Photo–Redox Properties of –SO₃H Functionalized Metal-Free g-C₃N₄ and Its Application in the Photooxidation of Sunset Yellow FCF and Photoreduction of Cr (VI)

Abstract

In this work, we synthesized a metal-free sulfonic functionalized graphitic carbon nitride using sulfuric acid through the wet impregnation technique. The functionalization of sulfonic groups (–SO₃H) on g-C₃N₄ will promote a high surface charge density and charge separation owing to its high electronegativity. The g-C₃N₄–SO₃H shows excellent optical/electronic and surface properties towards enhanced photo–redox reactions. The sulfonic groups also facilitate the availability of more separated charge carriers for photocatalytic oxidation and reduction reactions. The as-synthesized material has been characterized by different spectroscopic tools to confirm the presence of functionalized –SO₃H groups and optoelectronic possessions. The photocatalytic responses of g-C₃N₄–SO₃H result in 99.56% photoreduction of Cr (VI) and 99.61% photooxidation of Sunset Yellow FCF within 16 min and 20 min, respectively, of visible light irradiation. The g-C₃N₄–SO₃H catalyst exhibits a high apparent rate constant (K_app) towards the degradation of Cr (VI), and SSY, i.e., 0.783 min⁻¹ and 0.706 min⁻¹, respectively. The intense optical–electrochemical properties and potentially involved active species have been analyzed through transient photocurrent, electrochemical impedance, and scavenging studies. Consequently, the photocatalytic performances are studied under different reaction parameters, and the plausible photocatalytic mechanism is discussed based on the results.

1. Introduction

As we human beings tend to have the urge towards innovation and exploration in various fields, we are leaving behind a lot of detrimental effects on future generations via numerous hazardous outputs. We are responsible for making the world a better place to live. The major menaces to health are present in water and food products that entered through several insignificant ways and resulted in huge hazards. Nowadays, heavy metal contaminants and food additive poisoning are dominant in these environmental and health death traps. Heavy metals are widely utilized in chemical industries, mining, paint manufacturing, textile dyeing, and so on [1]. Among these, the usage of chromium in chrome electroplating [2] and leather tanning [3] leads to an extensive release of this particular heavy metal into the environment and causes severe ecological pollution.

In general, chromium exists in two different forms as trivalent [Cr(III)] and hexavalent [Cr (VI)] species; the former does not have adverse effects [4] and is immobile and less soluble and not widely absorbed by human cells. However, the hexavalent species of chromium (Cr⁶⁺) is mobile, soluble, and bioavailable under oxidizing conditions and can easily cross the cellular membranes through sulphate permeases, which disrupt the human health system. Hexavalent chromium is carcinogenic and has a deterioration effect on kidneys, the respiratory system [5], eyes, skin [6], and even DNA levels [7,8]. As per the World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA), the permissible limits of chromium are 0.05 ppm and 0.1 ppm in wastewater and soil, respectively [9,10]. The untreated discharge from industrial operations has made the presence of Cr⁶⁺ in drinking water inevitable [11]. Therefore, the chromium intake levels by human beings have surpassed the allowed limit. Chromium also accumulates in plants and enters animals and human biological systems [12,13]. Since the trivalent species is safer than the hexavalent form, this conversion is indispensable for all living organisms in the ecosystem [14,15].

Another primary concern is the hostile contaminants and additives in the food industry [16]. In look, taste, and marketing strategies, the addition of unwanted flavoring agents and coloring dyes is normalized nowadays. However, there is a presence of azo (N-N) and aromatic (>C₆) functional groups in these synthetic dyes. When the human body consumes them for a prolonged period, this can negatively affect distinct parts of the body [17,18]. According to the survey by the World Health Organization (WHO) and Food and Agricultural Organization (FAO), Sunset Yellow FCF (SSY), also known as Orange Yellow S (E110), is a petroleum-derived synthetic azo dye that has a long history as a food-coloring agent and is extensively used in various food products owing to its gratifying color, low manufacturing cost, water solubility, and stability over diverse pH ranges; consumption of this dye exceeds the acceptable daily intake (ADI). However this embryotoxic azo dye can cause diarrhoea, migraines, allergies, anxiety, immunosuppression, hyperactivity (ADHD), eczema, histopathological effects in the kidney and liver, and even cancer in humans based on the level of consumption [19,20]. It also seriously affects other animals by damaging their DNA and chromosomal levels [21,22,23,24,25] and aquatic plants by trapping sunlight and thereby hindering photosynthesis.

Consequently, it is elemental to design a competent and inexpensive approach to reduce these contaminants from effluents to a standard level. Many distinct systems such as electrocoagulation [26], nanofiltration and reverse osmosis [27], ion exchange [28], and cross-flow microfiltration [29] have actively participated in the removal of chromium from the natural ecosystem. However, these methods are often inefficient at lower concentrations and can be expensive. Some techniques that were dynamically employed to remove food colorants are Fenton’s oxidation process [30], membrane filtration, aerobic biotreatment [31], electrocoagulation [32], adsorption [33,34], and a few quantitative determination techniques such as electrochemical sensing [35,36]. Some of these methods require an additional separation system during the large-scale removal of these pollutants in discharges.

Over the past few decades, an efficacious and economical photocatalytic technique has been developed and used to remove numerous pollutants from the environment [37,38]. The utilization of various semiconducting and metal-free materials as a photocatalyst has substantially advanced lately [39,40]. Graphitic carbon nitride (g-C₃N₄) is a metal-free n-type semiconducting organic polymer, majorly explored in photocatalytic performances. Due to its astounding thermal and chemical stabilities, it is modified through metal/non-metal doping, composite preparation, and optimization of heterojunctions and is used as a photocatalyst for CO₂ reduction, solar cells, water splitting, and organic degradation, exhibiting astounding efficiency. It has a simplistic but effective synthesis technique using thermal polycondensation reactions from abundant nitrogen precursors such as urea, melamine, cyanamide, and dicyandiamide [41,42]. The position of the conduction band at −1.12 eV and the medium bandgap of 2.7 eV accounts for enhanced reduction capabilities without the presence of metal, which sustains the importance of this material [43]. In addition, it works as a promising host material for surface modifications considering its very stable tri-s-triazine allotropic form, which provides its unique stability and chemical resistance.

Though various methodologies have been used to improve the photocatalytic activity of g-C₃N₄, simple surface functionalization can make a substantial difference in its photocatalytic degradation ability [44,45]. In this paper, modest sulfonic acid functionalization of graphitic carbon nitride (g-C₃N₄–SO₃H) was carried out, and the enhancement in photocatalytic activity towards both the oxidation of SSY azo dye and reduction of hexavalent chromium was found to be phenomenal. This is due to the availability of more active sites on the surface that support fast photogenerated electron movement towards the catalyst surface and the electron-withdrawing nature of sulfate groups [46]. The functionalization of electronegative groups on the g-C₃N₄ surface with sulfonic groups facilitated electron half-reactions by regulating the electron distribution on the catalyst surface. In work by Patnaik et al., the enhanced vacant sites on the g-C₃N₄ surface by sulfate functionalization imparted extraordinary photocatalytic activity that was 35 times higher than that of pristine g-C₃N₄ [47].

The current work reports the facile synthesis of pristine g-C₃N₄ and its sulfonic-modified catalytic material for the visible light photocatalytic degradation of hexavalent chromium (photoreduction) and sunset yellow dye (photooxidation). The presence of abundant sulfonic groups on the surface of bulk g-C₃N₄ aided a successful functionalization. Additionally, the physicochemical properties of the prepared photocatalytic materials were characterized, and a feasible photocatalytic charge transfer mechanism was formulated. The involved electrochemical and photoelectrochemical (PEC) measurements were also analyzed to corroborate the proposed mechanism of photo-induced redox reactions. The photocatalytic ability of the synthesized g-C₃N₄–SO₃H material in various pH environments and its recycling capability were also investigated.

2. Results and Discussion

The chemical compositions of g-C₃N₄ and g-C₃N₄–SO₃H in their as-synthesized forms were investigated using FT-IR spectroscopy operated between 400 cm⁻¹ and 4000 cm⁻¹, and the obtained spectra are shown in Figure 1A. From Figure 1A, a sharp, significant characteristic peak appeared at 811 cm⁻¹, corresponding to the stretching vibration band of the triazine groups (C–N–C) present in the g-C₃N₄ structure. The standard vibration bands were traced between 1650 cm⁻¹ and 1200 cm⁻¹ and are attributed to the g-C₃N₄ containing C–N heterostructures in the heptazine cyclic arrangements. Different from this, distinct peaks located at 798 cm⁻¹ and 614 cm⁻¹ were ascribed to the C–N–S and –S = O groups due to their bending vibration modes related to the surface SO₃H group (dotted circle) [44]. In addition, the surface sulfonic group was affirmed by the presence of vibration bands at 1116 cm⁻¹ and 978 cm⁻¹. Further, surface-absorbed water with –OH peaks and –N–H, =N–H peaks from the unpolymerized free amino groups were observed as the broad vibration band between 3300 and 3050 cm⁻¹ [46].

The X-ray diffraction (XRD) spectra of pristine g-C₃N₄ and sulfonic acid functionalized g-C₃N₄–SO₃H were obtained and are exhibited in Figure 1B. Figure 1B displays the characteristic peak that appeared at 12.9°, corresponding to the interlayer structural packing (100), and the peak at 27.6° was attributed to the interplanar stacking (002) planes from the graphite-like structure exhibited by the g-C₃N₄ compound (JCPDS No. 87-1526). Similarly, the g-C₃N₄–SO₃H compound showed a similar diffraction pattern with a reduction in the peak intensity at 27.6° due to the increased interlayer distance resulting from surface functionalization by the sulfonic acid groups. The surface morphological analysis of the as-synthesized g-C₃N₄ and g-C₃N₄–SO₃H photocatalytic materials was scrutinized by Field-emission scanning electron microscopy (FE-SEM) analysis and is displayed in Figure 2A–F. The graphite-like stacked layers of bulk g-C₃N₄ can be observed in Figure 2A–C and the surface-functionalized g-C₃N₄–SO₃H in Figure 2D–F. The results show that both g-C₃N₄ and g-C₃N₄–SO₃H have graphitic stacked 2D sheet-like morphologies. The results also demonstrate that the sulfonic group (–SO₃H) functionalization did not affect the structural/morphological nature of pristine g-C₃N₄.

The energy dispersive X-ray (EDX) analysis with element composition and the overall elemental mapping of g-C₃N₄ and individual elements (C and N) are shown in Figure 3A–E, where Figure 3A,B corresponds to the EDX and Figure 3C–E represents the elemental mapping. Likewise, the EDX spectra and the overall elemental mapping (Figure 4A,B) and individual elements (C, N, S, and O) (Figure 4C–G) of sulfonic acid-functionalized g-C₃N₄–SO₃H are depicted in Figure 4A–G. Consequently, the obtained results confirm the formulated weight percentages of different elements and the uniform distribution of various elements (C, N, S, and O) from the functionalized sulfonic acid groups.

Photoluminescence (PL) emission occurs during the radiative recombination of charge carriers (electrons/holes); this was measured using the PL emission spectra and is shown in Figure 5A. In this work, PL spectra were obtained using a 266 nm laser as an exciting source for the photocatalytic materials and were measured in the range of 350 nm to 800 nm at room temperature. From Figure 5A, it is evident that the PL intensity of g-C₃N₄–SO₃H decreased significantly compared with g-C₃N₄ due to the reduced recombination effect exhibited by the functionalized photocatalytic material. In addition, the slight blue shift shown by g-C₃N₄–SO₃H was due to the electron-withdrawing ability of the –SO₃H present on the surface, which was confirmed by the UV-DRS. The efficiency of the catalytic materials was correlated using the full-width half maximum (FWHM) values of the PL peaks. This affirmed the extended lifetime of the photogenerated carriers (h⁺/e⁻), which facilitates better electron-hole separation and enhanced photocatalytic efficacy.

Furthermore, Raman spectra were obtained for the pure g-C₃N₄ and functionalized g-C₃N₄-SO₃H to measure the surface structural defects through molecular vibrations, as illustrated in Figure 5B. From Figure 5B, the graphitic nature and degree of graphitization of g-C₃N₄ and g-C₃N₄–SO₃H were ensured by establishing the position of the D (red line) and G (black line) bands concentrated at 1405 cm⁻¹ and 1545 cm⁻¹ [48]. The intensity ratio (I_D/I_G) of the D and G bands, indicating the surface defect values of the materials, was calculated to be 1.00 and 1.005 for g-C₃N₄ and g-C₃N₄–SO₃H, respectively. A slight increase in the intensity ratio owing to the acid treatment for the surface functionalization indicated the increased defects on the surface of the g-C₃N₄–SO₃H photocatalytic material.

The chemical environment on the surface and the elemental composition of the as-synthesized photocatalytic materials were bolstered by the X-ray photoelectron spectroscopy (XPS) analysis. The XPS survey spectra of the g-C₃N₄ and g-C₃N₄–SO₃H materials operating in the binding energy range between 0 and 1200 eV were obtained and are shown in Figure 6A–F. Figure 6A,B displays the complete spectrum of g-C₃N₄ and g-C₃N₄–SO₃H. The high-resolution deconvoluted spectra of individual elements were adjusted with respect to the adventitious carbon peak at 284.6 eV. Figure 6C shows the C_1s spectra of g-C₃N₄–SO₃H, deconvoluted to two separate peaks owing to the sp²-hybridized carbon on the surface at 284.6 eV and the C–(N)₃ carbon present in the heptazine ring structure [49].

From Figure 6D, three distinct peaks of the high-resolution N_1s spectrum were obtained for the C–N = C, N–(C)₃, and C–NH groups at 398.7, 399.6, and 400.9 eV, respectively [50]. The C_1s and N_1s spectra of g-C₃N₄–SO₃H that shifted towards higher binding energies than the unmodified g-C₃N₄ (Figure S1) demonstrated the increased shielding effect owing to the high electron density caused by the surface –SO₃H groups. In addition, the high-resolution spectra of O_1s were deconvoluted into two distinct peaks located at 531.3 and 532.1 eV, attributed to S = O and S–OH, respectively [51]. On the other hand, the XPS core-level spectra of S_2p deconvoluted into two discrete peaks located at 164.1 and 167.9 eV were assigned to the H–S and N–SO₃ groups, respectively [52] (Figure 6E,F).

The electronic bandgap values and photo-absorption characteristics of the as-synthesized g-C₃N₄ and g-C₃N₄–SO₃H photocatalytic materials were calculated and measured using the UV–visible diffuse reflectance spectroscopy (UV-DRS) method (Figure 7A,B). Figure 7A shows that the optical absorption band edges of the synthesized g-C₃N₄ and g-C₃N₄–SO₃H materials were positioned at ~471 nm and ~459 nm, respectively. This slight blue shift observed in the spectra was due to the quantum confinement effect exerted by the g-C₃N₄–SO₃H material in its layered structure [53]. Additionally, the electronic bandgap energy of the materials of interest was calculated using Tauc plots by employing the Kubelka–Munk formula.

$$\mathsf{\alpha }\mathrm{h}\mathsf{\nu }=\mathrm{A} {(\mathrm{h}\mathsf{\nu } -{ \mathrm{E}}_{\mathrm{g}})}^{\mathrm{n}}$$

(1)

where α = the absorption coefficient; h = Planck’s constant; ν = frequency; A = proportionality constant; E_g = bandgap energy, with n = ½ for direct allowed transition and n = 2 for indirect allowed transition [54]. From the obtained values of UV-DRS, the Tauc plot was drawn and is shown in Figure 7B; the bandgap energies of g-C₃N₄ and g-C₃N₄–SO₃H were 2.61 eV and 2.64 eV, respectively. The g-C₃N₄–SO₃H photocatalytic material had slightly increased bandgap energy compared with the pristine g-C₃N₄ photocatalyst.

The flat band potentials (V_FB) of the photocatalysts, g-C₃N₄ and g-C₃N₄–SO₃H, were obtained using Mott–Schottky (M-S) analysis in the presence of the supporting electrolyte (0.1 M Na₂SO₄) under dark conditions. In Figure 8A,B, the M-S plots of pure g-C₃N₄ and functionalized g-C₃N₄–SO₃H are illustrated, and from the positive slope obtained, the n-type semiconducting nature of g-C₃N₄ due to the n-type nitrogen donor atoms was affirmed. Based on the linear fitting of the obtained data, the conduction bands for the g-C₃N₄ and g-C₃N₄–SO₃H materials were measured with the intercept, and the values were −0.95 eV and −1.09 eV, respectively. Thus, the Mott–Schottky analysis confirmed that the CB of g-C₃N₄–SO₃H (−1.09 V) shifted to a more negative potential than the bulk g-C₃N₄ (−0.95 V), resulting in g-C₃N₄–SO₃H being the capable candidate for both photocatalytic oxidation and reduction reactions.

2.1. Photocatalytic Reduction of Cr (VI) and Oxidation of SSY

The photocatalytic reduction reactions were performed using the 100 ppm aqueous solution (0.1 g L⁻¹) of potassium dichromate (K₂Cr₂O₇) as the source of hexavalent chromium (Cr⁶⁺), and 40 mL of this aqueous solution (pH = 2.58) was used in the presence of 10 mg of the g-C₃N₄ and g-C₃N₄–SO₃H catalysts. The photocatalytic reaction was accomplished under visible light (≥400 nm) irradiation and also in the presence of an organic sacrificial agent, citric acid (10 mg), to trap the holes from recombination. Likewise, the photocatalytic oxidation of the food colorant Sunset Yellow FCF (SSY) was performed using the aqueous solution (50 ppm) of Sunset Yellow FCF (pH = 2.92) under visible light irradiation in the presence of 10 mg of the g-C₃N₄ and g-C₃N₄–SO₃H photocatalytic materials. The entrapment of holes with the sacrificial agent was restricted to oxidize with the h⁺ charge carriers. The photodegradation processes of Cr⁶⁺ and SSY were monitored by the UV–visible spectrophotometer at regular intervals, and the degradation percentage was also calculated using the equation given below (Equation (2)):

$$\mathrm{Photodegradation} \mathrm{percentage} (\%)=\frac{\mathrm{C}-{\mathrm{C}}_{\mathrm{o}}}{\mathrm{C}}\times 100\%$$

(2)

where C_o = initial concentration; C = concentration at specific intervals.

Figure 9A,B shows the UV–Vis profiles for the photocatalytic reduction of hexavalent chromium (100 ppm) using bulk g-C₃N₄ and g-C₃N₄–SO₃H in the presence of citric acid (scavenger). Based on a comparison of the catalytic activity of the unmodified g-C₃N₄ (19.15%) and surface-functionalized g-C₃N₄–SO₃H (99.56%) photocatalytic material, the activity of the sulfonic acid-functionalized material was more than five times that of pristine carbon nitride, and the highest degradation rate was 99.56% within 16 min of visible light irradiation. The photooxidation activity by the g-C₃N₄–SO₃H (99.61%) photocatalyst towards the SSY was almost double that of the bulk g-C₃N₄ (53.74%), as shown in Figure 9C,D. The final concentrations of Cr (VI) and SSY after the photoredox reactions over the g-C₃N₄–SO₃H catalyst were 0.36 mg/L, and 0.15 mg/L, respectively. The enhanced performance of the –SO₃H-modified g-C₃N₄ was nearly ~5.2 times higher than that of the pristine g-C₃N₄ sample; the increase in photocatalytic efficiency was mainly due to the high photogenerated electron-hole separation efficiency, higher surface electron transportability, and reduced charge recombination rates due to the high electronegativity of the surface-functionalized –SO₃H groups.

Under the same experimental conditions, the hexavalent chromium species and SSY did not degrade without the catalytic materials. Their comparative photolysis study is illustrated in Figure 10A,B. The surface-modified g-C₃N₄–SO₃H material had fivefold activity towards the photoreduction of Cr (VI) and twofold activity towards the SSY compound compared to the bulk pristine g-C₃N₄, indicating the immense enhancement in the photocatalytic activity by the –SO₃H modification, which facilitated electron absorption and increased charge separation. In the meantime, the sulfonic acid group present in the modified carbon nitride material intensified the concentration of photogenerated electrons, enhancing the photoreduction and permitting the covering of the photogenerated holes during the photooxidation process.

2.2. The Possible Mechanism for the Photoreduction of Cr (VI) and Photooxidation of SSY

Under visible light irradiation, the g-C₃N₄–SO₃H photocatalyst underwent a photoexcitation reaction, and the photogenerated charge carriers (e⁻/h⁺) were separated. The photoexcited electrons moved towards the conduction band. They were readily available for the photoreduction reaction (Cr (VI)), where the recombination of excited species was hindered by the addition of a sacrificial agent (citric acid). In contrast, when the photocatalytic reactor was added with catalytic material (SSY) under visible light irradiation, the photogenerated h⁺ species present in the valence band underwent a photooxidation reaction since the sacrificial hole trapping agent was not added. The enhanced photoactivity of the g-C₃N₄–SO₃H material was mainly influenced by the surface-functionalized sulfonic groups. Due to the strong electronegativity of the sulfonic groups, the photoexcited electrons migrated rapidly to the catalyst surface. This process can increase the surface electron density, which is beneficial for the photoreduction reactions. Furthermore, it effectively separates the photo-induced charge carriers and reduces recombination, which is also helpful for the photooxidation reactions. The photo–redox reactions over the g-C₃N₄–SO₃H surface can be ascribed as the following steps (Equations (3)–(5)):

$$\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4-{\mathrm{SO}}_3\mathrm{H}+\mathrm{hv} \to { \mathrm{e}}_{\mathrm{CB}}^{-}+{ \mathrm{h}}_{\mathrm{VB}}^{+}$$

(3)

$${\mathrm{e}}_{\mathrm{CB}}^{-}+\mathrm{Cr}\left(\mathrm{VI}\right)+{ \mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_7+{ \mathrm{h}}_{\mathrm{VB}}^{+} \to \mathrm{Cr}\left(\mathrm{III}\right)+{\mathrm{CO}}_2+{ \mathrm{H}}_2\mathrm{O}$$

(4)

$${\mathrm{h}}_{\mathrm{VB}}^{+}+\mathrm{SSY} \to \mathrm{Degradation} \mathrm{products}$$

(5)

with the compilation of the photocatalytic behaviors of the g-C₃N₄ and g-C₃N₄–SO₃H semiconductor materials obtained and mentioned above, the relationship between the ln(C/C_o) and the degradation time was formulated (Figure 11A,B) [55], following pseudo-first-order kinetics (Equation (6)) for the photodegradation of Cr (VI) and SSY in the presence of photocatalytic materials.

$$-\ln \left(\frac{\mathrm{C}}{{\mathrm{C}}_{\mathrm{o}}}\right)={\mathrm{K}}_{\mathrm{app}}\times \mathrm{t}$$

(6)

where C is the concentration of SSY and Cr (VI) at time (t); C_o is the initial concentration of Cr (VI) and SSY (t = 0); K_app is the rate constant of the redox reactions; and t is the irradiation time or reaction time. For the photoreduction reaction of hexavalent chromium (Cr (VI)), g-C₃N₄–SO₃H showed a rate constant (K_app) of 0.783 min⁻¹, which was 313.2- and 142.36-fold higher compared to that of the blank (0.0025 min⁻¹) and g-C₃N₄ (0.0055 min⁻¹), respectively. The rate constant for the photooxidation of SSY was 0.706 min⁻¹ for the g-C₃N₄–SO₃H photocatalytic material, which was 117.67 times higher than that of the blank (0.0060 min⁻¹) and 16.37 times higher than that of the g-C₃N₄ (0.0431 min⁻¹) photocatalytic material.

Interpreting the results mentioned above, the plausible photocatalytic mechanism for the photo–redox reactions for the degradation of the Cr (VI) and SSY contaminants was formulated and is displayed in Scheme 1. The bandgap energies of the g-C₃N₄ (2.61 eV) and g-C₃N₄–SO₃H (2.64 eV) semiconductor photocatalysts were derived by Tauc plots obtained from the UV–diffuse reflectance spectra and the position of the conduction band (CB) established by the Mott–Schottky measurements. The valence band (VB) position was calculated using the formula mentioned below (Equation (7)).

$${\mathrm{E}}_{\mathrm{CB}}={\mathrm{E}}_{\mathrm{VB}}-{\mathrm{E}}_{\mathrm{g}}$$

(7)

where E_VB = valence band energy, E_CB = conduction band energy, and E_g = bandgap energy. The valence band (VB) and conduction band (CB) energies of the g-C₃N₄ and g-C₃N₄–SO₃H photocatalytic materials were measured to be +1.66 eV/+1.55 eV and −0.95 eV/−1.09 eV, respectively. These obtained values were confirmed by the valence band X-ray photoelectron spectroscopic (VB-XPS) data and are shown in Figure S2. The CB potential of g-C₃N₄–SO₃H was more negative than that of unmodified g-C₃N₄, which is more beneficial for the photoreduction of Cr (VI). The surface-functionalized, high electron-withdrawing sulfonic groups dramatically increased the surface electron densities. This could be the reason for the prolonged lifetime of the photo-excited charge (e⁻/h⁺) carriers.

2.3. Influence of pH and Scavenging Species and Recycling Stability

Predominantly, the pH of the reaction environment plays a vital role in the catalyst’s absorption–desorption ability and photocatalytic redox activity. The photoreduction of Cr(VI) and photooxidation of SSY under the influence of a wide pH range were determined in the presence of modified g-C₃N₄–SO₃H material and are illustrated in Figure 12A. Figure 12A demonstrates the photodegradation activity of Cr(VI) and SSY over the pH range of 3.0–11.0, and the acquired results show that the photoreduction of Cr(VI) was most favorable in the acidic environment; a drastic drop in the degradation ability was observed when the reaction mixture adjusted to the basic nature. This is due to the reduced attraction of reacting species towards the negatively charged catalytic surface under the basic conditions, and the pH of the photoreduction of Cr (VI) was found to be around ~2.5 throughout the reactions. The photooxidation of SSY was also promising under acidic conditions, and the pH of the reaction environment was measured at nearly ~2.9 for all other reactions throughout the work.

Likewise, to determine the active reacting species involved and to reaffirm the photodegradation mechanism, an active radical trapping experiment was performed, and the results are shown in Figure 12B and Figure S3. The active photogenerated species such as the hydroxyl radical (•OH), superoxide radicals (•O²⁻), holes (h⁺), and electrons (e⁻) can be trapped using scavenging species such as isopropyl alcohol (IPA), acrylamide (AA), triethanolamine (TEOA), and silver nitrate (SN), respectively. The previous experimental result affirms that the presence of silver nitrate had the greatest influence in the photodegradation of Cr (VI) (10.3%), indicating the significance of photogenerated electrons (e⁻) in the photoreduction reaction. On the other hand, photodegradation of the SSY contaminant was hampered by the trapping of holes (h⁺) in the photooxidation reaction by TEOA (54.19%). The trend of the influence of scavenging species for the photoreduction of Cr (VI) was e⁻ > •OH > •O²⁻ > h⁺, compared with h⁺ > •O²⁻ > e⁻ > •OH for the photooxidation of SSY. These radical trapping experiments showed that the results supported the predicted photodegradation mechanism explained earlier in the discussion.

The recycling stability of the surface-functionalized g-C₃N₄–SO₃H photocatalytic material was examined in three consecutive repeated cycles. The utilized photocatalyst in the reaction was recovered by centrifugation and drying at 60 °C in between each successive step, and the acquired results are displayed in Figure 13 and Figure S4. At the end of the three successful photo–redox reactions, there was only 3.18% and 1.33% reduction in the degradation ability for the photoreduction of Cr (VI) and photooxidation of SSY, respectively. This proves the better repeating stability and high sustainability of the g-C₃N₄–SO₃H photocatalytic material for effective environmental remediation applications.

2.4. Comparative Study of the Photodegradation of Cr (VI) and SSY

Some of the previous distinctive works on the photocatalytic reduction of hexavalent chromium (Cr (VI)) and photocatalytic degradation of SSY azo dye were compared with this current work based on their different concentrations of contaminants and duration of irradiation of visible light or time taken for >92% degradation, shown in

Table 1. Comparative analysis of the photocatalytic ability of various catalytic materials across different contaminant concentration levels.

S. No.	Material/	Contaminant	Concentration	Duration	Reference
	Composite
1.	ZnFe2O4/CdS	Cr (VI)	100 ppm	120 min	[56]
2.	CPVA/MoS2-OH	Cr (VI)	25–75 ppm	105 min	[40]
3.	Nano-sized red phosphorus	Cr (VI)	20 ppm	36 min	[57]
4.	AgI/TiO2	Cr (VI)	160 µmol/	60 min	[58]
			47 ppm
5.	TiO2-P25	Cr (VI)	10 ppm	60 min	[15]
6.	TiO2-rGO	Cr (VI)	10 ppm	180 min	[59]
7.	g-C3N4-SO3H	Cr (VI)	100 ppm	16 min	This work
8.	SeO2/TiO2	SSY	120 ppm	90 min	[60]
9.	HoAG/g-C3N4	SSY	5 ppm	60 min	[41]
10.	CAC/TiO2	SSY	300 µmol/	80 min	[61]
			135.7 ppm
11.	Se NPLs	SSY	5 ppm	600 min	[62]
12.	g-C3N4-SO3H	SSY	50 ppm	20 min	This work

.

2.5. Photoelectrochemical Measurements

The interfacial charge transfer resistance (R_ct), photoexcitation, and the stability of the photogenerated charge carriers (h⁺/e⁻) of the g-C₃N₄ and g-C₃N₄–SO₃H semiconductor photocatalysts under visible light irradiation were confirmed by the photo(electro)chemical impedance spectroscopic (EIS) studies and transient-photocurrent (TPC) analysis, respectively, shown in Figure 14A,B. The charge transferability behavior and the photo-induced charge transfer resistance (R_ct) can be determined from the EIS Nyquist plots of the photoactive catalytic materials shown in Figure 14A. Based on a comparison of the R_ct values of the unmodified g-C₃N₄ and sulfonic acid group-enriched g-C₃N₄–SO₃H, the functionalized catalytic material (78.35 Ω) had significantly less charge transfer resistance than the pristine g-C₃N₄ material (92.38 Ω). This decrease in the R_ct is due to the presence of sulfonic acid groups, which account for the more electronegative nature of the material. In addition, it aids the catalytic activity by providing induction to the photo-induced e⁻ and lowering the resistance by the charge carrier density of the sulfonic acid groups in the photoelectrochemical EIS measurements. Furthermore, the equivalent circuit diagram of the EIS-Nyquist plots is provided as the inset of Figure 14A, where Z_w = Warburg impedance; R_s = electrolyte resistance; R_ct = charge transfer resistance; and C_dl = double layer capacitance.

Furthermore, the transient-photocurrent (TPC) measurements were performed in NaOH electrolyte solution with the applied potential of −0.1 V in the presence (On) and absence (Off) of visible light irradiation, displayed in Figure 14B. The obtained data showed that the photocurrent response exerted by the surface-functionalized g-C₃N₄–SO₃H (9.8824 µA) was almost ~4000 times the current response of pure g-C₃N₄ (2.4629 nA) under irradiation. Notably, the sulfur effect exercised by the functionalized photocatalytic material on charge separation and carrier density was the reason for the enhanced photocurrent response. This photocurrent response shows the boosted mobility and lifetime of the charge carriers and thus the photocatalytic activity.

3. Materials and Methods

3.1. Materials

Dicyandiamide (C₂N₄H₄), sulfuric acid (H₂SO₄), Sunset Yellow FCF (C₁₆H₁₀N₂Na₂O₇S₂), potassium dichromate (K₂Cr₂O₇), citric acid (C₆H₈O₇), isopropyl alcohol (C₃H₈O), acrylic acid (C₃H₈O₂), silver nitrate (AgNO₃), triethanolamine (C₆H₁₅NO₃), potassium ferricyanide (K₃[Fe(CN)₆]), sodium hydroxide (NaOH), potassium chloride (KCl), and potassium ferrocyanide (K₄[Fe(CN)₆]) were procured from Sigma Aldrich, Darmstadt, Germany, as analytical grade and were used without any further purification for the synthesis. Deionized water (DI) was used as a solvent in all experiments.

3.2. Characterizations

The photoluminescence (PL) studies were performed using a UniRAM Micro-PL spectrometer acquired from UniNanoTech Co., Ltd., Gyeonggi-do, South Korea, that has a 266 nm exciting laser as a source at room temperature. The electronic bandgap energies from the Tauc plots were assessed using a UV–Vis spectrophotometer (Cary 5000 series, Agilent Technologies Inc., CA, USA), and optical absorbance spectra were measured using a UV–Vis-NIR spectrometer coupled with a 150 mm UV–Vis–NIR integrating Sphere, JASCO V-770, Tokyo, Japan. The chemical composition and oxidation states and valence band (VB) spectra of the synthesized materials were scrutinized using a Fourier-transform infrared spectrometer (FT-IR) from PerkinElmer frontiers, Spotlight 200i, Buckinghamshire, UK, and X-ray photoelectron spectrometer (XPS) made by JPS 9030, JEOL Ltd., Tokyo, Japan. The structural and surface morphological characterizations of the synthesized materials were examined utilizing Raman spectroscopy using a Confocal Micro-Raman Spectrum ACRON, UniNano Tech Ltd., Gyeonggi-do, South Korea, and field-emission scanning electron microscopy with an energy-dispersive X-ray spectroscope (FESEM and EDX) made by JSM 7610F, JEOL Ltd., Tokyo, Japan. The crystalline configuration of the prepared materials was examined using X-ray diffraction spectroscopy (XRD) with CuKα radiation (λ = 1.5406 Å; Malvern PANalytical X’pert Pro, EA Almelo, The Netherlands). A 350 W mercury-xenon lamp XE350 made by Prosper Electronics, Yilan, Taiwan, was employed for all photocatalytic degradation applications, which simulated the solar light source with 388 mW cm⁻² photonic flux. All of the electrochemical and photoelectrochemical (PEC) experiments were done using an electrochemical workstation (CHI1205B) with a standard three-electrode system made by CH Instruments Inc., USA. A Pt wire, material-coated ITO plate, and Ag/AgCl (sat. KCl) functioned as the counter, working, and reference electrodes, respectively.

3.3. Synthesis of Pristine g-C₃N₄

The bulk graphitic carbon nitride (g-C₃N₄) was synthesized using dicyandiamide as a precursor material by a thermal polycondensation reaction. In short, 5 g of finely ground dicyandiamide was placed in a silica crucible, covered with the lid, placed in a muffle furnace, and heated to 550 °C for 3 h with a heating ramp of 3 °C/min. The obtained pale-yellow g-C₃N₄ was ground into a fine powder using an agate mortar and stored for further reactions.

3.4. Synthesis of Sulfonic Group-Functionalized g-C₃N₄

The surface functionalization of pristine g-C₃N₄ was achieved by the complete solvent evaporation and impregnation method using sulfuric acid. In this process, 500 mg of as-synthesized g-C₃N₄ was placed in a beaker containing 50 mL of DI water. This mixture was sonicated for 1 hr to obtain a completely dispersed suspension, and 0.25 M of H₂SO₄ was added to the mixture under stirring conditions at 60 °C. This condition was maintained until all of the solvent evaporated, and the obtained powder was washed with DI water and ethanol and dried for 12 h at 80 °C. Consequently, the obtained sulfonic group-functionalized g-C₃N₄–SO₃H was heated to bond at 200 °C for 2 h using a muffle furnace.

3.5. Photocatalytic Redox Degradation, Electrochemical, and PEC Measurements

In this study, the degradation of potassium dichromate (Cr⁶⁺) and Sunset Yellow FCF was accomplished using photocatalytic reactions in the presence of as-synthesized g-C₃N₄ and g-C₃N₄–SO₃H. For the photocatalytic reduction of hexavalent chromium into trivalent species, 40 mL DI water containing an initial concentration of 100 ppm K₂Cr₂O₇ solution and 10 mg of citric acid equivalent to 10 mg of g-C₃N₄ and g-C₃N₄–SO₃H was sonicated in a bath for 2 min for complete dispersion of the materials. The photocatalytic reactor was made of Pyrex glass and was fitted with a water circulatory system to maintain the room temperature conditions (24 ± 2 °C) and was placed 40 cm from the light source on the top of the reactor. In another part, 50 ppm of SSY in 40 mL of DI water was prepared in the reactor container with 10 mg of as-synthesized catalytic materials and kept under the conditions mentioned above for the photocatalytic oxidation reactions. The reaction mixture was stirred under dark conditions for 15 min before the irradiation of the light source to obtain catalyst–analyte adsorption equilibria. To evaluate the photodegradation of the analyte of interest using a UV spectrophotometer, 3 mL of aliquots was taken from the reaction mixture at designated time intervals. For the brief electrochemical impedance spectroscopy (EIS) and photocurrent evaluations, 5 mM [Fe (CN)₆]^3-/4- mixture dispersed in 100 mM KCl and 0.1 M NaOH were used as electrolyte solutions. The overall synthesis of the catalytic materials and their photocatalytic activity under the solar spectrum is pictorially represented in Scheme 2.

4. Conclusions

In summary, the g-C₃N₄–SO₃H was synthesized through thermal polycondensation and wet impregnation techniques. The infrared and XPS results confirm the functionalized/impregnated sulfonic groups on the g-C₃N₄ surface. The TPC and PL results also ensured the respective high photostability and reduced recombination process of the photoexcited charge carriers (h⁺/e⁻). The Mott–Schottky analysis confirmed that the CB of g-C₃N₄–SO₃H (−1.09 V) shifted to a more negative potential than the bulk g-C₃N₄ (−0.95 V), which allowed the g-C₃N₄–SO₃H to be the capable candidate for both oxidation and reduction reactions. The metal-free g-C₃N₄–SO₃H photocatalyst exhibited excellent photo–redox activities towards Cr (VI) and SSY under visible light illumination. The photoreduction of Cr (VI) in the presence of g-C₃N₄–SO₃H (99.56%) was 5.19 times higher than that of the bulk g-C₃N₄; on the other hand, the photooxidation of SSY in the presence of g-C₃N₄–SO₃H (99.61%) was 1.85 times higher than that of the bulk g-C₃N₄. The photoredox activities of the g-C₃N₄–SO₃H material under different pH values and in the presence of different scavenging species were also investigated; the results showed that higher degradation efficiencies occurred at low pH (pH = 3), and e^– and h⁺ were the major reactive species for the respective Cr (VI) photoreduction and SSY photooxidation reactions. The considerable photoactivity and recycling stability of metal-free g-C₃N₄–SO₃H shows that this is a robust photocatalyst for prospective energy applications.