Highly Porous SnO2/TiO2 Heterojunction Thin-Film Photocatalyst Using Gas-Flow Thermal Evaporation and Atomic Layer Deposition

Abstract

Highly porous heterojunction films of SnO₂/TiO₂ were prepared using gas-flow thermal evaporation followed by atomic layer deposition (ALD). Highly porous SnO₂ was fabricated by introducing an inert gas, Ar, during thermal evaporation. To build heterogeneous structures, the TiO₂ layers were conformally deposited on porous SnO₂ with a range of 10 to 100 cycles by means of ALD. The photocatalytic properties for different TiO₂ thicknesses on the porous SnO₂ were compared using the degradation of methylene blue (MB) under UV irradiation. The comparisons showed that the SnO₂/TiO₂-50 heterostructures had the highest photocatalytic efficiency. It removed 99% of the MB concentration, and the decomposition rate constant (K) was 0.013 min⁻¹, which was approximately ten times that of the porous SnO₂. On the other hand, SnO₂/TiO₂-100 exhibited a lower photocatalytic efficiency despite having a TiO₂ layer thicker than SnO₂/TiO₂-50. After 100 cycles of TiO₂ ALD deposition, the structure was transferred from the heterojunction to the core–sell structure covered with TiO₂ on the porous SnO₂, which was confirmed by TEM analysis. Since the electrons photogenerated by light irradiation were separated into SnO₂ and produced reactive oxygen, O₂⁻, the heterojunction structure, in which SnO₂ was exposed to the surface, contributed to the high performance of the photocatalyst.

1. Introduction

Water contamination with dye discharges from various industries is considered a significant threat to the environment and public health [1,2]. Moreover, the scarcity of water resources has also become a major problem. Additionally, conventional wastewater treatment technologies have been problematic because of their long operation times and high costs [3,4]. Among the various dye-contaminated water treatment methods, photocatalytic degradation has attracted much attention for the removal of organic residues from wastewater. In particular, metal oxide heterojunctions employing advanced oxidation processes (AOPs) have been widely explored because of their rapid purification time and environmentally benign processes [5,6]. Under light irradiation, the heterojunction of a metal oxide semiconductor facilitates the separation of the generated electron–hole (e⁻/h⁺) pairs. These generated excitons produce reactive oxygen species in aqueous media, such as hydroxyl radicals (·OH) and superoxides (·O₂⁻), which are known to be photocatalytically active for the decomposition of organic contaminants, thereby purifying dye-contaminated water [7,8,9]. Among various metal oxides, TiO₂ has been considered an attractive photocatalyst material for dye degradation because of its excellent catalytic activity and high chemical stability [10,11,12,13,14,15]. Under UV irradiation, the electrons generated by light absorption form a radical on the surface of TiO₂ near the energy level of the valence band, thus removing the dye from the water. However, the large band gap, ~3.2 eV, and fast recombination of the exciton of TiO₂ limit the photocatalytic efficiency. To resolve the low light adsorption and fast recombination of excitons in a single metal oxide, the heterostructures of TiO₂ with other metal oxide semiconductors, such as ZnO, and SnO₂, etc., have been explored using various synthesis methods. Metal oxide semiconductors with lower valence bands than TiO₂, such as SnO₂, facilitate the transfer of photogenerated holes in the valence band to TiO₂ in the heterojunction, thus hindering charge recombination and leading to efficient photocatalysis [16,17]. In addition to the selection of appropriate materials in heterojunctions, most research efforts in the field of photocatalysts have focused on optimizing microstructures to increase light absorption efficiency and to enlarge the active surface area using particle-type metal oxides. Since nanoparticle-type photocatalysts enlarge the active surface area and enhance light absorption, the synthesis of metal oxide nanoparticles has been widely investigated as a high-performance photocatalyst. However, despite their high performance, particle-type photocatalysts require additional separation processes in the suspension system, leading to high costs. Additionally, they cause secondary contamination resulting from the organic and inorganic binders in the synthesis and deposition process, thereby limiting their application [18,19,20]. Furthermore, most fabrication methods for particle-type heterojunctions involve complicated preparation processes and exhibit lower microstructure and chemical composition reproducibility [21]. Therefore, the development of a film-type photocatalyst with reproducibility is necessary to extend the usability and to improve the efficiency of the photocatalyst.

In this study, we fabricated a nanoporous SnO₂/TiO₂ heterojunction on a planar substrate using gas-flow thermal evaporation followed by atomic layer deposition (ALD) without binders. The flow of an inert gas, Ar, with a pressure of 0.2 Torr during thermal evaporation, allows the formation of a nanoporous structure. The conformal growth of TiO₂ on porous SnO₂ was demonstrated with a different cycle of ALD. In order to not cover the entire surface of SnO₂, TiO₂ was deposited with a lower number of ALD cycles, from 10 to 100 cycles, corresponding to a thickness of less than 2 nm. Chemical and microstructural analyses revealed the formation of the SnO₂/TiO₂ heterostructure. The evaluation of the photocatalytic activities of the as-prepared highly porous SnO₂/TiO₂ structures used the degradation of a methylene blue (MB) dye solution with a low UV irradiation intensity. Therefore, the results reveal that the heterostructure, not the core-shell structure of SnO₂/TiO₂, induces high photocatalytic activity, improves charge separation, and utilizes separated electrons and holes for photocatalysis in TiO₂ and SnO₂, respectively.

2. Results and Discussion

The highly porous heterojunction photocatalyst SnO₂/TiO₂ was fabricated by gas-flow-modified thermal evaporation followed by ALD. The plan-view and cross-sectional SEM images, Figure 1a,b, respectively, clearly show that the 3D nanoform microstructure of SnO₂ is well deposited with gas-flow-modified thermal evaporation at a pressure of 0.2 Torr. The surface area of the porous SnO₂ and SnO₂/TiO₂ was measured as about 74 m²/g and 72.5 m²/g with the Brunauer–Emmett–Teller method, respectively. The 50 cycles of ALD on the SnO₂ also did not significantly change the porosity of the SnO₂, as shown in Figure 1c,d, since the corresponding thickness by the ALD process is about 3 nm. On the other hand, the EDX elemental mapping on the SnO₂/TiO₂-50 clearly shows that the Ti atom was deeply distributed inside the matrix of the SnO₂ porous structure, indicating that the TiO₂ layer was conformally deposited on the porous nanostructure, as shown in Figure S1. In addition, transmission electron microscopy (TEM) analysis with electron energy loss spectroscopy showed that the TiO₂ layers were well deposited on the surface of the porous SnO₂ by ALD (Figure 2). As shown in Figure 2a, the nanoform of SnO₂/TiO₂-100 shows that the TiO₂ layer is coated on the SnO₂ structure, with a thickness of approximately 1.3 nm. The growth rate of the TiO₂ on the porous SnO₂ structure is as low as one-third of that on the planar substrate because more precursors are required to cover the large surface area of the porous structure (Figure S2). Moreover, the electron energy loss spectroscopy spectrum showed that the TiO₂ was uniformly decorated on the whole surface of SnO₂, indicating that the TiO₂ deposition in 100 ALD cycles could fabricate a core-shell structure (Figure 2b–d. On the other hand, after ALD with less than 50 cycles, the particular structure of SnO₂ was not entirely covered by TiO₂ (Figure S3). The effect of microstructural changes with the number of ALD cycles on the photocatalytic performance will be further discussed in detail with the results of MB decomposition. The EDXRF analysis also showed that the Ti density increased with the number of ALD cycles, implying that the gradual growth of the TiO₂ layer with the number of ALD cycles was achieved on the porous structure (Figure 3). Note that the Ti density of sample after 100 cycles of ALD dramatically increases compared to other samples. Since the porous structure requires more cycles to move from the nucleation stage to the growth stage of ALD due to the larger surface area, it is expected that the SnO₂ nanoforms deposited through less than 50 cycles of ALD exhibit a different growth rate compared to SnO₂/TiO₂-100. The ALD as-synthesized TiO₂ layer was analyzed as the anatase phase using XRD, corresponding to JCPDS no. 21-1272 (Figure S4).

The elemental chemical states of the constituent element, SnO₂/TiO₂, were determined using XPS for the different numbers of ALD cycles. As shown in Figure 4a, the two peaks of the Sn 3d spectra, 486.3 eV and 494.7 eV, assigned as Sn 3d_5/2 and Sn 3d_3/2, respectively, were observed in the spectra of the SnO₂/TiO₂-50 nanocomposite, implying that the porous SnO nanostructure was converted to the SnO₂ morphology after the annealing process. The two peaks of 458.4 eV and 464.1 eV corresponding to the peaks of Ti 2p_3/2 and Ti 2p_1/2 indicated successful TiO₂ deposition for the typical Ti⁴⁺-O bond on the porous SnO₂ matrix, which is in good agreement with the EDXRF and EELS results (Figure 4b). As the amount of TiO₂ on the surface of the porous SnO₂ increased with the number of ALD cycles, a gradual shift of the Sn 3d peak toward a lower binding energy was observed, which was attributed to the donation of electrons to TiO₂ via the formation of Sn-O-Ti at the interface (Figure 4c,d) [22,23,24]. The introduction of TiO₂ on the surface of the SnO₂ induced an electron-rich environment at the interface, thereby lowering the binding energy of the SnO₂ 3d electrons [25]. As shown in Figure 5, the XPS spectra for O 1s of pristine SnO₂ can be deconvoluted into two peaks with binding energies of 530.4 eV and 531.2 eV for the lattice oxygen (Lo) and oxygen vacancy or defect (Vo) of SnO₂, respectively [26]. The O 1s binding energy (530.4 eV) of SnO₂/TiO₂ gradually decreased with number of TiO₂ ALD cycles (Figure 5). Since TiO₂ has lower binding energy (529.8 eV) in the oxygen state than SnO₂, the formation of a TiO₂/SnO₂ heterojunction led to the formation of Sn-O-Ti bonding, thus exhibiting the lower binding energy of the Lo State. Hence, the Lo binding energy was substantially decreased in the SnO₂/TiO₂ heterostructure. The Vo state, implying an oxygen vacancy, on the other hand, slightly decreased with increasing TiO₂ layers, indicating that the deposition of TiO₂ reduced the oxygen vacancies and defects on the SnO₂ nanoform. Therefore, XPS analysis revealed that the ALD process not only induced the strongly bound heterojunction of SnO₂/TiO₂ but also reduced the surface defects of SnO₂, leading to efficient exciton separation and charge transfer.

The light absorption and band gap with different TiO₂ ALD cycles were measured using UV-vis absorption spectra, as shown in Figure 6. The SnO₂/TiO₂ samples showed strong absorption intensity at 320 nm with the increasing thickness of the TiO₂ nanoscale layers [27]. The nanoform of the porous SnO₂/TiO₂ induces more light absorption owing to the greater light pathway of the nanoporous structure. Additionally, the formation of a semiconducting heterojunction structure with TiO₂ enhances the absorption intensity at approximately 400 nm. For better understanding, the optical band gaps of the SnO₂/TiO₂ samples were experimentally determined using the linear portion of the Kubelka–Munk function. The measured optical band gap of the porous SnO₂ nanoform was 3.2 eV whereas all the SnO₂/TiO₂ photocatalysts had lower band gaps. Further, the SnO₂/TiO₂-10, 25, 50, and 100 samples were slightly shifted toward shorter band gap, with the increasing TiO₂ layer at 3.03, 2.93, 2.88, and 3.00 eV, respectively. However, the SnO₂/TiO₂-100 samples displayed a wavelength shift toward a larger band gap, which indicated that the thick layer of TiO₂ caused less light scattering in the pores and decreased light reflection pathways [28,29].

The photocatalytic performance of the highly porous SnO₂/TiO₂-10, 25, 50, and 100 heterojunction films was evaluated using the degradation of the MB solution under UV light irradiation (Figure 7). While the MB solution was barely degraded without the catalysts, the porous SnO₂ nanofoam exhibited a 60% MB degradation after 300 min, and the presence of all of the porous SnO₂/TiO₂ nanofoams enhanced photocatalytic activity compared to the bare SnO₂ nanofoam due to the synergistic effect between the SnO₂ and TiO₂ heterojunction. The time-dependent UV-vis absorbance spectra of MB degradation displayed a significant reduction in the MB concentration, as shown in Figure S5. The photocatalytic efficiency (η)% of the photocatalyst was calculated from η = (C₀ − C)/C × 100, where C₀ is the initial concentration of MB, and C is the remaining concentration of MB after photocatalysis. As confirmed in Figure 6 and Figure S6, the SnO₂/TiO₂-50 heterostructure photocatalyst displays the highest photocatalytic efficiency (99%) compared to SnO₂/TiO₂-10 (73%), SnO₂/TiO₂-25 (91%), SnO₂/TiO₂-100 (81%), and SnO₂ (60%) after photocatalysis. Comparing the photocatalytic reaction rate, which follows pseudo first order reaction kinetics and is expressed as ln(C₀/C) = −Kt, where K is apparent rate constant, and t is the time interval, the decomposition rate constant K values were 0.0030, 0.0042, 0.0078, 0.013, and 0.0056 min⁻¹ for the SnO₂, SnO₂/TiO₂-10, 25, 50, and 100 photocatalysts, respectively (Table 1). The rate constant of SnO₂/TiO₂-50 was approximately two times higher than that of the core-shell SnO₂/TiO₂-100 and had the highest reaction rate compared to the as-fabricated photocatalysts. It is generally acknowledged that photoexcited electrons and holes play a key role in photocatalytic reduction reactions, as they generate lots of main reactive species that are involved in photocatalytic oxidation, such as ·O₂⁻ and ·OH. The low photocatalytic efficiency of the porous SnO₂ films is attributed to conduction band of the SnO₂, which has lower electric potential compared to the O₂/O₂⁻ oxidation potential, although the holes in valence band of SnO₂ oxidized the organic pollutant directly using the ·OH radical to contribute to the performance [30]. On the contrary, the SnO₂/TiO₂ heterojunction gives rise to charge separation due to the potential difference, hence increasing the lifetime of the charge carrier and improving the interfacial charge transfer to the adsorbed surface. The photogenerated electrons and holes of the SnO₂/TiO₂ heterojunction produced strong oxidizing radicals of ·O₂⁻ and ·OH from the water or oxygen on the surface of SnO₂ and TiO₂, respectively, thereby degrading the organic pollutant (Figure 8) [31,32]. Since the ·OH radical generated in the valence band of TiO₂ is a stronger oxidant, the introduction of the TiO₂ layer can improve the photocatalytic efficiency. In addition, light absorption was enhanced by increasing the thickness of the TiO₂. Therefore, more ALD cycles of TiO₂ induce more active sites for TiO₂ and light absorption, thereby improving the photocatalytic performance, as shown in Figure 6. Note that the SnO₂/TiO₂-100 photocatalyst had a lower MB degradation performance than the SnO₂/TiO₂-50 despite the thicker TiO₂ layers and greater light absorption. As shown in Figure 2, 100 cycles of the ALD process produced a core-shell structure and removed the surface of SnO₂, which was used to generate ·O₂⁻ oxidizing radicals from the transfer of photogenerated electrons. Furthermore, the thicker TiO₂ layer of SnO₂/TiO₂-100 hinders the charge transfer to the surface of the TiO₂ layer to degrade MB. Therefore, the SnO₂/TiO₂-50 heterojunction structure possessing both the TiO₂ and SnO₂ surfaces is advantageous for efficient photocatalysis since it has enhanced light absorption and more oxidizing radicals from both the photogenerated electrons and holes.

3. Experimental Section

3.1. Chemical and Substrate

Pure tin oxide (SnO) (0.2 g, 99.995% pure, LTS Research Laboratories, Inc., USA) was used as the thermal evaporation material. The titanium tetra-isopropoxide (TTIP) precursor for ALD was purchased from UP Chemical Co., Ltd., Pyeongtaek, Korea. The porous SnO films were fabricated on Si (100) wafers or ITO substrates with the dimensions of 2 × 2 cm². The substrate was cleaned thoroughly with deionized water, acetone, and ethanol for 10 min and were dried by blowing N₂ over the sample. The UV-vis absorption spectra were measured using a SnO₂/TiO₂ photocatalyst on an indium-doped tin oxide (ITO) substrate (7–10 ohm/sq, transmittance: 80–85%).

3.2. Preparation of Porous SnO₂ Nanoform

The highly porous SnO nanoform was fabricated by Ar-assisted modified thermal evaporation, as previously reported [33,34]. The SnO source was kept in a Ta boat, which was subsequently placed at the center of the chamber, and the Si/SiO₂ substrate was placed at the upper site, with the cooling system at 23 °C. During thermal evaporation, the highly pure Ar gas (99.9999%) flowed with 100 sccm to reach a pressure of 0.2 Torr after evacuation to a 5 × 10⁻⁶ Torr pressure, and the SnO source was rotated at a speed of 8 rpm in order to form a uniform nanoporous structure. After thermal deposition, the porous SnO nanoform was annealed at 700 °C for 1 h with 50 sccm pure air in a tube furnace to obtain the SnO₂ crystallite structure.

3.3. Preparation of the Porous SnO₂/TiO₂ Heterostructure

The highly porous SnO₂/TiO₂-10, 25, 50 and 100 heterostructure films was synthesized by atomic layer deposition with a different number of cycles on the porous SnO₂ nanofoam. The TTIP and deionized water were used as the metal precursor and oxygen source, respectively. Highly pure N₂ gas (99.9999%) was used for the purge process with a gas flow rate of 200 sccm. The as-synthesized porous SnO₂ film was kept at 300 °C in a reaction chamber, and the base pressure was maintained at 1.5 × 10⁻² torr using a dry pump (Edwards, IQDP80, Edwards. Co., United Kingdom). In order to control the uniformity and thickness of the TiO₂ layer, it was coated with a highly porous SnO₂ nanocomposite with 10, 25, 50, and 100 cycles of ALD, denoted as SnO₂/TiO₂-10, SnO₂/TiO₂-25, SnO₂/TiO₂-50, and SnO₂/TiO₂-100, respectively.

3.4. Evaluation of Photocatalytic Activity

The photocatalytic properties of the as-synthesized SnO₂/TiO₂-10, 25, 50, and 100 films were evaluated by the degradation of MB under UV light irradiation (265 nm) in a cooling bath. MB (1.2 mg) was dissolved in 1 L of deionized water and was stirred for 2 h in the dark. Before evaluating the photocatalytic properties, the porous SnO₂/TiO₂ heterostructured films were kept in 50 mL of MB solution and were stirred for 30 min in the dark to achieve adsorption–desorption equilibrium. After UV irradiation with a power of 16 W, the solution was characterized by UV-vis absorbance spectroscopy (UV-2500 UV-vis spectrophotometer, Shimadzu. Co., Japan) to evaluate the photodegradation of MB.

3.5. Materials Characterization

The microstructure of the SnO₂/TiO₂ nanoform was characterized by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7100F, JEOL Co., Japan) with energy dispersive X-ray spectroscopy (EDS) mapping. Elemental mapping analysis of SnO₂/TiO₂ from electron energy loss spectroscopy (EELS) were conducted using a ZEISS Libra 200 HT Mc Cs, Zeiss, Co., Germany (200 kV)/JEOL ARM 200 F, JEOL. Co., Japan (200 kV). The chemical binding energy of the SnO₂/TiO₂ nanoform with the number of TiO₂ deposition cycles was analyzed through X-ray photoelectron spectroscopy (XPS) using a K-alpha plus (ESCALAB 250Xi, Thermo Fisher Scientific. Co., USA) instrument utilizing monochromatic Al Kα (1486.6 eV) radiation. The surface composition of Ti in SnO₂/TiO₂ was determined using a Rigaku ZSX Primus II energy-dispersive X-ray fluorescence (EDXRF) spectrometer (Rigaku. Co., Japan).

4. Conclusions

A highly porous SnO₂/TiO₂ heterojunction structure was synthesized using gas flow thermal evaporation and ALD, which controlled the TiO₂ thickness on the porous SnO₂ structure. Owing to the charge separation that leads to the formation of radicals, the heterojunction structure with a TiO₂ layer on SnO₂ exhibited a higher photocatalytic efficiency than bare SnO₂. The effect of TiO₂ thickness on the photocatalytic activity was also investigated. According to the results, SnO₂/TiO₂-50, which was analyzed to have TiO₂ deposited sparsely on the porous SnO₂, had the highest kinetic constant of 0.013 min⁻¹, which was two times higher than that of the core-shell structure of SnO₂/TiO₂-100. The facile separation of the electron/hole pairs and corresponding oxygen radicals on the surface of both TiO₂ and SnO₂ can improve the photocatalytic activity in the heterojunction compared to the core-shell structure.

Author Contributions

Conceptualization, S.K. and Y.M.P.; methodology, S.K., H.-K.C. and H.-J.K.; validation, S.K., K.B.K. and H.-N.L.; formal analysis, S.K.; investigation, S.K., K.B.K., T.J.P. and Y.M.P.; data curation, S.K.; writing—original draft preparation, S.K. and Y.M.P.; writing—review and editing, S.K. and Y.M.P.; project administration, Y.M.P.; All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Basic Research Program of the Korea National Research Foundation (Project No. NRF-2020R1F1A1067830).

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. FE-SEM analysis of the porous SnO2 and SnO2/TiO2-50. The surface section of (a) SnO2 and (c) SnO2/TiO2-50 and the cross section of (b) SnO2 and (d) SnO2/TiO2-50.

Figure 2. (a) The TEM image of SnO2/TiO2-100 and the EELS elemental mapping showing the elemental distribution of (b) Sn, (c) Ti, and (d) their overlay.

Figure 3. The Ti density of the SnO2/TiO2 photocatalysts analyzed using EDXRF. The Ti density increased with the number of TiO2 ALD cycles.

Figure 4. The XPS spectrum of the (a) Sn 3d and (b) Ti 2p states of the SnO2/TiO2-50 photocatalyst. The peak shift of (c) Ti 2p and (d) Sn 3d with the increase of TiO2 ALD cycles.

Figure 5. Comparison of the O 1s state of the SnO2/TiO2 photocatalysts with different TiO2 ALD cycles from 10 to 100 cycles. The blue line was the chemical state of O 1s spectra and deconvoluted of lattice oxygen (Lo) and oxygen vacancy (Vo) which was indicated the red and green line, respectively.

Figure 6. (a) UV-vis absorption spectra of the porous SnO2 and all the SnO2/TiO2 photocatalysts on the ITO substrate. (b) Calculation of the optical bandgap from the UV-vis absorption spectra of the porous SnO2 nanofoam and all of the the SnO2/TiO2 photocatalysts.

Figure 7. (a) Decomposition of the MB concentration ratio (C/C0) and (b) the kinetics of the photocatalytic degradation of MB by all the SnO2/TiO2 photocatalysts with different TiO2 ALD cycles.

Figure 8. Band diagram of the SnO2/TiO2 heterojunction and photocatalysis mechanism.

Table 1

Comparison of the photocatalytic efficiency (%) and decomposition reaction rate (K) of the porous SnO₂ nanofoam and SnO₂/TiO₂-10, 25, 50, and 100 heterojunction films.

	MB	SnO2	SnO2/TiO2-10cyc	SnO2/TiO2-25cyc	SnO2/TiO2-50cyc	SnO2/TiO2-100cyc
Photo-catalytic efficiency (%)	<1	60	73	91	99	81
K (min−1)	N/A	0.0030	0.0042	0.0078	0.013	0.0056

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11101144/s1, Figure S1: EDX mapping for SnO₂/TiO₂-50: (a) lateral SEM image of SnO₂/TiO₂-50 of information collection area, (b) O element, (c) Si element (d) Sn element, and (e) Ti element, Figure S2: Growth per cycle (GPC) of TiO₂ thin film at 300 °C on Si substrate, Figure S3: EELS spectrum of SnO₂/TiO₂-50. Sn and Ti clearly reveal presence as heterojunction structure, Figure S4: XRD pattern of as-deposited TiO₂ layer by ALD cycles process on Si substrate, Figure S5: UV-Visible absorbance spectra of photodegradation of methylene blue (a–e) by SnO₂ and all SnO₂/TiO₂ with different ALD cycles, Figure S6: The photocatalytic efficiency (%) with increasing degradation time by SnO₂ and all SnO₂/TiO₂-50 photocatalysts.

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