Oxygen source-oriented control of atmospheric pressure chemical vapor deposition of VO 2 for capacitive applications

Vanadium dioxides of different crystalline orientation planes have successfully been fabricated by chemical vapor deposition at atmospheric pressure using propanol, ethanol and O2 gas as oxygen sources. The thick a-axis textured monoclinic vanadium dioxide obtained through propanol presented the best electrochemical response in terms of the highest specific discharge capacity of 459 mAh g with a capacitance retention of 97 % after 1000 scans under constant specific current of 2 A g.


Introduction
Vanadium dioxide (VO 2 ) exists in more than 10 polymorphs, however, rutile, monoclinic (described as a slightly distorted rutile structure) and metastable phases have mainly attracted interest because of their interesting chemical and physical properties for catalytic and electrochemical applications [1][2][3].The crystal structure of VO 2 consists of sheets of edge-sharing VO 6 octahedra linked by corner sharing to adjacent sheets along the c-direction of the unit cell [4].This corner-sharing structure strengthens the structural stability and the resistance to lattice shearing during cycling in lithium ion batteries [5].In that context, VO 2 was found to show better electrochemical performance compared with the well-known V 2 O 5 [6].
To date, various VO 2 forms have been synthesized such as nanocrystalline VO 2 (B) [4], nanothorn VO 2 (B) hollow microspheres [7], three dimensional hierarchical microflowers and microspheres [8,9] as well as VO 2 /reduced graphene oxide [10,11], VO 2 (B)/carbon nanotubes [12] and hydrogen treated VO 2 [13] composite powder materials with sufficient capacitive characteristics.But, most of the above mentioned materials showed poor cycling stability after 500 cycles.Hence, it is important to improve the long-term performance and high rate capability of VO 2 .
In this paper, the APCVD of VO 2 on SnO 2 -precoated glass substrate was studied using VO(acac) 2 and propanol, ethanol, O 2 gas as oxygen sources from the perspective of the potential application of such coatings as cathodes.We have demonstrated the differences in the resulting coatings obtained from a structure-orienting role played by the oxygen sources utilized and possible explanation of this role is examined.

Preparation of VO 2 coatings
The APCVD reactor utilized in this work was an in-house design as also reported previously [3,23,24].The vanadium source was the VO(acac) 2 (98 %, Sigma-Aldrich), which was placed in a bubbler at 200 o C, while the gas lines were kept at 220 o C to avoid any condensation or blocking.The carrier gas was N 2 (99.9 %), which was passed through the reactor during all depositions.The N 2 flow rate through the vanadium precursor bubbler was 1.4 L min -1 , for growth temperature and period of 500 o C and 7.5 min, respectively.Additionally, the flow rate of propanol (99.5 %, Sigma-Aldrich), ethanol (≥ 99.5 %, Sigma-Aldrich) and O 2 gas (99.9 %) was 0.8 L min -1 .Finally, the total N 2 flow rate was kept at 12 L min -1 in all CVD experiments.
The substrates were commercial SnO 2 -precoated glass (Uniglass, Greece), all of dimensions 2×2×0.4cm.Prior to deposition, they were all cleaned with H 2 O and detergent, rinsed thoroughly with H 2 O and deionised H 2 O, and allowed to dry in air.Once the allotted time was complete, the reactor temperature was turned off and the substrate allowed cooling at 100 o C under an atmosphere of N 2 .Then it was removed from the reactor, handled and stored in air.

Structural and morphological characterization of coatings
The structure of the coatings was examined in a Siemens D5000 Diffractometer for 2-theta = 15.00 -60.00 o , step size 0.05 o and step time 5 min/ o .Additionally, their morphology was evaluated in a Jeol JSM-7000 microscope.In this case, coatings were over-coated with a thin film of gold prior to analysis to avoid charging.
Finally, the coating's thickness was estimated using a profilometer A-step TENCOR.A step was done by etching the vanadium oxide coatings off the SnO 2 -precoated glass substrate in 1:3, H 2 O 2 (30 %):HCl.Tin dioxide remained intact after this procedure and the thickness was obtained from the measured step height.

Electrochemical evaluation
Cyclic voltammetry experiments were performed using a three electrode electrochemical cell as reported previously [32][33][34] for a potential range of -1 V to +1 V, a scan rate of 10 mV s -1 and a number of scans up to 1000.In particular, Pt, Ag/AgCl and vanadium oxides on SnO 2 -precoated glass substrates were used as counter, reference and working electrodes, respectively.All measurements were carried out in 1 M LiOH, which acted as electrolyte.The chronoamperometric measurements were done at -1 V and +1 V for a step of 200 s and a total period of 2000 s.Additionally, the chronopotentiometric curves were obtained at a constant specific current of 2 A g -1 and a potential range of +0.1 V to +0.6 V.

Results and discussion
Presented here are investigations into APCVD VO 2 coatings grown from VO(acac) 2 and three oxygen sources: propanol, ethanol and O 2 gas.It will be shown that a level of control can be exerted over orientation and morphology through the different sources.All coatings produced were stable in air for over six months and resistant to H 2 O, acetone and toluene.Additionally, they passed the "scotch tape test"; a piece of sticky tape was placed on the coating and then removed without lifting off the coating.

Structure
Figure 1 presents the x-ray diffraction (XRD) patterns of the APCVD coatings using propanol, ethanol and O 2 gas.As shown in the case of propanol and ethanol, one diffraction peak is observed at 18.21 o , which matches to monoclinic a-axis textured VO 2 coatings [30,31,35].This peak can be indexed to 100 plane showing the preferred orientation growth of these coatings.Furthermore, the XRD pattern of the as-grown coatings using O 2 gas show two peaks at 55. 4 and 57.6 o with Miller indices 022 and 220 due to monoclinic VO 2 phase indicating that is a 022-oriented single phase [36][37][38].Finally, peaks at 26.5, 33.7, 37.1, 51.7 and 54.7 o with respective Miller indices 110, 101, 211 and 220 (indicated with asterisk in Figure 1) are due to SnO 2 -precoated glass substrate [39].The preferential orientation of the VO 2 on SnO 2 -precoated glass substrates at angles other than 27.8 o is not clearly understood.Nevertheless, it seems to be more sensitive to O 2 source during the growth rather than to glass substrate.The reaction mechanism for the formation of VO 2 from VO(acac) 2 in the presence of O 2 gas has previously been studied in the literature [40].The possible decomposition routes of VO(acac) 2 species involve a simple intramolecular rearrangement of the VO(acac) 2 precursor resulting in the release of two C 3 H 4 molecules, followed by the decomposition of VO(CH 3 COO) 2 to yield (CH 3 CO) 2 O and VO 2 .On the other hand, in the case of alcohols, the VO 2 deposits possibly act as heterogeneous catalytic sites for their oxidation to propanal (for propanol) and acetaldehyde (for ethanol).A similar behavior is also observed in the presence of methanol [41,42].The active oxygen required for their oxidation comes from the VO(acac) 2 itself, since it can be regarded as a source of excess oxygen (there are 5 oxygen atoms to 1 V atom, while only 2 oxygen atoms are required for VO 2 formation).
Other researchers have also attempted to control the crystalline orientation of VO 2 .Gary et.al. reported the a-axis textured VO 2 deposited on R-plane sapphire and suggested that the cause could be a stress developing on the interface between the substrate and the coating [30].Muraoka et.al. studied the epitaxial growth of VO 2 001-oriented single phase on TiO 2 001 substrates and 110-oriented phase on TiO 2 110, respectively [29].Ngom et.al. indicated that the crystalline orientation of the VO 2 thin films was drastically changed because of the formation of an interface layer between the VO 2 and the soda lime glass [35].Chiu et.al. also attempted to grow VO 2 on glass using a 5 nm ZnO buffer layer.In the case of the direct VO 2 growth on amorphous glass, polycrystalline films formed, while only VO 2 011 peaks located at 27.90 o were observed for the growth on ZnO [43].

Morphology
Figure 2 presents the field-emission scanning electron microscope (FE-SEM) images of vanadium oxide coatings grown at 500 o C on SnO 2 -precoated glass substrate for 0.8 L min -1 flow rate of propanol, ethanol and O 2 gas.For propanol and ethanol, compact grains of nearly round shape are mainly observed with their sizes being 160 nm and 40 nm, respectively.It was observed that thinner coatings grown using ethanol (95 nm) were denser, while as thickness increased for propanol (120 nm), the surface appeared less dense with growth at specific sites, suggestive of a Stranski-Krastanov type of growth mechanism [44].Regarding the O 2 gas, agglomeration of grains forming rod-like structures is shown with thickness of 80 nm.

Electrochemical characteristics
In order to study the effect of oxygen source on the electrochemical performance of the coatings, cyclic voltammetry curves were obtained as indicated in Figure 3.The potential range was -1 V to +1 V at a scan rate of 10 mV s -1 .All curves are normalized to the mass of the working electrode.The mass was measured by a 5-digit analytical grade scale and found to be 0.00002 g, which was obtained by measuring the glass substrate before and after the growth.It can be observed that the as-grown vanadium oxide coatings using propanol present two anodic peaks at -0.05 V / +0.52 V and two cathodic peaks at -0.15 V / ≈+0.64 V (vs.Ag/AgCl), which are accompanied by color changes from green, blue to yellow and then yellow, blue to green.Since, the electrochemical cell is made up of glass, we have observed these color changes during the measurements.One may then assume that V +5 ions are reduced to V +4 and V +3 , since two anodic peaks are observed.A similar explanation can be given for the oxidation peaks, i.e.V +3 ions oxidize into V +4 and V +5 .These color changes are attributed to Li + intercalation and deintercalation [45].On the other hand, the shape of the curve for the vanadium oxide coating using O 2 gas is different indicating one anodic peak at +0.11 V and one cathodic peak at -0.34 V accompanied by color changes from green to yellow and vice versa.This may be due to the existence of different VO 2 orientation planes compared with the one observed for alcohols.Furthermore, the specific current of the as-grown coatings using alcohols as oxygen source is the highest presenting an enhanced electrochemical activity.We then suggest that this is correlated to both the 100 plane and the increased thickness, which incorporate more active material for the insertion of Li + .Chronoamperometry measurements were also performed to calculate the specific charge during Li + intercalation / deintercalation.It is estimated by integration of excess current measured upon switching the bias potential with time [32] as shown in Figure 4 for the as-grown vanadium oxide using 0.8 L min -1 of propanol.The amount of specific charge for propanol found to be 120 C g -1 , which is three times higher than that of O 2 gas.  Figure 6 presents the specific discharge capacities of the as-grown coatings at 500 o C for 7.5 in using 0.8 L min -1 flow rate of propanol, ethanol and O 2 gas under a constant specific current of 2 A g -1 .The propanol's curve indicates two plateaus at approximately 0.25 V and 0.5 V, which present the two-step Li + intercalation process as also observed in cyclic voltammetry analysis.The specific discharge capacity was 459 mAh g -1 with a capacitance retention of 97 % after 1000 scans (Figure 6 inset) keeping the staircase shape, which is promising for lithium ion batteries.The specific discharge capacity was higher than the APCVD metastable [3] and 022-oriented monoclinic VO 2 [24].On the other hand, the ethanol and O 2 gas samples lack of staircase-like shape probably due to the less defined phase transition associated with Li + .This result may arise due to the largest thickness of the propanol sample, which facilitates larger number of Li + within the vanadium oxide lattice.
Figure 6.The chronopotentiometric curves for the as-grown sample at 500 o C for 7.5 min using 0.8 L min -1  propanol, ethanol and O 2 gas under a constant specific current of 2 A g -1 and potential ranging from 0.1 V to 0.6 V.The 1000 th scan of 0.8 L min -1 propanol is also included as inset.

Figure 1 .
Figure 1.XRD of APCVD vanadium oxides at 500 o C for 0.8 L min -1 flow rate of propanol, ethanol and O 2 gas.

Figure 2 .
Figure 2. FE-SEM of APCVD vanadium oxides at 500 o C for 0.8 L min -1 flow rate of propanol (a), ethanol (b) and O 2 gas (c).

Figure 3 .
Figure 3. Cyclic voltammograms of the first scan for the APCVD vanadium oxide coatings for 0.8 L min -1 flow rate of propanol, ethanol and O 2 gas and an electrode geometrical active area of 1 cm 2 .Maximized cyclic voltammogram curve for the region of -0.2 V -+1 V of the grown vanadium oxide coating using propanol as inset.

Figure 4 .
Figure 4.The chronoamperometric response of the first scan recorded at -1 V and +1 V for an interval of 200 s of the as-grown coatings at 500 o C for 0.8 L min -1 flow rate of propanol.

Figure 5 .
Figure 5. Intercalated and deintercalated specific charge as a function with oxygen source utilized.