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Phosphomolybdic acid-modified highly organized TiO2 nanotube arrays with rapid photochromic performance
Journal of Materials Science & Technology 35 (2019) 1951–1958
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Research Article
Phosphomolybdic acid-modified highly organized TiO2 nanotube arrays with rapid photochromic performance Yuanyuan Wei a , Bing Han b , Zhaojun Dong a , Wei Feng a,b,∗ a Key Lab of Groundwater Resources and Environment Ministry of Education, College of New Energy and Environment, Jilin University, Changchun 130021, China b Key Laboratory of Songliao Aquatic Environment Ministry of Educatio, Jilin Jianzhu University, Changchun 130118, China
a r t i c l e
i n f o
Article history: Received 21 March 2019 Received in revised form 23 April 2019 Accepted 26 April 2019 Available online 17 May 2019 Keywords: Phosphomolybdic acid TiO2 nanotube array Rapid Photochromic performance
a b s t r a c t TiO2 nanotube arrays were prepared by means of an electrochemical anodization technique in an organic electrolyte solution doped with polyvinyl pyrrolidone (PVP) and were subsequently modified with phosphomolybdic acid (PMoA) to obtain PMoA/TiO2 nanotube arrays. The microstructure and photochromic properties were investigated via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible spectroscopy (UV–vis), and X-ray photoelectron spectroscopy (XPS). The results indicated that the Keggin structure of PMoA and the nanotube structure of TiO2 were not destroyed, and there was a strong degree of interaction between PMoA and TiO2 at the biphasic interface with lattice interlacing during the compositing process. The XPS results further indicated that there was a change in the chemical microenvironment during the formation process of the composite, and a new charge transfer bridge was formed through the Mo-O-Ti bond. Under visible light irradiation, the colorless PMoA/TiO2 nanotube array quickly turned blue and exhibited a photochromic response together with reversible photochromism in the presence of H2 O2 . After visible light irradiation for 60 s, the appearance of Mo5+ species in the XPS spectra indicated a photoreduction process in accordance with a photoinduced electron transfer mechanism. © 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction Since their discovery, photochromic materials have received an extensive amount of attention due to their potential application prospects in high-tech fields, such as data storage equipment, biological imaging, sensor applications and biomedical research [1–5]. Polyoxometalates (POMs), a class of molecularly defined inorganic metal-oxide clusters [6–9], are one of the most studied photochromic materials due to their important characteristic of accepting one or more electrons to produce mixed-valence colored species (heteropoly blues or heteropoly browns) [1,4,6,9]. Among six basic structures of POMs, including Keggin structure, Dawson structure, Derson structure, Waugh structure, Silverton structure and Lindqvist structure, Keggin structure is the most representative one [10,11]. In all POMs with kegging structure, phosphomolyb-
∗ Corresponding author at: Key Lab of Groundwater Resources and Environment Ministry of Education, College of New Energy and Environment, Jilin University, Changchun 130021, China. E-mail address: [email protected] (W. Feng).
dic acid (PMoA) and phosphotungstic acid are two typical one. The lower redox potential than phosphotungstic acid [10] make PMoA is more prone to electron transfer. Henceforth, recently researches on organic-inorganic photochromic materials mainly focused on PMoA [6–13]. Currently, photochromic materials are mainly prepared using noncovalent bonds to embed PMoA into various organic or inorganic polymer networks, such as polyacrylamide (PAM) [7,13], polyvinylpyrrolidone (PVP) [14,15], polyvinyl alcohol (PVA) [16,17], polyether chains (PEs) [18], WO3 [19–21] and TiO2 [22–25], to construct composite films, thus greatly improving the physical and chemical properties of the hybrid films. Among these composite, inorganic-inorganic composite photochromic materials containing titanium oxide have attracted attention. TiO2 is widely used in the field of photochemistry due to its advantages of environmental friendliness, significant dielectric effect, nontoxicity and complete mineralization [22,26–28]. Specifically, compared to other structures, TiO2 nanotube arrays have a highly ordered and vertically oriented tubular structure with a large specific surface area and high adsorption capacity [29–33], which is favorable for the separation of photogenerated electron-hole pairs to improve the efficiency of charge collection [30,32]. In addition,
https://doi.org/10.1016/j.jmst.2019.05.014 1005-0302/© 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
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the highly ordered nanotube structure can also reduce the loss from light reflection to improve the light absorption rate because the photons entering the nanotube array are unlikely to escape due to the multiple scattering of radiation by the nanotubes [22]. Therefore, TiO2 nanotube arrays are highly suited for use as electron donors in photochromic reactions [33,34]. To ensure the excellent photoelectric properties of the nanotubes, it is necessary to control the physical and chemical properties of the TiO2 nanotube array during the preparation process [33–39]. The preparation methods for TiO2 nanotube arrays include templated, hydrothermal, and anodic oxidation reactions [38,40–42]. Among them, the anodic oxidation method has been widely used because of its simple process, easily controlled reaction rate, and preparation of nanotubes that are highly ordered and easy to industrialize [37,38]. Therefore, the TiO2 nanotube arrays in this experiment were prepared by an anodic oxidation method. According to previous results [34–38,43–46], the composition of the electrolyte, anodization parameters and annealing temperature during the preparation of TiO2 nanotube arrays by electrochemical anodization all affect the morphology of the array, thus affecting the photochromic properties. As a synthetic water-soluble polymer, PVP has the typical properties of water-soluble polymer compounds, such as colloidal protection, film-forming ability, adhesive, hygroscopic, solubility or condensability, but its most characteristic properties are its excellent solubility and biocompatibility [47,48]. Recently, Zhou et al. [47] studied the PL properties of SiC nanotubes prepared from electrospun polymer templates and found that the addition of a PVP solution prevented the growth of nanocrystals and stabilized the particles against aggregation, resulting in a high number of surface defects and greatly improved PL performance. However, there are few reports on the application of PVP in the fabrication of TiO2 nanotube arrays from anodization methods. Therefore, it would be significant to improve the morphology of TiO2 nanotube arrays by doping PVP in organic electrolyte solutions. In this work, we prepared a TiO2 nanotube array by an electrochemical anodization technique with an organic electrolyte solution doped with polyvinyl pyrrolidone (PVP) and modified the TiO2 nanotube array with PMoA to obtain a PMoA/TiO2 nanotube array. Then, through instrumental analysis, the microstructure and composition of the PMoA/TiO2 nanotube array were studied. The photochromic properties and mechanism were investigated in detail via ultraviolet-visible (UV–vis) spectra and X-ray photoelectron spectroscopy (XPS). The results indicated that the photoresponsive rate of the PMoA/TiO2 nanotube system was very fast, and a new charge transfer bridge was formed through the Mo-O-Ti bond during the photoreduction process. The formation of the Mo-O-Ti bond reduced the excitation energy of the Ti-O bond and enabled the composite nanotubes to change color under visible light excitation. The photoreduction process was in accordance with the photoinduced electron transfer mechanism.
2. Experimental 2.1. Materials Titanium foil (1.2 cm × 3.0 cm, 0.2 mm thickness, 99.7% purity) was purchased from Shenzhen Fuxin Titanium Technology Co., Ltd. Phosphomolybdic acid (PMoA) was purchased from Tianjin Kermel Chemical Reagent Development Center and recrystallized twice before use. Polyvinylpyrrolidone (PVP, Mw = 50,000) was obtained from Aldrich and purified by fractional distillation before use. All other chemical reagents were of analytical grade, and deionized water was used in all experiments.
2.2. Preparation The anodization method was used to fabricate the TiO2 nanotube arrays. Before anodization, the Ti foils (1.2 cm × 3.0 cm, 0.2 mm thickness, 99.7% purity) were mechanically polished with different abrasive papers (400#, 800# and 1200#) and then cleaned by sonication with acetone, ethanol and purified water for 30 min. The titanium foils were polished with a mixed acid solution (HF:HNO3 :H2 O = 1:4:5 volumetric ratio) for 30 s to produce a smooth surface. Finally, the titanium was washed with purified water and then dried in an oven at 60 ◦ C. All the other chemicals were of analytical grade and used as received without further purification. The anodization was carried out using a conventional two-electrode system with the Ti sheet as the anode and an untreated Ti sheet (2.5 cm × 3.5 cm) as the cathode. Both electrodes were immersed into 100 mL of the organic electrolyte. The organic electrolyte was composed of 0.5 wt% NH4 F, 0.1 wt% PVP and an ethylene glycol system with a mixed volume ratio of 2% water. The samples were anodized at 30 V for 4 h. After anodization, the samples were annealed in a laboratory tube furnace at 450 ◦ C for 2 h with a heating rate of 1 ◦ C/min in ambient air and then naturally cooled to induce crystallization. The as-prepared samples were immersed in a 10 mg/ml solution of phosphomolybdic acid in ethanol for 2 h and then ultrasonically washed in ultrapure water, ethanol, and acetone for 30 min. 2.3. Instrumental analysis Diffraction patterns were obtained with an X-ray diffractometer (XRD) with a monochromatic Cu K␣ radiation source ( = 0.15418 nm) operating at 40 kV and 100 mA with a scan rate of 0.05◦ over the 2 range of 20◦ to 80◦ . FTIR spectra were determined from samples deposited on KBr pellets at room temperature with a Nicolet Impact 410 FTIR spectrometer in the wavenumber range of 500–4000 cm–1 . The surface morphologies of the prepared samples were observed by scanning electron microscopy (SEM) on a JEOLJSM-6700 F. Transmission electron microscopy (TEM) observations were performed on a JEOL JEM-2100 operating at 200 kV to characterize the microstructures of the PMoA/TiO2 nanotube arrays. Absorbance curves were measured on a UV–Vis spectrophotometer (JASCO V-550) with an optical resolution of 1 nm in the range of 350–900 nm. X-ray photoelectron spectroscopy (XPS) valence band spectra were obtained with an ESCA LAB-MKII photoelectron spectrometer. Photochromic experiments were carried out using a 300 W xenon lamp filtered by UV filter as the visible light source under the condition of complete light avoidance. The distance between the lamp and composite nanotube arrays was 150 mm. The composite nanotube arrays were exposed to air during the visible light irradiation process. After irradiating for a certain time, the in situ absorbance curve was obtained. The irradiation time was increased until the obtained curve was consistent with the previous curve. While exposed to air, the composite nanotube arrays were protected from light, and the absorption spectra were regularly measured to monitor the bleaching process. All measurements were carried out at room temperature. 3. Results and discussion The XRD patterns of the TiO2 nanotube array (a), PMoA (b), and PMoA/TiO2 nanotube array (c) are shown in Fig. 1. The results indicate that the TiO2 nanotube arrays are well crystallized after annealing at 450 ◦ for 2 h and the pure PMoA also has obvious crystal characteristic peak. However, in the XRD pattern of the PMoA/TiO2 nanotube array, it is hardly to observe the characteristic
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Fig. 1. XRD patterns of PMoA (a), TiO2 nanotube array (b) and PMoA/TiO2 nanotube array without visible light irradiation.
Fig. 2. FTIR spectra of PMoA (a), TiO2 (b) and the PMoA/TiO2 nanotube array before (c) and after (d) visible light irradiation. The inserted picture of the Fig. 2 is the FTIR spectra of PMoA/TiO2 nanotube array before (c) and after (d) visible light irradiation in the range of 650–850 cm−1 .
peaks of PMoA because of the coverage of the characteristic peaks of TiO2 . The characteristic peaks of the TiO2 nanotube array are well matched with the reflection planes of the anatase phase, where the diffraction peaks at 2 values of 25.3, 37.8, 48.0, 53.9, 55.06, 62.7 and 70.5 can be ascribed to the reflections of the (101), (004), (200), (105), (211), (204) and (220) planes of the anatase phase, respectively. No rutile phase can be detected. Moreover, compared with the XRD spectra of the TiO2 nanotube array, there is no discernible change in the phase of the TiO2 nanotube array when modified with PMoA (Fig. 1(c)), indicating that PMoA only occurred on the surface of the TiO2 nanotube array and that the crystalline structure of the TiO2 nanotube array has not been destroyed. To confirm the structural characteristics of the composite nanotubes, FTIR spectra of pure PMOA (a), TiO2 nanotubes (b), and the PMOA /TiO2 nanotube array before (c) and after (d) visible light irradiation in the range of 500–1500 cm−1 were obtained, and the results are shown in Fig. 2. In the FTIR spectra of the pure PMoA crystals, there are four characteristic bands at 1064, 968, 876 and 789 cm–1 , corresponding to the stretching vibrations of (P-Oa), (Mo-Od), (Mo-Ob-Mo) and (Mo-Oc-Mo), respectively
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[14], which represent the Keggin structure [25]. The spectrum corresponding to PMoA is shown in Fig. 2(a). For the PMoA/TiO2 nanotube array, the above characteristic vibration bands are similar to those of pure PMoA except for having shifted by a few wavenumbers (Fig. 2(c,d)). This indicates that the Keggin structure of PMoA is still preserved and that a strong interfacial interaction between PMoA and TiO2 occurs. Comparing pure PMoA with PMoA/TiO2 , as indicated by curve a and curve c in Fig. 2, the Mo-Od asymmetrical stretching frequency in PMoA/TiO2 has a redshift of 4 cm−1 , which is mainly caused by TiO2 increasing the distance between adjacent heteropoly anions and weakening the electrostatic interactions between heteropoly anions. Compared with pure PMoA, the (Mo Ob Mo) and (Mo Oc Mo) vibrations in the PMoA/TiO2 nanotube array have blueshifts of 14 and 18 cm−1 , respectively, and the results show that a Mo-O-Ti bond was formed between PMoA and TiO2 [25], which is further proven by the XPS results. In Fig. 2(b), a characteristic bond of TiO2 can be observed, which represents the stretching vibration of (Ti O). For the PMoA/TiO2 nanotube array, the characteristic vibration band of (Ti O) for TiO2 still exists and has a shift of a few wavenumbers (Fig. 2(c,d)), indicating that the basic structure of TiO2 is not destroyed and that there is a strong interfacial interaction between PMoA and TiO2 . After visible light irradiation for 60 s, the Mo Ob Mo and Mo Oc Mo vibrations in the PMoA/TiO2 nanotube array redshift by 10 and 8 cm−1 (Fig. 2(d)), proving that the heteropolyacids that accept electrons are transformed into heteropoly blue, and correspondingly, the PMoA/TiO2 nanotube array changes from being colorless to blue. Fig. 3 shows SEM images of the TiO2 nanotube array (a) and PMoA/TiO2 nanotube array (b). Fig. 2(a) clearly shows that the TiO2 nanotube arrays were highly ordered and uniform over the whole Ti substrate. The average inner diameter and tube length are approximately 77 nm and 5.21 m, respectively. Compared with the TiO2 nanotube array, the PMoA/TiO2 nanotubes maintain the nanotube structural integrity, and the inner diameter and length of the tubes are almost constant; however, the tube walls and orifices became rough, which can be attributed to the passivation of PMoA on the surface during the compounding process. The microstructures of the TiO2 nanotube array and PMoA/TiO2 nanotube array were observed by TEM and the resulting images are shown in Fig. 4. As shown in Fig. 4(a), the TiO2 nanotube array reveals a highly ordered tubular structure, and its tube diameter is approximately 100 nm, which is consistent with the SEM results. In Fig. 4(b), the PMoA/TiO2 nanotube array still presents a uniform tubular structure, but the surface of the wall and mouth of the tube are rough because of the etching effect of PMoA, which is consistent with the SEM results. Fig. 4(c) shows the HRTEM image of the PMoA/TiO2 nanotube array. The PMoA layer, interfacial layer and TiO2 , from outside-to-inside can clearly be observed in image Fig. 4(c), and the lattice spacing of TiO2 is 0.352 nm (101). To understand the changes in the chemical microenvironment of Ti, Mo and O before and after PMoA/TiO2 nanotubes compositing, XPS study of Ti 2p (a), Mo 3d (b), and O 1s (c) was conducted for pure TiO2 , pure PMoA and the PMoA/TiO2 nanotube array without visible light irradiation. The results are shown in Fig. 5. In Fig. 5(a), the binding energy (BE) values for TiO2 at 458.64 and 464.35 eV, which are assignable to the 2p3/2 and 2p1/2 of Ti4+ in TiO2 , can be observed. After compositing with PMoA, the BE values of the Ti 2p double peaks of Ti4+ are 458.38 and 464.08 eV, respectively, in PMoA/TiO2 , decreasing by 0.26 and 0.27 eV, respectively, compared to those in TiO2 , which leads to a weakening of the Ti-O bond so that TiO2 can more easily be excited. Through Gaussian deconvolution, the Mo 3d spectra of PMoA and the PMoA/TiO2 nanotube array before visible light irradiation can be well resolved into a 3d5/2 and 3d7/2 doublet caused by spin-
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Fig. 3. SEM images of TiO2 nanotube array (a) and PMoA/TiO2 nanotube array (b).
Fig. 4. TEM images of TiO2 nanotube array (a), PMoA/TiO2 nanotube array (b) and HRTEM images of PMoA/TiO2 nanotube array (c).
orbit coupling. For pure PMoA, only the BE values at 232.44 and 235.62 eV, which are assignable to the 3d5/2 and 3d7/2 levels of Mo6+ , are observed in Fig. 5(b). For the PMoA/TiO2 nanotube array without irradiation, the BE values of the Mo 3d doublet peak of Mo6+ were 232.65 and 235.82 eV, increasing by 0.21 and 0.20 eV, respectively, and those of Mo5+ were 231.78 and 234.99 eV, as seen in Fig. 5(b). The increase in the BE values of Mo6+ are due to the interactions between PMoA and TiO2. Combined with the FTIR results, the XPS data further demonstrates the existence of the Mo-O-Ti bridge bond in the composite. The appearance of the characteristic signal of Mo5+ prior to irradiation can be attributed to the excitation caused by the X-ray inspection, which indicates that a photoreduction reaction occurred under X-ray excitation and that Mo6+ was converted into Mo5+ to generate heteropoly blue. In addition, two chemical valence degenerate peak areas were calculated from the XPS spectra by integration, and the Mo5+ /Mo ratio was 0.38. This indicates that the PMoA/TiO2 nanotube array is exceptionally easy to rapidly excite by X-ray irradiation and is a rapid photochromic material that responds very quickly to ultraviolet light. In Fig. 5(c), the BE values of the O 1s peaks in TiO2 are 529.80 and 532.12 eV. The BE value at 529.80 eV (OL ) is assigned to the lattice oxygen of O-Ti4+ , while the BE value at 532.12 (Oc ) is assigned to chemisorbed oxygen in hydroxyl-like groups. For the PMoA/TiO2 nanotube array without irradiation, shown in Fig. 5(c), the BE values of the O 1s peaks are 529.68 and 531.50 eV. The BE value at 529.68 eV (OL ) is assigned to the lattice oxygen in PMoA/TiO2 , while the BE value at 531.50 eV (Oc ) is assigned to chemisorbed oxygen in hydroxyl-like groups. Compared with the BE values of the O 1s in TiO2 and PMoA/TiO2 , the BE values of the lattice oxygen in the composite are decreased by 0.12 eV compared to that of O-Ti4+ , which illustrates that the Ti O bond has weakened and that a new charge
transfer bridge (the Mo O Ti bond) has been formed during the composition process. This result is consistent with the FTIR results. The UV–Vis absorption spectra of the PMoA/TiO2 nanotube array after different visible light irradiation times were investigated in this study (Fig. 6). Before visible light irradiation, there was no significant absorption by the PMoA/TiO2 nanotubes in the UV–Vis region. After visible light irradiation, the nanotubes changed from being colorless to having a blue color, and a characteristic absorption band appeared at 715–728 nm, which can be attributed to an intervalence charge transfer (IVCT) transition from Mo6+ to Mo5+ and is the characteristic spectral band of heteropoly blue. The intensity increases, and the peak position of the IVCT undergoes a blueshift from 728 nm to 715 nm with increasing irradiation time, indicating that the degree of anion reduction increases with increasing irradiation time. The absolute absorbance of the PMoA/TiO2 nanotubes reached saturation with a maximum absorbance of 0.127 after 60 s of visible light irradiation, indicating that the excitation time required for the absorbance of PMoA/TiO2 nanotubes to reach saturation is short and the color generation rate is fast. The different bleaching processes of the PMoA/TiO2 nanotube array are shown in Fig. 7. After removing the visible light irradiation and storing the colored nanotubes in the dark in nitrogen or under vacuum, the colored nanotubes can retain their blue color for 7 d. However, when the colored nanotubes were stored under oxygen or ambient air in the dark, the colored nanotubes began to discolor, which indicates that oxygen plays a decisive role in the bleaching process and oxidizes Mo5+ to Mo6+ . Moreover, the absorbance of the colored nanotubes at 715 nm decreases to 70% and 30% of the saturation absorbance after storing for 2 h and 20 h, respectively, under these conditions. The results showed that the discoloration process
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Fig. 6. UV–Vis adsorption spectra of the PMoA/TiO2 nanotube array after different visible light irradiation times.
Fig. 7. UV–Vis spectra of the PMoA/TiO2 nanotube array without visible light irradiation (a), irradiation for 60 s (b), 7 d in nitrogen or under vacuum after irradiation for 60 s (c), 2 h in oxygen after irradiation for 60 s (d), 20 h in oxygen after irradiation for 60 s (e) and bleaching process (f).
Fig. 5. Gaussian deconvolution of Ti 2p (a), Mo 3d (b) and O 1s (c) level spectra of pure TiO2 , pure PMoA and the PMoA/TiO2 nanotube array without visible light irradiation.
of the colored nanotubes is slow, and the discoloration speed gradually decreases with increasing bleaching time. However, when a 15% H2 O2 solution is added dropwise onto the surface of the colored nanobutes, the blue nanotubes quickly fade to their original color, which indicates that the coloring and discoloring processes together form a reversible process. The reversibility of the coloration-discoloration cycles of the PMoA/TiO2 nanotube array at 715 nm is shown in Fig. 8. The coloration process of the PMoA/TiO2 nanotube array was conducted by irradiation with visible light, and the discoloration process was carried out by adding two drops of a 15% H2 O2 solution onto the surface of the colored nanotubes. The initial absorbance of each
Fig. 8. Coloration-discoloration cycles of the PMoA/TiO2 nanotube array at 715 nm.
cycle increases with increasing number of irradiation cycles, owing to the intensity of the excitation light. However, the difference in absorbance between coloration and bleaching is in the range of 0.124 to 0.127 during the 7 cycles. This indicates that the PMoA/TiO2
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Fig. 9. UV absorption edges of the TiO2 nanotube array and PMoA/TiO2 nanotube array.
Fig. 10. Gaussian deconvolution of Mo3d level spectra of PMoA/TiO2 nanotube array after visible light irradiation for 60 s.
nanotube array presents good photochromic properties, including a rapid photochromic response, high stability and good reversibility. To further understand the influence of PMoA modification on the optical response performance of the PMoA/TiO2 nanotube array, Fig. 9 shows the UV absorption edge of the TiO2 nanotube array and the PMoA/TiO2 nanotube array in the range of 350–900 nm before visible light irradiation. Compared to the TiO2 nanotube array, the absorption edge of the PMoA/TiO2 nanotube array redshifts from 377 to 420 nm, accompanied by a decrease in the forbidden band width from 3.08 eV to 2.86 eV, which indicates that after PMoA modification, the nanotubes can be excited by visible light. To illustrate the photochromic mechanism of PMoA/TiO2 composite nanotubes, XPS was used to investigate the variations in the electronic structure of the PMoA/TiO2 nanotubes before and after visible light irradiation (Fig. 10).
Through Gaussian deconvolution, the Mo 3d spectra of the PMoA/TiO2 nanotube array before and after visible light irradiation can be well resolved into 3d5/2 and 3d7/2 doublets caused by spin-orbit coupling, and the binding energies (BE) are listed in Table 1. Compared with the PMoA/TiO2 nanotube array before irradiation in Fig. 5(b), the BE values after 60 s visible light irradiation for the Mo 3d doublet peak of Mo6+ are 232.64 and 235.75 eV, and those of Mo5+ are 231.83 and 234.92 eV, as shown in Fig. 9(b). After visible light irradiation for 60 s, the BE values of the 3d5/2 peak of Mo6+ decrease and that of Mo5+ increases, while the BE values of 3d7/2 peak for both Mo6+ and Mo5+ decreased indicating that the chemical microenvironment of Mo in the PMoA/TiO2 nanotube array has changed. As shown in Table 1, the Mo5+ /Mo ratio increases from 0.38 to 0.42 after visible light irradiation, indicating that a photoreduction reaction occurs. The content of Mo5+ does not increase significantly after visible light irradiation, which shows that the coloration rate of the PMoA/TiO2 nanotube system is very fast and can quickly reach saturation. Based on the above analysis, the photochromic process of the PMoA/TiO2 nanotubes can be considered to be conducted according to an electron transfer mechanism. The possible photochromic mechanism for the composite nanotubes irradiated by visible light is given in Fig. 11. Due to the highly ordered nanotube structure, the TiO2 nanotube array has a larger specific surface area and higher adsorption capacity than other structures of TiO2 , which is favorable for the separation of photoelectrons and holes and improves the efficiency of charge collection. In addition, the highly ordered nanotube structure can improve the light absorption rate by reducing the loss from light reflection because the photons entering the nanotube are unlikely to escape due to the multiple scattering of radiation by the nanotube. After PMoA modification, the nanotube surface becomes rough such that the specific surface area becomes larger, which is more favorable for the separation of photoelectrons and holes. Moreover, a new Mo-O-Ti charge bridge was formed in the resulted PMoA/TiO2 nanotube between TiO2 and PMoA and weakened the Ti-O bond. This caused the forbidden band width of TiO2 decreased from 3.08 eV to 2.86 eV. The decline of the forbidden band width make the absorption edge of the modified nanotube exhibited redshift from 377 nm to 420 nm, which make it possible for the nanotubes to be excited by visible light. So in this paper, TiO2 acted as a visible light absorber and electron acceptor and PMoA as electron acceptor and chromogenic group. Under visible light excitation, TiO2 forms conduction band electrons and valence band holes. Photoproduced electronics are excited from the TiO2 valence band (O 2p orbit), migrate to the surface of the TiO2 nanotubes and transferred to the PMoA conduction band (Mo 3d orbit) attached to the surface of the TiO2 nanotubes through the Mo-O-Ti bond. Mo6+ is reduced to Mo5+ , forming heteropoly blue. Therefore, the photochromic process of the PMoA/TiO2 film is conducted in accordance with an electron transfer mechanism. Under the action of oxygen, heteropoly blue can be oxidized and restored to the colorless state. The photochromic mechanism of the PMoA/TiO2 composite film is shown in Fig. 11.
Table 1 Binding energies (eV) of the Mo 3d peak of the PMoA/TiO2 nanotube array before and after 60 s of visible light irradiation. Sample
PMoA/TiO2 (before) PMoA/TiO2 (after)
Mo6+
Mo5+
3d5/2
3d7/2
3d5/2
3d7/2
232.65 232.64
235.82 235.75
231.78 231.83
234.99 234.92
Mo5+ / Mo ratio 0.38 0.42
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Fig. 11. Schematic diagram of photochromic mechanism of PMoA/TiO2 nanotube array.
4. Conclusion A novel photochromic nanotube array was prepared by modifying a TiO2 nanotube array with PMoA. The prepared PMoA/TiO2 nanotube array preserved the Keggin structure of PMoA and the nanotube structure of the TiO2 nanotube array. Modification with PMoA had a significant impact on the photochromic properties of the PMoA/TiO2 nanotube array. The highly ordered nanotube structure enabled the PMoA/TiO2 nanotube array to more easily generate photoelectrons and holes and have a higher charge collection efficiency and light absorption rate. Modification with PMoA enabled the composite nanotubes to be easily and rapidly excited by visible light, which was related to the formation of an Mo-OTi bond between PMoA and TiO2 . The photochromic process of the PMoA/TiO2 nanotubes occurred in accordance with an electron transfer mechanism. Acknowledgments This work was supported by the National Natural Science Foundation of China (No.61774073), Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University (No. 2016-25) and Science and Technology Development Program of Jilin province (No. 20170101086JC). References [1] T. Yamase, Catal. Surv. Asia 7 (2003) 203–217. [2] J.J. Walsh, A.M. Bond, R.J. Forster, T.E. Keyes, Coord. Chem. Rev. 306 (2016) 217–234. [3] S.H. Lee, H.T. Bui, T.P. Vales, S. Cho, H.J. Kim, Dyes Pigm. 145 (2017) 216–221. [4] T. Yamase, Polyhedron 5 (1986) 79–86. [5] L. Wen, P. Il-Soo, K. Seong-Kyun, L. Myongsoo, Chem. Commun. 48 (2012) 8796–8798. [6] W. Sun, Y. Si, H. Jing, Z. Dong, C. Wang, Y. Zhang, L. Zhao, W. Feng, Y. Yan, Chem. Res. Chin. Univ. 34 (2018) 464–469. [7] H.F. Bao, X.Y. Wang, G.Q. Yang, H.Y. Li, F.J. Zhang, W. Feng, Colloid Polym. Sci. 292 (2014) 2883–2889. [8] X.F. Jing, D.L. Zou, Q.L. Meng, W. Zhang, F.J. Zhang, W. Feng, X.K. Han, Inorg. Chem. Commun. 46 (2014) 149–154. [9] H.X. Xiao, C.L. Teng, Q. Cai, S.Q. Sun, T.J. Cai, Q. Deng, Solid State Sci. 58 (2016) 122–128. [10] Y. Sun, Preparation and Visible-light Photochromism of Hybrid Films Based on Polyoxometalates, Master.Thesis, Jilin University, 2018 (in Chinese).
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Research Article
Phosphomolybdic acid-modified highly organized TiO2 nanotube arrays with rapid photochromic performance Yuanyuan Wei a , Bing Han b , Zhaojun Dong a , Wei Feng a,b,∗ a Key Lab of Groundwater Resources and Environment Ministry of Education, College of New Energy and Environment, Jilin University, Changchun 130021, China b Key Laboratory of Songliao Aquatic Environment Ministry of Educatio, Jilin Jianzhu University, Changchun 130118, China
a r t i c l e
i n f o
Article history: Received 21 March 2019 Received in revised form 23 April 2019 Accepted 26 April 2019 Available online 17 May 2019 Keywords: Phosphomolybdic acid TiO2 nanotube array Rapid Photochromic performance
a b s t r a c t TiO2 nanotube arrays were prepared by means of an electrochemical anodization technique in an organic electrolyte solution doped with polyvinyl pyrrolidone (PVP) and were subsequently modified with phosphomolybdic acid (PMoA) to obtain PMoA/TiO2 nanotube arrays. The microstructure and photochromic properties were investigated via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible spectroscopy (UV–vis), and X-ray photoelectron spectroscopy (XPS). The results indicated that the Keggin structure of PMoA and the nanotube structure of TiO2 were not destroyed, and there was a strong degree of interaction between PMoA and TiO2 at the biphasic interface with lattice interlacing during the compositing process. The XPS results further indicated that there was a change in the chemical microenvironment during the formation process of the composite, and a new charge transfer bridge was formed through the Mo-O-Ti bond. Under visible light irradiation, the colorless PMoA/TiO2 nanotube array quickly turned blue and exhibited a photochromic response together with reversible photochromism in the presence of H2 O2 . After visible light irradiation for 60 s, the appearance of Mo5+ species in the XPS spectra indicated a photoreduction process in accordance with a photoinduced electron transfer mechanism. © 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction Since their discovery, photochromic materials have received an extensive amount of attention due to their potential application prospects in high-tech fields, such as data storage equipment, biological imaging, sensor applications and biomedical research [1–5]. Polyoxometalates (POMs), a class of molecularly defined inorganic metal-oxide clusters [6–9], are one of the most studied photochromic materials due to their important characteristic of accepting one or more electrons to produce mixed-valence colored species (heteropoly blues or heteropoly browns) [1,4,6,9]. Among six basic structures of POMs, including Keggin structure, Dawson structure, Derson structure, Waugh structure, Silverton structure and Lindqvist structure, Keggin structure is the most representative one [10,11]. In all POMs with kegging structure, phosphomolyb-
∗ Corresponding author at: Key Lab of Groundwater Resources and Environment Ministry of Education, College of New Energy and Environment, Jilin University, Changchun 130021, China. E-mail address: [email protected] (W. Feng).
dic acid (PMoA) and phosphotungstic acid are two typical one. The lower redox potential than phosphotungstic acid [10] make PMoA is more prone to electron transfer. Henceforth, recently researches on organic-inorganic photochromic materials mainly focused on PMoA [6–13]. Currently, photochromic materials are mainly prepared using noncovalent bonds to embed PMoA into various organic or inorganic polymer networks, such as polyacrylamide (PAM) [7,13], polyvinylpyrrolidone (PVP) [14,15], polyvinyl alcohol (PVA) [16,17], polyether chains (PEs) [18], WO3 [19–21] and TiO2 [22–25], to construct composite films, thus greatly improving the physical and chemical properties of the hybrid films. Among these composite, inorganic-inorganic composite photochromic materials containing titanium oxide have attracted attention. TiO2 is widely used in the field of photochemistry due to its advantages of environmental friendliness, significant dielectric effect, nontoxicity and complete mineralization [22,26–28]. Specifically, compared to other structures, TiO2 nanotube arrays have a highly ordered and vertically oriented tubular structure with a large specific surface area and high adsorption capacity [29–33], which is favorable for the separation of photogenerated electron-hole pairs to improve the efficiency of charge collection [30,32]. In addition,
https://doi.org/10.1016/j.jmst.2019.05.014 1005-0302/© 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
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the highly ordered nanotube structure can also reduce the loss from light reflection to improve the light absorption rate because the photons entering the nanotube array are unlikely to escape due to the multiple scattering of radiation by the nanotubes [22]. Therefore, TiO2 nanotube arrays are highly suited for use as electron donors in photochromic reactions [33,34]. To ensure the excellent photoelectric properties of the nanotubes, it is necessary to control the physical and chemical properties of the TiO2 nanotube array during the preparation process [33–39]. The preparation methods for TiO2 nanotube arrays include templated, hydrothermal, and anodic oxidation reactions [38,40–42]. Among them, the anodic oxidation method has been widely used because of its simple process, easily controlled reaction rate, and preparation of nanotubes that are highly ordered and easy to industrialize [37,38]. Therefore, the TiO2 nanotube arrays in this experiment were prepared by an anodic oxidation method. According to previous results [34–38,43–46], the composition of the electrolyte, anodization parameters and annealing temperature during the preparation of TiO2 nanotube arrays by electrochemical anodization all affect the morphology of the array, thus affecting the photochromic properties. As a synthetic water-soluble polymer, PVP has the typical properties of water-soluble polymer compounds, such as colloidal protection, film-forming ability, adhesive, hygroscopic, solubility or condensability, but its most characteristic properties are its excellent solubility and biocompatibility [47,48]. Recently, Zhou et al. [47] studied the PL properties of SiC nanotubes prepared from electrospun polymer templates and found that the addition of a PVP solution prevented the growth of nanocrystals and stabilized the particles against aggregation, resulting in a high number of surface defects and greatly improved PL performance. However, there are few reports on the application of PVP in the fabrication of TiO2 nanotube arrays from anodization methods. Therefore, it would be significant to improve the morphology of TiO2 nanotube arrays by doping PVP in organic electrolyte solutions. In this work, we prepared a TiO2 nanotube array by an electrochemical anodization technique with an organic electrolyte solution doped with polyvinyl pyrrolidone (PVP) and modified the TiO2 nanotube array with PMoA to obtain a PMoA/TiO2 nanotube array. Then, through instrumental analysis, the microstructure and composition of the PMoA/TiO2 nanotube array were studied. The photochromic properties and mechanism were investigated in detail via ultraviolet-visible (UV–vis) spectra and X-ray photoelectron spectroscopy (XPS). The results indicated that the photoresponsive rate of the PMoA/TiO2 nanotube system was very fast, and a new charge transfer bridge was formed through the Mo-O-Ti bond during the photoreduction process. The formation of the Mo-O-Ti bond reduced the excitation energy of the Ti-O bond and enabled the composite nanotubes to change color under visible light excitation. The photoreduction process was in accordance with the photoinduced electron transfer mechanism.
2. Experimental 2.1. Materials Titanium foil (1.2 cm × 3.0 cm, 0.2 mm thickness, 99.7% purity) was purchased from Shenzhen Fuxin Titanium Technology Co., Ltd. Phosphomolybdic acid (PMoA) was purchased from Tianjin Kermel Chemical Reagent Development Center and recrystallized twice before use. Polyvinylpyrrolidone (PVP, Mw = 50,000) was obtained from Aldrich and purified by fractional distillation before use. All other chemical reagents were of analytical grade, and deionized water was used in all experiments.
2.2. Preparation The anodization method was used to fabricate the TiO2 nanotube arrays. Before anodization, the Ti foils (1.2 cm × 3.0 cm, 0.2 mm thickness, 99.7% purity) were mechanically polished with different abrasive papers (400#, 800# and 1200#) and then cleaned by sonication with acetone, ethanol and purified water for 30 min. The titanium foils were polished with a mixed acid solution (HF:HNO3 :H2 O = 1:4:5 volumetric ratio) for 30 s to produce a smooth surface. Finally, the titanium was washed with purified water and then dried in an oven at 60 ◦ C. All the other chemicals were of analytical grade and used as received without further purification. The anodization was carried out using a conventional two-electrode system with the Ti sheet as the anode and an untreated Ti sheet (2.5 cm × 3.5 cm) as the cathode. Both electrodes were immersed into 100 mL of the organic electrolyte. The organic electrolyte was composed of 0.5 wt% NH4 F, 0.1 wt% PVP and an ethylene glycol system with a mixed volume ratio of 2% water. The samples were anodized at 30 V for 4 h. After anodization, the samples were annealed in a laboratory tube furnace at 450 ◦ C for 2 h with a heating rate of 1 ◦ C/min in ambient air and then naturally cooled to induce crystallization. The as-prepared samples were immersed in a 10 mg/ml solution of phosphomolybdic acid in ethanol for 2 h and then ultrasonically washed in ultrapure water, ethanol, and acetone for 30 min. 2.3. Instrumental analysis Diffraction patterns were obtained with an X-ray diffractometer (XRD) with a monochromatic Cu K␣ radiation source ( = 0.15418 nm) operating at 40 kV and 100 mA with a scan rate of 0.05◦ over the 2 range of 20◦ to 80◦ . FTIR spectra were determined from samples deposited on KBr pellets at room temperature with a Nicolet Impact 410 FTIR spectrometer in the wavenumber range of 500–4000 cm–1 . The surface morphologies of the prepared samples were observed by scanning electron microscopy (SEM) on a JEOLJSM-6700 F. Transmission electron microscopy (TEM) observations were performed on a JEOL JEM-2100 operating at 200 kV to characterize the microstructures of the PMoA/TiO2 nanotube arrays. Absorbance curves were measured on a UV–Vis spectrophotometer (JASCO V-550) with an optical resolution of 1 nm in the range of 350–900 nm. X-ray photoelectron spectroscopy (XPS) valence band spectra were obtained with an ESCA LAB-MKII photoelectron spectrometer. Photochromic experiments were carried out using a 300 W xenon lamp filtered by UV filter as the visible light source under the condition of complete light avoidance. The distance between the lamp and composite nanotube arrays was 150 mm. The composite nanotube arrays were exposed to air during the visible light irradiation process. After irradiating for a certain time, the in situ absorbance curve was obtained. The irradiation time was increased until the obtained curve was consistent with the previous curve. While exposed to air, the composite nanotube arrays were protected from light, and the absorption spectra were regularly measured to monitor the bleaching process. All measurements were carried out at room temperature. 3. Results and discussion The XRD patterns of the TiO2 nanotube array (a), PMoA (b), and PMoA/TiO2 nanotube array (c) are shown in Fig. 1. The results indicate that the TiO2 nanotube arrays are well crystallized after annealing at 450 ◦ for 2 h and the pure PMoA also has obvious crystal characteristic peak. However, in the XRD pattern of the PMoA/TiO2 nanotube array, it is hardly to observe the characteristic
Y. Wei et al. / Journal of Materials Science & Technology 35 (2019) 1951–1958
Fig. 1. XRD patterns of PMoA (a), TiO2 nanotube array (b) and PMoA/TiO2 nanotube array without visible light irradiation.
Fig. 2. FTIR spectra of PMoA (a), TiO2 (b) and the PMoA/TiO2 nanotube array before (c) and after (d) visible light irradiation. The inserted picture of the Fig. 2 is the FTIR spectra of PMoA/TiO2 nanotube array before (c) and after (d) visible light irradiation in the range of 650–850 cm−1 .
peaks of PMoA because of the coverage of the characteristic peaks of TiO2 . The characteristic peaks of the TiO2 nanotube array are well matched with the reflection planes of the anatase phase, where the diffraction peaks at 2 values of 25.3, 37.8, 48.0, 53.9, 55.06, 62.7 and 70.5 can be ascribed to the reflections of the (101), (004), (200), (105), (211), (204) and (220) planes of the anatase phase, respectively. No rutile phase can be detected. Moreover, compared with the XRD spectra of the TiO2 nanotube array, there is no discernible change in the phase of the TiO2 nanotube array when modified with PMoA (Fig. 1(c)), indicating that PMoA only occurred on the surface of the TiO2 nanotube array and that the crystalline structure of the TiO2 nanotube array has not been destroyed. To confirm the structural characteristics of the composite nanotubes, FTIR spectra of pure PMOA (a), TiO2 nanotubes (b), and the PMOA /TiO2 nanotube array before (c) and after (d) visible light irradiation in the range of 500–1500 cm−1 were obtained, and the results are shown in Fig. 2. In the FTIR spectra of the pure PMoA crystals, there are four characteristic bands at 1064, 968, 876 and 789 cm–1 , corresponding to the stretching vibrations of (P-Oa), (Mo-Od), (Mo-Ob-Mo) and (Mo-Oc-Mo), respectively
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[14], which represent the Keggin structure [25]. The spectrum corresponding to PMoA is shown in Fig. 2(a). For the PMoA/TiO2 nanotube array, the above characteristic vibration bands are similar to those of pure PMoA except for having shifted by a few wavenumbers (Fig. 2(c,d)). This indicates that the Keggin structure of PMoA is still preserved and that a strong interfacial interaction between PMoA and TiO2 occurs. Comparing pure PMoA with PMoA/TiO2 , as indicated by curve a and curve c in Fig. 2, the Mo-Od asymmetrical stretching frequency in PMoA/TiO2 has a redshift of 4 cm−1 , which is mainly caused by TiO2 increasing the distance between adjacent heteropoly anions and weakening the electrostatic interactions between heteropoly anions. Compared with pure PMoA, the (Mo Ob Mo) and (Mo Oc Mo) vibrations in the PMoA/TiO2 nanotube array have blueshifts of 14 and 18 cm−1 , respectively, and the results show that a Mo-O-Ti bond was formed between PMoA and TiO2 [25], which is further proven by the XPS results. In Fig. 2(b), a characteristic bond of TiO2 can be observed, which represents the stretching vibration of (Ti O). For the PMoA/TiO2 nanotube array, the characteristic vibration band of (Ti O) for TiO2 still exists and has a shift of a few wavenumbers (Fig. 2(c,d)), indicating that the basic structure of TiO2 is not destroyed and that there is a strong interfacial interaction between PMoA and TiO2 . After visible light irradiation for 60 s, the Mo Ob Mo and Mo Oc Mo vibrations in the PMoA/TiO2 nanotube array redshift by 10 and 8 cm−1 (Fig. 2(d)), proving that the heteropolyacids that accept electrons are transformed into heteropoly blue, and correspondingly, the PMoA/TiO2 nanotube array changes from being colorless to blue. Fig. 3 shows SEM images of the TiO2 nanotube array (a) and PMoA/TiO2 nanotube array (b). Fig. 2(a) clearly shows that the TiO2 nanotube arrays were highly ordered and uniform over the whole Ti substrate. The average inner diameter and tube length are approximately 77 nm and 5.21 m, respectively. Compared with the TiO2 nanotube array, the PMoA/TiO2 nanotubes maintain the nanotube structural integrity, and the inner diameter and length of the tubes are almost constant; however, the tube walls and orifices became rough, which can be attributed to the passivation of PMoA on the surface during the compounding process. The microstructures of the TiO2 nanotube array and PMoA/TiO2 nanotube array were observed by TEM and the resulting images are shown in Fig. 4. As shown in Fig. 4(a), the TiO2 nanotube array reveals a highly ordered tubular structure, and its tube diameter is approximately 100 nm, which is consistent with the SEM results. In Fig. 4(b), the PMoA/TiO2 nanotube array still presents a uniform tubular structure, but the surface of the wall and mouth of the tube are rough because of the etching effect of PMoA, which is consistent with the SEM results. Fig. 4(c) shows the HRTEM image of the PMoA/TiO2 nanotube array. The PMoA layer, interfacial layer and TiO2 , from outside-to-inside can clearly be observed in image Fig. 4(c), and the lattice spacing of TiO2 is 0.352 nm (101). To understand the changes in the chemical microenvironment of Ti, Mo and O before and after PMoA/TiO2 nanotubes compositing, XPS study of Ti 2p (a), Mo 3d (b), and O 1s (c) was conducted for pure TiO2 , pure PMoA and the PMoA/TiO2 nanotube array without visible light irradiation. The results are shown in Fig. 5. In Fig. 5(a), the binding energy (BE) values for TiO2 at 458.64 and 464.35 eV, which are assignable to the 2p3/2 and 2p1/2 of Ti4+ in TiO2 , can be observed. After compositing with PMoA, the BE values of the Ti 2p double peaks of Ti4+ are 458.38 and 464.08 eV, respectively, in PMoA/TiO2 , decreasing by 0.26 and 0.27 eV, respectively, compared to those in TiO2 , which leads to a weakening of the Ti-O bond so that TiO2 can more easily be excited. Through Gaussian deconvolution, the Mo 3d spectra of PMoA and the PMoA/TiO2 nanotube array before visible light irradiation can be well resolved into a 3d5/2 and 3d7/2 doublet caused by spin-
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Fig. 3. SEM images of TiO2 nanotube array (a) and PMoA/TiO2 nanotube array (b).
Fig. 4. TEM images of TiO2 nanotube array (a), PMoA/TiO2 nanotube array (b) and HRTEM images of PMoA/TiO2 nanotube array (c).
orbit coupling. For pure PMoA, only the BE values at 232.44 and 235.62 eV, which are assignable to the 3d5/2 and 3d7/2 levels of Mo6+ , are observed in Fig. 5(b). For the PMoA/TiO2 nanotube array without irradiation, the BE values of the Mo 3d doublet peak of Mo6+ were 232.65 and 235.82 eV, increasing by 0.21 and 0.20 eV, respectively, and those of Mo5+ were 231.78 and 234.99 eV, as seen in Fig. 5(b). The increase in the BE values of Mo6+ are due to the interactions between PMoA and TiO2. Combined with the FTIR results, the XPS data further demonstrates the existence of the Mo-O-Ti bridge bond in the composite. The appearance of the characteristic signal of Mo5+ prior to irradiation can be attributed to the excitation caused by the X-ray inspection, which indicates that a photoreduction reaction occurred under X-ray excitation and that Mo6+ was converted into Mo5+ to generate heteropoly blue. In addition, two chemical valence degenerate peak areas were calculated from the XPS spectra by integration, and the Mo5+ /Mo ratio was 0.38. This indicates that the PMoA/TiO2 nanotube array is exceptionally easy to rapidly excite by X-ray irradiation and is a rapid photochromic material that responds very quickly to ultraviolet light. In Fig. 5(c), the BE values of the O 1s peaks in TiO2 are 529.80 and 532.12 eV. The BE value at 529.80 eV (OL ) is assigned to the lattice oxygen of O-Ti4+ , while the BE value at 532.12 (Oc ) is assigned to chemisorbed oxygen in hydroxyl-like groups. For the PMoA/TiO2 nanotube array without irradiation, shown in Fig. 5(c), the BE values of the O 1s peaks are 529.68 and 531.50 eV. The BE value at 529.68 eV (OL ) is assigned to the lattice oxygen in PMoA/TiO2 , while the BE value at 531.50 eV (Oc ) is assigned to chemisorbed oxygen in hydroxyl-like groups. Compared with the BE values of the O 1s in TiO2 and PMoA/TiO2 , the BE values of the lattice oxygen in the composite are decreased by 0.12 eV compared to that of O-Ti4+ , which illustrates that the Ti O bond has weakened and that a new charge
transfer bridge (the Mo O Ti bond) has been formed during the composition process. This result is consistent with the FTIR results. The UV–Vis absorption spectra of the PMoA/TiO2 nanotube array after different visible light irradiation times were investigated in this study (Fig. 6). Before visible light irradiation, there was no significant absorption by the PMoA/TiO2 nanotubes in the UV–Vis region. After visible light irradiation, the nanotubes changed from being colorless to having a blue color, and a characteristic absorption band appeared at 715–728 nm, which can be attributed to an intervalence charge transfer (IVCT) transition from Mo6+ to Mo5+ and is the characteristic spectral band of heteropoly blue. The intensity increases, and the peak position of the IVCT undergoes a blueshift from 728 nm to 715 nm with increasing irradiation time, indicating that the degree of anion reduction increases with increasing irradiation time. The absolute absorbance of the PMoA/TiO2 nanotubes reached saturation with a maximum absorbance of 0.127 after 60 s of visible light irradiation, indicating that the excitation time required for the absorbance of PMoA/TiO2 nanotubes to reach saturation is short and the color generation rate is fast. The different bleaching processes of the PMoA/TiO2 nanotube array are shown in Fig. 7. After removing the visible light irradiation and storing the colored nanotubes in the dark in nitrogen or under vacuum, the colored nanotubes can retain their blue color for 7 d. However, when the colored nanotubes were stored under oxygen or ambient air in the dark, the colored nanotubes began to discolor, which indicates that oxygen plays a decisive role in the bleaching process and oxidizes Mo5+ to Mo6+ . Moreover, the absorbance of the colored nanotubes at 715 nm decreases to 70% and 30% of the saturation absorbance after storing for 2 h and 20 h, respectively, under these conditions. The results showed that the discoloration process
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Fig. 6. UV–Vis adsorption spectra of the PMoA/TiO2 nanotube array after different visible light irradiation times.
Fig. 7. UV–Vis spectra of the PMoA/TiO2 nanotube array without visible light irradiation (a), irradiation for 60 s (b), 7 d in nitrogen or under vacuum after irradiation for 60 s (c), 2 h in oxygen after irradiation for 60 s (d), 20 h in oxygen after irradiation for 60 s (e) and bleaching process (f).
Fig. 5. Gaussian deconvolution of Ti 2p (a), Mo 3d (b) and O 1s (c) level spectra of pure TiO2 , pure PMoA and the PMoA/TiO2 nanotube array without visible light irradiation.
of the colored nanotubes is slow, and the discoloration speed gradually decreases with increasing bleaching time. However, when a 15% H2 O2 solution is added dropwise onto the surface of the colored nanobutes, the blue nanotubes quickly fade to their original color, which indicates that the coloring and discoloring processes together form a reversible process. The reversibility of the coloration-discoloration cycles of the PMoA/TiO2 nanotube array at 715 nm is shown in Fig. 8. The coloration process of the PMoA/TiO2 nanotube array was conducted by irradiation with visible light, and the discoloration process was carried out by adding two drops of a 15% H2 O2 solution onto the surface of the colored nanotubes. The initial absorbance of each
Fig. 8. Coloration-discoloration cycles of the PMoA/TiO2 nanotube array at 715 nm.
cycle increases with increasing number of irradiation cycles, owing to the intensity of the excitation light. However, the difference in absorbance between coloration and bleaching is in the range of 0.124 to 0.127 during the 7 cycles. This indicates that the PMoA/TiO2
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Fig. 9. UV absorption edges of the TiO2 nanotube array and PMoA/TiO2 nanotube array.
Fig. 10. Gaussian deconvolution of Mo3d level spectra of PMoA/TiO2 nanotube array after visible light irradiation for 60 s.
nanotube array presents good photochromic properties, including a rapid photochromic response, high stability and good reversibility. To further understand the influence of PMoA modification on the optical response performance of the PMoA/TiO2 nanotube array, Fig. 9 shows the UV absorption edge of the TiO2 nanotube array and the PMoA/TiO2 nanotube array in the range of 350–900 nm before visible light irradiation. Compared to the TiO2 nanotube array, the absorption edge of the PMoA/TiO2 nanotube array redshifts from 377 to 420 nm, accompanied by a decrease in the forbidden band width from 3.08 eV to 2.86 eV, which indicates that after PMoA modification, the nanotubes can be excited by visible light. To illustrate the photochromic mechanism of PMoA/TiO2 composite nanotubes, XPS was used to investigate the variations in the electronic structure of the PMoA/TiO2 nanotubes before and after visible light irradiation (Fig. 10).
Through Gaussian deconvolution, the Mo 3d spectra of the PMoA/TiO2 nanotube array before and after visible light irradiation can be well resolved into 3d5/2 and 3d7/2 doublets caused by spin-orbit coupling, and the binding energies (BE) are listed in Table 1. Compared with the PMoA/TiO2 nanotube array before irradiation in Fig. 5(b), the BE values after 60 s visible light irradiation for the Mo 3d doublet peak of Mo6+ are 232.64 and 235.75 eV, and those of Mo5+ are 231.83 and 234.92 eV, as shown in Fig. 9(b). After visible light irradiation for 60 s, the BE values of the 3d5/2 peak of Mo6+ decrease and that of Mo5+ increases, while the BE values of 3d7/2 peak for both Mo6+ and Mo5+ decreased indicating that the chemical microenvironment of Mo in the PMoA/TiO2 nanotube array has changed. As shown in Table 1, the Mo5+ /Mo ratio increases from 0.38 to 0.42 after visible light irradiation, indicating that a photoreduction reaction occurs. The content of Mo5+ does not increase significantly after visible light irradiation, which shows that the coloration rate of the PMoA/TiO2 nanotube system is very fast and can quickly reach saturation. Based on the above analysis, the photochromic process of the PMoA/TiO2 nanotubes can be considered to be conducted according to an electron transfer mechanism. The possible photochromic mechanism for the composite nanotubes irradiated by visible light is given in Fig. 11. Due to the highly ordered nanotube structure, the TiO2 nanotube array has a larger specific surface area and higher adsorption capacity than other structures of TiO2 , which is favorable for the separation of photoelectrons and holes and improves the efficiency of charge collection. In addition, the highly ordered nanotube structure can improve the light absorption rate by reducing the loss from light reflection because the photons entering the nanotube are unlikely to escape due to the multiple scattering of radiation by the nanotube. After PMoA modification, the nanotube surface becomes rough such that the specific surface area becomes larger, which is more favorable for the separation of photoelectrons and holes. Moreover, a new Mo-O-Ti charge bridge was formed in the resulted PMoA/TiO2 nanotube between TiO2 and PMoA and weakened the Ti-O bond. This caused the forbidden band width of TiO2 decreased from 3.08 eV to 2.86 eV. The decline of the forbidden band width make the absorption edge of the modified nanotube exhibited redshift from 377 nm to 420 nm, which make it possible for the nanotubes to be excited by visible light. So in this paper, TiO2 acted as a visible light absorber and electron acceptor and PMoA as electron acceptor and chromogenic group. Under visible light excitation, TiO2 forms conduction band electrons and valence band holes. Photoproduced electronics are excited from the TiO2 valence band (O 2p orbit), migrate to the surface of the TiO2 nanotubes and transferred to the PMoA conduction band (Mo 3d orbit) attached to the surface of the TiO2 nanotubes through the Mo-O-Ti bond. Mo6+ is reduced to Mo5+ , forming heteropoly blue. Therefore, the photochromic process of the PMoA/TiO2 film is conducted in accordance with an electron transfer mechanism. Under the action of oxygen, heteropoly blue can be oxidized and restored to the colorless state. The photochromic mechanism of the PMoA/TiO2 composite film is shown in Fig. 11.
Table 1 Binding energies (eV) of the Mo 3d peak of the PMoA/TiO2 nanotube array before and after 60 s of visible light irradiation. Sample
PMoA/TiO2 (before) PMoA/TiO2 (after)
Mo6+
Mo5+
3d5/2
3d7/2
3d5/2
3d7/2
232.65 232.64
235.82 235.75
231.78 231.83
234.99 234.92
Mo5+ / Mo ratio 0.38 0.42
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Fig. 11. Schematic diagram of photochromic mechanism of PMoA/TiO2 nanotube array.
4. Conclusion A novel photochromic nanotube array was prepared by modifying a TiO2 nanotube array with PMoA. The prepared PMoA/TiO2 nanotube array preserved the Keggin structure of PMoA and the nanotube structure of the TiO2 nanotube array. Modification with PMoA had a significant impact on the photochromic properties of the PMoA/TiO2 nanotube array. The highly ordered nanotube structure enabled the PMoA/TiO2 nanotube array to more easily generate photoelectrons and holes and have a higher charge collection efficiency and light absorption rate. Modification with PMoA enabled the composite nanotubes to be easily and rapidly excited by visible light, which was related to the formation of an Mo-OTi bond between PMoA and TiO2 . The photochromic process of the PMoA/TiO2 nanotubes occurred in accordance with an electron transfer mechanism. Acknowledgments This work was supported by the National Natural Science Foundation of China (No.61774073), Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University (No. 2016-25) and Science and Technology Development Program of Jilin province (No. 20170101086JC). References [1] T. Yamase, Catal. Surv. Asia 7 (2003) 203–217. [2] J.J. Walsh, A.M. Bond, R.J. Forster, T.E. Keyes, Coord. Chem. Rev. 306 (2016) 217–234. [3] S.H. Lee, H.T. Bui, T.P. Vales, S. Cho, H.J. Kim, Dyes Pigm. 145 (2017) 216–221. [4] T. Yamase, Polyhedron 5 (1986) 79–86. [5] L. Wen, P. Il-Soo, K. Seong-Kyun, L. Myongsoo, Chem. Commun. 48 (2012) 8796–8798. [6] W. Sun, Y. Si, H. Jing, Z. Dong, C. Wang, Y. Zhang, L. Zhao, W. Feng, Y. Yan, Chem. Res. Chin. Univ. 34 (2018) 464–469. [7] H.F. Bao, X.Y. Wang, G.Q. Yang, H.Y. Li, F.J. Zhang, W. Feng, Colloid Polym. Sci. 292 (2014) 2883–2889. [8] X.F. Jing, D.L. Zou, Q.L. Meng, W. Zhang, F.J. Zhang, W. Feng, X.K. Han, Inorg. Chem. Commun. 46 (2014) 149–154. [9] H.X. Xiao, C.L. Teng, Q. Cai, S.Q. Sun, T.J. Cai, Q. Deng, Solid State Sci. 58 (2016) 122–128. [10] Y. Sun, Preparation and Visible-light Photochromism of Hybrid Films Based on Polyoxometalates, Master.Thesis, Jilin University, 2018 (in Chinese).
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