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Enhanced photocatalytic CO2-reduction activity of electrospun mesoporous TiO2 nanofibers by solvothermal treatment† Junwei Fu,a Shaowen Cao,a Jiaguo Yu,*a,c Jingxiang Lowa and Yongpeng Leib Photocatalytic reduction of CO2 into renewable hydrocarbon fuels using semiconductor photocatalysts is considered as a potential solution to the energy deficiency and greenhouse effect. In this work, mesoporous TiO2 nanofibers with high specific surface areas and abundant surface hydroxyl groups are prepared using an electrospinning strategy combined with a subsequent calcination process, followed by a solvothermal treatment. The solvothermally treated mesoporous TiO2 nanofibers exhibit excellent photocatalytic performance on CO2 reduction into hydrocarbon fuels. The significantly improved photocatalytic activity can be attributed to the enhanced CO2 adsorption capacity and the improved charge separation
Received 19th January 2014, Accepted 24th March 2014
after solvothermal treatment. The highest activity is achieved for the sample with a 2-h solvothermal
DOI: 10.1039/c4dt00181h
treatment, showing 6- and 25-fold higher CH4 production rate than those of TiO2 nanofibers without solvothermal treatment and P25, respectively. This work may also provide a prototype for studying the effect
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of solvothermal treatment on the structure and photocatalytic activity of semiconductor photocatalysts.
Introduction Driven by the management of fossil fuel shortage and increasing carbon dioxide (CO2) emissions, photocatalytic reduction of CO2 into renewable hydrocarbon fuels using semiconductor photocatalysts is considered as an attractive way for solving the energy deficiency and reducing the greenhouse effect.1–3 Since the pioneering study by Inoue et al. on the photoreduction of CO2 in semiconductor involved aqueous suspensions,4 various semiconductors such as TiO2,5 Ga2O3,6 Zn2GeO4,7 ZnGa2O4,8 ZnS9 and CdS10 have been studied for the photocatalytic reduction of CO2. Among them, TiO2 has proven to be one of the most promising photocatalysts because of its low cost, non-toxicity, chemical inertness and long-term photostability.11 However, several drawbacks still restrict the photocatalytic efficiency of TiO2. For instance, TiO2 nanocrystals tend to aggregate into larger particles, resulting in reduced surface area and higher charge recombination rate.12,13 As such, tremendous efforts have been made to modify the
a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China. E-mail: [email protected]; Fax: +86-27-87879468; Tel: +86-27-87871029 b College of Basic Education, National University of Defense Technology, Changsha 410073, P. R. China c Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4dt00181h
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physicochemical property of TiO2 with good crystallinity, small grain size and large surface area, etc.13,14 in order to improve its photocatalytic performance. As a candidate, one-dimensional (1D) TiO2 such as nanorods,15 nanotubes16 and nanofibers,12,17–19 has shown extraordinary properties with high electron mobility and excellent charge separation ability, giving rise to the breakthrough of its application in several fields, especially photocatalysis.15–19 To date, a great deal of methods have been developed to prepare 1D TiO2, including sol–gel,20 hydrothermal,21 electrospinning method,22 etc. In particular, electrospinning of TiO2 nanofibers has been demonstrated to be a simple, low-cost, and effective strategy to prepare ultrafine 1D TiO2. The electrospun TiO2 nanofibers have closely aligned nanocrystals which notably enhance the electron diffusion coefficient, leading to a higher electron–hole separation rate.18,23,24 Sung and coworkers have found that the electrospun TiO2 exhibits superior photocatalytic activity, due to the efficient charge separation through interparticle charge transfer along the nanofiber frameworks.19 Moreover, the electrospun TiO2 nanofibers present intimately stacked hierarchical porous structures which minimize the light reflection on the surface and enable the light to transmit deeply into the TiO2 nanofibers. Chuangchote et al. have demonstrated that such light scattering effect of TiO2 nanofibers can result in an increased photocatalytic activity.24 In this study, mesoporous TiO2 nanofibers are fabricated in a large scale via the simple electrospinning strategy combined
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with the subsequent calcination treatment. After that, the TiO2 nanofibers are treated by a solvothermal process with different time periods. This solvothermal treatment provides the mesoporous TiO2 nanofibers not only with well crystallized and closely packed grains for improving the transfer of photogenerated charge carriers, but also released adsorption sites for enhancing the adsorption capacity of CO2. As a result, the solvothermally treated TiO2 nanofibers show significantly improved photocatalytic performance for CO2 reduction. At an optimal solvothermal treatment time of 2 h, the as-obtained TiO2 nanofibers exhibit 6- and 25-fold higher photocatalytic efficiency on CO2 reduction than those of TiO2 nanofibers without solvothermal treatment and P25, respectively.
Experimental All chemicals were analytical grade without further purification. Tetrabutyl titanate (Ti(OC4H9)4, TBOT) was purchased from Shanghai Kefeng Industry Co., Ltd. Polyvinylpyrrolidone (PVP K-90) was purchased from Tianjin Bodi Chemical Co., Ltd. Glacial acetic acid (CH3COOH) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Ethanol (C2H5OH) and hydrogen Chloride (HCl) were purchased from Shanghai Chemical Regent Factory of China. Sodium bicarbonate (NaHCO3) was purchased from Shanghai Avision Chemical Factory. Deionized (DI) water was used in all experiments. Preparation of photocatalysts Electrospinning and calcination process: TiO2 nanofibers were prepared according to an electrospinning method modified from that described by Xia et al.22 A solution was first prepared by dissolving 1 g of PVP (K-90) in 10 mL of ethanol. Then 2 mL of glacial acetic acid and 3 mL of TBOT were added into the above solution. The mixture was stirred continuously for 12 h to yield a homogeneous precursor solution. Prior to electrospinning onto a ground aluminium foil electrode, the precursor solution was sonicated for 10 min and loaded into a 20 mL plastic syringe with a stainless steel needle (diameter of 0.7 mm) at the tip. Electrospinning was performed at 26 °C for 3 h at an applied potential of 15 kV with a flow rate of 3 mL h−1, and a distance of ∼10 cm from the tip to the collector. The relative humidity of the chamber was kept at ∼30%. The as-obtained nonwoven mats of composite nanofibers consisting of amorphous TiO2 and PVP were then annealed at 400 °C for 6 h to eliminate the PVP. The heating and cooling rates were both set at 2 °C min−1. Solvothermal treatment: In a typical procedure, 20 mL of ethanol and 0.4 g of the as-prepared TiO2 nanofibers were loaded into a Teflon-lined stainless steel autoclave with a capacity of 50 mL, and heated at 180 °C for 1, 2, 3 and 6 h, respectively. The white precipitate was collected and thoroughly washed with deionized water for six times to remove the residual impurity on the surface of the samples. The samples were denoted as S0 (un-treated), S1, S2, S3, and
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S6, corresponding to the different solvothermal treatment time, respectively. Characterization X-ray diffraction (XRD) patterns were obtained on a D/Max-RB X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation at a scan rate (2θ) of 0.05° s−1. Field emission scanning electron microscopy (FESEM) images were examined by a Hitachi S-4800 scanning electron microscope equipped with a cold field emission electron gun. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis were conducted by a JEM-2100F transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed on a VG ESCALAB210 XPS system with Mg Kα source and all the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The nitrogen adsorption and desorption isotherms were measured using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All the as-prepared samples were degassed at 150 °C prior to nitrogen adsorption measurements. The Brunauer–Emmett–Teller (BET) specific surface area (SBET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.3. The pore size distribution was determined using adsorption data via the Barret–Joyner–Halender (BJH) method. The average pore size was obtained from the nitrogen adsorption volume at P/P0 of 0.97. Fourier transform infrared (FTIR) spectra were obtained from a Shimadzu IR Affinity-1 FTIR spectrometer. The CO2 adsorption was measured by using a Micromeritics ASAP 3020 carbon dioxide adsorption apparatus (USA). Photoluminescence (PL) spectra were taken with a fluorescence spectrophotometer (F-7000, Hitachi, Japan). The excitation wavelength was 375 nm, the scanning speed was 1200 nm min−1, and the PMT voltage was 700 V. The width of excitation slit and emission slit were both 5.0 nm. Photocatalytic reduction of CO2 The photocatalytic reduction of CO2 was carried out in a 200 mL double-neck flask with a groove in one neck, at ambient temperature and atmospheric pressure.10b A 300 W Xe arc lamp (Changzhou Siyu Science Co. Ltd, China), providing UV/visible-light irradiation, was used as light source and positioned ∼22 cm away from the photocatalytic reactor. Typically, 100 mg of photocatalyst was dispersed in 10 mL DI water by sonication and transferred to the reaction vessel. After evaporation of water at 80 °C, a smooth film was formed at the bottom. Before irradiation, the reactor was sealed and blown with nitrogen for 30 min to remove air and ensure that the reaction system was under anaerobic conditions. CO2 and H2O vapor were produced by the reaction of NaHCO3 (120 mg, introduced into the reactor before seal) and HCl aqueous solution (0.25 mL, 4 M), which was introduced into the reactor by syringe. The flask was then kept under xenon lamp irradiation. The gas product composition was analyzed by a gas chromato-
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graph (GC-2014C, Shimadzu) equipped with a flame ionization detector (FID). Products were calibrated with a standard mixture gas and determined by the retention time.
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Results and discussion Phase structure and morphology The phase structure, crystallite size and crystallinity of typical samples were determined by XRD analysis. Fig. 1 shows the XRD patterns of the TiO2 nanofibers solvothermally treated at different time periods. The characteristic diffraction peaks for S0 at 25.3°, 37.8°, 48.2°, 54.1° and 62.5° can be assigned to the (101), (004), (200), (105) and (204) crystal planes of anatase TiO2 (JCPDS, No. 21-1272), respectively. These peaks do not show any shift for other samples, indicating the quite stable phase structure of anatase TiO2 during the solvothermal process. It is noted that the main diffraction peak of anatase (101) becomes gradually narrower and stronger with increasing solvothermal treatment time, as shown by the enlarged view of the peak (101) in Fig. S1.† We have estimated the average crystallite size of the samples with the Scherrer formula based on this main (101) peak. As listed in Table 1, the average crystallite size slightly increases from 10.1, to 10.3 and 10.5 nm by the sequence of S0, S2, and S6. These results indicate that the crystallinity of electrospun TiO2 nanofibers is further improved by solvothermal treatment at high temperature and pressure conditions. There are only small structural changes for electrospun TiO2 nanofibers. However, it may have a significant effect
on the photocatalytic properties, as the resulting bigger crystallite size will lead to improved structural connections with close-packing of grains, which is beneficial for the charge separation in photocatalytic reactions.25 The morphology and microstructure of the TiO2 nanofibers were examined by SEM and TEM (Fig. 2). As shown in Fig. 2a, the S0 sample appears as a common cylindrical morphology with a diameter from 300 to 500 nm and a length of dozens of micrometers. The high-magnification FESEM image in Fig. 2b clearly shows the surface of the nanofibers with aggregated nanocrystals. The overall morphology of sample S2 does not change obviously. Fig. 2c indicates that single nanofibers in sample S2 are constructed from many polycrystalline nanoscaled grains. An in-depth observation of the microstructure was conducted by HRTEM. Fig. 2d confirms that the average size of these nanoscaled grains is ∼10 nm. The lattice fringes with d spacing of 0.35 nm can be assigned to the (101) crystal plane of anatase TiO2. It is worth noting that the TiO2 nanograins are intimately contacted with each other. During the electrospinning process, the TBOT was hydrolyzed to form amorphous TiO2 particles in the PVP matrix. These TiO2 particles were uniform in size due to the space confinement of the PVP matrix.26 The PVP matrix was then removed by the calcination process. Meanwhile, the amorphous TiO2 particles were crystallized and turned into uniform and closely contacted anatase grains during the calcination process. Such intimate contact enables the electrons to move easily from one grain to another and provides an effective electron transfer pathway on TiO2,18 which can remarkably improve the separation of photogenerated electron–hole pairs. Since the TiO2 grains are already arranged closely, they are only able to grow slightly bigger upon further solvothermal treatment, due to the mutual restriction of the neighboring grains. However, this slight growth is enough to improve the connections of the closely contacted grains, which may result in a significant effect on the photocatalytic performance of TiO2 nanofibers. XPS characterization
Fig. 1 XRD patterns of TiO2 nanofibers with different solvothermal treatment time.
Table 1
The XPS survey spectra (Fig. S2†) indicate the existence of Ti, O and C in the S0 and S2 samples, and their photoelectron peaks emerge at binding energies of 458 (Ti 2p), 531 (O 1s) and 285 eV (C 1s), respectively. High-resolution XPS spectra of C 1s, O 1s, and Ti 2p were further examined and are shown in Fig. 3a–c. The C 1s spectrum of S0 (Fig. 3a) can be deconvoluted into three peaks at 284.8, 286.3, and 288.8 eV. The main peak at 284.8 eV can be assigned to the carbon atoms in the
Physical properties and photocatalytic performance of the typical samplesa
Sample
Solvothermal time (h)
Crystallite size (nm)
SBET (m2 g−1)
Pore volume (cm3 g−1)
Average pore size (nm)
CO2 adsorption (mmol g−1)
CH4 production rate (μmol h−1 g−1)
S0 S2 S6
0 2 6
10.1 10.3 10.5
59 55 49
0.072 0.064 0.055
4.84 4.68 4.40
0.14 0.19 0.16
3.15 19.55 3.08
a
The CO2 adsorption amount obtained at P/P0 = 1.
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Fig. 2 in (c).
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(a, b) FESEM images of the S0 sample; (c) TEM image of single nanofiber selected from the S2 sample; (d) HRTEM image of the circled area
C–C, CvC, and C–H bonds, which are caused by carbonization of organic groups or adventitious carbon.27 The peak at 286.3 eV is related to the hydroxyl carbon (C–OH) and the peak at 288.8 eV is attributed to carboxyl carbon (O–CvO), originating from the residual of the precursor.28 In comparison, the C 1s spectrum (Fig. 3a) of S2 only shows two peaks at 284.8 and 288.8 eV, with a dramatic decrease for the relative intensity of the peak at 288.8 eV. This is due to the fact that the solvothermal treatment largely reduces the residual of the organic precursor, thereby decreasing the amount of hydroxyl carbon and carboxyl carbon in the sample. Fig. 3b shows the O 1s spectra of S0 and S2. Three distinct peaks are observed for S0 at 529.7, 531.8, and 533.2 eV. The peak at 529.7 eV can be assigned to TiO2 lattice oxygen (Ti–O–Ti).29 The peak at 531.8 eV is attributed to the oxygen atoms in the Ti–OH (surface hydroxyl groups) and overlapped CvO bonds (residual from precursor).30,31 And the peak at 533.2 eV are related to the oxygen atoms in the CvO bonds.30 Nevertheless, the O 1s spectrum of S2 does not show the peak at 533.2 eV, owing to the dramatic decrease of the organic species after solvothermal treatment. Moreover, the peak corresponding to TiO2 lattice oxygen shifts to 530.0 eV. The Ti 2p spectra of S0 (Fig. 3c) shows two peaks centred at 458.5 and 464.2 eV which can be ascribed to Ti 2p3/2 and Ti 2p1/2, respectively, while these two peaks shift to 458.8 and 464.5 eV for sample S2. The observed spin–orbit splitting between the Ti 2p3/2 and Ti 2p1/2 is 5.7 eV, which is in good agreement with the values of Ti4+ state in TiO2.32 The results indicate that there are a lot of organic species on the surface of S0. After solvothermal treatment, these organic species are mostly removed and a cleaner surface of TiO2 is achieved. That is why the binding energies of Ti 2p and O 1s
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of TiO2 lattice oxygen for S2 are both positively shifted by 0.3 eV. It suggests that the surface properties of the electrospun TiO2 nanofibers do vary after solvothermal treatment. The resulting cleaner surface can help the charge immigration for better photocatalytic properties.33 Meanwhile, the removal of organic residual will release more adsorption sites for CO2 capture. BET surface areas and pore size distribution Fig. 4 shows the nitrogen adsorption–desorption isotherms and pore size distributions of samples S0, S2 and S6. According to Brunauer–Deming–Deming–Teller classification, the isotherms of all samples are of type IV, indicating the presence of mesopores (2–50 nm).34 Furthermore, the shapes of the hysteresis loops are of type H2 at the relative pressure (P/P0) range of 0.4–0.8, suggesting the formation of pores with narrow necks and wider bodies (ink-bottle pores).34 The pore size distributions (inset of Fig. 4) show a range of 2–40 nm with a peak pore diameter of ∼5 nm for all the samples, further confirming the existence of mesopores. However, slightly reduced specific surface areas from 59 (S0) to 55 (S2) and 49 m2 g−1 (S6) are obtained, and the pore volume and average pore size also decrease from S0 to S2 and S6, as listed in Table 1. This is due to the slightly increased size and confirms that the grains are packed more closely after solvothermal treatment. FTIR spectroscopy measurement The FTIR spectra of samples S0, S2 and S6 are shown in Fig. S3.† The broad peak at 3400 cm−1 and the peak at 1650 cm−1 are attributed to the surface-adsorbed water and hydroxyl groups.35 It suggests that all TiO2 nanofiber samples
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Fig. 3
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High-resolution XPS spectra for C 1s (a), O 1s (b), and Ti 2p (c) of S0 and S2.
Fig. 4 N2 adsorption and desorption isotherms and pore size distributions (the inset) of S0, S2 and S6.
are with a number of surface hydroxyl groups that can favour the CO2 capture. The main peaks located in the range of 400–700 cm−1 can be assigned to Ti–O stretching and Ti–O– Ti bridging stretching modes.36 The peak lying in 1380–1400 cm−1 region corresponds to carboxyl (CvO) groups of the organic species,35,37 The peak in the 2300–2370 cm−1 region can be ascribed to CO2.38 Note that the CO2 peaks of samples S2 and S6 are stronger than sample S0, implying the larger CO2 adsorption capacity of S2 and S6.
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Fig. 5
CO2 adsorption isotherms of S0, S2 and S6.
CO2 adsorption test It is well-known that the adsorption of CO2 onto the photocatalyst surface is the first essential step for the photocatalytic CO2 reduction. Thus we further investigated the CO2 adsorption performance of samples S0, S2 and S6 using CO2 adsorption isotherms. As shown in Fig. 5, all samples exhibited a rapid rise in CO2 uptake along with elevated CO2 pressure at a relatively low level (P/P0 < 0.2), indicating specific interactions
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between CO2 and the surface hydroxyl groups of TiO2 nanofibers. Especially, both the CO2 adsorption capacity of samples S2 and S6 is higher than that of S0, due to released adsorption sites after the removal of organic residual by solvothermal treatment. It is noted that as the relative pressure increases, sample S2 shows better CO2 uptake ability than S6. This is because at high P/P0 condition, more CO2 is adsorbed into the interparticle space of the nanofibers, whereas S6 has a lower specific surface area and smaller pore volume, resulting in less surface CO2 adsorption sites. Detailed CO2 adsorption capacity is listed in Table 1. Photoluminescence spectra The PL spectra can be used to reveal the separation and recombination tendency of the photogenerated electron–hole pairs within the TiO2 nanofibers.39 Fig. S4† shows a comparison of PL spectra of the S0 and S2 samples in the wavelength range 375–550 nm. Two main emission peaks appear at about 396 and 468 nm wavelengths. The former is ascribed to the emission of bandgap transition with the energy of light approximately equal to the bandgap energy of anatase,40 while the latter is the emission signal originating from the charge transfer transition from Ti3+ to oxygen anions in a TiO68− complex.41,42 Slightly weaker emission intensity is observed for S2, as compared to that for S0, implying a lower recombination rate of photogenerated electron–hole pairs. This is because the solvothermal environment can accelerate the crystallization and generate a cleaner surface of TiO2, resulting in the reduction of the number of defects and impurity in TiO2 crystals, thus the chance of photogenerated electron–hole recombination can be reduced. Photocatalytic activity The photocatalytic activity of the prepared samples is evaluated by using the reaction of photocatalytic CO2 reduction into hydrocarbon fuels. The original chromatogram for the reduction of CO2 on sample S2 is shown in Fig. 6a. Control experiments indicated that no hydrocarbon compound was detected in the absence of either photocatalyst or light irradiation, suggesting that photocatalyst and light irradiation are two indispensable factors for photocatalytic reduction of CO2, and production of hydrocarbon compounds. In our study, CH4 is detected as the main product, with a small amount of CH3OH and HCHO. As such, possible reactions for the photocatalytic CO2-reduction are proposed as below. CO2 þ 4Hþ þ 4e ! HCHO þ H2 O
E0redox ¼ 0:48 V
CO2 þ 6Hþ þ 6e ! CH3 OH þ H2 O
E0redox ¼ 0:38 V
CO2 þ 8Hþ þ 8e ! CH4 þ 2H2 O
E0redox ¼ 0:24 V
Fig. 6b shows the comparison of photocatalytic CH4 production rates on different samples. P25 is used as a reference and exhibits much lower activity than all the TiO2 nanofiber
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Fig. 6 (a) The original chromatograms of S2 after 1 h irradiation and (b) CH4 generation of the samples with different solvothermal time periods (S0, S1, S2, S3, and S6).
samples, demonstrating the superiority of the electrospun TiO2 nanofibers for photocatalytic CO2 reduction. It is clearly seen that the CH4 production rate increases dramatically from S0 to S2. This is mainly due to the improved CO2 adsorption promoted by the solvothermal treatment. Furthermore, the slightly growing and more closely packed grains after solvothermal treatment can lead to improved structural connections of TiO2 crystals, thus to accelerate the separation of photogenerated electron–hole pairs in terms of fast movement along the grain boundaries.25 In addition, the reduction of defects and impurity can suppress the recombination of photogenerated electron–hole pairs. However, a gradual decrease of CH4 production rate is observed for the samples with increasing solvothermal treatment time (from 3 to 6 h), due to the decreased CO2 adsorption ability and surface area. Therefore, it is necessary to modulate the solvothermal treatment time at an optimal value. As for our study, the best activity is obtained for sample S2, with 6- and 25-fold higher CH4 production rate than those of S0 and P25, respectively. Actually we have measured the UV-vis spectra of all the TiO2 nanofiber samples, but no difference was observed. This
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21177100). Also, this work was financially supported by the Fundamental Research Funds for the Central Universities (2013-VII-030) and Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1).
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Notes and references
Fig. 7 Schematic illustration for a mesoporous TiO2 nanofiber and its photocatalytic CO2 reduction mechanism.
means the band gap of TiO2 nanofibers are not affected by solvothermal treatment, thus there is no contribution from band gap alteration for the photocatalytic performance. Based on the above experimental results, a possible photocatalytic mechanism of CO2 reduction over TiO2 nanofibers is proposed and illustrated in Fig. 7. Firstly, CO2 is largely adsorbed on the surface of TiO2 nanofibers, through the bonding between CO2 and surface hydroxyl groups, as well as the physisorption into the interparticle space. Upon light irradiation, photogenerated electrons and holes can easily transfer to the nanofiber surface along the well crystallized and closely packed grains with a clean surface. As a result, the photogenerated electrons are capable of reducing CO2 into hydrocarbon fuels, meanwhile the separated holes can oxidize H2O into O2 and H+.
Conclusions In summary, we have successfully prepared mesoporous TiO2 nanofibers on a large scale through a simple electrospinning strategy combined with a subsequent calcination process. Further solvothermal treatment for the TiO2 nanofibers enables them with well crystallized and closely packed grains with a clean surface. The significantly improved efficiency of photocatalytic CO2 reduction into hydrocarbon fuels is obtained due to the improved structural connections for charge separation, and the released adsorption sites for enhanced CO2 adsorption capacity after solvothermal treatment. At an optimal solvothermal time of 2 h, the treated TiO2 nanofibers exhibit 6- and 25-fold higher CH4 production rate than those of TiO2 nanofibers without solvothermal treatment and P25, respectively. This work may not only indicate the effect of solvothermal treatment on the photocatalytic CO2 reduction over TiO2, but also provide a prototype for studying the effect of solvothermal treatment on the structure and photocatalytic activity of semiconductor photocatalysts.
Acknowledgements This work was supported by the 973 program (2013CB632402), and NSFC (51272199, 51320105001, 51372190, 51203182 and
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23 M. Adachi, Y. Murata, J. Takao, J. T. Jiu, M. Sakamoto and F. M. Wang, J. Am. Chem. Soc., 2004, 126, 14943. 24 S. Chuangchote, T. Sagawa and S. Yoshikawa, Appl. Phys. Lett., 2008, 93, 033310. 25 P. S. Archana, R. Jose, C. Vijila and S. Ramakrishna, J. Phys. Chem. C, 2009, 113, 21538. 26 Y. Q. Dai, C. M. Cobley, J. Zeng, Y. M. Sun and Y. N. Xia, Nano Lett., 2009, 9, 2455. 27 J. G. Yu, Y. Wang and W. Xiao, J. Mater. Chem. A, 2013, 1, 10727. 28 J. J. Fan, S. W. Liu and J. G. Yu, J. Mater. Chem., 2012, 22, 17027. 29 L. H. Nie, J. G. Yu, X. Y. Li, B. Cheng, G. Liu and M. Jaroniec, Environ. Sci. Technol., 2013, 47, 2777. 30 G. P. Dai, J. G. Yu and G. Liu, J. Phys. Chem. C, 2011, 115, 7339. 31 S. D. Gardner, C. S. K. Singamsetty, G. L. Booth and G. R. He, Carbon, 1995, 33, 587. 32 G. Laan and I. W. Kirkman, J. Phys.: Condens. Matter, 1992, 4, 4189. 33 J. Z. Y. Tan, D. Kong, Q. Xiang, W. Wang, L. T. Zhan, J. L. Zeng and X. W. Zhang, Curr. Org. Chem., 2013, 17, 1382.
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34 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouqerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603. 35 Z. H. Xu, J. G. Yu and W. Xiao, Chem. – Eur. J., 2013, 19, 9592. 36 A. M. Peiro, J. Peral, C. Domingo, X. Domenech and J. A. Ayllon, Chem. Mater., 2001, 13, 2567. 37 S. Biniak, G. Szymanski, J. Siedlewski and A. Swiatkowski, Carbon, 1997, 35, 1799. 38 (a) C. Knofel, C. Martin, V. Hornebecq and P. L. Llewellyn, J. Phys. Chem. C, 2009, 113, 21726; (b) Y. Le, D. Guo, B. Cheng and J. Yu, J. Colloid Interface Sci., 2013, 408, 173; (c) Y. Le, D. Guo, B. Cheng and J. Yu, Appl. Surf. Sci., 2013, 274, 110. 39 Q. J. Xiang, K. L. Lv and J. G. Yu, Appl. Catal., B, 2010, 96, 557. 40 J. G. Yu, L. F. Qi and M. Jaroniec, J. Phys. Chem. C, 2010, 114, 13118. 41 J. G. Yu, H. G. Yu, B. Cheng, X. J. Zhao, J. C. Yu and W. K. Ho, J. Phys. Chem. B, 2003, 107, 13871. 42 Z. K. Zheng, B. B. Huang, J. B. Lu, X. Y. Qin, X. Y. Zhang and Y. Dai, Chem. – Eur. J., 2011, 17, 15032.
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Enhanced photocatalytic CO2-reduction activity of electrospun mesoporous TiO2 nanofibers by solvothermal treatment† Junwei Fu,a Shaowen Cao,a Jiaguo Yu,*a,c Jingxiang Lowa and Yongpeng Leib Photocatalytic reduction of CO2 into renewable hydrocarbon fuels using semiconductor photocatalysts is considered as a potential solution to the energy deficiency and greenhouse effect. In this work, mesoporous TiO2 nanofibers with high specific surface areas and abundant surface hydroxyl groups are prepared using an electrospinning strategy combined with a subsequent calcination process, followed by a solvothermal treatment. The solvothermally treated mesoporous TiO2 nanofibers exhibit excellent photocatalytic performance on CO2 reduction into hydrocarbon fuels. The significantly improved photocatalytic activity can be attributed to the enhanced CO2 adsorption capacity and the improved charge separation
Received 19th January 2014, Accepted 24th March 2014
after solvothermal treatment. The highest activity is achieved for the sample with a 2-h solvothermal
DOI: 10.1039/c4dt00181h
treatment, showing 6- and 25-fold higher CH4 production rate than those of TiO2 nanofibers without solvothermal treatment and P25, respectively. This work may also provide a prototype for studying the effect
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of solvothermal treatment on the structure and photocatalytic activity of semiconductor photocatalysts.
Introduction Driven by the management of fossil fuel shortage and increasing carbon dioxide (CO2) emissions, photocatalytic reduction of CO2 into renewable hydrocarbon fuels using semiconductor photocatalysts is considered as an attractive way for solving the energy deficiency and reducing the greenhouse effect.1–3 Since the pioneering study by Inoue et al. on the photoreduction of CO2 in semiconductor involved aqueous suspensions,4 various semiconductors such as TiO2,5 Ga2O3,6 Zn2GeO4,7 ZnGa2O4,8 ZnS9 and CdS10 have been studied for the photocatalytic reduction of CO2. Among them, TiO2 has proven to be one of the most promising photocatalysts because of its low cost, non-toxicity, chemical inertness and long-term photostability.11 However, several drawbacks still restrict the photocatalytic efficiency of TiO2. For instance, TiO2 nanocrystals tend to aggregate into larger particles, resulting in reduced surface area and higher charge recombination rate.12,13 As such, tremendous efforts have been made to modify the
a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China. E-mail: [email protected]; Fax: +86-27-87879468; Tel: +86-27-87871029 b College of Basic Education, National University of Defense Technology, Changsha 410073, P. R. China c Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4dt00181h
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physicochemical property of TiO2 with good crystallinity, small grain size and large surface area, etc.13,14 in order to improve its photocatalytic performance. As a candidate, one-dimensional (1D) TiO2 such as nanorods,15 nanotubes16 and nanofibers,12,17–19 has shown extraordinary properties with high electron mobility and excellent charge separation ability, giving rise to the breakthrough of its application in several fields, especially photocatalysis.15–19 To date, a great deal of methods have been developed to prepare 1D TiO2, including sol–gel,20 hydrothermal,21 electrospinning method,22 etc. In particular, electrospinning of TiO2 nanofibers has been demonstrated to be a simple, low-cost, and effective strategy to prepare ultrafine 1D TiO2. The electrospun TiO2 nanofibers have closely aligned nanocrystals which notably enhance the electron diffusion coefficient, leading to a higher electron–hole separation rate.18,23,24 Sung and coworkers have found that the electrospun TiO2 exhibits superior photocatalytic activity, due to the efficient charge separation through interparticle charge transfer along the nanofiber frameworks.19 Moreover, the electrospun TiO2 nanofibers present intimately stacked hierarchical porous structures which minimize the light reflection on the surface and enable the light to transmit deeply into the TiO2 nanofibers. Chuangchote et al. have demonstrated that such light scattering effect of TiO2 nanofibers can result in an increased photocatalytic activity.24 In this study, mesoporous TiO2 nanofibers are fabricated in a large scale via the simple electrospinning strategy combined
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with the subsequent calcination treatment. After that, the TiO2 nanofibers are treated by a solvothermal process with different time periods. This solvothermal treatment provides the mesoporous TiO2 nanofibers not only with well crystallized and closely packed grains for improving the transfer of photogenerated charge carriers, but also released adsorption sites for enhancing the adsorption capacity of CO2. As a result, the solvothermally treated TiO2 nanofibers show significantly improved photocatalytic performance for CO2 reduction. At an optimal solvothermal treatment time of 2 h, the as-obtained TiO2 nanofibers exhibit 6- and 25-fold higher photocatalytic efficiency on CO2 reduction than those of TiO2 nanofibers without solvothermal treatment and P25, respectively.
Experimental All chemicals were analytical grade without further purification. Tetrabutyl titanate (Ti(OC4H9)4, TBOT) was purchased from Shanghai Kefeng Industry Co., Ltd. Polyvinylpyrrolidone (PVP K-90) was purchased from Tianjin Bodi Chemical Co., Ltd. Glacial acetic acid (CH3COOH) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Ethanol (C2H5OH) and hydrogen Chloride (HCl) were purchased from Shanghai Chemical Regent Factory of China. Sodium bicarbonate (NaHCO3) was purchased from Shanghai Avision Chemical Factory. Deionized (DI) water was used in all experiments. Preparation of photocatalysts Electrospinning and calcination process: TiO2 nanofibers were prepared according to an electrospinning method modified from that described by Xia et al.22 A solution was first prepared by dissolving 1 g of PVP (K-90) in 10 mL of ethanol. Then 2 mL of glacial acetic acid and 3 mL of TBOT were added into the above solution. The mixture was stirred continuously for 12 h to yield a homogeneous precursor solution. Prior to electrospinning onto a ground aluminium foil electrode, the precursor solution was sonicated for 10 min and loaded into a 20 mL plastic syringe with a stainless steel needle (diameter of 0.7 mm) at the tip. Electrospinning was performed at 26 °C for 3 h at an applied potential of 15 kV with a flow rate of 3 mL h−1, and a distance of ∼10 cm from the tip to the collector. The relative humidity of the chamber was kept at ∼30%. The as-obtained nonwoven mats of composite nanofibers consisting of amorphous TiO2 and PVP were then annealed at 400 °C for 6 h to eliminate the PVP. The heating and cooling rates were both set at 2 °C min−1. Solvothermal treatment: In a typical procedure, 20 mL of ethanol and 0.4 g of the as-prepared TiO2 nanofibers were loaded into a Teflon-lined stainless steel autoclave with a capacity of 50 mL, and heated at 180 °C for 1, 2, 3 and 6 h, respectively. The white precipitate was collected and thoroughly washed with deionized water for six times to remove the residual impurity on the surface of the samples. The samples were denoted as S0 (un-treated), S1, S2, S3, and
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S6, corresponding to the different solvothermal treatment time, respectively. Characterization X-ray diffraction (XRD) patterns were obtained on a D/Max-RB X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation at a scan rate (2θ) of 0.05° s−1. Field emission scanning electron microscopy (FESEM) images were examined by a Hitachi S-4800 scanning electron microscope equipped with a cold field emission electron gun. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis were conducted by a JEM-2100F transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed on a VG ESCALAB210 XPS system with Mg Kα source and all the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The nitrogen adsorption and desorption isotherms were measured using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All the as-prepared samples were degassed at 150 °C prior to nitrogen adsorption measurements. The Brunauer–Emmett–Teller (BET) specific surface area (SBET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.3. The pore size distribution was determined using adsorption data via the Barret–Joyner–Halender (BJH) method. The average pore size was obtained from the nitrogen adsorption volume at P/P0 of 0.97. Fourier transform infrared (FTIR) spectra were obtained from a Shimadzu IR Affinity-1 FTIR spectrometer. The CO2 adsorption was measured by using a Micromeritics ASAP 3020 carbon dioxide adsorption apparatus (USA). Photoluminescence (PL) spectra were taken with a fluorescence spectrophotometer (F-7000, Hitachi, Japan). The excitation wavelength was 375 nm, the scanning speed was 1200 nm min−1, and the PMT voltage was 700 V. The width of excitation slit and emission slit were both 5.0 nm. Photocatalytic reduction of CO2 The photocatalytic reduction of CO2 was carried out in a 200 mL double-neck flask with a groove in one neck, at ambient temperature and atmospheric pressure.10b A 300 W Xe arc lamp (Changzhou Siyu Science Co. Ltd, China), providing UV/visible-light irradiation, was used as light source and positioned ∼22 cm away from the photocatalytic reactor. Typically, 100 mg of photocatalyst was dispersed in 10 mL DI water by sonication and transferred to the reaction vessel. After evaporation of water at 80 °C, a smooth film was formed at the bottom. Before irradiation, the reactor was sealed and blown with nitrogen for 30 min to remove air and ensure that the reaction system was under anaerobic conditions. CO2 and H2O vapor were produced by the reaction of NaHCO3 (120 mg, introduced into the reactor before seal) and HCl aqueous solution (0.25 mL, 4 M), which was introduced into the reactor by syringe. The flask was then kept under xenon lamp irradiation. The gas product composition was analyzed by a gas chromato-
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graph (GC-2014C, Shimadzu) equipped with a flame ionization detector (FID). Products were calibrated with a standard mixture gas and determined by the retention time.
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Results and discussion Phase structure and morphology The phase structure, crystallite size and crystallinity of typical samples were determined by XRD analysis. Fig. 1 shows the XRD patterns of the TiO2 nanofibers solvothermally treated at different time periods. The characteristic diffraction peaks for S0 at 25.3°, 37.8°, 48.2°, 54.1° and 62.5° can be assigned to the (101), (004), (200), (105) and (204) crystal planes of anatase TiO2 (JCPDS, No. 21-1272), respectively. These peaks do not show any shift for other samples, indicating the quite stable phase structure of anatase TiO2 during the solvothermal process. It is noted that the main diffraction peak of anatase (101) becomes gradually narrower and stronger with increasing solvothermal treatment time, as shown by the enlarged view of the peak (101) in Fig. S1.† We have estimated the average crystallite size of the samples with the Scherrer formula based on this main (101) peak. As listed in Table 1, the average crystallite size slightly increases from 10.1, to 10.3 and 10.5 nm by the sequence of S0, S2, and S6. These results indicate that the crystallinity of electrospun TiO2 nanofibers is further improved by solvothermal treatment at high temperature and pressure conditions. There are only small structural changes for electrospun TiO2 nanofibers. However, it may have a significant effect
on the photocatalytic properties, as the resulting bigger crystallite size will lead to improved structural connections with close-packing of grains, which is beneficial for the charge separation in photocatalytic reactions.25 The morphology and microstructure of the TiO2 nanofibers were examined by SEM and TEM (Fig. 2). As shown in Fig. 2a, the S0 sample appears as a common cylindrical morphology with a diameter from 300 to 500 nm and a length of dozens of micrometers. The high-magnification FESEM image in Fig. 2b clearly shows the surface of the nanofibers with aggregated nanocrystals. The overall morphology of sample S2 does not change obviously. Fig. 2c indicates that single nanofibers in sample S2 are constructed from many polycrystalline nanoscaled grains. An in-depth observation of the microstructure was conducted by HRTEM. Fig. 2d confirms that the average size of these nanoscaled grains is ∼10 nm. The lattice fringes with d spacing of 0.35 nm can be assigned to the (101) crystal plane of anatase TiO2. It is worth noting that the TiO2 nanograins are intimately contacted with each other. During the electrospinning process, the TBOT was hydrolyzed to form amorphous TiO2 particles in the PVP matrix. These TiO2 particles were uniform in size due to the space confinement of the PVP matrix.26 The PVP matrix was then removed by the calcination process. Meanwhile, the amorphous TiO2 particles were crystallized and turned into uniform and closely contacted anatase grains during the calcination process. Such intimate contact enables the electrons to move easily from one grain to another and provides an effective electron transfer pathway on TiO2,18 which can remarkably improve the separation of photogenerated electron–hole pairs. Since the TiO2 grains are already arranged closely, they are only able to grow slightly bigger upon further solvothermal treatment, due to the mutual restriction of the neighboring grains. However, this slight growth is enough to improve the connections of the closely contacted grains, which may result in a significant effect on the photocatalytic performance of TiO2 nanofibers. XPS characterization
Fig. 1 XRD patterns of TiO2 nanofibers with different solvothermal treatment time.
Table 1
The XPS survey spectra (Fig. S2†) indicate the existence of Ti, O and C in the S0 and S2 samples, and their photoelectron peaks emerge at binding energies of 458 (Ti 2p), 531 (O 1s) and 285 eV (C 1s), respectively. High-resolution XPS spectra of C 1s, O 1s, and Ti 2p were further examined and are shown in Fig. 3a–c. The C 1s spectrum of S0 (Fig. 3a) can be deconvoluted into three peaks at 284.8, 286.3, and 288.8 eV. The main peak at 284.8 eV can be assigned to the carbon atoms in the
Physical properties and photocatalytic performance of the typical samplesa
Sample
Solvothermal time (h)
Crystallite size (nm)
SBET (m2 g−1)
Pore volume (cm3 g−1)
Average pore size (nm)
CO2 adsorption (mmol g−1)
CH4 production rate (μmol h−1 g−1)
S0 S2 S6
0 2 6
10.1 10.3 10.5
59 55 49
0.072 0.064 0.055
4.84 4.68 4.40
0.14 0.19 0.16
3.15 19.55 3.08
a
The CO2 adsorption amount obtained at P/P0 = 1.
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Fig. 2 in (c).
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(a, b) FESEM images of the S0 sample; (c) TEM image of single nanofiber selected from the S2 sample; (d) HRTEM image of the circled area
C–C, CvC, and C–H bonds, which are caused by carbonization of organic groups or adventitious carbon.27 The peak at 286.3 eV is related to the hydroxyl carbon (C–OH) and the peak at 288.8 eV is attributed to carboxyl carbon (O–CvO), originating from the residual of the precursor.28 In comparison, the C 1s spectrum (Fig. 3a) of S2 only shows two peaks at 284.8 and 288.8 eV, with a dramatic decrease for the relative intensity of the peak at 288.8 eV. This is due to the fact that the solvothermal treatment largely reduces the residual of the organic precursor, thereby decreasing the amount of hydroxyl carbon and carboxyl carbon in the sample. Fig. 3b shows the O 1s spectra of S0 and S2. Three distinct peaks are observed for S0 at 529.7, 531.8, and 533.2 eV. The peak at 529.7 eV can be assigned to TiO2 lattice oxygen (Ti–O–Ti).29 The peak at 531.8 eV is attributed to the oxygen atoms in the Ti–OH (surface hydroxyl groups) and overlapped CvO bonds (residual from precursor).30,31 And the peak at 533.2 eV are related to the oxygen atoms in the CvO bonds.30 Nevertheless, the O 1s spectrum of S2 does not show the peak at 533.2 eV, owing to the dramatic decrease of the organic species after solvothermal treatment. Moreover, the peak corresponding to TiO2 lattice oxygen shifts to 530.0 eV. The Ti 2p spectra of S0 (Fig. 3c) shows two peaks centred at 458.5 and 464.2 eV which can be ascribed to Ti 2p3/2 and Ti 2p1/2, respectively, while these two peaks shift to 458.8 and 464.5 eV for sample S2. The observed spin–orbit splitting between the Ti 2p3/2 and Ti 2p1/2 is 5.7 eV, which is in good agreement with the values of Ti4+ state in TiO2.32 The results indicate that there are a lot of organic species on the surface of S0. After solvothermal treatment, these organic species are mostly removed and a cleaner surface of TiO2 is achieved. That is why the binding energies of Ti 2p and O 1s
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of TiO2 lattice oxygen for S2 are both positively shifted by 0.3 eV. It suggests that the surface properties of the electrospun TiO2 nanofibers do vary after solvothermal treatment. The resulting cleaner surface can help the charge immigration for better photocatalytic properties.33 Meanwhile, the removal of organic residual will release more adsorption sites for CO2 capture. BET surface areas and pore size distribution Fig. 4 shows the nitrogen adsorption–desorption isotherms and pore size distributions of samples S0, S2 and S6. According to Brunauer–Deming–Deming–Teller classification, the isotherms of all samples are of type IV, indicating the presence of mesopores (2–50 nm).34 Furthermore, the shapes of the hysteresis loops are of type H2 at the relative pressure (P/P0) range of 0.4–0.8, suggesting the formation of pores with narrow necks and wider bodies (ink-bottle pores).34 The pore size distributions (inset of Fig. 4) show a range of 2–40 nm with a peak pore diameter of ∼5 nm for all the samples, further confirming the existence of mesopores. However, slightly reduced specific surface areas from 59 (S0) to 55 (S2) and 49 m2 g−1 (S6) are obtained, and the pore volume and average pore size also decrease from S0 to S2 and S6, as listed in Table 1. This is due to the slightly increased size and confirms that the grains are packed more closely after solvothermal treatment. FTIR spectroscopy measurement The FTIR spectra of samples S0, S2 and S6 are shown in Fig. S3.† The broad peak at 3400 cm−1 and the peak at 1650 cm−1 are attributed to the surface-adsorbed water and hydroxyl groups.35 It suggests that all TiO2 nanofiber samples
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Fig. 3
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High-resolution XPS spectra for C 1s (a), O 1s (b), and Ti 2p (c) of S0 and S2.
Fig. 4 N2 adsorption and desorption isotherms and pore size distributions (the inset) of S0, S2 and S6.
are with a number of surface hydroxyl groups that can favour the CO2 capture. The main peaks located in the range of 400–700 cm−1 can be assigned to Ti–O stretching and Ti–O– Ti bridging stretching modes.36 The peak lying in 1380–1400 cm−1 region corresponds to carboxyl (CvO) groups of the organic species,35,37 The peak in the 2300–2370 cm−1 region can be ascribed to CO2.38 Note that the CO2 peaks of samples S2 and S6 are stronger than sample S0, implying the larger CO2 adsorption capacity of S2 and S6.
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Fig. 5
CO2 adsorption isotherms of S0, S2 and S6.
CO2 adsorption test It is well-known that the adsorption of CO2 onto the photocatalyst surface is the first essential step for the photocatalytic CO2 reduction. Thus we further investigated the CO2 adsorption performance of samples S0, S2 and S6 using CO2 adsorption isotherms. As shown in Fig. 5, all samples exhibited a rapid rise in CO2 uptake along with elevated CO2 pressure at a relatively low level (P/P0 < 0.2), indicating specific interactions
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between CO2 and the surface hydroxyl groups of TiO2 nanofibers. Especially, both the CO2 adsorption capacity of samples S2 and S6 is higher than that of S0, due to released adsorption sites after the removal of organic residual by solvothermal treatment. It is noted that as the relative pressure increases, sample S2 shows better CO2 uptake ability than S6. This is because at high P/P0 condition, more CO2 is adsorbed into the interparticle space of the nanofibers, whereas S6 has a lower specific surface area and smaller pore volume, resulting in less surface CO2 adsorption sites. Detailed CO2 adsorption capacity is listed in Table 1. Photoluminescence spectra The PL spectra can be used to reveal the separation and recombination tendency of the photogenerated electron–hole pairs within the TiO2 nanofibers.39 Fig. S4† shows a comparison of PL spectra of the S0 and S2 samples in the wavelength range 375–550 nm. Two main emission peaks appear at about 396 and 468 nm wavelengths. The former is ascribed to the emission of bandgap transition with the energy of light approximately equal to the bandgap energy of anatase,40 while the latter is the emission signal originating from the charge transfer transition from Ti3+ to oxygen anions in a TiO68− complex.41,42 Slightly weaker emission intensity is observed for S2, as compared to that for S0, implying a lower recombination rate of photogenerated electron–hole pairs. This is because the solvothermal environment can accelerate the crystallization and generate a cleaner surface of TiO2, resulting in the reduction of the number of defects and impurity in TiO2 crystals, thus the chance of photogenerated electron–hole recombination can be reduced. Photocatalytic activity The photocatalytic activity of the prepared samples is evaluated by using the reaction of photocatalytic CO2 reduction into hydrocarbon fuels. The original chromatogram for the reduction of CO2 on sample S2 is shown in Fig. 6a. Control experiments indicated that no hydrocarbon compound was detected in the absence of either photocatalyst or light irradiation, suggesting that photocatalyst and light irradiation are two indispensable factors for photocatalytic reduction of CO2, and production of hydrocarbon compounds. In our study, CH4 is detected as the main product, with a small amount of CH3OH and HCHO. As such, possible reactions for the photocatalytic CO2-reduction are proposed as below. CO2 þ 4Hþ þ 4e ! HCHO þ H2 O
E0redox ¼ 0:48 V
CO2 þ 6Hþ þ 6e ! CH3 OH þ H2 O
E0redox ¼ 0:38 V
CO2 þ 8Hþ þ 8e ! CH4 þ 2H2 O
E0redox ¼ 0:24 V
Fig. 6b shows the comparison of photocatalytic CH4 production rates on different samples. P25 is used as a reference and exhibits much lower activity than all the TiO2 nanofiber
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Fig. 6 (a) The original chromatograms of S2 after 1 h irradiation and (b) CH4 generation of the samples with different solvothermal time periods (S0, S1, S2, S3, and S6).
samples, demonstrating the superiority of the electrospun TiO2 nanofibers for photocatalytic CO2 reduction. It is clearly seen that the CH4 production rate increases dramatically from S0 to S2. This is mainly due to the improved CO2 adsorption promoted by the solvothermal treatment. Furthermore, the slightly growing and more closely packed grains after solvothermal treatment can lead to improved structural connections of TiO2 crystals, thus to accelerate the separation of photogenerated electron–hole pairs in terms of fast movement along the grain boundaries.25 In addition, the reduction of defects and impurity can suppress the recombination of photogenerated electron–hole pairs. However, a gradual decrease of CH4 production rate is observed for the samples with increasing solvothermal treatment time (from 3 to 6 h), due to the decreased CO2 adsorption ability and surface area. Therefore, it is necessary to modulate the solvothermal treatment time at an optimal value. As for our study, the best activity is obtained for sample S2, with 6- and 25-fold higher CH4 production rate than those of S0 and P25, respectively. Actually we have measured the UV-vis spectra of all the TiO2 nanofiber samples, but no difference was observed. This
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21177100). Also, this work was financially supported by the Fundamental Research Funds for the Central Universities (2013-VII-030) and Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1).
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Notes and references
Fig. 7 Schematic illustration for a mesoporous TiO2 nanofiber and its photocatalytic CO2 reduction mechanism.
means the band gap of TiO2 nanofibers are not affected by solvothermal treatment, thus there is no contribution from band gap alteration for the photocatalytic performance. Based on the above experimental results, a possible photocatalytic mechanism of CO2 reduction over TiO2 nanofibers is proposed and illustrated in Fig. 7. Firstly, CO2 is largely adsorbed on the surface of TiO2 nanofibers, through the bonding between CO2 and surface hydroxyl groups, as well as the physisorption into the interparticle space. Upon light irradiation, photogenerated electrons and holes can easily transfer to the nanofiber surface along the well crystallized and closely packed grains with a clean surface. As a result, the photogenerated electrons are capable of reducing CO2 into hydrocarbon fuels, meanwhile the separated holes can oxidize H2O into O2 and H+.
Conclusions In summary, we have successfully prepared mesoporous TiO2 nanofibers on a large scale through a simple electrospinning strategy combined with a subsequent calcination process. Further solvothermal treatment for the TiO2 nanofibers enables them with well crystallized and closely packed grains with a clean surface. The significantly improved efficiency of photocatalytic CO2 reduction into hydrocarbon fuels is obtained due to the improved structural connections for charge separation, and the released adsorption sites for enhanced CO2 adsorption capacity after solvothermal treatment. At an optimal solvothermal time of 2 h, the treated TiO2 nanofibers exhibit 6- and 25-fold higher CH4 production rate than those of TiO2 nanofibers without solvothermal treatment and P25, respectively. This work may not only indicate the effect of solvothermal treatment on the photocatalytic CO2 reduction over TiO2, but also provide a prototype for studying the effect of solvothermal treatment on the structure and photocatalytic activity of semiconductor photocatalysts.
Acknowledgements This work was supported by the 973 program (2013CB632402), and NSFC (51272199, 51320105001, 51372190, 51203182 and
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