Fabrication of porous TiO2 nanosheets assembly for improved photoreactivity towards X3B dye degradation and NO oxidation

Journal Pre-proofs Full Length Article Fabrication of porous TiO2 nanosheets assembly for improved photoreactivity towards X3B dye degradation and NO Oxidation Xiaofang Li, Heng Yang, Kangle Lv, Lili Wen, Yi Liu PII: DOI: Reference:

S0169-4332(19)32896-X https://doi.org/10.1016/j.apsusc.2019.144080 APSUSC 144080

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Applied Surface Science

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30 April 2019 17 August 2019 17 September 2019

Please cite this article as: X. Li, H. Yang, K. Lv, L. Wen, Y. Liu, Fabrication of porous TiO2 nanosheets assembly for improved photoreactivity towards X3B dye degradation and NO Oxidation, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144080

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Fabrication of porous TiO2 nanosheets assembly for improved photoreactivity towards X3B dye degradation and NO Oxidation

Xiaofang Lia,1, Heng Yangb,c,1, Kangle Lvb,*, Lili Wenc, Yi Liua,d,*

a

College of Chemistry and Chemical Engineering, Wuhan University of Science and

Technology, Wuhan, 430081, China Tel: +86-27- 6875 6667 Email: [email protected] (Y. Liu) b

Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of

Education & Hubei Key Laboratory of Catalysis and Materials Science, College of Resources and Environmental Science, South-Central University for Nationalities, Wuhan 430074, China Tel: +86-27-67841369, Fax: +86-27-67843918 Email: [email protected] (K.L. Lv) c

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College

of Chemistry, Central China Normal University, Wuhan, 430079, China d

College of Chemistry and Molecular Sciences, Wuhan University, 430072, China

1

These authors contributed equally.

1

Abstracts Porous nanosheet can enhance the adsorption performance and light-harvesting ability of the photocatalyst due to the enlarged surface area and improved multi-reflection of the incident light between nanosheets. In this work, porous TiO2 nanosheets (TiO2-NSs) assembly was obtained by transformation TiO2 nanocubes in NaOH solution via hydrothermal reaction and followed by acid wash and heat treatment. The effect of reaction time (t) -dependent on photocatalytic activity of the obtained TiO2-NSs (St sample) was systematically investigated. It was found that TiO2-NSs exhibits superior catalytic activity for X3B dye degradation and NO oxidation when compared to pristine TiO2 nanocubes (S0 sample). The optimized photoreactivity of TiO2-NSs improved 8.1 and 5.0 times for X3B degradation (S1.5 sample) and NO oxidation (S3.0 sample), respectively. The enhanced photoreactivity of TiO2-NSs assembly is ascribed to the combined effects of enlarged surface area and unique structure of nanosheets assembly which facilitates the absorption of incident light. Both hydroxyl radicals and superoxide radicals are important reactive oxygen species (ROSs) responsible for the degradation of organic dye and NO oxidation. The present study provides a novel way to prepare high efficient semiconductor photocatalyst for water treatment and air purification.

Keywords: TiO2 nanosheets; Photocatalytic oxidation; X3B; NO.

2

1. Introduction. Semiconductor photocatalytic oxidation is considered to be a key subject for environmental pollution and energy crisis [1]. As a typical semiconductor photocatalyst, TiO2 has the merits of low toxicity, good chemical stability and excellent biocompatibility, which therefore is regarded as one of the highly desirable materials applying for organic pollutant degradation [2-4], air purification [5-7], water cleaning [8], water splitting to produce hydrogen [9-12], as well as energy conversion and storage [13-17]. Despite these impressive advantages, TiO2 is still restricted in large-scale applications due to its wide band gap, low quantum efficiency and poor adsorption to pollutants. Therefore, intense research efforts have been made for optimizing

its

optical/electrical

properties

to

stimulate

the

separation

of

photogenerated carriers, including element doping [18-20], noble metal depositing [21-23], surface modification [24-26], and semiconductor coupling [27-30]. Not this merely but also high surface area and large pore volume of photocatalyst exhibit significant effect [31]. Reducing the particle size often results in an enlarged specific surface area to achieve more active sites, facilitating the adsorption to pollutant, as well as an excellent capacity for carrying and conducting electrons and holes to improve photocatalytic activity [32]. Up to now, many nanostructured TiO2 materials were reported, including nanoparticles [33], nanowires [34, 35], nanotubes [36, 37] and nanosheets [38]. However, the use of nanostructures is not a panacea. TiO2 nanomaterials usually suffer from aggregation, uncontrolled size and low uniformity because of the high surface energy. Furthermore, when particles size of TiO2 is too small, it becomes transparent, without harvesting the incident light [39]. Based on these deficiencies, TiO2 nanomaterials with complex structures have attracted much 3

attention due to their exciting performances. The 3D porous nanostructure is accredited to be one of the most effective systems applied in catalysis considering, due to the high specific surface area providing abundant active sites and the porosity reducing the bulk to surface diffusion/transfer distance to accelerate the separation efficiency of photo-generated electron hole pairs [40-42]. Our previous results showed that TiO2 hollow microspheres self-assembly from nanosheets have larger BET surface area and exhibit stronger capacity of light absorption when compared to discrete TiO2 particles [43-45]. It was also reported that the catalytic process 3D porous nanostructured photocatalyst shows improved reactivity compared to bulk catalyst [46]. Although self-assembly techniques have well-established, which provides possibilities for producing diverse architectures, it still remains a great challenge to fabricate high efficient semiconductor photocatalyst with complex nanostructure. Herein, 3D porous TiO2 nanosheets assembly is fabricated by transforming TiO2 nanocubes in an alkaline solution via hydrothermal reaction. The surface area and pore structure can be tailored by adjusting the hydrothermal reaction time. Owing to the unique structure, the prepared TiO2 nanosheets assembly exhibits superior photocatalytic activity for both Reactive Brilliant Red X3B (X3B) dye degradation and NO oxidation.

2. Experiment. 2.1 Synthesis of TiO2 nanocubes. All the reagents with analytical grade were purchased from Sinopharm Chem. (China) and used without any further purification. TiO2 nanocubes were prepared by a modified hydrothermal method according to our previous report [47]. Typically, 15 mL of tetrabutyl titanate (TBT) alcoholic solution 4

containing 3.4 g of TBT was dropwise added into 30 mL of NH4HF2 aqueous solution (0.285 g NH4HF2) under magnetic stirring. Subsequently, 30 mL of H2O2 (40 wt%) was added to form deep orange solution. Finally, the solution was transferred to a 100 mL Teflon-lined autoclave and kept in an oven at 150 °C for 10 h. After cooling down to room temperature naturally, the resultant white precipitate was filtered, washed with H2O and allowed to dry in oven at 60 °C for 12 h to obtain TiO2 nanocubes (S0 sample). 2.2 Synthesis of TiO2 nanosheet assembly. TiO2 nanosheets assembly was obtained by hydrothermal reaction of the TiO2 nanocubes in a basic solution. Specifically, 1.0 g of TiO2 nanocubes were dispersed in 100 mL of NaOH solution (10 molL-1) and then transferred into a Teflon autoclave for hydrothermal reaction at 120 oC for 1.5 h. After cooling down to room temperature, the as-prepared samples were dispersed into 1.2 L of HCl solution (0.1 molL-1) and kept magnetic stirring for 12 h. After that, the solution was filtered and washed with distilled water. After drying in air at 60 °C for 12 h, the as-obtained powders were collected and calcined at 400 °C for 1 h to obtain the 3D TiO2 nanosheets assembly (S1.5 sample). In order to systematically study the effect of hydrothermal reaction time, a serial of samples were obtained by otherwise identical conditions but with varying reaction time (0, 0.5, 1.0, 1.5, 2.0 and 3.0 h). The resulting samples were denoted as Sx, where x represents reaction time (see Table 1). 2.3 Characterization. The phase and crystal structure of the products are detected by X-ray diơraction (XRD) patterns using D8-advance X-ray diơractometer (German Bruker). Morphologies and structures are obtained by Field scanning electron microscopy (FESEM) (S-4800, Hitachi, Japan) and transmission electron microscopy (TEM) (Tacnai G20, USA). The components and valence states of the obtained 5

samples are analyzed by X-ray photoelectron spectroscopy (XPS) performing with Kratos XSAM800 XPS system. UV-vis diffuse reflectance spectra (DRS) were performed on UV-vis spectrophotometer (UV-2550, Shimadzu, Japan). The Brunauer-Emmett-Teller (BET) special surface areas were obtained by nitrogen adsorption apparatus (ASAP 2020). Photoluminescence (PL) spectra were detected on a fluorescence spectrophotometer (F-7000, Hitachi, Japan) with an excitation wavelength of 310 nm. The DMPO spin-trapping ESR spectras are measured under illumination by an Xe lamp (DMPO-·OH detection are carried out in aqueous dispersion, while DMPO-·O2- in methanol dispersion). The electrochemical measurement was performed on a standard three-electrode system (CHI760E, Chen Hua electrochemical workstation, Shanghai). 2.4 Photocatalytic activity measurements. The photocatalytic activity of the obtained TiO2 nanosheet assembly was evaluated by photocatalytic degradation of organic X3B dye and photocatalytic oxidation of NO, a typical vehicle exhausted pollutant in air. As for X3B degradation, 50 mg of the TiO2 nanosheets were dispersed into the X3B aqueous solution (1.0 g·L-1) in a 50 mL of cyclindrical Pyrex flask, and the suspension was ultrasonicated and stirred in the dark for overnight to reach adsorption-desorption equilibrium. Then the equilibrium system was illuminated by an Xe lamp (300 W). During the given space of time, 3.0 mL of suspension was sampled and centrifuged, the X3B concentration of the clear supernatant was then monitored by UV-visible spectroscopy at 554 nm. Photocatalytic oxidation of NO was carried out in a 4.5 L flow reactor with a visible LED lamp. Typically, 20 mg of the photocatalyst was dispersed in 30 mL H2O, and the resulted suspensions were ultrasonicated for 30 min, which was then 6

transferred into a culture dish (diameter of 11.5 cm) and followed by heating at 70 oC for 8 h to thoroughly evaporate all the water. After cooled down to room temperature, the culture dish deposited with TiO2 photocatalyst film was transferred into the reactor. Then the reactor was sealed and followed by supplying NO gas from a compress gas (N2 balance) diluted to 600 ppb by air. After the gas system reached adsorption-desorption equilibrium, the LED lamp was turned on to begin the photocatalytic reaction. The concentrations of the NO and the NO2 from the exit were online detected by a chemiluminescence NOx analyzer (setup in Scheme 1).

3. Results and discussion. 3.1 Phase structure and morphology evolution. Fig. 1 compares XRD patterns of the obtained photocatalysts. It can be seen that all samples have similar X-ray diffraction peaks, corresponding to anatase phase of TiO2 (JCPDS No. 71-1167) [48]. The diffraction peaks at 2θ = 25.3o, 37.8o, 48.1o, 53.9o and 55.1 o are assigned to the (101), (004), (200), (105) and (211) crystal planes of anatase TiO2. Carefully view shows that the widths of the peaks increase while the intensity gradually attenuated along with reaction time extension. This infers that the crystalline size of TiO2 decreases with increasing the reaction time. Although smaller particle size means large BET surface area, it also causes surface defects, which is detrimental to the photoreactivity of TiO2 photocatalyst. Fig. 2 and Fig. 3 illustrate the morphology evolution of TiO2 photocatalyst by TEM and SEM, respectively. The precursors of TiO2 are relatively monodisperse cubes in sidelength of about 100 nm (Fig. 2a and Fig. 3a). After hydrothermal reaction in alkaline solution for 1.5 h, the cubic TiO2 nanoparticles transform into multi-layered TiO2 nanosheets along each side (Fig. 2b and Fig. 3b). When compared 7

nanocubes, the formed TiO2 nanosheets should possess large BET surface area, which is beneficial to the photocatalytic activity of TiO2 photocatalyst. It has been reported [46, 49, 50] that the commercial TiO2 (P25) nanoparticles can transform into sodium titanate (Na2TiO3) nanosheets in alkaline solution under hydrothermal reaction (Eq. (1)), then sheet-like hydrogen titanate (H2TiO3) will generate after acid wash (Eq. (2)), which then converts into anatase TiO2 nanosheets after calcination (Eq. (3)). It is worth noting that the morphology of TiO2 nanosheets assembly has slightly changed during the phase transformation from H2TiO3 to TiO2 (Fig. 2c and Fig. 3c). TiO2 (nanoparticle) + 2NaOH = Na2TiO3 (nanosheet) + H2O

(1)

Na2TiO3 (nanosheet) + 2HCl = H2TiO3 (nanosheet) + 2NaCl

(2)

H2TiO3 (nanosheet) + heat = TiO2 (nanosheet) + H2O

(3)

To account for the morphology evolution of TiO2 nanosheets assembly, the time-dependent hydrothermal reaction of TiO2 nanocubes in NaOH solution was carried out, and the results were shown in Fig. 4 (TEM images) and Fig. 5 (SEM images). It can be seen that the anatase TiO2 gradually alters from nanocubes to nanosheets which self-assembled into 3D porous structure. When the TiO2 nanocubes were treated into NaOH solution for 0.5 h, the surface of the TiO2 nanocubes seem to be wrapped with small size nanosheets (Figs. 4a and 5a). These nanosheets should be composed of sodium titanate, which forms after TiO2 nanocubes interaction with NaOH aqueous solution at high temperature (Eq. (1)). As the extension of reaction time to 1.0 h, more and more small size of nanosheets formed and intercrossed with each other (Figs. 4b and 5b). When the reaction time reaches to 2 h, the formed Na2TiO3 nanosheets sharply increased (Figs. 4c and 5c). At last, almost all the precursor TiO2 nanocubes completely transforms into Na2TiO3 nanosheets which 8

further self-assembled into 3D structures when hydrothermal reaction is as long as 3 h (Figs. 4d and 5d). Base on the above analysis, a scheme illustrating the formation processes of TiO2-NSs assembly from TiO2 nanocubes is proposed (Scheme 2). Firstly, on the surface of TiO2 nanocubes, Ti-O-Ti bonds are broken up to form Ti-O-Na bonds as soon as the precursor TiO2 nanocubes are dispersed in NaOH solution. Secondly, the formed sodium titanate (Na2TiO3) crystal nuclei will further grow to form nanosheets through Ostwald ripening on the expense of the sacrifice of TiO2 nanocubes. Finally, all the TiO2 nanocubes are dissolved, and the formed Na2TiO3 nanosheets will aggregate with each other. After acid wash and calcination, the TiO2 nanosheets assembly is formed. 3.2 Nitrogen adsorption-desorption and UV-visible absorption. Fig. 6 compares the nitrogen sorption isotherms and corresponding pore size distribution curves (inset) of the photocatalysts. All the isotherms curves are type of IV with the match of Brunauer-Deming-Deming-Teller (BDDT) classification, corresponding well with the mesoporous (2~50 nm) [47]. The hysteresis loop of S0 in the relative pressure of 0.4-1.0 is of type H2, which indicates the ink-bottle-like mesopores form by aggregation of TiO2 nanocrystals [51], consistent with its cubic structure. Different form S0, the hysteresis loop of S0.5 sample is of type H3 at relative pressure of 0.9-1.0. These slit-shaped pores are generally associated with plate-like particles [52], corresponding to the sheet-like aggregate structure of products. Table 1 summarizes the BET surface areas and pore structures of the photocatalysts. It can be seen that the BET surface area of TiO2-NSs assembly steady increases with increase in the reaction time. S3.0 sample gives a specific surface area of 168 m2·g-1, which is 3.8 times larger than pristine TiO2 nanocubes (44 m2·g-1). The enlargement of surface area means 9

more active sites can be used for the photocatalytic reaction. The pore volume of the photocatalyst also increases when compared with TiO2 nanocubes. Large pore volume can facilitate the diffusion of the substrate during photoreaction, enhancing the photoreactivity [53]. Light harvesting ability is also important for the photoreactivity of the photocatalyst. Therefore, we compare the UV-visible diffuse reflectance spectra between pristine TiO2 nanocubes (S0 sample) and TiO2-NSs assembly (S3.0 sample). From Fig. 7a, we can see that the absorption of S3.0 sharply increases in UV region when compared with S0. The enhanced light-harvesting ability of the obtained TiO2-NSs assembly is probably ascribed to the multi-reflection of the incident light between nanosheets [5]. The bandgap of TiO2 photocatalyst can be obtained from the corresponding M-K function (Fig. 7b). It can be seen that the badgap of TiO2-NSs (3.38 eV) assembly slightly increases when compared with TiO2 nanocubes (3.30 eV). 3.3 XPS analysis. Fig. 8a shows the XPS survey spectra of S0 and S1.5 samples. It can be seen that both samples contains Ti, O and C elements with binding energies of 458.6 eV (Ti 2p), 529.9 eV (O 1s) and 284.6 eV (C 1s), respectively. The C element should come from the residual carbon from the precursor solution and adventitions hydrocarbon from the XPS equipment [54]. Careful view shows that pristine TiO2 nanocubes contain F element with binding energy of 683.8 eV (F 1s). Note that TiO2 nanocubes are synthesized in the presence of HF solution. Therefore, it is understandable the presence of F element in S0 sample. The high-resolution XPS spectra of S0 and S1.5 samples in the regions of Ti 2p and O 1s are shown in Fig. 8b and Fig. 8c, respectively. As for S1.5 sample, the peak assigned to Ti 2p3/2 increases from 458.4 eV to 458.6 eV (Fig. 8b), and the peak assigned to O 1s also increases from 529.6 eV to 529.9 eV (Fig. 8c) when compared 10

with S0 sample. Both the binding energies of Ti 2p and O 1s for TiO2-NSs assembly present a slight upshift after hydrothermal reaction. This can be explained by the stronger electronegativity of F than that of O element [55]. From Fig. 8d, we can see the binding energy of F1s for S0 sample is 683.8 eV, which can be atrributed to the surface adsorbed F ions instead of lattice doped fluoride ions (binding energy of 688.4 eV) [56]. After hydrothermal reaction in NaOH for 1.5 and followed with calcination at 400 oC for 1 h, almost all of the adsorbed fluoride ions are removed (Fig. 8d). 3.4 Photocatalytic activity. X3B is a typical organic azo dye, which is very stable even under the irradiation of UV light in the absence of photocatalyst [45]. From Fig. 9a, it can be seen that significant degradation of X3B is observed in the presence of TiO2 photocatalyst, and the S1.5 sample exhibits the highest photocatalytic activity among all these different samples. The degradation curves of X3B obey the pseudo 1st-order kinetics, and Fig. 9b compares the corresponding degradation rate constants. It can be seen that the reactivity of S1.5 (0.203 min-1) is 8.1 times higher than that of S0 (0.025 min-1). The photocatalytic activity of TiO2-NSs assembly is much superior compared with previously reported studies (Table 2). It can be attributed to the increase of BET surface area pore volume, which facilitated the adsorption and diffusion of the substrate. However, further increase in the hydrothermal reaction time, the photoreactivity of TiO2-NSs assembly begins to decrease. The corresponding degradation rate constant of S3.0 sample is 4.0 min-1, only 1/3 that of S1.5 sample. The reduced crystallinity may be responsible for to the photoreactivity of S3.0 sample (Fig. 1). The photocatalytic oxidation of NO is also used to further study the photoreactivity of TiO2-NSs assembly (Fig. 10a). Different from the results of X3B degradation, the photoreactivity of TiO2-NSs assembly steady increases with increase in the 11

hydrothermal reaction time. The S3 sample presents the highest photocatalytic activity with NO removal rate of as high as 55%, which is much higher than that of the S0 with a NO removal rate of 11%. The formation of NO2 after photocatalytic oxidation of NO is also detected, confirming the oxidation of NO (Fig. 10b). 3.5 Photoelectrochemical property. It has been reported that the efficient separation of photo-generated electron-hole pairs is very important to the photoreactivity of the photocatalyst [60]. To account for the superior photocatalytic activity of the porous TiO2-NSs assembly, we monitored the steady state photoluminescence (PL) and surface photovoltage of the TiO2 photocatalysts. As can be seen from Fig. 11a, the PL intensity the photocatalyst decreases with increase in the hydrothermal reaction time, which indicates that the formation of nanosheets can suppress recombination of electron-hole pairs, improving the photoreactivity. Consistent with the results of PL, TiO2-NSs assembly exhibits increased surface photovoltage spectrum (SPS). The surface photovoltage of TiO2-NSs assembly (S1.5 sample) is 0.586 mV, which is 21 times larger than that of TiO2 nanocubes (S0 sample) with a surface photovoltage of 0.028 mV (Fig. 11b). Both hydroxyl radicals (·OH) or super oxygen radicals (·O2-) are important reactive oxygen species (ROSs) that are responsible for the photocatalytic oxidation of organics. To study the oxidation mechanism, the DMPO spin-trapping ESR spectra are used to detected the ·OH (Fig. 12a) and ·O2- (Fig. 12b) in aqueous and ethanol dispersion, respectively. From Fig. 12, we can see that both the signal intensities of the trapped ·OH and ·O2- radicals are sharply increased in illuminated TiO2-NSs assembly (S1.5 sample) when compared with these of irradiated TiO2 nanocubes (S0 sample). Therefore, it is not strange to see the enhanced photoreactivity of TiO2-NSs assembly when compared with TiO2 nanocubes precursor. 12

3.6 Reusability of the photocatalyst. From the viewpoint of practical applications, photo-stability of TiO2-NSs assembly is very important. From the reusability curves of S1.5 in degradation of X3B and S3.0 in oxidation of NO (Fig. 13b), we can see that both the degradation of X3B and NO oxidation are almost keep unchanged even after continuous use for 5 times, indicating the excellent stability. The high stability of TiO2 nanosheets assembly makes it promising to be used in water treatment and air purification.

4. Conclusions. In summary, porous TiO2 nanosheet assembly was successfully obtained by transformation of TiO2 nanocubes in alkaline solution. The optimized photoreactivity of TiO2-NSs improved 8.3 and 5.0 times in X3B dye degradation and NO oxidation when compared with pristine TiO2 nanocubes. The enhanced photocatalytic activity of porous TiO2-NSs assembly can be attributed to the combined effects of (1) enlarged surface area that provides more active sites and pore volume that facilitates the diffusion of the substrate, and (2) improved light-harvesting ability that caused by multi-reflection of the incident light between nanosheets. Both ·OH and ·O2- are important ROSs that are responsible for the degradation of organic dye and NO abatement. This work provides novel ways to fabricate high efficient semiconductor photocatalyst.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (51672312, 21373275 and 51808080) and the Fundamental Research Funds for the Central University, South-Central University for Nationalities (CZP19006). 13

References: [1] J.X. Low, L.Y. Zhang, T. Tong, B.J. Shen, J.G. Yu, TiO2/MXene Ti3C2 composite with excellent photocatalytic CO2 reduction activity, J. Catal. 361 (2018) 255-266. [2] Q. Li, T. Zhao, M. Li, W. Li, B. Yang, D. Qin, K. Lv, X. Wang, L. Wu, X. Wu, J. Sun, One-step construction of Pickering emulsion via commercial TiO2 nanoparticles for photocatalytic dye degradation, Appl. Catal. B-Environ. 249 (2019) 1-8. [3] X. Wang, R. Xia, E. Muhire, S.B. Jiang, X.J. Huo, M.Z. Gao, Highly enhanced photocatalytic performance of TiO2 nanosheets through constructing TiO2/TiO2 quantum dots homojunction, Appl. Surf. Sci. 459 (2018) 9-15. [4] A. Mbiri, D.H. Taffa, E. Gatebe, M. Wark, Zirconium doped mesoporous TiO2 multilayer thin films: Influence of the zirconium content on the photodegradation of organic pollutants, Catal. Today 328 (2019) 71-78. [5] Y.Y. Duan, L. Liang, K.L. Lv, Q. Li, M. Li, TiO2 faceted nanocrystals on the nanofibers: Homojunction TiO2 based Z-scheme photocatalyst for air purification, Appl. Surf. Sci. 456 (2018) 817-826. [6] L.F. Qi, B. Cheng, J.G. Yu, W.K. Ho, High-surface area mesoporous Pt/TiO2 hollow chains for efficient formaldehyde decomposition at ambient temperature, J. Hazard. Mater. 301 (2016) 522-530. [7] X.F. Zhu, B. Cheng, J.G. Yu, W.K. Ho, Halogen poisoning effect of Pt-TiO2 for formaldehyde catalytic oxidation performance at room temperature, Appl. Surf. Sci. 364 (2016) 808-814. [8] Y.K. Lai, Y.X. Tang, J.J. Gong, D.G. Gong, L.F. Chi, C.J. Lin, Z. Chen, 14

Transparent

superhydrophobic/superhydrophilic

TiO2-based

coatings

for

self-cleaning and anti-fogging, J. Mater. Chem. 22 (2012) 7420-7426. [9] X. Cheng, Y.J. Zhang, Y.P. Bi, Spatial dual-electric fields for highly enhanced the solar water splitting of TiO2 nanotube arrays, Nano Energy 57 (2019) 542-548. [10] Y.H. Hu, A Highly Efficient PhotocatalystHydrogenated Black TiO2 for the Photocatalytic Splitting of Water, Angew. Chem. Int. Edit. 51 (2012) 12410-12412. [11] M.Z. Ge, Q.S. Li, C.Y. Cao, J.Y. Huang, S.H. Li, S.N. Zhang, Z. Chen, K.Q. Zhang, S.S. Al-Deyab, Y.K. Lai, One-dimensional TiO2 nanotube photocatalysts for solar water splitting, Adv. Sci. 4 (2017) n/s. [12] P.P. Wan, Z.D. Hood, S.P. Adhikari, Y.F. Xu, S.Z. Yang, S.J. Wu, Enhancing the photoresponse and photocatalytic properties of TiO2 by controllably tuning defects across {101} facets, Appl. Surf. Sci. 434 (2018) 711-716. [13] S. Shi, Y. Chen, J. Lee, Z. Jiang, X. Cui, Direct fabrication of anatase TiO2 hollow microspheres for applications in photocatalytic hydrogen evolution and lithium storage, J. Solid State Electr. 22 (2018) 705-715. [14] Z.X. Pei, M.S. Zhu, Y. Huang, Y. Huang, Q. Xue, H.Y. Geng, C.Y. Zhi, Dramatically improved energy conversion and storage efficiencies by simultaneously enhancing charge transfer and creating active sites in MnO x/TiO2 nanotube composite electrodes, Nano Energy 20 (2016) 254-263. [15] E.J. Lee, I. Nam, J. Yi, J.H. Bang, Nanoporous hexagonal TiO 2 superstructure as a multifunctional material for energy conversion and storage, J. Mater. Chem. A 3 (2015) 3500-3510. [16] W. Wang, D. Xu, B. Cheng, J. Yu, C. Jiang, Hybrid carbon@TiO2 hollow spheres with enhanced photocatalytic CO2 reduction activity, J. Mater. Chem. A 5 (2017) 15

5020-5029. [17] J.G. Yu, J.X. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced photocatalytic CO2-reduction activity of anatase TiO2 by coexposed {001} and {101} facets, J. Am. Chem. Soc. 136 (2014) 8839-8842. [18] K. Li, S.M. Gao, Q.Y. Wang, H. Xu, Z.Y. Wang, B.B. Huang, Y. Dai, J. Lu, In-situ-reduced synthesis of Ti3+ self-doped TiO2/g-C3N4 heterojunctions with high photocatalytic performance under LED light irradiation, ACS Appl. Mater. Interfaces 7 (2015) 9023-9030. [19] A.A. Isari, A. Payan, M. Fattahi, S. Jorfi, B. Kakavandi, Photocatalytic degradation of rhodamine B and real textile wastewater using Fe-doped TiO2 anchored on reduced graphene oxide (Fe-TiO2/rGO): Characterization and feasibility, mechanism and pathway studies, Appl. Surf. Sci. 462 (2018) 549-564. [20] M.S. Akple, J.X. Low, S.W. Liu, B. Cheng, J.G. Yu, W.K. Ho, Fabrication and enhanced CO2 reduction performance of N-self-doped TiO2 microsheet photocatalyst by bi-cocatalyst modification, J. CO2 Util. 16 (2016) 442-449. [21] A.Y. Meng, L.Y. Zhang, B. Cheng, J.G. Yu, TiO2-MnOx-Pt Hybrid multiheterojunction

film

photocatalyst

with

enhanced

photocatalytic

CO2-reduction activity, ACS Appl. Mater. Inter. 11 (2019) 5581-5589. [22] Y. Wang, J.G. Yu, W. Xiao, Q. Li, Microwave-assisted hydrothermal synthesis of graphene based Au-TiO2 photocatalysts for efficient visible-light hydrogen production, J. Mater. Chem. A 2 (2014) 3847-3855. [23] W. Wang, M. Lai, J.J. Fang, C.H. Lu, Au and Pt selectively deposited on {001}-faceted TiO2 toward SPR enhanced photocatalytic Cr(VI) reduction: The influence of excitation wavelength, Appl. Surf. Sci. 439 (2018) 430-438. [24] J.X. Low, B. Cheng, J.G. Yu, Surface modification and enhanced photocatalytic 16

CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658-686. [25] A. Hankhuntod, E. Kantarak, W. Sroila, T. Kumpika, P. Singjai, W. Thongsuwan, alpha-Fe2O3 modified TiO2 nanoparticulate films prepared by sparking off Fe electroplated Ti tips, Appl. Surf. Sci. 477 (2019) 116-120. [26] M.G. Mendez-Medrano, E. Kowalska, A. Lehoux, A. Herissan, B. Ohtani, D. Bahena, V. Briois, C. Colbeau-Justin, J.L. Rodriguez-Lopez, H. Remita, Surface modification of TiO2 with Ag nanoparticles and CuO nanoclusters for application in photocatalysis, J. Phys. Chem. C 120 (2016) 5143-5154. [27] G.M. Jiang, J.W. Cao, M. Chen, X.M. Zhang, F. Dong, Photocatalytic NO oxidation on N-doped TiO2/g-C3N4 heterojunction: Enhanced efficiency, mechanism and reaction pathway, Appl. Surf. Sci. 439 (2018) 430-438. [28] L. Pan, S.B. Wang, J.W. Xie, L. Wang, X.W. Zhang, J.J. Zou, Constructing TiO2 p-n homojunction for photoelectrochemical and photocatalytic hydrogen generation, Nano Energy 28 (2016) 296-303. [29] M.G.Wang, H. Zhang, H.L. Zu, Z.G. Zhang, J. Han, Construction of TiO2/CdS heterojunction photocatslysts with enhanced visible light activity, Appl. Surf. Sci. 455 (2018) 729-735. [30] X.B. Wei, C.L. Shao, X.H. Li, N. Lu, K.X. Wang, Z.Y. Zhang, Y.C. Liu, Facile in situ synthesis of plasmonic nanoparticles-decorated g-C3N4/TiO2 heterojunction nanofibers and comparison study of their photosynergistic effects for efficient photocatalytic H2 evolution, Nanoscale 8 (2016) 11034-11043. [31] S.L. Wang, M. Xu, T.Y. Peng, C.X. Zhang, T. Li, I. Hussain, J.Y. Wang, B.E. Tan, Porous hypercrosslinked polymer-TiO2-graphene composite photocatalysts for visible-light-driven CO2 conversion, Nat. Commun. 10 (2019) 1-10. 17

[32] J.Y. Liao, J.W. He, H.Y. Xu, D.B. Kuang, C.Y. Su, Effect of TiO2 morphology on photovoltaic performance of dye-sensitized solar cells: nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres, J. Mater. Chem. 22 (2012) 7910-7918. [33] Z.G. Mou, Y.J. Wu, J.H. Sun, P. Yang, Y.K. Du, C. Lu, TiO2 nanoparticles-functionalized N-doped graphene with superior interfacial contact and enhanced charge separation for photocatalytic hydrogen generation, ACS Appl. Mater. Inter. 6 (2014) 13798-13806. [34] L.L. Lai, W. Wen, B. Fu, X.Y. Qian, J.B. Liu, J.M. Wu, Surface roughening and top opening of single crystalline TiO2 nanowires for enhanced photocatalytic activity, Mater. Design. 108 (2016) 581-589. [35] F.Y. Xu, K. Meng, B. Cheng, J.G. Yu, W.K. Ho, Enhanced photocatalytic activity and selectivity for CO2 reduction over a TiO2 nanofibre mat using Ag and MgO as Bi-cocatalyst, ChemCatChem 11 (2019) 465-472. [36] Z.B. Dong, D.Y. Ding, T. Li, C.Q. Ning, Ni-doped TiO2 nanotubes photoanode for enhanced photoelectrochemical water splitting, Appl. Surf. Sci. 443 (2018) 321-328. [37] J.X. Low, B.Z. Dai, T. Tong, C.J. Jiang, J.G. Yu, In situ irradiated X-ray photoelectron spectroscopy investigation on a direct Z-scheme TiO2/CdS composite film photocatalyst, Adv. Mater. 31 (2019) 1802981. [38] A.Y. Meng, L.Y. Zhang, B. Cheng, J.G. Yu, Dual-cocatalysts in TiO2 photocatalysis, Adv. Mater. 31 (2019) 1807660. [39] K.X. Wang, C.L. Shao, X.H. Li, X. Zhang, N. Lu, F.J. Miao, Y.C. Liu, Hierarchical heterostructures of p-type BiOCl nanosheets on electrospun n-type TiO2 nanofibers with enhanced photocatalytic activity, Catal. Commun. 67 (2015) 6-10. 18

[40] X.J. Zhou, C.L. Shao, X.H. Li, X.X. Wang, X.H. Guo, Y.C. Liu, Three dimensional hierarchical heterostructures of g-C3N4 nanosheets/TiO2 nanofibers: Controllable growth via gas-solid reaction and enhanced photocatalytic activity under visible light, J. Hazard. Mater. 344 (2018) 113-122. [41] I. Hashemizadeh, V.B. Golovko, J. Choi, D.C.W. Tsang, A.C.K. Yip, Photocatalytic reduction of CO2 to hydrocarbons using bio-templated porous TiO2 architectures under UV and visible light, Chem. Eng. J. 347 (2018) 64-73. [42] S.M. Yong, D.S. Kim, K. Jung, K.K. Bharathi, D.K. Kim, Freeze-casted TiO2 photoelectrodes with hierarchical porous structures for efficient light harvesting ability in dye-sensitized solar cells, Appl. Surf. Sci. 449 (2018) 405-411. [43] Y. Zheng, J.H. Cai, K.L. Lv, J. Sun, H.P. Ye, M. Li, Hydrogen peroxide assisted rapid synthesis of TiO2 hollow microspheres with enhanced photocatalytic activity, Appl. Catal. B-Environ. 147 (2014) 789-795. [44] L. Liang, K.N. Li, K.L. Lv, W.K. Ho, Y.Y. Duan, Highly photoreactive TiO2 hollow microspheres with super thermal stability for acetone oxidation, Chin. J. Catal. 38 (2017) 2085-2093. [45] X.F. Li, K.L. Lv, K.J. Deng, J.F. Tang, R. Su, J. Sun, L.Q. Chen, Synthesis and characterization of ZnO and TiO2 hollow spheres with enhanced photoreactivity, Mat. Sci. Eng. B-Adv. 158 (2009) 40-47. [46] R.W. Yang, J.H. Cai, K.L. Lv, X.F. Wu, W.G. Wang, Z.H. Xu, M. Li, Q. Li, W.Q. Xu, Fabrication of TiO2 hollow microspheres assembly from nanosheets (TiO2-HMSs-NSs) with enhanced photoelectric conversion efficiency in DSSCs and photocatalytic activity, Appl. Catal. B-Environ. 210 (2017) 184-193. [47] J.X. Low, S.Q. Qiu, D.F. Xu, C.J. Jiang, B. Cheng, Direct evidence and enhancement of surface plasmon resonance effect on Ag-loaded TiO2 nanotube 19

arrays for photocatalytic CO2 reduction, Appl. Surf. Sci. 434 (2018) 423-432 [48] Y.C. Jiao, B. Zhao, F. Chen, J.L. Zhang, Insight into the crystal lattice formation of brookite in aqueous ammonia media: the electrolyte effect, CrystEngComm 13 (2011) 4167-4173. [49] J. Sun, X. Yan, K. Lv, S. Sun, K. Deng, D. Du, Photocatalytic degradation pathway for azo dye in TiO2/UV/O3 system: Hydroxyl radical versus hole, J. Mol. Catal. A-Chem. 367 (2013) 31-37. [50] X.F. Wu, J.S. Cheng, X.F. Li, Y.H. Li, K.L. Lv, Enhanced visible photocatalytic oxidation of NO by repeated calcination of g-C3N4, Appl. Surf. Sci. 465 (2019) 1037-1046. [51] A.Y. Meng, S. Wu, B. Cheng, J.G. Yu, J.S. Xu, Hierarchical TiO2/Ni(OH)2 composite fibers with enhanced photocatalytic CO2 reduction performance, J. Mater. Chem. A 6 (2018) 4729-4736. [52] J.S. Cheng, Z. Hu, Q. Li, X.F. Li, S. Fang, X.F. Wu, M. Li, Y.B. Ding, B. Liu, C.J. Yang, L.L. Wen, Y. Liu, K.L. Lv, Fabrication of high photoreactive carbon nitride nanosheets by polymerization of amidinourea for hydrogen production, Appl. Catal. B-Environ. 245 (2019) 197-206. [53] H.Y. Zhu, Y. Lan, X.P. Gao, S.P. Ringer, Z.F. Zheng, D.Y. Song, J.C. Zhao, Phase transition between nanostructures of titanate and titanium dioxides via simple wet-chemical reactions, J. Am. Chem. Soc. 127 (2005) 6730-6736. [54] K.L. Lv, S. Fang, L.L. Si, Y. Xia, W.K. Ho, M. Li, Fabrication of TiO 2 nanorod assembly grafted rGO (rGO@TiO2-NR) hybridized flake-like photocatalyst, Appl. Surf. Sci. 391 (2017) 218-227. [55] K.L. Lv, Q.J. Xiang, J.G. Yu, Effect of calcination temperature on morphology and photocatalytic activity of anatase TiO2 nanosheets with exposed {001} facets, 20

Appl. Catal. B-Environ. 104 (2011) 275-281. [56] K.L. Lv, B. Cheng, J.G. Yu, G. Liu, Fluorine ions-mediated morphology control of anatase TiO2 with enhanced photocatalytic activity, Phys. Chem. Chem. Phys. 14 (2012) 5349-5362. [57] H.P. Ye, S.M. Lu, Effect of substrate on evaluation of the photocatalytic activity of TiO2 nanocrystals with exposed {001} facets, Appl. Surf. Sci. 270 (2013) 741-745. [58] X.F. Wu, S. Fang, Y. Zheng, J. Sun, K.L. Lv, Thiourea-modified TiO2 nanorods with enhanced photocatalytic activity, Molecules 21 (2016) 181. [59] D. Wang, S.Y. Zheng, J. Yao, Y.W. Wang, Y. Huang, C.M. Fan, W. Huang, A novel c Ti3+ self-doped TiO2 for photocatalytic removal of NO, Chem. Phys. Lett. 716 (2019) 215-220. [60] L. Hao, L. Kang, H.W. Huang, L.Q.Ye, K.L. Han, S.Q. Yang, H.J. Yu, M. Batmunkh, Y.H. Zhang, T.Y. Ma, Surface-halogenation-induced atomic-site activation and local charge separation for superb CO2 photoreduction, Adv. Mater. (2019) 1900546.

21

Scheme 1.

Schematic illustration for the continuous photocatalytic reactor of NO

removal.

22

Scheme 2.

Schematic diagram depicts the morphology evolution as per reaction

time to form 3D porous TiO2 nanosheets assembly photocatalyst.

23

Table 1.

Physical property of the photocatalyst.

Reaction time

SBET

Average pore

Pore volume

(h)

(m2·g-1)

size (nm)

(cm3·g-1˅

S0

0

44

14.0

0.18

S0.5

0.5

52

37.2

0.41

S1.0

1.0

94

18.8

0.44

S1.5

1.5

120

13.3

0.41

S2.0

2.0

124

13.1

0.39

S3.0

3.0

168

2.35

0.1

Sample

24

Table 2.

Comparison of the photoreactivity of TiO2 nanosheets assembly with that

of other photocatalyst. Removal rate of

Kapp (min )a

Reference

-1

Photocatalyst NO (%) TiO2 nanosheets assembly

55

0.203

This work

TiO2 nanocrystals

-

0.020

[57]

TiO2 Nanorods

-

0.015

[58]

TiO2 nanorod assembly

-

0.039

[54]

N-doped TiO2/g-C3N4

46

-

[27]

TiO2 hollow nanoboxes

20

-

[59]

Ti3+ self-doped TiO2

60

-

[60]

a

degradation rate constant of X3B.

25

Fig. 1. XRD patterns of the TiO2 samples .

26

Fig. 2.

TEM images of the TiO2 photocatalysts for S0 (a1 and a2), S1.5 before (b1

and b2) and after calcination (c1and c2).

27

Fig. 3. SEM images of the TiO2 photocatalysts for S0 (a1 and a2), S1.5 before (b1 and b2) and after calcination (c1and c2).

28

Fig. 4.

TEM images of S0.5 (a1 and a2), S1.0 (b1 and b2), S2.0 (c1 and c2) and

S3.0 (d1 and d2).

29

Fig. 5.

SEM images of S0.5 (a1 and a2), S1.0 (b1 and b2), S2.0 (c1and c2) and S3.0

(d1 and d2).

30

Fig. 6.

Nitrogen sorption-desorption isotherms of the S0, S0.5 and S1.5 samples

(insert: corresponding pore size distribution curves).

31

Fig. 7.

UV-visible diffuse reflectance spectra (A) and the corresponding M-K

functions (B) of the S0 and S1.5 samples.

32

Fig. 8.

XPS survey spectra of the S0 and S1.5 samples (a), and the corresponding

high resolution curves in Ti 2p (b), O 1s (c), and F 1s (d) regions, respectively.

33

Fig. 9.

Degradation curves of X3B under the UV light irradiation (a) and the

corresponding rate constants (b).

34

Fig. 10.

Photocatalytic oxidation curves of NO (a) and the corresponding emission

curves of NO2 detected in the exhaust port (b).

35

Fig. 11. Comparison of the photoluminescence spectra (a) and surface photovotage spectra (b) of the photocatalysts (λex = 310 nm).

36

Fig. 12.

ESR signals of the DMPO-·OH (a) and DMPO-·O2- (b) adducts formed in

the suspensions of TiO2 photocatalysts.

37

Fig. 13.

Durable test for the photocatalytic degradation of X3B in the presence of

S1.5 (a), and the oxidation of NO using S3.0 as photocatalyst.

38

Graphical abstract

Highlights z

Porous TiO2 nanosheets assembly was prepared in alkaline solution.

z

BET surface area of TiO2 nanosheets assembly increase from 44 to 168 m2·g-1.

z

TiO2 nanosheets assembly exhibit superior photocatalytic activity.

z

Improved adsorption and increased multi-reflection are responsible for the enhanced photoreactivity.

39