In Situ Study of Thermal Stability of Copper Oxide Nanowires at Anaerobic Environment

Manymetal oxides with promising electrochemical properties were developed recently. Before those metal oxides realize the use as an anode in lithium ion batteries, their thermal stability at anaerobic environment inside batteries should be clearly understood for safety. In this study, copper oxide nanowires were investigated as an example. Several kinds of in situ experimentmethods including in situ optical microscopy, in situ Raman spectrum, and in situ transmission electron microscopy were adopted to fully investigate their thermal stability at anaerobic environment. Copper oxide nanowires begin to transform as copper(I) oxide at about 250C and finish at about 400C.The phase transformation proceeds with a homogeneous nucleation.


Introduction
Energy issues become more and more important today than those at any time in the past.Lithium ion batteries (LIBs), as one of the most promising technologies, have obtained high attentions in both industry and science societies.Important advances have been made for further improving the battery properties recently, such as the developments of Li-O 2 and Li-S batteries, Na-ion batteries, and double-walled silicon nanotube batteries [1][2][3][4][5][6].
Metal oxides, expected to be important anode materials in the future, have high theoretical capacity.SnO 2 , Co 3 O 4 , RuO 2 , and TiO 2 anodes [18,[21][22][23][24][25][26] have several advantages compared to others.The research team headed by Huang et al. revealed the charge-discharge lithium mechanism of metal oxide nanowires by in situ lithiation-delithiation in transmission electron microscopy (TEM) [27,28].Those researchers have greatly enhanced our understanding of the charge-discharge lithium mechanism of metal oxides and promoted the development of science and technology.
Metal oxides have high thermal stability in the oxygen environment.However whether they have high thermal stability at anaerobic environment in LIBs is still an open question.The accidents of "Boeing 787s" overheated batteries and "Sony" notebooks remind us that the thermal problems should never be an afterthought or dealt with lightly.The temperature in the batteries would significantly increase when the batteries are working at high current density.Some local areas may reach a rather high temperature due to the complicated and inhomogeneous microstructure of the anodes.The situation would be more significant for the Na/S and Na/NiCl 2 systems operating at higher temperatures (300-350 ∘ C) [29].However, it is nearly a fresh area for the research on thermal stability of metal oxides in such a high temperature anaerobic environment.
Copper oxide nanomaterials have attracted great attentions and have demonstrated significant performances as anode materials [30][31][32].It was reported that the capacity of the attenuation was less than 10% after 70 cycles and the cyclic capacity maintained at 400 mAh/g when using copper oxide nanowires as anodes [33].Further improvement has been obtained that the cyclic capacity of copper oxide film could remain at 608 mAh/g with a current density of 100 mA/g after 275 cycles, which reached 90% of the theoretical capacity [34][35][36][37].
In this study, copper oxide nanowires were investigated as an example.Several in situ tests were adopted to explore the thermal stability of copper oxide nanowires at anaerobic environment directly.In situ optical microscopy, in situ Raman, and in situ TEM were carried out to study both the morphology and structure evolutions during in situ heating.

Materials and Methods
Cu foils (99.9% purity, Alfa Aesar Company) with a thickness of 25 m were cleaned in an aqueous 1.0 mol/L HCl solution for 5∼10 s, followed by repeated 2-3 times rinsing with deionized water.Cu foils were dried under N 2 gas flow, then placed in a rapid thermal processing system furnace (JET FIRST-200), and then heated to 500 ∘ C for 5 h in air ambient with a heating rate of 40 ∘ C/s.
The crystal structure of copper oxide was analyzed by X-ray diffraction (XRD, X'Pert PRO) with Cu-K radiation from 10 ∘ to 90 ∘ .The microstructure of the copper oxides was observed by scanning electron microscope (SEM, Hitachi S3700) and TEM (FEI F30).Some copper oxide nanowires were heated to a maximum temperature of 400 ∘ C with nitrogen atmosphere protection.The morphology change of the copper oxide nanowires was in situ observed under the optical microscope equipped in the Raman test system and the structure change of the copper oxide nanowires was in situ analyzed by Raman test during heating.The Raman spectra were recorded by using a LabRamHRUV instrument equipped with a 532 nm laser excitation.The microstructure change of the copper nanowires was in situ investigated in the TEM equipped with an in situ heating holder (Gatan 652).

Results and Discussion
High-density copper oxide nanowires are obtained when copper foils were oxidized in air ambient at 500 ∘ C for 5 h (Figure 1(a)).The average diameter of the nanowires is about 65 nm and the length of the nanowires is up to 20 m.XRD pattern of the sample in Figure 1(a) indicates the coexistence of CuO phase of monoclinic structure (JCPDS80-1917) and Cu 2 O phase of cubic structure (JCPDS78-2076) in the oxide layer (Figure 1(b)).The majority phase is the Cu 2 O, which is in well accordance with previous studies [38].Copper is always first oxidized into the Cu 2 O phase and the subsequent formation of CuO phase would be very slow due to the cover of Cu 2 O layer.
Figure 2(a) shows the TEM image of an individual nanowire with a diameter of about 60 nm.There is an interface obviously in the middle of nanowire which divides the nanowire into two planes along the longitudinal axis.The interface is confirmed to be twin boundary by select area electron diffraction (SAED) pattern analysis as shown in Figure 2(b).The nanowire is confirmed to be CuO nanowire by measuring the distance and angle of the diffraction spots in the SAED pattern.The SAED pattern corresponds to the two sets of lattice.One set diffraction spot of lattice is observed along the [110] direction while the other is observed along [110].High resolution TEM image clearly shows that the twin boundary is parallel with the (001) plane in Figure 2(c).
The morphology changes of the CuO nanowire at different temperatures at anaerobic environment were investigated by in situ heating an individual CuO nanowire with nitrogen atmosphere protection under an optical microscope.CuO nanowire has no obvious change when temperature is below 250 ∘ C (Figure 3(a)).When heating temperature reaches 300 ∘ C, some changes occur as pointed by the arrow in Figure 3(b).More changes appear as the temperature increases to 350 ∘ C (Figure 3(c)).Almost the entire nanowire changes when the temperature increases to 400 ∘ C. The morphology changes of the nanowire indicate that there should be some transformation of the structure of CuO phase during the in situ heating process with a homogeneous nucleation behavior.
To analyze the structure change of the CuO nanowire during in situ heating process, in situ Raman test was carried out for the CuO nanowire heated at the same temperatures with nitrogen atmosphere protection (Figure 4).There are three Raman peaks at 296 cm −1 , 345 cm −1 , and 630 cm −1 for the test nanowire from 25 ∘ C to 250 ∘ C, which correspond to the A g , B 1g , and B 2g modes [39] of CuO phase, respectively.When heating temperature reaches 300 ∘ C, two small new Raman peaks emerge at 154 cm −1 and 218 cm −1 , which consists with the Raman peaks of Cu 2 O phase [40].   is three orders higher than that of CuO [41], the intrinsic resistivity of the LIBs would be greatly enhanced once some of CuO phase transformed into Cu 2 O phase when the CuO is used as cathode.As a result, overheating or even explosion may be triggered when such LIBs are charged.

Conclusions
In this study, copper oxide nanowires were grown by a simple thermal oxide method.The average diameter of nanowires is 65 nm and the length is up to 20 m.In situ Raman and optical microscope tests reveal that copper oxide nanowires gradually change into copper(I) oxide particles during in situ heating at anaerobic environment.In situ TEM test further confirms that copper oxide begins to transform into copper(I) oxide at 250 ∘ C and nearly finish at 400 ∘ C with a homogeneous nucleation behavior.

Figure 1 :
Figure 1: (a) SEM image of Cu foil oxide layer and (b) XRD pattern of Cu foil oxide layer.

Figure 2 :
Figure 2: (a) TEM image of an individual CuO nanowire, (b) SAED pattern of the single nanowire, and (c) corresponding high resolution TEM image.

Figure 5 :
Figure 5: TEM images and SAED patterns corresponding to the white circle of an individual CuO nanowire in situ heated to different temperatures.