Microwave Irradiation Induces Oxygen Vacancy in Metal Oxides based Materials and Devices: A Review

A mini-review of defects formation and engineering in energy and sensor materials through intentional doping and microwave irradiation is presented. Intensive research interest has been observed in the development of Transition Metal Oxides (TMOs) energy and sensor materials for industrial applications such as energy generation, energy storage and sensor devices. A variety of selected notable reports are organized into sections depicting structural, microstructural, luminescent and electronic properties of TMO materials and devices, regardless of their deep synthetic chemistry. There are three types of Oxygen Vacancies (VO) related to TMO materials among which neutrally ionized V 0, singly ionized Vo1+, and doubly ionized Vo2+. Oxygen vacancies (V) have been characterized using several experimental techniques such as X-ray diffraction, Thermo-Gravimetric Analysis (TGA), Ultra-Violet Visible Spectroscopy (UV-Vis), Photoluminescence (PL) spectroscopy, Raman spectroscopy, Electron Spin Resonance (ESR), Electron Energy Loss Spectroscopy (EELS) and X-ray Photoelectron spectroscopy (XPS). High resolution XPS O 1s core levels analysis is one of the most accurate analytical techniques to characterize VO in conjunction with other techniques. The deconvolution of O 1s core levels using a Gaussian function into three peaks Lattice oxygen (OL), surface hydroxyl oxygen (O-OH) and adsorbed oxygen (OS) has been widely used to correlate XPS to XRD, UV-Vis, PL and Raman data. This review summarizes the representative reports of VO formation via intentional doping or microwave irradiation in TMOs.


Introduction
Over the past two decades, oxygen vacancies (VO) have been widely reported to dramatically alter structural, luminescent, catalytic and electronic properties of materials applied in a variety of applications such as energy, sensors and display technologies [1][2][3][4][5][6][7][8][9]. These defects classified as point defects are till now the subject of a hot controversy on their natural origin among the scientific community worldwide due to the inherent challenges related to materials synthesis and characterization management in the vast majority of experimental fields. Oxygen vacancies exist in three states, namely neutrally ionized V 0 , singly ionized V 1+ and doubly ionized V 2+ , the first and the former are stable (10,11) ( Figure 1). Hitherto, fundamental studies have been devoted to elucidate the factors characterizing point defects, most of them have been investigated by first principle calculations [11,12]. The main factors are the formation energy and defect concentration, the former is well described in thermodynamic equilibrium by a Boltzmann distribution while at the same time it is closely related to the formation energy, defects with high formation energy are formed in low concentration [11]. The formation energy of point defects should not be considered as a constant due to its dependence to growth conditions. Previous studies have revealed that the formation energy of an oxygen vacancy depends on the abundance of the parent atoms found in the material and becomes more complex when the oxygen vacancy is charged because of the non-negligible effect of the electron chemical potential [11].
pressure, temperature and gas treatment environment. Moreover, the exposure time to electromagnetic radiations sources such as Deep UV lamps, induction sources, and microwave reactors have been found to also contribute to the alteration of materials primitive lattice structure. It should be noted that these lattice structure alterations may have non-negligible direct contribution to the variation of oxygen vacancy content as it can be achieved elsewhere from simple conventional doping processes. Owing to the close correlation between materials lattice structure and electronic band-structure properties, one should have a precise understanding of the material band-structure in order to constructively and optimally describe research observations and findings.
Through structural doping the formation of oxygen defective sites has been successfully achieved, while the resulting tensile strain was revealed by the expansion of material lattice structure [67]. In fact, Xray diffraction analysis of the lattice structure quality and crystallographic fingerprint may lead to the detection of possible alteration of the lattice structure resulting from foreign atoms insertion [68]. In their report, Kaur et al [68] demonstrated that doping a transition metal oxide host material with foreign atoms resulted in a tensile strain observed from XRD peak shift toward lower angles which leads to better materials properties later on. Moreover, the slight alteration observed in the lattice structure may have dramatic impact in the materials performance enhancement or degradation, depending of the targeted application.
Interestingly, Kumar et al. [69], have recently achieved lattice structure expansion via microwave irradiation, this process leads to the realization of high performing electrochemical spinel cathode materials for energy storage applications. Likewise, Mesfin et al. [70] and Jafta et al. [71] reported the same phenomenon and achieved exceptional electrochemical properties. Several factors have been identified by these authors to be responsible for these unique observations but oxygen vacancy increase should be seriously considered as the most plausible cause from a fundamental understanding [71].
Hence, the selection of a particular material synthetic method is of particular importance to the formation and careful control of oxygen vacancy. To date synthesis assisted with microwave treatment has proven to be one of the most effective approaches to meet such performance [70,71]. Microwave irradiation has demonstrated numerous advantages over other conventional materials processing such as fast and effective heating during chemical reactions, high product yield and environmental benign [72][73][74]. During microwave irradiation, a direct diffusion of electromagnetic wave energy to the material occurs via molecular interaction with the electric and magnetic field vectors of microwaves generated by a magnetron which produces waves in the frequency range of 300 MHz -300 GHz (λ:1m-1mm) ( Figure 2). It is plausible that during the interaction of microwaves with the material nonequilibrium processes occur, the formation of oxygen deficient sites are then promoted in the material lattice structure consecutively to entropy-driven desorption of lattice oxygen in the lattice structure [75,76] (Figure 3). Seminal studies have established that TMOs are among the best microwave absorbers and that their electrical resistance decreases with temperature increase and that microwave energy heats TMOs better than metals due to their low electric field penetration [77].
This review covers recent studies that focus on spectroscopic investigation of the versatile effects of oxygen vacancy on intrinsic properties of TMO materials for energy conversion, energy storage, sensors and solid state lighting. Special attention was devoted to XPS O 1s core level analysis of oxygen vacancy using GAUSSIAN function deconvolution.  : Proposed schematic diagram describing the mechanism of oxygen vacancy formation via microwave irradiation and its effect on lattice structure and electronic band-structure. Eg and ΔE represent the energy gap and the quantified red-shift resulting from microwave irradiation which is shown by the red arrow.

Structural doping
In a recent study, it has been demonstrated that using dopants one can tune oxygen vacancy in TMO base devices, precisely studies revealed that p-type dopants introduced holes into the system to lower significantly the formation energy of oxygen vacancy [85]. Several other methods have been successfully used to dope transition metal oxide based materials and devices and hence generate oxygen vacancies, such as combustion synthesis, solution-combustion, solid state reaction, sol-gel [86][87][88], co-precipitation, hydrothermal [89], sonochemical, chemical vapor deposition, plasma enhanced chemical deposition, RF magnetron sputtering, evaporation, electrochemical synthesis, spin coating, spray pyrolysis, and spray coating, etc. Park et al. [90] have successfully doped ZnO with Co 2+ ions and remarkably analyzed the variation of VO in the undoped and Co-doped ZnO samples via Maximum entropy method (MEM) using electron density distribution derived from Rietveld refinement. The authors found that the ZnO unit cell volume was increased with Co 2+ doping concentration and that consecutively the amount of VO increased up to a certain limit before dropping for higher doping concentration. The author faced a challenge to completely assigned the abrupt increase observed in the lowest Co 2+ doping concentration and stated that further analysis were needed. It is plausible that Co 2+ ions may have reached its saturation limit in the ZnO crystal lattice below 5 mol% and above 1 mol% and this could not lead to a continuous increase in VO content probably due to the segregation of Co 2+ ions which are very likely to have formed islands of dopants in the crystal lattice instead of bonding.

Microwave irradiation
Material and device properties have been successfully tailored via microwave irradiation in a very recent past [81,[91][92]. Microwave irradiation is a rapid and efficient heating way for highly homogenous materials processing which is eco-friendly and easy to manipulate as compared to conventional synthetic approaches. This approach has been used for large scale and cost-effective synthesis of supercapacitor electrode materials at desired temperatures up to 1500°C and controlled pressure (≤ 199 bar) at relatively short reaction time [93]. This technique has been used independently or in conjunction with other processing methods to complement them and achieve high yield innovative materials in the so-called microwave-assisted synthesis [94][95][96][97][98][99][100][101][102]. In a seminal research work, Newnham and co-workers investigated the microwave-matter interactions ; they reported that subjecting a material to microwave which has two vector components, namely, magnetic and electric field which induces in the material an electric polarization P(C/m 2 ) equivalent to the dipole moment (C -m ) per unit volume (m -3 ), hence generating thermodynamic nonequilibrium entropy mechanism [77]. The authors identified several polarization mechanisms in solids among which three major leads to loss in the microwave region, namely, space charges arising from localized electrical conduction, rotating electrical dipoles and ionic polarization associated with far-infrared vibrations [77].

Characterization of Oxygen Vacancy
Several analytical techniques have been used to characterize oxygen vacancies in transition metal oxides, namely, X-ray diffraction (XRD), Thermo-Gravimetric Analysis (TGA), Ultra-Violet Visible Spectroscopy (UV-Vis), Photoluminescence (PL) spectroscopy, Raman spectroscopy, Electron Spin Resonance (ESR), Electron Energy Loss Spectroscopy (EELS) and X-ray Photoelectron spectroscopy (XPS) [9, 90,[103][104][105][106][107][108]. The detailed analysis of XPS O 1s core level which consist of GAUSSIAN function deconvolution has been widely accepted among the analytical techniques used to qualitatively analyze VO and was carefully adopted to get more insights into O 1s medium peak located at ~531 eV presented in Figure 4ab. This peak has been ascribed to surface hydroxyl oxygen (O-OH) related to O 2ions that are localized in the oxygen deficient regions within the ceramic lattice [109,110]. Moreover, Yoshida et al. successfully investigated the effect of microwave irradiation on electronic band-structure of ZnO via absorption and emission spectroscopy (92). The authors systematically demonstrated via PL spectroscopy that microwaves generated a new deep level defect which lowers the bandgap of ZnO as illustrated in Figure 4cd, and they assigned this defect level to zinc and oxygen plasma or to the thermal effect induced by microwave. However, particular attention should be given to the former assertion because the nonequilibrium processes resulting from thermal effect may have created oxygen vacancies which can be described via energy bandstructure as shown in Figure 4e. It is important to note that for detailed quantitative analysis of oxygen vacancy Rutherford backscattering spectroscopy and X-Ray absorption spectroscopy using synchrotron radiation are more appropriate [111,112].

Oxygen Vacancy Enhanced Electrochemical Performances for Energy Storage
Very recently exceptional class of materials have been uncovered, these multifunctional materials have tremendous advantages and exceptional performances in energy storage applications, namely, Layered-Double Hydroxides (LDHs), Metal Organic Frameworks (MOFs), Multishelled and nanostructured materials; among various morphologies obtained hollow-spheres are the most efficient in energy storage [113][114][115][116].
Oxygen vacancy plays an important role in tuning the physicochemical properties of these innovative materials applied in supercapacitors, lithium ions batteries, lithium sulfur batteries, sodium ion batteries, magnesium ion batteries and zinc air batteries. Some of these innovative materials were produced via microwave assisted synthesis. Copper and Nickel have been associated to produce via microwave synthesis a Hollow-sphere MOFs material for enhanced lithium battery performances [117]. Moreover, the authors revealed that the matched composition of CuO@NiO resulted in an efficient step-by-step lithium insertion reaction which finally contributed to the excellent electrochemical properties as presented in Figure 5. Furthermore, the core-shell architecture allowed easy lithium/electron diffusion while the volume change was accommodated in the microsphere pores of the bimetallic oxides [117]. The concept has been widely adopted and improved by several groups using different combination of TMOs and organic linkers to achieve better storage properties in supercapacitors and batteries as presented in Figures 6 and 7, respectively. In the same spirit, Li et al. produced NiO/ZnO hierarchical double-shelled hollow spheres with exceptional cycling stability up to 117% after 2000 cycles at a current density of 5.2 A.g-1 [118]. Bruce Dunn group on the other hand remarkably demonstrated the strategic influence of oxygen vacancy on faster promotion of charge storage kinetics which makes it easy for Li ions intercalation and de-intercalation [9]. That leads to enhanced capacity up to ~ 320 mA hg -1 as shown in Figure 8. Teng Zhai and coworkers have exceptionally demonstrated that oxygen vacancies could enhance both conductivity and capacitance of nanorods based wearable asymmetric supercapacitor [119]. The remarkable strategy consisted of a simple hydrogenation treatment of MnO2 electrodes which displayed exceptional rate capability and cycling with only a moderate concentration of oxygen vacancies [119].

Oxygen Vacancy Dynamic in Photovolatiac Solar Cells
The increasing demand of clean and environmental benign energy production sources is today in the heart of public and private sector research institutes strategy development plans worldwide due to environmental concerns related to conventional energy sources based on fossil fuels and CO 2 production. Researchers are currently driven to achieve innovative solutions to the betterment of mankind life through the development of pollution-free energy sources such as photovoltaic solar cells. Regardless of the type of mechanism exploited to generate and dissociate electron-hole pairs in solar cells, TiO 2 has been extensively used as a counter electrode and also a hole conducting layer mainly in quantum dot and dye-sensitized solar cells [122][123][124][125][126][127][128][129][130].
In an astonishing investigation Su et al. [131] have for the first time experimentally demonstrated using Electron Paramagnetic Resonance (EPR) that oxygen vacancy could modulate photoelectrical conversion efficiency in a TiO 2 dye-sensitized solar cell grown via hydrogenation at low temperature and mild pressure. Their exceptional findings are summarized and presented in Figure 9. The effect of oxygen vacancy on the band gap corroborates the results found in previous studies [25]. Microwave synthesis has been used to synthesize highly crystalline TiO 2 with a shorter treatment time as compared to conventional heating sources when used in solar cells application  [132]. Hence, microwave heating has been demonstrated to be more efficient than conventional heating sources such as muffle furnace, nowadays a worldwide clue. Recently Ghosh and co-workers [133] have systematically designed and fabricated via microwave a high efficient counter electrode based on CuxS and grapheme oxide for quantum dot sensitized solar cells. Owing to the challenges involved in photoanode operation, particular care was devoted to its design in order to effectively reduce the oxidized electrolyte, hereafter achieving a 6.81% record efficiency of which corresponding results are depicted in Figure 10.

Sensor Sensitivity Derived Oxygen Vacancy
From medical applications, namely, diseases detection and environmental protection in the mining sector, there are rooms for improvements. The growing momentum observed in the design and fabrication of electrochemical, gas and chemical sensors drive researchers and technologists to meet the agenda of the next generation of sensor devices which are sought to be flexible, highly selective and sensitive than ever. TMOs based sensors have been widely investigated for the past two decades and a lot of issues related to their practical application have been exposed and much still need to be done for easy and safe domestic usage. Among the vast majority of synthetic methods used to produce TMOs, microwave has a place of choice due to the extraordinary variety of materials morphology which also exhibit unique physic-chemical properties such as high surface area and exceptional conductivity to cite few. It should be noted that oxygen vacancy plays a critical role in materials conductivity as demonstrated by several authors (9,25,92). From a fundamental point of view, it is well agreed that the targeted metal oxide sensors resistance modulation is the result of a surface mechanism of oxidation-reduction reaction (ORR) involving chemisorbed oxygen with the molecules of the detected gases [134]. This implies that there exist in metal oxides based sensors a relative correlation between their sensitivity and the concentration of surface adsorbed oxygen [134]. In the same spirit, several authors have reported results in agreement of this understanding, and in particular it should be pointed out that the samples investigated in these studies were produced via microwave-assisted synthesis [89,[135][136][137][138][139]. Via oxygen vacancy modulation investigated using XPS, Wang and colleagues [134] enhanced sensing performance and mechanism of hydrogenated NiO particles. The innovative concept investigated based on hydrogenation consisted in increasing the density of unsaturated transition metal atoms with dangling bonds on the surface, this concept was investigated elsewhere using Raman spectroscopy [104]. In the study by Wang et al., for which the sensing reaction mechanism is described and presented in the Figure 11, the XPS analysis revealed that the relative amount (%) of oxygen vacancy increased with increase hydrogenated time as shown in Figure 11. The authors found reasonable to conclude that the sensing properties enhancement resulting from hydrogenation may originate from the increase of the relative percentage of OV and OC GAUSSIAN components in the XPS O 1s core levels and also the decrease in the amount of -OH group [134]. Due to its role on the electronic band gap decrease and conductivity increase discussed earlier in section 3, oxygen vacancy increase could be considered as the main factor responsible for enhanced sensing activity. The response and recovery along with the sensors resistance are summarized in the Figure 12.

Oxygen Vacancy Modulation for Efficient Control of Field Emission and Light Emitting Devices Properties
Since the past two decades, the display technology industry is experiencing a considerable investment of scientists and technologists to the realization of white light emitting materials and devices. It has been however demonstrated that controlling these devices calorimetric performances is closely dependent on their oxygen vacancy content [140][141][142]. A number of research groups have devoted their efforts to synthesize these light emitting materials using microwave approach [143][144][145][146]. Reproduced with permission from ref. [134]. Copyright 2017 Elsevier Ltd.
Two years ago a thorough investigation by Zhang et al. [147] was conducted to elucidate on the role of oxygen vacancy on the persistent luminescence of a terbium doped light emitting material. The investigators focused their effort on unraveling the origin of defects involved in the enhancement of persistent luminescence for samples annealed in an oxygen-poor atmosphere. They observed via thermoluminescence spectroscopy analysis that the increase of persistent luminescence was accompanied by oxygen vacancy content increase. Moreover, a comparison of defect energy levels was conducted between their experimental findings and theoretical calculations derived from Khon-Sham levels studies by Freysoldt and co-worker [148]. They proposed a re-evaluation of the theoretical result obtained for the depth of doubly charged VO2+ at 0.69-0.74 eV as compared to 1 eV below the conduction band [147]. In the same line, Xu et al. [149] investigated red phosphorescence of Sr 2 SnO 4 :Sm 3+ phosphor successively sintered in air and 10 -2 Torr vacuum atmosphere. The later treatment option contributed to the increase in oxygen vacancies amount which according to authors acted as the sensitizer and electron traps for effective energy transfer from the host matrix Sr2SnO4 to the dopant Sm3+. The findings of their study are summarized in Figure 13. Finally, remarkable enhanced phosphorescence properties were achieved by the authors after vacuum sintering process [149]. In a very recent research work, Ai-Zhen Liao et al. [150] successfully increased field emission properties of a device based on α-Fe 2 O 3 nanorod arrays via conductivity increase and work function decrease through polaronic hopping mechanism owing to oxygen vacancy. The significant findings of the study among which band-structure modification similarly reported by earlier authors are summarized in the Figure 14 shown below [25,92]. Unambiguously the investigation revealed according to the authors that oxygen deficient HNAs were potential candidates for nanoelectronic applications and more

Conclusion
This mini-review paper provides an overview of microwave irradiation tuning of oxygen vacancy and its role in tailoring materials properties. The key performances of energy storage, sensor and light emitting materials have been widely assigned to oxygen vacancy. Various approaches have been reported to account for oxygen vacancy control among which microwave irradiation is one of the most accessible and cost effective. It has been demonstrated that at some extent oxygen vacancy increase is responsible of the p-type conductivity observed in these materials and plays a considerable role in the exceptional performances documented in the literature. Due to thermodynamic nonequilibium processes that occur during microwave irradiation which contribute to the formation of oxygen deficient sites in the lattice structure, microwave treatment should be considered as an effective approach to boost electrochemical, sensing and optical properties via protonic conductivity enhancement among others. A consistent observation of the microwave effect on the band structure modification has been established, which in most cases resulted in band-gap decrease due to the creation of defects levels above the valence band highest unoccupied level and below the conduction band lowest occupied level. These lead to the overall agreement that oxygen vacancy enhance electronic conductivity and carriers mobility in most materials and devices leading to easy transport of injected electrons through oxygen deficient sites. Further thermodynamic nonequilibrium processes can be studied from other electromagnetic radiation sources such as UV light to achieve same or better results. However, in devices where exciton dissociation and recombination processes occur, challenging control of interfacial oxygen vacancy is required in order not to degrade their physicochemical performances. Finally, the attention of the reader should also be directed to the importance of XPS analysis which is one of the most accessible analytical techniques needed for effective oxygen vacancy characterization.