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Interfacial engineering of nanoporous architectures in Ga2O3 film towards self-aligned tubular nanostructure with an enhanced photocatalytic activity on water splitting Nabeen K. Shrestha, Hoa Thi Bui, Taegweon Lee, and Yong-Young Noh Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00670 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Interfacial engineering of nanoporous architectures in Ga2O3 film towards self-aligned tubular nanostructure with an enhanced photocatalytic activity on water splitting

Nabeen K. Shrestha,*,a Hoa Thi Bui,b Taegweon Lee,a Yong-Young Noh*,a a

Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea.

b

Department of Chemistry, Hanynag University, Seoul-04763, Republic of Korea.

Abstract The present work demonstrates the formation of self-aligned nanoporous architecture of gallium oxide by anodization of gallium metal film controlled at -15 °C in aqueous electrolyte consisting of phosphoric acid. SEM examination of the anodized film reveals that by adding ethylene glycol to the electrolyte and optimizing the ratio of phosphoric acid and water, chemical etching at the oxide/electrolyte interfaces can be controlled, leading to the formation of aligned nanotubular oxide structures with closed bottom. XPS analysis confirms the chemical composition of the oxide film as Ga2O3. Further, XRD and SAED examination reveals that the as-synthesized nanotubular structure is amorphous, and can be crystalized to β-Ga2O3 phase by annealing the film at 600 °C. The nanotubular structured film, when used as photoanode for photo-electrochemical splitting of water, achieved a higher photocurrent of about two folds than that of the nanoporous film, demonstrating the rewarding effect of the nanotubular structure. In addition, the work also demonstrates the formation of highly organized nonporous Ga2O3 structure on a non-conducting glass substrate coated with thin film of Ga-metal, highlighting that the current approach can be extended for the formation of self-organized nanoporous Ga2O3 thin film even on non-conducting flexible substrates.

Keywords: thin film, interfacial chemical etching, self-aligned, nanoporous, nanotubular, Ga2O3.


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■ INTRODUCTION Owing to the high specific area with abundant catalytic sites, designing of 3-D hierarchical structured nanomaterials with smart morphology, such as the material consisting of assembly from 0-D, 1-D or 2-D, or their combinations, can be an effective approach for enhancing the photocatalytic activities of semiconductor photocatalysts in solar energy conversion technology.1-5 In addition, constructing heterojunctions with suitable band alignments can further enhance the photocatalytic performance of the nanomaterials.6-9 Among different hierarchical architectures, 1-D hollow tubular structures can provide a larger surface area with abundant catalytic active sites together with facile charge transportation via thin tube-walls, thereby promoting the catalytic performance. Among various strategies for designing 1-D hollow tubular structures, electrochemical anodization of transition metals is a facile way of achieving self-organized hollow tubular structures of metal oxides. After the pioneering works on self-organization of porous oxide nanostructures via anodization of aluminium by Masuda et al., and titanium by Assefpour-Dezfuly et al. / Zwilling et al., electrochemical anodization of metals and their alloys has been investigated extensively for formation and potential applications of self-organized one dimensional nanotubular or nanoporous metal oxide thin films.10-24 In addition to the specific functionality of the metal oxides derived through their intrinsic properties, the anodically achieved oxide films further aid an enhanced functionality. This is, in particular, owing to the facile electron transportation through the resulting one-dimensional porous structures, and, in general, due to the beneficial low contact resistance of the film on the respective metal substrates working as back-contact electrodes.18,25,26 Being a wide band gap semiconducting material with high reduction potential of conduction band electrons (-7.75 eV) and high oxidation potential of valence band holes (-2.95 eV), Ga2O3 possesses a high thermodynamic driving force for overall splitting of water, and for oxidizing a wide variety of organic toxics under open-circuit condition.27-37 Apart from that Ga2O3 has also been employed as a promising candidate in gas sensors, optoelectronic devices, resistance random access memory devices, and TCO applications.38-43 The performance of Ga2O3 film on those applications can be enhanced by employing self-organized nonporous structured film. Although various methods have been reported for the formation of Ga2O3 thin film, to the best of our knowledge, to date there are 2 ACS Paragon Plus Environment

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only two reports on formation of self-organized nonporous Ga2O3 film employing anodization route.44,45 Nevertheless, despite the potentially high application of self-organized Ga2O3 nanostructures and their formation simply by anodization of Ga- metal, they have been hardly studied. This could be mainly due to the inappropriately low melting point of Ga-metal (29.77 °C), thereby hurdling in handling and anodization of the Ga-metal at room temperature. Previous study on anodization of Ga-metal at much lower freezing temperatures demonstrated the formation of self-organized porous Ga2O3 films.45 On the other hand, some studies show that water in electrolyte is essential for the growth and self-organization of the anodically obtained oxide nanostructures, and depending upon the water content, tubular to porous transition could be observed.46-48 In this communication, we demonstrate towards the formation of self-ordered one dimensional Ga2O3 nanotubular structure by optimizing the anodization conditions, particularly by controlling the chemical etching of the already grown nanoporous structured Ga2O3. To the best of our knowledge, this is the first report on selfaligned Ga2O3 nanotubular structured film. Further, as compared to the nanoporous structure, the nanotubular structured film demonstrated a relatively facile charge transportation, leading to the better photocatalytic performance on water oxidation.

■ MATERIALS AND METHODS Ga-film substrates for anodization were prepared by melting Ga-metal (99.9995%, SigmaAldrich) at 45 °C followed by doctor blading the liquid Ga on Ti-foil or plain glass as substrates. Before using, the Ga-films and all electrolytes were cooled and stored in a refrigerator. A cell consisting of two-electrode system with a Pt cathode was used for the anodization. Before and during anodization, the substrate was cooled and maintained to -15 °C using a Peltier element (Conrad electronics). The anodically grown nanostructures were characterized using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800), energy dispersed X-ray analyzer (EDX, EM912), X-ray diffractometer (XRD, Rigaku D/MAX 2600 V, Cu Kα = 0.15418 nm), X-ray photoelectron microscope (XPS, VG scientific ESCALAB MK II), transmission electron microscope (TEM, Omega EM ) and selected area electron diffraction (SAED) analysis.


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The photo-electrochemical splitting of water was performed in degassed 1M KOH aqueous electrolyte using a three-electrode electrochemical cell with Pt counter and Ag/AgCl reference electrodes. The photo-anode was illuminated using a solar simulator (PEC-L01, Peccell)

under the condition of 100 mW cm−2 (1 sun, 1.5 AM) illumination, and chopping the illumination at regular intervals during under an external bias at the speed of 1 mV s-1 using an electrochemical workstation (IVIUM, COMPACTSAT.e). Further, an electrochemical impedance spectroscopy of the cell was studied in the frequency range of 10-2 - 106 Hz under the 1 sun condition illumination at an external bias of 1.5 V vs RHE.

■ RESULTS AND DISCUSSION Fig. 1a shows the top SEM view of Ga-metal film coated on a Ti-foal via the doctor blade technique. In a high magnification mode, the film showed some pin holes and hairy lines on the coating surface. The cross-sectional SEM view in the inset of Fig. 1a reveals a uniform coating of the Ga- metal film with a thickness of about 800 nm. Owing to the fact that Al and Ga are the adjacent elements in the same column of periodic table, and thus they could experience similar behaviours during anodization, acidic electrolytes such as oxalic and phosphoric acids used usually for anodization of Al-metal for obtaining self-organized Al2O3, were investigated as ideal electrolytes in the present case. In addition, based on our previous work for achieving self-aligned porous Ga2O3 film, anodization was primarily performed by reducing and controlling the substrate temperature to -15 °C.45 Fig. 1b shows the top SEM view of a typical self-aligned nanoporous structure obtained on the surface of Ga-metal film at the optimized condition of anodization at 40 V for 4 h in 0.5 M oxalic acid electrolyte. Under these anodization conditions, SEM image shows the formation of oxide with some randomly oriented porous and compact layer structures on the anodized surface (Fig. 1b). The cross-section of the anodized film shown in the inset image reveals columnar feature. In contrast, when the electrolyte was switched to phosphoric acid electrolyte, a better ordered porous structure was observed (Fig. 1c) under the anodization voltage of 40 V. However, some pin holes, directional lines of the pore growth and some island of oxide growth were observed while carefully examining the anodized surface with SEM (Fig. S1, supporting Information). These undesirable defects on the porous structure are originally initiated from the defects present on the Ga-metal film substrate. For example, directional line of the porous 4 ACS Paragon Plus Environment

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growth is due to the oxide growth directed by the hairy lines present initially on the surface of the Ga-metal film whereas the island oxide growth is owing to the presence of Ga-metal grains present initially on the coating surface. In order to further improve the degree of uniformity of the porous structure, keeping all other anodization conditions as they were, the content of phosphoric acid in the aqueous electrolyte is gradually increased from 12.5 wt % to 25, 37.5, and finally to 50 wt %. Under the increasing concentration of electrolyte, the SEM topography views of the anodized surface show the gradual enlargement of the pores. This could be due to the corrosive nature of the acid, and the degree of etching of the porous oxide is obviously observed higher while increasing the concentration of the acid. The etching of the porous oxide on the other hand has contributed towards gradual removal of the directional line of the pore growth. The cross-section of the anodized film obtained from the 50 wt% content of phosphoric acid shows the porous surface and channel extending from top to bottom of the film (Fig. 1f, inset). In an attempt to further improve the degree of selforganization of the porous channels, anodization was prolonged to 8 h under the same anodization conditions. Under this circumstance, etching of oxide, thereby forming nanofibers from the top peripheral surface of the pores, and even from the wall of the channels was observed as evident in Fig. 2a and b. The etching of oxide structure is attributed to corrosive action of the electrolyte, leading to the dissolution of the already grown oxides at the oxideelectrolyte interfaces.49 This dissolution phenomenon is purely chemical in nature. However, when the anodization voltage and anodization time were increased, SEM examination of the anodized surface exhibited more severe dissolution of the anodically grown porous oxide structure as shown in Fig. 2c, d and Fig. S2. This phenomenon is attributed to the chemical as well as field-assisted chemical dissolution.

An interesting and noteworthy result was obtained when a thin film of Ga-metal coated on a plain non-conducting glass substrate was anodized at 40 V for 1 h. The SEM top views of the anodized surface are shown in Fig. S3, which reveal the formation of self-organized nanopores distributed uniformly on the entire anodized surface. The cross-sectional SEM view of the film shown in the inset of Fig. S3 (d) reveals the thickness of about 500 nm. This outcome suggests that the present approach demonstrated here can be applied to the formation of self-organized gallium oxide nanoporous film on any non-conducting substrates including


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plastics, and the technique highlights for its application to the next generation flexible devices used in various fields. It should be noted that the comparative investigation of the aqueous electrolytes with different phosphoric acid content suggests that the porous nanostructure obtained in the case of 50 wt% acid content are highly ordered (Fig. 1f). However, chemical etching of the channels leading to wall thinning particularly at the top part, which was actually exposed from the beginning of the anodization in the corrosive electrolyte, can be clearly realized from Fig. 2 and S2. Based on the above results of anodization in the most favorable concentration of phosphoric acid (i.e., 50 wt%) in aqueous electrolyte, and meanwhile considering the observed phenomenon of etching of the already grown porous oxide structure in the same electrolyte, further optimization of anodization was realized for the electrolyte consisting of 50 wt% of phosphoric acid. In order to prevent the etching of oxide structure, attempt in controlling the water content of the electrolyte was carried out. For this, ethylene glycol was used as an additive in the electrolyte, because this - (i) lowers the freezing point of aqueous electrolyte, thereby facilitating electrochemical reactions involving in anodization even at reduced temperature of –15 °C; (ii) increases the viscosity of the electrolyte, thereby lowering the chemical etching of oxide structure; and (iii) lowers the electrical conductivity of the electrolyte, thereby reducing the field assisted etching phenomenon. At first, the original 85 wt% phosphoric acid was diluted with mixture of water and ethylene glycol so that the resulting electrolyte contained 50 wt% acid and 20 vol% ethylene glycol (Table S1). Unlike the case of pure aqueous electrolyte, anodization of the Ga-film substrate in this electrolyte produced self-organized pores only at relatively higher anodization voltage. After several trials, the anodization in this ethylene glycol mixed electrolyte was optimized at 100 V, and the top SEM view of the anodized surface obtained by anodizing for 4 h shows self-organized pores with relatively thicker pore wall (Fig. 3a). Although the SEM topography of this top anodized surface does not show etching of oxide structure from the top surface, perforated channels can be observed in the cross-sectional SEM view of the anodized film (Fig. 3b). In order to further suppress the chemical etching of the oxide nanostructure, water content in the electrolyte was reduced by elevating ethylene glycol content to 30 vol% (Table S1). The top surface and cross-sectional SEM views of the anodized Ga-film surface in this electrolyte shows better self-organization of the pores (Fig. 3c and d). However, the number of pore density on the surface decreased, thereby thickening 6 ACS Paragon Plus Environment

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the pore wall. Thus, deceleration of the chemical etching of the porous oxide structure by decreasing the water content in the electrolyte is evident from the above results. The crosssection SEM view in Fig. 3d clearly shows that the bottom part of the porous channels are open and each channel has sheared the walls of surrounding pores.

It should be noted that the self-organized tubular structures consist of bundles of porous channels with each channel growing from top to bottom without shearing the walls of other channels. For instance, if we drill a metal block forming a number of holes in equidistance, each hole in this case shears the wall of the adjacent holes. This structure represents the selforganized porous structure. In contrast, if we assemble a large number of hollow pipes or test tubes, the resulting assembly represents the self-organized tubular structure with closed or opened bottom, respectively. In this case, the individual porous channels have their own walls enclosing the channel. Even more striking feature of the oxide structure appeared when the porous film was grown by further increasing the ethylene glycol content in the electrolyte to 35 vol%. The SEM topography of the anodized surface in this electrolyte shows that each porous channel has open bottom, and still sheared the walls of the adjacent channels. However, each channel wall has a clear diving line on the channel wall as shown by arrows in Fig. 3e and f, suggesting that further reduction in water content of the electrolyte would be beneficial toward producing the tubular structure. Accordingly, the water content in the electrolyte was finally reduced to 14.9 vol% without external addition of water as shown in Table S1, and the anodization was performed under the similar condition of 100 V for 4 h. In this case, the SEM top surface view of the anodized surface shows a thin initiation porous oxide film15 with relatively smaller number of self-organized pores (Fig. 4a and Fig. S4). Surprisingly better than expected, when the initiation oxide film was scratched away, tubular structure that was buried underneath the top initiation porous layer, was obtained as evident in Fig.4b and Fig. S4. While fracturing the film and examining carefully, a complete view of the nanotubular structures with opened top and closed bottom was obtained as shown in Fig. 4c. It is noteworthy to be highlighted that the closed bottom part shows an assembly of individual cell like structure suggesting that the nanotubes are grown with their own individual walls. Meanwhile, the large-scale view of the bottom part of the film shows the uniform and highly dense distribution of the nanotubes (Fig. 4d).


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As the Ga-metal film coated on a Ti-foil was only about 800 nm thick (Fig. 1a, inset), all of the Ga-metal was found to be completely anodized in the above anodization condition of 4 h. Therefore, to know the real growth of the nanotubular oxide, Ga-metal block (c.a. 4 mm thick) obtained by frizzing the melt was used as the Ga-substrate for anodization. After anodizing for 4 h at 100 V under the above discussed optimized condition for growing nanotubular structure, the anodized surface was scratched away with a sharp edged knife and examined with SEM. The cross-sectional SEM views shown in Fig. 5 reveal the formation of self-aligned tubular structure with ca. 3 µm long. The SEM images (Fig. 5) further shows the bundle of broken nanotubes bending slightly inwards. The bending could be due to the pressure from the top together with the instant melting of the Ga-meal laying immediate underneath of the nanotubes owing to the generation of heat during scratching. It is most likely that the melted Ga-metal was flown as shown by the arrow in Fig. 5a to the anodized surface during scratching and formed Ga-metal film on the top surface.

To study the chemical composition of the nanotubular oxide film, EDX analysis was performed and the results are shown in Fig. S5. In addition to Ga and O signals, the EDX spectra also reveal the presence of P. The quantitative analysis shows the presence of about 5.7 at% and 9.7 at% of P, respectively together some amount of C from the EDX analysis of the top and bottom parts of the nanotubular structured film. The relatively larger amount of P shown at the bottom part could be due to the accumulation of electrolyte, which is hard to leach out during washing the film. Further, to understand the detail chemical composition of the nanotubular oxide film, and study the interaction of phosphoric acid with the oxide structure, XPS analysis of the film was performed. The high resolution XPS Ga 2p spectrum shows the photoemission doublet (2p1/2, 2p3/2) with separation of 26.8 eV and the Ga 2 p3/2 peak centred at 1118.9 eV (Fig. S6a). This suggests that the Ga in the film has 3+ oxidation state.43In addition, the O 1s XPS spectrum shows the photoemission peak centred at 531.5 eV (Fig. S6b), and Ga to O atomic ratio was found to be ca. 0.64:1. This finding suggests that the Ga and O are bounded chemically in the form of Ga2O3, constituting the oxide film. The P 2p XPS spectrum recorded for the phosphorus showed only one photoemission peak centered at 133.1 eV (Fig. S6c). This peak can be ascribed to the 2p 3/2 of simply phosphate (PO43−) 8 ACS Paragon Plus Environment

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species, suggesting that the above phosphorous content revealed by the EDX analysis in the nanotubular structure is only due to adsorption of phosphoric acid from the electrolyte.50 In line with the SEM topography, TEM examination of the nanotubular structured Ga2O3 film also shows hollow tube, and the SAED patterns reveals the amorphous characteristic in the asprepared state of the film (Fig. 6a). The XRD patterns of the as-prepared film also reveal the amorphous feature. However, when the film was annealed at 600 °C in air, the film showed polycrystalline with β-Ga2O3 phase (Fig. 6b). As depicted in Fig. S7, the SEM and TEM examinations of the Ga2O crystalline film reveal that the initial nanotubular structure of the amorphous film is completely uninterrupted even after annealing at 600 °C for 3 h.

Further, to demonstrate the merit of the Ga2O3 nanotubular structured film, the photo-activity of the tubular structure vs porous structure of the film was studied. For this, as a concept-ofdemonstration, the crystalline Ga2O3 films on Ti-substrate were employed as photo-anodes, and their performance on photo-electrochemical water splitting was evaluated in 1 M KOH electrolyte. Fig. 7a shows the comparative photo-current density on photo-assisted oxygen evolution reaction (OER), revealing about 2 folds higher current density in the case of tubular structured film based photo-anode. The photo-current density could be further improved to some extent by increasing the film thickness. Note that the optimization on film thickness to maximize the current density is, however, not the goal of the present work. Nevertheless, the performance obtained for the un-optimized Ga2O3 film based electrodes on photoelectrochemical water splitting is reasonably appreciable, particularly in the case of tubular structured film. The optimization on film thickness, and other aspect, such as construction of heterojunction by self-doping or via deposition of suitably band aligned photocatalytic materials,6-9,51-54 will be followed in our future works. Nevertheless, it is noteworthy to underline that the photo-current density demonstrated by the anodically obtained Ga2O3 films of this study is significantly higher than that exhibited by the RF sputtered Ga2O3 films.55 It can also be noticed an apparent dark-current exhibited by the RF sputtered Ga2O3 films.55 In the present case, when the Ga2O3 film electrodes was polarized at dark in the potential window as shown in Fig. 7a, no evolution of oxygen gas was detected at the electrode surface. However, as in the case of RF sputtered Ga2O3 films, an apparent dark-current can be observed. Although, we do not have concrete clue at the moment, one possibility of this dark-


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current could be associated with the intercalation of charges, as large number of porous structured oxides have been studied as intercalating materials.

To understand the comparatively higher photo-current demonstrated by the tubular structure, interfacial charge transportation of the two morphologically distinct Ga2O3 films was studied using an electrochemical impedance spectroscopy. Fig. 7b shows the Nyquist plot obtained from the measured impedance data, which reveals the relatively smaller charge transfer resistance in the case of the tubular structured film based electrode. This finding reveals that, in contrast to the structurally deformed irregular walls of porous film (Fig. 1f, and 2b), the tubular structures having smooth wall (Fig. 5b) can reduce electron trapping, leading to a facile transportation of the photo-generated electrons to the back-contact anodic electrode (i.e., Ti-substrate). Hence, upon illumination, the photo-generated electrons at the conduction band of the Ga2O3 are smoothly transported through the tubular structure to the back-contact anodic electrode, while the holes at the valance band are transported to the Ga2O3/electrolyte interface. Consequently, water oxidation is facilitated, thereby producing a higher photocurrent, as apparent in Fig. 7 a. In contrast, owing to the poorer charge transportation in the case of porous structure, charge carrier recombination events are more likely to occur, thereby reducing the photocurrent. Being a wide band gap material (4.8 eV), it can be believed that no photo-corrosion of the Ga2O3 photo-anode takes place. However, in harsh electrolyte (i.e., 1 M KOH), it is essential to examine the stability of the photo-anode. Inset of Fig. 7a shows the long-run stability test of the photo-anode consisting of nanotubular structured Ga2O3 film under the illumination of 100 mWcm-2, and an external bias of 2.5 V vs RHE. It is evident that the photo-current shown by the device is fairly constant throughout the stability test for 5 h. This finding demonstrates that the anodically obtained nanotubular structured Ga2O3 film is absolutely stable against photo-chemical and electrochemical reactions taking place during photo-electrochemical water splitting. ■ CONCLUSIONS The present work demonstrates the electrochemical anodization route for the formation of vertically oriented self-aligned nanoporous film of Ga2O3 using a frozen Ga-substrate in aqueous phosphoric acid, and aqueous phosphoric acid/ethylene glycol mixed electrolytes under the substrate temperature controlled at -15 °C. The SEM topography of the anodized 10 ACS Paragon Plus Environment

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surface revealed the chemical etching of the already grown oxide nanostructure in pure aqueous electrolyte, suggesting water content in the electrolyte is the key factor for the observed etching phenomenon. The control experiments on reduction of water content by adding ethylene glycol to the electrolyte demonstrates the inhibition of the chemical etching on the oxide nanostructures, leading toward the formation nanotubular structure. Thus, by controlling ethylene glycol to water volume ratio to 1:0.36 in 50 wt% phosphoric acid, the anodized film consisting of aligned nanotubular structure of Ga2O3 was achieved. The nanotubular structured film demonstrated a comparatively facile charge transportation, thereby leading to the better photocatalytic performance on water oxidation with high photochemical and electrochemical stability. The work also highlights the potential application of the current approach for the formation of self-organized nanoporous Ga2O3 thin film even on non-conducting flexible substrates, which may find various applications in the next generation flexible devices.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/. Table showing composition of electrolytes, additional SEM and TEM images, EDX spectra, and XPS spectra.

■ AUTHOR INFORMATION Corresponding Authors *N. K. Shrestha: E-mail: [email protected] *Y.Y. Noh: E-mail:[email protected]

ORCID Nabeen K. Shrestha: 0000-0002-4849-4121 Yong-Young Noh:

0000-0001-7222-2401

Notes


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The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea and the Ministry of Science & ICT of the Republic of Korea.

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Shrestha, N.K.; Lee, K.; Hahn, R.; Schmuki, P. Anodic growth of hierarchically

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Shinde, D.V.; Lee, D.Y.; Patil, S.A.; Lim, I.; Bhande, S.S.; Lee, W.; Sung, M.M.;

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Figure captions

Fig. 1. SEM top surface views of (a) Ga-metal thin film coated on a titanium substrate showing the uniformity of the coating all over the surface; the same Ga-metal thin film after anodization at 40 V for 4 h in aqueous electrolyte containing (b) 0.5 M oxalic acid, (c) 12.5 wt% H3PO4, (d) 25 wt% H3PO4, (e) 37.5 wt% H3PO4, and (f) 50 wt% H3PO4. Inset images show the corresponding cross-section SEM views.

Fig. 2. SEM views of the Ga-metal film after anodization in 50 wt% H3PO4 aqueous electrolyte. (a) top surface, and (b) cross-sectional views obtained at 40 V for 8 h. Top surface views obtained at (c) 80 V for 4 h, and (d) 80 V for 15 h. Fig. 3. SEM views of the Ga-metal film after anodization in mix electrolyte containing 50 wt% H3PO4 and various volume ratios of ethylene glycol and water. (a) top surface, and (b) cross-sectional views obtained at 100 V for 4 h in 20 vol% of ethylene glycol, 36 vol% water; (c) top surface, and (d) cross-sectional views obtained at 100 V for 4 h in 30 vol% of ethylene


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glycol, 26 vol% water; (e) top surface and side, and (f) cross-sectional views obtained at 100 V for 4 h in 35 vol% of ethylene glycol, 21 vol% water.

Fig. 4. SEM views of the Ga-metal film after anodization in mix electrolyte containing 50 wt% H3PO4, 41.2 vol% of ethylene glycol and 14.9 vol% of water at 100 V for 4 h. (a) Top surface, (b) surface after scratching the top initiation porous layer, (c) complete side views of nanotubular structure showing top opened and bottom closed feature, (d) large-scale view of bottom part of nanotubular structure.

Fig. 5. SEM cross-sectional views of Ga2O3 nanotubular structure obtained by anodizing Gametal block (5 mm thick) in mix electrolyte containing 50 wt% H3PO4, 41.2 vol% of ethylene glycol and 14.9 vol% of water at 100 V for 4 h. (a) Complete cross-sectional view showing tube growth of ca. 3 µm, (b) magnified view of the same cross-sectional part of the Ga2O3 film.

Fig. 6. (a) TEM view of Ga2O3 nanotubular structure obtained by anodizing Ga-metal block (5 mm thick) in mix electrolyte of ethylene glycol and water containing 50 wt% H3PO4 , 41.2 vol% of ethylene glycol and 14.9 vol% of water at 100 V for 4 h. Inset image shows SAED pattern of the same nanotubular structure. (b) XRD patterns of Ga2O3 nanotubular structured film (i) as-prepared, and (ii) after annealing at 600 °C for 3 h.

Fig. 7. (a) Polrarization curve of the porous and tubular structured Ga2O3 film based electrodes in 1 M KOH aqueous electrolyte under light (100 mW cm-2 1.5 AM). Illumination was chopped at regular interval, and bias was swept at 1 mVs-1. Inset shows the long-run stability test of the tubular structured Ga2O3 film based electrode for 5 h under 100 mW cm-2 illumination at the external bias of 2.5 V vs RHE. (b) Nyquist plots of the same Ga2O3 film based electrodes under illumation, and at the external bias of 1.5 V vs RHE. 18 ACS Paragon Plus Environment

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Fig. 1


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Fig. 2


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Fig. 4


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Fig. 5


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Fig. 7


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