Dynamics of Charge-Transfer Behavior in a Plasmon-Induced Quasi-Type-II p-n/n-n Dual Heterojunction in Ag@Ag3PO4/g-C3N4/NiFe LDH Nanocomposites for Photocatalytic Cr(VI) Reduction and Phenol Oxidation.

In this work, a series of heterostructure Ag@Ag3PO4/g-C3N4/NiFe layered double hydroxide (LDH) nanocomposites were prepared by a combination of an electrostatic self-assembly and in situ photoreduction method. In this method, positively charged p-type Ag3PO4 was electrostatically bonded to the self-assembled negatively charged surface of the n-n-type g-C3N4/NiFe (CNLDH) LDH hybrid material with partial reduction of Ag+ to metallic Ag nanoparticles (NPs) by the photogenerated electrons and available surface -OH groups of LDH under visible light irradiation. The presence of Ag3PO4 as a p-type semiconductor, the surface plasmon resonance (SPR) effect of metallic Ag NPs, and oxygen vacancies as Ov-type defects in NiFe LDH could greatly achieve the quasi-type-II p-n/n-n dual heterojunctions, which was revealed by the shifted conduction band and valence band potentials in Mott-Schottky (M-S) analysis. Among all the optimized heterostructures, CNLDHAgP4 could achieve the highest photocatalytic Cr(VI) reduction rate of 97% and phenol oxidation rate of 90% in 2 h. The heterostructure CNLDHAgP4 photocatalyst possesses a unique morphology consisting of cubic phases of both Ag NPs and Ag3PO4, which adhered to the thin and curvy layers of the CNLDH hybrid for smooth electronic and ionic charge transport. Furthermore, the intimate Schottky barriers formed at the interface of quasi-type-II p-n/n-n dual heterojunctions were verified by the photoluminescence, linear sweep voltammetry, M-S, electrochemical impedance study, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy studies. The SPR effect of Ag NPs and oxygen vacancies as Ov-type defect in NiFe LDH can effectively accelerate the threshold of charge separation and be the main reason for the enhanced activity achieved by the as-fabricated heterostructure photocatalyst.

Introduction

Environmental pollution by heavy metals and stable organic effluents from industrial discharge has awakened the public concern over the last decade.[1] Hexavalent chromium symbolized as Cr(VI) is a highly toxic and hazardous metal.[2] Accordingly, Cr(VI) is one of the major toxic waste metals associated with many industrial activities such as in leather tanning industries and chromate manufacturing and mining units, which are then released into the environment, causing severe contamination in productive land as well as in aquatic system.[3] Alternatively, the toxicity and carcinogenicity of phenolic pollutants in water bodies have caused serious damage to both human health and aquatic living organisms.[4] The numerous application of phenol ranges from cosmetics, disinfectant products to chemical reagents, dyes and artificial resins, and so forth. However, phenolic compounds are ecotoxic and are also an organic waste discharged into the aquatic environment by various industries such as pharmaceuticals, petroleum refineries, food, and paint and dye industries.[5] Therefore, an effective and simultaneous removal of both heavy metal cation Cr(VI) and stable pollutant phenol from various wastewater resources is of primary importance for saving the earth. Recently, the photocatalytic reduction and oxidation processes using the versatile photocatalyst have emerged as the most effective techniques toward the elimination of Cr(VI) and phenol in aquatic environments.[6,7]

Among the developed photocatalytic materials, n-type NiFe layered double hydroxide (LDH) can be considered as one of the ideal semiconductor photocatalysts on account of their low cost, nontoxicity, suitable redox potential, and excellent optical absorption behavior.[8−10] The most inherent properties of the NiFe LDH system is their optical absorption capability in terms of multiple interelectronic excitation paths via metal-to-metal charge transfer (MMCT) through oxo-bridged bimetallic linkages (Ni2+–O–Fe3+), d–d transitions of Ni2+, and ligand-to-metal charge transfer (LMCT) O → Ni2+/Fe3+ similar to that in Ni/Zn–Cr LDH.[10,11] However, the inherent relatively low conductivity in terms of carrier efficiency of NiFe LDH certainly hinders their catalytic performances.[12] To achieve a high-performance NiFe LDH-based material, diverse efforts have been attempted by various scientific groups from all over the globe to design the NiFe LDH-based heterostructure material. By the formation of a structurally modulated heterostructure material, intrinsic surface defects can be created on the surface of LDH, which can significantly promote the catalytic performances.[13] Examples of various works on the NiFe LDH-based heterostructure material include carbon quantum dots/NiFe LDH,[14] carbon nanotube/NiFe LDH,[15] graphene/NiFe LDH,[16] NiCo2S4/NiFe LDH,[17] NiCo2O4 nanowire/NiFe LDH,[18] Cu nanowires/NiFe LDH,[19] NiFe LDH/molybdate anions,[20] and FeOOH/NiFe LDH.[21]

Our previous work reports that the exfoliated and electrostatic self-assembled hybrid heterostructures of n–n-type g-C3N4/NiFe LDH (CNLDH) nanocomposites are ideal energy materials for the photocatalytic water oxidation and reduction reactions because of a similar layered structure.[10] Therefore, it is rational that coupling g-C3N4 with NiFe LDH would result in an eco-friendly material for the mitigation of environmental pollutants.[22] Unfortunately, fabrication of heterostructures using exfoliated LDH nanosheets is still a challenge because they are prone to form strong irreversible agglomerates, which can decrease the photoactive sites as well as the inaccessibility of the inner surfaces of the resultant materials for easy charge transport, thus badly affecting the electron-transfer kinetics.[23] Moreover, still the poor stability of powdered LDH nanosheets further restrains their extensive applications in photocatalytic fields as in ZnCr LDH/graphene oxide.[24]

To address these issues and for further effective charge transport, one feasible strategy is to incorporate noble metal plasmonic Ag NPs (nanoparticles abbreviated as NPs) together with Ag3PO4 at the interface of the self-assembled CNLDH hybrid. In particular, Ag3PO4 is a highly active p-type semiconductor and possesses a conduction band (CB) located at +0.45 V and a very deep valence band (VB) located at +2.9 V versus normal hydrogen electrode (NHE) well aligned for its visible light active photocatalytic oxidation reactions.[25−27] Additionally, the presence of metallic Ag NPs induces surface plasmon resonance (SPR), in which the collective oscillations of conductive electrons on the surface of metallic Ag NPs interact with electromagnetic radiation or offer hot electrons to Ag3PO4 via a plasmonic effect, which can intensify the visible light absorption capability and consequently improve the catalytic activity.[28−30] Gholami et al. and Xu et al. reported the synthesis and photocatalytic activity of Ag3VO4/Ag3PO4/Ag and Ag3PO4/AgBr/Ag plasmonic photocatalysts under visible light.[31,32] Among the reported Ag3PO4, modified LDH includes ortho Ag3PO4/flaky ZnCr LDH composites reported by Xianlu Cui et al.[33] In another report, Sun et al. prepared Fe3O4@LDH@Ag/Ag3PO4 submicrosphere as a magnetically separable visible light photocatalyst for the photocatalytic degradation of methylene blue.[34] Recently, our group has reported Ag@Ag3VO4/ZnCr LDH heterostructure systems toward photocatalytic water oxidation and phenol degradation reaction.[35] However, no reports on the study of the heterostructure Ag@Ag3PO4/CNLDH nanocomposite as the photocatalyst and how the SPR electrons of Ag NPs together with Ag3PO4 can participate in effective charge separation and protect the structural stability of exfoliated NiFe LDH nanosheets in this system have still been scarcely developed up to now.

Herein, we report the design and development of a novel plasmonic heterostructure photocatalyst, namely, Ag@Ag3PO4/g-C3N4/NiFe LDH (CNLDHAgPx, x = 2, 4, 6, and 8 wt % of Ag3PO4 on 10 wt % of the CNLDH hybrid) fabricated by the combination of an electrostatic self-assembly and in situ photoreduction method. In this method, positively charged p-type Ag3PO4 and Ag NPs were electrostatically bonded to the self-assembled negatively charged surface of the n–n-type CNLDH hybrid material. The copious amount of surface hydroxyl groups of NiFe LDH and photogenerated electrons of the heterostructure reduces Ag+ to metallic Ag NPs. By introducing plasmonic Ag NPs and p-type Ag3PO4 over the electrostatically assembled surface of n–n-type NiFe LDH/g-C3N4, a quasi-p–n/n–n dual heterojunction is established in Ag/Ag3PO4@g-C3N4/NiFe LDH for the effective charge separation. The role of the plasmonic effect of Ag NPs in protecting the stability of p-type Ag3PO4 as well as the exfoliated NiFe LDH nanosheets and presence of Ov-type defects (Ov as oxygen vacancies) in NiFe LDH for electron-trapping sites were well discussed in this study. By this, the heterostructure Ag@Ag3PO4/g-C3N4/NiFe LDH nanocomposite possesses a strong ability for the reduction of Cr(VI) to Cr(III) and for the oxidation of phenol to nontoxic products. Furthermore, the effects of pH solution on the photocatalytic performance were well investigated in this study.

Results and Discussion

Generally, the photocurrent measurement study reveals the dynamics of interfacial charge transfer and separation in a heterostructure material. Figure represents the current density versus potential (I–V) measurement plot by using the linear sweep voltammetry (LSV) study for the as-prepared electrodes (NiFe LDH, g-C3N4, CNLDH, Ag3PO4, and CNLDHAgPx), under both dark and light illumination (λ ≥ 400 nm). In dark, NiFe LDH, g-C3N4, and CNLDH generate a minimum current density in the potential range of −1.5 to +1.2 V. Under light illumination, NiFe LDH, g-C3N4, and CNLDH exhibit an anodic photocurrent behavior with photocurrent densities of +0.0010 μA cm–2, +16.40 μA cm–2, and +0.0010 mA cm–2 at potentials of −0.60, −1.13, and −0.96 V, respectively (Figure A–C). Figure D shows the LSV curve of Ag3PO4 under dark and light irradiation. Ag3PO4 shows an incremental cathodic photocurrent response with the applied bias. The cathodic photocurrent response of Ag3PO4 reveals its p-type semiconductor property of the material.[38] Ag3PO4 generates a maximum cathodic current density of −0.9 mA cm–2 at −1.5 V. In dark, the current density of Ag3PO4 was approximately 1.5 times lower than that of Ag3PO4 under light illumination. To confirm a dual heterojunction of p–n/n–n type established among Ag3PO4, g-C3N4, and NiFe LDH in CNLDHAgPx, LSV measurements of the heterostructure CNLDHAgPx were taken into account and shown in Figure E. The heterostructure CNLDHAgPx was synthesized by varying different weight percentage loadings of Ag3PO4 over 10 wt % of the CNLDH hybrid. The weight percentages of Ag3PO4 to CNLDH were 2, 4, 6, and 8% in the heterostructure CNLDHAgPx and named as CNLDHAgP2, CNLDHAgP4, CNLDHAgP6, and CNLDHAgP8, respectively. The photocurrent measurement study of all the heterostructure materials were carried out under the exposure of visible light. However, not surprisingly, the cathodic current density of CNLDHAgPx is significantly increased at a negative bias of −1.89 V, which signifies its p-type semiconductor properties. Also, the photocurrent measurements of all the heterostructure CNLDHAgPx exhibit asymmetric photocurrent in the reverse and forward direction, which was divided into two regions, that is, maximum reverse current density (region I) at negative bias and maximum forward current density (region II) at positive bias. The photocurrent displayed a marked difference at the reverse bias region I and forward bias region II. In a conducting state, CNLDHAgP4 shows the highest current density of −7.90 mA cm–2 at a reverse bias of −1.72 V and +8.30 mA cm–2 at a forward bias of +1.79 V, which demonstrates the potential of electrochemical behavior of the heterostructure CNLDHAgP4 nanocomposite. The I–V curve of CNLDHAgPx also exhibits a fine rectifying behavior of p–n- and n–n-type heterojunctions. The current can only pass and complete a cycle when p-type Ag3PO4 is positively biased. Therefore, the rectification behavior is predominantly originated from the formation of p–n heterojunction interaction between p-type Ag3PO4 and n-type g-C3N4 in CNLDHAgP4. When an equilibrium was established during the formation of a junction, electrons in Ag3PO4 were transferred to g-C3N4 because of the SPR effect of Ag NPs and then to NiFe LDH. At the same time, hole transfer takes place in opposite order from NiFe LDH to g-C3N4 and then to Ag3PO4 in order to achieve the balance between the electric potential of a three-component semiconductor in CNLDHAgP4. Finally, when they form a p–n/n–n dual heterojunction, a built-in potential difference at the interface will be created. Under forward bias, majority charge carriers flow freely because of the reduced built-in potential, resulting in the ON state of the p–n/n–n dual heterojunctions. However, when the p–n/n–n dual heterojunctions were reverse-biased, the built-in potential increases, and then the reverse current was minimal and the junction is in an OFF state. Additionally, CNLDHAgP2, CNLDHAgP6, and CNLDHAgP8 generate +7.55, +8.04, and +1.33 mA cm–2 n-type currents at forward bias while −7.49, −7.70, and −3.50 mA cm–2 p-type current at reverse bias, respectively. Therefore, the photocurrent measurements concluded that a double heterojunction, that is, p–n and n–n junction, has been established between the p-type Ag3PO4, n-type g-C3N4, and n-type NiFe LDH. The decrease in the photocurrent density of CNLDHAgP8 may be due to the increase wt % loading of Ag3PO4 NPs at the interfacial area of CNLDH, which leads to photocorrosion, causes instability of the material, and affects the photocurrent density.[38]

Figure 1

LSV curves of (A) NiFe LDH, (B) g-C3N4, (C) CNLDH, and (D) Ag3PO4 under dark and light irradiation in 0.1 M Na2SO4 aqueous solutions at pH 6.5 and (E) LSV curves of the heterostructure CNLDHAgPx nanocomposites [(a) CNLDHAgP2, (b) CNLDHAgP4, (c) CNLDHAgP6, and (d) CNLDHAgP8] under visible light irradiation in 0.1 M Na2SO4 aqueous solutions at pH 6.5.

The crystal phases of all the heterostructure nanocomposites named as CNLDHAgP2, CNLDHAgP4, CNLDHAgP6, and CNLDHAgP8 were analyzed through the X-ray diffraction (XRD) measurement and compared with NiFe LDH, g-C3N4, Ag3PO4, and CNLDH, respectively (Figure ). Generally, pristine LDHs exhibit characteristic intense diffraction peaks of the (00l) plane because of the high periodicity and favorable crystallographic orientation along the direction of c-axis, whereas the diffraction peak intensity of other planes is much reduced. All diffraction patterns of pristine NiFe LDH can be well indexed into the three-layer 3R polytypic rhombohedral symmetry of the space group r3̅m with basal reflections of the (003), (006), and (012) planes referring to JCPDS card no. 38-0715.[39] It has been well-known that isomorphous substitution of Fe3+ into the crystal lattice of Ni(OH)2 could replace Ni2+ to form a stable NiFe LDH structure. The development of excess cationic charge due to the incorporation of Fe3+ was balanced by the intercalation of various anions inside the interlayer of LDH.[10,39] The basal spacings (d) were calculated by using the Bragg law, nλ = 2d sin(θ), where n is an integer = 1, λ is the wavelength of the incident light, and θ is the angle of incidence.[40] The d value of NiFe LDH was calculated to be 0.79 nm for NO3– as an intercalating anion. Similarly, the value of lattice parameter “a”, which represents the minimum distance between two different types of cations in the brucite-like layers of NiFe LDH, was estimated to be ∼0.31 nm. The main characteristic diffraction peak of neat g-C3N4 appeared at 2θ = 27.3° and another one with a less intense peak appeared at 2θ = 13.1°, both of which can be indexed into the (002) and (100) planes of g-C3N4, respectively.[10] CNLDH composites show characteristic peaks of both NiFe LDH and g-C3N4 phases without the interference of crystal growth of one another.[10] The diffraction peaks of Ag3PO4 matched well with the long-range structural order body-centered cubic phases of Ag3PO4 (JCPDS file no. 06-0505) with the space group P4̅3n. Moreover, the high intense peaks of Ag3PO4 suggested its high degree of crystalline nature. By using the Debye–Scherer formula[41] and from the full width at half-maxima of the (210) diffraction peak of Ag3PO4, the crystallite sizes of Ag3PO4 in CNLDHAgP2, CNLDHAgP4, CNLDHAgP6, and CNLDHAgP8 were estimated to be ∼5.5, 6.4, 8.4, and 8.8 nm, respectively (Table S1). This reveals that the size of the Ag3PO4 nanocrystals in the CNLDHAgPx heterostructure can be controlled by changing the initial concentration of AgNO3. For CNLDHAgPx, the peaks located at 2θ values of 20.9, 29.7, 33.3, 36.58, 53.3, 55.0, and 57.3 can be indexed into the (110), (200), (210), (222), (211), (320), and (321) crystal planes with cubic phases of Ag3PO4 (JCPDS card no. 06-0505).[42] The weaker intense peaks found at 38.1, 44.2, and 64.3 corresponding to the (111), (200), and (220) basal planes for metallic Ag NPs of face-centered cubic symmetry (JCPDS no. 04-0783).[42] In sharp contrast, the low diffraction peak intensity of the (003), (006), (009), and (110) planes of NiFe LDH with the (002) plane of g-C3N4 gradually decreases, whereas the diffraction intensity of the (210) and (211) planes of Ag3PO4 progressively increases in CNLDHAgPx. These results confirmed the lesser periodicity in the normal direction toward the layered growth of NiFe LDH,[43] which is also the first indication of exfoliation of NiFe LDH, resulting in strong interfacial coupling of Ag3PO4 cubic phase over the electrostatic self-assembled surface of exfoliated NiFe LDH sheets and g-C3N4. This reflects the synergistic interaction among the most intense (003) and (006) planes of NiFe LDH, the (002) plane of g-C3N4, and (210) and (211) planes of Ag3PO4 in the heterostructure CNLDHAgPx nanocomposite. The unit cell directions “a” and “c” of NiFe LDH in CNLDHAgP4 were calculated to be 0.31 and 50 nm, respectively. The changes in lattice parameter “c” reveal the continuous growth and dispersion of Ag NPs and Ag3PO4 phases over the stacked layer-by-layer self-assembled structure of positively charged exfoliated NiFe LDH sheets with negatively charged g-C3N4. The systematic structural data of the as-synthesized materials are given in Table S1.

Figure 2

XRD patterns of NiFe LDH, g-C3N4, CNLDH, Ag3PO4, CNLDHAgP2, CNLDHAgP4, CNLDHAgP6, and CNLDHAgP8.

The formation of the heterostructure CNLDHAgPx nanocomposite synthesized under visible light irradiation was further confirmed from the Fourier transform infrared (FTIR) spectra (Figure ). For this, CNLDHAgP4 prepared with and without irradiation was observed to be notably unusual. The spectra reveal the position of −OH groups at 3375 cm–1 and −C–N and −C=N stretching vibration modes of CN at 1637 and 1243 cm–1, respectively. The Fe–O stretching modes of vibrations were found at 776 and 555 cm–1, respectively. The Ni–O and Ni–O–Fe lattice vibrations were found at around 500–900 cm–1. The comparison of the above-mentioned stretching and lattice vibration frequencies of both the heterostructures prepared without (Figure a) and with irradiation (Figure b) confirms the existence of phosphate and oxygen functional groups in CNLDHAgP4. However, in the case of CNLDHAgP4 (prepared with irradiation), the asymmetric vibration of the P–O peak at 1018 cm–1 is shifted to 1068 cm–1, and in the case of CNLDHAgP4 (prepared without irradiation), the peak is more prominent. The bending mode of vibrations of the six-membered heterocyclic rings of triazine units at 808 cm–1 significantly decreased as compared to the CNLDHAgP4 heterostructure (prepared without irradiation). Again, in the case of CNLDHAgP4 (prepared with irradiation), a small shoulder peak at 3621 cm–1 and a prominent peak at 2250 cm–1 suggested the presence of a second type of −OH stretching vibration of weakly H-bonded to −OH groups of H2O molecules as found in the CeO2/MgAl-LDH work of the group of authors.[44] This observation shows that a large quantity of phosphate and oxygen functional groups was present in CNLDHAgP4 (with irradiation) and also confirms the formation of heterostructures under visible light illumination.

Figure 3

FTIR spectra of the CNLDHAgP4 heterostructure prepared (a) without irradiation and (b) with irradiation.

In the morphological study of the catalysts fabricated by the way of liquid-state reaction under photoirradiation, the morphology exhibited a different and unique collection of spherical Ag NPs and Ag3PO4 adhered to the thin and slightly curvy layers of NiFe LDH and g-C3N4 hybrid materials (Figure ). In the photoirradiation method, as CH3OH was used as a polar solvent, there might be a chance of exfoliation of NiFe LDH and g-C3N4 in this synthetic process and also an equal chance of formation of metallic Ag NPs with the growth of Ag3PO4. In this way, the reaction was carried out for the growth of the Ag NPs and Ag3PO4 catalyst over the thin layers of the self-assembled CNLDH hybrid, and this provides an extra savor to the morphology in the matter of the heterostructure CNLDHAgP4 nanocomposite. The transmission electron microscopy (TEM) image in Figure a–c reveals that the coherent interface and lamellar morphology of g-C3N4 were beneficial for Ag NPs and Ag3PO4 to grow along the surface of the self-assembled CNLDH nanohybrid with strong electrostatic bonding interactions. As expected in the TEM results (Figure c), a thin and transparent-like g-C3N4 intimately dispersed and stabilized over the NiFe LDH matrix without obvious aggregation. However, in the magnified high-resolution TEM (HRTEM) image as shown in Figure d,e, no lattice fringe of g-C3N4 was visualized. The lattice spacing of 0.234 nm corresponded to the (111) crystallographic plane of Ag NPs (JCPDS file no. 01-071-3762), which is in accordance with the XRD result. The lattice spacings of 0.269 nm and 0.260 could be indexed as the (210) and the (012) crystallographic plane of Ag3PO4 (JCPDS file no. 06-0505) and NiFe LDH (JCPDS file no. 38-0715), respectively. Nevertheless, the energy-dispersive X-ray (EDX) elemental spectrum clearly shows the well-defined spatial distribution of C, N, O, Fe, Ni, Ag, and P elements (Figure g), which indicate the homogeneous distribution of the Ag NPs and Ag3PO4 over the interfacial area of the CNLDH hybrid for recuperating the surface redox reactions. The tight adhesion between the Ag3PO4, Ag NPs, g-C3N4, and NiFe LDHs would greatly favor the interfacial charge transfer and promote the photocatalytic activities. In sum, TEM and HRTEM results confirmed that Ag NPs and Ag3PO4 were successfully formed over the surface of the CNLDH hybrid after in situ liquid-state reaction under visible light irradiation. The interfaces between the lattice fringes of Ag, Ag3PO4, and NiFe LDH were clearly distinguished in the HRTEM image (Figure e), which indicated the formation of heterostructures and promoted the efficiency of electron transfer within the heterojunction nanocomposite. Furthermore, the crystallinity of the heterostructure CNLDHAgP4 was confirmed by the selected area electron diffraction (SAED) pattern (Figure f). The ring patterns in the heterostructure CNLDHAgP4 correspond to the (210) and (012) crystallographic planes of Ag3PO4 and NiFe LDH, respectively.

Figure 4

(a–c) TEM images and (d,e) HRTEM images, (f) SAED pattern, and (g) EDX spectra of the CNLDHAgP4 sample.

In the growth process of the designed p–n/n–n dual heterojunctions, an improved in situ deposition photoreduction synthetic route is used to deposit Ag NPs and p-type Ag3PO4 over the self-assembled surface of n-type g-C3N4/n-type NiFe LDH. Finally, the plasmonic heterostructure Ag@Ag3PO4/g-C3N4/NiFe LDH nanocomposite is obtained by the photoreduction of Ag+ by the photogenerated electrons on the surfaces of p-type Ag3PO4 using the AgNO3 precursor. In the presence of methanol, which acts as a dispersing as well as a reducing agent, Ag+ ions get attached onto the negatively charged surface of the CNLDH hybrid and partially get reduced. Initially, Ag2O was precipitated from the solution of AgNO3 because of copiously available surface −OH groups of LDH. However, the obtained brown colored precipitate dissolves in excess of (NH4)2HPO4 precursor to form an [Ag (NH3)2]+ complex, which releases Ag+ ions into the solution (Ag+/Ag = 0.7996 V and Ag2O/Ag+OH– = 0.342 V vsNHE, pH = 0).[35] The chemical reaction between these species in solution under visible light irradiation results in the formation of Ag NPs and Ag3PO4. The whole reduction process is depicted by the following equations:

The X-ray photoelectron spectroscopy (XPS) technique is used to confirm the surface chemical composition and electronic environment of elements present in the as-synthesized materials. Figure S1 shows the survey scan of CNLDH and the heterostructure CNLDHAgP4. The signals of Ni, Fe, Ag, N, P, O, and C were detected in the survey XPS spectrum, confirming the formation of heterostructures. The erratic oxidation state of all the elements present in the material was further analyzed by the deconvolution of the high-resolution XPS peak of each element using the help of CASAXPS and Origin software. This deconvolution provides sufficient evidence about the interaction among the constituent components present in the heterostructure. For the Ni 2p spectrum, Ni 2p3/2 and Ni 2p1/2 peaks at 856.0 and 873.8 eV were fitted with two spin–orbit doublets, followed by two prominent shake-up satellites at 856.0 and 873.8, which are characteristic of high-spin Ni2+ state and manifested as Ni(OH)2 in CNLDHAgP4 (Figure a).[45] In sharp contrast, the Fe 2p spectrum (Figure b) shows two prominent peaks located at 712.9 and 725.9 eV, which are assigned to Fe 2p3/2 and Fe 2p1/2, respectively.[45] The high-resolution O 1s spectra of CNLDHAgP4 (Figure c) reveal four distinct peaks attributed to the surface hydroxyl groups attached to metal center-O1 (531.5 eV),[46] lattice oxygen-O2 (530.5 eV),[47] under coordinated lattice oxygen related to oxygen vacancies-O3 (531.6 eV),[48] and absorbed water-O4 (532.8),[49] respectively. Moreover, the N 1s wide peak of CNLDHAgP4 (Figure d) could be fitted into three peaks and interpreted as (I) sp2 N present in triazine ring units corresponding to the C–N=C group and is a vital part of sp2-bonded graphitic carbon nitride (398.3 eV),[50] (II) N–(C)3 as bridged nitrogen atoms (399.8 eV),[50] and (III) the presence of chemisorbed nitrogen species as NH4+ ions (403.4 eV),[51] respectively.

Figure 5

Comparison results of the deconvoluted XPS spectra of CNLDH and CNLDHAgP4 for (a) Ni 2p, (b) Fe 2p, (c) O 1s, and (d) N 1s.

The high-resolution Ag 3d XPS spectrum of Ag3PO4 present in CNLDHAgP4 (Figure a) consists of Ag 3d5/2 and Ag 3d3/2 states, which could be divided into four characteristic peaks, of which the first two peaks at 368.0 and 367.4 eV are assigned to Ag 3d5/2 and the other two peaks at 373.8 and 374.2 eV are assigned to Ag 3d3/2, respectively.[38,52−54] The peaks at 374.2 and 368.0 eV can be attributed to the Ag0 species, whereas the peaks at 373.8 and 367.4 eV can be attributed to the Ag+ ions of Ag3PO4 in CNLDHAgP4.[52,53] The photocatalytic activity is not affected to a greater extent because of slight increment of the metallic Ag NP content in the material. To further prove the presence of metallic Ag NPs in the heterostructure CNLDHAgP4, the XPS analysis of the peak of the Ag region was taken into account. The relative concentration of metallic Ag NPs was estimated to be 46% by analyzing the deconvoluted areas of both Ag+ and Ag0 peaks in the XPS spectra of the Ag region using the following equation:

Figure 6

Deconvoluted XPS spectra of CNLDHAgP4 for (a) Ag 3d, (b) P 2p, and (c) C 1s.

The binding energy (BE) of P 2p is determined to be 134.6 eV, corresponding to P+5 in Ag3PO4 (Figure b). The deconvoluted C 1s XPS spectrum (Figure c) corresponds to the C–C (284.9 eV), C–O (286 eV), C–N (287.6 eV), and O–C=O (288.9 eV) linkages,[50] which validate the existence of g-C3N4 in CNLDHAgP4.

To further confirm the electron transfer from Ag3PO4 to NiFe LDH, the chemical states and positions of the BE region of Ni 2p, Fe 2p, N 1s, and O 1s in CNLDH were examined by XPS (Figure a–d). These results indicated the negative shift, that is, (∼0.6, 0.3 eV), of BE positions of Ni and N in CNLDHAgP4 as compared to that of CNLDH,[10] whereas a positive shift, that is, (∼0.8, 0.1 eV), of BE of Fe 2p and O 1s was noticed in comparison to that of CNLDH. Furthermore, the peak of oxygen vacancies (O3) remains more prominent in CNLDHAgP4, which proves the presence of oxygen vacancy-type defects (Ov) induced because of exfoliation of NiFe LDH during the fabrication process of CNLDHAgP4. Similarly, from Figure a, the Ag 3d peaks in CNLDHAgP4 shifted positively ∼0.4 eV as compared to the reported values of Ag peaks in Ag3PO4 (367.2 and 373.1 eV).[54] The shifting in the BE peaks of Ni, Fe, Ag, N, and O in CNLDHAgP4 demonstrates the strong interaction and migration of the electron cloud from p-type Ag3PO4 to n-type g-C3N4 and finally to n-type NiFe LDH, which facilitate the formation of dual interfaces in the heterostructure CNLDHAgP4.

UV–visible diffuse reflectance spectroscopy (UV–vis DRS) was used to assess the photoabsorptive behaviors of the studied photocatalyst (Figure ). Bare NiFe LDH shows absorption bands in both the UV and visible regions, where absorption bands were located within 200–300, 300–600, and 600–800 nm. The intrinsic absorption band within 200–300 nm in the UV region could be assigned to the LMCT from the O 2p → Ni 3dt2g orbital, while the bands within 300–800 nm corresponded to d–d transitions and are characteristic geometry of Ni2+ ions in an octahedral field.[55,56] Another intense absorption band found within 520 nm was due to the transition of Ni2+–O–Fe3+ to Ni+–O–Fe4+, originating from the induced MMCT for the oxo-bridged bimetallic linkage. These MMCT absorption bands most likely originated from the crystal field splitting of the oxo-bridged bimetallic LDH system.[10,57] The absorption bands located at 380 and 740 nm corresponded to spin-allowed transitions of 3A2g(F) → 3T1g(P) and 3A2g(F) → 3T1g(F), which result from the characteristic d8 configuration geometry of Ni2+ ions in an octahedral field. Likewise, the bands located at 420 and 645 nm corresponded to spin-forbidden transitions 3A2g(F) → 1T2g(D) and 3A2g(F) → 1Eg(D), respectively.[58] Neat g-C3N4 displays a strong absorption band edge at 460 nm, which is attributed to the n−π* transitions related to the involvement of lone pairs on the edge of N atoms of the triazine/heptazine ring units and corresponds to the band gap energy value of ca. 2.7 eV.[10,28] The CNLDH composite material exhibits a blue-shift absorption edge at 441 nm, which is due to the quantum confinement effect of CN over LDH with the absorption band covering a wider visible region.[10,59] The spectral behavior of Ag3PO4 indicated that it can absorb solar light corresponding to a band gap energy of 2.43 eV, which utilizes the efficiency of solar light and consequently enhances the photocatalytic activity.[38,54] In the heterostructure CNLDHAgPx nanocomposite, incorporation of Ag3PO4 to the CNLDH hybrid can significantly affect their optical properties with enhanced absorption intensity covering a wider visible region. Notably, four absorption edges can be detected in the spectral curves of the CNLDHAgPx material, for example, in CNLDHAgP4, at 460, 560, 620, and 650 nm (Figure ). The absorption edges at 460 and 560 nm were assigned to g-C3N4 and the SPR effect of plasmonic Ag NPs (inset of Figure ), respectively.[10,60] The SPR absorption peak was dominated over the absorption peak of Ag3PO4 and MMCT absorption edges of LDH and showed a broad shoulder band at 500–600 nm, which was due to the narrowing of the band gap. Clearly, the absorption edges at 620 and 650 are due to the d–d transition of Ni2+ of NiFe LDH in CNLDHAgP4. Overall, the enhanced absorption intensity of the heterostructure CNLDHAgP4 could be ascribed to the dominant plasmonic effect of Ag NPs,[48] during the growth of Ag3PO4 over the electrostatic self-assembled surface of CNLDH.[47] The phenomenon was in well agreement with the color of the samples, which changed from pale yellow to deep yellow and then to pale brown with the content of Ag3PO4. The optical studies strongly confirm the synergism charge transfer between Ag NPs, Ag3PO4, g-C3N4, and NiFe LDH.

Figure 7

UV–vis DR spectra of NiFe LDH, g-C3N4, Ag3PO4, CNLDH, CNLDHAgP2, CNLDHAgP4, CNLDHAgP6, and CNLDHAgP8.

The band gap energies (Eg) of a semiconductor material were calculated by fitting the absorption data with Kubelka–Munk absorbance and Tauc’s plot via the following equation:where α is the absorption coefficient, h is the Planck constant, ν is the energy of incident lights, A is an arbitrary constant, and Eg is the band gap energy of a semiconductor material. In Tauc’s expression, the value of n signifies the kind of optical transition of a semiconductor (n = 1/2 for direct transition and n = 2 for indirect transition).[10,35,44,56] The values of Eg for different materials were determined by extrapolating the linear portion of the (αhν)1/2 curve versus photon energy hν. From Figure , the Eg values of NiFe LDH, g-C3N4, CNLDH, Ag3PO4, and CNLDHAgP4 were calculated to be 2.20, 2.70, 2.35, 2.43, and 2.38 eV, respectively. The most interesting finding in optical studies is the red shifting of absorption band from 500 to 800 nm in all the as-synthesized heterostructure CNLDHAgPx, which could be especially due to the dominant SPR effect of metallic Ag NPs along with the added contribution of d–d transition of Ni2+, MMCT (MII–O–MIII), and LMCT of NiFe LDH.

Figure 8

Band gap energy values estimated from UV–vis DRS of (a) NiFe LDH, (b) g-C3N4, (c) CNLDH, (d) Ag3PO4, and (e) CNLDHAgP4.

Photocatalytic Reduction and Oxidation Potential of the Catalyst

Assessment of Cr(VI) Reduction Potential

The photocatalytic activities of NiFe LDH, CN, CNLDH, and heterostructure CNLDHAgPx were measured for the reduction of Cr(VI) to Cr(III) in an aqueous solution of K2Cr2O7 under solar light exposure (Figure ). A blank experiment without a photocatalyst or sunlight was also performed to verify the concentration of Cr(VI), and the results were found to remain constant, which reflects the insignificant photolysis and relatively stable content of K2Cr2O7. Before being exposed to light, the Cr(VI) solution was stirred in the dark at about 30 min to ensure the establishment of an adsorption–desorption equilibrium between the photocatalyst and the Cr(VI) solution. As shown in Figure a, the adsorption rates were found to be 5.5% (Ag3PO4), 7% (g-C3N4), 10.0% (NiFe LDH), 12.0% (CNLDH), 15.5% (CNLDHAgP2), 25.8% (CNLDHAgP4), 18.0% (CNLDHAgP6), and 12.2% (CNLDHAgP8), respectively. After the exposure of sunlight for 120 min (Figure a), there was apparent decrease in the concentration of Cr(VI) for pristine NiFe LDH (45%), g-C3N4 (38%), Ag3PO4 (20%), and CNLDH (65%). However, the heterostructure CNLDHAgPx possesses enhanced Cr(VI) reduction activities. Thus, the modification of Ag3PO4 together with plasmonic Ag NPs dispersed over the CNLDH hybrid was an advantage for the photocatalytic reduction of Cr(VI). The Cr(VI) reduction activities of various CNLDHAgPx samples follow the order as CNLDHAgP8 (70%) < CNLDHAgP2 (78%) < CNLDHAgP6 (84%) < CNLDHAgP4 (97%). On the basis of the erratic results of the samples corresponding to reaction times of Cr(VI) reduction over a period of time, many factors such as size, morphology, electronic, electrochemical, and interfacial charge transfer should be accountable for the photocatalytic activity. Therefore, it can be believed that the heterojunction interaction between Ag NPs, Ag3PO4, g-C3N4, and NiFe LDH plays a major role in the photocatalytic reduction of Cr(VI) to Cr(III). In fact, the dominant SPR effect of Ag NPs together with Ag3PO4 extends the visible light reaction, and the availability of electrons contributes a lot to a higher Cr(VI) reduction. Furthermore, a large quantity of Ag3PO4 on the surface of CNLDHAgPx may hinder the light penetration in the materials and may confine interfacial charge transfer between NiFe LDH, g-C3N4, and Ag3PO4, which limit the contraction of Cr(VI). Therefore, high content of Ag3PO4 in CNLDHAgPx cannot exhibit high photocatalytic activity. This is because in the case of CNLDHAgP8, Ag3PO4 might act as a recombination center for photogenerated electron–hole pairs.[61] The lowest photoluminescence (PL) intensity, largest photocurrent density, and reduced arc in the Nyquist plot also supported that the heterostructure CNLDHAgP4 is the best photocatalyst in this finding. The spectral changes of absorbance during Cr(VI) removal using different catalysts were examined at different intervals of time and are shown in Figure b. Typically, in the presence of the CNLDHAgP4 catalyst and under solar light, the intensity of the absorption peak at λmax = 540 for Cr(VI) in the UV–vis nanometer slowly flattened at a time period of 120 min. This can also be monitored by visualizing the color of the analyte solution, which slowly changes from yellow color to transparent because of reduction of Cr(VI) to Cr(III).[7]

Figure 9

(a) Photocatalytic adsorption vs Cr(VI) reduction (%), (b) monitoring of changes in spectral absorption during Cr(VI) reduction over the as-prepared catalyst at 120 min of time interval, (c) second-order kinetic plot of Cr(VI) reduction over CNLDHAgP4, and (d) spectral absorption changes during Cr(VI) reduction over CNLDHAgP4 at different pHs.

Kinetics of the Cr(VI) Reduction Potential

To evaluate the kinetics of Cr(VI), the zero-order (eq ), first-order (eq ), and second-order models (eq ) were used to analyze the experimental data and the fitting results. The rate constants (k), average relative errors, and coefficient of determinations (R2) for kinetics of Cr(VI) were tabulated in Table .

Table 1

Fitted Data Results of Cr(VI) Reduction Kinetics Using Different As-Prepared Catalysts at pH 5

catalyst	NiFe LDH	g-C3N4	Ag3PO4	CNLDH	CNLDHAgP2	CNLDHAgP4	CNLDHAgP6	CNLDHAgP8
zero-order R2	0.98	0.78	0.94	0.84	0.89	0.94	0.91	0.80
slope (k0)	0.01	0.004	0.004	0.005	0.005	–0.006	0.004	0.0041
standard error	0.07	0.066	0.030	0.070	0.051	0.046	0.04406	0.0590
first-order R2	0.97	0.88	0.98	0.86	0.95	0.99	0.955	0.87
slope (k1)	0.007	0.008	0.007	0.0117	0.010	0.0157	0.009	0.007
standard error	0.03	0.096	0.030	0.137	0.067	0.038	0.059	0.086
second-order R2	0.98	0.96	0.91	0.86	0.89	0.94	0.97	0.91
slope (k2)	0.013	0.012	0.011	0.027	0.026	0.050	0.018	0.014
standard error	0.064	0.179	0.074	0.325	0.0038	0.0056	0.090	0.1352

The reaction is best fitted for the second-order relationship 1/C = 1/C0 + k2 × t, where C0 is the initial concentration of the solution, t is the reaction time, and C is the solution concentration at reaction time t. The kinetics of Cr(VI) reduction plots for different samples are shown in Figure c. The rate constants of the reaction were obtained by the polynomial fitting and entered in Table . The rate constant (k2) of the reaction rate of CNLDHAgP4 (0.05049 L/mg min) was 1.85-fold to that of CNLDH (0.02742 L/mg min), 4.16-fold to that of g-C3N4 (0.01253 L/mg min), 3.84-fold to that of NiFe LDH (0.01363 L/mg min), and 4.54-fold to that of Ag3PO4 (0.01164 L/mg min), which indicates an increased rate of photocatalytic reduction of Cr(VI) over the heterostructure CNLDHAgP4.

Effect of pH on the Photocatalytic Reduction of Cr(VI)

The pH of a solution plays a crucial role in the adsorption and reduction reaction of Cr(VI) to Cr(III). Generally, the Cr(VI) reduction process decreases at higher pH because of the negatively charged surface of the photocatalyst, which repels the Cr2O72– anions. At lower pH, the surface of a photocatalyst becomes positively charged because of the availability of protons and tends to attract Cr2O72– anions, which facilitates the adsorption process. To study these effects, a series of experiments were performed at different pHs ranging from 5 to 8, and the results are shown in Figure d. The highest reduction rate of Cr(VI) was possible under an acidic pH 5. At higher pH, the redox reaction was carried out in the solution, which inhibits the reduction of Cr(VI).[62] Furthermore, with the increment of pH, there is a chance that the soluble metal cations (i.e., Ni2+, Fe3+, and Cr3+) would be removed away from the solution by forming Ni(OH)2 (Ksp of 5.5 × 10–16) and Fe(OH)3 (Ksp of 2.79 × 10–39).[62] Alternatively, in very strong acidic or basic conditions, the dissolution of oxygen-containing functional groups of the NiFe LDH phase present in the heterostructure would be possible and decrease the adsorption process as well as the rate of reduction. Therefore, the suitable pH, that is, 5, was effective for the reduction reaction of Cr(VI) to Cr(III) in the heterostructure CNLDHAgP4. Hence, the removal of Cr(VI) in CNLDHAgP4 was achieved at a suitable pH of 5 with the availability of protons in the solution and electrons at the CB of NiFe LDH because of the SPR effect of Ag NPs and interfacial charge transfer by the formation of quasi-type-II dual heterojunctions. The reduction reaction of Cr(VI) to Cr(III) follows the following equation:

Effect of Scavengers on the Reduction of Cr(VI)

Generally, it is known that the CB electrons of a photocatalyst play a vital role during the reduction of Cr(VI) to Cr(III). To prove the active role of electrons, CH3OH was used as a hole scavenger during the Cr(VI) reduction process, which then consumed the photogenerated holes in the VB of the photocatalyst so that the reduction percentage of Cr(VI) was significantly enhanced. A typical experimental method involves addition of different quantities of CH3OH and citric acid to a specific parts per million concentration of Cr(VI) solution maintaining pH 5. As shown in Figure S2a, the addition of CH3OH (2 mL) decreases the photoreduction time together with increase in the percentage of Cr(VI) reduction. Furthermore, aqueous citric acid solution (1 mL, 5 mM) was also used for the complete reduction of Cr(VI) within 80 min. When a mixed solution containing both CH3OH (2 mL) and citric acid (1 mL) was added to the parts per million concentration of aqueous Cr(VI) solution, then complete reduction was found in 60 min. This suggests that the electron is the major active species for the reduction of Cr(VI). For strong support of the participation of electrons, dimethyl sulfoxide (DMSO, 4 mM) and AgNO3 (0.4 mM) as electron scavengers were added to the reaction medium. However, the rate of photoreduction of Cr(VI) was negligible by the addition of both DMSO and AgNO3. Consequently, the Cr(VI) photoreduction mechanism proceeded with a direct one-step reduction reaction by photogenerated electrons in the presence of both CH3OH and citric acid. Also, Fu et al. reported the mechanism of the photocatalytic reduction of Cr(VI), proposing that Cr(VI) captures the photoexcited CB electrons for the reduction process and that H2O gets adsorbed by the VB holes.[63]

Photocatalytic Oxidation of Phenol

The versatile photocatalytic activities of the heterostructure CNLDHAgPx nanocomposites were also evaluated for the oxidation of phenol as the model organic contaminant under direct natural solar light irradiation. The effect of different pHs over phenol oxidation was also examined, and afterward pH 7 was found to be optimum pH for the oxidation of phenol. The efficiency of oxidation percentage was found to increase at acidic pH, whereas the degradation efficiency was found to decrease at basic pH. In very strong basic and acidic solutions, the oxygen functional groups of NiFe LDH in the heterostructure CNLDHAgPx would be dissolved and quite unstable. Therefore, the catalyst has a tendency to lose its activity, and consequently, the degradation efficiency was hampered. The highest efficiency for phenol degradation under solar light irradiation was found to be at pH 7 at the time interval of 120 min. The sequence of degradation activity in different pH studies follows the order: pH 7 > pH 6 > pH 8 > pH 4 > pH 11 (Figure a). In the absence of both the photocatalyst and solar light, a negligible amount of oxidation was noticed, which also shows the stability of organic pollutant phenol and sensitivity of the photocatalyst toward light. However, about 90% of phenol oxidation was achieved in 2 h under direct solar light irradiation. The photocatalytic oxidation ability of the as-prepared catalysts follows the order: CNLDHAgP4 (90%) > CNLDHAgP6 (85%) > CNLDHAgP2 (80%) > CNLDHAgP8 (75%) > CNLDH (51%) > Ag3PO4 (50%) > NiFe LDH (37%) > g-C3N4 (27%). Figure b displays the variation in UV–vis absorption spectral curves of phenol oxidation. Phenol exhibits an absorption peak at λmax of 269.5 nm, which was found to fade away gradually with the increase in exposure time of natural sunlight. Generally, the phenol oxidation reactions were preceded with the absorbance peak at 269.5 nm, which first slightly faded without the shifting in absorption band. Afterward, the absorption peaks of some intermediate aromatic species such as catechol (278–280 nm) and p-benzoquinone (p-BQ) (249 nm) simultaneously appeared and then finally converted to CO2 and H2O.[35] The reduction reaction of Cr(VI) and phenol oxidation activities clearly reveal the importance of Ag3PO4 (4%) as the optimum loading in CNLDHAgP4. The synergistic interaction among Ag NPs, Ag3PO4, g-C3N4, and NiFe LDH was mainly accountable for the larger photocatalytic accomplishment of CNLDHAgP4. Furthermore, the superior photocatalytic performance of CNLDHAgP4 was also attributed to some fundamental reasons of light absorption and effective charge separation by the subsequent points as (i) the dominant SPR effect of metallic Ag NPs over the oxo-bridge bimetallic linkages in NiFe LDH was responsible for the enhanced absorption of light in CNLDHAgP4 (from the UV–vis DRS study), (iii) Ov-type defect as oxygen vacancies in NiFe LDH acts as trapping sites for electrons and suppressed the charge recombination (from the XPS and PL study), and (iv) effective charge separation because of the establishment of a quasi-type II p–n/n–n dual heterojunction between Ag3PO4, g-C3N4, and NiFe LDH (from the electrochemical study).

Figure 10

(a) Photocatalytic oxidation (%) of phenol at different pHs, (b) spectral changes during the oxidation of phenol over CNLDHAgP4 at different intervals of time, and (c) pseudo-first-order kinetics, followed by the oxidation of phenol over CNLDHAgP4.

Kinetics of Photocatalytic Phenol Oxidation

The phenol oxidation kinetics of all the as-prepared catalyst was analyzed by a UV–visible spectrophotometer (Figure c). The kinetics study was carried out by using 20 ppm phenol concentration (20 mL) with a catalyst dose of 0.02 g at an optimum pH of 7. The kinetics results of phenol oxidation follow Langmuir–Hinshelwood pseudo-first-order kinetics, which is expressed as follows:where k is the rate constant for the degradation reaction in min–1, C0 is the initial concentration of the substrate, and C is the final concentration at time t. The plot of ln(C0/C) versus time results a straight line with slope kapp estimated via linear fitting of the regression curve. The apparent oxidizing rate constant (kapp) and correlation coefficient (R2) of phenol oxidation over different as-prepared catalysts were determined and listed in Table S2. It was calculated that CNLDHAgP4 holds a higher rate constant value of 0.00751 min–1, which was 2.28, 5.28, 1.73, and 1.20 times higher than that of NiFe LDH (0.00329 min–1), g-C3N4 (0.00142 min–1), Ag3PO4 (0.00433 min–1), and CNLDH (0.00625 min–1), respectively.

Effect of Scavengers on the Photooxidation of Phenol

To evaluate the participating active species in the mechanism of photooxidation of phenol over CNLDHAgP4, the scavenger experiment was carried out under sunlight irradiation. In a typical experiment, 1 mM isopropyl alcohol (IPA) (•OH), BQ (•O2–), DMSO (e–), and ethylenediaminetetraacetic acid (EDTA) (h+) were added to a solution that contained phenol (20 ppm) and the CNLDHAgP4 catalyst (0.02 g). The mixture was subjected to sunlight irradiation for 120 min. Figure S2b shows the effect of different scavengers on the photooxidation of phenol. The result indicated that a reduction in the photooxidation activities of 23, 36, 80, and 88% was found for IPA, EDTA, p-BQ, and DMSO, respectively. These results confirms that the primary reactive species responsible for the photooxidation of phenol was the •OH and h+ as the secondary reactive species. The role of •O2– and e– was considered to be negligible in the photooxidation process of phenol. Furthermore, the quantification experiments of •OH were carried out under sunlight irradiation for 2 h by measuring the fluorescence intensity of 2-hydroxyterephthalic acid (TAOH) in a PL probing technique (Figure S3). Terephthalic acid acted as a probe molecule that reacts with •OH radical to generate a fluorescence active product TAOH with an emission peak at 426 nm. The intensity of the PL spectrum of TAOH is directly proportional to the quantity of the •OH radical. The highest PL intensity of the heterostructure CNLDHAgP4 indicates the higher amount of the formation of •OH radicals as compared to neat NiFe LDH, g-C3N4, and Ag3PO4 materials. Additionally, the presence of surface −OH groups in NiFe LDH further promotes in producing •OH radicals, which play a number of roles in the photocatalytic oxidation process. The difference in the fluorescence intensity of the TAOH measurement experiment sturdily favors the best catalytic activities of CNLDHAgP4, which was achieved due to the better interfacial charge separation at the dual heterojunction interface. Table represents a state-of-the-art for the comparison of Cr(VI) reduction and phenol oxidation potential over heterostructure CNLDHAgP4 nanocomposites with other reported materials.

Table 2

State-of-the-Art for the Comparison of Cr(VI) reduction and Phenol Oxidation Potential over Heterostructure CNLDHAgP4 with Other Reported Materials

catalytic system	concentration of Cr(VI) or phenol	light source	preparation method	catalytic activity time (h)	pH	results (%)	refs
N, S co-doped CeO2	50 ppm Cr(VI)	visible light	in situ co pyrolysis	2	2	93	(64)
P-doped/g-C3N4 nanosheets	20 ppm Cr(VI)	visible light	element doping and thermal exfoliation method	2	2.13	75	(65)
α-MnO2@RGO	10 ppm Cr(VI)	visible light	in situ hydrothermal	2	2	97	(36)
Ag@Ag3PO4/g-C3N4/NiFe LDH	20 ppm Cr(VI)	visible light	electrostatic self-assembly and in situ photoreduction method	2	5	97	present work
CeO2/Mg–Al LDH	20 ppm phenol	visible light	impregnation	3	natural pH suspension	50	(66)
Zn2+Me3+ (Me = Al/Ga) LDHs doped with Ga2O3 and In2O	25 ppm phenol	180 W Unnasol US 800 solar simulator	calcinations followed by coprecipitation	4	NA	77	(67)
Ag@Ag3VO4/ZnCr LDH	20 ppm phenol	solar light	in situ hydrothermal	3	7	93	(35)
Ag@Ag3PO4/g-C3N4/NiFe LDH	20 ppm phenol	solar light	electrostatic self-assembly and in situ photoreduction method	2	7	90	present work

Recyclability Study of the Heterostructure CNLDHAgP4

It is important to test the recyclability of a photocatalyst for practical application as its efficiency may change after the interaction with a particular pollutant in aqueous medium under sunlight irradiation. To confirm the photostability and reusability of the catalyst for real-time applications, the heterostructure CNLDHAgP4 nanocomposite was evaluated for five different recycle experiments for the reduction reaction of Cr(VI) and phenol oxidation activities (Figure S4). Average 95% of Cr(VI) reduction and 89% oxidation of phenol were maintained at the end of five repeating cycles over heterostructure CNLDHAgP4. Figure S5a represents the FTIR analysis of the CNLDHAgP4 sample after Cr(VI) reduction activities. The spectra prominently show a peak shifting toward the lower wave number region. These results are indicative of good interaction between CNLDHAgP4 and Cr(VI), and the characteristic spectral peaks are still preserved even after five repeated cycles of the reduction reaction process. Additionally, available UV–vis DRS spectra after Cr(VI) reduction (Figure S5b) concluded a little change in the absorption intensity of CNLDHAgP4. The amount of leached silver ion concentration was also investigated by using pure Ag3PO4 and Ag@Ag3PO4/g-C3N4/NiFe LDH during phenol oxidation over a time study of 60 min and analyzed through inductively coupled plasma mass spectrometry (ICP-MS) as shown in Figure S6 (Supporting Information). The plot in Figure S6 shows the released silver ion concentration measured through ICP-MS versus time. At the beginning, a sharp increase of released silver ion concentration was found in the pure Ag3PO4 sample, whereas the release of silver ion concentration was relatively slow in Ag@Ag3PO4/g-C3N4/NiFe LDH. Also, the concentration of silver ion released from the pure Ag3PO4 sample was more than that of Ag@Ag3PO4/g-C3N4/NiFe LDH. After 30 min of irradiation, the silver ion concentration was found to decrease in both the photocatalysts, which could be ascribed to the reduction of Ag+ to metallic Ag0 NPs by the photogenerated electrons on the surface of the photocatalyst. The slow release of silver ion in Ag@Ag3PO4/g-C3N4/NiFe LDH nanocomposite indicates slower decomposition of Ag3PO4, which firmly demonstrate the excellent stability of Ag@Ag3PO4/g-C3N4/NiFe LDH nanocomposite than pure Ag3PO4 during photocatalytic reactions. However, the reusable efficiency of phenol oxidation is far below than Cr(VI) reduction with the increase of repeating cycle, which could be explained by the subsequent reasons. First, the leached silver ions were present during the first 30 min reuse of the catalyst (first cycle), which directly results in the loss of the silver ions of the photocatalyst in the subsequent reactions with further cycles. Second, the formation of phenol intermediate products such as catechol (278–280 nm) and p-BQ (249 nm) blocks the active sites, thus decreasing the photocatalytic efficiency of CNLDHAgP4 during the phenol recycle studies. In addition, the decreasing efficiency trend of phenol oxidation would be in reverse order with the increase in repeating cycle, when the intermediate products completely mineralize into CO2 and H2O. This illustrates that the heterostructure CNLDHAgP4 could retain excellent photocatalytic activity even after five repeated cycles of use, which established its high stability and reusability for the management of wastewater contaminated with Cr(VI) and phenol.

In-Depth Charge Separation and Photocatalytic Mechanism behind the Construction of a Quasi-Type-II p–n/n–n Dual Heterojunction in the CNLDHAgP4 System

The higher photocatalytic activities of CHLDHAgP4 were mainly credited to the establishment of a quasi-type-II p–n/n–n dual heterojunction consisting of n-type NiFe LDH, n-type g-C3N4, p-type Ag3PO4, and plasmon-excited Ag NPs. The mechanistic path of photoinduced charge separation and transfer in an interfacial region of dual heterojunctions is a vital factor to not only enhance the photocatalytic efficiencies of a material but also affect its durability to a greater extent. For better understanding of the carrier charge separation at the interfacial region of p–n/n–n dual heterojunctions, various characteristic techniques such as PL, electrochemical impedance spectroscopy (EIS), and Mott–Schottky (M–S) results were discussed.

PL Study

The charge carrier transport across the heterostructure CNLDHAgPx nanocomposite was further revealed through PL emission spectroscopy excited at 380 nm and compared with NiFe LDH, g-C3N4, CNLDH, and Ag3PO4 (Figure ). Pristine NiFe LDH exhibited three types of emission band at 442 (strong), 460 (strong), and 575 nm (medium). The centered peaks at 442 and 460 nm were associated with the radiative recombination of charge carriers associated with the ligand field transitions of Ni2+ octahedrons.[68] The continuous steady emission at 575 nm was due to the surface defects of NiFe LDH.[10] g-C3N4 shows a distinct emission peak centered at 460 nm, which could be ascribed to the band-to-band PL emission related to the approximate band gap energy of g-C3N4.[10] The emission peak intensity of the CNLDH composite was sharply decreased as compared to that of NiFe LDH and g-C3N4 because of the suppression of charge carriers, and the main reason was the strong quantum confinement effect caused by the highly extended g-C3N4 on NiFe LDH.[10] In Ag3PO4, a strong emission peak at 440 (blue) and a weak emission peak at around 550 nm (green) were observed in the spectra. According to Liang et al., the origin of luminescence properties in Ag3PO4 originated from the recombination of charge carrier transition between O 2p → Ag 5d orbital or owing to the self-trapped excitons in the PO4 oxyanion complex.[61] Botelho et al. also demonstrated the origin of PL properties of Ag3PO4 because of the charge transition in [PO4] and [AgO4] clusters.[69] The blue emission peak in Ag3PO4 arises because of the [PO4] clusters and the green emission peak arises because of the highly distorted tetrahedral [AgO4] clusters.[69] The PL spectra of the heterostructure CNLDHAgPx were significantly lower than that of NiFe LDH, g-C3N4, Ag3PO4, and CNLDH. Especially in CNLDHAgP4, significant suppression of charge carrier was found, which was ∼2 times reduced than that of CNLDH and ∼4 times reduced than that of NiFe LDH, g-C3N4, and Ag3PO4. CNLDHAgP4 exhibited a blue-shifted emission peak at 460, a green-shifted emission peak at 540, and a red-shifted emission peak at 575 nm, respectively. The emission in the blue-shifted region of CNLDHAgP4 indicated the presence of the quantum confinement effect of g-C3N4 and a high contribution of [PO4] clusters of Ag3PO4. The presence of the [PO4] cluster was further confirmed by the presence of the asymmetric stretching vibration peak of P–O at 1068 cm–1 in FTIR spectra (Figure b). The green emission at 540 nm was due to the presence of Ov-type defects (oxygen vacancy defect complex) CNLDHAgP4.[70] The red-shifted emission peak found at 575 nm was associated with the combined results of the plasmonic effect of Ag NPs and intermediate energy levels of [AgO4] clusters of Ag3PO4 in CNLDHAgP4. In this way, PL spectroscopy greatly revealed the effective charge separation in CNLDHAgP4, which proves the incremental antirecombination process of exciton pairs in the as-synthesized heterostructure material for superior photocatalytic activity.

Figure 11

PL spectra of NiFe LDH, g-C3N4, Ag3PO4, CNLDH, and heterostructure CNLDHAgPx samples measured at room temperature with an excitation energy of 380 nm.

EIS Measurement Study

EIS has been proven to be a significant tool for the investigation of the charge carrier separation and charge-transfer process across the electrode and electrolyte interfacial contact area in a three-electrode electrochemical workstation. The EIS spectra of NiFe LDH, g-C3N4, Ag3PO4, CNLDH, and CNLDHAgP4 of the as-prepared electrodes are shown in Figure . The equivalence fitted model circuit for the CNLDHAgP4 electrode is shown in the inset in Figure . R1 can be assigned as the series resistance of the system. R2 can be assigned as the charge-transfer resistance across the Pt counterelectrode/electrolyte interface, which represents the first semicircle in the high-frequency region of 25–40 Ω for the CNLDHAgP4 electrode system. The second semicircle in the middle frequency region of 15–25 Ω could be assigned to the charge-transfer resistance (R2) of the as-prepared CNLDHAgP4 electrode and electrolyte interface. The R3 represents the charge-transfer resistance in the Helmholtz double layer, whereas CPE1 and CPE2 represent the chemical capacitance. The EIS spectra clearly indicate the reduced diameter of the arc radius of the CNLDHAgP4 electrode than those of the other as-prepared electrodes, which reveals the faster interfacial charge transfer and separation efficiency of the CNLDHAgP4 electrode system. The sequential arrangement of the charge-transfer resistance value of the as-prepared electrode follows the order: CNLDHAgP4 (37.5 Ω) < CNLDH (52.3 Ω) < g-C3N4 (59.1 Ω) < Ag3PO4 (60.8 Ω) < NiFe LDH (154.2 Ω).

Figure 12

EIS spectra of NiFe LDH, g-C3N4, Ag3PO4, CNLDH, and heterostructure CNLDHAgP4 samples.

M–S Analysis

Generally, the basic principles of M–S measurements were based on the Schottky barrier generated during the contact of semiconductor with electrolyte solution and was employed to determine the carrier density of a semiconductor material.[71,72] The slopes of the M–S plots are very much crucial in judging the n-type semiconductor properties with a positive slope and p-type semiconductor properties with a negative slope tilting toward the X-axis.[71,72] To approximate the flat band potential (Vfb), the following equation is used:where ε, ε0, Nd, Vapp, and Vfb symbolize the dielectric constant of a semiconductor, vacuum permittivity, donor density, applied potential, and flat band potential and kT/q is a temperature-dependent term in the M–S equation, respectively. The intercept value of the linear plot at 1/C2 = 0 gives the flat band potential (Vfb). The Vfb is approximately equal to the CB potential (ECB) for n-type semiconductors and VB potential (EVB) for p-type semiconductors. In accordance with the intercept of the M–S plots (Figure a,b), the Vfb for NiFe LDH, g-C3N4, CNLDH, Ag3PO4, and CNLDHAgP4 were estimated to be about −0.01, −0.54, −0.37, +2.88, and +2.49 V versus NHE, respectively. Therefore, the ECB potential of NiFe LDH, g-C3N4, and CNLDH were estimated to be −0.01, −0.54, and −0.37 V, respectively. In our previously reported work,[10] the ECB levels of g-C3N4 were calculated to be −1.13 V versus Ag/AgCl by using LSV plots (Figure B), and in the present work, we have converted the values versus NHE to get a comparable energy level matching with the M–S plot for the proper description of the mechanism, and here, the estimated ECB value of g-C3N4 was −0.53 V versus NHE (Figure a). Notably, electrode potentials were converted to the NHE scale using the following equation:[73]E0(Ag/AgCl) = 0.197 at 25 °C, ECB(Ag/AgCl) is the experimentally deliberate potential against the Ag/AgCl reference electrode, and the measured pH value of the electrolyte was found to be approximately 6.5 for the 0.1 M Na2SO4 electrolyte.

Figure 13

M–S plots of (a) NiFe LDH, CN, and CNLDH and (b) Ag3PO4 and heterostructure CNLDHAgP4.

Furthermore, by analyzing these data in combination with the band gap energy (Eg) achieved from the Kubelka–Munk spectra (Figure ), the EVB positions of the NiFe LDH, g-C3N4, and CNLDH were calculated to be +2.19, +2.16, and +1.98 V, respectively. Likewise, the ECB and EVB potentials of Ag3PO4 were calculated to be +0.45 and +2.88 V, respectively. The ECB and EVB potentials of CNLDHAgP4 were calculated to be +0.11 and +2.49 V, respectively. Generally, the Fermi level is assumed to be present at 0.1 eV beneath the CB edge of n-type semiconductors and 0.1 eV above the VB edge of p-type semiconductors.[74] Earlier, it was confirmed from the LSV measurement study that both NiFe LDH and g-C3N4 belong to the n-type semiconductor, whereas CNLDH is a n–n-type semiconductor.[10] From the LSV measurement studies (Figure D), Ag3PO4 was confirmed to be a p-type semiconductor. Chen et al. also reported the p-type semiconductor properties of Ag3PO4 in the heterostructure Ag/Ag3PO4/TiO2.[38] Therefore, the respective Fermi level positions of NiFe LDH, g-C3N4, CNLDH, Ag3PO4, and CNLDHAgP4 were estimated to be +0.11, −0.44, −0.27, +2.78, and +0.2 V versus NHE, respectively.[74]

The M–S plot of the heterostructure CNLDHAgP4 (Figure b) clearly displayed an inverted “V-shaped” curve with two flat band potentials, which reveals the existence of p–n/n–n dual heterojunctions and corresponds to the existence of both positive and negative charge carriers.[71,72] A slightly intense slope in the M–S curves of CNLDHAgP4 than that of NiFe LDH and g-C3N4 indicates lower donor density and reveals the presence of p-type Ag3PO4 in CNLDHAgP4. The heterostructure CNLDHAgP4 exhibits a negative shift in the CB edge (+0.11 V vs NHE) in comparison to p-type Ag3PO4 (+0.45 V vs NHE). It has been well-known that the higher Vfb (EVB) value of the p-type semiconductor signifies a higher degree of band bending, which resulted in a larger space charge region for charge separation. The higher EVB potential of +2.49 V in CNLDHAgP4 exerts a strong driving force for the separation of photogenerated charge pairs in the space charge region, which results in higher photocatalytic activities. This is due to the fact that the electron and hole pairs were trapped in the depletion layer of the electric field induced at the interface of p–n/n–n dual heterojunctions after equilibration of the Fermi level of all the constituent semiconductors in CNLDHAgP4. According to the semiconductor theory, the position of Fermi level and band edge potentials was an important criterion for the band alignment in a semiconductor heterojunction.

Given the above discussion, a mechanism of superior charge separation for the enhanced Cr(VI) reduction and phenol oxidation activities in the heterostructure CNLDHAgP4 was proposed and shown in Scheme . The possible band alignment of NiFe LDH, g-C3N4, Ag3PO4, and Ag NPs before and after contact was systematically described in Scheme . The entire photocatalytic reaction process was initiated by the photon energy absorption equal to or higher than the band gap energy, which causes the generation of electron–hole pairs from the photoexcited semiconductor components. Before contact (Scheme a), the band alignment of NiFe LDH, g-C3N4, Ag3PO4, and Ag NPs becomes messy, which is unable to support the charge separation of carriers. However, after contact, when p–n/n–n dual heterojunctions were formed, despite the lower CB edge potential of Ag3PO4, the electrons would flow from Ag3PO4 to g-C3N4 and then to NiFe LDH. It was due to the Fermi level alignment of the constituent semiconductor with the SPR effect of Ag NPs, which has a significant role in the charge separation of dual heterojunctions in CNLDHAgP4. After the contact (Scheme b), the CB and VB edge potential of g3PO4, g-C3N4, and NiFe LDH changes to reach the equilibration of Fermi levels (Ef). The Fermi level of Ag is situated at 0.4 V (vs NHE).[60] The SPR absorption of Ag NPs was found at 560 nm as verified from the UV–vis DRS spectra of the heterostructure CNLDHAgP4; consequently, the SPR electrons of Ag NPs were excited at an energy of 1.81 eV with regard to the Fermi level of Ag NPs. In order to maintain the energy band alignment during the course of formation of heterostructures, the Fermi level of Ag will shift to a position more negative than −1.81 V versus NHE in the presence of visible light irradiation. When the constituent phases acquire an equalized Fermi level, a built-in electric field directed from p-type Ag3PO4 to the n-type g-C3N4 and then to the n-type NiFe LDH will be established, which can stop the charge diffusion between them.[71,72,74] At the meantime, because of the SPR generated electronic drifting effect of Ag NPs, the energy bands of p-type Ag3PO4 shifted upward along with the Ef, whereas those of the n-type g-C3N4 and n-type NiFe LDH shifted downward to maintain an overall Fermi level equilibration in this process. The newly formed well-aligned band structures become to the interactive structure in CNLDHAgP4 interface (Scheme b).

Scheme 1

(a) Diagrammatical Representation of the Energy Band Positions of n-Type NiFe LDH, n-Type g-C3N4, p-Type Ag3PO4, and Ag NPs (before Contact), (b) Formation of Quasi-Type-II p–n/n–n Dual Heterojunctions and the Charge Separation Mechanism over Heterostructure CNLDHAgP4 under Visible Light Irradiation (after Contact)

Accordingly, the Fermi level of the excited electrons formed due to the effect of SPR excitation of the plasmonic-metal Ag NPs can be directly transferred to the CB of Ag3PO4, which flows toward the CB of the g-C3N4 semiconductor and then gets trapped by the oxygen vacancy (Ov) captured center of NiFe LDH. Yang et al. also proved that the Ag NP-based SPR generated electronic injection in the P/Ag/Ag2O/Ag3PO4/TiO2 system.[53] The green emission peak at 540 nm in PL spectra and the oxygen vacancy peak at the BE of 531.6 eV in the XPS spectra of CNLDHAgP4 confirm the defect type of Ov.[75] The energy level of Ov is about 0.9 V deeper than the CB of any semiconductor similar to that in NiFe LDH (−0.01 V).[76] Therefore, the photoelectron in g-C3N4 can be easily transferred to the Ov of NiFe LDH. Simultaneously, photogenerated holes in the VB of NiFe LDH could easily move to the VB of g-C3N4 and then to the VB of Ag3PO4. This process not only facilitates the charge separation but also accumulates electrons in the CB of NiFe LDH and holes in the VB of Ag3PO4, respectively. Moreover, the presence of the built-in electric field near the interface can also facilitate the separation of photogenerated electron–hole pairs. In this way, a quasi-type II p–n/n–n dual heterojunction has been established in the CNLDHAgP4 nanocomposite and greatly promotes the separation of charge carriers.

Under these circumstances, a large number of electrons were accumulated and generated on the CB of n-type NiFe LDH because of photoexcitation and transfer process. As the CB edge potential of n-type NiFe LDH (0.29 eV) was not sufficiently negative to reduce O2/•O2–(−0.33 eV vs NHE),[50,60] the electrons enriched on the surface of NiFe LDH could not be trapped by the molecular oxygen in the solution to form •O2–. Alternatively, the accumulated electrons at the CB edge of NiFe LDH exhibited more negative potential than the reduction potential of Cr(VI)/Cr(III), that is, +1.33 eV versus NHE, which indicates that more electrons were subsequently captured by Cr(VI) and helped in the reduction of Cr(VI) to Cr(III). This Cr(VI) reduction mechanism over CNLDHAgP4 was also well supported with the mechanism reported by Fu et al.[63] In addition, the scavenger test of the Cr(VI) reduction reaction proved that direct accumulated electrons at the bottom of CB are responsible for Cr(VI) reduction. Figure S2a depicts that the addition of mixed solution containing both CH3OH (2 mL) and citric acid (1 mL) as hole scavengers could react with the photogenerated holes at the VB of CNLDHAgP4 and decrease the photoreduction time of the parts per million concentration of Cr(VI) solution to 60 min for complete reduction process. Moreover, the VB potential of Ag3PO4 is +2.40 eV (after contact), which has a more positive potential than the redox potential of •OH/OH– (+1.99 eV vs NHE);[77] hence, OH– can be oxidized to •OH by holes. As such, the accumulated holes left in the VB of Ag3PO4 will directly oxidize phenol into nontoxic products[78] or oxidize H2O to form •OH active species, which subsequently oxidized phenol to CO2 and H2O. Similarly, from the scavenger test of phenol (Figure S2b), •OH and h+ were found to be the major and minor participating active species behind the mechanism of photooxidation of phenol. The production of •OH radicals was confirmed by the terephthalic acid test shown in Figure S3. On the basis of the above-mentioned discussion, the heterostructure CNLDHAgP4 nanocomposite follows the quasi-type-II dual p–n/n–n heterojunction mechanism for the photooxidation of phenol, which involves •OH and h+ as the reactive species and accumulated electrons as the reactive species for Cr(VI) reduction. Most importantly, Ag3PO4 can be protected from photocorrosion by transferring the photogenerated electrons to g-C3N4 and then to NiFe LDH, respectively. Therefore, the presence of Ag NPs could greatly improve the photostability of the CNLDHAgP4 nanocomposite. In addition, the structural stability of exfoliated NiFe LDH nanosheets can be well maintained because of the formation of a Schottky barrier across the n–n junction between g-C3N4 and NiFe LDH and follows a self-stability mechanism.[79] Therefore, the synergistic interaction of Ag3PO4, Ag NPs, g-C3N4, and NiFe LDH plays crucial roles in improving both the photocatalytic activity and stability of the as-prepared heterostructure CNLDHAgP4 nanocomposite.

Conclusions

Plasmonic heterostructure Ag@Ag3PO4/g-C3N4/NiFe LDH nanocomposites were successfully synthesized by the combination of an electrostatic self-assembly and in situ photoreduction method. The formation of quasi-type-II p–n/n–n dual heterojunctions between p-type Ag3PO4/n-type g-C3N4/n-type NiFe LDH with the SPR effect of Ag NPs leads to a remarkable enhancement in the photocatalytic performance of the heterostructure Ag@Ag3PO4/g-C3N4/NiFe LDH. The obvious enhancement of the photocatalytic performance of this heterostructure is mainly credited to the intimate Ag3PO4/g-C3N4/NiFe LDH interface created by its exceptional dual heterojunction architecture, which in turn allows rapid charge-transfer process. Furthermore, the quenching of PL intensity, highest photocurrent density (−7.90 mA cm–2), and reduced arc of the Nyquist plot (37.5 Ω) strongly manifest the effective charge separation and enhanced photocatalytic performance of CNLDHAgP4. Most importantly, the remarkable photocatalytic performance of CNLDHAgP4 for Cr(VI) reduction (97%) and phenol oxidation (90%) was far superior to that of NiFe LDH, g-C3N4, Ag3PO4, and CNLDH. Overall, the result of this finding was invigorating and might provide a new methodology for developing high-performance and cost-effective LDH-based plasmonic heterojunction photocatalysts for maximum photon absorption to thrash the challenging environmental remediation.

Experimental Section

Chemicals

Natural graphite powder (Aldrich, 99%), melamine (Aldrich, 99.0%), Ni(NO3)2·6H2O (Aldrich, 99.0%), Fe(NO3)3·9H2O (Aldrich, 99.0%), NaOH (Merck India, 99.5%), CH3OH (Merck India, 99%), AgNO3 (Merck India, 99.9%), 4-aminoantipyrene (Merck India, 98%), and potassium ferricyanide (Aldrich, 99.5%) were used as received. The metal salt solutions were prepared by using deionized water. The other reagents used in this research work are of analytically certified pure grade and directly used for the experimental process and analysis protocol.

Fabrication of Pristine NiFe LDH

Pristine NiFe LDH was synthesized according to our previous standardized method.[10] In a typical procedure, 0.5 M NaOH solution and 100 mL mixed metal nitrate solution consisting of 0.16 M Ni(NO3)2·6H2O and 0.033 M Fe(NO3)3·9H2O ([Ni2+ + Fe3+] = 0.2 M) were simultaneously added dropwise to a beaker containing 50 mL deionized water at a constant pH of 9.0. The obtained yellow brown colored precipitate was aged for 24 h at room temperature and then filtered, followed by washing with water and ethanol. The washed precipitate was thoroughly dried in an oven at 80 °C overnight and then grinded for further use.

Fabrication of Pristine g-C3N4

Pristine g-C3N4 was prepared by the heat treatment of melamine.[10] In a typical experiment, 5.0 g of melamine underwent calcination at 550 °C for 4 h with a heating rate of 2.5 °C/min. The product was naturally cooled down to room temperature, and then the as-obtained yellow color solid mass was grounded into a powder form, yielding light yellow color g-C3N4 designated as CN.

Preparation of Ag@Ag3PO4/g-C3N4/NiFe LDH

In a typical synthesis method of heterostructure Ag@Ag3PO4/g-C3N4/NiFe LDH nanocomposites, an appropriate 10 wt % of g-C3N4 with respect to NiFe LDH was dispersed in CH3OH (100 mL) and ultrasonicated for 1 h for complete dispersion in accordance with our previous report.[10] To this dispersion, approximately 1 g of freshly prepared NiFe LDH was introduced under vigorous stirring under an N2 atmosphere at room temperature. The mixed dispersion of both NiFe LDH and g-C3N4 was ultrasonicated for another 2 h to reach the electrostatic self-assembly between exfoliated positively charged sheets of NiFe LDH and negatively charged sheets of g-C3N4. A calculated amount of AgNO3 was added to the CNLDH dispersion to maintain the 3:1 molar ratio of Ag+/PO43–, followed by ultrasonication for another 15 min. Afterward, an aqueous solution of (NH4)2HPO4 was added dropwise to the CNLDH dispersion with vigorous stirring, followed by 15 min exposure of visible light irradiation. Finally, the whole precipitate was aged for 2 h. In this manner, different wt % (2, 4, 6, and 8) of the heterostructure Ag@Ag3PO4/g-C3N4/NiFe LDH was synthesized by varying Ag3PO4 over CNLDH and designated as CNLDHAgPx, x = 2, 4, 6, and 8 wt %, respectively. The precipitated solids were separated by centrifugation, followed by thorough washing with deionized water and then with ethanol. The solid materials were dried at 70 °C for further characterization, and the photocatalytic activities were tested. The formation of the heterostructure nanocomposite was clearly revealed from the color change from pale yellow to greenish brown. During the synthesis of heterostructure nanocomposites, the entire constituent semiconductor being the photocatalyst formed e––h+ pairs under visible light irradiation. The hole was trapped by the dry methanol as scavenger and the electron was used to reduce Ag+ to metallic Ag NPs, thereby producing a heterostructure Ag@Ag3PO4/g-C3N4/NiFe LDH nanocomposite. The pristine Ag3PO4 was prepared by following the same procedure but without the steps that involved LDH, g-C3N4, and irradiation of visible light. For the purpose of comparison, g-C3N4/NiFe LDH (CNLDH) composites were prepared following the same procedure similar to that of the heterostructure CNLDHAgPx but excluded the addition of AgNO3, (NH4)2HPO4, and visible light illumination. Besides, the exfoliated self-assembled CNLDH hybrid was aged in a fume hood for 24 h. The obtained powder was dried at 100 °C overnight, yielding CNLDH composites. The synthetic steps of the heterostructure CNLDHAgPx are illustrated in 2 2.

Scheme 2

Synthetic Steps of the Heterostructure CNLDHAgPx Nanocomposite

Characterization

The crystal phase purity of the prepared sample was measured by powder XRD using a Rigaku MiniFlex (set at 30 kV and 15 mA) instrument over the range of 5° < 2θ < 70° with a scan rate of 0.02° min–1. The stretching vibration mode of the catalyst was determined using an FTIR spectrometer (JASCO FT/IR-4600) within the range of 4000–400 cm–1 using KBr as the reference compound for the measurement process. The UV)–vis DR spectra of the materials were measured using a JASCO-V-750 UV–vis spectrophotometer attached with DR accessory within the range of 200–800 nm, and boric acid pellets were used as the reference compound. The PL spectra were measured and analyzed on a JASCO-FP-8300 fluorescence spectrophotometer with an excitation energy of 380 nm. The HRTEM images and EDX spectra were obtained on a FEI, Tecnai G220, TWIN, Philips system at an accelerating voltage of 200 kV. The XPS measurements were performed on a VG Microtech Multilab ESCA 3000 spectrometer with a nonmonochromatized Mg Kα X-ray source and an energy of 0.8 eV. The BE correction was performed by the C 1s reference peak of carbon atom at 284.9 eV. ICP-MS (Agilent 7500) was used for the quantification of leached silver ion concentration. The photoelectrochemical studies were carried out by preparing the working electrode of the synthesized catalyst (NiFe LDH, g-C3N4, Ag3PO4, CNLDH, and CNLDHAgP4) by the electrophoretic deposition method over a fluorine-doped tin oxide (FTO)-coated surface. The as-synthesized catalyst (30 mg) was mixed with 20 mg of iodine powder and 30 mL of acetone solution and sonicated for 15 min. Afterward, two FTO electrodes were dipped inside the solution facing parallel to each other with 10–20 mm of separation between them. A 60 V bias applied potential was fixed for 3 min through potentiostat. The uniformity of the coated area was fixed at 1 cm × 1 cm and then dried at 80 °C for 2 h. All the photoelectrochemical studies were carried out on an IVIUM-n-STAT electrochemical workstation. An aqueous solution containing 0.1 M Na2SO4 was used as an electrolyte with a standard three-electrode cell attached with a quartz pane and potentiostat–galvanostat. FTO-coated films were used as the working electrode, while the Pt electrode was used as the counter electrode and Ag/AgCl as the reference electrode. A 300 W Xe lamp (ORIEL) was used as the visible light source. EIS was measured at a frequency scan of 10 000–0.01 Hz at a potential of 0.1 V in 0.1 M Na2SO4 solution in an open-circuit potential. The M–S measurement was carried out at a constant frequency of 500 Hz under dark. The LSV plots were evaluated by a potential biasing of −2 to +3 V at a scan rate of 0.05 mV s–1 under both dark and light illuminations, respectively.

Photocatalytic Activity

The photocatalytic Cr(VI) reduction reaction of the samples was carried out by using an aqueous K2Cr2O7 solution. To access the photocatalytic activity, 0.02 g of catalysts was added to 20 mL of 20 ppm Cr(VI) solution taken in a Pyrex conical flask, and the reaction was continued for 2 h. Initially, the photocatalytic reduction of Cr(VI) samples was measured at different pH values of 5, 6, and 8, respectively. The suspension of the catalyst and Cr(VI) solution was kept under constant stirring in dark around 30 min in order to reach the adsorption and desorption equilibrium prior to sunlight exposure (100 000 lx). The residual concentration of Cr(VI) over a period of time was colorimetrically analyzed by the 1,5-diphenylcarbazide method using a JASCO-V-750 UV–vis spectrophotometer at λmax 540 nm.[36] In addition, the remaining Cr(VI) concentration in the sample was also determined after the photocatalytic activity.[37] The photocatalytic degradation of phenol was carried out by adding 0.02 g of the catalyst to 20 mL of phenol (20 ppm) taken in a 100 mL stoppered Pyrex conical flask. The experiments were continued for 2 h in the presence of sunlight. The suspension of the catalyst and phenol solution was kept under constant stirring in dark around 30 min to reach the adsorption and desorption equilibrium prior to sunlight exposure (100 000 lx). Furthermore, the photocatalytic degradation of phenol was measured at different pH values of 4, 6, 7, 8, and 11. After the photocatalytic activities, the residual concentration of phenol was measured by a JASCO-V-750 UV–vis spectrophotometer.