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Article

Nitrogen and Sulfur Co-Doped Graphene Quantum Dots Anchored TiO2 Nanocomposites for Enhanced Photocatalytic Activity

by
Jishu Rawal
1,
Urooj Kamran
1,2,
Mira Park
3,4,
Bishweshwar Pant
3,4,* and
Soo-Jin Park
1,*
1
Department of Chemistry, Inha University, 100 Inharo, Incheon 22212, Korea
2
Department of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 17104, Korea
3
Carbon Composite Energy Nanomaterials Research Center, Woosuk University, Wanju, Chonbuk 55338, Korea
4
Woosuk Institute of Smart Convergence Life Care (WSCLC), Woosuk University, Wanju, Chonbuk 55338, Korea
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(5), 548; https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050548
Submission received: 10 April 2022 / Revised: 10 May 2022 / Accepted: 11 May 2022 / Published: 17 May 2022
(This article belongs to the Special Issue Catalytic Processes for Water and Wastewater Treatment)
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Versions Notes

Abstract

:
Herein, nitrogen (N) and sulfur (S) co-doped graphene quantum dots (GQDs) using different one-dimensional (1-D) carbon nanomaterials as precursors were synthesized, followed by heterojunction formation with TiO2. GQDs exhibit unlike physiochemical properties due to the disproportionate ratio of N and S heteroatoms and dissimilar reaction parameters. Tailored type-II band gap (Eg) alignment was formed with narrowed Eg value that improves photogenerated electron transfer due to π-conjugation. GQDs-TiO2 nanocomposites exhibit remarkably high methylene blue (MB) degradation up to 99.78% with 2.3–3 times elevated rate constants as compared with TiO2. CNF-GQDs-TiO2 demonstrates the fastest MB degradation (60 min) due to the synergistic effect of nitrogen and sulfur doping, and is considered the most stable photocatalyst among prepared nanocomposites as tested up to three cyclic runs. Whereas, C–O–Ti bonds were not only responsible for nanocomposites strengthening but also provide a charge transfer pathway. Moreover, charge transport behavior, generation of active species, and reaction mechanism were scrutinized via free-radical scavenger analysis.

1. Introduction

Environmental catastrophes and energy crises have become the main challenges in recent decades because of their severe social and economic impact on humankind [1,2]. Heterogeneous photocatalysis has recently attracted enormous attention because of its green attributes and efficient applicability. Due to their favorable properties such as suitable bandgaps, good chemical stability, and strong oxidizing ability, semiconductor photocatalysts [3,4,5] (e.g., ZnO, WO3, CdS, CdSe, PbS, TiO2, and SnO2) and some biocomposites [6,7] have been extensively used to decompose organic pollutants [8]. Among the semiconductor photocatalysts, titanium dioxide (TiO2) appears to be a highly efficient, stable, nontoxic, and inexpensive photocatalyst [9,10]. TiO2 containing 80% anatase and 20% rutile, named Degussa P25 TiO2, is widely known to exhibit high performance because the mixed-phase material exhibits a slower recombination rate, higher photo efficiency, and lower light activation energy as compared with pure-phase anatase or rutile [11,12]. Despite its wide range of applications, good performance, and high stability, the use of TiO2 is restricted by its narrow ultraviolet light range response resulting from the intrinsic wide energy bandgap of the anatase phase [13] and its low quantum efficiency due to the high recombination rate of photogenerated charge carriers in the rutile phase [14].
Various methods have been used to further expand the optical UV-vis absorption region of TiO2 and to enhance the efficient light-induced charge separation. Numerous approaches to modifying TiO2 via doping [15,16], semiconductor coupling [17], composite formation with carbon nanomaterials [18,19,20,21], and chemical treatment [22] have been explored to overcome the aforementioned shortcomings [23]. The best option reported thus far for spanning the UV-vis spectrum is combining TiO2 with typical narrow-bandgap quantum dots (QDs) of different semiconductors and carbon quantum dots (CQDs) [24,25,26,27,28]. To establish a proper channel among the QDs and TiO2 for effective induced charge separation, heterojunction formation is critical. Despite the incorporation of QDs, photocatalytic activity has not been remarkably improved because of the high instability, rich surface traps, and inevitable photo-oxidization of QDs [29]. Moreover, the use of heavy-metal ions (e.g., Zn, Cu, Cd, and Pb) which are toxic, raises concerns about environmental and human safety [30]. Therefore, the development of an eco-friendly, highly efficient, and recyclable catalyst that can efficiently harvest solar energy and form heterojunctions under suitable conditions is a formidable challenge.
Recently, CQDs have emerged as materials that introduce new possibilities for the development of heterojunctions because of their novel electronic and optical properties with improved photocatalytic activity. As a unique class of CQDs, graphene quantum dots (GQDs) are a nontoxic metal-free zero-dimensional (0D) material usually composed of one or a few layers of graphene and with diameters in the nanometer range [31]. GQDs have a wide range of applications in energy conversion, bio-imaging, drug delivery, sensing, solar energy conversion [32], and light-emitting diodes [33]. Moreover, they exhibit strong fluorescence and semiconducting properties that arise from their pronounced edge effects, quantum confinement, and functionalities [34]. The substantial UV-vis absorption capability of GQDs is provided by the π-plasmon absorption effect [35]. GQDs possess a tunable wide energy bandgap that can be altered by changing the size and functionalities on edge sites [36]. In addition to these merits, pure GQDs have a large exciton binding energy (0.8 eV calculated for 2 nm GQDs), which enhances the recombination rate, resulting in poor catalytic behavior [37]. Doping of different atoms such as nitrogen (N) and sulfur (S) into the lattice of GQDs has been reported to be the most effective approach to modifying the electronic, optical, and transport properties of GQDs to achieve fast electron transfer [38,39]. Shen et al. have reported that coupling of N- and S-GQDs with other materials results in the formation of an advanced hybrid photocatalyst that promotes photocatalysis [40]. Previous reports have indicated that TiO2 nanoribbons and carbon nanotube composites exhibit faster photocatalytic degradation of methylene blue (MB) than TiO2 alone [41]. Graphene and titania interfaces as Schottky-like nanocomposites for efficient photocatalysis have been reported [42]. Zhu et al. suggested a self-assembly method for preparing WSe2-graphene-TiO2 for dye degradation and hydrogen production [43]. C–O–Ti bond formation has been reported in TiO2-graphene composites as a result of free-electron interactions [44]. However, to the best of our knowledge, a pathway for different synthesis routes of doped GQDs and forming TiO2 heterojunctions has not yet been addressed. Thus, we propose an essential mechanism, the simplest method of doping, and oxygen functionalities’ role on edge states of GQDs via acid treatment and the formation of GQDs nanocomposite with TiO2 as a promising stable photocatalyst.
Here, a novel top-down synthesis of N, S co-doped GQDs via simple cutting methods using different one-dimensional carbon nanomaterials (carbon nanotubes (CNT), reflux carbon nanotubes (RCNT) [45,46], and carbon nanofibers (CNF) [47,48]) as precursors is reported. In the present study, we elucidated the comparative behavior of N, S co-doped GQDs and their respective GQDs-TiO2 nanocomposites, named as (CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO2) as a highly robust visible-light sensitizer with a tunable bandgap, optical, and capacitive parameters. As-prepared GQDs were used to sensitize a TiO2 photocatalyst for degradation of MB as a model pollutant dye. The mechanism of heterojunction formation between the GQDs and TiO2 for charge separation at the interface was proposed through MB degradation and cyclic runs of samples to express the photocatalytic properties of nanocomposites. To elucidate the role of free radicals in the photodegradation of MB, scavenger tests on a complete set of as-prepared nanocomposites were also performed. The degradation pathway, interfacial charge transfer mechanism, and significance of the metal-free carbon nanomaterial-based photocatalyst are discussed.

2. Results and Discussion

2.1. Physiochemical Properties

Figure 1 shows the morphology of GQDs and GQDs-TiO2 nanocomposites via images captured by FE-TEM and FE-SEM instruments. FE-SEM and FE-TEM image of the CNT-GQDs shows ring formation due to strong sp2 bonding of carbon atoms, indicated by a white hexagonal outline of the shape of GQDs exhibit an average diameter of 8 nm. Similarly, FE-TEM images for the RCNT-GQDs and CNF-GQDs shows average diameters of 15 nm and 100 nm, respectively. Particle size of CNT-GQDs varied from 3 to 8 nm and average particle size is approximately 5 nm. Whereas, RCNT-GQDs size range lies between 8 to 20 nm with 13 nm average size and CNF-GQDs size is between 70–120 nm with average size of 90 nm. Agglomeration and overlap phenomena are observed because the high surface-area-to-volume ratio favors large surface energy and instability in the GQDs, especially in the case of CNT-GQDs and RCNT-GQDs [49]. To prepare a nanocomposite in which these effects of GQDs are diminished, we strongly sonicated GQDs and stirred them with TiO2 to balance the surface energy and stability of the nanocomposites, which also promotes the formation of heterojunctions between the GQDs and TiO2. Well distributed and firmly decorated GQDs are observed on TiO2 surface. CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO2 exhibit lattice spacings of 0.2873 nm, 0.3368 nm, and 0.4275 nm corresponding to the (004), (112) and (101) planes, respectively. The FE-TEM images of nanocomposites demonstrate that the GQDs are amorphous phases and the TiO2 is crystalline phases; these phases are clearly seen, where the GQDs (amorphous) lie on the TiO2 (crystalline) lattice to form the well-structured nanocomposites.
Pristine phases of the crystal and structural behavior of as-prepared GQDs and GQDs-TiO2 nanocomposites were characterized by XRD on the basis of Bragg’s law, 2dsinθ = nλ (n = 1, λ = 0.154 nm). Detailed XRD patterns of GQDs and GQD-TiO2 nanocomposites are shown in Figure 2a. Intense peaks of CNFs, CNTs at 2θ ≈ 25° and 27.95° are predominant GQDs data (JCPDS-ICDD file No. 75-1621), these peaks represent the (002) and (101) planes of sp2-hybridized hexagonal carbon atoms, respectively [50]. Whereas, crystalline phases of TiO2 represented by 2θ ≈ 25° and 48° corresponding to (110) and (200) planes, respectively and indicating good agreement with the standard spectrum (JCPDS-ICDD file No. 21-1272. Traces of S in the CNF-GQDs with 40% crystallinity and the RSWCNT-GQDs with 22% crystallinity are confirmed by the (220) plane with an interlayer spacing of 0.39 nm. Planes (110) represent oxygen abundance in GQDs. Peaks at 2θ ≈ 24.9° and 27.2° correspond to the anatase phase of TiO2 and the graphitic structure of the GQD-TiO2 nanocomposite, with d-spacings of 0.35 nm and 0.32 nm, respectively [51]. An intense (101) peak represents a high weight percentage of TiO2 in the prepared nanocomposite as compared with the GQDs. Prominent peaks for GQDs in the XRD of the GQDs-TiO2 were absent because of the lesser proportion of GQDs and the high surface coverage by TiO2. We concluded that the GQDs’ peaks are obscured by the dominant TiO2 peaks. Similarly, peaks related to the presence of S and N in the GQDs are also diminished in the patterns of the nanocomposite samples. Additional peaks observed at 2θ ≈ 33.5° and 37.4° denote strong C–O–Ti bonding [52]. Because of interface formation and proper dispersion of the GQDs over the TiO2 surface, the crystallinity of samples appears to increase to approximately 37% for the CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and 47% for the CNF-GQDs-TiO2 but cannot be increased further because of the abundance of O and other impurities.
TGA was carried out on GQDs-TiO2 nanocomposites to clarify the structural thermal stability, bonding with GQDs, and the significance of the nanocomposite properties. Thermograph temperature in the range of 50–150 °C, 150–350 °C, 350–550 °C, and above 550 °C denote physio-adsorbed water, dehydration–decarboxylation of bonded water and functional groups, oxidation of amorphous groups, and oxidation of graphitic structures, respectively [53]. Figure 2b shows the TGA curve of the GQDs-TiO2 nanocomposites. Interestingly, these thermographs show nanocomposite formations that denote low weight loss and indicate strong bonding of the GQDs onto the TiO2 surface. The CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO2 demonstrate 6.81%, 20.81%, and 2.09% mass loss, respectively. These results undoubtedly predict the formation of thermally stable structures of nanocomposites. Minor weight loss in CNF-GQDs-TiO2 indicates strongest covalent bonding between CNF-GQDs and TiO2. Specifically, it indicates chemisorption in the investigated temperature range, rearrangement of lattices when in-plane (C–O) functional groups were transferred to oxygen-deficient vacancies in the TiO2 plane, and the formation of stable C–O–Ti bonds. This explanation reflects the roles of functional groups in GQDs and oxygen vacancies in TiO2 in determining the configuration of nanocomposites.
The heterostructure interfacial chemical bonding and interaction in GQDs-TiO2 samples were investigated by XPS analysis. Peaks appeared at binding energies of 531, 284, 406, and 168 eV in the spectra of the GQDs, corresponding to O 1s, C 1s, N 1s, and S 2p, respectively. Identical peaks with a slight change in intensity were observed in the spectrum of the GQDs-TiO2 nanocomposites, along with an additional peak of Ti 2p at 458 eV, as shown in Figure 3a–c. High-resolution C 1s spectra were fitted with two Gaussian peaks with maxima at 284.28 and 287.48 eV in the spectrum of CNT-GQDs-TiO2 as shown in Figure 3d, signifying C=C bonds of a graphitic honeycomb structure and other show a Ti–O–C bond peak associated with titanium carbonate and strong C=C peak of GQD, respectively. Deconvoluted C 1s peaks for the spectrum of the RCNT-GQDs-TiO2 are attributed to titanium carbide and C=C as mentioned in Figure 3e. These results strongly evidence the formation of C–Ti bonds and C–O–Ti bonds. Ti–C bonds exhibit greater stability and better interfacing for RCNT-GQDs-TiO2 nanocomposite. However, the C 1s peak fitting in Figure 3f results for CNF-GQDs-TiO2 that are identical to RCNT-GQDs-TiO2, which indicates C–Ti and C–O–Ti bond formation. C 1s range of GQDs-TiO2, C=C bond formation indicates that the synthesis of nanocomposites using high sonication of GQDs and extended stirring times at a certain temperature led to a well-decorated GQDs-TiO2 interface with strong bonding between Ti–O and Ti–O–C.
Meanwhile, a core-level study of the high-resolution O 1s spectra of the GQDs-TiO2 nanocomposites as shown in Figure 3g–i revealed relevant peaks centered at 529.18, 530.6, and 532.08 eV, which are attributed to Ti–O–Ti, C=O, and Ti–O–C interfacial bonds, respectively [54]. The Ti–O–Ti bond peaks are prominent because the TiO2 content of nanocomposites was much greater than their GQD and O2− contents. After analyzing the spectroscopic data, it can be concluded that reaction parameters such as temperature, chemical treatment, and time cause changes in the ratio of N, S- heteroatom doping extent, oxygen functionalities, and physiochemical properties of as-prepared GQDs that affects overall bonding strength of the nanocomposites. N and S heteroatom dopant traces can be clearly seen in CNF-GQDs-TiO2 XPS data, whereas absence or negligence of S doping in CNT-GQDs-TiO2 and N doping in RCNT-GQDs-TiO2 were observed. Due to which it can be stated that CNF-GQDs-TiO2 can experience the synergistic effect that leads to the better performance of nanocomposite because of strong absorption capability and photosensitizer ability, as discussed in the next sections. These XPS results are fully consistent with the XRD analyses.

2.2. Optical Study and Photoelectrochemical Study

Figure 4a shows the UV-Vis absorption spectra for GQDs-TiO2 nanocomposites. Figure 4c inset represents a UV-Vis plot of the GQD absorption spectra. All the GQDs show characteristic π–π* transitions below 300 nm within the sp2 C=C hybridized framework of carbon domains [55]. The absorption tail extending to ~350 nm in the visible region was assigned to the n → π* transitions of nonbonding electrons of O, N, and S atoms [56]. Greater absorption, specifically in the case of CNF-GQDs, was due to these extra electrons caused by N, S doping. PL spectra of as-prepared GQDs samples tested in powder form by excitation of a He–Cd laser at 325 nm. For better understanding of GQDs band gap, Tauc plot of respective GQDs was shown in Figure 4c. Bandgap corresponding to CNT-GQDs, RCNT-GQDs, and CNF-GQDs as obtained by the formulation, were 3.3, 2.2, and 2.8 eV, respectively. for composites tailored band gap, Tauc plot of respective GQDs-TiO2 nanocomposites was shown in Figure 4b. The bandgap of the samples can be easily determined by the intersection point of the energy axis with the slope of the curve in graph. Bandgap corresponding to CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO2 nanocomposites as obtained by the formulation, were 2.21, 2.23, and 2.29 eV, respectively. Ti–O–C interfacial bond formation and intimate interaction in nanocomposites could decrease the bandgap value of nanocomposites. Interaction between TiO2 and GQDs was because of chemical coupling and influence of C atoms on the molecular orbitals of TiO2, as a result, the bottom band of the nanocomposite lowers itself as compared with that of TiO2. Incorporation of GQDs will enhance the lifetime of electron-hole pairs and result in faster photogenerated electron transfer in TiO2 due to π-conjugation of the GQDs.
Figure 4d depicts the photoluminescence (PL) spectra of three GQDs, CNF-GQDs clearly show the strongest PL emission in the orange-red region of the visible spectra, with a bandgap energy of 2.01 eV; similarly, the RCNT-GQDs emit in the yellow-orange region, with an equivalent range of photon energy of 2.09 eV but with lesser intensity than the emission of the CNF-GQDs. By contrast, the CNT-GQDs exhibit totally different PL emission at 408.75 nm with a 3.03 eV bandgap energy in the violet region but with the same intensity as the emission of the RCNT-GQDs. PL phenomena were observed in GQDs when the excitation wavelength was comparable to the transition wavelength. Long wavelengths were absorbed and emitted by C=S and C=N chromophores, which resulted in red-yellow emission; by contrast, n → π* excitation of C=O chromophores were assigned to short wavelengths with violet and blue emission [57].
Table 1 denotes the atomic % of elements clearly shows that the CNF-GQDs and RCNT-GQDs were rich in S-group content, whereas the CNT GQDs were rich N-group content. Because of the abundance of S groups and some traces of N in conjugated molecules of CNF-GQDs, the n → π* transition of the C=S group dominates with overlapping of N and O groups in CF-GQDs [58], which led to the synergistic effect of these three groups in lattice formation, as confirmed from the XRD and XPS analysis. Hence, orange–red emission was observed upon excitation of CNF-GQDs. The proposed mechanism responsible for the CNF-GQDs strong PL emission is S π* orbital insertion between C n and N π* orbitals and the formation of a low-electronegative S state. Remarkable difference in PL intensities of GQDs strongly justifies the CNF-GQDs efficiency, these property variations experienced by GQDs was based on synthesis method. On comparing the key parameters of synthesis process, we conclude that simple oxidative cutting with comparatively low temperature range (110–120 °C) using CNF is more feasible and effective for GQDs synthesis because refluxing process and followed hydrothermal reaction at high temperature range proved to be harsh on the surface and also it distorts the edges and structure of the GQDs which hinders the oxygen vacancies, graphitic properties and anchoring properties those can be inherited by GQDs from CNTs. This proves the CNF-GQDs with higher PL intensity as a potential contender for photocatalytic process.
Separation of photogenerated charge carriers of as-synthesized GQDs-TiO2 nanocomposites can be investigated by photoelectrochemical study at room temperature using an Ivium electrochemical workstation under solar simulator. The electrolyte used through the experiments was 2 M KOH aqueous solution. A typical three-electrode setup consisting of a working electrode, a Pt coil as a counter electrode, and a saturated calomel electrode (SCE) as the reference electrode was used. Fluorine doped tin oxide coated (FTO) glass electrode was prepared by spin coating technique at 20,000 rpm for 10 s in three runs with exposed area of 1 × 1 cm2. Coating slurry was prepared by dispersing 5 mg of photocatalyst in 1 mL ethanol containing 10 µL Nafion. Electrodes were oven dried for 3 h and calcinated at 200 °C in nitrogen atmosphere for 2 h to augment adhesion.
Photoelectrochemical analysis of GQDs-TiO2 nanocomposites as shown in Figure 4e,f transient photocurrent and EIS graphs respectively. While referring to transient current curves, it can be clearly observed that TiO2 has negligible current response as compared to prepared nanocomposites. CNT-GQDs-TiO2 and RCNT-GQDs-TiO2 electrodes exhibit high photocurrent density, well stable and longstanding photoresponse under visible light projection. CNF-GQDs-TiO2 electrode produces much larger photocurrent density than other electrodes, this indicates that CNF-GQDs-TiO2 possess highest charge transfer rate and improved photogenerated charge carriers’ separation efficiency. Meanwhile, more effective way to reveal electron transfer efficiency of as prepared sample electrodes is through EIS (electrochemical impedance spectroscopy). In Figure 4f, clear comparative analysis can be done among the nanocomposites based on EIS curve radius. Shorter the radius implies better mobility and well separated photogenerated charge carriers. CNF-GQDs-TiO2 demonstrates the smallest radius curve among all, results in better charge transfer at electrode interface that enhance the separation of carriers. These electrochemical results prove that the GQDs-TiO2 heterojunctions specially CNF-GQDs-TiO2 serves as electron harvester for improved degradation performance and separation of charge carriers, this phenomenon can be thoroughly observed in next sections.

2.3. Visible-Light Photocatalysis and Mechanism Studies

Figure 5 presents the visible-light photocatalytic degradation kinetics for different nanocomposites samples toward the degradation of MB as a model pollutant dye. Figure 5a shows the photocatalytic degradation behavior of different compositions of GQDs-TiO2 nanocomposites. MB degradation first-order-rate-constant behavior was fitted using the Langmuir-Hinshelwood rate formalism, which represents the kinetics of photocatalytic reaction in different catalysts and was expressed as ln(C/C0) = ka, where ka is the pseudo-first-order-reaction rate constant and C and C0 denote the MB concentrations after and before irradiation time t, respectively [59]. The rate constant for different catalysts was obtained from the slope of the straight-line plot shown in Figure 5b. All the as-prepared nanocomposites exhibit an excellent degradation percentage, as shown in Figure 5e. The CNT-GQDs-TiO2 sample showed a dramatic increase in degradation percentage to 99.78% in 90 min; similar results were obtained with the RCNT-GQDs-TiO2 and CNF-GQDs-TiO2 nanocomposites. Meanwhile, CNF-GQDs-TiO2 exhibited the fastest degradation kinetics, with 99.39% degradation in 60 min of light irradiation. Graphical representation of the rate constants for different nanocomposites is shown in Figure 5f. These data for samples show that GQDs-TiO2 exhibit an approximately 2.3 to 3-fold higher degradation rate constant than the commercial P25 photocatalyst. The highest rate constant recorded for the CNT-GQDs-TiO2 was 6.75 × 10−2 min−1, whereas that of the TiO2 was 2.48 × 10−2 min−1. The characteristic absorption peak of MB rapidly decreased in intensity in the experiments with all of the prepared nanocomposites; also, this decrease to a certain level was also obtained with TiO2 (79% degradation). As-prepared GQDs show strong optical absorption capabilities in a wide light spectrum but with poor photocatalytic properties ascribed to their corresponding large binding energy. The key role of a stable heterojunction interface is evident when the GQDs and TiO2 are blended with each other to form an optimum nanoscale composition containing well-dispersed soluble GQDs on the surface of TiO2 nanoparticles using a simple sonication and stirring method. The content of GQDs cannot be increased further because of their strong tendency to aggregate on the TiO2 surface, which retards the photocatalytic process remarkably. The construction of well-dispersed nanoparticle heterojunctions is critical and can be effectively achieved through bandgap narrowing, as shown by the Tauc plot previously. By contrast, C–O–Ti bond formation ascribed to chemisorption of GQDs on the TiO2 surface plays a strong role in strengthening the interaction between nanocomposites [60]. This strong coupling of GQDs with TiO2 forms the hot-carrier injection pathway, whereas N and S doping causes substantial bandgap reduction. Because of this heterojunction formation, efficient charge separation occurred on the surface, results in a higher photodegradation rate. Comparatively slight variation among the degradation rates of the samples was observed because of the difference in the bandgap value of nanocomposites, where the smallest bandgap value was reported for CNT-GQDs-TiO2, which exhibited the highest degradation rate of 99.78%, confirming that the bandgap is an essential factor in dye degradation. CNT-GQDs-TiO2 exhibits slightly higher degradation from CNF-GQDs-TiO2 because of its smaller bandgap as it can be seen in Tauc plot, but based on TGA analysis and repetitive cyclic runs, it is not as stable as CNF-GQDs-TiO2. Therefore, CNF-GQDs-TiO2 is considered as the best suited composite among all due to strong covalent bonding between CNF-GQDs and TiO2 that indicates stable C–O–Ti bonds formation. Also, CNF-GQDs experienced the N and S dopant synergistic effect that results in better performance of nanocomposite with strong absorption capability and photosensitizer ability.
pH effect is an important parameter for the dye degradation. MB degradation was tested in the pH range between 3 to 10, pH values were adjusted using H2SO4 and 0.1 mol/L NaOH. Experimental condition was identical as mentioned in the experimental section. Figure 5c,d shows that photodegradation rate was improved by increasing the pH values. At pH 3, degradation efficiency was dropped to 84%. At pH 10 and pH 7, degradation efficiency is same ~99.23% but with the increase of pH value the process of degradation become fast. At pH 10, 99.23% degradation was achieved in 45 min. This shows that alkaline conditions increase the reaction rate due to the abundance of hydroxyl ions that helps in increasing the degradation reaction. Whereas at low pH, reaction rate reduced due to excess of H+ ions [61]. Hence, it can be stated that alkaline medium is favorable for MB degradation using CNF-GQDs-TiO2 nanocomposites.
GQDs-TiO2 composites were studied by many groups with TiO2 nanostructures (nanotubes, nanoparticles, and nanosheets) for the degradation of organic dyes to find the best suited one as shown in Table 2. Various chemical composition and formation methods were used to synthesize GQDs and composites with desired physiochemical properties that results in fastest, stable, and complete organic dye degradation. GQDs, N-GQDs, and N, S-GQDs based TiO2 composites were studied previously but results in slow and lesser degradation efficiency [62,63,64]. Whereas, Tian et al. synthesized the N, S-GQDs based reduced graphene oxide (rGO)-TiO2 nanotubes (NTs) using hydrothermal and stirring process for degradation methyl orange (MO). This results in MO degradation in 4 h upto 90% [65]. This work, have presented the three types of GQDs formation with different N, S doping amount due to the different precursors and process of synthesis. Best results were obtained by TiO2 composites formed using CNF-GQDs due to strong covalent bonding between CNF-GQDs and TiO2 defining its stability, also it indicates chemisorption and rearrangement of lattices when in-plane (C-O) functional groups were transferred to oxygen-deficient vacancies in the TiO2 plane to form stable C–O–Ti bonds. This reflects the role of functional groups in GQDs and oxygen vacancies in TiO2 for determining the configuration of nanocomposites. CNF-GQDs experienced the N and S dopant synergistic effect that leads to the better performance of nanocomposite with strong absorption capability and photosensitizer ability. Based on this, we concluded that CNF-GQDs-TiO2 has shown the fastest degradation (60 min) of methylene blue (MB) with highest efficiency of 99.23% as compared with the previously reported articles.
Another essential factor for photocatalysts is reusability and stability. The reusability of the nanocomposites was tested over three repeated cyclic runs of photocatalysis, as shown in Figure 6a. These stability results provide important facts about catalysts. After three cyclic runs, the CNT-GQDs-TiO2 demonstrated a 0.54% decrement. By contrast, the RCNT-GQDs-TiO2 exhibited only 55.5% degradation, with a 44.11% decrement compared with its initial degradation results. The CNF-GQDs-TiO2 catalyst was the most stable for three repeated cyclic runs. Reason behind CNF-GQDs reusability and long-term stability was N, S doping, thermally stable structure and oxygen mediated vacancies which was responsible for C–O–Ti bonding. Poor stability of the RCNT-GQDs-TiO2 and strong bonding of the CNF-GQDs-TiO2 were confirmed by the TGA results.
To investigate the role of the electron, transfer mechanism and the reaction process in a typical photocatalysis system, active species such as holes (h+), hydroxyl radicals (•OH), and superoxide radicals (O2•−) were studied. Solutions of IPA, BQ, and EDTA scavengers were used to study the roles of •OH, O2•−, and h+, respectively [72]. To investigate and explore their contribution in visible-light photocatalysis, a series of scavenger tests were performed on CNT-GQDs-TiO2 nanocomposites. The dosage addition of scavengers inhibits the degradation efficiency of MB to various degrees, which refers to the involvement of all oxidative species in the photodegradation system. Changes in relative concentration of MB in different scavengers for all of the as-prepared nanocomposite systems are shown in Figure 6b–d. In the case of IPA addition, the MB was degraded by 61.74% for CNT-GQDs-TiO2, implying the vital role of •OH radical, and RCNT-GQDs-TiO2 was quenched to 47.55% in EDTA, indicating the important of h+ species in the degradation process. When BQ was added to the CNF-GQDs-TiO2 system, 58.26% MB degradation was observed, indicating that the major reactive species involved in the process was O2◦-. Figure 6e shows a comparative analysis of the nanocomposite systems with and without scavenger addition and respective values can be seen in Table 3. All studies of samples show the highest quenching of MB degradation in different scavengers. These results indicate that •OH, O2•−, and h+ are the major reactive species in MB degradation for CNT-GQDs-TiO2, CNF-GQDs-TiO2, and RCNT-GQDs-TiO2, respectively.
Based on the aforementioned results, the degradation mechanism can occur through various steps, implying direct electron transfer. Figure 6f shows the proposed mechanism of free radicals involved in the photodegradation process. The phenomena that lead to the formation of the Schottky barrier between photoexcited electrons in GQDs and the TiO2 lattice are thermionic emission and quantum tunneling. Achieving thermionic emission was difficult because the complete experimental procedure was carried out at room temperature. Hence, the most likely factor behind electron transport in the GQD–TiO2 composite was quantum tunneling, which forms the heterojunction type II band alignment.
Degraded methylene blue (MB) results in some intermediate volatile end products of MB solution. When N-S heterocyclic compound of methylene blue was separated 2-Amino-5-dimethylamino-benzenesulfonic acid anion was formed. Degradation intermediate of 2-Amino-5-dimethylamino-benzenesulfonic acid was 4-Amino-benzenesulfonic acid and 2-Amino-5-hydroxy-benzenesulfonic acid. Oxidization of 4-Amino-benzenesulfonic acid results in 4-Nitro-benzenesulfonic acid anion. Products formed after hydroxyl radical attack on the N–S heterocyclic of MB molecules were Dimethyl-(4-nitro-phenyl)-amine and p-Dihydroxybenzene. Tiny molecular products of degradation correspond to succinic acid generated by open-loop benzene [73]. On the basis of these intermediate products, we can deduce the degradation pathway of MB. The photogenerated electrons in the TiO2 with the in situ excited electrons of GQDs reacts with free radicals. Hence, electrons from the GQDs transfer to the conduction band of TiO2 to trap the oxygen (O2) molecule to generate the O2•− radicals, and h+ react with an electron donor (water molecules or hydroxide ions) to yield oxidizing hydroxyl •OH radicals. These generated radicals of O2•− and •OH are the essential substances to decompose the dye molecules into CO2 and H2O. Thus, GQDs with high electron mobility are a perfect electron-transfer tank for enhanced electron–hole separation at the interface of TiO2. The high degradation of MB in several scavengers can be interpreted as the conversion of O2•− radical to •OH radical. The possible pathways for MB degradation are expressed by the following reactions:
TiO 2 − GQD + h υ   → TiO 2   e − − GQD + TiO 2   h + − GQD
TiO 2   e − − GQD → TiO 2 − GQD e −
GQD   e − + O 2 → GQD + O 2 • −
TiO 2   h + +   H 2 O → h + +   OH •
O 2 • − + OH • + MB → water +   CO 2

3. Materials and Experimental

3.1. Chemicals

Vapor-grown carbon nanofibers were purchased from Carbon Nano-material Technology. Co. Ltd., Pohang, Korea. Single-walled carbon nanotubes (SWCNTs (75%)) with an average diameter of 1.8 ± 0.4 nm and length greater than 5 µm were supplied by TuballTM-OCSiAl Pvt. Ltd., Incheon, Korea. TiO2 powder, sulfuric acid (99.99%), nitric acid (ACS reagent, 70%), methylene blue (MB), 2,3-dichloro-5,6-dicyano-p-benzoquinone (98%-BQ), ethylenediaminetetraacetic acid (EDTA, >98%), and isopropyl alcohol solution (70% in H2O–IPA) were purchased from Sigma-Aldrich Inc., Seoul, Korea. All reagents were of analytical grade and were used without further purification. Distilled water (18.2 MΩ·cm) was used throughout the experiments.

3.2. Synthesis of GQDs from CNT (CNT-GQDs)

A hydrothermal etching method was used to prepare oxygen-rich GQDs with some traces of N. A total of 300 mg of SWCNTs was refluxed with 150 mL of HNO3 for 12 h at 100 °C [49]. The suspension was then cooled and filtered to remove the supernatant. The collected residue was dispersed in 50 mL of DI water and heated hydrothermally in a Teflon-lined autoclave at 200 °C for 12 h. The same etching procedure (i.e., refluxing with HNO3, followed by filtering and hydrothermal heating) was subsequently repeated. After two cycles of the etching process, nitrogen-doped GQDs named as CNT-GQDs were obtained for further use.

3.3. Synthesis of GQDs from RCNT (RCNT-GQDs)

300 mg of SWCNTs was refluxed with H2SO4 and HNO3 (3:1) for 24 h at 120 °C. The mixture was then cooled and diluted with DI water (1:5) while maintaining the pH at ~7. Stepwise gradual heating (100 °C, 150 °C, and 200 °C) of the suspension was conducted at 4 h intervals to obtain sulfur-doped GQDs named as RCNT-GQDs. The as-prepared sample of GQDs was washed with DI water using the filtration technique.

3.4. Synthesis of GQDs from CNF (CNF-GQDs)

Oxygen-rich GQDs doped with nitrogen (N) and sulfur (S) atoms were synthesized using a simple oxidative cutting method. A total of 900 mg of CFs was treated with a mixture of conc. H2SO4 and HNO3 (3:1) under continuous stirring for 24 h at 120 °C. The mixture was diluted with DI water (1:5) after being cooled to room temperature. To maintain the acidity of the mixture, its pH was adjusted to ~7 using NaOH. Finally, a dialysis process of the end product was conducted in a dialysis bag for 3 days and the dialyzed solution was heated at 80 °C to obtain CNF-GQDs.

3.5. Synthesis of TiO2 and GQD Composite

Different as-prepared samples of GQDs (2 mg/mL) were dispersed in DI water to obtain a uniform suspension using the ultrasonication method. A total of 200 mg of TiO2 was added to 50 mL of GQD suspension and stirred at 60 °C for 10–12 h, until it dries completely and turned into powder. The GQDs-TiO2 nanocomposite was obtained in powder form. The nanocomposite samples were named CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO2. The synthesis scheme for respective GQDs is shown in Figure 7.

3.6. Characterization Techniques

The structure, crystal facets, and phase of the as-prepared samples were examined by powder X-ray diffraction (XRD, Bruker D2 PHASER, Gyeonggi-do, Korea). The morphological features, crystallinity, and chemical composition were studied by field-emission scanning electron microscopy (FE-SEM, Hitachi High-Tech Korea Co., Ltd., S-4300, Seoul, Korea), field-emission transmission electron microscopy (FE-TEM JEM-2100F/JEOL, Seoul, Korea), and energy-dispersive X-ray spectroscopy (EDX) composition analysis. The X-ray photoelectron spectroscopy data were also obtained (XPS, VG Scientific Co., ESCA LAB MK-II, Korea) using a monochromatic Al Kα X-ray beam with a current of 20 mA. Quantitative analysis of the solid organic elements present in the samples was performed with an elemental analyzer (Thermo Fisher Scientific, Inc. EA1112, Korea). The specific surface area measurements were performed using an adsorption analyzer (BEL BELSORP, Inc. Japan), and thermogravimetric analysis (TGA, NETZSCH, TG 209 F3, Germany) was carried out for the as-prepared samples at a heating rate of 10 °C/min and with N2 gas as a purge gas. Photoluminescence (PL) emission spectra were acquired under excitation at 325 nm (RAM Boss, Maple Dongwoo Optron). The UV-vis absorption was performed using a commercial UV-vis spectrophotometer (Scinco, S-3100, Seoul, Korea). Electrochemical analysis was carried out using an Ivium electrochemical analyzer.

3.7. Photodegradation Test

Photocatalytic studies of the GQDs-TiO2 nanocomposites were conducted using a solar simulator (SUN 2000, Abet Technologies, Inc. Gyeonggi-do, Korea) fitted with a 440 nm cutoff filter. The photocatalytic experiments were carried out in the presence of MB as an organic dye pollutant. A total of 20 mg of catalyst were dispersed in 50 mL of MB aqueous solution with an initial concentration of 10 mg/L. Prior to irradiation, the as-prepared solution was stirred continuously in the dark for 1 h at room temperature to achieve adsorption-desorption equilibrium. At 15 min irradiation intervals, a 5 mL aliquot was withdrawn and analyzed via UV-vis spectrophotometry with a maximum absorption peak at 665 nm.

3.8. Free-Radical Scavenger Test

A free-radical scavenger experiment was conducted to study the involvement of reactive species in the photodegradation of MB. Isopropyl alcohol (IPA) solution, BQ, and EDTA were used as scavengers to analyze the influence of hydroxyl radicals (•OH), superoxide radicals (O2•−), and holes (h+), respectively. In the scavenging experiments, 10 mg of a scavenger was mixed with the dye–catalyst solution (20 mg, 10 mg/L). Afterward, the same procedure mentioned in previous section was conducted under identical conditions.

4. Conclusions

A novel approach for the synthesis of N, S doped GQDs from 1-D carbon nanomaterial with high photocatalytic and photoelectrochemical response was suggested. The mechanism of interfacial charge transfer in GQDs-TiO2 heterojunction is elucidated through the possible C–O–Ti bonds formation. TGA and XPS analyses revealed that N and S doping with in-plane functional groups of GQDs were responsible for stable interface formation between GQDs and TiO2. Photoelectrochemical measurements provide insights into photocurrent and impedance characteristics of as-prepared GQDs-TiO2 nanocomposites. Remarkably high degradation percentages of MB under visible-light illumination were observed for CNT-GQDs-TiO2 (99.78%), RCNT-GQDs-TiO2 (99.39%), and CNF-GQDs-TiO2 (99.23%) nanocomposites. Rapid interfacial charge transfer from the GQDs to TiO2 is the key phenomenon for the enhanced photodegradation of MB. Consecutive degradation cyclic runs to test the highest efficiency and stability of nanocomposites result that the CNF-GQDs-TiO2 nanocomposites have the fastest photodegradation rate and proved to be the most stable among all. CNT-GQDs-TiO2, RCNT-GQDs-TiO2, and CNF-GQDs-TiO2 nanocomposites show different characteristic radicals •OH, h+, and O2•−, respectively, as revealed by radical quenching experiments. This study is considered to be a platform for nonmetallic GQDs and metal oxide semiconductor nanocomposites for green-environment, sustainable energy, and optoelectronic applications.

Author Contributions

J.R.: Conceptualization, Investigation, Methodology, Validation, Data curation, Writing—original draft, Writing—review and editing. U.K.: Conceptualization, Visualization, Formal analysis, Writing—review and editing. M.P.: Writing—review and editing. B.P.: Writing—review and editing. S.-J.P.: Supervision, Funding acquisition, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Korea Electric Power Corporation (Grant number: R21XO01-5). This work was also supported by Nano-Convergence Foundation (www.nanotech2020.org (accessed on 1 May 2021)) funded by the Ministry of Science and ICT (MSIT, Korea) & the Ministry of Trade, Industry and Energy (MOTIE, Korea). [Project Name: Development of high-efficiency activated carbon filter for removing indoor harmful elements (VOCs, radon, bacteria, etc.)].

Data Availability Statement

The data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. FE-SEM micrographs and FE-TEM images of as-synthesized GQDs with size distribution and their respective GQDs-TiO2 nanocomposites.
Figure 1. FE-SEM micrographs and FE-TEM images of as-synthesized GQDs with size distribution and their respective GQDs-TiO2 nanocomposites.
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Figure 2. (a) XRD pattern of as-synthesized GQDs and their respective GQDs-TiO2 nanocomposites. (b) TGA analysis of as-synthesized GQDs and their respective GQDs-TiO2 nanocomposites.
Figure 2. (a) XRD pattern of as-synthesized GQDs and their respective GQDs-TiO2 nanocomposites. (b) TGA analysis of as-synthesized GQDs and their respective GQDs-TiO2 nanocomposites.
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Figure 3. Synoptic XPS survey scan of (a) CNT-GQDs and CNT-GQDs-TiO2 nanocomposites, (b) RCNT-GQDs and RCNT-GQDs-TiO2 nanocomposites, (c) CNF-GQDs and CNF-GQDs-TiO2 nanocomposites. High resolution XPS spectra of C1s peak (d) CNT-GQDs-TiO2, (e) RCNT-GQDs-TiO2, (f) CNF-GQDs-TiO2. High resolution XPS spectra of O1s peak (g) CNT-GQDs-TiO2, (h) RCNT-GQDs-TiO2, (i) CNF-GQDs-TiO2.
Figure 3. Synoptic XPS survey scan of (a) CNT-GQDs and CNT-GQDs-TiO2 nanocomposites, (b) RCNT-GQDs and RCNT-GQDs-TiO2 nanocomposites, (c) CNF-GQDs and CNF-GQDs-TiO2 nanocomposites. High resolution XPS spectra of C1s peak (d) CNT-GQDs-TiO2, (e) RCNT-GQDs-TiO2, (f) CNF-GQDs-TiO2. High resolution XPS spectra of O1s peak (g) CNT-GQDs-TiO2, (h) RCNT-GQDs-TiO2, (i) CNF-GQDs-TiO2.
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Figure 4. (a) UV-vis absorption of as-synthesized GQDs-TiO2 nanocomposites, (b) Tauc plot of respective GQDs-TiO2 nanocomposites, (c) Tauc plot of as-synthesized GQDs with respective UV-vis absorption as inset image, (d) Photoluminescence (PL) spectra of as-synthesized GQDs excited at 325 nm by He-Ne laser. Photoelectrochemical analysis (e) Transient photocurrent response of the as-prepared GQDs-TiO2 nanocomposites, (f) EIS measurements.
Figure 4. (a) UV-vis absorption of as-synthesized GQDs-TiO2 nanocomposites, (b) Tauc plot of respective GQDs-TiO2 nanocomposites, (c) Tauc plot of as-synthesized GQDs with respective UV-vis absorption as inset image, (d) Photoluminescence (PL) spectra of as-synthesized GQDs excited at 325 nm by He-Ne laser. Photoelectrochemical analysis (e) Transient photocurrent response of the as-prepared GQDs-TiO2 nanocomposites, (f) EIS measurements.
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Figure 5. Visible light photocatalytic degradation of MB for as-synthesized GQDs-TiO2 nanocomposites (a) Change in relative concentration (C/C0) of MB in GQDs-TiO2 nanocomposites as a function of irradiation time up to 90 min, (b) Derived plot from same data to study ln(C/C0) variations with irradiation time. pH variation effect on the (c) Change in relative concentration (C/C0) as a function of irradiation time up to 60 min using CNF-GQDs-TiO2 nanocomposites, (d) Respective derived plot of ln(C/C0) variations with irradiation time. (e) Comparison of MB degradation in different nanocomposites, (f) Comparative study of first order rate constants of as-prepared GQDs-TiO2 nanocomposites.
Figure 5. Visible light photocatalytic degradation of MB for as-synthesized GQDs-TiO2 nanocomposites (a) Change in relative concentration (C/C0) of MB in GQDs-TiO2 nanocomposites as a function of irradiation time up to 90 min, (b) Derived plot from same data to study ln(C/C0) variations with irradiation time. pH variation effect on the (c) Change in relative concentration (C/C0) as a function of irradiation time up to 60 min using CNF-GQDs-TiO2 nanocomposites, (d) Respective derived plot of ln(C/C0) variations with irradiation time. (e) Comparison of MB degradation in different nanocomposites, (f) Comparative study of first order rate constants of as-prepared GQDs-TiO2 nanocomposites.
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Figure 6. (a) Cyclic runs of MB degradation under visible light exposure for GQDs-TiO2 nanocomposites. MB degradation concentration in absence and presence of different scavengers (scavengers: IPA~•OH, BQ~ O2•−, EDTA~ h+) by (b) CNT-GQDs-TiO2, (c) RCNT-GQDs-TiO2, (d) CNF-GQDs-TiO2. (e) Comparative analysis of effect of scavengers on MB degradation. (f) Schematic illustration of free radical generation and interfacial charge separation at GQDs-TiO2 nanocomposite junction for MB degradation.
Figure 6. (a) Cyclic runs of MB degradation under visible light exposure for GQDs-TiO2 nanocomposites. MB degradation concentration in absence and presence of different scavengers (scavengers: IPA~•OH, BQ~ O2•−, EDTA~ h+) by (b) CNT-GQDs-TiO2, (c) RCNT-GQDs-TiO2, (d) CNF-GQDs-TiO2. (e) Comparative analysis of effect of scavengers on MB degradation. (f) Schematic illustration of free radical generation and interfacial charge separation at GQDs-TiO2 nanocomposite junction for MB degradation.
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Figure 7. Schematic for the synthesis of N, S co-doped GQDs named as (a) CNT-GQDs, (b) RCNT-GQDs, (c) CNF-GQDs.
Figure 7. Schematic for the synthesis of N, S co-doped GQDs named as (a) CNT-GQDs, (b) RCNT-GQDs, (c) CNF-GQDs.
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Table
Table 1. Atomic percentage analysis with statistical error using XPS spectrometer for as-prepared GQDs and GQDs-TiO2 nanocomposites.
Table 1. Atomic percentage analysis with statistical error using XPS spectrometer for as-prepared GQDs and GQDs-TiO2 nanocomposites.
ElementsCNT-GQDsCNT-GQDs-TiO2RCNT-GQDsRCNT-GQDs-TiO2CNF-GQDsCNF-GQDs-TiO2
Atomic percentage
C44.14 ± 2.027.41 ± 0.262.51 ± 3.251.07 ± 5.224.51 ± 0.737.76 ± 4.7
O47.61 ± 1.053.11 ± 1.033.51 ± 3.538.11 ± 8.562.99 ± 4.144.55 ± 0.7
N6.53 ± 0.82.45 ± 0.50.21 ± 0.10.12 ± 0.54.55 ± 2.91.46 ± 0.9
S1.72 ± 0.10.15 ± 0.043.69 ± 0.092.52 ± 0.67.95 ± 0.41.75 ± 0.3
Ti-17.22 ± 0.08-8.19 ± 2.2-14.49 ± 2.6
Table
Table 2. Comparative data analysis of dye degradation using GQDs based TiO2 nanocomposites.
Table 2. Comparative data analysis of dye degradation using GQDs based TiO2 nanocomposites.
PhotocatalystPollutantIrradiation Time (min)Degradation Efficiency (%)References
GQD/a-TiO2Rhodamine B3597[62]
GQDs/TiO2Methylene Blue12096.70[63]
GQDs/TiO2Rhodamine B6070[66]
3DGA@CDs-TNsRhodamine B6097.60[67]
N-GQDs/TiO2Rhodamine B12094[64]
N-GDQs/TiO2Methylene Blue150100[68]
N-GDQs/TiO2Methylene Blue7085[69]
Ti3+-TiO2/GQD NSsMethylene Blue6090[70]
NSTGMethylene Blue24098.40[40]
N, S-GQDs/rGO/TiO2 NTMethyl orange24090[65]
TiO2/rGOMethylene Blue12091.48[71]
CNF-GQDs-TiO2Methylene Blue6099.23This work
Table
Table 3. Role of quenchers in UV-Vis photocatalysis of MB degradation.
Table 3. Role of quenchers in UV-Vis photocatalysis of MB degradation.
QuencherCNT-GQDs-TiO2
(% Degradation)		RCNT-GQDs-TiO2
(% Degradation)		CNF-GQDs-TiO2
(% Degradation)
EDTA 70.8947.5564.97
IPA 61.7456.1793.94
BQ76.9880.1558.26
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Rawal, J.; Kamran, U.; Park, M.; Pant, B.; Park, S.-J. Nitrogen and Sulfur Co-Doped Graphene Quantum Dots Anchored TiO2 Nanocomposites for Enhanced Photocatalytic Activity. Catalysts 2022, 12, 548. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050548

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Rawal J, Kamran U, Park M, Pant B, Park S-J. Nitrogen and Sulfur Co-Doped Graphene Quantum Dots Anchored TiO2 Nanocomposites for Enhanced Photocatalytic Activity. Catalysts. 2022; 12(5):548. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050548

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Rawal, Jishu, Urooj Kamran, Mira Park, Bishweshwar Pant, and Soo-Jin Park. 2022. "Nitrogen and Sulfur Co-Doped Graphene Quantum Dots Anchored TiO2 Nanocomposites for Enhanced Photocatalytic Activity" Catalysts 12, no. 5: 548. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050548

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