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Fabrication of porous TiO2-RGO hybrid aerogel for high-efficiency, visible-light photodegradation of dyes
Journal Pre-proof Fabrication of porous TiO2-RGO hybrid aerogel for high-efficiency, visible-light photodegradation of dyes Xiaoqiang Sun, Shangdong Ji, Minqiang Wang, Jinjuan Dou, Zhi Yang, Hengwei Qiu, Song Kou, Yongqiang Ji, Hui Wang PII:
S0925-8388(19)34279-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.153033
Reference:
JALCOM 153033
To appear in:
Journal of Alloys and Compounds
Received Date: 11 August 2019 Revised Date:
22 October 2019
Accepted Date: 14 November 2019
Please cite this article as: X. Sun, S. Ji, M. Wang, J. Dou, Z. Yang, H. Qiu, S. Kou, Y. Ji, H. Wang, Fabrication of porous TiO2-RGO hybrid aerogel for high-efficiency, visible-light photodegradation of dyes, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153033. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Fabrication of porous TiO2-RGO hybrid aerogel for high-efficiency, visible-light photodegradation of dyes Xiaoqiang Sun#, Shangdong Ji#, Minqiang Wang*, Jinjuan Dou, Zhi Yang, Hengwei Qiu, Song Kou, Yongqiang Ji, Hui Wang Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi'an 710049, China E-mail: [email protected] #
X. S. and S. J. contributed equally.
Abstract In recent years, TiO2-graphene composites of various forms have attracted considerable attention as high-performance photocatalysts because of their desirable attributes including high adsorptivity of pollutants, extended light absorption range, and efficient charge separation. However, TiO2-graphene powders prepared via the classic hydrothermal method is faced with the obstacle of heavy agglomeration between graphene layers, which can cause a huge loss in adsorption and reaction sites. To improve this, we have employed graphene aerogel, which was synthesized via thermal reduction of graphene oxide (GO) aerogel, as an auxiliary material for TiO2. Herein, a highly porous TiO2-reduced graphene oxide (RGO) aerogel with high specific volume and enlarged specific surface area was presented, exhibiting considerably higher adsorption rate and capacity as well as improved photocatalytic efficiency, compared with classic hydrothermal-synthesized TiO2-graphene powders.
The TiO2-RGO aerogels also showed high visible light absorption, endowing it with higher utilization of sunlight. This work presents a new approach for the synthesis of high-performance TiO2-graphene photocatalyst, and also provides a general graphene aerogel-based strategy for the improvement of various other photocatalysts.
Key words: Photocatalysis degradation; TiO2-graphene catalyst; Graphene aerogel; Chemical adsorptivity 1. Introduction TiO2 is a low-cost, non-toxic and high-efficient photocatalyst which has been widely used in pollutant decomposition and hydrogen generation in recent years [1-5]. However, pure TiO2 can only be excited by the ultraviolet (UV) light because of its wide bandgap (~3.2 eV), leading to poor utilization of sun light (3-5%). The easy recombination of photogenerated electron-hole pairs in TiO2 also restricts its catalytic efficiency [6,7]. Meanwhile, graphene, which has desirable properties such as high electron mobility, large specific surface area and high pollutant adsorptivity, is an ideal auxiliary material for photocatalysts. Zhang et al. firstly developed a one-pot hydrothermal method to prepare chemically bonded TiO2-graphene composite photocatalyst [8], which exhibited prominent photocatalytic properties including high adsorptivity of dye pollutants, extended light absorption range and easy charge separation/transportation [2,9,10]. Based on such hydrothermal method, great efforts have been devoted to prepare various TiO2-graphene composites. To date, TiO2-graphene composite photocatalysts of diverse forms have been widely
investigated, including TiO2 nanoparticle-graphene [8,10], TiO2 nanotube-graphene [7,11,12], TiO2-graphene oxide [13,14], and noble metal-doped TiO2-graphene composites [9,15]. Nevertheless, most of the above hydrothermal-synthesized TiO2-graphene composites were in powder form, and they are usually faced with the problem of serious agglomeration between graphene layers during the solution-phase reaction process, which is due to the van der Waals force between adjacent graphene layers [16]. The Brunauer-Emmett-Teller (BET) surface areas of such powder-form TiO2-graphene composites are very limited (40-50 m2/g) [8,10,17], significantly lower than the theoretical specific surface area of graphene (2630 m2/g) [18]. Such agglomeration can render a heavy loss in photocatalytic active sites and pollutant adsorption capacity, which can be highly detrimental to the photocatalytic efficiency of the TiO2-graphene catalysts. In order to suppress the agglomeration of TiO2-graphene composites, graphene aerogel, generally prepared from graphene aqueous dispersions by sol-gel processing and subsequently by supercritical fluid drying or by freeze drying, is employed in this work as a supportive material for TiO2 due to its unique characteristics including high specific surface areas, large pore volumes and tunable porosity [19]. Fig. 1 displays the structure of the TiO2-RGO aerogel, and the mechanisms of selective adsorption and photodegradation. Generally, the efficiency of photocatalysts is determined by three crucial factors: (i) the pollutant molecule adsorption, (ii) the light absorption, and (iii) the charge separation and transportation [20,21]. Interestingly, all the above factors can be improved with the introduction of graphene aerogel. The large
theoretical specific surface area of graphene gives it huge potential adsorption capacity for pollutants, and the highly porous aerogel structure of such TiO2-RGO composites can make the best of such high adsorptivity. Specially, aromatic dyes such as MB and RhB can be very efficiently and selectively adsorbed onto graphene because of the π-π stacking effect between aromatic parts of target molecules and aromatic regions of graphene sheets [22], as illustrated in Fig. 1. Graphene is also an excellent electron acceptor material owning to its two-dimensional π-conjugation structure [22,23], and has been successfully applied in TiO2-graphene composites to suppress the recombination of photogenerated electron-hole pairs [8,24]. In addition, graphene also possesses ultra-high conductivity because of the two-dimensional planar structure [25], thereby the photogenerated energy electrons can be facilely transferred to reaction sites. Given the above characteristics, graphene, especially graphene aerogel, can be an excellent candidate to improve the photocatalysis performance of TiO2. In this work, RGO aerogel was prepared by directly freeze-drying GO aqueous dispersion, and subsequently heating the obtained GO aerogel in a tube furnace for thermal reduction, after which RGO aerogel was obtained. It should be noted that GO has far better dispersity in water than RGO owning to the presence of multiple hydrophilic function groups [26], thereby the graphene aerogels synthesized via this route could have a more uniform microstructure than solution-reduced graphene aerogels. By facilely mixing P25 with the GO precursor, TiO2-RGO hybrid aerogel can be obtained with the same process. When used in the visible light photodegradation of dyes, such TiO2-RGO aerogel
exhibited high pollutant adsorptivity, high visible light utilization and enhanced photocatalytic efficiency. To better illustrate the superior photocatalysis performance of the TiO2-RGO aerogel, TiO2-RGO powder with the same TiO2:RGO ratio (10:1) was also synthesized via the reported hydrothermal method [8] as a contrast. Compared with its hydrothermal-synthesized powder-form counterparts, such TiO2-RGO hybrid aerogel showed enlarged BET surface area and significantly enhanced adsorptivity, which effectively promoted its photocatalytic efficiency. This work presents a new approach for the synthesis of high-performance TiO2-graphene photocatalyst, and is anticipated to provide a general graphene aerogel-based strategy for the improvement of diverse photocatalysts, as well as to facilitate their application in various environmental protection issues. 2. Experimental 2.1 Materials Graphite powder (99.9%, 325 mesh) was purchased from Beijing Wodetai Co., Ltd. P25 (TiO2, 20% rutile and 80% anatase) was purchased from Degussa. Sodium nitrate (NaNO3, AR), potassium permanganate (KMnO4, AR), concentrated sulfuric acid (H2SO4, 98wt%), concentrated hydrochloric acid (HCl, 37wt%), hydrogen peroxide (H2O2, 30wt%), methylene blue (MB, C16H18ClN3S, AR) and rhodamine B (RhB, C28H31ClN2O3, AR) were purchased from different companies. All the reagents were used as received without further purification. The deionized (DI) water used throughout this work was filtered with a Millipore E-pure filtration system at >18 MΩ·cm.
2.2 Synthesis of GO GO was synthesized by the modified Hummers’ method [27]. Briefly, 3 g graphite powder and 1.5 g NaNO3 were added to 70 mL of concentrated H2SO4 (98 wt%) with stirring at room temperature. The mixture was then cooled to 0 °C, followed by the slow addition of 9 g KMnO4. Note that the temperature of the mixture should not exceed 20 °C. Successively, the mixture was kept at 35-40 °C for 0.5 h, after which 140 mL of DI water was added and stirred for another 15 min. With the addition of water, the viscous, dark green fluid turned yellowish-brown, and much heat was generated. After that, 20 mL of H2O2 aqueous solution (30 wt%) and 500 mL of DI water were added to the mixture, and the color turned golden yellow while generating lots of bubbles. After the reaction, the mixture was filtered and rinsed with 250 mL of HCl aqueous solution (10 wt%) to eliminate excess MnO2, and washed repeatedly with H2O to remove acid residues until the pH of the filtrate was neutral. The resulting GO was dispersed in DI water and preserved at room temperature. 2.3 Synthesis of TiO2-RGO aerogel In a typical fabrication of the TiO2-RGO hybrid aerogel, 2 g P25 was added to 100 mL of GO aqueous dispersion (2 mg/mL) and treated with 30 min of ultrasonication and 30 min of magnetic stirring. The ultrasonication and stirring was conducted twice (2 h in total). Thereafter, the mixture was dropwise added into liquid nitrogen very slowly, and the resulting frozen “beads” were subsequently freeze-dried in vacuum for 24 h. The obtained spherical TiO2-GO aerogels were then heated in a tube furnace at 120 °C for 3 h under inert (Ar) atmosphere. After the thermal
treatment, the color of TiO2-GO aerogel “beads” turned from yellowish to dark gray (Supporting information Fig. S1) and easily collapsed into “fluffy” powders. 2.4 Photodegradation experiments The photocatalytic activities of TiO2, TiO2-RGO powder and TiO2-RGO aerogel was measured by the photodegradation of RhB under simulated solar irradiation (Xe lamp, 150 W) as model reaction. Briefly, 40 mg of catalyst was added to 100 mL of RhB aqueous solution (10 mg/mL) under ultrasonication for 10 min. Before illumination, the mixture was magnetically stirred in dark for 60 min to establish an adsorption-desorption equilibrium of RhB molecules with the catalyst [28,29]. Afterwards, the mixture solution was placed under the light source (10 cm below the Xe lamp) for photodegradation. All other lights were insulated during the photocatalysis. At given intervals, 5 mL of the suspension was withdrawn and centrifuged to remove the dispersed catalysts. The concentrations of the clean, transparent RhB solutions were determined by measuring the 554 nm absorbance of RhB. The MB dark adsorption experiments were conducted under the same conditions, and the concentrations of MB solutions were characterized with its 664 nm absorbance. 2.5 Characterization The surface morphologies of all samples were characterized with a FEI Quatan FEG 250 field-emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS). The structural information was obtained with a JEOL JEM-2100 field-emission transmission electron microscope (TEM). The
Diffuse reflectance spectra (DRS) and UV-visible absorption spectra were measured with a Jasco V-570 UV/VIS/NIR spectrophotometer. The X-ray diffraction (XRD) patterns were characterized using a D/max-2400 diffraction spectrometer. The SERS measurements were performed on a HR Evolution-800 Raman microscope system (HORIBA) equipped with standard 633 and 532 nm lasers. The photoluminescence (PL) spectra were measured with a SENS-9000 fluoroSENS fluorescence spectrometer. The X-ray photoelectron spectra (XPS) were measured on an AXIS Ultrabld X-ray photoelectron spectrometer using Al-K α as the exciting source and its binding energy calibration was based on C 1s at 284.8 eV. The N2 adsorption-desorption measurements were performed on an ASAP 2460 surface area and porosity analyzer. The photodegradation experiments were performed with a 150 W Xe lamp (Sciencetech Inc., SS-150). 3. Results and discussions SEM images of the as-synthesized GO aerogel, TiO2-GO aerogel, RGO aerogel and TiO2-RGO aerogel are shown in Fig. 2. It is observed that GO/RGO aerogels contain a large amount of macro pores formed during freeze-drying. The pore volumes and density can be facilely regulated by adjusting the GO dispersion concentration. Such highly porous structure can effectively avoid the agglomeration of GO/RGO, and usually have a large specific surface area [19], which is beneficial for pollutant adsorption. By adding P25 into the GO precursor, TiO2-GO/RGO aerogels can be obtained with the same procedure. The TiO2-GO aerogel consists of GO network frame and dense TiO2 particles loaded on GO layers, as shown in Fig. 2b.
After thermal treatment, the TiO2-GO aerogel transformed into TiO2-RGO aerogel, and its structure turned “fluffier”, exhibiting a huge number of mesopores and macropores (Fig. 2d). In contrast, serious aggregation was observed in the SEM images of TiO2-GO powder synthesized via the hydrothermal method (Fig. S2). EDS element mapping (Fig. 2e) of such TiO2-RGO aerogel revealed that the C, O and Ti elements have a very uniform distribution, indicating that P25 particles are uniformly incorporated into the RGO network. TEM analysis (Supporting information Fig. S3) of the TiO2-RGO aerogel also shows that TiO2 particles are loaded on RGO layers, and lattice fringes of TiO2 particles can be observed in the HRTEM image. The interplanar spacings of 0.315 nm and 0.345 nm corresponds with rutile (101) and anatase (101) planes of TiO2, respectively [30]. UV-vis diffuse reflectance spectra (DRS) were used to investigate the absorption band of P25 and TiO2-RGO powder/aerogel (Fig. 3a). Compared with pure P25, the UV-vis diffuse reflectance spectra of TiO2-RGO powder/aerogel showed an obvious red-shift in the absorption edge and higher absorbance in the whole visible region, which can effectively promote visible light absorption of the catalysts. Based on the DRS results, the bandgaps of pure P25, TiO2-RGO aerogel and TiO2-RGO powder were roughly estimated to be 3.04 eV, 2.75 eV and 2.60 eV, respectively. Such narrowing of TiO2 optical bandgap is attributed to the formation of Ti-O-C bonds [31], and gives the TiO2-RGO composites higher utilization of the solar energy. XRD patterns of TiO2, TiO2-RGO powder and TiO2-RGO aerogel are shown in Fig. 3b. Typical anatase and rutile diffraction peaks can be observed in all samples, indicating
that the crystal phase of TiO2 did not change after hydrothermal/heating treatment. However, the diffraction peak of graphene was not observed. The diffraction peak of graphene (~23°) [32] is likely to be covered by TiO2 (101) peak located at 25.3° because of its low content (10 wt%) and low intrinsic diffraction intensity. Besides, the diffraction peak of GO (~11°) [33] was not observed due to the reduction of GO in the hydrothermal/heating process. Given this, Raman spectra of TiO2-RGO powder/aerogel (Fig. 3c) were used to demonstrate the presence of graphene. Apparently, TiO2-RGO powder and aerogel showed very similar Raman spectra, in which both TiO2 and RGO Raman bands can be clearly observed. To better verify the thermal reduction of GO, XRD patterns of pure GO aerogel before and after heating were shown in Fig. S4. It can be observed that the GO diffraction peak (10.5°) disappeared after the heating treatment, and a broad diffraction peak centered at ~23.5° appeared, which is typical of RGO, indicating the successful thermal reduction. Oxygen functionalities in GO can be reduced when heated at 120 ℃, and oxygen-containing species like COx and H2O are generated during the dihydroxylation process [34]. Such thermal reduction avoids the introduction of various reducing agents such as hydrazine/dimethylhydrazine, L-ascorbic acid and NaBH4 [35-37], thus is a “clean” route. In addition, the solution-free reduction process effectively avoids the agglomeration of graphene layers, and the microstructure can be well preserved. Unlike the TiO2-GO aerogel, which is easily dissolved in water, the as-prepared TiO2-RGO aerogel exhibited poor solubility in water. Such promoted stability is on account of the gradually restored strong π-π
interaction during the thermal reduction process [38] and the removal of hydrophilic groups. Fig. 3d shows the PL spectra of P25, TiO2-RGO powder and TiO2-RGO aerogel aqueous dispersion with the same TiO2 content (0.18 mg/mL). PL quenching effect is an efficient method to characterize the charge transfer within composite materials [39]. It can be observed that with the introduction of RGO, the PL intensity of TiO2 showed a remarkable decrease, indicating the charge transfer from TiO2 to RGO. This is highly desirable because during the photocatalysis process, such charge transportation can greatly suppress the energy electron-hole recombination, and the transferred electrons can take part in the degradation reaction of pollutants adsorbed on RGO, thus the photocatalysis efficiency can be greatly improved [23,40]. Notably, TiO2-RGO powder exhibited stronger PL quenching effect as well as narrower optical bandgap than aerogel, which may indicate that the reduction reaction was more complete in the hydrothermal reduction than in the thermal reduction. The chemical compositions and elemental valence states of the TiO2-RGO aerogel were investigated by X-ray photoelectron spectroscopy (XPS). High-resolution XPS spectra of Ti 2p and C 1s are shown in Fig. 3e-f. As can be observed, the pseaks located at 459.4 and 465.1 eV are characteristic of TiO2 [41]. The peak located at 285.1 eV in the C 1s spectrum is assigned to C-C bond in the aromatic rings of graphene. The peaks at 287.0 and 288.6 eV are respectively assigned to the carbon in C-O and C=O bonds [42]. According to the XPS results, a considerable quantity of oxygen-containing functional groups still existed after the heating treatment, indicating that the thermal reduction of GO was incomplete. Those oxygen-containing groups present on the
incompletely reduced GO can interact with TiO2 nanoparticles, forming hydrogen bonds between them [43], and enhancing the structural stability of the TiO2-RGO aerogel. The specific surface area and pore structure of the TiO2-RGO powder/aerogel were characterized with the N2 adsorption-desorption measurements at 77 K. The Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to respectively characterize the surface area and pore size. As shown in Fig. 4, TiO2-RGO aerogel exhibits a typical type-IV isotherm with a high adsorption capacity in the high relative pressure, suggesting the presence of abundant meso/macropores, as is consistent with its SEM image (Fig. 2d). According to the BET analysis, such TiO2-RGO aerogel has a specific surface area of 83.5 m²/g. As a contrast, hydrothermal-synthesized TiO2-graphene composites with the same graphene content (10 wt%) showed significantly smaller specific surface area of 53.1 m²/g. The enlarged surface area of TiO2-RGO aerogel is on account of the “fluffy”, porous structure. However, the surface area of the TiO2-RGO aerogel is still significantly smaller than pure graphene aerogel (which can be up to 500 m2/g [19,44]), because its dominant component is P25 nanoparticles. The pore size distribution of TiO2-RGO aerogel is calculated with the BJH method. According to the BJH equation from the desorption branch of the isotherm, the TiO2-RGO aerogel showed a broad pore size distribution centered at ~50 nm (inset of Fig. 4). The large specific area and porous structure can greatly promote the pollutant adsorptivity of the TiO2-RGO aerogel. Fig. 5a displays the photograph of 80 mg TiO2-RGO powder synthesized via the
common hydrothermal method, and our TiO2-RGO aerogel. Evidently, the TiO2-RGO aerogel exhibited significantly larger specific volume than mass-equal TiO2-RGO powder. To further characterize their adsorptivity, the two samples were both used for the dark adsorption of MB, and their absorption intensities were checked periodically. According to the Lambert-Beer Law, the concentration of dyes can be characterized with their absorbance: A=ɛcl where A represents the absorbance, ɛ for the molar absorbing coefficient, c for the component concentration, and l for the length of absorbing layer. Obviously, the normalized concentration (C/C0) of dyes was proportional to the normalized maximum absorbance (A/A0). According to the results shown in Fig. 5b, TiO2-RGO aerogel reached the adsorption saturation within 2 min, while it took >10 min for TiO2-RGO powder to reach the adsorption saturation (Detailed absorption spectra were shown in Supporting information Fig. S5). This can be directly observed from the change of their color (Fig. 5c-d). The adsorption rate of catalysts is of great importance during the photodegradation process, because while the adsorbed molecules are continuously degraded, the adsorption-desorption equilibrium should be reestablished, therefore a high adsorption rate is the prerequisite for higher catalytic efficiency [45]. To characterize the photocatalytic activities of TiO2, TiO2-RGO powder and TiO2-RGO aerogel, the photodegradation of RhB under simulated solar irradiation (Xe lamp, 150 W) was used as the model reaction. The above samples were added
into RhB solutions and stirred in dark for 1 h to reach the adsorption-desorption equilibrium. As a result (Fig. 6a), when reaching the adsorption saturation, pure TiO2 can only adsorb 4% of the RhB molecules, while TiO2-RGO powder and aerogel can respectively adsorb 58.7% and 65.3%. Obviously, TiO2-RGO aerogel displayed the highest adsorption capacity due to its large surface area. It should be noted that the TiO2-RGO composites showed better performance for the adsorption of MB, in which TiO2-RGO aerogel can adsorb nearly 90% of the MB molecules, indicating a selective adsorption: RGO exhibits considerably higher adsorptivity towards MB than RhB. After the dark treatment, the mixed solution was exposed to Xe lamp irradiation, and the degradation degree of RhB was checked every 30 min, as shown in Fig. 6a (Detailed absorption spectra were shown in Fig. S6). After 3 h of light illumination, the degraded rates of RhB using TiO2, TiO2-RGO powder and TiO2-RGO aerogel as the catalyst were respectively 62.7%, 77.7% and 84.6%, while there was no degradation without catalysts. Photodegradation experiments without absorption in dark showed similar results (Fig. S7). As described above, both TiO2-RGO powder/aerogel exhibited extended absorption band and promoted visible light absorption, therefore they can better make use of visible light and thus showed higher photocatalytic efficiency. In contrast, pure TiO2 can only absorb ultra-violet light, and the photocatalytic efficiency was significantly lower. As anticipated, the TiO2-RGO aerogel displayed higher photodegradation activity than its powder-form counterpart, which is mainly attributed to its promoted adsorption rate and capacity towards target molecules. To further check the stability of such TiO2-RGO aerogel, it was collected
via centrifugation and rinsed with DI water and ethanol for reuse after the photodegradation process, and repeatedly underwent the same procedures (1 h of dark treatment and 3 h of light illumination) for 5 cycles in total. After each cycle, the final degradation rate was recorded, as shown in Fig. 6b. As a result, the TiO2-RGO aerogel exhibited no obvious decrease in its photocatalytic efficiency after 5 cycles of reuse, thus the TiO2-RGO aerogel also showed high stability. 4. Conclusions In conclusion, TiO2-RGO aerogel was prepared by freeze-drying the mixed P25 and GO aqueous dispersion and subsequently by heating in a tube furnace. The TiO2-RGO aerogel exhibited red-shifted absorption band and enhanced absorbance in the visible region, which led to higher utilization of visible light. PL quenching effect verified the energy electron transfer from TiO2 to RGO, which helps to suppress the recombination of photogenerated electron-hole pairs in TiO2. Compared with the TiO2-RGO powder prepared from the classic hydrothermal method, the TiO2-RGO aerogel showed enlarged specific surface area, which resulted in greatly enhanced adsorption rate and capacity and finally led to higher photodegradation efficiency. The TiO2-RGO aerogel also showed high stability for recyclable use. Such graphene aerogel-based strategy may have potential for the improvement of various catalysts. Acknowledgements This work was financial supported by the Natural Science Foundation of China (NSFC, 61774124, 51572216 and 61604122), the Fundamental Research Funds for the Central Universities (1191329876 and 1191329152), the 111 Program (B14040)
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Figures
Fig. 1. Schematic: Structure of the TiO2-RGO aerogel, mechanisms of the selective adsorption and photodegradation.
Fig. 2. (a-d) SEM images of GO aerogel, TiO2-GO aerogel, RGO aerogel and TiO2-RGO aerogel. (e) SEM image of the TiO2-RGO aerogel and EDS element mapping of the corresponding area.
Fig. 3. (a) Diffuse reflectance spectra of pure TiO2 and TiO2-RGO powder/aerogel. (b) XRD patterns of pure TiO2 and TiO2-RGO powder/aerogel. (c) Raman spectra of the TiO2-RGO powder/aerogel. (d) Photoluminescence spectra of pure TiO2 and TiO2-RGO powder/aerogel with the excitation wavelength of 285 nm. (e-f) High-resolution XPS spectra of Ti 2p and C 1s of the TiO2-RGO aerogel.
Fig. 4. N2 adsorption-desorption isotherm of the TiO2-RGO powder/aerogel at 77 K and the pore size distribution of TiO2-RGO aerogel measured by the BJH method (inset).
Fig. 5. (a) Photograph of 80 mg TiO2-RGO powder (left) and aerogel (right). (b) MB dark adsorption results of the TiO2-RGO aerogel/powder. (c-d) Color changing of MB solution during dark adsorption with TiO2-RGO powder (c) and aerogel (d).
Fig. 6. (a) Dark adsorption and photodegradation of RhB using the above samples under simulated sunlight. (b) The final degradation rate after each cycle.
·Graphene aerogel was employed for the fabrication of TiO2-graphene hybrid catalysts. ·TiO2-RGO aerogel exhibited high pollutant adsorptivity, extended absorption range and efficient charge separation. ·TiO2-RGO aerogel showed higher adsorptivity and improved catalytic efficiency compared with traditional TiO2-RGO powders.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34279-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.153033
Reference:
JALCOM 153033
To appear in:
Journal of Alloys and Compounds
Received Date: 11 August 2019 Revised Date:
22 October 2019
Accepted Date: 14 November 2019
Please cite this article as: X. Sun, S. Ji, M. Wang, J. Dou, Z. Yang, H. Qiu, S. Kou, Y. Ji, H. Wang, Fabrication of porous TiO2-RGO hybrid aerogel for high-efficiency, visible-light photodegradation of dyes, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153033. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Fabrication of porous TiO2-RGO hybrid aerogel for high-efficiency, visible-light photodegradation of dyes Xiaoqiang Sun#, Shangdong Ji#, Minqiang Wang*, Jinjuan Dou, Zhi Yang, Hengwei Qiu, Song Kou, Yongqiang Ji, Hui Wang Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi'an 710049, China E-mail: [email protected] #
X. S. and S. J. contributed equally.
Abstract In recent years, TiO2-graphene composites of various forms have attracted considerable attention as high-performance photocatalysts because of their desirable attributes including high adsorptivity of pollutants, extended light absorption range, and efficient charge separation. However, TiO2-graphene powders prepared via the classic hydrothermal method is faced with the obstacle of heavy agglomeration between graphene layers, which can cause a huge loss in adsorption and reaction sites. To improve this, we have employed graphene aerogel, which was synthesized via thermal reduction of graphene oxide (GO) aerogel, as an auxiliary material for TiO2. Herein, a highly porous TiO2-reduced graphene oxide (RGO) aerogel with high specific volume and enlarged specific surface area was presented, exhibiting considerably higher adsorption rate and capacity as well as improved photocatalytic efficiency, compared with classic hydrothermal-synthesized TiO2-graphene powders.
The TiO2-RGO aerogels also showed high visible light absorption, endowing it with higher utilization of sunlight. This work presents a new approach for the synthesis of high-performance TiO2-graphene photocatalyst, and also provides a general graphene aerogel-based strategy for the improvement of various other photocatalysts.
Key words: Photocatalysis degradation; TiO2-graphene catalyst; Graphene aerogel; Chemical adsorptivity 1. Introduction TiO2 is a low-cost, non-toxic and high-efficient photocatalyst which has been widely used in pollutant decomposition and hydrogen generation in recent years [1-5]. However, pure TiO2 can only be excited by the ultraviolet (UV) light because of its wide bandgap (~3.2 eV), leading to poor utilization of sun light (3-5%). The easy recombination of photogenerated electron-hole pairs in TiO2 also restricts its catalytic efficiency [6,7]. Meanwhile, graphene, which has desirable properties such as high electron mobility, large specific surface area and high pollutant adsorptivity, is an ideal auxiliary material for photocatalysts. Zhang et al. firstly developed a one-pot hydrothermal method to prepare chemically bonded TiO2-graphene composite photocatalyst [8], which exhibited prominent photocatalytic properties including high adsorptivity of dye pollutants, extended light absorption range and easy charge separation/transportation [2,9,10]. Based on such hydrothermal method, great efforts have been devoted to prepare various TiO2-graphene composites. To date, TiO2-graphene composite photocatalysts of diverse forms have been widely
investigated, including TiO2 nanoparticle-graphene [8,10], TiO2 nanotube-graphene [7,11,12], TiO2-graphene oxide [13,14], and noble metal-doped TiO2-graphene composites [9,15]. Nevertheless, most of the above hydrothermal-synthesized TiO2-graphene composites were in powder form, and they are usually faced with the problem of serious agglomeration between graphene layers during the solution-phase reaction process, which is due to the van der Waals force between adjacent graphene layers [16]. The Brunauer-Emmett-Teller (BET) surface areas of such powder-form TiO2-graphene composites are very limited (40-50 m2/g) [8,10,17], significantly lower than the theoretical specific surface area of graphene (2630 m2/g) [18]. Such agglomeration can render a heavy loss in photocatalytic active sites and pollutant adsorption capacity, which can be highly detrimental to the photocatalytic efficiency of the TiO2-graphene catalysts. In order to suppress the agglomeration of TiO2-graphene composites, graphene aerogel, generally prepared from graphene aqueous dispersions by sol-gel processing and subsequently by supercritical fluid drying or by freeze drying, is employed in this work as a supportive material for TiO2 due to its unique characteristics including high specific surface areas, large pore volumes and tunable porosity [19]. Fig. 1 displays the structure of the TiO2-RGO aerogel, and the mechanisms of selective adsorption and photodegradation. Generally, the efficiency of photocatalysts is determined by three crucial factors: (i) the pollutant molecule adsorption, (ii) the light absorption, and (iii) the charge separation and transportation [20,21]. Interestingly, all the above factors can be improved with the introduction of graphene aerogel. The large
theoretical specific surface area of graphene gives it huge potential adsorption capacity for pollutants, and the highly porous aerogel structure of such TiO2-RGO composites can make the best of such high adsorptivity. Specially, aromatic dyes such as MB and RhB can be very efficiently and selectively adsorbed onto graphene because of the π-π stacking effect between aromatic parts of target molecules and aromatic regions of graphene sheets [22], as illustrated in Fig. 1. Graphene is also an excellent electron acceptor material owning to its two-dimensional π-conjugation structure [22,23], and has been successfully applied in TiO2-graphene composites to suppress the recombination of photogenerated electron-hole pairs [8,24]. In addition, graphene also possesses ultra-high conductivity because of the two-dimensional planar structure [25], thereby the photogenerated energy electrons can be facilely transferred to reaction sites. Given the above characteristics, graphene, especially graphene aerogel, can be an excellent candidate to improve the photocatalysis performance of TiO2. In this work, RGO aerogel was prepared by directly freeze-drying GO aqueous dispersion, and subsequently heating the obtained GO aerogel in a tube furnace for thermal reduction, after which RGO aerogel was obtained. It should be noted that GO has far better dispersity in water than RGO owning to the presence of multiple hydrophilic function groups [26], thereby the graphene aerogels synthesized via this route could have a more uniform microstructure than solution-reduced graphene aerogels. By facilely mixing P25 with the GO precursor, TiO2-RGO hybrid aerogel can be obtained with the same process. When used in the visible light photodegradation of dyes, such TiO2-RGO aerogel
exhibited high pollutant adsorptivity, high visible light utilization and enhanced photocatalytic efficiency. To better illustrate the superior photocatalysis performance of the TiO2-RGO aerogel, TiO2-RGO powder with the same TiO2:RGO ratio (10:1) was also synthesized via the reported hydrothermal method [8] as a contrast. Compared with its hydrothermal-synthesized powder-form counterparts, such TiO2-RGO hybrid aerogel showed enlarged BET surface area and significantly enhanced adsorptivity, which effectively promoted its photocatalytic efficiency. This work presents a new approach for the synthesis of high-performance TiO2-graphene photocatalyst, and is anticipated to provide a general graphene aerogel-based strategy for the improvement of diverse photocatalysts, as well as to facilitate their application in various environmental protection issues. 2. Experimental 2.1 Materials Graphite powder (99.9%, 325 mesh) was purchased from Beijing Wodetai Co., Ltd. P25 (TiO2, 20% rutile and 80% anatase) was purchased from Degussa. Sodium nitrate (NaNO3, AR), potassium permanganate (KMnO4, AR), concentrated sulfuric acid (H2SO4, 98wt%), concentrated hydrochloric acid (HCl, 37wt%), hydrogen peroxide (H2O2, 30wt%), methylene blue (MB, C16H18ClN3S, AR) and rhodamine B (RhB, C28H31ClN2O3, AR) were purchased from different companies. All the reagents were used as received without further purification. The deionized (DI) water used throughout this work was filtered with a Millipore E-pure filtration system at >18 MΩ·cm.
2.2 Synthesis of GO GO was synthesized by the modified Hummers’ method [27]. Briefly, 3 g graphite powder and 1.5 g NaNO3 were added to 70 mL of concentrated H2SO4 (98 wt%) with stirring at room temperature. The mixture was then cooled to 0 °C, followed by the slow addition of 9 g KMnO4. Note that the temperature of the mixture should not exceed 20 °C. Successively, the mixture was kept at 35-40 °C for 0.5 h, after which 140 mL of DI water was added and stirred for another 15 min. With the addition of water, the viscous, dark green fluid turned yellowish-brown, and much heat was generated. After that, 20 mL of H2O2 aqueous solution (30 wt%) and 500 mL of DI water were added to the mixture, and the color turned golden yellow while generating lots of bubbles. After the reaction, the mixture was filtered and rinsed with 250 mL of HCl aqueous solution (10 wt%) to eliminate excess MnO2, and washed repeatedly with H2O to remove acid residues until the pH of the filtrate was neutral. The resulting GO was dispersed in DI water and preserved at room temperature. 2.3 Synthesis of TiO2-RGO aerogel In a typical fabrication of the TiO2-RGO hybrid aerogel, 2 g P25 was added to 100 mL of GO aqueous dispersion (2 mg/mL) and treated with 30 min of ultrasonication and 30 min of magnetic stirring. The ultrasonication and stirring was conducted twice (2 h in total). Thereafter, the mixture was dropwise added into liquid nitrogen very slowly, and the resulting frozen “beads” were subsequently freeze-dried in vacuum for 24 h. The obtained spherical TiO2-GO aerogels were then heated in a tube furnace at 120 °C for 3 h under inert (Ar) atmosphere. After the thermal
treatment, the color of TiO2-GO aerogel “beads” turned from yellowish to dark gray (Supporting information Fig. S1) and easily collapsed into “fluffy” powders. 2.4 Photodegradation experiments The photocatalytic activities of TiO2, TiO2-RGO powder and TiO2-RGO aerogel was measured by the photodegradation of RhB under simulated solar irradiation (Xe lamp, 150 W) as model reaction. Briefly, 40 mg of catalyst was added to 100 mL of RhB aqueous solution (10 mg/mL) under ultrasonication for 10 min. Before illumination, the mixture was magnetically stirred in dark for 60 min to establish an adsorption-desorption equilibrium of RhB molecules with the catalyst [28,29]. Afterwards, the mixture solution was placed under the light source (10 cm below the Xe lamp) for photodegradation. All other lights were insulated during the photocatalysis. At given intervals, 5 mL of the suspension was withdrawn and centrifuged to remove the dispersed catalysts. The concentrations of the clean, transparent RhB solutions were determined by measuring the 554 nm absorbance of RhB. The MB dark adsorption experiments were conducted under the same conditions, and the concentrations of MB solutions were characterized with its 664 nm absorbance. 2.5 Characterization The surface morphologies of all samples were characterized with a FEI Quatan FEG 250 field-emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS). The structural information was obtained with a JEOL JEM-2100 field-emission transmission electron microscope (TEM). The
Diffuse reflectance spectra (DRS) and UV-visible absorption spectra were measured with a Jasco V-570 UV/VIS/NIR spectrophotometer. The X-ray diffraction (XRD) patterns were characterized using a D/max-2400 diffraction spectrometer. The SERS measurements were performed on a HR Evolution-800 Raman microscope system (HORIBA) equipped with standard 633 and 532 nm lasers. The photoluminescence (PL) spectra were measured with a SENS-9000 fluoroSENS fluorescence spectrometer. The X-ray photoelectron spectra (XPS) were measured on an AXIS Ultrabld X-ray photoelectron spectrometer using Al-K α as the exciting source and its binding energy calibration was based on C 1s at 284.8 eV. The N2 adsorption-desorption measurements were performed on an ASAP 2460 surface area and porosity analyzer. The photodegradation experiments were performed with a 150 W Xe lamp (Sciencetech Inc., SS-150). 3. Results and discussions SEM images of the as-synthesized GO aerogel, TiO2-GO aerogel, RGO aerogel and TiO2-RGO aerogel are shown in Fig. 2. It is observed that GO/RGO aerogels contain a large amount of macro pores formed during freeze-drying. The pore volumes and density can be facilely regulated by adjusting the GO dispersion concentration. Such highly porous structure can effectively avoid the agglomeration of GO/RGO, and usually have a large specific surface area [19], which is beneficial for pollutant adsorption. By adding P25 into the GO precursor, TiO2-GO/RGO aerogels can be obtained with the same procedure. The TiO2-GO aerogel consists of GO network frame and dense TiO2 particles loaded on GO layers, as shown in Fig. 2b.
After thermal treatment, the TiO2-GO aerogel transformed into TiO2-RGO aerogel, and its structure turned “fluffier”, exhibiting a huge number of mesopores and macropores (Fig. 2d). In contrast, serious aggregation was observed in the SEM images of TiO2-GO powder synthesized via the hydrothermal method (Fig. S2). EDS element mapping (Fig. 2e) of such TiO2-RGO aerogel revealed that the C, O and Ti elements have a very uniform distribution, indicating that P25 particles are uniformly incorporated into the RGO network. TEM analysis (Supporting information Fig. S3) of the TiO2-RGO aerogel also shows that TiO2 particles are loaded on RGO layers, and lattice fringes of TiO2 particles can be observed in the HRTEM image. The interplanar spacings of 0.315 nm and 0.345 nm corresponds with rutile (101) and anatase (101) planes of TiO2, respectively [30]. UV-vis diffuse reflectance spectra (DRS) were used to investigate the absorption band of P25 and TiO2-RGO powder/aerogel (Fig. 3a). Compared with pure P25, the UV-vis diffuse reflectance spectra of TiO2-RGO powder/aerogel showed an obvious red-shift in the absorption edge and higher absorbance in the whole visible region, which can effectively promote visible light absorption of the catalysts. Based on the DRS results, the bandgaps of pure P25, TiO2-RGO aerogel and TiO2-RGO powder were roughly estimated to be 3.04 eV, 2.75 eV and 2.60 eV, respectively. Such narrowing of TiO2 optical bandgap is attributed to the formation of Ti-O-C bonds [31], and gives the TiO2-RGO composites higher utilization of the solar energy. XRD patterns of TiO2, TiO2-RGO powder and TiO2-RGO aerogel are shown in Fig. 3b. Typical anatase and rutile diffraction peaks can be observed in all samples, indicating
that the crystal phase of TiO2 did not change after hydrothermal/heating treatment. However, the diffraction peak of graphene was not observed. The diffraction peak of graphene (~23°) [32] is likely to be covered by TiO2 (101) peak located at 25.3° because of its low content (10 wt%) and low intrinsic diffraction intensity. Besides, the diffraction peak of GO (~11°) [33] was not observed due to the reduction of GO in the hydrothermal/heating process. Given this, Raman spectra of TiO2-RGO powder/aerogel (Fig. 3c) were used to demonstrate the presence of graphene. Apparently, TiO2-RGO powder and aerogel showed very similar Raman spectra, in which both TiO2 and RGO Raman bands can be clearly observed. To better verify the thermal reduction of GO, XRD patterns of pure GO aerogel before and after heating were shown in Fig. S4. It can be observed that the GO diffraction peak (10.5°) disappeared after the heating treatment, and a broad diffraction peak centered at ~23.5° appeared, which is typical of RGO, indicating the successful thermal reduction. Oxygen functionalities in GO can be reduced when heated at 120 ℃, and oxygen-containing species like COx and H2O are generated during the dihydroxylation process [34]. Such thermal reduction avoids the introduction of various reducing agents such as hydrazine/dimethylhydrazine, L-ascorbic acid and NaBH4 [35-37], thus is a “clean” route. In addition, the solution-free reduction process effectively avoids the agglomeration of graphene layers, and the microstructure can be well preserved. Unlike the TiO2-GO aerogel, which is easily dissolved in water, the as-prepared TiO2-RGO aerogel exhibited poor solubility in water. Such promoted stability is on account of the gradually restored strong π-π
interaction during the thermal reduction process [38] and the removal of hydrophilic groups. Fig. 3d shows the PL spectra of P25, TiO2-RGO powder and TiO2-RGO aerogel aqueous dispersion with the same TiO2 content (0.18 mg/mL). PL quenching effect is an efficient method to characterize the charge transfer within composite materials [39]. It can be observed that with the introduction of RGO, the PL intensity of TiO2 showed a remarkable decrease, indicating the charge transfer from TiO2 to RGO. This is highly desirable because during the photocatalysis process, such charge transportation can greatly suppress the energy electron-hole recombination, and the transferred electrons can take part in the degradation reaction of pollutants adsorbed on RGO, thus the photocatalysis efficiency can be greatly improved [23,40]. Notably, TiO2-RGO powder exhibited stronger PL quenching effect as well as narrower optical bandgap than aerogel, which may indicate that the reduction reaction was more complete in the hydrothermal reduction than in the thermal reduction. The chemical compositions and elemental valence states of the TiO2-RGO aerogel were investigated by X-ray photoelectron spectroscopy (XPS). High-resolution XPS spectra of Ti 2p and C 1s are shown in Fig. 3e-f. As can be observed, the pseaks located at 459.4 and 465.1 eV are characteristic of TiO2 [41]. The peak located at 285.1 eV in the C 1s spectrum is assigned to C-C bond in the aromatic rings of graphene. The peaks at 287.0 and 288.6 eV are respectively assigned to the carbon in C-O and C=O bonds [42]. According to the XPS results, a considerable quantity of oxygen-containing functional groups still existed after the heating treatment, indicating that the thermal reduction of GO was incomplete. Those oxygen-containing groups present on the
incompletely reduced GO can interact with TiO2 nanoparticles, forming hydrogen bonds between them [43], and enhancing the structural stability of the TiO2-RGO aerogel. The specific surface area and pore structure of the TiO2-RGO powder/aerogel were characterized with the N2 adsorption-desorption measurements at 77 K. The Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to respectively characterize the surface area and pore size. As shown in Fig. 4, TiO2-RGO aerogel exhibits a typical type-IV isotherm with a high adsorption capacity in the high relative pressure, suggesting the presence of abundant meso/macropores, as is consistent with its SEM image (Fig. 2d). According to the BET analysis, such TiO2-RGO aerogel has a specific surface area of 83.5 m²/g. As a contrast, hydrothermal-synthesized TiO2-graphene composites with the same graphene content (10 wt%) showed significantly smaller specific surface area of 53.1 m²/g. The enlarged surface area of TiO2-RGO aerogel is on account of the “fluffy”, porous structure. However, the surface area of the TiO2-RGO aerogel is still significantly smaller than pure graphene aerogel (which can be up to 500 m2/g [19,44]), because its dominant component is P25 nanoparticles. The pore size distribution of TiO2-RGO aerogel is calculated with the BJH method. According to the BJH equation from the desorption branch of the isotherm, the TiO2-RGO aerogel showed a broad pore size distribution centered at ~50 nm (inset of Fig. 4). The large specific area and porous structure can greatly promote the pollutant adsorptivity of the TiO2-RGO aerogel. Fig. 5a displays the photograph of 80 mg TiO2-RGO powder synthesized via the
common hydrothermal method, and our TiO2-RGO aerogel. Evidently, the TiO2-RGO aerogel exhibited significantly larger specific volume than mass-equal TiO2-RGO powder. To further characterize their adsorptivity, the two samples were both used for the dark adsorption of MB, and their absorption intensities were checked periodically. According to the Lambert-Beer Law, the concentration of dyes can be characterized with their absorbance: A=ɛcl where A represents the absorbance, ɛ for the molar absorbing coefficient, c for the component concentration, and l for the length of absorbing layer. Obviously, the normalized concentration (C/C0) of dyes was proportional to the normalized maximum absorbance (A/A0). According to the results shown in Fig. 5b, TiO2-RGO aerogel reached the adsorption saturation within 2 min, while it took >10 min for TiO2-RGO powder to reach the adsorption saturation (Detailed absorption spectra were shown in Supporting information Fig. S5). This can be directly observed from the change of their color (Fig. 5c-d). The adsorption rate of catalysts is of great importance during the photodegradation process, because while the adsorbed molecules are continuously degraded, the adsorption-desorption equilibrium should be reestablished, therefore a high adsorption rate is the prerequisite for higher catalytic efficiency [45]. To characterize the photocatalytic activities of TiO2, TiO2-RGO powder and TiO2-RGO aerogel, the photodegradation of RhB under simulated solar irradiation (Xe lamp, 150 W) was used as the model reaction. The above samples were added
into RhB solutions and stirred in dark for 1 h to reach the adsorption-desorption equilibrium. As a result (Fig. 6a), when reaching the adsorption saturation, pure TiO2 can only adsorb 4% of the RhB molecules, while TiO2-RGO powder and aerogel can respectively adsorb 58.7% and 65.3%. Obviously, TiO2-RGO aerogel displayed the highest adsorption capacity due to its large surface area. It should be noted that the TiO2-RGO composites showed better performance for the adsorption of MB, in which TiO2-RGO aerogel can adsorb nearly 90% of the MB molecules, indicating a selective adsorption: RGO exhibits considerably higher adsorptivity towards MB than RhB. After the dark treatment, the mixed solution was exposed to Xe lamp irradiation, and the degradation degree of RhB was checked every 30 min, as shown in Fig. 6a (Detailed absorption spectra were shown in Fig. S6). After 3 h of light illumination, the degraded rates of RhB using TiO2, TiO2-RGO powder and TiO2-RGO aerogel as the catalyst were respectively 62.7%, 77.7% and 84.6%, while there was no degradation without catalysts. Photodegradation experiments without absorption in dark showed similar results (Fig. S7). As described above, both TiO2-RGO powder/aerogel exhibited extended absorption band and promoted visible light absorption, therefore they can better make use of visible light and thus showed higher photocatalytic efficiency. In contrast, pure TiO2 can only absorb ultra-violet light, and the photocatalytic efficiency was significantly lower. As anticipated, the TiO2-RGO aerogel displayed higher photodegradation activity than its powder-form counterpart, which is mainly attributed to its promoted adsorption rate and capacity towards target molecules. To further check the stability of such TiO2-RGO aerogel, it was collected
via centrifugation and rinsed with DI water and ethanol for reuse after the photodegradation process, and repeatedly underwent the same procedures (1 h of dark treatment and 3 h of light illumination) for 5 cycles in total. After each cycle, the final degradation rate was recorded, as shown in Fig. 6b. As a result, the TiO2-RGO aerogel exhibited no obvious decrease in its photocatalytic efficiency after 5 cycles of reuse, thus the TiO2-RGO aerogel also showed high stability. 4. Conclusions In conclusion, TiO2-RGO aerogel was prepared by freeze-drying the mixed P25 and GO aqueous dispersion and subsequently by heating in a tube furnace. The TiO2-RGO aerogel exhibited red-shifted absorption band and enhanced absorbance in the visible region, which led to higher utilization of visible light. PL quenching effect verified the energy electron transfer from TiO2 to RGO, which helps to suppress the recombination of photogenerated electron-hole pairs in TiO2. Compared with the TiO2-RGO powder prepared from the classic hydrothermal method, the TiO2-RGO aerogel showed enlarged specific surface area, which resulted in greatly enhanced adsorption rate and capacity and finally led to higher photodegradation efficiency. The TiO2-RGO aerogel also showed high stability for recyclable use. Such graphene aerogel-based strategy may have potential for the improvement of various catalysts. Acknowledgements This work was financial supported by the Natural Science Foundation of China (NSFC, 61774124, 51572216 and 61604122), the Fundamental Research Funds for the Central Universities (1191329876 and 1191329152), the 111 Program (B14040)
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Figures
Fig. 1. Schematic: Structure of the TiO2-RGO aerogel, mechanisms of the selective adsorption and photodegradation.
Fig. 2. (a-d) SEM images of GO aerogel, TiO2-GO aerogel, RGO aerogel and TiO2-RGO aerogel. (e) SEM image of the TiO2-RGO aerogel and EDS element mapping of the corresponding area.
Fig. 3. (a) Diffuse reflectance spectra of pure TiO2 and TiO2-RGO powder/aerogel. (b) XRD patterns of pure TiO2 and TiO2-RGO powder/aerogel. (c) Raman spectra of the TiO2-RGO powder/aerogel. (d) Photoluminescence spectra of pure TiO2 and TiO2-RGO powder/aerogel with the excitation wavelength of 285 nm. (e-f) High-resolution XPS spectra of Ti 2p and C 1s of the TiO2-RGO aerogel.
Fig. 4. N2 adsorption-desorption isotherm of the TiO2-RGO powder/aerogel at 77 K and the pore size distribution of TiO2-RGO aerogel measured by the BJH method (inset).
Fig. 5. (a) Photograph of 80 mg TiO2-RGO powder (left) and aerogel (right). (b) MB dark adsorption results of the TiO2-RGO aerogel/powder. (c-d) Color changing of MB solution during dark adsorption with TiO2-RGO powder (c) and aerogel (d).
Fig. 6. (a) Dark adsorption and photodegradation of RhB using the above samples under simulated sunlight. (b) The final degradation rate after each cycle.
·Graphene aerogel was employed for the fabrication of TiO2-graphene hybrid catalysts. ·TiO2-RGO aerogel exhibited high pollutant adsorptivity, extended absorption range and efficient charge separation. ·TiO2-RGO aerogel showed higher adsorptivity and improved catalytic efficiency compared with traditional TiO2-RGO powders.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: