Ultra-fine carbon decorated TiO₂/C/g-C₃N₄ hybrid for strong physical adsorption and efficient photodegradation of pollutants

1 Introduction

Recently, photocatalysis has been attracted tremendous interest due to their wide spread applications. This interest arises from an increasing understanding of their real-world benefits in fields such as renewable energy, environmental remediation, and water purification. Using solar energy, photocatalysis shows great potential in producing chemical fuels and cleaning the environment by decontaminating various pollutants. These catalysts have attracted attention due to their efficacy in degrading complex organic pollutants, presenting promising prospects for diverse practical applications (Dalanta et al., 2022; Shi et al., 2023; Wang et al., 2010; Lu et al., 2010; Wang et al., 2019; Yan et al., 2023; Li et al., 2022; Xin et al., 2021; Yin et al., 2022; Osterloh, 2013; Wang and Wang, 2022). Since the pioneering report by Fujishima and Honda in 1979 (Cao et al., 2013; Inoue et al., 1979) various photocatalysts have been developed, such as, TiO₂, g-C₃N₄, WO₃, SiC, ZnO, and ZnS for various photocatalytic application. Due to outstanding properties, TiO₂ remains the most widely studied photocatalyst (Liyanaarachchi et al., 2023; Bashir et al., 2022; Wang and Wang, 2022; Bao et al., 2023; Ismael, 2020; Sun et al., 2020; Wu et al., 2023; Han et al., 2023; Cui and Sun, 2006). The g-C₃N₄ (graphitic carbon nitride) has promising characteristics, including 2.7 eV band gap, non-toxicity, affordability, and potential for industrial and combustion applications (Wudil et al., 2023; Ismael, 2020; Sun et al., 2020; Que et al., 2021; Wang et al., 2023; Guo et al., 2021). It also has exciting electrical and electrochemical properties which make it, a candidate material for applications into optoelectronic, photonic, photocatalysis, electrocatalysis, batteries and for degradation of organic pollutants (Nguyen et al., 2020; Ismael, 2020; Que et al., 2021; Wang et al., 2023). According to measurements with normal hydrogen electrode (NHE), it has been determined that the potential difference between the conduction and valence bands in graphitic carbon nitride is 1.3 eV and 1.4 eV. These values are particularly favorable for facilitating efficient water splitting (Ismael, 2020; Wang et al., 2023). However, the current visible light photocatalytic activity of pure g-C₃N₄ is still low, mainly due to the unoptimized electronic structure of g-C₃N₄ material, which leads to a high rate of photocarrier recombination and a large bandgap width (∼2.8 eV), preventing the effective utilization of sunlight. Various approaches have been developed to overcome this challenge such as element doping, modified with other composites semiconducting materials. These strategies in photocatalysts increase the diffusion length of photogenerated carriers that prolong the lifetime of charges, inhibiting the internal recombination rate of charges. In addition, element doping favors the photons with small energy excited electrons (e^-) and holes (h⁺) captured at doping levels, which improves the utilization rate of photons and optimize the visible light absorption of photocatalysts (Djurišić et al., 2014). One promising research direction is the hybrid formation of g-C₃N₄ with other semiconductor composites, which facilitate the negative charge flow and reducing their recombination with positive charges (Sun et al., 2020; Cui and Sun, 2006; Djurišić et al., 2014; Xin et al., 2021; Yin et al., 2022; Xu et al., 2024). TiO₂ has been used widely to construct hybrid, effectively separating and transferring photogenerated carriers (Shaban, 2019; Xin et al., 2021; Yin et al., 2022; Jonathan et al., 2023; Chen et al., 2023; Guo et al., 2020). Jin Luo et al. reported the development of two innovative Z-scheme photocatalysts: a LaMnO₃/g-C₃N₄ hybrid, constructed for efficient visible-light-driven degradation of tetracycline (TC), and an AgI/Zn₃V₂O₈ hybrid, where AgI nanoparticles were immobilized on flower-like Zn₃V₂O₈ nanosheets via a simple precipitation method. Both designs utilize the Z-scheme mechanism to enhance photocatalytic performance (Luo et al., 2019; Luo et al., 2021). Similarly, Lin et al. synthesized a CdS/ZnS core/shell structure,where zinc vacancies transformed the type-I heterojunction into a Z-scheme hybrid, enabling the ZnS shell to effectively capture photogenerated holes from the CdS core (Lin et al., 2020). TiO₂ shows potential for forming a hybrid structure with g-C₃N₄. However, its small surface area has limited its photocatalytic efficiency. Co-doping in TiO₂/C₃N₄ has demonstrated significant enhancement in catalytic performance attributed to the availability of more active sites (Huang et al., 2021).

Jiao et al, developed an ultrasmall (∼5 nm) visible-light-responsive TiO₂/C photocatalyst with a hybrid structure via chemical vapor deposition. The monolayer carbon shell enhances charge separation, stabilizes Ti³⁺, and narrows the bandgap (2.83 eV), leading to superior photocatalytic performance for water oxidation, dye degradation, and hydrogen evolution. This design provides a promising approach for industrial applications requiring stable, visible-light-responsive photocatalysts (Jiao et al., 2022). In comparison, the TiO₂/C/g-C₃N₄ photocatalyst, developed via a facile heat-treatment method, incorporates an ultrafine carbon layer to enhance electron mobility, improving catalytic performance through both physical adsorption and chemical degradation. Both designs focus on enhancing photocatalytic efficiency, with one emphasizing structural stability and bandgap tuning, and the other excelling in adsorption capacity and recyclability. The hybrid of TiO₂ with C₃N₄ facilitates to separate charges at junction interface. This configuration control the hole recombination with electron because of very low valence band edge potential of TiO₂. But electrons in C₃N₄ still do not have enough potentials to trigger the chemical reactions with the organic pollutant and to reduce it. Before, the reaction initiates the electrons recombine with holes in the host material and decomposes the material. Therefore, an external facilitator is needed to boost up the electron transfer mechanisms in the staggered band alignment and hence increase the degradation of organic pollutant. Based on a detailed literature survey, the TiO₂/C/g-C₃N₄ hybrid photocatalyst was first developed through a facile heat-treatment process, incorporating an ultrafine carbon layer to enhance electron mobility for more efficient catalytic reactions. Secondly, this facile heat-treatment method in air is valuable for the large-scale preparation of photocatalysts, up to 1 kg, employing an air calcination process to deposit graphitic nitride (g-C₃N₄) onto a carbon-modified TiO₂ surface. While many researchers have studied how photocatalyst performance can be enhanced by either physical adsorption or chemical degradation of pollutants. Thirdly, the innovative aspect of the present work lies in coupling both physical adsorption and chemical degradation to improve photocatalytic performance.

In this study, we have applied a straightforward air calcination technique to fabricate a unique hybrid structure with a carbon (C) layer acting as a separator. This design not only facilitates efficient electron transfer but also hinders recombination with holes. These advancements significantly elevate the quantum efficiency, thereby augmenting the degradation of both atmospheric and organic pollutants. This work is really important for cleaning up the environment and can be used in many different industries. This study introduces a photocatalyst called TiO₂/C/g-C₃N₄, which is really effective in decomposing methyl orange and formaldehyde. The surface engineering of air calcination process to design the ultrafine carbon layer to bridge TiO₂ and g-C₃N₄ with strong physical absorption as well as enhanced photocatalytic performance.

2 Experimental section

3 Results and discussion

The conventional approach to photocatalytic degradation of pollutants primarily involves structural and energy level modulation to extend catalytic responsiveness from ultraviolet (UV) to visible light spectra, thereby facilitating the decomposition of pollutants in air or water. In this study, a novel strategy is proposed, integrating physical adsorption and photocatalysis in a hybrid framework. This 3D carbon layer bridging structures effectively exert the physical adsorption function of carbon materials, and also regulate the energy level structure of the materials, achieving a dual functional combination of physical adsorption and photocatalysis. The hybrid photocatalyst, denoted as TiO₂/C/g-C₃N₄, is synthesized through a straightforward heating process under normal atmospheric pressure, as illustrated in Fig. 1a. Urea mixed with TiO₂/C₇H₈O was stirred for 1 h then heated in the furnace inset covered crucible at 500 °C for 2 h, yielding the TiO₂/C/g-C₃N₄ hybrid with desired photocatalytic properties. Moreover, characterization of the morphologies of the graphitized carbon nitride, TiO₂, and (TiO₂/C/g-C₃N₄) samples is performed using high-resolution transmission electron microscopy (HRTEM), as depicted in Fig. 1(b–e). Fig. 1c clearly shows that g-C₃N₄ has a layered structure, indicated by the yellow lines. The HRTEM images of g-C₃N₄ reveal an amorphous layer structure, while the observed lattice fringe of 0.35 nm in the TiO₂/C/g-C₃N₄ hybrid corresponds to the (1 0 1) planes of TiO₂ (Chen et al., 2017). During the CVD process, the residual organic material underwent decomposition into carbon (C) atoms that are highly reactive, which were doped directly onto TiO₂ surface. The surface of TiO₂/C/g-C₃N₄ displays disordered carbon layers indicated by black dotted line, and potential defects indicated by red circle as shown in Fig. 1e. Elemental mapping for C, N, Ti and O elements in the TiO₂/C/g-C₃N₄ hybrid formation is presented in Fig. 1(f-j), respectively.

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Fig. 1

(a) Graphical representation of the TiO₂/C/g-C₃N₄ hybrid preparation by air calcination method at 500 °C. (b–c) TEM results of g-C₃N₄ and (d–e) TiO₂/C/g-C₃N₄ (f–j) Mapping of elements; C (g), N (h), Ti (i) and O (j) elements of TiO₂/C/g-C₃N₄.

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The crystal phase of TiO₂/C/g-C₃N₄ hybrid photocatalyst was identified using XRD (X-ray diffraction) technique and the results are given in Fig. 2a. All diffraction peaks of TiO₂/C₇H₈O and TiO₂/C/g-C₃N₄ correspond to the TiO₂ anatase phases, which mean that the TiO₂/C₇H₈O and TiO₂/C/g-C₃N₄ hybrids belong to the anatase phases of TiO₂. Additionally, the spectra (XRD) of TiO₂/C₇H₈O and TiO₂/C hybrids exhibits four well-defined diffraction peaks at the lattice planes of (1 0 1), (004), (200), and (105) respectively, in agreement with previous investigations (Zhang et al., 2023; Xiao et al., 2023). This indicates that the crystal structure of TiO₂/C₇H₈O remains unchanged following high-temperature calcination and the addition of carbon capsules. Actually, the carbon capsules act as a protective barrier during high-temperature calcination, preventing excessive grain growth and limiting agglomeration of TiO₂ nanoparticles, thereby preserving a well-defined crystalline structure. Additionally, they stabilize the photo-catalytically active anatase phase by buffering against phase transitions to rutile at elevated temperatures, ensuring the catalyst retains its optimal photocatalytic performance. Moreover, the observed peaks at (1 0 0) and (004) align with those characteristics of g-C₃N₄, consistent with findings in existing literature (Chen et al., 2023).

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Fig. 2

(a) XRD patterns, (b) Raman spectra, (c) TGA, and (d) FTIR of TiO₂/C₇H₈O, g-C₃N₄, TiO₂/C/g-C₃N₄-1, TiO₂/C/g-C₃N₄-2 and TiO₂/C/g-C₃N₄-3, respectively.

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The structure and molecular composition of TiO₂/C/g-C₃N₄ hybrids were analyzed using the Raman technique, which revealed that the TiO₂/C₇H₈O and TiO₂/C/g-C₃N₄ hybrids have peaks at 152, 392, 515, and 631 cm⁻¹ as given in Fig. 2b. These peaks correspond to the four basic modes of titanium dioxide (E_g, B1_g, A1_g, and Eg), indicating that the anatase phase of TiO₂ is not affected by the in-situ addition of carbon capsule in TiO₂/C/g-C₃N₄ (Cui, 2006; Yao et al., 2021).

Thermogravimetric analysis (TGA) was performed under open platform conditions, encompassing from room temperature to 800 °C with a heating rate of 30 °C/min, aimed at elucidating the thermostability of the hybrid. The TGA results (Fig. 2c) illustrate notable weight loss phases occurring primarily between ambient temperature and 200 °C. This phenomenon is attributed to the dissociation of adsorbed and intercalated water molecules. Notably, g-C₃N₄ undergoes fast weight loss within the temperature interval of 400 to 780 °C, resulting in substantial weight reduction during this range. At 750 °C, pure g-C₃N₄ experiences complete weight loss, indicative of its compositional transformation during decomposition. The second stage of weight loss reveals that the g-C₃N₄ quantity present in the composites based on their weight. The measured quantity of g-C₃N₄ in the photocatalysts were determined as 93.6 %, 88.8 %, and 77.7 % by weight for samples TiO₂/C/g-C₃N₄-1, TiO₂/C/g-C₃N₄-2, and TiO₂/C/g-C₃N₄-3, respectively. Noteworthy is the lower initial weight loss temperature observed in the TiO₂/C/g-C₃N₄-1 sample, attributed to the elevated ratio of TiO₂. This higher TiO₂ ratio facilitates catalytic oxidation of g-C₃N₄, even at temperatures below 600 °C.

FTIR spectroscopy was employed to further analyze the components of prepared catalysts. As depicted in Fig. 2d, both g-C₃N₄ and TiO₂/C/g-C₃N₄-(1, 2,3) hybrids display typical peaks (absorption) for g-C₃N₄. The sharp peak of g-C₃N₄, located at approximately 800 cm⁻¹ corresponds to the inherent vibrational modes, specifically the breathing motions of s-triazine molecules. Conversely, the minor peaks observed in the TiO₂/C/g-C₃N₄ hybrids within the range of 1200 to 1600 cm⁻¹ are attributed to the typical N=C-N stretching mode inherent to heterocycles in g-C₃N₄. The broad band peaks between 3000 and 3500 cm⁻¹ were attributed to N–H stretching vibration of the amine groups. These spectroscopic features collectively indicate effective coupling of g-C₃N₄ with the TiO₂/C/g-C₃N₄ hybrids (1, 2, 3). The strong bonding between g-C₃N₄ and the TiO₂/C/g-C₃N₄ hybrids (1, 2, 3) facilitates efficient charge transfer at the interface, resulting in enhanced photocatalytic activity. This bond promotes energetic electron transfer and generates synergistic interfacial effects, ultimately boosting the overall performance of the photocatalyst.

The X-ray photoelectron spectroscopy (XPS) was utilized to examine the different composition of the TiO₂/C/g-C₃N₄ hybrid photocatalysts. The energy band spectra were obtained using a C 1 s peak at (284.8 eV) for correcting the position of the peak. Fig. 3a-e shows the XPS analysis results for both pristine graphitic nitride and TiO₂/C/g-C₃N₄. The contents of C, N, Ti, and O components in the TiO₂/C/g-C₃N₄ hybrid sample are shown in Fig. 3a, indicating the integration of TiO₂ into g-C₃N₄. The C 1 s pattern of the TiO₂/C/g-C₃N₄ hybrid is displayed in Fig. 3b. The spectrum exhibits two distinct peaks related to carbon species. The strong peak in the higher energy range (287.2 eV to 288.19 eV) assigned to N-C=C bonding, while the second peak within the energy range (284.79 eV to 284.82 eV) represents C–C coordination, which is due to the presence of g-C₃N₄ (Wang et al., 2016). Fig. 3c demonstrates the high accuracy (N 1 s) spectra of TiO₂/C/g-C₃N₄ have peak in energy range (398.5 to 398.72) eV which assigned to C⚌N–C bond, while peaks in energy range (400.09–400.15) eV corresponds to N-(C3) bond (Wang et al., 2016) and the peaks appear in (401.10 eV 403.65 eV) energy range are designated to C–N–H bond (Wang et al., 2016; Li et al., 2014; Wang et al., 2023). In Fig. 3d, the Ti 2p spectrum has characteristic configurations, the sharpest of which are 464.68 eV is due to the Ti-2p^1/2 while 459 eV is assigned to Ti-2p^3/2 (Wang et al., 2023). The XPS results of (O 1s) are given Fig. 3e, which ascribed two peaks, 530.19 eV assigned to Ti-O bond and 532.46 eV to O–H bond (Jiang et al., 2018; Zou et al., 2017; Niu et al., 2012). The existence of a OH intermediates or water molecule upon that interface of the TiO₂/C/g-C₃N₄ hybrid structure is related to the presence of an O 1s peak at 532.06 eV. The XPS analysis demonstrates the effective formation of a heterostructure photocatalyst.

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Fig. 3

XPS spectrum of (a) g-C₃N₄ and TiO₂/C/g-C₃N₄, XPS spectra with high resolution for C 1s (b), N 1s (c), Ti 2p (d), (e) O 1s for graphitic nitride (g-C₃N₄) and multi-junction (TiO₂/C/g-C₃N₄).

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The catalytic activity of TiO₂/C/g-C₃N₄ hybrids in methyl orange (MO) degradation under visible light irradiation is depicted in Fig. 4a. When studying the performance of TiO₂/C/g-C₃N₄ hybrids catalyst, this research motivation is to combine the dual functions of chemical photocatalysis and physical adsorption. After physically adsorbing pollutants, there is a significant change of catalyst powders and MO solution in color. After absorption saturation, the color changed to light yellow due to MO (yellow) covering on catalyst. From Fig. 4a, it is true that the absorption of pollutants on catalyst surface is related to physical adsorption in dark. But it is NOT really degradation for MO pollutants based on the color-change observation. This process is mainly divided into two steps: physical adsorption and further chemical photodegradation. In Fig. 4a the step-I explained the physical adsorption of MO onto catalyst surface in dark region mentioned in the corresponding Fig. 4a. The step-II confirmed the photocatalytic degradation of MO under light irradiation, which explained the photocatalytic properties. The results indicate a significant increase in MO adsorption capacity, with values rising from 18.0 % for g-C₃N₄ to 61.2 % for TiO₂/C/g-C₃N₄-1. Additionally, as the light-driven reaction proceeded, TiO₂/C/g-C₃N₄-1 hybrids exhibited a notable enhancement in the degradation rate of MO. Within a 180-minute period of visible light exposure, the MO photo-degradation efficiencies were measured as 93.6 %, 32.4 %, and 37.6 % for TiO₂/C/g-C₃N₄-1, TiO₂/C/g-C₃N₄-2, and TiO₂/C/g-C₃N₄-3, respectively. Notably, TiO₂/C/g-C₃N₄-1 demonstrated superior photocatalytic degradation efficiency compared to the other two hybrids. This enhanced degradation capability was attributed to the presence of oxygen vacancies, defects, and primarily to the improved charge separation resulting from the hybridization between TiO₂, g-C₃N₄, and the carbon layer.

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Fig. 4

Photocatalytic performance of (a) MO and (b) HCHO relative value MO concentration (C/C₀) of TiO₂/C hybrids. (c) UV–Vis absorbance spectra in the range of 200–800 nm and (d) Tauc plot by Kubelka-Munk equation of Pristine TiO₂/C₇H₈O, g-C₃N_4, TiO₂/C/g-C₃N₄-1, TiO₂/C/g-C₃N₄-2, TiO₂/C/g-C₃N₄-3.

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Furthermore, to assess the catalytic degradation activity, a photocatalytic device was utilized to evaluate the degradation of formaldehyde (HCHO) as a pollutant. The photocatalyst was applied onto cotton cloth, encapsulating HCHO within a closed chamber. The amount of HCHO evaporating from the chamber over a specified time period was monitored under LED illumination. These findings of this investigation are depicted in Fig. 4b. Notably, there was only a slight decrease in HCHO concentration within 8 h during the dark and blank experiments, likely attributed to HCHO evaporation through wiring pores. For TiO₂/C₇H₈O, the HCHO degradation rate after 8 h was only 42.2 %. However, the degradation rates of HCHO by TiO₂/C/g-C₃N₄-1 (93.7 %) exhibited significant improvement compared to TiO₂/C/g-C₃N₄-2 (83 %) and TiO₂/C/g-C₃N₄-3 (85 %). The formation of a hybrid incorporating an active carbon layer, type-II band alignment, and the introduction of oxygen vacancies contributes to the enhanced catalytic activity of TiO₂ materials within the visible light spectrum.

Using UV–Vis optical spectroscopy, energy gap values were estimated for TiO₂/C₇H₈O, TiO₂/C/g-C₃N₄-1, TiO₂/C/g-C₃N₄-2, and TiO₂/C/g-C₃N-3 hybrid materials. Notably, the TiO₂/C/g-C₃N₄-1 hybrids displayed absorption with a high-energy edge compared to TiO₂/C₇H₈O, indicating their superior ability to harness visible light. The absorption edges of TiO₂/C/g-C₃N₄-1, TiO₂/C/g-C₃N₄-2, and TiO₂/C/g-C₃N-3 were observed at 412 nm, 403 nm, and 396 nm, respectively, revealing clear red shifts in comparison to the 374 nm absorption edge of TiO₂/C₇H₈O. Remarkably, TiO₂/C/g-C₃N₄-1 exhibited a significant blue shift up to 412 nm.

The Tauc diagram, derived using the Kudelka-Munk formula Ahѵ1/2 = A(hѵ-Eg), provided insights into the bandgap energy (Eg) values. The Eg of TiO₂/C₇H₈O, g-C₃N₄, TiO₂/C/g-C₃N₄-1, TiO₂/C/g-C₃N₄-2, and TiO₂/C/g-C₃N₄-3 were determined as 3.08 eV, 2.79 eV, 2.76 eV, 2.87 eV, and 2.96 eV, respectively, as illustrated in Fig. 4d. Notably, TiO₂/C₇H₈O and TiO₂/C/g-C₃N₄-1 exhibited the lowest bandgap energies among the materials examined.

This observation correlates with the enhanced degradation efficiency for formaldehyde (HCHO), suggesting that the band structure of TiO₂/C₇H₈O is effectively tailored through doping materials to exploit the energy of visible light. This the importance of uniform carbon layer deposition on TiO₂ and the formation of novel hybrid structures, which effectively reduce the band gap of TiO₂. Ultimately, this modification positively impacts both physical adsorption and chemical degradation processes.

Furthermore, the study aimed to investigate the continuous cyclic degradation of organic dye pollutants, specifically HCHO and MO, in the presence of TiO₂/C/g-C₃N₄-1 under optical stimulation. The stability of TiO₂/C/g-C₃N₄-1 during the degradation process of MO and HCHO in light was examined, as shown in Fig. 5. It is found that the concentration of MO in the TiO₂/C cyclic degradation remained relatively stable after four cycles, indicating a consistent photo-degradation efficiency of approximately 93 %. This finding suggests that TiO₂/C/g-C₃N₄-1 maintained its high photocatalytic performance and stability throughout the cyclic degradation process, with no significant damage observed to the active sites on its surface. In Fig. 5b, it is observed that the degradation efficiency of TiO₂/C/g-C₃N₄-1 for HCHO gradually declined with each subsequent cycle, suggesting a reduction in its photocatalytic activity over time. This decline is attributed to irreversible modifications on the surface state of TiO₂/C/g-C₃N₄ during the cyclic degradation of MO, leading to a gradual decrease in its photocatalytic performance. TiO₂/C/g-C₃N₄-1 exhibits remarkable activity and stability in the degradation of MO. Its surface, characterized by an amorphous structure and the presence of oxygen vacancy defects, facilitates efficient acceptance and transfer of photogenerated electrons (e^-) while inhibiting recombination with photogenerated holes (h⁺). Several experiments were carried out to identify the active groups involved in the photodegradation process using MO as the target organic pollutant. The ascorbic acid (AA), triethanolamine (TEOA) and Isopropyl alcohol (IPA) were introduced as scavengers to capture ·O₂–, h⁺, and *OH respectively. As shown in Fig. 5c, the addition of IPA had minimal impact on the degradation process, indicating that ·OH is not a major active species. However, the addition of AA significantly reduced degradation efficiency from 93.4 % (Blank sample) into 37.2 %, showing that O2⋅−, is the primary active species. When TEOA was added, the degradation efficiency decreased into 51.7 %, suggesting that h + is a secondary active species. In conclusion, O2⋅−, plays a major role while h + plays a minor role in the photocatalytic degradation of MO. The adsorption and desorption isotherms of N₂ were analyzed for TiO₂/C/g-C₃N₄ and TiO₂/C₇H₈O within a pressure range (0.0–1.0). The findings revealed that TiO₂/C/g-C₃N₄ exhibited a significantly higher specific surface area of approximately 181.2 m²/g compared to TiO₂/C₇H₈O, which had a specific surface area of only about 100.96 m²/g. This increased the specific surface area, creating more contact space between the TiO₂/C/g-C₃N₄ hybrid and the target pollutants. As a result, more active sites are available for efficient catalytic reactions.

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Fig. 5

Photocatalytic activity degradation cycle test of (a) HCHO and (b) MO by TiO₂/C/g-C₃N₄-1, (c) Photocatalytic degradation of MO by TiO₂/C/g-C₃N_4-1 with different scavengers under visible light, (d) N₂ adsorption/desorption isotherm TiO₂/C/g-C₃N₄ and TiO₂/C₇H₈O.

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The combination of dual functions of chemical photocatalysis and physical adsorption for TiO₂/C/g-C₃N₄ hybrid catalyst is developed. In terms of physical adsorption in dark, MO undergoes physical adsorption onto the hybrid catalyst surface, resulting in a slight darkening of the MO solution from dark yellow, reddish yellow into brown without degradation. The results highlight only physical adsorption it’s not effective for MO degradation in dark. In terms of photocatalytic process, the schematic band alignment calculated using density functional theory (DFT) is shown in Fig. 6, highlighting the heterojunction between g-C₃N₄, carbon layer, and TiO₂, enabling effective charge transfer and separation of electrons and holes. Upon light irradiation, electrons are excited from the valence band (VB) to the conduction band (CB) of both g-C₃N₄ and TiO₂. The energy positions are crucial in directing charge carriers; g-C₃N₄’s CB at −1.16 eV and TiO₂’s at −0.021 eV create a favorable pathway for electrons to migrate from g-C₃N₄ to TiO₂ via the carbon layer interface. Carbon layer acts as a conductive bridge, facilitating rapid electron transfer and minimizing electron-hole recombination. The carbon layer ensures the efficient transfer of electrons to the TiO₂ side for reduction reactions. Simultaneously, the holes generated in the valence band of g-C₃N₄ remain within the g-C₃N₄ layer, where they participate in oxidation reactions. This spatial separation of electrons and holes enhances the overall photocatalytic efficiency by reducing the likelihood of recombination. On the TiO₂ side, the electrons engage in the reduction of formaldehyde (HCHO), while on the g-C₃N₄ side, the holes are responsible for the oxidation of methyl orange (MO). The synergistic interaction between g-C₃N₄, carbon, and TiO₂ ensures that both oxidation and reduction reactions are occurring efficiently and in different zones. The alignment of the energy bands in this heterostructure enhances light absorption and charge transport, with the larger bandgap of TiO₂ (3.1 eV) enabling it to drive reduction reactions, while the narrower bandgap of g-C₃N₄ (1.49 eV) promotes hole-driven oxidation reactions. This arrangement ensures spatial separation of redox processes, significantly improving photocatalytic performance and pollutant degradation.

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Fig. 6

The staggered band alignment and charge carrier separation in TiO₂/C/g-C₃N₄ composite structures.

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The advantage of g-C₃N₄/C/TiO₂ hybrid lies in their rapid physical adsorption of MO pollutants, which allows photocatalysts to degrade pollutants fast. This is due to the increased local concentration of pollutants on the catalyst surface. Besides, the g-C₃N₄/C/TiO₂ heterostructure’s ability to absorb light energy efficiently, maintain charge separation, and optimize reaction zones makes it highly effective for the degradation of pollutants like MO and HCHO.

4 Conclusion

A novel hybrid photocatalyst of TiO₂/C/g-C₃N₄ is developed through an efficient air calcination technique and precursory methodology. This hybrid photocatalyst demonstrates remarkable efficacy in catalyzing the degradation of organic compounds and air pollutants. Notably, among the synthesized samples, the g-C₃N₄/C/TiO₂ hybrid photocatalyst exhibits that the catalyst efficiently adsorbs MO in dark without degradation. Combining physical adsorption with photocatalytic activity, it degrades 94 % of MO and HCHO within 180 min at an initial concentration of 0.2 mol/L, achieving almost complete decolorization under visible light irradiation. The observed enhancement in catalytic performance is attributed to the staggered band alignment within the TiO₂/C/g-C₃N₄ hybrid photocatalyst, facilitating efficient charge separation and promoting electron transfer at the interface, thereby accelerating catalytic reactions. Furthermore, the incorporation of carbon layers speeds up electron transport, contributing to the enhanced degradation of MO and HCHO within the TiO₂/C/g-C₃N₄ hybrid photocatalyst. This study introduces a straightforward and cost-effective approach for fabricating hybrid TiO₂/C/g-C₃N₄ photocatalysts, with potential scalability for the practical applications.

CRediT authorship contribution statement

Azim Khan: Writing – review & editing, Writing – original draft, Data curation, Conceptualization. Ruhumuriza Jonathan: Writing – original draft, Methodology, Data curation. Shafiq Ur Rehman: Writing – review & editing, Data curation. Muhammad Shoaib: Writing – review & editing, Data curation. Feng Cao: Writing – review & editing, Investigation, Data curation. Sajjad Ali: Writing – review & editing, Data curation, Conceptualization. Mohamed Bououdina: Writing – review & editing, Data curation, Conceptualization. Pir Muhammad Ismail: Writing – review & editing, Investigation, Formal analysis. Junwei Wang: Writing – review & editing, Formal analysis, Data curation, Conceptualization. Hazem Abu-Farsakh: Writing – review & editing, Funding acquisition, Formal analysis, Data curation. Yifan Liu: Writing – review & editing, Formal analysis, Data curation. Xian Jian: Supervision, Project administration, Funding acquisition, Formal analysis, Conceptualization.