Self-Assembled Silver Nanoparticles Decorated on Exfoliated Graphitic Carbon Nitride/Carbon Sphere Nanocomposites as a Novel Catalyst for Catalytic Reduction of Cr(VI) to Cr(III) from Wastewater and Reuse for Photocatalytic Applications.

Silver nanoparticles decorated on an exfoliated graphitic carbon nitride/carbon sphere (AgNP/Eg-C3N4/CS) nanocomposites were synthesized by an adsorption method with a self-assembled process. These nanoparticles were characterized by different techniques like UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Raman spectroscopy, scanning electron spectroscopy (SEM), transmission electron spectroscopy (TEM), electrochemical impedance spectroscopy (EIS), and ζ potential. AgNP/Eg-C3N4/CS nanocomposites showed a higher catalytic reduction activity for the conversion of Cr(VI) into Cr(III) with formic acid (FA) at 45 °C when compared to bulk graphitic carbon nitride (Bg-C3N4, Eg-C3N4, CS, and Eg-C3N4/CS). The kinetic rate constants were determined as a function of catalyst dosage, concentration of Cr(VI), pH, and temperature for the AgNP/Eg-C3N4/CS nanocomposite. This material showed higher reduction efficiency (98.5%, k = 0.0621 min-1) with turnover frequency (0.0158 min-1) for the reduction of Cr(VI) to Cr(III). It also showed great selectivity and high stability after six repeated cycles (98.5%). Further, the reusability of the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite was also investigated for the photocatalytic degradation of methylene blue (MB) under visible light irradiation with various time intervals and it showed good degradation efficiency (α = 97.95%). From these results, the AgNP/Eg-C3N4/CS nanocomposite demonstrated higher catalytic activity, improved environmental friendliness, lower cost for the conversion of toxic Cr(VI) to Cr(III) in solutions, and also good reusability.

Introduction

Chromium(VI) is an inorganic pollutant that contaminates water and soil and produces carcinogenic and mutagenic effects in human and animals.[1−3] Most Cr(VI) is discharged into the environment by different industrial processes like metallurgy, dyeing, textiles, wood, paint, pulp, paper, leather, and metal finishing.[4−6] Chromium contains two oxidation states that are environmentally stable, and these are Cr(VI) and Cr(III), in which Cr(VI) is more toxic than Cr(III) due to better solubility and the existence of different anionic forms with pH (CrO42–, Cr2O72–, and HCrO4–).[7,8] By contrast, Cr(III) is less toxic and inert and trace quantities are utilized in biological applications.[9] The discharge of Cr(VI) into the environment causes various diseases like cancer, skin disorders, kidney problems, vomiting, and ulcerations.[10] The World Health Organization (WHO) permitted a maximum allowable level of 0.05 mg/L for Cr(VI) in water and manages environmental release of Cr(VI).[11] Therefore, the reduction of Cr(VI) to Cr(III) with a suitable green approach and eco-friendly materials is extremely important. Various methods have been developed for the reduction of Cr(VI) to Cr(III) and include adsorption,[12] photocatalytic reduction,[13,14] electrochemical reduction,[15] ion-exchange,[16] liquid–liquid extraction,[17] biosorption,[18] electrocoagulation,[19] and precipitation.[5] These methods have however shown some disadvantages like high cost and lower concentration detection.[20,21] Hence, the researchers are significantly challenged to develop a potential environmentally conducive method. However, the catalytic reduction method has shown promise for reinforcing greater conversion of Cr(VI) to Cr(III) due to easy operation, good competence, and eco-friendliness.

It is important to overcome the drawbacks associated with the catalytic reduction of Cr(VI) to Cr(III) by various materials such as H2S, Na2S2O4, and Fe(II).[22−24] Nowadays, noble metals (Pd, Pt, Ag, and Au),[25] noble bimetallic nanoparticles,[26,27] Pd nanoparticles,[28] core–shell AuPd@Pd nanocrystals,[29] core–shell Pd nanocrystals,[30] core–shell Pt@Pd nanowires,[31] Au@Pd heterostructures,[32] and PtAu nanospheres/reduced graphene oxide (RGO) have been utilized.[33] These nanomaterials were strongly recommended as efficient catalysts for the catalytic reduction of Cr(VI) to Cr(III).[34] However, the above materials are complicated to prepare, and consequently, their synthesis on an industrial scale is time-consuming.

Graphitic carbon nitride (g-C3N4) may be a metal-free catalyst with light harvesting ability for photocatalysis for the generation of hydrogen due to better conductivity of electronic transition sheet structures, physical and chemical stability, and easy availability.[35−37] However, g-C3N4 has demonstrated lower catalytic activity because of a higher electron–hole recombination rate with light absorption, less density of active sites, low electrical conductivity, and a high wavelength region.[38−40] Additionally, g-C3N4 with carbon materials possessed enhanced properties such as mechanical strength, electrical conductivity, and thermal stability.[41,42] The composite materials of carbon@g-C3N4, carbon dots/g-C3N4, graphene/g-C3N4, and carbon nanotube (CNT)/g-C3N4 have enhanced properties like conductivity, hydrogenation evolution, and photo-electrochemical activity.[43−46] Among them, carbon@g-C3N4 composite material has shown high surface area, good stability in physicochemical processes, low electron–hole recombination rate, good electronic transition in band structure, and better visible (vis) light photocatalyst activity.

Different nanocomposites such as Pd/graphene oxide (GO),[47] Pd@granular activated carbons (GACs),[48] Pd-CNTs,[49] Ni@C-dots,[50] Ag–Au/RGO,[51] Ni@GR-Cu,[52] Pd@SiO2–NH2,[53] polyaniline (PANI)/MnO2/TiO2,[54] Pt/Pd nanoparticles,[55] palladium (Pd) tetrapods,[56] AgCl@Ag/CS–NCs,[57] Ag/SnO2/NiO,[58] CdS/RGO,[59] g-C3N4/ZnFe2O4,[60] Fe3O4/RGO,[61] CuO/ZrO2-MCM-4,[62] α-MnO2@RGO,[63] G-Fe3O4 and Au/G-Fe3O4,[64] α-FeOOH nanorod/RGO,[65] Gd(OH)3 nanorod/RGO,[66] p–n heterojunction of MCeO2–BiFeO3,[67] Bi/Bi2MoO6,[68] and Ag@Ag3PO4/g-C3N4/NiFe layered double hydroxide (LDH)[69] have been used for the photocatalytic and catalytic reduction of Cr(VI) to Cr(III). Nowadays, formic acid (FA, HCOOH) is used in the reduction of Cr(VI) to Cr(III) and it acts as a reducing agent for the generation of hydrogen and carbon dioxide during the reaction.[70] Generally, the noble metals (Ag, Au, Pd, Pt, etc.) are coated on a hybrid composite to enhance the catalytic activity and reduce the electron–hole recombination rate and they have shown surface plasmon resonance (SPR) for charged electrons to hybrid composites and hydrogen production for the reduction of Cr(VI) to Cr(III).[58,71,72] Among these, Ag showed low toxicity, high surface area, better optical and electrical conductivities, and low band gap energy when compared to those of Pt and Pd.[73−75]

Herein, for the first time, the preparation of a novel silver nanoparticles decorated on exfoliated graphitic carbon nitride/carbon spheres (AgNP/Eg-C3N4/CS) nanocomposite has been reported and used for the catalytic reduction of Cr(VI) to Cr(III). Various precursors of melamine and d-dextrose were used for the synthesis of bulk graphitic carbon nitride (Bg-C3N4) and carbon sphere (CS) using thermal and hydrothermal methods, respectively. The exfoliated graphitic carbon nitride (Eg-C3N4) was prepared by an ultrasonication method with H2SO4 treatment. The preparation of the Eg-C3N4/CS composite was synthesized by an adsorption method. To the best of our knowledge, there are no reports on the AgNPs coated on the Eg-C3N4/CS nanocomposite. This nanocomposite was therefore employed in a chemical reduction method with NaBH4 as a reducing agent. This AgNP/Eg-C3N4/CS nanocomposite was characterized by various instrumental methods and used for the catalytic reduction of Cr(VI) to Cr(III) in water with formic acid as a reducing agent at 45 °C. Thereafter, the reuse application for the photocatalytic degradation of methylene blue (MB) was conducted with the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite after reduction under visible light irradiation with various time intervals.

Results and Discussion

Detailed Mechanism Formation of Self-Assembled AgNP/Eg-C3N4/CS Nanocomposites

An adsorption method was described for the synthesis of AgNP/Eg-C3N4/CS nanocomposite photocatalysts, and the preparation method is explained, as shown in Scheme . First, g-C3N4 was obtained by a thermal polycondensation method with melamine and it was dispersed in sulfuric acid with sonication to get Eg-C3N4. Then, CS was added into the mixture, which is obtained from d-dextrose by a hydrothermal method. The CS was coated on Eg-C3N4 to form a Eg-C3N4/CS composite due to electrostatic attraction between the negative charge of CS and positive charge of Eg-C3N4. Then, the addition of AgNO3 and sodium borohydrate (reducing agent) provided electrons for the formation of AgNPs from the AgNO3 solution to form the AgNP/Eg-C3N4/CS nanocomposite under stirring conditions.[76] Finally, the mixture was centrifuged to remove impurities and water and dried at room temperature to obtain the AgNP/Eg-C3N4/CS nanocomposite. In this method, Eg-C3N4 and CS were used as materials to form the self-assembled nanocomposite and they hindered the aggregation of AgNPs. Finally, the AgNP/Eg-C3N4/CS nanocomposite has shown good photocatalytic activity.

Scheme 3

Schematic Diagram of the Self-Assembled AgNP/Eg-C3N4/CS Nanocomposite

Characterization

UV–Visible Spectroscopy Study

Figure A(a–e) shows the UV–visible spectra of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites in the solution state. The spectrum of Bg-C3N4 showed peaks at 321 and 390 nm,corresponding to n−π* and π–π* of C=N in the triazine group.[77,78] The spectrum of Eg-C3N4 showed a shift to higher wavelength at 335 nm (π–π*) with a lower intensity peak at 390 nm (n−π*) when compared to that of Bg-C3N4, and this change was due to single-layer protonation of Eg-C3N4 with acid treatment, as shown in Figure A(b).[79,80] Bare CS does not show any distinct absorption peaks, as shown in Figure A(c). Figure A(d) shows that the peak moved to a blue shift at 328 nm (π–π*) and there was a minimized peak intensity at 390 nm (n−π*) due to sp2 hybridization of Eg-C3N4, which contributed to greater absorption of light and also lowered the band gap energy.[41,81]Figure A(e) shows two peaks at 411 and 358 nm, which corresponded to surface plasmon resonance (SPR) of AgNPs and electronic transitions in Eg-C3N4 groups during the self-assembly formation of the AgNP/Eg-C3N4/CS nanocomposite.[82]Figure B(a,b) shows the UV–visible diffuse reflectance spectra, which were used to calculate the band gap energy before and after reduction of Cr(VI) with the AgNP/Eg-C3N4/CS nanocomposite. Figure B(a) shows a peak at 444 nm due to the SPR effect of AgNPs in the AgNP/Eg-C3N4/CS nanocomposite.[83] The Cr(III)-AgNP/Eg-C3N4/CS nanocomposite showed a slightly red shift with reduced peak intensity at 454 nm, which confirmed the successful reduction Cr(VI) to Cr(III) and Cr(III) coating on the AgNP/Eg-C3N4/CS nanocomposite, as shown in Figure B(b). The band gap energies of the AgNP/Eg-C3N4/CS nanocomposite (2.79 eV) and Cr(III)-AgNP/Eg-C3N4/CS nanocomposite (2.73 eV) were obtained by the Kubelka–Munk plot of band gap energy (eV) vs (αhν)1/2, as shown in Figure C(a,b). Therefore, lower band gap energy also confirmed the effective reduction of Cr(VI) to Cr(III) with the AgNP/Eg-C3N4/CS nanocomposite.

Figure 1

(A) UV–vis spectra of Bg-C3N4 (a), Eg-C3N4 (b), CS (c), Eg-C3N4/CS (d), and AgNP/Eg-C3N4/CS nanocomposites (e); (B) DRS UV–vis spectra of the AgNP/Eg-C3N4/CS nanocomposite (a) and Cr(III)-AgNP/Eg-C3N4/CS nanocomposite (b); (C) Kubelka–Munk transformation of UV–vis spectra of the AgNP/Eg-C3N4/CS nanocomposite and Cr(III)-AgNP/Eg-C3N4/CS nanocomposite; and (D) photoluminance spectra of Bg-C3N4 (a), Eg-C3N4 (b), Eg-C3N4/CS (c), and AgNP/Eg-C3N4/CS nanocomposites (d).

Photoluminescence (PL) Study

The photoluminescence (PL) spectrum was used to determine the charge separation efficiency of photogenerated electrons and holes in catalyst materials.[84]Figure D(a–d) shows that the PL spectra of Bg-C3N4, Eg-C3N4, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites were measured with the recombination of electron and holes. The PL spectra of Bg-C3N4 showed a high-intensity peak at 422 nm, while the excitation wavelength was 360 nm, which shows additionally high recombination of electron–hole pairs, as shown in Figure D(a). Eg-C3N4 increased the peak intensity and slightly shifted the peak position at 428 nm at an excitation wavelength of 360 nm, and it also displayed the highest recombination of electron–hole pairs. By comparison of AgNP/Eg-C3N4/CS with Eg-C3N4/CS, the peak intensity for the AgNP/Eg-C3N4/CS nanocomposite decreased when compared to that of Eg-C3N4/CS. This is due to the fact that the AgNPs self-assembled on Eg-C3N4/CS lowered the recombination rate of electron–hole charge carriers, as shown in Figure D(d) (inset).[85] The reduced recombination rate of photogenerated electron–hole pairs was due to the higher catalytic activity when compared to Eg-C3N4/CS, Eg-C3N4, and Bg-C3N4. In this study, CS was negligible for PL analysis.

Fourier Transform Infrared (FT-IR) Study

The FT-IR spectra of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites were recorded to identify functional groups in the as-prepared materials, as shown in Figure A(a–e). Figure A(a) shows that the spectrum of Bg-C3N4 has several peaks at around 1687–969 cm–1, which corresponded to C=N, C–N, and C–NH–C stretching vibrations of tri-s-triazine molecules.[86,87] A sharp peak at 815 cm–1 was recorded and ascribed to the out-of-plane bending vibration of triazine units.[64] Two broad peaks were noted at 3373 and 3123 cm–1, and these are attributed to the N–H stretching vibration of the amine group and O–H stretching vibrations of water molecules on the Bg-C3N4 surface.[88]Figure A(b) shows that the sulfuric acid treatment of the Eg-C3N4 sheet also resulted in a peak with increasing intensity of the broad region of N–H amine and O–H stretching vibration and tri-s-triazine units of C–N and C–NH–C stretching vibration groups.[89] The triazine unit showed reduced peak intensity at 815 cm–1, which confirmed the acidification of Eg-C3N4, as shown in Figure A(b). CS shows a broad peak with high intensity at 3401 cm–1, which appeared due to the O–H stretching vibration. Peaks at 2986 and 2917 cm–1 were assigned to the C–H stretching vibration of sp2 (=CH2) and sp3 (−CH3) hybridization, respectively. The bands at 1700 and 1623 cm–1 were denoted by the C=O and C=C stretching vibrations, respectively, which implied the aromatization of d-dextrose.[90] The bands at 1049 and 866 cm–1 indicated the C–OH and C–H bending vibrations, respectively, which contained a OH group and an aromatic moiety on the CS, as shown in Figure A(c). Eg-C3N4/CS of N–H, O–H, C=O, and C=C groups showed decreased peak intensity, and the peaks associated with C–H and C–OH disappeared without a change in position, as shown in Figure A(d). The triazine unit was shifted to a lower wavenumber at 807 cm–1, and it is evident that Eg-C3N4 was enclosed on CS.[43,91,92]Figure A(e) shows that the majority of the peaks were mostly increased because AgNPs self-assembled on Eg-C3N4/CS as a result of electrostatic forces between AgNPs and Eg-C3N4/CS nanocomposites.[82,93]

Figure 2

(A) FT-IR spectra of Bg-C3N4 (a), Eg-C3N4 (b), CS (c), Eg-C3N4/CS (d), and AgNP/Eg-C3N4/CS nanocomposites (e); (B) X-ray diffraction (XRD) spectra of Bg-C3N4 (a), Eg-C3N4 (b), CS (c), Eg-C3N4/CS (d), and AgNP/Eg-C3N4/CS nanocomposites (e); and (C) thermal gravimetric analysis (TGA) spectra of Bg-C3N4 (a), Eg-C3N4 (b), CS (c), Eg-C3N4/CS (d), and AgNP/Eg-C3N4/CS nanocomposites (e).

XRD Study

The XRD pattern was used to determine the crystalline and amorphous nature of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites, as shown in Figure B(a–e). Bg-C3N4 showed two diffraction peaks at 13.06 and 27.77°, which corresponded to (100) and (002) planes, and these were related to diffraction planes of Bg-C3N4 (JCPDS Card no. 46-1088),[94] as shown in Figure B(b). The high-intensity peak of the (002) plane was dependent on interlayer stacking in the aromatic structure of graphite materials.[95] The low-intensity peak of the (100) plane was based on the in-planar repeating motifs of the tri-s-triazine unit.[96] Eg-C3N4 showed a changed peak position with low-intensity peaks at 12.43 and 27.55°, as shown in Figure B(c). Figure B(a) shows that two peaks were observed at 2θ = 23.15 and 44.58° for CS and were attributed to (002) and (101) planes, respectively, with a hexagonal graphite lattice structure. These diffraction patterns were correlated to CS (JCPDS no. 75-1621).[36]Figure B(d) shows that Eg-C3N4/CS exhibited a broad and sharp peaks at 25.01 and 19.36°, which were assigned to the (002) plane of CS and (002) plane of Eg-C3N4.[97] This XRD spectrum unambiguously confirmed that Eg-C3N4 was self-assembled on CS. The AgNP/Eg-C3N4/CS nanocomposite showed several diffraction peaks at 37.95, 44.08, 64.27, and 77.33°, which are related to the (111), (200), (220), and (311) planes of the FCC structure of AgNPs, respectively.[98−100] These were correlated with JCPDS card no. 01-089-3722 of AgNPs. The other diffraction peaks of CS (002) and Eg-C3N4 (002) disappeared due to the strong self-assembly of AgNPs on the Eg-C3N4/CS nanocomposite, as shown in Figure B(e). The mean crystallite size D was calculated using the predominant peak related to the AgNPs at 37.95° along the (111) plane and Scherrer’s formula D = Kλ/(B cos θ) (where K = 0.9 is a dimensionless shape factor, λ(CuKR) = 0.15418 nm, θ is the Bragg angle, and B is the full width at half-maximum of the diffraction peak, in radian); the average grain size of the AgNPs is calculated to be about D = 10 nm based on the observable four crystal directions and is consistent with the transmission electron spectroscopy (TEM) observations.

TGA Study

Thermal stabilities of Bg-C3N4, Eg-C3N4, CS, and Eg-C3N4/CS and AgNP/Eg-C3N4/CS nanocomposites were determined by thermogravimetric analysis, as shown in Figure C(a–e). Bg-C3N4 showed a weight loss at 550–750 °C, which means that the decomposition of carbon nitride occurred with 63.67% weight loss, as shown in Figure C(a).[101,102]Figure C(b) shows that Eg-C3N4 exhibited two weight losses at 500–710 °C (29.6%) and 95–250 °C (2.67%) corresponding to the removal of carbon nitride and water molecules, respectively. As can be seen from the spectra, Eg-C3N4 showed less weight loss than Bg-C3N4 due to high thermal stability of Eg-C3N4.[103] CS showed three curves at the 25–125 °C, 125–378 °C, and 378–690 °C temperature ranges, which corresponded to weight losses of 1.6, 27.67, and 8.83%, respectively, as shown in Figure C(c). The first curve was formed due to elimination of moisture, the second curve was due to the decomposition of oxygen groups, and the third curve was due to the dehydration of oxygen-containing groups on CS. Eg-C3N4/CS showed a clear bending curve at 95–225 °C, which was based on the elimination of water, another weight loss at 225–375 °C, which was attributed to the decomposition of CS, and the last bending curve at 375–650 °C, which was due to the decomposition of carbon nitride in Eg-C3N4, as shown in Figure C(d). The AgNP/Eg-C3N4/CS nanocomposite showed large curves at 115–335 °C with 10.06% weight loss and at 375–725 °C with 7.94% weight loss, as shown in Figure C(e). From the results, the self-assembled AgNP/Eg-C3N4/CS nanocomposite showed higher thermal stability than Eg-C3N4/CS, CS, Eg-C3N4, and Bg-C3N4.[104]

Raman Study

Raman spectra were recorded to investigate the crystalline nature of carbon materials of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites, as shown in Figure S1A–E, Supporting Information. Figure A shows two bands, which appeared at 1445 and 1556 cm–1 and corresponded to the D-band and G-band of amorphous Bg-C3N4 atoms, respectively.[105] The D-band represented the disordered phase and defects of the graphite layer of sp3 atoms in ring modes of the A1 symmetry, and the G-band was attributed to the graphite structure of the E2g mode of sp2 carbon–carbon bonds atoms in the ring and chains.[106] The peaks at 577, 785, and 1115 cm–1 corresponded to the A1′ vibration symmetry of tri-s-triazine units of Bg-C3N4.[107,108] The peaks at 673 and 978 cm–1 were assigned to the 2 mode and 1 mode of traizine rings of Bg-C3N4 atoms, respectively, as shown in Figure S1A, Supporting Information.[109] The tri-s-triazine units of Eg-C3N4 showed both high-intensity and low-intensity peaks at 547 and 1115 cm–1, respectively, and the peak at 651 cm–1 together with some other peaks disappeared. The peaks of triazine units reduced in intensity as well and the peak positions changed to 651 and 993 cm–1, respectively, which confirmed that Eg-C3N4 contained sulfuric acid. The D-band and G-band also decreased in peak intensity at 1458 and 1551 cm–1, respectively, as shown in Figure S1B, Supporting Information. The ratio of ID/IG showed the defects and disorders in the graphite material. The ID/IG value of Eg-C3N4 (1.04) was higher than that of Bg-C3N4 (0.98), which provided evidence that the structure of Eg-C3N4 was more disordered and contained more defects.[110] The Raman spectra of CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites showed the two important vibrations of the D-band and G-band, as shown in Figure S1C–E, Supporting Information. The comparison of ID/IG ratio values for CS (1.07), Eg-C3N4/CS (1.01), and AgNP/Eg-C3N4/CS nanocomposite (0.96) showed that the AgNP/Eg-C3N4/CS nancomposite contained less disorder in its structure when compared to Bg-C3N4, Eg-C3N4, Eg-C3N4/CS, and CS.[111,112] As can be seen from Figure S1A–E, Supporting Information, the ID/IG value of the AgNP/Eg-C3N4/CS nanocomposite was lowered, which means that the AgNPs preferred the degree of graphite carbon atoms.[113,114]

Figure 3

(A) ζ potentials of (a) Bg-C3N4, (b) Eg-C3N4, (c) CS, (d) Eg-C3N4/CS, and (e) AgNP/Eg-C3N4/CS nanocomposites and (B) electrochemical impedance spectroscopy (EIS) Nyquist plot of (a) Bg-C3N4, (b) Eg-C3N4, (c) CS, (d) Eg-C3N4/CS, and (e) AgNP/Eg-C3N4/CS nanocomposites.

Brunauer–Emmett–Teller (BET) Study

The surface area, pore volume, and pore size were observed by the Brunauer–Emmett–Teller (BET) method with nitrogen adsorption–desorption processes.[115]Figure S2A–E (Supporting Information) shows the surface areas of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/C3N4/CS nanocomposites, which were determined by isothermal N2 adsorption–desorption BET methods at 10 °C/min.[116−119] These obeyed the type H3 hysteresis loop model and also assumed mesoporous and macroporous properties by Barrett–Joyner–Halenda (BJH) plots with pore size distribution range 10–100 nm, as shown in Figure S2A–E (inset), Supporting Information. Among them, the AgNP/Eg-C3N4/CS nanocomposite showed the higher surface area and lower size because of electrostatic attractions between AgNPs and Eg-C3N4/CS nanocomposites, which were favored for catalytic applications.

Analysis

The surface charge of self-assembly processes was also investigated by ζ potential measurements. The surface charges of Bg-C3N4, Eg-C3N4, CS, and Eg-C3N4/CS and AgNP/Eg-C3N4/CS nanocomposites were measured. The ζ potentials corresponded to −31.8, 12.1, −38.6, −34.0, and −35.4 mV, respectively, as shown in Figure A(a–e). Bg-C3N4 indicated a negative charge ζ potential at −31.8 mV due to the amine groups of heptazine rings.[120] The ζ potential of Eg-C3N4 showed a positive potential at 12.1 mV, which is due to the protonation of Eg-C3N4 with H2SO4. CS displayed a higher negative charge potential at −38.6 mV due to the several oxygen atoms of COOH and OH groups. After self-assembly of Eg-C3N4/CS, a less negative potential at −34.0 mV was noted when compared to the potential of CS because of the positive charge of Eg-C3N4 coated on the negative charge of CS with electrostatic interactions.[121,122] The AgNP/Eg-C3N4/CS nanocomposite also increased the negative charge to about −35.4 mV due to electrostatic repulsion forces between AgNPs and the Eg-C3N4/CS nanocomposite.[123]

EIS Study

Electrochemical impedance spectroscopy (EIS) measurement was used to determine the electron-transfer behavior of the catalyst. The radius of the arc is related to the charge-transfer behavior at the electrode–electrolyte interface.[124] The conductivity behavior and resistance of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites were modified on a glassy carbon electrode (GCE) in a supporting electrolyte solution of 0.1 M KCl and 10 mM K3[Fe(CN)6]3–/4– with a frequency range of 0.1–100 kHz, as shown in Figure B(a–e). The EIS Nyquist plot of CS spheres showed a large arc radius due to less charge separation and low electron transfer at the electrode and electrolyte interface, as shown in Figure B(c). Bg-C3N4 exhibited higher resistance and lower conductivity because of a large semicircle arch, which blocks the electron transfer, as shown in Figure B(a).[125] The semicircle arch was slightly decreased and also the conductivity slightly increased with reduced resistance of Eg-C3N4, as shown in Figure B(c).[126] Eg-C3N4/CS was also reduced in diameter due to Eg-C3N4 self-assembled on CS, which increased the conductivity and improved the electron transfer, as shown in Figure B(d).[127] The AgNP/Eg-C3N4/CS nanocomposite semicircle of the Nyquist plot showed a smaller arc because of AgNPs hindering the recombination of electron–hole charge separation toward the catalytic reduction of Cr(VI) to Cr(III), as shown in Figure B(e). The AgNP/Eg-C3N4/CS nanocomposite catalyst conducted the charge transfer between the modified electrode and electrolyte solution more effectively than Eg-C3N4/CS, CS, Eg-C3N4, and Bg-C3N4.[128]

Surface Morphology Study

The surface morphologies of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites was determined by scanning electron spectroscopy (SEM), as shown in Figure S3A–E, Supporting Information. The image of multilayer with aggregated sheets of Bg-C3N4 was displayed at a high magnification of 10 μm, as shown in Figure S3A, Supporting Information. Eg-C3N4 showed several single-layer sheets at 10 μm magnification, which confirmed that Eg-C3N4 was acid-treated. Bg-C3N4 is shown in Figure S3B, Supporting Information. The uniform spherical shape was observed at a high magnification of 2 μm, which is due to the carbonization of d-dextrose to CS, as shown in Figure S3C, Supporting Information. Eg-C3N4 was coated on the smooth surface of CS. Eg-C3N4/CS at magnification of 2 μm is shown by the yellow arrow shown in Figure S3D, Supporting Information. The white particles of AgNPs coated on the Eg-C3N4/CS composite (AgNP/Eg-C3N4/CS nanocomposite) at 2 μm are shown in Figure S3E, Supporting Information, which is formed by the electrostatic interaction between the AgNPs and Eg-C3N4/CS. The chemical composition of the AgNP/Eg-C3N4/CS nanocomposite presented C, N, O, and Ag, and they were evaluated by energy-dispersive X-ray analysis (EDAX), as shown in Figure S3F, Supporting Information.

TEM Study

The TEM images further confirmed the sizes and shapes of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites, as shown in Figure A–N. The Bg-C3N4 showed sheetlike structures with black dots at various magnifications of 500, 200, and 100 nm, as shown in Figure A–C. At a high magnification of 100 nm, Bg-C3N4 revealed multilayer sheets, as shown in Figure C. The single-layer sheets with dispersed images were displayed at 200, 100 and 100 nm magnifications, as shown in Figure D–F. Figure F shows that a clear single sheet was recorded at 100 nm magnification, which confirmed the formation of Eg-C3N4 with ultrasonication. Eg-C3N4 was coated on the CS layer at low magnifications of 0.5 μm and 500 nm, as shown in Figure G,H. Eg-C3N4/CS was clearly coated on the CS layer at 1 μm, as shown in Figure I, which proved the strong forces of electrostatic interaction between the amine group of Eg-C3N4 and hydroxyl group of the CS layer. The AgNP/Eg-C3N4/CS nanocomposite confirmed that the AgNPs were coated on the negatively charged surface of the Eg-C3N4/CS composite, and it was analyzed by TEM with different magnifications of 200, 100, 100, 100, and 50 nm, as shown in Figure J–N. At the highest magnification of 50 nm, AgNPs were clearly coated on Eg-C3N4/CS, as shown Figure N. Further, the crystallinity of AgNPs in the AgNP/Eg-C3N4/CS nanocomposite was determined by selected area electron diffraction (SAED) pattern, as shown in Figure O. The particle size distribution in the histogram plot is shown in Figure O (inset). The spacing distance of the lattice border of AgNPs was clearly shown in the high-resolution TEM (HRTEM) images in Figure P,Q. From the TEM and HRTEM images, it was evident that AgNPs successfully self-assembled on the surface of the Eg-C3N4/CS composite, and this confirmed the formation of the AgNP/Eg-C3N4/CS nanocomposite and enhanced the catalytic activity.

Figure 4

TEM images of Bg-C3N4 at (A) 500 nm, (B) 200 nm, and (C) 100 nm; Eg-C3N4 at (D) 200 nm, (E) 100 nm, and (F) 100 nm; Eg-C3N4/CS at (G) 0.5 μm, (H) 500 nm, and (I) 1 μm; AgNP/Eg-C3N4/CS nanocomposite at (J) 200 nm, (K) 100 nm, (L) 100 nm, (M) 100 nm, and (N) 50 nm. (O) SAED pattern of the AgNP/Eg-C3N4/CS nanocomposite (inset: histogram of the particle size distribution). (P, Q) High-resolution TEM images of AgNPs on the AgNP/Eg-C3N4/CS nanocomposite.

XPS Study

X-ray photoelectron spectroscopy (XPS) was conducted to investigate the binding energy, chemical composition, and oxidation states of elements in the as-synthesized materials. The survey spectra of CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites were recorded, as shown in Figure A(a–c). The survey spectrum of CS showed two peaks of C 1s and O 1s, as shown in Figure A(a). Gaussian software was used to fit the C 1s into three peaks such as C=C, C–C, and O–C=C, as shown in Figure S4A, Supporting Information. The O 1s peak was assigned to the three peaks of C=O, C–O, and O–C=C, as shown in Figure S4B, Supporting Information. These fitted curves confirmed the formation of CS by the hydrothermal method. The survey spectra of Eg-C3N4/CS exhibited three peaks of C 1s, N 1s, and O 1s, as shown in Figure A(b). Figure S5A–C, Supporting Information, shows three peaks at 284.5, 286.8, and 288.5 eV with defects of sp2-hybridized C=C bond, sp3-hybridized C–N bond in Eg-C3N4, and C=O and HO–C=O bonds in carbon spheres.[85,129] The O 1s fitting curve demonstrated two peaks at 529.5 and 532.4 eV of N–C–O and −C–O–C=O bonds due to formation of the Eg-C3N4/CS composite, as shown in Figure S4B, Supporting Information.[130] The N 1s fitted spectrum displayed three peaks at 396.2, 398.4, and 399.1 eV, which corresponded to tertiary nitrogen of N–(C)3, triazine units of C=N–C, and the amine group of C–N–H, respectively, and were protonated with the positive charge of the −C–N bond, as shown in Figure S5C, Supporting Information.[46,131] These results showed that the self-assembled Eg-C3N4 on CS formed the Eg-C3N4/CS composite. Figure A(c) shows the spectrum of the AgNP/Eg-C3N4/CS nanocomposite in which the four peaks of C 1s, N 1s, O 1s, and Ag 3d appeared. The C 1s spectrum was centered at 282.5 eV for the C=C bond, 284.2 eV for the C–N bond in Eg-C3N4, and C=O and O–C=O bonds in CS, as shown in Figure B. The fitting curve of O 1s was splitted into three peaks at 536.5 and 537.2 eV, which corresponded to C–OH/C–O–C bonds in CS and N–C–O bonds in Eg-C3N4, as shown in Figure C. Figure D shows that the N 1s fitted curve had three peaks at 397.6 eV for the sp2 C=N–C bond and 398.5 and 399.8 eV of N–(C)3 bonds in Eg-C3N4.[132] The deconvoluted spectrum of AgNPs was splitted into four peaks at 365.2 eV, 368.4 eV (Ag 3d5/2) and 371.6 eV and 374.4 eV (3d3/2), as shown in Figure E. From these binding energies, 368.4 and 374.4 eV indicated the presence of metallic silver (Ag0) on the AgNP/Eg-C3N4/CS nanocomposite.[82,133]

Figure 5

(A) XPS survey spectra of CS (a), Eg-C3N4/CS (b), and AgNP/Eg-C3N4/CS nanocomposite (c) and fitting curves of (B) C 1s, (C) O 1s, (D) N 1s, and (E) Ag 3d and Ag 5d of the AgNP/Eg-C3N4/CS nanocomposite.

Catalyst Testing for Conversion Cr(VI) to Cr(III)

Catalytic reduction of Cr(VI) to Cr(III) (K2Cr2O7, 20 ppm) was determined by Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites (15 mg) as catalysts in the presence of HCOOH (2 mM, 50 mL) as a reducing agent at 45 °C at different time intervals of 0–28 min. Similar experiments were conducted without the respective catalysts. The catalytic performance for Cr(VI) to Cr(III) reduction was monitored by UV–visible spectroscopy with a peak at 350 nm.[134]Figure A–E shows that the catalytic reduction of Cr(VI) to Cr(III) was recorded in the presence of HCOOH without and with a catalyst at various time intervals of 0–28 min. The conversion of Cr(VI) to Cr(III) absorption peaks was not altered at 350 nm at different time intervals of 0–28 min due to high kinetic barriers between Cr2O72– and FA without a catalyst, as shown in Figure A–E. The catalysts of Bg-C3N4, Eg-C3N4, CS, and Eg-C3N4/CS had not completely reduced Cr(VI) to Cr(III), as shown in Figure A–D. The yellow Cr(VI) regularly turned colorless due to the formation of Cr(III) in the presence of FA with the AgNP/Eg-C3N4/CS nanocomposite as the catalyst with increasing times of 0–28 min, as shown in Figure E. The AgNP/Eg-C3N4/CS nanocomposite therefore showed higher catalytic activity when compared to Bg-C3N4, Eg-C3N4, CS, and Eg-C3N4/CS. This AgNP/Eg-C3N4/CS nanocomposite’s capability of reduction of Cr(VI) to Cr(III) was also confirmed with the addition of excess NaOH to produce the green precipitate of Cr(OH)63–, as shown in Figure E, inset.

Figure 6

UV–vis spectra for catalytic reduction of Cr(VI) to Cr(III) in FA with different catalysts: 15 mg/mL (A) Bg-C3N4, (B) Eg-C3N4, (C) CS, (D) Eg-C3N4/CS, and (E) AgNP/Eg-C3N4/CS nanocomposite for 0–28 min at 45 °C and pH 2; (F) plot of the reduction efficiency % vs time; and (G) plot of ln(C/C0) vs time of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites. Digital images of additional excess of 10 M NaOH to green precipitate of Cr(OH)63– ((E) inset).

Figure F shows the catalytic reduction efficiency of Cr(VI) to Cr(III), which was evaluated with a plot of α = CI – CF/CI × 100 vs time by eq . The catalytic detoxification of Cr(VI) to Cr(III) is illustrated in the plot of α = CI – CF/CI × 100 vs time with different catalysts of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites, as shown in Figure F. From Figure F, the AgNP/Eg-C3N4/CS nanocomposite exhibited 98.51% conversion of Cr(VI) to Cr(III) when compared to the other catalysts of CS/Eg-C3N4 (33.35%), CS (5.59%), Eg-C3N4 (12.60%), and Bg-C3N4 (8.51%) because of the synergistic effect of the AgNP/Eg-C3N4/CS nanocomposite and Cr2O72– in the presence of FA. Therefore, AgNPs were an effective nanocatalyst in the AgNP/Eg-C3N4/CS nanocomposite for the catalytic reduction of Cr(VI) to Cr(III) due to the initial adsorption of FA on the AgNP/Eg-C3N4/CS nanocomposite to form CO2 and H2 and then the catalytic conversion of Cr(VI) to Cr(III) due to intermolecular interaction, charge-transfer reaction, physisorption, and electrostatic interaction forces between Cr(VI) and the AgNP/Eg-C3N4/CS nanocomposite.[135,136]

The catalytic reduction rate constant of Cr(VI) to Cr(III) was computed from the plot of ln(C/C0) vs time (t) with the various catalysts of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites, as shown in Figure G. The rate of reaction was dependent on the reducing action of FA and not Cr(VI) ions because of the higher concentration of FA vs Cr(VI) ions. Hence, the reaction rate constant might be pseudo-first-order kinetic, as shown in eq where C0 and C represent the intensities of the absorbance at initial time (t = 0) and time t, respectively. The AgNP/Eg-C3N4/CS nanocomposite showed a higher rate constant (k = 0.0621 min–1) when compared to the other catalysts Eg-C3N4/CS ((k = 0.0146 min–1), Eg-C3N4 (k = 0.0019 min–1), Bg-C3N4 (k = 0.0036 min–1), and CS (k = 0.0021 min–1), as shown in Figure G. It was observed that the AgNP/Eg-C3N4/CS nanocomposite showed better catalytic conversion of Cr(VI) to Cr(III) in the presence of FA. The catalytic activity of the AgNP/Eg-C3N4/CS nanocomposite was compared with other nanocomposites, as mentioned in Table . Furthermore, the turnover frequency (TOF) was evaluated for the catalytic activity of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites by eq . The AgNP/Eg-C3N4/CS nanocomposite showed a higher TOF value of 0.01587 min–1, as shown in Table . Among them, the AgNP/Eg-C3N4/CS nanocomposite recorded excellent catalytic activity when compared to the other catalyst materials.

Table 1

Comparison of Catalytic Reduction of Cr(VI) into Cr(III) on the AgNP/Eg-C3N4/CS Nanocomposite with Other Reported Nanocomposites

catalysts	dosage	Cr(VI) concentration	TOF (min–1)	pH	efficiency (%)	rate constant (k) (min–1)	ref
Pd/GO	0.05	2.0 mM	3.6				(47)
Pd@GAC	2.0	0.8 mM	358.1			0.4210	(48)
Pd-CNTs	1.0 mol %	1.0 mM				0.31	(49)
Ni@C-dots	0.5 mL	1 mM					(50)
Ag–Au/RGO	0.5 mg	0.1 mM	186.2		99.6	2.5793 × 10–2	(51)
Ni@GR-Cu	4 mg	100 mg/L	0.024	2.0		0.344	(52)
Pd@SiO2–NH2	15.00 mg/mL	6.0 mM	258	3.0	>85.0	0.094–0.363	(53)
PANI/MnO2/TiO2	1.0 mg/mL	2 mM	0.3333	2	99.90	15.97 × 10–2	(54)
Pt/Pd NPs	15 mg	20 mM	1.0 × 1018			0.133	(55)
palladium (Pd) tetrapods	2.0 g	2 mM				0.571	(56)
AgCl@Ag CS–NCs	20 mg	0.2 mM		2.0		0.125	(57)
Ag/SnO2/NiO	1 g/L	20 mg/L		2.0	98	0.023	(58)
CdS/RGO	2.5 × 10–2 g L–1	1 × 10–5 M				4.36 × 10–3	(59)
PANI-sensitized g-C3N4/ZnFe2O4	0.2 g	20 ppm		2.0	74.0	0.0326	(60)
g-Fe3O4/RGO	1 g/L	50 ppm		2.0	97		(61)
mesoporous CuO/ZrO2-MCM-41	1 g/L	20 mg/L		4.0	99		(62)
α-MnO2@RGO nanorods	1 g/L	10 mg /L			97		(63)
G-Fe3O4 and Au/G-Fe3O4	0.02 g	10 ppm				0.0313	(64)
α-FeOOH nanorod/RGO	1 g/L	10 mg/L			94		(65)
Gd(OH)3 nanorod/RGO	1 g/L	10 mg/L			83		(66)
p–n heterojunction of MCeO2–BiFeO3	20 mg	10 ppm			98.2	0.06533	(67)
Bi/Bi2MoO6	0.02 g	100 mg/L		2	97	0.014	(68)
Ag@Ag3PO4/g-C3N4/NiFe layered double hydroxide	0.02 g	20 ppm		5	97	0.0041	(69)
AgNP/Eg-C3N4/CS nanocomposite	15 mg	20 ppm	0.01587	2.0	98.5	6.21 × 10–2	this work

Comparison Study with the 1,5-Diphenylcarbazide (DPC) Method

Further, the catalytic activity of the AgNP/Eg-C3N4/CS nanocomposite was also investigated for the reduction of Cr(VI) by measuring the accurate concentration between the absorption peak of DPC–Cr(VI) at 540 nm and simulated formic acid at various time intervals. During the catalytic reduction method, the absorption spectra at 540 nm was regularly reduced due to the concentration of Cr(VI)-DPC decreasing gradually, and it was completely decolorized after 28 min, which demonstrates that the catalytic reduction of Cr(VI) to Cr(III) using the AgNP/Eg-C3N4/CS nanocomposite has occurred, as shown in Figure A. In the meantime, the Cr(VI) concentration was reduced with different time intervals from 0 to 28 min.

Figure 7

(A) UV–visible spectra of catalytic reduction of Cr(VI)-DPC with the AgNP/Eg-C3N4/CS nanocomposite, (B) calibration plot of (C/C0) and ln(C/C0) vs time for reduction of Cr(VI) with the AgNP/Eg-C3N4/CS nanocomposite, (C) comparison plot of kinetic rate constant vs methods, and (D) digital image of successive reduction of Cr(VI) by the Cr(VI)-DPC method.

The catalytic activity of the AgNP/Eg-C3N4/CS nanocomposite at various time intervals and concentrations of Cr(VI) is determined by a plot of C/C0 vs time, as shown in Figure B(a). According to eq , the pseudo-first-order reaction kinetic rate constant of the AgNP/Eg-C3N4/CS nanocomposite is k = 0.0599 min–1, as shown in Figure B(b). The k value (0.0599 min–1) for the Cr(VI)-DPC method was slightly below the value obtained for the Cr(VI)-AgNP/Eg-C3N4/CS nanocomposite method (k = 0.0621 min–1), as shown in Figure C. Figure D shows the digital image of the catalytic reduction of Cr(VI) by the Cr(VI)-DPC method with a color change from violet to colorless. Therefore, the AgNP/Eg-C3N4/CS nanocomposite was suitable for the catalytic reduction Cr(VI) to Cr(III).[57,137]

Effect of Catalyst Dosage

The optimized catalyst dosage was very important to identify the catalytic reduction of Cr(VI) to Cr(III) by changing the catalyst dosage of 5, 10, and 15 mg of AgNP/Eg-C3N4/CS nanocomposite in the presence of FA at 0–28 min, as shown in Figure S6A–C, Supporting Information. The results showed that the catalytic reduction percentages of 45.6, 76.8, and 98.5% corresponded to different catalyst weights of 5, 10, and 15 mg of AgNP/Eg-C3N4/CS nanocomposite, respectively as shown in Figure S6D, Supporting Information. The kinetic rate constant was obtained from the plot of ln(C/C0) vs time, k = 0.0184, 0.452, and 0.0620 min–1, which corresponded to 5, 10, and 15 mg of AgNP/Eg-C3N4/CS nanocomposite, as shown in Figure S6E, Supporting Information. The results confirmed that 15 mg of AgNP/Eg-C3N4/CS nanocomposite diminished the yellow Cr(VI) to colorless Cr(III) within 0–28 min with FA. This higher catalyst dosage of AgNP/Eg-C3N4/CS nanocomposite resulted in the rapid reduction of Cr(VI) to Cr(III) within 0–28 min due to increased active sites and a better reducing agent like FA to produce H2 and CO2.[138] Further, the kinetic studies were performed for the conversion of Cr(VI) to Cr(III) with 15 mg of AgNP/Eg-C3N4/CS nanocomposite as the catalyst.

Effect of Concentration

Different Cr(VI) concentrations (5, 10, 15, and 20 ppm) were used with AgNP/Eg-C3N4/CS nanocomposite and FA, as shown in Figure S7A–D, Supporting Information. The catalytic reduction of Cr(VI) to Cr(III) was rapid (within 0–8 min) for the lower concentration of Cr(VI) (5 ppm) and higher catalytic activity sites of 15 mg of AgNP/Eg-C3N4/CS nanocomposite, as shown in Figure S7A, Supporting Information. The Cr(VI) concentration was increased to about 10 and 15 ppm and the catalytic reduction time was also increased 0–12 and 0–18 min, respectively, under the same conditions, as shown in Figure S6B,C, Supporting Information. The higher concentration of 20 ppm of Cr(VI) uniformly decreased the peak intensity and also decolorized Cr(VI) with the AgNP/Eg-C3N4/CS nanocomposite at 0–28 min, as shown in Figure S7D, Supporting Information. Concentrations above 20 ppm reduced the catalytic reduction of Cr(VI) to Cr(III) under the same conditions because of higher quantities of Cr(VI) ions that block the active sites and enhance the shielding effect of the AgNP/Eg-C3N4/CS nanocomposite, as a result of minimized decomposition of FA and less conversion of Cr(VI) to Cr(III).[139] From the results, 20 ppm of Cr(VI) was suitable for the catalytic study.

Figure S6E,F (Supporting Information) shows the percentage of catalytic reduction and kinetic rate constants of Cr(VI) to Cr(III) reductions with various concentrations of 5, 10, 15, and 20 ppm. The highest percentage conversion of Cr(VI) to Cr(III) was at 20 ppm (98.5%) when compared to 15 ppm (96.90%), 10 ppm (97.16%), and 5 ppm (96.5%), as shown in Figure S7E, Supporting Information. The rate constants of Cr(VI) to Cr(III) reduction were k = 0.0620, 0.163, 0.285, and 0.4023 min–1 with corresponding concentrations of 20, 15, 10, and 5 ppm, respectively, as shown in Figure S7F, Supporting Information. The data confirmed that the optimized concentration of Cr(VI) was 20 ppm.

Effect of Temperature

The effect of temperature (25, 35, and 45 °C) was studied for the catalytic reduction of Cr(VI) to Cr(III) with 15 mg of AgNP/Eg-C3N4/CS nanocomposite in the presence of FA at 0–28 min, as shown in Figure S8A–C, Supporting Information. Figure S8A shows that the catalytic reduction of Cr(VI) did not show complete decolorization at 25 °C with different time intervals of 0–28 min. The conversion of Cr(VI) to Cr(III) was partially conducted at 35 °C with 0–28 min, as shown in Figure S8B, Supporting Information. At a higher temperature of 45 °C, the catalytic reduction of Cr(VI) to Cr(III) gradually reduced the peak intensity at 350 nm and 0–28 min, as shown in Figure S8C, Supporting Information. The catalytic conversion of Cr(VI) to Cr(III) was well performed at a higher temperature of 45 °C with the AgNP/Eg-C3N4/CS nanocomposite at 0–28 min due to better decomposition of FA to CO2 and H2 followed by the reduction of Cr(VI).[140] The reduction percentage of Cr(VI) to Cr(III) is as follows: 45 °C (90.0%), 35 °C (44.54%), and 25 °C (39.67%), as shown in Figure S8D, Supporting Information. The kinetic rate constant k values were derived from the plot ln (C/C0) vs time, as shown in Figure S8E, Supporting Information. Higher conversion percentage (90.0%) and kinetic rate constant (0.0762 min–1) were evaluated at a higher temperature of 45 °C due to heat energy required for the catalytic reduction of Cr(VI) to Cr(III).[141−144]

Effect of pH

The effective catalytic reduction of Cr(VI) to Cr(III) was dependent on the pH of different solutions because of the surface charge of the catalyst and different species of chromium(VI) at pH 2.0 (H2Cr2O7), pH 2.0–6.0 (Cr2O72– and HCrO4–), and pH 6.8 (CrO42–) in solution.[145,146] The pH of solutions of Cr(VI) (20 ppm) was varied at 2, 4, and 6, and the catalytic activity for the detoxification of Cr(VI) was evaluated with 15 mg of AgNP/Eg-C3N4/CS nanocomposite in the presence of FA at 0–28 min, as shown in Figure S9A–C, Supporting Information. Figure S9A shows that pH 2.0 exhibited the highest catalytic reduction of Cr(VI) to Cr(III) because Cr(VI) dissociates into various types of ions such as HCrO4– and Cr2O72– at lower pH.[147] The maximum absorption peak of Cr(VI) at 350 nm was observed under acidic conditions (pH 2) because of the protonation of CrO42– to form HCrO4–, and this result is consistent with that reported in the literature.[148]Figure S9B,C shows that at pH 4 and 6 incomplete reduction of Cr(VI) to Cr(III) occurred due to electrostatic repulsion between the negative charge of the catalyst of AgNP/Eg-C3N4/CS nanocomposite and negative charge of Cr(VI).[149] At high pH 4–6, the major species is CrO42– in solution and the Cr(VI) solution color changed from light yellow to intense yellow, which corresponded to a shift in the absorption peak position with pH.[150] This effect was also observed in a previous report.[151] The percentage reduction of Cr(VI) to Cr(III) was calculated at pH 2 (80.3%), pH 4 (69.7%), and pH 6 (59.4%), as shown in Figure S9D, Supporting Information. In highly acidic solutions, Cr(VI) to Cr(III) displayed higher reduction efficiency (80.3%) and rate constant (0.0488 min–1) due to the positive charge of the AgNP/Eg-C3N4/CS nanocomposite and negative charge of HCrO4– and Cr2O72–, as shown in Figure S9E, Supporting Information.[152] Under basic conditions, the rate constant decreased because of the precipitation of CrO42– to Cr(OH)3, and it created a negative charge on the AgNP/Eg-C3N4/CS nanocomposite. Hence, the catalytic reduction of Cr(VI) to Cr(III) was favored at acidic pH 2 with the AgNP/Eg-C3N4/CS nanocomposite than at pH 4 and 6.[28] The catalytic reduction of Cr(VI) to Cr(III) is shown with acid, neutral, and base conditions in eqs –4.[153,154]

Acidic conditionNeutral conditionBasic condition

Interference and Stability Studies

The interference study was investigated using different inorganic (NaCl, NaF, KCl, and K2HPO4) and organic (starch, d-dextrose, cellulose, urea, and citric acid) compounds with the AgNP/Eg-C3N4/CS nanocomposite under the same conditions. This showed promising performance of the AgNP/Eg-C3N4/CS nanocomposite for catalytic reduction of Cr(VI) to Cr(III) with FA in the presence of inorganic and organic compounds in wastewater. Figure A shows the plot of reduction efficiency (%) vs various interferences was slightly changed in the conversion of Cr(VI) to Cr(III) with AgNP/Eg-C3N4/CS nanocomposite because of low interaction of inorganic and organic substances during the reduction of Cr(VI). The stability test is very important for the reuse and efficiency of the AgNP/Eg-C3N4/CS nanocomposite after reduction of Cr(VI) to Cr(III). The nanocomposite was washed with distilled water in every process and dried at room temperature, and it was applied into a newly prepared Cr(VI) solution. The AgNP/Eg-C3N4/CS nanocomposite did not reduce the catalytic activity much after six cycles, as shown in Figure B. Further, it maintained the same morphology as shown by TEM and FT-IR after reuse for the reduction of Cr(VI) to Cr(III), as shown in Figure A–C. The above test confirmed that the AgNP/Eg-C3N4/CS nancomposite was a better catalyst with good stability.

Figure 8

(A) Plot of catalytic conversion % vs interference study of catalytic reduction Cr(VI) to Cr(III) after addition of (1) NaCl, (2) KCl, (3) NaF, (4) K2HPO4, (5) starch, (6) d-dextrose, (7) cellulose, (8) urea, and (9) citric acid with the AgNP/Eg-C3N4/CS nanocomposite and (B) stability of the AgNP/Eg-C3N4/CS nanocomposite for catalytic reduction of Cr(VI) to Cr(III) in FA after six repeated recycles.

Figure 9

(A–C) TEM images of the AgNP/Eg-C3N4/CS nanocomposite after six recycles at (A) 100 nm and (B) 50 nm and (C) FT-IR spectra of the AgNP/Eg-C3N4/CS nanocomposite before and after reduction of Cr(VI) to Cr(III).

To study the catalytic reduction mechanism of the reduction of Cr(VI) to Cr(III), a H2 generation experiment was conducted. According to the reaction pathway of Cr2O72– + 14H+ + 6e– → 2Cr3+ + 7H2O,[155] the catalytic reduction of Cr(VI) to Cr(III) was illustrated by FA (HCOOH → CO2 + H2) as a strong reducing agent and generated H2 on the metal catalyst surface.[156−158] The suitable decomposition of the FA reaction was conducted on the surface of the AgNP/Eg-C3N4/CS nanocomposite.[159] FA was initially adsorbed on the AgNP/Eg-C3N4/CS nanocomposite surface followed by decomposition to CO2 and H2.[160] Then, electrons were transferred from Cr(VI) to Cr(III) on the surface of the AgNP/Eg-C3N4/CS nanocomposite under the H2 reaction pathway. Similar results have been reported for the reduction of Cr(VI) to Cr(III) on Pd3Ag1/GO–NH2 and Pd-on-Au bimetallic catalysts.[25,161] Finally, Cr(III) was adsorbed from the AgNPs on the Eg-C3N4/CS surface. Reduced Cr(III) was confirmed by an excess amount of NaOH to obtain a green color of [Cr(OH)6]3–.[162] Further, Cr(III) was also evident using H2O2 as a oxidizing agent to produce a yellow solution of Cr(VI) ions. The catalytic reduction mechanism of Cr(VI) to Cr(III) is displayed in Scheme .

Scheme 1

Catalytic Reduction of Cr(VI) to Cr(III) with the AgNP/Eg-C3N4/CS Nanocomposite Using FA and Digital Images of Reduction of Cr(VI) to Cr(III) Colorless Solution and Green Color Solution of [Cr(OH)6]3– and Excessive Addition of NaOH Solution for Confirmation of Cr(III) Reduction

Cyctotoxicity study of before (left) and after (right) reduction of Cr(VI) with mung bean germination images after 5 days.

Reusability Evaluation of Photocatalytic Activity

The reuse of the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite (100 mg) was analyzed for the photocatalytic degradation of MB (1 mM in 100 mL of H2O) under visible light irradiation with various time intervals and also compared with that of the AgNP/Eg-C3N4/CS nanocomposite. The photocatalytic degradation of MB was conducted with the AgNP/Eg-C3N4/CS nanocomposite and Cr(III)-AgNP/Eg-C3N4/CS nanocomposite under visible light irradiation at different time intervals, as shown in Figure A,B. First, MB was adsorbed before and after reduction of Cr(VI) nanocomposites under dark conditions at 30 min and irradiated with visible light at different time intervals. Figure A shows that the photocatalytic degradation of MB was achieved with the AgNP/Eg-C3N4/CS nanocomposite under visible light irradiation at 0–90 min but the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite speedily degraded MB under visible light irradiation within 0–70 min, as shown in Figure B. In total, 97.95% degradation of MB was obtained using the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite as compared to that using the AgNP/Eg-C3N4/CS nanocomposite (94.89%) due to the large surface area after doping with Cr(III) thereby resulting in lower electron–hole recombination rate, as shown in Figure C. The degradation rate constants of the AgNP/Eg-C3N4/CS nanocomposite (k = 0.0306 min–1) and Cr(III)-AgNP/Eg-C3N4/CS nanocomposite (k = 0.0589 min–1) were calculated by the pseudo-first-order kinetic equation (eq ), and these were obtained from the plot of ln(C/C0) vs times, as shown in Figure D. From these results, the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite promised great photocatalytic activity and better reusability than the AgNP/Eg-C3N4/CS nanocomposite. This Cr(III)-AgNP/Eg-C3N4/CS nanocomposite was also compared with other nanocomposites for MB photocatalytic degradation efficiency, as shown in Table S1, Supporting Information.[163−168] In addition, there was almost little to no conversion of Cr(III) to Cr(VI) observed after the photocatalytic degradation process. The Cr(VI) concentration is negligible and will be difficult to be determined in solution. Thus, reuse in the photocatalytic degradation process does not result in any further or undesirable environmental hazards.[169] Moreover, the Eg-C3N4 and CS components of the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite are supported to absorb Cr(III) into their active sites from the solution, which enables the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite to enhance the photocatalytic degradation activity during the reaction.

Figure 10

UV–vis spectra of (A) before AgNP/Eg-C3N4/CS nanocomposite and (B) after Cr(III)-AgNP/Eg-C3N4/CS nanocomposite the photocatalytic degradation of MB under visible light irradiation at different time intervals, (C) calibration plot of the percentage of degradation efficiency vs time before and after, and (D) plot of ln(C/C0) vs time before and after.

Photocatalytic Degradation Mechanism of MB with the Cr(III)-AgNP/Eg-C3N4/CS Nanocomposite

Based on the data and discussion above, the photocatalytic degradation mechanism of MB on the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite is described, as shown in Scheme . When the visible light was irradiated on CS (3.54 eV), the electrons were excited from the valence band (VB) to the conduction band (CB), and this transferred the conduction band electrons to Eg-C3N4 due to lower band gap energy (2.7 eV).[170] Additionally, the conduction band (CB) electrons in Eg-C3N4 were immediately transferred to AgNPs because of the lower band gap energy of Eg-C3N4 compared to that of CS and then the accumulated electrons rapidly moved to the metallic AgNPs that act as electron-scavenging centers.[171,172] Again, the same CB electrons of AgNPs moved to Cr(III) present at the interface of CS because of a reduction in the electron–hole pair recombination rate. The electrons in the CB of Cr(III) have a strong reduction power, whereas the holes in the VB of Ag display a strong oxidation ability. The high reducing electrons located on the CB of Cr(III) would react with O2 to form O2•–.[173] This can further oxidize MB. The holes located on the VB of Ag would photocatalytically oxidize H2O, giving rise to an •OH radical, which in turn will oxidize MB.[174,175] Simultaneously, parts of photoinduced electrons on the CB of CS are involved in the reduction of O2 to yield O2•– due to more negativity.[176] These superoxide radical anions and hydroxyl radicals were highly oxidant to degrade the MB dyes, as shown in Scheme . The Cr(III)-AgNP/Eg-C3N4/CS nanocomposite efficiently exhibited the photocatalytic activity for MB degradation and also displayed reusability. The various possible reaction steps involved in the photocatalytic degradation of dyes are illustrated through the following equations (eqs –12)Furthermore, the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite photocatalyst was compared with other literature-reported nanocomposites. This reusable nanocomposite has improved the photocatalytic degradation of MB when compared to the results in previously reported works.

Scheme 2

Photocatalytic Degradation Mechanism of MB by the Reusable Cr(III)-AgNP/Eg-C3N4/CS Nanocomposite under Visible Light Irradiation

A reactive species investigation was conducted to determine the role of the reactive species created during the photocatalytic degradation of methylene blue with Cr(III)-AgNP/Eg-C3N4/CS under visible light irradiation at various time intervals of 0–90 min. The superoxide radicals (•O2–), hydroxide radicals (•OH), and holes (h+) were generated, and these corresponded to isopropyl alcohol (IPA), p-benzoqunione (BQ), and ethylenediaminetetraacetic acid (EDTA) as the scavengers, respectively.[177] The degradation efficiency of MB was 97.95% in the absence of scavengers after 90 min. IPA, BQ, and EDTA scavengers were added to the reaction mixture, and the degradation efficiency of MB was 96.82, 90.76, and 65.25%, respectively, as shown in Figure A. From the results, the superoxide radicals (IPA) reactive species was effectively involved in the photocatalytic degradation of MB when compared to the other systems of BQ and EDTA.[178,179] Hence, the photocatalytic degradation of MB was significantly reduced due to the addition of IPA to photogenerated hydroxide radicals (•OH) and the degradation efficiency was slightly varied after the addition of BQ and EDTA scavengers.[180]

Figure 11

(A) Determination of radical scavengers for the photodegradation of MB on the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite and (B) band structure and reaction mechanism of MB photodegradation on the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite under visible light irradiation.

Further, the potential of conduction band (ECB) and potential of valence band (EVB) of CS, Eg-C3N4, AgNPs, and Cr were also calculated from the following equations[181]where χ is the electronegativity of CS (2.5 eV), Eg-C3N4 (4.72 eV), Ag (1.93 eV), and Cr (1.7 eV); Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV); and Eg is the band gap energy of CS (3.42 eV), Eg-C3N4 (2.7 eV), AgNPs (2.73 eV), and Cr (3.2 eV). By the calculation, EVB and ECB potentials of CS were found to be 2.79 and −0.63 eV, respectively. EVB and ECB potentials of Eg-C3N4 were found to be 2.52 and −0.18 eV, respectively. EVB and ECB potentials of AgNPs were found to be 1.98 and −0.75 eV, respectively, and EVB and ECB potentials of Cr(III) were found to be 2.63 and −0.59 eV, respectively.

Holes in EVB of AgNPs could move to Eg-C3N4, where electrons were often transferred from Eg-C3N4 (ECB) to AgNPs (ECB) simultaneously. These electrons and holes were effectively separated to enhance the photocatalytic activity. Even though the electrons in ECB of AgNPs were transferred to O2 to make superoxides (•O2–) due to the higher positivity of AgNPs (0.75 eV vs normal hydrogen electrode (NHE)) than O2/•O2– (−0.33 eV vs NHE),[182] this process was against the prior-reported formation of the •O2– radicals and had an important effect on the photocatalytic reaction process. Therefore, an additional method would more properly reproduce the actual mechanism. The photogenerated electrons in the ECB of AgNPs were suddenly moved to the ECB of Cr and followed by recombination with h+. The highly negative position in the ECB of Cr than the O2/•O2– potential (−0.33 eV vs NHE) produced the •O2– radicals, which can directly lead to the degradation of MB to nontoxic products. Meanwhile, holes accumulated on the VB of AgNPs also enable direct oxidization of MB rather than oxidizing H2O to create •OH because the EVB positions of both AgNPs and Eg-C3N4 were less positive than the H2O/•OH potential (2.73 eV vs NHE). Furthermore, the holes in VB of AgNPs (1.98 eV vs NHE) have shown a more positive potential than the redox potential of E0 (OH–/•OH) = 1.9 eV vs NHE).[183,184] Based on the above analysis, it indicated that –OH can be oxidized by holes. As such, the accumulated holes left in the VB of AgNPs will directly oxidize MB to degraded products and lead to the oxidation of H2O to create •OH reactive species, which consequently oxidize MB to CO2 and H2O.[130] The IPA scavenger was mostly involved in the degradation of MB when compared to other BQ and EDTA scavengers, as shown in Figure B. In general, the typical Cr(III)-AgNP/Eg-C3N4/CS system was suitable to enhance the photocatalytic degradation mechanism of MB. It was noted that the Cr(III)-AgNP/Eg-C3N4/CS nanocomposite system maintained a high potential band to improve the photocatalytic degradation performance.

Cytotoxicity Study In Vivo

The cytotoxicity was tested with mung green bean sprouts to determine the toxicity before and after reduction of Cr(VI) and Cr(III) solutions, respectively, in vivo after 10 days of culturing. The Cr(VI) (20 mL) and Cr(III) (20 mL) solutions were individually taken in 100 mL beakers and the mung green bean sprouts were added, as shown in Figure S10A,B, Supporting Information. The mung bean sprouts were completely damaged in the Cr(VI) solution after 5 days due to high toxicity, as shown in Figure S10A, Supporting Information. Figure S10B shows that the roots and stems were germinated in the Cr(VI) solution after 5 days. The maximum roots and stems of mung green bean were significantly germinated in Cr(III) solution because of less toxicity and also confirmed that the AgNP/Eg-C3N4/CS nanocomposite effectively reduced Cr(VI) to Cr(III) at pH 2.

Conclusions

A self-assembled AgNP/Eg-C3N4/CS nanocomposite was prepared by an adsorption method under room temperature. This showed good optical properties, high surface area, and large pore size. This development of the AgNP/Eg-C3N4/CS nanocomposite with AgNPs can result in improved catalytic activity due to surface plasmon resonance. Minimized photoluminescence intensity and electrochemical impedance plots with small arcs provided evidence for better charge separation and improved catalytic performance of the AgNP/Eg-C3N4/CS nanocomposite. The catalytic reduction activity of the AgNP/Eg-C3N4 nanocomposite for conversion of Cr(VI) to Cr(III) (98.5%) was significantly higher than those of Bg-C3N4 (8.51%), CS/Eg-C3N4 (33.35%), Eg-C3N4 (12.60%), and CS (5.59%). The AgNP/Eg-C3N4/CS nanocomposite was used for the catalytic reduction of Cr(VI) to Cr(III) with formic acid as a reducing agent at 45 °C and showed higher kinetic rate constant (0.0621 min–1). Effects of different catalyst dosages, concentrations of Cr(VI), pH levels, and various temperatures on the catalytic reduction of Cr(VI) to Cr(III) were also studied. Under all conditions, the AgNP/Eg-C3N4/CS nanocomposite showed higher catalytic activity than Bg-C3N4, Eg-C3N4, CS, and Eg-C3N4/CS for the reduction of Cr(VI) in aqueous solutions. Moreover, it was also investigated in selectivity and stability studies with various interferences and repeated for six cycles (96.5%), respectively. The eco-friendly and economic nature of the AgNP/Eg-C3N4/CS nanocomposite will enable it to be prepared in large quantities for the reduction of toxic metals into nontoxic forms and can serve as a catalyst with industrial applications and good reusability and a photocatalyst as well.

Experimental Section

Chemicals and Reagents

Melamine, K2Cr2O7, HCOOH, AgNO3, H2SO4, and d-dextrose were purchased from Sigma-Aldrich. HCl, NaOH, urea, NaCl, KCl, d-dextrose, sucrose, cellulose acetate, methylene blue, Na2SO4, K2HPO4, and ethanol were brought from Merck, South Africa. Milli-Q water was used for all experiments.

Synthesis of Eg-C3N4 by Thermal Polycondensation

Thermal polycondensation was used to prepare Bg-C3N4 with melamine as precursors. Melamine (6.0 g) was crushed for 5 min and transferred in a closed ceramic crucible at 550 °C for 2 h at a heating rate of 5 °C/min in a muffle furnace. Bulk g-C3N4 was obtained as a yellow compound after 2 h.[185] Eg-C3N4 was prepared by 3.0 g of Bg-C3N4 and 5 M H2SO4 in 100 mL of water in a 250 mL beaker with sonication for 5 h. A yellow soultion formed, and it was centrifuged with water until pH 7. The precipitate was kept in a hot air oven at 60 °C for 12 h for drying. The light yellow powder confirmed the formation of protonated Eg-C3N4, and the formation mechanism is shown in Scheme .

Preparation of CS by Hydrothermal Method

The carbon spheres (CSs) were prepared by a hydrothermal method, where 6.0 g of d-dextrose in 70 mL of water was stirred for 30 min at room temperature. It was poured into a Teflon-lined autoclave at 180 °C for 10 h. The blackish-brown precipitate was centrifuged with water several times and dried at 60 °C for 12 h to get a black-brownish solid as carbon spheres (CSs).[91]

Preparation of the Eg-C3N4/CS Composite by Self-Assembly

The Eg-C3N4/CS composite was prepared by a self-assembly process. Eg-C3N4 (1.0 g) and 2.0 g of CS were added to 50 mL of distilled water while stirring for 12 h at room temperature. It was collected after centrifugation with water and ethanol and dried at 60 °C for 12 h.

Preparation of the AgNP/Eg-C3N4/CS Nanocomposite by Self-Assembly

The AgNP/Eg-C3N4/CS nanocomposite was synthesized by a self-assembly method. The Eg-C3N4/CS composite (1.0 g) was dispersed in 30 mL of distilled water while stirring for 30 min at room temperature. Ten milligrams of (0.01 M, 10 mL) AgNO3 was mixed with the C3N4/CS composite solution while stirring at 30 min. Then, 0.25 M (10 mL) of NaBH4 solution was added dropwise to obtain a dark gray solution and the formation of AgNPs on the Eg-C3N4/CS nanocomposite was confirmed. The AgNP/Eg-C3N4/CS nanocomposite was centrifuged with water and ethanol five times and dried in a hot air oven at 60 °C for 12 h, and the detailed mechanism is shown in Scheme .

Catalytic Reduction Method

Fifty milliliters of 20 ppm of K2Cr2O7 in a 100 mL beaker and 1 mL of 2 mM formic acid (85%) were added to 15 mg of AgNP/Eg-C3N4/CS nanocomposite with stirring at 45 °C. Following the reaction, 2.5 mL of solution was taken at each time interval for monitoring the catalytic reduction efficiency of Cr(VI) to Cr(III) by a UV–visible spectrophotometer. The catalytic reduction of Cr(VI) to Cr(III) was also tested with other materials like Eg-C3N4/CS composites, CS, Eg-C3N4, and Bg-C3N4 under the same conditions. Additionally, sodium hydroxide (10 M, NaOH) was excessively added to the reduced (Cr(III)) solution, which appeared as a light green solution due to the reduction of Cr(VI) to Cr(III).

Catalytic Reduction Efficiency (%)

The catalytic reduction efficiency α (%) of Cr(VI) to Cr(III) was calculated as given in eq 15.[186]where CI is the initial concentration of reactants at tI and CF is the final concentration after reactants reacted with the catalyst at time tF.

Turnover Frequency of the Heterogeneous Catalyst

The turnover frequency (TOF) is defined as the number of moles of reactants (Cr(VI)) reduced per mole of the AgNP/Eg-C3N4/CS nanocomposite catalyst sites per unit time. The TOF was investigated by following eq with a unit of mol/mol × min.[187]where TON is equal to the number of moles of the reactant/number of moles of catalyst sites.

Characterization of Materials

The optical properties of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites were determined by a UV–visible spectrophotometer (Shimadzu UV-1208 model) and a fluorescence spectrometer (PerkinElmer spectrometer). Fourier transformation infrared (FT-IR) spectroscopy was used to investigate the functionalization of prepared materials by a PerkinElmer PE 1600, in the range of 4000–400 cm–1, and KBr was also used to make the pellet. The crystalline nature of materials was investigated with a Philips PW1729 X-ray diffractometer, with Cu Kα radiation (λ = 1.5406 Å) working at 45 kV and 40 mA. Raman spectra were recorded with a PerkinElmer spectrometer and a laser excitation line of 532 nm. ζ potentials of Bg-C3N4, Eg-C3N4, CS, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites were calculated using the surface charge with a Malvern Zetasizer NanoZS 90. The surface morphology of the prepared materials was also confirmed by a scanning electron microscope (TESCAN, VEGA SEM, electron acceleration voltage 20 kV). The sizes of Bg-C3N4, Eg-C3N4, Eg-C3N4/CS, and AgNP/Eg-C3N4/CS nanocomposites were obtained by a transmission electron microscope (TEM JEOL JEM-2100F, electron accelerating voltage of 200 kV). Electrochemical impedance spectroscopy (EIS) was performed with a GAMRY electrochemical instrument (Interface 1000, potentiostat/galvanostat/ZRA, 11085, working electrode glassy carbon electrode (GCE), platinum wire auxiliary electrode, and Ag/AgCl reference electrode). The catalytic reduction of the K2Cr2O7 solution was monitored using a UV–visible spectrophotometer (Shimadzu UV-1208 model), and the pH adjustment was conducted with an OHAUS starter 2100. The reuse application for the photocatalytic degradation of methylene blue with Cr(III)-AgNP/Eg-C3N4/CS was investigated by a 500 mL capacity glass quartz reactor and a 250 W xenon lamp as a visible light supply with filter glasses (λ > 420 nm).

Confirmation of Catalytic Reduction of Cr(VI) to Cr(III) by the DPC Method

Additionally, the catalytic reduction of Cr(VI) to Cr(III) over a period of time was determined by a spectrophotometric method with 1,5-diphenylcarbazide (DPC) using a Shimadzu UV-2550 double-beam UV–visible spectrophotometer with a 1 cm quartz cell at λ = 540 nm.[188] Cr(VI) was sensitively reacted with DPC to produce a reddish-violet complex. Then, the DPC method was used to perform for Cr(VI) after catalytic reduction for over a period of time, which did not change the color of solution, and this result confirmed the complete reduction of Cr(VI) to Cr(III).[189]

Reagents and Standard Procedures for the DPC Method

A 20 ppm solution was prepared by dilution of the standard stock solution of 1000 ppm K2Cr2O7 in 50 mL of deionized water in a 100 mL beaker. One milliliter of 0.2 N H2SO4 was added to 100 mL of distilled water. 1,5-Diphenylcarbazide (0.25 g) was dissolved in 100 mL of acetonitrile, and 200 mL of distilled water was added. The standard solutions of Cr(VI) of 5, 10, 15, 20, and 25 ppm were prepared from the stock solution. The solution was maintained at pH 2 with dilute H2SO4 acid before the DPC test. The correlation coefficient R2 = 0.999 was obtained from the absorbance vs concentration plot.

Scavenger Trapping Methods

The scavenger trapping methods were used to determine the active species in the photocatalytic degradation of MB. Isopropanol (IPA), p-benzoquinone (BQ), and ethylenediaminetetraacetic acid (EDTA) were used as scavengers for quenching •OH, •O2–, and h+, respectively. This method was conducted during the photocatalytic degradation of MB by 1 mM scavengers.