 Composites

Figure 1 shows the X-ray diffraction (XRD) spectra of the as-synthesized composites compared to those of as-synthesized SnS2 and TiO2 as references. The XRD spectrum of the as-synthesized SnS2 nanoparticles exhibits the characteristics of hexagonal SnS2 structure (JCPDS no. 01-1010) [17]. Additionally, as-synthesized TiO2 nanosheets give an XRD spectrum with the characteristic peaks of anatase phase TiO2 without the presence of a rutile phase (JCPDS card no. 21-1272) [18]. The intense (200) diffraction of anatase TiO2 compared to (004) originates from the shape of the nanosheets with the large (001) facets of anatase TiO2 [19,20], which is due to the surfactant-mediated preferential growth mediated by surfactant toward directions perpendicular to (001). The XRD patterns of the SnS2/TiO2 composites display the characteristic peaks of the anatase TiO2 and hexagonal SnS2 only, indicating the selective formation of both phases in the compounds with varying ratios of SnS2. The intensity ratios of the peak SnS2 (001) to the TiO2 (101) or (004) change in accordance with the mole ratio of SnS2 to TiO2 with increasing %SnS2 content. Figure S1 contains the XRD patterns of all the prepared composites. The results indicate that the microwave process can generate composites based on both the hexagonal SnS2 and the anatase TiO2 phases over a wide range of SnS2 mole ratios.
The absorbance of the composites was measured by UV–vis differential reflectance spectroscopy (DRS), as shown in the inset of Figure 2. The optical bandgap of the composites was determined using the Kubelka–Munk function (Figure S2), and the results are shown as a plot as a function of %SnS2 in Figure 2. The measured band gap of the TiO2 nanosheets agreed well with the value in the literature (3.2 eV) for the anatase TiO2 with an absorption edge at around 400 nm [21]. SnS2 has an absorption edge of around 560 nm, which corresponds to a bandgap of 2.02 eV [14]. For the composites, the bandgap rapidly decreased from 3.2 eV as the %SnS2 increased to 20%. At a higher %SnS2, it slowly approaches that of SnS2 (2.02 eV). This agrees well with the presence of the SnS2 phase in the composites and the absorption by SnS2 in the visible region with increasing %SnS2.
Figure 3 shows the SEM images of the SnS2/TiO2 composites with various %SnS2 ratios (c)–(e) compared to those of SnS2 (a) and TiO2 (b), respectively. The as-synthesized TiO2 nanosheets show SEM images with characteristic square-shaped nanosheets (Figure 3a), and the shape of the as-synthesized SnS2 nanoparticles is characterized by irregularly shaped platelets (Figure 3f), confirming the 2D nature of the material. At low %SnS2 ratios (<20%), the square-shaped TiO2 nanosheets appear to have small nanoparticles at both the large facets and the edges of the nanosheets. It appears that the edges of the square-shaped TiO2 nanosheets are more populated with small SnS2 nanoparticles than the large {001} facets. In the composites with higher %SnS2 ratios (>33%), nanoplates of different shapes that can be assigned to TiO2 and SnS2, respectively, are identified from the SEM images Figure 3e). It also shows that square-shaped TiO2 nanoplates are in close contact with 2D SnS2 platelets. With increasing %SnS2 ratios (Figure S3), the abundant square-shaped nanosheets gradually transform into more irregularly shaped platelets, suggesting the population of SnS2 nanoparticles increases. The EDS composition of the composites also confirms that the stoichiometric phases of SnS2 and TiO2 change in ratio in accordance with the varying SnS2 mole ratios (Figure S4 and Table S1). The high oxygen content is attributed to the result of some adsorbed oxygen on the samples from the atmosphere [22]. The unique preferential distribution of SnS2 nanoparticles at the edges of TiO2 nanosheets may be advantageous in that the photoexcited electrons transported to the edges of TiO2 can be consumed by the SnS2 nanoparticles for enhanced charge separation.
From Figure 4a,b, the lattice fringe of (200) planes with a lattice spacing of 0.19 nm confirms the assignment of the nanosheets to anatase TiO2, as reported previously [23]. The TEM images of SnS2 can be distinguished from those of TiO2 (Figure 4a) from the characteristic irregular shapes (Figure 4c) and the distinct interplanar distance of 0.59 nm (Figure 4d) which corresponds to the d-spacing of (001) planes of hexagonal SnS2 [24]. For the case of SnS2/TiO2 composites, a preferential distribution of small particles at the edges of square-shaped nanosheets can be seen (Figure 4e), suggesting that there exists a preferential chemical bonding of SnS2 nanoparticles at the edges of the TiO2 nanosheets. The SnS2 nanoparticles at the edges are confirmed from the lattice fringes with an interplanar distance of 0.59 nm (Figure 4f as well as Figure S5), corresponding to the (001) facet for the 2D SnS2 nanoparticles and the spacing of 0.35 nm which agrees well with the assignment of the {101} facet of anatase TiO2 [5,25]. The {001} facets of TiO2 are observed to be most exposed to the surface without being in contact with SnS2. The edges of the anatase TiO2 nanosheets are suggested to be the {101} facets, and the large planes are photoactive {001} facets [7,26,27,28]. From Figure 4e,f (as well as Figure S6), the amount of SnS2 nanoparticles on the {001} facets of TiO2 increases with an increase in %SnS2. Such a change in the distribution of SnS2 nanoparticles with increasing SnS2 coverage is expected to have a significant influence on the overall charge separation efficiency of the composites, as well as the charge separation mechanism at the heterojunctions.
Figure 5 shows the Raman spectra of the as-synthesized SnS2/TiO2 composites in comparison with those of TiO2 and SnS2. The characteristic modes of the anatase TiO2 (Eg, B1g, A1g, and Eg) and that of the SnS2 phase (A1g) can be identified from the spectra [14,29,30,31], which confirms the presence of the anatase TiO2 and SnS2 phases in the composites and the relative intensity ratios of the two phases vary according to the SnS2 ratio. With increasing %SnS2, the intensity of Eg for anatase TiO2 decreases, while that of the A1g mode for SnS2 increases. The Raman spectra of all the composites are shown in Figure S7.
The photoluminescence (PL) spectra shown in Figure 6 indicate that the TiO2 nanosheets show high PL emission at 520–550 nm under the illumination of a 325 nm laser, which is attributed to the recombination of photoexcited e−–h+ pairs. From Figure 6b, it can be seen that the PL intensities of the composites are dramatically lower than those of TiO2. The PL emission rapidly drops from the very low SnS2 ratio and approaches the minimum value at the SnS2 ratio of about 33%. This can be interpreted as the result of the charge separation at the heterojunction between SnS2 and TiO2. If we assume no charge transfer between the two, the PL emission can be expected to decrease linearly from that of TiO2 to that of pure SnS2 (the dashed line in Figure 6b). Then, the difference between the dashed line and the actual emission can be related to the efficiency of charge separation (Figure 6c), which is the maximum up to the SnS2 ratio of about 33%. This result demonstrates that the composites are not just a mixture of anatase TiO2 nanosheets and SnS2 nanoparticles but have heterojunctions that can transfer charges between them to lead to the separation of photoexcited e−–h+ pairs, which can lead to enhanced photocatalytic activity.
XPS was employed to further interrogate the chemical bonding states in the composites. The survey spectra (Figure S8) confirm the presence of elements O, Ti, S, and Sn in the composites. The core level spectra of O 1s in Figure 7a indicate the presence of two oxygen components at 529.7 and 531.5 eV, respectively, for TiO2, which can be assigned to lattice oxygens with direct Ti–O bonds and surface oxygen atoms in the forms of OH, respectively [32]. For the SnS2/TiO2 composite, the same oxygen components are observed at 530.6 and 532.4 eV, respectively, which are shifted towards higher binding energies compared to those of TiO2. The Ti 2p core-level spectra in Figure 7b show Ti 2p3/2 and Ti 2p1/2 features at 458.6 and 464.3 eV, respectively, for TiO2 nanosheets. The Ti 2p3/2 peak at 458.6 eV is indicative of the Ti4+ chemical state of stoichiometric TiO2 [33]. The binding energies of the Ti 2p core level for SnS2/TiO2 also appear at 459.3 and 465.0 eV, respectively, which are shifted towards higher binding energies. Detailed fitting results of the composites further disclose the presence of an additional small Ti 2p3/2 component at lower binding energies of 456.6 eV. This may be due to Ti species with a direct Ti–S bond at the heterojunction between SnS2 and TiO2 [34,35]. The formation of a direct Ti–S bond at the interface can also be seen in the S 2p core-level spectra in Figure 7, which reveals a small S 2p component at the binding energies of 162.4 and 163.6 eV with the spin-orbit splitting of 1.18 eV [34,35] in addition to the major component assigned to S2− of SnS2 at 161.5 and 162.7 eV (S 2p3/2 and S 2p1/2, respectively) for SnS2/TiO2. For the case of SnS2, the binding energies of S2− are shown to appear at 161.8 eV and 162.9 eV for S 2p3/2 and S 2p1/2, respectively. Therefore, the binding energies of the sulfur component for SnS2/TiO2 are shifted towards lower binding energies than those for SnS2. This may be the result of the heterojunction formation between SnS2 and TiO2 and the resulting downward (upward) shift of the electronic bands of TiO2 (SnS2) through an electron migration between TiO2 and SnS2 in the heterojunction [33]. Sn 3d core-level spectra in Figure 7d reveal the characteristics of Sn 3d3/2 and Sn 3d5/2 features at 486.7 and 495.2 eV, respectively, for SnS2 and SnS2/TiO2, which are assigned to Sn4+ of SnS2. No distinct shift in the Sn 3d features between SnS2 and SnS2/TiO2 can be attributed to the small shift for SnS2 and the broad nature of the Sn 3d. In addition, the broad Sn 3d spectral shape of the SnS2/TiO2 may suggest a possible contribution of Sn–O bonding even though it is not resolved. Overall, the XPS results further indicate the heterojunction formation between SnS2 and TiO2 nanosheets with the formation of a direct Ti–S bond, which can be beneficial for the efficient charge transfer at the heterojunction for improvement in photoactivity.
BET (Brunauer, Emmett, and Teller) N2 adsorption isotherm was obtained (Figure S9a) for all the composites to evaluate the surface area since it can also provide insight into the resulting photocatalytic properties [36,37]. The measured surface areas (m2g−1) for the TiO2 nanosheets and the SnS2 nanoparticles were 61.2 and 51.2 m2g−1, respectively, which are in the range expected for nanoparticles of about 100 nm in diameter [38,39]. The surface area of the SnS2/TiO2 composites was found to be strongly dependent on the SnS2 ratio (Figure S9b) and was found to be the highest (~118 m2g−1) at the SnS2 ratios of around 33%. This result is consistent with the observation of the morphology of the composites (Figure 3 and Figure 4), which are characterized by small SnS2 nanoparticles distributed on the TiO2 nanosheets. The large surface area may act as a promotional effect on the overall photocatalytic performance by providing more reaction sites.

2.2. Photoactivity of SnS2/TiO2 Composite

The photocatalytic activity of the composites was evaluated using the photodegradation rates of MB and RhB. The characteristic absorption bands of MB and RhB decrease in intensity as the degradation proceeds, as shown in Figure S10. From the measured variation in the intensities of the absorption bands, the C/C0 was calculated and plotted against the irradiation time for MB and RhB, respectively, in Figure 8a,c. The changes in the concentration of both dye solutions with the catalyst present in the dark were limited to 3–14%, possibly due to the adsorption capacity of the catalysts (Figure S11). Assuming a first-order reaction, the initial rate constants of the composites were calculated, and the results were plotted as a function of %SnS2 in Figure 8b,d for MB and RhB, respectively. The results of Figure 8 indicate that the composites exhibit higher photocatalytic performances in the photodegradation of MB and RhB, especially when the %SnS2 is in the range of approximately 30%. The highest rate constant was 4.13 × 10−2 min−1 for MB and 9.6 × 10−3 min−1 for RhB, respectively. These values for the composites are 2–3 times higher than those for anatase TiO2 nanosheets and SnS2 nanoparticles. The enhanced photodegradation performances of the composites are interpreted to be the result of efficient charge separation at the heterojunction of SnS2 and TiO2, which would result in reduced recombination rates as measured in the PL measurements (Figure 6). The large contact area between SnS2 and TiO2 expected for the SnS2/TiO2 composites with a large surface area (Figure S9) is expected to facilitate the charge transfer rate at the heterojunction to reduce the recombination rates and enhance the photodegradation rate. Photogenerated electrons within TiO2 may be collected more efficiently by SnS2 nanoparticles at the edges of TiO2 due to the preferential diffusion of electrons towards the {101} facets of TiO2 [40,41] and react with holes in the valence band (VB) of SnS2 via the Z-scheme. On the other hand, small particles of SnS2 may also provide recombination sites for charge carriers and limit light absorption of the anatase TiO2 nanosheets to contribute to reduced photodegradation rates as well [42,43]. Therefore, the compensation between the promotional and inhibiting roles of SnS2 nanoparticles may give an optimal SnS2 ratio (~30%), as observed in this study. The detailed size distribution and morphologies of the SnS2 nanoparticles with increasing SnS2 ratios are expected to play an important role in determining the optimum SnS2 ratio.

2.3. Proposed Mechanism for the Enhanced Photoactivity of SnS2/TiO2 Composite

To further investigate the degradation mechanism and the role of SnS2/TiO2 heterojunction, we employed scavengers such as 1,4 benzoquinone (for •O2−) [44,45], isopropyl alcohol (for •OH) [46,47], sodium oxalate (for h+) [21,48], and silver nitrate (for e−) [49,50] and the changes in the photodegradation rates were evaluated (Figure S12) and compared as shown in Figure 9. The decrease in the degradation rate with the addition of the scavenger indicates that the associated hot carrier plays a dominant role in the photodegradation reaction. For the SnS2/TiO2 composites with the optimized SnS2 content (~30%), the results indicate that the rate constant generally decreases when scavengers are added, suggesting that all active species play a role in the photodegradation of the dyes. The impact of adding 1,4 benzoquinone (for •O2−) and silver nitrate (for e−) was found to be a dramatic decrease of approximately 70–90%, while the influences of isopropyl alcohol (for •OH) and sodium oxalate (for h+) were relatively limited to a small decrease of approximately 10–30%. Such trends were found to be consistent for both MB and RhB, as can be seen in Figure 9, suggesting that e− and •O2− play a major role in the photodegradation of the dyes.
Considering that the ECB and EVB for SnS2 (−0.82 eV and 1.17 eV vs. NHE, respectively) are more negative than those of anatase TiO2 nanosheet (−0.31 eV and 2.93 eV vs. NHE, respectively), the heterojunction could be either type II heterojunction or direct Z-scheme [51]. Composite systems with SnS2 and TiO2 heterojunctions listed in Table 1 were also proposed to follow either type II heterojunction or direct Z-scheme, depending on the detailed preparation and evaluation conditions. Nonetheless, the EVB of SnS2 is more negative than the standard redox potential of E°(·OH/OH−) = 1.99 eV (vs. NHE) [52], implying that the photogenerated holes of SnS2 may not be able to oxidize OH− to yield ·OH radicals [52,53]. To confirm this, trapping experiments were carried out for TiO2 nanosheet and SnS2 using sodium oxalate (for h+) and silver nitrate (for e−) (Figure S13). The hot carriers (h+ and e−) of TiO2 were found to play an important role in the degradation of the dye, as can be expected from the proposed energy diagram of Figure 10. The effect of trapping e− was also a significant reduction in the degradation rate, as can be expected from the sufficiently high reducing power of the e− in the CB of SnS2. However, the capture of h+ in SnS2 was found to slightly increase the degradation rate, which can be interpreted as the role of the h+-scavenger was the suppression of the recombination rate within SnS2. This is consistent with the above energy diagram and confirms that the photogenerated holes of SnS2 may not be effective in oxidizing OH− to yield ·OH radicals. Contrary to the case of SnS2, the scavenger experiments for the SnS2/TiO2 composites (Figure 9) clearly show that the h+ and ·OH radicals contribute to the photodegradation of the dyes. Therefore, the most likely charge transfer pathway at the heterojunction is likely to be the direct Z-scheme, as has been reported for studies of 2D−2D SnS2/TiO2 [42]. SnS2 nanoparticles preferentially bound at the {101} edges of TiO2 nanosheets can collect electrons more favorably from TiO2 to promote Z-scheme interaction. Similar charge transfer mechanisms have been reported for other composite systems, such as SnS2/BiOBr [53] and g-C3N4/Vo-ZnO [52].
That being the case, under light irradiation, both the TiO2 nanosheet and SnS2 absorb photons and generate electron−hole pairs with electrons excited to their respective conduction bands. The electrons accumulated in the conduction band of SnS2 with a strong reduction potential can react with O2 to form •O2− because ECB of SnS2 is more negative than E°(O2/•O2−) = −0.046 eV (vs. NHE) [52]. Meanwhile, the photoinduced electrons in the TiO2 conduction band can recombine with the holes in the valence band of SnS2. Finally, the holes in the valence band of the TiO2 nanosheet with its high positive potential (EVB = 3.02 eV) can oxidize MB. This charge transfer pathway would effectively enhance charge separation while retaining its great oxidation and reduction abilities [51]. Although the degradation products were not determined in this study, the subsequent degradation of both dyes was suggested to undergo the following pathways. The degradation of RhB was suggested to be initiated by the removal of ethyl groups, resulting in intermediate products such as phthalic acid, benzyloxyamine, and benzoic acid [54]. Similarly, the degradation of MB was suggested to begin with the removal of methyl groups, resulting in the formation of intermediates such as N-methyl formamide, azure A, B, and C, as well as thionine [55,56,57].
Table 1. Proposed type of heterojunction and degradation performances of SnS2/TiO2 photocatalysts.
Table 1. Proposed type of heterojunction and degradation performances of SnS2/TiO2 photocatalysts.
SystemType of
Heterojunction		Catalyst
Concentration (g/L)		Dye (Concentration)Light SourceRate
Constant
(min−1)		Reference
SnS2/TiO2Z-scheme0.02MB (7.5 µM)
and RhB (3.75 mg/L)		Halogen lamb (100 mW/cm2)0.052 (MB)
0.008 (RhB)		This work
SnS2/TiO2Z-scheme0.15MB (12 µM) and RhB (10 mg/L)Xe lamp (200 W, 200–800 nm)0.02 (MB)
0.022 (RhB)		Gao et al. (2021) [58]
SnS2/TiO2Type II0.2MB (20 µM)Mercury lamp (250 W)0.03Zhang et al. (2017) [56]
TiO2/SnS2/MoS2Z-scheme-MB (10 mL, 5 mg/L)Artificial sunlight (AM 1.5 G, 150 mW)0.0175Gao et al. (2022) [59]
SnS2/TiO2Type II1RhB (10 mg/L)Xe lamp (300 W, 420 nm)0.035Yan et al. (2017) [60]
SnS2/BiOBrZ-scheme0.625RhB (10 mg/L)Xe lamp (400 W, 420 nm)0.1203Qiu et al. (2017) [53]
SnS2/TiO2Z-scheme50CO2Xe lamp (300 W)-She et al. (2019) [42]
TiO2/CdSZ-scheme2CO2Xe lamp (350 W)-Wang et al. (2020) [61]

3. Materials and Methods

3.1. Synthesis of TiO2 Nanosheets

The detailed procedures for the preparation of the anatase TiO2 nanosheet were described in our previous work [27]. Butanol (25 mL) and tetrabutyl titanate (TBT, 25 g) were added into a Teflon beaker and kept in an ice bath under stirring. Then, 4 mL of HF was subsequently added dropwise, and the resulting solution was transferred into a Teflon-lined autoclave, which was firmly screwed tight and kept in an oven at 200 °C for 24 h. After the reaction, the precipitate was filtered, washed with deionized water, and dried at 80 °C.

3.2. Synthesis of SnS2/TiO2

The as-synthesized TiO2 nanosheet (0.2 g) was dispersed in 40 mL of deionized water and stirred for 1 h using a high-power sonicator (VC505, Sonics & Materials, Newtown, CT, USA). Then, an aqueous solution of SnCl4·5H2O and CH3CSNH2 (with the Sn-to-S mole ratio of 1:2) was mixed with the aqueous suspension of TiO2 with initial mixing mole ratios of Sn from 9 to 80%. The mixture was stirred for 1 h and placed on a microwave synthesizer (Monowave 400, Anton Paar, Graz, Austria) operating at 850 W, 200 °C, and stirring at 900 rpm for 1-h synthesis. The schematic of the whole procedure is shown in Scheme 1.

3.3. Characterization of As-Synthesized Photocatalysts

The detailed morphologies of the as-synthesized composites were examined by transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin, Igualada, España) and scanning electron microscopy (SEM, JSM-7900F, JEOL, Tokyo, Japan). Diffuse reflectance spectra were measured with a UV–visible spectrophotometer (Scinco, Neosys-2000, Seoul, Republic of Korea) that was equipped with an integrating sphere and BaSO4 as the reference. X-ray diffraction (XRD) patterns of our samples were measured with Cu Kα radiation from an X-ray diffractometer (Rigaku, Ultima III, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurement was performed using K-alpha (Thermo VG, Altrincham, UK) with monochromated Al X-ray sources (Al Kα line: 1486.6 eV). All binding energies are calibrated to the C 1s peak at 284.8 eV of the adventitious carbon on the surface.

3.4. Evaluation of Photocatalytic Properties

The photodegradation characteristics of the composites were evaluated using MB and RhB as model dyes. 10 mL of the aqueous solution of MB or RhB (6 × 10−5 M) was added to 40 mL of the aqueous solution with SnS2/TiO2 composites (0.04 g/L), which were diluted with 30 mL of distilled H2O to result in 80 mL aqueous solution. The solution was stirred in the dark for 60 min before light illumination to achieve adsorption–desorption equilibrium of the MB and RhB dyes. Photocatalytic reactions were initiated with a halogen lamp (100 mW/cm2). During light illumination, the air was constantly blown by a fan to mitigate any temperature rise from room temperature. For the investigation of the role of hot carriers, the aqueous solution (5 mL, 20 mM) of the scavengers (1,4 benzoquinone (for •O2−), isopropyl alcohol (for •OH), sodium oxalate (for h+), and silver nitrate (for e−)) was added to the solution before shining light. The solution was sampled during the reaction to determine the concentration of MB (or RhB) from the absorbance at 664 (555) nm for MB (RhB). Initial rate constants for the photodegradation reactions were obtained by assuming first-order reaction kinetics.

4. Conclusions

A microwave-assisted hydrothermal method was employed to synthesize SnS2/TiO2 composites for photocatalytic applications. Both TiO2 nanosheets and SnS2 nanoparticles showed activities in the photodegradation of MB and RhB. Such activities were found to be further enhanced for the SnS2/TiO2 composites, especially when the SnS2 ratio in the composite was around 30%, giving an overall volcano-type change in the graph of the photodegradation performances of the composites vs. the SnS2 ratio. The origin of the enhanced photocatalytic activity is attributed to the formation of SnS2–TiO2 heterojunctions, which can facilitate the charge separation to reduce the recombination rate. The composites with the maximum photocatalytic performances (in the SnS2 ratio of about 30%) have small SnS2 nanoparticles in contact with the TiO2 nanosheets, especially at the edges of the nanosheets, which can result in a large surface area of the composites (~118 m2g−1) and the large interfacial heterojunction for efficient charge separation following the Z-scheme towards enhanced photocatalytic performances. The preferential diffusion of electrons toward the edges of the TiO2 nanosheets may facilitate the charge separation at the SnS2 heterojunctions at the {101} edges of TiO2 via Z-scheme further. Scavenger experiments further corroborate the charge transfer mechanism at the interfaces following Z-scheme, suggesting that the SnS2/TiO2 composites can be good examples of promising photocatalysts for the photodegradation of dyes with enhanced redox ability.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14070442/s1, Figure S1. XRD patterns of as-prepared SnS2, TiO2 nanosheet, and SnS2/TiO2 heterojunction. Figure S2. Bandgap energy plot Kubelka–Munk function for synthesis samples. Figure S3. a–i shows SEM images of SnS2/TiO2 heterojunctions. Figure S4. a–i shows EDS images of SnS2/TiO2 heterojunctions. Figure S5. Profile of inverse fast Fourier transformation (IFFT) for (a) anatase TiO2 nanosheet (200), (b) SnS2 nanoparticles (001), (c) SnS2/TiO2 43% (TiO2 (101) and SnS2 (001), respectively). Figure S6. Increase content of SnS2 nanoparticles for (a) SnS2/TiO2 33% and (b) SnS2/TiO2 43%. Figure S7. Raman spectra patterns of as-synthesis TiO2 nanosheets, SnS2 nanoparticles, and SnS2/TiO2 heterojunctions. Figure S8. Survey scan spectra of synthesis samples. Figure S9. (a) BET N2 adsorption isotherms and (b) the measured surface area of the SnS2/TiO2 composites, SnS2, and TiO2. Figure S10. Typical absorbance for a) MB and b) RhB degradation under halogen lamp irradiation in the presence of SnS2/TiO2 shows a decrease in absorbance peak with time. Figure S11. Photodegradation rates are shown as plots of C/Co vs. time in the dark for (a) MB and (b) RhB. Figure S12. C/C0 for trapping experiment using MB (SnS2/TiO2 29%) and RhB (SnS2/TiO2 33%). Figure S13. Rate constant for trapping experiment for using MB TiO2 and SnS2. Table S1. %SnS2 calculations from EDS data. References [62,63,64] are cited in the Supplementary Materials.

Author Contributions

N.C.: writing—original draft, investigation, visualization, formal analysis. M.F.: visualization, formal analysis. H.-S.O.: investigation, formal analysis. Y.-K.K.: conceptualization, formal analysis, writing—original draft, writing—review and editing, project administration, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1A2C1007227 and NRF-2021R1A6A1A10044950), Learning & Academic research institution for Master’s·PhD students and Postdocs (LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS-2023-00285390), and the H2KOREA funded by the Ministry of Education (2022Hydrogen fuel cell-002, Innovative Human Resources Development Project for Hydrogen Fuel Cells).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. XRD patterns of selected SnS2/TiO2 composites in comparison with those of as-prepared SnS2 and TiO2.
Figure 1. XRD patterns of selected SnS2/TiO2 composites in comparison with those of as-prepared SnS2 and TiO2.
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Figure 2. Bandgap of the composites as a function of %SnS2. Also shown in the inset are the absorbance spectra of the composites.
Figure 2. Bandgap of the composites as a function of %SnS2. Also shown in the inset are the absorbance spectra of the composites.
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Figure 3. SEM images of (a) TiO2 nanosheets, (b–e) SnS2/TiO2 composites with the %SnS2 ratio of 9–75%, and (f) SnS2 nanoparticles.
Figure 3. SEM images of (a) TiO2 nanosheets, (b–e) SnS2/TiO2 composites with the %SnS2 ratio of 9–75%, and (f) SnS2 nanoparticles.
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Figure 4. TEM images of TiO2 nanosheets (a,b), SnS2 nanoparticles (c,d), and SnS2/TiO2 composites with a SnS2 ratio of 33% (e) and 43% (f).
Figure 4. TEM images of TiO2 nanosheets (a,b), SnS2 nanoparticles (c,d), and SnS2/TiO2 composites with a SnS2 ratio of 33% (e) and 43% (f).
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Figure 5. Raman spectra of SnS2/TiO2 composites in comparison with those of as-synthesis TiO2 nanosheets and SnS2 nanoparticles.
Figure 5. Raman spectra of SnS2/TiO2 composites in comparison with those of as-synthesis TiO2 nanosheets and SnS2 nanoparticles.
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Figure 6. (a) PL spectra for SnS2/TiO2 composites with the excitation of a 325 nm laser, (b) the plot of maximum PL intensities at wavelengths of 520–550 nm as a function of % SnS2, and (c) the plot of the difference between the dashed line (Figure 6b) and the actual emission.
Figure 6. (a) PL spectra for SnS2/TiO2 composites with the excitation of a 325 nm laser, (b) the plot of maximum PL intensities at wavelengths of 520–550 nm as a function of % SnS2, and (c) the plot of the difference between the dashed line (Figure 6b) and the actual emission.
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Figure 7. (a) O 1s, (b) Ti 2p, (c) S 2p, and (d) Sn 3d core-level spectra of the SnS2/TiO2 composite (%SnS2 = 33%) in comparison with those of SnS2 and TiO2.
Figure 7. (a) O 1s, (b) Ti 2p, (c) S 2p, and (d) Sn 3d core-level spectra of the SnS2/TiO2 composite (%SnS2 = 33%) in comparison with those of SnS2 and TiO2.
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Figure 8. Photodegradation rates are shown as plots of C/Co vs. irradiation time for (a) MB and (c) RhB, along with initial rate constants vs. %SnS2 in the composites for (b) MB and (d) RhB.
Figure 8. Photodegradation rates are shown as plots of C/Co vs. irradiation time for (a) MB and (c) RhB, along with initial rate constants vs. %SnS2 in the composites for (b) MB and (d) RhB.
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Figure 9. Comparison of rate constants with the use of trapping agents for (a) MB (SnS2/TiO2 29%) and (b) RhB (SnS2/TiO2 33%).
Figure 9. Comparison of rate constants with the use of trapping agents for (a) MB (SnS2/TiO2 29%) and (b) RhB (SnS2/TiO2 33%).
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Figure 10. Energy diagram of the valence band (VB) and conduction band (CB) of the SnS2/TiO2 composite and a plausible mechanism for the charge separation and the generation of radical species. (a) Band alignment diagram for the SnS2/TiO2 and (b) Proposed charge separation mechanism at the heterojunction via Z-scheme. The energy levels for the CB (ECB) and the VB (EVB) can be calculated following Equations (S1) and (S2) listed in the Supplementary Materials.
Figure 10. Energy diagram of the valence band (VB) and conduction band (CB) of the SnS2/TiO2 composite and a plausible mechanism for the charge separation and the generation of radical species. (a) Band alignment diagram for the SnS2/TiO2 and (b) Proposed charge separation mechanism at the heterojunction via Z-scheme. The energy levels for the CB (ECB) and the VB (EVB) can be calculated following Equations (S1) and (S2) listed in the Supplementary Materials.
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Catalysts 14 00442 sch001
Scheme 1. The schematic diagram for the synthesis of SnS2/TiO2 heterojunction.
Scheme 1. The schematic diagram for the synthesis of SnS2/TiO2 heterojunction.
Catalysts 14 00442 sch001
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MDPI and ACS Style

Carl, N.; Fiaz, M.; Oh, H.-S.; Kim, Y.-K. Enhanced Photocatalytic Performances of SnS2/TiO2 Composites via a Charge Separation Following Z-Scheme at the SnS2/TiO2{101} Facets. Catalysts 2024, 14, 442. https://doi.org/10.3390/catal14070442

AMA Style

Carl N, Fiaz M, Oh H-S, Kim Y-K. Enhanced Photocatalytic Performances of SnS2/TiO2 Composites via a Charge Separation Following Z-Scheme at the SnS2/TiO2{101} Facets. Catalysts. 2024; 14(7):442. https://doi.org/10.3390/catal14070442

Chicago/Turabian Style

Carl, Nkenku, Muhammad Fiaz, Hyun-Seok Oh, and Yu-Kwon Kim. 2024. "Enhanced Photocatalytic Performances of SnS2/TiO2 Composites via a Charge Separation Following Z-Scheme at the SnS2/TiO2{101} Facets" Catalysts 14, no. 7: 442. https://doi.org/10.3390/catal14070442

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