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Review

Atomic Level Interface Control of SnO2-TiO2 Nanohybrids for the Photocatalytic Activity Enhancement

by
Hiroaki Tada
1,* and
Shin-Ichi Naya
2
1
Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
2
Environmental Research Laboratory, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(2), 205; https://doi.org/10.3390/catal11020205
Submission received: 8 January 2021 / Revised: 18 January 2021 / Accepted: 19 January 2021 / Published: 3 February 2021
(This article belongs to the Section Photocatalysis)
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Abstract

:
This review article highlights atom-level control of the heterojunction and homojunction in SnO2-TiO2 nanohybrids, and the effects on the photocatalytic property. Firstly, a comprehensive description about the origin for the SnO2-TiO2 coupling effect on the photocatalytic activity in the conventional SnO2-TiO2 system without heteroepitaxial junction is provided. Recently, a bundle of thin SnO2 nanorods was hetero-epitaxially grown from rutile TiO2 seed nanocrystals (SnO2-NR#TiO2, # denotes heteroepitaxial junction). Secondly, the heterojunction effects of the SnO2-NR#TiO2 system on the photocatalytic activity are dealt with. A novel nanoscale band engineering through the atom-level control of the heterojunction between SnO2 and TiO2 is presented for the photocatalytic activity enhancement. Thirdly, the homojunction effects of the SnO2 nanorods on the photocatalytic activity of the SnO2-NR#TiO2 system and some other homojunction systems are discussed. Finally, we summarize the conclusions with the possible future subjects and prospects.

1. Introduction

Nanohybrid photocatalysts consisting of metals and semiconductors is the key material for solar-driven chemical transformations [1,2,3,4,5]. The enhancement in the photocatalytic activity stems from the effective interplay between the components depending on the interface quality. Among the nanohybrid photocatalysts, the system consisting of SnO2 and TiO2 is the representative one [6]. TiO2-SnO2 (or fluorine-doped SnO2, FTO) is also the basic electrode for various photoelectrochemical devices for solar-to-electric and chemical conversions [7,8,9,10]. From a view of practical point, the SnO2-TiO2 coupling system is a very promising material owing to the robustness, harmlessness, and inexpensiveness. The remarkable SnO2-TiO2 coupling effect on the photocatalytic activity is well recognized for various reactions as reported in recent papers on degradations of phenol [11] and dyes [12,13,14,15]. However, the fundamental mechanism has not been fully understood so far. Further, the effects of atomically commensurate junctions in the SnO2-TiO2 coupling system on the photocatalytic activity have recently been clarified [16,17].
This article reviews atomic level control of the heterojunction and homojunction in the SnO2-NR#TiO2 system, and the effects on the photocatalytic property. Section 2 describes the origin for the remarkable SnO2-TiO2 coupling effect on the photocatalytic activity. Section 3 deals with the heterojunction effects of the SnO2-NR#TiO2 system on the photocatalytic activity. Section 4 discusses the homojunction effect of the SnO2-NR#TiO2 system on the photocatalytic activity. Finally, in Section 5, the conclusions are summarized with the future subjects and prospects. Recently, the research of homojunction photocatalysts including the p-n homojunction type [18,19,20] and the morphological homojunction type [21,22,23,24,25] is currently in rapid progress. This article would also contribute to the development of the homojunction photocatalysts.

2. Origin for the SnO2-TiO2 Coupling Effect

This section describes the general features of the conventional SnO2-TiO2 nanohybrid photocatalysts without atomically commensurate junction (Scheme 1). The photocatalytic activity of TiO2 can be greatly boosted by coupling with SnO2 for various reactions [11,12,13,14,15,16,17]. It is worth noting that the electron acceptor in common with these reactions is molecular oxygen.
As an example, here we take gas-phase decomposition of acetaldehyde by a patterned TiO2/SnO2 bilayer type photocatalyst [26]. Samples nonpatterned and patterned TiO2 films on SnO2/soda lime (SL)-glass are designated as TiO2/SnO2/SL-glass and pat-TiO2/SnO2/SL-glass, respectively. In the pat-TiO2/SnO2/SL-glass, 1 mm wide stripes of TiO2 film regularly appeared on the SnO2/SL-glass substrate in a 1 mm pitch. In this study, acetaldehyde was used as a model for harmful organic gas (Figure 1).
This reaction is categorized as photocatalytic on the basis of the fact that both illumination and TiO2 are needed for the decomposition to occur (a) and a turnover number of >103. The photocatalytic activity of TiO2/SnO2/SL-glass (c) is higher than that of TiO2/quartz (b). However, the rate in the former system decreases with irradiation time, while the rate in the latter system is almost constant. Strikingly, the patterning of the TiO2 film (d) drastically increases the photocatalytic activity without causing the decay. The high photocatalytic activity of the SnO2-TiO2 coupling system partly stems from effective charge separation by the interfacial electron transfer from TiO2 to SnO2, which was substantiated by labeling and visualizing the reduction sites using the Ag photodeposition method [26]. Consequently, SnO2 and TiO2 act as reduction and oxidation sites, respectively, in the SnO2-TiO2 coupling system. However, the conduction band (CB) minimum of SnO2 is situated at −4.92 eV vs. vacuum at pH 0 [26], which is too low to cause one-electron oxygen reduction reaction (ORR) (Equation (1), Scheme 1).
O2 + e− → O2−
To determine the electron number of ORR (n), linear sweep voltammograms (LSVs) were measured at electrode potential (E vs. hydrogen electrode potential, SHE) from −0.8 V to +0.2 V for FTO and TiO2 film-coated FTO (TiO2/FTO) electrodes in argon-bubbled and aerated electrolyte solutions (Figure 2a). In the LSV for TiO2/FTO, the cathodic current flows at E < −0.4 V regardless of the absence and presence of O2, and the current is ascribable to the reduction of TiO2 [27]. On the other hand, in the LSV for FTO with O2, the current-onset potential shifts to approximately 0 V with the magnitude of current drastically increased, while no current is observed without O2. The current was measured as a rotating rate of the FTO electrode in aerated electrolyte solution. The Koutecky–Levich plot of the FTO electrode for the current in the presence of O2 at E = −0.8 V provides a straight line from the slope of which the n value was calculated to be 1.6 (Figure 2b). This finding indicates that two-electron ORR can partially occur on FTO (or SnO2), whereas TiO2 is electrocatalytically inactive for ORR. The electrons in the CB of SnO2 also has a potential sufficient to proceed two-electron ORR (Equation (2), Scheme 1). Thus, another reason for the effective SnO2-TiO2 coupling effect is ascribable to the electrocataltyic activity of SnO2 for two-electron ORR. On the basis of this scheme, the photocatalytic activity of pat-TiO2/SnO2/SL-glass much higher than that of nonpatterned sample is rationalized in terms of the balanced areas of the surfaces where oxidation (TiO2) and reduction (SnO2) sites occur in the former system.
O2 + 2H+ + 2e− → H2O2

3. Atom-Level Heterojunction Effect

This section deals with the hetero-epitaxial junction effect on the charge separation and photocatalytic activity of the SnO2-TiO2 nanohybrid system [16]. The effective charge separation can arise from the smooth interfacial electron transfer from TiO2 to SnO2-NR through the high-quality junction and subsequent efficient charge separation due to the lattice strain-induced unidirectional potential gradient of the CB minimum in the SnO2-NR. This nanoscale band engineering presents a novel methodology for the effective charge separation to enhance the activity of the nanohybrid photocatalysts.
SnO2 NRs were grown from rutile TiO2 seed nanocrystals in an alkaline SnCl4 solution by a hydrothermal process for a given time (tHT). Low- and high-magnification scanning electron microscopy (SEM) images of the sample prepared at tHT = 72 h (Figure 3a,b) shows that NRs are grown from every TiO2 particle with a specific orientation by the hydrothermal reaction. The powder X-ray diffraction (XRD) pattern (Figure 3c) has the diffraction peaks of rutile TiO2, and after the hydrothermal reaction, new peaks appear at 2θ = 26.52°, 34.42°, and 52.46° assigned to the diffraction from the (110), (101), and (211) crystal planes of SnO2, respectively. The high resolution-transmission electron microscopy (HR-TEM) image of SnO2-NR/TiO2 (Figure 3d) shows an SnO2(110) lattice fringe parallel to its growth direction. Clearly, SnO2-NRs grow in the [001] direction from the rutile TiO2 surface. Conversely, the solvothermal preparation of rutile TiO2 nanowire arrays with the [001] orientation on FTO electrode has recently been reported [28].
In the bulk system, an a-axis mismatch of 3.11% is present between SnO2 and rutile TiO2. Surprisingly, a heteroepitaxial junction was formed in the nanoscale system [16] in spite that the formation of heteroepitaxial junction is limited to the systems with the lattice mismatch smaller than 0.1% in the bulk systems [29]. This finding suggests that even if the heteroepitaxial junction cannot be formed in the bulk state, it is possible in the nanoscale [30]. A junction model was presented with the SnO2-NR grown in the [001] direction from the rutile TiO2 surface having a heteroepitaxial relation of SnO2{001}/TiO2{001} (SnO2-NR#TiO2). Previously, oriented SnO2 nanowire arrays were formed on rutile TiO2(001) single crystal by a chemical vapor deposition method [31].
In today’s time, acetaldehyde is industrially produced by the Wacker oxidation of ethylene using a PdCl2-CuCl2 catalyst at ~1 MPa and ~400 K [32], and, then, the development of the green process for the selective synthesis of acetaldehyde from biomass-derived ethanol under mild conditions is very significant (Equation (3)) [33].
CH3CH2OH (g) + 1/2O2 (g) → CH3CHO (g) + H2O (l)
We have recently found that rutile TiO2 exhibits high photocatalytic activity for the partial oxidation of ethanol to acetaldehyde at ambient temperature and pressure, whereas ethanol is completely oxidized to carbon dioxide in the anatase TiO2 photocatalytic system [34,35].
The photocatalytic activity of various samples was studied for gas-phase oxidation of ethanol to acetaldehyde (Figure 4a). The loading amount and rod length of SnO2 are denoted as xSnO2 and lNR, respectively. UV-irradiation of TiO2 produces acetaldehyde of which amount increases with an increase in irradiation time, while SnO2 is completely inert. Rutile TiO2 shows much higher photocatalytic activity than anatase TiO2, and the former activity is further enhanced by mixing with 10.0 mass% SnO2. Strikingly, SnO2-NR#TiO2 (lNR = 61.4 nm, xSnO2 = 11.0 mass%) exhibits a high level of photocatalytic activity far exceeding even that of the physical mixture of rutile TiO2 and SnO2, whereas no reaction proceeded in the dark or under UV-light irradiation without O2. The apparent quantum yield or external quantum yield (ϕex) defined by Equation (4) reached 25.6% at λex = 365 nm in the SnO2-NR#TiO2 system.
ϕex (%) = {2 × (number of acetaldehyde molecules)/number of incident photons} × 100
This value surpasses the values reported for the TiO2 photocatalytic oxidation of ethanol to acetaldehyde (<~10%) [34,36]. These findings evince the importance of the junction state between SnO2 and TiO2 for the activity in the hybrid photocatalyst.
Further, the initial photocatalytic activity of SnO2-NR#TiO2 (v0/mol h−1) increases with an increase in xSnO2 (Figure 4b). In addition, there is a clear trend that the photocatalytic activity increases with increasing lNR. On the other hand, there is linear relations between the amount of acetaldehyde produced and the amount of ethanol consumed in the SnO2-NR#TiO2 and unmodified rutile TiO2 systems. The selectivity was calculated from the slope in the SnO2-NR#TiO2 (lNR = 61.4 nm) system to be ~100%. The photocataltyic decomposition of acetaldehyde was further examined with SnO2-NR#TiO2, anatase and rutile TiO2 particles. Rutile TiO2 and SnO2-NR#TiO2 exhibit much lower photocatalytic activity than anatase TiO2 (Figure 4c). Ethanol oxidation has so far been reported for the photocatalsyts of anatase TiO2 and P-25 (anatase-rutile mixture). These studies reported that ethanol undergoes complete oxidation to carbon dioxide, and the selectivity to acetaldehyde is lower than 50% [34,37,38,39]. Therefore, the high selectivity in the rutile TiO2 and SnO2-NR#TiO2 systems results from the suppression of the ethanol overoxidation.
The insight into the charge separation in the hybrid photocatalysts can be gained by photoluminescence (PL) measurements [40]. TiO2 has a broad PL band arising from the emission from vacancy levels around 520 nm (Figure 4d) [41]. In the spectra for SnO2-NR#TiO2, the emission band extremely weakens. In addition, selective TiO2 excitation of SnO2-NR#TiO2 in AgNO3 aqueous solution led to preferential deposition of Ag NPs on SnO2-NR [26]. Evidently, UV-light irradiation of SnO2-NR#TiO2 induces smooth interfacial electron transfer from TiO2 to SnO2-NR followed by the effective charge separation through the high-quality heterojunction.
The SnO2(110) d-spacing in the NR was determined as a function of the distance from the interface with TiO2 (dFIF) from the HR-TEM analysis [16]. The a-axis length calculated from the (110) d-spacing gradually increased with an increase in dFIF from 4.52 Å at dFIF = 1 nm to 4.73 Å at dFIF = 75 nm, which is equal to the value for bulk SnO2. The formation of the heteroepitaxial junction causes the shrinkage of the a-axis near the interface to relax in the [001] direction of SnO2-NR from the root to the tip. Density functional theory (DFT) simulations were performed for model slabs of the SnO2-NR hetero-epitaxially grown from TiO2. The energy diagram created by using the calculated values qualitatively reproduced the increase in the band gap with decreasing rod length. More importantly, the energy diagram showed that a significant downward bending in the CB minimum potential is induced in the direction from the root to the tip of SnO2 NR. A recent paper has reported that in the SnO2 thin film epitaxially grown on the Al2O3(0001) substrate, the interfacial tensile strain generated in the SnO2 lattice conversely lowers the band gap [42].
The action mechanism of SnO2-NR#TiO2 in the photocatalytic gas-phase selective oxidation of ethanol to acetaldehyde can be explained on the basis of the energy diagram in Scheme 2, where the energy levels are shown with respect to the vacuum level (at pH 0). The flat band potentials of rutile TiO2 and SnO2 electrodes were previously determined to be −4.50 V [43] and −4.92 V [26] by the Mott–Schottky plots. SnO2-NRs are grown on rutile TiO2 with a heteroepitaxial relation of SnO2{001}/TiO2{001} by the present hydrothermal reaction. The positive a-axis mismatch generates a compressive strain in the SnO2-NR near the interface to induce the continuous increase in the a-axis length extending over 60 nm towards the SnO2[001] direction from the root to the tip. As a result, a downward band bending is formed in the interior of the SnO2-NR. UV-light irradiation of SnO2-NR#TiO2 promotes the electrons in the VB of TiO2 to the CB. The excited electrons in the CB with ECBM = −4.50 eV are smoothly transferred to the CB of SnO2-NR with the ECBM = −4.92 eV through the atomically commensurate interface, while the holes are left in the VB of TiO2 because the VB maximum of SnO2 is located much lower with respect to that of TiO2. In addition, the electrons can be separated from the VB-holes in TiO2 due to the unidirectional downward potential gradient in the CB minimum in the SnO2-NR. The CB-electrons in the SnO2-NR have sufficient energy to cause a two-electron oxygen reduction reaction (E0(O2/H2O2) = −5.14 V), while the VB-holes in TiO2 selectively oxidize ethanol to acetaldehyde because of the suppression of the overoxidation. The enhancement of the photocatalytic activity with increasing rod length can be rationalized in terms of the long-range charge separation. The groups of Majima and Choi have recently shown that the reactive oxygen species photogenerated on rutile TiO2 are the surface bound OH radicals (or surface trapped holes, ∙OHs) limiting the oxidation mainly to the surface, whereas the oxidation on anatase TiO2 can occur at the place far away from the surface in the former system because of the diffusion of OH radicals from the surface (or free OH radicals) [44]. Further, we have found that the adsorption of acetaldehyde on rutile TiO2 is suppressed in the presence of adsorbed water [45]. Consequently, in the present SnO2-NR#TiO2 photocatalytic reaction system, the highly selective and efficient oxidation of ethanol to acetaldehyde proceed.

4. Atom-Level Homojunction Effect

4.1. SnO2-TiO2 Homojunction Systems

In the as-grown SnO2-NR#TiO2, the apparent single SnO2-NR is actually composed of a bundle of thin SnO2 NRs. This section discusses the effect of the formation of homojunction between the thin SnO2 NRs on the photocatalytic activity [17]. The photocatalytic activity-heating temperature (Tc) curve in the SnO2-NR#TiO2 system shows a volcano-shaped profile with the maximum activity at Tc = 500 °C. The increase in the photocatalytic activity by the heating at Tc = 500 °C results from the high electron mobility in the SnO2 NRs with the fusion of the thin SnO2 NRs.
SEM observation was carried out for SnO2-NR#TiO2 prepared at varying Tc in air for 1 h (Figure 5a,b). As-grown SnO2-NR#TiO2 consists of a bundle of thin SnO2 NRs. The heat treatment at 500 °C induces fusion of the bundle forming a monolithic SnO2 NR.
Further, the HR-TEM image shows that the SnO2 moiety of SnO2-NR#TiO2 (Tc = 500 °C) has good crystallinity (Figure 5c). On the other hand, heating at 700 °C causes many disjuncture in the lattice fringe due to the fine segmentation of the SnO2 NRs (Figure 5d).
The specific surface area (SBET) of SnO2-NR#TiO2 was measured by Brunauer–Emmett–Teller method. As a result of an increase in Tc, the SBET gradually decreases with fusion of the bundle of thin SnO2 NRs (Figure 6a). In addition, the crystallite size of SnO2 (D) was estimated using the Scherrer equation from the full-width at half maximum of the SnO2(110) diffraction peak. In the plot of D versus Tc, the D value of ~25 nm for the as-grown sample increases with an increase in Tc, going through a maximum of ~80 nm at Tc = 500 °C to steeply decrease above 600 °C (Figure 6a). Clearly, the growth of crystallites with increasing Tc at ≤500 °C increases the D value, which then decreases due to the segmentation at Tc ≥ 600 °C.
The Tc-dependence of the photocatalytic activity for the gas-phase oxidation of ethanol to acetaldehyde was studied. The reaction apparently follows the first-order rate law in every system, and the pseudo-first-order rate constant (k) was calculated from the plots of ln [C0/(C0-C)] versus t, where C0 and C are the initial concentration of EtOH and the concentration of acetaldehyde at the irradiation time t, respectively. The plot of k vs. Tc exhibits a volcano-shaped curve with the peak at Tc = 500 °C (Figure 6b), which well resembles the D-Tc one (Figure 6a).
The heating effect on the photocatalytic activity can be explained in terms of the change in the homojunction state in the SnO2 NRs (Scheme 3) [17]. As explained in Section 3, SnO2-NR#TiO2 can work as an excellent charge separator owing to the smooth interfacial electron transfer from TiO2 to SnO2 through the high-quality interface and the subsequent electron transport in the SnO2-NR from the interface to the tip by the assistance of the lattice strain-induced unidirectional potential gradient in the CB [16]. In this case, the heat treatment at Tc ≤ 500 °C causes the fusion of the bundle of the SnO2 NRs to decrease the resistance for the electron transport and enhance the charge separation. At Tc ≥ 600 °C, the heat treatment incurs the segmentation of the SnO2 crystal, which is also evidence of the presence of the hetero-epitaxial junction-induced lattice distortion in the SnO2 NR. The many boundaries generated in the SnO2 NR would scatter electrons to interfere with the electron transport or charge separation. As a result of the balance between them, there exists an optimum heating temperature around 500 °C.
Zeng and co-workers constructed a branched rutile TiO2 NR array on FTO substrate using a two-step route involving a hydrothermal synthesis and a chemical bath deposition [22]. In this method, the length of the branches was controlled by the chemical bath deposition time (tCBD). A coherent interface was observed between the TiO2 NR and the branch by HR-TEM. The sample prepared at tCBD = 36 h exhibits a high level of photocatalytic activity for gas-phase decomposition of benzene under UV-light irradiation (200 nm < λex < 400 nm). The striking photocatalytic activity was ascribable to the branch-to-NR interfacial electron transfer and subsequent charge separation due to the smooth electron transport along the single-crystal TiO2 NR.

4.2. Other Homojunction Systems

Among various homojunction systems, those with the formation of coherent interface confirmed are only limited. In addition to those, the works on the homojunction between the identical crystals with only different morphologies are described in this section.
Zou and co-workers prepared a homojunction system consisting of n-type oxygen-defected TiO2 and p-type titanium-defected TiO2 by a multi-step process involving liquid-phase synthesis and calcination [18]. The formation of a somewhat atomically commensurate interface was confirmed by HR-TEM observation. PL and electrochemical impedance spectroscopy (EIS) analyses indicated that effective charge separation occurs in this system. Consequently, Pt nanoparticle-loaded p-n homojunction TiO2 (Pt/p-n homojunction TiO2) showed higher photocatalytic activity than Pt/p-TiO2 and Pt/n-TiO2 by factors of 2.3 and 10.8, respectively, for H2 generation from a methanol aqueous solution under UV-light irradiation. The simultaneous interfacial electron transfer from p-type TiO2 to n-type TiO2 and hole transfer in the opposite direction were assumed.
Chen, Zhou, and co-workers prepared p-n Bi4V2O11 homojunction through Bi5+ self-doping (Bi5+-BVO) [19]. Electrochemical analysis of the Bi5+-BVO electrode showed a p-n junction character, while a nondoped BVO electrode has an n-type character. PL, EIS, and time-resolved fluorescence decay spectroscopy indicated that the p-n homojunction suppresses the electron-hole recombination. Bi5+-BVO exhibited significantly larger photocatalytic activity than nondoped BVO for Cr(VI) reduction in the presence of citric acid under visible-light irradiation. The high photocatalytic activity was ascribable to the effective charge separation through the p-n junction although no information about the interface at an atomic level was provided.
Lyu and co-workers fabricated a homojunction between anatase TiO2 nanoparticles (NPs) and a microporous anatase TiO2 layer by a two-step process involving vapor-induced hydrothermal synthesis and subsequent photothermocatalytic treatment [23]. No information about the junction state was provided; however, the homojunction sample provided significantly higher mineralization efficiency in the gas-phase decomposition of toluene than microporous and nonporous TiO2 NPs under UV-light irradiation (λex = 254 nm). On the basis of the data on surface photovoltage spectroscopy (SPS) measurements, the remarkable photocatalytic activity of the homojunction system was ascribable to effective charge separation in addition to the large specific surface area.
Yang and co-workers proposed a p-n TiO2 homojunction involving amorphous and anatase TiO2 prepared by controlling the heating temperature (Tc) of the latter around 350 °C [20]. The structure of the interface between amorphous and anatase TiO2 was shown at an atomic level resolution. The homojunction sample showed photocatalytic activity significantly larger than amorphous TiO2 and anatase TiO2, but the reason remains unclear.
Ren, Li, and co-workers formed a homojunction between anatase TiO2 nanosheets (NSs) and anatase TiO2 NPs by a two-step process involving vapor-induced hydrothermal synthesis and subsequent photothermocatalytic treatment [25]. HR-TEM image showed that the anatase TiO2 NS and NPs possess dominant {001} and {101} facets although the junction state is unclear. The homojunction sample afforded much higher photocatalytic activity than TiO2 NSs and NPs for gas-phase decomposition of acetone under UV-light irradiation (300 nm < λex). The reduction and oxidation reactions were reported to primarily occur on the {001} and {101} facets, respectively, in faceted anatase TiO2 photocatalyst. The authors proposed a crystal facet-induced charge separation mechanism to explain the high photocatalytic activity.

5. Conclusions and Future Prospects

Most importantly, this review article points to the general possibility of the hetero-epitaxial junction formation between the components of the nanoscale hybrids even if it is inhibited in the bulk state because of significant lattice mismatch. This is also valid for the SnO2-TiO2 coupling system with an a-axis mismatch over 3%.
The improvement in the photocatalytic activity of TiO2 by the hybridization with SnO2 for the oxidative reactions originates from the charge separation because of the interfacial electron transfer from TiO2 to SnO2 and the electrocatalytic activity of SnO2 for two-electron ORR. A novel nanoscale band engineering of heteronanostructured photocatalysts for the charge separation and activity enhancement is presented in a hybrid consisting of SnO2 NR and TiO2 with heteroepitaxial junction (SnO2-NR#TiO2). In addition, fusion of the homojunctions between a bundle of thin SnO2 NRs in SnO2-NR#TiO2 further increases the photocatalytic activity due to the lowering in the resistance of the electron transport in the SnO2 NR. In this manner, the enhancement in the photocatalytic activity of the SnO2-TiO2 nanohybrid can be achieved through the atomic-level control of the heterojunction and homojunction.
SnO2-NR#TiO2 has wide and high potentials as the photocatalytic for various reactions, and the exploitation should be a coming subject. In addition, they only respond to UV-light, and the visible-light response is of crucial importance for the effective utilization of the sunlight as the driving force. A promising approach to achieve this is the surface modification by plasmonic metals such Au [46] and Ag [47,48]. Finally, the development of various nanohybrids with atomically commensurate junctions other than the SnO2-NR#TiO2 system can bring wide and fruitful applications.

Author Contributions

H.T. chose the topic; H.T. and S.-I.N. wrote and revised the article. All authors have read and agreed to the published version of the manuscript.

Funding

The authors greatly acknowledge financial support from JST Adaptable and Seamless Technology Transfer Program through Target-driven R&D, JSPS KAKENHI Grant-in-Aid for Scientific Research (C) no. 18K05280 and 20K05674, the Futaba Foundation, and Nippon Sheet Glass Foundation for Materials Science and Engineering.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All links are given to publications that are in the open press and are easily available.

Acknowledgments

The authors acknowledge Emeritus H. Kobayashi for DFT calculations.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Energy diagram of the SnO2-TiO2 hybrid-photocatalyzed reaction system.
Scheme 1. Energy diagram of the SnO2-TiO2 hybrid-photocatalyzed reaction system.
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Figure 1. Time courses of decomposition of CH3CHO upon illumination in the presence of SnO2/SL-glass (a), TiO2/quartz (b), TiO2/SnO2/SL-glass (c), and pat-TiO2/SnO2/SL-glass (d). The figure is taken from ref. [26].
Figure 1. Time courses of decomposition of CH3CHO upon illumination in the presence of SnO2/SL-glass (a), TiO2/quartz (b), TiO2/SnO2/SL-glass (c), and pat-TiO2/SnO2/SL-glass (d). The figure is taken from ref. [26].
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Figure 2. (a) Linear sweep voltammogram obtained with the TiO2/FTO and FTO electrodes in argon gas-bubbled and aerated 0.1 M NaClO4 electrolyte solution (pH 5.70) with a potential sweep rate of 20 mVs−1. Jre expresses the current density per real surface area of the electrode. (b) Koutecky-Levich plots of the FTO electrode for the current as E = −0.8 V.
Figure 2. (a) Linear sweep voltammogram obtained with the TiO2/FTO and FTO electrodes in argon gas-bubbled and aerated 0.1 M NaClO4 electrolyte solution (pH 5.70) with a potential sweep rate of 20 mVs−1. Jre expresses the current density per real surface area of the electrode. (b) Koutecky-Levich plots of the FTO electrode for the current as E = −0.8 V.
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Figure 3. Low (a) and high (b) magnification SEM images for SnO2-NR/TiO2 (tHT = 72 h). The inset in (a) shows SEM image for pristine rutile TiO2. (c) XRD patterns for SnO2-NR/TiO2, and pristine TiO2 for comparison. (d) HR-TEM image for SnO2-NR/TiO2. The figure is taken from ref. [16].
Figure 3. Low (a) and high (b) magnification SEM images for SnO2-NR/TiO2 (tHT = 72 h). The inset in (a) shows SEM image for pristine rutile TiO2. (c) XRD patterns for SnO2-NR/TiO2, and pristine TiO2 for comparison. (d) HR-TEM image for SnO2-NR/TiO2. The figure is taken from ref. [16].
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Figure 4. (a) Time courses for gas-phase photocatalytic oxidations of ethanol to acetaldehyde under UV-light irradiation (λex > 300 nm). The amount of acetaldehyde is normalized by the specific surface area of the photocatalyst. (b) Relation between the photocatalytic activity and SnO2 loading amount. The corresponding SnO2-NR length is shown in the figure. (c) Time courses for gas-phase photocatalytic decomposition of acetaldehyde under UV-light irradiation (λex > 300 nm) in the SnO2-NR#TiO2 (lNR = 61.4 nm) system, and anatase and rutile TiO2 system for comparison. (d) PL spectra for authentic rutile TiO2 and SnO2-NR#TiO2 with varying mean SnO2-NR length under irradiation of light with wavelength of 340 nm at 77 K. The figure is taken from ref. [16].
Figure 4. (a) Time courses for gas-phase photocatalytic oxidations of ethanol to acetaldehyde under UV-light irradiation (λex > 300 nm). The amount of acetaldehyde is normalized by the specific surface area of the photocatalyst. (b) Relation between the photocatalytic activity and SnO2 loading amount. The corresponding SnO2-NR length is shown in the figure. (c) Time courses for gas-phase photocatalytic decomposition of acetaldehyde under UV-light irradiation (λex > 300 nm) in the SnO2-NR#TiO2 (lNR = 61.4 nm) system, and anatase and rutile TiO2 system for comparison. (d) PL spectra for authentic rutile TiO2 and SnO2-NR#TiO2 with varying mean SnO2-NR length under irradiation of light with wavelength of 340 nm at 77 K. The figure is taken from ref. [16].
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Scheme 2. A proposed action mechanism of the SnO2-NR#R-TiO2-photocatalyst in the oxidation of ethanol to acetaldehyde. The scheme is taken from ref. [16].
Scheme 2. A proposed action mechanism of the SnO2-NR#R-TiO2-photocatalyst in the oxidation of ethanol to acetaldehyde. The scheme is taken from ref. [16].
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Figure 5. SEM images of SnO2-NR#TiO2: (a) as-grown (b) Tc = 500 °C. HR-TEM image of the SnO2-NR moiety of SnO2-NR#TiO2: (c) Tc = 500 °C, (d) Tc = 700 °C. The figure is taken from ref. [17].
Figure 5. SEM images of SnO2-NR#TiO2: (a) as-grown (b) Tc = 500 °C. HR-TEM image of the SnO2-NR moiety of SnO2-NR#TiO2: (c) Tc = 500 °C, (d) Tc = 700 °C. The figure is taken from ref. [17].
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Figure 6. (a) Plots of specific surface area and SnO2 crystallite size as a function of heating temperature (Tc). (b) Relation between the photocatalytic activity and Tc. The figure is taken from ref. [17].
Figure 6. (a) Plots of specific surface area and SnO2 crystallite size as a function of heating temperature (Tc). (b) Relation between the photocatalytic activity and Tc. The figure is taken from ref. [17].
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Scheme 3. Heating effect of the SnO2-NR#TiO2 photocatalyzed on the oxidation of ethanol to acetaldehyde.
Scheme 3. Heating effect of the SnO2-NR#TiO2 photocatalyzed on the oxidation of ethanol to acetaldehyde.
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Tada, H.; Naya, S.-I. Atomic Level Interface Control of SnO2-TiO2 Nanohybrids for the Photocatalytic Activity Enhancement. Catalysts 2021, 11, 205. https://doi.org/10.3390/catal11020205

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Tada H, Naya S-I. Atomic Level Interface Control of SnO2-TiO2 Nanohybrids for the Photocatalytic Activity Enhancement. Catalysts. 2021; 11(2):205. https://doi.org/10.3390/catal11020205

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Tada, Hiroaki, and Shin-Ichi Naya. 2021. "Atomic Level Interface Control of SnO2-TiO2 Nanohybrids for the Photocatalytic Activity Enhancement" Catalysts 11, no. 2: 205. https://doi.org/10.3390/catal11020205

APA Style

Tada, H., & Naya, S. -I. (2021). Atomic Level Interface Control of SnO2-TiO2 Nanohybrids for the Photocatalytic Activity Enhancement. Catalysts, 11(2), 205. https://doi.org/10.3390/catal11020205

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