Structural design, biomimetic synthesis, and environmental sustainability of graphene-supported g-C3N4/TiO2 hetero-aerogels

As an important semiconductor photocatalytic material, titanium oxide (TiO2) has attracted increasing attention inphotocatalytic degradation of organic pollutants due to its high oxidation ability, long-term photostability, and non-toxicity.In this work, we propose a green, facile, efficient, and sustainable strategy to mineralize TiO2 nanocrystals on biomoleculesusing biomimetic synthesis. Different from previous reports, we used short peptides with different functional sequences tocarry out the biomimetic growth of TiO2 in the process of peptide self-assembly, in order to obtain peptide nanofiber (PNF)- TiO2 nanohybrids with controllable structure. In subsequent applications, the biomimetic synthesized PNF-TiO2 nanohybridswere non-covalently cross-linked with graphene oxide (GO) and graphitic carbon nitride (g-C3N4) and freeze-dried to obtainnanohybrids with heterostructured aerogels (hetero-aerogels). The GO/PNF/g-C3N4-TiO2 hetero-aerogel exhibits goodphotocatalytic activity after thermal annealing, and exhibits rapid photocatalytic degradation for methylene blue (MB) andrhodamine B (RhB). Finally, the sustainability of the hetero-aerogels is evaluated using the ranking efficiency product (REP)method. The strategy for preparing TiO2-functionalized hetero-aerogels via the biomimetic synthesis in this study is green,economic, and efficient, and the obtained hetero-aerogels are expected to reveal wide applications in the fields of energystorage, wearable devices, environmental science, and analytical science. More importantly, in future research, we canprepare TiO2 nanohybrids with different requirements by controlling the structure and morphology of biological templateself-assembly for application in more fields.

1.Introduction Titanium dioxide (TiO2) has attracted much attention because of its wide application in catalysts, solar cell modules, catalyst supports, etc. Since TiO2 is currently the only substance actually used as a photocatalyst, there have been many related reports. 1-3 Many methods for synthesizing TiO2 nanoparticles have been developed, such as the hydrothermal synthesis,4 solvothermal synthesis,5 sol-gel preparation,6 biomimetic mineralization,7 and so on. As a green, mild, and low-cost synthesis method, biomimetic mineralization has great application potential, in which some biomolecules, such as enzymes, proteins, peptides, and other can be used as templates and active precursors for biomineralization to grow TiO2 nanoparticles.8, 9 Titanium(IV) bis(ammonium lactate) dihydroxide (TBALDH) is an excellent precursor for the biomimetic synthesis of TiO2 because it can be mixed in aqueous solution without significant hydrolysis or condensation reactions. Previously, Nonoyama et al. selected peptides with riched cationic lysine residues on the side chains to capture TBALDH anions, and presented a β-sheet conformation by designing alternating hydrophilic and hydrophobic peptides as catalytic organic templates.10 When hydrophobic amino acids drive self-assembly through hydrophobic interactions, the resulting bilayer peptide nanofibers entangled and formed a 3D network-like structure. The results showed that controlling the self-assembly process of sequence-designed peptides can control the morphology of mineralized inorganic substances, thereby improving the performance of TiO2 nanoparticles. In the work of Ahn and co-workers,11 the effect of continuous lysine on the TiO2 precipitation was investigated. They found that when the oligo(L-lysine) contained more than three consecutive lysines, it caused a noticeable rapid precipitation, and the TiO2 precipitation increased with the length of L-lysine. In a similar study, Park et al. explored the effect of peptide conformation on TiO2 biomineralization,12 and proved that the biomineralization activity of peptides depends on the spatial proximity of amino groups, which was affected by the secondary structure of peptides. With the continuous development of textile, printing, cosmetics, food science, and other industries, the amount of organic dyes is expanding day by day. As a classic organic dye, methylene blue (MB) has been widely used in textile, paper production, and pharmaceutical fields. Its high toxicity, carcinogenicity, and non-biodegradability will cause great harm to the environment and the health of human being.13-15 In order to prevent MB from flowing into the natural ecosystem, TiO2 is used as a typical photocatalyst to degrade organic pollutants in water.16-18 TiO2 can produces photogenerated holes and some oxygen-containing free radicals under the light of a certain wavelength, which can completely mineralize organic macromolecular dyes into harmless substances, such as water and carbon dioxide. For example, Shaban et al. successfully synthesized TiO2 nanotubes with Ni doping and Ni/Cr co-doping using a novel hydrothermal method. The co-doping of Ni/Cr will shift the light absorption edge of TiO2 nanotubes to higher wavelengths, and 6Ni/4Cr co-doped TiO2 shows efficient catalytic degradation of MB under visible light.19 And Krishnan et al. incorporated chlorophyll into TiO2 to enhance its photocatalytic activity by incipient wetness impregnation method. The bandgap of this TiO2/chlorophyll was measured to be 2.83 eV, which enabled an optimal degradation of 85% of MB under blue LED illumination.20 In addition, in the work of Kurniawan et al., GO was used as a carrier of TiO2, and TiO2 was attached to the surface of GO nanosheets. Under the irradiation of UV-visible light, MB was converted into oxidation byproducts, which were adsorbed on the surface of GO.21 The incorporation of GO enhances the adsorption capacity of hybrid materials and can effectively increase the photocatalytic efficiency of TiO2. Although TiO2 has strong photocatalytic activity, its wide bandgap (~3.2 eV) makes it only play a photocatalytic activation role under the irradiation of ultraviolet light, and the utilization efficiency of sunlight is very low. To improve the photocatalytic efficiency of TiO2, a photocatalyst with strong visible-light photocatalytic activity can be introduced. For example, graphitized carbonitride (g-C3N4), which is a two-dimensional (2D) semiconductor with a lower energy band gap, can show strong photocatalytic performance as well as physical and chemical stability under the irradiation of visible light.22 Tian et al. constructed a two-dimensional layered heterostructure gC3N4/MoS2/graphene by in situ adsorption method, in which gC3N4 was doped between graphene and MoS2 as an intermediate layer.23 This two-dimensional layered structure can not only provide abundant active sites for photocatalysis, but also increase charge transport, reduce the recombination probability of photoexcited carriers, and have excellent photocatalytic activity under visible light. The work of Bao et al. reported the LaNiO3/g-C3N4/MoS2 Z-type heterostructure for the first time.24 The formation of the Z-type heterostructure and the synergistic mechanism of the co-catalyst change the charge transfer mode, which greatly enhances the photocatalytic performance of the nanocomposite. Similarly, this twodimensional nanosheet with excellent performance and TiO2 also have a well-matched energy level, and the heterostructure formed by the coupling of the two can improve the performance of their respective actions, thereby obtaining higher photocatalytic activity and efficiency. Therefore, the coupling of g-C3N4 and TiO2 has been of great concern to design and synthesis of functional photocatalytic nanomaterials.25-27 For example, Sheng et al. used a one-step calcination method to construct a 3D hybrid photocatalyst with g-C3N4/TiO2 heterojunction. In the static system, the degradation efficiency of MB and phenol was 4.0 and 4.5 times that of pure g-C3N4. 28 Similarly, Zhang et al. used gC3N4-TiO2-GA composite airgel materials casted by hydrothermal method. The heterojunction composed of g-C3N4 and TiO2 can effectively improve the catalytic ability of the photocatalyst, and in the experiment of removing rhodamine B dye, it shows a degradation ability far exceeding that of a single catalyst.29 Therefore, using g-C3N4 hybridized TiO2 to improve the photocatalytic ability of TiO2 is undoubtedly a very promising method. Figure 1. Schematic model of the design and biomimetic synthesis of GO/PNF/g-C3N4-TiO2 hetero-aerogels. Based on the inspiration of previous work, in this work, we did not adopt the general synthesis method of TiO2, but expected to use the self-assembled biological template to control the crystal form and morphology of TiO2 through the strategy of biomimetic synthesis and preliminarily explore its application in photocatalytic degradation of dyes. We use the sequence-designed peptide KIIIIKYWYAF as a biotemplate for TiO2 biomineralization. As shown in Figure 1, TBALDH is used as a titanium source, and peptide self-assembly under mild conditions is carried out to obtain peptide nanofibers (PNFs) loaded with TiO2 nanoparticles. Then 2D g-C3N4 and graphene oxide (GO) nanosheets are introduced, assembled under noncovalent interactions, and 3D hetero-aerogels with network, porous structure are obtained by freeze-drying and the crystal structure of TiO2 isfixed underthe treating of high temperature. The formed GO/PNF/g-C3N4-TiO2 hetero-aerogels have an ultralight mass and a porous structure, shows a faster photocatalytic degradation efficiency of MB towards the dye degradation, and can be quickly recovered after the catalysis is completed. This work is different from previous biomimetic mineralization method of peptide conformation such as polylysine, however using a short peptide designed with a small amount of lysine sequence to obtain nanofibers with a better mineralization degree by adjusting the self-assembly conditions of peptide molecules. This strategy will improve the utilization of biomolecular templates of different structures with TiO2 nanoparticles through supramolecular self-assembly and biomimetic mineralization in the future. In addition, the synthesized hetero-aerogels are green and sustainable, showing great advantage for the removal of organic dyes from water systems. 

2. Materials and methods 2.1. Materials and reagents GO suspension (concentration 10 mg/g) was purchased from the Hangzhou Gaoxi Technology (Hangzhou, China). The KIIIIKYWYAF sequence peptide was purchased from the SynPeptide Biotechnology Co., Ltd. (Nanjing, China). Urea (99%) was purchased from the Sinopharm Chemical Reagent Co., Ltd., TBALDH was bought from the Ron Reagents, and all reagents were of analytical grade. Alumina crucibles (with lids) were provided by the Yingdun Laboratory Supplies, the photocatalytic sleeve reactors were purchased from the Huasheng Quartz Glass Factory, and ultra-micro cuvettes were purchased from the Deen Optics. 

2.2. Self-assembly of peptides and biomimetic mineralization ofTiO2 In brief,10 mg of KIIIIKYWYAF peptide monomer was dissolved in 20 mL of water, and then sonicated for 10 minutes to get dispersed solution without aggregation, to which 100 μL of TBALDH was added to produce a small amount of white precipitate aftershaking evenly. A vial containing 20 mL of liquid was then placed in a water bath at 70 °C for reaction, and 10 μL sample was taken out every once in a while, and dropped onto the freshly cleaved mica substrate for characterization. After 48 h, the mixed solution was taken out, and after the temperature was dropped to room temperature, it was centrifuged at 12,000 rpm to remove the supernatant, and the light-yellow precipitate was collected for later use after washing three times repeatedly. 

2.3. Preparation of g-C3N4 nanosheets 15 g of urea was weighed and added into an alumina crucible with a cover, which wasthen placed in a muffle furnace, and the temperature was increased from room temperature to 550 °C at a rate of 10 °C/min, and the high temperature was kept for 1 h. After cooling down to room temperature, the pale-yellow gC3N4 powder was collected and characterized. 

2.4. Preparation and high temperature sintering of GO/PNF/gC3N4-TiO2 hybrid aerogels In order to synthesize hybrid hetero-aerogels with a 3D structure, 100 mg of weighed g-C3N4 powder was put into 10 mL of water for vibration and ultrasonication for 2 h, then the white precipitate formed by co-assembly with 10 mg of peptide was added to 10 mL of water, which was then mixed with 20 mL of GO under stirring for 30 minutes. After that, the g-C3N4 suspension was slowly added into the GO/PNF mixed solution and stirred for 2 hours. The mixed suspension was introduced into the prepared plastic mold, and placed in a refrigerator at - 18 °C to freeze. After 12 hours, the frozen samples were put it into a freeze dryer for 24 hours to freeze-dry. Finally, the prepared hybrid hetero-aerogels were placed in a tube furnace, and in a nitrogen atmosphere the temperature was raised from room temperature to 580 °C at a rate of 1 °C/min and kept for 4 hours. The final samples were cooled down to room temperature to obtain the ultralight black hybrid aerogels. 

2.5. Photocatalytic tests The photocatalytic activity of the as-synthesized GO/PNF/gC3N4-TiO2 was investigated according to the reaction of MB degradation under simulated sunlight. All photocatalytic tests were carried out in a photocatalytic reactor with a light source of 500 W xenon lamp without any filter. Briefly, 10 mg of prepared GO/PNF/g-C3N4-TiO2 hetero-aerogel was placed in a 100 mL photocatalytic reactor, and 20 mL of MB dye solution was added under a dark environment. After 1 minute, the reactor was placed under the xenon lamp irradiation, and a small amount of reaction solution was taken out every 20 minutes and the absorbance was measured to calculate the removal rate of MB. When the removal rate remained stable, the reaction was terminated and lasted for about 120 minutes. 

2.6. Sustainability analysis The REP analysis of hybrid hetero-aerogels was performed by considering eight factors, such as the preparation simplicity (P), natural waste utilization (U), Power consumption (W), photocatalytic activity (A), cost (C), environmental friendliness (E), degradability (D), and recyclability (R). According to previous reports, the REP value can be calculated by the following equation