Epitaxial Growth of Cobalt Oxide Thin Films on Sapphire Substrates Using Atmospheric Pressure Mist Chemical Vapor Deposition

: This study demonstrated the epitaxial growth of single-phase (111) CoO and (111) Co 3 O 4 thin ﬁlms on a -plane sapphire substrates using an atmospheric pressure mist chemical vapor deposition (mist-CVD) process. The phase structure of the grown cobalt oxide ﬁlms was manipulated by controlling the growth temperature and process ambient, conﬁrmed through X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Furthermore, the electrical properties of Co 3 O 4 ﬁlms were signiﬁcantly improved after thermal annealing in oxygen ambient, exhibiting a stable p-type conductivity with an electrical resistivity of 8.35 Ohm cm and a carrier concentration of 4.19 × 10 16 cm − 3 . While annealing CoO in oxygen atmosphere, the Co 3 O 4 ﬁlms were found to be most readily formed on the CoO surface due to the oxidation reaction. The orientation of the atomic arrangement of formed Co 3 O 4 was epitaxially constrained by the underlying CoO epitaxial layer. The oxidation of CoO to Co 3 O 4 was largely driven by outward diffusion of cobalt cations, resulting in the formation of pores in the interior of formed Co 3 O 4 ﬁlms.


Introduction
Metal oxide semiconductors are regarded as promising, next-generation, functional materials due to their various functionalities, including transparency, electrical conductivity, ferroelectricity, ferromagnetism, and superconductivity [1][2][3][4].Thus, they are in demand for a wide range of applications, including transparent electronics, gas sensors, energy storage, thin film transistors, photodetectors, solar cells, catalysis, memory devices, etc.Most metal oxide semiconductors exhibit n-type conductivity, such as Al-doped ZnO, Sn-doped In 2 O 3 , and Sn-doped Ga 2 O 3 .However, several metal oxide semiconductors demonstrate p-type conductivity, including cobalt oxide, nickel oxide, and copper oxide [5].Among these materials, cobalt oxide, studied in this work, has attracted intensive attention in the view of its applications in various fields, including photovoltaics devices [6,7], catalysis [8], gas sensors [9], water splitting [10], resistive switching [11], rechargeable batteries [12], supercapacitors [13], photodetectors [14], and spintronic devices [15].There are two stable cobalt oxide phases: CoO (rock-salt structure, a = 0.426 nm) and Co 3 O 4 (normal spinel structure, a = 0.808 nm).Both phases are direct band gap p-type semiconductors.CoO has a cubic rock-salt structure with high-spin Co 2+ cations occupying the octahedral sites.Co 3 O 4 , on the other hand, has a normal spinel structure, with the mixed Co 2+ and Co 3+ cations occupying the tetrahedral and octahedral sites, respectively.

Epitaxial Growth of CoO and Co 3 O 4 Thin Films on Sapphire Substrates
Based on previous successful work on the growth of ZnO epitaxial films using mist-CVD using the solution of zinc acetate dissolved into a solution of deionized water and acetate acid [28,29], we replaced zinc acetate with cobalt acetate in this work.The cobalt oxide thin films were grown at 450 and 500 • C using mist-CVD under nitrogen and oxygen ambient, respectively.We performed XRD to identify the phase of the grown films.Figure 1 depicts the 2θ-ω scan profiles of cobalt oxide thin films grown under various conditions.Figure 1b,c show the XRD profiles of the films grown under nitrogen ambient at 500 • C (Figure 1b) and oxygen ambient at the temperature of 450 • C (Figure 1c), respectively, revealing that the diffracted peaks at 2θ positions of 36.62 • and 77.83 • correspond to the (111) and (222) planes of the CoO phase (JCPDS No.  and the diffracted peak at 38.59 • and 59.42 • corresponds to the (222) and (333) plane of the Co 3 O 4 phase (JCPDS No. 42-1467), together with intense peaks of sapphire substrate (2θ = 37.7 • ).It is shown that CoO and Co 3 O 4 phases coexisted in both grown cobalt oxide films.Notably, the cobalt oxide film grown under nitrogen ambient at the temperature of 450 • C solely exhibited a single CoO phase, as shown in Figure 1a.On the other hand, only the Co 3 O 4 phase was detected in the cobalt oxide film grown at 500 • C under oxygen ambient, as shown in Figure 1d.Nevertheless, all of the cobalt oxide thin films grown on sapphire substrates under various conditions exhibited the <111> texture.The formation of CoO and Co 3 O 4 significantly depends on the growth temperature and ambient.In general, the usage of metalorganic precursor containing only divalent Co 2+ tends to favor the formation of a less stable CoO phase containing cobalt cations in a lower oxidation state (Co 2+ ) [21,30].However, they can still be used to grow the single-phase Co 3 O 4 films comprising cobalt cations in two different oxidation states, Co 2+ and Co 3+ , by adjusting CVD growth conditions (temperature and ambience).According to the abovementioned results, the use of divalent cobalt (cobalt (II) acetate) precursors for mist-CVD growth facilitated the growth of CoO films at relatively low temperatures in nitrogen (oxygen-deficient) atmosphere.On the other hand, the relatively high temperature and oxygen-rich atmosphere were beneficial for maintaining cobalt in its higher oxidation state, leading to the growth of single-phase Co 3 O 4 .The crystallite sizes of grown CoO and Co 3 O 4 films can be estimated using the Scherrer method based on the results of the 2θ-ω scan [18,31].Utilizing the Scherrer equation and the full-width at half-maximum (FWHM) of diffracted peaks corresponding to CoO (222) and Co 3 O 4 (333), the average crystallite sizes of CoO and Co 3 O 4 were determined to be 53 nm and 30 nm, respectively.In addition, the out-of-plane strains in both films were calculated from the 2θ positions of the diffracted peaks corresponding to CoO (222) and Co 3 O 4 (333), and they were found to be −0.32% and −0.1%, respectively.These negative strains along the out-of-plane direction resulted from the presence of tensile in-plane strain in both of the grown epitaxial films.Furthermore, we carried out Raman analysis to explore the structural details in both of the cobalt oxide films grown at 450 • C in nitrogen ambient for CoO and 500 • C in oxygen ambient for Co 3 O 4 , respectively.Figure 2a,b display the corresponding Raman spectra of CoO and Co 3 O 4 films.In Figure 2a, due to the NaCl-type centrosymmetric lattice structure, CoO exhibited a weak Raman scattering [32,33].Except for the sharp peaks of sapphire substrates, as indicated by an asterisk, a broad asymmetric band with weak intensity appeared at 530 cm −1 , indicated by a square mark.Although the first-order phonon scattering is forbidden based on the selection rules for the NaCl-type lattice structure, the presence of cobalt vacancy or other structural defects in the CoO lattice can lead to the emergence of the first-order phonon scattering.The broad band observed at 530 cm −1 can be attributed to this defect-induced one-phonon longitudinal optical (LO) Raman scatter [32].The Raman spectrum of the Co 3 O 4 film (Figure 2b) reveals the presence of five sharp peaks, which are located at 194, 482, 521, 619, and 691 cm −1 .These peaks are in good agreement with the five Raman active phonon modes (A 1g + 3F 2g + E g ) associated with the spinel Co 3 O 4 reported by Hadjiev et al. [34].In comparison to CoO, the Co 3 O 4 film exhibits lower optical transmittance at 633 nm, leading to the absorption of most incident lasers.However, CoO has higher optical transmittance and weaker Raman scattering due to its NaCl-type centrosymmetric lattice structure.This behavior is likely responsible for the appearance of Raman scattering corresponding to the underlying sapphire substrate.Based on the results of the XRD 2θ-ω scan and Raman scatter analysis, single-phase CoO and Co 3 O 4 films can be obtained by controlling growth temperature and ambience during the mist-CVD process.
for the NaCl-type lattice structure, the presence of cobalt vacancy or other structural defects in the CoO lattice can lead to the emergence of the first-order phonon scattering.The broad band observed at 530 cm −1 can be attributed to this defect-induced one-phonon longitudinal optical (LO) Raman scatter [32].The Raman spectrum of the Co3O4 film (Figure 2b) reveals the presence of five sharp peaks, which are located at 194, 482, 521, 619, and 691 cm −1 .These peaks are in good agreement with the five Raman active phonon modes (A1g + 3F2g + Eg) associated with the spinel Co3O4 reported by Hadjiev et al. [34].In comparison to CoO, the Co3O4 film exhibits lower optical transmittance at 633 nm, leading to the absorption of most incident lasers.However, CoO has higher optical transmittance and weaker Raman scattering due to its NaCl-type centrosymmetric lattice structure.This behavior is likely responsible for the appearance of Raman scattering corresponding to the underlying sapphire substrate.Based on the results of the XRD 2θ-ω scan and Raman scatter analysis, single-phase CoO and Co3O4 films can be obtained by controlling growth temperature and ambience during the mist-CVD process.broad band observed at 530 cm −1 can be attributed to this defect-induced one-phonon longitudinal optical (LO) Raman scatter [32].The Raman spectrum of the Co3O4 film (Figure 2b) reveals the presence of five sharp peaks, which are located at 194, 482, 521, 619, and 691 cm −1 .These peaks are in good agreement with the five Raman active phonon modes (A1g + 3F2g + Eg) associated with the spinel Co3O4 reported by Hadjiev et al. [34].In comparison to CoO, the Co3O4 film exhibits lower optical transmittance at 633 nm, leading to the absorption of most incident lasers.However, CoO has higher optical transmittance and weaker Raman scattering due to its NaCl-type centrosymmetric lattice structure.This behavior is likely responsible for the appearance of Raman scattering corresponding to the underlying sapphire substrate.Based on the results of the XRD 2θ-ω scan and Raman scatter analysis, single-phase CoO and Co3O4 films can be obtained by controlling growth temperature and ambience during the mist-CVD process.however, the triangular-shaped grains could be observed on the surfaces of the Co 3 O 4 films.The thicknesses of the CoO and Co 3 O 4 films, which were grown for one hour, were approximately 294 nm and 180 nm, respectively, as shown in Figure 3c,d.In this work, the growth rates of single-phase CoO and Co 3 O 4 epitaxial films were 294 nm and 180 nm/h, respectively, which were slower than those grown using other CVD processes (230-920 nm/h) [21,35].The cross-sectional SEM image shown in Figure 3d reveals the presence of the columnar grain growth of Co 3 O 4 on the sapphire substrate.The grain sizes measured from the cross-sectional SEM image ranged from 57 to 150 nm.Remarkably, the grain sizes observed through SEM are not equivalent to crystallite sizes estimated from XRD peak widths using the Scherrer equation because a grain may be composed of two or more crystallites.
The corresponding surface morphologies of the CoO and Co3O4 films were observed through SEM, as shown in Figure 3a,b; both as-grown cobalt oxide films possessed uniform surface morphologies.The CoO film exhibited a flat and smooth surface morphology; however, the triangular-shaped grains could be observed on the surfaces of the Co3O4 films.The thicknesses of the CoO and Co3O4 films, which were grown for one hour, were approximately 294 nm and 180 nm, respectively, as shown in Figure 3c,d.In this work, the growth rates of single-phase CoO and Co3O4 epitaxial films were 294 nm and 180 nm/h, respectively, which were slower than those grown using other CVD processes (230-920 nm/h) [21,35].The cross-sectional SEM image shown in Figure 3d reveals the presence of the columnar grain growth of Co3O4 on the sapphire substrate.The grain sizes measured from the cross-sectional SEM image ranged from 57 to 150 nm.Remarkably, the grain sizes observed through SEM are not equivalent to crystallite sizes estimated from XRD peak widths using the Scherrer equation because a grain may be composed of two or more crystallites.Based on the results of XRD 2θ-ω scans, regardless of the growth conditions (growth temperature or ambient), the cobalt oxide films grown on a-plane sapphire substrates using mist-CVD exhibited <111> preferred orientation.To characterize the orientation relationship between cobalt oxide films and a-plane sapphire substrates, the XRD φ-scan measurement of CoO and Co3O4 films was performed, respectively.Figure 4a,b show the XRD φ-scan profiles of CoO and Co3O4 films grown on a-plane sapphire substrates.Remarkably, the XRD φ-scan profiles of {200} reflection of CoO film as well as {400} reflection of Co3O4 film exhibited six peaks separated by 60°, consisting of two sets of three-fold symmetric peaks separated by 60°, due to the crystal structures of rock salt CoO and spinel Co3O4 having three-fold symmetry along the <111> direction.This confirms the presence of 60°rotated twin domains in either CoO or Co3O4 films grown on a-plane sapphire substrates.A single grain of CoO or Co3O4 can contain multiple twin domains, resulting in the small crystallite sizes determined using the Scherrer equation.Based on the results of XRD φscans, the obtained CoO films were epitaxially grown on sapphire substrate with the Based on the results of XRD 2θ-ω scans, regardless of the growth conditions (growth temperature or ambient), the cobalt oxide films grown on a-plane sapphire substrates using mist-CVD exhibited <111> preferred orientation.To characterize the orientation relationship between cobalt oxide films and a-plane sapphire substrates, the XRD ϕ-scan measurement of CoO and Co 3 O 4 films was performed, respectively.Figure 4a 222) plane, respectively.It was found that the FWHM values of both XRCs were 0.25 • , determined by fitting a single pseudo-Voigt function, indicating that the epitaxial quality of the cobalt oxide films grown using atmospheric pressure mist-CVD was comparable to those grown using the conventional vacuum-based processes [18,20,[35][36][37][38].The threading dislocation densities of epitaxial films can be determined from the XRD rocking curves.The dislocation density (D) can be calculated using the measured FWHM of the XRC with the following equation [39]: where b is the Burgers vector of dislocation.The Burgers vectors in both rock-salt and spinel structures are a 0 /2 110 [40,41]; hence, the Burgers vectors of CoO and Co 3 O 4 are 0.304 nm and 0.5716 nm, respectively.Thus, the dislocation densities in the CoO and Co 3 O 4 epitaxial films were estimated to be 4.7 × 10 9 cm −2 and 1.3 × 10 9 cm −2 , respectively.
Coatings 2023, 13, x FOR PEER REVIEW 6 of 17 following epitaxial orientation relationship of (111) [11 0]CoO||(112 0)[0001]Al2O3; however, the epitaxial relationship between Co3O4 film and sapphire substrate was determined as (111)[112 ]Co3O4||(112 0)[0001]Al2O3, indicating the rotation of the in-plane axis of Co3O4 film on sapphire substrate through an angle of 30° with respect to that of the CoO film.Moreover, the epitaxial qualities of cobalt oxide films were evaluated through the XRD rocking curve (XRC).Figure 4c,d show the XRC profiles of the CoO (111) plane and Co3O4 (222) plane, respectively.It was found that the FWHM values of both XRCs were 0.25°, determined by fitting a single pseudo-Voigt function, indicating that the epitaxial quality of the cobalt oxide films grown using atmospheric pressure mist-CVD was comparable to those grown using the conventional vacuum-based processes [18,20,[35][36][37][38].The threading dislocation densities of epitaxial films can be determined from the XRD rocking curves.The dislocation density (D) can be calculated using the measured FWHM of the XRC with the following equation [39]: where b is the Burgers vector of dislocation.The Burgers vectors in both rock-salt and spinel structures are  2 ⁄ 〈110〉 [40,41]; hence, the Burgers vectors of CoO and Co3O4 are 0.304 nm and 0.5716 nm, respectively.Thus, the dislocation densities in the CoO and Co3O4 epitaxial films were estimated to be 4.7 10 cm −2 and 1.3 10 cm −2 , respectively.CoO film in the visible region was in the range of 30-40%.For the Co 3 O 4 epitaxial film, the corresponding UV-Vis spectrum (Figure 5c) shows an increase in the transmittance as the wavelength increases, with two absorptions at approximately 600 nm and 800 nm, respectively.The optical absorption coefficient (α) can be estimated from the transmittance (T) by the following equation: where d is the film thickness.The optical bandgap (E g ) can be determined from (αhν) 2  versus the hν plot (Tauc Plot), as shown in Figure 5b,d, where hν is the photon energy.
The extrapolation of the linear region of the Tauc plot led to the optical bandgaps of the thin films.Based on Figure 5b, the optical bandgap of the CoO film was estimated to be 2.81 eV, consistent with that previously reported [18,22].In contrast, the linearly fitted segments in the Tauc plot of the Co 3 O 4 film (Figure 5d) prove the presence of two energy levels of the direct allowed transitions at 1.41 eV and 2.03 eV, respectively.As mixed Co 2+ and Co 3+ cations occupy the tetrahedral and octahedral sites in the spinel Co 3 O 4 lattice, the charge transfer process associated with the Co 3+ -Co 2+ and an oxide ligand (O 2− ) to metal (Co 2+ ) transition, respectively, were identified.The bandgap of 1.41 eV corresponds to the Co 3+ -Co 2+ charge transfer transition, reported within the range of 1.4-1.5 eV [16,21,22,42].The bandgap of 2.03 eV is interpreted as an oxide ligand (O 2− ) to metal (Co 2+ ) charge transfer transition, in agreement with the values (1.9-2.4 eV) reported in earlier studies [16,21,22,42].
Coatings 2023, 13, x FOR PEER REVIEW 7 of 17 increases, with two absorptions at approximately 600 nm and 800 nm, respectively.The optical absorption coefficient (α) can be estimated from the transmittance (T) by the following equation: where d is the film thickness.The optical bandgap (Eg) can be determined from (αhν) 2  versus the hν plot (Tauc Plot), as shown in Figure 5b,d, where hν is the photon energy.
The extrapolation of the linear region of the Tauc plot led to the optical bandgaps of the thin films.Based on Figure 5b, the optical bandgap of the CoO film was estimated to be 2.81 eV, consistent with that previously reported [18,22].In contrast, the linearly fitted segments in the Tauc plot of the Co3O4 film (Figure 5d) prove the presence of two energy levels of the direct allowed transitions at 1.41 eV and 2.03 eV, respectively.As mixed Co 2+ and Co 3+ cations occupy the tetrahedral and octahedral sites in the spinel Co3O4 lattice, the charge transfer process associated with the Co 3+ -Co 2+ and an oxide ligand (O 2− ) to metal (Co 2+ ) transition, respectively, were identified.The bandgap of 1.41 eV corresponds to the Co 3+ -Co 2+ charge transfer transition, reported within the range of 1.4-1.5 eV [16,21,22,42].The bandgap of 2.03 eV is interpreted as an oxide ligand (O 2− ) to metal (Co 2+ ) charge transfer transition, in agreement with the values (1.9-2.4 eV) reported in earlier studies [16,21,22,42].The electrical properties of CoO films were evaluated using the Hall-effect measurement.The electrical resistivities of the as-grown CoO and Co3O4 films were 1.8 × 10 5 Ω-cm and 2.3 × 10 2 Ω-cm, respectively, which are consistent with the values reported by other studies (>10 4 Ohm cm for CoO; 1~10 3 Ohm cm for Co3O4) [22].Based on the results reported by previous studies, the electrical performance of various metal-oxide semiconductor films can be further improved via annealing at the temperature higher than that used in the film growth process [43,44].Therefore, the annealing temperature of 650 °C, studies (>10 4 Ohm cm for CoO; 1~10 3 Ohm cm for Co 3 O 4 ) [22].Based on the results reported by previous studies, the electrical performance of various metal-oxide semiconductor films can be further improved via annealing at the temperature higher than that used in the film growth process [43,44].Therefore, the annealing temperature of 650 • C, being higher than that used in the mist-CVD growth process, was chosen to further modify the electrical properties of the grown films.Notably, the electrical properties of CoO and Co 3 O 4 films can be further improved using a post-thermal annealing treatment at 650 • C in oxygen ambient for 30 min.After thermal annealing, the electrical properties of annealed Co 3 O 4 film were significantly enhanced, showing that the electrical resistivity was 8. 35 Ohm cm, and the carrier concentration, with a positive Hall coefficient sign, increased from 1.5 × 10 15 cm −3 to 4.19 × 10 16 cm −3 .In contrast, the as-grown CoO film behaved almost as an electrical insulator due to its extremely high electrical resistivity and low carrier concentration (2.3 × 10 12 cm −3 ).When annealing the sample of CoO film in oxygen ambient, the sample showed p-type conductivity with the electrical resistivity and carrier concentration of 14.2 Ohm cm and 4.37 × 10 16 cm −3 .Annealing in an oxygen atmosphere significantly improves the electrical properties of both cobalt oxide thin films, which can increase carrier concentration to the order of magnitude of 10 16 cm −3 .When annealing in an oxygen-rich environment, the cobalt vacancies are the dominant point defects in cobalt oxide lattices and act as electron acceptors, which are the origin of hole carriers and p-type conductivity [45].For the annealing of CoO film, the oxidation of CoO to Co 3 O 4 readily occurred while heating at 300-650 • C in the presence of oxygen [46].Therefore, the possible reason for improved electrical conductivity in the samples of annealed CoO film is attributed to the transformation into the Co 3 O 4 phase through an oxidation process.

Thermal Annealing of CoO Epitaxial Films
Although improved electrical conductivity in the samples of annealed CoO film can possibly be attributed to the formation of the Co 3 O 4 phase, the details of microstructural and chemical information of Co 3 O 4 converted from CoO was unknown.Thus, the main aim of this study in this section is to explore the microstructural evolution during phase transition from CoO to Co 3 O 4 .To further prove the phase transition of CoO to Co 3 O 4 during thermal annealing, XRD and Raman measurements were implemented, respectively, as shown in Figure 6.The result of the 2θ-ω scan profile revealed that the phase corresponding to the annealed sample of CoO film was Co 3 O 4 (Figure 6a).The average crystallite size of the corresponding Co 3 O 4 film was 35 nm, which was estimated from the diffraction peak width using the Scherrer equation.In addition, the Raman spectrum recorded from the annealed sample was consistent with the typical characteristic Raman peaks of spinel Co 3 O 4 (Figure 6b).From the above results, it is shown that cobalt monoxide (CoO) was fully converted to tricobalt tetroxide (Co 3 O 4 ) through an oxidation reaction, labeled as Co 3 O 4(Oxid) , during the annealing process in oxygen ambient.Figure 7 shows the top-view and cross-sectional SEM images of the annealed CoO film (Co 3 O 4(Oxid) ).In contrast to the flat surface of the CoO film before the annealing process (Figure 3a), the surface of the annealed CoO film exhibited an undulating and uneven morphology (Figure 7a).The average thickness of the annealed CoO film was approximately 300 nm.The density of CoO (6.45 g/cm 3 ) is higher than that of Co 3 O 4 (6.11g/cm 3 ).Thus, while CoO films were transformed into Co 3 O 4 during thermal annealing, the volume (thickness) of the formed Co 3 O 4 films was slightly increased.Remarkably, the pore-like structures present in the interiors of the films can be observed from the cross-sectional SEM image (Figure 7b).The details regarding the microstructure need to be further characterized through TEM and will be discussed in the following paragraphs.To further investigate the valence and chemical state, X-ray photoelectron spectroscopy was used to characterize as-grown CoO, Co3O4 converted from CoO (Co3O4(Oxid)), and CVD-grown Co3O4 films.Figure 8 shows survey spectra of as-grown CoO, Co3O4 converted from CoO (Co3O4(Oxid)), and CVD-grown Co3O4 films.These XPS spectra dominantly show the element of Co and O along with the signal of carbon impurity (C 1s).The corresponding atomic concentrations (at.%) of cobalt were 48.3, 35.7, and 35.1 at.%, respectively, which were lower than those of cobalt oxides with stoichiometry (CoO: 50 at.%;Co3O4: 42.9 at.%).This confirms that all of the cobalt oxide films were metal-deficient-type oxides, indicating that cobalt vacancies were the dominant point defects in cobalt oxide lattices.The Co 2p spectra of CoO, Co3O4(Oxid), and CVD-grown Co3O4, are shown in Figure 8a-c.The Co 2p core level splits into 2p 3/2 and 2p 1/2 due to spin-orbit coupling.For asgrown CoO film (Figure 9a), the spin-orbit doublet was observed at a binding energy (B.E.) of 780 and 795.8 eV with a spin-orbit splitting of 15.8 eV between Co 2p 3/2 and 2p 1/2 core levels, along with an intense shake-up satellite peak at 786 eV, consistent with the characteristic XPS spectrum of CoO reported by other literatures [22,47].The Co 2p spectrum of the Co3O4(Oxid) film (Figure 9b) was similar to that of the CVD-grown Co3O4 film (Figure 9c).The two main peaks corresponding to Co 2p 3/2 and Co 2p 1/2 core levels were located at 780 and 795.1 eV with spin-orbit splitting of 15.1 eV, together with a weak and broadened shake-up satellite peak located at 789 eV, consistent with the characteristic XPS spectrum of Co3O4 reported by other studies [10,22].The Co 2+ and Co 3+ cations coexist in Co3O4; therefore, the Co 2p doublet peaks can be deconvoluted into four subpeaks, where the fitting peaks at 779.To further investigate the valence and chemical state, X-ray photoelectron spectroscopy was used to characterize as-grown CoO, Co3O4 converted from CoO (Co3O4(Oxid)), and CVD-grown Co3O4 films.Figure 8 shows survey spectra of as-grown CoO, Co3O4 converted from CoO (Co3O4(Oxid)), and CVD-grown Co3O4 films.These XPS spectra dominantly show the element of Co and O along with the signal of carbon impurity (C 1s).The corresponding atomic concentrations (at.%) of cobalt were 48.3, 35.7, and 35.1 at.%, respectively, which were lower than those of cobalt oxides with stoichiometry (CoO: 50 at.%;Co3O4: 42.9 at.%).This confirms that all of the cobalt oxide films were metal-deficient-type oxides, indicating that cobalt vacancies were the dominant point defects in cobalt oxide lattices.The Co 2p spectra of CoO, Co3O4(Oxid), and CVD-grown Co3O4, are shown in Figure 8a-c.The Co 2p core level splits into 2p 3/2 and 2p 1/2 due to spin-orbit coupling.For asgrown CoO film (Figure 9a), the spin-orbit doublet was observed at a binding energy (B.E.) of 780 and 795.8 eV with a spin-orbit splitting of 15.8 eV between Co 2p 3/2 and 2p 1/2 core levels, along with an intense shake-up satellite peak at 786 eV, consistent with the characteristic XPS spectrum of CoO reported by other literatures [22,47].The Co 2p spectrum of the Co3O4(Oxid) film (Figure 9b) was similar to that of the CVD-grown Co3O4 film (Figure 9c).The two main peaks corresponding to Co 2p 3/2 and Co 2p 1/2 core levels were located at 780 and 795.1 eV with spin-orbit splitting of 15.1 eV, together with a weak and broadened shake-up satellite peak located at 789 eV, consistent with the characteristic XPS spectrum of Co3O4 reported by other studies [10,22].The Co 2+ and Co 3+ cations coexist in Co3O4; therefore, the Co 2p doublet peaks can be deconvoluted into four subpeaks, where the fitting peaks at 779.6 and 794.8 eV are assigned as the Co 3+ state and the fitting peaks at 781 and 796 eV are ascribed to the Co 2+ state.The signals of Co 3+ and Co 2+ in the Co 2p  8a-c.The Co 2p core level splits into 2p 3/2 and 2p 1/2 due to spin-orbit coupling.For as-grown CoO film (Figure 9a), the spin-orbit doublet was observed at a binding energy (B.E.) of 780 and 795.8 eV with a spin-orbit splitting of 15.8 eV between Co 2p 3/2 and 2p 1/2 core levels, along with an intense shake-up satellite peak at 786 eV, consistent with the characteristic XPS spectrum of CoO reported by other literatures [22,47].The Co 2p spectrum of the Co 3 O 4(Oxid) film (Figure 9b) was similar to that of the CVD-grown Co 3 O 4 film (Figure 9c).The two main peaks corresponding to Co 2p 3/2 and Co 2p 1/2 core levels were located at 780 and 795.1 eV with spin-orbit splitting of 15.1 eV, together with a weak and broadened shake-up satellite peak located at 789 eV, consistent with the characteristic XPS spectrum of Co 3 O 4 reported by other studies [10,22] The contribution of oxygen bonded to cobalt in the lattice was estimated to be 72.8%,72.0%, and 75.6%, respectively, indicating that all of the cobalt oxide films retain more structural oxygen in the lattice.Considering a water/acetate solution of cobalt acetate used as the cobalt source, the incorporation of carbon from the source is still possible [49].Therefore, the residual impurities, such as carbon, in grown films were also detected through XPS.Although the signal of carbon 1s core level emission was not detected from the sample after argon ion sputtering, as shown in Figure S1 in Supplementary information, it is possible that the carbon content was below the instrument detection limit.In future work, secondary ion mass spectroscopy (SIMS) is needed for the detection of the residual impurity with low concentration.
Coatings 2023, 13, x FOR PEER REVIEW 10 of 17 spectra were numerically integrated to obtain the ratios of Co 3+ /Co 2+ cations of Co3O4(Oxid) and CVD-grown Co3O4 films, respectively, estimated to be 0.50 and 0.89, respectively, which are both lower than the theoretical ratio of Co 3+ /Co 2+ in Co3O4 (2:1).The Co 3+ vacancies (VCo3+) at the octahedral sites in spinel Co3O4 were indicated to dominate in both samples [45].Remarkably, compared to CVD-grown Co3O4, Co3O4(Oxid) thin films, converted from CoO to Co3O4, revealed a lower Co 3+ /Co 2+ ratio, possibly due to the initial CoO phase solely comprising divalent cobalt.Theoretical and experimental studies have proven that the formation of the cation vacancies in metal oxides is favorable in an oxygen-rich environment [7,45,48].Thus, the Co vacancies in Co3O4 were regarded as the major origin of their p-type semiconducting properties.The O 1s spectra of CoO, Co3O4(Oxid), and CVDgrown Co3O4, as shown in Figure 9d, are deconvoluted into three components at approximately 530, 531.2, and 532.2 eV, which is attributed to oxygen bonded with cobalt in the lattice (Olat), hydroxyl groups bonded to surface defects, such as oxygen vacancies (OV), and weakly absorbed molecular water (Oabs), respectively.The difference between the three O 1s core level XPS spectra recorded from CoO, Co3O4(Oxid), and CVD-grown Co3O4 films was not obvious.The contribution of oxygen bonded to cobalt in the lattice was estimated to be 72.8%,72.0%, and 75.6%, respectively, indicating that all of the cobalt oxide films retain more structural oxygen in the lattice.Considering a water/acetate solution of cobalt acetate used as the cobalt source, the incorporation of carbon from the source is still possible [49].Therefore, the residual impurities, such as carbon, in grown films were also detected through XPS.Although the signal of carbon 1s core level emission was not detected from the sample after argon ion sputtering, as shown in Figure S1 in Supplementary information, it is possible that the carbon content was below the instrument detection limit.In future work, secondary ion mass spectroscopy (SIMS) is needed for the detection of the residual impurity with low concentration.According to the studies reported on the oxidation of CoO to Co3O4, the formation of the Co3O4(Oxid) layer initially occurs on the surface of CoO [50,51].Therefore, the crystallographic alignment of the Co3O4(Oxid) film formed through the oxidation reaction is closely related to the orientation of the underlying CoO epitaxial film [50,51].The X-ray diffraction φ-scan and XRC analysis were employed to assess the relationship and the crystalline quality of the formed Co3O4(Oxid) film.Figure 10a shows the XRD φ-scan profiles for the Co3O4(Oxid) film detecting Co3O4 (400) and Al2O3 (112 0) reflections.The profile of the Co3O4(Oxid) film revealed six-fold symmetry with a spacing of 60°, indicating that the Co3O4(Oxid) film was in epitaxy with the sapphire substrate.Similarly, the presence of 60°rotated twin domains in the Co3O4(Oxid) film was detected from the X-ray diffraction φ-scan profile.Interestingly, the crystallographic relationship between the Co3O4(Oxid) film and sapphire is (111) [ 4b.In Figure 10b, the Co3O4(Oxid) presents a broader FWHM (0.62°) of XRC for Co3O4 (222) diffraction.According to the cross-sectional SEM observation (Figure 7b), there were several pore-like structures in the interiors of the Co3O4(Oxid) films.These pore-like structures affected the growth of crystallites during the annealing process and were possibly responsible for the broader width of the XRC.The formation of the Co3O4 layer initially occurred on the surface of CoO.Sequentially, the entire CoO layer was gradually converted into Co3O4 during the oxidation process, as illustrated in Figure 11.The oxygen sublattices of the CoO and Co3O4, both being fcc sublattices, are nearly identical, with a lattice mismatch of about 5% [50].According to the studies reported on the oxidation of CoO to Co 3 O 4 , the formation of the Co 3 O 4(Oxid) layer initially occurs on the surface of CoO [50,51].Therefore, the crystallographic alignment of the Co 3 O 4(Oxid) film formed through the oxidation reaction is closely related to the orientation of the underlying CoO epitaxial film [50,51].The Xray diffraction ϕ-scan and XRC analysis were employed to assess the relationship and the crystalline quality of the formed Co 3 O 4(Oxid) film.Figure 10a    We performed cross-sectional TEM to gain insight into the microstructure of the Co3O4(Oxid) film converted from CoO. Importantly, several pores present in the interior of Co3O4(Oxid) films were observed from the low-magnification bright-field TEM image (Figure 12a).During the oxidation of CoO to Co3O4, the growth of Co3O4 is initiated on the surface of CoO.Then, the Co3O4 layer grows inward and outward simultaneously [52][53][54].In the case of outward growth, the formation of Co3O4 occurs due to the outward diffusion of cobalt cations and electrons as a result of the following reaction [52][53][54]:   We performed cross-sectional TEM to gain insight into the microstructure of the Co3O4(Oxid) film converted from CoO. Importantly, several pores present in the interior of Co3O4(Oxid) films were observed from the low-magnification bright-field TEM image (Figure 12a).During the oxidation of CoO to Co3O4, the growth of Co3O4 is initiated on the surface of CoO.Then, the Co3O4 layer grows inward and outward simultaneously [52][53][54].In the case of outward growth, the formation of Co3O4 occurs due to the outward diffusion of cobalt cations and electrons as a result of the following reaction [52][53][54]: In the case of outward growth, the formation of Co 3 O 4 occurs due to the outward diffusion of cobalt cations and electrons as a result of the following reaction [52][53][54]: where Co •• i and e denote the double ionized interstitial cobalt cation and a free electron, respectively.The migrating interstitial cobalt cations reacted with oxygen on the surface to form Co 3 O 4 .In the case of inward growth, the reaction occurred at the interface between CoO and Co 3 O 4 , according to the displacement reaction [52][53][54]: CoO and Co3O4, according to the displacement reaction [52][53][54]: Based on the marker experiment for studying the oxidation of CoO, it was observed that cobalt cations continued to diffuse outward through the Co3O4 layer during the oxidation reaction [53,54].Consequently, this resulted in the excessive concentration of cobalt vacancies inside the film layer due to the directional diffusion cobalt cations (the so-called Kirkendall effect).Thus, the thickness of formed Co3O4 films became thicker after the annealing process.The formation of pores was the consequence of a supersaturation of vacancies in the lattice brought about by the coalescence of cobalt vacancies [52,54].In Figure 12b, the pattern of the selected area electron diffraction (SAED) recorded from the interface region shows the presence of two twinned variants in the formed Co3O4(Oxid) film, designated by green and yellow dashed lines, both indexed as the normal and 60°-rotated twin variants, respectively.They were oriented epitaxially with respect to the sapphire substrate.The epitaxial relationship between Co3O4(Oxid) and the sapphire substrate was identified as (111)[11 0]Co3O4(Oxid)||(112 0)[0001]Al2O3, which is consistent with the result of the XRD φ-scan, as shown in Figure 10a.Despite the presence of numerous pore structures in the formed Co3O4(Oxid) layer, the diffraction pattern of the Co3O4(Oxid) film remained consistent throughout the entire film layer.The microstructure near the film/substrate heterointerface was visualized using HRTEM, as shown in Figure 13a.This view is along the sapphire [0001] zone axis.Remarkably, the interface between the formed Co3O4(Oxid) layer and the sapphire substrate is not atomically abrupt; instead, it appears intermixed.It is possible that during the annealing process at 650 °C for 30 min, the occurrence of slight interdiffusion between the formed Co3O4(Oxid) film and the sapphire substrate is not ignored.Furthermore, the corresponding fast Fourier transformation pattern (Figure 13b) of the HRTEM image was consistent with the SAED pattern (Figure 12b).In Figure 13c, the Fourier-filtered image, reconstructed Based on the marker experiment for studying the oxidation of CoO, it was observed that cobalt cations continued to diffuse outward through the Co 3 O 4 layer during the oxidation reaction [53,54].Consequently, this resulted in the excessive concentration of cobalt vacancies inside the film layer due to the directional diffusion cobalt cations (the so-called Kirkendall effect).Thus, the thickness of formed Co 3 O 4 films became thicker after the annealing process.The formation of pores was the consequence of a supersaturation of vacancies in the lattice brought about by the coalescence of cobalt vacancies [52,54].In Figure 12b, the pattern of the selected area electron diffraction (SAED) recorded from the interface region shows the presence of two twinned variants in the formed The microstructure near the film/substrate heterointerface was visualized using HRTEM, as shown in Figure 13a.This view is along the sapphire [0001] zone axis.Remarkably, the interface between the formed Co 3 O 4(Oxid) layer and the sapphire substrate is not atomically abrupt; instead, it appears intermixed.It is possible that during the annealing process at 650 • C for 30 min, the occurrence of slight interdiffusion between the formed Co 3 O 4(Oxid) film and the sapphire substrate is not ignored.Furthermore, the corresponding fast Fourier transformation pattern (Figure 13b) of the HRTEM image was consistent with the SAED pattern (Figure 12b).In Figure 13c, the Fourier-filtered image, reconstructed using the Co 3 O 4 222 and sapphire 2110 reflections, reveals a lattice-matched interface, accompanied by several of the extra-half planes of misfit dislocations located at the interface, indicated by black triangles, to accommodate the lattice mismatch.The misfit dislocations at the interface region were not arranged in a periodic or evenly spaced manner.It is evident that the paradigm of the domain match epitaxy does not seem to be applicable to this intermixed interface.Limited by the spatial resolution of conventional HRTEM, the atomic arrangement of different elements located near the interface cannot be clearly identified.To explore the explicit mechanism governing this interface, achieving sub-angstrom resolution high angle annular dark field (HAADF) imaging using scanning transmission electron microscopy (STEM) with a spherical aberration corrector is highly necessary in future research.Such imaging will help identify the position of atomic columns at the interface region.Despite the presence of pores in the films, the lattice-matched interface between the formed Co 3 O 4(Oxid) film and sapphire substrate was confirmed by the HRTEM image.
Coatings 2023, 13, x FOR PEER REVIEW 14 of 17 using the Co3O4 222 and sapphire 21 1 0 reflections, reveals a lattice-matched interface, accompanied by several of the extra-half planes of misfit dislocations located at the interface, indicated by black triangles, to accommodate the lattice mismatch.The misfit dislocations at the interface region were not arranged in a periodic or evenly spaced manner.It is evident that the paradigm of the domain match epitaxy does not seem to be applicable to this intermixed interface.Limited by the spatial resolution of conventional HRTEM, the atomic arrangement of different elements located near the interface cannot be clearly identified.To explore the explicit mechanism governing this interface, achieving sub-angstrom resolution high angle annular dark field (HAADF) imaging using scanning transmission electron microscopy (STEM) with a spherical aberration corrector is highly necessary in future research.Such imaging will help identify the position of atomic columns at the interface region.Despite the presence of pores in the films, the lattice-matched interface between the formed Co3O4(Oxid) film and sapphire substrate was confirmed by the HRTEM image.

Conclusions
In this study, we demonstrated selective epitaxial growth of the single-phase rocksalt CoO and spinel Co3O4 thin films on sapphire substrates using mist-CVD by controlling growth temperature and ambient, as confirmed by XRD, Raman, and XPS spectroscopy measurements.The surface morphologies of the obtained CoO films exhibited a flat and smooth surface morphology; in contrast, the triangular-shaped grains were observed on the surfaces of the Co3O4 films.The epitaxial relationship of CoO and Co3O4 on sapphire substrates was determined to be (111) [

Conclusions
In this study, we demonstrated selective epitaxial growth of the single-phase rock-salt CoO and spinel Co 3 O 4 thin films on sapphire substrates using mist-CVD by controlling growth temperature and ambient, as confirmed by XRD, Raman, and XPS spectroscopy measurements.The surface morphologies of the obtained CoO films exhibited a flat and smooth surface morphology; in contrast, the triangular-shaped grains were observed on the surfaces of the Co

Figure 1 .
Figure 1.X-ray diffraction 2θ-ω scan profiles of the films grown under nitrogen ambient at (a) 450 °C and (b) 500 °C and under oxygen ambient at temperatures of (c) 450 °C and (d) 500 °C, respectively.

Figure 2 .
Figure 2. The Raman spectra of (a) CoO and (b) Co3O4 films.The asterisk and square in (a) indicate Raman scattering peaks of sapphire substrate and CoO film, respectively.

Figure 1 .
Figure 1.X-ray diffraction 2θ-ω scan profiles of the films grown under nitrogen ambient at (a) 450 • C and (b) 500 • C and under oxygen ambient at temperatures of (c) 450 • C and (d) 500 • C, respectively.

Figure 1 .
Figure 1.X-ray diffraction 2θ-ω scan profiles of the films grown under nitrogen ambient at (a) 450 °C and (b) 500 °C and under oxygen ambient at temperatures of (c) 450 °C and (d) 500 °C, respectively.

Figure 2 .
Figure 2. The Raman spectra of (a) CoO and (b) Co3O4 films.The asterisk and square in (a) indicate Raman scattering peaks of sapphire substrate and CoO film, respectively.

Figure 2 .
Figure 2. The Raman spectra of (a) CoO and (b) Co 3 O 4 films.The asterisk and square in (a) indicate Raman scattering peaks of sapphire substrate and CoO film, respectively.The corresponding surface morphologies of the CoO and Co 3 O 4 films were observed through SEM, as shown in Figure 3a,b; both as-grown cobalt oxide films possessed uniform surface morphologies.The CoO film exhibited a flat and smooth surface morphology;
,b show the XRD ϕ-scan profiles of CoO and Co 3 O 4 films grown on a-plane sapphire substrates.Remarkably, the XRD ϕ-scan profiles of {200} reflection of CoO film as well as {400} reflection of Co 3 O 4 film exhibited six peaks separated by 60 • , consisting of two sets of three-fold symmetric peaks separated by 60 • , due to the crystal structures of rock salt CoO and spinel Co 3 O 4 having three-fold symmetry along the <111> direction.This confirms the presence of 60 • -rotated twin domains in either CoO or Co 3 O 4 films grown on a-plane sapphire substrates.A single grain of CoO or Co 3 O 4 can contain multiple twin domains, resulting in the small crystallite sizes determined using the Scherrer equation.Based on the results of XRD ϕ-scans, the obtained CoO films were epitaxially grown on sapphire substrate with the following epitaxial orientation relationship of (111)[110] CoO ||(1120)[0001] Al2O3 ; however, the epitaxial relationship between Co 3 O 4 film and sapphire substrate was determined as (111)[112] Co3O4 ||(1120)[0001] Al2O3 , indicating the rotation of the in-plane axis of Co 3 O 4 film on sapphire substrate through an angle of 30 • with respect to that of the CoO film.Moreover, the epitaxial qualities of cobalt oxide films were evaluated through the XRD rocking curve (XRC).Figure 4c,d show the XRC profiles of the CoO (111) plane and Co 3 O 4 (

Figure 4 .
Figure 4. X-ray diffraction ϕ-scan profiles of (a) CoO {200} and sapphire {1120} reflections for the CoO sample, and (b) Co 3 O 4 {400} and sapphire {1120} reflections for the Co 3 O 4 sample.Rocking curve profiles of (c) CoO (111) and (d) Co 3 O 4 (222) reflections.The optical properties of CoO and Co 3 O 4 epitaxial films were examined through UV-visible (UV-Vis) spectroscopy.Figure 5a,c are the UV-Vis transmission spectra of CoO and Co 3 O 4 epitaxial films.The transmittance spectrum of the CoO film shows an absorption edge between approximately 400 and 500 nm.The average transmittance of

Figure 5 .
Figure 5. UV-visible transmission spectra of (a) CoO and (c) Co 3 O 4 films; corresponding Tauc plots of (αhν) 2 versus photo energy for (b) CoO and (d) Co 3 O 4 films.The electrical properties of CoO films were evaluated using the Hall-effect measurement.The electrical resistivities of the as-grown CoO and Co 3 O 4 films were 1.8 × 10 5 Ω-cm and 2.3 × 10 2 Ω-cm, respectively, which are consistent with the values reported by other

Figure 6 .
Figure 6.(a) X-ray diffraction 2θ-ω scan profile and (b) Raman spectrum of the CoO film sample annealed at 650 °C in oxygen ambience.

Figure 7 .
Figure 7. (a) Top-view and (b) cross-sectional SEM micrographs of the CoO film annealed at 650 °C in oxygen ambience.
6 and 794.8 eV are assigned as the Co 3+ state and the fitting peaks at 781 and 796 eV are ascribed to the Co 2+ state.The signals of Co 3+ and Co 2+ in the Co 2p

Figure 6 . 17 Figure 6 .
Figure 6.(a) X-ray diffraction 2θ-ω scan profile and (b) Raman spectrum of the CoO film sample annealed at 650 • C in oxygen ambience.

Figure 7 .
Figure 7. (a) Top-view and (b) cross-sectional SEM micrographs of the CoO film annealed at 650 °C in oxygen ambience.

Figure 7 .
Figure 7. (a) Top-view and (b) cross-sectional SEM micrographs of the CoO film annealed at 650 • C in oxygen ambience.To further investigate the valence and chemical state, X-ray photoelectron spectroscopy was used to characterize as-grown CoO, Co 3 O 4 converted from CoO (Co 3 O 4(Oxid) ), and CVD-grown Co 3 O 4 films.Figure 8 shows survey spectra of as-grown CoO, Co 3 O 4 converted from CoO (Co 3 O 4(Oxid) ), and CVD-grown Co 3 O 4 films.These XPS spectra dominantly show the element of Co and O along with the signal of carbon impurity (C 1s).The corresponding atomic concentrations (at.%) of cobalt were 48.3, 35.7, and 35.1 at.%, respectively, which were lower than those of cobalt oxides with stoichiometry (CoO: 50 at.%;Co 3 O 4 : 42.9 at.%).This confirms that all of the cobalt oxide films were metal-deficient-type oxides, indicating that cobalt vacancies were the dominant point defects in cobalt oxide lattices.The Co 2p spectra of CoO, Co 3 O 4(Oxid) , and CVD-grown Co 3 O 4 , are shown in Figure8a-c.The Co 2p core level splits into 2p 3/2 and 2p 1/2 due to spin-orbit coupling.For as-grown CoO film (Figure9a), the spin-orbit doublet was observed at a binding energy (B.E.) of 780 and 795.8 eV with a spin-orbit splitting of 15.8 eV between Co 2p 3/2 and 2p 1/2 core levels, along with an intense shake-up satellite peak at 786 eV, consistent with the characteristic XPS spectrum of CoO reported by other literatures[22,47].The Co 2p spectrum of the Co 3 O 4(Oxid) film (Figure9b) was similar to that of the CVD-grown Co 3 O 4 film (Figure9c).The two main peaks corresponding to Co 2p 3/2 and Co 2p 1/2 core levels were located at 780 and 795.1 eV with spin-orbit splitting of 15.1 eV, together with a weak and broadened shake-up satellite peak located at 789 eV, consistent with the characteristic XPS spectrum of Co 3 O 4 reported by other studies[10,22].The Co 2+ and Co 3+ cations coexist in Co 3 O 4 ; therefore, the Co 2p doublet peaks can be deconvoluted into four subpeaks, where the fitting peaks at 779.6 and 794.8 eV are assigned as the Co 3+ . The Co 2+ and Co 3+ cations coexist in Co 3 O 4 ; therefore, the Co 2p doublet peaks can be deconvoluted into four subpeaks, where the fitting peaks at 779.6 and 794.8 eV are assigned as the Co 3+ state and the fitting peaks at 781 and 796 eV are ascribed to the Co 2+ state.The signals of Co 3+ and Co 2+ in the Co 2p spectra were numerically integrated to obtain the ratios of Co 3+ /Co 2+ cations of Co 3 O 4(Oxid) and CVD-grown Co 3 O 4 films, respectively, estimated to be 0.50 and 0.89, respectively, which are both lower than the theoretical ratio of Co 3+ /Co 2+ in Co 3 O 4 (2:1).The Co 3+ vacancies (V Co3+ ) at the octahedral sites in spinel Co 3 O 4 were indicated to dominate in both samples [45].Remarkably, compared to CVD-grown Co 3 O 4 , Co 3 O 4(Oxid) thin films, converted from CoO to Co 3 O 4 , revealed a lower Co 3+ /Co 2+ ratio, possibly due to the initial CoO phase solely comprising divalent cobalt.Theoretical and experimental studies have proven that the formation of the cation vacancies in metal oxides is favorable in an oxygen-rich environment [7,45,48].Thus, the Co vacancies in Co 3 O 4 were regarded as the major origin of their p-type semiconducting properties.The O 1s spectra of CoO, Co 3 O 4(Oxid) , and CVD-grown Co 3 O 4 , as shown in Figure 9d, are deconvoluted into three components at approximately 530, 531.2, and 532.2 eV, which is attributed to oxygen bonded with cobalt in the lattice (O lat ), hydroxyl groups bonded to surface defects, such as oxygen vacancies (O V ), and weakly absorbed molecular water (O abs ), respectively.The difference between the three O 1s core level XPS spectra recorded from CoO, Co 3 O 4(Oxid) , and CVD-grown Co 3 O 4 films was not obvious.
shows the XRD ϕscan profiles for the Co 3 O 4(Oxid) film detecting Co 3 O 4 (400) and Al 2 O 3 (1120) reflections.The profile of the Co 3 O 4(Oxid) film revealed six-fold symmetry with a spacing of 60 • , indicating that the Co 3 O 4(Oxid) film was in epitaxy with the sapphire substrate.Similarly, the presence of 60 • -rotated twin domains in the Co 3 O 4(Oxid) film was detected from the X-ray diffraction ϕ-scan profile.Interestingly, the crystallographic relationship between the Co 3 O 4(Oxid) film and sapphire is (111)[110]Co 3 O 4(Oxid) ||(1120)[0001] Al2O3 , rotated through an angle of 30 • with respect to that of CVD-grown Co 3 O 4 film on sapphire substrate ((111)[112] Co3O4 ||(1120)[0001] Al2O3 ), as shown in Figure 4b.In Figure 10b, the Co 3 O 4(Oxid) presents a broader FWHM (0.62 • ) of XRC for Co 3 O 4 (222) diffraction.According to the cross-sectional SEM observation (Figure 7b), there were several pore-like structures in the interiors of the Co 3 O 4(Oxid) films.These pore-like structures affected the growth of crystallites during the annealing process and were possibly responsible for the broader width of the XRC.The formation of the Co 3 O 4 layer initially occurred on the surface of CoO.Sequentially, the entire CoO layer was gradually converted into Co 3 O 4 during the oxidation process, as illustrated in Figure 11.The oxygen sublattices of the CoO and Co 3 O 4 , both being fcc sublattices, are nearly identical, with a lattice mismatch of about 5% [50].Therefore, the oxygen sublattice of newly formed Co 3 O 4(Oxid) inherited the original framework of CoO and was epitaxially constrained by the lattice planes of the underlying CoO, as illustrated in Figure 10.Coatings 2023, 13, x FOR PEER REVIEW 12 of 17 Therefore, the oxygen sublattice of newly formed Co3O4(Oxid) inherited the original framework of CoO and was epitaxially constrained by the lattice planes of the underlying CoO, as illustrated in Figure 10.

Figure 11 .
Figure 11.Schematic representation Co3O4(Oxid) layer formation through the oxidation of the CoO epitaxial layer and lattice match between (111) planes of the CoO and Co3O4 oxygen sublattices.

Figure 11 .
Figure 11.Schematic representation Co3O4(Oxid) layer formation through the oxidation of the CoO epitaxial layer and lattice match between (111) planes of the CoO and Co3O4 oxygen sublattices.

Figure 11 .
Figure 11.Schematic representation Co 3 O 4(Oxid) layer formation through the oxidation of the CoO epitaxial layer and lattice match between (111) planes of the CoO and Co 3 O 4 oxygen sublattices.We performed cross-sectional TEM to gain insight into the microstructure of the Co 3 O 4(Oxid) film converted from CoO. Importantly, several pores present in the interior of Co 3 O 4(Oxid) films were observed from the low-magnification bright-field TEM image (Figure 12a).During the oxidation of CoO to Co 3 O 4 , the growth of Co 3 O 4 is initiated on the surface of CoO.Then, the Co 3 O 4 layer grows inward and outward simultaneously [52-54].

Figure 12 .
Figure 12.(a) Low-magnification bright-field transmission electron micrograph of the Co3O4(Oxid.)film converted from the CoO epitaxial layer via oxidation reaction; (b) Corresponding selected area electron diffraction (SAED) patterns recorded at the interfacial regions and viewed along the sapphire [0001] zone axis.The normal and 60°-rotated twin variants are indicated by green and yellow dashed lines, respectively.

Figure 12 .
Figure 12.(a) Low-magnification bright-field transmission electron micrograph of the Co 3 O 4(Oxid.)film converted from the CoO epitaxial layer via oxidation reaction; (b) Corresponding selected area electron diffraction (SAED) patterns recorded at the interfacial regions and viewed along the sapphire [0001] zone axis.The normal and 60 • -rotated twin variants are indicated by green and yellow dashed lines, respectively.
Co 3 O 4(Oxid) film, designated by green and yellow dashed lines, both indexed as the normal and 60 • -rotated twin variants, respectively.They were oriented epitaxially with respect to the sapphire substrate.The epitaxial relationship between Co 3 O 4(Oxid) and the sapphire substrate was identified as (111)[110]Co 3 O 4(Oxid) ||(1120)[0001] Al2O3 , which is consistent with the result of the XRD ϕ-scan, as shown in Figure 10a.Despite the presence of numerous pore structures in the formed Co 3 O 4(Oxid) layer, the diffraction pattern of the Co 3 O 4(Oxid) film remained consistent throughout the entire film layer.

Figure 13 .
Figure 13.(a) High-resolution transmission electron microscopic image of the Co3O4(Oxid) film-sapphire interface visualized along the sapphire [0001] zone axis.(b) Corresponding fast Fourier transformation pattern of (a).(c) Fourier-filtered image reconstructed using Co3O4 222 and sapphire 21 1 0 reflections.The black triangles in the Fourier-filtered image indicate the position of misfit dislocations.
1 1 0]CoO||(11 2 0)[0001]Al2O3 for CoO and (111)[112 ]Co3O4||(112 0)[0001]Al2O3 for Co3O4.The crystalline quality of both CoO and Co3O4 epitaxial films grown using atmospheric pressure mist-CVD was comparable to those grown by the other vapor phase epitaxy in a vacuum environment.The CoO films had a direct bandgap of 2.81 eV, and the two direct bandgap values at 1.41 eV and 2.03 eV were

Figure 13 .
Figure 13.(a) High-resolution transmission electron microscopic image of the Co 3 O 4(Oxid) filmsapphire interface visualized along the sapphire [0001] zone axis.(b) Corresponding fast Fourier transformation pattern of (a).(c) Fourier-filtered image reconstructed using Co 3 O 4 222 and sapphire 2110 reflections.The black triangles in the Fourier-filtered image indicate the position of misfit dislocations.
3 O 4 films.The epitaxial relationship of CoO and Co 3 O 4 on sapphire substrates was determined to be (111)[110] CoO ||(1120)[0001] Al2O3 for CoO and (111)[112] Co3O4 ||(1120)[0001] Al2O3 for Co 3 O 4 .The crystalline quality of both CoO and Co 3 O 4 epitaxial films grown using atmospheric pressure mist-CVD was comparable to those grown by the other vapor phase epitaxy in a vacuum environment.The CoO films had a direct bandgap of 2.81 eV, and the two direct bandgap values at 1.41 eV and 2.03 eV were observed for Co 3 O 4 .The electrical properties of Co 3 O 4 epitaxial films could be enhanced through thermal annealing in oxygen ambient, exhibiting a stable p-type conductivity with an electrical resistivity of 8.35 Ohm cm and a carrier concentration of 4.19 × 10 16 cm −3 .According to XPS analysis, cobalt vacancies, which functioned as acceptors and were predominantly present in Co 3 O 4 films, were found to facilitate the generation of hole carriers.In addition, the phase transition of CoO to Co 3 O 4 during the annealing process was evidenced by XRD, Raman, and XPS spectroscopy measurements.Therefore, the oxidation of CoO to Co 3 O 4 primarily accounts for the improved electrical properties observed in the annealed samples of CoO films.The crystallographic orientation of the Co 3 O 4 film formed via the oxidation reaction was epitaxially constrained by the lattice planes of the underlying CoO epitaxial film, primarily because CoO and Co 3 O 4 share a nearly identical oxygen sublattice.The formation of Co 3 O 4 films during the oxidation of CoO was predominantly governed by the outward diffusion of cobalt cations.Therefore, the formation of the pores within the interiors of Co 3 O 4 films was attributed to the accumulation of excessive cobalt vacancies.Despite the presence of several pores within the interiors of Co 3 O 4 films, the lattice-matched interface between the Co 3 O 4 film and the sapphire substrate was confirmed by the HRTEM image.