Shell Nanorod Array for Efficient

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Constructed TiO2/NiO Core/Shell Nanorod Array for Efficient Electrochromic Application Guofa Cai,† Jiangping Tu,*,†,‡ Ding Zhou,† Lu Li,† Jiaheng Zhang,† Xiuli Wang,†,‡ and Changdong Gu†,‡ †

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and Department of Materials Science and Engineering and ‡Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, China

ABSTRACT: A TiO2/NiO core/shell nanorod array film is prepared by the combination of hydrothermal and chemical-bath deposition. Compared to the NiO nanoflake film, the TiO2/NiO core/shell nanorod array exhibits better electrochromism with reversible color changes bewteen transparency and dark brown, shows larger optical modulation (83%), higher coloration efficiency (60.6 cm2 C1−) and better cycling performance. The enhancement of electrochromic performances is attributed to the synergetic contribution from the single crystalline TiO2 nanorod core and the ultrathin NiO nanoflake shell, as well as the ordered array geometry, which can all offer direct electrical pathways for electrons and increase the electron transport rate.

1. INTRODUCTION Electrochromism is broadly defined as optical property that can be altered reversibly when a small external potential is applied. Electrochromic materials have received extensive attention in recent years because they can be used as energy-efficient glazing, large area displays, automobile sunroofs, smart mirrors, and military camouflage.1−11 The challenge for commercially electrochromic devices is to prepare the electrochromic materials with long-term cyclic stability, large optical modulation, and short switching time. A number of organic and inorganic materials exhibit electrochromism. Almost all of the interesting inorganic electrochromic materials are transition metal oxides, such as WO3, Co3O4, MoO3, V2O5, NiO, and TiO2.12−27 Among these metal oxides, NiO is one of the most exhaustively investigated materials due to its high electrochromic efficiency, large modulation range, natural abundance, and low material cost.28−30 Numbers of methods have been used to prepare the NiO electrochromic films including chemical vapor deposition,31 sputtering,32 pulsed laser deposition,33 electrodeposition,34 spray pyrolysis,35 chemical bath deposition (CBD),36 and so forth. Of all, CBD is an advantageous technique because of its low-temperature synthesis process and convenience for large-area deposition. However, poor cycling durability and slow switching speed limit its commercialization in current NiO film prepared by CBD.37 © 2014 American Chemical Society

Aligned TiO2 nanorods have high chemical stability in alkaline electrolyte and can be easily fabricated by hydrothermal methods.38−40 In addition, the TiO2 nanorods provide the vertically aligned nanostructure with a high surface area that allows the electrolyte to penetrate and shorten the ions diffusion length within the bulk TiO2. Moreover, the TiO2 nanorod can reduce the refractive index and enhance optical transparency.20,41 In addition, the n-type TiO2 core/p-type NiO shell heterostructure can enhance the separation of electron and proton by the electric junction field, favor the interfacial charge transfer, and subsequently could improve reaction reversibility and electrochemical activity.42 Therefore, it would be very interesting to integrate NiO and TiO2 into a single electrode in order to enhance the electrochromic properties of the composite film. To the best of our knowledge, there are no reports dedicated to the electrochromic performance of TiO2/NiO core/shell nanorod array. Herein, a TiO2/NiO core/shell nanorod array film was prepared by a combination of hydrothermal and CBD methods. The improved electrochemical and electrochromic properties of the TiO2/NiO core/shell nanorod array are investigated in comparison with pure NiO nanoflake film. Received: January 21, 2014 Revised: March 4, 2014 Published: March 7, 2014 6690

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2. EXPERIMENTAL METHODS All solvents and chemicals were of analytical grade and used without further purification. Titanium n-butoxide, nickel sulfate, potassium persulfate, aqueous ammonia, and hydrochloric acid were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). FTO were purchased from Zhuhai Kaivo Electronic Components Co., Ltd. All aqueous solutions were freshly prepared with deionized water. 2.1. Preparation of TiO2 Nanorod Array. TiO2 nanorod array was grown on a fluorine-doped tin oxide (FTO) glass substrate using a previously reported hydrothermal method.39,43 In a typical synthesis, 30 mL of deionized water was mixed with 30 mL of concentrated hydrochloric acid in a 100 mL beaker. The resulting solution was stirred at ambient temperature for 15 min before the addition of 1 mL of titanium n-butoxide. After stirring for another 15 min, the clear solution was transferred to a Teflon-lined stainless steel autoclave (100 mL in volume). Then, a clean FTO substrate (2 × 4 cm2 in size and sheet resistance Rs = 10 Ω) was submerged in the solution and placed at an angle against the wall of the Teflon lined with the conducting side facing down. The hydrothermal process was conducted at 150 °C for 3.5 h in a vacuum oven and then cooled down to room temperature under flowing water. After rinsed extensively with deionized water and dried in ambient air, a TiO2 nanorod array film was uniformly coated on the FTO glass substrate. 2.2. Preparation of TiO2/NiO Core/Shell Nanorod Array. A solution for CBD was made by mixing 300 mL of 0.5 M nickel sulfate, 300 mL of 0.15 M potassium persulfate, and 60 mL of aqueous ammonia (25−28%) in a 1000 mL pyrex beaker at 50 °C. The as-obtained TiO2 nanorod array coated on FTO was used as the substrate and the back side was masked with polyimide tape to prevent the deposition on the nonconductive side. The substrate was vertically supported on the wall of the bath container for 2 min to deposit the precursor film. After removing the tape masks, the precursor film was rinsed with distilled water and finally annealed at 300 °C in argon for 2 h to obtain the TiO2/NiO core/shell nanorod array. For comparison, a pure NiO nanoflake film on FTO substrate was also prepared with the same parameters. 2.3. Characterization. The microstructure, surface morphology, and composition of the as-prepared films were characterized using X-ray diffraction (XRD, RIGAKU D/ MAX 2550/PC with Cu Kα radiation), X-ray photoelectron spectroscopy (XPS, AXIS UTLTRADLD equipped with a dual Mg Ka-Al Kaanode for photoexcitation), field emission scanning electron microscopy (FESEM, Hitachi SU-70), and transmission electron microscopy (TEM, FEI tecnai G2 F20). The transmission spectra of the films in the fully colored and fully bleached states were measured over the wavelength range from 300 to 1000 nm with a SHIMADZU UV-3600 spectrophotometer. The UV−vis absorption spectra of these films were also conducted on this spectrophotometer. The bandgap energies of the films were obtained using their UV−vis absorption spectra. The cyclic voltammetry (CV) and chronoamperometry (CA) measurements were conducted on a CHI660E electrochemical workstation using a three-electrode electrochemical cell with 1 M KOH as the electrolyte, platinum foil as the counter- electrode and Hg/HgO as the reference electrode. During the CA process, Raman spectra were recorded on LABRAM HR-800 following the evolution of the films using an excitation wavelength of 514 nm.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. Figure 1 shows the XRD patterns of bare TiO2 nanorod array, NiO, and TiO2/NiO

Figure 1. XRD patterns of (a) TiO2 nanorod array, (b) NiO film, (c) TiO2/NiO core/shell nanorod array.

core/shell nanorod array films. After subtracting the diffraction peaks of FTO glass, all the diffraction peaks observed in pattern (a) match the tetragonal rutile TiO2 (JCPDS No. 21-1276). In pattern (b), the film exhibits three diffraction peaks at 2θ degrees of 37.2, 43.3, and 62.9° except the diffraction peaks of FTO glass, which can be indexed as (111), (200), and (220) crystal planes of a cubic NiO phase (JCPDS No. 47-1049). From pattern (c), the film displays all the diffraction characteristics of FTO, TiO2, and NiO. In addition, given that the thinness and porous structure of the NiO layer, it is difficult to obtain high intensity peaks of NiO XRD pattern. In order to further investigate the chemical composition and the oxidation state of NiO on the TiO2 nanorods, XPS measurements were performed on the TiO2/NiO core/shell nanorod array film. Figure 2a shows the Ni 2p XPS spectrum of the TiO2/NiO core/shell nanorod array film. The main peaks of Ni 2p3/2 and Ni 2p1/2 are located at 853.3 and 871.0 eV, respectively, which are in agreement to those reported in the literature for NiO.44−47 The Ti 2p XPS spectrum of the core/ shell nanorod array film is shown in Figure 2b, which can be fitted with two doublets. Because the NiO shells are uniformly coated on the TiO2 nanorods, the peaks are not well resolved. The major peak of first doublet is at 459.0 eV and satellite peak is at 464.5 eV, which corresponds to Ti 2p3/2 and Ti 2p1/2, respectively. These values are in accordance with those reported in the literatures.48−50 Figure 3 shows the surface and cross-sectional morphologies of the TiO2, TiO2/NiO core/shell nanorod array and NiO films. Before deposition of NiO, the entire surface of the FTO substrate is covered uniformly with TiO2 nanorods with a rectangular cross section for the TiO2 film (Figure 3a,b). The TiO2 nanorods are nearly perpendicular to the substrate with diameters of 30−60 nm. After coated with NiO, the nanoflakes uniformly cover the whole surfaces of TiO2 nanorods, and the alignment of TiO2 nanorods is preserved (Figure 3c,d). The thicknesses of both the TiO2 and TiO2/NiO core/shell nanorod array films are about 500 nm, as shown in the insets of Figure 3b,d, respectively. If NiO is directly deposited on the FTO substrate by CBD method, the film has a porous and interconnecting network structure. The interconnecting network is made up of flaky NiO with thicknesses of 15−20 nm (Figure 3e,f). The thickness of the NiO film is about 300 nm, as shown in the inset of Figure 3f. 6691

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The detailed structural features of the TiO2 and TiO2/NiO core/shell nanorods are further examined by TEM. Figure 4a

Figure 2. XPS spectra of TiO2/NiO core/shell nanorod array film. (a) Ni 2p and (b) Ti 2p.

Figure 4. TEM images of (a,b) TiO2 nanorod (inset corresponding SAED pattern) and (c,d) TiO2/NiO core/shell nanorod. (e) EDS mapping results from a single core/shell nanorod, demonstrating the TiO2 core/NiO shell hierarchical structure.

displays the TEM image of a single TiO2 nanorod with a diameter of about 50 nm. The high-magnification TEM (HRTEM) image shows that the nanorod is crystalline along its entire length and reveals clear lattice fringes with interplanar spacings of 0.32 nm, which is consistent with the d-spacings of (110) planes of rutile TiO2 (Figure 4b). Selected area electron diffraction (SAED) pattern of the TiO2 nanorod verifies its single-crystalline nature with [001] orientation (inset of Figure 4b). The single-crystalline TiO2 nanorods can offer direct electrical pathways for electrons and increase the electron transport rate, and subsequently improve the electrochromic performance.39 For a representative core/shell nanorod, the TiO2 nanorod core is uniformly covered by interconnected NiO nanoflakes (Figure 4c). The HRTEM image shows the lattice fringe characteristics of both the NiO and TiO2 (Figure 4d). Energy dispersive X-ray spectrometry (EDS) mapping analysis, shown in Figure 4e, of a single hybrid nanorod

Figure 3. SEM images of (a,b) TiO2 nanorod array, (c,d) TiO2/NiO core/shell nanorod array, and (e,f) NiO nanoflake film (cross-sectional view presented in insets).

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NiO + OH− ↔ NiOOH + e−

unambiguously confirms the TiO2 core/NiO shell heterostructure. 3.2. Electrochemical and Electrochromic Performances. In order to investigate the band structure properties of the TiO2 nanorod array, the TiO2/NiO core/shell nanorod array, and pure NiO films on FTO substrate, Tauc plots of all the films are obtained by extracting their UV−vis absorbance spectra to estimate their bandgap energies. The optical bandgaps are obtained by dropping a line from the maximum slope of the curve to the x-axis.51 Figure 5 shows that the

(1)

or NiO + H 2O ↔ NiOOH + H+ + e−

(2) 2+

3+

During the anodic scan, the oxidation of Ni to Ni causes coloration of the film. In the reverse cathodic scan, the reduction of Ni3+ to Ni2+ leads to the bleaching of the film. As shown in Figure 6, the reduction and oxidation peaks of the NiO film shift to lower and higher potentials than that of the TiO2/NiO core/shell nanorod array film, respectively, leading to a larger potential separation between the oxidation and reduction peaks. Besides, the cathodic and anodic peak currents of the TiO2/NiO core/shell nanorod array are much higher than those of the NiO film. All of these indicate that the TiO2/ NiO core/shell nanorod array has an improved reaction reversibility and electrochemical activity. In order to quantitatively compare the electrochromic performances of NiO and TiO2/NiO core/shell nanorod array films, the transmittance spectra are measured after the film electrodes had been subjected to CV test for 10 cycles in 1 M KOH. The film electrodes are colored by applying step voltages of 0.8 V for coloration and 0 V (vs Hg/HgO) for bleaching. The color of the thin films changes from brown (colored state) to transparent (bleached state). It can be seen from Figure 7 that the TiO2/NiO core/shell nanorod array

Figure 5. Tauc plots of the bandgap of TiO2 nanorod, NiO, and TiO2/ NiO core/shell nanorod array on FTO substrate.

bandgaps for TiO2 nanorod, the TiO2/NiO core/shell nanorod array, and pure NiO film are 3.12, 3.0, and 3.67 eV, respectively. Obviously, the TiO2/NiO core/shell nanorod array possesses the smallest bandgap of 3.0 eV among the three films. Generally, films with small bandgaps can improve the charge transfer and subsequently enhance the electrochromic performance. The enhanced electrochemical activity is also confirmed by CV tests. Figure 6 compares the 10th CV curve of the NiO and

Figure 7. Optical transmittance spectra of the NiO and TiO2/NiO core/shell nanorod array films at their colored and bleached states.

shows a larger transmittance modulation than that of the NiO film. The transmittance variation of the TiO2/NiO core/shell nanorod array reaches about 83% at 550 nm, while the NiO film exhibits 73% at 550 nm. The transmittance modulation value of 83% is also larger than those of the NiO films in previously reported works.52−55 The photographs of the TiO2/ NiO core/shell nanorod array film in the colored and bleached states are shown in Figure 8. The coloration and bleaching times of the NiO and TiO2/ NiO core/shell nanorod array are investigated by CA and the corresponding in situ transmittance at 550 nm, as shown in Figure 9. The switching time is defined as the time required for a system to reach 90% of its full modulation. The coloration and bleaching time are 12.2/7.4 and 6.8/14.8 s for the NiO and the TiO2/NiO core/shell nanorod array film, respectively. Apparently, the coloration time of the TiO2/NiO core/shell nanorod array film is shorter than that of the NiO but the bleaching time is longer. However, the total response times are almost the same for both of the films. The structural evolution of the TiO2/NiO core/shell nanorod array at coloration and bleaching state is recorded

Figure 6. The 10th CV curves of NiO and TiO2/NiO core/shell nanorod array film.

TiO2/NiO core/shell nanorod array films carried out in 1 M KOH solution in the potential region of 0−0.8 V at a scan rate of 20 mV s−1. Just one redox process of NiO is seen in the CV curves. No electrochemical current peaks of TiO2 are observed in this potential range. It indicates that the NiO plays a leading role in this potential range for electrochromism, and the TiO2 mainly acts as a scaffold. The coloration process of the films corresponds to the oxidation peak before the oxygen evolution reaction, whereas the bleaching process is associated with the reduction peak, which can be attributed to the following electrochemical reactions 6693

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A parameter often used to characterize an electrochromic material is its coloration efficiency (CE), which is defined as the change in optical density (ΔOD) per unit of charge (Q) inserted into (or extracted from) the film. The change of optical density is obtained as follows CE(λ) =

ΔOD(λ) Q

ΔOD(λ) = log

Tb Tc

(3)

(4)

where Tb and Tc denotes transmittance in bleached and colored states, respectively. Both OD and CE present the ability of optical modulation during the coloration−bleaching process, but the CE value is under the consideration of energy consumption. A high value of CE indicates that the electrochromic material exhibits a large optical modulation with a small intercalation charge density. The CE values are 33.9 and 60.6 cm2 C−1 at 550 nm for the NiO and TiO2/NiO core/shell nanorod array film, respectively. The high CE of the porous TiO2/NiO nanorod array film is attributed to the synergetic contribution from the single crystalline TiO2 nanorod core, the ultrathin NiO shell, as well as the ordered array geometry. The durability of the NiO and TiO2/NiO core/shell nanorod array films is evaluated by CA measurements and corresponding in situ transmittance at 550 nm. Figure 11 shows the

Figure 8. Photographs of the TiO2/NiO core/shell nanorod array film with a size of 2 × 4 cm2 in the bleached and colored states.

Figure 9. In situ transmittance response between the colored and bleached states for the NiO and TiO2/NiO core/shell nanorod array films measured at 550 nm.

by Raman spectroscopy (Figure 10). In the colored state, two remarkable Raman peaks observed at 473 and 550 cm−1 are

Figure 11. Durability tests of NiO and TiO2/NiO core/shell nanorod array films for 2400 cycles at 550 nm.

transmittance discrepancies of the films during a 2400 cycle test. After subjected for 1000 cycles, the transmittance modulations decay about 89.6 and 13% for the NiO and TiO2/NiO core/shell nanorod array films, respectively. The transmittance modulation decays only about 31.5% even when subjected to 2400 cycles for the TiO2/NiO core/shell nanorod array films. In addition, the TiO2/NiO core/shell nanorod array can effectively keep the structure stable after 2400 cycles (Figure 12), while almost all the NiO nanosheet fell off from FTO glass after 1000 cycles for the NiO film. These results indicate that the core/shell nanorod array possesses quite good cycling durability.

Figure 10. Raman spectra of TiO2/NiO core/shell nanorod array film at the colored and bleached states.

attributed to the Ni−O bending vibrations and Ni−O stretching vibrations, respectively, which are the characteristic peaks of NiOOH. In the bleaching state, the film exhibits characteristic peaks of both TiO2 and NiO. The bands at 240, 445, and 607 cm−1 can be assigned to rutile TiO2. In addition, the peaks located at about 494 and 695 cm−1 are observed in the spectra, corresponding to the shaking peaks of NiO. The results are in agreement with the electrochemical reactions 1 and 2.

4. CONCLUSION A TiO2 /NiO core/shell nanorod array film has been successfully prepared by combining hydrothermal and CBD methods. Compared to the NiO nanoflake film, the TiO2/NiO core/shell nanorod array film exhibits improved electrochromic 6694

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Figure 12. (a) Low magnification and (b) high magnification SEM images of the TiO2/NiO core/shell nanorod array film after 2400 cycles.

properties with larger optical modulation, higher coloration efficiency, and better cycling stability. The improved electrochromic performance is mainly attributed to the synergetic contribution from the core/shell heterostructure, as well as the ordered array geometry, which can facilitate the ion diffusion and increasing the electron transport rate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel.: +86 571 87952856. Fax: +86 571 87952573. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), Key Science and Technology Innovation Team of Zhejiang Province (2010R50013), and a support program of the Ministry of Education of China.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp500699u | J. Phys. Chem. C 2014, 118, 6690−6696