简介概要

Controllable synthesis and tunable photocatalytic activity of TiO₂ nanowires via alcohol-thermal method

来源期刊：Rare Metals2019年第5期

论文作者：Zheng-Xia Xu An-Qi Wang Yong-Fa Zhu

文章页码：390 - 396

摘要：Titania nanowires（TiO₂-NW） with tunable aspect ratios and morphologies were directly synthesized using a simple alcohol-thermal technique. Specifically,ethanol and acetic acid were used as solvents and lithium ion was used as the capping agent to promote the conversion of titanium butoxide into TiO₂-NW. The morphologies and crystal phases of TiO₂-NW were determined by the molar ratio of solvents and the content（mol%） of lithium ion. The band gap of TiO₂-NW with pure anatase phase is slightly bigger than that of TiO₂-NW with a mixture of anatase and rutile phases. All TiO₂-NW could achieve effective decolorization of methyl blue（the decolorization rate is over 95%） after 35-min ultraviolet（UV） irradiation.

详情信息展示

稀有金属 (英文版) 2019,38(05),390-396

Controllable synthesis and tunable photocatalytic activity of TiO₂ nanowires via alcohol-thermal method

Zheng-Xia Xu An-Qi Wang Yong-Fa Zhu

School of Civil Engineering and Mechanics, Yanshan University

Department of Chemistry, Tsinghua University

作者简介：*Yong-Fa Zhu e-mail: zhuyf@tsinghua.edu.cn;

收稿日期：23 December 2018

基金：financially supported by the National Natural Science Foundation of China (No. 51408528);the Natural Science Foundation of Hebei Province, China (No. E2014203089);

Controllable synthesis and tunable photocatalytic activity of TiO₂ nanowires via alcohol-thermal method

Zheng-Xia Xu An-Qi Wang Yong-Fa Zhu

School of Civil Engineering and Mechanics, Yanshan University

Department of Chemistry, Tsinghua University

Abstract：

Titania nanowires (TiO₂-NW) with tunable aspect ratios and morphologies were directly synthesized using a simple alcohol-thermal technique. Specifically, ethanol and acetic acid were used as solvents and lithium ion was used as the capping agent to promote the conversion of titanium butoxide into TiO₂-NW. The morphologies and crystal phases of TiO₂-NW were determined by the molar ratio of solvents and the content (mol%) of lithium ion. The band gap of TiO₂-NW with pure anatase phase is slightly bigger than that of TiO₂-NW with a mixture of anatase and rutile phases. All TiO₂-NW could achieve effective decolorization of methyl blue (the decolorization rate is over 95%) after 35-min ultraviolet (UV) irradiation.

Keyword：

Nanowires; TiO₂; Photocatalysis; Degradation; Photocurrent;

Received： 23 December 2018

1 Introduction

One-dimensional titanium dioxide, or TiO₂, nanomaterials, such as nanowire, nanorod, nanotube and nanobelt, have drawn greatly attention because of their unique physicochemical properties and a broad range of applications [ 1, 2, 3, 4, 5, 6, 7, 8] .Geometrically, one-dimensional nanostructures with controllable dimensions and aspect ratios make them have unusual properties that are difficult to achieve with nanoparticles and two-dimensional nanosheets, such as maximum charge transport along the columnar axis [ 9, 10, 11] and enhanced mechanical properties [ 12] .The most widely used methods for producing one-dimensional TiO₂are template-directed synthesis [ 13, 14] , seeded growth [ 15, 16, 17, 18] and electrochemical etching [ 19, 20, 21] .However, the process of these methods is mostly consisted of several steps that are difficult to handle.Recently, anatase TiO₂nanowires have been directly synthesized via hydrothermal method, using N, N-dimethylformamide (DMF) and acetic acid as solvents and lithium acetate as additive.Lithium ions play an important role in transformation of polyhedral nanocrystals to hierarchical nanowires [ 22, 23, 24] .One-step formation of one-dimensional TiO₂is not favorable under normal conditions because the requirement to obtain anisotropic growth is not naturally satisfied due to the isotropic property of TiO₂crystal structures.Thus, the synthesis of one-dimensional TiO₂with controllable aspect ratios and morphologies remains a significant challenge.

In this work, we presented an alcohol-thermal method for the synthesis of one-dimensional TiO₂with controllable aspect ratios and morphologies.Solvent was an important factor influencing the sizes and morphologies of the formed crystallites in solvothermal method, since it controlled the state of active titanium atoms during the crystal growth.Here, we used alcohol as an environmentally friendly solvent, replacing the traditional and toxic solvents, such as DMF [ 22, 23] and toluene [ 25] .The results demonstrated that the molar ratio of solvents played an important role in controlling the morphologies and aspect ratios of one-dimensional TiO₂.

2 Experimental

2.1 Materials

All solvents and reagents were of analytical reagent grade or better and were used as received.Titania nanowires (TiO₂-NW) were synthesized in a quaternary solution containing titanium butoxide (TB) , absolute ethanol (ET) , acetic acid (HAc) and lithium acetate (LiAc) , as modified from previous methods [ 22, 23] .The morphology and crystal phase of TiO₂-NW changed dramatically after replacing DMF with ET.Typically, TB (25 mmol) was added to a mixture of ET, HAc and LiAc in a dried Teflon cup.After stirred for 10 min, the solution in Teflon cup was transferred into a 100-ml stainless steel autoclave.Then, the autoclave was kept at 200°C for 20 h.The white precipitates were washed three times with ethanol and water, respectively, and then vacuum freeze-dried.Samples which were prepared from solution with molar ratios of TB/ET/HAc/LiAc at 1:X:Y:Z were denoted as TiO₂-NW-X:Y:Z.

2.2 Characterization

Transmission electron microscopy (TEM) images of titania nanowires were obtained from a transmission electron microscope (Hitachi Model HT7700, Japan) operated at100 k V.High-resolution transmission electron microscopy (HRTEM) was recorded on JEM-2011F field emission transmission electron microscope (FESEM) with an accelerating voltage of 200 k V.Powder X-ray diffraction (XRD) patterns of the products were recorded on a diffractometer (Rigaku D/MAX2500, Japan) equipped with Cu Ka radiation at the steps of 2h=0.02°.Using BaSO₄as a reflectance standard, ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS) of titania nanowires were acquired with a spectrophotometer (Shimadzu UV-3600, Japan) in the wavelength range of 200–800 nm.Fourier transform infrared spectra (FTIR) of titania nanowires were recorded with an E55+FRA106 spectrometer (Bruker, Karlsruhe, Germany) in the wave number ranging from 400to 4000 cm^-1.Photoelectrochemical properties were measured by a CHI660B electrochemical workstation with a three-electrode system in which indium tin oxide (ITO) glass slide (1 cm × 2 cm) coated with titania nanowires acted as the working electrode, platinum wire as a counter electrode and the standard calomel electrode as a reference electrode.The electrochemical impedance spectroscopy (EIS) was carried out at on a direct current bias of 0.0 V with an alternating current voltage of 5 mV in the frequency range from 100 kHz to 0.05 Hz.The exposed area of glass electrodes was kept at 1.0 cm², and the loading mass of titania nanowires on electrode was 2 mg·cm^-2.Na₂SO₄ (0.1 mol·L^-1) aqueous solution was used as the electrolyte.A 500-W xenon lamp (center wavelength420 nm) was used as the light source.

2.3 Photocatalysis measurement

The photocatalytic activity of titania nanowires was evaluated for decomposition rates of methyl blue in the presence of a photocatalyst.In the photocatalysis measurement, 100 mg of photocatalysts was added to 200 ml methyl blue aqueous solution (5× 10^-5mol·L^-1) , and then, the solution was stirred with a magnetic stirrer in the dark for30 min to reach adsorption equilibrium.5.0 ml adsorption equilibrium solution was taken out to determine the amount of methyl blue absorbed by photocatalysts.Then, the solution was exposed to UV irradiation using a 250-W high pressure mercury lamp placed 8 cm above the liquid surface.5.0 ml solution was taken out for every 5 min, and the photocatalysts were separated from the solution by centrifugation at 8000 r·min^-1for 5 min.UV–Vis absorption spectra of the upper liquid were measured with a spectrophotometer (Shimadzu UV-3550, Japan) .

The photocatalytic decolorization rates (DR) were calculated by the following formula:

where A₀is the initial absorbance of methyl blue solution before adding the photocatalyst, A_tis the absorbance of methyl blue solution after adsorption and then ultraviolet irradiation for time t.

3 Results and discussion

The morphology of the TiO₂-NW was characterized by TEM.Figure 1 shows TEM images of the TiO₂-NW synthesized by varying the molar ratio of ET/HAc or the content of LiAc (mol%) , while keeping the molar ratio of TB to ET and HAc as constant (TB/ (ET+HAc) =1:40) .When the molar ratio of ET/HAc/LiAc is 20:20:1, TiO₂-NW-20:20:1 with length from 30 to 200 nm and diameter from 5 to 10 nm are obtained (Fig.1a) .Some bended nanowires are observed.By increasing the content of LiAc (mol%) and keeping the molar ratio of ET/HAc unchanged, TiO₂-NW-20:20:5 with bimodal size distribution is produced (Fig.1b) .Some large-size nanowires with diameter bigger than 5 nm and lots of small-size nanowires with length lesser than 10 nm are both observed.If the molar ratio of ET/HAc/LiAc is changed to 30:10:1, TiO₂-NW-30:10:1 with nanoparticles embedded in is formed, as shown in Fig.1c.A further change of the ET/HAc/LiAc molar ratio to 10:30:1 leads to the formation of TiO₂-NW-10:30:1 with aspect ratios bigger than 20 (Fig.1d) .It should be pointed out that two kinds of TiO₂-NW can be observed in TiO₂-NW-10:30:1 straight and V-shaped.These V-shaped nanowires display different bending angles (e.g., 105°, 114°, and 126°) .To the best of our knowledge, the synthesis of V-shaped TiO₂-NW has rarely been achieved before.Some researchers [ 26, 27] have reported that V-shaped silver nanowires have been prepared by polyol-thermal synthesis method.The formation of V-shaped silver nanowires is caused by matching a crystal lattice or sharing a twinned crystal plane.The exact mechanism for the formation of V-shaped TiO₂-NW is still unclear.Finally, the morphologies of TiO₂-NW change from flower-like assemblies to bended, straight or V-shaped structures by replacing previously used DMF [ 22] with ET.

Fig.1 TEM images of a TiO₂-NW-20:20:1, b TiO₂-NW-20:20:5, c TiO₂-NW-30:10:1 and d TiO₂-NW-10:30:1

The TiO₂nanowires were further characterized by HRTEM technology to elucidate the crystalline structure and growth direction of these nanowires.Figure 2a, c displays the magnified HRTEM images of TiO₂-NW-20:20:1 and TiO₂-NW-10:30:1, which are taken from the square regions of typical nanowires in Figs.S1 and S2A, respectively.The line profiles from the trace (white line) in Fig.2a, c are given in Fig.2b, d, respectively.Two sets of lattice fringes with plane spacing of 0.237 and 0.324 nm calculated from the line profiles are attributed to (004) plane of the anatase phase and (110) plane of rutile phase.The lattice fringes that correspond to (110) plane of rutile phase are parallel to the longitudinal axis of TiO₂-NW (Fig.2c) .Combining the crystal structure of rutile TiO₂and the above observation, this typical nanowire is growing along the (110) direction of rutile phase to form anisotropic structure.Furthermore, V-shaped nanowires are formed by the fusion of two nanowires, as shown in Fig.S2B.

Fig.2 a HRTEM image, b line profile from trace of TiO₂-NW-20:20:1;c HRTEM image, d line profile from trace of TiO₂-NW-10:30:1

The crystallographic structure of the synthesized TiO₂-NW was verified by XRD.Figure 3a gives XRD patterns of TiO₂-NW prepared with different molar ratios of TB/ET/HAc/LiAc.All diffraction peaks can be assigned to those of standard diffraction pattern for anatase (bottom, JCPDS No.21-1272) or rutile (top, JCPDS No.21-1276) TiO₂.XRD patterns also indicate that the crystal phase of TiO₂-NW-30:10:1 is pure anatase phase, while the crystal phases of other TiO₂-NW exist as a mixture of anatase and rutile crystal forms.The calculated anatase/rutile weight ratios for TiO₂-NW-20:20:1, TiO₂-NW-20:20:5 and TiO₂-NW-10:30:1 are 79/21, 18/82 and 23/77, respectively [ 28, 29] .UV–Vis DRS of TiO₂-NW are given in Fig.3b.The diffuse reflectance rate for all TiO₂-NW is nearly the same in the visible region, while the diffuse reflectance rate for TiO₂-NW-20:20:5 is apparently higher than that for the other TiO₂-NW in the ultraviolet region.Furthermore, TiO₂-NW-30:10:1 exhibits a slight blue shift in the absorption edge.The estimated band gaps for TiO₂-NW-20:20:1, TiO₂-NW-20:20:5, TiO₂-NW-30:10:1 and TiO₂-NW-10:30:1 are 3.00, 2.98, 3.20 and 2.98 eV, respectively (Fig.S3 and Table S1) .

Fig.3 a XRD patterns and b UV–Vis DRS of TiO₂-NW prepared with different molar ratios of TB/ET/HAc/LiAc

The TiO₂-NW samples are photoactive, as confirmed by the photoresponse evaluation.Figure 4 shows the transient photocurrent responses for TiO₂-NW-20:20:1, TiO₂-NW-20:20:5, TiO₂-NW-30:10:1 and TiO₂-NW-10:30:1 photoelectrodes with ten on–off cycles of intermittent irradiation.When all of the photoelectrodes are irradiated by simulated light, the photogenerated electron and hole are instantaneously produced.So, the photocurrent is reproducibly generated under the exerted electric field.The photocurrent value rapidly declines to nearly zero after the cessation of the irradiation.The transient photocurrent of TiO₂-NW-30:10:1 is sharply rising and then rapidly decaying during the light-on period, which may result from charge recombination in process of electron transmission.The photocurrent of TiO₂-NW-30:10:1 is gradually decreasing by*40%after ten on–off cycles of intermittent irradiation.There is no obvious change on the photocurrent of Ti O₂-NW-20:20:5 and TiO₂-NW-10:30:1during on–off cycles.Moreover, the EIS study reveals that the arc radii of the Nyquist plots for TiO₂-NW under irradiation (Fig.S4) are smaller than those in the dark (Fig.S5) .For the TiO₂-NW samples under irradiation, the radii of the curve are in the order of TiO₂-NW-30:10:1[TiO₂-NW-20:20:1[TiO₂-NW-20:20:5[TiO₂-NW-10:30:1 (Fig.S4) , while for the same samples without irradiation, the radii of the curve corresponding to TiO₂-NW-20:20:1 and TiO₂-NW-30:10:1 are smaller than those for TiO₂-NW-20:20:5 and TiO₂-NW-10:30:1 (Fig.S5) .The smaller radii of Nyquist plots indicate a more efficient charge transfer, which results in the enhancement of conductivity properties and then the improvement in photocatalytic activities (Fig.5) .

Although the surface of TiO₂-NW is absorbed by some organic species, such as HAc (Fig.S6) , these TiO₂-NW possess excellent photocatalytic activity for degradation of methyl blue (MB) .The photocatalytic activity of all TiO₂-NW was investigated by degradation of MB (5× 10^--5mol·L^-1) in aqueous solution (Figs.S7–S10) .Figure 5a displays the comparison of photocatalytic degradation rate for TiO₂-NW prepared with different molar ratios of TB/ET/HAc/LiAc.After adsorption (30 min) [ 22, 30] and ultraviolet irradiation (25 min) , over 95%MB is removed from aqueous solution.The self-degradation rate of MB without photocatalyst is about 30%under ultraviolet irradiation.The adsorption capacity of MB on TiO₂-NW is21–26 mg·g^-1, and 26%–32%MB is adsorbed after reaching equilibrium.About 40%MB reduced from aqueous solution is caused by photocatalytic degradation, after deduction of adsorption and self-degradation.Although the decolorization rates of all TiO₂-NW are almost the same after 30-min adsorption and 35-min UV irradiation, the kinetic constants of different photocatalysts are varied (Fig.5b and Table S1) .The fitted kinetic constants of Ti O₂-NW-20:20:1, TiO₂-NW-20:20:5, TiO₂-NW-30:10:1 and TiO₂-NW-10:30:1 are-0.26, -0.11, -0.20and-0.11 min^-1, respectively.The kinetic constant of TiO₂-NW-20:20:1 is 2.4 times as high as that of TiO₂-NW-10:30:1 and 16.6 times as high as that of self-degradation.The photocatalytic activity of TiO₂-NW is not only influenced by their morphology and optical properties (diffuse reflectance and band gap) , but also determined by their crystal phases (pure and mixture) and conductivity properties (radii of the Nyquist plots) .Their photocatalytic activities are in the order of TiO₂-NW-20:20:1 (anataserich) [TiO₂-NW-30:10:1 (pure anatase) [TiO₂-NW-20:20:5 (rutile-rich) ≈TiO₂-NW-10:30:1 (rutile-rich) , which is consistent with the previous studies [ 30, 31] .

Fig.4 Transient photocurrent density of TiO₂-NW prepared with different molar ratios of TB/ET/HAc/LiAc in 0.1 mol·L^-1Na₂SO₄under simulated sunlight irradiation:a TiO₂-NW-20:20:1, b TiO₂-NW-20:20:5, c TiO₂-NW-30:10:1 and d TiO₂-NW-10:30:1

Fig.5 Comparison of a photocatalytic rate and b first-order kinetics plot of MB degradation for TiO₂-NW prepared with different molar ratios of TB/ET/HAc/LiAc

4 Conclusion

This study shows the preparation of TiO₂-NW with tunable aspect ratios and morphologies by a simple alcohol-thermal method.The molar ratio of ET and HAc (constant total molar number is 40 mmol) and the content (mol%) of lithium ion in reaction solution were found to have a profound impact on the morphologies and crystal phase of TiO₂-NW.TiO₂-NW with different morphologies, such as bended, straight and V-shaped structures, were obtained by changing the molar ratio of reagents.The photocatalytic activity of these samples was influenced by optical properties (diffuse reflectance and band gap) , crystal phase (pure or mixture) and morphology.