Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High

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Atomic Layer Deposited of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors Qing-Hua Ren, Yan Zhang, Hong-Liang Lu, Yong-Ping Wang, WenJun Liu, Xin-Ming Ji, Anjana Devi, An Quan Jiang, and David Wei Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13392 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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ACS Applied Materials & Interfaces

Atomic Layer Deposited of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors †

†

†

†

†

Qing-Hua Ren , Yan Zhang , Hong-Liang Lu ,*, Yong-Ping Wang , Wen-Jun Liu , Xin-Ming Ji†, Anjana Devi ‡,*, An-Quan Jiang†, David Wei Zhang†,* †

State Key Laboratory of ASIC and System, Shanghai Institute of Intelligent

Electronics & Systems, Fudan University, Shanghai 200433, China

‡

Inorganic Materials Chemistry, Ruhr-University Bochum, 44780 Bochum, Germany

*E-mail: [email protected] (H. L. Lu), [email protected] (A. Devi), and [email protected] (D. W. Zhang)

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT A novel hybrid core-shell structure of ZnO nanowires (NWs)/Ni as pseudocapacitor electrode was successfully fabricated by atomic layer deposited of nickel shell and its capacitive performance was systemically investigated. Transmission electron microscopy and x-ray photoelectron spectroscopy results indicated the NiO was formed at the interface between ZnO and Ni where the Ni was oxidized by ZnO during the ALD of Ni layer. Electrochemical measurement results revealed that the Ti/ZnO NWs/Ni (1500 cycles) electrode with a 30 nm thickness of Ni-NiO shell layer had the best supercapacitor properties including ultrahigh specific capacitance (~2440 F g-1), good rate capability (80.5%) under high current charge-discharge conditions, and relatively better cycling stability (86.7% of the initial value remained after 750 cycles at 10 A g-1). These attractive capacitive behaviors are mainly attributed to the unique core-shell structure and the combined effect of ZnO NW arrays as short charge transfer pathways for ion diffusion and electron transfer, and conductive Ni serving as channel for the fast electron transport to Ti substrate. This high performance Ti/ZnO NWs/Ni hybrid structure is expected to be one of a promising electrodes for high-performance supercapacitor applications.

KEYWORDS: supercapacitors, nickel oxide, ZnO nanowire arrays, hybrid core-shell structure, pseudocapacitor electrode, atomic layer deposition, high specific capacitances


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INTRODUCTION The escalating problems of energy sources depletion and environment pollution stimulate the society to pay close attention to exploring sustainable and renewable resources.1,2 As a result, the most important energy solution strategies are to develop the devices, which can effectively harvest and convert renewable energy.3-8 The supercapacitor, also known as electrochemical capacitor, has attracted a lot of attention as a promising energy storage device due to its unique properties such as low cost, high power density, short charging time, superior rate capability, and excellent stability.6,7,9,10 It can be divided into two types according to their energy storage mechanism, electrical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are electrochemical capacitors in which energy storage predominantly is obtained by double-layer capacitance. They generally operate with stable performance even after many charge-discharge cycles. The pseudocapacitors rely on the reversible redox reactions during the charge-discharge process. Usually, the pseudocapacitor can provide 10-100 times more capacitance than the EDLCs due to their high energy transfer during the faradic reaction.11 Transition metal oxides have been always employed as electrode materials for pseudocapacitor applications because they all have various oxidation states, which benefit the faradic redox reactions.12-16 Among them, NiO has been considered as one of the most attractive pseudocapacitive materials because of its high theoretical capacitance, high chemical and thermal stability,17-19 distinct redox reaction and controllable morphology.20 The charge storage ability for the NiO-based pseudocapacitors strongly depends on the properties



of

electrode

including

morphologies,

surface-to-volume

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areas,

and

band

alignments.17-20 It is reported that the highest specific capacitance of 439 F g−1 (50 mV s−1) can be obtained by using a nanocrystalline NiO layer as electrode.18 A specific capacitance of 266 F g−1 (0.1 A g−1) was achieved when NiO nanotubes were used to be the electrodes.21 Zhang et al. obtained a specific capacitance of 390 F g−1 (5 A g−1) by using porous NiO nanocolumns.23 However, all of the reported specific capacitance values are much less than the theoretical values of the NiO-based pseudocapacitors. This may be ascribed to their poor electrical conductivity, which will slow electrons transport at high electrical rates. It will degrade the capacitive performance of the device and limit its practical application.24 In addition, a high weight percentage loading of NiO on the electrode will lead to the dense package on the ZnO surface. It will decrease the reactive areas for the efficient faradic redox reaction and increase the contact resistances. Both of them will degrade the specific capacitance values for the corresponding devices. To improve the performance of the devices, it is essential to design and fabricate novel hybrid electrodes with large surface area and high rate of ion diffusion and electron transportation.25,26 The nanostructures such as nanofibers, carbon nanotubes, core-shell nanowires have been employed to improve the performance of NiO based pseudocapacitors.27-29 The ZnO nanowires (NWs) have been reported to be the common support nanostructure for coating active electrode nanomaterials due to their large specific area, good mechanical flexibility, and chemical stability,.30-32 They can also provide efficient electron transfer pathway. Unfortunately, the specific capacitance value for the


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reported NiO supercapacitors based on the ZnO NWs is relatively low.33,34 As mentioned above, the quality of the electrode materials affects the performance of the capacitance. Exploring a proper approach to fabricate the novel composite electrodes is very important. Recently, the Ni-NiO composites have attracted increasing interest for their application in the supercapacitors. The Ni in the composite can improve the electrical conductivity of NiO and provide channels for the electrons transporting efficiently to the Ti current collector. The pseudocapacitor can be enhanced accordingly.35,36 Generally, there are two methods to prepare Ni-NiO composite. One is the metal Ni layer deposited and then calcinated in the oxygen atomosphere to obtain Ni/NiO composites. The other one is the NiO layer fabricated and then reduced to Ni in the hydrogen atmosphere.37-39 Obviously, both of them are relatively complicated. Herein, a novel hybrid ZnO NWs/Ni core-shell structure have been achieved by a combined method. The ZnO NWs were grown by a typical hydrothermal growth on a titanium (Ti) substrate with atomic layer deposited (ALD) ZnO seed layer. An ultrathin Ni shell layer was then deposited onto the ZnO NWs by a plasma enhanced ALD technique. The ALD technique can prepare an extremely uniform and conformal hybrid core-shell structure with low defect density even on high aspect ratio structures through a sequence of self-limiting surface chemistry reactions.40-42 The hybrid electrode was used as electrochemical anodes for the supercapcitor. The electrochemical measurements show that the fabriacted electrode exhibits excellent capacitive performance due to its large surface areas and fast charge transfer pathways



of ZnO NW arrays for faradic reactions.

EXPERIMENTAL SECTION Reagents The Ti sheets with 99.99+% pure and 0.1 mm thick were used as substrates. Diethyl zinc (Sigma Aldrich, 99.999%), zinc nitrate hexahydrate (Sigma Aldrich, 99.999%), hexamethylenetetramine (Merck, 99.5%), bis (cyclopentadienyl) nickel (Sigma Aldrich, 99.999%), high-purity NH3 (99.999%) were employed as reactants or precursors. All other chemical reagents were of analytical grade and were used without further purification. High pure N2 (99.999%) ware used as carrier and purge gas during the ALD process. The deionized water (DI water) was provided by a Millipore Q purification system with a resistivity value over 18 MΩ.cm.

Preparation of Ni/NiO on ZnO NWs on Ti sheets Firstly, the Ti sheets (1×1 cm2) were polished by a SiC abrasive paper. They were then dipped in HCl (5%) solution for 5 min. After that, they were cleaned ultrasonically with acetone, ethanol, and DI water for 10 min, respectively. A 20 nm thick ZnO seed layer was deposited on the cleaned Ti sheets using ALD. The ZnO NWs were then obtained using a typical hydrothermal method described in our previous works.42 The ultrathin Ni layer was prepared at 260 °C on the ZnO NWs by plasma-assisted ALD with NiCp2 and NH3 plasma as precursors. The temperature of NiCp2 was kept at 80 °C and the NH3 plasma was generated under a power of 3000 W.


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High pure N2 was used as carrier and purge gases. The flow rate of N2 was kept at 50 sccm. An ALD of Ni growth cycle consisted of a 2 s NiCp2 pulse, a 5 s N2 purge, a 20 s NH3 plasma pulse, and a 6 s N2 purge. The base pressure in the deposition chamber was about 1200 Pa during the growth process. The thickness of of Ni shell layers can be controlled via changing the ALD cycles. In this work, 800 and 1500 ALD cycles were used to prepare the corresponding Ni layer. Samples Ti/ZnO NWs/Ni-800 cys and Ti/ZnO NWs/Ni-1500 cys are denoted as Z-N 800 and Z-N 1500, respectively. For comparison, ZnO NWs without Ni layer was also prepared.

Characterization Characterization of materials A GES-5E spectroscopic ellipsometer was used to characterize the thickness of ALD films. The crystalline structures of the prepared samples were determined by a Bruker D8 Advance X-Ray Diffractometer using Cu Kα (λ=1.54 Å) radiation with the voltage and current of 40 kV and 40 mA, respectively. The morphology and microstructure were investigated using a field-emission scanning electron microscope (SEM, JEOL JSM-6700F) and a transmission electron microscope (TEM, Tecnai F20) at 200 kV. The chemical bonding states were characterized using a PHI 5000 ESCA X-ray photoelectron spectroscopy (XPS) with a monochromated Al Ka X-ray source (1486.6 eV). The binding energy was calibrated by using the C 1s peak at 284.8 eV as the reference. To obtain the mass of the active Ni-NiO, thermogravimetric analysis and an inductively coupled plasma atomic emission spectroscopy were carried out. The



thermogravimetric analysis was performed using a Pyris 1 TGA system in air with a flow rate of 40 mL/min. The heating temperatures were ranging from 25 to 600 °C with a ramping rate at 5 °C/min. The total masses of Ni were measured by using an inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 8000). The specific capacitance of the two electrodes was calculated based on the total weight of the active Ni-NiO layer without ZnO NWs.

Electrochemical measurements All the electrochemical measurements were carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China). A standard three-electrode cell with the sample on Ti sheet was directly used as the working electrode. The Pt sheet and Hg/HgO electrode were selected as the counter electrode and reference electrode, respectively. The as-grown Z-N 1500 sample and the corresponding test device were shown in Figure S1 (Electronic Supplementary Materials (ESI), Figure S1). The electrolyte used was a 1 M KOH aqueous solution. Cyclic voltammetry (CV) experiments were operated at a voltage range from 0 to 0.6 V vs. Hg/HgO at a scan rate of 10-100 mV s-1. The galvanostatic charge-discharge (GCD) processes were tested by cycling the potential from 0 to 0.6 V at different current densities. The working electrodes for the prepared samples were immersed in the electrolyte for 40 min before the measurement. Electrochemical impedance spectroscopy (EIS) analyses were also conducted to measure the conductivity of composites in a frequency range from 10 Hz to 10 kHz with AC amplitude of 5 mV.


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RESULTS AND DISCUSSION The crystal phases of ZnO NWs coated with or without Ni shell grown on Ti substrates were analyzed firstly. As shown in Figure 1, a prominent ZnO (002) peak located at 34.8° was observed, indicating that the prepared ZnO NWs have a highly c-axis-oriented hexagonal wurtzite structure. A new peak with a 2θ of 44.5° can be found after the ultrathin Ni layer depositing on ZnO NWs. It is assigned to Ni (111) plane of the face-centered-cubic structure of Ni crystal (JCPDS No. 04-0850). In addition, no NiO related peak can be detected. Figure 2 shows the SEM images of ZnO NWs/Ni samples. As can be seen from Figure 2a, the bare ZnO NWs are distributed homogeneously on Ti substrates which have a relatively smooth surface with an average diameter of 40 nm and a planar hexagonal faceted top morphology (the inset of Figure 2a). The mean diameter increases to around 70 nm and 100 nm for the Z-N 800 and Z-N 1500 samples (Figure 2b,c), respectively. The core of ZnO NWs was uniformly coated by the Ni shell layer. It demonstrates the merit of ALD method to prepare uniform and conformal film on the high aspect ratio surface. The average growth rate of the Ni layer is determined to be 0.2 Å per ALD cycle. In addition, not only the hybrid arrays become dense but also the top hexagonal facets of ZnO NWs turn to be circular after the growth of the Ni shell layer. The cross-section morphologies of as-grown ZnO NWs and Z-N 800 were showed in Figure S2 (ESI, Figure S2). Figure 2d shows the cross-section morphologies of the Z-N 1500 sample. Typically, a spiral structure can be observed in



inset of Figure 2d. To further study the microstructures of the prepared ZnO/Ni core/shell arrays, TEM measurements were carried out. Figure 3a shows a typical bright-field TEM image for a single ZnO NWs. The diameter for the bare ZnO NWs depicted in the low-magnification image is around 43 nm. Figure 3b is a high-resolution TEM (HRTEM) image of the same ZnO NW, which indicates that ZnO exhibited ordered fringes. The d-spacings of 0.261 nm and 0.280 nm marked in Figure 3b correspond to ZnO (002) and (100) planes of wurtzite structure, respectively.42 The corresponding selected area electron diffraction (SAED) pattern was given in Figure 3c. It further testifies that the obtained ZnO NWs exhibit single-crystal characteristics with a wurtzite structure and are directed along the [0001] direction. Figure 3d gives a low-magnification TEM image of Z-N 800 sample. The diameter of the ZnO NW “core” is detected to be around 46 nm in the hybrid nanostructures. It was uniformly wrapped a Ni “shell” (~16 nm). Figure 3e shows HRTEM image of the same sample. The marked interplanar d-spacing of 0.20 nm in the shell region corresponds to the (111) plane of Ni. In addition, the lattice fringes of 0.24 and 0.21 nm are derived from the (111) and (200) crystalline planes of cubic NiO, respectively. It suggests the coexistence of Ni and NiO in the shell layer. The formed NiO was proposed to result from the oxidation by ZnO NWs during the process of ALD Ni. Therefore, a Ni-NiO shell layer can be fabricated through a single ALD process. Moreover, the SAED pattern corresponding to Figure 3e is shown in Figure 3f. It further confirmed that the hybrid electrode are composed of a single-crystalline ZnO NWs core and a


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polycrystalline NiO and Ni shell layer. Figure 4a gives the high-angle annular dark field (HAADF) scanning image of a single Z-N 800 sample. The insert shows the corresponding TEM image. The mass contrast between ZnO core and the Ni-NiO shell is not obvious in the HAADF image as the mass of Ni is similar to Zn. However, the interface can be clearly distinguished from the inserted TEM image. The elemental composition and spatial elemental distributions of the Z-N 800 sample is also investigated by scanning elemental mapping and x-ray energy dispersive x-ray spectroscopy (EDS). Figures 4b-d present the elemental distribution maps for the line scanning along the red line in Figure 4a. The elemental maps demonstrate the consistency of the distribution for the elements Zn, Ni. and O. It can be found that the Zn element is mainly distributed at the core region which resulting from the ZnO NW core while the Ni element exhibits two prominent peaks in the shell layer region attributed to ALD deposited Ni. The O shows a broad spectrum throughout the entire cross section of the core-shell NW. It is interesting to find that the O distribution at the interface between ZnO core and Ni shell increases sharply in a small range. The increased O is resulting from the NiO in shell layer. Thus, this result further confirms the coexistence of Ni and NiO in the shell layer by a single ALD process. The XPS measurements were further performed to investigate the elemental composition and chemical bonding state of the obtained ZnO/Ni hybrid structure. The survey X-ray photoelectron spectra of Z-N 200 and Z-N 800 are showed in Figure S3 (ESI, Figure S3), which indicates that C, O, Ni existed in both the Z-N 200 and Z-N



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800 samples. The absence of Zn element in Z-N 800 samples was ascribed to the detection limitation. Figure 5a-b shows the Ni 2p and O 1s XPS spectra for the prepared Z-N 800 sample, respectively. The Ni 2p spectrum (Figure 5a) can be deconvoluted into six peaks. The two peaks located at 852.2 and 869.4 eV (red line) are assigned to Ni 2p 3/2 and Ni 2p 1/2 of Ni0 state, respectively. Other four peaks (green line) are the characteristics of Ni2+ state.43,44 The XPS spectra also confirm the coexistence of Ni0 and Ni2+ in the shell layer. Figure 5b gives the O 1s spectrum, which can be deconvoluted into two peaks. The peak located at 530.7 eV is derived from O2- in O-Ni bond18 and the other one located at 532.1 eV is probably resulted from physically and chemically absorbed H2O at or near the surface of Ni-NiO composite.45 Therefore, all the above XPS results fully validate the coexistence of Ni and NiO in the shell layer. The trochemical performance of the prepared Z-N 800 and Z-N 1500 electrodes was tested in a typical three-electrode cell at room temperature. Figure 6a presents the cyclic voltammetry (CV) curves of ZnO NWs, Z-N 800 and Z-N 1500 electrodes collected at scan rate of 50 mV s-1. It can be seen that there is no redox peaks occur on the Ti/ZnO NWs sample, suggesting the capacitance contribution from the Ti substrate and ZnO NWs is negligible. In contrast, the CV curves for both samples show a pair of well-defined redox peaks, indicating the good redox transitions due to the reversible faradaic reactions of Ni2+ ↔ Ni3+ on the electrode surface. The electrochemical reaction can be expressed by,46 NiO + OH- ↔ 2NiOOH + e-


(1)

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In addition, the forward and reverse scan curves are highly symmetrical, suggesting the good pseudocapacitive characteristics of both electrodes. Figure 6b and c present the CV curves of Z-N 800 and Z-N 1500 electrode at various scan rates ranging from 10 to 100 mV s-1, respectively. Each of the CV curves shows a pair of distinct redox peaks during the sweeps. The oxidation peak results from the conversion of NiO to NiOOH (Anodic process), whereas the reduction peak is simply due to the reverse reaction of Ni3+ to Ni2+ (Cathodic process).The appearance of these redox peaks is involved in the above Equation (1). When the voltage scan rate changes from 10 to 100 mV s−1, the potentials and the peak current intensities of both oxidation peaks and reduction peaks shift to larger absolute values. This demonstrates the kinetics of faradic redox reactions that the ZnO/Ni core/shell structure exhibits larger activated areas and the rapid electronic/ionic transportation rate.47 Furthermore, the shape of cathodic and anodic peak curves over the entire range of scan rates has not changed significantly, indicating the excellent reversibility of the electrodes. 17,48 The Z-N 1500 electrode exhibited the similar results as the Z-N 800 electrode discussed above but presented a larger enclosed area due to the large contact surface between the thick Ni-NiO and electrolyte. It is known that the specific capacitance (Csp, F g-1) is one of the most crucial aspects to evaluate the performance of capacitive electrode. Therefore the Csp of Z-N 800 and Z-N 1500 have been calculated from the enclosed area of their respective CV curves according to the following equations,49,50

C =

ν

I VdV


(2)


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where Csp (F g-1), m (g), ν (V s−1), Vc and Va (V), and I (A) are the specific capacitance, the mass of the active materials loaded in the electrode (here is the mass of Ni-NiO), potential scan rate, high and low potential limit of the CV tests, and the instant current on CV curves, respectively. Therefore, the specific capacitance of the two electrodes as a function of scan rate is calculated from the CV curves in Figure 6b and Figure 6c. The obtained values are presented in Figure 6d. The Z-N 1500 electrode exhibits a higher specific capacitances than the Z-N 800 electrode at the same scan rate and yields the highest specific capacitance of ~2440 F g-1 at the scan rate of 10 mV s -1, which is very close to the theoretical value.19 In addition, the value decreases at first and then remains stable as the scan rate increases. This decrease is ascribed to the limitation of the ion diffusion process at higher scan rates. At the higher scan rate of 100 mV s-1, the specific capacitance value of Z-N 1500 electrode remains almost ~1573 F g-1 and ~1497 F g-1 for the Z-N 800 electrode. In order to further evaluate the electrochemical performance and confirm the superior supercapacitive properties, GCD measurements of the fabricated electrodes were recorded at various current densities over a potential range of 0-0.6 V. The charge-discharge curves of the Z-N 800 and the Z-N 1500 electrodes are presented in Figure 7a and 7b, respectively. The upward lines correspond to charging process and downward ones stands for discharging process. Evidently, all of the charge-discharge curves

exhibited

symmetric

shapes,

indicating

a

typical

pseudocapacitive

characteristics and excellent reversible redox reaction.50 This result is in line with the result from the CV curves (Figure 6a). Moreover, the specific capacitance value Csp of


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both electrodes can further be derived from the discharging times using the following equation:51

C =

∆

(3)

∆

where Csp (F g-1) is the specific capacitance, I (A) stands for the discharge current, and m (g), ∆V (V) and ∆t (s) represent the mass of active materials, the potential range and total discharge time required for the entire voltage range of ∆V. The value of I/m is the discharge current density. The specific capacitance (Csp) as a function of discharge current density for Z-N 800 electrode and Z-N 1500 electrode calculated from Figure 7a and 7b are illustrated in Figure 7c. According to Eq. (3), the average Csp values of Z-N 1500 electrode are calculated to be

~2079, ~2054, ~2030, ~1902,

~1727 and ~1674 F g-1 at a discharge current density of 6.41, 10.7, 21.4, 42.7, 107 and 214 A g-1, respectively. For the Z-N 800 sample, the corresponding values are ~1757, ~1756, ~1707, ~1640, ~1533 and ~1433 F g-1 at the discharge current density of 10, 12, 20, 40, 100 and 200 A g-1, respectively. It is obvious that the Z-N 1500 electrode has a higher specific capacitance than the Z-N 800 electrode at the same discharge current. The increment of active Ni loading is supposed to increase the contact area between active NiO and electrolyte. The maximum specific capacitance value from CV in this work is greater than the reported values for the NiO-based materials prepared by other methods.52-58 In addition, the Csp for both electrodes decreases as the current density increases, which might be due to low current density allowing ions to have sufficient time to diffuse and migrate into the active materials for capacitance. However, at higher current density, only a part of active material is



utilized in redox reactions due to slow diffusion and limited migration of the electrolytic ions.59 It is worth noting that the Csp value of Z-N 1500 electrode decreases to ~1674 F g-1 as the discharge current density increases to 214 A g-1. Similarly, the value turns to be ~1433 F g-1 at the discharge current density of 200 A g-1 for the Z-N 800 electrode. Nearly 80.5% and 81.6% capacity retentions have been obtained for both electrodes, respectively. This result indicated our electrodes have excellent rate capability or capacitive behaviors under high current charge-discharge conditions. Figure S4 (ESI, Figure S4) also presents the EIS for the two samples. The Z-N 1500 electrode displays lower Warburg impedance, suggesting its better capacitive behavior. Figure 7d shows the cycling stability of the Z-N 1500 electrode as a function of the cycle numbers, which was investigated using a long-term GCD process in 1 M KOH aqueous electrolyte for 750 cycle. Noticeably, the specific capacitance of the Z-N 1500 electrodes increased slightly at first and then decreased after 500 cycles. The maximum values increased to 103.6% for Z-N 1500 at a current density of 10 A g-1. In addition, the first ten cycles and the last ten cycles of 750 GCD curves are depicted in the inset of Figure 7d. To be clearly, the first and the 750th GCD curves are depicted in Figure S5 (ESI, Figure S5). It can be seen that the Z-N 1500 electrode remained at 1531 F g-1, 86.7 % of the initial capacitance, which indicates relatively better cycling stability of the electrode. The shapes of the first and the last ten GCD cycle curves of the sample changed a little, suggesting a high columbic efficiency of the electrode.


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Based on the above results, the supercapacitors based on the Ti/ZnO NWs/Ni-NiO core/shell electrodes exhibit large Csp, excellent rate capability. The inherent mechanism can be attributed to the following factors. First, the ZnO films directly grown on Ti sheet have outstanding chemical stability and good electrical conductivity. It can strengthen the structural integrity of the core during the charge-discharge process. The provided charge transfer pathways help speed up the ion diffusion and electron transfer, which will improve the rate capability. Secondly, the Ni-NiO shell layer grown on the ZnO NWs with a large surface area can act as excellent conductive highways for charge accumulation and electrons transfer. It will enhance the transportation of electrolyte ions, which is very crucial to the high-power energy storage devices. Most importantly, the Ni-NiO shell layer can be uniformly grown on the whole ZnO NWs by ALD. It will provide enough active materials NiO for faradaic redox reactions. What’s more, the prepared Ni can enhance the electron transporting efficiently to the current collector (Ti), which will increase the pseudocapacitance of the device.

CONCLUSIONS In summary, a supercapacitor electrode based on ZnO NWs/Ni was successfully prepared onto a Ti substrate by a facial hydrothermal approach combined with ALD technique. Electrochemical investigations revealed the Z-N 1500 hybrid electrode with a 30 nm thickness of Ni-NiO shell layer has an ultrahigh specific capacitance value of ~2440 F g-1 at the scan rate of 10 mV s -1. It can be retained about ~1573 F



g-1 even at the scan rate of 100 mV s -1. While this value is ~2079 F g-1 at the discharge current density of 6.41 A g-1 and can be retained ~1674 F g-1 at the discharge current density of 214 A g-1 obtained from the GCD curves. The obtained electrode also shows good rate capability at high current density and relatively better cycling stability even after continuous charge-discharge process for 750 cycles at the high current density of 10 A g-1. These attractive capacitive behaviors is attributed to the ZnO NWs, which serve as the large platform areas for Ni-NiO loading. Moreover, they provide fast charge transfer pathways for charge accumulation, electrons transfer and ion diffusion. In addition, the pseudocapacitance can be enhanced by the conductive Ni in the Ni-NiO composite.

AUTHOR INFORMATION Corresponding Authors *H. L. Lu. E-mail: [email protected]. Tel./Fax: +86-21-65642457. *A. Devi. E-mail: [email protected]. Tel.: +49-234-3224150. *D.W. Zhang. E-mail: [email protected]. Tel./Fax: +86-21-65642389. Author Contributions All authors have given approval to the final version of the paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No.


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61376008, U1632121, and 61674044), National Key Basic Research Program of China (No. 2014CB921004), the Project supported by State Key Laboratory of Luminescence and Applications (SKLA-2016-16), the Innovation Program of Shanghai Municipal Education Commission (14ZZ004), and the Program for Professor of Special Appointment (Eastern Scholar) in Shanghai.

Electronic

Supplementary

Material:

Supplementary

material (the

optical

photographs of the Ti/ZnO NWs/Ni-NiO composite electrode, representative SEM images of the cross-section of ZnO NWs and Z-N 800 samples, survey XPS spectra, Nyquist plots of electrochemical impedance spectroscopy for the two electrodes, and the first and the 750th GCD curve is available in the online version of this article.



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Figure Captions Figure 1. XRD patterns of Ti substrate, Ti/ZnO NWs, and Ti/ZnO NWs/Ni-NiO with different ALD cycles for Ni-NiO shell layer. Figure 2. SEM images of (a)Ti/ZnO NWs, Ti/ZnO NWs/Ni-NiO with different ALD cycles for Ni-NiO shell layer (b)800, (c)1500, and (d) cross section corresponding to (c). The insets show the corresponding enlarged images, respectively. Figure 3. (a) Low-magnification and (b) high-resolution TEM images of the bare ZnO NWs, (d) low-magnification and (e) high-resolution TEM images of the Z-N 800 electrode, and the SEAD patterns (c) and (f) corresponding to (b) and (e), respectively. Figure 4. (a) HAADF image of the Z-N 800 sample. Inset image shows the TEM image corresponding to (a). (b-d) EDS line scanning profiles of the elements of Zn, Ni, and O across the core-shell NW (red line in the HAADF). Figure 5. XPS spectra of (a) Ni 2p and (b) O 1s of the Z-N 800 electrode. Figure 6. (a) CV curves of the Ti/ZnO NWs, Z-N 800 and Z-N 1500 electrodes collected at a scan rate of 50 mV s-1 in a three-electrode system and CV curves of (b) Z-N 800 electrode, (c) Z-N 1500 electrode at various scan rates, and (d) Csp of the two electrodes as a function of the scan rate. Figure 7. Galvanostatic charge-discharge curves of (a) Z-N 800 and (b) Z-N 1500 electrodes at various current densities, (c) Csp of the two electrodes as a



function of current density, and (d) cyclic stability performance of the Z-N 1500 electrode. Insert shows the first and the last ten cycles of 750 GCD cycles.


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Figure-1 130x105mm (300 x 300 DPI)



Figure-2 71x54mm (300 x 300 DPI)


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Figure-3 351x234mm (300 x 300 DPI)



Figure-4 196x167mm (300 x 300 DPI)


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Figure 5 158x61mm (300 x 300 DPI)



Figure 6 160x122mm (300 x 300 DPI)


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Figure 7 162x121mm (300 x 300 DPI)



Abstract Graphic, TOC 35x10mm (300 x 300 DPI)


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Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High

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