Semiconductor photocatalysis is a sustainable technology for solving the problems related to environmental pollution and energy crisis, as it can be used to oxidize organic pollutants [1-6], reduce CO2 into hydrocarbon fuels [7-10], produce H2 by splitting water [11-15], and convert solar energy to electricity [16-19]. Among all the semiconductor photocatalysts, TiO2 is the most widely studied owing to its strong oxidation power, excellent photoelectrochemical stability, and good biocompatibility [20-22]. However, the photoreactivity of TiO2 is not high enough to satisfy the requirements for practical applications, which is attributed to its low quantum efficiency [23-25]. It is generally accepted that the photoreactivity of TiO2 is a function of many parameters such as Brunauer-Emmett-Teller (BET) surface area, crystallinity, crystalline phases, morphology, and pore structure [26]. As these parameters are usually in conflict of each other, optimizing the photoreactivity of TiO2 is very complex. For example, reducing the particle size of TiO2 can facilitate the adsorption of photosensitizers, which improves the photoelectric conversion efficiency of a dye-sensitized solar cell (DSSC). However, if the TiO2 particle size is too small, the photoanode film will become transparent with little light scattering, which is not good for the light absorption properties of TiO2 photoanode film [27, 28].
Although zero-dimensional (0D) TiO2 nanoparticles usually exhibit a high photoreactivity owing to the large BET surface area, they are very hard to collect and reuse [29]. Therefore, much attention has been paid to the synthesis of one-dimensional (1D) TiO2 nanowires [17] or nanotubes [30, 31], two-dimensional (2D) TiO2 nanosheets [32, 33] or arrays [18], and even three-dimensional (3D) TiO2 hollow microspheres [28, 34] and hollow nanoboxes [3, 35].
Electrospinning provides a technique to fabricate 1D structured semiconductors such as nanofibers from discrete 0D nanoparticles, which can improve the photoreactivity of the photocatalyst by retarding the recombination of carriers as the photogenerated electrons and holes are supposed to migrate along different directions [29, 36-38]. In addition, the gas permeability of the nonwoven materials obtained from electrospinning facilitates photocatalytic purification of air owing to the high porosity.
To further increase the BET surface area and porosity of the TiO2 nanofibers (TiO2-NFs) synthesized by electrospinning, they were treated in NaOH solution. In the form of anatase, TiO2 nanoparticles (TiO2-NPs) can react with NaOH to produce sodium titanate nanosheets (Reaction (1)), which can then transform into anatase nanosheets (TiO2-NSs) after acid washing and calcination (Reactions (2) and (3)) [28, 39]. Using this strategy, TiO2-NF assembly can be obtained from TiO2-NSs (TiO2-NFs-NSs).
The study by Lin et al. [40] showed that, when applied in a DSSC, TiO2-NFs-NSs exhibited a higher photoelectric conversion efficiency than pristine TiO2-NFs. However, the dependence of the photoelectric efficiency of TiO2-NFs-NSs on hydrothermal reaction time was not systematically studied. In addition, the performance of a DSSC is based on a photosensitization mechanism, whereby only the surface adsorbed photosensitizer (dye) in TiO2 photoanode film is excited under visible light irradiation to produce high-energy electrons, which then transfer to the conduction band of TiO2. However, in photocatalytic oxidation, reactive oxygen species (ROS) such as hydroxyl radicals (·OH) and super oxygen radicals (·O2–) form on the surface of UV-illuminated TiO2 photocatalyst, and these ROS can be used to oxidize organic pollutants. Therefore, in this research, we systematically studied the effect of hydrothermal reaction time on the photocatalytic performance and photoelectric conversion efficiency of TiO2-NFs-NSs.
TiO2-NFs was fabricated by electrospinning technique [29]. Briefly, 3.4 g of tetrabutyl titanate was added dropwise into a beaker containing 15.0 g of absolute ethanol and 4.0 g of glacial acetic acid under vigorous magnetic stirring. Then, 1.0 g of polyvinylpyrrolidone (Mw = 1, 300, 000) was added into the mixed solution. After further stirring for 6 h, the produced viscous solution was spun using an electrospinning device (QZNT-E01, Foshan Lepton, China) with a flow rate of 0.1 ml min-1 and an applied voltage of 22 kV. The distance between the needle tip and the Al foil board used for collecting fibers was kept at about 4.0 cm. The collected nanofibrous membrane was further calcined at 500 ℃ for 2 h to obtain TiO2-NFs (T0 sample)
Roller brush-like TiO2-NF assembly was synthesized from nanosheets by hydrothermal treatment of pristine TiO2-NF (T0 sample) nanofibrous membranes in an alkaline solution, which was followed by acid washing and calcination. Typically, 2.5 g of the as-spun nanofibrous membrane was immersed in 160 ml of NaOH solution (10 M), which was then transferred into a 200 ml Teflon-lined autoclave and kept in an oven at 120 ℃ for 3 h. After cooling to room temperature, the supernate was removed, while the collected white powders were re-dispersed in 900 ml of diluted HCl solution (0.1 M). After magnetic stirring for 12 h, the suspensions were filtrated, and the resulted cake was washed with distilled water until the pH became neutral. Finally, the cake was calcined at 400 ℃ for 1 h to obtain TiO2-NFs-NSs (T3.0 sample).
For comparison, a series of TiO2-NFs-NS samples were prepared under identical conditions, except for the change in hydrothermal reaction time. The prepared TiO2-NFs-NS samples were denoted as "Tx", where "x" represents the hydrothermal reaction time (h) in NaOH solution (Table 1).
The morphology of TiO2 nanofibers was observed by field emission scanning electron microscopy (SEM, Hitach, Japan) at 10 kV acceleration voltage and by transmission electron microscopy (TEM; Tecnai G20, USA) at 200 kV acceleration voltage. The phase structure of the photocatalyst was identified by X-ray powder diffraction (XRD; Bruker) at 15 kV acceleration voltage and 20 mA applied current. The light-harvesting ability of the photocatalyst was measured by UV-visible diffuse reflectance spectroscopy (UV-2550, Shimadzu) by using BaSO4 as the reference. The BET specific surface area and pore structure of the photocatalyst were analyzed with a nitrogen adsorption apparatus (ASAP2020, Micromeritics, USA). The calculation of the BET surface area was based on the adsorption data obtained by multipoint BET method in the relative pressure range 0.05–0.3, whereas Barret-Joyner-Halender method was used to determine the pore size distribution of the photocatalyst from the data of the adsorption isotherm. To obtain information on the separation and migration of photogenerated carriers in the photocatalyst, the photoluminescence (PL) spectrum was recorded by a fluorescence spectrophotometer (F-7000, Hitachi, Japan) at the excitation wavelength of 315 nm.
The photocurrent response of the photocatalyst was measured by an electrochemical workstation (CHI760e, Shanghai, China) with the help of the standard three-electrode photoelectrochemical cell, where a Pt-wire electrode, Ag/AgCl electrode, and the prepared samples were used as the counter electrode, reference electrode, and working electrodes, respectively. A 365 nm LED (3 W; Shenzhen Lamplic, China) was used as the light source, and Na2SO4 solution (0.4 M) was employed as the electrolyte. Before measurement of the photocurrent, the working electrode was prepared by coating TiO2 slurry onto the surface of indium tin oxide conductive glasses. First, 30 mg of TiO2 nanofibers was dispersed in 0.75 mL of ethanol solution (50 v/v%), which was followed by sonication for 30 min. Secondly, 20 μL of Nafion, which was used as the agglomerant, was added into the above dispersion, which was followed by ultrasonication for 30 min to obtain a homogeneous slurry.
The photocatalytic activity of TiO2 nanofibers was evaluated for photocatalytic oxidation of acetone [33]. The volume of the reactor was 15 L and an UV lamp that mainly emitted 365 nm radiation was used as the light source [41]. 0.3 g of the photocatalyst well-dispersed in 30 mL distilled water was distributed evenly in three watch glasses. Those watch glasses coated with burr-like TiO2 nanofibers were dried overnight at 60 ℃ and then placed under the UV lamp. After that, 10 μL of acetone was injected into the reactor, in which the system was sealed. After about 30 min, the adsorption of acetone reached equilibrium and its concentration reached about 300 ppm. The concentrations of acetone and carbon dioxide in the reactor could be detected on-line through a photoacoustic IR multigas monitor (INNOVA Air Tech Instruments, Model 1412).
The photoelectric conversion efficiency of TiO2-NFs was measured based on the photovoltaic current density-voltage (I-V) curve. First, we prepared a TiO2-NF film-based photoanode by pasting TiO2-NF slurry onto the surface of FTO glass by screen printing [28, 42]. In order to evaporate the organic solvent, the air-dried FTO glass coated with a layer of TiO2-NF film was placed inside an oven and heated at 80 ℃ for 10 min. After cooling to room temperature, the film was placed in a tubular muffle furnace to completely remove the organics from the substrate by calcination at 450 ℃ for 30 min. Secondly, the calcined FTO glass coated with 10-layered TiO2-NF film was immersed in N719 dye anhydrous ethanol solution (0.5 mM) for 24 h to adsorb the sensitizer. Finally, the photoanode film was rinsed with ethanol to remove the physically adsorbed sensitizer, which was then heated in an oven at 80 ℃ for 2 h to evaporate the solvent.
To assemble DSSCs, the sensitized TiO2-NF film-based photoanode was clamped together with the Pt counter electrode. After injection of the electrolyte, the photoelectric conversion efficiency was immediately determined.
Fig. 1 shows the SEM images of T0 sample, which was not treated with NaOH solution. It can be seen that T0 sample contains aggregates of nanofibers with diameters of about 80–100 nm (Fig. 1(a)). From the enlarged SEM image (Fig. 1(b)), we can see that these TiO2-NFs are assembled from nanoparticles. After treatment in NaOH solution for 0.5 h, it appears that the morphology of the TiO2 nanofibers (T0.5 sample) almost remains unchanged (Fig. 2(a)). However, careful inspection reveals that the surface of T0.5 sample is coarse. Some nanosheets are attached to the surface of TiO2-NFs (Fig. 2(b)); they should have originated from the transformation of TiO2 nanoparticles (Eq. (1)–(3)) [28].
The surface of the obtained TiO2-NFs becomes much coarser when the reaction time is extended to 1.0 h (Fig. 2(c)), which is ascribed to the growth of TiO2-NSs (Fig. 2(d)). These TiO2-NFs become curly brush-like upon further increasing the reaction time to 1.5 h (Fig. 2(e)), 2.0 h (Fig. 2(f)), and 2.5 h (Fig. 2(g)). The fibrous structure of TiO2-NFs can hardly be observed for T3.0 sample owing to the overgrowth of TiO2-NSs (Fig. 2(h)).
The transformation of TiO2 nanoparticles to TiO2 nanosheets after the treatment in NaOH solution can also be more clearly observed in the corresponding TEM images of T0 sample (Fig. 3(a) and Fig. 3(b)) and T3.0 sample (Fig. 3(c) and Fig. 3(d)). Elemental mapping was also used to record the transformation processes from anatase (Fig. 4(a)) to Na2TiO3 (Fig. 4(b)), H2TiO3 (Fig. 4(c)), and anatase TiO2 (Fig. 4(d)). From Fig. 4, we can see that the NaOH-treated TiO2 sample contains a high concentration of Na element, which is due to the formation of Na2TiO3 (Eq. (1)). After acid washing, the concentration of Na sharply decreases because of the production of H2TiO3 (Eq. (2)). Therefore, by using this strategy, we successfully transformed primary TiO2-NFs into TiO2-NFs-NSs. This transformation can not only sharply increase the BET surface area of TiO2-NFs to provide more active sites for adsorption and photoreaction, but also facilitate the light absorption owing to the multi-reflection of light between nanosheets [43], which will be shown below.
The XRD characterization results show that all the nanofibers are mainly anatase, with a small amount of rutile phase (Fig. 5). Generally speaking, anatase exhibits a higher photoreactivity than rutile phase, and the presence of a small amount of rutile is believed to be beneficial to the photocatalytic activity of anatase owing to the formation of a TiO2 homojunction [44].
From Fig. 5, we can also see that the XRD diffraction peaks, particularly the (101) facet of anatase, become wider and wider with the increase in the hydrothermal reaction time, which indicate reduced crystallization. By using Scherrer equation, we calculated the average crystalline sizes of the TiO2 samples, and the results are shown in Table 1. It can be seen that the crystallite size of TiO2 steady decreases from 17.1 nm (T0 sample) to 13.4 nm (T3.0 sample) with the increase in the hydrothermal time from 0 to 3.0 h. Poor crystallization means more defects, which is detrimental to the photoreactivity of photocatalytic TiO2 materials.
Specific surface area and pore structure are important factors affecting the photoreactivity of a photocatalyst. Therefore, nitrogen sorption-desorption isotherms are obtained and compared. In this research, from Fig. 6, we can see that the adsorption isotherm steadily shifts upward with an increase in the hydrothermal reaction time, which indicates an increase in the BET surface area of TiO2-NFs-NSs [28]. The BET surface area of TiO2-NFs (T0 sample) is only 28 m2 g–1, which increases to 62, 91, and 106 m2 g–1 after treatment in NaOH solution for 1.0, 2.0, and 3.0 h, respectively (Table 1). Therefore, the increase in the BET surface area of TiO2-NFs was achieved by treating TiO2-NFs in NaOH solution, which was followed by acid washing and calcination.
Careful inspection suggests that the hysteresis loop of the adsorption-desorption isotherm in the relative pressure range 0.6–0.9 evolves from type H2, which originates from bottle-like pores, to type H3, which is ascribed to the formation of narrow slit-shaped pores. This is consistent with the shape evolution of the building blocks of TiO2-NFs from nanoparticles (T0) to nanosheets (T3.0) [45, 46]. The inset of Fig. 6 compares the pore size distribution curves of the photocatalyst. It can be seen that the pore volume of the sample steady increases with the increase in the hydrothermal reaction time (Table 1), which is ascribed to the growth of TiO2-NSs. The pore volume of T3.0 sample is as high as 0.54 cm3 g–1, which is 6.0 times that of T0 sample (0.09 m3g-1). The increased pore volume can facilitate the penetration of gas during the photocatalytic reaction, which is beneficial to the photoreactivity [47].
Acetone is a widely used organic solvent in industry that is also a typical hazardous volatile organic compound (VOC) that can cause nervous disorders [48, 49]. Here, photocatalytic oxidation of acetone under UV irradiation was carried out to evaluate the photocatalytic activity of TiO2-NFs. The oxidation of acetone is based on the following equation (Eq. (4)).
Fig. 7(A) displays the photocatalytic oxidation curves of acetone over different TiO2-NFs. It can be seen, by using T0 sample as an example, that 32 ppm of acetone is decomposed within 30 min, and simultaneously, 77 ppm of CO2 is produced, which indicate that the reduced acetone is mineralized instead of being adsorbed on the surface of the photocatalyst upon irradiation.
The oxidation curve of acetone can be fitted with the pseudo-zero-order kinetic equation (Eq. (5)).
where C (ppm) is the concentration of acetone at irradiation time t (min), and K (ppm min–1) is the rate constant.
Then, the relative photocatalytic activity of TiO2-NFs can be compared with the corresponding rate constant. From Fig. 7(B), we can clearly see that the photoreactivity of TiO2-NFs steady increases with the increase in the hydrothermal reaction time from 0 to 2.5 h. The photoreactivity of T2.5 sample reaches 3.41 ppm min–1, which is 3.1 times that of T0 sample (1.11 ppm min–1). However, upon further increasing the hydrothermal reaction time to 3.0 h, the reactivity of TiO2-NFs reduces to 3.32 ppm min–1. The drop in the photoreactivity for T3.0 sample is possibly a result of its reduced TiO2 crystallite size (Table 1).
To account for the improved photoreactivity of TiO2-NFs-NSs, we compared the light-harvesting abilities of the photocatalyst. From the UV-visible absorption spectra shown in Fig. 8(A), we can see that the absorption of TiO2-NFs in the UV region steadily increases with increasing hydrothermal reaction time, which is possibly due to the multi-reflection of light between the nanosheets. Further observation shows that all the samples exhibit weak absorptions at wavelengths above 410 nm. This is possibly caused by the impurities present in TiO2, as the precursor of TiO2-NFs contains some organics such as polyvinylpyrrolidone that may not have been completely removed during calcination. The absorption of TiO2-NFs in the visible region is found to steadily decrease as the hydrothermal reaction time increases, which indicates that the hydrothermal reaction between TiO2-NFs and NaOH facilitates the removal of impurities from the TiO2-NFs.
From the corresponding PL spectra shown in Fig. 8(B), we can see that the PL intensity of TiO2-NFs also steadily increases with the increase in the hydrothermal reaction time. These PL peaks in the visible region, ranging from 400 to 500 nm, originate from surface O vacancies and defects [50]. The increased PL intensity of TiO2-NFs-NSs can also be attributed to the improved light-harvesting ability in the UV region.
Fig. 9 compares the transient PL spectra of pristine TiO2-NFs (T0 sample) and TiO2-NFs-NSs (T3.0 sample). The average lifetime of the carriers for TiO2-NFs-NSs is calculated to be 4.3 ns, which is slightly shorter than that of pristine TiO2-NFs (6.7 ns). Because only those pairs that are charged very fast can recombine, the shorter average carrier lifetime of TiO2-NFs-NSs implies enhanced exciton dissociation [11]. Therefore, the transformation of TiO2 nanoparticles to TiO2 nanosheets is beneficial to the charge dissociation and migration.
Considering that the separation/migration rate of photogenerated carriers is of great importance to the photoreactivity of semiconductor photocatalysts, we further compared the photocurrents of the photocatalyst. It is well accepted that, the larger the photocurrent, the more efficient is the separation and migration of photogenerated carriers [51]. The photocurrent of T2.0 sample reaches 0.156 μA cm–2, which is almost 2 times that of T0 sample (0.085 μA cm–2). Therefore, it is not strange to observe a higher photoreactivity for TiO2-NFs-NSs, compared with that of pristine TiO2-NFs (Fig. 10).
The photoreactivity of TiO2-NFs was also evaluated in terms of the photoelectric conversion efficiency of DSSCs.
The photoelectric conversion efficiency (η) was calculated according to Eq. (6) [22, 27]:
where VOC, ISC, FF, and Pin are the open circuit voltage, short circuit current density, fill factor of the film, and energy of incident light, respectively.
Fig. 11 compares the photovoltaic current density-voltage (I-V) curves of TiO2-NF film-based DSSCs, and Table 1 summarizes the corresponding photovoltaic parameters. We can see that all the samples reveal similar open circuit voltages (of about 0.75–0.78 V), whereas the short circuit current densities of the TiO2-NF-based film cells steadily increase from 2.25 to 4.87 mA cm–2 with the increase in the hydrothermal reaction time from 0 to 2.5 h. Upon further increasing the hydrothermal reaction time to 3.0 h, the short circuit current density of T3.0 sample reduces to 2.75 mA cm–2. Therefore, the photoelectric conversion efficiency of T2.5 sample-based film DSSC is the highest (2.65%), which is 2.3 times that of pristine TiO2-NF-based DSSC (1.14%). This trend is consistent with the photoreactivity of the photocatalyst toward acetone oxidation (Fig. 7).
Further study shows that the absorption of sensitized TiO2-NF film in the visible region steadily increases with the increase in the hydrothermal reaction time of the photocatalyst (Fig. 12), which can be ascribed to the improved adsorption of the dye (sensitizer) over TiO2-NFs-NSs. Since the BET surface area of TiO2-NFs is positively related to the hydrothermal reaction time (Table 1), TiO2-NFs-NSs (such as T3.0 sample) can provided more sites for adsorption of the dye, compared with pristine TiO2-NFs (T0 sample), which facilitates the harvesting of visible light [52], thus improving the photoelectric conversion efficiency (Fig. 11).
Highly photoreactive TiO2-NFs-NSs were prepared by simple hydrothermal treatment of electrospun TiO2-NFs in NaOH solution followed by acid washing and calcination. By using this strategy, TiO2 nanoparticles can be transformed into TiO2 nanosheets, which sharply increases the BET surface area and pore volume of TiO2-NFs. Compared with those of pristine TiO2-NFs, the photocatalytic activity of the hierarchical TiO2-NFs-NSs (T2.5 sample) toward acetone oxidation increases 3.1 times and the photoelectric conversion efficiency of the film-based DSSC improves 2.3 times. The enhanced photoreactivity of hierarchical TiO2-NFs-NSs is ascribed to the combined effects of an enlarged BET surface that provides more active sites for substrate adsorption and photoreaction, increased pore volume that is beneficial to the diffusion/penetration of gas, improved light-harvesting ability owing to multi-reflection of light between the nanosheets, and a more efficient separation of the photogenerated carriers of TiO2 nanosheets, compared to the case of TiO2 nanoparticles.
2020,
Vol. 41 Issue (1): 209-218 DOI: S1872-2067(19)63470-4 
PDF
2020,
Vol. 41 Issue (1): 209-218 DOI: S1872-2067(19)63470-4 PDF