Fabrication of a Z-Scheme g-C₃N₄/Fe-TiO₂ Photocatalytic Composite with Enhanced Photocatalytic Activity under Visible Light Irradiation

Abstract

In the present study, a nanocomposite material g-C₃N₄/Fe-TiO₂ has been prepared successfully by a simple one-step hydrothermal process and its structural properties were thoroughly studied by various characterization techniques, such as X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, electron paramagnetic resonance (EPR) spectrum, X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectrometry (UV-vis DRS). The performance of the fabricated composite material towards the removal of phenol from aqueous phase was systematically evaluated by a photocatalytic approach and found to be highly dependent on the content of Fe³⁺. The optimum concentration of Fe³⁺ doping that showed a dramatic enhancement in the photocatalytic activity of the composite under visible light irradiation was observed to be 0.05% by weight. The separation mechanism of photogenerated electrons and holes of the g-C₃N₄/Fe-TiO₂ photocatalysts was established by a photoluminescence technique in which the reactive species generated during the photocatalytic treatment process was quantified. The enhanced photocatalytic performance observed for g-C₃N₄-Fe/TiO₂ was ascribed to a cumulative impact of both g-C₃N₄ and Fe that extended its spectrum-absorptive nature into the visible region. The heterojunction formation in the fabricated photocatalysts not only facilitated the separation of the photogenerated charge carriers but also retained its strong oxidation and reduction ability.

1. Introduction

For the past several decades, many semiconducting materials have been employed as photocatalysts and their photocatalytic performance was proved to be appropriate for organic pollutant degradation, hydrogen production from water splitting, and the reduction of CO₂ into fuels [1,2,3]. Among the studied materials, titanium dioxide (TiO₂) has been widely investigated owing to its excellent photocatalytic performance, viability, nontoxic nature, and good chemical stability. However, TiO₂ badly suffers from its wide band gap (3.0–3.2 eV) and low quantum efficiency, which limits its efficiency in practical applications. The conventional drawback of TiO₂ as a photocatalyst is that it can be activated only in the ultraviolet light region. Hence, work on extending its absorptive behavior into the visible range and reducing the photoexcited electron–hole pair recombination rate has been carried out by several strategies, such as the doping of metal (Fe, Cu, V) [4,5,6] and nonmetal (N, S) [7,8] elements into the lattice of TiO₂, the deposition of a noble metal (Pt, Au) [9,10] on its surface as a cocatalyst, and coupling it with an another semiconductor to form a heterojunction structure that narrows the band gap of TiO₂.

The doping of transition metal into a semiconductor (TiO₂) is one of the effective approaches to extend its absorptive behavior into the visible range besides improving the quantum efficiency. In particular, Fe has been considered to be a suitable candidate as the radius of both Fe³⁺ and Ti⁴⁺ (Fe³⁺: 0.69 Å; Ti⁴⁺: 0.745 Å) is almost the same, so that the incorporation of Fe into the crystal lattice of TiO₂ becomes easier [11]. In addition, as the energy level of Fe²⁺/Fe³⁺ is much closer to that of Ti³⁺/Ti⁴⁺, Fe³⁺ could provide a shallow trap for photo-generated charge carriers that favors charge separation, which in turn improves the quantum yield efficiency [11].

In an another approach to enhance the efficiency of charge separation in TiO₂, a heterojunction structure consisting of two different semiconductors has been demonstrated [12]. Once the heterojunction has formed between TiO₂ and the coupled semiconductor material of a suitable band gap, the photoexcited electron of the lower conduction band (CB) potential of TiO₂ will be promoted to the CB potential of the coupled semiconductor material, and similarly, the photoexcited hole of the higher valence band (VB) potential of TiO₂ will be transferred to the VB of the coupled semiconductor material [13]. The oxidation and reduction abilities of the composite comes from those of the transferred respective photoexcited carriers, which are weaker than those of the original counterparts. As a result, though the charge separation efficiency of the composite is improved by the heterojunction, the oxidation and reduction abilities of the composite are decreased considerably [14,15,16]. Nevertheless, a coupling of two different semiconductors could lead to a formation of a typical Z-scheme system, in which the photoexcited electrons from the semiconductor with a less negative CB will transfer to the VB of the coupled semiconductor and combine with the photoexcited holes over there [17,18]. A composite following the Z-scheme system exhibits a higher redox capability than either of the components alone, thereby enhancing the charge separation efficiency and increasing the lifetime of charge carries as well. Owing to the above-mentioned advantages, the work on developing TiO₂-based Z-scheme photocatalysts has emerged as an important research area in the recent past.

Ever since the debut work carried out on graphite carbon nitride (g-C₃N₄) in 2009 [19], the metal-free semiconductor has attracted the attention of scientists working in the photocatalytic domain due to its narrow band gap (2.7 eV), extreme negative CB position (−1.12 eV versus Normal Hydrogen Electrode (NHE)), structural flexibility, and good chemical stability. Although the activation of pristine g-C₃N₄ can be achieved in the visible light region up to 460 nm, its photocatalytic efficiency is limited due to the high recombination probability of photoexcited electron–hole pairs [20]. It is expected that coupling g-C₃N₄ with TiO₂ would form a Z-scheme photocatalytic system and solve the problems normally encountered when using each of the semiconducting materials individually. In order to further improve the photocatalytic performance of the g-C₃N₄/TiO₂ composite under visible-light irradiation, attempts on developing composites, such as g-C₃N₄-Ti³⁺/TiO₂ and S-, N-, or Fe³⁺-doped TiO₂/g-C₃N₄, have been made [21,22,23,24]. However, very few works have been done on the fabrication of g-C₃N₄ and Fe-doped TiO₂ nanocomposite structures. Phenol is one of the most common organic water pollutants, because it is toxic even at low concentrations, and also its presence in natural waters can lead further to the formation of substituted compounds during disinfection and oxidation processes. Additionally, phenol is a model non-dye pollutant and a typical refractory aromatic compound considered to be a good probe molecule in testing photocatalytic activity for environmental purposes. The photocatalytic abatement of phenol vapors on anatase TiO₂ and g-C₃N₄-Ti³⁺/TiO₂ nanotubes has been the object of a study [25,26], and the mineralization process is complete in about 3–4 h and 7–8 h, respectively.

The present work focused on preparing a photocatalytic nanocomposite g-C₃N₄/Fe-TiO₂ by a simple one-step hydrothermal process followed by a complete characterization using instrumental techniques such as X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Electron Paramagnetic Resonance (ESR), and UV-vis Diffused Reflectance spectrophotometry (UV-vis DRS). The photocatalytic activity of the as-prepared g-C₃N₄/Fe-TiO₂ composites was investigated under the visible light region by preparative degradation experiments using phenol as model pollutant. To further confirm the enhanced activity of the as-prepared g-C₃N₄/Fe-TiO₂ composites, a comparison experiment was carried out with an Fe-TiO₂ particle photocatalyst.

2. Results and Discussion

2.1. Phase Structures and Morphology

Figure 1 shows the XRD patterns of the TiO₂, Fe-TiO₂, g-C₃N₄/TiO₂ (CT), 0.05Fe-CT, and g-C₃N₄ samples. It can be seen that all of the TiO₂-based samples exhibit identical diffraction patterns. The 2θ peaks observed at 25.3°, 37.8°, 48.0°, 54.0°, and 62.4° were well-matched with the standard data and correspond to the (101), (004), (200), (204), and (211) crystal planes of anatase TiO₂, respectively [27]. The two prominent diffraction peaks observed at 13.6° and 27.7° for pure g-C₃N₄ could be attributed to the diffraction patterns of the (100) and (002) crystal planes, respectively [28]. No peak corresponding to the characteristics of g-C₃N₄ was observed in either the CT or 0.05Fe-CT samples, and this might be due to the relatively poor crystallization and less content of g-C₃N₄ within the composites [29]. In addition, there is no obvious change in the peaks of anatase TiO₂ in both the composites, which indicates that neither the coupling of g-C₃N₄ nor the Fe-doping affects the phase structure of TiO₂.

The images of SEM and TEM taken for the synthesized samples are displayed in Figure 2 and Figure 3, respectively. The pure g-C₃N₄ sample exhibits a layered structure with a smooth surface that can be clearly seen in Figure 2a. This layered structure is expected to provide more sites for the growth of Fe-TiO₂ nanoparticles. Figure 2b reveals that Fe-TiO₂ materials consist of the aggregation of small nanocrystals. From the result shown in Figure 2c, it is found that the layered structure remains intact upon the incorporation of Fe-TiO₂ nanoparticles; moreover, the surface of g-C₃N₄ becomes slightly rough due to the formation of Fe-TiO₂ nanoparticles, suggesting that at least two semiconductors are in absolute physical contact with each other, which is the premise for the probable formation of either heterojunction or Z-scheme composites. Additionally, Energy Dispersive X-ray Spectroscopy (EDX) mapping of the composite shown in Figure 2d–h confirms the presence of Ti, O, C, N, and Fe elements in the 0.05Fe-CT sample. These results along with those of XRD and SEM suggest that Fe-TiO₂ nanoparticles are successfully loaded on the surface of g-C₃N₄.

From the high resolution transmission electron microscopy (HR-TEM) image of the 0.05Fe-CT sample as shown in Figure 3a,b, a distribution of TiO₂ nanoparticles with the size of ~5 nm on the surface of g-C₃N₄ was confirmed. The lattice spacing of TiO₂ nanoparticles was found to be 0.351 nm, which matches with the (101) plane. The corresponding EDX pattern shows the existence of C, N, Ti, O, and Fe elements, which was in accordance with the results of the EDX mapping discussed earlier.

As seen in Figure 4a, the Fourier Transform Infrared (FTIR) spectrum for the g-C₃N₄, TiO₂, CT, and 0.05Fe-CT samples shows strong bands in the region of 450–4000 cm⁻¹. For g-C₃N₄, the bands observed around 1100–1650 cm⁻¹ could be assigned to C-N and C=N stretching vibrations; the band at 810 cm⁻¹ corresponds to s-triazinering vibrations; and the band around 3000–3300 cm⁻¹ is correlated to N-H stretching vibration modes [30,31,32,33]. For the TiO₂ sample, the band observed around 500–700 cm⁻¹ could be accounted for with Ti-O stretching and Ti-O-Ti bridging stretching modes; and the bands at 1630 and 3400 cm⁻¹ are correlated to the H-O-H bending stretch of surface-adsorbed water and its hydroxyl groups, respectively [34]. The characteristic bands observed for g-C₃N₄ and TiO₂ appeared in both the CT and 0.05Fe-CT composite samples too.

As seen in Figure 4b, the optical property of the 0.05Fe-CT, CT, pure TiO₂, and g-C₃N₄ samples was measured by UV-vis diffuse reflectance spectroscopy. The UV-vis DRS spectra of 0.05Fe-CT and CT are quite similar to that of pure TiO₂, except for a slight movement of their main absorption edges toward the visible light region. In addition, upon the incorporation of g-C₃N₄, i.e., for the CT sample, a red shift moving up to 443 nm was observed, which indicates a reduced bandgap absorption edge of 2.80 eV estimated from 1240/λ (λ describes wavelength) [35]. Further, upon the doping of Fe to the CT sample, the light absorption of 0.05Fe-CT was extended to a still longer wavelength region and the appearance of the highest red shift, to a maximum of 461 nm (2.69 eV), was observed. These observations clearly indicate that the incorporation of composite material and the doping of Fe could graft a photocatalyst with the ability to utilize visible light effectively.

EPR as a highly sensitive spectroscopic technique for examining paramagnetic species can give valuable information about the lattice site wherein a paramagnetic doping ion is located. This technique can detect Fe ions to an extent of even less than 0.01 wt % in metal-oxide matrices [36]. The EPR spectra of TiO₂, CT, 0.01Fe-CT, 0.03Fe-CT, 0.05Fe-CT, and 0.06Fe-CT are depicted in Figure 5. At high magnetic field, a symmetrical EPR signal is observed at g = 2.004 for both TiO₂ and CT as well, which is an identification of the trapping of electrons on oxygen vacancies [37]. In addition, the EPR signal of CT is in accordance with that of TiO₂, which strongly indicates that the presence of CNs has no influence on the phase structure of the TiO₂. For the xFe-CT samples, unsymmetrical signals are observed at g = 1.99, which can be assigned to the fact that the Fe³⁺ is substituted for Ti⁴⁺ in the octahedral surroundings/atmosphere [37,38], otherwise it could simply be an overlapping of the two kinds of EPR signals. Further, as no signal at other g values was observed, the existence of Fe ions as Fe₂O₃-type clusters (g = 2.16) could not be possible [39,40]. It is worth noting that the intensity of signals at g = 1.99 of xFe-CT samples as shown in Figure 5a increases with increasing Fe³⁺ content, which indicates that the substitution of Fe³⁺ for Ti⁴⁺ in the TiO₂ lattice was effectively accomplished by a hydrothermal approach of simply increasing the iron content in the solution mixture. As seen in Figure 5b, the EPR spectra at low magnetic field exhibited very weak signals of g value at 4.29, which suggests that Fe³⁺ was located in a strongly distorted rhombic environment [40]. It is clear that the specific signals of EPR spectra at both high and low magnetic field confirmed the successful incorporation of Fe³⁺ into the crystal lattice of TiO₂ by a one-step hydrothermal method.

In order to examine the chemical states of elements involved in the as-prepared samples, XPS measurements were performed. The comparison of the Ti 2p spectra for samples TiO₂, Fe-TiO₂, and 0.05Fe-CT is shown in Figure 6a. The Ti 2p_3/2 and Ti 2p_1/2 peak positions of the TiO₂ sample were 458.55 eV and 464.25 eV, whereas they shifted to a higher binding energy of 458.65 eV and 467.35 eV for the Fe-TiO₂ and 0.05Fe-CT samples, respectively. The small shifts of binding energy might be due to the effect of the Fe³⁺ in the interstitial and/or substitutional site in the TiO₂ crystal lattice and formed the Ti-O-Fe bonds in the crystal lattice [41,42]. Due to the low doping level, the signals of Fe were too weak to be observed (not shown). As for the O 1s spectra presented in Figure 6b, two peaks of the binding energy at 529.95 and 532 eV for the 0.05Fe-CT sample were associated with the O₂⁻ in TiO₂ and the -OH terminal on the surface [42]. For the 0.05Fe-CT sample, the formation of the new Ti-O-Fe bonds in the crystal lattice might change the electron densities of Ti⁴⁺ cations and O₂⁻ anions, which caused a slightly higher shift of O 1s peaks compared to those for TiO₂ at 529.75 and 531.65 eV, respectively, and which might be a cause for the enhanced photocatalytic activity [42,43].

From the XPS spectra of C 1s in Figure 6c, three peaks centered at 285, 286.2, and 288.9 eV can be observed in all three samples. The main C 1s corresponds to adventitious carbon species presenting a band located at 284.4 eV [44]. The small shoulder at 286.4 and 288.9 eV could be accounted for with the C-N-C and C-(N)₃ groups of g-C₃N₄, respectively [26,45,46]. In addition, the regional spectrum of N 1s for 0.05Fe-CT as seen in Figure 6d could be fitted into two peaks at 399.2 and 400.4 eV, with the former ascribed to C-N=C [34] and the latter to the N-(C)₃ of the g-C₃N₄ [26]. No peak concerning chemical interaction between Ti and C (Ti-C) or N (Ti-N) is seen for the 0.05Fe-CT sample in the XPS (Figure 6c,d) spectra. Taking into account of the results of FTIR, SEM, and DRS studies together, the deposition of g-C₃N₄ could only be on the surface of the Fe-TiO₂, and there was a chemical reaction between them as no apparent characteristic Ti-C(N) coordination peaks were seen.

2.2. Photocatalytic Activity Test

The photocatalytic activity of the prepared samples was evaluated in terms of photodegradation of phenol with a concentration of 10 mg dm⁻³ under visible light irradiation. Phenol was chosen as a model pollutant because it is the basic molecule of phenolic compounds, which are known to be highly toxic, persistent, and biorecalcitrant, widely used in preservative, herbicide, and pesticide products, and being considered to be a grave threat to the health of humankind [47]. The study was carried out under similar experimental conditions using the respective photocatalytic materials.

The change of concentration of phenol during the photodegradation process under visible light irradiation is shown in Figure 7a. The decomposition of phenol was achieved to an extent of 38.6% and 72% at 80 min for pure g-C₃N₄ and TiO₂, respectively. The effective photodegradation of phenol observed (79.2%) in the case of g-C₃N₄ (CT) could account for the impact of the hybrid structure on enhancing the photocatalytic activity of the material. Upon the incorporation of Fe³⁺ into CT, i.e., xFe-CT, the observed photocatalytic activity was higher compared to that of TiO₂ and CT, separately. Furthermore, the photocatalytic activity of the xFe-CT increased as the content of Fe³⁺ increased initially from 0.01% to 0.05%, and thereupon there was a decline trend up to 0.06%. Additionally, the 0.05Fe-CT sample showed a highest photocatalytic activity of complete degradation of phenol at 80 min under visible light irradiation. These findings confirmed that the enhanced photocatalytic activity of the composite materials cause a synergetic effect of both g-C₃N₄ and Fe³⁺.

The degradation of phenol follows a pseudo-first-order reaction [28,48] of $ln(\frac{c_t}{c_0})=-kt$, where C₀ is the initial concentration of phenol, C_t is the concentration of phenol at time t, and k is kinetic constant. The kinetic constants (k) of all samples were calculated and are given in Figure 7b. It is 0.019 min⁻¹ for CT composites higher than that of either pure g-C₃N₄ (the k value was 0.0044 min⁻¹) or TiO₂ (the k value was 0.016 min⁻¹). When Fe³⁺ ions were introduced, an enhanced photocatalytic activity was observed from all of the g-C₃N₄/Fe-TiO₂ composites and the rate constants were 0.016 min⁻¹, 0.034 min⁻¹, 0.035 min⁻¹, 0.04 min⁻¹, and 0.029 min⁻¹ for the samples of 0.01Fe-CT–0.06Fe-CT, respectively. It was found that the incorporation of Fe³⁺ enhanced the photocatalytic activity of the obtained heterojunctions, and the highest performance was observed in 0.05Fe-CT.

The stability of 0.05Fe-CT was examined by catalyst recycling experiments under similar operating conditions for the photodegradation of phenol. After each cycle, the catalyst was separated by centrifugation, washed with ethanol and millipore deionized water, then dried and reused for a fresh run of photodegradation of phenol with a concentration of 10 mg dm⁻³. As seen in Figure 7c, no obvious decline in the degradation efficiency was observed after five cycles, suggesting that the combination of g-C₃N₄ and Fe-TiO₂ has high-level photocatalytic stability for phenolic compound degradation.

2.3. Photocatalytic Mechanism

In order to elucidate the photocatalytic mechanism of Fe-CT composites, the main active species generated over g-C₃N₄, CT, 0.05Fe-CT, and TiO₂ were quantified by adding a suitable scavenger during the photocatalytic degradation of phenol. The reactive species corresponding to both g-C₃N₄ and Fe-TiO₂ were nullified as references, so that the reactive species of 0.05Fe-CT alone could be figured out. The scavengers used in this study were sodium oxalate (OA, 0.5 mmol dm⁻³), p-benzoquinone (BQ, 0.5 mmol dm⁻³), and isopropanol (IPA, 1 mmol dm⁻³) against photogenerated holes (h⁺) [49], superoxide anion radicals (•O₂⁻) [18], and hydroxyl radicals (•OH) [49], respectively. It needs to be mentioned that the applied concentration of each scavenger did not cause any removal of phenol in the respective control experiment [23].

As shown in Figure 8, for pure g-C₃N₄, the addition of OA caused a slight decrease of the photocatalytic efficiency from 38.6% to 37.1% at 80 min, which indicates that h⁺ was not the major reactive specie; the introduction of IPA caused a decrease to 32.5%, which indicates that •OH made a considerable contribution towards the photocatalytic degradation. When BQ was added into the reaction solution, the degradation efficiencies of phenol showed a significant fall to 10.1%. From these results, it is very clear that •O₂⁻ is the major reactive specie in the photocatalytic reaction of pure g-C₃N₄. The order of influence was •O₂⁻ > •OH > h⁺.

For the TiO₂, the efficiency of phenol degradation was shown to be 72.1% when no scavenger was added. With the addition of OA and IPA into the reaction solution of a separate run, the photocatalytic efficiency of phenol degradation decreased to 23.8% and 52.9%, respectively. In the presence of BQ, the degradation rate of phenol was slightly decreased to 71%. Obviously, the major reactive species for pure TiO₂ are h⁺ and •OH.

As seen in Figure 8, for the g-C₃N₄/TiO₂ photocatalytic system, the degradation efficiency of phenol was inhibited in the order BQ > OA > IPA when these three scavengers were added in the separate run, which indicates that •O₂⁻, •OH, and h⁺ were all of the active species generated in the g-C₃N₄/TiO₂ photocatalytic system.

It is clear in Figure 8 that the photocatalytic efficiency of phenol for the 0.05Fe-CT photocatalyst was 100% at 80 min without any scavengers. With the addition of scavenger IPA, BQ, and OA in the separate run, the photocatalytic efficiencies of phenol decreased to 90.1%, 58.5%, and 17.6%, respectively. The inference is that both g-C₃N₄/TiO₂ and Fe-CT showed an identical trend in the presence of scavengers, the major reactive species were •O₂⁻ as well as h⁺ in the photocatalytic reaction of g-C₃N₄/Fe-TiO₂, and the order of influence was •O₂⁻ > h⁺ >•OH.

To further determine the photocatalytic mechanism of Fe-CT composites, a quantitative estimation of •OH was carried out by the photoluminescence (PL) method using terephthalic acid (TA) as a probe molecule during the photocatalysis process. The PL signals of g-C₃N₄, TiO₂, Fe-TiO₂, CT, and 0.05Fe-CT samples recorded at 80 min of the photocatalysis process are shown in Figure 9a. It could be easily understood that no •OH was generated during the photocatalysis process using g-C₃N₄ as there was no corresponding PL signal. The absence of •OH radicals in the photocatalysis process of g-C₃N₄ could be well-explained by taking into account the position of the VB edges of g-C₃N₄ and the actual potential of the OH⁻/•OH couple (+1.83 V/+2.7 V) (versus NHE) formation. Thus, the photogenerated holes on the surface of g-C₃N₄ were not strong enough to oxidize the OH⁻ or H₂O into •OH [26,50]. However, the formation of •OH was observed in the TiO₂, Fe-TiO₂, CT, and 0.05Fe-CT samples, among which 0.05Fe-CT showed the greatest quantity of •OH generation, which confirms the Z-scheme of transferring photoexcited charge carriers between g-C₃N₄ and Fe-TiO₂. Otherwise, if 0.05Fe-CT worked only under the general heterojunction system, the oxidation ability of the composite would have been the same as that of g-C₃N₄, wherein the production of •OH is not possible.

As seen in Figure 9b, the linear potential part of the Mott–Schottky plot based on impedance measurements was used to determine the flat-band positions of the samples [51]. The positive slope of the straight lines indicates that both TiO₂ and Fe-TiO₂ are n-type semiconductors, i.e., the flat-band potential [52] of the samples approximately equates to the lowest potential of the CB. Thus, the CB level of TiO₂ and Fe-TiO₂ are measured to be ca +0.05 V (versus NHE) and −0.01 V, respectively. The negative shift of flat-band potentials (E_fb) after Fe doping suggests a similar shift of the Fermi level, which facilitates the charge separation at the semiconductor/electrolyte interface [53].

These accumulated electrons in the CB of TiO₂ (Figure 10a) could not effectively reduce the O₂ to yield •O₂⁻ due to its CB being less negative than that of •O₂⁻/O₂ potential (−0.28 V versus NHE) [54]. The VB of TiO₂ is more positive than that of H₂O/•OH potential (+2.7 V versus NHE) and capable enough to oxidize H₂O to form •OH [34]. Our experiment mentioned above has also confirmed that h⁺ but not the •O₂⁻ generated was the major reactive specie in the photocatalytic degradation of the phenol molecule for pure TiO₂; on the other hand, the VB levels (ca. +1.58 V) of g-C₃N₄ are not positive enough to drive the oxidation of H₂O to form •OH, but its CB level (ca. −1.12 V) is negative enough to reduce O₂ to produce •O₂⁻ [18,49]. Also, it has been observed that the •O₂⁻ was a major reactive specie in the photocatalytic reaction for pure g-C₃N₄. For the composite of CT, the formation of •OH and the significant contribution of both h⁺ and •O₂⁻ in the photocatalytic reaction showed that the composite followed the Z-scheme. If the charge carriers of the CT were transferred as per the so-called usual model, the electrons in the CB of g-C₃N₄ would have migrated to the CB of TiO₂ and accumulated over there, which could possibly not reduce the O₂ to yield •O₂⁻; holes in the VB of TiO₂ would migrate to the VB of g-C₃N₄, which could not oxidize ⁻OH/H₂O to give •OH.

For the Fe-doped TiO₂ (Fe-TiO₂), a prominent decrease in the band gap and a red shift of the threshold absorption were observed in UV-vis DRS analysis. In addition, the extent of doping of Fe³⁺, which actually existed in the form of O^••_v in the band gap of TiO₂, could enhance the photocatalytic activity of the material in the visible region [55,56]. As a result, the Fe-TiO₂ showed higher photocatalytic activity and a greater quantity of •OH generation than those of TiO₂.

Based on the above results, the Z-scheme mechanism of the g-C₃N₄/Fe-TiO₂ composites is illustrated in Figure 10b in detail. Due to their narrow band gaps, both g-C₃N₄ and Fe-TiO₂ can be easily excited to yield photogenerated electron–hole pairs under visible-light irradiation. Since both the CB and VB positions of Fe-TiO₂ are lower than those of g-C₃N₄, the photogenerated electrons (e⁻) in the CB of Fe-TiO₂ tend to transfer and recombine with the photogenerated holes (h⁺) in the VB of g-C₃N₄. The photogenerated holes left behind in the VB of Fe-TiO₂ can directly oxidize phenol into harmless metabolite products. Simultaneously, the remaining photogenerated electrons in the CB of g-C₃N₄ can reduce the adsorbed O₂ to yield •O₂⁻, which is again a powerful oxidative species for phenol degradation. The g-C₃N₄/Fe-TiO₂ composites following a Z-scheme mechanism enable a fast separation and transfer of the photogenerated electron−hole pairs and in turn show strong oxidation and reduction abilities for the efficient photocatalytic degradation of organic pollutants.

3. Materials and Methods

3.1. Chemical and Material

Analytical grade (AR) chemicals viz. Ferric nitrate (Fe(NO₃)₃·9H₂O), tetra-butyl titanate (TBOT), absolute ethyl alcohol (C₂H₆O), nitric acid (HNO₃), melamine, isopropanol, 5% Nafion, phenol sodium oxalate (OA), p-benzoquinone (BQ), and isopropanol were purchased from Sinopharm Chemical Reagent CO. Ltd., Shanghai, China and used as received. Millipore deionized water was used for preparing the stock solutions and the entire experimental part.

3.2. Catalyst Preparation

The g-C₃N₄ was synthesized from melamine by a direct heating step. Five grams (5 g) of melamine powder, taken in an alumina crucible, was placed in a muffle furnace and heated at 500 °C for 2 h. After cooling down to room temperature, the yellowish product was ground into powder form and again heated in a muffle furnace at 500 °C for another 2 h.

The composite particles were synthesized through a one-step hydrothermal process. A 20 mL volume of TBOT was gradually dropped into a mixture containing 167.5 mL of C₂H₆O, 5.0 mL specific concentration of Fe(NO₃)₃, 1.25 mL of HNO₃, and 0.047 g of g-C₃N₄ under vigorous stirring, and the stirring process continued for another 1 h. Then, the mixture was transferred into a 500 mL teflon-lined stainless steel autoclave vessel and it was kept at 200 °C for 6 h. After the hydrothermal process, the precipitate was centrifuged, washed several times with ethanol and water, dried at 80 °C overnight, and ground well. The as-prepared samples were denoted as xFe-CT, where x stands for the weight percentage of Fe (x = 0.01, 0.03, 0.04, 0.05, 0.06) with respect to TiO₂ content, and CT denotes the g-C₃N₄/TiO₂.

3.3. Characterization

The phase purity and crystal structure of the as-obtained samples were examined by the XRD technique using Rigaku Ultima IV X-ray diffraction (Rigaku Corporation, Tokyo, Japan) equipped with Cu Kα radiation (40 kV, λ = 1.5406 Å). The 2θ scanning angle range was 20–80° with a step of 0.05 s⁻¹. The morphology was examined using a field emission scanning electron microscope (FE-SEM, NANOSEM 450, FEI Corporation, Eindhoven, The Netherlands) operating at an accelerating voltage of 30 kV. TEM characterizations were done using an H-7000FA microscope (Hitachi, Tokyo, Japan) operating at the accelerating voltage of 75 kV. The UV-visible spectrum was obtained on a UV-2550 UV-visible spectrophotometer (Shimadzu Corporation, Kyoto, Japan) at room temperature and the spectrum range analyzed was 200–800 nm. The infrared absorption spectra were measured in a frequency range from 500 cm⁻¹ to 4000 cm⁻¹ on a Bruker V-70 FTIR spectrophotometer (Bruker, Karlsruhe, Germany). The X-band electron paramagnetic resonance (EPR) spectra were recorded at room temperature using a Bruker A300-10/12 EPR spectrophotometer (Bruker Corporation, Karlsruhe, Germany). The microwave frequency was fixed at 100 KHz, the power was 10 mW, and the field modulation ranged between 1.3–1.9 G and 3.2–3.7 G. The X-ray photoelectron spectroscopy (XPS) data were collected by an Axis Ultra instrument (Kratos Analytical, Manchester, UK) under an ultra-high vacuum (<10⁻⁸ Torr) using a monochromatic Al Ka X-ray source (hν = 1486.6 eV) operating at 150 W.

Mott–Schottky plots have been investigated on an electrochemical workstation (CS310, CorrTest, Wuhan, China) under a three-electrode configuration by employing TiO₂ or Fe-TiO₂, Ag/AgCl, and Pt mesh as the working, reference, and counter electrode, respectively. Herein, the working electrodes had undergone a two-step treatment. Initially, 5 mg of synthesized sample was mixed with 800 μL of isopropanol, followed by 200 μL of millipore deionized water, and finally 20 μL of 5% Nafion, and then the mixture underwent an ultrasonication treatment; 6 μL of the mixture was dropped onto glassy carbon electrode and dried in an air environment. The supporting electrolyte used was Na₂SO₄ with a concentration of 0.5 mol dm⁻³. The potential scanning measurements for the electrode were performed from −0.2 V to 0.8 V in dark conditions, and the impedance-potential characteristics of the electrode were recorded at a frequency of 10 Hz.

3.4. Photocatalytic Activity Measurement

The photocatalytic experiment on phenol degradation under visible light was carried out in a glass container having a volume capacity of 200 mL to evaluate the activity of the g-C₃N₄/Fe-TiO₂ composites. The light source was a 300 W PLS-SXE 300 xenon lamp (Perfect light, Wuhan, China) with a 400 nm cut filter to remove the UV irradiation that was suspended over a height of 10 cm from the reaction solution surface. Typically, 5 mg of as-prepared photocatalyst was added into 50 mL of phenol-contaminated (10 mg dm⁻³) working solution. The glass container was placed in an ice-water bath, and the entire setup was placed on a magnetic stirrer operated at a constant stirring rate of 380 rpm. Prior to light irradiation, the suspension was stirred for 1 h to establish an adsorption/desorption equilibrium between phenol and photocatalyst under dark conditions. After visible light irradiation for a defined period of time (every 10 min), the reaction solution (1 mL) for analysis was siphoned out, and then the suspensions were removed by centrifugation and the clear supernatant solution was used for analysis. The concentration of phenol was measured by high performance liquid chromatography (HPLC) (Shimadzu, Kyoto, Japan) equipped with a UV detector and a C₁₈ reverse-phase column (4.6 mm i.d. ×150 mm, Agilent, CA, USA). The mobile phase used in the HPLC was water and methanol (volumetric ratio of 50:50), and the injection volume of the sample was 20 μL with a flow rate of 0.5 mL min⁻¹. The wavelength of the UV absorbance detector was fixed at 270 nm.

The quantity of •OH in the photocatalytic process was determined by the photoluminescence (PL) technique using terephthalic acid as a probe molecule. Terephthalic acid reacts with •OH to produce a highly fluorescent product 2-hydroxy terephthalic rapidly and specifically, which reflects as the PL signal at 425 nm excited by 315 nm of light. Detailed experimental information is given in our previous work [57].

4. Conclusions

The photocatalytic g-C₃N₄/Fe-TiO₂ composite was successfully synthesized by a one-step hydrothermal process and found to exhibit excellent photocatalytic activity and stability for the photocatalytic degradation of organic pollutants. The composite with an optimum content of Fe³⁺ of 0.05 wt % exhibits the highest photocatalytic activity towards phenol degradation owing to a stronger spectral absorption of visible light wavelengths, an enhancement in the carrier density, and a decrease in the charge transfer resistance between the interface of solid and electrolyte. The formation of Z-scheme g-C₃N₄/Fe-TiO₂ heterojunctions possesses a higher efficiency of charge separation and transfer as well as stronger oxidation and reduction abilities. This work may give new insight into the development of Z-scheme composite photocatalysts, which is of a great interest to the scientific community for photocatalysis.