Enhanced Photocatalytic Degradation of 2-Butanone Using Hybrid Nanostructures of Gallium Oxide and Reduced Graphene Oxide Under Ultraviolet-C Irradiation

Abstract

Hybrid nanostructures made of gallium oxide (Ga₂O₃) and reduced graphene oxide (rGO) are synthesized using a facile hydrothermal process method, where the Ga₂O₃ nanostructures are well dispersed on the rGO surface. The formed Ga₂O₃-rGO hybrids are characterized via Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), a diffuse reflectance Ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer, Brunauer–Emmett–Teller (BET), and photoluminescence (PL). Gas chromatography mass spectrometry (GC-MS) was used for analyzing volatile organic compounds (VOCs). The photocatalytic activity of the hybrid nanostructures is evaluated via the degradation of the 2-butanone, representing the VOCs under 254-nm radiation in the atmosphere. That activity is then compared to that of the Ga₂O₃ and commercial TiO₂-P25. The Ga₂O₃-rGO hybrid shows enhanced photocatalytic degradation of 2-butanone compared to Ga₂O₃ and TiO₂-P25, which is attributed to the enhanced specific surface area. The results indicate that the Ga₂O₃-rGO hybrid could be a promising method of enhancing photocatalytic activity and thereby effectively degrading VOCs, including the 2-butanone.

1. Introduction

Volatile organic compounds (VOCs) with high vapor pressures at room temperature are everywhere because they are emitted from a wide range of consumer products, building materials and adhesives. Many VOCs are toxic and adversely affect the human body in both indoor and outdoor environments [1,2,3]. As human beings spend more time at home and in the work place, the control of VOCs in indoor environments is a critical concern [4]. While absorption and incineration methods have traditionally been used to remove VOCs, photocatalytic oxidation (PCO) using metal oxide semiconductors has emerged as an attractive alternative due to their low cost and non-toxicity [5]. The photo-induced generation of electron-hole pairs promotes reduction and oxidation upon the surface of the semiconducting transition metal oxides [6,7]. Among various semiconductors, TiO₂ is preferred in terms of its photocatalytic activity under UV light absorption [8]. It has been widely applied to photocatalysts due to its nontoxicity, low cost and high reactivity. Nevertheless, its long-term instability greatly limits the practical applications, due to the deactivation of the photocatalytic sites resulting from the accumulation of a stable intermediate on the TiO₂ surface from the oxidation of aromatics in a dry atmosphere [9,10]. Recently, interest has increased in Ga₂O₃ (E_g = 4.8 eV) as a wide band gap semiconductor with its high redox ability and long-term stability, compared to commercial TiO₂ (E_g = 3.2 eV), for the degradation of aromatic pollutants, such as the benzene series and toluene [11,12,13,14,15]. In addition to its strong redox ability, due to a higher conduction band (E_cb = −1.55 eV) and lower valence band (E_vb = 3.25 eV) than TiO₂ (E_cb = −0.5 eV, E_vb = 2.7 eV) [16], Ga₂O₃ is composed of a valence band of O 2p orbitals and a dispersive conduction band of s and p orbitals from the p-block metal oxide atom, which promotes the mobility of photo-induced electrons in the Ga₂O₃ [13,17,18]. This improves the separation efficiency of the photo-induced electrons and holes, resulting in an increase in photocatalytic activity. Advanced strategies to enhance photocatalytic activity have been reported via the formation of a heterogeneous structure with noble metals, semiconductors and/or graphene allotropes [19,20,21,22]. Among these materials, the addition of graphene oxide has been demonstrated to be an effective method due to its large specific surface area and excellent charge carrier mobility [5,21,22,23,24,25,26]. However, to the best of our knowledge, few studies thus far have looked at the photocatalytic degradation of gas-phase contaminants for the purification of enclosed atmospheres with the use of Ga₂O₃-reduced graphene oxide (rGO) hybrids under ultraviolet C (UVC) irradiation. The present study focuses on the photocatalytic degradation of 2-butanone, which is used in adhesives and cleaning agents, as an indoor VOC source [27,28]. Herein, we report on the synthesis of the Ga₂O₃-rGO hybrids formed using the hydrothermal method, and the photocatalytic degradation of 2-butanone is discussed as compared to TiO₂ and Ga₂O₃.

2. Results and Discussion

Figure 1a shows the Field emission scanning electron microscope (FESEM) images of the rGO, Ga₂O₃-rGO hybrids and Ga₂O₃. The results showed that the rGO flakes were wrinkled, partially folded and stacked. This was attributed to the weakening of the hydrogen bonding network between the flakes and the generation of various structural defects during the hydrothermal process [29,30]. The images of the Ga₂O₃-rGO hybrids showed that the Ga₂O₃ nanorods were well dispersed and uniformly anchored on the rGO. The chemical bonds between the Ga₂O₃ and rGO were discussed via the Fourier transform infrared spectra (FTIR) analysis, thereby promoting the charge transfer of photo-excited carriers from the Ga₂O₃ to rGO and eventually enhancing the photocatalytic activity. The magnified image (inset of Figure 1a) of the Ga₂O₃ revealed that the Ga₂O₃ possessed a mesoporous surface that could contribute to the improvement of photocatalytic performance [31,32]. Figure 1b shows the X-ray diffraction (XRD) patterns of the pristine GO, rGO, Ga₂O₃-rGO hybrids and Ga₂O₃ nanorods. The pristine GO showed a very strong intensity at 2θ = 10.55°, representing a typical inter-planar d-spacing (0.84 nm) of the GO stacks [33]. After the GO was reduced to rGO via the hydrothermal process, the peaks originating from the GO d-spacing diminished, but the broad peak at 2θ = 24° appeared, corresponding to the inter-planar d-spacing (0.37 nm) of the rGO [34]. The decreased d-spacing in the rGO compared to the GO was attributed to the removal of oxygen-containing functional groups, which also indicated the reestablishment of the sp² carbon network in the rGO [35]. The results also revealed that the hydrothermal process was an effective method for the reduction of a GO without strong acidic conditions [25,36]. For Hybrid-2, the most noticeable peaks were indexed to the β-Ga₂O₃ with a broad rGO peak. As the ratio of Ga₂O₃ over rGO increased, the peaks representing Ga₂O₃ increased while the broad peak representing rGO diminished, as shown with Hybrid-1. These results showed that the Ga₂O₃ retained its crystal structure even after the hydrothermal process and dispersion on the rGO.

Figure 2 shows the FTIR spectra in the wavenumber range from 4000 to 650 cm⁻¹, along with a schematic drawing representing the responsible chemical bonds of the pristine GO, rGO, Ga₂O₃-rGO hybrids and Ga₂O₃. The spectrum of the pristine GO showed typical transmittance peaks. The bands in the range of 1700 to 1000 cm⁻¹ were associated with oxygen functional groups such as the carboxyl groups, hydroxyl groups and epoxy groups on both sides of the GO surface [24,36,37]. These bands of oxygen functional groups in the GO significantly decreased after the formation of rGO via the hydrothermal process [34,38]. This revealed that most oxygen functional groups had been removed from the GO surface while the GO was reduced [29]. These eliminated peaks associated with the oxygen functional group were recovered in the spectra of the Ga₂O₃-rGO hybrids. The presence of these oxygen functional groups indicated the formation of the chemical bonds between the Ga₂O₃ nanorods and the rGO, which allowed the carrier transfer from the Ga₂O₃ to rGO, and thereby enhanced the photocatalytic activity.

The Ultraviolet-visible-near infrared (UV-Vis-NIR) absorption of the Ga₂O₃ and hybrids were characterized in the wavelength range of 240 to 1000 nm, and the results are shown in Figure 3a. These results showed that the Ga₂O₃ was transparent in the visible light and ultraviolet C. As the ratio of the rGO over Ga₂O₃ increased, the overall absorption from the visible and near infrared (400–1000 nm) into the UV wavelength regions (260 nm ~) increased. In detail, a red shift in the absorption edge was observed in Hybrid-1 compared with Ga₂O₃, indicating a narrowing of the Ga₂O₃ band gap due to the coupling in the Ga₂O₃–rGO hybrids. The inset in Figure 3a shows the Tauc plot for the Ga₂O₃ and Ga₂O₃–rGO hybrids, where (ahv) ² was plotted against the photo energy (hv), and the band gap energies were calculated from the extrapolated lines as shown. The optical energy band gap of the Ga₂O₃ and Hybrid-1 was calculated as 4.63 and 4.25 eV, respectively. However, it was difficult to calculate the exact optical energy band gap for Hybrid-2 because the large amount of rGO induced a very significant background absorption in the range of 400 to 800 nm. Also, while the absorption of Hybrid-1 around 254 nm was comparable to that of Ga₂O₃, the absorption of Hybrid-2 (a greater rGO ratio) was greatly reduced. This indicated that the photogenerated electrons from the Ga₂O₃ in Hybrid-1 were able to move along the rGO and then interact with the absorbed O₂ to produce OH^·, which facilitated the strong oxidative ability to degrade the organic pollutants. In this process, the rGO acted as an electron collector and transporter to effectively interfere with the electron-hole recombination and extend the lifetime of the photogenerated charge carriers formed by Ga₂O₃, leading to the higher photocatalytic efficiency in Hybrid-1. However, with the excessive addition of rGO (Hybrid-2), the rGO nanosheets blocked the UVC light from reaching the surface of the Ga₂O₃, resulting in the lower generation rate of the charge carriers. Figure 3b shows the PL emission spectra of the Ga₂O₃, Hybrid-1 and -2 when excited at 260 nm. A broad emission band ranging from 300 to 500 nm was observed in the Ga₂O₃ spectrum, while considerable PL quenching was observed in the hybrids with higher ratios of rGO over Ga₂O₃. The decreased intensity of the PL emission might have been due to the reduction in the recombination process of electrons and holes because of the charge carrier transfer from the excited Ga₂O₃ to the rGO [23,39,40]. To further discuss the charge carrier transfer from the excited Ga₂O₃ to the rGO, the PL decay characteristics of the Ga₂O₃ and hybrids were investigated, as shown in the inset of Figure 3b. The results were fitted to multi-exponential curves to derive the PL decay parameters that were obtained using Equations (1) and (2) [41]. The lifetime, pre-exponential factors and average lifetime of the Ga₂O₃ and hybrids are summarized in

Table 1. Summary of the fitting parameters for the PL decay components of the Ga₂O₃ and hybrids.

	A1	τ1 (ns)	A2	τ2 (ns)	A3	τ3 (ns)	A4	τ4 (ns)	$\langle \mathbf{\tau }\rangle \left(\mathbf{ns}\right)$
Ga2O3	8973	2.5	0		603	110	201	2876	68.3
Hybrid-1	9538	2.4	0		236	126	81	3081	30.7
Hybrid-2	9597	1.6	186	43.6	59	419	49	4171	25.7

.

$$f\left(t\right)=A_1{e}^{\frac{t}{{\tau }_1}}+A_2{e}^{\frac{t}{{\tau }_2}}+A_3{e}^{\frac{t}{{\tau }_3}}+A_4{e}^{\frac{t}{{\tau }_4}}$$

(1)

$$\langle \mathsf{\tau }\rangle ={\displaystyle \sum }_{i=1}^n\frac{A_i{\tau }_i^2}{A_i{\tau }_i}$$

(2)

where $A_i$ denotes the pre-exponential factor, ${\tau }_i$ is the lifetime decay constant, and $\langle \mathsf{\tau }\rangle$ is the average lifetime.

For the Ga₂O₃, the fast and slow component of the PL decay had a lifetime of 2.5 ns and 2876 ns, respectively. As the ratio of the rGO over Ga₂O₃ increased, the fast lifetime and average lifetime decreased, but the slow lifetime increased. The analysis revealed that the charge carrier transfer between the excited Ga₂O₃ and rGO was dominated by the fast component. Furthermore, from the fast decay component, the electron-transfer rate could be estimated using Equation (3).

$$k_{et}=\frac{1}{\tau }\left(G{a}_2{O}_3-rGO\right)-\frac{1}{\tau }\left(G{a}_2{O}_3\right)$$

(3)

The electron-transfer rate of Hybrid-1 was $0.017\times {10}^9 s^{-1}$, while that of Hybride-2 was $0.225\times {10}^9 s^{-1}$. This showed that the electron-transfer rate of Hybrid-2 was 13.5 times higher than that of Hybrid-1, while the number of electron-hole pairs was lower in Hybrid-2. Based on this analysis, it was concluded that the chemical bonds between the Ga₂O₃ and rGO were very effective in transferring the photo-excited electrons before the recombination process.

The identification of the adhesive component was measured with a GC-MS analysis, which was identified by comparing retention times and mass spectra with library spectra. Figure 4a shows that air, water and 2-butanone were detected before the photodegradation. After UVC irradiation of the 2-butanone for 30 min using a Ga₂O₃-rGO hybrid (Figure 4b), it was found that air and water were the main by-products. Figure 4c shows the degradation of 2-butanone as a function of exposure time under the 254 nm UVC irradiation time, at intervals of 30 min when different catalysts were used. The TiO₂-P25, which is known to be a good photocatalyst, was also compared with its counterparts under the identical reaction conditions. With conventional TiO₂-25, the 2-butanone degraded to 62% within 1 h. On the other hand, the 2-butanone degraded to 93% within 1 h when Ga₂O₃ was used as a catalyst. The hybrids showed a much-enhanced photodegradation efficiency compared to the Ga₂O₃ and TiO₂. The 2-butanone degraded to 99.9% and 98.2% under the identical conditions when Hybrid-1 and Hybrid-2 were used, respectively. The additional rGO in the Ga₂O₃ played a role as a transfer pathway of the photo-generated carriers. In addition, the rGO provided a much higher specific surface area, improving the photocatalytic performance of the Ga₂O₃. It was found that Hybrid-1 had a higher specific surface area (S_BET = 149 m²·g⁻¹) than the pure Ga₂O₃ (S_BET = 3.4 m²·g⁻¹) and Hybrid-2 (S_BET = 36 m²·g⁻¹), using the Brunauer–Emmett–Teller (BET) surface area measurement [42,43]. While the additional rGO in the Ga₂O₃ improved the photocatalytic performance via the reasons above mentioned, it adversely degraded the photocatalytic performance due to the lower UVC absorption blocked by excessive rGO [20,34], which could be observed in Figure 3a. This explained the experimental results; the degradation performance of different catalysts was in the order of Hybrid-1 (1 wt. % of rGO) > Hybrid-2 (5 wt. % of rGO) > Ga₂O₃ > TiO₂-P25. In addition to the photocatalytic activity, the stability and repeatability of the 2-butanone degradation was investigated using Hybrid-1, with the results shown in Figure 4d. The results showed that Hybrid-1 retained its photocatalytic activity even after prolonged operation.

Figure 5 shows the optical transmittance of Hybrid-1 before and after 2-butanone degradation. The transmittance decreased after 2-butanone degradation particularly around 1710 cm⁻¹ and 1560 cm⁻¹, representing C=O bonding [12] and C=C bonding [44], respectively. This indicated that the decomposed residue of the 2-butanone remained on the Hybrid-1 surface. These residual byproducts on the surface need to be investigated further.

3. Materials and Methods

3.1. Synthesis of Hybrids

The GO was prepared using a modified Hummers’ method [45], and the Ga₂O₃ powder (particle diameter 2.5–6 μm, 99.999%) was purchased from Taewon Scientific Co., Ltd. (Republic of Korea). Ga₂O₃-rGO hybrids were synthesized via a conventional hydrothermal method. First, a GO solution in which the GO (lateral size > 3 μm, 99%) was dispersed in distilled water (DI water, 50 mL) was ultrasonicated for 3 h to obtain uniform dispersions of exfoliated GO. Next, Ga₂O₃ powders were added into the prepared GO solutions. After being vigorously stirred for 2 h, the mixture of Ga₂O₃ and GO solutions was transferred to a Teflon-lined autoclave for the hydrothermal process. The mixed solution was heated in an oven at 150 °C for 6 h (6 °C min⁻¹) and cooled at room temperature. The unreacted solvent and the resulting precipitate were separated by centrifugal force at 4000 rpm for 30 min. The collected precipitate was washed and rinsed with DI water and ethanol several times and then dried at 60 °C for 12 h (6 °C min⁻¹). Finally, two Ga₂O₃-rGO hybrids with 1 wt. % and 5 wt. % of rGO were formed, which were called Hybrid-1 and Hybrid-2, respectively. It was noted that the GO was successfully converted into rGO without strong acidic conditions via the hydrothermal process.

3.2. Materials Characterization

The microstructures of the synthesized Ga₂O₃-rGO hybrids were observed using a field emission scanning electron microscope (FESEM, JEOL JSM-7100F, Japan) at 15 kV acceleration voltage; the samples were distributed on the sticky-carbon tape followed by a Pt coating. The crystallographic structures of the synthesized hybrids were determined using a powder X-ray diffraction system (XRD, Rigaku Ultima IV XRD, Japan) over the 2θ range of 5–80° with Cu Kα radiation (λ = 0.15405 nm). Middle-infrared (4000–650 cm⁻¹) transmission spectra were collected with a Fourier-transform infrared spectra (FTIR, Agilent Cary 670, U.S.A.) spectrometer with an ATR accessory (ZnSe and Ze crystal). UV-Vis-NIR spectroscopy (DRS, Shimadzu SolidSpec-3700, Japan) was measured in the range from 240 to 1000 nm using BaSO₄ as a reference. Photoluminescence (PL) spectra were recorded under 260 nm UV laser excitation (PL, HORIBA LabRAM HR-800, Japan). Furthermore, Time-resolved PL measurements were performed on a HORIBA Fluorolog-3 via a time-correlated single photon counting (TCSPC) spectrometer equipped with a Pulsed Nano LED 260-nm excitation source. The specific surface area of the product was measured using a nitrogen adsorption-desorption isotherm on a BELSORP-max instrument and calculated via the Brunauer–Emmett–Teller (BET) method. Prior to measurement, the samples were degassed at 473 K for 4 h.

3.3. Photocatalytic Activity

The photocatalytic performance of the Ga₂O₃-rGO hybrids with different rGO wt. % were evaluated via the photodegradation of 2-butanone using a commercial VOC air detector (KHALDER: KD-001) with TVOC values ranging between 0.00 to 9.99 mg/m³. For every experiment, 200 mg of the photocatalyst powder and 18.2 mg of a commercial adhesive (as a 2-butanone source) were used with a UVC source (λ = 254 nm, 4W, irradiance of 340 μW·cm⁻² with a distance of 15 cm, Model: LF-104.S). No noticeable degradation of the 2-butanone was observed in the absence of a photocatalyst under the UVC irradiation. In the presence of a photocatalyst under UVC exposure, the photodegradation efficiency was calculated using Equation (4):

Degradation efficiency (%) = (C₀ − C)/C₀ × 100

(4)

where C₀ is the initial concentration and C is the final concentration of the 2-butanone. Changes in the 2-butanone concentration were measured at intervals of 30 min. For comparison, photodegradation efficiency of Ga₂O₃ and TiO₂-P25 was also carried out. To further explore the long-term stability of the Ga₂O₃-rGO hybrids, repeated recycling experiments were carried out using Hybrid-1 under identical conditions. The gaseous products generated before and after the photocatalytic degradation were collected and analyzed with a gas chromatograph (GC, Agilent 5975C, U.S.A.) equipped with a HP-5 MS column (with dimensions of 60 m × 0.25 μm × 0.25 mm). A product sample was injected into the GC column with a split ratio 20:1. Helium was used as the carrier gas at a flow rate of 1.0 mL·min⁻¹. The column temperature program was as follows: 50 °C for 10 min, increasing at a rate of 10 °C min⁻¹ to 300 °C. The product identification was confirmed with a gas chromatograph-mass spectrometer. Major reaction products were identified by comparing retention times and mass spectra with reference compounds.

4. Conclusions

Ga₂O₃-rGO hybrids were synthesized using a facile hydrothermal process method without using any additives or surfactants. The Ga₂O₃ nanorods were well dispersed on the surface of the rGO nanosheet through strong coupling between the Ga₂O₃ and rGO. Compared with the pure Ga₂O₃ and conventional TiO₂-P25, the photocatalytic activity of the hybrids was significantly enhanced. The enhanced photocatalytic activity of the hybrids was attributed to the effective carrier transfer from the Ga₂O₃ to rGO and enhanced specified surface area. The best photocatalytic activity was observed with Hybrid-1 (1 wt. % rGO) instead of Hybrid-2 (5 wt. % rGO), indicating that the presence of rGO with Ga₂O₃ can also degrade the photocatalytic performance via the lower absorption of UVC light blocked by the excessive rGO. The Ga₂O₃-rGO hybrid offers a promising method of enhancing photocatalytic activity and thereby effectively degrading 2-butanone.