nanomaterials Article
A Gallium Oxide-Graphene Oxide Hybrid Composite for Enhanced Photocatalytic Reaction Seungdu Kim 1,2 , Kook In Han 1 , In Gyu Lee 1 , Won Kyu Park 2,3 , Yeojoon Yoon 2 , Chan Sei Yoo 2 , Woo Seok Yang 2, * and Wan Sik Hwang 1, * 1 2 3
*
Department of Materials Engineering, Korea Aerospace University, Goyang 412-791, Korea; [email protected] (S.K.); [email protected] (K.I.H.); [email protected] (I.G.L.) Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Korea; [email protected] (W.K.P.); [email protected] (Y.Y.); [email protected] (C.S.Y.) School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea Correspondence: [email protected] (W.S.Y.); [email protected] (W.S.H.); Tel.: +82-31-789-7256 (W.S.Y.); +82-2-300-0293 (W.S.H.)
Academic Editors: Ming-Tsang Lee and Te-Hua Fang Received: 1 June 2016; Accepted: 23 June 2016; Published: 1 July 2016
Abstract: Hybrid composites (HCs) made up of gallium oxide (GaO) and graphene oxide (GO) were investigated with the intent of enhancing a photocatalytic reaction under ultraviolet (UV) radiation. The material properties of both GaO and GO were preserved, even after the formation of the HCs. The incorporation of the GO into the GaO significantly enhanced the photocatalytic reaction, as indicated by the amount of methylene blue (MB) degradation. The improvements in the reaction were discussed in terms of increased surface area and the retarded recombination of generated charged carriers. Keywords: gallium oxide; ultraviolet radiation
graphene oxide;
photocatalytic reaction;
methylene blue;
1. Introduction Various metal oxides have been investigated with respect to their photocatalytic reactions, and the resulting technologies can already be found in self-cleaning and anti-fogging/antibacterial fields [1,2]. These products work via the photo-induced redox reactions of adsorbed materials and/or photo-induced hydrophilic conversion of materials [3–5]. The acceleration of the photoreaction occurs on a material’s surface, so the introduction of nano-materials and nano-technologies can significantly enhance catalytic efficiency [6–9]. Meanwhile, two-dimensional (2D) materials, such as graphene and graphene oxide, have been intensively studied in various areas due to their extraordinary high surface-area to volume ratio [10]. Theoretically, a hybrid composite (HC) made up of a metal oxide and 2D materials could facilitate an enhanced photocatalytic reaction and thus further extend and widen the applications of these materials [4]. Although HCs of several metal oxides and graphene/graphene oxide have been explored [4], an HC using gallium oxide (GaO) and graphene oxide (GO) has not yet been investigated. In fact, GaO is an important metal oxide showing a larger energy bandgap (4.8~5.1 eV) than conventional metal oxides, such as TiO2 (3.0 eV) and ZnO (3.2 eV) [11,12]. In addition, GaO has recently garnered more attention for use with power electronics [13] and the extension of their applications to various other areas. In this work, a GaO-GO HC is presented, and its material and photocatalytic reaction properties are investigated. Photo-induced charged carriers are generated from the GaO, and these carriers spread out across the GO, which has a large surface area, resulting in a boost to the photocatalytic reaction. In addition, the GO that attaches to the GaO is able to retard the recombination of generated electron-hole pairs.
Nanomaterials 2016, 6, 127; doi:10.3390/nano6070127
www.mdpi.com/journal/nanomaterials
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2. Experimental Section 2. Experimental Section HCs made up of GaO and GO were synthesized using a conventional hydrothermal method that HCs made up of GaO andoxide GO were synthesized using a conventional hydrothermal method is often used to produce metal powder [14]. GaCl 3 (99.999%) was diluted in deionized (DI) that is often used to produce metal oxide powder [14]. GaCl in deionized 3 (99.999%) water to form a synthesis precursor, and the GO was prepared in the DI was waterdiluted as a solvent. After (DI) water to form a synthesis precursor, and the GO was prepared in the DI water as a solvent. mixing the GaCl3 and GO solutions, ammonium hydroxide (NH4(OH) 28% in H2O) was also added After mixing the balance GaCl3 and GO8solutions, ammonium hydroxide (NH in H2 O) was also 4 (OH) to maintain a pH above since the reaction necessary to form Ga oxide28% nanoparticles would added to maintain a pH balance above 8 since the reaction necessary to form Ga oxide nanoparticles not occur at a pH value below 8 [15]. All of the processes were conducted in a three-neck roundwould flask not occur pH to value belowa 8temperature [15]. All ofofthe processes wereand conducted a three-neck bottom in an at iceabath maintain 0 °C. The solvent HC wereinseparated via ˝ C. The solvent and HC were round-bottom flask in an ice bath to maintain a temperature of 0 centrifugal force at 4000 rpm for 30 min. The collected HCs were then washed and rinsed to remove separated viaand centrifugal at 4000 rpm for min. Thedried collected HCs were then washed and the ammonia chlorineforce residues. Finally, the 30 HCs were in a conventional freeze-dryer at rinsed to remove the ammonia and chlorine residues. Finally, the HCs were dried in a conventional −80 °C for 48 h. The formed HCs and GaO were characterized via scanning electron microscopy (SEM) ˝ C for 48 h. The formed HCs and GaO were characterized via scanning electron freeze-dryer at ´80 (Busan, Korea), X-Ray Diffraction (XRD) (Busan, Korea) and Raman analyses (Seoul, Korea). microscopy (SEM) (Busan, Korea), X-Ray Diffraction (Busan, Korea) and analyses (Seoul, Furthermore, the photocatalytic reactions of the (XRD) HCs were evaluated viaRaman the degradation of Korea). Furthermore, the photocatalytic reactions of the HCs were evaluated via the degradation methylene blue (MB; C16H18N3SCl) at 245 nm of exposure. Multiple 100-mg HCs were added to 100of methylene blue (MB; H218 N3The SCl)mixed at 245solutions nm of exposure. Multiple 100-mg HCs mercury were added to 16H mL MB solutions (0.3 g/LCin O). were exposed under a 245-nm lamp 100-mL MB solutions in H solutions were under a 245-nm mercury 2 O). (Seongnam, Korea) at(0.3 0.4 g/L mW/s for 0 The min,mixed 60 min, 120 min andexposed 180 min, respectively. For the lamp (Seongnam, Korea) at 0.4 mW/s for 0 min, 60 min, 120 min and 180 min, respectively. For the absorbance spectrum analysis, 5 mL was collected from each mixed solution, and the absorbance absorbance spectrum analysis, 5 mL was collected from each mixed solution, and the absorbance spectrum was obtained as a function of wavelength in the range of 450–800 nm. The entire experiment spectrum was obtained as atofunction of wavelength the range of 450–800 nm. The entire experiment was conducted in the dark avoid any light sourceindisturbance. was conducted in the dark to avoid any light source disturbance. 3. Results and Discussion 3. Results and Discussion The HCs formed via the aforementioned processes are shown in Figure 1. The SEM images in The HCs formed via the aforementioned processes are shown in Figure 1. The SEM images in Figure 1a,b shows grain-shaped GaO, and Figure 1b also reveals that the formed GO connected to Figure 1a,b shows grain-shaped GaO, and Figure 1b also reveals that the formed GO connected to the GaO in a fishing net configuration. The GaO samples with higher GO concentrations showed the GaO in a fishing net configuration. The GaO samples with higher GO concentrations showed higher surface areas, which eventually led to enhancements in the photocatalytic reaction. The GaO higher surface areas, which eventually led to enhancements in the photocatalytic reaction. The GaO only sample was made using a 5 g GaCl3 solution. However, the 4% GO-GaO and 10% GO-GaO only sample was made using a 5 g GaCl solution. However, the 4% GO-GaO and 10% GO-GaO samples were made using 0.2 g of GO and30.5 g of GO as well as a 5 g GaCl3 solution. In addition, it samples were made using 0.2 g of GO and 0.5 g of GO as well as a 5 g GaCl solution. In addition, it is is interesting to note that the height to width ratio of the GaO with a GO 34% solution decreased in interesting to note that the height to width ratio of the GaO with a GO 4% solution decreased in the the GaO with a GO 10% solution. It was presumed that the presence of GO might have hindered the GaO with a GO 10% solution. It was presumed that the presence of GO might have hindered the GaO GaO growth for a certain structural plane More detailed structure and crystallinity information growth for a certain structural plane More detailed structure and crystallinity information regarding regarding the GaO and GO was obtained via XRD and Raman analyses. the GaO and GO was obtained via XRD and Raman analyses.
Figure 1. Schematic illustrations of hybrid composite (HC) growth depending on (a) a 4% graphene Figure 1. Schematic illustrations of hybrid composite (HC) growth depending on (a) a 4% graphene oxide (GO) solution and (b) a 10% GO solution. The scale bar in the scanning electron microscopy oxide (GO) solution and (b) a 10% GO solution. The scale bar in the scanning electron microscopy (SEM) is 1 µm. (SEM) is 1 μm.
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Figure Figure 2a 2a shows shows the the XRD XRD spectra spectra of of GaO-GO GaO-GO HCs, HCs, as as well well as as GaO GaO for for comparison. comparison. It It shows shows that that an orthorhombic structure was preferred in the GaO, and the crystallinity of the GaO was an orthorhombic structure was preferred in the GaO, and the crystallinity of the GaO was preserved preserved even even when when forming forming HCs HCs with with GO GO [16]. [16].
Figure Figure 2. 2. (a) (a) X-Ray X-Ray Diffraction Diffraction (XRD) (XRD) spectra spectra of of different different HCs HCs made made up up of of gallium gallium oxide oxide (GaO) (GaO) and and with/without GO; (b) Full-width at half maximum (FWHM) variation for XRD peaks from (a) and with/without GO; (b) Full-width at half maximum (FWHM) variation for XRD peaks from (a) and grain grain size size of of the the HCs HCs as as aa function function of of GO GO concentration concentration in in the the solution; solution; (c) (c) Raman Raman of of different different HCs HCs using the same proportions as in (a). using the same proportions as in (a).
In addition, the full-width at half maximum (FWHM) of {110} was measured, and the grain size structures was was extracted extracted from the the FWHM FWHM at at different different GO GO concentrations concentrations [17]. [17]. The GaO grain of the structures 45 nm nm continued continued to decrease decrease as the the GO GO concentration concentration increased in the HC. It was presumed presumed size of 45 that the functional groups such as hydroxyl, hydroxyl, epoxide, carbonyl and carboxyl at the GO surface surface can serve as as nucleation nucleationsites. sites.Accordingly, Accordingly,there thereshould shouldbebemore morenucleation nucleation sites the GaO when there serve sites forfor the GaO when there is is a higher GO concentration because this leads to a smaller GaO grain size in an HC [18]. This reveals a higher GO concentration because this leads to a smaller GaO grain size in an HC [18]. This presence of of GO GO during during the the Ga Ga oxide oxide formation formation affects affects the the grain grain size size of of the the Ga Ga oxide oxide in a nano that the presence scale range. The crystallinity of the HC was further investigated via Raman analyses, as shown in Figure 2c. The representing GaO were observed in the GaO particles. As Theresults resultsshow showthat thatthe thepeaks peaks representing GaO were observed in the GaO particles. thethe GOGO concentration increased inin the HC, As concentration increased the HC,the theGO GOpeak peakbecame becamemore moreprominent. prominent. D-band D-band (D) occurs at 1350 region because of oxide. The G-band (G) is generated at 1350 region because of graphite [17]. The D/G D/G ratio conventional GO. The results The ratio of of the the GO GO peak peak in the HC was comparable to that of a conventional high quality quality GO GO was was preserved preserved even even in in the the GO GO in in the the HC. HC. show that high The photocatalytic reaction of the GaO with/without GO was through the photophotocatalytic reaction of the GaO with/without GO investigated was investigated through the degradation of theofMB UV UV exposure. Figure 3 shows time evolution photo-degradation theunder MB under exposure. Figure 3 shows time evolutionofofthe theMB MB absorbance spectrum depending dependingon onnanoparticle nanoparticlesample, sample,i.e., i.e.,only onlyGaO, GaO,GaO GaO a 4% GO solution and oxide spectrum inin a 4% GO solution and GaGa oxide in in a 10% GO solution. Before the spectra analysis, the samples were exposed at 254 nm for 0 min, 60 a 10% GO solution. Before the spectra analysis, the samples were exposed at 254 nm for 0 min, 60 min, min,min 120and min180 and 180respectively. min, respectively. The show results show that the absorbance theonly MB 120 min, The results that the absorbance spectrumspectrum of the MBof with with remained only GaO remained constant. However,intensity the spectrum intensity significantly decreased GaO constant. However, the spectrum significantly decreased according to time according to time and This GO concentration. reveals that proportional the MB degradation to i.e., the and GO concentration. reveals that theThis MB degradation to the GOproportional concentration, GOtotal concentration, i.e., the surface area inthe thetotal MB. surface area in the MB. A schematic illustration of the charged carrier transfer and MB degradation mechanism of the GaO (Figure 4a) and the HCs (Figure 4b) made up of Ga oxide and GO is described in Figure 4. The HCs made up of GaO and GO were able to slow the recombination of generated electron-hole pairs and enhance the reaction potential with the MB.
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Figure 3. The absorbance spectra of the methylene blue (MB) solution where (a) GaO; (b) an HC of GaO and 4% GO; and (c) an HC of GaO and 10% GO were added to the MB. Each sample was exposed to 254 nm of radiation at different times.
A schematic illustration of the charged carrier transfer and MB degradation mechanism of the GaO Figure (Figure3.4a) the HCsspectra (Figureof4b) up ofblue Ga oxide and GOwhere is described Figure Theand absorbance themade methylene (MB) solution (a) GaO;in(b) an HC4.ofThe Figure 3. The absorbance spectra of the methylene blue (MB) solution where (a) GaO; (b) an HC of HCs GaO made up of GaO and GO were able to slow the recombination of generated electron-hole pairs and 4% GO; and (c) an HC of GaO and 10% GO were added to the MB. Each sample was exposed GaO and 4% GO; and (c) an HC of GaO and 10% GO were added to the MB. Each sample was exposed and enhance the reaction potential with the MB. to 254 nm of radiation at different times. to 254 nm of radiation at different times.
A schematic illustration of the charged carrier transfer and MB degradation mechanism of the GaO (Figure 4a) and the HCs (Figure 4b) made up of Ga oxide and GO is described in Figure 4. The HCs made up of GaO and GO were able to slow the recombination of generated electron-hole pairs and enhance the reaction potential with the MB.
Figure 4. 4. Schematic Schematicillustrations illustrationsofofcharged chargedcarrier carriergeneration generationand andtransfer, transfer,asaswell wellasasthe thewater water Figure splitting mechanism mechanismof of(a) (a)the theGa Gaoxide oxideand and(b) (b)the theHC HCofofGa Gaoxide oxideand andGO. GO.hνhνisisultraviolet ultraviolet light splitting light constant and and νν isisfrequency. frequency. energy, where h is Planck’s constant
4. Conclusions Conclusions Hybrid composites (HCs) of and GO investigated facilitate Hybrid composites (HCs) made made up ofGaO GaO andgeneration GOwere wereand investigated facilitate enhanced Figure 4. Schematic illustrations ofup charged carrier transfer, astoto well as the enhanced water photocatalytic reactions under ultraviolet (UV) improvements the were photocatalytic reactions of under ultraviolet (UV) radiation. The improvements thereaction reaction were splitting mechanism (a) the Ga oxide and (b)radiation. the HC of The Ga oxide and GO. hν in isin ultraviolet light to the increased surface area when The generated energy, where h is Planck’s constant and is frequency. attributed increased surface areathat thatνresulted resulted whenthe theGO GOattached attachedtotothe theGaO. GaO. The generated charged carriers carriers in in the the GaO GaO under under UV UV radiation radiationspread spreadout outthrough throughthe theGO GOand andretarded retardedthe the 4. Conclusions recombination of significantly enhancing the photocatalytic reaction with the MB. recombination ofelectron-hole electron-holepairs, pairs, significantly enhancing the photocatalytic reaction with the The material properties of both the the GaO-GO were preserved even after thethe formation of of HCs made MB. The material properties of both GaO-GO were after formation HCs made Hybrid composites (HCs) made up of GaO andpreserved GO wereeven investigated to facilitate enhanced up of GaO and GO. The HCs developed in this study can be used in various applications requiring up of GaO and GO. The HCs developed in this study can be used in various applications requiring photocatalytic reactions under ultraviolet (UV) radiation. The improvements in the reaction were enhanced photocatalytic reactions. enhanced reactions. attributedphotocatalytic to the increased surface area that resulted when the GO attached to the GaO. The generated charged carriers in the GaO under UV radiation spread out through the GO and retarded the work waswas supported by thebyBasic ResearchResearch Program through Nationalthe Acknowledgments: This This work supported the Science Basic Science Programthe through recombination of Foundation electron-hole pairs, enhancing the photocatalytic reaction with the MB. National Koreasignificantly (NRF) the Ministry of Science, Future Planning Research Research Foundation of Koreaof (NRF) fundedfunded by thebyMinistry of Science, ICT ICT & & Future Planning The material properties both the GaO-GO were preserved even after the formation of HCs made (2014R1A1A1004770), by theofTechnology Innovation Program (No. 10044410) funded by the Korea Government (2014R1A1A1004770), by the Technology Innovation Program (No. 10044410) funded by the Korea Government Ministry of Knowledge an Energyin Efficiency & Resources of theinKorea Institute of Energy requiring Technology up of GaO and GO. Economy, The HCs by developed this study can be used various applications Ministry of and Knowledge by anfunded Energyby Efficiency & Government Resources ofMinistry the Korea Institute of Economy Energy Evaluation PlanningEconomy, (KETEP) grant the Korea of Knowledge enhanced photocatalytic reactions. Technology Evaluation andbyPlanning (KETEP) grant funded by the Korea Ministry (No. 20142010102690) and a grant from the Advanced Technology CenterGovernment R&D Program fundedofbyKnowledge the Ministry of Trade, Industry & Energy ofand Korea Korea government Ministry of Knowledge Economy. Economy (No. 20142010102690) by a(10048475), grant from by thethe Advanced Technology Center R&D Program funded by the Acknowledgments: This work was supported by the Basic Science Research Program through the National MinistryContributions: of Trade, IndustryS.K., & Energy of Korea by conceived the Korea government Ministry of Knowledge Economy. Author K.I.H., W.S.Y.(10048475), and W.S.H. and designed the experiments, W.K.P. and Research Foundation of Korea (NRF) funded by the Ministry of and Science, & Future Planning Y.Y. contributed to ultraviolet measurements and interpretation, I.G.L. C.S.Y. ICT provided and interpreted (2014R1A1A1004770), byS.K., the Technology Program (No. 10044410) the Koreathe Government the morphologic model, W.S.Y. and Innovation W.S.H. wrote the manuscript. All funded authorsby discussed results and commented the manuscript at all stages. Ministry ofon Knowledge Economy, by an Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant of funded by the Korea Government Ministry of Knowledge Conflicts of Interest: The authors declare no conflict interest. Economy (No. 20142010102690) and by a grant from the Advanced Technology Center R&D Program funded by the Ministry of Trade, Industry & Energy of Korea (10048475), by the Korea government Ministry of Knowledge Economy.
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A Gallium Oxide-Graphene Oxide Hybrid Composite for Enhanced Photocatalytic Reaction Seungdu Kim 1,2 , Kook In Han 1 , In Gyu Lee 1 , Won Kyu Park 2,3 , Yeojoon Yoon 2 , Chan Sei Yoo 2 , Woo Seok Yang 2, * and Wan Sik Hwang 1, * 1 2 3
*
Department of Materials Engineering, Korea Aerospace University, Goyang 412-791, Korea; [email protected] (S.K.); [email protected] (K.I.H.); [email protected] (I.G.L.) Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Korea; [email protected] (W.K.P.); [email protected] (Y.Y.); [email protected] (C.S.Y.) School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea Correspondence: [email protected] (W.S.Y.); [email protected] (W.S.H.); Tel.: +82-31-789-7256 (W.S.Y.); +82-2-300-0293 (W.S.H.)
Academic Editors: Ming-Tsang Lee and Te-Hua Fang Received: 1 June 2016; Accepted: 23 June 2016; Published: 1 July 2016
Abstract: Hybrid composites (HCs) made up of gallium oxide (GaO) and graphene oxide (GO) were investigated with the intent of enhancing a photocatalytic reaction under ultraviolet (UV) radiation. The material properties of both GaO and GO were preserved, even after the formation of the HCs. The incorporation of the GO into the GaO significantly enhanced the photocatalytic reaction, as indicated by the amount of methylene blue (MB) degradation. The improvements in the reaction were discussed in terms of increased surface area and the retarded recombination of generated charged carriers. Keywords: gallium oxide; ultraviolet radiation
graphene oxide;
photocatalytic reaction;
methylene blue;
1. Introduction Various metal oxides have been investigated with respect to their photocatalytic reactions, and the resulting technologies can already be found in self-cleaning and anti-fogging/antibacterial fields [1,2]. These products work via the photo-induced redox reactions of adsorbed materials and/or photo-induced hydrophilic conversion of materials [3–5]. The acceleration of the photoreaction occurs on a material’s surface, so the introduction of nano-materials and nano-technologies can significantly enhance catalytic efficiency [6–9]. Meanwhile, two-dimensional (2D) materials, such as graphene and graphene oxide, have been intensively studied in various areas due to their extraordinary high surface-area to volume ratio [10]. Theoretically, a hybrid composite (HC) made up of a metal oxide and 2D materials could facilitate an enhanced photocatalytic reaction and thus further extend and widen the applications of these materials [4]. Although HCs of several metal oxides and graphene/graphene oxide have been explored [4], an HC using gallium oxide (GaO) and graphene oxide (GO) has not yet been investigated. In fact, GaO is an important metal oxide showing a larger energy bandgap (4.8~5.1 eV) than conventional metal oxides, such as TiO2 (3.0 eV) and ZnO (3.2 eV) [11,12]. In addition, GaO has recently garnered more attention for use with power electronics [13] and the extension of their applications to various other areas. In this work, a GaO-GO HC is presented, and its material and photocatalytic reaction properties are investigated. Photo-induced charged carriers are generated from the GaO, and these carriers spread out across the GO, which has a large surface area, resulting in a boost to the photocatalytic reaction. In addition, the GO that attaches to the GaO is able to retard the recombination of generated electron-hole pairs.
Nanomaterials 2016, 6, 127; doi:10.3390/nano6070127
www.mdpi.com/journal/nanomaterials
Nanomaterials2016, 2016,6,6,127 127 Nanomaterials
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2. Experimental Section 2. Experimental Section HCs made up of GaO and GO were synthesized using a conventional hydrothermal method that HCs made up of GaO andoxide GO were synthesized using a conventional hydrothermal method is often used to produce metal powder [14]. GaCl 3 (99.999%) was diluted in deionized (DI) that is often used to produce metal oxide powder [14]. GaCl in deionized 3 (99.999%) water to form a synthesis precursor, and the GO was prepared in the DI was waterdiluted as a solvent. After (DI) water to form a synthesis precursor, and the GO was prepared in the DI water as a solvent. mixing the GaCl3 and GO solutions, ammonium hydroxide (NH4(OH) 28% in H2O) was also added After mixing the balance GaCl3 and GO8solutions, ammonium hydroxide (NH in H2 O) was also 4 (OH) to maintain a pH above since the reaction necessary to form Ga oxide28% nanoparticles would added to maintain a pH balance above 8 since the reaction necessary to form Ga oxide nanoparticles not occur at a pH value below 8 [15]. All of the processes were conducted in a three-neck roundwould flask not occur pH to value belowa 8temperature [15]. All ofofthe processes wereand conducted a three-neck bottom in an at iceabath maintain 0 °C. The solvent HC wereinseparated via ˝ C. The solvent and HC were round-bottom flask in an ice bath to maintain a temperature of 0 centrifugal force at 4000 rpm for 30 min. The collected HCs were then washed and rinsed to remove separated viaand centrifugal at 4000 rpm for min. Thedried collected HCs were then washed and the ammonia chlorineforce residues. Finally, the 30 HCs were in a conventional freeze-dryer at rinsed to remove the ammonia and chlorine residues. Finally, the HCs were dried in a conventional −80 °C for 48 h. The formed HCs and GaO were characterized via scanning electron microscopy (SEM) ˝ C for 48 h. The formed HCs and GaO were characterized via scanning electron freeze-dryer at ´80 (Busan, Korea), X-Ray Diffraction (XRD) (Busan, Korea) and Raman analyses (Seoul, Korea). microscopy (SEM) (Busan, Korea), X-Ray Diffraction (Busan, Korea) and analyses (Seoul, Furthermore, the photocatalytic reactions of the (XRD) HCs were evaluated viaRaman the degradation of Korea). Furthermore, the photocatalytic reactions of the HCs were evaluated via the degradation methylene blue (MB; C16H18N3SCl) at 245 nm of exposure. Multiple 100-mg HCs were added to 100of methylene blue (MB; H218 N3The SCl)mixed at 245solutions nm of exposure. Multiple 100-mg HCs mercury were added to 16H mL MB solutions (0.3 g/LCin O). were exposed under a 245-nm lamp 100-mL MB solutions in H solutions were under a 245-nm mercury 2 O). (Seongnam, Korea) at(0.3 0.4 g/L mW/s for 0 The min,mixed 60 min, 120 min andexposed 180 min, respectively. For the lamp (Seongnam, Korea) at 0.4 mW/s for 0 min, 60 min, 120 min and 180 min, respectively. For the absorbance spectrum analysis, 5 mL was collected from each mixed solution, and the absorbance absorbance spectrum analysis, 5 mL was collected from each mixed solution, and the absorbance spectrum was obtained as a function of wavelength in the range of 450–800 nm. The entire experiment spectrum was obtained as atofunction of wavelength the range of 450–800 nm. The entire experiment was conducted in the dark avoid any light sourceindisturbance. was conducted in the dark to avoid any light source disturbance. 3. Results and Discussion 3. Results and Discussion The HCs formed via the aforementioned processes are shown in Figure 1. The SEM images in The HCs formed via the aforementioned processes are shown in Figure 1. The SEM images in Figure 1a,b shows grain-shaped GaO, and Figure 1b also reveals that the formed GO connected to Figure 1a,b shows grain-shaped GaO, and Figure 1b also reveals that the formed GO connected to the GaO in a fishing net configuration. The GaO samples with higher GO concentrations showed the GaO in a fishing net configuration. The GaO samples with higher GO concentrations showed higher surface areas, which eventually led to enhancements in the photocatalytic reaction. The GaO higher surface areas, which eventually led to enhancements in the photocatalytic reaction. The GaO only sample was made using a 5 g GaCl3 solution. However, the 4% GO-GaO and 10% GO-GaO only sample was made using a 5 g GaCl solution. However, the 4% GO-GaO and 10% GO-GaO samples were made using 0.2 g of GO and30.5 g of GO as well as a 5 g GaCl3 solution. In addition, it samples were made using 0.2 g of GO and 0.5 g of GO as well as a 5 g GaCl solution. In addition, it is is interesting to note that the height to width ratio of the GaO with a GO 34% solution decreased in interesting to note that the height to width ratio of the GaO with a GO 4% solution decreased in the the GaO with a GO 10% solution. It was presumed that the presence of GO might have hindered the GaO with a GO 10% solution. It was presumed that the presence of GO might have hindered the GaO GaO growth for a certain structural plane More detailed structure and crystallinity information growth for a certain structural plane More detailed structure and crystallinity information regarding regarding the GaO and GO was obtained via XRD and Raman analyses. the GaO and GO was obtained via XRD and Raman analyses.
Figure 1. Schematic illustrations of hybrid composite (HC) growth depending on (a) a 4% graphene Figure 1. Schematic illustrations of hybrid composite (HC) growth depending on (a) a 4% graphene oxide (GO) solution and (b) a 10% GO solution. The scale bar in the scanning electron microscopy oxide (GO) solution and (b) a 10% GO solution. The scale bar in the scanning electron microscopy (SEM) is 1 µm. (SEM) is 1 μm.
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Figure Figure 2a 2a shows shows the the XRD XRD spectra spectra of of GaO-GO GaO-GO HCs, HCs, as as well well as as GaO GaO for for comparison. comparison. It It shows shows that that an orthorhombic structure was preferred in the GaO, and the crystallinity of the GaO was an orthorhombic structure was preferred in the GaO, and the crystallinity of the GaO was preserved preserved even even when when forming forming HCs HCs with with GO GO [16]. [16].
Figure Figure 2. 2. (a) (a) X-Ray X-Ray Diffraction Diffraction (XRD) (XRD) spectra spectra of of different different HCs HCs made made up up of of gallium gallium oxide oxide (GaO) (GaO) and and with/without GO; (b) Full-width at half maximum (FWHM) variation for XRD peaks from (a) and with/without GO; (b) Full-width at half maximum (FWHM) variation for XRD peaks from (a) and grain grain size size of of the the HCs HCs as as aa function function of of GO GO concentration concentration in in the the solution; solution; (c) (c) Raman Raman of of different different HCs HCs using the same proportions as in (a). using the same proportions as in (a).
In addition, the full-width at half maximum (FWHM) of {110} was measured, and the grain size structures was was extracted extracted from the the FWHM FWHM at at different different GO GO concentrations concentrations [17]. [17]. The GaO grain of the structures 45 nm nm continued continued to decrease decrease as the the GO GO concentration concentration increased in the HC. It was presumed presumed size of 45 that the functional groups such as hydroxyl, hydroxyl, epoxide, carbonyl and carboxyl at the GO surface surface can serve as as nucleation nucleationsites. sites.Accordingly, Accordingly,there thereshould shouldbebemore morenucleation nucleation sites the GaO when there serve sites forfor the GaO when there is is a higher GO concentration because this leads to a smaller GaO grain size in an HC [18]. This reveals a higher GO concentration because this leads to a smaller GaO grain size in an HC [18]. This presence of of GO GO during during the the Ga Ga oxide oxide formation formation affects affects the the grain grain size size of of the the Ga Ga oxide oxide in a nano that the presence scale range. The crystallinity of the HC was further investigated via Raman analyses, as shown in Figure 2c. The representing GaO were observed in the GaO particles. As Theresults resultsshow showthat thatthe thepeaks peaks representing GaO were observed in the GaO particles. thethe GOGO concentration increased inin the HC, As concentration increased the HC,the theGO GOpeak peakbecame becamemore moreprominent. prominent. D-band D-band (D) occurs at 1350 region because of oxide. The G-band (G) is generated at 1350 region because of graphite [17]. The D/G D/G ratio conventional GO. The results The ratio of of the the GO GO peak peak in the HC was comparable to that of a conventional high quality quality GO GO was was preserved preserved even even in in the the GO GO in in the the HC. HC. show that high The photocatalytic reaction of the GaO with/without GO was through the photophotocatalytic reaction of the GaO with/without GO investigated was investigated through the degradation of theofMB UV UV exposure. Figure 3 shows time evolution photo-degradation theunder MB under exposure. Figure 3 shows time evolutionofofthe theMB MB absorbance spectrum depending dependingon onnanoparticle nanoparticlesample, sample,i.e., i.e.,only onlyGaO, GaO,GaO GaO a 4% GO solution and oxide spectrum inin a 4% GO solution and GaGa oxide in in a 10% GO solution. Before the spectra analysis, the samples were exposed at 254 nm for 0 min, 60 a 10% GO solution. Before the spectra analysis, the samples were exposed at 254 nm for 0 min, 60 min, min,min 120and min180 and 180respectively. min, respectively. The show results show that the absorbance theonly MB 120 min, The results that the absorbance spectrumspectrum of the MBof with with remained only GaO remained constant. However,intensity the spectrum intensity significantly decreased GaO constant. However, the spectrum significantly decreased according to time according to time and This GO concentration. reveals that proportional the MB degradation to i.e., the and GO concentration. reveals that theThis MB degradation to the GOproportional concentration, GOtotal concentration, i.e., the surface area inthe thetotal MB. surface area in the MB. A schematic illustration of the charged carrier transfer and MB degradation mechanism of the GaO (Figure 4a) and the HCs (Figure 4b) made up of Ga oxide and GO is described in Figure 4. The HCs made up of GaO and GO were able to slow the recombination of generated electron-hole pairs and enhance the reaction potential with the MB.
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Figure 3. The absorbance spectra of the methylene blue (MB) solution where (a) GaO; (b) an HC of GaO and 4% GO; and (c) an HC of GaO and 10% GO were added to the MB. Each sample was exposed to 254 nm of radiation at different times.
A schematic illustration of the charged carrier transfer and MB degradation mechanism of the GaO Figure (Figure3.4a) the HCsspectra (Figureof4b) up ofblue Ga oxide and GOwhere is described Figure Theand absorbance themade methylene (MB) solution (a) GaO;in(b) an HC4.ofThe Figure 3. The absorbance spectra of the methylene blue (MB) solution where (a) GaO; (b) an HC of HCs GaO made up of GaO and GO were able to slow the recombination of generated electron-hole pairs and 4% GO; and (c) an HC of GaO and 10% GO were added to the MB. Each sample was exposed GaO and 4% GO; and (c) an HC of GaO and 10% GO were added to the MB. Each sample was exposed and enhance the reaction potential with the MB. to 254 nm of radiation at different times. to 254 nm of radiation at different times.
A schematic illustration of the charged carrier transfer and MB degradation mechanism of the GaO (Figure 4a) and the HCs (Figure 4b) made up of Ga oxide and GO is described in Figure 4. The HCs made up of GaO and GO were able to slow the recombination of generated electron-hole pairs and enhance the reaction potential with the MB.
Figure 4. 4. Schematic Schematicillustrations illustrationsofofcharged chargedcarrier carriergeneration generationand andtransfer, transfer,asaswell wellasasthe thewater water Figure splitting mechanism mechanismof of(a) (a)the theGa Gaoxide oxideand and(b) (b)the theHC HCofofGa Gaoxide oxideand andGO. GO.hνhνisisultraviolet ultraviolet light splitting light constant and and νν isisfrequency. frequency. energy, where h is Planck’s constant
4. Conclusions Conclusions Hybrid composites (HCs) of and GO investigated facilitate Hybrid composites (HCs) made made up ofGaO GaO andgeneration GOwere wereand investigated facilitate enhanced Figure 4. Schematic illustrations ofup charged carrier transfer, astoto well as the enhanced water photocatalytic reactions under ultraviolet (UV) improvements the were photocatalytic reactions of under ultraviolet (UV) radiation. The improvements thereaction reaction were splitting mechanism (a) the Ga oxide and (b)radiation. the HC of The Ga oxide and GO. hν in isin ultraviolet light to the increased surface area when The generated energy, where h is Planck’s constant and is frequency. attributed increased surface areathat thatνresulted resulted whenthe theGO GOattached attachedtotothe theGaO. GaO. The generated charged carriers carriers in in the the GaO GaO under under UV UV radiation radiationspread spreadout outthrough throughthe theGO GOand andretarded retardedthe the 4. Conclusions recombination of significantly enhancing the photocatalytic reaction with the MB. recombination ofelectron-hole electron-holepairs, pairs, significantly enhancing the photocatalytic reaction with the The material properties of both the the GaO-GO were preserved even after thethe formation of of HCs made MB. The material properties of both GaO-GO were after formation HCs made Hybrid composites (HCs) made up of GaO andpreserved GO wereeven investigated to facilitate enhanced up of GaO and GO. The HCs developed in this study can be used in various applications requiring up of GaO and GO. The HCs developed in this study can be used in various applications requiring photocatalytic reactions under ultraviolet (UV) radiation. The improvements in the reaction were enhanced photocatalytic reactions. enhanced reactions. attributedphotocatalytic to the increased surface area that resulted when the GO attached to the GaO. The generated charged carriers in the GaO under UV radiation spread out through the GO and retarded the work waswas supported by thebyBasic ResearchResearch Program through Nationalthe Acknowledgments: This This work supported the Science Basic Science Programthe through recombination of Foundation electron-hole pairs, enhancing the photocatalytic reaction with the MB. National Koreasignificantly (NRF) the Ministry of Science, Future Planning Research Research Foundation of Koreaof (NRF) fundedfunded by thebyMinistry of Science, ICT ICT & & Future Planning The material properties both the GaO-GO were preserved even after the formation of HCs made (2014R1A1A1004770), by theofTechnology Innovation Program (No. 10044410) funded by the Korea Government (2014R1A1A1004770), by the Technology Innovation Program (No. 10044410) funded by the Korea Government Ministry of Knowledge an Energyin Efficiency & Resources of theinKorea Institute of Energy requiring Technology up of GaO and GO. Economy, The HCs by developed this study can be used various applications Ministry of and Knowledge by anfunded Energyby Efficiency & Government Resources ofMinistry the Korea Institute of Economy Energy Evaluation PlanningEconomy, (KETEP) grant the Korea of Knowledge enhanced photocatalytic reactions. Technology Evaluation andbyPlanning (KETEP) grant funded by the Korea Ministry (No. 20142010102690) and a grant from the Advanced Technology CenterGovernment R&D Program fundedofbyKnowledge the Ministry of Trade, Industry & Energy ofand Korea Korea government Ministry of Knowledge Economy. Economy (No. 20142010102690) by a(10048475), grant from by thethe Advanced Technology Center R&D Program funded by the Acknowledgments: This work was supported by the Basic Science Research Program through the National MinistryContributions: of Trade, IndustryS.K., & Energy of Korea by conceived the Korea government Ministry of Knowledge Economy. Author K.I.H., W.S.Y.(10048475), and W.S.H. and designed the experiments, W.K.P. and Research Foundation of Korea (NRF) funded by the Ministry of and Science, & Future Planning Y.Y. contributed to ultraviolet measurements and interpretation, I.G.L. C.S.Y. ICT provided and interpreted (2014R1A1A1004770), byS.K., the Technology Program (No. 10044410) the Koreathe Government the morphologic model, W.S.Y. and Innovation W.S.H. wrote the manuscript. All funded authorsby discussed results and commented the manuscript at all stages. Ministry ofon Knowledge Economy, by an Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant of funded by the Korea Government Ministry of Knowledge Conflicts of Interest: The authors declare no conflict interest. Economy (No. 20142010102690) and by a grant from the Advanced Technology Center R&D Program funded by the Ministry of Trade, Industry & Energy of Korea (10048475), by the Korea government Ministry of Knowledge Economy.
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