Facile synthesis of tunable carbon modified mesoporous TiO2 for visible light photocatalytic application

Accepted Manuscript Title: Facile synthesis of tunable carbon modified mesoporous TiO2 for visible light photocatalytic application Authors: Xiao-Na Wei, Hui-Long Wang, Xin-Kui Wang, Wen-Feng Jiang PII: DOI: Reference:

S0169-4332(17)30984-4 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.283 APSUSC 35659

To appear in:

APSUSC

Received date: Revised date: Accepted date:

16-1-2017 25-3-2017 30-3-2017

Please cite this article as: Xiao-Na Wei, Hui-Long Wang, Xin-Kui Wang, Wen-Feng Jiang, Facile synthesis of tunable carbon modified mesoporous TiO2 for visible light photocatalytic application, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.03.283 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of tunable carbon modified mesoporous TiO2 for visible light photocatalytic application Xiao-Na Wei, Hui-Long Wang*, Xin-Kui Wang, Wen-Feng Jiang Department of Chemistry, Dalian University of Technology, Dalian 116023, China

Author for correspondence: Hui-Long Wang Department of Chemistry, Dalian University of Technology Dalian 116023, China Tel.: +86-411-84706303; fax: +86-411-84708590 E-mail address: [email protected] (H.-L. Wang)

1

Graphical abstarct

Research Highlights  Combined hydrothermal-calcination steps were used to prepare mesoporous C-TiO2.  Polyacrylate was employed as the carbon source.  XPS revealed the interstitial carbon modifying mode through carbonate-like species.  C-TiO2 exhibited visible light activity towards dinitro butyl phenol degradation.

ABSTRACT In this paper, we describe a simple and novel approach for preparing tunable carbon-modified mesoporous TiO2 photocatalysts by combining the in-situ carbonization of PAA-Ti/TiO2, hydrothermal reaction process and post-calcination treatment. The synthesized carbon-modified mesoporous TiO2 powders were of high crystallinity, large specific surface area and good visible light response. The carbon 2

species were formed by the carbonization of polyacrylate (PAA). The presence of carbonates was subsequently confirmed by the XPS spectra, which significantly narrow down the band gap of TiO2. The organic group in polyacrylate served as the carbon source and carbon resulted from in-situ carbonization treatment could help to inhibit the excessive growth of TiO2 grain and enlarge the pore structure of TiO2. The amount of carbon species could be feasibly modulated by adjusting the post-calcination temperature and the surface area of the photocatalyst was enlarged further after the partial removal of carbon species. The carbon-modified mesoporous TiO2 powders exhibit excellent reproducibility and photocatalytic performance under visible light irradiation. Keywords:

Mesoporous

C-TiO2;

Carbon

modification;

Polyacrylate;

Carbonization-calcination; Photocatalysis; Dinitro butyl phenol.

1. Introduction Titanium dioxide (TiO2), the most extensively studied material for photocatalysis, is widely used for water splitting and environmental treatment [1-9]. The band gap excitation on TiO2 leads to the generation of hole (hvb+)-electron (ecb) pairs that can initiate red-ox reactions during photocatalysis. However, the band gap (Eg, 3.0-3.2 eV) of TiO2 is wide and can only be excited by the UV light, which accounts for only a 3

small percentage (3-5%) of the usable solar energy [10]. Thus, attentions were drawn to improve the photocatalytic activity under the irradiation of visible light by modifying pure TiO2 [11,12], which was mainly achieved by introducing either cation or anion dopants (transition metals [13-15], or rare earth metals [16,17], main group elements [7,18-23]) into the TiO2 structure. Among them, modification by carbon species has received increasing attention and has been shown to be able to widen the light absorption range of TiO2 to visible region by either introducing a localized electronic state into the band gap or forming carbonate-like species that act as photo-sensitizers by enabling a widened absorption band in the visible-light region [24-29]. Diverse methods are available for the preparation of carbon-modified TiO2 materials [25, 30-34]. However, to prevent the decomposition of carbon species under air atmosphere, calcination treatment is often performed at relatively low temperature and TiO2 of poor crystallinity in such conditions was usually resulted. TiO2 of mesoporous structure was advantageous in photocatalytic applications because large surface area can improve the photocatalytic activity by introduing more active sites. Most approaches for the fabrication of mesoporous TiO2 are based on hard templating methods [35-37] (porous inorganic or organic materials) or soft templating methods (block copolymers and surfactants) [38,39]. During the process of removing template, the mesoporous structure is prone to be damaged due to sintering and crystallization at high temperature. Moreover, in the process of fabricating TiO2 of mesoporous strucuture, it is not easy to achieve the doping simultaneously. Thus, it was still a difficult task to synthesize TiO2 with high crystallinity, satisfying 4

mesoporosity and excellent visible-light response. Herein, we present a novel approach for the feasible synthesis of tunable carbon-modified mesoporous TiO2 powders with high crystallinity, large specific surface area and good visible light response. The carbon-modified mesoporous TiO2 was prepared by combining the in-situ carbonization (sulfuric acid curing at low temperature)

of

polyacrylate

(PAA)-Ti/TiO2,

hydrothermal

treatment

and

post-calcination. Without the addition of external carbon precursor, the organic group in PAA served as the carbon source and carbon species resulted from in-situ carbonization treatment could help to inhibit the excessive growth of TiO2 grain and can enlarge the pore structure. The amount of carbon species can be feasibly modulated by controlling the post-calcination temperature and the surface area was enlarged further after the partial removal of carbon species. The carbon species and the structure of carbon-modified mesoporous TiO2 have been systematically investigated by various characterizations. Performances of the C-meso-TiO2 materials are evaluated under the illumination of visible light. The synthesized carbon-modified mesoporous TiO2 powders show excellent performance for photocatalytic degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) and can be reused repeatedly without significantly losing the photocatalytic efficiency. Thus, our study presents a novel approach for the fabrication of tunable carbon-modified mesoporous TiO2, which shows excellent photocatalytic ability under visible light irradiation and could be possibly used in the future industrial applications.

2. Experimental section 5

2.1. Fabrication of carbon-modified mesoporous TiO2 The PAA-Ti/TiO2 composite was synthesized following our previously reported method [40], and the detailed proceduce can be found in the summlemetary material. Then the carbonization of PAA-Ti/TiO2 composite was conducted according to previous report [41,42]. Detailly, the as-prepared PAA-Ti/TiO2 nanocomposite was evenly dispersed in dilute H2SO4 (2%) (pH = 0.1) and stirred for 30 min, the resulting mixture were then heated at 100°C for about 12 h and subsequently heated at 160°C for another 12 h to evaporate the water and accomplish the carbonization. The resulting material was washed with water to remove H2SO4. Then the carbon-modified mesoporous TiO2 was obtained with the hydrothermal treatment followed by partial calcination. In detail, the C-TiO2 obtained in above procedure was evenly dispersed in solution consisting 20 mL deionized water and 40 mL EtOH with the aid of sonication. Then, the mixture was moved to Teflon-lined stainless steel autoclave and heated at 160°C for about 24 h. After cooling the autoclave, the collected materials were washed with H2O for three times and with EtOH for two times. After being dried in a vacuum oven, the resulting material was heated under air at 200, 250, 300, 350, 400 and 500°C for 3 h, respectively. The resulting samples were denoted as C-meso-TiO2-X (X being the calcination treatment temperature). As a comparison, the meso-TiO2 was produced from Ti(SO4)2 with NH3·H2O as precipitant and was then synthesized under the same hydrothermal condition. 2.2. Evaluation of photocatalytic activity of C-meso-TiO2-X materials 2-sec-butyl-4,6-dinitrophenol

(DNBP) 6

has

been

commonly

used

as

polymerization inhibitor for vinyl aromatics in chemical industry and herbicide in agriculture [43]. However, it has been found to be toxic, carcinogenic and mutagenic. Thus, DNBP is chosen as a typical model pollutant to evaluate photocatalytic activity of the as-prepared C-meso-TiO2-X materials under the illumination of visible light. The photocatalytic degradation experiment was carried out in a photochemical reactor (XPA-7, Xujiang Electromechanical). A 500 W xenon lamp was used as light source and the short wavelength components (  400 nm) of the light were cut off by a glass optical filter. Typically, 10 mg of the C-meso-TiO2-X catalysts were dispersed in an aqueous DNBP solution (20 mg L-1). After stirring for about 2 h in the dark, the suspensions were treated with the light source for different time. The amount of DNBP was monitored by measuring the absorption intensity at 375 nm using a Double Beam UV-Vis Spectrophotometer. P25 (Degussa), a typical reference TiO2, was used for comparison with C-meso-TiO2-X photocatalyst under the same irradiation condition. To determine the active species in the degradation of DNBP, controlled experiments were conducted by adding different radical scavengers (1 mmol/L) to the reaction system.

3. Results and discussion 3.1. Preparation and characterization of carbon-modified mesoporous TiO2 The process for the fabrication of carbon-modified mesoporous TiO2 can be divided into the following parts (Scheme 1): (i) PAA-Ti/TiO2 composite was obtained through a facile one-pot method using PAAS as coupling molecule, and was then subjected to carbonization by H2SO4 pre-treatment. The H2SO4 used tends to promote 7

the dehydration reactions and facilitate the carbonization process, which was also used in producing carbon species from P123, sucrose and other polymers [44,45]. (ii) The carbonized material was then subjected to a hydrothermal treatment, which enabled the formation of crystallized anatase TiO2 with mesoporous structure. (iii) The resulting materials were calcined at different temperatures to partially remove carbon species, which can not only modulate the amount of the carbon species, but also increase the surface area of C-meso-TiO2 further. Fig. 1 displays the XRD patterns of carbonized PAA-Ti/TiO2 after H2SO4 treatment and C-meso-TiO2-X (X = 200, 250, 300, 350, 400, and 500) samples prepared by the solvothermal treatment followed by calcination at different temperatures. It can be found that crystallization of the amorphous TiO2 occurred in the process of hydrothermal treatment and the XRD patterns of all the C-meso-TiO2-X samples exhibit diffraction peaks that could be ascribed to the phase of anatase TiO2. No signals induced by other polymorphs of titania were noticed and the sharp diffraction peaks indicate the well crystallized structure of the C-meso-TiO2-X. The average crystallite sizes (Table 1) of above samples were calculated according to the Scherrer equation (D = 0.89λ/Bcosθ). The results indicated that the average particle size of samples increased slightly with the raise of post-treatment temperature, which may be ascribed to the secondary growth of TiO2 grain. Compared with the meso-TiO2 obtained without the presence of carbon species, the size of C-meso-TiO2-X were significantly smaller, indicating the presence of carbon species could restrain the excessive growth of TiO2 grain during the solvothermal process. 8

Fig. 2 presents the TEM image of C-meso-TiO2-300. The HRTEM image of C-meso-TiO2-300 (Fig. 2b) reveals a lattice fringe of ~0.35 nm that could be ascribed to the (101) planes of the tetragonal anatase. On the surface of TiO2 grains, carbonaceous species can be seen clearly. Fig. 3 presents N2 adsorption-desorption isotherm of C-meso-TiO2-X samples. The nitrogen adsorption-desorption isotherm shows a type IV isotherm with an H2 hysteresis loop, suggesting the existence of mesopores in the particle matrix. The distribution of pore size present a bimodal shape and it has been reported that a multimodel pore-size distribution was usually caused by the different aggregates in the powders [46,47]. The small mesopore is related to the intra-aggregated pore resulted by intra-agglomeration of primary TiO2 nanocrystals, while the large mesopore arises from larger inter-aggregated pore formed between inter-agglomerated secondary particles. The BET surface area of C-meso-TiO2 obtained by the hydrothermal treatment is 92.7 m2 g-1 and is larger than that of the meso-TiO2 (76.6 m2 g-1), which may be caused by the restrained growth of TiO2 grains in the presence of carbon species during the solvothermal treatment and is in accordance with the smaller grain size (12.0 nm) compared with that of meso-TiO2 (19.6 nm). After subjected to heat-treatment at 200°C, the BET surface area increases to 135.7 m2 g-1, which can be ascribed to the removal of carbon species from the inside of the channel. However, the crystallite growth may occur simultaneously during the calcination process. Therefore, the gradual decrement of the BET surface area of C-meso-TiO2-X (X = 250, 300, 350, 400, and 500) might be due to the further crystallization with the 9

increase of treatment temperature. As the presence of carbon species can restrain the coarsening of TiO2 grain during calcinations, therefore the BET surface areas of C-meso-TiO2-X (X = 200, 250 and 300) samples do not change much. Calcination at a temperature higher than 400°C results in a significant decrease of the BET surface area due to the collapse of the mesoporous framework at high temperature. Overall, the above results indicate that the porous structure could be preserved with the modification of carbonaceous species by controlling the post-treatment temperature. The fabricated C-meso-TiO2-X materials exhibit an open mesoporous structure, which tends to provide more active sites for the adsorption of organic pollutants. Fig. S4 shows the FT-IR patterns of C-meso-TiO2-X samples. The intense band at 1630 cm-1 and broad band located at 3400 cm-1 may be related to O-H bending and stretching vibration of adsorbed water, respectively [48]. The absorption peak at 500-900 cm-1 could be attributed to the stretching vibration of Ti-O-Ti [49,50]. Noticeably, the FT-IR spectra of C-meso-TiO2-X samples (X = 200, 250, 300, 350) have a same peak at the position of 1415 cm-1, which can be attributed to a carbon-related species [51,52] and weakened gradually with increase in calcination temperature. However, this peak was not seen in the FT-IR profile of C-meso-TiO2-400 and C-meso-TiO2-500 samples, indicating the decomposition of most carbonaceous species at above 400°C. And this is also in accordance with the result of Raman spectra analysis (Fig. S5). The broad peaks introduced by the amorphous carbon-related substrate in Raman spectra decreased gradually with the increase of treatment temperature and can be hardly seen in C-meso-TiO2-400 and 10

C-meso-TiO2-500 samples. Fig. 4 shows the wide survey spectrum and the high resolution XPS spectrum of the C-meso-TiO2-300 sample. The full survey spectrum (Fig. 4a) indicated the presence of titanium (Ti 2p), oxygen (O 1s), and carbon (C 1s) in the C-meso-TiO2-300 sample. Fig. 4b displays the high resolution XPS spectrum of the binding energy for Ti 2p. Two intense peaks located at 458.5 and 464.2 eV could be attributed to the Ti 2p3/2 and Ti 2p1/2 respectively, which indicate the existence of Ti4+ ions in TiO2. [10,53]. Fig. 4c shows the C 1s spectra of C-meso-TiO2-300. The major peak located at 284.6 eV was ascribed to the neutral C–C bond and the large intensity indicates the signal was from carbon species loaded onto TiO2. The peaks located at 286.3 eV and 288.6 eV could be related to C–O and C=O bonds, respectively [25,33], whose concurrent presence suggests the presence of carbonate-like species [54]. The peak at 282 eV could not be observed, which indicates that the oxygen species in the TiO2 lattice were not substituted by the carbon species by forming a Ti-C bond [55]. As shown in Fig 4d, the O 1s spectra of C-meso-TiO2-300 display two peaks located at 529.7 eV and 531.5 eV, respectively. The peak at 529.7 eV is associated with oxygen atoms in the TiO2 lattice, while the peak located at 531.5 eV is related to the oxygen atoms in C–O and C=O bonds in carbonate groups [20,56]. Based on the analysis of C 1s and O 1s XPS spectra, the existence of carbonates was confirmed. Fig. 5 and Fig. S6 displayed the diffuse reflectance UV-visible spectra of C-meso-TiO2-X with different carbon amounts and Degussa P25, respectively. 11

Compared with Degussa P25, the spectra of the C-meso-TiO2-X samples shift to the longer wavelength, indicating the decrease in their band gap. It is clear that the visible light absorption from 400 to 800 nm decreased systematically with the raise in heating temperature from 200 to 500°C for C-meso-TiO2-X samples, which is caused by the decrease in carbonaceous content. In the post-treatment process, the amount of carbon species in C-meso-TiO2-X were modulated by the calcination at different temperature under air atmosphere, i.e., the light absorption property of C-meso-TiO2-X could be regulated by the calcination temperature. As presented in the thermogravimetric analysis (Fig. S7 and S8), the weight loss occurred below 100°C could be attributed to the removal of trapped solvent and water. Heat treatment ranging from 100 to 400°C leads to additional weight reduction, which might be related to the decomposition of carbon related species [20]. The removal of carbon species was further confirmed by the elemental analysis (Table 1) and the decomposition of the carbon species occurring at low temperature suggests that most carbonaceous species were loaded on TiO2 surface rather than TiO2 lattice [57]. According to the method reported in the literature [48,58], the band gap energy of reference Degussa P25 was calculated to be 3.05 eV, while that of C-meso-TiO2-300 is 2.83 eV. Additionally, valence band (VB) XPS of Degussa P25 and C-meso-TiO2-300 were also measured, as exhibited in Fig.6. The characteristic VB density of states (DOS) of TiO2 was displayed in Degussa P25, with the band edge located at around 1.93 eV below the Fermi energy. As the band gap of Degussa P25 is about 3.05 eV, the conduction band minimum (CBM) of Degussa P25 should be 12

located at -1.12 eV. Compared with that of Degussa P25, a notable difference observed in the VB-XPS of C-meso-TiO2-300 is the presence of a band tail. The main absorption onset in C-meso-TiO2-300 is located at 1.56 eV, therefore, its corresponding CBM would occur at -1.27 eV. The maximum energy related to the band tail is around 0.72 eV, consequently, remarkable band gap narrowing of C-meso-TiO2-300 (1.99 eV) could be resulted by the substantial shifts of VB tails. The decrease of band gap in C-meso-TiO2-300 can be attributed to the surface modification with carbon species in TiO2. It has been recognized that modifying TiO2 with carbon species can significantly narrow down the band gap of TiO2 [10,20]. As a result, the obtained carbonate-modified mesoporous TiO2 exhibit strong visible light absorption. Additionally, the incorporation of carbonaceous species with coke-like structure could act as sensitizer by extending the absorption edge of TiO2 toward visible light [25,59]. The red shift observed in UV-vis/DRS is also in accordance with the color change of the samples (Fig. S6, Supporting information). The separation efficiency of charge carrier pairs of C-meso-TiO2-X samples and Degussa P25 were examined by photo-current tests, electrochemical impedance spectroscopy (EIS) and fluorescence spectroscopy measurements, respectively. The photocurrent-time (I-t) curves presented in Fig. 7a were obtained by several on-off treatment cycles of intermittent irradiation. Clearly, under the visible light irradiation, the optimum C-meso-TiO2-300 sample displayed the highest photocurrent transient response, indicating the superior separation efficiency of the photo-excited electron-hole pairs within this material. The enhanced transfer rate of charge carriers 13

in C-meso-TiO2-300 sample could also be verified by the analysis of EIS. As can be seen in Fig. 7b, the order of the frequency semicircles decreases is C-meso-TiO2 > Degussa P25 > C-meso-TiO2-500 > C-meso-TiO2-400 > C-meso-TiO2-200 > C-meso-TiO2-250 > C-meso-TiO2-350 > C-meso-TiO2-300. In the EIS Nyquist diagram, the smaller frequency semicircle of the arc is associated with the smaller resistance at the interface and the smaller charge transfer resistance on the electrode surface. Additionally, as shown in Fig.8, C-meso-TiO2-300 shows the lowest PL intensity, which suggests the lowest recombination level of electrons and holes. The intensities of PL signals of C-meso-TiO2-X (X = 200 and 250) are higher because the excess carbonaceous species can act as recombination center for the photo-generated electron-hole pairs. Besides, the higher PL intensities in C-meso-TiO2-X (X = 350, 400 and 500) can be also observed, which is due to the significant decomposition of carbonaceous species at higher treatment temperatures. The PL results are consistent with the photo-current tests and EIS measurements, and this result further indicates that the efficient separation of charge carriers can only be achieved by keeping the carbonaceous species at appropriate level. 3.2. Photocatalytic degradation of DNBP under visible light irradiation The photocatalytic performance of carbon-modified mesoporous TiO2 was examined by the degradation of DNBP under the illumination of visible light. Fig. 9a presents changes of the relative concentrations of DNBP with irradiation time. It can be found that C-meso-TiO2-X exhibited higher photocatalytic efficiency than Degussa P25 in the degradation of DNBP under visible light irradiation, which is in accordance 14

with the red shift of the C-meso-TiO2-X materials. As can be seen in Fig. 9b, the photocatalytic removal of DNBP follows a pseudo first-order kinetic, and the photocatalytic process can be described using the equation ln (C0/C) = kt, in which C0 and C are the initial and actual concentration of DNBP, and k denotes the apparent degradation reaction rate constant. As shown in Fig. 9c, among the C-meso-TiO2-X samples, C-meso-TiO2-300 presented the highest photocatalytic efficiency. Though C-meso-TiO2-X (X = 200 and 250) have more carbon related species and thus stronger absorption in the visible light range, the excess amount of carbon species have negative impact on the improvement of photocatalytic performance and similar conclusion was also made in other studies [60-64]. Excess carbon species in the C-meso-TiO2-X materials can hamper the photocatalytic activity by acting as recombination centers. Besides, the excess amount of carbon might affect the light reaching the TiO2, which also results in decreased photocatalytic performance. To the C-meso-TiO2-X (X = 350, 400 and 500) samples, with the elevation of heat treatment temperature, the surface area become smaller and deposited carbon contents become lesser, which lead to a poorer visible light response and thus reduced photocatalytic performance. That is to say, the presence of suitable amount of carbonaceous materials modified on the TiO2 nanocrystals is beneficial to improve the photocatalytic activity under visible light irradiation. Fig. 9d shows the typical absorption spectra of DNBP aqueous solution under visible-light illumination for different irradiation time in the presence of C-meso-TiO2-300 sample. A progressive absorption decrease at wavelength of 375 nm (characteristic absorption peak for 15

DNBP molecule) was observed, which indicates that the decomposition of DNBP was occurring under visible light illumination. The absorption at wavelength of 375 nm almost completely disappears after 3 h of irradiation, indicating the complete degradation of DNBP. The photostability is also a crucial parameter for the practical application of the C-meso-TiO2-X

photocatalysts.

Therefore,

repetitive

usage

experiment

of

C-meso-TiO2-300 was also performed. Conspicuously, as shown in Fig. 10, the catalysis efficiency of C-meso-TiO2-300 photocatalysts was still higher than 90% after being used for six cycles, which suggests that the synthesized C-meso-TiO2-300 photocatalyst has good repeatability. The slight reduction in the photocatalytic degradation efficiency among the cycles may be ascribed to the formation and accumulation of by-products in the cavities and on the active surface sites of the photocatalyst during the degradation reaction process. Overall, the high photocatalytic activity, excellent structural stability and good reproducibility make the synthesized photocatalysts suitable to be used in wastewater treatment. The active species, including holes (h+), hydroxide radical (OH) and superoxide radical (O2-), etc. play important roles during the photocatalytic degradation process. In order to evaluate the importance of these primary actives in the degradation of DNBP, several controlled experiments were conducted by adding different scavengers. Fig. 11 exhibits the photocatalytic performance of C-meso-TiO2-300 in presence of different scavengers, i.e., ammonium oxalate (AO) scavenger for h+, benzoquinone (BQ) scavenger for O2- and tert-butyl alcohol (TBA) scavenger for OH, respectively 16

[9]. Obviously, the photocatalytic degradation of DNBP could be inhibited by the presence of AO, TBA and BQ, and the importance of these active species in the degradation of DNBP follows the sequence of OH ˃ O2- ˃ h+. The OH and O2play major role in the photocatalytic degradation of DNBP, while holes (h+) play a relatively minor role. 3.3. Mechanism of the photocatalysis Based on the characterizations of C-meso-TiO2-X, the possible band structure of the C-meso-TiO2-X and photocatalytic mechanism under visible light irradiation was proposed (Fig. 12). Two possible pathways that can generate active species to degrade DNBP or other organic pollutants under the irradiation of visible light: Firstly, the C-doping existing at the interface between the carbon part and the TiO2 core could narrow the band gap of TiO2 to make it to be excited by visible light [26]. Under the illumination of visible light, electron in valence band would be excited and transferred to conduction band, while the hole remains in the valence band. The photogenerated hole could oxidize the adsorbed water molecule (OH) to form the OH radical with strong oxidation power. Secondly, upon visible light irradiation, carbonate species modified on the surface of TiO2 can play a role of photo-sensitizer (P) by absorbing visible light and transferring electrons into the conduction band of TiO2 [25,65,66]. The injected electrons can be captured by the adsorbed oxygen molecule [O2(ads)], leading to the formation of O2. During the process of photocatalysis, the organic pollutants can be attacked by OH radicals and O2 produced through above two pathways, which would be decomposed into CO2 and H2O after a series of reactions. 17

However, if the layer of coated carbon is too thick to allow light from irradiating on the photocatalyst, the photocatalysis reaction would be restrained.

4. Conclusions In summary, carbon-modified mesoporous TiO2 (C-meso-TiO2) photocatalysts have been successfully synthesized by combining the in-situ carbonization of PAA-Ti/TiO2, hydrothermal process and post-calcination treatment. The amount of carbon species can be feasibly modulated by adjusting the calcination temperature and the surface area could be further enlarged after the partial removal of carbon species. The deposited carbon species can narrow down the band gap of anatase TiO2 and act as photosensitizer to absorb visible light irradiation, which can remarkably enhance the visible light photocatalytic activity. The obtained C-meso-TiO2 powders present excellent performance for photocatalytic decomposition of DNBP under the illumination of visible light. The C-meso-TiO2 catalyst synthesized in this study has good repeatability and no obvious decline in efficiency of the catalyst was observed after six consecutive runs, which made the application of the C-meso-TiO2 catalysts for water pollution treatment more practical.

Acknowledgements We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Project No.21377018) and the Fundamental Research Funds for the Central Universities (DUT15ZD240).

18

References [1] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891-2959. [2] S.G. Kumar, K.S.R.K. Rao, Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO2, WO3 and ZnO), Appl. Surf. Sci. 391 (2017) 124-148. [3] C. Lai, M.M. Wang, G.M. Zeng, Y.G. Liu, D.L. Huang, C. Zhang, R.Z. Wang, P. Xu, M. Cheng, C. Huang, H.P. Wu, L. Qin, Synthesis of surface molecular imprinted TiO2/graphene photocatalyst and its highly efficient photocatalytic degradation of target pollutant under visible light irradiation, Appl. Surf. Sci. 390 (2016) 368-376. [4] L.J. Luo, Y. Yang, A. Zhang, M. Wang, Y.J. Liu, L.C. Bian, F.Z. Jiang, X.J. Pan, Hydrothermal synthesis of fluorinated anatase TiO2/reduced graphene oxide nanocomposites and their photocatalytic degradation of bisphenol A, Appl. Surf. Sci. 353 (2015) 469-479. [5] C.P. Sajan, S. Wageh, A.A. Al-Ghamdi, J.G. Yu, S.W. Cao, TiO2 nanosheets with exposed {001} facets for photocatalytic applications, Nano Res. 9 (2016) 3-27. [6] X. Li, J.G. Yu, J.X. Low, Y.P. Fang, J. Xiao, X.B. Chen, Engineering heterogeneous semiconductors for solar water splitting, J. Mater. Chem. A 3 (2015) 2485-2534. [7] M. Myilsamy, M. Mahalakshmi, V. Murugesan, N. Subha, Enhanced 19

photocatalytic activity of nitrogen and indium co-doped mesoporous TiO2 nanocomposites for the degradation of 2,4-dinitrophenol under visible light, Appl. Surf. Sci. 342 (2015) 1-10. [8] E.M. Samsudin, S.B. Abd Hamid, J.C. Juan, W.J. Basirun, Influence of triblock copolymer (pluronic F127) on enhancing the physico-chemical properties and photocatalytic response of mesoporous TiO2, Appl. Surf. Sci. 355 (2015) 959-968. [9] F.J. Wu, W. Liu, J.L. Qiu, J.Z. Li, W.Y. Zhou, Y.P. Fang, S.T. Zhang, X. Li, Enhanced photocatalytic degradation and adsorption of methylene blue via TiO2 nanocrystals supported on graphene-like bamboo charcoal, Appl. Surf. Sci. 358 (2015) 425-435. [10] J. Liu, Q. Zhang, J. Yang, H. Ma, M.O. Tade, S. Wang, J. Liu, Facile synthesis of carbon-doped mesoporous anatase TiO2 for the enhanced visible-light driven photocatalysis, Chem. Commun. 50 (2014) 13971-13974. [11] J.Q. Wen, X. Li, W. Liu, Y.P. Fang, J. Xie, Y.H. Xu, Photocatalysis fundamentals and surface modification of TiO2 nanomaterials, Chinese J. Catal. 36 (2015) 2049-2070. [12] J.X. Low, B. Cheng, J.G. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658-686. [13] P. Bouras, E. Stathatos, P. Lianos, Pure versus metal-ion-doped nanocrystalline titania for photocatalysis, Appl. Catal. B-Environ. 73 (2007) 51-59. 20

[14] S.M. Chang, W.S. Liu, Surface doping is more beneficial than bulk doping to the photocatalytic activity of vanadium-doped TiO2, Appl. Catal. B-Environ. 101 (2011) 333-342. [15] N.F. Jaafar, A.A. Jalil, S. Triwahyono, Visible-light photoactivity of plasmonic silver supported on mesoporous TiO2 nanoparticles (Ag-MTN) for enhanced degradation of 2-chlorophenol: Limitation of Ag-Ti interaction, Appl. Surf. Sci. 392 (2017) 1068-1077. [16] A.W. Xu, Y. Gao, H.Q. Liu, The preparation, characterization, and their photocatalytic activities of rare-earth-doped TiO2 nanoparticles, J. Catal. 207 (2002) 151-157. [17] Z.M. El-Bahy, A.A. Ismail, R.M. Mohamed, Enhancement of titania by doping rare earth for photodegradation of organic dye (Direct Blue), J. Hazard. Mater. 166 (2009) 138-143. [18] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269-271. [19] X. Chen, C. Burda, The electronic origin of the visible-light absorption properties of C-, N- and S-doped TiO2 nanomaterials, J. Am. Chem. Soc. 130 (2008) 5018-5019. [20] B. Liu, L.-M. Liu, X.-F. Lang, H.-Y. Wang, X.W. Lou, E.S. Aydil, Doping high-surface-area mesoporous TiO2 microspheres with carbonate for visible light hydrogen production, Energ Environ Sci, 7 (2014) 2592-2597. [21] Y.C. Li, Y.Q. Wang, J.H. Kong, H.X. Jia, Z.S. Wang, Synthesis and 21

characterization of carbon modified TiO2 nanotube and photocatalytic activity on methylene blue under sunlight, Appl. Surf. Sci. 344 (2015) 176-180. [22] D. He, Y.L. Li, I.S. Wang, J.S. Wu, Y.L. Yang, Q.E. An, Carbon wrapped and doped TiO2 mesoporous nanostructure with efficient visible-light photocatalysis for NO removal, Appl. Surf. Sci. 391 (2017) 318-325. [23] X.J. Wang, J.K. Song, J.Y. Huang, J. Zhang, X. Wang, R.R. Ma, J.Y. Wang, J.F. Zhao, Activated carbon-based magnetic TiO2 photocatalyst codoped with iodine and nitrogen for organic pollution degradation, Appl. Surf. Sci. 390 (2016) 190-201. [24] Y. Park, W. Kim, H. Park, T. Tachikawa, T. Majima, W. Choi, Carbon-doped TiO2 photocatalyst synthesized without using an external carbon precursor and the visible light activity, Appl. Catal. B-Environ. 91 (2009) 355-361. [25] L. Zhang, M.S. Tse, O.K. Tan, Y.X. Wang, M.D. Han, Facile fabrication and characterization of multi-type carbon-doped TiO2 for visible light-activated photocatalytic mineralization of gaseous toluene, J. Mater. Chem. A 1 (2013) 4497-4507. [26] S. Wang, L. Zhao, L.N. Bai, J.M. Yan, Q. Jiang, J.S. Lian, Enhancing photocatalytic activity of disorder-engineered C/TiO2 and TiO2 nanoparticles, J. Mater. Chem. A 2 (2014) 7439-7445. [27] J. Matos, A. Garcia, L. Zhao, M.M. Titirici, Solvothermal carbon-doped TiO2 photocatalyst for the enhanced methylene blue degradation under visible light, Appl. Catal. A-Gen. 390 (2010) 175-182. 22

[28] L.-W. Zhang, H.-B. Fu, Y.-F. Zhu, Efficient TiO2 photocatalysts from surface hybridization of TiO2 particles with graphite-like carbon, Adv. Funct. Mater. 18 (2008) 2180-2189. [29] M.A. Mohamed, W.N.W. Salleh, J. Jaafar, M.S. Rosmi, Z.A.M. Hir, M. Abd Mutalib, A.F. Ismail, M. Tanemura, Carbon as amorphous shell and interstitial dopant in mesoporous rutile TiO2: Bio-template assisted sol-gel synthesis and photocatalytic activity, Appl. Surf. Sci. 393 (2017) 46-59. [30] S.U. Khan, M. Al-Shahry, W.B. Ingler, Jr., Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243-2245. [31] X.X. Wang, S. Meng, X.L. Zhang, H.T. Wang, W. Zhong, Q.G. Du, Multi-type carbon doping of TiO2 photocatalyst, Chem. Phys. Lett. 444 (2007) 292-296. [32] H. Liu, A. Imanishi, Y. Nakato, Mechanisms for photooxidation reactions of water and organic compounds on carbon-doped titanium dioxide, as studied by photocurrent measurements, J. Phys. Chem. C 111 (2007) 8603-8610. [33] J.H. Park, S. Kim, A.J. Bard, Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting, Nano Lett. 6 (2006) 24-28. [34] M.E. Hassan, L.C. Cong, G.L. Liu, D.W. Zhu, J.B. Cai, Synthesis and characterization

of

C-doped

TiO2

thin

films

for

visible-light-induced

photocatalytic degradation of methyl orange, Appl. Surf. Sci. 294 (2014) 89-94. [35] D. Chen, L. Cao, F. Huang, P. Imperia, Y.B. Cheng, R.A. Caruso, Synthesis of monodisperse mesoporous titania beads with controllable diameter, high surface areas, and variable pore diameters (14-23 nm), J. Am. Chem. Soc. 132 (2010) 23

4438-4444. [36] L. Zhao, X. Chen, X. Wang, Y. Zhang, W. Wei, Y. Sun, M. Antonietti, M.M. Titirici, One-step solvothermal synthesis of a carbon@TiO2 dyade structure effectively promoting visible-light photocatalysis, Adv. Mater. 22 (2010) 3317-3321. [37] J.H. Pan, X. Zhang, A.J. Du, D.D. Sun, J.O. Leckie, Self-etching reconstruction of hierarchically mesoporous F-TiO2 hollow microspherical photocatalyst for concurrent membrane water purifications, J. Am. Chem. Soc. 130 (2008) 11256-11257. [38] D.G. Shchukin, R.A. Caruso, Template synthesis and photocatalytic properties of porous metal oxide spheres formed by nanoparticle infiltration, Chem. Mater 16 (2004) 2287-2292. [39] Y. Ren, L.J. Hardwick, P.G. Bruce, Lithium intercalation into mesoporous anatase with an ordered 3D pore structure, Angew Chem. Int .Ed. 49 (2010) 2570-2574. [40] X.-N. Wei, H.-L. Wang, Facile fabrication of hydrophilic PAA-Ti/TiO2 nanocomposite for selective enrichment and detection of phosphopeptides from complex biological samples, Anal. Chim. Acta 949 (2017) 67-75. [41] H. Qin, Z. Hu, F. Wang, Y. Zhang, L. Zhao, G. Xu, R. Wu, H. Zou, Facile preparation of ordered mesoporous silica-carbon composite nanoparticles for glycan enrichment, Chem. Commun. 49 (2013) 5162-5164. [42] G. Xu, S. Liu, J. Peng, W. Lv, R. Wu, Facile synthesis of gold@graphitized mesoporous

silica

nanocomposite 24

and

its

surface-assisted

laser

desorption/ionization for time-of-flight mass spectroscopy, ACS Appl. Mater. Interfaces 7 (2015) 2032-2038. [43] M. Matsumoto, C. Poncipe, M. Ema, Review of developmental toxicity of nitrophenolic herbicide dinoseb, 2-sec-butyl-4,6-dinitrophenol, Reprod. Toxicol. 25 (2008) 327-334. [44] J. Kim, J. Lee, T. Hyeon, Direct synthesis of uniform mesoporous carbons from the carbonization of as-synthesized silica/triblock copolymer nanocomposites, Carbon 42 (2004) 2711-2719. [45] P. Valle-Vigón, M. Sevilla, A.B. Fuertes, Synthesis of uniform mesoporous carbon capsules by carbonization of organosilica nanospheres, Chem. Mater. 22 (2010) 2526-2533. [46] J.G. Yu, L.J. Zhang, B. Cheng, Y.R. Su, Hydrothermal preparation and photocatalytic activity of hierarchically sponge-like macro-/mesoporous titania, J. Phys. Chem. C 111 (2007) 10582-10589. [47] J.G. Yu, G.H. Wang, B. Cheng, M.H. Zhou, Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders, Appl. Catal. B-Environ. 69 (2007) 171-180. [48] X.N. Wei, X. Shi, Mesoporous Zirconium phenylphosphonates for selective enrichment of phosphopeptides, J. Phys. Chem. C 118 (2014) 4213-4221. [49] X.-N. Wei, H.-L. Wang, Z.-D. Li, Z.-Q. Huang, H.-P. Qi, W.-F. Jiang, Fabrication of the novel core-shell MCM-41@mTiO2 composite microspheres with large specific surface area for enhanced photocatalytic degradation of dinitro butyl 25

phenol (DNBP), Appl. Surf. Sci. 372 (2016) 108-115. [50] M. Ye, Q. Zhang, Y. Hu, J. Ge, Z. Lu, L. He, Z. Chen, Y. Yin, Magnetically recoverable core-shell nanocomposites with enhanced photocatalytic activity, Chem.-Eur. J. 16 (2010) 6243-6250. [51] X.Y. Wu, S. Yin, Q. Dong, C.S. Guo, H.H. Li, T. Kimura, T. Sato, Synthesis of high visible light active carbon doped TiO2 photocatalyst by a facile calcination assisted solvothermal method, Appl. Catal. B-Environ. 142 (2013) 450-457. [52] R. Miao, Z. Luo, W. Zhong, S.Y. Chen, T. Jiang, B. Dutta, Y. Nasr, Y.S. Zhang, S.L. Suib, Mesoporous TiO2 modified with carbon quantum dots as a high-performance visible light photocatalyst, Appl. Catal. B-Environ. 189 (2016) 26-38. [53] S.K. Parayil, H.S. Kibombo, C.M. Wu, R. Peng, J. Baltrusaitis, R.T. Koodali, Enhanced photocatalytic water splitting activity of carbon-modified TiO2 composite materials synthesized by a green synthetic approach, Int. J. Hydrogen Energ. 37 (2012) 8257-8267. [54] J.D. Zhuang, Q.F. Tian, H. Zhou, Q. Liu, P. Liu, H.M. Zhong, Hierarchical porous TiO2@C hollow microspheres: one-pot synthesis and enhanced visible-light photocatalysis, J. Mater. Chem. 22 (2012) 7036-7042. [55] D.E. Gu, Y. Lu, B.C. Yang, Y.D. Hu, Facile preparation of micro-mesoporous carbon-doped TiO2 photocatalysts with anatase crystalline walls under template-free condition, Chem. Commun. (2008) 2453-2455. [56] M. Bou, J.M. Martin, T.L. Mogne, L. Vovelle, Chemistry of the interface between 26

aluminium and polyethyleneterephthalate by XPS, Appl. Surf. Sci. 47 (1991) 149-161. [57] J.

Zhong,

F.

Chen,

J.L.

Zhang,

Carbon-deposited

TiO2:

Synthesis,

characterization, and visible photocatalytic performance, J. Phys. Chem. C 114 (2010) 933-939. [58] H.-L. Wang, Y. Li, L. Pang, W.-Z. Zhang, W.-F. Jiang, Preparation and application of thermosensitive poly(NIPAM-co-MAH--CD)/(TiO2-MWCNTs) composites for photocatalytic degradation of dinitro butyl phenol (DNBP) under visible light irradiation, Appl. Catal. B-Environ. 130 (2013) 132-142. [59] C. Lettmann, K. Hildenbrand, H. Kisch, W. Macyk, W.F. Maier, Visible light photodegradation of 4-chlorophenol with a coke-containing titanium dioxide photocatalyst, Appl. Catal. B-Environ. 32 (2001) 215-227. [60] Y. Cong, J.L. Zhang, F. Chen, M. Anpo, Synthesis and characterization of nitrogen-doped TiO2 nanophotocatalyst with high visible light activity, J. Phys. Chem. C, 111 (2007) 6976-6982. [61] M.Y. Xing, J.L. Zhang, F. Chen, Photocatalytic performance of N-doped TiO2 adsorbed with Fe3+ ions under visible light by a redox treatment, J. Phys. Chem. C, 113 (2009) 12848-12853. [62] M.Y. Xing, D.Y. Qi, J.L. Zhang, F. Chen, One-step hydrothermal method to prepare carbon and lanthanum co-doped TiO2 nanocrystals with exposed {001} facets and their high UV and visible-light photocatalytic activity, Chem.-Eur. J. 17 (2011) 11432-11436. 27

[63] D.Y. Qi, M.Y. Xing, J.L. Zhang, Hydrophobic carbon-doped TiO2/MCF-F composite as a high performance photocatalyst, J. Phys. Chem. C 118 (2014) 7329-7336. [64] S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: Selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211-13241. [65] X.P. Wang, Y.X. Tang, M.Y. Leiw, T.T. Lim, Solvothermal synthesis of Fe-C codoped TiO2 nanoparticles for visible-light photocatalytic removal of emerging organic contaminants in water, Appl. Catal. A-Gen. 409 (2011) 257-266. [66] D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR, J. Phys. Chem. B 107 (2003) 4545-4549.

28

Scheme and Figure Captions Fig.1. XRD spetra of (a) the carbonized PAA-Ti/TiO2 after H2SO4 treatment, (b) C-meso-TiO2

(without

calcinations

treatment),

(c)

C-meso-TiO2-200,

(d)

C-meso-TiO2-250, (e) C-meso-TiO2-300, (f) C-meso-TiO2-350, (g) C-meso-TiO2-400 and (h) C-meso-TiO2-500. Fig.2. (a) TEM and (b) HRTEM image of C-meso-TiO2-300 sample. Fig.3.

Nitrogen

adsorption-desorption

isotherms

of

C-meso-TiO2

(without

calcinations treatment), C-meso-TiO2-X (X = 200, 300, 400 and 500) and meso-TiO2. Fig.4. XPS spectra of (a) survey, (b) Ti 2p, (c) C 1s and (d) O 1s in C-meso-TiO2-300. Fig.5. UV-vis diffuse reflectance spectra of C-meso-TiO2-X and Degussa P25 (the inset displays the photographs of C-meso-TiO2-300 and Degussa P25). Fig.6. Valence band (VB) XPS of Degussa P25 and C-meso-TiO2-300 sample. Fig.7. (a) The transient photocurrent responses and (b) electrochemical impedance spectroscopy of C-meso-TiO2-X (X = 200, 250, 300, 350, 400, and 500) and Degussa P25 under visible light irradiation. Fig.8. Fluorescence spectroscopy of C-meso-TiO2-X (X = 200, 250, 300, 350, 400, and 500) and Degussa P25. Fig.9. (a) The change of concentration of DNBP treated with different materials, (b) rate constants of C-meso-TiO2-300 and Degussa P25, (c) rate constant of photocatalytic degradation of DNBP as a function of post-calcination temperature and (d) typical absorption spectra of DNBP aqueous solution treated for different time in 29

the presence of C-meso-TiO2-300 under visible light irradiation. Fig.10. Results of recycling studies of C-meso-TiO2-300 catalyst. Fig.11. Photocatalytic activity of C-meso-TiO2-300 sample for the photodegradation of DNBP using different scavengers under exposure to visible light irradiation. Fig.12. Postulated mechanism of the photocatalytic reaction for C-meso-TiO2 under visible light irradiation. Scheme 1. Schematic illustration of synthetic procedure of carbon-modified mesoporous TiO2.

30

Figures

Fig.1

31

Fig.2

32

Fig.3

33

Fig.4

34

Fig.5

35

Fig.6

36

Fig.7

37

Fig.8

38

Fig.9

39

Fig.10

40

Fig.11

41

Fig.12

Scheme 1

42

Table Table 1 BET specific surface area, phase composition, particle size and carbon amount of the as-prepared samples. BET surface area (m2/g)

Phase

Particle size (nm)

Carbon amount (wt %)

C-meso-TiO2

92.7

Anatase

12.0

13.87

C-meso-TiO2-200

135.7

Anatase

12.1

6.77

C-meso-TiO2-250

132.6

Anatase

12.3

4.25

C-meso-TiO2-300

131.6

Anatase

12.5

2.48

C-meso-TiO2-350

111.7

Anatase

13.1

0. 84

C-meso-TiO2-400

89.1

Anatase

13.7

0.14

C-meso-TiO2-500

64.7

Anatase

14.5

0.067

Meso-TiO2

76.6

Anatase

19.6

N/A

Degussa P25

49.0

Anatase & Rutile

21.5

N/A

Sample

43