Ultrasonochemical-Assisted Synthesis of CuO Nanorods with High Hydrogen Storage Ability

Uniform CuO nanorods with different size have been synthesized in a water-alcohol solution through a fast and facile ultrasound irradiation assistant route. Especially, the as-prepared CuO nanorods have shown a strong size-induced enhancement of electrochemical hydrogen storage performance and exhibit a notable hydrogen storage capacity and big BET surface area. These results further implied that the as-prepared CuO nanorods could be a promising candidate for electrochemical hydrogen storage applications. The observation of the comparison experiments with different concentrations of NaOH, ethanol, CTAB, and HTMA while keeping other synthetic parameters unchanged leads to the morphology and size change of CuO products.


Introduction
It is widely accepted that the properties of nanomaterials are not only closely related to their sizes but also to their shapes. Therefore, controlling the morphologies of nanomaterials is one of the most important issues and effective ways to obtain desirable properties. one-dimensional (1D) nanoscaled materials such as carbon nanotubes (CNTs) [1], semiconductor nanowires, and nanobelts [2][3][4][5][6][7] exhibit interesting and useful properties and may be applied as building blocks for the integration of the next generation of nanoelectronics, ultrasmall optical devices, biosensors, and so forth. As a p-type semiconductor with a narrow band gap (1.2 eV), CuO nanomaterials have been widely exploited for diverse applications, such as heterogeneous catalysts [8][9][10], photoelectrochemical materials [11], gas sensors [12], lithium ion electrode materials [13], electrochemical hydrogen storage materials [14], and field emission (FE) emitters [15,16]. Many recent efforts have been directed toward the fabrication of nanostructured CuO to enhance its performance in currently existing applications. In particular, a variety of 1D CuO nanostructures have been prepared by high-temperature approaches [17][18][19][20][21][22][23][24][25] and low-temperature wet chemical approaches [26][27][28][29][30][31][32]. Generally, the abovementioned methods for 1D CuO nanomaterials require high temperature or additional templates to act as a support and are constrained by expense and complex. Comparatively, the ultrasonic approach was more attractive for both its simplicity and commercial feasibility. Different from other traditional chemical methods, the sonochemistry route is based on acoustic cavitations through the formation, growth, and collapse of bubbles in the liquid. The implosive collapse is an adiabatic process, which generates localized hot spots with transient temperatures of 5000-25000 K, pressures of about 1800 atm [33,34], and heating and cooling rates in excess of 1010 K/s [35,36]. These extreme conditions have been applied to prepare amorphous metals, carbides, oxides, sulfides, and so forth in various media [37][38][39][40][41].
Herein, we present a fast and facile ultrasound (US) irradiation assistant route to mass-synthesize uniform CuO nanorods with different size in a water-alcohol solution. Especially, the as-prepared CuO nanorods have shown a strong size-induced enhancement of electrochemical hydrogen storage performance and exhibit a notable hydrogen storage capacity and big BET surface area. These results further implied that the as-prepared CuO nanorods could be

Experimental
In a typical experiment, all of the chemicals were of analytical grade and were used as received. Aqueous solutions were prepared using distilled water. Firstly, 0.11 g CuAC 2 ·H 2 O was dissolved in the solution mixed with 15 mL ethanol and 35 mL distilled water in a 100 mL beaker. Then 8 mL NaOH aqueous solution (0.5 M) and 0.3 g CTAB were added slowly. After that, the solution was kept under US irradiation at room temperature for 45 minutes. Finally, the obtained precipitates were collected and washed several times with absolute ethanol and distilled water. The comparison experiments with different concentrations of NaOH, ethanol, CTAB, and HTMA have also been conducted as listed in Table 1. X-ray powder diffraction (XRD) analysis was carried out with a Japan Rigaku D/max-rA X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 1.54178Å). The scan rate of 0.05 • /s was used to record the patterns in the 2θ range of 20-70 • . SEM image was obtained by a JSM-6700 F field emission scanning electron microanalyser (JEOL, Japan), whereby the resulting powder was mounted on a copper slice. HRTEM images were recorded on a JEOL-2010 TEM at an acceleration voltage of 200 KV. The porous nature of the nanorods was further confirmed by the measurement of the pore size distribution, which was obtained by the nitrogen adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) methods on an OMNISORP-100CX accelerated surface area and porosimetry system. The electrochemical measurements were carried out following the method reported in [14] with slight modification. Briefly, the electrode was fabricated by directly pressing the synthesized CuO powders on a sheet of nickel foam at 50 MPa. All of the experiments were performed in a three-electrode cell in 6 M KOH at 25 • C under normal atmosphere. The CuO nanostructures were used as the working electrode, Ni(OH) 2 /NiOOH as the counterelectrode, and Ag/AgCl as a reference electrode. The CuO nanostructures electrode was charged for 3 h at a current density after a 2 s rest. All of the electrochemical hydrogen storage experiments were carried out using the Land battery system (CT2001A) at room temperature.

Results and Discussion
The overall crystallinity and purity of the as-synthesized samples were investigated by XRD and HRTEM measurement. As shown in Figure 1 Figures 1(b) and 1(c) show the large-scale CuO nanorods obtained, from which it can be seen that the nanorod is more than 200 nm long and 10 nm in diameter, and a high yield (>95%) of this 1D form can be easily observed. The HRTEM image in Figure 1(d) indicates that the nanorod is of single crystal and can be indexed as the monoclinic CuO phase, which is in accord with the XRD result. As shown in Figure 1(d), the lattice interplanar spacing has been determined to be 2.76Å, corresponding to the (110) plane of monoclinic CuO, which suggests that the nanorods grow preferentially through (110) plane stacking.
The measurement of BJH methods showed that the BET surface area of CuO nanorods was 49.8 m 2 /g, which implied that the CuO nanorods obtained here were a kind of potential porous material, as shown in Figure 2(a). Further study of the pore size distributions of the sample is illustrated in Figure 2(b). The Curve shows the relative pore volume distributions according to the average size of pores, in which there is a distribution centered around 40 nm. Interestingly, the electrochemical study demonstrated that the discharge capacity of CuO nanorods displayed a noticeable electrochemical hydrogen storage ability (∼ 165 mAh/g), which amounts to the 32.8% hydrogen storage capacity of SWNTs, whose discharge capacity is 503 mAh/g, corresponding to 1.84 wt% hydrogen [42]. In the charge curve of CuO nanorods, as shown in Figure 2(c), with the increase of the electrochemical capacity, the potential increases quickly but remains unchanged when the charge capacity reaches 3 mAh/g. One new obvious plateau of potential is observed between 5 mAh/g and 170 mAh/g. This indicates that two different hydrogen adsorption sites [42] exist in the synthesized CuO nanorods; in other words, Journal of Nanomaterials It is assumed that the H was first adsorbed onto the surface of each nanorod and then diffused into the interstitial sites among CuO. The discharge curve also displays two different hydrogen release processes, which further confirms the above results. The cycle life of CuO nanorod electrode is shown in Figure 2(d). After being cycled 50 times at the charge-discharge current density of 30 mA/g, the discharging capacities of CuO nanobelts remain over 20 mAh/g. Compared with the capacities (100 mAh/g and 130 mAh/g) of other 1D CuO nanostructures obtained by us, as shown in Figures 2(e) and 2(f) and Figures 3(a), 3(c), and 3(d), the as-prepared CuO nanorods exhibited higher capacity. The relatively high capacity was considered to be pertinent to the enhanced electrocatalytic activity of the highly porous and layered structures of the synthesized CuO nanorods. And we believe that the investigations of electrochemical hydrogen storage of CuO nanostructures help us to understand the relationship between morphology, size, and properties and thus inspire us to explore new nanostructures with higher hydrogen uptake.
Comparison experiments with different concentrations of NaOH, ethanol, and CTAB while keeping other synthetic parameters unchanged leads to the morphology and size change of CuO products. As shown in Figures 3(a) and 3(b), when the NaOH concentrations were reduced, CuO nanorod bundles composed of rods with smaller size (several nanometers) were obtained. When the ethanol concentrations were increased, short CuO nanorods (∼50 nm long) formed, as shown in Figures 3(c) and 3(d). CuO nanorod bundles were also produced when the CTAB mount was reduced or free, as shown in Figures 3(e) and 3(f). When using other surface active reagent (such as hexamethylene tetramine (HTMA)) instead of CTAB, different CuO 1D nanostructures have also been prepared, as shown in Figure 4. Therefore, suitable thermodynamic experimental conditions favor the oriented crystalline growth process of the CuO nanorods. It is also implied that different shapely CuO nanostructures can be controllably synthesized through adjusting the reaction parameters of this US chemical reaction process.    Table 1.

Conclusions
In summary, CuO nanorods were synthesized through a fast and facile ultrasound irradiation assistant route. The products exhibit excellent hydrogen storage capacity and big BET surface area. Different shapely CuO nanostructures have been controllably synthesized. The comparison experiments show that the reactant concentrations are critical to the formation of 1D CuO nanostructures. Further research will be performed on more novel cupreous 1D nanostructures exhibiting different electrochemical hydrogen storage performances, in which more excellent hydrogen storage materials might be found.