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Article

Novel Anodic TiO2 Synthesis Method with Embedded Graphene Quantum Dots for Improved Photocatalytic Activity

by
Ainars Knoks
*,
Liga Grinberga
and
Janis Kleperis
Institute of Solid State Physics, University of Latvia, LV-1063 Riga, Latvia
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(11), 1407; https://doi.org/10.3390/coatings14111407
Submission received: 29 September 2024 / Revised: 24 October 2024 / Accepted: 2 November 2024 / Published: 5 November 2024
(This article belongs to the Special Issue Advanced Research on Energy Storage Materials and Devices)
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Abstract

:
Photocatalytic degradation of pollutants have a high potential for sustainable and renewable uses. TiO2 is a widely studied photocatalyst due to its high chemical and photochemical stability and wide range of applications. However, the wide band gap and low capacity of photo-induced charge separation provide lower catalytic activity; thus, improvement of these properties must be found. The doping of TiO2 with other elements, such as carbon nanoparticles (CNP) in a quantum dot form, offers a promising pathway to improve the aforementioned properties. In addition, in situ doping methods should be investigated for practical scalability, as they offer the advantage of integrating dopants directly during material synthesis, ensuring a more uniform distribution and better interaction between the dopant and the host material, in turn leading to more consistent photocatalytic properties. Current technologies primarily involve nanoparticle combinations. This work focuses on the development of a novel in situ synthesis methodology by the introduction of three different graphene-based quantum nanodots into anodic TiO2 and the following investigation of structural, morphological, and photocatalytic properties. Results indicate that the introduction of CNP allows for the shift of a set of parameters, such as the optical band gap, increased photo-induced charge carrier density of TiO2/CNP composite, and, most importantly, the change of crystalline phase composition depending on added CNP material. Research indicates that not only a higher concentration of added CNP enhances higher photocatalytic activity as tested by the degradation of methylene blue dye, but also the type of CNP determines final crystalline phase. For the first time brookite and rutile phases were obtained in anodic titania synthesized in inorganic electrolyte by introducing hydrothermally treated exfoliated graphene.

1. Introduction

It is not a secret that the majority of our energy needs are met by fossil fuels [1], which are a major source of anthropogenic CO2 released into the atmosphere. The rapidly increasing concentration of CO2 is believed to be one of the causes of climate change [2], as evidenced by the rise in sea level [3] and by changes in natural habitats and the environment. These changes include decreasing oxygen content in the global ocean, leading to change in pH and in CO2 absorption capability, which, in worst-case scenarios, can lead to anoxia [4]. Additionally, there are other aspects that need attention, such as the overall impact on treatability, e.g., pollution’s influence on antibiotic resistance [5], and, more directly, on organic waste, including drug and medical byproducts and pesticides [6].
One foreseen way to mitigate the aforementioned problems is to decrease the energy demand in various aspects of life. Water treatment is energy demanding [7], especially in areas with fossil-fuel-based electricity production such as China, whether it be via direct [8] or hidden [9] emissions. The use of robust materials [10,11], particularly the reduction of various pollutants via the use of abundant energy sources, e.g., the photocatalytic degradation of pollutants using sunlight, would shift energy demand from an active demand to a more passive demand, thus, saving a substantial amount of energy/emissions. TiO2 is a suitable material due to its wide availability, low toxicity, high chemical stability [12], and its large area of application, i.e., in CO2 reduction [13], batteries [14,15], microbial fuel cells [16,17] and others. However, TiO2 is known to have relatively low efficiency as a photocatalyst due to its wide band gap [18,19]. Specifically, titania has two major obstacles for wide photocatalytic use: a wide band gap, which requires irradiation with high energy photons (in the UV region), and a fast recombination of photo-induced charge carriers [20,21,22], which reduces photoactivity.
To improve the photocatalytic activity of TiO2, its properties and parameters should be changed. Firstly, the optical absorption edge should be shifted to longer wavelengths, i.e., those in the visible range. Secondly, improving the separation of photo-induced charge carriers is crucial for increasing their lifetime. The enhancement can be achieved by introducing dopant materials into TiO2 [23,24,25]. In addition, the dopant material should be accessible and abundant. Carbon nanomaterials, such as graphene and carbon nanoparticles (CNPs), have attracted attention. However, the production of graphene at scale faces technological obstacles, and is thus still under development. Some studies have reported methods for the electrochemical production of graphene and graphene quantum dots (GQDs) [26]. We can utilize the unique properties of GQDs to enhance the photocatalytic activity of the catalyst surface i.e., by increasing visible light response and utilizing hot electrons [27,28]. GQDs have applications in photocatalysis [29], optoelectronics [30,31], sensors [32], medicine, bio-imagining [33], and others. GQDs were initially discussed in the context of carbon materials and carbon nanodots, as described in [34]. Graphene is a zero bandgap semiconductor, and its band gap can be determined/tuned by the substrate [35,36]. GQDs graphene sheets composed of several layers and are a few nanometers wide. In addition, they have edge site effects, as well as quantum confinement, which may improve the photocatalytic properties of TiO2. Similarly to graphene, GQD production/synthesis is not trivial. Current synthesis methods include hydrothermal and solvothermal techniques [32], electrochemical exfoliation, and thin layer techniques. Each method provides various challenges. For example, after exfoliation, graphene must be separated, and, to achieve true GQDs, these graphene sheet stacks require further separation and dispersion. Zhao et al. have demonstrated a three-step hydrothermal synthesis requiring high concentrations of HNO3 [37]. Other methods include the oxidation of GO with a variety of agents, such as O3 [38], hydrothermal cutting, [26,39], the photo-Fenton method [40] and ultrasonication [41]. As summarized by Pirsaheb et al. [42], the introduction of GQD enhances the photocatalytic properties of TiO2.
The successful implementation of GQDs into anodic TiO2 has yet to demonstrate its full utility. A successful integration of GQDs in titania nanoparticles has been demonstrated previously [43,44,45]. NP synthesis and use has advantages but it also has drawbacks, such as possible leeching into the environment, and technological complications in terms of filtration and recovery. These problems can be solved with nanostructured coatings, though the wide implementation of CNPs for coatings has not yet been demonstrated. A simplified synthesis process of catalyst with modifications is essential for large-scale application. Incorporating GQDs during the synthesis would streamline production and enhance scalability. In this work, a novel anodic TiO2 composite synthesis method is developed to incorporate carbon nanoparticles into titania, thereby improving its photocatalytic activity. Titania has been synthesized and CNPs have been incorporated using a unique combination of anodization process and electrophoretic deposition. Morphology, structure, and photoelectrochemical properties were then investigated to compare synthesized and commercial GQD influence on TiO2 properties.

2. Materials and Methods

All chemicals used in this research, including Ti foil, were purchased from Sigma-Aldrich. In this work, three carbon material additives were used in a developed in situ methodology that combines anodization and electrophoretic methods. The first additive was reduced graphene sheet stacks (for simplicity, referred to here as G), synthesized by exfoliating metallurgically used graphite crucibles, followed by reduction in an inert atmosphere. The electrochemical exfoliation was performed in a H2SO4 solution, with a frequency-modulated voltage alternating every 10 s between two graphite electrodes to increase the yield, assuming SO42− ions are responsible for the exfoliation process. After collecting a substantial amount of material, it was washed with deionized water, vigorously ultrasonicated (UP200St—Ultrasonic Lab Homogenizer, Hielscher Ultrasonics, Teltow, Germany) to break apart agglomerates, and dried in an ambient atmosphere at 60 ℃. However, graphite used in metallurgy, as well as sulfur-based exfoliation electrolyte, introduced impurities into exfoliated material. Therefore, it was annealed in an Ar/H2 atmosphere (Linde Gas SIA, Riga, Latvia) for 4 h at 900 °C (Snol tube furnace, SnolTherm, UAB, Narkunai, Lithuania). After annealing, the obtained G particles were dispersed in a nonpolar solvent (DMF) and ultrasonicated to break down particle size further until a homogeneous dispersion was achieved. An extensive description of the method, though with different electrolytes, is discussed in the work of Olins et al. [46]. To further reduce the particle size, the G material emulsion was placed in a stainless-steel autoclave with a Teflon chamber and heated at 140 °C for 8 h, followed by 200 °C for an additional 8 h for hydrothermal synthesis. The obtained material was designated as H. As a result, an emulsion with carbon nanoparticles, with an approximate size under 10 nm, was achieved. For comparison, commercial graphene quantum dots (Q) were purchased (Sigma-Aldrich, Saint Louis, MO, USA, green luminescence GQD in water 1mg/mL). These three carbon nanomaterials—G, H, and Q—were further used as additives in TiO2 synthesis, with the samples then denoted as G, H, and Q appropriately.
TiO2 was synthesized through the electrochemical anodization of Ti foil in F− ion-containing water/H3PO4-based electrolyte, as described in [40]. The synthesis was modified to include two steps—anodizing at 25 V for 25 min and then again for 90 min. In the second step, CNP additives were introduced into the anodization electrolyte at various concentrations by adding 0.5, 1, 2, and 4 mL (concentration 1mg/mL) to 60 mL electrolyte. Before the introduction of additive CNP, the solutions were sonicated for even dispersion. After the addition, electrolyte was mixed throughout the synthesis process, which helped to prevent sedimentation and agglomeration. The method of composite synthesis in the organic electrolyte is detailed in our previous work, where commercial Pt-doped carbon and exfoliated N-doped graphene particles were used as additives, though no additional description of the synthesis process was elaborated [47].
Three sets of samples were synthesized and treated using identical parameters, differing only in the addition of carbon material at various amounts. The samples were designated as follows: G-material-added samples were identified as G 0, 1, 2, 3, 4; hydrothermal-synthesis-material-added samples were identified as H 0, 1, 2, 3, 4; and commercial graphene-quantum-dot-added samples were identified as Q 0, 1, 2, 3, 4. After synthesis, all samples were annealed in an Ar/H2 atmosphere in the tube furnace at 500 °C for 120 min to achieve crystallization of TiO2.
Photoelectrochemical properties were measured using on–off modulation cycles of light irradiation to measure both the open-circuit potential (OCP) and the photocurrent response (PCR). After reaching a steady state, the flat band potential (EFb) was determined in darkness by the Mott–Schottky method (MS) [48,49,50,51]. For light modulation, a 150W Xenon lamp (with UV cut off) was used, a VoltaLab potentiostat (Radiolab Analytica PGZ 301, Lyon, France) was exploited for the photoelectrochemical (PEC) measurements in 1 M NaOH, and a saturated calomel electrode (SCE) was used as a reference electrode. Morphology was investigated using scanning electron microscopy (Phenom Pro SEM, Phenom-World, Eindhoven, The Netherlands) and atomic force microscopy (AFM NT-MDT, model Smena, Moscow, Russia). The structural analysis was performed using X-ray diffraction (XRD RIGAKU MiniFlex 600 X-ray diffractometer, Tokyo, Japan) and Raman spectroscopy (TriVista CRS confocal Raman microscope, Spectroscopy & imaging GmbH, Bördestr, Germany). To estimate the band gap value of the samples, recorded reflectance spectra (Shimadzu SpecCord 9000 with integrating sphere, Kyoto, Japan) and Kubelka–Munk transformation [52,53] was used. TiO2 has an indirect allowed band gap; therefore, the transformation coefficient n = ½ was used.
The photocatalytic properties of synthesized samples were investigated through the degradation of methylene blue (MB). The 0.03 mM MB solution was prepared by dissolving MB in deionized water. Then, the sample was immersed in the MB and left to reach a steady state. Samples were irradiated using the same light source as in PEC measurements. The investigation of solution absorbance was achieved with chosen time intervals measured using the absorbance kit and software (OceanOptics HR4000, Ostfildern, Germany). Absorbance was measured before irradiation started and after reaching a steady state (chosen time for sample relaxation in MB solution allowing MB adsorption). The degradation coefficient (k) was then calculated from the linear slope coefficient of the plot
l n A t A 0 = − k t

3. Results

The synthesized and purchased CNPs were investigated with AFM. In order to ensure comparable particle size evaluation, solutions had to be diluted. All samples for AFM were prepared in the same manner by dispersing small quantities, around 0.001 mL of material (H, G, and Q) solution in 20 mL of deionized water, then sonicated for 5 min for higher dispersion and to ensure a very low concentration. Then, using a precision paint spray gun for even distribution, the solution was sprayed on a silica wafer and allowed to dry. The AFM investigation results are depicted in Figure 1.
As is seen from Figure 1, H particles measured approximately 2.0 nm in height and 53 nm in width, G measured 2.9 nm in height and 51 nm in width, and Q particles were measured to be 6.6 nm in height and 32 nm in width. The large width suggests particle agglomeration after the deposition for testing during the drying process.
To incorporate CNP into titania during anodization, the z potential of the particles should be negative. The zeta potential measurements for used CNPs are shown in Table 1, with measurements carried out with the LitesizerTM 500 (Anton Paar, Graz, Austria). Both the commercial and synthesized NPs have similar potentials, whereas G shows a more positive potential, indicating that G has a lower incorporation efficiency and possibly a higher agglomeration rate compared with other CNPs.
A structural investigation of synthesized composite samples was conducted using XRD and Raman spectroscopy (Figure 2). Table 2 shows the vibrational modes of synthesized samples, their comparison with the known TiO2 anatase, and the rutile vibrational mode values. It is noteworthy that, for the H series, there are shifts of vibrational modes and additional shoulders, indicating possible phase mixtures, such as brookite or rutile, in addition to anatase. In the presence of brookite, the modes would be at A1g (127, 154, 194, 247, 412, 640 cm−1), B1g (133, 159, 215, 320, 415, 502 cm−1), B2g (366, 395, 463, 584 cm−1) and B3g (452 cm−1) [54,55].
As seen in Figure 3, XRD shows the presence of TiO2 in the anatase phase as seen by the peaks corresponding to (101), (200), (105), (107), (215), (301), (224) in samples G and Q. The same peaks at various intensities are visible for other samples and at various added CNP concentrations. The estimation of crystallite size for G samples is roughly the same size, similar to Q, where A (101) crystallites are the same size with an increase of added material.

Optical and Electrochemical Properties

Introducing carbon materials into TiO2 shifts the optical absorption edge to the visible spectrum from 3.2 eV of anatase or 3.0 eV of rutile up to 2.7 eV, as seen in Table 3 and in Figure 4a. In the Q series, there is a slight shift of absorption edge to higher wavelengths, as seen in Figure 4a, while, on the other hand, G and H showed a large decrease of Egap. Spectrophotometric data show that composite layers experience a shift of Egap, depending on the amount added—from 3.21 eV to 3.06, 2.60, and 2.98 eV, respectively, in the case of Q. In comparison, synthesized nanoparticles of the H samples provide a larger shift in the Egap value. A comparison of the light-induced charge carrier separation as a photocurrent response to light chopping (PCR) can be seen in Figure 4b.
The addition of H and Q increases OCP and PCR values compared with pristine TiO2, where PCR change is shown in Figure 4b. On the other hand, G decreases both OCP and PCR values, indicating reduced electron transport and an increased recombination rate, which corresponds with the higher zeta potential values measured. The increase in carbon material lowers the amount of noise and should increase the charge separation and transfer. Additionally, EFb was determined, along with charge carrier density (ND), in order to evaluate the proposed charge separation. The G, H, and Q samples show that an increase in the loading of CNP changes not only the ND but also the EFB, as seen in Figure 5. A moderate addition of G increases the ND, but a further increase of additive can bring it below the pristine TiO2 level, while EFb shifts to more positive potential. On the other hand, Q and H provide an EFb shift towards more negative values, while charge carrier density increases with both additive materials. A summary of these values can be seen in Table 3.
A well-established evaluation of photocatalytic activity is MB degradation. The decrease in solution absorbance was investigated with chosen time intervals. Concentration decrease in time, as well as calculated degradation coefficient results are depicted in Figure 6. Adding G lowers the photocatalytic activity of anodic TiO2, which corresponds with the higher zeta potential and the lower PCR values, as shown in Table 1 and in Figure 4 and Figure 6, respectively. Adding Q increases the activity at low added amounts, but further increasing the loading decreases the activity, as seen in Figure 6. Thus, a moderate amount of this type of carbon material is necessary for an optimal increase in photocatalytic activity. Only the addition of H increases the photocatalytic activity with an increase in the loading.

4. Discussion

It was identified that, for the Q samples, there is an additional shift of TiO2 vibration modes from 398 cm−1 for Q0 to 395.1 for Q1, 395.1 for Q2, and 395.1 for Q3, and that the similarity mode at 516.5 for Q0 shifted to 514.7 for Q1, 512.8 for Q2 and 514.7 for Q3. The highest mode shift can be seen from 636.3 for Q0 to 632.6 for Q1 and Q2, and 630.8 cm−1 for Q3; a broad low-intensity rutile mode (at 446.6 cm−1) emerged from Q0. For series G the Raman vibration shift is not as visible for the mode at 144, which for G0, G1 and G2 stands at 144.4 cm−1, for G3 stands at 142.5, and for G4 stands at 146.3 cm−1. As the A1g&B1g vibration mode is present in all samples. The shift is more evident in the E3g vibrational mode, where G0 is at 611.4, G1 is at 620.4, G2 is at 631.3, G3 is at 631.3 and G4 is at 609.6 cm−1. This indicates that there is another phase presence in the samples, especially considering that, e.g., the rutile mode at 447 cm−1 is visible in these samples. Regarding carbon vibrational modes at 1350 and 1590 cm−1, there are additional shifts visible in comparison with graphene quantum dot D and G vibrational modes at 1350 and 1585 cm−1, respectively [62].
As for the H sample series, it is clear that only sample H0 is in pure anatase form, as seen by classic E1g, E2g, B1g, as well as A1g&B1g, but it is noteworthy that E3g has a slight shift from 640 to 631.3 cm−1. Other samples show some mixture of anatase and rutile with a great increase in rutile content as seen by the appearance of B1g (144), Eg (445 and further increase of H addition shifts this vibrational mode to 441 cm−1) and A1g (615 and 606 cm−1).
The introduction of carbon material initiating a blue shift effect on Raman vibrational modes has been shown before [13] and single crystal heating in argon also shows blue shift of the anatase Eg(1) mode [61], which is attributed to the Ti-O vibration and, thus, to oxygen vacancy defects. Furthermore, the G and Q samples are mostly anatase with possibly small amounts of rutile for Q, as seen in XRD. The H samples are clearly transforming from anatase (at H0) to a brookite/rutile mixture (see Figure 3 H4); thus, increasing the addition of this H material promotes the phase transition. For H series we see clear changes of crystalline structure by the appearance of new peaks corresponding to the brookite phase [63,64] in addition to a strong development of a rutile peak R(110), ascribed to the thermal oxide layer by Albu et al. [65] and which should hinder photocatalytic activity. However, the brookite/rutile combination ensures larger charge carrier separation and increases catalytic activity, contrary to the study of Albu et al., in which an anatase/rutile composition was found. It is noteworthy, that in the anodic titania, thermal oxide was identified in air annealed samples rather than as a result of annealing in reducing atmospheric conditions, as in our case. It has been shown that in some cases the addition of carbon material has proven to promote the formation of rutile, as shown by Varnagiris et al. for thin films [66].
As we can see, the crystalline structure, as well as the phase composition, changes with the addition of carbon NP, but that there is a dependence on the type of carbon material as seen from clear differences in XRD and Raman data. It has been noted that carbon as a dopant can be a strong reduction agent if TiO2 with carbon is heated in an inert atmosphere, as it was undertaken here, by defect creation such as oxygen vacancies. In combination with the inert atmosphere itself, the presence of carbon could be a promoter of phase transformation [67].
In addition, increasing the amount of CNP in the Q samples slightly lowers the amount of anatase and increases the amount of rutile, as shown by the decrease and disappearance of the A(200) peak at 2θ = 48° as well as the introduction of R(101) at 2θ = 36°. Thus, the introduction of carbon materials and of graphene-based carbon nanoparticles of small size in combination with a reduced atmosphere, as per this study, promotes phase transition. In addition, and for the first time, we have shown a brookite phase in anodic TiO2 synthesized in an inorganic electrolyte. Reducing atmosphere creates oxygen vacancies and, in combination with carbon as a reducing agent, more bonds are broken; as a result, the phase transformation can be said to be possible at lower temperatures. A proposed depiction of the process is shown in Figure 7.
As is seen in Table 3, G influences the properties of TiO2 in a mostly negative way, which can be ascribed to the loss of electrons on G particles. As the amount of CNP increases, more electrons are trapped and lost to CNP, decreasing the overall activity due to electron recombination within G particles, which corresponds with the lower determined values of OCP, PCR, and MB degradation. On the other hand, further increase, as seen by G4, CNP seems to overtake the MB via reduction of adsorbed oxygen, which, in turn, allows the creation of •OH radicals for MB degradation. It should be noted that decreasing the flat-band potential (to more positive values) is a desirable outcome but is outweighed by the decrease of other determined parameters. On the other hand, commercial graphene quantum dots were found to increase the photocatalytic performance of TiO2, as can be seen by the increase in the measured parameters, including the degradation coefficient shown in Figure 6, but it is noteworthy that only small additive amounts provide a positive effect. This could be explained by increased charge separation and transfer, as well as increased light absorption as seen by the optical absorption edge that was redshifted in the Q samples, which can be attributed to changes in crystalline structure. The absorption edge redshift has been noted previously by Martins et al. [68] for titania nanoparticles. Hydrothermally treated exfoliated graphene nanoparticles have shown a positive increase in properties related to photo-catalytic activity, especially charge carrier density and MB degradation coefficient, providing much higher gains. We attribute this activity to the highest crystalline structure change and the increase of brookite phase in the samples, which is partially supported by available literature such as that describing the hydrothermal synthesis of titania nanoparticles with phase mixture [69] or the study of Kang et al. exploring anodic nanotubes in organic electrolyte and their phase transition due to annealing atmosphere [70], though we have not seen this type of phase transition empirically. Thus, the combination of structure and carbon presence allows higher charge carrier separation and light absorption. On the other hand, we have demonstrated changes in anodic titania using novel composite coating synthesis method of in situ CNP introduction.

5. Conclusions

This work investigated the influence of carbon nanoparticle introduction into anodic titania for changes in photocatalytic activity. Reduced graphene sheet stacks (G) and hydrothermally treated exfoliated graphene nanoparticles (H) were compared with commercial graphene quantum dots (Q) on the basis of changes in the photoelectrochemical properties of anodic TiO2. The addition of carbon nanoparticles was carried out with a novel composite synthesis method via the combined electrophoretic-anodic growth of TiO2, providing an in situ additive introduction and nanotubular titania coating. The addition of carbon material changed the structural, optical, and photoelectrochemical properties of TiO2. For the first time, the brookite phase was discovered in samples using hydrothermally treated particles, moreover, this material showed highest increase in photocatalytic activity. Introducing carbon materials in anodic TiO2 is possible through the developed composite synthesis method and anodization growth process.

Author Contributions

Conceptualization, A.K. and J.K.; methodology, A.K. and L.G.; validation, A.K., J.K. and L.G.; formal analysis, A.K.; investigation, A.K.; data curation, J.K.; writing—original draft preparation, A.K.; writing—review and editing, L.G.; visualization, A.K.; supervision, J.K.; project administration, A.K. and L.G.; funding acquisition, A.K. and L.G. All authors have read and agreed to the published version of the manuscript.

Funding

The research is funded by the Latvian Council of Science (project “Smart materials, photonics, technology and engineering ecosystem, project No. VPP-EM-FOTONIKA-2022/1-0001), Institute of Solid State Physics, University of Latvia, as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2. Materials were funded by Scientific Research Project for Students and Young Researchers Nr. SJZ/2018/9 realized at the Institute of Solid State Physics, University of Latvia.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to ongoing research.

Acknowledgments

The author expresses gratitude to Astrida Berzina for AFM measurements.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Coatings 14 01407 g001
Figure 1. AFM investigation of synthesized (H, G) and purchased (Q) carbon nanomaterials.
Figure 1. AFM investigation of synthesized (H, G) and purchased (Q) carbon nanomaterials.
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Figure 2. Raman investigation, clear TiO2 anatase phase and carbon material, high-intensity modes at 144 and low-intensity modes at 394, 518, and 634 cm−1.
Figure 2. Raman investigation, clear TiO2 anatase phase and carbon material, high-intensity modes at 144 and low-intensity modes at 394, 518, and 634 cm−1.
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Figure 3. Influence of CNP additives on sample crystalline structure investigated with XRD for the G, Q, H series of samples.
Figure 3. Influence of CNP additives on sample crystalline structure investigated with XRD for the G, Q, H series of samples.
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Figure 4. (a) Influence of nanocarbon additives on TiO2 optical absorption edge (relative change calculated as the ratio of value with added materials vs. as anodized sample). (b) Comparison of the light-induced PCR (right) value changes in series G (●), H (▲) and Q (■).
Figure 4. (a) Influence of nanocarbon additives on TiO2 optical absorption edge (relative change calculated as the ratio of value with added materials vs. as anodized sample). (b) Comparison of the light-induced PCR (right) value changes in series G (●), H (▲) and Q (■).
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Figure 5. Flat-band potential (EFB) and charge carrier density (ND) calculated from Mott–Schottky plots for sample G (●), H (▲) and Q (■) in dependency on loading.
Figure 5. Flat-band potential (EFB) and charge carrier density (ND) calculated from Mott–Schottky plots for sample G (●), H (▲) and Q (■) in dependency on loading.
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Figure 6. MB degradation graphs and dependence of the MB degradation coefficient dependence on the loading of sample H (●), Q (▲) and G (■).
Figure 6. MB degradation graphs and dependence of the MB degradation coefficient dependence on the loading of sample H (●), Q (▲) and G (■).
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Figure 7. Phase transition model of the anatase to rutile transition in reducing atmosphere and CNP presence.
Figure 7. Phase transition model of the anatase to rutile transition in reducing atmosphere and CNP presence.
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Table 1. Measured z potential and electrophoretic mobility.
Table 1. Measured z potential and electrophoretic mobility.
CNPZeta Potential, mVNegative Electrophoretic Mobility, µm·cm·Vs−1
Q−35.7 ± 5.41.79 ± 0.01
G−5.0 ± 0.60.25 ± 0.01
H−46.8 ± 2.32.35 ± 0.01
Table 2. Raman mode position of samples.
Table 2. Raman mode position of samples.
SampleAnataseRutileReferences
Eg(1)Eg(2)B1gA1g & B1gEg(3)B1gEgA1g
147.0198.0398.0518.0640143.0447.0612[56]
144.0197.0399.0513, 519639 [57,58]
144.0197.0397.0516.0641143.0447.0612[59]
147.0198.0398.0515.0640144.0448.0612[60]
143.0196.0394.0512.0630 [61]
H0144.4199.3395.7515.1631.3---This work
H1140.5218.2393.8515.1631.3140.5445.5615This work
H2144.4210.7---144.4441.8606This work
H3144.4----144.4--This work
H4144.4220.1---144.4441.8606This work
G0148.2199.3392.0527.9609.6 This work
G1144.4199.3392.0515.1611.4 This work
G2146.3199.3393.8527.9620.4 This work
G3144.4197.4393.8515.1631.3 This work
G4142.5197.4392.0517.0631.3 This work
Q0145.2199.2398.9514.7636.3 This work
Q1145.2199.2395.1514.7632.6 This work
Q2147.1199.2395.1516.5632.6 This work
Q3145.2199.2395.1512.8630.8 This work
Q4145.2199.2395.1512.8630.7 This work
Table 3. Summary of measured values and parameters.
Table 3. Summary of measured values and parameters.
AdditiveSampleOCP, mVPCR, µA/cm2Egap, eVEFb, mVND, cm−3‣1017k, h−1
QQ0−29.1 ± 0.52.1 ± 0.13.21 ± 0.02−908.0 ± 5.015.4−0.00323
Q1−58.71.43.06−1004.617.8−0.00505
Q2−26.31.53.06−1034.018.2−0.00549
Q3−126.11.82.61−1015.028.2−0.00434
Q4−156.12.92.98−1036.027.8−0.00299
GG0−46.51.53.13−942.597.8−0.01541
G1−33.81.43.05−1011.121.5−0.01512
G2−22.60.53.06−959.23.95−0.00746
G3−53.20.73.11−947.94.99−0.00192
G4−18.70.63.14−704.268.5−0.01021
HH0−109.91.73.08−725.09.17−0.0085
H1−238.211.22.95−988.51.97−0.01212
H2−256.812.62.59−1059.830.1−0.00966
H3−19.57.12.83−1080.9126.0−0.00959
H4−108.715.32.84−1060.0133.0−0.01269
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Knoks, A.; Grinberga, L.; Kleperis, J. Novel Anodic TiO2 Synthesis Method with Embedded Graphene Quantum Dots for Improved Photocatalytic Activity. Coatings 2024, 14, 1407. https://doi.org/10.3390/coatings14111407

AMA Style

Knoks A, Grinberga L, Kleperis J. Novel Anodic TiO2 Synthesis Method with Embedded Graphene Quantum Dots for Improved Photocatalytic Activity. Coatings. 2024; 14(11):1407. https://doi.org/10.3390/coatings14111407

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Knoks, Ainars, Liga Grinberga, and Janis Kleperis. 2024. "Novel Anodic TiO2 Synthesis Method with Embedded Graphene Quantum Dots for Improved Photocatalytic Activity" Coatings 14, no. 11: 1407. https://doi.org/10.3390/coatings14111407

APA Style

Knoks, A., Grinberga, L., & Kleperis, J. (2024). Novel Anodic TiO2 Synthesis Method with Embedded Graphene Quantum Dots for Improved Photocatalytic Activity. Coatings, 14(11), 1407. https://doi.org/10.3390/coatings14111407

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