Engineering S-scheme Ag₂CO₃/g-c₃N₄ heterojunctions sonochemically to eradicate Rhodamine B dye under solar irradiation

Abstract

The use of natural solar radiation is a low-cost significant technology for water pollution remediation and production of clean energy. In this work, S-scheme Ag₂CO₃/g-C₃N₄ heterojunctions were engineered for carefully eradicating Rhodamine B dye under natural sunlight irradiation. Solid thermal decomposition reactions generate g-C₃N₄ sheets by annealing urea at 520 °C. Ag₂CO₃ nanoparticles are directed and localized sonochemically to the active centers of g-C₃N₄ sheets. The physicochemical properties of the solid specimen were determined by PL, DRS, XRD, HRTEM, mapping, EDX, N₂-adsorption–desorption isotherm and XPS analyses. As elucidated by HRTEM, PL and DRS analyses, 5 wt% of spherical Ag₂CO₃ nanoparticles deposited on the g-C₃N₄ sheet surface and nearly equidistant from each other elevate the electron–hole separation efficiency and broaden the absorption capacity of photocatalysts. Rhodamine B dye was degraded at a rate of 0.0141 min⁻¹ by heterojunctions containing 5 wt% Ag₂CO₃ and 95 wt% g-C₃N₄, which is three-fold higher than that on pristine g-C₃N₄ nanosheets. Free radical scrubber experiments revealed the contribution of charge carriers and reactive oxygen species to the decomposition of RhB dye with a preferential role of positive holes and superoxide species. PL measurements of terephthalic acid and scrubber trapping experiments provide confirmatory evidence for charge diffusion via the S-scheme mechanism that accounts for the production of electron–hole pairs with strong redox power. This novel research work is contributory to manipulate the S-scheme heterojunction for efficient and low-cost wastewater treatment under natural solar irradiation.

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1. Introduction

Water pollution is an environmental crisis that urgently needs to be addressed to meet the increasing demand for fresh water. Organic dyes are the major toxic pollutants discharged from different industries causing serious problems to human health and aquatic life.^1–4 Adsorption, reverse osmosis, electrochemical process, coagulation, enzymatic treatment and ion-exchange processes are very expensive, complicated techniques that produce a second generation of pollutant materials.^5–10 Photocatalysis on the surface of semiconductors is a green technology for the destruction of organic wastes into eco-friendly species. The photocatalytic reaction requires two factors to achieve optimization conditions. The first one is a low-cost and broadband solar radiation source that is sufficient to decompose millions of molecules of pollutants. The second requirement is an appropriate semiconductor with tunable band gap energy and extremely high surface area that generates huge amounts of reactive oxygen species, which can destruct toxic pollutants in a short time under light irradiation. Natural sunlight in our country is very abundant in all seasons, particularly in summer days and acts as a low-cost radiation source. The natural sunlight contains a minor amount of UV radiation (5%), visible radiation (43%) and near infra-red (NIR) radiation (50%). Most of the previous research studies have focused on investigating the photocatalytic process under UV and visible light irradiation. In recent years, NIR is very effective in decomposing organic pollutants via photocatalytic and thermal routes. An appropriate semiconductor from the perspective of thermodynamics and kinetics must have the advantages of low cost, environmentally benign nature, high separation rate and fast transfer of charge carriers. Efficient semiconductors with wide bandgap structures as TiO₂, SnO₂, ZnO and CeO₂ show high rate of organic pollutant decomposition^11–16 under UV irradiation that constitutes only 5% of the natural solar radiation. Narrow band gap energy semiconductors absorb visible light radiation; however, the ultra-fast electron–hole pair coulombic attraction decreased the life time required to generate reactive oxygen species.^17–20 In conclusion, a single semiconductor cannot achieve the optimization properties for decomposing the organic pollutants. To overcome the limitations of single photocatalysts, heterojunctions composed of wide band gap energy with high efficiency for charge carrier separation and narrow band gap energy that harvests solar radiation are more favored. Recently, graphitic carbon nitride (g-C₃N₄), has gained great interest in photocatalytic reactions due to its good stability in basic and acidic media, low synthesis costs, non-toxicity without side effects, sufficient biocompatibility, thermal stability, and a narrow band gab energy of about 2.7 eV and promote the photocatalytic activity under visible light.^21–25 Low specific surface area (∼50 m² g⁻¹) and ultra-fast attraction of electron–hole pairs inhibit the prolonged time of charge carriers in the photocatalytic process. The low valence band potential (E_VB = +1.5 eV) of g-C₃N₄ fails to generate hydroxyl radicals (·OH), which are essential species for the destruction of organic pollutants. The hybridization of wide and narrow band gap energy semiconductors is a promising issue for successfully generating heterojunctions with appropriate solar radiation absorbability and high separation and transportation efficiency of electron–hole pairs. Silver-based photocatalysts such as AgCl, Ag₃PO₄, AgIO₄, AgVO₃ and Ag₂CO₃ exhibit strong photocatalytic reactivity in the decomposition of organic pollutants.^26–31 Much attention is drawn towards Ag₂CO₃ nanoparticles due to their insolubility in water, low toxicity, adjustable bandgap structures and simple synthesis routes.^32–41 The successful construction of heterojunctions requires the appropriate adjustment of the band energy structure of the two semiconductors to generate charge carriers with an auspicious redox power.^42–44 Recent research studies have focused on coupling Ag₂CO₃ with g-C₃N₄ to generate a successful heterojunction for removing organic containments from wastewater.^45–51 Konglin et al. reported the successful photodegradation of Rhodamine B and methylene blue dyes over g-C₃N₄/Ag₂CO₃ containing 3.5 wt% g-C₃N₄, which was ascribed to the predominant role of g-C₃N₄ in limiting the electron–hole recombination.⁴⁵ Lei et al. synthesised g-C₃N₄/Ag₂CO₃ containing 25 wt% g-C₃N₄ via a sonochemical route for destructing RhB dye under a 300 W xenon lamp, and the experimental results manifested that g-C₃N₄ increases the life time for charge carriers, which is responsible for the degradation process.⁴⁶ Shugang et al. prepared Ag₂CO₃/g-C₃N₄ containing 40% Ag₂CO₃ by a co-precipitation process for removing methyl orange and methylene dyes. The remarkable reactivity of the nanocomposite was ascribed to the influence of Ag₂CO₃ in enhancing the electron–hole transportation and separation.⁴⁸ Yun Feng et al. prepared Ag₂CO₃/g-C₃N₄ containing 30% Ag₂CO₃ by a precipitation method for the removal of methyl orange and methylene blue. The exceptional reactivity is attributed to the role of Ag₂CO₃ in enhancing the efficiency of charge carrier separation. Previous research studies concentrated on doping silver carbonate as a major constituent with g-C₃N₄, which is considered a high-cost route for water treatment. In this work, we made an attempt to construct S-scheme Ag₂CO₃/g-C₃N₄ with a minimum amount of silver carbonate. The previous research explores the construction of Ag₂CO₃/g-C₃N₄ via type (II) heterojunctions and direct Z-scheme mechanism. The type (II) heterojunction fails from dynamic, thermodynamic and energetic points of view to explain the actual charge transportation. The electron transfer from high to low conduction band is accompanied by repulsion force between the existing and the transferring electrons. Concurrently, the electron diffusion from high to low energy level dissipates electrons with a strong reducing power. Direct Z-scheme fails to explain the precise analysis of charge migration between the two semiconductors. The construction of S-scheme Ag₂CO₃/g-C₃N₄ heterojunctions has not been explored in previous research studies. Hybridizing Ag₂CO₃ with high positive oxidative potential and g-C₃N₄ with high negative reductive potential generate successful S-scheme heterojunctions. The Fermi level and conduction band of g-C₃N₄ is higher than those of Ag₂CO₃; however, the work function of Ag₂CO₃ is greater than that of g-C₃N₄. Upon light illumination, the Fermi levels of Ag₂CO₃ and g-C₃N₄ jump upward and downward until the two Fermi levels are contacted at the interface region. At this contact point, the electrons and holes with low redox potential are attracted toward each other, and vanished leaving a strong internal electric field. Concurrently, the holes and electrons in the higher valence and conduction bands with a strong redox power are consumed in the photocatalytic process. The production of S-scheme systems is more favorably engineered via a solution-based approach to control and tune the particle structure and pore matrix. The sonochemical route is a professional solution process for the synthesis of Ag₂CO₃/g-C₃N₄ heterojunctions with the homogeneous location of Ag₂CO₃ on g-C₃N₄ active centers. The as-synthesized heterojunction was subjected to decomposition of Rhodamine B under solar irradiation to utilize UV, visible and near infra-red radiations. Exclusively, NIR light elevates the reaction temperature through the photothermal effects, which enhances the photocatalytic activity. In this work, we aimed to synthesise g-C₃N₄ from microcrystalline urea via a thermal decomposition process. Ag₂CO₃ nanoparticles were hybridized with g-C₃N₄ sheets in an ultrasonic bath of 300 W intensity. The physicochemical properties of the solid specimens were characterized by XRD, FTIR, HRTEM, EDX, XPS, mapping, N₂ adsorption–desorption isotherm, DRS and PL analyses. The photocatalytic activity of the heterojunctions was explored by following the degradation of Rhodamine B dye under natural sunlight radiation of 500 W intensity. The key role of the oxygen radicals and charge carriers was elaborated by carrying out various trapping scavenger experiments and following the PL analysis of terephthalic acid as a probe radical material. On the basis of DRS, PL and trapping scavenger analyses, a proposed mechanism for the transportation of charge carriers between Ag₂CO₃ and g-C₃N₄ semiconductors in the circuit of the heterojunction is illustrated.

2. Materials and methods

2.1. Material

Isopropanol, urea, sodium carbonate, methanol, silver nitrate, benzoquinone, ammonium oxalate, Rhodamine B dye and terephthalic acid were supplied by Sigma-Aldrich Company.

2.2. Preparation of g-C₃N₄ nanosheets

g-C₃N₄ sheets were synthesized from urea after purification via recrystallization by 200 mL of methanol (99%) with continuous stirring at 150 °C. The solution was allowed to stand at room temperature to get rid of volatile methanol carefully. Then, 100 g of recrystallized urea was annealed in a reactor made of aluminum metal at 520 °C for 3 hours and the obtained solid was washed with distilled water and dried at 80 °C. Finally, a yellow solid specimen was obtained and ground in a porcelain mortar to generate g-C₃N₄ nanosheets.

2.3. Preparation of Ag₂CO₃ nanoparticles

First, 6 g AgNO₃ dissolved in 100 mL distilled water was added to a solution containing 2 g Na₂CO₃ dissolved in 100 mL distilled water with continuous stirring for 1 hour to ensure homogenous distribution. The solution was sonicated for 30 min in an ultrasonic bath at 300 W intensity followed by filtration, washing with distilled water and drying at 110 °C. A dark yellow solid was obtained, ground and stored in a falcon tube.

2.4. Preparation of Ag₂CO₃/g-C₃N₄ nanocomposites

Fig. 1 illustrates the plausible scheme for the synthesis of S-scheme Ag₂CO₃/g-C₃N₄ heterojunctions via a sonochemical route. Typically, a definite amount of silver carbonate nanoparticles dispersed in 20 mL of distilled water were sonicated with g-C₃N₄ nanoparticles for 30 min by certain ratio to obtain (5, 10 and 20 w/w%) Ag₂CO₃/g-C₃N₄. After a while, each of the solution mixture was stirred for 1 h, filtered, washed with distilled water and dried at 80 °C. The photocatalysts are denoted as g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20 for pristine g-C₃N₄, pristine Ag₂CO₃ and the heterojunctions containing 5, 10 and 20 wt% Ag₂CO₃, respectively.

Fig. 1 Scheme for the synthesis of Ag₂CO₃/g-C₃N₄ nanocomposites.

2.5. Material characterization

A PANalytical X’PERT MPD diffractometer with Cu [Kα₁/Kα₂] radiation was employed to investigate the crystalline properties of the as-synthesized heterojunctions. FTIR spectroscopy revealed the functional groups of the nanocomposite samples. Adsorption isotherms of N₂ at 77 K precisely investigated the surface parameters and pore structure of the solid specimens. HRTEM (JEOL 6340) visualized the particle size distribution of Ag₂CO₃ on the sheets of g-C₃N₄. A K-ALPHA (Themo Fisher Scientific, USA) instrument with monochromatic X-ray Al Kα radiation in the range of 10–1350 eV was used for XPS analysis. JASCO spectroscopy (V-570) was performed to analyze the diffuse spectrum of the photocatalyst and determine the band energy structure. A lumina fluorescence spectrometer (Thermo Fisher Scientific) was used to analyze the efficiency of photogenerated electron–hole separation.

2.6. Photocatalytic degradation of RhB dye

Typically, 0.1 g of the photocatalyst is mixed with 100 mL of 10 mg L⁻¹ RhB dye in a solar reactor. The suspension was continuously stirred for 1 hour in the darkness until adsorption/desorption equilibrium was reached. After a while, the mixture solution was exposed to natural sunlight (450 W) at about 3 p.m. (Aug. 2021) for two hours. Then, 4 mL of the mixture was collected at definite time intervals, and then the photocatalyst was separated by centrifugation at 4000 rpm for 10 min. The photocatalytic degradation progress was followed by tracing of the reduction of dye color intensity using a UV-visible spectrophotometer. The photocatalytic mechanism and nature of reactive species were investigated using free radical scrubbers such as AgNO₃, isopropanol, ammonium oxalate and benzoquinone for detecting conduction band electrons, hydroxyl radicals, holes and superoxide radicals. The photoluminescence of terephthalic acid was investigated to monitor the production of hydroxyl radicals at an excitation wavelength of 325 nm.

3. Results and discussions

XRD spectrum explores the crystalline structure of pristine g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20 nanocomposites [Fig. 2]. Prevailing diffraction peaks of g-C₃N₄ were recorded at 13.2 and 27.59°, which were ascribed to the (100) and (002) diffraction planes (JCPDS no. 87–1526). However, the diffraction peaks recorded at 17.8°, 20.1°, 31.9°, 32.7°, 36.5°, 38.8°, 41.1°, 43.7°, 46.5° and 50.9° were ascribed to Ag₂CO₃ (JCPDS No. 23–0339) in the monoclinic crystalline structure. The Debye–Scherrer equation records that the crystalline size is 23.5, 30.7, 8.5, 7.3 and 6.5 nm for g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20 nanoparticles, respectively. The XRD pattern of the CNAg5 nanocomposite containing 5 wt% Ag₂CO₃ resembled the diffraction pattern of g-C₃N₄ with complete missing of the diffraction peaks assigned to Ag₂CO₃, revealing the homogeneous dispersion of Ag₂CO₃ between g-C₃N₄ sheets. The prevailing diffraction peaks assigned to monoclinic Ag₂CO₃ were vividly recorded in the nanocomposites containing 10 and 20 wt% Ag₂CO₃. The chemical interaction between g-C₃N₄ and Ag₂CO₃ was investigated by the FTIR spectrum. In Fig. 3a, the spectrum of g-C₃N₄ shows various peaks in the range of 1100–1700 cm⁻¹ assigned to the stretching vibration of heterocyclic C–N and C=N bonds and the peak at 808 cm⁻¹ is attributed to the triazine units, and the broad peak in the range of 3000–3500 corresponds to N–H and O–H bonds of physically adsorbed water. For the spectrum of Ag₂CO_3, four peaks were found at 720, 800, 1320, and 1430 cm⁻¹, indicating the presence of the CO₃²⁻ group. N₂-adsorption–desorption isotherms of pristine g-C₃N₄ and CNAg5 were classified as type (II) with a closed hysteresis loop, which was ascribed to the non-porous structure [Fig. 4]. The surface area of g-C₃N₄ and CNAg5 is 55.5 and 44.3 m² g⁻¹ according to the BET equation in its normal range of applicability. The existence of such open surface prohibits the photocatalytic reaction with unrestricted accommodation of pollutant molecules. However, porous systems restrict the diffusion and transportation of RhB molecules due to the pore constrictions. The HRTEM image of the CNAg5 heterojunction is represented in Fig. 5, which records the generation of g-C₃N₄ sheets with laminar structure containing various wrinkle points. On careful exploration of HRTEM images at different magnifications, one can notice the successful deposition of spherical Ag₂CO₃ nanoparticles on the active sites of g-C₃N₄ sheets at equidistant positions in the homogeneous arrangement, which reveals the strong chemical interaction between Ag₂CO₃ nanoparticles and g-C₃N₄ sheets. The particle size distribution was elucidated by constructing histograms that reveal that major nanoparticles exhibit a size varying between 15 and 20 nm. HRTEM records the existence of lattice fringes of spacing 0.315 nm, which were ascribed to the (100) plane of Ag₂CO₃. SAED analysis manifests the existence of different rings ascribed to the crystalline planes of g-C₃N₄ and Ag₂CO₃ nanoparticles. Mapping and EDX elemental analysis results are illustrated in Fig. 6, revealing the uniform distribution of C, N, Ag and O in the CNAg5 heterojunction. The production of porous graphitic carbon nitride sheets with spongy structure was clearly observed with the homogeneous distribution of Ag₂CO₃ on the localized active sites on g-C₃N₄ sheets. The oxidation state and elemental distribution of the heterojunction constituent was investigated by XPS analysis [Fig. 7]. The spectrum indicates the existence of Ag, O, C and N with a perspective binding energy, revealing the high purity of the nanocomposite without the existence of any contaminant from the preparation medium. At binding energies of 367 eV and 373 eV, Ag (3d) was detected, which deconvoluted into Ag 3d_5/2 and Ag 3d_3/2, respectively, having a spin–orbit separation of ca. 6.0 eV as detected in the previous research studies. The C 1s signal was resolved into three different peaks at 284.28 eV, 287.68 eV and 293.28 eV, corresponding to the C–C bonding states of surface carbon, sp²-bonded C (N–C=N) groups and carbonate (CO₃²⁻) groups from Ag₂CO₃. The broad peak of the N 1s spectra was resolved into four peaks at 398.6, 399.8, 400.4 and 403.9 eV, assigned to C–N–C, N–(C)₃, C–NH_x, and π excitation, respectively. Moreover, the characteristic peak of O 1s was deconvoluted into three peaks at 530.1 eV, 531.8 and 532.8 eV which were ascribed to the C–O and C=O bonds in Ag₂CO₃ and the surface of composite adsorbed –OH groups. The color of the nanocomposite became darker upon the incorporation of various contents of silver carbonate, revealing the shifting in the solid specimen's response to the visible light absorbability. Fig. 8 illustrates the DRS analysis of pristine g-C₃N₄, Ag₂CO₃ and Ag₂CO₃/g-C₃N₄ heterojunctions. The band gap energy was calculated according to the Tauc equation to determine the type of the electronic transition. The shifting in light absorbability of g-C₃N₄ from 420 to 470 nm is sufficient to enhance the photocatalytic reactivity of the solid specimen in the visible light region. The valence and conduction band potentials were calculated on the basis of band gap energy and the electronegativity of g-C₃N₄ and Ag₂CO₃. The valence and conduction band potentials of g-C₃N₄ are +1.57 and −1.13 eV. However, the valence and conduction band potentials of Ag₂CO₃ are +2.55 and +0.45 eV. The electron–hole separation efficiency for single-phase g-C₃N₄ and the as-synthesized heterojunctions was explored by constructing the PL spectrum of solid specimens [Fig. 9]. A fantastic emission signal at an intensity of 445 nm belongs to the electron–hole coulombic attraction recorded in the PL spectrum. About 40, 45, 68 and 73% reduction in the PL peak intensity was detected with the introduction of 5, 10, 15 and 20 wt% Ag₂CO₃ on the g-C₃N₄ surface. These results clearly indicated the significant role of Ag₂CO₃ in decreasing the electron–hole coulombic attraction force as well as promoting strong interactions between Ag₂CO₃ and g-C₃N₄.

Fig. 2 (a) XRD pattern of g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20.

Fig. 3 FTIR spectra of g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20 nanocomposites.

Fig. 4 N₂-adsorption–desorption isotherms of g-C₃N₄ and CNAg5.

Fig. 5 HRTEM image and SAED pattern of CNAg5.

Fig. 6 Mapping and EDX spectrum of CNAg5.

Fig. 7 XPS spectra of C, O, N and Ag in CNAg5.

Fig. 8 DRS spectra of (a) g-C₃N₄, (b) Ag₂CO₃, and (c) CNAg5 nanoparticles. Tauc plots of (d) g-C₃N₄, (e) Ag₂CO₃, and (f) CNAg5 nanoparticles.

Fig. 9 PL spectra of g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10, and CNAg20.

4. Photocatalytic degradation of Rhodamine B dye

The ubiquitous existence of dyes and antibiotics in wastewater is an alarming environmental issue that affects human health and aquatic life. Among the industrial dyes, Rhodamine B is one of the most employed dyes in industries. Rhodamine B dye (RhB) is toxic, carcinogenic, and non-biodegradable that poses a serious threat to human health. Various routes were developed for the safe destruction of RhB dye into eco-friendly species without producing a secondary generation of pollutants. Photocatalytic destruction of RhB under natural solar irradiation into CO₂, water and small molecules is an auspicious route for expelling RhB dye from wastewater. The efficiency of the as-synthesized heterojunctions in degrading RhB was tested and compared under natural sunlight irradiation [Fig. 10] to investigate the influence of change in silver carbonate concentration on the efficiency of g-C₃N₄ sheets. The decomposition of Rhodamine B dye under natural sunlight irradiation in the absence of photocatalysts is negligible due to the strong chemical stability of the dye solution. Pristine g-C₃N₄ sheets exhibit poor efficiency in degrading RhB dye due to the ultra-fast recombination rate of electron–hole pairs and mild absorption responsibility under solar irradiation. On the contrary, pristine Ag₂CO₃ shows high efficiency in degrading RhB dye, which results from the matching in band gap energy with sunlight absorbability. However, the high cost of the silver precursor hampers the industrialization of Ag₂CO₃ for wastewater treatment. The incorporation of small amounts of Ag₂CO₃ on the surface of low-cost semiconductors is a promising route for the manipulation of efficient heterojunctions with a strong redox power. Compared with pristine g-C₃N₄, the heterojunctions containing 5, 10, and 20 wt% recorded a fast rate of dye decomposition that reached 95% of the dye initial concentration on the surface of heterojunctions containing 5 wt% Ag₂CO₃ [Fig. 11a]. The decomposition of RhB dye passes through several intermediates generated by the expelling of the ethyl groups one by one. This process is trusted by observing various absorption peaks at different wavelengths arranged in descending order. The process of de-ethylation of the fully N,N,N⁻,N⁻-tetra-ethylated Rhodamine molecule generates different intermediates as N,N,N⁻-tri-ethylated Rhodamine, N,N⁻-di-ethylated Rhodamine, and N-ethylated Rhodamine at 540, 522 and 502 nm, respectively. The rate of pseudo first order of the dye degradation is 0.0012, 0.0434, 0.0141, 0.0123 and 0.0098 over g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20, respectively [Fig. 11b]. The kinetic results indicated that the rate of dye degradation over CNAg5 is fourfold higher than that on pristine g-C₃N₄. Scrubber trapping experiments using benzoquinone, isopropanol, ammonium oxalate and silver nitrate were carried out to explore the role of superoxide and hydroxyl radicals besides the positive hole and electron conduction band on the destruction of RhB dye. The experimental result recorded a preferential retardation in RhB dye mineralization in the presence of benzoquinone, isopropanol and ammonium oxalate, which directed the attention toward the positive role of reactive oxygen species and positive hole in the photocatalytic reaction [Fig. 11c]. The production of OH˙ groups under solar irradiation on the CNAg5 surface was recorded by following the intensity of PL emission signals of hydroxyl terephthalic acid at 424 nm [Fig. 11d]. The peak intensity elevates with the increase in the irradiation time, which is taken as evidence for increase in the production of OH˙ species.

Fig. 10 Absorption spectrum of photodegradation of Rhodamine B dye over g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20.

Fig. 11 (a) Variation in the removal of RhB dye over the surface of g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20 and the time of irradiation. (b) Pseudo-first order plot for the photodegradation of RhB over the surface of g-C₃N₄, Ag₂CO₃, CNAg5, CNAg10 and CNAg20. (c) Effect of various scavengers on the photocatalytic degradation of RhB dye over CNAg5 nanocomposites. (d) PL spectrum of terephthalic acid at 315 nm excitation wavelength over CNAg5 nanocomposites against the time of irradiation.

The exceptional photocatalytic reactivity of the as-synthesized samples was attributed to the construction of S-scheme Ag₂CO₃/g-C₃N₄ heterojunctions. This novel heterojunction harvests the full broad spectrum and enhances the efficiency of electron–hole pair separation and transportation under solar irradiation. The S-scheme heterojunction is composed of a Ag₂CO₃ oxidative photocatalyst and a g-C₃N₄ reductive photocatalyst. Upon light illumination, electrons are transferred from g-C₃N₄ with a greater Fermi level and more negative conduction band potential to Ag₂CO₃ with a low Fermi level [Fig. 12]. Concurrently, the Fermi levels of Ag₂CO₃ and g-C₃N₄ photocatalysts jump upward and downward until the two Fermi levels are equalized. The fruitless holes and electrons of g-C₃N₄ and Ag₂CO₃ respectively are attracted to each other by coulombic attraction force and vanished. On the contrary, the hot holes of Ag₂CO₃ and electrons of g-C₃N₄ with a higher redox potential were consumed in the photocatalytic process. This mechanism is manifested from the scrubber experiments and the emission PL analysis of hydroxyterephthalic acid. The holes of the valence band of g-C₃N₄ (E_VB = 1.5 eV) and electrons of the conduction band of Ag₂CO₃ (E_CB = +0.5 eV) are useless charge carriers with a weak redox potential and are attracted toward each other by coulombic attraction force and removed leaving a strong built-in internal electric field. On the contrary, the valence band holes of Ag₂CO₃ (E_VB = +2.5 eV) and electrons in the conduction band of g-C₃N₄ (E_CB = −1.3 eV) were consumed readily in the photocatalytic process. Ag₂CO₃ positive holes with a potential of 2.55 eV generate hydroxyl radicals (E_OH⁻/OH˙ = + 2.4 eV) and electrons in the CB of g-C₃N₄ with a potential of −1.13 eV generate superoxide radicals . The classical staggered type (II) heterojunction cannot give a true pathway for charge diffusion between oxidative Ag₂CO₃ and reductive g-C₃N₄ sheets. The electrons jumping from the conduction band of g-C₃N₄ to the conduction band of Ag₂CO₃ fail to produce superoxide radicals and the transfer of holes from the Ag₂CO₃ valence band to the valence band of g-C₃N₄ cannot oxidize water to produce hydroxyl radicals . On the basis of the aforementioned results, the S-scheme heterojunction is the actual mechanism for explaining the precise charge transportation in the Ag₂CO₃/g-C₃N₄ heterojunction. Table 1 provides the comparative study of the decomposition of various organic dyes on the surface of Ag₂CO₃/g-C₃N₄ prepared via various approaches such as hydrothermal, co-precipitation and sonochemical routes. The results recorded in the previous research studies manifest the requirement of a large proportion of Ag₂CO₃ in the nanocomposite, which is a negative economic factor due to the large cost of silver precursors. The mechanism of charge transportation is discussed through the type (II) heterojunction or direct Z-scheme, which fail together in explaining the actual charge migration on the heterojunction surface. On the contrary, our research developed a low-cost and simple sonochemical route for developing Ag₂CO₃/g-C₃N₄ with 5 wt% Ag₂CO_3, which is efficient to destruct 95% of RhB dye via S-scheme heterojunctions.

Fig. 12 Scheme for the charge transfer between g-C₃N₄ and Ag₂CO₃ under natural solar irradiation adopting (a) Step S-scheme mechanism and (b) Type (II) heterojunction.

Table 1 Comparative study

Composite	wt%	Preparation	Pollutant	Source of light	Degradation %	Time (min)	Reference
Ag2CO3/g-C3N4	5%	Sonochemical	Rhodamine B 100 mL, 10 mg L^−1	Natural sunlight	Rhodamine B 100%, 10 mg of catalyst	100	Our research
g-C3N4/Ag2CO3	3.5%	Sonochemical	Rhodamine B, methylene blue, 30 mL, 10 mg L^−1	Xe lamb 500 W	Rhodamine B 100%, methylene blue 100%, 20 mg of catalyst	60	45
g-C3N4/Ag2CO3	25%	Sonochemical	Rhodamine B, 100 mL 5 mg L^−1	Xe lamb 300 W	Nearly 100%, 100 mg of catalyst	40	46
Ag2CO3@/g-C3N4 core-shell	5%	Co-precipitation	Rhodamine B, methylene blue, methyl orange, phenol, 100 mL, 10 mg L^−1	250 W halide lamp	Rhodamine B above 95%	54	47
					Methylene blue 96%	54
					Methyl orange 96.7%	54
					Phenol 85.5%, 0.1 g of catalyst	40
Ag2CO3/g-C3N4	40%	Co-precipitation	Methyl orange, methylene blue, 50 mL 10 mg L^−1	Xe lamb 500 W	Methyl orange 93.9%	180	48
					Methylene blue 62.8%, 20 mg of catalyst	240
Ag2CO3/g-C3N4	10%	Co-precipitation	Rhodamine B, 250 mL, 5 mg/L	Natural sunlight	100%, 0.1 g of catalyst	30	49
Ag2CO3/g-C3N4	7%	Facile precipitation	Methyl orange, 50 mL, 10 mg L^−1	500 W Xe lamp	90%, dosage of catalyst 1g L^−1	10	50
Ag2CO3/g-C3N4-MN	30%	Facile precipitation	Methyl orange, rhodamine B, 90 mL, 10 mg L^−1	300 W Xe lamp	Methyl orange, above 95%, 30 mg of catalyst	30	51

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5. Conclusions

In this research work, S-scheme Ag₂CO₃/g-C₃N₄ heterojunctions have been generated by hybridizing different concentrations of spherical Ag₂CO₃ nanoparticles and g-C₃N₄ sheets sonochemically for expelling RhB dye. The experimental analysis indicates that the localized deposition of Ag₂CO₃ on g-C₃N₄ sheets, deep absorption of solar radiation, better electron hole–electron separation and transportation are the main parameters for optimizing the photocatalytic efficiency of the heterojunction composed of 5 wt% Ag₂CO₃ and 95% g-C₃N₄ sheets. The destruction of Rhodamine B dye passes through a series of intermediate organic compounds that finally decompose completely into CO₂ and H₂O. The charge transportation via the S-scheme photocatalytic mechanism was manifested from scrubber trapping experiments and PL analysis of terephthalic acid. Superoxide radicals in addition to positive holes demonstrate a predominant contribution role in degrading Rhodamine B dye under natural sunlight of 500 W intensity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research has been funded by Researchers Supporting Project number (RSP2023R78) King Saud University, Riyadh, Saudi Arabia.

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