Fabrication of multilayer porous structured TiO2–ZrTiO4–SiO2 heterostructure towards enhanced photo-degradation activities

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Fabrication of multilayer porous structured TiO2–ZrTiO4–SiO2 heterostructure towards enhanced photo-degradation activities Changqing Liu∗, Xu Li, Yuanting Wu, Luyue Zhang, Xiaojing Chang, Xiaoxiao Yuan, Xiufeng Wang School of Material Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science & Technology, Xi'an, 710021, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Sol-gel process Interface Optical properties TiO2–ZrTiO4–SiO2 Functional application

Multilayer porous structured TiO2–ZrTiO4–SiO2 photocatalyst with built-in TiO2/ZrTiO4 heterojunction and oxygen vacancies was synthesized by sol-gel method combined with template method using colloidal polystyrene spheres as templates. Results show that the multilayer porous structure can be fabricated by controlling the calcination system and the amount of template, and the fabrication of which can also contribute to the generation of oxygen vacancies by creating an anoxic environment. During the photodegradation process, high efficiency of visible light utilization can be achieved due to the slow photon effect of the multilayer porous structure, which can also increase the probability of the contact between Rhodamine B (RhB) with the active sites of the catalyst. Also, the synergistic effect of the generated TiO2/ZrTiO4 heterojunction and built-in oxygen vacancy defects jointly promoted the separation of photogenerated carriers. Thus excellent adsorption rate (75.6%, 60 min) and photodegradation rate (95%, 90 min) of the catalyst were obtained. Furthermore, the upshifted conduction band and valence band maximum positions are beneficial for the mobility of photogenerated holes and inhibit their reaction with H2O to generate ·OH, while the photogenerated electrons can react with O2 to form ·O2−, resulting that the holes and ·O2− participated in the photodegradation of RhB over the as-prepared catalyst.

1. Introduction Semiconductor photocatalysis technology has become one of the most effective methods to solve the environmental and energy-related problems, due to its cost-effective, clean and environmental protecting properties [1–6]. Numerous studies have shown that the main factors affecting the photocatalytic performance could be summarized into two categories: efficiencies of light-harvesting and utilization, and the separation rate of photogenerated carriers [7,8]. In recent years, development of a photocatalyst with efficient visible light response, high carrier mobility and separation rate has attracted great attentions. TiO2 as one of the most promising semiconductor photocatalysts has been widely applied in much areas, such as organic pollutants degradation, carbon dioxide reduction, photocatalytic water splitting and so on, because of its chemical stability, nontoxicity and efficient utilization of solar energy [9–11]. However, further application of TiO2 is limited because its poor visible light absorption and high recombination rate of photogenerated carriers [12,13]. Till date, enormous strategies, such as morphology and structure control [14], surface sensitization [15], ion ∗

doping [16] and so on, have been expanded to modify its intrinsic insufficient by improving its light-harvesting and photogenerated carriers separation efficiency. Among various methods, constructing a heterostructure was proved to be an effective way to improve the photocatalytic performance of TiO2, in which the utilizing of visible light and the separation rate of photogenerated electrons and holes can be effectively enhanced [17–19]. Besides, the physicochemical properties of TiO2, including its optical behavior, electronic structure, dissociative adsorption properties and catalytic performance, can be significantly influenced by the defect states in the materials. Especially, oxygen vacancy defects in the material can not only change the material's electronic structure, but also can work as adsorption sites due to its abundant localized electron, thus resulting in excellent photocatalytic performance [20,21]. Furthermore, increasing the specific surface area is also an useful approach to increase the contact area of the dye with improved degradation efficiency [22]. Therefore, inert SiO2 working as the catalyst support has been widely used because of its large specific surface area, transparency to light, and its ability to stable the crystal phase and restrain the grain

Corresponding author. E-mail address: [email protected] (C. Liu).

https://doi.org/10.1016/j.ceramint.2019.08.285 Received 17 July 2019; Received in revised form 29 August 2019; Accepted 30 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Changqing Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.285

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respectively, were applied. Based on the analysis of the TG-DSC curves of the obtained precursor in air (Fig. S1), the 600 °C was chosen to pyrolysis the PS template, and the 800 °C here was to crystallize the ceramic phases. Moreover, various amounts of PS sphere suspension were used to prepare the hybrid sol, which was 0, 5, 10, 15 and 20 mL, respectively. For the convenience of discussion, the prepared catalyst with optimized multilayer porous structure was labeled as MPC (multilayer porous structured catalyst).

growth of TiO2 catalyst [23]. Photonic crystal (PC) is the artificial periodic dielectric structures with photonic band gap characteristics [24–26]. It has attracted great attention since it was discovered in 1987. Since the scattering effect, intrinsic photonic band gap and slow photon effect of PC structure, fabrication of photonic crystal materials has become one of the most powerful approaches to improve photocatalytic activity [27]. Specifically, as the typical photonic crystal skeleton, three-dimensionally ordered macroporous (3DOM) structure exhibiting strong light harvesting effect was extensively applied for improving the photocatalytic efficiencies [28]. Therefore, it is hopeful to enhance the capability of photo-catalyst by combining the heterostructure catalyst with the strong light harvesting effect of 3DOM structure. In this work, multilayer porous structured TiO2–ZrTiO4–SiO2 heterostructure with built-in oxygen vacancies exhibiting excellent adsorption capability and photocatalysis degradation ability was fabricated. This specific multilayer porous structure possessed strong lightharvesting ability due to its scattering effect and slow photon effect, which was similar to that of PC structure. On the other hand, strong capacity of adsorption was obtained for the prepared catalyst, which can not be achieved for PC structured materials. Specifically, structure control of the multilayer porous structure, and mechanism of the adsorption and photo-degradation of the prepared TiO2–ZrTiO4–SiO2 catalyst were investigated and discussion in detail.

2.3. Characterization of photocatalysts The powder X-ray diffraction patterns were obtained by X-ray diffractometer (XRD, Rigaku D/max2200PC), using Cu Kα radiation (λ = 1.541 Å) over a 2θ ranging from 10° to 70°. Morphologies and structures of the samples were determined by scanning electron microscopy (SEM, FEI Verios 460), as well as transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN) equipped with energydispersive X-ray spectroscopy (EDS). Textural property of the sample was investigated on a BET surface analyzer (ASAP2460) by N2 adsorption performed at −196 °C. Composition and chemical state of the sample were analyzed by X-ray photoelectron spectrometer (XPS, AXIS SUPRA) with Al Kα X-ray source (10 mA, 15 kV). Photo-absorption of the samples was analyzed by the UV–vis diffuse reflectance spectra (DRS) recorded on a Cary 5000 spectrophotometer equipped with an integrating sphere. Moreover, the photocatalytic activities and the active species trapping experiments were analyzed by a UV–Vis spectrophotometer (UV2800-A, UNICO).

2. Materials and methods 2.1. Materials and reagents

2.4. Photodegradation procedure All the reagents used in the experiment, including Zirconyl chloride octahydrate (ZrOCl2·8H2O, AR), Tetraethyl orthosilicate (TEOS, AR), Titanium sulfate (Ti(SO4)2, CP), Citric acid monohydrate (C6H8O7·H2O, AR), Polythylene glycol 600 (PEG-600, CP), Styrene (C8H8, CP), Methacrylic acid (C4H6O2, AR), Potassium peroxydisulfate (K2S2O8, AR) and Rhodamine B (RhB) were purchased from Sinopharm Chemical Reagent and were used without further purification.

Photocatalytic ability of the prepared MPC was investigated by degradation of RhB under visible light irradiation (300 W Xe lamp). Typically, 30 mg photocatalyst was suspended in 30 mL RhB (10 mg/L) solution. Before irradiation, the solution was kept in the dark under constantly stirring to ensure adsorption-desorption equilibrium. Then the residue RhB was monitored by UV–Vis spectrophotometer using a wavelength of 554 nm. Moreover, scavenging experiments were carried out to understand the mechanism during the RhB dye degradation, in which Isopropyl alcohol (IPA), Triethanolamine (TEOA), and Benzoquinone (BQ) were used as scavengers. When the photocatalytic degradation experiment was finished, the catalyst was washed thoroughly and centrifugated for three times using deionized water as solvent, then collected and dried overnight at 60 °C before recycled. The collected catalyst was used for another two cycles for RhB degradation under the same condition.

2.2. Preparation of photocatalysts The samples were prepared through a simple sol-gel method combined with the template method using colloidal polystyrene (PS) spheres as templates, which can be removed by heat-treated at temperatures higher than 600 °C. The PS suspension was synthesized using a typical emulsion polymerization process using 6 mL Styrene, 0.2 mL C4H6O2, 0.14 g K2S2O8 and 100 mL deionized water as raw material and heated at 75 °C under oil bath agitation for 8 h. Besides, zirconium containing precursor (Zr-sol) was prepared using ZrOCl2·8H2O (0.001 M) as zirconium source, C6H8O7·H2O (4 g) as the complexing agent, deionized water (20 mL) and ethanol (20 mL) as the mixed solvent, PEG-600 (0.1 mL) as dispersant under constantly stirring for 1 h. Meanwhile, Si-sol (0.06 M) and Ti-sol (0.05 M) were obtained under similar procedure by changing the zirconium source into silicon source and titanium source, respectively. Typically, the prepared Si-sol was added into various amounts of PS sphere suspension under continuous magnetic stirring, followed by the adding of Zr-sol and Ti-sol to obtain hybrid sol precursor, and the molar ration of Si: Zr: Ti was set as 6:1:5. After being fully dried at 60 °C and calcined in a muffle furnace, the aimed powder was obtained and saved for the following analysis. To obtain the multilayer porous structured materials, the key procedure that the self-assemble of PS spheres in the preparation of photonic crystal structured materials to prepare ordered and periodic porous structure was omitted in our work deliberately. Furthermore, in order to achieve the structure control and analysis the formation mechanism of the multilayer porous structure, various calcination systems i.e. T1 (600 °C-0 h + 800 °C-2 h), T2 (600 °C-2 h + 800 °C-2 h), T3 (600 °C-3 h + 800 °C-2 h) and T4 (600 °C-4 h + 800 °C-2 h),

2.5. Data evaluation Adsorption kinetics were investigated by analyzing 30 mL RhB solution (known initial concentration) with 30 mg of adsorbent for different time intervals performed at 25 °C, at pH of 7 ± 0.3, and at 120 rpm. The adsorption amount (qe, mg/g) at equilibrium time was obtained by:

qe =

(C0 − Ce ) ∗ V w

(1)

The adsorption amount (qt, mg/g) at time t was calculated by the following equation:

qt =

(C0 − Ct ) ∗ V w

(2)

Where, C0, Ce and Ct represent the initial, equilibrium dye concentrations and dye concentrations at any time t (mg/L), respectively. V (L) is the solution volume, and w (g) is the mass of catalyst used in the adsorption experiment. Langmuir adsorption isotherm model considers that the adsorption 2

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600 were used in our experiment, working as template agent, the crosslinking reagent and dispersant, respectively. As is known, the Ti4+, Zr4+ and Si4+ ions are readily hydrolyzed in the presence of water, and then crosslinked with citric acid and PEG-600, thus dispersed evenly in the hybrid polymer network [31]. To obtain the aimed structured catalyst, controlling the procedure of calcination is another key step, during which the TiO2–ZrTiO4–SiO2 ceramic phases can be crystallized accompanied by the formation of the porous structure by removing PS spheres. To clarify the effect of calcination process on the microstructure evolution of MPC, the prepared gel was heat-treated at various calcination systems. SEM images of the resulted samples (Fig. S2) and simulation diagram of the microstructure evolution are presented in Fig. 2. As for T1, because of the short calcination time, pyrolysis of the precursor and removal of the residue carbon were incomplete, with the increasing of temperature, the generated glassy SiO2 covered the surface of the material showing a caking morphology without obvious pores. By extending the holding time to T2, multilayer porous structure with uniformly distributed pores can be obtained. And also, granular morphology with even particle size can be easily noted. However, pores are interconnected with each other, and the multilayer porous structure begins to collapse (T3). Moreover, the skeleton network collapsed completely into accumulated particles when the holding time was increased to 4 h (T4). To further put light on the formation process of the multilayer porous structure, different amounts of template were used and the SEM images (Fig. S3) and simulation diagram of resulted samples are shown in Fig. 3. As can be observed, the adding of PS template exhibited significant effect on the microstructure of the resulted samples. Without the addition of template, the obtained powder composed of irregular particles with large particle size. By adding template, the particle size was refined. While, the powder shows the morphology of particle accumulation, indicating the porous structure cannot be formed with low template amounts (PS, 5 mL and 10 mL). And it was profitable to increase the template content for constructing the multilayer porous structure, as the aimed structure was observed in the sample adding 15 mL template agent. However, high template content (PS, 20 mL) showed negative effect on the aimed structure, since the multilayer pores were destroyed by the interconnection between the adjacent pores.

sites are homogenous within the adsorbent, mainly applying for monolayer adsorption activities. The equation of this model is expressed as follows [29]:

qe =

qm K a Ce 1 + K a Ce

(3)

Freundlich adsorption isotherm model assumes that the adsorption surface of the material is heterogeneous with unequal available sites. The Freundlich adsorption isotherm model is presented as [29]: 1

qe = Kf Cen

(4)

Where, qe, qm and Ce represent the amount of adsorbed dye at equilibrium time, the maximum adsorption capacity, and the equilibrium dye concentration (mg/g), respectively. Ka (L/mg) is the isotherm constant for the Langmuir adsorption isotherm model. Kf (mg/g(L/mg)n) is the adsorption capacity, n is the adsorption constant representing the adsorption intensity. The pseudo-first-order rate expression is described as [30]:

qt = qe [1 − exp (Kf t )]

(5)

The pseudo-second-order equilibrium is given as [30]:

qt =

Ks qe2 t 1 + Ks qe t

(6)

Where, qt (mg/g) represents the adsorbed dye amount at time t, qe (mg/ g) is the maximum adsorption capacity, Kf (min−1) and Ks (min−1) is the pseudo-first-order and pseudo-second-order rate constant, respectively, and t (min) is the contact time. 3. Results and discussion 3.1. Fabrication of MPC The procedure to synthesize the aimed structured material mainly includes two steps: in-situ polymerization of complex to prepare precursor gel with uniformly distributed PS spheres and the calcination process to generate the multilayer porous structured TiO2–ZrTiO4–SiO2 photocatalyst by removing the template, as is shown in Fig. 1. As for the preparation of the precursor, PS sphere suspension, citric acid and PEG-

Fig. 1. Simulation diagram of the preparation process of MPC. 3

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Fig. 2. SEM images and simulation diagram of the samples obtained at different calcination systems.

template, in which the multilayer porous structure was constructed. As for the performance of adsorption, the capability was reduced after the first increase in the trend for the samples prepared at different calcination systems, which was also fit for the obtained samples using different amounts of template. Therefore, enhanced adsorption capacity and photo-degradation ability were achieved for the optimized porous structured sample, i.e. 75.6% of adsorption during the dark reaction and 95.6% of degradation after visible light irradiation for 150 min. Combined with the microstructure analysis above, the decreased adsorption capability was due to the unformed or destroyed multilayer

To verify the effect of the multilayer porous structure on the properties of the catalyst, photocatalytic performance of the obtained samples were studied by degradation of RhB under visible light irradiation. The photodegradation kinetic curves for samples obtained at different calcination systems and with various amounts of PS templates are exhibited in Fig. 4a and c, respectively. The corresponding histograms are presented in Fig. 4b and d, respectively. With the increasing of holding time and the amount of template, the degradation rate for the as-prepared samples increased first and then decreased, and the best performance can be observed for sample heat-treated at T2 using 15 mL of

Fig. 3. SEM images and simulation diagram for the samples prepared using different amounts of template. 4

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Fig. 4. Photocatalytic degradation kinetic curves (a, c) and the corresponding histogram (b, d) of the samples prepared using different calcination systems and different amounts of template.

[33]. In the case of as-prepared MPC, because of the uneven surface, it is unable to form the continuous liquid film. RhB solution can infiltrate into the inner of pores, thus contributes to the improvement of adsorption capability and photocatalytic activity by enhancing the effective areas of catalysis reaction.

porous structure. What's more, the significantly decreased capability of adsorption was caused by the complete collapse of the multilayer porous structure. Based on the above analysis, the prepared MPC possess excellent adsorption and degradation ability. However, the strong capacity of adsorption observed in the prepared MPC cannot be achieved for PC structured TZS with three-dimensionally ordered macroporous structure (Fig. S4), which is consistent with other reports [28]. The reason can be explained as below, shown in Fig. 5. For PC structured TZS that possess highly ordered periodic porous structure and smooth surface [32], due to the existence of surface tension, when the RhB solution flowed over, a covered liquid film would be formed at the surface, which prevented the solution from infiltrating into the inner of pores

3.2. Adsorption mechanism of MPC N2 adsorption-desorption study was conducted to investigate the BET surface area and pore size distribution of the as-prepared MPC, as shown in Fig. 6a. Based on the Brunauer-Deming-Teller classification, the obtained isotherm belongs to the type IV, and the hysteresis loops belong to H3 type, indicating a hierarchical porous structure of as-

Fig. 5. The proposed adsorption process of MPC compared with PC structured TZS. 5

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Fig. 6. (a) N2 adsorption/desorption isotherm plots and pore size distribution pattern, (b) adsorption isotherm models fitted to experimental adsorption of RhB, and (c) adsorption rate curves of MPC, (d) photocatalytic degradation kinetic curves and (e) the corresponding histogram of as-prepared MPC and P25, (f) repetitive experiment for as-prepared MPC, (g) the degradation evolution. Table 1 Parameters for different isotherm models for the adsorption study of RhB onto as-prepared MPC. Sample

MPC

qe (mg/g)

10.5756

Langmuir Model

Freundlich Model

qm (mg/g)

Ka (L/mg)

R2

n

KF mg/g(L/mg)n

R2

11.90358

0.10305

0.99538

0.21515

0.8081

0.98567

6

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Table 2 Pseudo-first-order model and Pseudo-second-order model constants and correlation coefficients for RhB adsorption onto as-prepared MPC. Sample

MPC

qe (mg/g)

10.5756

Pseudo first-order model

Pseudo second-order model

qe (mg/g)

Kf (min

10.2762

0.1148

−1

)

R

2

0.999

qe (mg/g)

Kf (min−1)

R2

11.3158

0.0151

0.994

Fig. 7. (a) XRD pattern, (b) Raman spectrum, (c) UV–Vis diffuse reflection spectra and plots of (αhν)1/2 versus energy, (d) XPS valence-band spectrum for as-prepared MPC.

adsorption isotherm model due to the higher correlation coefficient values (R2 > 0.995), indicating a homogeneous distribution of active sites within the catalyst. The maximum capacity at equilibrium (qm) calculated from the Langmuir model is 11.90 mg/g, slightly higher than the experimental data showing a high consistence. The adsorption kinetics were studied and concluded to determine the adsorption rate of the catalyst. Fig. 6c displays the variation of the adsorbed amount (qt) as a function of time. The result of fitting is listed in Table 2. In Fig. 6c, it can be seen that the initial adsorption is quite rapid due to the available uncovered surface of the catalyst, the rate of adsorption becomes slower and finally reaches a constant value at equilibrium time. Further, the adsorption process of the catalyst could be fitted by the Pseudo first-order kinetics model since the higher correlation coefficient values (R2 > 0.999). And also, the q value calculated from the model was in high consistent with the experimental q value.

Fig. 8. The proposed mechanism of slow photon effect for as-prepared MPC.

prepared MPC [34,35], which confirms the multilayer porous structure of the catalyst. Besides, the sample shows a wide pore size distribution of 10–100 nm. Further, the BET and average pore diameter of the MPC are 121.41 m2/g and 16.3 nm, respectively. The surface properties and the adsorption capacity of the prepared catalyst were revealed by fitting the equilibrium data with Langmuir and Freundlich models. A comparison of the fitted curves with the experimental data is shown in Fig. 6b. The fitting result is listed in Table 1. The result indicates that all the experimental result showed better compliance with the Langmuir

3.3. Photocatalytic performance of MPC In order to test the photocatalytic performance of as-prepared MPC, RhB was chosen as the target pollutants. The photocatalytic reactions in the presence of commercial P25, MPC and no catalyst under the same conditions were performed, as well as the blank reactions carried out in the dark. The results are presented in Fig. 6d and e. No obvious adsorption and degradation of RhB can be observed for P25 in given experimental conditions. The system without catalyst or visible-light 7

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the RhB molecules were adsorbed onto MPC. After visible light irradiated for 60 min, the adsorbed RhB molecules were decomposed, followed by the residue RhB in the solution moving onto the MPC surface (Fig. 6g–3). All the RhB were removed from the system after 90 min visible-light irradiation (Fig. 6g–4). 3.4. Photodegradation mechanism of the MPC XRD patterns of the synthesized MPC are shown in Fig. 7a, and all the other XRD patterns of the prepared samples using various procedures can be found in Fig. S5. In Fig. 7a, the diffraction peaks assigned to anatase TiO2 (PDF 21–1272) and ZrTiO4 (PDF 34–0415), as well as the wide peak ascribing to amorphous SiO2 can be detected [36], indicating the composition of the prepared catalysts were anatase TiO2, ZrTiO4 and SiO2, which also fit for the other samples in Fig. S5. This result shows that fabrication of the multilayer porous structure has no obvious effect on the composition of the catalyst. As is known, 6-modes (A1g + 2B1g + 3Eg) and 3-modes (A1u + 2Eu) can be detected for anatase TiO2, among which the former are Raman active and the latter are IR active [37,38]. Accordingly, Raman spectrum of the Eg modes appear at 639, 197 and 144 cm−1, the B1g mode occurs at 399 cm−1, and the doublet of the A1g and B1g mode is at 516 cm−1, respectively [39]. As for the Raman spectra of as-prepared MPC (Fig. 7b), all the peaks appeared at 144, 196, 394, 516 and 638 cm−1, respectively, indicating the pure anatase structure, which is in complete agreement with the XRD studies. UV–Vis diffuse reflection spectra of the catalyst are shown in Fig. 7c. It reveals that MPC exhibited fairly strong photoadsorption for UV light and near-visible light region but a weak photoabsorption for visible-light [40]. Considering the weak visible light absorption for as-prepared MPC and the unexpected enhanced visible light photocatalysis ability, the multilayer porous structure may play a role in improving the visible light utilization, which can be explained as the slow photonic effect, as is shown in Fig. 8. When the light enters a multilayer porous structure, the photons propagate through the material by reflection and refraction, and the group velocity is extensively reduced, which will cause a delay and residence of light inside the material, which can effectively improve the efficiency of the light utilization. Moreover, the calculated band gap energy of the sample was 2.54 eV (shown in the insert figure of Fig. 7c), which was estimated through the equation αhν = A(hν-Eg)n⁄2 (n = 1), where, α is the absorption coefficient, hν is the photon energy, and A is a constant related to the material [41]. The other DRS spectra and calculated band energies of the prepared samples using various procedures can be found in Fig. S6, which no obvious difference in the visible light absorption and band gap energies can be observed. This result indicates that the optimized multilayer porous structure shows little effect on improving the range of visible light absorption, but can enhance the efficiency of visible light utilization (Fig. 8). Besides, XPS valence-band spectrum was recorded for as-prepared MPC to estimate the position of VB, and the result was displayed in Fig. 7d. The obtained VB value for MPC was +1.55 eV, considering a corrected value of a standard hydrogen electrode (0.63 eV, pH = 7) [42]. Based on the equation ECB = EVB - Eg, the calculated CB maximum was −0.99 eV. Also, Eg (TiO2) was 2.60 eV and Eg (ZrTiO4) was 2.72 eV as were calculated from the DRS of the pure phase prepared under the same condition. The related experimental data can be found in the supplement materials (Fig. S7). Furthermore, according to equations ECB = X - EC - Eg/2 and EVB = ECB + Eg [43], the calculated VB and CB potentials of monophase TiO2 and ZrTiO4 were 2.712 and 0.012 eV, 2.678 and −0.042 eV, respectively. Obviously, the up-shifted VB maximum (VBM) position of obtained MPC (1.55 eV) was observed compared to that of the pure TiO2 (2.712 eV) and ZrTiO4 (2.678 eV), implying higher mobility of the photo-generated holes [44]. What's more, the upshifted VB and CB edges of as-prepared MPC indicate the enhanced reduction ability of the conduction electrons to react with oxygen molecules to generate more superoxide radicals, thus

Fig. 9. (a) TEM image, and (b) HRTEM image of as-prepared MPC, (c) the atomic model of the lattice interface between ZrTiO4 and TiO2, (d) EDS elements spectra of as-prepared MPC, (e–h) EDS elemental mapping images of O, Si, Ti, Zr.

irradiation shows no obvious degradation of RhB. The as-prepared MPC shows both the excellent adsorption and degradation capability of RhB, i.e., 75.6% of was adsorbed (dark reaction, 30 min), and 95.6% of RhB was decomposed (visible light irradiation, 90 min). Compared with P25, the performance of the MPC was quite remarkable, nearly six times of photodegradation and adsorption efficiency than P25 at identical conditions. Fig. 6f shows the cycling photodegradation curves for as-prepared MPC. As can be seen, there is no obvious reduction observed in the photodegradation of RhB after three experimental runs, indicating the stability and recyclability of as-prepared MPC is superior. Fig. 6g shows the color changes of the solution in the degradation of RhB. Fig. 6g–1 shows the moment when MPC was added, and after stirring in the dark place for 60 min (Fig. 6g–2), during which most of 8

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Fig. 10. XPS spectra of as-prepared MPC: (a) wide spectra, (b) fitting spectra of O1s, (c) fitting spectra Ti2p, (d) fitting spectra of Zr3d.

Fig. 11. Illustration of the roles of the oxygen vacancy introduced in TiO2–ZrTiO4 heterostructure and the mechanism of improving photocatalytic.

Fig. 12. The proposed mechanism for photo-degradation of RhB over as-prepared MPC.

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maximum position of the MPC was up-shifted to −0.99 eV, considering that the reduction potential of O2 E◦ (O2/•O2−−) is −0.33 eV, the photogenerated electron in MPC can be trapped to produce •O2−−. Therefore, holes and •O2−− worked as the major active sites during the photocatalytic degradation of RhB over the as-prepared MPC, which is in good agree with the free radical capture experiment result (Fig. S8).

enhance the separation of photo-generated charge carriers and improve the photocatalytic activity [45,46]. TEM analysis was performed to determine the interfacial structure of MPC and the results are shown in Fig. 9. As is verified in Fig. 9a, the multilayer porous structure can be observed for as-prepared MPC. The EDS spectrum (Fig. 9d) confirms the composition of MPC was O, Si, Ti and Zr elements, and elemental mapping data in Fig. 9(e-h) shows the uniform distribution of the ceramic phases, indicating the generation of TiO2–ZrTiO4 heterojunction. The lattice spacing distance of 0.348 nm and 0.293 nm depicted in the HRTEM images are shown in Fig. 9b, which are in good agreement with the (101) lattice plane of the anatase TiO2 and the (110) spacing of ZrTiO4, respectively. Besides, the lattice interface between TiO2 and ZrTiO4 and the amorphous SiO2 can be easily noted, verifying the formation of TiO2–ZrTiO4 heterojunction. To be clear, the atomic model of TiO2–ZrTiO4 heterojunction is described in Fig. 9c. XPS analysis of the prepared MPC is shown in Fig. 10. In Fig. 10a, the XPS survey spectra show the material are composed of Ti (3.16%), Zr (0.85%), Si (17.65%), C (32.24%) and O (46.09%), indicating a carbon-rich composition of MPC. In the pyrolysis process of the precursor with added PS spheres, due to the increasing oxygen consumption, the low oxygen concentration leaded to the high amount of residue carbon and oxygen vacancy defects in obtained MPC. Three oxygen states positioned at 529.6 eV, 531.9 eV and 532.5 eV were observed in the fitting spectra of O1s (Fig. 10b), which are assigned to the oxygen within the bulk of TiO2 crystal lattice [47], the oxygen vacancies [48], and the carbonate group, respectively [49]. This result verifies the existence of oxygen vacancy defects in the prepared MPC, which play an important role in restricting the recombination of photogenerated charge carriers, since the excited electrons in CB are easily trapped by these oxygen vacancies [20]. Therefore, it can be concluded that fabrication of the multilayer porous structure contributed to the generation of the internal oxygen vacancy defects by creating the anoxic environment. Furthermore, as the resulted reduced recombination of charge carriers, the electrons can easily transfer from the adsorbed O2 on the surface of MPC to O2 working as the reactive species, as is illustrated in Fig. 11. As for the XPS fitting spectra of Ti2p in Fig. 10c, only two peaks positioned at around 458.5 eV (Ti2p3/2) and 464.1 eV (Ti2p1/2) are fitted, further confirming that the Ti element is present as Ti4+ state and the TiO2 possess an anatase structure [50]. Fig. 10d shows the Zr3d XPS fitting spectra, the peaks at the binding energies of 181.6, 182.1, 184.2 and 185.1 eV belong to Zr–O (Zr3d5/2), Zr–O–Ti (Zr3d5/2), Zr–O (Zr3d3/2) and Zr–O–Ti (Zr3d3/2), respectively, confirming the formation of ZrTiO4 and the formation of TiO2/ZrTiO4 interface [51,52]. Fig. 12 describes the proposed mechanism for photo-degradation of RhB over as-prepared MPC. In particular, the enhanced photodegradation activity can be ascribed to the high efficiency in the utilizing of visible light, which is achieved due to the slow photon effect of the multilayer porous structure. The multilayer porous structure with high specific surface area can improve the probability of the contact between RhB with the active sites of the MPC. Furthermore, the synergistic effect of the generated TiO2/ZrTiO4 heterojunction and builtin oxygen vacancy defect jointly promote the separation of photogenerated carriers. The role of the oxygen vacancy in restraining the recombination rate of excited electrons and holes has been discussed in the above section. As for the TiO2/ZrTiO4 heterojunction, the detailed mechanism is shown in Fig. 12. Since the CB position of ZrTiO4 is more negative, and its VB position is more positive than that of TiO2, the photogenerated electron on the CB of ZrTiO4 can transfer to that of TiO2, and the holes on the VB of TiO2 will transfer to the VB of ZrTiO4, thus restrain the recombination of photogenerated charge carriers. Furthermore, the VB maximum position of the MPC was up-shifted to 1.55 eV, which is beneficial for the mobility of photo-generated holes and also inhibits their reaction with H2O to generate ·OH, according to the oxidation potential of E (•OH/H2O) (2.8 eV). Besides, since the CB

4. Conclusions Multilayer porous structured TiO2–ZrTiO4–SiO2 heterostructure catalyst was prepared through simple sol-gel using PS spheres as templates by calcination at optimized heating system (600 °C - 2h + 800 °C - 2h) using 15 mL PS sphere solution. As for the prepared MPC, the upshifted VB and CB values were observed, while TiO2/ZrTiO4 heterojunction and oxygen vacancies were formed in as-prepared MPC. The obtained MPC exhibited outstanding adsorption, photodegradation ability and recyclability. During the photodegradation process, the high efficiency in the utilizing of visible light can be achieved due to the slow photon effect of the multilayer porous structure. The multilayer porous structure can increase the probability of the contact between RhB with the active sites of the MPC. Furthermore, the synergistic effect of the generated TiO2/ZrTiO4 heterojunction, built-in oxygen vacancy defects and the up-shifted CB and VB maximum positions jointly promote the separation of photogenerated carriers and the generation of •O2−− active species, thus improving the photocatalytic activities. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers 51702194, 51302161); the Natural Science Foundation of Shaanxi Province (grant numbers 2018JQ5055, 2018GY-106); and the Research Startup Foundation of Shaanxi University of Science and Technology (grant number 2016GBJ09). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.08.285. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] J.W. Fu, Q.L. Xu, J.X. Low, C.J. Jiang, J.G. Yu, Ultrathin 2D/2D WO3/g-C3N4 stepscheme H2-production photocatalyst, Appl. Catal. B Environ. 243 (2019) 556–565. [3] Y.W. Feng, L.L. Ling, J.H. Nie, K. Han, X.Y. Chen, Z.F. Bian, H.X. Li, Z.L. Wang, Selfpowered electrostatic filter with enhanced photocatalytic degradation of formaldehyde based on built-in triboelectric nanogenerators, ACS Nano 11 (2017) 12411–12418. [4] M. Nasr, C. Eid, R. Habchi, P. Miele, M. Bechelany, Recent progress on titanium dioxide nanomaterials for photocatalytic applications, ChemSusChem 11 (2018) 3023–3047. [5] Y.W. Feng, H. Li, L.L. Ling, S. Yan, D.L. Pan, H. Ge, H.X. Li, Z.F. Bian, Enhanced photocatalytic degradation performance by fluid-induced piezoelectric field, Environ. Sci. Technol. 52 (2018) 7842–7848. [6] Y.J. Yuan, Z.J. Ye, H.W. Lu, B. Hu, Y.H. Li, D.Q. Chen, J.S. Zhong, Z.T. Yu, Z.G. Zou, Constructing anatase TiO2 nanosheets with exposed (001) facets/layered MoS2 twodimensional nanojunctions for enhanced solar hydrogen generation, ACS Catal. 6 (2016) 532–541. [7] X. Zhang, X. Li, D. Zhang, N.Q. Su, W. Yang, H.O. Everitt, J. Liu, Product selectivity in plasmonic photocatalysis for carbon dioxide hydrogenation, Nat. Commun. 8 (2017) 14542. [8] X. Guo, Y.B. Chen, Z.X. Qin, J.Z. Su, L.J. Guo, Facet-selective growth of CdS nanorods on ZnO microrods: intergrowth effect for improved photocatalytic performance, ChemCatChem 10 (2018) 153–158. [9] L. Irina, K. Marcela, J.R.M. Juan, M.A. Javier, A.M. Manuel, Antimicrobial activity of printed composite TiO2/SiO2 and TiO2/SiO2/Au thin films under UVA-LED and natural solar radiation, Appl. Catal. B Environ. 239 (2018) 609–618. [10] H. Yu, Y. Zhao, C. Zhou, L. Shang, Y. Peng, Y. Cao, L.Z. Wu, C.H. Tung, T. Zhang, Carbon quantum dots/TiO2 composites for efficient photocatalytic hydrogen evolution, J. Mater. Chem. A. 2 (2014) 3344–3351.

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