Boron doped graphene oxide with enhanced

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Jun 4, 2018 - oxygen functional groups can be present in or on the surface of boron doped reduced graphene oxide (BR-GO) as shown in Fig. 1. BR-GO is.
Journal of Photochemistry & Photobiology A: Chemistry 364 (2018) 130–139

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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Boron doped graphene oxide with enhanced photocatalytic activity for organic pollutants Manmeet Singha, Sandeep Kaushala, Pritpal Singha, Jeewan Sharmab, a b

T



Department of Chemistry, Sri Guru Granth Sahib World University, Fatehgarh Sahib 140407, Punjab, India Department of Nanotechnology, Sri Guru Granth Sahib World University, Fatehgarh Sahib 140407, Punjab, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Photocatalysis Boron doped reduced graphene oxide Dye degradation Graphene oxide

In present study, boron doped reduced graphene oxide (BR-GO) is synthesized by reducing graphene oxide (GO) in the presence of boric acid, and employed as a photocatalyst for the degradation of organic pollutants methyl orange (MO) and methylene blue (MB). GO and BR-GO are systematically characterized by X-ray diffraction, Raman spectroscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and UV–vis spectroscopy. The modifications in the structure of GO are confirmed. The BR-GO has sheet structure with various graphene islands and disordered regions. The electrical conductivity has been measured using twoprobe method. The conductance is explained by two-dimensional variable range hopping model. The photocatalytic activity of GO is enhanced on doping with boron due to increase in the density of states value near Fermi level. The adsorption mechanism and kinetic studies for the degradation of MB and MO dyes suggest that the photocatalytic degradation follows a pseudo-second order mechanism.

1. Introduction Graphene oxide (GO) is a graphene sheet with carboxylic groups at its edges, and phenol, hydroxyl and epoxide groups on its basal plane [1,2]. GO has been used as a precursor for the synthesis of graphene based composites due to its flexible solution processibility [3]. The graphene based composites have wide potential applications like conductive thin film, nano sensor, super capacitors, nano electronics and nano medicine, due to their unique mechanical, thermal and electronic properties [4]. The electronic properties of graphene based conductive materials have been tailored by chemical doping. Various non-metals such as S [5], P [6], Si [7], I [8], and many different metals have been doped in graphene [9]. In comparison with other dopants, N or B doped materials have revealed pronounced improvement in electrical properties in comparison to those of materials without doping [10]. Due to size compatibility of boron and carbon atoms, boron can be easily doped by substituting carbon atom in the lattice structure of GO. Boron has electron deficiency and thus, its doping is anticipated to enhance p-type conducting behavior of pure graphene, leading to its applications in nano electronic devices [11], sensors [12], energy generation [13], storage [14], biomedicine [15] and photocatalysis [16]. During reduction or doping with boron, some defects are introduced and the oxygen functional groups can be present in or on the surface of boron



Corresponding author. E-mail address: [email protected] (J. Sharma).

https://doi.org/10.1016/j.jphotochem.2018.06.002 Received 25 April 2018; Received in revised form 19 May 2018; Accepted 1 June 2018 Available online 04 June 2018 1010-6030/ © 2018 Elsevier B.V. All rights reserved.

doped reduced graphene oxide (BR-GO) as shown in Fig. 1. BR-GO is advantageous with many potential applications [17], apart from photocatalytic degradation usability. Gualdron et al. studied the photocatalytic and photoelectrocatalytic activity of B-Doped graphene modified TiO2 thin films [17a]. Mombeshora and Nyamori explored the physical and chemical transformation of reduced graphene oxide samples with regards to use of different reducing agents and their appropriateness in electrochemical capacitors [17b]. Sahoo et al. used boron doped graphene–SnO2 as anode in Li ion batteries [17c]. The waste water containing dyes from rubber, textile, paper, cosmetic, plastic and pharmaceutical industries is harmful for aquatic life, plants, animals and humans. The ever increasing volume of such toxic and non-biodegradable effluents has become a major threat to humans and ecology. Methyl orange (MO) and methylene blue (MB) are the two widely used dyes and are a major source of pollution in industrial activities [18]. Various physiochemical methods such as advanced oxidation and biological processes, membrane separation, coagulation, ozonation, electrochemical and adsorption techniques are used for the removal of dyes from waste water [19]. Photocatalytic degradation is one of the promising methods for removal of dyes and has gained much attention, owing to its high efficiency and energy economy [20]. Recently, use of BR-GO for photo reduction of CO2 is highlighted in literature [21]. Xing and co-workers synthesized boron doped graphene nanoribbons and investigated their photocatalytic activity for

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Fig. 1. Structural transformation of GO to BR-GO.

for all films presented here. Typically, the substrate was completely covered with sufficient amount of GO and BR-GO solution, allowed to stand for 60 s and spin coated at 500, 800, and 1600 rpm for 30 s each using Hind HIVAC spin coater (BC-300). After spin coating the GO and BR-GO films were dried at 100 °C in a vacuum oven and stored in ambient conditions in sealed plastic containers.

degradation of Rhodamine B dye. A complete degradation of dye in 120 min was observed [22]. Zang et al. provide detail study on graphene-based photocatalysis. They discussed about multifarious roles of graphene in enhancing the photocatalytic performance of TiO2 [23]. Han et al. reviewed the structural diversity of graphene materials and their multi-functionality in heterogeneous photocatalysis [24,25]. In the present study, nanosized BR-GO has been synthesized by modified Hummer’s method and its photocatalytic activity was studied after doping with boron. BR-GO exhibits higher photocatalytic activity than pure GO for degradation of cationic dye as BR-GO has semi-metallic and p-type properties which are more favorable for the degradation of MB (cationic dye) than MO(anionic dye).

2.4. Characterization X-ray diffraction pattern of synthesized samples were obtained using X’pert PRO X-ray diffractrometer in 2θ range from 10° to 90° with CuKα radiations (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) is carried out using ESCA+ (omicron nanotechnology, Oxford Instrument Germany) equipped with monochromator aluminum Source (Al ka radiation hv = 1486.7 eV). The instrument was operated at 15 kV and 20 mA. Angle between analyzer to source is 90°. Resolution is confirmed by FWMH about 0.60 eV. Raman spectroscopy was used to monitor structural changes during oxidation reduction process and degree of defects in GO and BR-GO. Dry powdered samples were analyzed on a laser Raman spectrometer (in-via confocal Raman microscope, Renishaw plc, UK). The spectra were acquired using a green laser with a microscope focused beam of 532 nm wavelength and 50 mW laser power with single continuous acquisition and 80 s exposure time. The specific surface area and textural parameters of as-prepared GO and BR-GO samples were investigated by BET method using Nitrogen adsorption system (Microtrac Belsorp Mini-II). Elemental composition analysis of the samples was carried out using FE-SEM equipped with an energy dispersive X-ray (EDS) spectrometer (HITACHI, SU8010 electron microscope). The sample morphology, microstructure and measurement of lattice-fringe spacing were investigated using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HR-TEM, Tecnai G2 20, 200 KeV FEI). The UV–vis spectrometer was operated between 200–800 (nm) wavelength (Model. UV2600, SHIMADZU copp 01197).

2. Experimental details 2.1. Materials Natural graphite powder was used as precursor for GO preparation. Potassium permanganate (KMnO4), sodium hydroxide (NaOH), hydrogen peroxide (H2O2, 30%), sodium nitrate (NaNO3) and hydrochloric acid (HCl) were used as oxidizing and reducing agents. H3BO3 was used as boron dopant. Organic dyes, MB and MO were used for photocatalytic degradation study. Aqueous solutions were prepared using double distilled water. All the chemicals were purchased from Sigma-Aldrich, USA (Analytical grade). 2.2. Synthesis of GO and BR-GO GO was synthesized from natural graphite using modified Hummer’s method [26]. As prepared GO was used for the synthesis of BR-GO. 5 mL GO (0.416 M) was dispersed in 20 mL of double distilled water and ultra sonicated for 1 h. After sonication, 0.4 mg H3BO3 was added in the solution under vigorous stirring for another 1 h. Then the solution was heated in oven at 60 °C for 12 h. The sample turned into brown colored flakes after drying, the flakes were annealed under vacuum at 300 °C for 3 h. After annealing, the color of the flakes changed from brown to black. The black flakes were dispersed in 20 mL double distilled water and ultra sonicated for 1 h. To wash out boron oxide, 30 mL of HCl (1 M) was added to sonicated solution under vigorous stirring for 15 h. The flakes were washed with double distilled water. Finally, the sample was dispersed in 5 mL double distilled water and stored.

2.5. Photocatalytic activity measurements The photocatalytic degradation of dye has been investigated using UV–vis spectrophotometer. The photocatalytic activities of GO and BRGO were investigated in terms of the degradation of MO and MB in water in UV–vis region. In this process, 25 mg sample of GO and BR-GO each was added separately in 100 ml of 10 ppm solution of each dye. These mixtures of dye solutions and catalyst suspensions were stirred with magnetic stirrer for 30 min and placed in dark to attain the adsorption–desorption equilibrium. The experiments were performed in a reactor of 1000 mL capacity. A 175 W metal halide lamp (Philips) positioned inside a cylindrical vessel surrounded by a jacket was used as light source. For sieving the UV emission of the lamp and cooling the

2.3. Synthesis of thin films Thin film deposition by spin-coating, drop-casting and solvent-induced precipitation revealed that spin coating produces more uniform and thinner films than the other methods, and this technique was used 131

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‘B 1s’ peak detected for BR-GO indicates that GO is successfully doped with boron. The presence of oxygenated functional groups such as carbonyl (eC]O), hydroxyl (eOH), carboxyl (eCOOH) and epoxide (CeOeC) is well-known in functionalized GO. Xu et al. observed a loss of oxygen containing functional groups in XPS of reduced graphene in comparison to functionalize GO. Similar XPS results of GO and BR-GO are also obtained earlier by other researchers (Xu et al.) [30]. Abundant oxygen functionalized groups are observed for GO in the previous study [31]. However, in the present study, a significant loss of oxygen functionalized peaks of C]O, CeOeC, CeO is observed. The comparison with literature clearly confirms significant reduction of GO after born doping. Fig. 3d shows the XPS survey spectra of BR-GO which confirms the doping of boron via hydrothermal process. Raman spectroscopy has been used to characterize the structural changes occurring during the synthesis of GO (oxidation) and BR-GO (reduction). As reported earlier, the pristine graphite has a prominent G band peak at 1566 cm−1 and negligibly small D band peak at 1340 cm−1 [32]. Fig. 4 shows the Raman spectra of GO and BR-GO. In spectrum of GO, D band peak at 1347 cm−1 is intensified and G band peak is shifted to 1592 cm−1. This behavior can be attributed to structural defects induced by oxygen functional groups on oxidation. Kudin et al. suggested that this blue shift in G band might be due to activation and merging of Raman-inactive D band with G band [33]. The spectrum of BR-GO show D band peak at 1343 cm−1 and G band peak at 1578 cm−1. Clearly, there is a red shift in G band w.r.t. GO, confirming the reduction of GO. This type of red shift has been observed by other researchers also [34]. The relative intensity of D and G band (ID/IG) gives an estimation of disorder level in graphene type materials. In GO, the intensity of D band is less than G band and ID/IG = 0.78. This confirms that attachment of oxygen functional groups to graphite layer affects the structural properties of graphite. In BR-GO, the intensity of D band is greater than G band with ID/IG = 1.06. By reducing the defective GO, the ID/IG value increases due to elimination of defects induced by oxygen functional groups. The morphology and structure of GO and BR-GO are investigated through TEM and HR-TEM observations. A typical TEM image of GO nanostructures prepared by modified Hummer’s method is shown in Fig. 5a [35]. It reveals two-dimensional, wrinkled and undulated structure due to deformation upon exfoliation and restacking processes [36]. The TEM image of BR-GO (Fig. 5b) represents a thin flexible sheet-like structure with more corrugations than GO. Energy dispersive X-ray spectroscopic (EDS) analysis has been performed to estimate the elemental composition of BR-GO. The EDS spectrum confirms the occurrence of C, O and doping of boron in GO (Inset Fig. 5b). The elemental composition obtained by EDS has been provided as supplement (Figure S1). The absence of other adhering impurities in the samples is also evident. As displayed in the HR-TEM image (Fig. 5c), a BR-GO sheet contains boron doped graphene islands separated by disordered regions. Others have also observed similar type of islands in boron doped graphene oxide [37]. The magnified view of one such island (Fig. 5d) reveals highly oriented crystal structure with an interplanar spacing of 0.33 nm, corresponding to that of graphene (002) planes. (Fig. 5e) shows the corresponding ring like SAED patterns, confirming the polycrystalline nature of BR-GO. Fig. 6 shows the UV–vis spectra of GO and BR-GO. Pure GO shows absorption peak at ∼ 232 nm corresponding to the π→π* transition of the CeC bonds along with another peak at ∼ 303 nm, which corresponds to the n→π* transition of the CeO bonds [38]. However, in BRGO, the peak corresponding to the π→π* transition has been shifted to ∼ 268 nm suggesting the restoration of electronic conjugation on reduction [39]. The band gap values of GO and BR-GO had been calculated from absorbance data using Tauc-plot (Fig. S2). The indirect band gap of GO and BR-GO is 2.8 eV and 3.00 eV [40]. It is clear from the above discussion that good a quality GO and BR-GO have been synthesized and successfully reduced using boron.

reactor to 20 °C, NaNO2 solution was circulated through the jacket. The test solutions were then exposed to ultraviolet radiation. The samples from exposed test solution were taken out at regular intervals, centrifuged for 10 min at 3500–4000 rpm, and their absorbance was recorded for MO and MB dyes using spectrophotometer. Degradation efficiency for dye was calculated by using the following formula:

Percentage of dye degraded =

(C0 − Ct ) × 100 C0

(1)

where, Co and Ct are the initial and final concentrations of dye, respectively. 2.6. Electrical measurements The electrical measurements were carried out in a metallic sample holder in which two-probe geometry was used. Planar geometry of films with breadth 2.5 cm and electrode gap 0.5 cm was used and thick indium electrodes were deposited on films. Digital Picoammeter (DPM – 111 Model) and a constant voltage supply (EHT – 11 Model) were used for these measurements. 3. Results and discussion The XRD spectra of GO and BR-GO have been given in Fig. 2. For comparison, the standard intensities of pristine graphite are also shown in this figure (JCPDS 01-075-2078). Graphite shows a sharp peak at 2θ = 26.55° due to (002) planes with basal spacing of 3.35 Å. In GO, there is an additional dominant peak at lower angle (2θ = 10.24°), which refers to an interlayer spacing of 8.64 Å. A small peak very close to (002) peak of pristine graphite at 2θ = 26.59° with d spacing of 3.35 Å has also been observed. The increase in d spacing (3.35 Å–8.64 Å) can be attributed to intercalation of water molecules and to oxygen containing functional groups introduced on oxidation [27]. In contrast to these, BR-GO has a broad and low intensity peak around 2θ = 25.25°, corresponding to d spacing of 3.52 Å. The disappearance of peak at 2θ = 11.71° as compared to GO and decrease in d values close to pristine confirms that oxygen containing functional groups have been removed in BR-GO. Some researchers have also suggested that in reduced GO, graphene nanosheets exfoliate into few layers or a monolayer, resulting in significantly different structure than pristine and GO [28]. To investigate the elemental composition of BR-GO, X-ray photoelectron spectroscopy (XPS) has been performed (Fig. 3). It is evident from detailed ‘C 1s’ spectra that it decomposes into the dominant peak at CeC (283.81 eV) followed by C]O (288.02 eV), OeC]O (290.70 eV) and CeO at 285.01 eV. As shown in Fig. 3a, the results are in agreement with previous literature reports [29]. In Fig. 3b, the peaks of BC3 (191.77 eV), BC2O (193.01 eV) and BCO2 (193.61 eV) clearly indicate the bonding of doped boron to sp2, sp3 carbon atoms. The XPS

Fig. 2. Diffraction patterns of BR-GO, GO and graphite. 132

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Fig. 3. C 1s, B 1s, O 1s and survey spectrum XPS of BR-GO.

decomposition effect at dark state has been observed for both the cases. This may be due to an adsorption effect on the catalyst. It has been observed that the decomposition effect is more prominent in case of BRGO that can be attributed to enhancement in adsorption due to more disordered regions in BR-GO as compared to GO. To support this effect, N2 gas adsorption and desorption isotherms of GO and BR-GO have been represented in Fig. S5. The photocatalysis is a surface reaction and mostly occurs on the surface of catalyst. The texture of the material plays important role in its adsorption ability [42]. The comparison of photocatalytic behavior of GO and BR-GO with commercial P25 TiO2, under UV light, has been represented in Table 1. It is clear that the degradation efficiency of GO is better than P25 for both MB and MO dyes [40,41]. The efficiency is further enhanced when GO is doped with B. Generally, the open space between the two dimensional nano-sheets is responsible for the maximum specific surface area, pore volume and pore diameter of GO and BR-GO. GO has specific surface area, pore volume and average pore size equals to 49.3 m2g−1, 0.064 cm3g−1 and 4.94 nm, respectively. These parameters improved for boron incorporated graphene oxide. BR-GO exhibits specific surface area, pore volume and average pore size equals to 56.6 m2g−1, 0.197 cm3g−1 and 19.98 nm, respectively. These results reflect the increase in adsorption of BR-GO, which is the cause for good photocatalytic behavior of BR-GO for MB and MO dyes. To study the photocatalytic behavior of samples the degradation rate was normalized. The degradation of MO has been shown in Fig. 8. These results indicate that the efficiency of the prepared GO catalyst is enhanced by doping of boron in case of donor cationic dyes. During the photocatalytic reaction over GO and BR-GO, the absorption intensity of MO and MB decreases and no other absorption peak appears, indicating the successful degradation of MO and MB dyes. As shown in Fig. 7b, MB dye degraded completely in just 50 min over BR-GO and is 70% degraded in the same time period with sole GO under identical reaction conditions. Under UV light irrediation (Fig. 8b), almost 100% of the MO dye has been degraded within 100 min by BR-GO whereas only 50% of dye has been degraded by GO under similar reaction conditions. There is an enhancement in photocatalytic activity for both anionic

Fig. 4. Raman spectra of GO and BR-GO.

3.1. Photocatalytic degradation of MO and MB The dye degradation process of MB and MO was investigated by measuring the time-dependent UV–vis absorption spectra of the dyecatalyst solution. Complete degradation of MB is shown in Fig. S3, ESI. Similarly, complete time-dependent degradation of MO is shown in Figure S4, ESI. Fig. 7 shows the MB degradation results for light exposure duration ranging from 0 to 70 min and 0 to 50 min in presence of GO and BR-GO, respectively. The degree of photocatalytic MB degradation as a function of time (Ct/Co) was calculated with respect to the maximum absorbance at 664 nm. To check the self-photosensitization process, blank experiment for dyes was performed in the absence of catalyst under UV–vis light. During the blank experiment, slight degradation was observed as shown in Fig. 7a and 8a [41]. In comparison, when the catalyst was added to the solution, noticeable dye degradation was observed, which confirms that the degradation of MB can be attributed mostly to a photocatalytic process. Similar results are observed in the case of MO dye when it was exposed to light for 160 min and 100 min in presence of GO and BR-GO, respectively (Fig. 8). A 133

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Fig. 5. (a, b) TEM images of GO; (c, d) HR-TEM images of BR-GO; (e) SAED patterns of BR-GO and Inset (b) EDS spectrum of BR-GO.

photocatalytic activity than undoped reduced graphene oxide towards Rhodamine B (RhB) degradation under visible light irradiation [50]. Stability of the samples towards photocatalytic degradation of MB has also been examined by the successive recycle test over the optimal sample BR-GO. As shown in Fig. 9a, after each test the photocatalyst was reused after washing with ethanol and dried at 60 °C while keeping other factors identical. The time course of MB decolorization during three consecutive cycles under UV-light is shown in Fig. 9a and it is clear that no obvious loss of the activity for MB decolorization is observed during three cycles, indicating that the BR-GO possesses excellent long term stability. The structural changes of BR-GO before and after three consecutive cycles are also recorded by XRD (Fig. 9b), which shows that no structural transformation has happened [51]. Fig. 6. UV–vis absorption spectra of GO and BR-GO.

3.2. Mechanism The charge separation occurs after the exposure, of the surface, of catalyst (BR-GO) by UV-light. The electrons and holes may migrate to the catalyst surface where they participate in redox reaction with sorbed species (dye particles). As BR-GO is a p-type semiconductor, there is a majority of holes (h+) in valance band (VB) and less number of electrons (e−) in conduction band (CB). Holes at valence band may react with surface bounded H2O or OH− to produce the hydroxyl radicals, and electrons at conduction band are picked up by oxygen to generate superoxide radical anions (O2−) as indicated in the following equations:

(MO) and cationic (MB) dyes by BR-GO in comparison to GO. It is reported that the electronic properties of GO are similar to those of graphite oxide and both of them behave as semiconducting materials. Similarly, the BR-GO is a form of reduced GO and it also behaves as a semiconductor owing to its incomplete reduction in vacuum. The low transfer efficiency of photo-produced electrons in GO is responsible for its low photocatalytic activity in comparison to BR-GO. Electrical conductivity and work function of graphene can increase with doping of boron due to increase in density of state value near Fermi level [49], which is useful for the photo-generated charge carrier transfer in graphene-based photocatalyst. Tang et al. reported that the boron-doped reduced graphene oxide is able to exhibit significantly higher

BR-GO + UV light → e− (CB) + h+ (VB) 134

(2)

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Fig. 7. (a) Variation of concentration and (b) degradation efficiency with irradiation time for MB dye.

O2 + e− (CB) → O2• −

(3)

(H2 O⇔ H+ + OH −)ads + h+ (VB) → •O H+ H+

(4)

R + •O H→ R• + + H2 O

(5)

R + h+ → R• − → degradation products

(6)

3.3. Hopping model The room temperature (310 K) conductivity of GO and BR-GO has been calculated to be 2.84 × 10−7 Ώ−1cm−1and 1.07 × 10−6 Ώ−1cm−1, respectively. Clearly the conductivity of BR-GO is larger than that of GO. This may be attributed to various domains formed on doping. To study the conduction behavior in these materials, the variation of dark conductivity (σd) with temperature can be represented by Arrhenius equation:

When BR-GO is irradiated under UV light, the formation of superoxide radical anions takes place followed by neutralization of OH– group into %OH by the holes. It has been suggested that the hydroxyl radical (%OH) and superoxide radical anions (O2−) are the primary oxidizing species. The difference in donor acceptor nature of both dyes, MB and MO, is the main cause of difference in their degradation behavior and time. As MB is an electron donor and reacts properly with holes of VB which are formed in large quantity due to p-type behavior of the catalyst. Whereas, MO is an electron acceptor dye and reacts with electrons at the CB. Attributed to the different behavior of both dyes, they show different photocatalytic degradation by catalysts. These oxidative reactions would result in the degradation of pollutants as shown in the following Eqs. (5) and (6). Then, oxidation reaction of the organic pollutants takes place via successive attack by %OH radicals (Fig. 10). The enhancement in photocatalytic degradation of dyes was also contributed by the photosensitization of dyes towards catalysts. After the absorption of UV-light by dye molecule, sensitization occurs. The dye electrons were excited from HOMO to LUMO and these electrons were transferred to conduction band of catalysts (GO and BR-GO). It eventually induced degradation of dyes on GO and BR-GO surface, due to the formation of superoxide radicals. These radicals produced during dye sensitization mechanism, enhance the degradation of dye molecules [52]. These results are in well agreement with the findings of Koh et al. They demonstrated photosensitization mechanism of MB dye towards TiO2 which contributed in the photocatalytic degradation of dye [53].

E σd = σ0 exp ⎛− a ⎞ ⎝ kT ⎠

(7)

where, Ea represents activation energy and k symbolizes Boltzmann’s constant. σd is temperature dependent and this variation has been presented in Fig. 11 for GO and BR-GO thin films. There are two linear portions in lnσd vs. 1000/T plots of BR-GO. The activation energy in dark (Ea) is estimated utilizing the slopes of (Fig. 11). The lower values of activation energy (in BR-GO) for lower temperatures indicate hoping as the most probable mechanism of the transport [54]. The BR-GO sheet has various graphene islands and disordered regions. As reported earlier, the carriers in these islands behave as delocalized state carriers, but for disordered regions with localized states, conduction is mainly realized by hopping. As shown in Fig. 5d, there are graphene islands in BRGO. These islands have delocalized states and the conductance in these regions can be realized by two dimensional variable range hoping model [55]. The conduction of a BR-GO sheet can be explained by the two-dimensional variable range hopping (2D VRH) mechanism with expression:

B σd = σ0 exp ⎛− 1/3 ⎞ ⎝ T ⎠

(8)

where, B is hopping parameter which depends on the density of states N (Ef) near the Fermi level and localization length Ll of the electronic wave functions. For 2D VRH, the dependence is given by following relation:

Fig. 8. (a) Variation of concentration and (b) degradation efficiency with irradiation time for MO dye. 135

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Table 1 Comparison of GO and BR-GO photocatalyst with commercial P25 TiO2. S. No.

Photocatalyst

1 2 3 4 5 6 7 8 9 10 11

Degussa Degussa Degussa Degussa Degussa Degussa Degussa GO BR-GO GO BR-GO

TiO2 TiO2 TiO2 TiO2 TiO2 TiO2 TiO2

P25 P25 P25 P25 P25 P25 P25

Degradation under UV- light (%)

Time (min)

Dye

Dye Conc.

Photocatalyst Dose

Ref.

10 47 99 91 89 98 25 98 98 99 99

180 360 80 180 120 40 120 160 100 70 50

MO MB 1,4-dioxane MO MB MO R6G MO MO MB MB

20 mg/L 10−5 M 27.8 mg/L 50 mg/L 25 mg/L 20 mg/L 10 mg/0.5 L 10 ppm 10 ppm 10 ppm 10 ppm

0.04 g/150 ml 0.0375 g 7.0g 0.8 g/1L 0.12 g/0.2L 2 g/1 L 5 mg/0.05 L 25 mg/0.1L 25 mg/0.1 L 25 mg/0.1 L 25 mg/0.1L

[43] [26] [44] [45] [46] [47] [48] Present Present Present Present

study study study study

Fig. 9. (a) Three repeated processes of BR-GO catalyst for degradation of MB dye under UV-light irradiation and (b) XRD pattern for BR-GO before and after reaction.

Fig. 10. Schematic representation of the complete degradation process. 1/3

3 ⎞ B = ⎜⎛ ⎟ 2 kN ( E F ) L1 ⎠ ⎝

value of B has been calculated to be 203.13. (Kaiser et al. also explained hopping method) [56].

(9)

3.4. Kinetics of photocatalytic degradation process

where, k is Boltzmann‘s constant. With increase in temperature, increased carrier mobility is expected, which is the dominant factor responsible for enhancement in photocatalytic degradation. Inset of Fig. 11 represents variation of Lnσd with T−1/3. The straight fitting of this plot also confirms VRH in BR-GO. From the slopes of graph the

Different kinetic models can be applied to study the kinetics of adsorption process. These models include pseudo-first order model given by Langmuir and pseudo-second order model given by Mckey. 136

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Fig. 11. Variation of lnσd with temperature. Inset: Lnσd vs. T−1/3 plot for BRGO thin film.

Fig. 13. Pseudo-second order reaction kinetics for MO and MB dyes with different catalysts.

The value of rate constant K1 is obtained from the slope of the curve and the value of qe is calculated from the intercept of the curve. The coordination coefficient R2 is obtained from the linear fitting of the curves. The calculated values of K1, qe and R2 are given in Table 2. The pseudo-second order rate equation has been applied to MO and MB dye systems. The pseudo-second order rate equation given by Mckey can be represented as:

t 1 t = + qt k2 (qe )2 qe

(13)

where, k2 is the second order rate constant. Fig. 13 shows a graph between t vs t for MB and MO dye systems in which straight lines are qt

obtained for both MB and MO dyes. The value of second order rate constant is calculated from the intercept and the value of qe is calculated from the slope of the curve. The value of coordination coefficient R2 was calculated from the linear fitting of the curve. The calculated values of K1, qe and R2 are represented in Table 2. It is clear that the value of coordination coefficient R2 for first order is in the range 0.9174 to 0.9852 while the value of R2 for second order kinetics is in the range 0.9517 to 0.9989. The low value of coordination coefficient for the first order kinetics suggests that the photocatalytic degradation of both the dyes follows a pseudo-second order mechanism rather than pseudo-first order mechanism.

Fig. 12. Pseudo-first order reaction kinetics for MO and MB dyes with different catalysts.

The adsorption kinetics has been studied using these two models [57]. The Langmuir’s pseudo first order rate equation is represented as:

ln(qe−qt ) = lnqe−k1 t

(10)

where, qe and qt denote the amount of dye adsorbed at equilibrium and at any time t, respectively and k1 is first order rate constant. The values of qe and qt were calculated by the following formulae:

qt =

qe =

4. Conclusions

(C0−Ct ) × v w

(11)

n (C0−Ce ) × v w

(12)

In the present study, modified Hummer’s method is applied to prepare two-dimensional, wrinkled and undulated structured GO from natural graphite. Boron doped graphene oxide is confirmed by XRD, EDX, XPS and HR-TEM results. Optical study shows the restoration of electronic conjugation on reduction. BR-GO has uniform particle distribution with an average lateral size of few nm. BR-GO sheet contains graphene islands separated by disordered regions. The reason for better photocatalytic activity of BR-GO catalyst for both organic pollutant dyes is due to the majority of electron holes in the valence band of BRGO because of p-type behavior. Degradation of MO dye was enhanced

where C0 is the initial concentration of the dye in solution, Ct (mg/L−1) is the concentration of the dye at any time, Ce (mg/L−1) is the concentration of dye at equilibrium, V is the volume of the solution in liters and W is the mass of dry adsorbent in grams. The plots of ln (qe−qt) in the ordinate axis versus time t for pseudofirst order kinetics of dye degradation have been represented in Fig. 12.

Table 2 Values of different parameters in pseudo-first order and pseudo-second order reaction. Organic dye

MB MO

Catalyst

GO RGO GO RGO

qe (expt)×10−3

1.2362 1.2387 1.2672 1.2684

Pseudo-first order Reaction

Pseudo-second order Reaction −3

2

K1

qe (calc) × 10

R

0.0612 0.0844 0.0144 0.0339

1.2713 1.1150 1.4922 2.6860

0.9596 0.9852 0.9425 0.9174

137

K2

qe (calc) × 10−3

R2

50.6944 101.1732 13.2075 16.3483

1.4731 1.4097 1.5357 1.7027

0.9988 0.9989 0.9517 0.9607

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M. Singh et al.

from 50% for sole solution to 100% with boron doping of GO in 100 min time. However, for MB dye, degradation is increased from 70% to complete degradation after 50 min. Doping cause increase in electrical conductivity and the electrical conductivity in BR-GO follows 2D VRH model. BR-GO has potential to be used in photocatalytic applications. [18]

Appendix A. Supplementary data

[19]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.06. 002. References

[20]

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