Photocatalytic performance of TiO₂ modified with graphene derivatives and Fe (Ⅲ) at different thermal reduction temperatures

In recent years, persistent organic pollutants (POPs) and antibiotics have become a growing focus of attention. Emerging contamination (EC) such as persistent organic pollutants (POPs), pharmaceuticals and personal care products (PPCPs) enter the environment through multiple pathways and place a huge burden on the environment (Katsoyiannis & Samara 2004; Yanei et al. 2023; Die Ling et al. 2024). Research shows that 15% of persistent organic dyes in the wastewater produced by the textile industry are discharged into the water without treatment and 30% of antibiotics are discharged into the environment without being absorbed and utilized by animals and plants (Karthikeyan & Meyer 2006; Channei et al. 2014). Unfortunately, these pollutants are difficult to degrade by traditional biological treatment methods and cause further pressure on the surroundings (Guo Dong 2019; Chen et al. 2023). Thus, it is necessary to find a more efficient treatment process (Kurniawan et al. 2020).

The development of photocatalytic oxidation processes utilizing highly reactive oxygen species (ROS) such as H₂O₂, ·OH, and ·OOH for the efficient treatment of wastewater has attracted widespread attention in recent years. Photocatalytic oxidation, a green, mild and environmental-friendly technology, has been one of the important ways to control environmental problems in recent years (Fujishima et al. 2000; Anpo & Kamat 2010; Nosaka & Nosaka 2013). Photocatalytic oxidation processes can degrade nearly all of the organic pollutants in the wastewater (Michael & Henderson 2011). Of all of the photocatalysts, TiO₂ is considered to be one of the most promising catalysts due to its good chemical stability, biocompatibility, availability and low cost (Drunka et al. 2015). However, anatase TiO₂ has a narrow light absorbance range (<380 nm) because of the large band gap of 3.2 eV. The high recombination efficiency of photogenerated electrons and hole has a negative effect on the formation of ROS (Asahi et al. 2001; Jin & Zhen 2002; Christian et al. 2014). Limited visible light utilization and low electron–hole separation efficiency are the main disadvantages that hinder its further application.

In order to improve the photocatalytic activity of TiO₂, a series of feasible approaches mainly including doping and surface sensitization have been proposed (Ruohang et al. 2023; Zhifeng et al. 2023). Among them, the surface modification of TiO₂ by Fe (Ⅲ) and graphene have been found to be of particular interest because of their synergistic effect as electron co-catalysts. As a common transition metal, Fe (Ⅲ) has been proven to be a very effective co-catalyst. On one hand, Fe (Ⅲ) doped in TiO₂ lattice changes its crystalline shape, which improves the photocatalytic efficiency and promotes the utilization of visible light by shortening the band gap and broadening the light absorption range. On the other hand, Fe³⁺/Fe²⁺ (0.771 V) has more positive potential than the conduction band of TiO₂. It generates electron traps on the semiconductor surface. Meanwhile, Fe (Ⅱ) can reduce oxygen rapidly by a possible multi-electron reduction mechanism (4Fe²⁺ + O₂ + 4H⁺→ 4Fe³⁺ + 2H₂O or 4Fe²⁺ + O₂ + 2H₂O → 4Fe³⁺ + 2OH⁻), which eventuates the efficient transfer of photogenerated electrons with Fe (Ⅲ) surfaced modification of the catalyst (Li et al. 2008; Hemmati Borji et al. 2014; Yu et al. 2015; Shufang et al. 2023). However, the electron migration capability is restricted because the contact between the Fe (Ⅲ) co-catalyst and the catalyst is very limited, while graphene and its derivatives have a large specific surface area. The large number of contact sites between graphene derivatives and catalysts will further accelerate the electron migration process (Yu et al. 2015). In recent years, the field of two-dimensional material graphene nanocomposites has developed rapidly due to the highly conjugated hexagonal honeycomb structure (Zhao & Yang 2019), which has a wide range of applications in photocatalysis. Graphene has a large specific surface area, photochemical stability and adsorption capacity. The zero band gap of graphene allows photogenerated electrons to be excited at the Fermi energy level under visible light irradiation, thus broadening the light absorption range. The delocalized conjugated π electron system of graphene can be used as a mediator for photogenerated electron transfer to improve the separation efficiency of semiconductor photogenerated electron–hole pairs (Neto et al. 2009; Mak et al. 2010; Bo et al. 2018; Zhang et al. 2019; Shufang et al. 2023). It has been widely demonstrated that the surface modification of graphene derivatives greatly improves the photocatalytic performance of various photocatalysts. In numerous studies by scholars, however, rGO is often used as a co-catalyst, because the synthesis temperature of graphene-based photocatalysts is often higher than the thermal reduction temperature of graphene oxide (GO) in numerous preparation methods (Shen et al. 2011). In fact, the hydrophilic capacity of GO and rGO differ greatly due to the different number of oxygen-containing functional groups, which bring different degrees of influence on the process of assisting photocatalysts to catalyze organic compounds. The effect of different oxidation degrees of graphene derivatives on photocatalysis has not been mentioned.

Here, graphene derivatives and Fe-modified titanium dioxide with different oxidation degrees are successfully prepared by a simple one-step solvothermal synthesis method, where titanium dioxide nanoparticles and GO are used as precursors. The high-temperature and high-pressure conditions of the solvothermal method not only combine GO with TiO₂ and Fe (Ⅲ) easily but also have the advantage of being industrialized. The varying exothermic degradation of oxygen-containing groups in GO occurs under 150–300 °C (Yap et al. 2023). Thus, various types of graphene oxygenated derivatives can be prepared by thermal reduction with varying temperatures. The prepared catalysts showed significantly better performance than TiO₂ in experiments for the degradation of rhodamine B (RhB) and sulfamethadiazole (SMX). More importantly, with the increase of oxygen-containing functional groups, it was found that adsorption and sensitized photolysis have a non-negligible role in promoting the photocatalytic performance of the materials. In this paper, the relationship between the sensitized photolysis and the synergistic effect of graphene and Fe (Ⅲ) co-catalysts will be discussed in depth on the basis of existing studies.

Nano titanium dioxide, Fe(NO₃)₃·9H₂O, polyvinyl pyrrolidone (M ≈ 8,000, K = 16 ∼ 18) and absolute ethanol were purchased from Aladdin. Multilayer GO was obtained from Shenzhen, China. Rhodamine B (RhB) and sulfamethoxazole (SMX) as degradation substrates were purchased from Macklin. A hydrochloric acid solution was used to regulate pH. All the chemicals were analytical grade and all the solutions were made using deionized water.

During the synthetic process, TiO₂ was used as a precursor to prepare graphene-based photocatalytic by one-step solvothermal method. 0.5000 g GO, 0.5000 g TiO₂, 2.0 g polyvinyl pyrrolidone and 5.0 mL Fe³⁺ solution (100 mg/L, pH = 2) were dispersed in 80 mL ethanol/water (1:3) solution by using ultrasonic (360 W, 40 kHz) for 30 min. The gray suspension was synthesized by polytetrafluoroethylene (PTFE)-lined high-pressure reactor in the container at 105 °C for 24 h. The product was washed with deionized water, filtered and dried at 60 °C, and the GO and Fe (Ⅲ) modified TiO₂ labeled as GOFT-105 was prepared. the catalysts GOFT-150, rGOFT-180, rGOFT-210 and rGOFT-270 were obtained by calcining GOFT-105 at 150, 180, 210 and 270 °C for 1 h, respectively. The abbreviations ‘F’ and ‘T’ are defined as Fe (Ⅲ) and TiO₂ load on the substrate GO or reduced graphene oxide (rGO).

Scanning electron microscopy (SEM) images of the detail morphology of materials were captured by a microscope (MIRA LMS, TESCAN, Czech). The Fourier transform infrared (FTIR) spectra of the samples with wavelengths ranging from 500 to 4,000 cm⁻¹ for analyzing the information on oxygen-containing functional groups were obtained by Plastic Analyzer (Shimadzu, Japan). The thermogravimetric data of the catalyst were obtained by thermogravimetric analysis (TG, SDT Q600, America). The information on the structural integrity of sp² in graphene derivative was obtained by Raman spectroscopy (alpha300R, WITec, Germany) by using a 532 nm, 6.0 mW laser. An X-ray photoelectron spectroscopy (XPS, Nexsa, Thermo Scientific, America) was applied to study the chemical valences and the oxidation degree of graphene derivative. UV-vis diffuse reflectance spectra ranging from 200 to 800 nm for calculating the band gap energy of catalysts were taken from a UV-vis spectrophotometer (PE Lambda 950, PERKINELMER, America). The electron paramagnetic resonance (EPR) spectrum of the material under ultraviolet light was obtained by the EPR test (Bruker EMX PLUS, Germany).

Thermogravimetric analysis of GOFT-105 in a simulated preparation environment reveals two peaks in the first derivative of the thermogravimetric curve (Figure 3(c)). The mass loss observed between 60 and 130 °C can be attributed to the evaporation of molecular water between GO layers, while that observed between 170 and 250 °C is mainly due to the pyrolysis of oxygen-containing functional groups, resulting in the conversion of GO into rGO (Yap et al. 2023). As the calcination temperature increases, the number of oxygen-containing functional groups on the surface of each sample's graphene derivatives gradually decreases, which is consistent with the results obtained from infrared spectroscopy.

The Raman spectra (Figure 4(e)) exhibited five characteristic peaks at 150, 201, 391, 510 and 632 cm⁻¹, which were assigned to the E_g, E_g, B_1g, A_1g + B_1g and E_g modes of the anatase phase titanium dioxide (Gupta et al. 2010). The inset of the figure displayed the Raman spectra of GOFT-105 and rGOFT-180, which showed the characteristic D band at 1,342 cm^–1 and G band at 1,591 cm^–1. It is well known that the relative intensity of the D band and the G band is indicative of the proportions of sp³ C atom and sp² C atom in graphene (Yan et al. 2022). The thermal decomposition of oxygen-containing functional groups converts GO into rGO, resulting in the restoration of sp² domains. However, both samples exhibit similar ratios (Figure 4(f)), indicating the occurrence of carbon vacancies (V_C) formation, which introduces additional sp³ defects (Shen et al. 2013).

Additionally, an increase in oxidation degree led to an increase in the photocatalytic reaction rate constant. Therefore, oxygen-containing functional groups on the surface of graphene derivatives play a crucial role. On one hand, this is because these functional groups aid in the connection between TiO₂ and graphene derivatives. The π–d electron coupling facilitates the rapid transport of photogenerated electrons between graphene derivatives and TiO₂, effectively inhibiting the recombination of photogenerated electron-hole pairs in TiO₂, resulting in a high-performance photocatalyst. On the other hand, an increase in oxygen-containing functional groups enhances the material's hydrophilicity, improving the chemical adsorption of pollutants in water and increasing the bonding ability with various pollutant molecules. Furthermore, graphene derivatives act as photosensitizers to sensitize photolysis, with n-type semiconductor GO exhibiting a more potent sensitization effect on pollutants than p-type semiconductor rGO (Figure 8). This sensitization effect further improves the photocatalytic efficiency of graphene derivatives.

Through a comprehensive analysis of morphological and chemical characterization results, it is evident that the GO precursor was effectively dispersed using polyvinyl pyrrolidone (PVP) dispersant, and titanium dioxide catalysts modified with graphene derivatives containing Fe (III) and varying degrees of oxidation were successfully prepared. The formation of p–n or n–n heterojunctions between graphene derivatives and TiO₂ leads to alterations in the catalyst's band structure, resulting in a narrower band gap and an expanded light absorption range for the material. This study demonstrates that photocatalysts modified with graphene derivatives exhibiting higher oxidation levels exhibit superior photocatalytic performance. The synergistic interplay of electron transport between these graphene derivatives and Fe (III), as well as the sensitization photocatalytic effect of the graphene derivatives themselves, collectively contribute to the heightened efficiency of photocatalysis. Additionally, the synergistic impact of graphene derivatives and Fe (III) dual co-catalysts was further amplified.

The authors gratefully acknowledge the financial support provided by Hainan Province Key Scientific Research Project (ZDYF2021GXJS026); National Natural Science Foundation of China (Grant No. 52360002). We also appreciate the editors' valuable comments very much, which are helpful to improve the quality of our present study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.