Received: 25 Aug 2018
Accepted: 29 Nov 2018
Published online: 30 Nov 2018
Siyao Guo,1,2 Jun Shang,1,2 Tiejun Zhao,1,2 Dongshuai Hou,3* Zuquan Jin1,2 and Guoxing Sun,1,2*
1School of Civil Engineering, Qingdao Technological University, Qingdao 266033, China
2Collaborative Innovation Center of Engineering Construction and Safety in Shandong Blue Economic Zone, Qingdao 266033, China
3Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau, China
A novel TiO2/β-cyclodextrin (TiO2/β-CD) organic-inorganic hybrid nano-material with alveolate structure is fabricated using a facile one-step strategy. The alveolate TiO2/β-CD hybrid nano-material exhibits outstanding photocatalytic activity and recyclability on photocatalytic water splitting. The production of H2 evolution of TiO2/β-CD hybrid reaches to 5800 μmolg-1 after 8 h without noble metal. Meanwhile, the recyclability of the TiO2/β-CD photocatalyst is found to be no obvious decrease with the H2 generation after third successive runs. The formation of the oxygen defects occurred on coordinatively unsaturated Ti-sites by the introduction of β-CD at the outer surface of TiO2 could be the predominant active species in the photocatalytic system. Due to the alveolate heterogeneous structure of TiO2/β-CD hybrid play a role as “channel” for photocatalytic activity. The channel can capture more electrons and light, provides a larger area for reaction. This work provides a promising approach to construct noble metal-free and more stabilized organic-inorganic nanocomposite photocatalysts applied in the photocatalytic water splitting.
Table of Content
A novel alveolate-structured Titania/β-cyclodextrin organic-inorganic hybrid nano-material with high hydrogen production in photocatalytic water splitting.
Keywords: Organic-inorganic hybrid; Tinatia; β-cyclodextrin; Photocatalytic H2-production
Photocatalytic water splitting of hydrogen production is an attractive way for utilization of inexhaustible and clean solar energy.1 Many photocatalysts have been used to improving the hydrogen production of water splitting, especially heterostructured nanoparticles play an important role in photocatalytic water splitting due to the advantages of improving the quantum yield, inhibiting photo-generated carriers recombination and providing reaction active sites, etc. In the last years many hybrid catalysts have been designed and applied for photocatalytic hydrogen production.2-4
TiO2 nanoparticles (NPs) have been extensively investigated as promising state-of-the-art photocatalysts due to their strong oxidizing power, non-toxic and simple synthesis.5 However, the poor adsorption performance, low cycle utillzation and difficult recycling also limited its application.6,7 In recent years, many methods have been used to improve the photoactivity of TiO2, such as metal loading, dye sensitization, composite semiconductor, and anion doping.8-17 But it is found that these methods are not effective routes to imporve the photocatalytic water splitting of TiO2 nanoparticles. Moreover, as an efficient co-catalysts, noble metals or noble oxides are commonly used in photocatalytic hydrogen generation due to their promotional effect on electrone-hole pair separation. However, the high cost and scarcity of noble metals hampered their application in the water splitting of hydrogen production.7,18-20,29
Hybrid organic-inorganic materials (HOIMs) aroused wide concern due to their inherent advantages of structure. One of the appealing feature for HOIMs is their unique property which is difficult to achieve in either inorganic material or organic material alone.21 Compared with homogenous materials, the emerging organic-inorganic material have the advantages of dimensional stability which is stemming from the organic phase, as well as the reliability and high catalytic performance which is deriving from the inorganic phase.22-26 Although a variety of organic-inorganic materials have been synthesized with superior performance, it is still hard to realize controllable synthesis, and the special heterogeneous structure is difficult to synthesis. Meanwile, using cyclodextrin (CD) to modify nanomaterial caused concern because it can provide a stable skeleton structure for nanomaterial, and easy to form a unique heterostructure.27 Nanomaterial can be endowed with cyclodextrin structure after by modification with CD, which result in more efficient functions and guest-targeting of the TiO2 for cyclodextrin, and cyclodextrin play a role as a ‘‘bridge” and “channel’’ on the surface of the TiO2 nanomaterial.28
In this work, a kind of alveolate TiO2/β-cyclodextrin (TiO2/β-CD) organic-inorganic hybrid nano-material has been developed. Notably, the novel TiO2/β-CD nano photocatalyst exhibits outstanding performance on photocatalytic H2 generation. It also exhibits extremely high cycle performance and recyclability. The recyclability of the TiO2/β-CD photocatalyst was found to be no obvious decrease with the H2 generation after third successive runs. We conclude that this alveolate TiO2/β-CD organic-inorganic hybrid nano-material could be expected to be applicable in photocatalytic H2 generation due to the advantages of strong adsorption, simple systhesis procedure and high cycle utilization performance.
β-cyclodextrin was recrystallized twice and then dried before use. All other chemicals were of the analytical grade and used without further purification. Tetrabutyl orthotitanate (20 ml) was added in deionized water (100 ml) dropwise with vigorous stirring. The solutions were stirred for 24 hours at room temperature (25 ℃). And then the white precipitate was washed with deionized water and separated from the liquid phase by centrifugation. The product was dried at 80 ℃ overnight and ground into powders. The titania (2 g) and β-CD (2 g) were added in deionized water (150 ml) with vigorous stirring. After homogenization for several hours, the mixed solution obtained was transferred into a teflon-lined autoclave for crystallization at 160 ℃ for 12 h. The resulting product was washed with deionized water by centrifugation. The final product was dried at 80 ℃ for 12 hours.
The procedure for synthesize of TiO2/β-CD organic-inorganic hybrid nano-material was depicted in Fig. 1a. XRD patterns of TiO2/β-CD nano-material are presented in Fig. 1b. The XRD analysis of hybrid reveals that the nano-material exhibits single-phase which belongs to anatase-type TiO2, it is identified by comparing the above spectra with the JCPDS file #21-1272. Diffraction peaks at 25.28º, 37.80º, 48.05º, 53.89º, 55.06º and 62.69º, which is corresponding to (101), (004), (200), (105), (211) and (204) planes of TiO2, respectively.29 The relatively high intensity of the peak for (101) plane is an indicative of anisotropic growth, implying a preferred orientation of the crystallites. Meanwhile, the synthesized TiO2/β-CD samples presented XRD patterns similar to pure TiO2, there is no obvious characteristic peak of β-CD could be found, implying highly uniform dispersion β-CD nanoparticles in TiO2 matrix. The similiar result is also reported by Zhang et al.30
Fig. 1 (a) Schematic diagram of the synthesize of TiO2/β-CD organic-inorganic hybrid nano-material (b) X-ray powder diffraction patterns of TiO2- β-CD
The TiO2/β-CD hybrid nano-material was studied by FE-SEM to research its structure and morphology, as shown in the Fig. 2. The image revealed that TiO2/β-CD has an alveolate heterogeneous structure with similiar aperture of 40 nm. This kind of porous structure is easy to provide more active sites for the photocatalytic performance, and there are many spatial distribution of the hydroxyl groups in the β-CD structure, Thus β-CD could help a lot in capturing the photo generated carriers.33
Fig. 2 FE-SEM of the TiO2/β-CD sample
Fig. 3 (a) Hydrogen evolution of the samples under the xenon lamp irradiation (b) Cycling runs for the photocatalytic hydrogen evolution
The photocatalytic water splitting of hydrogen evolution activity over pure TiO2 and TiO2/β-CD samples was evaluated under visible light illumination (λ＞420nm). Fig. 3a shows a typical time course of hydrogen evolution for the photocatalytic water splitting of the prepared samples. Stoichiometric evolution of hydrogen is evident from the start of the reaction, and there is a steady hydrogen increase throughout the entire run. TiO2/β-CD has significantly higher H2 evolution rates of 5800 μmol/g, which demonstrates it is quite effective to employ β-CD as cocatalyst for improving efficiency of TiO2 photocatalytic activity. with no observable photocatalyst activity decay. Fig. 3b shows the stability of photocatalytic H2 evolution using the TiO2/β-CD photocatalyst as the representative sample. The recyclability of the TiO2/β-CD photocatalyst was found to be high with the H2 generation at 70% of the initial value after third cycles, revealing the superior long-term stability of TiO2/β-CD nanocomposites.
The oxidation level for H2O to H2O2 or O2 is above the valence band (VB), and the the conduction band (CB) is high than the reduction level of hydrogen.34,35 These bands respectively are easily to allow migration of photo-induced holes and electrons of photocatalytic water splitting.36 In this study, we used hydrothermal synthesis method to prepare TiO2/β-CD hybrid organic-inorganic structure, so it is easy to form binding between cyclodextrin and TiO2 surface due to the adhesion of the hydroxyl functional groups on the surface. Cyclodextrin would capture holes on active TiO2 surface resulting in the formation of stable organic-inorganic hybrid composites. On the other hand, β-CD could play a role as “bridge” or “channel” for capturing more photo-induced electrons and light, and also provide a larger area for reaction. When TiO2/β-CD organic-inorganic structure is irradiated by solar light, electrons will be photoexcited into the conduction band (CB), this process will result in the generation of holes on the valence band (VB). In the presence of β-CD, the photo-induced electrons will quickly transfer to the CB of TiO2 rapidly, and the H+ of water will be reduced into H2. The result implying that the role of cyclodextrin on TiO2 nanoparticle acts not only as a molecular transfer channel but also as an electron donator. The synergy between TiO2 and β-CD is in favor of the energy transfer from the TiO2 to the β-CD and lead to a high photocatalytic acitivity.37 Hence, the photocatalytic water splitting of TiO2/β-CD for hydrogen production are remarkably improved under the condition of solar light irradiation.
The hydrogen/oxygen generation of the basic principle of photocatalytic reactions is depicted in the Fig. 4. In the process of photocatalytic water splitting, the e−/h+ pairs photogenerated on the TiO2 particles will move to the β-CD surface where the redox reaction will take place under the photoexcitation progress. The oxidation of water is slow than the speed of oxidation of methanol. On the oxidative side, the Ti–OH groups of the β-CD surface will react with the photogenerated holes which result in producing trapped holes, in another way, β-CD may adsorb water molecules and form adsorbed •OH radicals. Moreover, it seems indicate that methanol may play an important role in the production of hydrogen production. Sometimes it seems difficult to calculate the exact source of the protons yielding H2 from methanol or water, or rather whether source of H+ is belonged to water or CH3OH.
Fig. 4 The basic principle of photocatalytic reactions for hydrogen/oxygen generation using electron donors/acceptors as the sacriﬁcial reagent
In this work, we demonstrated a kind of TiO2/β-CD organic-inorganic hybrid nano-material with alveolate structure via a facile one-step strategy in low temperature. Because of its better adsorption capability, efficient separation and migration of photo-induced electron-hole pairs and larger BET surface area, the supramolecular TiO2/β-CD hybrid exhibited considerably enhanced photocatalytic activity and good cycle performance towards the H2 evolution. This TiO2/β-CD nano photocatalyst could be expected to be applicable in photocatalytic water splitting under solar light due to its merits of simple procedure, recyclability and high catalytic activity. And it opens up new possibilities and opportunities for developing other hybrid organic-inorganic materials by incorporating a large array of organic materials for a variety of technological applications in the solar hydrogen production or environmental cleaning.
This work was supported by the Fundamental Research Funds for the Central Universities (DL11EB02), the National Natural Science Foundation of China (51508293), China Postdoctoral Science Foundation Funded Project (2016M600527). The China Ministry of Science and Technology under Grant 2015CB655100, and Major International Joint Research Project under Grant 51420105015 are gratefully acknowledged. The first author also would like to acknowledge the fellowship support received from the Tai Shan Scholar Program.