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Construction of nanocomposite film of Dawson-type polyoxometalate and TiO2 nanowires for electrochromic applications
Accepted Manuscript Title: Construction of nanocomposite film of Dawson-type polyoxometalate and TiO2 nanowires for electrochromic applications Authors: Shuping Liu, Xiaoshu Qu PII: DOI: Reference:
S0169-4332(17)30941-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.244 APSUSC 35620
To appear in:
APSUSC
Received date: Revised date: Accepted date:
30-12-2016 6-3-2017 26-3-2017
Please cite this article as: Shuping Liu, Xiaoshu Qu, Construction of nanocomposite film of Dawson-type polyoxometalate and TiO2 nanowires for electrochromic applications, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.03.244 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Construction of nanocomposite film of Dawson-type polyoxometalate and TiO2 nanowires for electrochromic applications Shuping Liua*, Xiaoshu Qub
(aCollege of Tourism and Cuisine, Chemistry Center, Harbin University of Commerce, Harbin 150076, China; bJiLin Institute of Chemical Technology, JiLin City, 132073, P. R. China.)
Shuping Liu: [email protected]
*Correspongding author: No.138, Tongda Road, Harbin, China Tel: +86-451-84839474 Fax: +86-451-84866022 E-mail address: [email protected]
1
GRAPHICAL ABSTRACT
Highlights
A film containing polyoxometalate and TiO2 nanowires was successfully fabricated.
The composite film exhibits improved electrochemical and electrochromic properties.
The maximum optical contrast of 45.1% and coloration efficiency of 69.0 cm2/C.
Abstract: This paper shows a nanocomposite film of Dawson-type polyoxometalate (POM) K6P2W18O62 (P2W18) and TiO2 nanowires by combining hydrothermal and Layer-by-Layer (LbL) assembly methods. The electrochemical and electrochromic (EC) performances were examined and compared with P2W18 and TiO2 composite film as well as individual P2W18 and TiO2 structures. The nanocomposite film exhibits improved EC properties with high transmittance modulation (45.1% at 650 nm), fast switching times (1.9 s and 6.7 s) and outstanding coloration efficiency (69.0 cm2/C at 650 nm). The enhancement can be attributed to the contribution from P2W18 and TiO2, which displays large real active area and increase the capabilities for EC material. 2
Keywords: Electrochromic; Polyoxometalates; TiO2 nanowire; Nanocomposite
1. Introduction Electrochromic (EC) materials can be changed optical properties (transmittance, absorbance and reflectance) continuously and reversibly during electrochemical oxidation and reduction [1-5]. Recently, EC materials can be applied for rear-view mirrors, displays and antiglare rear-view mirrors in car [6-10]. The EC materials have been widely studied such as conducting polymers, Prussian blue and inorganic metal oxides (e.g. WO3, TiO2, NiO) [11-15]. They can be generally divided into two groups, cathodic (coloring under negative potential) and anodic (coloring under positive potential) electrochromic materials [16]. Two cathodic films (or anodic films) can be incorporated into a composite film, in which both the films can be colored or bleached under a certain potential. Titanium dioxide (TiO2) is a typical cathodic EC material and has shown particular potential as an electro-active material due to its high activity, superior chemical stability and strong oxidation capability [17-19]. Recently, it is widely recognized that one-dimensional (1D) TiO2 nanostructures have attracted great interest owing to superior charge transport properties and facile processing [20,21]. Lee et al. prepared TiO2 rutile nanowires along [001] direction grown on FTO by a simple one step solvothermal process, which exhibited high coloration efficiency compared with P25-coated and rutile nanowires-coated electrodes [22]. Chen et al. constructed a highly transparent electrochromic device based on anatase TiO2 nanowires, which exhibited high color density, good coloration efficiency and fast color-switching time [23]. However, long nanowires or nanotubes cannot be fabricated easily, and therefore the incorporation of another cathodic EC material could improve the EC properties of TiO2 film. Reyes-Gil et al. [24] fabricated WO3/TiO2 film using TiO2 nanotubes as template for WO3 electrodeposition, which displayed higher ion storage capacity, better stability, enhanced EC contrast compared with the pure WO3 and TiO2. Jia et al. [25] displayed the TiO2/V2O5 core/shell structure hybrid film by electrodepositing the V2O5 film on a TiO2 nanorod substrate, which has better electrochemical and EC properties compared with the single V2O5 film. Polyoxometalates (POMs), a well-known class of transition metal oxide nanoclusters with 3
intriguing structures and diverse properties, have been used candidate material for fabricating EC devices [26-30]. However, there have been very few studies on the EC performance of TiO2 nanowire composite film via the addition of POMs. In this paper, we attempted to design a composite material containing TiO2 nanowires (NWs) and Dawson type K6P2W18O62 (P2W18) using a two-step synthesis method. The TiO2/POM electrode displayed a higher real active surface area than that of the single film. In addition, the composite nanostructure exhibits enhanced electrochromic performance including large transmittance modulation, high coloration efficiency and good fast switching speed compared to the pure TiO2 and POM film. Therefore, the combination of both cathodic EC materials is expected to improve the EC performance of materials.
2. Experimental Section 2.1. Chemical Materials All reagents were of analytical grade and used without further treatment. Tetrabutyl titanate (98%) was purchased from Beijing Chemical Works. 3-Aminopropyltrimethoxysilane (APS) and Polyetherimide (PEI) were obtained from Aladdin. P2W18 was prepared according to the literature method [31-32]. Fluorine doped tin oxide (FTO) were purchased from Yingkou OPV Tech New Energy Co., Ltd (7-8 -1). 2.2. Preparation of TiO2 nanowires The synthetic routes of TiO 2 nanowires (TiO2 NW) have been described in detail in our previous report [33]. Firstly, the FTO substrates were cleaned with a detergent solution, washed with deionized water and ethanol and finally dried in a nitrogen stream. The precursor solution was obtained by mixing 0.45 mL tetrabutyl titanate, 18 mL HCl in 18 mL deionized water. The precursor solution was transferred into a Teflon-lined stainless steel autoclave, containing a vertically oriented FTO substrate. Then the autoclave was heated to 150 ℃for 4 h and cooled to room temperature under flowing water. The samples were rinsed with deionized water, and then annealed in air at 450 ℃for 30 min.
2.3. Preparation of composite films The composite film was synthesized by growing single-crystalline nanowires on the FTO substrates using a hydrothermal method, followed by depositing negatively charged P2W18 and 4
positively PEI onto the TiO2 nanowires by Layer-by-Layer (LbL) method. The composite films were assembled on the substrates by first immersing them into APS solution for 8 h. After the treatment, the substrates were dipped into HCl (pH = 2.0) for 20 min to form a positively charged surface. Then they were alternately dipped into the P2W18 (5 × 10-3 mol/L in 0.2mol/L HOAc-NaAc pH = 4) and PEI (5 × 10 -3 mol/L pH = 4) for 8 min. This cycle was repeated until the desired number of [P2W18/PEI] cycles was obtained. After each layer was deposited, the films were washed with deionized water and dried in nitrogen. All adsorption processes were performed at room temperature. The TiO 2-[P2W18/PEI]40 film thus obtained was designated as NW-P2W18 film. For comparison, the [P2W18/PEI]40 film (PEI-P2W18) was prepared on FTO substrates by the same method.
2.4. Characterization UV-vis absorption spectra were performed by using a TU-1901 PERSEE UV-vis spectrophotometer. The surface morphologies were investigated using scanning electron microscopy (SEM, Hitachi SU8010), high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100) and atomic force microscopy (AFM, Icon Bruker Company). A three-electrode cell was used for electrochemical measurement with 0.2 mol/L HOAc-NaAc buffer solution (pH = 3.4) as the electrolyte. All the electrochemical experiments were performed on the CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Factory, China). A conventional three electrodes system was used, with Ag/AgCl (3 mol/L KCl) as the reference electrode, platinum coil as the counter electrode and FTO electrode coated by the films as the working electrode. Spectroeletrochemical measurements of the nanowires were performed by combining the in situ UV-vis spectrophotometer with the electrochemical workstation.
3. Results and discussion 3.1 Fabrication and structure analyses Fig. 1a illustrates the schematics of the fabrication of NW-P2W18 film. As shown in Fig. 1b, the fabrication process of NW-P2W18 (number of cycle: 2-20) was monitored by UV-vis spectroscopy. It exhibits strong absorption of P2W18 with the bands at 204 and 286 nm. The former is due to the terminal oxygen to tungsten charge transfer transitions, and the latter is owing to charge transfer transitions from bridge-oxygen to tungsten [34]. The 5
inset of Fig. 1b shows the plots of the absorbance values at 204 and 286 nm as a function of the layer number. The linear growth of absorbance indicates that almost the same amount of P2W18 was deposited in each cycle, confirming the film growth for each deposition cycle is uniform. In addition, the stepwise growth of the films can be evaluated by cycle voltammetry (CV). As shown in Fig. 1c, the redox chemistry of the NW-P2W18 (number of cycle: 0, 2, 6, 10, 14, 18) film was examined which swept in the potential range of 0.2 to -0.9 V at the scan rate of 100 mV/s. The films display four couples of redox waves, which is similar to the CV curve of the P2W18 solution. This indicates that P2W18 is effectively incorporated into the composite film. With the increase in the number of cycles, the cathodic peak potentials shift slightly toward negative potential values and the anodic peak potentials shift toward positive potential values, which probably relates to the uncompensated resistance [35]. As shown in the inset of Fig. 1c, the current density of peak III increases linearly with the number of layers, indicating that growth of each deposition cycles is reproducible. Figure 1
3.2 Morphology analysis Fig. 2a and SI-1a displays the tope-view and cross-sectional SEM image of the TiO2 nanowires grown on FTO substrates. The images clearly indicate the surface of the FTO substrate is covered with randomly oriented TiO2 nanowires. The HR-TEM image of TiO2 nanowires shows lattice fringes with interplanar spacings d 110 = 0.325 and d001 = 0.293 nm, which are consistent with the tetragonal rutile phase (Fig. SI-1b in the ESI†). As shown in Fig. 2b, the PEI-P2W18 film surface is relatively smooth over a large area with small islands, which are evidences for the aggregation of P 2W18 anions. As shown in Fig. 2c-d, the nanowires in the NW-P2W18 film grow wider and more compact. It is possibly that TiO2 nanowires are enclosed by depositing P2W18 anions and PEI. In addition, the thickness of the NW-P2W18 film is approximately 600 nm. Fig. 3 shows energy dispersive X-ray spectrometry (EDS) mapping for Ti, P and W of the NW-P2W18 film. The uniform signal coverage of the three elements is observed, suggesting the presence of TiO2 and P2W18. 6
Figure 2 Figure 3
AFM can provide the detailed information concerning the surface morphology and homogeneity of the films. Fig. 4 displays the 2D and 3D AFM images of TiO2 NW, PEI-P2W18 and NW-P2W18 films. As shown in Fig. 4a, the PEI-P2W18 film displays relatively smooth surface with multitude of grains due to the aggregation of P 2W18 anions. Fig. 4c shows a regular globular microstructure, suggesting the presence of TiO 2 nanowires. From Fig. 4e, the surface of the NW-P2W18 film is similar to that of the TiO2 NW. The root mean square (RMS) roughness is calculated from an area of 5 × 5 μm2 in AFM image. The RMS roughnesses are 4.1, 79.3 and 50.6 nm for PEI-P2W18, TiO2 NW and NW-P2W18 films, respectively. Obviously, the RMS roughness decreased after P2W18
anions were deposited on the surface of TiO2 nanowires. In addition, the 3D AFM images (shown in Fig. 4d and 4f) further confirmed that the RMS roughness of the NW-P2W18 film is smoother than that obtained for the TiO2 NW. Apparently, such a surface of the NW-P2W18 film may provide a larger surface area for EC performance. Figure 4 3.3 Electrochemical behavior of the composite film Fig. 5a shows the CVs the NW-P2W18 film at different scan rates in the potential range from -0.9 to 0.2 V. When the scan rate increases from 20 to 100 mV/s, the cathodic peak potentials shift to negative potential values while the anodic peak potentials shift to positive potential values, which is consistent with a reversible but nonideal redox process [36]. As shown in the set of Fig. 5a, the peak current has a good linear relationship with the square root of the scan rate, indicating that the electrochemical behavior is diffusion confined process. Fig. 5b exhibits CV curves of NW-P2W18, PEI-P2W18 and TiO2 NW films at a scan rate of 20 mV/s. It can be seen that the maximum of peak current was obtained for NW-P2W18 film in comparison to TiO2 NW and PEI-P2W18 films. Furthermore, the real active surface area of the films can be calculated by employing the Randles-Sevcik equation. 7
ip = (2.69 × 105) A D1/2n3/2v1/2C Where ip is the peak current, A is the real area active surface area of the working electrode (cm2), n is the electron number in this reaction, v is the scan rate (V s -1) and C is the concentration of Li + ions. It is found that the real active surface area for the NW-P2W18 film is higher than those of individual TiO2 NW and P2W18 films, suggesting that it has the ability to support larger sites for Li + ions intercalation and extraction. Figure 5
3.4 Electrochromic properties of the composite film Spectroelectrochemistry was used to evaluate the optical properties of the composite film. Fig. 6a displays the visible spectrum of the NW-P2W18 film-modified FTO glass electrode at different potentials. As the potential decreases from -0.5 to -0.9 V, a maximum transmittance peak appeared at 650 nm owing to the intervalence charge transfer band of P2W18 anions. On increasing the potential to -1.5 V, the peak at 600 nm increases obviously because of the reduced TiO 2 nanowires. Fig. 6b-d shows the transmittance spectra of NW-P2W18, PEI-P2W18 and TiO2 NW films in the colored and bleached states at wavelength from 400 to 800 nm. When the electrode is cathodically polarized (from +1.5 to -1.5 V), the NW-P2W18 film changes from transparent to dark blue. When the electrode is anodically polarized (from -1.5 to +1.5V), the NW-P2W18 film is bleached. It can be seen that the NW-P2W18 film shows a larger transmittance modulation than those of individual POM and TiO2 films, which was due to the combination of two cathodic EC materials. Figure 6 Quantitative EC response analysis was based on the chronoamperometry (CA) and the corresponding in situ transmittance measurements. As shown in Fig. 7a, the NW-P2W18 film displays a higher transient current density than pure P 2W18 and TiO2 films, suggesting that more trap sites for cation insertion/extraction were supplied. As can be seen in Fig. 7b, the NW-P2W18 film has the largest transmittance modulation (45.1%), as compared to 31.2% for PEI-P2W18 and 3.4% for TiO2 NW. The obtained transmittance modulation of NW-P2W18 film is higher than that of TiO 2-WO3 composite film (41.8%) [37], and single
8
cathodic EC films [38,39]. The enhanced transmittance modulation is mainly due to high active real area and simultaneous modulation of both P 2W18 and TiO2 electrodes. The switching time between the colored and bleached states is a very important factor for EC materials. The coloration and bleaching times are calculated as the time required for 90% changes in the whole transmittance modulation at 650 nm, as displayed in Fig. 7c. The NW-P2W18 film exhibits coloration and bleaching times for 1.9 s and 6.7 s, whereas the PEI-P2W18 film needs 5.3 s and 10.5 s, respectively. The fast coloration/bleaching kinetics may be attributed to the large active surface of both the P2W18 and TiO2 films, facilitates the penetration of electrolyte into the films and shortens the ion diffusion path. The TiO2 NW film displays 18.4 s for coloring and 4.3 s for bleaching, which probably relates to the decrease of optical contrast of the TiO 2 NW film. The coloration efficiency (CE) is a key parameter for the comparison of various EC materials, which are evaluated by the following equations: CE (η) =ΔOD (λ)/Q = log(Tb/Tc)/Q Where ΔOD is the optical contrast at a given wavelength λ, Q is the injected/ejected electronic charge density, Tb and Tc is the transmittances in the bleached and colored states, respectively. As shown in Fig. 7d, the calculated CE value of the NW-P2W18 film was 69.0 cm2/C, which represents an approximate improvement compared with the CE value of the PEI-P2W18 film (22.1 cm2/C), and the value is also superior to that of TiO 2 NW (1.1 cm2/C). The calculated CE value is larger than that obtained for TiO 2/V2O5 hybrid film (28 cm2/C) [25], and POM/CdS film (38.29 cm 2/C) [40]. The higher CE value indicates that the NW-P2W18 film could provide large transmittance modulation with small changes in amount of inserted or extracted charge. The improved CE value is a direct consequence of large active surface area providing good ion access and a higher modulation range of transmittance. Figure 7
4. Conclusions In summary, we have synthesized the NW-P2W18 film assembled P2W18 and PEI films on FTO substrate pre-coated with TiO2 nanowires by LbL method. Compared with the 9
individual films, the NW-P2W18 film exhibited higher real active area, leading to an enhanced EC performance. The NW-P2W18 film demonstrated larger transmittance modulation (45.1% at 650 nm), faster switching times (t c = 1.9 s and t b = 6.7 s) and superior coloration efficiency (69.0 cm2/C) than pure P2W18 and TiO2 films. The improved EC properties are mainly due to the combination of two cathodic EC materials. These advantages can make the composite film very attractive for potential application in EC device.
Acknowledgment
This work was supported by the Natural Science Foundation of China (Grant No. 21301041), Postdoctoral Scientific Research Starting Foundation of Heilongjiang Province, China (No. LBH-Q15072), Harbin University of Commerce Doctor Start-up Fund Research (No. 12DW030), the Department of Education of Jilin Province (No. 2014349 and 2015431), Jilin Science and Technology Bureau (No. 20156418), Jilin Institute of Chemical Technology (No. 201343 and 2015031) and Natural Science Foundation of Heilongjiang Province of China (No. B201409).
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Figure Captions
Fig. 1. (a) Schematics of formation of a NW-P2W18 film. (b) UV-vis absorption spectra of NW-P2W18 film (number of cycle: n = 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20) film. Inset: plots of the absorbance values at 204 and 286 nm. (c) CV curves of NW-P2W18 film at a scan rate of 100 mV/S (from inner to outside: n = 0, 2, 6, 10, 14, 18). Inset: the relationship of the peak III as a function of cycle. Fig. 2. SEM images of (a) TiO2 NW, (b) PEI-P2W18 and (c) NW-P2W18 films; (d) Cross-sectional SEM image of NW-P2W18 film. Fig. 3. EDS mapping of the NW-P2W18 film for Ti, P and W, respectively. Fig. 4. 2D AFM images of (a) PEI-P2W18, (c) TiO2 NW and (e) NW-P2W18 films; 3D AFM images of (b) PEI-P2W18, (d) TiO2 NW and (f) NW-P2W18 films. Fig. 5. (a) Cycle voltammgtams of the NW-P2W18 film at different scan rates (20, 40, 60, 80 and 100 mV/s). Inset: Cathodic/anodic peak current density as a function of the square root of the scan rate. (b) CV curves of NW-P2W18, PEI-P2W18 and TiO2 NW films at a scan rate of 20 mV/s. Fig. 6. (a) Visible transmittance spectrum of NW-P2W18 film at different potentials. Visible spectra of (b) NW-P2W18 (c) PEI-P2W18 and (d) TiO2 NW films at colored and bleached state. Fig. 7. (a) Chronoamperometry measurements and (b) corresponding in situ optical transmittance curves for NW-P2W18, PEI-P2W18 and TiO2 NW films at 650 nm. (c) Coloration/bleaching time extracted for a 90% transmittance change. (d) Plot of optical contrast versus charge density for NW-P2W18, PEI-P2W18 and TiO2 NW films.
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S0169-4332(17)30941-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.244 APSUSC 35620
To appear in:
APSUSC
Received date: Revised date: Accepted date:
30-12-2016 6-3-2017 26-3-2017
Please cite this article as: Shuping Liu, Xiaoshu Qu, Construction of nanocomposite film of Dawson-type polyoxometalate and TiO2 nanowires for electrochromic applications, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.03.244 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Construction of nanocomposite film of Dawson-type polyoxometalate and TiO2 nanowires for electrochromic applications Shuping Liua*, Xiaoshu Qub
(aCollege of Tourism and Cuisine, Chemistry Center, Harbin University of Commerce, Harbin 150076, China; bJiLin Institute of Chemical Technology, JiLin City, 132073, P. R. China.)
Shuping Liu: [email protected]
*Correspongding author: No.138, Tongda Road, Harbin, China Tel: +86-451-84839474 Fax: +86-451-84866022 E-mail address: [email protected]
1
GRAPHICAL ABSTRACT
Highlights
A film containing polyoxometalate and TiO2 nanowires was successfully fabricated.
The composite film exhibits improved electrochemical and electrochromic properties.
The maximum optical contrast of 45.1% and coloration efficiency of 69.0 cm2/C.
Abstract: This paper shows a nanocomposite film of Dawson-type polyoxometalate (POM) K6P2W18O62 (P2W18) and TiO2 nanowires by combining hydrothermal and Layer-by-Layer (LbL) assembly methods. The electrochemical and electrochromic (EC) performances were examined and compared with P2W18 and TiO2 composite film as well as individual P2W18 and TiO2 structures. The nanocomposite film exhibits improved EC properties with high transmittance modulation (45.1% at 650 nm), fast switching times (1.9 s and 6.7 s) and outstanding coloration efficiency (69.0 cm2/C at 650 nm). The enhancement can be attributed to the contribution from P2W18 and TiO2, which displays large real active area and increase the capabilities for EC material. 2
Keywords: Electrochromic; Polyoxometalates; TiO2 nanowire; Nanocomposite
1. Introduction Electrochromic (EC) materials can be changed optical properties (transmittance, absorbance and reflectance) continuously and reversibly during electrochemical oxidation and reduction [1-5]. Recently, EC materials can be applied for rear-view mirrors, displays and antiglare rear-view mirrors in car [6-10]. The EC materials have been widely studied such as conducting polymers, Prussian blue and inorganic metal oxides (e.g. WO3, TiO2, NiO) [11-15]. They can be generally divided into two groups, cathodic (coloring under negative potential) and anodic (coloring under positive potential) electrochromic materials [16]. Two cathodic films (or anodic films) can be incorporated into a composite film, in which both the films can be colored or bleached under a certain potential. Titanium dioxide (TiO2) is a typical cathodic EC material and has shown particular potential as an electro-active material due to its high activity, superior chemical stability and strong oxidation capability [17-19]. Recently, it is widely recognized that one-dimensional (1D) TiO2 nanostructures have attracted great interest owing to superior charge transport properties and facile processing [20,21]. Lee et al. prepared TiO2 rutile nanowires along [001] direction grown on FTO by a simple one step solvothermal process, which exhibited high coloration efficiency compared with P25-coated and rutile nanowires-coated electrodes [22]. Chen et al. constructed a highly transparent electrochromic device based on anatase TiO2 nanowires, which exhibited high color density, good coloration efficiency and fast color-switching time [23]. However, long nanowires or nanotubes cannot be fabricated easily, and therefore the incorporation of another cathodic EC material could improve the EC properties of TiO2 film. Reyes-Gil et al. [24] fabricated WO3/TiO2 film using TiO2 nanotubes as template for WO3 electrodeposition, which displayed higher ion storage capacity, better stability, enhanced EC contrast compared with the pure WO3 and TiO2. Jia et al. [25] displayed the TiO2/V2O5 core/shell structure hybrid film by electrodepositing the V2O5 film on a TiO2 nanorod substrate, which has better electrochemical and EC properties compared with the single V2O5 film. Polyoxometalates (POMs), a well-known class of transition metal oxide nanoclusters with 3
intriguing structures and diverse properties, have been used candidate material for fabricating EC devices [26-30]. However, there have been very few studies on the EC performance of TiO2 nanowire composite film via the addition of POMs. In this paper, we attempted to design a composite material containing TiO2 nanowires (NWs) and Dawson type K6P2W18O62 (P2W18) using a two-step synthesis method. The TiO2/POM electrode displayed a higher real active surface area than that of the single film. In addition, the composite nanostructure exhibits enhanced electrochromic performance including large transmittance modulation, high coloration efficiency and good fast switching speed compared to the pure TiO2 and POM film. Therefore, the combination of both cathodic EC materials is expected to improve the EC performance of materials.
2. Experimental Section 2.1. Chemical Materials All reagents were of analytical grade and used without further treatment. Tetrabutyl titanate (98%) was purchased from Beijing Chemical Works. 3-Aminopropyltrimethoxysilane (APS) and Polyetherimide (PEI) were obtained from Aladdin. P2W18 was prepared according to the literature method [31-32]. Fluorine doped tin oxide (FTO) were purchased from Yingkou OPV Tech New Energy Co., Ltd (7-8 -1). 2.2. Preparation of TiO2 nanowires The synthetic routes of TiO 2 nanowires (TiO2 NW) have been described in detail in our previous report [33]. Firstly, the FTO substrates were cleaned with a detergent solution, washed with deionized water and ethanol and finally dried in a nitrogen stream. The precursor solution was obtained by mixing 0.45 mL tetrabutyl titanate, 18 mL HCl in 18 mL deionized water. The precursor solution was transferred into a Teflon-lined stainless steel autoclave, containing a vertically oriented FTO substrate. Then the autoclave was heated to 150 ℃for 4 h and cooled to room temperature under flowing water. The samples were rinsed with deionized water, and then annealed in air at 450 ℃for 30 min.
2.3. Preparation of composite films The composite film was synthesized by growing single-crystalline nanowires on the FTO substrates using a hydrothermal method, followed by depositing negatively charged P2W18 and 4
positively PEI onto the TiO2 nanowires by Layer-by-Layer (LbL) method. The composite films were assembled on the substrates by first immersing them into APS solution for 8 h. After the treatment, the substrates were dipped into HCl (pH = 2.0) for 20 min to form a positively charged surface. Then they were alternately dipped into the P2W18 (5 × 10-3 mol/L in 0.2mol/L HOAc-NaAc pH = 4) and PEI (5 × 10 -3 mol/L pH = 4) for 8 min. This cycle was repeated until the desired number of [P2W18/PEI] cycles was obtained. After each layer was deposited, the films were washed with deionized water and dried in nitrogen. All adsorption processes were performed at room temperature. The TiO 2-[P2W18/PEI]40 film thus obtained was designated as NW-P2W18 film. For comparison, the [P2W18/PEI]40 film (PEI-P2W18) was prepared on FTO substrates by the same method.
2.4. Characterization UV-vis absorption spectra were performed by using a TU-1901 PERSEE UV-vis spectrophotometer. The surface morphologies were investigated using scanning electron microscopy (SEM, Hitachi SU8010), high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100) and atomic force microscopy (AFM, Icon Bruker Company). A three-electrode cell was used for electrochemical measurement with 0.2 mol/L HOAc-NaAc buffer solution (pH = 3.4) as the electrolyte. All the electrochemical experiments were performed on the CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Factory, China). A conventional three electrodes system was used, with Ag/AgCl (3 mol/L KCl) as the reference electrode, platinum coil as the counter electrode and FTO electrode coated by the films as the working electrode. Spectroeletrochemical measurements of the nanowires were performed by combining the in situ UV-vis spectrophotometer with the electrochemical workstation.
3. Results and discussion 3.1 Fabrication and structure analyses Fig. 1a illustrates the schematics of the fabrication of NW-P2W18 film. As shown in Fig. 1b, the fabrication process of NW-P2W18 (number of cycle: 2-20) was monitored by UV-vis spectroscopy. It exhibits strong absorption of P2W18 with the bands at 204 and 286 nm. The former is due to the terminal oxygen to tungsten charge transfer transitions, and the latter is owing to charge transfer transitions from bridge-oxygen to tungsten [34]. The 5
inset of Fig. 1b shows the plots of the absorbance values at 204 and 286 nm as a function of the layer number. The linear growth of absorbance indicates that almost the same amount of P2W18 was deposited in each cycle, confirming the film growth for each deposition cycle is uniform. In addition, the stepwise growth of the films can be evaluated by cycle voltammetry (CV). As shown in Fig. 1c, the redox chemistry of the NW-P2W18 (number of cycle: 0, 2, 6, 10, 14, 18) film was examined which swept in the potential range of 0.2 to -0.9 V at the scan rate of 100 mV/s. The films display four couples of redox waves, which is similar to the CV curve of the P2W18 solution. This indicates that P2W18 is effectively incorporated into the composite film. With the increase in the number of cycles, the cathodic peak potentials shift slightly toward negative potential values and the anodic peak potentials shift toward positive potential values, which probably relates to the uncompensated resistance [35]. As shown in the inset of Fig. 1c, the current density of peak III increases linearly with the number of layers, indicating that growth of each deposition cycles is reproducible. Figure 1
3.2 Morphology analysis Fig. 2a and SI-1a displays the tope-view and cross-sectional SEM image of the TiO2 nanowires grown on FTO substrates. The images clearly indicate the surface of the FTO substrate is covered with randomly oriented TiO2 nanowires. The HR-TEM image of TiO2 nanowires shows lattice fringes with interplanar spacings d 110 = 0.325 and d001 = 0.293 nm, which are consistent with the tetragonal rutile phase (Fig. SI-1b in the ESI†). As shown in Fig. 2b, the PEI-P2W18 film surface is relatively smooth over a large area with small islands, which are evidences for the aggregation of P 2W18 anions. As shown in Fig. 2c-d, the nanowires in the NW-P2W18 film grow wider and more compact. It is possibly that TiO2 nanowires are enclosed by depositing P2W18 anions and PEI. In addition, the thickness of the NW-P2W18 film is approximately 600 nm. Fig. 3 shows energy dispersive X-ray spectrometry (EDS) mapping for Ti, P and W of the NW-P2W18 film. The uniform signal coverage of the three elements is observed, suggesting the presence of TiO2 and P2W18. 6
Figure 2 Figure 3
AFM can provide the detailed information concerning the surface morphology and homogeneity of the films. Fig. 4 displays the 2D and 3D AFM images of TiO2 NW, PEI-P2W18 and NW-P2W18 films. As shown in Fig. 4a, the PEI-P2W18 film displays relatively smooth surface with multitude of grains due to the aggregation of P 2W18 anions. Fig. 4c shows a regular globular microstructure, suggesting the presence of TiO 2 nanowires. From Fig. 4e, the surface of the NW-P2W18 film is similar to that of the TiO2 NW. The root mean square (RMS) roughness is calculated from an area of 5 × 5 μm2 in AFM image. The RMS roughnesses are 4.1, 79.3 and 50.6 nm for PEI-P2W18, TiO2 NW and NW-P2W18 films, respectively. Obviously, the RMS roughness decreased after P2W18
anions were deposited on the surface of TiO2 nanowires. In addition, the 3D AFM images (shown in Fig. 4d and 4f) further confirmed that the RMS roughness of the NW-P2W18 film is smoother than that obtained for the TiO2 NW. Apparently, such a surface of the NW-P2W18 film may provide a larger surface area for EC performance. Figure 4 3.3 Electrochemical behavior of the composite film Fig. 5a shows the CVs the NW-P2W18 film at different scan rates in the potential range from -0.9 to 0.2 V. When the scan rate increases from 20 to 100 mV/s, the cathodic peak potentials shift to negative potential values while the anodic peak potentials shift to positive potential values, which is consistent with a reversible but nonideal redox process [36]. As shown in the set of Fig. 5a, the peak current has a good linear relationship with the square root of the scan rate, indicating that the electrochemical behavior is diffusion confined process. Fig. 5b exhibits CV curves of NW-P2W18, PEI-P2W18 and TiO2 NW films at a scan rate of 20 mV/s. It can be seen that the maximum of peak current was obtained for NW-P2W18 film in comparison to TiO2 NW and PEI-P2W18 films. Furthermore, the real active surface area of the films can be calculated by employing the Randles-Sevcik equation. 7
ip = (2.69 × 105) A D1/2n3/2v1/2C Where ip is the peak current, A is the real area active surface area of the working electrode (cm2), n is the electron number in this reaction, v is the scan rate (V s -1) and C is the concentration of Li + ions. It is found that the real active surface area for the NW-P2W18 film is higher than those of individual TiO2 NW and P2W18 films, suggesting that it has the ability to support larger sites for Li + ions intercalation and extraction. Figure 5
3.4 Electrochromic properties of the composite film Spectroelectrochemistry was used to evaluate the optical properties of the composite film. Fig. 6a displays the visible spectrum of the NW-P2W18 film-modified FTO glass electrode at different potentials. As the potential decreases from -0.5 to -0.9 V, a maximum transmittance peak appeared at 650 nm owing to the intervalence charge transfer band of P2W18 anions. On increasing the potential to -1.5 V, the peak at 600 nm increases obviously because of the reduced TiO 2 nanowires. Fig. 6b-d shows the transmittance spectra of NW-P2W18, PEI-P2W18 and TiO2 NW films in the colored and bleached states at wavelength from 400 to 800 nm. When the electrode is cathodically polarized (from +1.5 to -1.5 V), the NW-P2W18 film changes from transparent to dark blue. When the electrode is anodically polarized (from -1.5 to +1.5V), the NW-P2W18 film is bleached. It can be seen that the NW-P2W18 film shows a larger transmittance modulation than those of individual POM and TiO2 films, which was due to the combination of two cathodic EC materials. Figure 6 Quantitative EC response analysis was based on the chronoamperometry (CA) and the corresponding in situ transmittance measurements. As shown in Fig. 7a, the NW-P2W18 film displays a higher transient current density than pure P 2W18 and TiO2 films, suggesting that more trap sites for cation insertion/extraction were supplied. As can be seen in Fig. 7b, the NW-P2W18 film has the largest transmittance modulation (45.1%), as compared to 31.2% for PEI-P2W18 and 3.4% for TiO2 NW. The obtained transmittance modulation of NW-P2W18 film is higher than that of TiO 2-WO3 composite film (41.8%) [37], and single
8
cathodic EC films [38,39]. The enhanced transmittance modulation is mainly due to high active real area and simultaneous modulation of both P 2W18 and TiO2 electrodes. The switching time between the colored and bleached states is a very important factor for EC materials. The coloration and bleaching times are calculated as the time required for 90% changes in the whole transmittance modulation at 650 nm, as displayed in Fig. 7c. The NW-P2W18 film exhibits coloration and bleaching times for 1.9 s and 6.7 s, whereas the PEI-P2W18 film needs 5.3 s and 10.5 s, respectively. The fast coloration/bleaching kinetics may be attributed to the large active surface of both the P2W18 and TiO2 films, facilitates the penetration of electrolyte into the films and shortens the ion diffusion path. The TiO2 NW film displays 18.4 s for coloring and 4.3 s for bleaching, which probably relates to the decrease of optical contrast of the TiO 2 NW film. The coloration efficiency (CE) is a key parameter for the comparison of various EC materials, which are evaluated by the following equations: CE (η) =ΔOD (λ)/Q = log(Tb/Tc)/Q Where ΔOD is the optical contrast at a given wavelength λ, Q is the injected/ejected electronic charge density, Tb and Tc is the transmittances in the bleached and colored states, respectively. As shown in Fig. 7d, the calculated CE value of the NW-P2W18 film was 69.0 cm2/C, which represents an approximate improvement compared with the CE value of the PEI-P2W18 film (22.1 cm2/C), and the value is also superior to that of TiO 2 NW (1.1 cm2/C). The calculated CE value is larger than that obtained for TiO 2/V2O5 hybrid film (28 cm2/C) [25], and POM/CdS film (38.29 cm 2/C) [40]. The higher CE value indicates that the NW-P2W18 film could provide large transmittance modulation with small changes in amount of inserted or extracted charge. The improved CE value is a direct consequence of large active surface area providing good ion access and a higher modulation range of transmittance. Figure 7
4. Conclusions In summary, we have synthesized the NW-P2W18 film assembled P2W18 and PEI films on FTO substrate pre-coated with TiO2 nanowires by LbL method. Compared with the 9
individual films, the NW-P2W18 film exhibited higher real active area, leading to an enhanced EC performance. The NW-P2W18 film demonstrated larger transmittance modulation (45.1% at 650 nm), faster switching times (t c = 1.9 s and t b = 6.7 s) and superior coloration efficiency (69.0 cm2/C) than pure P2W18 and TiO2 films. The improved EC properties are mainly due to the combination of two cathodic EC materials. These advantages can make the composite film very attractive for potential application in EC device.
Acknowledgment
This work was supported by the Natural Science Foundation of China (Grant No. 21301041), Postdoctoral Scientific Research Starting Foundation of Heilongjiang Province, China (No. LBH-Q15072), Harbin University of Commerce Doctor Start-up Fund Research (No. 12DW030), the Department of Education of Jilin Province (No. 2014349 and 2015431), Jilin Science and Technology Bureau (No. 20156418), Jilin Institute of Chemical Technology (No. 201343 and 2015031) and Natural Science Foundation of Heilongjiang Province of China (No. B201409).
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multilayers with polyoxometalate nanoclusters, J. Am. Chem. Soc. 124 (2002) 12279-12287. [37] N.M. Vuong, D. Kim, H. Kim, Electrochromic properties of porous WO3-TiO2 core-shell nanowires, J. Mater. Chem. C 1 (2013) 3399-3407. [38] Z.Q. Tong, H.W. Yang, L. Na, H.Y. Qu, X. Zhang, J.P. Zhao, Y. Li, Versatile displays based on a 3-dimensionally ordered macroporous vanadium oxide film for advanced electrochromic devices, J. Mater. Chem. C 3 (2015) 3159-3166. [39] V.V. Kondalkar, S.S. Mali, R.R. Kharade, R.M. Mane, P.S. Patil, C.K. Hong, J.H. Kim, S. Choudhury, P.N. Bhosale, Langmuir–Blodgett self organized nanocrystalline tungsten oxide thin films for electrochromic performance, RSC Adv. 5 (2015) 26923-26931. [40] D. Zhang, Y.Y. Chen, H.J. Pang, Y. Yu, H.Y. Ma, Enhanced electrochromic performance of a vanadium-substituted tungstophosphate based on composite film by incorporation of cadmium sulfide nanoparticles, Electrochim. Acta 105 (2013) 560-568.
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Figure Captions
Fig. 1. (a) Schematics of formation of a NW-P2W18 film. (b) UV-vis absorption spectra of NW-P2W18 film (number of cycle: n = 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20) film. Inset: plots of the absorbance values at 204 and 286 nm. (c) CV curves of NW-P2W18 film at a scan rate of 100 mV/S (from inner to outside: n = 0, 2, 6, 10, 14, 18). Inset: the relationship of the peak III as a function of cycle. Fig. 2. SEM images of (a) TiO2 NW, (b) PEI-P2W18 and (c) NW-P2W18 films; (d) Cross-sectional SEM image of NW-P2W18 film. Fig. 3. EDS mapping of the NW-P2W18 film for Ti, P and W, respectively. Fig. 4. 2D AFM images of (a) PEI-P2W18, (c) TiO2 NW and (e) NW-P2W18 films; 3D AFM images of (b) PEI-P2W18, (d) TiO2 NW and (f) NW-P2W18 films. Fig. 5. (a) Cycle voltammgtams of the NW-P2W18 film at different scan rates (20, 40, 60, 80 and 100 mV/s). Inset: Cathodic/anodic peak current density as a function of the square root of the scan rate. (b) CV curves of NW-P2W18, PEI-P2W18 and TiO2 NW films at a scan rate of 20 mV/s. Fig. 6. (a) Visible transmittance spectrum of NW-P2W18 film at different potentials. Visible spectra of (b) NW-P2W18 (c) PEI-P2W18 and (d) TiO2 NW films at colored and bleached state. Fig. 7. (a) Chronoamperometry measurements and (b) corresponding in situ optical transmittance curves for NW-P2W18, PEI-P2W18 and TiO2 NW films at 650 nm. (c) Coloration/bleaching time extracted for a 90% transmittance change. (d) Plot of optical contrast versus charge density for NW-P2W18, PEI-P2W18 and TiO2 NW films.
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