Progress in Natural Science: Materials International 2013;23(2):164–169 Chinese Materials Research Society Progress in Natural Science: Materials International www.elsevier.com/locate/pnsmi www.sciencedirect.com ORIGINAL RESEARCH CNTs/TiO2 composites and its electrochemical properties after UV light irradiation Bin Zhanga, Rui Shia, Yupeng Zhanga, Chunxu Pana,b,n a School of Physics and Technology, and MOE Key Laboratory of Artificial Micro- and Nano-structures, Wuhan University, Wuhan, 430072, China b Center for Electron Microscopy, Wuhan University, Wuhan, 430072, China Received October 2012; accepted January 2013 Available online April 2013 KEYWORDS Carbon nanotubes (CNTs); TiO2; Composite; Supercapacitor Abstract Due to the unique structure and special physical and chemical properties, carbon nanotubes (CNTs) have potential applications in supercapacitors Recently, CNTs and their composites as a kind of supercapacitor electrode material have been made many achievements In this paper, a CNTs/TiO2 composite was prepared successfully with hydrothermal method, and was used as a supercapacitor electrode material After the tests on surface chemistry and electrochemical property, it was found that: (1) the capacitance of the CNTs/TiO2 composite electrode increased by 56%, compared with pure CNTs electrode, (2) after UV light irradiation pretreatment, due to the special photoelectric effect of TiO2 which improves the interfacial property and electrochemical property of the composite electrode, the capacitance further increased by 53% when compared with the electrode without the pretreatment, and meanwhile, the cycle life also increased significantly, i.e., the capacitance was up to 97%, after 100 cycles of charge and discharge, (3) due to the improvement of the interfacial property, the ion transport in the composite electrode became smoother, and the pore utilization was also effectively enhanced during high-current charge and discharge, and (4) due to the generation of a large amount of oxygen-containing groups on the TiO2 surface after UV pretreatment, the CNTs/TiO2 composite electrode earned extra large pseudo capacitance, and therefore the capacitance of the composite electrode was further increased Based on the experimental results in the present study a new process to improve surface character and electrochemical n Corresponding author at: School of Physics and Technology, and MOE Key Laboratory of Artificial Micro- and Nano-structures, Wuhan University, Wuhan, 430072, PR China Tel.: ỵ86 27 68752969; fax: ỵ86 27 68752003 E-mail address: cxpan@whu.edu.cn (C Pan) Peer review under responsibility of Chinese Materials Research Society 1002-0071 & 2013 Chinese Materials Research Society Production and hosting by Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.pnsc.2013.03.002 CNTs/TiO2 composites and its electrochemical properties after UV light irradiation 165 property of the electrode has been developed by using a metal oxide as both pseudo capacitive material and surface modification material of the composites with a UV light irradiation & 2013 Chinese Materials Research Society Production and hosting by Elsevier B.V All rights reserved Introduction Today, when the whole world is facing the growing shortage of energy, the development of new energy and new energy materials is necessary, and the related research work has been a high degree of international attention Supercapacitor, as a new type of energy storage device, earns more and more attention for its unique physical and chemical properties Since the discovery of carbon nanotubess (CNTs) by Iijima in 1991 [1], a lot of research work have been done in the world Up to now, the study of CNTs is still at the forefront of nanotechnology research It is expected to take advantage of this special one-dimensional nanomaterial in application areas including composites [2], energy storage devices [3], field emission [4] as well as molecular devices [5], due to its unique physical and chemical properties In the field of energy storage, CNTs have been considered to be an excellent electrode material for supercapacitor, especially high power supercapacitor, due to its unique hollow structure, good electrical conductivity, large specific surface area, porosity suitable for electrolyte ion transport, and nano-scaled multi-stage pore structure by interactive wound [6] Currently, there are many reports on supercapacitors by directly using CNTs as an electrode material Numerous studies have shown that the pseudocapacitive of capacitor based on Faraday oxidation–reduction reaction exhibited much higher capacity than that of the electric double layer capacitor In general, deposit a layer of materials which have redox activity, such as metal oxide or conductive polymer, on the surface of CNTs would be helpful to effectively increase the capacitance [7] TiO2 has been widely used in photocatalysis, solar cells, electrochromic devices, gas sensors and other areas, because of its superior optical performance and environmental friendliness [8] It also shows a potential application in the field of energy storage, such as on lithium battery electrode [9] In this work, CNTs/TiO2 composite was synthesized by hydrothermal method, and its surface morphology, surface chemical and electrochemical properties, especially the effect of the UV light irradiation on the interfacial and electrochemical properties of the electrodes were studied systematically It shows a great improvement on the electrochemical property and expects expanding the application of TiO2 in supercapacitors Experimental The CNTs/TiO2 composite were prepared by using hydrothermal method CNTs were purchased from Shenzhen Nanotech Port Co Ltd., China The detailed experimental process were described as follows: (1) 300 mg CNTs was placed in an autoclave with 15 mL deionized water, and ultrasonic vibration for dispersing CNTs uniformly, (2) 300 mg titanium sulfate and 300 mg urea were added to the CNTs suspension, and ultrasonic vibration again for 40 for mixing uniformly, (3) The autoclave was placed in an oven and baked at 150 1C for h, (4) After the reaction, the resultant mixture was centrifugation for 10 in the speed of 3500 rpm, and washed respectively with water and ethanol for three times, (5) The last product CNTs/TiO2 composite was dried at 80 1C The morphology of the CNTs/TiO2 composite was observed by using a scanning electron microscope (SEM) (Sirion, FEI, The Netherlands) The crystal structure of the composite was analyzed by using X-ray diffraction (XRD) (D8 Advanced, Bruker AXS, Germany) with parameters involving X-ray source Cu Kα target with wavelength of 1.5406 Å, 2θ angle range 20–801, and scanning speed deg/min For comparison, pristine CNTs were also used The electrodes were made by pressing and the detailed steps were: (1) cut the nickel foam into small pieces of 10 mm  10 mm as a collector, (2) mixed CNTs or the CNTs/TiO2 composites with PTEF at quality ratio of 10:1, and then added ethanol into the mixture and milled for hr, (3) coated the mixure on nickel foam, and pressed them together by using a mold The thickness of the electrodes was controlled within about 0.7 mm In order to study the effect of UV light irradiation on the surface chemical properties, the CNTs/TiO2 composite electrode was placed in a black-box-type UV analyzer After irradiation under 254 nm and 365 nm UV light for h, the contact angle of deionized water on the electrodes sheet were measured, and the results were compared with the samples without UV light irradiation The tests on electrochemical properties were carried out by using an electrochemical workstation (Shanghai Chenhua CHI666C series electrochemical analyzer), including cyclic voltammetry, chronopotentiometry and AC impedance method The working electrode was CNTs electrode The reference electrode was saturated calomel electrode (SCE) The counter electrode was a platinum wire electrode, and the electrolyte was mol/L KOH solution Results and discussion Fig shows SEM morphologies of the electrode chip surfaces of pristine CNTs and the CNTs/TiO2 composite Obviously, the pristine CNTs exhibited a well structure and dispersed evenly, in the CNTs/TiO2 composite, both CNTs and TiO2 nanoparticles were basically well-mixed XRD analysis revealed the apparent diffraction peaks of graphite-state carbon and anatase TiO2, as shown in Fig 2, which meant that the composite has been successfully prepared It is well-known that the narrow band gap (about 3.2 eV) of anatase TiO2 results in its absorption in the ultraviolet light region to produce photo-induced charge carriers, which further generates hydrophilic hydroxyl free radical ( Á OH) In the work, it was found that when compared with the non-irradiation sample, after the UV light irradiation for an hour, the hydrophilicity of the CNTs/TiO2 composite electrode was significantly improved, as shown in Fig 3, and the contact angle decreased from 581 to 211 This result could be ascribed to the formation of a large amount of 166 B Zhang et al Fig SEM morphologies of the supercapacitor electrode (a) and (b) pristine CNTs; (c) and (d) CNTs/TiO2 composite Fig XRD pattern of the CNTs/TiO2 composite Without UV irradiation After UV pretreatment Fig Contact angle test of the CNTs/TiO2 composite electrodes oxygen-containing groups on the TiO2 surface during the UV light irradiation [10–12] That is to say, the compounded TiO2 nanoparticles not only provided the redox pseudocapacitance, but also played a role for modifying the hydrophobicity of the CNTs surface by improving the infiltration of aqueous electrolyte on electrode interface, which enhanced the utilization rate of the electrode material surface and thus improved the electrochemical property of the supercapacitor electrode Although TiO2 itself had a good hydrophilicity, the UV light irradiation could further increase its hydrophilicity at the electrode interface This process exhibited a great importance for obtaining a supercapacitor electrode with more excellent electrochemical performance Fig illustrates the cyclic voltammeter (C–V) curves of the pristine CNTs electrode, CNTs/TiO2 electrode and UV light irradiated CNTs/TiO2 electrode, respectively The curves were measured at parameters involving potential window −0.9 V–0 V and scanning speed 5, 20, 50, 100, 200 mV/s, respectively Obviously, all electrodes exhibited perfect symmetry at different scanning speeds with relatively smooth curves, and a good rectangular shape, especially in the case of low speed It was implied that the electrodes were of excellent electric double layer capacitor characteristics Fig shows cyclic voltammeter curves of three electrodes in a scanning speed of 20 mV/s From comparison of the area surrounded by the curves, the capacitance of the CNTs/TiO2 composite electrode had a larger increasing than that of the pristine CNTs electrode, and the capacitance of UV light irradiated composite electrode were further improved It was noted that there were some redox peaks in the cyclic voltammeter curves Especially, at the low scanning speed, the redox peaks apparently appeared in the ranges of −0.3 V–−0.2 V and −0.6 V to −0.5 V, and the peaks was particularly prominent in the UV light irradiated CNTs/TiO2 electrode curve Obviously, the presence of some oxide groups and TiO2 particles on the surface of CNTs played a key rule on inducing redox reaction and contributing to the part of the pseudo capacitive From Fig 6, we could see that compared with the pristine CNTs electrode, the electrochemical capacity of the composite has been greatly improved, which meant that TiO2 nanoparticles provided a big pseudocapacitance Fig shows the capacitance variations of three electrodes at different scanning speeds With increase of the scanning speeds, the charge–discharge rate raised and the capacitance reduced, which particularly became more obvious at the low scanning speed For example, compared with the value at mV/s, the capacitance of the CNTs/TiO2 composites and its electrochemical properties after UV light irradiation 167 Fig Capacitance variations of the electrodes at different scanning speeds Fig Cycle life of the electrodes Fig Cyclic voltammeter curves of the electrodes (a) pristine CNTs electrode; (b) CNTs/TiO2 composite electrode; and (c) UV light irradiated CNTs/TiO2 composite electrode Fig Cyclic voltammeter curves of the electrodes at scanning speed 20 mV/s pristine CNTs electrode at 200 mV/s was only 46%, which was the worst property among three electrodes The capacitance of the regular CNTs/TiO2 composite electrode maintained 54% value of the original capacitance (200 mV/s) exhibiting a better electrochemical performance It was importance that after the UV light irradiation, the capacitance of the CNTs/TiO2 composite electrode exhibited an obvious decrease at high scanning speed (about 48% of capacitance at low scanning speed) This is because at high scanning speed the oxygen-containing functional groups on the TiO2 surface made it difficult to have a redox reaction, and therefore, the pseudo capacitive could not be presented The cycle lives of three electrodes were measured by using cyclic voltammetry At scanning speed of 200 mV/s, the charge– discharge cycles were run for 100 times The specific capacitances (F/g) were calculated at the 1st, 10th, 20th, 50th, 80th, 100th cycles, respectively, as shown in Fig It was revealed that the UV light irradiated composite electrode exhibited the most superior specific capacitance by all cycles In addition, the capacitances of the pristine CNTs electrode and the CNTs/TiO2 composite electrode reduced gradually along with the cycle number increase, while the capacitance of the UV light irradiated CNTs/TiO2 composite electrode almost kept in stable, except a decreased at the beginning, which demonstrated its excellent long cycle life It was found that even after 100 times charge–discharge cycles, the capacitance of the UV light irradiated electrode was up to 97% of the original value Fig shows the constant current charge–discharge curves of the electrodes by chronopotentiometry method The current density was 0.25 A/g, and the range of voltages was from −0.9 V to V The curves were of good symmetry and indicated a typical double layer capacitor The calculation from the curves 168 B Zhang et al Fig 10 Fig Constant current charge–discharge curves of the electrodes Nyquist plots of the electrodes electrode/electrolyte interface, increasing hydrophilicity of the electrode, and thus accelerating the ion transmission, which effectively increased the utilization of the electrode porosity under large current charge–discharge cycles Fig Relationships between capacitance and current density of the electrodes revealed that the specific capacitances of the UV light irradiated CNTs/TiO2 composite electrode, regular CNTs/TiO2 electrode and pristine CNTs electrode were 6.89 F g−1, 4.57 F g−1 and 2.5 F g−1, respectively Clearly, the UV light irradiation effectively enhanced the electrochemical properties of the CNTs/TiO2 composite electrode, which was consistent with the results from the cyclic voltammetry Fig illustrates the curves of the specific capacitance (F/g) which were calculated at different current density 0.25 A/g, 0.5 A/g, 2.5 A/g and A/g In general, with increasing of the current density, the specific capacitance of the electrodes gradually reduced This was because of the greater the current density, the faster the rate of charging and discharging, and the bigger the resistance on ion transport, which therefore resulted in difficulty for the ion entering into the deeper pores, and led to decrease of the electrode capacitance However, the following phenomena were found 1) From 0.25 A/g to A/g, the pristine CNTs electrode exhibited the slowest decrease, while the CNTs/TiO2 composite electrodes declined fast The reason was that the pristine CNTs electrode was based upon an electric double layer capacitor with a fast charge-discharge rate, and the CNTs/ TiO2 composite electrodes produced a pseudocapacitive reaction with a slow response during large current chargedischarge process 2) For the CNTs/TiO2 composite electrodes, the specific capacitance of the non-irradiated electrode at A/g current density was only 24% of the value at 0.25 A/g; while the irradiated electrode was 32% Obviously, its charge-discharge performance was improved at large current These results indicated that the UV light irradiation provided advantages for improving the Fig 10 illustrates the Nyquist plots of the electrodes with real part Z' of impedance as X-axis and the inverse of imaginary part –Z" as Y axis It shows a typical AC impedance of supercapacitor with semicircular curves in high frequency region and inclined straight lines in low frequency region The diameter of the semicircle in high frequency region represented the polarization resistance Rp, which generally associated with the Faraday pseudocapacitive reaction and pore structure of electrode materials [13] From the plots, the UV light irradiated CNTs/TiO2 composite electrode had the largest polarization resistance, because of its largest Faradic pseudo capacitance from the oxidation-reduction reaction of TiO2, and also the oxygen-containing groups on the TiO2 surface In above experiments, different methods were used tested the electrochemical properties of the electrodes including cyclic voltammetry, constant current charge-discharge and AC impedance Each method had its own characters on characterization of the electrochemical properties of supercapacitor electrodes from different physical and chemical aspects, and the values may be varied with different parameters However, all the data showed the same trends of variations for different materials, as listed in Table Overall, the average capacitances of the pristine CNTs electrode, regular CNTs/TiO2 composite electrode and UV light irradiated CNTs/TiO2 composite electrode were 4.1 F/g; 6.4 F/g and 9.8 F/g, respectively Conclusions A novel and effective route to get a high-capacity supercapacitor by UV light irradiating the CNTs/TiO2 composite electrode was introduced in this paper This may be attributed to that the UV light irradiated TiO2 had a good hydrophilicity and provided a rule for interface modification, and hence a simple UV light irradiation could achieve a significant improvement on electrochemical performance of the CNTs/TiO2 composite electrode In addition to the supercapacitor electrode material, the present process also presents a great potential application in the fields of lithium-ion batteries, fuel cells, dye-sensitized solar cells, and photoelectric catalytic CNTs/TiO2 composites and its electrochemical properties after UV light irradiation Table 169 Capacitances calculated from different measures (F/g) CNTs CNTs/TiO2 UVỵ CNTs/TiO2 Cyclic voltammetry (2 mV/s) Chronopotentiometry (0.25 A/g) AC Impedence method (0.1 Hz) Average value 5.8 7.9 11.9 2.5 4.6 6.9 4.1 6.6 10.7 4.1 6.4 9.8 Acknowledgments This research was supported by the National Natural Science Foundation of China (No 11174227) and the Fundamental Research Fund for the Central Universities (2011202020003) References [1] S Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58 [2] R.H Baughman, A.A Zakhidov, W.A de Heer, Carbon nanotubes— the route toward applications, Science 297 (2002) 787–792 [3] A Kiebele, G Gruner, Carbon nanotube based battery architecture, Applied Physics Letters 91 (2007) 144104 [4] W.A De Heer, A Chatelain, D.A Ugarte, Carbon nanotube fieldemission electron source, Science 270 (1995) 1179–1180 [5] Q Bao, J Zhang, C Pan, J Li, C Li, D Tang, recoverable photoluminescence of flame-synthesized multiwalled carbon nanotubes and its intensity enhancement at 240 K, Journal of Physical Chemistry C 111 (2007) 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