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Materials and Interfaces

Suppression of TiO2 Photocatalytic Activity by Lowtemperature Pulsed CVD-grown SnO2 Protective Layer Yangyang Yu, Yingming Zhu, Jing Guo, Hairong Yue, Hegui Zhang, Changjun Liu, Siyang Tang, and Bin Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00270 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Industrial & Engineering Chemistry Research

Suppression of TiO2 Photocatalytic Activity by Lowtemperature Pulsed CVD-grown SnO2 Protective Layer Yangyang Yu1, Yingming Zhu2, Jing Guo3, Hairong Yue1,2, Hegui Zhang1, Changjun Liu1,2, Siyang Tang1, Bin Liang1,2*

1

Multi-phases Mass Transfer and Reaction Engineering Laboratory, School of Chemical

Engineering, Sichuan University, Chengdu 610065, China 2

Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610207,

China 3

Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of

China, Taiyuan, Shanxi 030051, China

*Corresponding author: [email protected], TEL/FAX: (+86)-28-85460556

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Abstract: TiO2 pigments are widely used in paint industries. Inert coating layers were usually deposited on the TiO2 pigments to suppress the photocatalytic activity of TiO2, which can prevent the degradation of surrounding polymer molecules. However, the traditional wet chemical methods normally form thick films, which would impair the pigment properties of TiO2. In this work, SnO2 and SiO2 protective layers were grown on the TiO2 particle surface by low-temperature pulsed chemical vapor deposition. At temperatures 3 nm to ensure the suppression of TiO2 photocatalytic activity.34 However, SiO2 coating layers with the thickness of >3nm may also result in a significant decrease in pigment properties such as lightening power of TiO2 because the refractive index of SiO2 (1.46) is much lower than that of anatase TiO2 (2.55).10, 35, 36 SnO2 has attracted increasing attention due to its high transparency (in the 0.4–1.2 µm range ), high thermal and chemical stability, excellent optical and electrical properties.37, 38 In addition, the refractive index of SnO2 (2.00) is similar to that of TiO2. Rutile TiO2-SnO2 composite powders were reported to have high brightness, whiteness, and light scattering index.39, 40 Moreover, SnCl4 has a relatively low boiling point (114 °C) and can be easily hydrolyzed to form SnO2. Thus, in this work, SnO2 films can be prepared by low-temperature pulsed CVD method. Although the effects of surface coatings on the photocatalytic activity of TiO2 have been extensively reported, the mechanism is still unclear. Hughes41 reported the suppressive effect of

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Al2O3 coating and suggested that Al2O3 served as a strong electron acceptor to prevent the formation of hydroxyl and peroxide free radicals. Wiseman41 argued that the pseudo-boehmite structure of hydrated alumina had a relatively large amount of surface hydroxyl groups and claimed that compact coatings of Al2O3 or SiO2 led to the photocatalytic activity decrease of TiO2 pigments due to their low surface area and porosity. Young Soo Kang suggested that the hydrated oxide groups can capture the radicals which were formed during the migration of the electron-hole pairs.42 In addition, Palomares claimed that Al2O3, ZrO2 and SiO2 coating layers acted as a barrier to prevent the migration of electrons/holes from the TiO2 bulk phase to the surface because they had larger bandgap, more negative conduction bands and more positive valence bands than TiO2.43 However, the mechanism of photocatalytic activity suppression property caused by CoO26 and SnO2 coating layers was not discussed. In this work, SnCl4 reacted with surface –OH groups of both anatase TiO2 particles and rutile TiO2 particles to grow a SnO2 protective layer by the low-temperature pulsed CVD method. A thin and uniform amorphous SnO2 film (1.1±0.3 nm) was obtained, and the pigment properties of the SnO2-coated TiO2 particles (TiO2/SnO2 core−shell particles) were studied. The effects of SnO2 and SiO2 coatings on the photocatalytic activity and pigment properties of TiO2 were compared. The photocatalytic activity suppression mechanism was investigated by transient fluorescence spectroscopy, photocurrent measurements and electrochemical impedance spectroscopy.

2. Experimental Section 2.1. Materials SnCl4 (analytical reagent (AR)) and SiCl4 (AR) were purchased from Aladdin Chemistry Co. and used without any further treatment. The precursors were placed in a 50 ml glass vial and kept at 60 °C during the coating experiments. Anatase TiO2 powders with a diameter of 100–300 nm (Figure S1) were received from Taihai TiO2 pigment Co. (Panzhihua, China), Rhodamine B (RhB, AR) was purchased from Kelong Chemistry Co. The water content in the air is about 2.8 wt %. 2.2. Preparation of TiO2/SnO2 core−shell particles As shown in Figure 1a, the volume of the reaction vessel was about 1 L (d=0.12 m, L=0.10 m), and the reactor was heated with an electric heating jacket. TiO2 powders (1.50 g), pretreated at 120 °C for 2 h, were spread over a porous distributor plate with a thickness of 2 mm. At this thickness and with a long dosing time of up to 60 min, the diffusion limitation of the gas molecules to the


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bottom of the layer can be eliminated.44 SnCl4 (liquid) was placed in a glass vial, and valve V3 was connected to a vacuum pump. After sealing, the reactor was vacuumed to 50 mbar by closing valves V1 and V2 and opening V3, to create a differential pressure between the reactor and the SnCl4 vapor precursor. Next, V3 was closed, V1 was opened, and 1.1 × 10−3, 1.8× 10−3 and 2.5× 10−3 mol SnCl4 was vaporized and fed into the reactor space at the reaction time of 20 min, 40 min, and 60 min, and the amount of SnCl4 gas was excessive during the reaction (see section 1.1 of the Supporting Information). The reactor was heated to 60 °C for reaction with the surface hydroxyl groups of the TiO2 particles (Figures 1b-d) for 20, 40, and 60 min (to obtain TiO2/SnO2 20 min coating, TiO2/SnO2 40 min coating, and TiO2/SnO2 60 min coating core−shell particles, respectively). After reaction, a vacuum pump was connected with a NaOH wash bottle which was also connected with V3, and V3 was opened to remove by-products and unreacted molecules (Figure S2). Next, V2 was opened to introduce air into the reactor. Moisture in the air reacted with the surface Sn−Cl to form a hydroxyl-terminated surface (Figure 1e). After being exposed to air for 30 minutes, the TiO2/SnO2 particles were dried at 105 °C for 6 h, and the weight of TiO2/SnO2 40 min coating core−shell particles was 1.61g. It was calculated that the utilization rate of SnCl4 was 38.3% when deposition time was 40 min (see Figure S3 and section 1.2 of the Supporting Information). The thickness of the SnO2 layer increased by repeating this procedure, and in this case, a SnCl4 dosing time of 40 min was applied to all the depositions. TiO2/SiO2 core−shell particles were prepared according to our previous research,34 and the detail steps of the experiment are placed in section 1.3 of the Supporting Information. The preparation steps of SnO2- and SiO2coated rutile TiO2 are placed in section 1.4 of the Supporting Information. 2.3. Characterization of TiO2/SnO2 core−shell particles The surface morphology of the particles was observed by high-transmission electron microscopy (HRTEM) (FEI Titan Themis 200). The composition of the SnO2 and SiO2 layers was characterized by X-ray photoelectron spectroscopy (XPS) (Thermo Scientific ESCALAB 250Xi) and energydispersive X-ray spectroscopy (EDS) line scan (FEI Titan Themis 200). The crystalline phases of the particles were characterized by X-ray diffraction (XRD) (DX2700, Dandong Haoyuan) and high-transmission electron microscopy (HRTEM). The photoluminescence (PL) lifetime of particles was investigated by transient fluorescence spectroscopy (FLS 980). The photoelectric properties of particles were studied by photocurrent measurements and electrochemical impedance



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spectroscopy (EIS) (CHI 650E), and the whiteness of particles was determined by colorimetry (MODEL). The lightening power of particles was measured by colorimetry (SP60). 2.4. Photocatalytic activity The photocatalytic activity of the TiO2/SnO2 core−shell particles was evaluated by Rhodamine B (RhB) degradation, which was carried out in a 500 mL double-jacketed reactor. An aqueous RhB solution (6 mg/L, 150 mL) and TiO2 powders (20 mg) were magnetically stirred in the dark for 60 min to reach the adsorption equilibrium. The mixture was then irradiated by a Xenon lamp (300 W) for different time under stirring. After completion of the reaction, the powders were separated by centrifugation, and the RhB concentration of the solution was determined by UV-visible spectrophotometry (TU-1810, Persee, China) at a wavelength of 553 nm.

3. Results and discussion 3.1. SnO2 film growth mechanism It has been found that the growth of SiO2 film on TiO2 surface is due to the hydrolysis of SiCl4 vapor with the surface hydroxyl of TiO2.34 Similar growth mechanism was proposed for SnO2 film since SnCl4 can also react with hydroxyl groups.45 The sum reaction of the growth mechanism of SnO2 layer is the hydrolysis of SnCl4 , which can be described as:46 SnCl4 + 2H2O SnO2 + 4HCl

(1)

The reaction includes homogeneous combined heterogeneous processes.47, 48 The following surface heterogeneous reactions:47, 48 |−OH(s) + SnCl4(g)

|−O−Sn−Cl(s) + HCl(g)

(2)

|−O−Sn−Cl(s) + H2O(g)

|−O−Sn−OH(s) + HCl(g)

(3)

lead to saturation of the −OH groups, resulting in the formation of a uniform and compact surface film. On the other hand, homogeneous reactions between SnCl4 and water vapor lead to a fine powder deposition on the particle surface, resulting in a loose and porous surface layer. The water involved in these transformations is vaporous, which is introduced into the system with air and can also be formed from the condensation of hydroxyl groups. On the other hand, the condensation of −OH groups with Sn−Cl moieties forms cross-linked bonds (Sn−O−Sn) with release of HCl: |−O−Sn−OH(s) + |−O−Sn−OH(s) Sn−O−Sn(s) + H2O(g)

(4)

|−O−Sn−OH(s) + |−O−Sn−Cl(s) Sn−O−Sn(s) + HCl(g)

(5)


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3.2. Morphology and composition of the coating The surface morphologies of TiO2, TiO2/SnO2 and TiO2/SiO2 core−shell particles were investigated by HRTEM, and the images are shown in Figure 2, and the corresponding thickness data are shown in Table S2. Before coating, no surface layer was detected on the TiO2 particles (Figure 2a), and the coating thicknesses increased with increasing deposition time (Figures 2b–d). Different thin films with thicknesses of 0.5±0.3, 1.1±0.3 and 1.8±0.4 nm were observed on samples coated with SnCl4 under deposition time of 20, 40, and 60 min, respectively (Table S2). The film thickness increased to 2.0±0.4 nm when the sample was coated twice (Figure 2e and Table S2). With the coating time and coating cycles increasing, the standard deviations of 1 nm coating thickness decreased and the thicknesses of the SnO2 films increased, suggesting the increase of the uniformity and photocatalytic suppression ability of the films. (see section 1.5 of the Supporting Information). For TiO2/SiO2 core−shell particles, the SiO2 film obtained from a single coating operation with a SiCl4 deposition time of 40 min grew to a thickness of 4.2±0.8 nm (Figure 2f and Table S2), which was much thicker than the SnO2 film obtained by coating with SnCl4 for the same deposition time. Small standard deviations of 1nm coating thickness and large film thickness of SiO2 films might result in high photocatalytic suppression ability of films (see section 1.5 of the Supporting Information). The determinations of SnO2 and SiO2 film thickness between different particles from TEM images are shown in Table S3, and the conclusions obtained from Table S3 were consistent with those obtained from Table S2. The rutile TiO2 surface could also be coated with amorphous SnO2 and SiO2 films (Figure S4b–c). To determine the composition of the coating layer, the elemental distribution of the film on the TiO2/SnO2 core−shell particle surface obtained after two coating cycles was identified by EDS line scan. As shown in Figure 2g, light scanned 40 nm from the interior of the particle to the surface, and the chemical compositions corresponding to the locations are shown in Figure 2h. The Ti content gradually decreased from the core to the surface, where it was replaced by Sn. Moreover, the elemental contents demonstrate the presence of a film of about 2 nm thickness (see Figure S5 and section 1.6 of the Supporting Information). Next, the crystalline phases of the samples were investigated. As shown in Figure 3a, the XRD pattern of the anatase TiO2 pigments used in this work showed the typical diffraction peaks at 25°, 38°, 48°and 54°.49-51 All peaks were in good accordance with the standard spectrum of anatase TiO2 (JCPDS no: 88-1175 and 84-1286), indicating that TiO2 was in the 100wt% anatase phase.



Interestingly, the XRD patterns of both TiO2/SnO2 and TiO2/SiO2 core−shell particles were not affected by the coatings, and no diffraction peaks related to SnO2 or SiO2 were observed. These findings suggested that the surface layers were in amorphous form or their crystalline degrees were below the detectable limit. Similar conclusions could be reached for SnO2- and SiO2-coated rutile TiO2 particles (see Figure S4d and section 1.7 of the Supporting Information). The XRD patterns could not clearly determine the crystalline phases of SnO2 and SiO2 coating layers. However, some researchers reported that SiO2 coating layers on TiO2 were still amorphous even when calcined at a high temperature of up to 1100 °C.42 Thus, the SiO2 coating layers formed under our present experimental conditions should exist in amorphous phase. In order to study the crystalline phase of SnO2 coating layers, the HRTEM characterization of TiO2/SnO2 core−shell particle was carried out. As shown in Figure 3b, there were lattice fringes with the spacing of 0.235 nm in the core part of the particle, which was consistent with the lattice spacing of anatase (004), indicating that the composition of the core part of the particle was anatase TiO2.52 There were no lattice fringes in the SnO2 coating layer, indicating that the composition of the coating layer was amorphous SnO2. To further investigate the composition of the coating layers, XPS analysis was performed, and the spectra are shown in Figure 4. For TiO2, the O 1s spectra were deconvoluted into three peaks with binding energies (BEs) of 529.7, 531.0 and 532.2 eV (Figure 4a), corresponding to the lattice O2-, surface –OH groups and adsorbed H2O, respectively.53

54

For TiO2/SnO2 core−shell particles

(Figure 4a), the O 1s spectra were deconvoluted into four peaks corresponding to the O2- ions of the TiO2 and SnO2 lattices,55 Sn–OH groups and adsorbed H2O.56 It should be noted that the BE of lattice O2- in TiO2 surface phase shifted from 529.7 to 530.6 eV due to the effect of the higher electronegativity of Sn through the Ti–O–Sn bonds.40 On the other hand, in the O 1s spectra of TiO2/SiO2 core−shell particles (Figure 4a), the peaks at 530.3 and 532.7 eV were associated with the O2- ions in the TiO2 and SiO2 lattices respectively,57 and the peak at 533.3 eV was related to the Si–OH groups and adsorbed H2O.58 Thus, the O 1s peak shifted from 529.7 eV to 530.3 eV due to the formation of Ti–O–Si bond.5 The Ti 2p spectra of TiO2 showed two peaks at 458.5 and 464.2 eV (Figure 4b), corresponding to Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively.59 These BEs positively shifted to 459.5 and 465.2 eV in the spectra of TiO2/SnO2 core−shell particles (Figure 4b), and to 459.0 and 464.7 eV in that of TiO2/SiO2 core−shell particles (Figure 4b), respectively. Hence, both Sn and Si coatings resulted in


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an increase in the BEs of the Ti 2p inner shell electrons. Moreover, the Sn 3d spectra of TiO2/SnO2 core−shell particles showed two peaks related to Sn4+ 3d5/2 and Sn4+ 3d3/2 with BEs of 488.0 and 496.4 eV, respectively (Figure 4c),60-62 indicating that Sn element existed in the form of Sn4+. The Si 2p spectra of TiO2/SiO2 core−shell particles exhibited two peaks with BEs of 102.3 and 103.6 eV (Figure 4d), corresponding to Si 2p of SiO1.19 and SiO2, respectively.63 The XPS spectra of the samples indicated that SnO2 and SiO2 were combined onto the surface of TiO2 via Ti–O–Sn and Ti–O–Si bonds. In addition, only very small amount of Cl remained on the surface of TiO2/SnO2 40 min coating core−shell particles (Figure S3b), and Cl disappeared completely after washing (Figure S3c). As shown in Figure S3d, there was no considerable amount of Cl on the surface of TiO2/SiO2 40 min coating core−shell particles, which was consistent with our previous research.34 These results indicated that most of −Cl was consumed in the reaction process with water in the air. To evaluate the effect of the deposition time on the film composition, the surface elemental contents (Table 1), calculated from the XPS spectra, and the BEs of the photoemission peaks of the samples were compared (Table S4). The surface Sn content of TiO2/SnO2 core−shell particles changed with the variety of deposition time, increasing linearly up to 8.4% in the first 40 min and then slowing down, which suggested that the reaction pattern was heterogeneous and might reach a saturation point. With deposition time increasing, surface Ti was replaced by Sn, and the Ti content decreased from 23.9% to 21.1% in the range of 20−40 min, and to 20.2% from 40 to 60 min. These results were confirmed by the BEs of Ti 2p1/2, Ti 2p3/2, Sn 3d3/2, Sn 3d5/2, and Ti–O (O 1s), which shifted positively with deposition time increasing from 0 to 40 min but did not significantly change from 40 to 60 min, indicating that surface –OH groups might be saturated within the first 40 min. For the sample obtained by two coating cycles, the surface Sn content further increased to 11.9%, and a further 3.3% decrease in Ti was observed (Table 1), suggesting that a greater surface area was covered by the SnO2 film than TiO2/SnO2 1 cycle core−shell particles. Although the surface Sn coverage clearly increased (Table 1), the BEs did not increase significantly after two coating cycles (Table S4), which suggested that the surface chemical composition of TiO2 was almost not affected in the second cycle. These results indicated that the TiO2/SnO2 1 cycle core−shell particle surface was fully covered with hydrated SnO2 (Consistent with Figure S6 and section 1.8 of the Supporting Information). When repeating the coating process, the surface groups



were renewed during the sweeping operation in which vaporous water reacted with Sn−Cl to form Sn–OH groups, which further reacted with SnCl4 to deposit Sn on the surface. It should be noted that, for the same deposition time of 40 min, a greater amount of Si was loaded on TiO2/SiO2 core−shell particles as compared with the amount of Sn deposited on TiO2/SnO2 core−shell particles, with the Si content reaching 24.6% in a single coating cycle and the Ti content decreasing to 5.4%. This indicated that SiO2 coating layer was thicker than SnO2 coating layer, which was consistent with the results of HRTEM images (Figure 2). 3.3. Suppression of the photocatalytic activity The photocatalytic activity was evaluated by the degradation of RhB under UV irradiation, and the results are shown in Figure 5. In the absence of catalyst (blank experiment), RhB degraded slowly under UV irradiation through homogeneous oxidation. With the presence of TiO2, the degradation rate significantly increased, and RhB was almost completely consumed within 15 min. However, when TiO2 was coated with amorphous SnO2 films, RhB decaying rate was much slower, indicating a significant decrease in the photocatalytic activity of TiO2. Notably, this suppression effect was dependent on the deposition time. TiO2/SnO2 40 min coating and TiO2/SnO2 60 min coating core−shell particles gave degradation curves very similar to that of the blank experiment, showing effective suppression of the photocatalytic activity, whereas the degradation reaction was not significantly suppressed by TiO2/SnO2 20 min coating core−shell particles (Figure 5a). These results suggested that a deposition time of 20 min was not enough to form a compact film on the TiO2 surface. Using the first-order model to fit the reaction data, the following relationship between concentration and reaction time was obtained:64 −In = 6

where kapp is the apparent rate constant of the first-order reaction. The kapp constants were estimated from the slope of the lines obtained by plotting the data in equation (6) (Figure 5b), and are listed in Table S5. The results showed that the amorphous SnO2 coating could effectively suppress the photocatalytic activity of TiO2, and the influence of the deposition time was quantitatively presented. The suppression effect of TiO2/SnO2 core−shell particles was compared with that of TiO2/SiO2 core−shell particles prepared using the same deposition time of 40 min, as shown in Figure 5c. The


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kapp constants for TiO2/SnO2 and TiO2/SiO2 core−shell particles with different numbers of coating cycles were estimated and are listed in Table S5. For the single cycle-coated samples, the photocatalytic activity of TiO2/SnO2 core−shell particles was slightly higher than that of TiO2/SiO2 core−shell particles. However, after two coating cycles, the kapp values became similar. According to our previous work34, the photocatalytic activity of TiO2 could only be effectively suppressed with SiO2 film thickness higher than 3nm (4.2±0.8 nm in this experiment). However, the amorphous SnO2 thin film with a thickness of 1.1±0.3 nm could effectively suppress the photocatalytic activity of TiO2. The density of SnO2 (6.95 g/mL) is about 3 times higher than that of SiO2 (2.2 g/mL). Thus, similar mass of amorphous SnO2 and SiO2 films had similar suppression effect on the photocatalytic activity of TiO2. For SnO2- and SiO2-coated rutile TiO2 particles, amorphous SnO2 and SiO2 films could also effectively suppress the photocatalytic ability of rutile TiO2 (see Figure S7 and Table S6 of the Supporting Information). 3.4. Suppression mechanism of the photocatalytic activity by SnO2 and SiO2 coatings Under photoirradiation, TiO2 semiconductor is excited to generate holes and electrons,65 and the recombination of electron-hole pair produces photoluminescence.66 When the photoirradiation is stopped, the excited electrons are quickly quenched by electron-hole recombination, and the time which the excited electrons disappear after irradiation is referred to as PL lifetime. A long PL lifetime indicates a low electron-hole recombination rate, and therefore leads to a high density of excited electrons on the surface. Hence, the PL lifetime is often associated with the photocatalytic activity.67 Time-resolved photoluminescence was performed to study the migration of photo-generated carriers (Figure 6a-b). In order to compare the PL lifetimes of SnO2- and SiO2-coated samples, the PL profiles were normalized to the same maximum amplitude and fitted by an exponential twoparameter model, with amplitudes Ai and time constants τi (i = 1, 2). The regressed parameters are listed in Table 2, and the amplitude-weighted average lifetime, τave, which represents the average PL lifetime, is defined by the following equation:68 =

+ 7 +

where τ1 represents the PL lifetime originated from the direct formation of self-trapped excitons with free electrons and holes (bulk recombination), and τ2 represents the PL lifetime originated from the indirect formation of self-trapped excitons with trapped electrons (surface



recombination).69 For the same deposition time of 40 min, the PL lifetime decreased from 2.85 to 2.13 ns after SiO2 coating, and was further shortened to 1.64 ns from 2.85 ns for the TiO2/SnO2 core−shell particles, indicating that electrons and holes recombined rapidly beneath the amorphous SnO2 and SiO2 layers. The PL lifetime of TiO2/SnO2 core−shell particles was shorter than that of TiO2/SiO2 core−shell particles, which might be due to more defects existed in amorphous SnO2 than that in amorphous SiO2, resulting in larger charge-transfer resistance of amorphous SnO2 coating layers than that of amorphous SiO2 coating layers. In order to determine the surface electron density of the samples, the photocurrent density was measured under the intermittent illumination of a Xenon lamp at a reference electrode bias voltage of 0.24 V (Figure 6c). The photocurrent of TiO2 was clearly detected when the light was on, but rapidly decreased when the light was turned off. The sharp spike/dip in the photocurrent during the on/off illumination cycles indicated that the photo-generated electrons and holes were rapidly transferred from the bulk phase to the surface. On the other hand, the photocurrent density of TiO2/SnO2 core−shell particles was much lower than that of pure TiO2 and was almost zero for the TiO2/SnO2 60 min coating core−shell particles. This phenomenon was attributed to the coating layer preventing the electron migration from bulk phase to surface, which resulted in suppression of the photocatalytic activity. The same tendency was observed on the SiO2 coating samples. The photoelectric properties of particles were further investigated by EIS, and the Nyquist (Figure 6d) were used to analyze the impedance spectra. In the Nyquist plots, the imaginary component usually represents the capacitive parameter, the real component represents the ohmic parameter,70 and the radius of the semicircle is related to the charge transfer resistance.71 The impedance data were fitted to the equivalent circuit (Figure 6d), which included the solution resistance (Rs), charge-transfer resistance (R1) and capacitance (C1). The charge-transfer resistances of TiO2, TiO2/SnO2 and TiO2/SiO2 core−shell particles were 4.15, 34.16 and 18.94 k Ω, respectively. And this confirmed that the amorphous SnO2 and SiO2 protective layers provided a high resistance to the migration of the photo-generated electrons and holes to the TiO2 surface, thus suppressing the photocatalytic activity of TiO2. The charge-transfer resistance of TiO2/SnO2 core−shell particles was larger than that of TiO2/SiO2 core−shell particles, which was consistent with the results of time-resolved photoluminescence (Table 2). On the basis of the above results, the migration and recombination processes of photo-generated electrons and holes were proposed, as shown in Figure 7. Under UV light irradiation, photo-


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generated electrons migrated from the bulk phase to the surface of TiO2 and were absorbed by the amorphous SnO2 layer. Due to the large charge-transfer resistance of the amorphous SnO2 layer, the photo-generated electrons could not migrate to the surface and rapidly recombined with holes. The photo-generated holes follow the same migration/recombination process. 3.5. Pigment properties In order to evaluate the practical applicability of the TiO2/SnO2 core−shell particles, the pigment properties of the samples were measured, and the results are listed in Table S7. Both SnO2 and SiO2 coating layers improved the whiteness but reduced the lightening power of TiO2 pigment (see Table S7 and section 1.9 of the Supporting Information). Moreover, the lightening power of TiO2/SnO2 core−shell particles (1100) was higher than that of TiO2/SiO2 core−shell particles (1030) because of the higher refractive index of SnO2 (2.00) compared to that of SiO2 (1.43).

4. Conclusions The SnO2 protective layers were grown on the surface of TiO2 pigment particles by lowtemperature pulsed CVD method using SnCl4 as precursor. At the deposition time of 40 min, almost all hydroxyl groups on the TiO2 surface reacted with SnCl4, compact and uniform amorphous SnO2 thin film was obtained with a thickness of 1.1±0.3 nm, which possess good photocatalytic activity suppression property and high lightening power. For both SnO2 and SiO2 layers, the suppression of the photocatalytic activity of TiO2 was attributed to the electronic resistance: amorphous SnO2 and SiO2 layers have low electron mobility, which results in quick recombination of photo-generated electrons and holes, thus preventing their migration from the bulk phase to surface, leading to reduction of the photocatalytic activity of TiO2 for redox reactions. The amorphous SnO2 film proves to be an attractive coating material for TiO2 particles due to the significant improvement of the pigment properties.

ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the Key Project of the National Natural Science Foundation of China (No. 21236004).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:



The preparation method of the coated particles, TEM/EDS/XRD/Zeta potential analysis methods and results, analysis of the pigment performance.

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Figures and Tables Table 1. Surface elemental composition (atomic %) of samples determined by XPS analysis Sample

Ti

O

Sn

Si

uncoated TiO2

28.5

71.5

0

0

TiO2/SnO2 20 min Coating

23.9

71.9

4.2

0


21.1

70.6

8.4

0


20.2

70.2

9.1

0

TiO2/SnO2 2 cycles

17.8

70.3

11.9

0

TiO2/SiO2 1 cycle

5.4

70.1

0

24.6

TiO2/SiO2 2 cycles

4.0

67.5

0

28.5

For TiO2/SnO2 and TiO2/SiO2 core−shell particles prepared with different SnCl4 and SiCl4 deposition cycles, the time of each coating is 40 minutes.



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Table 2. Amplitudes Ai and time constants τi obtained by time-resolved photoluminescence

a

Sample

τ1 (ns)

A1

τ2 (ns)

A2

τavea (ns)

uncoated TiO2

0.75

0.48

4.79

0.52

2.85


0.60

0.59

3.59

0.41

1.83


0.57

0.62

3.41

0.38

1.64


0.55

0.62

3.27

0.38

1.58

TiO2/SnO2 2 cycles

0.50

0.61

2.95

0.39

1.46

TiO2/SiO2 1 cycle

0.62

0.52

3.76

0.48

2.13

TiO2/SiO2 2 cycles

0.69

0.57

3.84

0.43

2.04

average PL lifetime of photo-generated carriers.


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Figure 1. Schematic diagram of the reactor (a), and growth of the SnO2 layers (b-e)



Figure 2. HRTEM images of uncoated TiO2 particles (a); TiO2/SnO2 core−shell particles prepared with different SnCl4 deposition time of 20 min (b), 40 min (c), 60 min (d), and two coating cycles (e); TiO2/SiO2 core−shell particles with one coating cycle (f). EDS line scan across the TiO2/SnO2 core−shell particle surface layer obtained after two coating cycles (40 min coating for each cycle) (g) and the corresponding elemental distribution profiles (h).


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Figure 3. XRD pattern of the samples obtained with the deposition time of 40 min and two coating cycles (a). HRTEM images of TiO2/SnO2 core−shell particles obtained with the deposition time of 40 min and one coating cycle (b).



Figure 4. XPS spectra of O 1s (a), Ti 2p (b), Sn 3d (c), and Si 2p (d) for TiO2, TiO2/SnO2 and TiO2/SiO2 core−shell particles obtained with the deposition time of 40 min and one coating cycle.


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Figure 5. Photocatalytic degradation of RhB under 300 W UV light irradiation using TiO2/SnO2 core−shell particles prepared with different SnCl4 deposition time (a) and the corresponding kinetic plots (b). Photocatalytic degradation of RhB under 300 W UV light irradiation using TiO2/SnO2 and TiO2/SiO2 core−shell particles prepared with different SnCl4 deposition cycles (40 min coating for each cycle) (c) and the corresponding kinetic constants (d).



Figure 6. Time-resolved photoluminescence of TiO2 and TiO2/SnO2 core−shell particles prepared with different SnCl4 deposition time (a), and TiO2/SnO2 and TiO2/SiO2 core−shell particles prepared with different SnCl4 deposition cycles (40 min coating for each cycle) (b). Photocurrent density of TiO2, TiO2/SnO2 and TiO2/SiO2 core−shell particles (c). Nyquist plots and the equivalent circuit of the samples obtained with the deposition time of 40 min and two coating cycles (d).


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Figure 7. Migration and recombination processes of photo-generated electrons and holes.



Abstract Graphics


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Suppression of TiO2 Photocatalytic Activity by Low ... - ACS Publications

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