Catalyst-free synthesis of a three-dimensional nanoworm-like gallium oxide–graphene nanosheet hybrid structure with enhanced optical properties

Abstract

We here report the synthesis and growth of catalyst-free three-dimensional β-gallium oxide nanoworm-like nanostructures on graphene nanosheets (3D β-Ga₂O₃@GNSs) using a solid mixture of graphite oxide and gallium acetylacetonate by the microwave (MW)-assisted method for the first time. The MW-assisted synthesis of the 3D β-Ga₂O₃@GNSs hybrids contains 1D semiconducting β-Ga₂O₃ nanoworms (NWs) and 2D highly conducting graphene nanosheets (GNSs) materials. The β-Ga₂O₃ NWs have an average diameter of 200 nm and lengths of up to ∼1 μm grown on the GNSs. These 3D β-Ga₂O₃@GNSs hybrids have been synthesized in a very short time with scalable amounts. The controlling parameters such as MW irradiation time and power were found to greatly influence the structural morphology of the as-synthesized 3D β-Ga₂O₃@GNSs hybrid. This method for the synthesis of 3D β-Ga₂O₃@GNSs hybrids is imperative due to it allowing excellent control over experimental parameters, being low cost and having better reproducibility. Also, the catalyst-free MW-assisted method is a much more rapid and thus higher throughput alternative for effective and scalable growth over the conventional heating method. The crystallinity, structure, morphology, and optical analysis of the 3D β-Ga₂O₃@GNSs hybrids are carried out utilizing several techniques. The formation of the 3D β-Ga₂O₃@GNSs hybrids shows a band gap variation from 4.94 to 4.48 eV associated with the structural evolution. A suitable growth mechanism has been suggested for the formation of these 3D β-Ga₂O₃@GNSs hybrids.

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Introduction

Three-dimensional (3D) nanostructured materials are a focused research field both due to their importance in mesoscopic physics study and the increasing desire for applications of nano devices. The combination of two different types of nanomaterials containing different dimensions makes their properties attractive for electron transportation from one side to the other side with the help of their unique hybrid nanostructure. These types of hybrid nanostructures can be formed from the one-dimensional growth of semiconducting nanomaterials (nanorods, nanowires and nanotubes) on two-dimensional carbon-based materials like graphene nanosheets (GNSs). These 3D nanostructures have attracted much attention due to their novel importance in understanding fundamental physical concepts and their potential applications in building blocks for electronic, thermal and optical nano devices.^1–3 Gallium-based compound materials such as gallium nitride (GaN),⁴ gallium oxynitride (GaON),⁵ and gallium oxide (Ga₂O₃)⁶ are among the promising inorganic semiconductor compounds that provide numerous advantages over other organic materials for electronic and optoelectronic device applications.^7–13 Among the above Ga-based compounds, Ga₂O₃ possesses different crystalline phases, including α-, β-, γ-, δ- and ε-Ga₂O₃.¹⁴ Among these phases, the monoclinic structured β-Ga₂O₃ is the most stable form, and is a UV transparent semiconductor with a wide band gap of 4.9 eV.¹⁵ Ga₂O₃ also exhibits conduction and luminescence properties, and thus has potential applications in water splitting,^16,17 photocatalysis,^18,19 gas sensing,^20,21 nano-photonics,²² tunable phosphorescence²³ and plasmonic applications²⁴ etc. However, for the various application purposes Ga₂O₃ nanostructures have been synthesized by various methods, including laser ablation,²⁵ arc-discharge,²⁶ physical deposition via vapor–solid,^27,28 the vapor–liquid–solid method,²⁹ and so on. But from the above-mentioned synthesis methods, it is possible to synthesize only Ga₂O₃ nanostructures with no other binary elements present.

Graphene has a high theoretical surface area (2600 m² g⁻¹),³⁰ excellent thermal conductivity (4840–5300 W m⁻¹ K⁻¹),³¹ extremely high mechanical stiffness (tensile strength up to 130 GPa, modulus of 1000 GPa),³² and ultra high electron mobility (2 × 10⁵ cm² V⁻¹ s⁻¹)^33,34 due to its sp² hybridization with a one-atom thick and two-dimensional honeycomb lattice structure. These properties make it one of the most fascinating and peculiar materials for the preparation of high performance hybrids with other functional inorganic semiconductor nanostructures.³⁵ These hybrid nanostructures of inorganic semiconductors and graphene can offer additional flexibility, functionality and novelty for realizing advanced optoelectronic and electronic applications. The applications of hybrid nanostructures are shown as in nano generators, chemical sensors, photovoltaics, field emission devices, and sensitive biological and efficient energy conversion and storage devices.^36–39 Graphene, containing the weakly bonded layers of two-dimensional hexagonally arranged carbon atoms held together by strong triangular covalent σ-bonds of the sp²-hybridized orbitals, can allow us to transfer the grown inorganic semiconductor nanostructures or films onto the other arbitrary substrates such as glass, metal, and plastic easily.

In view of the significantly enhanced properties and applications of other wide band gap semiconductors coupled with graphene, a wide band gap semiconductor β-Ga₂O₃ has been synthesized. In sight of the attractive features of the chemically and thermally stable oxide semiconductor having a large direct band gap, β-Ga₂O₃ can be regarded as a conventional n-type oxide semiconductor with a large band gap of 4.5–5.0 eV.^40–42 Its unique monoclinic lattice (known as the β-gallium structure) leads to anisotropy in the optical and electrical properties. For the growth of high band gap inorganic semiconductors on graphene, the most common method used is the vapor-phase technique such as metal–organic vapor-phase epitaxy.^43,44 Several compounds and hybrids have also been synthesized using different methods with graphene using high band gap semiconductors such as graphene–ZnO composites,^45,46 graphene–ZnGa₂O₄ complexes,⁴⁷ and graphene–SiC hybrids^48,49 etc. These reported methods require longer time and accuracy and are costly. Not only the time and cost but also the energy required to carry out such reactions can be saved with the MW irradiation route of syntheses.

The MW-assisted method has several advantages such as shorter reaction times, being an environmentally friendly and energy saving technique, giving uniform distribution of energy inside the sample, having better reproducibility and excellent control over experimental parameters etc. Here, we introduce a convenient MW-assisted exfoliation method to synthesize 3D β-Ga₂O₃@GNSs hybrids at a lower temperature and ambient pressure. To the best of our knowledge so far, there is no report available on the catalyst-free growth of such Ga-based compound materials on GNSs by a MW irradiation method. MW irradiation provides heating of the reaction mixture rapidly and homogeneously. Therefore, it opens up new options for energy and cost saving approaches towards 3D hybrid nanostructure production. This MW irradiation method seems to be a promising method to grow Ga₂O₃ inorganic semiconductor oxides on GNSs with superior controllability in terms of growth rates and structure dimensions. Also, this MW irradiation method is very attractive for large-scale synthesis of such unique 3D hybrids. The synthesized 3D nanomaterial shows significant band gap reducing properties.

Experimental section

Synthesis of 3D β-Ga₂O₃@GNSs hybrids

First, the starting material graphite oxide (GO) was synthesized by a modified Staudenmaier method.⁵⁰ In a typical synthesis route for the 3D β-Ga₂O₃@GNSs hybrids, GO dried powder (1.2 g) and gallium acetylacetonate (Ga(C₅H₇O₂)₃) powder (0.1 g) were added in ethanol (C₂H₅OH) (30 mL). The solution was then sonicated (5 min) and magnetically stirred (10 min) for homogeneous dispersion. During the stirring process, 2 mL NH₄OH (0.5 M) was added dropwise in the stirring solution. After stirring for 10 min, the solution was washed with DI water and dried at 30 °C. The dried powder was placed in a quartz cup and irradiated with MW power (P = 800 and 900 W) for different irradiation times (t = 1, 2 and 3 min). From a few trial experiments, it was found that a 3 min exposure time and 900 W was enough to ensure the completion of the reaction for the growth of the 3D β-Ga₂O₃@GNSs hybrids. During MW irradiation, the samples suddenly burned in the form of plasma and converted into black, highly porous materials. After MW irradiation at the above power and time, the black porous materials were collected and characterized for structural and morphological analysis.

Materials characterization

The microstructure and morphology of the as-synthesized 3D β-Ga₂O₃@GNSs hybrids were analyzed using X-ray diffraction (XRD, D/MAX-2500/PC, Rigaku Co., Tokyo, Japan), scanning electron microscopy (Philips XL 20) and transmission electron microscopy (TEM, FEI Tecnai G20, FEI Company, USA). The elemental compositions and defect information of the synthesized materials were analyzed using X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd, England) and Raman spectroscopy, respectively, whereas the optical absorption properties were investigated using a UV-visible diffuse reflectance spectrophotometer (U-41000, HITACHI, Tokyo, Japan). Band gap energies of the Ga₂O₃ and 3D β-Ga₂O₃@GNSs hybrids were calculated by analysis of the Tauc plots. Room temperature PL measurement of the powder samples was recorded with a Shimadzu RF-5301PC spectrofluoro-photometer using an excitation wavelength of 250 nm.

Results and discussion

The synthesis scheme of the 3D β-Ga₂O₃@GNSs hybrids is depicted schematically in Fig. 1. During the MW irradiation process, two different phenomena occur simultaneously. The first is MW-induced exfoliation and conversion of GO into few layer GNSs. This exfoliation shows the presence of gaseous products in GO, which release in the form of gases (CO₂, CO etc.) resulting in further expansion along the c-axis of the GNSs. The small amount of NH₄OH reacts with the oxygen moieties, carboxyl, hydroxyl, and carbonyl groups of the graphene oxide surfaces and grafts amine groups on it. The formation of β-Ga₂O₃ NWs is initiated from the decomposition of the gallium acetylacetonate compound under MW irradiation, resulting in generation of β-Ga₂O₃ nanoparticles. The complete formation of the 3D β-Ga₂O₃@GNSs hybrids takes place when the irradiation power and irradiation time are set at 900 W and 3 min, respectively. The Ga₂O₃ nanoparticles formed and anchored on the GNSs induce the growth of β-Ga₂O₃ NWs. Ga is a metal element, while the GNSs are highly conducting, so the former is more reductive, and easily gets oxygen from the GNS surface (oxygen-containing functional groups) to form Ga₂O₃ NWs.

Fig. 1 Schematic representation of the synthesis of the 3D β-Ga₂O₃@GNSs hybrids.

The crystal structure and crystalline phase of the as-synthesized powder materials were characterized by X-ray diffraction (XRD). A characteristic diffraction single and broad peak (002) at 2θ = 10° (Fig. 2(a)) clearly demonstrates the formation of GO. The inset of Fig. 2(a) shows XRD of the GNSs after the reduction and exfoliation of GO through MW irradiation. It clearly shows the shifting of the (002) peak from 2θ = 10° to 25° from GO to the GNSs and also shows the peak broadening in the GNSs which represents that the GNSs have the few layer characteristic. The XRD pattern of the as-synthesized 3D β-Ga₂O₃@GNSs hybrids, which was collected after 3 min of MW irradiation at 900 W (Fig. 2(b)), matches perfectly with the reported β-Ga₂O₃ structure.⁵¹ The XRD pattern of as-synthesized 3D β-Ga₂O₃@GNSs hybrids shows diffraction peaks at 2θ values of 30.1°, 31.6°, 33.4°, 35.2°, 37.4°, 38.4°, 43.1°, 45.4°, 48.4°, 49.6°, 54.6°, 57.6°, 59.9°, 60.9°, 62.7°, 64.7°, 69.4°, 70.3° and 72.4°, which can be indexed to (400), (002, 2̄02), (1̄11), (111), (401), (3̄11), (1̄12), (3̄12), (003), (402), (203), (3̄13), (113), (020), (710), (7̄12), (420), (2̄22, 022) and (6̄04), respectively to the monoclinic structure of Ga₂O₃ with cell constants of a = 12.21 Å, b = 3.03 Å, c = 5.79 Å, and β (angle) = 103.83° (JCPDS file no. 87-1901). We noticed that the relative intensities of the diffraction peak exhibit a significance difference between the present pattern and the standard pattern of the β-Ga₂O₃ phase as given in the JCPDS file (JCPDS file no. 87-1901). In the JCPDS file (101) is the first maximum and (002, 2̄02) is the second maximum but in our XRD pattern, the maximum intensity peak is (002, 2̄02), which is attributed to preferential growth of the worms in this particular direction. No peak associated with the other crystalline forms of the gallium oxides was detected in the pattern. The sharp diffraction peaks also reveal that the as-synthesized β-Ga₂O₃ NWs on the GNSs possess highly crystalline characteristics. The one broad and single peak at 2θ = 26.25° corresponds to the (002) plane of graphene due to the GNSs since GO has been effectively reduced and converted into GNSs during MW irradiation.⁵²

Fig. 2 XRD patterns of (a) graphite oxide and (b) 3D β-Ga₂O₃@GNSs hybrids. Inset shows XRD of the GNSs after reduction of GO through MW irradiation.

To analyze the changes in surface morphology of the hybrids during the synthesis process, SEM micrographs of the GO, exfoliated GNSs and 3D β-Ga₂O₃@GNSs hybrids are shown in Fig. 3. Fig. 3(a) shows the agglomerated structure with parallel flakes. After MW exfoliation, this GO gets converted into GNSs as shown in Fig. 3(b). These GNSs have several micron sized lateral graphene sheets with wrinkles on their surfaces. The surface morphology of the as-synthesized 3D β-Ga₂O₃@GNSs is shown in Fig. 3(c) and (d). SEM observations represent the 3D β-Ga₂O₃@GNSs hybrids with nearly vertical emerging β-Ga₂O₃ NWs on the surfaces of the GNSs and later they show a bent morphology. Fig. 3(c) and (d) show that the β-Ga₂O₃ NWs with low density are grown on the GNSs and distributed at a distance of ∼500 nm to 1 μm on the GNSs. The Fig. 3(c) SEM micrograph shows that the whole GNSs contain 1D β-Ga₂O₃ NWs standing on the 2D GNSs in different directions. Fig. 3(d) shows that the Ga₂O₃ nanoparticles are attached on the graphene surfaces. The high resolution SEM micrograph shown in Fig. 3(d) indicates that the NWs are firmly anchored on the graphene surface, and graphene provides support to the NWs. We can see that the as-synthesized β-Ga₂O₃ NWs are not grown vertically on the GNS surfaces and possess a bent structure which looks like earthworms on the graphene surfaces. The β-Ga₂O₃ NWs emerge out from the graphene surfaces showing a supporting platform for these NWs. The average diameter and length of the NWs are 200 nm and ∼1 μm, respectively.

Fig. 3 SEM micrographs of (a) graphite oxide, (b) MW exfoliated GNSs and (c and d) 3D β-Ga₂O₃@GNSs hybrids (P = 900 W, t = 3 min).

On the basis of the SEM micrographs we found that the different irradiation power and irradiation time have an effect on the morphology as shown in Fig. 4. When the experiment was conducted with the same MW irradiation power (900 W) for different irradiation times (t = 1 min), the β-Ga₂O₃ nanoparticles were decorated on the GNS surfaces and had diameters of 50–80 nm (Fig. 4(a)). After increasing the irradiation time (1 min < t < 2 min), we found that these β-Ga₂O₃ decorated nanoparticles start to agglomerate into larger sized nanoparticles. These agglomerated nanoparticles (inside circles) have diameters in the range 100–200 nm (Fig. 4(b)). For longer MW irradiation times (t = 2 min) we found small NW-like structures on the GNSs with lengths of 400–600 nm and some β-Ga₂O₃ nanoparticles can be seen on the graphene surfaces (Fig. 4(c)). This partial growth of the NWs and particles on the graphene surfaces suggests that this MW irradiation time is not sufficient for the growth of Ga₂O₃ nanoparticles. Due to this reason Ga₂O₃ nanoparticles could not grow as NWs.

Fig. 4 SEM micrographs of (a) β-Ga₂O₃ nanoparticles at GNSs (P = 900 W, t = 1 min), (b) β-Ga₂O₃ nanoparticles agglomerated at GNSs (P = 900 W, 1 < t < 2 min), (c) β-Ga₂O₃ NWs at GNSs (P = 900 W, t = 2 min) and (d) β-Ga₂O₃ NRs at GNSs (P = 800 W, t = 3 min).

A lower MW irradiation power (P = 800 W, t = 3 min) produces sub-micrometer sized irregularly shaped particles and short nanorods (NRs) with lengths of 200–400 nm on the GNSs (Fig. 4(d)). Fig. 4(d) shows the low magnification SEM image of the as-synthesized product and one can clearly see that the large yield Ga₂O₃ NR-like morphology is randomly grown on the GNSs. However, the majority of the NRs are comprised of different lengths with an agglomerate morphology with random growth. Occasionally, a small amount of nanoneedle-like Ga₂O₃ NRs can also be found. The structural variations in the as-synthesized hybrids (nanoparticles, NWs and NRs with GNSs) with respect to MW irradiation power and irradiation time are summarized in Table 1.

Table 1 Structural growth variations with MW irradiation power and time

S. No.	MW irradiation		Structural variation in hybrids
	Time (t) (min)	Power (P) (W)
1.	1	900	β-Ga2O3 nanoparticles anchoring on the GNSs (diameter: 50–80 nm)
2.	2	900	β-Ga2O3 NWs and nanoparticles on the GNSs (length ∼400–600 nm)
3.	3	900	Growth of β-Ga2O3 NWs on the GNSs and complete formation of 3D β-Ga2O3@GNSs (length ∼1 μm)
4.	3	800	β-Ga2O3 NR formations on the GNSs (length ∼200–400 nm)

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On the basis of the above observation we can conclude that the length of the NWs on the GNSs increases with the irradiation power with sufficient exposure time for the in situ chemical reaction for the growth. The β-Ga₂O₃ NWs grown with a short irradiation time show incomplete formation of the 1D nanostructure and the structure is dominated by both Ga₂O₃ NWs as well as Ga₂O₃ nanoparticles (Fig. 4(c)). Such types of structure seem to diminish with the increase in irradiation power. It is speculated that the increase of irradiation time as well irradiation power of the MW oven helps to promote the growth of Ga₂O₃ NWs on the GNSs and to complete the formation of the 3D β-Ga₂O₃@GNSs hybrids. When the irradiation time is increased (t = 1 min to 3 min) while keeping the irradiation power at 900 W, the grown structures are basically dominated by the β-Ga₂O₃ NWs. On the other hand, when the irradiation time is increased while keeping the irradiation power at 800 W, the Ga₂O₃ NR structure seems to dominate where their NRs and faceted particles arrangements co-exist in the nanostructure formation.

Fig. 5(a) is a typical TEM image of β-Ga₂O₃ NWs, which were grown on GNSs at 900 W for 3 min. This grown morphology shows not completely straight but bent structural β-Ga₂O₃ NWs on the GNSs. A high resolution TEM (HRTEM) image has been taken of the NWs as shown in Fig. 5(b). This HRTEM image gives further evidence of crystalline ordering within the walls of the β-Ga₂O₃ NWs. Also, it indicates the crystalline planes which have clearly-resolved inter-planer lattice spacings of different planes. The lattice spacing is approximately 0.28 nm corresponding to the distance between the (2̄02) planes of the β-Ga₂O₃ NWs.⁵³ Fig. 5(c) shows the energy dispersive spectroscopy (EDS) spectrum of the 3D β-Ga₂O₃@GNSs hybrids. The EDS data show that C, Ga and O are the only elements detected, indicating that the hybrids have a lower Ga content. Based on the obtained results, the atomic weight percentages of C, Ga and O were 81.83, 7.21 and 10.95, respectively. EDX analysis shows that the as-synthesized 3D β-Ga₂O₃@GNSs had an overall Ga : O ratio of 1 : 1.6 which is close to the expected 1 : 1.5 for Ga₂O₃. The existence of Ga and O with an approximate ratio of 2 : 3 indicates its stoichiometry.

Fig. 5 (a) TEM, (b) HRTEM and (c) EDS patterns of the 3D β-Ga₂O₃@GNSs hybrids.

Raman spectroscopy is a convenient and powerful tool to study the structural characteristics of materials and is used to distinguish the order and disorder/defect structures in hybridized carbon. The Raman characteristic peaks of the GNSs and the 3D β-Ga₂O₃@GNSs hybrids are shown in Fig. 6. There are two strong peaks centred at 1351 cm⁻¹ (D-band) and 1586 cm⁻¹ (G-band) in both samples. The peak at 1351 cm⁻¹ corresponds to the Raman-inactive A_1g in-plane breathing vibration mode, and is related to the defects and disorders in structures in carbonaceous solids. The G-band assigned to the Raman-active E_2g mode corresponds to the stretching vibration in the basal-plane of graphite, and is generally used to identify well-ordered GNSs. The D-band is connected with the defects and disorder in the hexagonal graphitic layers, while the G-band is associated with the Raman-active E_2g mode induced by the presence of sp² carbon-type structures.⁵⁴ The intensity ratio of the D-band and G-band (I_D/I_G) is used to estimate the graphitization quality of graphene-based materials. The I_D/I_G value for the GNSs and 3D β-Ga₂O₃@GNSs hybrids was found to be 0.72 and 0.97, respectively. This represents more plane defects in the β-Ga₂O₃@GNSs hybrids as compared to the GNSs due to the growth of the β-Ga₂O₃ NWs. Also, after formation of the 3D β-Ga₂O₃@GNSs hybrids, the D- and G-band peaks are broadened and the FWHM value is found to be 111 cm⁻¹ and 83 cm⁻¹ for the D-band and G-band respectively. However, in the case of the GNSs, the FWHM is 48 cm⁻¹ and 35 cm⁻¹ corresponding to the D- and G-band respectively. The 3D β-Ga₂O₃@GNSs hybrids exhibit five peaks at a lower Raman shift side at 176, 315, 419, 474 and 630 cm⁻¹, which match well with the Raman spectrum reported for the β-Ga₂O₃ phase.⁵⁵

Fig. 6 Raman spectra of the GNSs and 3D β-Ga₂O₃@GNSs hybrids.

To determine the elemental presence in the as-synthesized 3D β-Ga₂O₃@GNSs hybrids, X-ray photoelectron spectroscopy (XPS) was performed. Fig. 7(a) displays a full scan in the energy range from 0 to 1200 eV and the peaks corresponding to C 1s, O 1s, Ga 2p, Ga 3p, Ga 3s and Ga 3d as well as the O KLL and Ga LMM auger lines can be observed.⁵⁶ In Fig. 7(b), the gallium core levels Ga 2p_3/2 and Ga 2p_1/2 are observed at 1118 eV and 1145 eV, respectively, with a peak-to-peak separation of 27 eV which confirms the formation of β-Ga₂O₃ in the as-synthesized 3D β-Ga₂O₃@GNSs hybrids.⁵⁷ Fig. 7(c) shows the C 1s peak which can be deconvoluted into three peaks centred at 284.6, 286.3 and 288.5 eV, corresponding to the binding energies of the C=C (sp²), C–O and C=O bonds.⁵⁸ The C=C bond (sp²) is found to be the main contributor indicating the as-prepared material to be GNSs. Fig. 7(d) presents the O 1s peak located at 529.9 eV, which can be divided into two separate peaks located at 528.3 and 527.5 eV by deconvolution fitting. The key peak located at 530.8 eV can be assigned to the Ga–O bonding and the other weak peak at 532.2 eV is due to the C/O or OH adsorbed species on the surface. From the XPS spectra, the ratio of Ga and O is estimated to be about 1 : 1.6; the concentration of O is a little higher than the normal chemical composition possibly due to the existence of surface-adsorbed oxygen functionalities on the GNSs. This stoichiometry has been also confirmed by the EDS spectrum in Fig. 5. Therefore, the XPS results confirm that the synthesized hybrids contain the β-Ga₂O₃ phase with a stoichiometry ratio of ∼2 : 3.

Fig. 7 XPS spectra of the 3D β-Ga₂O₃@GNSs hybrids. (a) Complete survey, (b) Ga 2p spectrum, (c) C 1s spectrum and (d) O 1s spectrum.

UV-visible absorption spectroscopy was used as a tool to determine the band gap energy of the 3D β-Ga₂O₃@GNSs hybrids. The optical band gap for a semiconductor near the absorption band edge can be estimated from the following equation known as the Tauc plot:⁵⁹

(αhν) ∝ (hν − E_g)ⁿ

where α is the optical absorption coefficient, hν is the energy of the incident photon, E_g is the optical band and n = 1/2 and 2 corresponds to the direct allowed transition semiconductor and indirect allowed transition semiconductor respectively. The inset of Fig. 8 shows the Tauc plot of (αhν)² versus energy of the light photon (hv). By extrapolating the linear part of the (αhν)² curve, the direct band gap was determined for β-Ga₂O₃ and the 3D β-Ga₂O₃@GNSs hybrids. The band gap energy for the synthesized 3D β-Ga₂O₃@GNSs hybrids is 4.48 eV, which corresponds to an optical absorption edge of 264 nm estimated from its UV-visible absorption spectrum in Fig. 8. Also the band gap energy for the β-Ga₂O₃ nanostructure is 4.94 eV, corresponding to an optical absorption edge of 255 nm. With the formation of the 3D β-Ga₂O₃@GNSs hybrids, E_g decreases from 4.94 to 4.48 eV which shows the red shift of E_g and could be associated with the structural evolution, i.e., the NW formations on the GNSs. The surfaces of the GNSs consisting of the β-Ga₂O₃ rich phase of the self-assembled NWs on its surface intensively absorb light near the UV region. There are several reports regarding the red shift of E_g by metallic particles or clusters.⁶⁰

Fig. 8 UV-visible spectra of β-Ga₂O₃ and the 3D β-Ga₂O₃@GNSs hybrids.

The optical properties of the as-synthesized samples were also investigated by photoluminescence (PL) measurement. The PL spectra provide separation and recombination information for the photo-induced electrons and holes in the material.⁶¹ Fig. 9 shows the room temperature PL spectra of β-Ga₂O₃ and the 3D β-Ga₂O₃@GNSs hybrids with a UV fluorescent light excitation wavelength of 250 nm. For the as-prepared β-Ga₂O₃ nanoparticles, two luminescence peaks at 355 and 445 nm are seen. The intensity of the peak at 355 nm is considerably weaker than that of the peak at 445 nm. This weak peak at 355 nm is assigned to the recombinations due to self-trapped excitation.^61–63 The stronger luminescence emission peak centered at 445 nm can be attributed to the surface defects and oxygen vacancies in the β-Ga₂O₃ nanostructure lattice. This emission peak can be assigned to the electrons that are excited and recombined on the surface of the Ga₂O₃ nanoparticles. These combine radiatively to emit a blue photon. The blue emission occurring in β-Ga₂O₃ has been observed by other researchers also.^{28,29,63–66} It originates mainly from the recombination of an electron on a donor formed by oxygen vacancies and a hole on an acceptor formed by metal vacancies. The blue photon is emitted via the radiative recombination process.^61,64,67 In the case of the 3D β-Ga₂O₃@GNSs hybrids, the 435 nm peaks shows a decreased emission intensity as compared to the β-Ga₂O₃ nanostructure with slight broadening. The decreased intensity of the 3D β-Ga₂O₃@GNSs hybrids shows a blue shift of ∼10 nm compared with the peak at 445 nm of the β-Ga₂O₃ nanostructure. The decreased intensity is due to lower recombination of the photo-generated electron–hole pairs induced by the charge transfer between the GNSs and the β-Ga₂O₃ nanostructure. It is known that the β-Ga₂O₃ nanostructure is an electron donor and that carbon materials such as GNSs are known to be good electron acceptors.^68,69 Thus, the synergistic effects between these GNSs and the β-Ga₂O₃ nanostructure would effectively reduce the recombination of the photo-generated electron–hole pairs and this 435 nm peak gets suppressed. This indicates that the 3D β-Ga₂O₃@GNSs hybrids have a lower recombination rate of electrons and holes under UV light irradiation, which is mainly due to the fact that the electrons are excited from the valence band of β-Ga₂O₃ to the conduction band and then transferred to the GNSs, preventing a direct recombination of the electrons and holes. The observed slight blue shift (10 nm) in the 3D β-Ga₂O₃@GNSs hybrids is consistent with the presence of the defects.^62,63 Also this 435 nm decreased intensity in the 3D β-Ga₂O₃@GNSs hybrids tends towards quenching phenomena.⁴⁶

Fig. 9 Photoluminescence spectra of β-Ga₂O₃ and the 3D β-Ga₂O₃@GNSs hybrids.

Mechanism for the growth of the 3D β-Ga₂O₃@GNSs hybrids

The schematic diagram in Fig. 10 depicts the stepwise creation of the 3D β-Ga₂O₃@GNSs hybrids. On the basis of the obtained information, it can be concluded that the two parameter, irradiation time and power, played important roles in the formation of the different nanostructures, NRs and NWs, on the GNSs surfaces. The experimental results imply that the morphology of the as-synthesized 3D β-Ga₂O₃@GNSs hybrids is very sensitive and the key parameters governing the reaction conditions are irradiation power (900 W) and time (3 min). Reaction time is usually known to be an important influencing factor for morphological control.⁷⁰ The MW irradiation-induced chemical and thermal reactions at molecular level are responsible for the attachment of the nanoparticles on the GNSs and the growth of the NRs and NWs. When the MW irradiation is applied on the initial materials, extraction of the GNSs from GO occurs (GO carbonaceous chemicals are combusted, evolving some gases) and Ga is decomposed from its host material (Ga(C₅H₇O₂)₃). In this process, a significant amount of heat is released, and the local temperature of the sample becomes much higher than the actual room temperature. The GNS surface with good MW absorptivity decomposes the nearby Ga salt particles to promote the instantaneous formation of Ga oxide nanoparticles, decorating the GNS surfaces within minutes of MW exposure. Also, the GNSs have oxygen-containing functionalities (epoxy, hydroxyl, carbocyclic and carboxyl etc.) on their surfaces and edges that become negatively charged. The positive Ga ions interact with the functional groups via electrostatic attraction and are combined with these functionalities. These Ga ions attached on the surface of the GNSs get oxidized and convert into β-Ga₂O₃ nanoparticles which is the most stable phase by absorbing the thermal energy from MW irradiation. The decorated β-Ga₂O₃ nanoparticles on the GNS surfaces at 900 W and with a reaction time of 1 min are shown in Fig. 10 (Step: I). It was found that the high exposure of MW irradiation could result in the formation of more nanoparticles on the GNS surfaces (Step: II). The continuous MW irradiation seemed to have enhanced the local heating and initially formed β-Ga₂O₃ nanoparticles to yield agglomerated larger nanoparticles (Step: III). The decorated β-Ga₂O₃ nanoparticles start to assemble for a longer irradiation time acquiring high energy from the MW irradiation. In the subsequent process, to minimize the surface free energy of the system, the decorated β-Ga₂O₃ nanoparticles continuously assemble into cluster forms. The MW irradiation with a high heating rate and homogeneous volumetric heating suitably provides favourable conditions for nucleation and growth. The continuous MW irradiation of only 3 min induces speedy nucleation and growth of the NWs in one direction from the agglomerated β-Ga₂O₃ clusters on the GNS surfaces (Step: IV). The β-Ga₂O₃ NWs have a low density on the GNS surfaces because the agglomeration of β-Ga₂O₃ nanoparticles provides spaces at certain distances (500 nm to 1 μm) after the nucleation and growth. Further in the formation of larger size NWs (∼1 μm), more nuclei would readily aggregate. Also some agglomerated β-Ga₂O₃ nanoparticles are left on the GNS surfaces due to improper nucleation for the growth of the β-Ga₂O₃ NWs (Step: IV). At a lower irradiation power (800 W), the β-Ga₂O₃ nanoparticles were not able to agglomerate on the GNS surfaces and started to nucleate before agglomeration. These individual β-Ga₂O₃ nanoparticles form smaller sized NRs and also, initially the individual nucleated particles do not get sufficient energy and are kinetically not favoured for continuous 1D growth, and stop their straight growth after the desired growth in the form of short length NRs. These grown β-Ga₂O₃ NRs on the GNSs are dense because all individual β-Ga₂O₃ nanoparticles are responsible for the formation of the NRs on the GNSs.

Fig. 10 Schematic mechanism for the growth of the 3D β-Ga₂O₃@GNS hybrids.

Conclusions

We have employed a simple, eco-friendly, fast and cost effective MW irradiation method. The reaction involves 1–3 min of MW irradiation for the formation of 3D β-Ga₂O₃@GNSs hybrids under atmospheric pressure to produce NWs of β-Ga₂O₃ on the GNSs with diameters of 200 nm and lengths of up to ∼1 μm. This study also successfully provides a mechanism to synthesize β-Ga₂O₃ NRs on GNSs and β-Ga₂O₃ nanoparticle decorated GNS structure by simple and low-cost MW irradiation. Raman spectra clearly give evidence that the as-synthesized 3D β-Ga₂O₃@GNSs hybrids have a more defective structure due to the 1D NWs grown on the GNSs. The 3D β-Ga₂O₃@GNSs hybrids show band gap reducing phenomena and have a lower band gap (4.48 eV) in comparison to the pristine β-Ga₂O₃ (4.94 eV) structures. By choosing appropriate semiconducting metal oxides, we expect that the present method can be extended to synthesize other 3D hybrids comprising 1D nanowires, nanorods or nanoworm-like structures on graphene sheets.

Acknowledgements

RK, ARV and SAM would like to acknowledge CNPq and FAPESP (Brazil) for financial support.

Notes and references

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