Novel nanocomposites for environmental and energy applications

Based on NRGO/BaWO4/g-C3N4


Academic Paper, 2019
31 Pages, Grade: 10

Free online reading

TABLE OF CONTENTS PAGE NO

1.0 INTRODUCTION

2.0 EXPERIMENTAL
2.1 Materials and Methods
2.2 Synthesis of g-C3N
2.3 Synthesis of GO
2.4 Synthesis of BaWO
2.5 Synthesis of NRGO/BaWO4/g-C3N4 Nanocomposites
2.6 Characterization
2.7 Photocatalytic Measurements
2.8 Reduction Measurements
2.9 Electrocatalytic Measurements

3.0 RESULTS AND DISCUSSION
3.1 X-ray Diffraction Studies
3.2 Raman Studies
3.3 FTIR Studies
3.4 Surface Morphology Studies
3.5 XPS Studies
3.6 Optical Studies
3.7 PL Studies
3.8 Photocatalytic Studies
3.9 Hydrogenation Studies
3.10 Electrocatalytic Studies

4.0 CONCLUSIONS

REFERENCES

NOVEL NRGO/BaWO4/g-C3N4 BASED NANOCOMPOSITES FOR ENVIRONMENTAL AND ENERGY APPLICATIONS

M. Mohamed Jaffer Sadiq

Department of Chemistry, National Institute of Technology Karnataka Surathkal, Mangalore - 575025, India.

E-mail: sadiqmsc@gmail.com

1.0 INTRODUCTION

The abundance of existing major sources of energy such as coal, natural gas and other fossil fuels is limited and they are non-renewable too. With increase in the population, the global energy demand has increased and society is not able to cope with the demand. The quest for renewable clean energy sources has turned our attention towards hydrogen [1]. Hydrogen can be produced by water splitting through electrocatalysis. But most of the materials currently used for electrocatalytic water splitting are noble and expensive. Development of low cost and environmental friendly electrocatalyst is the need of the day now to commercialize hydrogen production [2]. Semiconductor photocatalysis has attracted interest of research community as it can solve both environmental and energy problems [2-3]. A good photocatalyst should have an extended excitation wavelength, low recombination rate of charge carriers and active sites on the surface to facilitate adsorption and reaction. Several strategies have been developed to achieve the above said properties like doping, coupling with semiconductors and compositing with layered materials to improve the surface area [3]. Despite these efforts commercialization of these materials is yet to be realized. Hence designing of materials with multifaceted catalytical activity such as, high performance towards electrolysis, photodegradation and reduction would help in solving the energy and environmental problems must be thoroughly considered. Such multifunctional catalysts will provide an economic way to serve several applications.

Graphene based architectures not only provide support to other materials but also have a potential to harness the electrical and redox properties [4-6]. The electronic property and the chemical reactivity of graphene can be tailored by doping nitrogen [7-8]. In N doped graphene ~0.5 electron per N atom is provided to carbon π conjugated system thus enhancing the photocatalytic efficiency. When composited with semiconductors, NRGO increases the rate of transfer of electrons from the conduction band of semiconductors in comparison with graphene.

Graphitic carbon nitride (g-C3N4), a metal-free polymeric semiconductor, with higher nitrogen content and porous structure has attracted significant attentions in the field of photocatalysis due to its good thermal-chemical stability, electronic and optical characteristics, low cost and non-toxicity. The g-C3N4 possesses a band gap of ~2.7 eV with the conduction band (CB) position at -1.1 eV and valence band (VB) position at +1.6 eV. As the CB potential of g-C3N4 is sufficiently negative, the strong reducing capability of the electrons in the CB of g-C3N4 surface can have great potential for photocatalytic studies [9-11]. Combining g-C3N4 with other appropriate semiconductors to form semiconductor heterostructure is considered as an effective method to enhance photocatalytic activity.

Semiconducting materials which possess narrow band gap have been used as photocatalysts, as their band gap energy lies in the energy range of the UV or visible light [12-13]. Tungsten oxides, hydrates and metal tungstates have been studied for their photocatalytic capabilities [14-15]. In particular, ZnWO4, CoWO4, FeWO4, NiWO4 have been extensively used for preparation of composites for water splitting [16-19]. On the other hand, BaWO4 which has a wide variety of applications has been studied meagerly in the field of photocatalysis due to its instability and slow electron transfer rate. To extend the absorption of BaWO4 from UV region to visible region of solar spectrum the band gap has to be decreased. Since smaller band gap leads to higher recombination rate, the transport property is tuned by doping carbon based material to improve the photocatalytic efficiency [20].

It is well known that use of the microwave irradiation over conventional heating has obvious advantages such as, uniform and volumetric heating, short reaction time and uniformity in the size and shape of formed nanoparticles without much need of high temperature and pressure. Several reports are available in support of the advantages of microwave assisted approach over conventional methods for the synthesis of nanocomposites [16-20].

Based on the above facts, we report the synthesis of NRGO/BaWO4/g-C3N4 nanocomposites via a facile, simple microwave method. To the best of our knowledge this is the first report on such kind of ternary nanocomposite with tri functional utility. The as synthesized material is thoroughly characterized using various advanced techniques. The reported material is highly efficient in catalyzing HER, degradation of MB dye and reduction of 4-NP to 4-AP. The reported approach is extendible to other metal tungstate nanocomposites as well.

2.0 EXPERIMENTAL

2.1 Materials and Methods

All the reagents and chemicals were of analytical grade and used without further purification. Deionized water was used throughout the study.

2.2 Synthesis of g-C3N4

The powder of g-C3N4 was prepared according to the previously reported technique [21]. Typically, 2 g of melamine was put into a semi-closed alumina crucible which was calcined at 550 °C with a heating rate of 10 °C/min for 4 hours in the air atmosphere in a muffle furnace. The obtained light-yellow g-C3N4 product was collected and ground into powder form, then thoroughly washed with deionized water, and dried at 60 °C overnight for further use.

2.3 Synthesis of Graphene Oxide (GO)

GO was prepared through chemical exfoliation of natural graphite powder by a modified Hummers’ method previously reported [22]. 5 g of graphite flakes and 2.5 g of NaNO3 was added in 150 mL of concentrated H2SO4 under constant stirring in a beaker immersed in an ice water bath. Then, 15 g of KMnO4 was added slowly and the mixture was stirred at the 30 °C for 2 hours and then for 30 minutes at 95 °C. Finally, the reaction mixture was diluted with distilled water and 10 mL of H2O2 was subsequently added. The GO obtained was separated from the yellow solution by centrifugation, washed with dilute HCl and water until the pH was 7. Later it was exfoliated by sonication.

2.4 Synthesis of BaWO4

The BaWO4 nanoparticles were prepared by simple microwave route. In a typical synthesis, solutions of barium chloride, ammonium tungstate and cetyl trimethyl ammonium bromide (CTAB) in H2O/ethanol were prepared with desired molar ratio and mixed together using ultrasonication for about 30 minutes. The resulting mixture was treated with microwave irradiation at 350 W for 10 minutes and then allowed to cool to room temperature. The obtained precipitate of BaWO4 was filtered and washed thoroughly with 10% ethanol several times to remove the impurities. Finally, the precipitate was dried at 60 °C overnight.

2.5 Synthesis of NRGO/BaWO4/g-C3N4 Nanocomposites

The NRGO/BaWO4/g-C3N4 nanocomposites were prepared using a simple microwave route. In a typical synthesis, a calculated amount of g-C3N4 and GO was dispersed in the water. Then solutions of barium chloride, ammonium tungstate, CTAB and urea in H2O/ethanol with desired molar ratios were added. The pH of the reaction mixture was maintained at 9 using ammonia. After 30 minutes of ultrasonication, the resulting mixture was treated with microwave irradiation at 350 W for 10 minutes and then allowed to cool to room temperature. The obtained precipitate of NRGO/BaWO4/g-C3N4 nanocomposite was filtered and washed thoroughly with 10% ethanol several times and finally dried at 60 °C overnight.

Similarly, g-C3N4/BaWO4 and NRGO/BaWO4 nanocomposites were prepared using appropriate starting materials. NRGO/BaWO4 nanocomposites with NRGO content varying from 0.5 to 5% were prepared and their catalytic efficiency for photodegradation of MB dye was tested. The nanocomposite with 2.5% NRGO showed highest activity and hence 2.5% NRGO was taken as the optimized composition for NRGO/BaWO4 nanocomposites. Similarly, in the case of NRGO/BaWO4/g-C3N4 ternary nanocomposites, keeping NRGO content fixed at 2.5%, g-C3N4 content was varied from 5 to 20%. The nanocomposite with 10% g-C3N4 showed maximum catalytic activity and hence it was taken as the optimized composition of g-C3N4 in the ternary nanocomposite (Figure 1). Schematic representation of the synthesis of NRGO/BaWO4/g-C3N4 nanocomposites is shown in the Figure 2.

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Figure 1. Catalytic activities of various compositions of NRGO/BaWO4/g-C3N4 nanocomposite.

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Figure 2. Schematic representation of the synthesis of NRGO/BaWO4/g-C3N4 ternary nanocomposite.

2.6 Characterization

The powder X-ray diffraction (XRD) patterns were recorded using X-ray diffractometer (Rigaku, Japan) with the Cu-Kα target (l=0.15406 nm) in the 2θ range 5° to 80° with a scan rate of 1° min-1. The specific surface area was obtained using SMART SORB 92/93 (Smart Instruments Company Private Limited) by Brunauer-Emmett-Teller (BET) method. The surface morphology was obtained using scanning electron microscopy (SEM, JEOL) and high-resolution transmission electron microscopy (HRTEM, Technai). Laser Raman microscope (Renishaw Invia) was used to obtain Raman spectrum using excitation laser wavelength source of 532 nm. X-ray photoelectron spectroscopy (XPS) was done to determine the chemical states of the samples with Multilab2000 (Thermo Scientific, UK) and the obtained data was calibrated using contaminant carbon at a binding energy of 284.8 eV. UV-Visible diffuse reflectance data were collected with a spectrophotometer (Analytik Jena). Photoluminescence (PL) spectra were obtained using Horiba Jobin Yvon spectrometer with excitation wavelength of 380 nm. The total organic carbon concentration (TOC) was measured using total organic carbon analyzer (TOC-V CSN, Shimadzu, Japan).

2.7 Photocatalytic Measurements

The photocatalytic activity of NRGO/BaWO4/g-C3N4 nanocomposites in the degradation of MB was evaluated as follows. Firstly, 0.02 g of NRGO/BaWO4/g-C3N4 nanocomposite was dispersed into a 200 mL MB (10 mg/L) aqueous solution in a Pyrex glass vessel and then the dispersion was kept in the dark for 30 minutes at room temperature to establish the adsorption equilibrium. The solution was irradiated using a Hg lamp (250 W with 400 nm cutoff filter). During the irradiation, samples were taken out at given time intervals, filtered and then the concentration of MB was determined at 664 nm using a UV-Visible spectrophotometer (Analytik Jena) from which percentage degradation was calculated. For comparison, the photocatalytic activities of the components of the composite were also tested following the same procedure. In the recycling experiment, the catalysts were collected and washed several times with 10% ethanol, before the next cycle.

To identify the nature of degradation process, the extent of mineralization of the dye during the photodegradation was estimated. The mineralization of the dye was calculated by using the total organic carbon content (TOC) analysis. The TOC was analyzed before the start of the experiment (TOCo) and at specified intervals during the photodegradation reaction (TOCt). The % mineralization of MB dye was calculated by using the following equation.

% Mineralization of the MB dye = [(TOCo - TOCt) / TOCo] x 100 ---------- (1)

where, TOCo is the initial concentration and TOCt is the concentration at a given interval time, of the MB dye solution, respectively.

In addition, to determine the mechanism of the photocatalytic activity, experiments were carried out using various radical scavengers like t-BuOH (TBA, 10 mM), KI (potassium iodide, 10 mM), BQ (benzoquinone, 1 mM) and AgNO3 (silver nitrate, 10 mM) which acted as the scavengers for hydroxyl radicals (.OH), holes (h+), superoxide radicals (.O2-) and electrons (e-), respectively as reported previously [23].

2.8 Reduction Measurements

The reduction of 4-NP to 4-AP by using NaBH4 at ambient temperature was chosen as a model reaction to test the catalytic activity of the NRGO/BaWO4/g-C3N4 nanocomposites. 0.3 mL of freshly prepared NaBH4 solution (0.1 M) was mixed with 2.7 mL of an aqueous 4-NP solution (0.1 mM) in a quartz cuvette, leading to an immediate color change from light yellow to yellow-green. Then, 0.0005 g of NRGO/BaWO4/g-C3N4 nanocomposites was added to start the reduction reaction and the reaction progress was monitored at 400 nm by using UV-Visible spectroscopy (Analytik Jena) at a regular time interval of 15 seconds. For comparison, the catalytic activities of components of the composite were also tested following the same procedure. In the recycling experiment, the catalysts were collected and washed several times with 10 % ethanol and then reused.

2.9 Electrocatalytic Measurements

All electrochemical measurements were performed using an IVIUM Potentiostat in a conventional three-electrode cell. A glassy carbon disk with a geometric area of 0.07065 cm2 modified with the catalysts was used as the working electrode, an Ag/AgCl (3 M KCl) as a reference electrode and a Pt wire as the counter electrode were employed for electrochemical measurements. 0.1 M KOH solution was used as the electrolyte. The reference electrode was calibrated with respect to the reversible hydrogen electrode (RHE). Prior to experiments, the glassy carbon electrode was polished with a polishing cloth using different alumina pastes (3.0 - 0.05 mm) to obtain a mirror-like surface, followed by ultrasonic cleaning in water. For electrochemical measurements, a catalyst ink was prepared by dispersing 2.0 mg/mL of the catalyst in water that contained 0.1 wt% Nafion under ultrasonication for 30 minutes. 5.0 µL of the catalyst suspension was drop-coated onto the polished glassy carbon electrode and dried in air at room temperature. Linear sweep voltammetry was done at a scan rate of 10 mV/s to evaluate the HER performance of the working electrode. The long-term stability was evaluated by chrono-potentiometry at a current density of -10 mA cm-2 using a glassy carbon disk (3mm diameter) modified with the catalyst. The catalyst loading for each electrode was 0.142 mg cm-2. All the measurements were carried out at room temperature. Also, before the experiment, electrolyte solution was purged with N2 for 30 minutes to remove the oxygen completely.

3.0 RESULTS AND DISCUSSION

3.1 X-ray Diffraction Studies

The XRD patterns of NRGO, g-C3N4, BaWO4, NRGO/BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposites are shown in Figure 3. The diffraction peak at 2θ = 24.4° corresponds to the (002) reflection plane of reduced phase of NRGO. The peaks at 2θ = 12.8° and 27.5° seen in pure g-C3N4, can be indexed to the (100) and (002) diffraction planes conforming to the JCPDS No. 87-1526 of the graphitic carbon nitride. The diffraction pattern of BaWO4 matches well with JCPDS No. 43-0646 suggesting that the material has the Scheelite-type tetragonal crystal structure with space group of I41/a symmetry [24]. The peaks found at 2θ values 17.3°, 26.5°, 28.1°, 31.9°, 43.0°, 45.7°, 48.7°, 53.6°, 54.5°, 66.6°, 67.7°, 69.4°, 73.0°, 73.8°, 75.8° and 76.8° can be ascribed to the (101), (112), (004), (200), (204), (220), (116), (312), (224), (400), (208), (411), (332), (404), (420) and (228) planes of BaWO4. NRGO/BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposite have similar diffraction peaks corresponding to BaWO4. These results specify that the introduction of NRGO and NRGO/g-C3N4 do not disturb the orientation and structure of Scheelite BaWO4.

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Figure 3. XRD patterns of the NRGO, g-C3N4, BaWO4, NRGO/BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposite.

3.2 Raman Studies

The as-synthesized nanocomposites are further characterized by using Raman spectroscopy. As shown in Figure 4, five peaks are exhibited at 334 cm-1, 478 cm-1, 794 cm-1, 830 cm-1 and 927 cm-1 in the pure BaWO4 sample. The bands positioned around 334 cm-1 is indexed to the stretching vibration of the BaO6 octahedra. The bands located at 478 cm-1, 794 cm-1, 830 cm-1 are assigned to the W-O stretching vibration of the WO4 tetrahedra. The band at 927 cm-1 is associated with the WO6 symmetric stretching vibration of the crystalline BaWO4 [25]. The Raman spectrum of the pure NRGO reveals two prominent peaks D band and G band at 1356 cm-1 and 1598 cm-1, respectively [7-8]. The Raman spectrum of the NRGO/BaWO4/g-C3N4 nanocomposites contains all the bands corresponding to BaWO4 and NRGO. The ratios of ID/IG in pure NRGO and NRGO/BaWO4/g-C3N4 nanocomposites are 1.09 and 1.13, respectively. The higher value for ternary composite may be due to the interaction of NRGO, g-C3N4, and BaWO4 leading to more defects and disorders in the NRGO/BaWO4/g-C3N4 nanocomposite. The typical bands of g-C3N4 appear at 421 cm-1, 605 cm-1, 897 cm-1, 1087 cm-1, 1219 cm-1 and 1339 cm-1 [10]. However, these bands of g-C3N4 are difficult to identify in the Raman spectrum of the NRGO/BaWO4/g-C3N4 nanocomposite. This may be due to the weak scattering ability of g-C3N4 on the surface of the nanocomposite [10].

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Figure 4. Raman spectra of the NRGO, g-C3N4, BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposite.

3.3 FTIR Studies

The presence of BaWO4, NRGO, and g-C3N4 in the as-synthesized nanocomposite can be identified using FT-IR spectra. As shown in Figure 5, the main peak at 819 cm-1 assigned to the stretching vibration of Ba-O, W-O, and W-O-W, is seen in the spectrum of the pure BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposites [26]. As shown in the inset of Figure 5, the peaks at 3406 cm-1, 2969 cm-1, 2884 cm-1, 1633 cm-1, 1465 cm-1 and 1248 cm-1 in the FT-IR spectra of NRGO and NRGO/BaWO4/g-C3N4 nanocomposites can be ascribed to the asymmetric and symmetric vibrations of O-H and C-H, respectively [27]. The spectrum of g-C3N4 shows peaks at 1248 cm-1, 1326 cm-1, 1415 cm-1, 1465 cm-1, 1575 cm-1 and 1633 cm-1 which are characteristic stretching modes of C-N heterocycles. The peaks at 808 cm-1 and 3184 cm-1 are attributed to the typical breathing mode of triazine units and the stretching mode of N-H in g-C3N4, respectively [27]. The FT-IR spectrum of NRGO/BaWO4/g-C3N4 nanocomposites reveals similar peaks as that of g-C3N4. All the peaks corresponding to the BaWO4, NRGO, and g-C3N4 are present in the spectrum of NRGO/BaWO4/g-C3N4 nanocomposites.

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Figure 5. FTIR spectra of the NRGO, g-C3N4, BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposite.

3.4 Surface Morphology Studies

The specific surface area of the as-synthesized nanocomposites was measured by BET analysis. The specific surface areas of BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposites are 3.51 and 25.02 m2 g-1, respectively. It is very clear that the specific surface area increases due to the introduction of the NRGO and g-C3N4 on the BaWO4 substrate. This increase in surface area facilitates the surface adsorption and creates more number of active sites resulting in enhanced catalytic activities [16].

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Figure 6. (a) HRTEM (inset showing SAED pattern), (b) particle size distribution, (c) EDX spectrum and Elemental mapping (d) C, (e) N, (f) Ba, (g) W and (h) O of the NRGO/BaWO4/g-C3N4 nanocomposite.

The HRTEM image (Figure 6a) reveals that BaWO4 particles are successfully deposited on the surface of the g-C3N4 and NRGO nanosheets. The spacing of the lattice fringes in the BaWO4 is about 0.51 nm which corresponds to the (101) plane of scheelite-type tetragonal crystal phase (JCPDS No. 43-0646). In addition, the lattice spacing of about 0.34 nm corresponds to the (002) plane of the graphitic carbon nitride (JCPDS No. 87-1526) and lattice spacing of 0.36 nm corresponds to the (002) planes of the NRGO nanosheets. The SAED pattern of the nanocomposite (inset of Figure 6a) shows ring pattern indicating the polycrystalline nature of the sample. The particle size distribution of the BaWO4 (Figure 6b) shows that the average particle sizes is around 12 nm.

The EDX analysis (Figure 6c) of NRGO/BaWO4/g-C3N4 nanocomposite further compliments the characterization and formation of the ternary composite. In addition to that, the distribution of elements in NRGO/BaWO4/g-C3N4 nanocomposites was determined by elemental mapping analysis (Figure 6d-h). The analysis infers the uniform distribution of the particles in the composite.

3.5 XPS Studies

XPS is used to investigate the elemental composition as well as the chemical environment on the surface of the as-prepared nanocomposites. XPS survey spectrum of NRGO/BaWO4/g-C3N4 nanocomposite is shown in Figure 7a. The elemental peaks of C, N, O, Ba and Ware found in the NRGO/BaWO4/g-C3N4 nanocomposites, which confirm the presence of NRGO, g-C3N4, and BaWO4 in the nanocomposite. High resolution C 1s spectra (Figure 7b) can be deconvoluted into four peaks. Peaks at 284.2 eV and 285.3 eV can be allocated to carbon (C=C) and typical sp2 carbon atoms bonded to N atoms in an aromatic ring (N-C=N), peak at 286.98 eV can be ascribed to C=N originated from NRGO and the formation of C-O-C bond between NRGO and g-C3N4 during microwave treatment. The peak at 288.03 eV can be ascribed to the interaction of C-N-C between g-C3N4 and NRGO. As shown in Figure 7c, five deconvoluted peaks in the high-resolution N 1s spectrum of NRGO/BaWO4/g-C3N4 nanocomposites are assigned to pristine g-C3N4 (396.8 eV), pyridinic N (397.9 eV), pyrrolic N (399.1 eV), graphitic N (400.8 eV) and terminal amino groups (403.9 eV), which is in agreement with the reported data [9]. The O 1s peaks in Figure 7d at 528.96 eV, 530.48 eV and 534.03 eV corresponds to Ba-O-W, N-C-O and N-C-O-W of the NRGO/BaWO4/g-C3N4 nanocomposites, revealing the chemical interaction between the NRGO, g-C3N4, and BaWO4 in the nanocomposites. As shown in Figure 7e, the doublet peaks at 37.1 eV and 35.0 eV belong to W 4f5/2 and W 4f7/2, which are the features of W6+ in pure BaWO4 [20]. The peaks at 794.6 eV and 779.2 eV correspond to Ba3d3/2 and Ba 3d5/2 (Figure 7f), which can be assigned to Ba2+ of pure BaWO4.

For comparison, survey and high resolution XPS spectra of as synthesized NRGO is given in Figure 8. XPS can be used to provide direct evidence for doping of N into RGO. The presence of N could be clearly detected in the XPS spectra of NRGO, and the high-resolution N1s XPS spectra could be fitted into four types of N doping, including pyridinic N (398.3 eV), pyrrolic N (399.6 eV), graphitic N (400.7 eV) and pyridinic N Oxide (402.5 eV). All the above results show that the N-doped RGO has been successfully synthesized. Further, the small differences in the binding energies observed for neat NRGO and NRGO in the nanocomposite can be attributed to the interactions of NRGO with other components of the composite.

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Figure 7. XPS spectrum (a) survey, high-resolution spectra (b) C1s, (c) N 1s, (d) O 1s, (e) W 4f and (f) Ba 3d of NRGO/BaWO4/g-C3N4 nanocomposite.

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Figure 8. Survey and high resolution N 1s spectra of NRGO

3.6 Optical Studies

The optical properties of NRGO, g-C3N4, BaWO4, NRGO/BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposite are determined via the UV-Visible DRS technique (Figure 9a). As shown in Figure 9a, the NRGO absorbs in the whole visible region. The NRGO/BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposites show higher absorption as well as wider wavelength window of absorption in the visible-light region compared with the pure g-C3N4 and BaWO4. Thus, the observation suggests that the NRGO/BaWO4/g-C3N4 nanocomposite can efficiently utilize visible-light and generate more electron–hole pairs under visible-light irradiation.

For a given semiconductor, its band gap energy (Eg) can be measured from the intercept of the tangents in the plot of (ahν)2/n vs photon energy (hν) based on Tauc relation given below in equation (2) and are shown in Figure 9b.

ahν = k(hν-Eg)n/2------------------------------ (2)

where a, h, ν, and k are absorption coefficient, Planck constant, frequency of light and a constant, respectively. In addition, n is a constant determined by the type of optical transition of a semiconductor, which is equal to 1 for a direct gap material, and 4 for an indirect gap material. According to equation 2, the intercept of the tangents to the plots of (ahν)2 vs photon energy could be employed to determine the band gaps of the given materials, due to their direct electronic transitions.

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Figure 9. (a) DRS spectrum, (b) Band gap plots and (c) PL spectrum of the NRGO/BaWO4/g-C3N4 nanocomposites.

The band gaps of pure g-C3N4, BaWO4, NRGO/BaWO4, and NRGO/BaWO4/g-C3N4 nanocomposite are 2.69 eV, 4.43 eV, 3.04 eV and 2.41 eV, respectively (Figure 9b). The band gaps of NRGO/BaWO4/g-C3N4nanocomposite are reduced compared to its components. The reduction in the band gap may be attributed to the delocalization of surface charges resulted by the interactions of the components in the composite. It is believed that interactions lead to the formation of new molecular orbitals of lower energy which in turn facilitates the reduction in the bandgap. Such observations for semiconductor composites are reported in the literature [14, 16]. Such reduction in the band gap of the materials is beneficial for enhancing both visible-light absorption and photocatalytic activity.

3.7 PL Studies

PL analysis represents the recombination performance of photogenerated charge carriers in a semiconductor. Higher rate of recombination results in higher intensity of the PL spectrum. The enhanced photocatalytic efficiency of NRGO/BaWO4/g-C3N4 nanocomposite is attributed to the effective separation of photogenerated charge carriers [14]. The nature of charge carrier recombination in BaWO4, NRGO/BaWO4, g-C3N4/BaWO4 and NRGO/BaWO4/g-C3N4 nanocomposite was investigated by PL analysis (Figure 9c). BaWO4 exhibits strong emission peak around 545 nm, which is due to the recombination of the photogenerated electron-hole pairs. After NRGO or g-C3N4 was introduced, the heterostructure samples show lower PL emission intensity compared with that of pure BaWO4, suggesting lower recombination of the photogenerated electron-hole pairs. The PL peak of NRGO/BaWO4/g-C3N4 nanocomposite shows the lowest intensity, which reveals higher separation between the photogenerated electron-hole pairs in the NRGO/BaWO4/g-C3N4 nanocomposite surface.

3.8 Photocatalytic Studies

The nanocomposite and its components were tested for their photocatalytic activities in the degradation of MB dye. In the absence of the catalyst the degradation was negligible. As shown in Figure 10a, the efficiencies of pure NRGO, BaWO4, g-C3N4, g-C3N4/BaWO4, NRGO/BaWO4, NRGO/g-C3N4 and NRGO/BaWO4/g-C3N4 nanocomposite are 13.10%, 21.06%, 46.99%, 65.47%, 74.49%, 79.63% and 99.44% for 120 minutes visible-light irradiation, respectively. Clearly, the photocatalytic degradation efficiency of these samples decreases in the following order: NRGO/BaWO4/g-C3N4>NRGO/g-C3N4>NRGO/BaWO4>g-C3N4/BaWO4>g-C3N4>BaWO4>NRGO. These results demonstrate that the NRGO/BaWO4/g-C3N4 nanocomposite shows higher photocatalytic activity for MB degradation than the pure NRGO, g-C3N4, BaWO4, g-C3N4/BaWO4 and NRGO/BaWO4 and NRGO/g-C3N4.

It is observed that the kinetics of photocatalytic degradation of MB follows Langmuir–Hinshelwood first-order kinetics model, which can be described by the following equation.

-ln(C/Co) = kt------------------------------ (3)

where Co is the initial concentration of dye, k is the first-order rate constant, C is the concentration of the dye at time interval ‘t’. The rate constants can be obtained from the slope of the plot of -ln(C/Co) vs irradiation time (Figure 10b).

The rate constant of NRGO/BaWO4/g-C3N4(0.01767 min-1) is about 11.70, 9.11, 4.09, 2.67, 2.04 and 1.63 times greater than those of the pure NRGO (0.00151 min-1), BaWO4 (0.00194 min-1), g-C3N4 (0.00432 min-1), g-C3N4/BaWO4 (0.00663 min-1) and NRGO/BaWO4 (0.00865 min-1) and NRGO/g-C3N4 (0.01087 min-1) respectively (Figure 10c). The enhanced performance of the ternary nanocomposite may be attributed to the factors such as narrow band gap, higher extent of energy absorption in the visible light region and efficient separation of charge carriers, contributing significantly towards improved photodegradation of MB in comparison to that of component materials.

The durability and stability of the photocatalysts are significantly important for their practical applications. The catalytic stability was determined by recycling the nanocomposite. The degradation rates changed from 99.44 % to 99.21 % to 98.82 % to 98.35 % to 97.44 % from 1st to 5th cycle, respectively. These results indicate that the NRGO/BaWO4/g-C3N4 nanocomposite is relatively stable during the photocatalytic process.

In order to understand the photocatalysis mechanism of the NRGO/BaWO4/g-C3N4 nanocomposite, the active species generated during the photocatalytic degradation process are identified through radical and hole trapping experiments. 10 mM Ternary butyl alcohol (TBA), 10 mM KI (potassium iodide), 1 mM BQ (benzoquinone) and 10 mM AgNO3(silver nitrate) acted as the scavengers for hydroxyl radicals (•OH), holes (h+), superoxide radicals (•O2–) and electrons(e-), respectively.

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Figure 10. (a) Degradation plots of MB over various catalysts, (b) First order kinetics plots of MB over various catalysts, (c) Rate constant of MB over various catalysts, (d) Plot depicting effects of different scavengers on degradation efficiency of NRGO/BaWO4/g-C3N4 nanocomposite under visible light irradiation.

Figure 10d displays the influence of different quenchers on the visible light photocatalytic activity of NRGO/BaWO4/g-C3N4 nanocomposite for the degradation of MB. With the addition of TBA, the photodegradation efficiency of MB did not change much indicating that the •OH is the not the active species generated in the NRGO/BaWO4/g-C3N4 photocatalytic reaction system. The degradation efficiency of MB changed slightly with the addition of BQ and AgNO3, implying that •O2– and e- was formed in the photocatalytic reaction system of NRGO/BaWO4/g-C3N4 under visible light irradiation. However, after addition of KI, the photodegradation efficiency of MB decreased significantly as a result of quenching of h+ in a reaction system, which indicated that h+ is the main active species generated in the NRGO/BaWO4/g-C3N4photocatalytic degradation of MB. The proposed reactions are shown in equations (4-7),

Abbildung in dieser Leseprobe nicht enthalten

where EVB and ECB are the VB and CB potentials, Eg is the band gap of the semiconductor, i.e., 4.43 eV and 2.69 eV for BaWO4 and g-C3N4, respectively. χ is the arithmetical mean of the absolute electronegativity of the component atoms in the semiconductor, and Ee is the free electron energy based on the hydrogen scale (4.5 eV). The χ values of BaWO4 and g-C3N4 are 5.6954 eV and 4.73 eV, respectively. The calculated VB potentials of BaWO4 and g-C3N4 are 3.4104 eV and 1.575 eV. The CB potentials are -1.0196 eV and -1.115 eV, respectively for BaWO4 and g-C3N4.

Based on the above discussions and analysis, a possible charge transfer mechanism for the NRGO/BaWO4/g-C3N4 nanocomposite is proposed, as shown in Figure 11. When the system is under visible irradiation, the VB electrons of g-C3N4 semiconductors are excited to the CB, leaving holes in the VB, thereby forming photoinduced electron-hole pairs. Because the CB potential of BaWO4 is slightly lower than that of the CB of g-C3N4, the CB electrons of g-C3N4 would easily jump to the CB of BaWO4 and from there to the highly conducting NRGO sheets, resulting in effective charge separation and transport. These electrons would then react with the oxygen to form superoxide radical anion. The holes will immediately get transported with the support of conductive NRGO matrix to react with MB to form products such as CO2, H2O, and other species. Thus, the entire process leads to efficient separation of the photogenerated electron-hole pairs, which in turn results in the enhancement of the photocatalytic activity of NRGO/BaWO4/g-C3N4 nanocomposite for the degradation of MB.

Abbildung in dieser Leseprobe nicht enthalten

Figure 11. Proposed mechanism for the photocatalytic degradation of dye by NRGO/BaWO4/g-C3N4 nanocomposite under visible light irradiation.

In order to understand the nature of products formed during photocatalytic degradation of MB, we have determined the total organic carbon content (TOC) of the reaction medium during the photo catalysis process. Using the TOC values, percentage mineralization of the dye was calculated employing the equation (1) given earlier. For comparison, the decrease in concentration of MB in terms of % decolorization was determined during catalytic photodegradation. The concentrations of solutions were calculated from absorbance data at 664 nm. Figure 12a shows absorbance versus wavelength plot at different intervals of time and Figure 12b shows variation of % mineralization and % decolorization with time in the form of a bar diagram. As can be observed from the plot, the absorbance at 664 nm decreased with time and reached 0.56% during scan at 120 minutes under visible light irradiation. The TOC value decreased to 14.36% or mineralization extent increased to 85.74% under visible light irradiation for 120 minutes of visible light irradiation. Thus, the results suggest that, the organic carbon is mostly converted to CO2 during the process. In view of this, it can be concluded that the nanocomposite is an eco-friendly photocatalyst.

Figure 12. (a) Absorbance versus wavelength and (b) Mineralization/Decolorization test bar diagram for photodegradation of MB catalyzed by NRGO/BaWO4/g-C3N4 nanocomposite at different intervals of time.

3.9 Hydrogenation Studies

The catalytic activity of NRGO/BaWO4/g-C3N4 nanocomposites was evaluated in the reduction of 4-NP using NaBH4 in an aqueous solution. In the absence of the catalyst the reduction was negligible. However, when NRGO/BaWO4/g-C3N4 nanocomposites were added into the 4-NP solution, the absorption of 4-NP found at 400 nm peak decreased immediately and the new absorption peak of 4-AP found at 300 nm was obtained, and then increased with time as shown in Figure 13a. The catalytic reduction of 4-NP into 4-AP was completed in 60 seconds. The complete change of 4-NP can be visually seen (inset Figure 13a) as the solution color changes from bright yellow to colorless. The observed results indicated that the NRGO/BaWO4/g-C3N4 nanocomposites are excellent catalyst in the reduction of 4-NP to 4-AP than that of pure NRGO, BaWO4, g-C3N4, g-C3N4/BaWO4 and NRGO/BaWO4 materials (Figure 13b).

Abbildung in dieser Leseprobe nicht enthalten

Figure 13. (a) UV-Vis absorption spectra for the reduction of 4-NP to 4-AP by NaBH4 in the presence of NRGO/BaWO4/g-C3N4 nanocomposite. (b) Catalytic reduction of 4-NP to 4-AP over various catalysts using NaBH4. (c) First order kinetic plots of -ln(C/Co) against the reduction time of 4-NP to 4-AP and (d) Rate constant values of 4-NP over various catalytic materials.

The reaction rate for the reduction process followed pseudo-first order kinetics with respect to 4-NP concentration similar to the one described for photodegradation previously. The first order kinetic plots of -ln(C/Co) vs. conversion time for all the catalyst materials are shown in Figure 13c. The reaction rate constant (k) is calculated from the slope of these plots and the same is given in the form of a bar diagram for comparison as shown in the Figure 13d. The above results clearly indicate that NRGO/BaWO4/g-C3N4 nanocomposites exhibit a significantly enhanced catalytic activity for the reduction of 4-NP to 4-AP than that of component materials.

The catalytic stability and reusability of the NRGO/BaWO4/g-C3N4 nanocomposites were tested after recovery from the previous reaction mixture (Figure 14). The recycled catalyst exhibited excellent catalytic activity even after 10 successive cycles, with nearly 100 % conversion within a time period of 92 seconds.

Abbildung in dieser Leseprobe nicht enthalten

Figure 14. Catalytic stability of NRGO/BaWO4/g-C3N4 nanocomposites for successive 10 cycles.

3.10 Electrocatalytic Studies

The electrocatalytic performance of the NRGO/BaWO4/g-C3N4 nanocomposite was investigated in 1.0 M KOH solution using a standard three-electrode system. For the sake of comparison, component materials and 20 wt % Pt/C were also tested under the same conditions. The polarization curves for the HER on various electrodes are shown in Figure 15a. NRGO/BaWO4/g-C3N4 nanocomposites demonstrated a remarkably high activity with an onset potential of ~83 mV vs RHE and a HER current density of 10 mA cm-2 at an overpotential of 211 mV. On the other hand, NRGO/BaWO4 and g-C3N4/BaWO4 had an onset potential of ~186 mV and ~205 mV and current density of 10 mA cm-2 at overpotential of 320 mV and 347 mV, respectively.

The linear regions of the Tafel plots (Figure 15b) were fitted into the Tafel equation (η =blog(j) + a, where b is the Tafel slope) [28-29], yielding 30 mV dec-1, 62 mV dec-1, 103 mV dec-1, 112 mV dec-1, 160 mV dec-1, 196 mV dec-1 and 227 mV dec-1 for 20 wt% Pt/C, NRGO/BaWO4/g-C3N4, NRGO/BaWO4, g-C3N4/BaWO4, BaWO4, NRGO and g-C3N4 electrode, respectively. This indicates that the NRGO/BaWO4/g-C3N4 nanocomposites electrode has much higher activity than the component materials electrode. Although its activity is still lower than the 20 wt.% Pt/C electrode, it may be considered significant because of the fact that it is a Pt-free catalyst. There are normally three principal steps for the HER in alkaline solutions namely, the Volmer reaction (electrochemical hydrogen adsorption: H2O + e- →Hads + OH-), and the Tafel reaction (chemical desorption: Hads + Hads → H2) or Heyrovsky process (electrochemical desorption: Hads + H2O + e- → H2 + OH-).29 Tafel slope of 120 mV dec-1, 40 mV dec-1 or 30 mV dec-1 is expected if the Volmer, Heyrovsky or Tafel reaction is the rate-determining step, respectively. Thus, the experimentally observed Tafel slope of 62 mV dec-1 indicated that the Heyrovsky process is the rate determining step for NRGO/BaWO4/g-C3N4nanocomposites.

Stability is one of the key factors in evaluating catalyst performance. In view of this, to assess the stability of NRGO/BaWO4/g-C3N4 nanocomposites during HER, continuous cyclic voltammograms to up to 2000 cycles, and galvanostatic polarization curves at a current density of -10 mA cm-2 were recorded. Figure 15c displays LSV curves recorded at a scan rate of 10 mV s-1 for NRGO/BaWO4/g-C3N4 nanocomposites before and after performing 2000 continuous cyclic voltammograms between 0.0 V and -0.5 V. Notably, no appreciable activity change was observed after 2000 cycles indicating excellent stability of the NRGO/BaWO4/g-C3N4 nanocomposites during HER. This observation further suggests that the NRGO/BaWO4/g-C3N4 nanocomposite structure and composition remain unchanged during the catalytic process. The potential required to deliver a current density of -10 mA cm-2 is an important figure of merit for the viability of a HER catalyst because it corresponds to the current density required to attain 10% efficiency in a solar-to-fuel conversion device [28-29]. Galvanostatic measurements also demonstrated the outstanding stability of the NRGO/BaWO4/g-C3N4 nanocomposites in basic media (Figure 15d), confirming that the ternary nanocomposite can be a very useful, efficient and promising candidate for HER.

Abbildung in dieser Leseprobe nicht enthalten

Figure 15. (a) LSV and (b) Tafel slope curve for different electrode materials. (c) Stability curve and (d) Chrono-potentiometry curves at -10 mA cm-2

4.0 CONCLUSIONS

In this work, NRGO/BaWO4/g-C3N4 nanocomposites were successfully synthesized by microwave irradiation method and characterized by different techniques like XRD, Raman, SEM, HRTEM, XPS, BET, PL and UV-Visible spectroscopy. The synthesized composite exhibited enhanced efficiency in HER, photodegradation of MB dye and catalytic reduction of 4-NP to 4-AP suggesting that the material can be a very promising multifunctional catalyst. The present approach provides fundamental insights which can be extended in future to other metal tungstate based ternary composites to act as multifunctional catalyst in the field of clean energy and environment.

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31 of 31 pages

Details

Title
Novel nanocomposites for environmental and energy applications
Subtitle
Based on NRGO/BaWO4/g-C3N4
College
National Institute of Technology Karnataka, Surathkal
Grade
10
Author
Year
2019
Pages
31
Catalog Number
V464544
ISBN (Book)
9783346091017
Language
English
Tags
novel, based, nrgo/bawo4/g-c3n4
Quote paper
Dr. Mohamed Jaffer Sadiq M (Author), 2019, Novel nanocomposites for environmental and energy applications, Munich, GRIN Verlag, https://www.grin.com/document/464544

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