146

Universidad de Sonora

ISSN: 1665-1456

146

Journal of biological and health sciences

http://biotecnia.unison.mx

Volume XXV, Issue 2

A simple solution method to prepare VO2:Co2+ precursors for

thin lm deposition by solution-processing method

Un método simple de solución para preparar películas delgadas de VO2:Co2+ para la deposición median-

te el método de procesamiento en solución

F. Hernandez-Guzmana, G. Suarez-Camposab*, D. Cabrera-Germana, M.A. Millan-Francoc, H. Huc, M.A. Quevedo-Lopez

a,d, M. Sotelo-Lermaa*

Department of Research in Polymers and Materials, University of Sonora, Hermosillo, Sonora, Mexico.

b Department of Chemical Engineering and Metallurgy, University of Sonora, Hermosillo, Sonora, Mexico.

Institute of Renewable Energies, National Autonomous University of Mexico, Temixco, Morelos, Mexico

d Materials Science and Engineering Department, University of Texas at Dallas, Richardson, United States.

*Author for correspondence: Guillermo Suarez Campos, Merida Sotelo-Lerma

e-mail: guillermo.suarez@unison.mx, merida.sotelo@unison.mx

Received: November 3, 2022

Accepted: March 6, 2023

ABSTRACT

Solution-processing is a low-cost solution method to prepa-

re a variety of organic or inorganic thin lms. For metal oxide

compounds, a solution-processing solution of an organo-

metallic compound is frequently used as a precursor to be

spin coated, followed by a thermal annealing to form metal

oxide. In this work, vanadium oxide powders are obtained

from a simple acid-base reaction, and then they are disper-

sed in isopropyl alcohol to form a solution for spin-coating.

Dierent amount of cobalt salt are also added together

with VOx into isopropyl alcohol to form VOx:Co2+ solutions.

After thermal annealing at 200 °C, continuous transparent

thin lms are obtained. Optical, structural, morphological

and chemical binding energies of those lms are analyzed.

It is found that amorphous VO2:Co2+ compound is formed

in those lms with V:Co atomic ratios between 6.6:1 and

1.6:1. Optical absorption onsets of those lms are around

2.3 eV. An interesting interconnected porous morphology is

observed when the atomic ratio of V:Co is around 4.9:1. It is

concluded that porous amorphous cobalt doped vanadium

oxide thin lms can be obtained from a spin-coating process

at low annealing temperature from a simple solution without

any complex agent.

Keywords: cobalt doped vanadium oxide, spin-coated thin

lms, porous morphology, solution processing without ligands.

RESUMEN

El procesamiento de soluciones es un método de bajo costo

para preparar una variedad de películas delgadas orgánicas

o inorgánicas. Para compuestos de óxidos metálicos, un pro-

cesamiento de solución de un compuesto organometálico

se usa con frecuencia como solución precursora para ser

recubierta por rotación, seguida de un tratamiento térmico

para formar el óxido metálico. En este trabajo se obtienen

polvos de óxido de vanadio a partir de una simple reacción

ácido-base, y luego se dispersan en alcohol isopropílico para

formar una solución para spin-coating. También se agregan

diferentes cantidades de sal de cobalto junto con VOx en

alcohol isopropílico para formar soluciones de VOx:Co2+. Des-

pués del tratamiento térmico a 200 °C, se obtienen películas

delgadas transparentes. Se analizan las propiedades ópticas,

estructurales, morfológicas y químicas. Se encontró que el

compuesto VO2:Co2+ es amorfo y se obtiene con una relación

atómica V:Co variada de 6.6:1-1.6:1. El material presenta una

absorción óptica alrededor de 2.3 eV. Se observa una intere-

sante morfología porosa interconectada cuando la relación

atómica de V:Co es ~4.9:1. Se concluye que se pueden obte-

ner películas delgadas amorfas porosas de VO2:Co2+ a partir

del spin-coating a una baja temperatura de tratamiento

utilizando una solución simple sin agente complejante.

Palabras clave: oxido de vanadio dopado con cobalto, pelícu-

las delgadas mediante recubrimiento centrifugo, morfología

porosa, procesamiento de la solución sin complejantes.

1. INTRODUCTION

Vanadium oxides (VOx) and VOx doped with dierent metal

oxides are interesting semiconductor materials because

of their abundance in the Earth’s crust, as well as its multi-

oxidation states and various crystalline structures (Silversmit

et al., 2004, 2006). As a semiconductor material, VOx shows a

band gap ~2.6-3.5 eV that makes it a good semiconductor

material for diverse electronic application. In the literature,

VOx thin lms have been deposited with dierent methods

for diverse potential applications employing techniques

such as: magnetron sputtering and spin-coating for thermo-

chromic applications (Ho et al., 2019; Zhan et al., 2020; Yuan et

al., 2021); pulsed laser deposition for cathodes in Lithium and

sodium-ion batteries (Petnikota et al., 2018); sprayed solution

for NO2 gas sensing (Khatibani, Abbasi and Rozati, 2016;

Mane and Moholkar, 2017); solution-processing method for

smart windows applications (Peng et al., 2018; Wang et al.,

2020; Shen et al., 2021).

On the other hand, modications of optical or structu-

ral properties of semiconductor thin lms are necessary to

improve application of these materials. Which by adding a

dierent type of metal ions into a VOx host matrix, known as

a doping process, is one of the most eective ways to modify

vanadium oxide structure, leading to dierent electrochemi-

DOI:10.18633/biotecnia.v25i2.1886

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147

cal, catalytic, and magnetic properties (Lu et al., 2019; Sharma

et al., 2021; Geng et al., 2022). Metal doped vanadium oxide

thin lms have been obtained with dierent deposition te-

chniques: Y-doped VOx thin lms by DC reactive magnetron

sputtering for electrical modulation applications (Zhou et al.,

2020); Cs-doped VOx thin lms deposited by spun-cast as a

hole extraction layer in perovskite solar cells (Yao et al., 2018);

Fe-doped VOx by sol-gel spin-coated for electrochromic devi-

ce applications (Bae, Koo and Ahn, 2019); In-doped VOx thin

lms obtained by spray pyrolysis coating system for optical

transparency modulation (Tabatabai Yazdi, Pilevar Shahri

and Shafei, 2021); Co-doped VOx thin lms by microwave irra-

diation process for battery-type supercapacitor applications

(Liu et al., 2020), among others (Ji et al., 2018; Li et al., 2019;

Xu et al., 2019).

Solution-processing spin-coating is an economical

method to obtain homogeneous pure or doped VOx thin

lms. Compared to other chemical solution deposition

methods, this method has the advantage that it enables the

deposition of metal oxide thin lms without seed layer on

substrates such as glass, uorine doped tin oxide (FTO), in-

dium doped tin oxide (ITO) or coated polyethylene (PET) for

transparent or exible electronics applications (Hajzeri et al.,

2012). In particular, metal ion doping in VOx thin lms could

be an in-situ process in a spin-coating method by adding the

metal source into the vanadium precursor solution. However,

expensive organometallic compounds are usually employed

to prepare metal precursor solutions. In this work, we propo-

se a simple method to prepare vanadium oxide and cobalt

doped vanadium oxide solutions, without the use of organo-

metallic compound or any complex agent that would result

in a slow metal cation release, and an increased time and

temperature deposition. Chemical photoelectron analysis

of vanadium oxide samples, with or without cobalt doping,

conrms the formation of a vanadium oxide VO2 compound

in the lm samples. Cobalt oxide CoO also prevails in VO2:Co2+

samples, with some neighboring atoms arranged in a short-

range order. Porous surface of VO2:Co2+ thin lms can be

obtained, giving an optical absorption onset at 2.3 eV.

2. EXPERIMENTAL PROCEDURE

2.1. Synthesis of VOx powders and solution

Precursor solutions for VOx powders were prepared by

using 0.1 M solution of vanadium (IV) oxide sulfate hydrate

(VOSO4·xH2O) and 1 M solution of sodium hydroxide (NaOH),

as previously suggested [25]. These two precursor solutions

were mixed, resulting in a blue color solution with a pH ~2

tested with a MColorHastTM. After that, the blue solution

was heated at 80 °C for 3 h to obtain VOx precipitates. These

precipitates were dispersed in methanol to dissolve possible

impurities and then centrifuged. After centrifugation, VOx

precipitates were dried at 80 °C for 2 h to obtain VOx powders.

To prepare a VOx sol-gel solution, VOx powders were

dispersed in isopropyl alcohol under ultrasonic vibration at

room temperature for 10 min. Then, the mixture solution was

ltered with a cotton lter to remove the undispersed preci-

pitates and obtain a transparent solution for thin lm depo-

sition by spin-coating. Pure VOx lms were called samples S1.

2.2. Preparation of VOx:Co solutions

To obtain cobalt doped VOx precursor solutions, dried VOx

powders and CoCl2·6H2O salt were dispersed/dissolved in

isopropyl alcohol. Then, the solutions were subjected to

ultrasonic vibration at room temperature for 10 min. Finally,

the solutions were ltered with a cotton lter to remove the

undispersed precipitates and obtain transparent VOx:Co so-

lutions for thin lm deposition by spin-coating. Four VOx:Co

solutions were prepared with VOx/CoCl2⋅6H2O weight ratios

of 0.7/0.3, 0.5/0.5, 0.3/0.7 and 0.1/0.9, called samples S2, S3, S4

and S5, respectively.

2.3. VOx and VOx:Co thin lm deposition

The ve precursor solutions, VOx and VOx/Co, were used to

form thin lms by spin-coating. Microscope glass slides from

VE-P10 Velab™, to be used as substrates for vanadium oxide

solutions, were washed with Alconox® detergent, rinsed

with distilled water and air-dried. The clean substrates were

placed in the Chemat Thecnology Spin-Coated KW-4A, 50 µL

of the VOx or VOx:Co solutions were deposited at 2500 rpm

for 2 min on the glass substrates. The obtained coatings were

thermally annealed in air at 200 °C for 1 h and became solid

thin lms.

2.4. Thin lms characterizations

X-ray photoelectron spectroscopy (XPS) was used to assess

the chemical composition of the lms. The tool employed

was PHI 5000 Versa Probe II, with a monochromatic Al Kα

(1486.6 eV) X-ray source with a 0.1 eV step size and a cons-

tant pass energy of 23.50 eV. The scale of binding energies

was corrected by calibrating the main C 1s peak at a binding

energy of 284.8 eV. Atomic compositions of VOx/Co lms

were estimated from the XPS results. Crystalline structures

of VOx/Co lms were probed with X-ray diraction (XRD)

employing a Rigaku DMAX-2200 diractometer equipped

with a Cu Kα (λ = 1.54 Å) X-ray source. A Jeol JSM-7800F eld

emission scanning electron microscope (FE-SEM) was used

to evaluate the surface morphology of the lms. The surface

morphology of each lm sample was analyzed by atomic

force microscopy (AFM) with a Veeco Dimension Icon, Bruker

instrument. Optical transmittance spectra of VOx/Co lms

were measured with a Perkin Elmer Lambda 20 UV–visible

spectrometer. Thicknesses of VOx and VOx:Co thin lms were

measured with an Ambios Technology XP 200 prolometer.

3. RESULTS AND DISCUSSION

3.1. Prolometry and main characteristics

The rst assessment of the possible formation of continuous

VOx and VOx:Co thin lms from the above mentioned precur-

sor solutions is lm thickness measurement by prolometry.

Table 1 lists the ve types of VOx:Co thin lms with their

respective thickness. In the Alpha-Step proles of all lm

samples, large grains of 100 to 400 nm of height were obser-

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148

ved and were embedded in continuous solid coatings with

thickness from 133 to 246 nm.

Table 1. Thickness, band gap and V:Co atomic ratio of VOx:Co lm samples.

Tabla 1. Grosor, brecha de banda y proporción atómica V:Co de muestras

VOx:Co.

Sample

name

Weight ratio

(VOSO4·xH2O / CoCl2·6H2O) Thickness (nm) Atomic ratio

of V:Co

S11/0 152 ± 44 1:0

S20.7/0.3 133 ± 39 6.6:1

S30.5/0.5 123 ± 19 4.9:1

S40.3/0.7 133 ± 26 2.9:1

S50.1/0.9 246 ± 11 1.6:1

3.2. Optical properties

Optical transmittance spectra of VOx:Co thin lms can be

appreciated in Figure 1a with transmittance percentage of

65 - 75 % in the 800 -1350 nm wavelength (λ) range. The

main inuence of Co2+ inclusion is observed in UV-A to blue

region, at 325 - 500 nm, as reported elsewhere (Hajzeri et

al., 2012; Martínez-Gil et al., 2020). Furthermore, samples

with larger Co2+ concentration, such as S4 and S5, show lower

transmittance at the visible region (400 - 700 nm). Optical

absorption coecient spectrum of each lm sample, α(λ)

(Figure 1b), is calculated from the simplied Beer-Lambert

equation: , where d is lm thickness and T(λ)

the transmittance spectrum. The optical absorption onset

(or band gap Eg) value is estimated by intersection of two

straight tting lines. The Eg values of pure vanadium oxide

and vanadium with dierent amounts of Co2+, are around 2.3

eV, which is very close to that previously reported for V2O5

[27]. It is also observed that the absorption coecients of all

the cobalt doped vanadium oxide thin lms (from S2 to S5)

are higher compared to the pure vanadium oxide sample (S1),

keeping the same forms of the spectra. Higher absorption

region (> 3.8 eV) in these samples should be related to the

glass substrate absorption.

3.3. X-ray diraction

X-ray diraction pattern of VOx lm sample (S1) is presented

in Figure 2, which appears to correspond to an amorphous

material due to the broad signal around 22°, plus a monocli-

nic phase of HNaV6O16•4H2O with the plane orientation (100)

at around 2θ ∼ 8° (JCPDS# 49-0996 (Channu et al., 2011)). This

crystalline compound could be a residual from VOSO4·xH2O

+ NaOH, reaction and was not be eliminated during the 200

°C annealing or the thin lm processing. Interestingly, after

Co2+ addition, the reection peak around 2θ ∼ 8° gradually

reduces. The higher the concentration of Co2+, the smaller

the intensity of that peak, until its complete elimination in

the sample with the highest concentration of Co2+ (sample,

S5). S5 shows the diraction pattern typical of an amorphous

lm, hence it is a predominantly amorphous material. Since

no diraction peaks of CoOx or VCoOx compounds appear

in the ve VOx:Co2+ samples, it seems that low temperature

annealing leads to amorphous materials, as indicated in (Hu

et al., 2017; Martínez-Gil et al., 2020).

300 450 600 750 900 1050 1200 1350

Transmittance (%)

Wavelength, λ(nm)

1 2 3 4

Absorption Coefficient, α (cm

-1

)

Photon Energy, hv (eV)

Figure 1. (a) Transmittance and (b) optical absorption coe-

cient spectra of vanadium oxide (S1) and cobalt doped vana-

dium oxide (S2 to S5) thin lms.

Figura 1. Espectros de transmitancia (a) y coeciente de ab-

sorción óptica (b) para películas delgadas de óxido de vanadio

(S1) y óxido de vanadio dopado (S2 to S5).

3.4. Thin lms surface morphology

Figure 3 shows AFM three dimensional surface topography

images in scales of 2 m x 2 µm of (a) S1, (b) S2, (c) S3, (d) S4 and

(e) S5 VOx:Co2+ lm samples. The root-mean-square roughness

of the same samples is plotted in Figure 3f. The surface topo-

graphy of pure VOx lm (S1) presents lumps and the lowest

roughness. As the cobalt ions are incorporated into VOx host,

variations in shape and roughness are observed in AFM

images. Notable changes are observed in samples S3 and S4

that exhibit a ring like morphology and increased roughness

(8 - 9 nm) in comparison with S1 and S2 (4 - 5 nm) samples.

sample S5 that contains the highest cobalt concentration

loses the ring pattern in its surface morphology and shows

the highest roughness (∼23 nm) among the ve samples.

Similar results are reported elsewhere, suggesting that the

inclusion of Co2+ in VOx host by solution methods modies

the supercial morphology of cobalt doped vanadium oxide

thin lms (Fuentes-Ríos et al., 2021). Such porous morphology

encourages potential use of VOx:Co2+ thin lms as a cathode

material for lithium battery applications (Wang et al., 2011).

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149

10 20 30 40 50 60

2θ (°)

JCPDS# 49-0996

Intensity (a.u.)

Figure 2. X-ray diractions patterns of vanadium oxide with

dierent Co2+ concentrations.

Figura 2. Patrones de difracción de rayos X de óxido de

vanadio con diferentes concentraciones de Co+2.

To complement the AFM results, SEM micrographs of

S1, S3 and S5 samples were analyzed and are shown in Figu-

re 4. Without cobalt addition, vanadium oxide sample S1

shows a granular ground with shallow leaves. The S3 sample

(VOx:Cobalt salt = 50:50), presents an interconnected rings or

porous arrangement, very similar to the AFM image of the

Figure 3. Three-dimensional AFM surface morphology images of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5 samples. (f) Roughness values of the

same samples.

Figura 3. Imágenes tridimensionales por AFM de la morfología supercial de las muestras (a) S1, (b) S2, (c) S3, (d) S4 y (e) S5. (f) Valores

de aspereza de las mismas muestras.

Figure 4. SEM micrographs with dierent magnications of a) S1, b) S3, c) S5

and d) histogram of S3 pores.

Figura 4. micrografías de microscopia electrónica de barrido con diferentes

magnicaciones de las muestras a) S1, b) S3, c) S5 y d) histograma de los

poros de la muestra S3.

same sample (Fig. 3c). The pore size distribution histogram

of this lm sample is in Figure 4d. It shows that the pore size

varies from 100 to 300 nm, and those with ~150 nm predo-

minates. By chemical element mapping in the SEM image of

sample S3, vanadium and cobalt elements have been detec-

ted to have a homogeneous distribution inside the pores.

Finally, the sample with the highest concentration of cobalt,

S5, shows an irregular porous granular pattern at the surface,

and under such pattern a continuous lm is observed.

3.5. Chemical composition

To assess the chemical state of the VOx:Co2+ lm samples, an

accurate peak-tting analysis of their X-ray photoelectron

spectra (XPS) (A. Herera-Gomez, no date) was carried on.

150 Volume XXV, Issue 2

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150

Figure 5 shows the photoelectron spectra of the ve samples

corresponding to the (a) Co 2p and (b) O 1s and V 2p core-

levels. From the Co 2p spectra (Figure 5a) we observe that the

experimental data are congruent to that of the Co2+ chemical

state, showing the main peak centered at 780.80 eV, but at

the same time, three satellite peaks at higher binding ener-

gies that are characteristics of the Co2+ state (Yang et al., 2010;

Biesinger et al., 2011; Martínez-Gil et al., 2020). It is important

to mention that the relative intensities of those satellite

peaks suggest that there are neighboring atoms arranged in

a short-range order, possibly having distortions that deviate

from the octahedral Co2+ symmetry. That is, the intensities of

the satellite peaks are proportional to the molecular defects

or amorphous state of the material, which correlates to the

amorphous XRD results for all samples.

The V 2p and the O 1s spectra share the same binding

energy range (Figure 5b), where we can observe the sole

presence of a vanadium oxide. Due to the large spread of the

reported binding energy values of this compound, to assess

the present lms we have employed the method proposed

by Hryha et al. (Hryha, Rutqvist and Nyborg, 2012), where ins-

tead of using only the energy position of the V 2p3/2 for che-

mical state determination, we have taken into account the

dierence in the binding energy between the O 1s and the

V 2p3/2 core levels (Δ= BE(O1s) – BE(V2p3/2)). The results show

that for all samples the energy separation is around 13.50 eV,

which suggests that the oxide obtained with our chemical

process is VO2 (Hryha, Rutqvist and Nyborg, 2012). Here it

is interesting to note that the energy position of the V 2p3/2

in our samples is at 516.50 eV, which is almost the average

value of those reported for VO2 (515.95 eV) and V2O5 (517.20

eV), however, due to also the large spread in binding energy

calibrations in the literature, we discard the possibility of the

V2O5 compound.

From peak intensities directly derived from the pho-

toelectron spectra, chemical compositions of our VO2:Co2+

lm samples have been determined. We have corrected the

peak intensities with the appropriate physical parameters

accounting for the photoemission signal attenuation due to

scattering (Cabrera-German, Gomez-Sosa and Herrera-Go-

mez, 2016). The results expressed in terms of atomic percen-

tage for each of the observed chemical species are presented

in Figure 6a. Here, we observe that as the Co2+concentration

in the precursor solution increases, the amount of Co2+ in the

4500

6000

7500

9000

10500

5500

11000

16500

22000

27500

4500

6000

7500

9000

10500

5500

11000

16500

22000

27500

4500

6000

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9000

10500

5500

11000

16500

22000

27500

4500

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9000

10500

5500

11000

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22000

27500

820 810 800 790 780

4500

6000

7500

9000

10500

540 535 530 525 520 515

5500

11000

16500

22000

27500

Intensity (a.u)

Co LMM

Auger

Co2+

Satellites

O 1sV 2p

O  met

Adventitious

C−O ; C=O

Intensity (a.u)Intensity (a.u)Intensity (a.u)

Intensity (a.u)

Binding energy (eV)

Co 2p

Binding energy (eV)

Figure 5. X-ray photoelectron spectra of VOx:Co2+ lms with dierent concentrations of Co2+.

Figura 5. Espectro de rayos X de películas de VOx:Co2+ con diferentes concentraciones de Co2+.

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151

deposited lms also increases, from 0 to 16.55 at %. On the

other hand, the V4+ atomic concentration in all ve samples

vary slightly, from 31 at % (sample S1) to 27 at % (sample S5),

which suggests that the VOx yield is high in all the probed

samples. We can also observe that the percentage of oxygen

atoms bonded to metals is coherent to the chemical species:

VO2 and CoO, reaching 69 at % for sample S1 and decreasing

up to 56.4 at % for sample S5 as the Co2+ in the lms is the

largest. The atomic ratio of vanadium versus cobalt (V:Co) in

each sample is listed in Table 1, changing from 6.6:1 (S2), 4.9:1

(S3), 2.9:1 (S4) to 1.6:1 (S5).

S1 S2 S3 S4 S5

1.0

1.2

1.4

1.6

1.8

2.0

2.2 b)

Atomic Percent (at%)

Co2+

V4+

O2-

O/Metal ratio

Samples

O/V

O/(V+Co)

Figure 6. XPS chemical composition assessment: (a) atomic percentage (at%)

of elements O, V and Co, and (b) O/V and O/(V+Co) atomic ratios of VOx:Co2+

thin lms.

Figura 6. Evaluación de la composición química por XPS: (a) porcentaje

atómico (at%) de los elementos O, V y Co, y (b) proporciones atómicas O/V y

O/(V+Co) de las películas delgadas de VOx:Co2+.

For a better assessment of metal oxide compositions

in the lm samples, the ratio between oxygen and metal

concentrations is plotted in Figure 6b. Here, the O/V atomic

ratio lies around the expected stoichiometry of VO2 for all

the samples, which further conrms our previous analysis.

Yet, if we determine the atomic ratio of oxygen to the sum

of the two metallic species, O/(V+Co), we observe that such

ratio decreases with increasing concentration of Co2+ in the

reaction solution, a trend that follows the expected stoichio-

metry of the CoO compound.

4. CONCLUSIONS

We demonstrate that amorphous thin lms of cobalt doped

vanadium dioxide can be prepared from spin-coating with a

simple solution method without using any ligand or surfac-

tant. Optical band gaps of those lms are of ∼2.3 eV, with hig-

her absorption coecients for cobalt doped samples. From

detailed quantitative photoelectron analysis, we conclude

that there is only one oxidation state of vanadium in all the

lm samples, V4+, giving predominantly VO2 products. The

formation of cobalt oxide is also conrmed by XPS, showing

a Co2+ oxidation state corresponding to possibly disordered

oxygen atoms in an octahedral, correlates to the amorphous

nature of the lms. Furthermore, the surface morphology

of the VO2:Co2+ thin lms is largely inuenced by the cobalt

concentration, giving interconnected circle pore network

when the V:Co atomic ratio is around 4.9:1. The wide band

gap properties with porous surface structure make these

semiconductor thin lms promising for cathode material in

lithium battery applications.

ACKNOWLEDGMENTS

The authors acknowledge the technical assistance of M.L.

Ramon-Garcia for XRD measurements, R. Moran-Elvira for

SEM analysis, G. Casarrubias-Segura for AFM analysis, and M.R.

Banda in XPS data acquisition. The project USO316007880

from División de Ingeniería at the University of Sonora is

also recognize. Francisco Hernandez Guzman gratefully

acknowledges PRODEP grant with the scholarship No. 511-

6/2019-13924.

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