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. 2022 Sep 1;7(36):32816–32826. doi: 10.1021/acsomega.2c05168

Crystallization, Phase Stability, Microstructure, and Chemical Bonding in Ga2O3 Nanofibers Made by Electrospinning

Nanthakishore Makeswaran †,‡,*, James P Kelly ‡, Jeffery J Haslam ‡, Joseph T McKeown §, Michael S Ross ‡, C V Ramana †
PMCID: PMC9476513  PMID: 36120052

Abstract

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We report on the crystal structure, phase stability, surface morphology, microstructure, chemical bonding, and electronic properties of gallium oxide (Ga2O3) nanofibers made by a simple and economically viable electrospinning process. The effect of processing parameters on the properties of Ga2O3 nanofibers were evaluated by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Thermal treatments in the range of 700–900 °C induce crystallization of amorphous fibers and lead to phase stabilization of α-GaOOH, β-Ga2O3, or mixtures of these phases. The electron diffraction analyses coupled with XPS indicate that the transformation sequence progresses by forming amorphous fibers, which then transform to crystalline fibers with a mixture of α-GaOOH and β-Ga2O3 at intermediate temperatures and fully transforms to the β-Ga2O3 phase at higher temperatures (800–900 °C). Raman spectroscopic analyses corroborate the structural evolution and confirm the high chemical quality of the β-Ga2O3 nanofibers. The surface analysis by XPS studies indicates that the hydroxyl groups are present for the as-synthesized samples, while thermal treatment at higher temperatures fully removes those hydroxyl groups, resulting in the formation of β-Ga2O3 nanofibers.

Introduction

There is an ever-increasing demand for functional materials and sensors that can operate in extreme environments and, hence, find potential applications in areas such as automation, homeland security, aerospace, healthcare, and energy and manufacturing industries. The need for sensors to work in extreme environments that often include high temperature, high radiation, thermal gradient extremes, high pressures, and toxic chemical environments places a stringent requirement on both the materials used in such sensors and the sensor architecture.1−4 For instance, modern combustion processes such as those found in energy generation systems or aircraft propulsion are heavily dependent on sensing, monitoring, and controlling oxygen flow in the system to maximize the efficiency.1−9 However, current gas sensor technology for energy and automotive technologies suffers from critical issues of response and recovery times and stability.1−15 Wide band gap oxide ceramics have been the focus of much research due to their applicability in a variety of applications, such as electronics, photonics, photo catalysis, and chemical sensing applications.1−6 Among these oxides, gallium oxide (Ga2O3) has multiple properties that make it a promising candidate for high-temperature gas sensors, power devices, optoelectronics, solar-blind photodetectors, and catalysts for dehydrogenation reactions, CO2 conversion, or CH4 conversion.7−23 β-Ga2O3, in particular, is a thermodynamically stable phase with a wide band gap of ∼4.9 eV, a melting point of approximately 1900 °C, a breakdown field of 8 MV/cm–1, and an electron mobility of 300 (cm2/vs).24,25 Incorporating Ga2O3 into a potentiometric gas sensor, which is dependent on the use of desired gas ion conducting materials as solid electrolytes, has been proposed and demonstrated based on thin-film structures.26,27

Novel structural approaches to create ceramic semiconductor nanostructures proved to be a viable solution to improve the durability (i.e., the long-term stability of the sensors) and improve the sensitivity and response times.8−17 One of the most promising nanoscale architectures or structured assemblies considered are nanofibers. These engineered ceramic nano-architectures display superior surface flexibility compared to other forms, including thin films previously used in such sensors. Such 1D nano-architectures provide an optimal material system to investigate the effects of dimensionality and size reduction on various properties such as electrical, optical, thermal, mechanical, and thermo-chemical properties.28−33 Additionally, recently, it is shown that gallium oxide nanofibers in the sub 50 nm diameter range exhibit high heat dissipation properties due to the comparatively large surface area of the structures.34Also, the feasibility of rare-earth ion incorporation, such as Ga2O3/Eu3+, into nanofibers by electrospinning to produce materials with variable morphology, structure, and photoluminescence properties has been demonstrated.36 Furthermore, the high surface to volume ratios of these fibers present an opportunity to improve chemical diffusion to aid in reducing response and recovery times.35,36 Inorganic nanowires/fibers have already been utilized as promising candidates for nanoscale electronics, opto-electronics, and sensor devices.37−40 In this context, herein, efforts were directed to synthesize Ga2O3 nanofibers with controlled morphology and phase stability. Various phases of Ga2O3 nanofibers with desired nanoscale features were produced by an electrospinning deposition process.

A widely used and effective means of producing nanofibers is the electrospinning process. Electrospinning involves applying a voltage bias between an emitter and collector. Under an appropriate bias, electric charge repulsion within a conductive, viscoelastic solution causes expulsion of a fiber jet from a Taylor cone that is drawn toward the collector.26,42 The solution in the jet is then drawn and stretched into a fiber by the electrostatic repulsion and other processes lead to the fiber being deposited in a random pattern on the collector.42,43 Primary advantages of the electrospinning technique are the flexibility to generate nanofibers of many different types of materials and to tune the resultant nanofibers via altering the spinning solution, or the processing parameters. For examples, Wu et al. demonstrated that the diameter of ZnO precursor fibers can be controlled by adjusting concentrations of zinc acetate in the solution.44,45 Zhou et al. have demonstrated enhanced yellow luminescence of amorphous Ga2O3 nanofibers by treating them in the temperature range of 450–750 °C.35 Thus, it seems reasonable to apply this simple and inexpensive method to design and develop Ga2O3 fibers with controlled phase, morphology, and properties to achieve enhanced device performance for a variety of applications. Furthermore, these types of fibers also show promising results when the nanoarchitecture is combined with suitable dopants, in terms of improved photocatalytic activity as demonstrated by Yoo et al.34,41 In their work, Si-doped β-Ga2O3 nanofibers were formed via the electrospinning method and photocatalytic properties were evaluated with variable Si concentrations.41 The photocatalytic performance of the Si-doped β-Ga2O3 nanofibers enhanced in direct correlation with the Si concentration. In fact, it was noted that the Si-dopant effect on the photocatalytic performance of the β-Ga2O3 nanofibers was more significant compared to those of TiO2 and ZnO with other dopants. These considerations, which clearly are promising to derive intrinsic and doped nanofibers of β-Ga2O3 for electronic device application, motivated us to utilize electrospinning for the deposition of β-Ga2O3 nanofibers.

The aim of this work was to fabricate Ga2O3 nanofibers by electrospinning and to understand thermally induced phase changes and the crystallization behavior. The goal is to enhance the ability to manipulate the specific crystal structures at nanoscale dimensions while optimizing the processing conditions to obtain desirable properties for the integration of Ga2O3 nanofibers in electronic device applications, specifically in integrated sensors for energy systems. The results obtained are presented and discussed in this paper to establish a correlation between processing conditions and the growth behavior, crystal structure and phase, phase transformations (if any), surface morphology, chemical bonding, microstructure, and optical properties of Ga2O3 nanofibers.

Materials and Methods

Synthesis

Gallium nitrate (99.999%, Alfa Aesar, Tewksbury, MA) was used as the gallium oxide precursor for nanofiber production. The electrospinning solution was prepared with a 1:1 weight ratio of ethanol (94–96%, Alfa Aesar, Tewksbury, MA) and deionized water to which varying weight percentages, 3.25–15%, of gallium nitrate were added. Finally, 10 wt % of polyvinylpyrrolidone (PVP) (M.W. 1,300,000, Alfa Aesar, Tewksbury, MA) was added to ensure sufficient viscosity for the electrospinning. For both the gallium nitrate and the PVP, each weight percentage is relative to prior-added components. Electrospinning was done on a commercial benchtop system (Fluidnatek LE-10, Bioinicia, Valencia, Spain). A potential of 25 kV and 15 cm collector distance was used to spin the fibers, resulting in an electric field strength of 1.7 kV/cm. Fibers were collected on a rotating, anodized aluminum drum with a diameter of 10 cm and length of 20 cm and that was covered with a mylar film to facilitate removal of the fibers. Flow rates for the solution varied and often had to be adjusted during each deposition. The electrospinner was run until 1 mL of solution was spun, at which point the mylar film was removed from the collector and the fibers were peeled off of the film for heat treatment. A schematic diagram of the electrospinning process is displayed in Figure 1. Heat treatments were performed at 700–900 °C in air for 2 h with a heating rate of 10 °C per minute to form gallium oxide.

Figure 1.

Figure 1

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Schematic diagram indicating the working principle of electrospinning to produce Ga2O3 precursor nanofibers.

Characterization

Viscosity/Surface Tension/Electrical Conductivity

The viscosity was measured with a viscometer (DV2T, Brookfield, Middleboro, MA, USA), surface tension was measured with a tensiometer (AquaPi, Kibron, Helsinki, Finland), and electrical conductivity was measured with a conductivity meter (FiveEasy, Mettler Toledo, Columbus, OH, USA).

Scanning Electron Microscopy

The surface morphology and microstructural features of Ga2O3 fibers are important and reveal information about the samples processed under variable synthetic conditions as well as post-processing effects. Therefore, scanning electron microscopy (SEM) was used to analyze Ga2O3 nanofibers. A Hitachi—4800 microscope was used to obtain the secondary electron imaging micrographs of as-synthesized as well as heat-treated Ga-oxide nanofibers. The nanofibers were mounted prior to imaging, the samples were sputter-coated with silver to avoid the charging effect, a common problem in imaging insulating samples. Secondary characterization was done using a NanoLab600i (FEI Company, Hillsboro, OR, USA). Prior to imaging, the samples were sputter-coated with gold to avoid charging. Fiber diameters and shrinkage during thermal treatment were determined from the secondary electron images.

Transmission Electron Microscopy

Transmission electron microscopy (TEM) was conducted using a ThermoFisher 80-300 Titan TEM operated at 300 kV. Measurements were made to analyze the surface morphology and crystal symmetry of Ga2O3 nanofibers. Bright-field (BF) and high-angle annular dark-field scanning TEM (HAADF STEM) images and selected area electron diffraction (SAED) patterns were recorded. Additional TEM measurements were performed using a JEOL JEM-2100F transmission electron microscope equipped with an Oxford AZtec energy-dispersive X-ray spectrometer and Gatan Tridiem GIF electron energy loss spectrometer was used to analyze the Ga2O3 nanocrystals. All the measurements and imaging analysis were made on the Ga2O3 nanofiber samples prepared under variable synthetic conditions. In order to prepare TEM samples, a small amount of Ga2O3 nanofiber sample was diluted with ethanol, the suspension was sonicated, and a single drop of the resulting suspension was placed onto a carbon-coated copper grid and allowed to dry in air.

X-ray Diffraction

The structural analysis was performed on a Rigaku Benchtop powder X-ray diffractometer (Mini Flex II) which provides excellent phase composition and crystal structure of the synthesized Ga2O3 nanofibers. Scanning parameters were as follows: 10–80° (2θ range), step size—0.02°, and scan rate—0.6̊/min. As-synthesized as well as heat-treated Ga-oxide nanofibers were analyzed. In addition, X-ray diffraction (XRD) data of pure Ga2O3 powder sample were analyzed for reference/calibration purpose.

Thermogravimetric Analysis

Thermogravimetric analysis (TGA/DSC3+, Mettler Toledo) was also performed to confirm that polymers were removed during heat treatment and that gallium oxide was forming properly. Samples were gradually exposed to a maximum temperature of 1050 °C with a constant ramp rate over 6 h.

X-ray Photoelectron Spectroscopy

We adopted the previously established procedures and methods to characterize intrinsic or doped Ga2O3 materials using X-ray photoelectron spectroscopy (XPS).28 For clarity purpose, the details of XPS measurements and analytical procedures performed are as follows. The XPS scans were obtained by employing a Kratos Axis Ultra DLD spectrometer and using an Al Kα monochromatic X-ray source (1486.6 eV). Survey scans and high-resolution spectra of Ga 2p and O 1s peak regions were obtained and analyzed to understand the effect of processing conditions on the chemistry of the nanofibers. The survey and high-resolution (HR) scans were carried out at pass energies of 160 and 40 eV, respectively. The charge neutralizer was set to a value of 3.5 eV as these are electrically insulating ceramic oxide samples. Raw XPS data were fitted with the help of CasaXPS software using a Gaussian/Lorentzian [GL(30)] line shape with a Shirley background correction. Survey scans were collected over the binding energy (BE) range of 1400–5 eV, whereas HR spectra of Ga 2p and O 1s peak regions were obtained with at least 8 number of sweeps for each of them depending on the clarity of the peaks. Though both the Ga peaks (i.e., Ga 2p and 3d) were collected for confirmation, only Ga 2p spectra is depicted in order to avoid confusion coming from the interference of Ga 3d peak with an overlapping O 2s peak. Detailed discussions on sample preparation techniques for XPS, precautions taken during sample transfer from the furnace atmosphere to the XPS analysis chamber and during XPS data collection, and particular instrumental parameters used for scanning have been used within our laboratory as well as widely in literature and found to be efficient to evaluate both intrinsic and doped Ga2O3 compounds.28

Raman Spectroscopy

Raman spectroscopic studies were performed on an InVia Micro RAMAN (Renishaw) spectrophotometer with 532 nm laser excitation. The peaks were fitted by the superposition of the Lorentzian function:

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where ω is phonon frequency, “ω0” is maximum phonon frequency, W is full width at half maxima (FWHM), “A” is normalization constant, and “I0” is intensity of the background.

Results and Discussion

Nanofiber Formation, Surface Morphology, and Microstructure

Solution parameters most often considered for the electrospinning process include viscosity, surface tension, and electrical conductivity. These parameters, measured as a function of gallium nitrate concentration variations used in this study, are shown in Figure 2. The apparent viscosity was found to increase with gallium nitrate. The formulation methodology is at least partially responsible for the viscosity increase because the polyvinylpyrrolidone concentration is also being increased as the gallium nitrate concentration increases and increasing polymer concentration in solution is well-known to increase viscosity. However, it is also possible that increasing gallium nitrate concentration also contributes to the viscosity increase through chelation between gallium ions and the carbonyl-nitrogen resonance structure in polyvinylpyrrolidone.46−49 The surface tension was not significantly affected by the gallium nitrate concentration. The one exception was with 15 wt % gallium nitrate, but this was most likely due to the high viscosity of the solution interfering with the measurement. The general surface tension constancy suggests that it is most likely being controlled by the water:ethanol solvent, which was held constant. The surface tensions of the feedstock solutions are intermediate between the surface tensions of water (72 dyn/cm) and ethanol (22 dyn/cm) and the mean surface tension value of 33 dyn/cm for a 50:50 wt % water/ethanol mixture is consistent with expectations.49 The electrical conductivities of the feedstock solutions increase linearly with gallium nitrate concentration, which is also expected based on the electrical conductivity dependence on charge carrier concentration.50

Figure 2.

Figure 2

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Apparent viscosity, surface tension, and electrical conductivity of electrospinning feedstock solutions as a function of Ga(NO3)3 concentration. Error bars on data points represent a 95% confidence interval from three measurements.

Initial fiber spinning of a solution containing 15 wt % of the gallium nitrate precursor led to erratic fiber formation and minimal fiber deposition on the rotating drum collector. This can most likely be attributed to either the high viscosity or the high electrical conductivity of this solution (see Figure 2). Poor fiber quality and microstructure has been observed when viscosity is above 1000 cP,48 but did not generally prevent deposition, so the high electrical conductivity is expected to be the major contributing factor. Electrospun fibers can remain electrically charged after the spinning process,51,52 and repulsion between fibers can be responsible for inhibiting fiber deposition. The initial electrospinning result suggests a gallium nitrate concentration of 15 wt % is too high for effective fiber collection. Reducing the gallium nitrate concentration below 9 wt % leads to successful fiber formation and collection.

Initial assumptions used in formulating the electrospinning solutions included the following: (1) viscosity is primarily controlled by the polymer concentration, (2) as-spun fiber diameter is primarily controlled by viscosity and so controlled by polymer concentration, and (3) the ceramic fiber diameter after heat treatment is primarily controlled by a combination of the initial fiber diameter and the ceramic precursor concentration. It was believed that if these assumptions were reasonable, then electrospinning the feedstock solutions using similar parameters would result in similar as-spun fiber diameters and the ceramic fiber diameters will depend on the gallium nitrate concentration. SEM images shown in Figure 3 demonstrate the fiber morphology of the as-spun fibers and of the Ga2O3 fibers after a heat treatment at 700 °C. The average fiber diameter of as-spun fibers with 3.25 wt % gallium nitrate was 118 ± 8 nm based on a 95% confidence interval from 40 measurements. This average fiber diameter is a statistically insignificant from the average fiber diameter of 126 ± 7 nm for the as-spun fibers with 7.50 wt % gallium nitrate based on analysis of variance (p = 0.15). In contrast, the difference between the two average fiber diameters of the Ga2O3 fibers from the as-spun fibers were statistically significant (p = 10–7), corresponding to 62 ± 5 nm for the fibers originally containing 3.25% wt % gallium nitrate and 83 ± 5 nm for the fibers originally containing 7.50 wt % gallium nitrate. The similar fiber diameters in the as-spun state and different fiber diameters in the heat-treated state confirm that our initial assumptions are reasonable over the concentration range of 3.25–7.50% gallium nitrate.

Figure 3.

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Secondary electron images of as spun fibers with (a) 3.25 wt % gallium nitrate and (b) 7.50 wt % gallium nitrate. The fibers with (c) 3.25 wt % gallium nitrate and (d) 7.50 wt % gallium nitrate after a thermal treatment at 700 °C are also shown.

The Ga2O3 nanofibers in Figure 3d have protrusions on the surfaces of the nanofibers. These protrusions are similar to prior observations by Zhou et al. when heat treating fibers at 600 °C,35 whereas smooth fibers were obtained when heat treating at either 550 °C, corresponding to fully amorphous fibers, or 750 °C and corresponding to fully crystalline fibers. Zhou proposed that this fiber morphology results from the onset of crystallization, whereby the protrusions are crystalline β-Ga2O3 growing on the amorphous fibers. The heat treatment at 700 °C is intermediate between the 600 and 750 °C temperatures used by Zhou et al.35 and is therefore consistent. However, an interesting feature of Figure 3 is that the fibers originally containing 3.25% gallium nitrate and heat-treated at 700 °C did not exhibit the same morphological features. This suggests different crystallization behaviors when using different gallium nitrate concentrations. Figure 4 displays fibers treated at 800 and 900 °C show a similar pattern of smooth fibers of reduced sized, even with higher concentrations of precursor, indicating a conversion to a fully crystalline structure at higher temperatures.

Figure 4.

Figure 4

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SEM images of Ga2O3 fibers formed from 7.5% precursor solution subjected to thermal annealing. The data shown are for Ga2O3 fibers annealed at 800 (A) and 900 °C(B), respectively.

X-ray Diffraction

The XRD patterns of pre-heat treated as well as post-processing annealed samples are shown in Figure 4. The data shown are for pre-heat treated samples and samples annealed at 700, 800, and 900 °C, respectively. While the pre-heat treatment samples show, predictable, no district peaks due to the amorphous nature of the sample, further heat treatment shows that both α and β phases of gallium oxide are present at 700 °C as well as peak initially consistent with GaO(OH). Increasing the temperature of calcining leads to a more complete transition to the beta phase with the one notable anomaly of a peak of alpha phase being measured within the 800 °C sample. This is unexpected as gallium oxide morphology is typically expected to shift entirely to the β phase when subjected to 800 °C temperatures when converted into thin films.

Thermal Stability Behavior

TGA and data obtained on the Ga-oxide nanofiber samples are shown in Figure 5. It is evident that TGA curves show an expected decrease in mass, which is associated with the decomposition of polymers within the fibers. Such a result would be expected with the conversion of the gallium nitrate precursor and polymer to amorphous gallium hydroxide (GaOOH) and then to crystalline gallium oxide (Ga2O3). Minimal weight change at higher temperatures indicates a high degree of thermal stability which would be expected from crystalline gallium oxide. The anomalies in the heat flow can likely be attributed to external factors influencing the instrument at the time of measurement.

Figure 5.

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XRD measurements for 7.5% precursor nanofibers heat-treated at 700, 800, and 900 °C.

Crystal Structure, Phase Stabilization, and Chemical Bonding

Transmission Electron Microscopy and X-ray Diffraction

To better understand the structure evolution in Ga-oxide nanofibers, the local microstructure of the selected samples is probed using TEM measurements. Figure 6 presents HAADF-STEM (Figure 6A,B) and BF TEM (Figure 6C,D) images of Ga2O3 fibers. Imaging analysis indicates that all the Ga2O3 fibers are nanocrystalline. While grain sizes (tens of nanometers) are too small to isolate a single grain for diffraction, SAED from a region allowed us to determine the crystal structure and specific phase of these materials. As is evident in the images, larger agglomerated regions are also present in the sample (Figure 6D).

Figure 6.

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TGA of nanofibers at varying precursor concentrations 3.25% (A) and 7.5%(B). The data recorded in the temperature range of 25–1050 °C shown.

The SAED patterns of Ga2O3 nanofibers are shown in Figure 7. The experimental and simulated patterns are shown to compare the data and to understand the phase stabilization in Ga2O3 nanofibers. First of all, it is evident that the SAED pattern exhibits a ring made up of bright spots that indicate that the nanofiber samples were composed of randomly oriented nanocrystalline Ga2O3. The pattern was successfully indexed with ICDD PDF4+ database software and the diffraction rings match with the simulated diffraction patterns of Ga2O3. The SAED analyses confirm the formation of nanocrystalline Ga2O3 fibers. Here, the simulated pattern is shown at approximately the same magnification as the experimental pattern, and the arrows indicate the positions of the non-overlapping alpha-phase diffraction rings (which appear to be absent from the experimental pattern). The three innermost diffraction rings match the expected diffraction rings for the beta phase. Thus, the electron diffraction analyses suggest that after the 900 °C heat treatment, only the β-phase is present. In the case of the 700 and 800 °C samples, the TEM results do not indicate an initial formation of alpha phase gallium oxide. However, there is evidence of both the more complete transition to the beta phase with increased heat treatment temperature as well as the improved crystallinity of the samples as indicated by the formation of distinct nanocrystals in the 800 °C fibers not present in the lower temperature samples (Figure 7E,F).

Figure 7.

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(A,B) High-angle annular dark-field scanning TEM (HAADF-STEM) and (C,D) bright-field (BF) TEM images of Ga-oxide nanofibers after 900 °C, 2 h heat treatment. The formation of nanocrystalline Ga-oxide fibers is evident in image A, while higher magnification images (B,C) show the fiber structure and arrangement. Microstructure is extensively nanocrystalline as indicated in image D. At the higher magnifications, the complete formation of distinct nano crystals can be seen for an 800 °C sample (F), this is not as the case for those fibers heat-treated at 700 °C that appear to partially retain an amorphous structure (E).

For Ga-oxide nanofibers, while structural analysis made by TEM is most reliable, for comparison and validation purposes, we also performed the XRD studies on the as-synthesized and heat-treated Ga-oxide nanofibers. The XRD patterns of pre-heat-treated as well as post-processing annealed samples are shown in Figure 5. The data shown are for Ga-oxide nanofibers samples pre-heat-treated and annealed at different temperatures. It can be seen that as-synthesized Ga-oxide nanofibers are amorphous. These results agree with those reported in the literature, where as-synthesized Ga-oxide nanofibers made by electrospinning are amorphous.36,41 The formation of the amorphous Ga2O3 fibers is due to the fact that no sufficient thermal energy is provided either by means of substrate heating or chemical reaction involved in the process of electrospinning. Thermal treatment at 800 °C transforms the amorphous Ga-oxide entirely into the β-phase Ga2O3. This is also seen in Raman spectroscopy data for the 800 °C samples as discussed in the section followed immediately after this. Overall, based on structural analysis, the as-deposited nanofibers are amorphous, which fully transform to crystalline β-phase Ga2O3 nanofibers, which is in good agreement with the reports in the literature for Ga2O3 nanofibers made by the electrospinning process.36,41 Zhao also reported that, in order to obtain crystalline nanofibers, samples were annealed at 900 °C in air for 3 h.36 Similarly, Yoo et al. obtained Si-doped β-Ga2O3 nanofibers via the electrospinning method by thermal treatment 900 °C in air for 6 h. Thus, comparing our results with intrinsic Ga2O3 nanofibers along with those reported on either intrinsic or doped Ga2O3 nanofibers, we can conclusively indicate that a temperature on the order of 900-1000 °C for 2–3 h is optimum to realize crystalline β-phase Ga2O3 nanofibers. However, while the precursor chemicals seems to be affecting the overall yield of the fibers and dimensions, slightly extended time of annealing and or higher temperatures are needed in the case of dopants employed to obtain nanofibers without perturbation to the parent β-phase of Ga2O3.

Raman Spectroscopy

To further assess the chemical bonding and structural quality of Ga-oxide nanofibers, we relied on the spectroscopic characterization, particularly Raman scattering, which provide direct information on the chemical bonding and dopant effect (if any), especially in nanomaterials and thin films.53,54 β-Ga2O3 exhibits monoclinic crystal symmetry with a C2/m space group. From group theory symmetry analysis, the number of expected optical vibrational modes of Ga2O3 is 27, and among these vibrational modes, 15 are Raman active, while 12 are infrared active.55 Raman spectra of Ga2O3 are classified into three regions: a high-frequency region (770–500 cm–1) which includes stretching and bending modes of GaO4 tetrahedra, phonon modes in a middle frequency region (480–310 cm–1) due to deformation of GaO6 octahedra, and phonon modes in a low-frequency region (below 200 cm–1) due to the oscillation and translation of Ga–O chains.41,55Figure 8 shows the Raman spectra of as-prepared and thermally annealed samples. Corroborating with structural data, as-prepared samples (prior to heat treatment) exhibit broader peaks, indicating the amorphous nature of the nanofibers. However, it should be noted that most of the broader peaks still correspond to the skeleton of Ga2O3. The peak evolution clearly indicates the structural transformations and microstructure changes. The most important observation is the fact that the sharp and high intense phonon modes reveal that samples are highly crystallized. The nanofibers prior to heat treatment exhibit a smaller number of the phonon modes, and the blue-shift in the frequency of phonon modes is clearly evident compared to bulk β-Ga2O3.55 A total of six phonon modes were found with two phonon modes in each region. These observations, particularly, the Raman peaks becoming very sharp and narrow indicating improved crystallinity of the samples under thermal treatment of Ga2O3 nanofibers was also reported by Zhao et al.36 On the other hand, despite the annealing temperature induces crystal growth and phase-stabilization, dopants/impurities can induce the degradation of the fibers’ structural quality, resulting in the Raman peak intensity decrease.36 Such effects are not seen in the present case accounting for the formation of β-Ga2O3 nanofibers with phase- and chemical purity upon thermal treatment.

Figure 8.

Figure 8

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A) Sample differences between α and β gallium oxide diffraction rings. (B) Observed diffraction rings indicating for both 700 and 800 °C samples show transition to the β without conclusive evidence of residual α phase (C) Diffraction pattern for 900 °C (D) index and d-spacing.

Chemical Analysis

The XPS core level spectra of the Ga 2p region were probed to understand the electronic structure difference (if any) between the Ga-oxide nanofibers synthesized under variable processing conditions. In all the samples, the Ga 2p3/2 peak was located with a positive Binding Energy (BE) shift relative to the Ga metallic state, indicating that the Ga ions exist in their higher valence state. Generally, the positive shift in the binding energy of Ga 2p3/2 core level peak is due to the redistribution of the electronic charge.56,57 On the other hand, very similar features and respective BE positions indicate that the Ga chemical state is stabilized in its highest oxidation state and is common in all of the Ga2O3 nanofibers. Specifically, no changes in BE location and/or peak shape were observed for the Ga 2p region as a function of either heat treatment of precursor concentration. This observation allows to conclude that the Ga ion chemical state is unaffected by any of these variable processing conditions or post processing thermal annealing and the Ga always exist in their highest chemical state (Ga3+). However, the critical analysis of the oxygen core level peak explained the key differences arising in the chemical quality of the samples.

To further understand the mechanism and differences in nanofibers stabilizing in a specific phase, the high-resolution XPS data of O 1s (Figure 9) in nanofibers are evaluated. It was noted that the O 1s peak is highly asymmetric for samples heat treated at lower temperatures, while it becomes symmetric in the samples heat treated at 900 °C. The asymmetric nature of the O 1s peak is, generally, indicative of oxygen bonded to different species (other than the host oxide matrix).56 Therefore, attempts were made to resolve the components by means of peak fitting. The asymmetric O 1s peak can be resolved into two component regions: (i) the first is the intense, main peak centered at BE ∼ 530.7 eV and (ii) two small shoulder peaks located at higher binding energies at 532.2 and 533.4 eV. These three component peaks indicate three different chemical states or chemical bonding environments around oxygen in the Ga-oxide ceramic nanofibers. The main O 1s peak component with a major intensity corresponds to oxygen bonded to gallium (Ga–O bonds) as expected in stoichiometric Ga2O3. It has been reported in the literature that the O 1s peak for the stoichiometric, undoped Ga2O3 occurs generally at BE ∼ 530.5–531.0 eV.56,57 Comparison of the O 1s peak data indicates that the O 1s peak position occurs at a well-defined BE. Also, there is no appreciable peak shift in the BE position. These factors indicate the formation of Ga–O bonds, which are the signature of stoichiometric Ga2O3, in all of the nanofibers. The shoulder peaks at BE 532.2 and 533.4 eV can be attributed to the surface oxygen bonded to carbon in the form of either carbonyl (oxygen bonded to carbon, C–O) or hydroxyl (oxygen bonded to hydrogen, O–H) groups.58 The presence of the carbonyl/hydroxyl component with a significant intensity for nanofibers at 700 °C compared to that of 900 °C indicates that the stabilization of alpha GaOOH may be due to the water incorporated into Ga2O3 via retention of hydroxyl and carbonyl groups within the crystalline structure.59−61 These functional chemical groups may be included into the sample during synthesis and retained during pyrolysis of the polymer, as the synthetic approach to these nanofibers is based on solution-based chemistry. However, when the samples are thermally annealed at higher temperatures (900 °C), the hydroxyl component may be fully eliminated leading to the formation of beta phase Ga2O3.

Figure 9.

Figure 9

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Raman spectra of samples as prepared and heat-treated Ga-oxide nanofibers.

Proposed Mechanism of Growth and Phase Stabilization

Based on the results obtained and discussed in the previous sections, a simple model can be postulated to explain the growth mechanism and phase stability of Ga2O3 nanofibers derived from electrospinning. It is widely accepted in the literature that the size, specific phase, and morphology of either α-GaOOH or β-Ga2O3 nanomaterials, such as nanorods and nanofibers, synthesized by solution-based chemical approaches are highly dependent on the synthetic conditions. Primarily, the solution-based chemistry, as determined by the precursor solutions and the ingredient concentrations, pH values, synthesis times, processing temperatures, reaction temperatures, and so forth, significantly influences the resulting materials’ microstructure, morphology, and phase stability, which in-turn govern the properties and performance.6−10,12−23 In the present approach, while the initial mechanism for Ga2O3 formation was assumed to be a direct decomposition of the Gallium nitrate precursor upon exposure to air at elevated temperatures leading to the initial formation of alpha phase Ga2O3, which in turn led to partial and then complete transformation into the beta phase polymorph a higher temperatures (800–900 °C). The sequence and proposed mechanism are schematically presented in Figure 10. Further evidence gathered from the Raman spectroscopy data indicating a secondary formation mechanism, namely, the presence of peaks indicative of GaO(OH) both in the as-prepared and lower temperature samples. This chemical signature is potentially the result of an unintentional forced hydrolysis upon application of the current for the electrospinning procedure.62 This, combined with the application of elevated calcining temperatures that would lead to a series of decomposition stages initiating with the formation of Ga(OH)2NO3 and then leading to further breakdown into Ga(OH)3 and Ga(NO3)O compounds before finally reaching the observed GaO(OH) and Ga2O3 phases present at the highest calcining temperatures.60−63

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Figure 10.

Figure 10

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XPS plots of O 1s core level for Ga-oxide nanofibers. The data shown are for the following samples: as synthesized (A), 800 (B), and 900 °C(C).

This proposed mechanism is also in agreement with some reports in the literature, where a gallium nitrate-based precursor was employed.61−63 It has been demonstrated that gallium oxide hydroxide (α-GaOOH), which crystallizes with a structure similar to α-AlOOH, undergoes transformation to Ga2O3 during thermal treatment. Figure 11 Thus, based on the present work and comparison with the literature, it can be conclusively stated that the synthesis of homogeneous Ga2O3 with phase stability can be obtained by thermal treatment.

Figure 11.

Figure 11

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Schematic diagram of proposed growth mechanism and phase stability of Ga-oxide nanofibers. The specific phase and transformation sequence as a function of temperature are as indicated in the figure.

Summary and Conclusions

The effect of processing parameters on the properties of Ga2O3 nanofibers was evaluated by SEM, TEM, XPS, and Raman spectroscopy. Thermal treatments in the range of 700–900 °C induces the crystallization of amorphous fibers and leads to the phase stabilization of α-Ga2O3 hydrate, β-Ga2O3, or mixtures of these phases. TEM SAED analyses indicate that the transformation sequence progresses by forming the amorphous GaOOH as synthesized and transitioning to α-GaOOH and β-Ga2O3 nanofibers at low to intermediate temperatures (700–800 °C) before fully transforming to β-Ga2O3 phase at higher temperatures (900 °C). Raman spectroscopic analyses corroborate the structural evolution and confirm high chemical quality of the β-Ga2O3 nanofibers. The electronic structure probed by XPS indicates that the hydroxyl groups present for the samples treated at lower temperatures account for the α-Ga2O3 phase formation. However, treatment at higher temperatures fully removes those hydroxyl groups, resulting in the formation of β-Ga2O3 nanofibers. Additional details and data of XRD and XPS are included..

Acknowledgments

The authors acknowledge support from the Nuclear Science and Security Consortium, the Nuclear Safety Research and Development Program for the U.S. Department of Energy, and the Nuclear Safety Research and Development Program for the National Nuclear Security Administration. The authors acknowledge, with pleasure, support from the National Science Foundation (NSF) with NSF-PREM Grant DMR-1827745. The authors would also like to acknowledge the assistance of Nick E. Teslich for the SEM imagery. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c05168.

  • XPS analysis of the core-level Ga 2p region (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c05168_si_001.pdf (125.3KB, pdf)

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