Bio-Inspired C/N/TiO2 Hybrid Composite Heterostructure: Enhanced Photocatalytic Activity under Visible Light

Department of Science and Humanities, Sumathi Reddy Institute of Technology forWomen,Warangal, Telangana 506371, India Department of Applied Sciences, National Institute of Technology Goa, Ponda 403401, India College of Engineering, National University of Science & Technology, P. O. Box 620, Seeb PC 130, Oman Department of Chemistry, School of Engineering, Presidency University, Bangalore, Karnataka 560064, India


Introduction
In recent years, photocatalysis has become increasingly popular in the development of energy-e cient, ecologically bene cial, and long-lasting processes [1][2][3][4][5][6]. Despite the fact that other photo-catalyst candidates have been investigated, titanium dioxide (TiO 2 ) has been the most thoroughly investigated material, and it is now considered to be the most likely photocatalyst for industrial-scale applications due to its high photoactivity e ciency, long-term stability, nontoxicity, and low price [7][8][9][10][11]. Recent research on the use of TiO 2 as a photocatalyst has concentrated on enhancing the material's photocatalytic activity under visible light and increasing its speci c surface area [12]. is is being done in an e ort to boost the e ciency with which solar energy is converted while simultaneously bringing down the associated costs. However, the large bandgap (∼3.2 eV) of TiO 2 restricts the photocatalytic activity in the ultraviolet wavelength range, which contributes less than 5% of the total energy of the overall solar spectrum [13][14][15]. It is also important to note that TiO 2 is limited by the rapid recombination of photo-generated electron-hole pairs, which is particularly noticeable in the absence of electron donors or acceptors, as it results in signi cant energy loss and an extremely small active photocatalytic reactive zone [15][16][17].
Numerous studies have sought to improve the photocatalytic activity of TiO 2 under visible light by doping it with metal/nonmetal anions [18][19][20][21]. In the case of metal ion doping, the photocatalytic activity failed due to insu cient interfacial electron transfer rates and charge carrier recombination rates, which resulted in poor photocatalytic activity [19,22,23]. On the other hand, nonmetal anion doping often increases the specific surface area of TiO 2 photocatalysts, limits the growth of crystallite size, and increases the percentage of anatase phase in the photocatalyst [12,19,24,25]. Additionally, nonmetal anions (C/N/ P/S, etc.) can enhance the photocatalyst activity by moving its wavelength sensitivity from the ultraviolet to the visiblelight area, which is made possible by the narrowing band gap of TiO 2 due to the presence of nonmetal anions [19,[26][27][28][29][30].
TiO 2 can be hydrogenated, doped with metals or nonmetals, combined with another semiconductor, or doped with nonmetals to enable it to be activated by visible light [1,16,19,21,31]. ese are only a few of the many possible activation methods. It is also vital to the synthesis of TiO 2 photocatalysts in order to produce cost-effective, efficient photocatalysts. A number of different doping procedures, such as annealing, hydrothermal, microwave-assisted, and sol-gel techniques, have been examined [16,[18][19][20]. Each of these doping procedures has its own set of benefits and drawbacks, such as the amount of time required for preparation and the amount of photocatalytic activity that is achieved as a result. In the field of photocatalysis, numerous in-depth reviews and studies have been reported on the modification of TiO 2 to increase its sensitivity to visible light. Comparing the doping methods of non-metal-doped TiO 2 and the parameters impacts the synthesis and photocatalytic reactions.
Furthermore, two structural design strategies for increasing the photocatalytic activity of TiO 2 are being considered: monolithic three-dimensional (3D) nanostructuring and heterostructure with nonmetal anions such as carbon, nitrogen, phosphorus, and other carbon nanomaterials [18,30,32,33]. A monolithic 3D nanostructure provides the most surface area possible in a given volume while also allowing for efficient reuse of the photocatalyst without the need for recollection. Heterostructuring with carbon nanomaterials aids in the efficient utilization of the solar spectrum and charge separation, as well as the production of TiO 2 photocatalysts with high efficiency [34,35]. erefore, it is critical to creating a simple, energy-efficient, and environmentally acceptable method for the synthesis of heterostructure carbon-based nanomaterials with high visible photocatalytic activity. Recently, bio-inspired carbon composites including TiO 2 nanoparticles have gained significant attention due to their ability to enhance the performance features of the individual components [36][37][38]. Moreover, when compared to bioinspired synthesized nanomaterials, chemically synthesized nanomaterials rely on their efficiency in producing pure products while minimizing the number of preparation steps, without using either excessively harmful reagents or unstable precursors, and without generating particularly toxic by-products. In this work, we develop a bio-inspired approach for visible-light photocatalytic material, and urea as both carbon and nitrogen sources, and tannic acid was used as a carbon source as well as a reducing agent for the preparation of C/N/TiO 2 hybrid composite via hydrothermal treatment.

Experimental Section
e hydrothermal technique was employed to prepare the TiO 2 and C/N/TiO 2 hybrid composites as previously reported [39,40], and the chemicals employed in this work were of analytical reagent grade (>99 percent purity) and acquired from SRL Chemicals, India. e following steps were taken in order to generate a bio-inspired C/N/TiO 2 hybrid composite: aqueous tannic acid solution (40 mg·L −1 ) and 45 mL of deionized water were immersed in for 5 minutes with shaking after being added to the solution with 1 g of TiO 2 and 0.0445 g of urea. Following that, 20 mL MOPS (3-(N-Morpholino) propanesulfonic acid) solution (pH 7.4, 0.4 mg·L −1 ) was added to the solution and then transferred to a stainless autoclave lined with Teflon. e autoclave was kept at 160 degrees Celsius for 12 hours before being allowed to cool to ambient temperature with the air. e finished product was collected and carefully cleaned with water and ethanol before being dried in an oven at 80 degrees Celsius for 10 hours; then, the dried product was placed in an alumina boat and moved to a tube furnace with Ar gas flowing through it for 3 hours at 500 degrees Celsius (5 degrees per minute).
In order to analyze the samples, X-ray diffraction (XRD) analysis was carried out on them using a Philips PANalytical X-ray diffractometer, which operated between 2θ � 20 and 60°and had a sampling step of 0.05°and a counting duration of 2 seconds. A monochromatic Cu K α radiation source was employed in this experiment, with a voltage of 40 kV and a current of 30 mA. e X'Pert HighScore Plus program was used to do both the quantitative analysis (weight percentages) and the phase determination of the samples. e morphology of the C/N/TiO 2 hybrid composite was studied using a scanning electron microscope (Zeiss GeminiSEM) operating at a 5 KeV accelerating voltage. e diffuse reflectance spectra of the materials were measured on a PerkinElmer Lambda 650 UV-Vis spectrophotometer in the ultraviolet-visible range. Fourier transformed infrared spectrophotometer (FTIR) carried out by using Bruker Optics, Germany (Model TENSOR 27, equipped with a KBr beam splitter), with an operating spectrum in the wavelength range between 4000 and 400 cm −1 . e carbon and nitrogen contents of the C/N/TiO 2 hybrid composite were determined using a TGA analysis (SII 6300 EXSTAR) system from Siemens. As a result of the photocatalytic dye degradation experiment, we measured the photodegradation of an RhB solution under xenon lamp (output power of 50 W) illumination to determine the rate of photodegradation of the solutions. A filter was utilized to select the range of the irradiated light, and only light with a wavelength of more than 400 nm was allowed to reach the sample. To summarize, 50.0 mg of sample and RhB solution (50 mg·L −1 ) were used to assess the photocatalytic activity of the catalyst. For the first 20 minutes of the experiment, the mixture was magnetically agitated in the dark to bring the dye and catalyst into an adsorption-desorption equilibrium, and then, it was exposed to room temperature light irradiation. It was necessary to place the experimental solution in a quartz cuvette that was 100 mm distant from the light source. e suspension was centrifuged to remove the scattered catalyst powder at regular intervals, with 5 mL of the suspension being removed at each interval. UV-Vis spectroscopy was used to evaluate the light absorption of the clear solutions at 553 nm (which corresponded to the absorbance of RhB).

Results and Discussion
As shown in Figure 1(a), the XRD profiles of pure TiO 2 and bio-inspired C/N/TiO 2 hybrid composites are well-indexed with the phase brookite (ICSD Reference code: 98-000-3869) and anatase (ICSD Reference code: 98-000-5224), and there are no further impurities in the samples. Pure TiO 2 has anatase (80%) and brookite (20%) phases composition, whereas bioinspired C/N/TiO 2 hybrid composites have complete tetragonal anatase (100%) phase composition due to the presence of carbon and nitrogen in the composite, as indicated by their XRD profiles. Furthermore, the peak at 2θ � ∼30.8°(1 2 1) was eliminated in the bio-inspired TiO 2 hybrid composite due to the fact that C/N prevents the formation of brookite [28]. Carbon and nitrogen spices that have been coated with TiO 2 are also present, probably as a result of the decrease of TiO 2 at the interfaces with surrounding carbon and nitrogen during the annealing process. From the sample's thermogravimetric analysis (TGA), it was possible to measure the amount of C/N present in the bio-inspired TiO 2 hybrid composite (Figure 1(b)). Considering how quickly surface-adsorbed water molecules in the sample may be removed, as well as how rapidly oxidation of TiO 2 can occur at very low temperatures, we hypothesize that the weight change of the sample up to 275°C is caused by both the loss of water and oxidation of TiO 2 . Carbon/nitrogen content in the bio-inspired TiO 2 hybrid composite was calculated to be approximately 23.3 weight percent based on this assumption. e SEM images in Figure 2(a) were taken to examine the surface morphology of the C-and N-doped bio-inspired C/N/TiO 2 hybrid photocatalyst. Figure 2(a) (left) appears to show TiO 2 nanotubes covering the C and N particles. Moreover, the TiO 2 nanotubes are arrayed in a radial pattern, indicating that they are composed of many microscopic C and N nanoparticles, which is consistent with their strong affinity for metal oxide surfaces and ability to generate C and N nanoparticles under controlled conditions of high-temperature sintering. Figure 2(a) (right) shows that the surface of TiO 2 became rougher and that interspace between nanotubes was partially occupied by carbon and nitrogen spices, indicating that carbon and nitrogen doping were successfully functionalized on the TiO 2 nanotube. Furthermore, micropores were observed in Figure 2(a) (right), which generate favorable surface properties to enhance photocatalyst activity over the bioinspired C/N/TiO 2 hybrid composite under visible light [41]. e specific surface area of bio-inspired C/N/TiO 2 hybrid photocatalyst was 47.8 m 2 g −1 , while that of doped TiO 2 with carbon and nitrogen was 53 m 2 g −1 . e slightly enhanced specific surface area of C-and N-doped TiO 2 is, without a doubt, connected to the smaller grain size and the existing pore structure of the material. e enhancement in photocatalytic performance may be attributable to an increase in photocatalytic activity brought about by an increase in specific surface area.
e UV-Vis absorption spectra of a bio-inspired C/N/ TiO 2 hybrid composite and pure TiO 2 are shown in Figure 2(b). While pure TiO 2 exhibits absorption exclusively in the ultraviolet range, the complete C-and N-doped bioinspired C/N/TiO 2 hybrid composite exhibits visible absorption. When C and N are doped into TiO 2 , the absorptions of the catalysts are enhanced and stronger in the wavelength range of 400-800 nm incorporation of carbon and nitrogen atoms into the lattice of TiO 2 . e photoresponse of C/N/TiO 2 hybrid composite in visible light can be ascribed to the presence of oxygen vacancy states, due to the formation of Ti 3+ species, between the valence and the conduction bands in the TiO 2 band structure. e vacant oxygen sites may have been substituted by carbon/nitrogen atoms, and these carbon/nitrogen sites are responsible for the visible-light sensitivity. ese encouraging results show that adding C and N doping via a bio-inspired method may increase the photocatalytic activity of TiO 2 under visible light. Aside from that, the energy band gaps were computed using the Tauc plots (Figure 2(c)), taking into consideration the absorbance transformed by the Kubelka-Munk function and the band gaps. Furthermore, Tauc plots of bio-inspired C/N/TiO 2 hybrid composite and pure TiO 2 demonstrate (Figure 2(c)) that the energy band gap decreases from TiO 2 (3.3 eV) to C-and N-doped bio-inspired C/N/TiO 2 hybrid composite (2.55 eV), which is less than previously reported carbon-and nitrogen-doped TiO 2 photocatalysts [42][43][44][45].
is showed that electrons in the valence band may more easily travel into the conduction band and create electronhole pairs, which could be advantageous in improving the redox characteristics of the catalysts when exposed to sunlight.
As can be seen in Figure 3(a), the FT-IR spectra of pure TiO 2 and C/N co-doped bio-inspired C/N/TiO 2 hybrid composite both have wavelengths in the 400-4000 cm −1 range. e spectra were obtained using different wavelengths of infrared radiation, which revealed the vibrations of different groups of molecules. e usual N-H properties of urea can be observed in Figure 3(a) between 3200-3600 cm −1 and 1650-1700 cm −1 , as illustrated in Figure 3(a) [46,47]. In particular, the related O-H stretching modes, which are derived from surface water molecules, are responsible for a prominent peak at 3335 cm −1 . It is possible to detect the bending vibration associated with the molecular water bending band at a wavelength of 1680 cm −1 as a result of TiO 2 nanoparticle water absorption at this wavelength [48]. e 1350 cm −1 band is caused by the in-plane skeletal vibrations of aromatic rings. e broad bands in the region 700 cm −1 and 550 cm −1 bands were assigned to Ti-O and Ti-O-Ti stretching vibrations of TiO 6 [49]. We believe that the main adsorption peak at 570 cm −1 in pure TiO 2 shifted to 650 cm −1 in C/N/TiO 2 hybrid composite due to the transformation of brookite to anatase crystal phase, as justified by XRD (Figure 1(a)). Figure 3(b) shows photoluminescence spectra of pure TiO 2 and a bio-inspired C/N/TiO 2 hybrid composite, which were used to explore the recombination of electron-hole pairs on TiO 2 surfaces. When comparing the luminescence efficiency of the hybrid C/N/TiO 2 and the pure TiO 2 , it is found that electron transfer from excited TiO 2 to carbon and nitrogen, which is known to hinder electron recombination, has had a detrimental effect on the composites' ability to recombine electron-hole pairs. is recombination occurs between bands at 364 nm, resulting in the photoluminescence peak at this wavelength (Figure 3(b)). A surface state and defect-related peak may be found in the 400-450 nm range, while a peak in the 460-470 nm range can be attributed to indirect recombination via oxygen defects.
Raman spectroscopy is a technique used to study the vibrational and rotational energy of molecules. When applied to nanomaterials, Raman spectroscopy can be used to gain insight into the crystallinity, composition, and structure of the material. In this study, Raman spectra were collected from a C/N/TiO 2 hybrid composite photocatalyst in order to investigate its activity. Figure 3(c) shows the Raman spectra of C/N/TiO 2 hybrid composite. In particular, the Raman peaks at roughly 144 (E g ), 203 (E g ), 399 (B1 g ), 519 (B1 g ), and 639 (E g ) cm −1 and the spectral pattern fit well with the results that have been reported in the literature for typical anatase TiO 2 [50,51]. In addition to this, Figure 3(c) displays an additional feature in the vicinity of 560 cm −1 [51]. e XRD patterns and the results of the spectrum show that anatase TiO 2 is the predominant phase in the C/N-doped TiO 2 sample. is finding is consistent with the findings of the spectrum [51]. e first-order scattering produced by a nonstoichiometric nitrogen/carbon-doped anatase TiO 2 structure is responsible for this characteristic's appearance. erefore, Raman spectra are evidence that the material is a C/N-doped TiO 2 compound. e photocatalytic activity was observed by the deterioration of RhB in an aqueous solution when exposed to visible light. RhB absorbs the most light at a wavelength of approximately 553 nm. During the photodegradation process, the primary absorption band experienced significant hypsochromic alterations.
e absorption peak gradually decreased as the visible light was irradiated, demonstrating the decomposition of the RhB compound in consideration. All rhodamine species were measured using the maximum absorption method, which was used to determine their total Journal of Nanotechnology concentrations. Figure 4(a) displays the photodegradation behavior of RhB dye in the absence of any light (dark test), and in the presence of the as-prepared TiO 2 , and C/N/TiO 2 hybrids after exposure to visible-light irradiation, where C is the concentration of RhB after different light irradiation times and C 0 is the initial concentration of RhB before dark adsorption. RhB content in aqueous solution decreases when C/N/TiO 2 hybrid composite or pure TiO 2 is present, and RhB self-degrades when exposed to ultraviolet light (λ > 400 nm) as depicted in Figure 4(a). It was discovered that the self-degradation of RhB was not readily apparent, indicating that RhB has been stabilized by exposure to visible-light irradiation. At 0 hours, in the presence of visible light, RhB deterioration on TiO 2 and C/N/TiO 2 hybrid composites has been observed. is is attributable to the selfsensitization of RhB, which causes the absorption of titania to extend into the visible-light range.
Furthermore, with the presence of as-prepared pure TiO 2 and bio-inspired C/N/TiO 2 hybrid composite photocatalysts, the degradation of RhB is observed as 9 and 16 percent after one hour and 35 and 70 percent after five hours under sunlight, respectively. RhB was used as a sensitizer in this instance, but it was also degraded at the same time. On the degradation of RhB, it was clear that the photocatalytic activity of the C/N/TiO 2 hybrid composite outperformed that of as-prepared pure TiO 2 as generated by the bio-inspired route. Even more importantly, the bio-inspired C/N/TiO 2 hybrid composite photocatalyst demonstrated significantly greater photocatalytic activity on the degradation of RhB when exposed to visible-light irradiation than commercial TiO 2 and P25 because of the lowered energy bandgap.
ere are three possible explanations for the strong visible-light activity of the hybrid composite of C/N-doped TiO 2 .
First, C/N-doped TiO 2 with a high surface area is able to supply more active sites and absorb more reactive species, which may have been the explanation for its increased photocatalytic activity. TiO 2 has a high atomic number. Because the TiO 2 had already been created prior to the doping process, it is believed that the carbon and nitrogen may be able to replace for some of the lattice titanium atoms that are close to or on the surface of the TiO 2 . As a result, a narrowing of the band gap occurred in the C/N/TiO 2 hybrid composite, which enabled it to absorb more visible light. Because the surface lattice titanium atoms of the TiO 2 sample had a certain amount, which resulted in a limited accommodation for carbon and nitrogen substitutions, these substitutions of surface lattice titanium atoms may explain why doped TiO 2 catalysts with different doping amounts of carbon and nitrogen showed similar photocatalytic activities. e reduction of urea and tannic acid during the hydrothermal process is thought to have resulted in the incorporation of carbonaceous and nitrogen species into the TiO 2 matrix, as the final hypothesis suggests. It is possible that this will result in the development of new active sites, which are also accountable for the increased photocatalytic activity that was observed. Figure 4(b) depicts the photodegradation of RhB over five cycles. As shown in Figure 4(b), after 5 cycles of photodegradation of RhB, the photocatalytic activity of the CNT/ TiO 2 nanohybrids shows a slight decrease due to partial remembrance and loss during washing. Despite the fact that the photocatalytic activity was initially quite high, this is the case. As a result, the photocatalyst C/N/TiO 2 nanohybrids developed are quite stable and have a significant amount of potential application in the field of water purification. e photocatalytic mechanism of bio-inspired C/N/TiO 2 hybrid composite heterostructure is a result of the combined effects of the light-induced charge separation at the semiconductor/oxide interface and the electron-transfer reactions between the catalyst and the substrate. In addition, photogenerated electrons and holes can be absorbed by adsorbed oxygen molecules and surface hydroxyl, respectively, both of which result in the creation of highly oxidative hydroxyl radical species (•OH) [12]. Because of their great oxidative capacity, (•OH) molecules attack dye molecules with minimal selectivity and are able to oxidize contaminants because of their high oxidative potential. Additionally, the bio-inspired C/N/TiO 2 hybrid composite heterostructure has a higher surface area and improved interfacial contact, which enhances the photocatalytic activity.
Our findings give evidence that the carbon in C/N-TiO 2 can facilitate its photocatalytic activity under visible light by maintaining the high reactivity of the photogenerated electrons and holes. is is demonstrated by the fact that the photocatalytic activity is enhanced. A novel precursor approach to manufacture C/N-doped TiO 2 modified by C components with high nitrogen concentration has been developed as a result of this investigation. It is hoped that the bioinspired hydrothermal precursor approach would throw light on the creation of doping materials that have been changed with carbon and nitrogen, with the end goal of increasing the visible-light photocatalytic activity of TiO 2 . We believe that the presence of nitrogen in the structure allows for more efficient electron transfer from the excited titanium dioxide to the organic molecule, resulting in a higher photocatalytic activity. Additionally, the bio-inspired structure may provide a more favorable surface area for light absorption and catalyst activation. us, it demonstrates the bio-inspired carbon-and nitrogen-doped C/N/TiO 2 hybrid composite is an excellent visible-light-driven photocatalyst.

Conclusions
With the use of a unique bio-inspired technique and carbon and nitrogen doping at 500 degrees Celsius, a visible-lightactive C/N/TiO 2 hybrid photocatalyst composite was successfully synthesized by the hydrothermal method. When compared to P25 and pure TiO 2 , carbon-and nitrogenmodified TiO 2 had a higher surface area and displayed more absorption in the visible-light range. It was discovered that carbon/nitrogen-doped TiO 2 prepared by treating it with tannic acid and urea had considerably better photocatalytic activity than pure TiO 2 when exposed to visible light (λ > 400 nm) for the degradation of RhB. Because of its simplicity, ecological, and sustainable energy-saving characteristics, this approach has the potential to be used in the industrial manufacture of visible-light-driven photocatalysts in the future.

Data Availability
All data have been included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.