Construction of Indium and Cerium Codoped Ordered Mesoporous TiO2 Aerogel Composite Material and Its High Photocatalytic Activity

Abstract In this study, ordered mesoporous In2O3‐CeO2/TiO2 aerogel composite material is fabricated via a sol–gel method. According to the preparation process of the aerogel, different weight percentages Ce(NO3)3 and In(NO3)3 are dissolved in the solvent, which would be completely dispersed in the porous gel when the system completely becomes gel. The prepared materials are used to degrade the Rhodamine B (Rh B) under visible light irradiation. 0.2 wt% In2O3‐0.2 wt% CeO2/TiO2 (In0.2‐Ce0.2/TiO2) sample has the highest degradation rate which reaches to 96.20%. When degradation time is continuously increased to 110 min, the degradation efficiency of In0.2‐Ce0.2/TiO2 sample is basically retained. The prepared In0.2‐Ce0.2/TiO2 sample has much better stability and reproducibility under visible light irradiation, the photocatalytic degradation efficiency of In0.2‐Ce0.2/TiO2 sample is still stable at more than 90% after the five times cycle.


DOI: 10.1002/gch2.201700118
pollution control becomes necessary. At present, the widest applied semiconductor photocatalysts mainly include titanium dioxide, zinc oxide, tin oxide and zinc sulfide.
TiO 2 active composite photocatalysts are often used to degrade organic pollutants in air and wastewater. Under the irradiation of ultraviolet light, the mesoscopic struc ture anatase TiO 2 could promote charge separation and improve the photocatalytic activity of materials. [7,8] Due to broad band width, TiO 2 photocatalytic activity is lim ited in ultraviolet light range. At present, the researches on TiO 2 photocatalyst are mainly focused on the increase of its photo catalytic activity in sunlight. Metal or non metallic ions doping is a much more effec tive method. Naldoni et al. [9] introduced Ti 3+ into TiO 2 , increased the oxygen vacancy concentration, decreased bandwidth of TiO 2 , and improved photocatalytic activity of TiO 2 in visible light range. The Au nanoparticles were deposited on mesostructure anatase TiO 2 by injection method, which could sig nificantly improve the photo catalytic activity of material in visible light range. [10] TiO 2 is a promising photocatalytic degradation cat alyst, [11] but the pure TiO 2 nanopowders have a small specific sur face area, poor adsorption and light absorption capacity, and poor separability and reusability. [12,13] CO 2 supercritical assisted liquid crystal soft template method was a new method, which could improve the separation and reusability of catalyst in a certain extent. However, the active site (TiO 2 ) was less, its catalytic activity was relatively low. [14] The matrix materials play a very important role in catalyst design. The matrix materials should have the higher adsorption performance, the greater TiO 2 loading quan tity, and the more active sites. The specific surface area can be enhanced by synthesizing nanostructured TiO 2 , [15][16][17] or load TiO 2 on a high specific surface area substrate. [18][19][20] TiO 2 was doped on fibrous SiO 2 substrate, which has higher photocatalytic activity than in Santa Barbara Amorphous (SBA)15 or mesoporous crys talline material (MCM)41 mesoporous materials. [21][22][23] CeO 2 is one of the most active rare earth metal oxides with a band gap of 2.92 eV. It has a high optical transparency in vis ible region, and possesses a high capacity to store oxygen. [24,25] CeO 2 can be used for photocatalytic degradation of organic pollutants in wastewater. [26,27] The metal ions were doped into TiO 2 , which could improve the electron-hole separation effi ciency, [28] and improved photocatalytic efficiency. [29][30][31][32][33][34] The introduction of CeO 2 into TiO 2 framework can effectively In this study, ordered mesoporous In 2 O 3 -CeO 2 /TiO 2 aerogel composite material is fabricated via a sol-gel method. According to the preparation process of the aerogel, different weight percentages Ce(NO 3 ) 3 and In(NO 3 ) 3 are dissolved in the solvent, which would be completely dispersed in the porous gel when the system completely becomes gel. The prepared materials are used to degrade the Rhodamine B (Rh B) under visible light irradiation. 0.2 wt% In 2 O 3 -0.2 wt% CeO 2 /TiO 2 (In 0.2 -Ce 0.2 /TiO 2 ) sample has the highest degradation rate which reaches to 96.20%. When degradation time is continuously increased to 110 min, the degradation efficiency of In 0.2 -Ce 0.2 /TiO 2 sample is basically retained. The prepared In 0.2 -Ce 0.2 /TiO 2 sample has much better stability and reproducibility under visible light irradiation, the photocatalytic degradation efficiency of In 0.2 -Ce 0.2 /TiO 2 sample is still stable at more than 90% after the five times cycle. Photocatalysis

Introduction
In recent years, the researches on various photocatalysts have attracted more and more attentions which has become a poten tial technical means in environmental and energy sustainable development, such as degradation of organic pollutants in waste water, organic synthesis, photolysis of water, photocata lytic treatment of desalination, and degradation of pharmaceu ticals. [1][2][3][4][5][6] With the improvement of living standards, human beings pay more and more attention on their health and living environment. Water is the source of life and an important material basis for achieving sustainable development. With the industrial accelerated development, water pollution becomes more and more serious, water shortage has become an important factor to restrict economic development, and water extend the visible light response of TiO 2 . Because of its low specific surface area, CeO 2 /TiO 2 catalyst usually has the lower photocatalytic activity. Many researchers have focused on preparation of mesostructured CeO 2 /TiO 2 , which have a large surface area and controllable pore size to improve its photo catalytic activity.
TiO 2 In 2 O 3 composite photocatalysts have been explored by many researchers. In 2 O 3 , a semiconductor with a direct band gap of 3.6 eV and an indirect band gap of 2.8 eV, is an efficient sensitizer to extend the absorption spectra of oxide semiconductor photocatalysts from the UV region to the vis ible region. [35] The electrochemical experiment showed that the molecular O 2 was reduced on the In 2 O 3 surface rather than on the TiO 2 surface at a markedly lower overvoltage. [36] The amount of hydroxyl groups on TiO 2 surface were greatly increased after doping with indium. The super hydrophilic In 3+ doped TiO 2 was also reported by Eshaghi et al. [37] Gonzaílez et al. [38] reported the synthesis, characterization and photo catalytic properties of In 2 O 3 TiO 2 catalysts. The large number of structural defects enhanced the acidity and adsorption of pollutants, and H 3 O + produced much more number of ·OH radicals.
In this study, we prepared indium and cerium oxides codoped ordered mesoporous CeO 2 In 2 O 3 /TiO 2 aerogel composite material by sol-gel without templates method. TiO 2 light absorption band edge distributed in ultraviolet region, the doping of indium and cerium dopants shifted the light absorption band edge to the visible light region, and enhanced the photocatalytic activity by efficiently sepa rating charge carriers (electrons/holes). This study was quite different from the past reported articles in preparation and photocatalytic. The aerogel technology was used to pre pare ordered mesoporous CeO 2 In 2 O 3 /TiO 2 aerogel material, which has never been report on indium and cerium oxides codoped ordered mesoporous TiO 2 . The structure of the pre pared material was characterized by Xray diffraction (XRD), scanning election microscope (SEM), transmission electron microscopy (TEM), N 2 adsorption-desorption method, energy dispersive Xray spectrometer (EDS), and UV-vis spectro photometer. The spectroscopic characterization of the CeO 2 In 2 O 3 /TiO 2 photocatalysts and their photocatalytic activities in degradation of Rhodamine B were also investigated under visible light irradiation. . Moreover, the (101) diffraction peak of the anatase phase TiO 2 was only observed in the samples, and (100) diffraction peaks of the rutile phase TiO 2 was not showed. It was concluded that the TiO 2 was pure anatase phase structure in prepared materials. Because of low dopant concentration, the characteristic diffraction peaks of CeO 2 and In 2 O 3 species were not observed. It was difficult for the eight coordinated Ce in cubic CeO 2 lattice to replace six coor dinated Ti in tetragonal TiO 2 lattice. [39] Hence, Ce ions might be highly dispersed in porous of ordered mesoporous TiO 2 in the form of metal oxides. From Figure 1b, all prepared samples exhibited three peaks near 2θ = 5.4°, 2θ = 8.1°, and 2θ = 10.0°, which could be attributed to (100), (110), and (200) crystal face diffraction peaks of the ordered mesoporous. The diffraction peaks of In 0.2 Ce 0.2 /TiO 2 sample slightly shifted to a higher angle, which revealed small shrinkage of skeleton structure. [40] SEM images of prepared 0.2 wt% CeO 2 /TiO 2 , 0.2 wt% In 2 O 3 / TiO 2 and In 0.2 Ce 0.2 /TiO 2 samples were shown in Figure 2a-c. The prepared composites showed a gel block structure, which were constituted with heterogeneous spherical gel particles. The single irregular spherical particle sizes were about 100, 82, and 50 nm, respectively. EDS images of the prepared samples are shown in Figure 2d-f. The signal of Si was introduced from silicon slice, which was used in test procedure. Ti, O, In, and Ce peaks were obviously found in the energy spectrum, which confirmed that In and Ce existed in TiO 2 matrix. From EDS analysis results, the atomic percentages of Ti, O, In, and Ce were shown in Table 1. Figure 3a-c shows TEM images of the prepared samples, which were taken with beam direction perpendicular to the pores. It could be seen that a large number of ordered porous structures were distributed on the surface of prepared samples.   Figure 4 shows N 2 adsorption-desorption isotherms and pore size distribution curve of the prepared samples. The iso therms were typeIV isotherm with a H1 hysteresis loop, which were the typical characteristic of mesoporous materials with cylindrical pores. The adsorption branch showed an uptake at relative pressure 0.4-0.8 range, it could be attributed to capil lary condensation of nitrogen into mesoporous, which was also a characteristic feature of mesoporous materials. [41] From Figure 4 illustration, the samples had a narrow porous size distribution. In this study, BET (Brunner-Emmett-Teller) method and BJH (Barrett-Joyner-Halenda) method were used to calculate the specific pore parameters. All data were derived from desorption branch of the nitrogen adsorption-desorption isotherm. The average pore size, specific surface area and pore volume were shown in Table 2. With increase of dopant concentration, the particle size decreased, and the surface area increased, which was contributed to refined crystal grain effect of rare earth. At higher dopant concentration, the highly dispersed In 2 O 3 and CeO 2 obstructed the pores of TiO 2 , which resulted in the decrease of porous size. [36] UV-vis absorption spectra of the prepared samples were showed in Figure 5. The prepared TiO 2 aerogel had much better absorption ability to ultraviolet light, which could not absorb the visible light. The absorption band appeared at 350-450 nm, which had an absorption in the visible range. For a semicon ductor, the band edge can be calculated via the follow formulas

Structural Characterizations
where α, ν, E g , and A are absorption coefficient, light frequency, band gap, and a constant, respectively. It should be noted that the n is determined by the type of optical transition of a semi conductor in Equation (1), which should be 1 for the TiO 2 due to its indirect semiconductor nature. Equation (2) Figure 6. In the spectrum of prepared material, the broad band around 3395 cm −1 was due to OH stretching vibration of surface adsorbed water. The bending vibration of water appeared at 1624 cm −1 . The CH 2 stretching vibration of template showed a small band at 2926 cm −1 . The intense sharp band at 1375 cm −1 was attributed to CH 2 bending vibration of the nhexane solution. The band at about 1100 cm −1 was due to CO stretching vibration of alcohol used in the synthesis. The broad band at 617 cm −1 was ascribed to the strong stretching vibration of TiOTi bonds. [42]    TiO 2 aerogel under the visible light. It is because that TiO 2 has only the absorption ability to ultraviolet light. From Figure 7d, it showed that the best doping concentration of Ce or In was both 0.2 wt%, and the degradation efficiency of In 0.2 Ce 0.2 /TiO 2 sample was better than In z Ce 0.2 /TiO 2 (z = 0, 0.1, 0.5, 1). Figure 8 shows Rh B degradation cycling stability test of In 0.2 Ce 0.2 /TiO 2 sample under visible light. During recycling experiment, the catalyst was collected by centrifugation, washed with deionized water, dried at 60 °C, and reused in the next cycles. The photocatalytic degradation efficiency was still stable at more than 90% after five times cycle. It indicated that the prepared sample had better stability and reproducibility under visible light irradiation.

Photocatalytic Mechanism
It is well known that indium and cerium doped titanium dioxide plays an important role in the visible light absorption. [43][44][45][46] The prepared samples can be activated by visible light, and result in the more electrons and holes in photocatalytic oxidationreduction reactions. The doping of indium ions results in the formation of the surface energy level at the electron energy level below the conduction band (CB) of TiO 2 at 0.3 eV. Wang et al. [44] reported the electron transition from the titania valence band (VB) to surface state level. The photocatalyst in visible region is activated, which can be attributed to the reduced bandgap. Photo generated electrons tend to accumulate at the surface state level, and photogenerated holes accumulate on the TiO 2 surface. The large mesoporous make the reactant molecules to easily access the active sites inside the pores, and pro vide the more pathways for the reactants to enter and the products to escape. Due to the charge imbalance on the TiO 2 solid sur face, the adsorption is also increased. When Ti 4+ cations are substituted by In 3+ cations in the titania lattice surface, a charge imbal ance is produced by TiOInOTi framework. The negative charges at the solid surfaces improved the adsorption of H 3 O + as well as cationic Rh B species. [38,45] The formation of surface energy level is a main reason to enhance the photocatalytic activity of visible light. With indium doping, the excess negative charges on the TiOInOTi surface induce the adsorption of H 3 O + , and introduces the Brønsted acidity. Thus, the surface OH groups has also been enhanced. During the photocatalytic reac tion, Rh B molecules adsorb on the surface active site of In 0.2 Ce 0.2 /TiO 2 , which are immediately oxidized by ·OH radicals. At the same time, the photogenerated electrons accumulate at the surface state energy level which are captured directly by Ce 4+ ions, and transfer to the adsorbed O 2 molecules on the surface to form O 2 · − active species. [46,47] The conduction band of TiO 2 or the surface state energy level is more negative than the reduction potential of Ce 4+ to Ce 3+ (+1.61 eV). The excitation energy of the electrons at CB or surface level can reduce Ce 4+ to Ce 3+ under visible light irra diation. The trapped electrons on Ce 4+ site can be easily trans ferred to adsorbed oxides (O 2 · − ) on the surface of In 0.2 Ce 0.2 / TiO 2 catalyst. Under the protonation, the superoxide anion radi cals (O 2 · − ) generates the hydroperoxyl radicals (HO 2 ·) which subsequently produces hydroxyl radicals (·OH). In terms of cerium doping, the relative mobility of oxygen on the surface of TiO 2 is much higher. [25,45] The VB of TiO 2 also has a poten tial of +2.9 eV, which is more positive than the oxidation poten tial of Ce 3+ to Ce 4+ (−1.61 eV). These cerium oxide ion species can carry the species of hydroxyl groups, and produce a large number of highly active hydroxyl radicals (·OH). [43] Ce 3+ and Global Challenges 2018, 2, 1700118   Ce 4+ coexist enhance the photocatalytic activity by inhibiting electron-hole recombination. The excitation of electrons and holes can be carried out by Ce 4+ and Ce 3+ ions through the fol lowing process [48] Ce e Ce 4 3 In addition, the potential of VB in TiO 2 (+2.9 V) is more positive than the reduction potential of OH − to ·OH (+1.9 eV). The VB of TiO 2 has the tendency to accept more electrons, and the photogenerated holes are easily oxidized by the adsorbent OH − to form ·OH radicals, which are a strong oxidizing agent to decompose of Rh B. The electron-hole recombination is sup pressed, and the quantum yield of photocatalysis is enhanced by indium and cerium doping. Visible light irradiation pro duces a large amount of ·OH radical, and improves the photo catalytic activity.
The proposed photocatalysis mechanism of In 0.2 Ce 0.2 / TiO 2 is shown in Figure 9. In 3+ and Ce 4+ /Ce 3+ play an impor tant role in improving the photocatalytic activity of TiO 2 matrix. The surface energy level is below conduction band of TiO 2 (0.3 eV), it dues to the O-In-Ce species on the sur face of the titanium dioxide, and the visible light stimulates the valence band electrons to the surface state level. The excited electrons are transferred by holes of Ce 4+ and Ce 3+ to the hydroxyl groups of the adsorbed oxygen molecules to produce a large amount of superoxide anion radicals and highly active hydroxyl radicals. The charge transfer efficiency and visible activity of the photocatalyst are enhanced by the doped Ce 4+ /Ce 3+ . In addition, the adsorbed H 3 O + and cationic Rh B species on TiO 2 surface are improved by In 3+ , which due to the negative charge on the solid surface. The large number of O 2 · − and ·OH efficiently degrade the adsorbed Rh B molecules, and ultimately enhance the photocatalytic activity.

Conclusions
In this study, the ordered mesoporous In 2 O 3 CeO 2 /TiO 2 aerogel composite material was fabricated via sol-gel method. The prepared In 2 O 3 CeO 2 /TiO 2 composite had anatase TiO 2 crystal structure. The monodisperse particle size was about 8-21 nm, and the prepared composite material retained the porous struc ture of the host material. The prepared materials were used to the degradation of the Rh B. Among all the synthesized photo catalysts with different dopant concentration, In 0.2 Ce 0.2 /TiO 2 nanocomposite showed the highest photocatalytic activity, which could be attributed to the increased surface area with sufficiently porous structure. The large number mesoporous provided the more active sites inside pores. The degradation efficiency of In 0.2 Ce 0.2 /TiO 2 sample reached 96.20% in 110 min, which had the better stability and reproducibility under visible light irradia tion, the photocatalytic degradation efficiency was still stable at more than 90% after the five times cycle.

Experimental Section
Materials: The tetrabutyl titanate (A.R., Shanghai Chemical Pharmaceutical Co., Ltd. China), cerium (III) nitrate hexahydrate (A.R., Tianjin Guangfu Fine Chemical Research Institute), and indium (III) nitrate hydrate (A.R., Sinopharm Chemical Reagent Co., Ltd) were used as the precursors of Ti, Ce, and In, respectively. Aceticacid (A.R., Shanghai Chemical Pharmaceutical Co., Ltd. China) was used as catalyzer. All reactants were analytical grade and used without further purification. Deionized water and absolute alcohol were used throughout. Rhodamine B (A.R., Tianjin Guangfu Fine Chemical Research Institute) was used as the model pollutant for the degradation study.

Synthesis of Ordered Mesoporous CeO 2 -In 2 O 3 /TiO 2 Aerogel Catalysts:
According to the prepared process of aerogel, the different weight percentages Ce(NO 3 ) 3 and In(NO 3 ) 3 were dispersed in the solvent. The solvent would be volatilized, and Ce 2 O 3 and In 2 O 3 would be almost completely dispersed in the porous of the obtained porous TiO 2 substrate when the material was heat treated. The specific synthesis process as follows: 8.7 mL tetrabutyl titanate and 6.2 mL ethanol were mixed with a certain percentage of Ce(NO 3 ) 3 ⋅ 6H 2 O and In(NO 3 ) 3 ⋅ 4.5H 2 O under stirring, the solution was signed as A solution. 3.1 mL acetic acid solution was mixed with 3.8 mL of distilled water with stirring, the solution was signed as B solution. The solution B was rapidly added into the solution A, and formed the gel. The gel was aged at room temperature for 24 h, and then continually aged in water at 70 °C for 2-4 d. The aged gel was soaked in n-hexane solution at 50 °C, and n-hexane solution was replaced three times in 48 h. The gel was dried at 100 °C for 24 h, and obtained the ordered mesoporous CeO 2 -In 2 O 3 /TiO 2 aerogel catalysts.
Photocatalytic Experiments: Photocatalytic activity of the prepared samples for degradation of Rh B was measured under visible light irradiation. The xenon lamp (500 W) was used as the visible light source, which distanced the solution surface about 5 cm. In course of experiment, 50 mg prepared samples were thrown into mixed solution of 5 mL Rh B solution (0.1 g L −1 ) and 95 mL distilled water, and the mixture was stirred in the dark for 1 h, and the photocatalyst with Rh B solution reached an adsorption-desorption equilibrium. Then, the solution was placed in visible light to irradiate, 4 mL of the mixed solution was removed every 5 min, and centrifuged to remove the photocatalyst whose concentration was determined. All of measurements were carried out at room temperature.
Characterization Method: X-ray powder diffraction (X-ray diffraction (XRD)) patterns were collected on a Siemens D5005 diffractometer using Cu-Kα radiation (λ = 1.5418 Å and operating at 30 kV and 20 mA). TEM images were taken on a JEOL 2010 TEM instrument. The SEM images were taken on a JEOL JSM-5600L instrument, the compositions of samples were examined by Oxford ISIS-300 EDS attached to the scanning electron microscope (SEM). The UV-vis absorption spectra for the evaluation of photocatalytic properties were measured through the Shimadzu UV-3600 spectrophotometer. The UVvis spectrophotometer was used to measure the degradation degree of Rh B through N4 spectrophotometer. All the measurements were tested at room temperature. Bet surface area (BET) and pores parameters of In 2 O 3 -CeO 2 /TiO 2 aerogel composites were identified by physical adsorption of nitrogen on a Micromeritics ASAP2010M volumetric adsorption analyzer. The In 2 O 3 -CeO 2 /TiO 2 aerogel composites were degassed in vacuum at 573 K for 3 h before measurement. Pore size was computed using the BJH method. The FT-IR spectrum was recorded on Perkin Elmer FT-IR spectrometer using KBr pellet technique. The pellet was scanned at 4 cm −1 resolution in the range of 4000-400 cm −1 .