Green synthesis of AgI nanoparticle-functionalized reduced graphene oxide aerogels with enhanced catalytic performance and facile recycling

Abstract

AgI nanoparticle-functionalized self-assembled reduced graphene oxide aerogels are constructed using vitamin C as the reducing agent. The obtained aerogels can be used as efficient catalysts for organic dye degradation, reduction of 4-nitrophenol, and synthesis of bis(indolyl)methane. A set of characterizations, including FESEM, TEM, XRD, XPS, Raman, FTIR, optical absorption, and photoluminescence techniques, confirm that the aerogel is formed from ultra-dispersed AgI nanocrystals and the self-assembly of reduced graphene oxide nanosheets into porous hydrogel structures. The obtained aerogels exhibit high photocatalytic degradation ability toward an organic dye (rhodamine-B) because of the high visible light-driven catalytic activity of AgI and the high specific surface area of graphene nanosheets with three-dimensional interconnected pores. The well-wrapped reduced graphene oxide nanosheets on AgI nanostructures could promote the transfer of photo-generated electrons, which not only effectively inhibits the recombination of electrons and holes but also suppresses the photocorrosion of AgI; this promotes the photocatalytic activity and stability. Moreover, these nanostructures show the best catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄ as a reducing agent. Furthermore, the AgI-reduced graphene oxide aerogel nanocomposites are active catalysts for the synthesis of bis(indolyl)methane under solvent-free conditions. The nanocomposites exhibit excellent catalytic activity and remarkable durability. This study brings a novel approach to the development of multi-responsive reduced graphene oxide aerogels via the co-assembly of various semiconductor nanocomponents for a variety of applications that involve sustained catalytic activity.

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Introduction

Among various carbon nanostructures, graphene has recently attracted wide and intense scientific interest because it comprises one atom-thick, two-dimensional (2D) atomic layers of sp²-hybridized carbon atoms.¹ Moreover, graphene possesses an excellent intrinsic mobility limit, good transparency, a large specific surface area, high thermal conductivity, and notable mechanical strength.² These unique properties have led to a growing interest in the application of graphene sheets for a variety of sustainable applications including energy storage, catalysis, and adsorption.³ However, 2D graphene layers tend to re-aggregate and restack with other graphene sheets in solution because of van der Waals forces; this results in the formation of much thicker multilayer graphene sheets, leading to poor accessibility to the surface area and limited electron and ion transport.⁴ All these factors hinder the industrialization and limit the applications of graphene in these fields. To overcome these issues, lightweight self-assembled 3D reduced graphene oxide aerogels (RGAs) have been generated and offer new possibilities by exploiting their high specific surface area, plethora of interconnecting macropores and mesopores, fast electron transport kinetics due to the continuous graphene backbone, and enhanced active sites.⁵ The green protocol for producing RGAs with hierarchical microstructures involves generating reduced graphene oxide hydrogels via in situ self-assembly of graphene oxide (GO) by mild chemical reduction under atmospheric pressure and subsequent freeze- or critical point-drying to replace the solvents in the wet gels with highly porous nanostructures with unique characteristics.⁶

However, pure graphene sheets are not suitable for visible light photocatalytic applications because they do not efficiently collect solar light because of their weak absorption abilities. Moreover, pure graphene nanosheets do not show any photocatalytic activity and serve only as electron acceptors or transport intermediates because of their zero band gap. In addition, at high GO loadings, a much thicker graphene layer forms and acts as a recombination center instead of providing an electron pathway thereby deteriorating the photocatalytic performance.⁷ Therefore, functionalization of photocatalytic semiconductor nanoparticles with 3D graphene sheets is often required to take full advantage of the unique properties of graphene sheets; this is an ideal strategy for maximizing the accessible surface area and developing high-performance recyclable photocatalysts. Considering this, researchers have recently made significant advances in generating various reduced graphene oxide-based nanohybrid systems containing a variety of immobilized nanoparticles such as TiO₂,⁸ P25,⁹ BiOBr,¹⁰ ZnS,⁵ Cu₂O,¹¹ and W₁₈O₄₉;¹² they found that the combination of semiconductor nanoparticles and graphene nanosheets with three-dimensional interconnected pores resulted in excellent catalytic activity for photooxidation of organic dyes and reduction of aromatic nitro groups to amino groups at room temperature. The widely accepted mechanism for the photocatalytic performance enhancement is increased light absorption, efficient separation of the photogenerated charge carriers, and extension of the lifetime of the electron–hole pairs because of the chemical interactions between the graphene sheets and semiconductor nanoparticles.⁸

Among these photocatalysts, silver iodide (AgI) is a novel photocatalyst with promising efficiency for decomposition of organic dyes by visible-light irradiation.¹³ Moreover, the photocatalytic efficiency of AgI nanostructures can be further improved by modulating the shape, morphology, and crystal faces of the AgI crystals.^14,15 However, pure AgI nanostructures are prone to photoreduction and decompose to weakly active silver during the photocatalytic reaction if no sacrificial reagent is supplied.¹⁶ This unsatisfactory durability is the main hindrance to the practical application of AgI as a recyclable and highly efficient photocatalyst. A variety of strategies, such as the formation of AgI composites^17,18 and surface modification,¹⁴ have been employed to reduce the recombination of charge carriers and enhance the photocatalytic activity and stability. Nevertheless, the new simple method that improves the durability of AgI while facilitating its recovery is still required for environmental applications. We believe that these issues may be fully resolved by embedding AgI nanostructures in reduced graphene oxide aerogels. Because this type of nanohybrid is lightweight and hygroscopic, it floats on top of the solution during the photodegradation process and can be easily separated for recycling without any loss of photocatalyst.

We have earlier reported¹⁹ enhanced catalytic activity in 2D AgI-RGO nanocomposites. In that report, it was observed that the rate of RhB degradation decreased more significantly with each cycle; only 72% RhB degradation was achieved on the second cycle. It is suggesting that the AgI-RGO nanostructures are unstable under light illumination; this may be due to their slight solubility in aqueous solutions and photocorrosion. This unsatisfactory durability of the AgI-RGO is the main hindrance for the practical application of AgI as a recyclable and highly efficient photocatalyst. With a view of looking for more stable and enhanced activity for AgI nanostructures, herein, for the first time, we demonstrate a facile green synthesis to fabricate 3D AgI-RGA nanocomposites using vitamin C as a reducing agent. Remarkably, AgI-RGA nanohybrids feature enhanced photocatalytic activity and durability for the photodecomposition of organic compounds (i.e., Rhodamine B (RhB)) under simulated sunlight and visible light irradiation compared to bare AgI nanostructures. In addition, AgI and AgI-RGAs are also catalytically active for the reduction of 4-nitrophenol in the presence of NaBH₄. Furthermore, the as-synthesized AgI-RGA nanocomposites are efficient catalysts for the synthesis of bis(indolyl)methane under solvent-free conditions; in the presence of the nanocatalyst, the yields of the corresponding products were moderate to good.

Results and discussion

The surface morphologies of the as-synthesized reduced graphene oxide aerogels, AgI nanostructures, and AgI-RGA nanocomposites were characterized by FESEM and are shown in Fig. 1(a–d). As shown in Fig. 1(a), the as-synthesized graphene aerogels exhibit well-defined and interconnected 3D porous network structures with continuous macro-pores in the micrometer size range. The pores of the 3D microstructures comprise intercalated graphene sheets, and the walls of the pores are thin; these factors indicate effective assembly of the graphene sheets. These unique 3D porous network structures can significantly prevent the aggregation of graphene sheets caused by π–π interactions and van der Waals forces, which may be beneficial for charge carrier transport. The as-synthesized AgI nanostructures exhibit a cuboidal-like morphology with rough surfaces. The rough surfaces indicate that the AgI nanostructures are composed of tiny AgI nanoparticles (Fig. 1(b)). Moreover, most of the nanoparticles seem to be agglomerated, which is characteristic of surfactant-free precipitation reactions in aqueous media. Fig. 1(c and d) present representative FESEM images of the AgI-RGA composites. The FESEM images clearly show that the AgI nanoparticles are anchored uniformly on both sides of the graphene sheets. It is noteworthy that no AgI particles that were disassociated from the graphene sheets were observed, suggesting efficient assembly of the AgI nanoparticles and graphene nanosheets. Such geometric confinement of semiconductor nanoparticles within the graphene layer can be expected to suppress dissolution and agglomeration of the nanoparticles, thereby improving the charge transport between the nanostructures and graphene sheets resulting in enhanced photocatalytic activity and stability of the composites. Furthermore, in the composite, the graphene nanosheets form interconnected 3D porous network structures with continuous macropores in the micrometer size range similar to those of bare graphene aerogel nanostructures; this indicates that the unique interconnected porous structure of the graphene sheets is not affected by functionalization with AgI nanoparticles.

Fig. 1 FESEM images of the graphene aerogel (a), AgI (b), and AgI-2 nanocomposites ((c) and (d)). TEM (e) and HRTEM (f) images of the AgI-2 nanocomposites.

Energy dispersion spectroscopy (EDS) analysis was carried out to examine the chemical compositions of the AgI nanostructures and AgI-RGAs. Typical EDS spectra of AgI and AgI-2 are shown in Fig. S2(a) and (b).† The EDS spectrum of the bare AgI nanostructures reveals that the nanostructures contain only Ag and I without any impurities, where as signals for carbon and oxygen are clearly evident in the EDS spectrum of AgI-RGA nanocomposites in addition to those for Ag and I. No traces of other elements are evident in the spectra, thereby confirming the purity of the samples. To further investigate the spatial homogeneity of these detected elements, X-ray EDS elemental mapping was carried out for the AgI-graphene nanocomposites, and the results are shown in Fig. S3(a–d).† It can be seen that C, Ag, and I are uniformly distributed over the entire matrix of the AgI-RGA nanocomposites.

To further investigate the structural information and microscopic morphology, TEM and HRTEM analysis of the AgI-2 nanocomposites was performed, and the obtained results are shown in Fig. 1(e) and (f). From these figures, it is clear that the AgI nanoparticles are anchored uniformly on both sides of the graphene sheets and the graphene nanosheets form interconnected 3D porous network structures. The HRTEM image of the AgI-2 nanocomposite (Fig. 1(f)) shows lattice fringes from the (002) crystal plane of AgI, apart from the stacked layers of reduced graphene oxide (RGO). The fringe widths were calculated to be 0.367 nm for AgI, and the stacking width was calculated to be 0.31 nm for the RGO layers.

The crystal structures and crystallinities of the as-obtained products were investigated by X-ray diffraction (XRD) measurements. Fig. 2 presents the XRD patterns of pure AgI and AgI-RGAs with different GO concentrations. The diffraction peaks of the pure AgI nanostructures can be readily assigned to the hexagonal phase of β-AgI (JCPDS File Card no. 09-0374), with characteristic diffractions of the (100), (002), (101), (102), (110), (103), (200), and (112) planes of hexagonal crystalline β-AgI. In addition to these peaks, a small diffraction peak at around 56.7°, which was ascribed to the (400) plane of γ-AgI, is evident. These results demonstrate the coexistence of β-AgI and γ-AgI phases in the as-synthesized nanocomposites.¹⁴ The intense and sharp peaks and narrow peak width elucidate that all the synthesized composites are well-crystallized. Moreover, no crystalline impurities related to reaction by-products, such as Ag metals or Ag clusters, were observed, which supports the high purity of the product. The obtained AgI-RGAs with different GO concentrations display similar XRD patterns to the AgI nanostructures, implying that the wrapped reduced graphene nanosheets do not influence the crystal phase of AgI. Moreover, typical diffraction peaks for graphene (002) were not observed; this suggests that the layer-stacking regularity of GO sheets would be inhibited after reduction by AgI nanocrystals as the spacers. The average nanocrystallite sizes (D) of the AgI and AgI-RGA composites were estimated using the Debye–Scherrer formula (D = 0.89λ/β cos θ, where λ is the wavelength of the Cu Kα irradiation, β is the FWHM of the diffraction peak, and θ is the diffraction angle for the (002) plane of hexagonal β-AgI); the calculated D values are in the range of 25–30 nm.

Fig. 2 X-Ray diffraction patterns of the AgI and AgI-RGA nanostructures.

To further investigate the surface composition, the valence states of the elements and purity of the as-synthesized products were determined using XPS. Fig. 3(a) shows a typical wide-scan survey XPS spectrum of GO. The survey spectrum indicates that the sample is composed of C and O elements, and no peaks for other elements are evident. Fig. 3(b) shows the narrow-scan C 1s XPS spectrum of GO. The spectrum can be fitted by three Gaussian peaks corresponding to carbon atoms in different oxygen-containing functional groups. The peak that appears at a binding energy of 284.8 eV is assigned to a non-oxygenated C–C bond containing sp²-hybridized carbon atoms. The spectral bands at binding energies of 286.2 and 288.8 eV were assigned to the C–OH and O=C–OH oxygen-containing functional groups, respectively.²⁰ Fig. 3(c) shows a typical wide-scan XPS spectrum of the AgI-RGA nanocomposite. In the spectrum, the photoelectron lines for Ag 3d, I 3d, Ag 3p, C 1s, and O 1s were observed at their respective standard binding energies, which confirm the existence of these elements in the synthesized nanocomposites. Fig. 3(d) shows the C 1s XPS core level spectrum of the AgI-RGA nanocomposite. In the spectrum, the de-convoluted peaks located at 284.9, 286.4, and 288.9 eV correspond to C–C, C–OH, and carboxylic groups (O–C=O), respectively. However, the intensities of these oxygen-containing functional groups are very low compared to that of the C 1s of GO, which indicates that the graphene in the prepared composite has a high degree of reduction.²⁰ Fig. 3(e) shows the high-resolution XPS spectra of Ag 3d. For Ag 3d, two peaks were observed at binding energies of about 368.0 and 374.0 eV, which correspond to Ag 3d_5/2 and 3d_3/2, respectively. The difference in the binding energies of Ag 3d_5/2 and 3d_3/2 is 6 eV, suggesting that the Ag ions exist in the +1 valence state in the matrix of the AgI product.¹⁴ No peaks related to Ag⁰ are evident at binding energies of 369.2 or 375.8 eV. The high-resolution XPS spectrum of the I 3d region is shown in Fig. 3(f) and displays two peaks at around 619.4 and 631.1 eV; these correlate with I 3d_5/2 and 3d_3/2, respectively. The XPS analysis results evidence the reduction and functionalization of RGO and confirm the components of the AgI-graphene nanocomposites.

Fig. 3 (a) and (c) XPS survey spectra of GO and the AgI-2 nanocomposites; (b) and (d) narrow-scan spectra of C 1s for GO and the AgI-2 nanocomposites; (e) and (f) narrow-scan spectra of Ag 3d and I 3d for the AgI-2 nanocomposites.

Raman spectroscopy was used to elucidate the degree of reduction of GO in the AgI-RGAs. Fig. 4(a) depicts room-temperature Raman spectra of GO, GA, and the AgI-2 nanocomposites. The spectra of all the samples have two vibrations at around 1589 and 1347 cm⁻¹, which are related to the D and G bands of RGO, respectively. The G band is assigned to the E_2g phonon of symmetric stretching of the sp² C–C bond, and the D band is a breathing mode of k-point phonons of A_1g symmetry, originating from disruption of the symmetrical hexagonal graphitic lattice. The ratio between the intensities of the D and G bands (I_D/I_G) is a measure of the disorder, as expressed by the sp²/sp³ carbon ratio. Compared with GO (0.93), GA (1.01) and the AgI-2 nanocomposite (1.04) have slightly higher I_D/I_G values, which are attributed to a decrease in the size of the sp² domains upon reduction and the interactions between the AgI nanostructures and graphene nanosheets.²¹

Fig. 4 (a) Raman spectra of GO, GA, and the AgI-2 nanocomposites. (b) FT-IR spectra of pure GO, graphene aerogel, and the AgI-2 nanocomposites. (c) Optical absorption spectra of AgI and AgI-RGAs with different GO concentrations. (d) Emission spectra of the AgI and AgI-RGA nanocomposites.

Furthermore, FTIR spectra of the GO, GA, and AgI-2 nanocomposites were measured over the range of 400–4000 cm⁻¹ to corroborate the XPS and Raman results. As illustrated in Fig. 4(b), in the FTIR spectrum of GO, the strong and broad absorption peak situated at about 3397 cm⁻¹ corresponds to stretching vibrations of the hydroxyl groups of GO, and the peak at ∼1630 cm⁻¹ is related to aromatic –C=C. The absorption peaks associated with oxygen-containing groups, such as C–O, phenolic C–OH, and carboxyl C=O stretching vibrations, are located at 1738, 1069, and 1239 and 1379 cm⁻¹, respectively.²² All these absorption bands have high intensities indicating successful oxidation of graphite to form GO sheets. In contrast, in the FTIR spectra of RGA and the AgI-2 nanocomposites, the intensities of the peaks for the oxygen-containing functional groups dramatically decrease, and the absorption peaks for C–O and C=O from COOH groups at 1738, 1239, and 1379 cm⁻¹ completely vanish. In addition, in the AgI-2 nanocomposites, new absorption bands appear in the range of 400–600 cm⁻¹ and were attributed to AgI. This feature further proves the reduction of GO, which is consistent with the XPS and Raman spectroscopy results.

The optical absorption properties of the AgI nanostructures and AgI-RGA nanocomposites were examined via UV-visible absorption spectroscopy in the spectral range of 230–800 nm (Fig. 4(c)). The pure AgI nanostructures show a sharp absorption edge with an absorption maximum at around 427 nm, which was attributed to the characteristic band of AgI induced by the forbidden transition (4d¹⁰ to 4d⁹5s¹) allowed by the tetrahedral symmetry of the Ag⁺ ion site.²³ The optical band gaps of the as-synthesized AgI nanostructures were deduced from the maximum absorbance using the band-gap relation. The estimated optical band gap of the AgI nanostructures is 2.90 eV, which is slightly higher than that of the bulk AgI material (2.82 eV).²⁴ Moreover, the band gap was slightly higher than that of pure γ-AgI²⁵ and lower than that of pure β-AgI. We believe that the change in the band gap of the synthesized nanostructure may be due to the coexistence of γ-AgI and β-AgI in the nanostructures.²⁶ However, for the AgI-RGA nanocomposites, two absorption bands were observed with absorption maxima at 280 and 429 nm. The absorption band at 280 nm was assigned to the π–π* transition of the aromatic C–C and C=C bonds related to graphene oxide. However, this band was at a higher wavelength than that of graphene oxide (235 nm), indicating the reduction of GO and restoration of the C–C bonds in the RGA composites.⁵ The other absorption peak is located at 429 nm and was attributed to the characteristic band of AgI induced by the forbidden transition (4d¹⁰ to 4d⁹5s¹) enabled by the tetrahedral symmetry of the Ag⁺ ion site.²³ Compared to the spectra of the AgI nanostructures, the absorbance peaks are more intense and the peak at 427 nm red-shifts slightly with increasing graphene content; this is ascribed to chemical bonding between the semiconductor photocatalyst and graphene nanosheets. These results imply that the as-synthesized AgI and AgI-RGA composites are suitable for use as visible light-induced photocatalysts.

The interface charge-separation and recombination properties of the as-synthesized nanostructures were investigated using photoluminescence (PL) emission spectroscopy. Fig. 4(d) presents the room-temperature PL spectra of the AgI nanostructures and AgI-RGA nanocomposites under 330 nm excitation. The spectra of the AgI nanostructures reveal a broad emission comprising multiple overlapped peaks that cover the visible region from 430 to 600 nm. The broadening of the emission peak is due to overlapping of emissions from various defects. The Gaussian curve-fitting for the broad emission of AgI shown in Fig. S4(a)† indicates that the broadening of the peak is due to overlapping of three emission peaks at 463, 502, and 575 nm; these three emissions were attributed to distant pair donor–acceptor (D–A) recombination mediated by the high density of deep trap states involving exciton–phonon interactions or crystalline defects or impurities.^27,28 Although the emission spectra of the AgI-RGA nanocomposites appear to be similar to those of AgI nanostructures, strong, preferential luminescence quenching was evident with increasing GO content. This is in accordance with the fact that graphene nanosheets have a higher tendency to accept electrons and the excited AgI nanostructures are good electron donors. The synergism between the AgI nanostructures and graphene sheets would effectively reduce electron–hole recombination leading to increased charge carrier separation. This means that the photo-excited electrons in the AgI nanostructures preferentially transfer to the RGO sheets instead of returning back to the AgI vacancy defect centers thereby prolonging the lifetimes of the photogenerated holes;¹⁹ this significantly inhibits the recombination rate of electrons and holes and consequently weakens the PL intensity. With all of the information mentioned above, one can envision that efficient charge separation could increase the lifetime of the charge carriers and enhance the efficiency of interfacial charge transfer to absorb the organic dyes, which would account for the higher photocatalytic activities of the AgI-RGA nanocomposites to some degree.

The nitrogen adsorption desorption isotherm of the as obtained AgI-2 reduced graphene aerogels were investigated and are presented in Fig. S5.† The samples exhibit type-IV isotherms with a distinct hysteresis loop in the relative pressure range of 0.7–1.0, which is a characteristic of the inhomogeneous mesoporous material with cylindrical pore geometry present within the nanostructures, and facile connectivity between the pores. The Brunauer–Emmett–Teller (BET) surface area of AgI-2 is 18.6970 m² g⁻¹ with an average pore diameter of 39.9055 nm. The observed BET surface area is higher than that of the pure AgI nanostructures and which can be attributed to the incorporation of RGO which has a large surface area, leading to more surface active sites and the increased adsorption of reactants.²¹

The photocatalytic activities of the as-synthesized AgI and AgI-RGA nanocomposites were probed by monitoring the degradation of organic dye, i.e., RhB, under simulated sunlight irradiation. The optical absorption spectra obtained for the photocatalysts, as a function of simulated sunlight irradiation time, are shown in Fig. S6(a–f).† The characteristic maximum absorbance intensity of RhB at 554 nm was used to assess the extent of degradation. The degradation of RhB was negligible under simulated sunlight irradiation in the absence of photocatalyst, whereas in the presence of any of the synthesized photocatalysts, the maximum absorbance intensity diminished sharply with illumination time. Among the synthesized nanocomposites, the AgI-2 nanocomposite exhibited the highest photocatalytic efficiency: the RhB dye completely degraded within 70 min under simulated sunlight irradiation, whereas 130, 90, 80, and 100 min were required to completely decompose the RhB using the AgI, AgI-1, AgI-3, and AgI-5 nanocomposites, respectively. Moreover, in the presence of the AgI nanostructures, the absorption maximum significantly shifts to a lower wavelength with irradiation time until it reaches zero absorbance, as shown in Fig. S6(a).† These gradual hypsochromic shifts of the absorption maxima indicate that RhB is first de-ethylated in a stepwise manner and then the remaining conjugated structure is destroyed. The hypsochromically shifted maximum absorption wavelengths of RhB of 554, 539, 522, 510, and 498 nm correspond to N,N,N¹,N¹-tetraethylated rhodamine, N,N,N¹-triethylated rhodamine, N,N¹-diethylated rhodamine, N-ethylated rhodamine, and rhodamine, respectively.^29,30 In the present study, five different intermediates form during the photoreaction over AgI nanostructures; the absorbances of the intermediates range from 554 to 497 nm in the UV-vis spectra. Moreover, the solution changed from pink to green and finally became almost colorless (Fig. S7†). Whereas only two intermediates could be identified as being formed in the presence of the AgI-1 composite nanostructures, the absorbance maxima of these intermediates are located from 554 to 530 nm (Fig. S6(b)†). However, for composites AgI-2 to AgI-5 (Fig. S6(c–e)†), no obvious blue-shift in the absorbance at 554 nm was observed and the color of the solution changed directly from pink to colorless (Fig. S7†), which suggests that degradation of RhB occurs mainly via direct mineralization yielding only gaseous end products such as CO₂, NO₃, or NO_x. In graphene-based AgI nanocomposites, the electron injection rate is much faster than in bare AgI, which causes the RhB to undergo direct mineralization instead of stepwise de-ethylation.³¹

Proton NMR analysis was performed to observe the intermediates of RhB during the degradation processes. Fig. S8† illustrates the ¹H NMR spectra of the pure RhB and final products of RhB photodegradation in the presence of AgI nanostructures at the time intervals of 0, 30, 90 and 120 min. The NMR signals from different aromatic hydrogen atoms and N-diethyl group were observed conspicuously at their respective standard values for pure RhB. As the photodegradation progresses, the intensity of the characteristic peaks associated with aromatic hydrogen atoms and ethyl group decreases, while new peaks appear at around δ 9.0–9.2 ppm, δ 3.0–3.1 ppm and δ 1.15–1.25 ppm. The decrease of characteristic peaks intensity indicates the breakdown of chromophore structures and simultaneous removal of ethyl groups during the degradation process. The new peaks in the spectrum are associated with the intermediate products formed during degradation. The NMR results further confirmed that the de-ethylation and degradative changes of RhB occurs during the photo degradation processes.^32,33

The time-dependent degradation ratios (i.e., C_t/C_o, where C_t is the concentration of RhB during the reaction and C_o is the initial concentration of RhB) of RhB for all the synthesized samples as a function of simulated sunlight irradiation were determined and are presented in Fig. 5(a). It is clearly evident that photolysis of RhB without any photocatalyst under simulated sunlight irradiation is negligible: the degradation efficiency was less than 4% after 130 min. This confirms that RhB essentially does not degrade in the absence of a photocatalyst with sunlight illumination. In the presence of the AgI nanostructures, RhB completely photodecomposed after 130 min of irradiation. The reaction mechanism for the degradation of the organic dye under simulated sunlight irradiation over AgI nanostructures is as follows: the photo-generated electrons (e⁻) in the valence band (VB) were excited to the conduction band (CB) by the AgI nanophotocatalyst, which was irradiated by simulated sunlight with energy greater than the threshold level. The recombined holes (h⁺) in the VB on the semiconductor surface were partially localized on the structurally defective centers of the crystalline lattice (eqn (1)). The reduction reaction occurred between the electrons in the conduction band and electron acceptors, such as adsorbed O₂ molecules, which yielded superoxide radical anions (eqn (2)). The resultant photo-induced holes either oxidized the organic compound directly or were trapped by electron donors (eqn (3)). H₂O₂ was formed by recombination of these oxidant radicals (eqn (4)). H₂O₂ may have then reacted with the superoxide radical anion to regenerate a hydroxyl radical (eqn (5)). These superoxide radical anions and hydroxyl radicals were responsible for decomposition of the RhB dye into non-toxic products, such as CO₂, H₂O, and mineral acids (eqn (6)), under simulated sunlight irradiation.¹⁵ From these mechanisms, plausible degradation reactions were as follows:

AgI + hν → AgI(e⁻) + AgI(h⁺) (1)

AgI(e⁻) + O₂ → ˙O₂⁻ (2)

AgI(h⁺) + H₂O→ ˙OH (3)

˙OH + ˙OH + H₂O₂ (4)

H₂O₂ + ˙O₂⁻ → OH⁻ + ˙OH + O₂ (5)

˙OH + RhB → H₂O + CO₂ + mineral acid (6)

Fig. 5 (a and d) Photocatalytic efficiency (C_t/C_o) as a function of simulated sunlight and visible light irradiation time of the AgI and AgI-RGA nanocomposites respectively. (b and e) Degradation efficiency in recycling of the AgI and AgI-2 composites, under simulated sunlight and visible light irradiation respectively. Time profiles of the photocatalytic degradation of RhB with different active species scavengers under simulated sunlight irradiation (c), under visible light irradiation (f). (BQ: benzoquinone, EDTA: disodiumethylenediaminetetraacetate, and TA: tert-butyl alcohol).

Compared with the as-synthesized AgI nanostructures, all the AgI nanoparticle-functionalized RGAs exhibit an appreciable degree of photocatalytic degradation ability. Among them, the AgI-2 nanostructures exhibit the fastest degradation rate: only 70 min was required for complete photodegradation of RhB, whereas 130, 100, 90, and 80 min were required to completely decompose RhB using AgI, AgI-5, AgI-4, and AgI-3, respectively. The photocatalytic activity gradually increases with increasing graphene content up to 2 mg mL⁻¹; beyond that, the photocatalytic performance deteriorates. This mainly occurs because of the formation of black reduced graphene oxide sheets with zero band gaps in the composite, which shield light from the surface of the AgI nanocomposites; this is a universal problem for graphene-based photocatalysts. Moreover, excessive graphene nanosheets act as recombination centers instead of providing an electron pathway, resulting in an intrinsically lower photo-generation rate.³⁴ All these factors lead to decreased photocatalytic performance at high contents of graphene. However, all AgI-RGAs proved to be far better than the AgI nanostructures alone. The enhanced photocatalytic activity of AgI-RGAs was attributed to the following factors: firstly, the AgI-RGAs show better adsorption capability towards RhB than the AgI nanostructures because of their higher surface area and sponge-like nature. Secondly, they more efficiently utilize visible light than the AgI nanostructures because they can float on the surface of the aqueous reaction system. Thirdly, they feature efficient charge separation and transportation due to the presence of graphene sheets, which effectively inhibit recombination of the photogenerated charge carriers resulting in high visible-light photocatalytic activity.³⁵ Finally, the photo-induced holes could either oxidize the organic compound directly or be trapped by electron donors, such as OH⁻, to produce ˙OH. The superoxide radical anions (O₂) and hydroxyl radicals are responsible for decomposition of the RhB dye into non-toxic products, such as CO₂, NO₃, NO_x, and mineral acids. The proposed mechanism for oxidation of RhB to mineralized compounds, CO₂, and H₂O over the AgI-RGA nanocomposites under solar irradiation is shown in Fig. 6.

Fig. 6 Illustration of the proposed reaction mechanism for photocatalytic degradation of RhB in an aqueous solution over AgI-RGA nanocomposites under simulated sunlight irradiation.

To further reveal the photocatalytic mechanism, disodium ethylenediaminetetraacetate (EDTA), tert-butyl alcohol (TA) and benzoquinone (BQ) were employed as scavengers of h⁺, ˙OH, and ˙O₂⁻ to investigate the specific reactive species involved in the RhB degradation over AgI-2 nanocomposite and are shown in Fig. 5(c). The photodegradation of AgI-2 was significantly suppressed by the introduction of EDTA anions, indicating that holes are the main reactive oxidative species involved in the photocatalysis. The addition of BQ provoked partial inhibition of the RhB degradation suggesting that ˙O₂⁻ radicals also play an important role in the photocatalytic reaction. When TA was added in the system as a scavenger for ˙OH, it did not obviously affect the decomposition rate at all over the photocatalytic system. Based on the results, it is clear that h⁺ and ˙O₂⁻ are the major reactive species in the AgI-2 photocatalysis reaction system.³⁶

To determine the mineralization ratio of the degraded RhB, total organic carbon (TOC) analysis was carried out. The obtained TOC values of the sample solutions for AgI and AgI-2 after 70 min sunlight irradiation were 2.242 mg L⁻¹, 8.341 mg L⁻¹ respectively, where the initial RhB concentration was 10 mg L⁻¹. The mineralization rate of RhB for AgI-2 sample is much higher than that of pure AgI nanostructures. This fact demonstrates that AgI-2 nanocomposites can efficiently degrade and mineralize organic pollutants under light irradiation, which is in agreement with the result of the photocatalytic test. However, after 70 min illumination time, no RhB could be detected by UV-Vis spectroscopy in the presence of AgI-2, but the result of TOC is 8.341 mg L⁻¹. This demonstrates that there are still some organic carbons left-likely aldehydes/carboxylic acids but no aromatic rings. So the reaction time for the complete degradation is longer than for complete disappearance of RhB in UV-vis spectra.³⁷

To elucidate the significant role of 3D interconnected graphene nanosheets and AgI nanostructures in enhancing the lifetime of electron–hole pairs and boosting photocatalytic ability, the transient photocurrent responses and electrochemical impendence spectra were recorded. Fig. S9(a)† shows the transient photocurrent responses during light-on and light-off cycles of AgI and AgI-2 nanocomposite electrodes under simulated sunlight irradiation. It is clear that the photocurrent of the AgI-2 nanocomposite electrode is much higher than that of the bare AgI electrode and the on–off cycles of the photocurrent are reproducible. The enhancement in the photocurrent probably results from the expanded range of light absorption and efficient production of photo-induced electrons and hole separation because of the synergetic effect of the interconnected graphene sheets and AgI nanostructures, which is consistent with the photocatalytic activities. Further, to evaluate the electron-transport ability of the AgI and AgI-2 nanocomposites, EIS was also carried out. Fig. S9(b)† displays the EIS results represented as Nyquist plots (Z_im vs. Z_re) for the pure AgI and AgI-2 nanocomposites under simulated sunlight irradiation. The Nyquist plot of the AgI-2 nanocomposite exhibits a smaller arc diameter than that of the pure AgI electrode, which suggests that the AgI-2 nanocomposite can efficiently transfer charge carriers and effect migration of photogenerated electron–hole pairs. The photocurrent and electrochemical impedance studies are in good accordance with the photocatalytic activities.

To quantitatively measure the photocatalytic efficiency, the photodecomposition rate constant (k) of RhB with respect of number of active sites on the materials over the AgI nanostructures and AgI-2 nanocomposites were calculated from a pseudo-first order reaction kinetic model using BET surface areas, i.e., ln(C_o/C_t) = kt, where C_o is the adsorption–desorption equilibrium concentration before light irradiation (mg L⁻¹), C_t is the RhB concentration in the aqueous solution at time t (mg L⁻¹), and k is the apparent pseudo-first order rate constant (min⁻¹).³⁸ The calculated apparent k values for AgI and AgI-2 nanocomposites are 0.005 and 0.0155 min⁻¹ m⁻², respectively. The k value is about 3.1 times higher than that of the bare AgI nanoparticles. This is clearly evidence of the significant synergistic effect of graphene coupling at enhancing the photocatalytic efficiency.

For efficient water remediation, in addition to high catalytic reactivity, the durability of the photocatalyst is important for practical applications. To evaluate the durability of the AgI and AgI-RGA nanocomposites, we performed another five successive photocatalytic degradations of RhB under the same experimental conditions. As shown in Fig. 5(b), with pure AgI, the rate of RhB degradation decreased more significantly with each cycle, and only 25.04% RhB degradation was achieved on the fifth cycle, suggesting that the pure AgI nanostructures are unstable under sun light illumination; this may be due to their slight solubility in aqueous solutions and photocorrosion. Moreover, we noticed that the yellow color of the AgI photocatalyst became darker after five successive photocatalytic treatments; this indicates the formation of metallic silver (Ag⁰) due to the reduction of Ag⁺ from the AgI nanostructures during the RhB degradation process under light irradiation. More importantly, as shown in Fig. 5(b), the photocatalytic efficiency of the AgI-RGA nanocomposites did not decline significantly even after five successive photocatalytic cycles: it exhibits a nearly constant photo-decomposition rate as that of the first cycle. The reasons for the enhanced durability of the AgI-RGA nanocomposites may be explained as follows: (i) the AgI-RGAs float on top of the liquid phase during the photo-degradation process, which means that there is no loss of photocatalyst and they can be easily separated for recycling; (ii) in the aerogels, the AgI nanoparticles are anchored uniformly on both sides of the graphene sheets, which protects the AgI nanostructures from dissolution into aqueous solution due to the strong chemical adsorption between the graphene nanosheets and AgI; and (iii) the high conductivity of the graphene sheets enables the photo-generated electrons formed in the conduction band of the AgI nanostructures to transfer to the surfaces of the graphene nanosheets to quickly participate in the photocatalytic reaction, so that the photogenerated carriers are efficiently separated. This efficient electron migration from AgI to the RGO sheets also promotes the stability of AgI-RGAs.

Further the morphology of the catalysts after repeated photocatalytic experiments under solar simulator was evaluated by FESEM. It can be seen that compared to before the catalytic reactions (Fig. 1(b)), numerous tiny Ag⁰ species are observed from the surfaces after the photocatalytic process as shown in Fig. S10(a).† Where as in AgI-2 nanocomposite there were no evident morphology changes after photocatalysis reaction (Fig. S10(b)†). These observations imply that our AgI-RGA nanocomposites are stable during the photocatalytic degradation process.

To evaluate the structural stability of the AgI and AgI-RGA nanocomposites after the photocatalytic experiments, we conducted XRD and XPS studies. Fig. S11† presents the XRD patterns of the AgI and AgI-2 samples before and after five runs under simulated sunlight irradiation for the degradation of RhB. The XRD patterns of the AgI nanostructures after five runs consist of two sets of diffraction peaks: in addition to those assigned to AgI, sharp diffraction peaks (marked by ♦) at 2θ values of 38.1°, 44.3°, and 64.4° were observed; these were indexed to the (111), (200), and (220) planes of metallic silver (JCPDS card no. 89-3722). These results confirm the formation of metallic silver through the reduction of Ag⁺ from the AgI nanostructures during the RhB degradation process under light irradiation. The XRD patterns of AgI-RGA show no notable differences after photocatalysis treatment compared to those of the corresponding samples before the photodegradation experiments, which suggests that the composites are stable during the photocatalytic degradation process.

Furthermore, XPS spectra were recorded to determine the surface Ag⁰ contents in the AgI and AgI-RGA nanocomposites and are shown in Fig. S12(a–d).† The calculated surface Ag⁰ contents of the AgI and AgI-RGA nanocomposite after five cycles are 19.38% and 5.21% respectively; this indicates that the photocorrosion rate of the AgI-graphene nanocomposites is far slower than that of the pure AgI nanostructures. Moreover, the C 1s deconvolution spectrum shown in Fig. S12(b)† demonstrates that the intensity of the peaks associated with the C–O and C=O moieties weaken during the degradation reactions, which indicates the continued occurrence of charge transfer and interactions between the graphene sheets and AgI even after five degradation reactions. As a result, both the XRD and XPS results confirm the favorable photostability of the AgI-RGA nanocomposites, in which photocorrosion of AgI is significantly suppressed.

The photocatalytic activity of AgI and AgI-RGAs nanocomposites were also carried out under visible light irradiation using λ ≥ 430 nm long pass filter. The time-dependent degradation ratios (C_t/C_o) of RhB for all the synthesized samples as a function of visible light irradiation were determined and are presented in Fig. 5(d). It is clearly evident that photolysis of RhB without any photocatalyst under visible irradiation is negligible: the degradation efficiency was less than 5% after 100 min. This confirms that RhB essentially does not degrade in the absence of a photocatalyst with visible light illumination. In the presence of the AgI and AgI-RGA nanostructures, the RhB was completely photodecomposed. Among all, AgI-2 nanostructures exhibit the fastest degradation rate: only 50 min was required for complete photodegradation of RhB, whereas 100, 60, 70, and 80 min were required to completely decompose RhB using AgI, AgI-3, AgI-4, and AgI-5, respectively. Further, to verify the practical applicability of the AgI-RGA, the reusability was further evaluated under visible light irradiation over 5 photocatalytic cycles. Fig. 5(e) demonstrates that the photocatalytic activity did not decline after five cycles. This indicates that stable and efficient performance of the AgI-RGA nanocomposites was maintained under visible light irradiation. Furthermore, reactive oxidative species trapping experiments shown in Fig. 5(f) demonstrate that h⁺ and ˙O₂⁻ are the major reactive species in the AgI-2 photocatalysis reaction system under visible light irradiation.

To evaluate the catalytic activity of the AgI nanostructures and AgI-RGA nanocomposite, reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄ was investigated. As shown in Fig. 7(a), 4-nitrophenol exhibits a peak at 318 nm in neutral aqueous solution; upon addition of NaBH₄, the absorption peak red-shifts to 400 nm. Accordingly, the solution changes from pale yellow to bright yellow because of the formation of 4-nitrophenolate ions under alkaline conditions.^39,40 Moreover, it is evident from the spectra, that in the absence of catalyst, the absorption peak at 400 nm remains unchanged, indicating that NaBH₄ alone cannot initiate the conversion reaction even after 30 min. In contrast, in the presence of AgI and AgI-2 catalysts, the characteristic absorption peak at 400 nm gradually declines as the reduction reaction time proceeds, while a new absorption peak appears at 295 nm and is associated with the formation of 4-aminophenol (Fig. 7(b) and (c)). This was also evident from discoloration of the solution. A reaction time of 60 s was required to achieve complete reduction of 4-nitrophenol using the AgI-RGA composites as catalysts, whereas 120 s was required to completely reduce 4-nitrophenol using the AgI nanostructures. The enhanced reduction activity of the AgI-RGAs was attributed to the following factors: (i) the high adsorption rate of 4-nitrophenol onto the nanocatalyst via π–π stacking interactions between the graphene sheets and AgI and 4-nitrophenol; (ii) the increased concentration of electrons due to efficient interfacial electron transfer from the graphene sheets to AgI, which cause efficient electron transfer from BH₄ (donor) to 4-nitrophenol (acceptor) through the nanocatalyst (AgI), resulting in highly efficient reduction activity; and (iii) graphene sheets prevent the aggregation of AgI nanostructures and improve reduction performance.⁴¹ The rate of reduction of the nanocatalysts can be described by pseudo-first order kinetics. The curves of −ln(C_t/C_o) versus irradiation time are linear, which is indicative of good correlation with first-order kinetics. The apparent reaction constant (k) of the AgI-RGAs nanocomposite is 0.0626 s⁻¹, which is 1.75 times higher than that of the bare AgI nanostructures (0.0356 s⁻¹). In addition to the high reduction capability, the reusability and stability of the catalyst is very important for practical applications. To investigate the reusability of the AgI-graphene nanocomposites, their activities were examined under the same conditions over five cycles, and the resultant reduction efficiencies are shown in Fig. 7(d). The obtained results indicate that the catalysts have good catalytic properties for the reduction of 4-nitrophenol and are readily reused.

Fig. 7 UV-visible spectra of 4-nitrophenol before and after adding NaBH₄ solution and after 30 min without catalysis (a) and in the presence of AgI (b) and AgI-2 nanocomposite (c). (d) Reusability of the AgI-2 composite for the reduction of 4-nitrophenol.

Furthermore, the as-synthesized AgI-RGAs nanocomposite was employed as a catalyst for the synthesis of bis(indolyl)methane under solvent-free conditions using indole (2 mmol) and aldehyde (1 mmol) as substrates. Scheme 1 demonstrates the overall synthetic procedure for the synthesis of bis(indolyl)methane using AgI-2 catalyst. From the control experiment conditions, we noticed that no coupled product was observed when the reaction was performed at room temperature. Then, we screened the experimental conditions and increased the temperature from room temperature to 40 °C and afforded a 50% yield for 1 h. In the case of reaction at 55 °C, the reaction was completed in 40 min with 68% yield; when the reaction was performed at 70 °C, the reaction was furnished within 25 min with 85% yield. Moreover, there was no difference in the conversion with reaction temperatures of 80 °C and above. Thus it was proved that 70 °C is the optimized temperature required for effecting this reaction by conventional heating. The resultant products were quantitatively analyzed using ¹H and ¹³C NMR, and the results are presented in Fig. S13.† The obtained spectral data match well with those in earlier reports.⁴² Scheme 2 shows a possible mechanism for the AgI-RGA-catalyzed synthesis of bis(indolyl)methane. The reaction was initiated by activation of the carbonyl function group by the Ag⁺ ion. Electrophilic substitution then followed at the 3-position of the indole. Dehydration resulted in the formation of the intermediate, which was activated again by the Ag⁺ ions, and finally, the participation of a second mole of indole resulted in the final product, i.e., bis(indolyl)methane.⁴³ In addition to its catalytic activity, the reusability of a photocatalyst is important for practical applications. The reusability of the AgI-RGAs nanocomposite was successfully examined four times; we noticed a very slight loss of catalytic activity, which suggests that the present catalysts have good stability and sustainability.

Scheme 1 AgI-RGA-catalyzed synthesis of bis(indolyl)methane.

Scheme 2 Proposed mechanism for AgI-RGA-catalyzed synthesis of bis(indolyl)methane.

Conclusions

We successfully prepared novel self-assembled graphene/AgI aerogels with robust interconnected 3D graphene networks embedded with AgI nanoparticles. The resulting reduced graphene oxide aerogel hybrids were used as catalysts for organic dye degradation, reduction of 4-nitrophenol, and synthesis of bis(indolyl)methane; for these reactions, they exhibited excellent catalytic activity and durability. The enhanced catalytic activity is mainly due to its superior adsorption capability and efficient charge separation and transportation between RGO and the AgI nanostructures. The charge transfer mechanism was supported by the photoluminescence and photoelectrochemical studies. Recycling results confirm the favourable photo stability of the AgI-RGA nanocomposites, in which photocorrosion of AgI was significantly suppressed. These findings may open new opportunities for the design of highly efficient and recyclable catalytic graphene semiconducting aerogels for a variety of applications that involve sustained catalytic activity.

Acknowledgements

This work was financially supported by National Research Foundation of Korea (NRF) grants, funded by the Korean government (MEST and MSIP) (2013S1A2A2035406, 2013R1A1A2009575 and 2014R1A4A1001690). This work was also supported by the 2015 Post-Doc. Development Program of Pusan National University. This work also supported by Max Planck POSTECH/Korea Research Initiative Program [Grant no. 2011-0031558] through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning.

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Footnote

† Electronic supplementary information (ESI) available: Experimental and characterizations, schematic mechanism of the formation of the AgI-graphene hydrogel, EDS and EDS mapping, PL and CIE analysis, BET analysis, UV-visible spectra of the photocatalytic activity, ¹H NMR of RhB with photodegradation time, photocurrent responses and EIS analysis, FESEM, XPS and XRD patterns of the recycled catalysts, ¹H and ¹³C NMR spectra of bis(indolyl)methane. See DOI: 10.1039/c5ra07267k

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