Photodegradation of Rhodamine-B using Gallium Hybrids as an Ecient Photocatalyst

Rhodamine B is a cationic xanthene dye widely used in the printing, textile, photography, and pharmaceutical industries. It is also well-recognized as water uorescent and dyes laser material because of its good stability. It is toxic to human beings irritating the skin, and eyes. The present study is designed to study the photodegradation of Rhodamine B using gallium oxide and gallium hybrids as photocatalyst. Precipitation coupled with the sonochemical method is adopted for the synthesis of gallium oxide while the post grafting method is adopted for the synthesis of gallium hybrids with the indole and its derivatives. FTIR spectra showed the characteristic absorption bands of gallium oxide and gallium hybrids at 400-700 cm -1 and 1400-1600 cm -1 . SEM and XRD showed the micro-sized rectangular rod-shaped gallium oxide with rhombohedral geometry. The BET isotherm revealed the adsorption type-IV and hysteresis loop (H3) proposing multilayer and mesoporous structures. XPS conrmed the presence of gallium, oxygen, nitrogen, and carbon. The gallium oxide and gallium hybrids showed 47-72% optimum degradation of Rhodamine B under two hours of illumination at pH 7 and 0.03mg/L. The degradation rate followed a Langmuir-Hinshelwood model with R 2 >0.9.


Introduction
Dyes are ionizing and aromatic organic compounds showed an a nity towards the substrate to which it is being applied. It contained compounds, such as benzene, xanthene, and aromatic amines, potentially hazardous to living organisms. It entered into water bodies from the e uent of textile and dye-producing industries such as printing, paper, carpet, leather during the nishing step which has not xed to the bers (Kansal et al. 2009). It is estimated that more than 0.7 million tons of dyes per annum are produced worldwide (Daneshvar et al. 2012) and around 10-15% of dyes are wasted into the environment upon completion of their use in the dyeing unit (Pengthamkeerati et al. 2008). Due to its complex structure, it is marked as a non-degradable, toxic, and recalcitrant pollutant. It is responsible for causing severe allergies, and skin irritation to human. Some of them were reported as carcinogens and mutagens (Wang 2008;Ling et al. 2010). It is not only concerned with human health but also a potential threat to the quality of water and disturb the aquatic life due to eutrophication, The treatment of wastewater before releasing is a major concern of the present day. Different physicochemical techniques such as chemical coagulation, oxidation, active sludge biochemical, adsorption, chlorination, and bio-degradation have been widely used for the removal of dyes. These established techniques are often unable to reduce contaminants adequately to the desired level effectively and economically. It also produces large amounts of toxic sludge, which further needs treatment. Photo-catalysis emerged as a promising technique that helped to oxidize and mineralize the organic pollutants into carbon dioxide, water, and inorganic anions successfully. Semiconductors such as titanium oxide (Sakatani et al. 2006), zinc oxide (Yu and Yu 2008, iron oxide (Wang 2007), cadmium sul de (Sakhthivel et al. 2003), and zinc sul de (Sharma et al. 2012) have been reported as potential catalysts for the successful photo-degradation of different dyes Reactive blue 19, Congo red, Eriochrome black (Kansal et.al 2007;Wang et al. 2015) has been reported. Rhodamine B is a cationic xanthene dye (Sharifzade et al. 2017), widely used in the printing, textile, photography, and pharmaceutical industries. It is also well-recognized as a water uorescent and dyes laser material because of its good stability (Al-Kahtani 2016). Photodegradation of Rhodamine B has been reported in the literature using different photocatalyst such as titanium oxide, zinc oxide, and titanium oxide-multiwall carbon nanotubes [15, (Cotto-Maldonado et al. 2013), titanium oxide and zinc oxide/β-Cyclodextrin (Rajalakshmi et al. 2017), Zinc-graphene-titanium (Nuengmatcha et al. 2016), and many others.
Gallium oxide offers several advantages being transparent, stable with remarkable thermal and electrical properties. It also exists in α, β, γ, δ, ε polymorphic forms (Stepanov et al.2016). Different methods such as sol-gel (Tsay et al. 2012), direct precipitation (Shan et al. 2017), wet chemical (Qian et al. 2008), forced hydrolysis (Kang et al. 2015), thermal evaporation (Zhang et al. 2013), hydrothermal , microwave-assisted (Deshmane et al. 2010), and electrochemical (Norizzawati et al. 2014) have been reported for the synthesis of gallium oxide. While sonochemical method  is reported as a facile, e cient, and environmentally benign route, where ultrasound waves interacted with liquid media and produce acoustic cavitation. Acoustic cavitation is the formation, growth, and collapse of microbubbles. When microbubbles collapsed generates physical (shock waves, microjets, turbulence, and shear forces) and chemical (active radical) effects. These physical and chemical changes can be utilized for different applications such as cleaning, emulsi cation, extraction, synthesis of metal oxide, nanomaterials, hybrids, polymers. Wongpisutpaisan et al. (2011) synthesized the copper oxide nanoparticle using sonication. Guo et al. (2011), synthesized titanium/graphene oxide under sonication (20 kHz) for one hour and used it as a photocatalyst for the photodegradation of methylene blue.
Similarly, Reheman et al. (2018) synthesized graphene/silver oxide quantum dot under sonication (20 kHz), and use it as a photocatalyst for the photodegradation of methylene blue. As Literature reported the remarkable application of the sonochemical method for the synthetic useful material. Therefore, the present research is designed to synthesize the gallium oxide and gallium hybrids using precipitation coupled with the sonochemical method and post-grafting method gallium hybrids. To the best of my knowledge, the present research is the rst attempt to synthesize gallium hybrid with indole group under sonication and tested as photocatalyst for photodegradation of Rhodamine B. Indole is the heterocyclic compound having excessive π-electron and have great potential for oxidative coupling with different metals such as cobalt (Liu et al. 2016), and gold ) via an electrophilic and nucleophilic substitution. It is also less toxic, and present in the structure of the natural product such as auxin, an amino acid.

Synthesis of Gallium oxide
Gallium oxide and was synthesized using precipitation coupled with the sonochemical method as reported in our previous research article (Parveen et al. 2018). Brie y, 2.5 g of gallium nitrate hydrate was dissolved in 100 mL of MilliQ water, and pH is adjusted at 8 using 10 % ammonia solution. The solution was ultra-sonicated for 1 h at room temperature under Branson Digital Soni er (S-250D, 20 kHz, and 40 W/cm 2 ). Then the obtained precipitates of gallium oxide were puri ed through washing, centrifuged (6500 rpm, 20 min), and calcined (500 °C , @10 °C /minute for 4 h). The synthesized gallium oxide was coded as G.
For surface activation, gallium oxide was charged with APTES as a bridging agent (Fernández et al. 2015;Yu et al. 2013;Iqbal et al. 2014;Devaraju et al. 2013) to enhance compatibility towards incoming indole groups. For this purpose, 0.2 g of gallium oxide was stirred with 10 % APTES for six-hour, ltered, washed repeatedly with isopropanol and ethanol to remove unreactive APTES, dried, and stored for hybrid synthesis. The activated gallium oxide was coded as AG.

Synthesis of gallium hybrids
The post-grafting method was used for the synthesis of gallium hybrids. For this purpose, 0.15 g of activated gallium oxide (AG) was dispersed in 20 mL of dichloromethane in a reagent ask with continuous stirring. An indole solution (0.30 g in 20 mL acetonitrile) was added to it and sonicate the solution for one hour using Branson Digital Soni er (S-250D, 20 kHz, and 40 W/cm 2 ). After that, the solution was left undisturbed for 60 minutes at room temperature (25 °C ). Thus gallium-indole hybrid was synthesized, ltered, dried in the air, and coded as GI. A similar procedure was adopted for the synthesis of other gallium hybrids with indole-2-carboxylic acid and 2-methylindole, separately, and coded as GCI and GMI. The prosed structure of gallium-indole, gallium-carboxylic indole, and gallium-methyl have shown in gure 1.

Photodegradation study of Rhodamine B
To test the potential application of gallium oxide and its hybrids as a photo-catalyst, the following benchscale photocatalysis setup was used (see Figure 2) with a specialized photocatalysis cell (double jacket beaker).
The photocatalysis specialized cell was equipped with a quartz window and cooling jacket. An ORIEL 100-1000 W Xenon arc lamp (wavelength =420nm; irradiance intensity = 500W/cm 2 ) was used as a visible light source and placed in front of the quartz window (radius=2.5cm) of the cell. To avoid the heating of the solution under illumination, cold water was circulated through the jacket, and the temperature of reaction solution was maintained at 22 °C . The variables such as pH (5, 7, and 9), concentration (0.01 mg/L, 0.03 mg/L and 0.05 mg/L) of adsorbate (Rhodamine B) under illumination time (120 minutes) were studied. A xed-dose of photocatalyst (30 mg) and xed volume (100 ml) of rhodamine B solution was used in each photocatalysis reaction. The qualitative analysis of Rhodamine B was monitored using a UV-Vis spectrophotometer (CARY 50 Bio UV-Visible) at regular intervals (10minutes) from 400-700 nm. The characteristic absorption peak of Rhodamine B was recorded at λ max 554 nm. The Photo-degradation rate was calculated using equation (1).
Whereas A i and A f is the initial and nal absorbance of the dye

Kinetic Study
The effect of different initial dye concentration on the degradation rate is explained well by kinetic model (Kumar et al. 2008). it is generally showed that the rate of reaction decreases with increase in concentration.
By taking the integration of above equation having C = C o at t=0 with C o being the initial concentration in the bulk solution and t the reaction time, lead to expected relation.
Where In [C 0 /C] versus time for different initial concentration of dye solution and Kapp is the apparent rst order rate constant.

Results And Discussion
The successful application of the sonochemical method opens a new window in the eld of material science for the synthesis of different novel materials. The powerful ultrasound radiation produces acoustic cavitation (formation, growth, and implosion of microbubbles) in a liquid medium. The implosion of these microbubbles generates extreme high temperature and pressure in the microscopic region (hot spots) with active radicles (hydrogen, oxygen, and hydroxyl O°, H°, OH°). These radicals are used to initiate the chemical reaction. In the present research work, gallium oxyhydroxide is synthesized when gallium hydroxide in the presence of ammonia solution sonicated for one hour. The sonication produces active dissolved oxygen and hydroxyl radicals (Bang & Suslick 2010) that react with gallium hydroxide and synthesized the gallium oxide. Similarly, sonication is also helpful in the initiation of reaction between indole groups and activated gallium oxide.
3.1. FTIR FTIR Spectrum of starting material (gallium oxide) showed characteristic oxygen-containing groups at 3440 cm -1 , 700-400 cm -1 assigned to stretching vibration of hydroxyl (O-H), and gallium oxide (Ga-O-Ga) present in the gallium oxide (Kang et a. 2016;Girija et al. 2015;Yang et al. 2009). The surface of gallium oxide is activated with 3-aminopropyltiethoxysilane (APTES) to make it compatible with the attachment of the incoming indole group. The IR spectrum of activated gallium oxide (see gure 3) showed distinct stretching vibrations of C-H, N-H, Si-O-Si at 2900 cm -1 , 3500-3300 cm -1 , and 1034 cm -1 region (Kim & Kown 2017). The broadening of the OH group occurred due to the overlapping of the amine group.
Furthermore, bending absorption bands of NH, and Si-C were also noticed at 1566 cm -1 and 1330 cm -1 .
The APTES is successfully attached to the surface of gallium oxide.
IR spectrum of gallium hybrids (see Figure 4) showed the distinct absorption bands of -CH (2900 cm -1 ), -C=C (1480-1400 cm −1 ), C-N (1340-1314 cm −1 ), C-O (1225 cm −1 ), C=O (2392-2341 cm −1 ), and -NH (1600-1400 cm −1 ). These additional absorption bands con rmed the attachment of indole with Ga-O (1132-1117 cm −1 ) and Si-O (1030-1008 cm −1 ) repeating units. Our study agrees with (Kumar 2015). It is also interesting to note that the benzene ring remains intact during hybrid synthesis . It means that metal bonding occurred at the active site (C 2 and C 3 ) of the pyrrole ring of indole (see gure 4). The coordination of different metals with indole is also investigated by other researchers (Liu et al. 2016) and(Gomez et al. 2012). However, methyl and carboxylic substituted indole group at C 2 position provided the additional attachment sites and gave more intense stretching and bending absorption bands between 1200-1500 cm −1 .

SEM
SEM images of gallium oxide showed the well-de ned rectangular-shaped micro-rods (see Figure 5) containing void spaces. These void spaces showed the sorption site where the catalytic reaction took place.
After activation with APTES, the surface of gallium oxide showed smoothness and homogeneity. A slight distortion has been noticed in the morphology after hybrid synthesis. It is likely to observe that the indole group alter the morphology of gallium oxide and arranged the particles in the form of a chain through silane bridging. The methyl and carboxylic group substituted indole further modi ed particles and stacking the particles together (see Figure 6). The reorientation of micro-rods resulted in the formation of the clumps. Hence, the effect of change in morphology is re ected in the functioning of the material. It showed that gallium hybrids adhere to the synergic property of gallium oxide and indole.

BET
Adsorption-desorption of nitrogen onto gallium oxide followed adsorption type-IV (see supplementary information, S1). BET isotherm proposing multilayer adsorption forming a hysteresis loop (H3) which is the signature of mesoporous particles of gallium oxide according to the IUPAC classi cation 1985 (Sing 1985). The maximum speci c surface area, pore-volume, and pore diameter calculated are 26 m 2 /g, 0.15 cm 3 /g, and 34 nm, respectively. While the BET isotherm of gallium hybrids showed similar characteristics as noted earlier for gallium oxide, i.e., hysteresis loop (H3), adsorption type-IV (Zhao et al. 2007) as shown in gure 8 b, c, d. However, an increase in surface area is noted from 26 m 2 /g (gallium oxide) to 31 m 2 /g (gallium-indole), 35 m 2 /g (gallium-methyl indole), and 37 m 2 /g (gallium-carboxylic indole).

TGA
TGA of gallium oxide showed two-step decomposition of 2 % and 10 % mass loss, whereas holding 88 % residue depicted the thermal stability of gallium oxide (see supplementary information, S2). The results are encouraging for its application under high temperatures. While the Thermal stability of gallium hybrids was also determined. TG curve of gallium hybrids (see supplementary information, S2) showed an initial mass loss due to volatile gases and water of hydration up to 100 °C , followed by dissociation of organic from inorganic substituents up to 600 °C . In particular, each indole group revealed different weight loss, i.e., 8 %, 10 %, and 8 % for gallium-indole (GI), gallium-carboxylic indole (GCI), and galliummethyl indole (GMI), respectively. The thermal stability sequence follows GCI > GI > GMI.
3.6. XPS XPS survey spectrum (see supplementary le S3) of gallium oxide showed the peak of gallium (Ga 2p 1/2 , Ga2p3 /2 ), and oxygen (O 1s) peak at 1180 eV and 531 eV, con rmed the successful synthesis of Ga 2 O 3 (Bharat et al. 2016). Whereas peak at 397 eV and 285 eV corresponds to N 1s and C 1s as traces (Colombo et al. 2017).The high resolution deconvoluted XPS spectrum showed gallium binding with and oxygen at ∼530.4 eV and ∼531.8 eV corresponding to Ga-O and Ga-OH. XPS survey spectra of gallium hybrids also displayed distinct peaks of O1s, C1s, N1s, and Ga2p at 531 eV, 285 eV, 399 eV, and 1180 eV with enhanced intensity (see supplementary le S3). The high-resolution deconvoluted XPS spectra of gallium hybrids showed distinct binding with its surrounding elements (see supplementary data, S4). The  Table 1 showed the elemental composition and percent weight of gallium oxide and gallium hybrids. The potential of gallium oxide and gallium hybrids as photocatalyst were tested in a batch mode as a function of concentration (0.01 mg/L, 0.03 mg/L, and 0.05 mg/L), pH (5 as acidic, 7 as neutral, 9 as basic), and illumination time (120 minutes) towards Rhodamine B (Rh-B).
Three preliminary experiments were also conducted for 20 minutes to clarify the role of light, and catalyst in the degradation process. (a) Dark reaction (gallium hybrids without visible light illumination), (b) Photolysis (only visible light illumination), (c) photo-catalysis reaction (gallium hybrids with visible light illumination). It has been observed that the better catalytic degradation of Rhodamine B was achieved in photo-reaction (18-36 %) over dark reaction (12-25 %) and photolysis (8-17 %) at pH 7 and 0.03 mg/L as shown in Figure 8. Thus, Photoreaction was further continued to evaluate the maximum degradation capability.

Gallium oxide as Photocatalyst
Gallium oxide showed 30 % photodegradation of Rh-B at 0.03 mg/L while 47 % at pH 7 as shown in gure 9. It was found that photodegradation of rhodamine B was gradually increased with an increase in illumination time. When more light falls on the catalyst surface, it enhanced the formation of more photo excited species, increased the photodegradation. The decrease in the photodegradation rate at high concentrations (0.05 mg/L) is due to less penetration of photons into the dye solution. The same trend was also observed in another study (Al-Kahtani 2016). pH is another important variable that affects the photodegradation process. It changed the surface chemistry of photocatalyst and dye molecule that shifts the photodegradation rate. The surface of gallium oxide and Rhodamine B at pH 5 gets protonated and acts as Lewis acid. So, less photodegradation (37 %) is achieved due to the electrostatic repulsion.
The deprotonation of Rhodamine B at pH 9 resulted in the formation of a zwitter (RB + ) ion that attracted by the surface of the catalyst, the presence of hydroxyl radicle is also facilitated the dye degradation up to 40%. The same trend was also observed by Hariprasad et al. (2013). The maximum photodegradation (48 %) is achieved at pH 7 because of the unlike charge on both catalyst surface and dye molecule responsible for strong electrostatic attraction as well as less hindrance for photo-excited species to attacked and degraded the dye easily (see gure 10).

Gallium hybrids as Photocatalyst
Gallium hybrids showed 44-58 % photodegradation of Rhodamine B at 0.03 mg/L within 120 min. while 59-72 % photodegradation of dye is achieved at pH 7(see gure 9). The photodegradation of Rh-B is enhanced by gallium hybrid as compared to gallium oxide. It is due to the binding of the indole group onto gallium oxide. Upon exposure to light, the indole group infused more photo-generated electrons into the conduction band of gallium oxide. Our nding is further supported by Kamil et al., (2018), who studied the degradation of Bismarck brown R using hybrid (multi-wall carbon nanotubes-titanium oxide) . He found that multi-wall carbon nanotubes as an electron promoter in the conduction band of titanium oxide-forming superoxide anion radicals. Rhodamine B showed optimal degradation at pH 7 due to electrostatic attraction between negatively charged catalyst surface and dye molecule. The deprotonation of Rhodamine B formed zwitter (RB + ) ion and the presence of hydroxyl radicle facilitates the degradation process (Nagaraja et al. 2012). However, in an acidic environment surface, both gallium hybrid surface of catalyst and dye molecule gets protonated and coulombic repulsion reduced the degradation process (see gure 10). Different researchers also mentioned the potential of gallium hybrid/composites for dye degradation (Banerjee et al. 2012;Kim et al. 2016;Das et al. 2019;Reddy et al. 2016).

Mechanism of Photocatalysis
When gallium hybrids were irradiated with visible light, the electron from the valence band of the catalysts promoted to the conduction band leaving holes in the valence band and excited electrons to the conduction band. The photoelectrons on the conduction band were scavenged by oxygen to produce reactive oxygen radicals (O°), whereas the holes in the valence band become trapped by the surface bounded hydroxyl radicals which produced on oxidation of either the surface hydroxyl group (OH) or water molecules (H 2 O). These hydroxyl radicals have high oxidation potential used to oxidation of dye [14] into non-hazardous by-products (CO 2 and H 2 O). The mechanisms of photodegradation are illustrated in Figure 11.
The photocatalytic activity of gallium oxide was enhanced after its binding with the indole group. Indole group helped to infuse more photo-generated electrons into the conduction band of gallium oxide upon exposure to light. Furthermore, the indole group also created defects that reduce the recombination of electrons (e -) and holes (h + ) that pronounced the photodegradation process. The band gap of all synthesized gallium hybrids was also calculated using the Tauc plot method (see the supplementary le, S6).

Kinetic Study
This study followed the pseudo rst order reaction (Kumar et al. 2007), the kinetic parameters are presented here in the tabulated form. The value of apparent rate constant is obtained from regression analysis of the linear curve of the plot.
The apparent rate constant clearly showed that the rate of degradation increases with the increase in initial concentration. While at high concentrations (0.05mg/L), the catalytic reaction becomes reduced due to less penetration of photons into the solution. The same trend was also observed in another study (Al-Kahtani 2016).

Conclusions
Synthesis of gallium oxide and gallium hybrids was successfully synthesized using precipitation coupled with the sonochemical method. The sonochemical method is emerged as a facile, and the time-e cient method which requires no additional chemicals during synthesis. Similarly, the post-grafting method is proved to be an e cient method for the synthesis of gallium-indole hybrids. Upon exposure to light, the indole group helped to infuse more photo-generated electrons into the conduction band of gallium oxide, which helps the photo-degradation of Rhodamine B up to 72 % at 0.03mg/L within 20 minutes under visible light. Pseudo rst order kinetic is tted well to experimental data with R 2 >0.9.

Declarations
Ethical Approval: The authors declare that there is no nancial and personal con ict of interest that could appear to in uence the work reported in this paper.
Consent to Participate: All Authors declare that they equally participated in the research study and development of article.
Consent to Publish: All the authors read the nal version of article and give consent to publish the article to be published in Environmental Science and pollution Research journal. Rawalpindi.
Competing interest: The authors have no competing interests to declare that are relevant to the content of this article.
Availability of data and materials: The authors con rm that the data supporting the ndings of this study are available within the article and its supplementary material. Raw data that supports the ndings of this study are available from the corresponding author, upon reasonable request.