Elsevier

Applied Surface Science

Volume 463, 1 January 2019, Pages 747-757
Applied Surface Science

Full Length Article
Metals free MWCNTs@TiO2@MMT heterojunction composite with MMT as a mediator for fast charges separation towards visible light driven photocatalytic hydrogen evolution

https://doi.org/10.1016/j.apsusc.2018.08.240Get rights and content

Highlights

  • MMT-decorated MWCNTs/TiO2 nanocomposite tested for H2 production from glycerol-water mixture.

  • H2 production efficiency depends on catalyst composition, reaction conditions and particle size.

  • H2 yield over MMT-MWCNTs/TiO2 photo catalyst was 3.05 and 7.12 folds higher than MMT/TiO2 andTiO2.

  • PTOP of H2 production over MMT fabricated MWCNTs-TiO2 was 5.94 folds higher than TiO2.

  • MMT-MWCNTs/TiO2 nanostructured catalyst gave higher stability in cyclic runs.

Abstract

Constructing heterojunctions with low-cost materials functional under solar energy is an important way for efficient hydrogen production for energy management applications. In this study, multiwall carbon nanotubes (MWCNTs) modified TiO2 1D/0D heterojunction dispersed in 2D montmorillonite (MMT) nanoclay was designed and fabricated using a facile sol-gel and wet impregnation approach. The samples were characterized by XRD, Raman, FESEM, HRTEM, FTIR, UV–vis and PL-spectroscopy techniques. Dispersing 1D/0D MWCNTs/TiO2 on the 2D MMT structure extends visible light absorption, shorten charges migration distance and provides abundant active sites. The performance of samples was investigated for dynamic photocatalytic H2 evolution with different sacrificial reagents under visible light irradiations. The MWCNTs/TiO2/MMT heterojunction composite exhibits the highest H2 evolution rate of 1888 ppm h−1, which is about 1.49, 3.07 and 7.12 folds higher than MWCNTs/TiO2, MMT/TiO2 and pure TiO2 samples, respectively. Among the different sacrificial reagents, highest H2 production was obtained using glycerol-water mixture due to the presence of α-hydrogen atoms attached to carbon atoms. Next, the photocatalytic turn-over productivity (PTOP) was estimated which demonstrated the molecular H2 production rate with the photon-energy utilization. The maximum PTOP for H2 production was achieved over MWCNTs/TiO2/MMT heterojunction composite, 5.94 times higher than using bare TiO2 NPs. The improved charges efficiency due to the synergistic effect between MWCNTs/TiO2/MMT in 1D/OD/2D heterojunctions is critical for enhanced and dynamic H2 evolution. The ternary composite exhibited excellent and stable performance for H2 production. This study suggests that constructing heterojunction of low cost coupling materials with TiO2 can provide an efficient way for H2 production under solar energy.

Introduction

Combination of light energy and nano-materials have been accounted as one of the most favorable technologies, which can mitigate the issues of global energy crisis and climate change [1], [2]. In particular, due to potential applications in eliminating environmental harmful contaminants, semiconductor photo-catalysis functional under visible light have gained considerable attentions [3], [4]. Among all, photo-catalytic H2 evolution is a clean, economical, reliable and environment friendly process and has invoked more attractions as an efficient approach for mitigating GHGs and recycling industrial effluents [5], [6]. In the pioneer work, Honda and Fujishima in 1972 [7] successfully demonstrated photocatalytic H2 generation by water splitting using semiconductor photo-catalysts. Instead of various research efforts, the stability and effectivity of semiconductor materials for photocatalytic water-splitting to molecular H2 product is not much impressive to fulfill the standards required for industrial applications [8]. However, H2 evolution rates can be enhanced by several times with the involvement of renewable sacrificing reagents such as methanol, ethanol, glycerol and glucose to gain more and more industry attention [9], [10], [11]. Among the alcohols, the glycerol could be used as an efficient sacrificing agent for dynamic H2 production to make the photocatalytic process more sustainable compared to pure water [12].

A large number of nanostructured photocatalysts such as Ag loaded ZnO nanosheets [13], CdS coupled g-C3N4 composite with RGO [14], WO3 coupled with g-C3N4 [15], ZnO [16], GO supported on ZnS [17], ZnS2 [18], and TiO2 [19] have been successfully investigated for efficient H2 production. Among all, TiO2 is recommended as a superior candidate because of its numerous advantages such as low-cost, non-corrosive, abundantly available and high chemical/thermal stability. However, high band gap energy (3.2 eV) and lower efficiency due to rapid recombination of photo-induced electron/hole (e/h+) are the limitations [20]. TiO2 efficiency can be improved under solar energy by band structuring, controlled morphology, photosensitizations with metals/non-metals and heterojunctions with low band gap materials [21], [22], [23]. TiO2 modification with various metals and semiconductors such as Pt doped graphene/TiO2 [24], Au loaded TiO2/CdS [25], Cu-TiO2 [26], Ag2O/TiO2, Pt doped TiO2/ZnO [27] and Au doped TiO2 [28] etc., have been reported with improved hydrogen production. Although, significant research has been conducted, yet low-cost and noble metals free materials as co-catalyst to design highly efficient TiO2 photo-catalyst functional under solar light are highly demanding.

Previously, carbon nano-structured have been explored as efficient coupling materials for TiO2 based nano-structured composites due to their electron-accepting and electron-transmitting properties [29]. For example, single and multi-walled carbon nanotubes (SWCNTs, MWCNTs), fullerene (C60) and graphene oxide (GO) have been coupled with TiO2 to get enhanced photocatalytic efficiency [30], [31], [32]. Among the carbon materials, MWCNTs exhibit extraordinary electrical, thermal, optical and chemical properties with high surface area that makes them conductive under visible spectrum [33], [34]. Due to high surface area, MWCNTs can be beneficial for suppression of electron/hole pairs recombination with efficient adsorption-desorption process. TiO2 nanoparticles loaded with MWCNTs would be fruitful to maximize visible light absorption, transmitting electrons and increased separation of charge carriers [35], [36]. Numerous photo-catalysts have been utilized for H2 production over MWCNTs such as Ag doped TiO2-MWCNTs [37], WO3-MWCNTs [38] and CdS-CNTs [39]. In addition, the efficiency of MWCNTs/TiO2 can be further improved by using layered natural clay due to their multifunctional properties, low cost, abundantly availability and environmental friendly to make sustainable photocatalytic process for hydrogen production.

Recently, multi-layered clay minerals are gaining much attractions due to their high stability, good conductivity, recyclability and large surface area [40], [41]. Among the available natural clays, montmorillonite (MMT) has gained more considerations due to pillared 2D layered structure, efficient adsorption due to microporous structure, remarkable charge capturing dexterity and excellent cation transfer potential [42]. These distinguished properties of MMT make it much attractive to be used as a co-catalyst in TiO2 surface modification. In this perspective, TiO2/MMT [43] and ZnS/MMT [44] composite catalysts for CO2 reduction applications have been reported with improved efficiency and selectivity. In the recent developments, Ni decorated over MMT/TiO2 nano-catalyst was successfully tested for photo-driven evolution of H2 and organic compounds via aqueous ethanol water solution under ultra-violet (UV) light irradiations [45]. The photoactivity of MMT modified TiO2 nano-composite can be further enhanced by loading with photo-sensitizers such as CNTs. Therefore, the dispersion of MWCNTs/TiO2 in the 2D layered structured of MMT would enable to develop 1D/0D/2D heterojunctions for enhanced photocatalytic H2 production under solar energy irradiations. To the best of our knowledge, MWCNTs/TiO2/MMT heterojunction composite has never been reported in photocatalytic water splitting applications.

Herein, the design and fabrication of novel 1D/0D MWCNTs/TiO2 structure dispersed in 2D layered MMT to synthesize 1D/0D/2D heterojunctions via single step sol-gel and wet impregnation method has been investigated. The performance of newly developed composite materials was analyzed for photocatalytic hydrogen generation from glycerol aqueous mixture in a slurry type photo-reactor system under visible light irradiations. Different operating parameters such as loading of MMT and MWCNTs over TiO2 nano-particles and different concentrations of feed mixtures were also studied. Moreover, photo-catalytic turn over productivity (PTOP) of the heterojunction composite was reported for the first time to estimate the concentration of molecular H2 produced based on photons energy and specific surface area. Stability analysis of heterojunction composite was carried out to estimate the duration and reliability of nanostructured composite in cyclic runs. Furthermore, the schematic mechanism for H2 evolution in the presence of photo-catalyst, solar light and glycerol aqueous mixture was also proposed.

Section snippets

Preparation of MMT/TiO2 composite sample

Commercially available Titanium (IV) iso-propoxide (sigma-aldrich) and montmorillonite (Bentonite, sigma-aldrich) were used to prepare MMT/TiO2 nano-structured catalysts. The crystalline structured and multilayered MMT dispersed TiO2 nano-structured composites were constructed via a single step sol-gel process. Typically, sample ‘A’ was synthesized by adding slowly 10 mL titanium solution in 25 mL 2-propanol (C3H8O) solution and continually stirred for 40 min in a round shaped bottom flask.

Materials characterization

To determine crystalline phase of synthesized TiO2, MWCNTs, MMT, MMT/TiO2 and MWCNTs/TiO2/MMT heterojunction composite, XRD analysis was conducted and the results are presented in Fig. 2. It was observed that TiO2 nanoparticles exhibited pure anatase and crystalline phase peaks positioned at 2-theta of 25.53°, 37.91°, 48.06°, 53.92°, 55.16°, 62.82°, which are aligned with (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) crystal planes. The diffraction peak of MMT at 2θ = 3.70°

Conclusions

MWCNTs/TiO2/MMT ternary heterojunction composite was synthesized via single step sol-gel and impregnation methods and tested in slurry phase photo-reactor using glycerol aqueous mixture for dynamic hydrogen evolution. The highest product rate of H2 was achieved with MWCNTs/MMT/TiO2 photocatalyst than several other composite photocatalysts, which exhibits 7.12 folds more H2 generation (1888 ppm/h) compared to bare TiO2 (250 ppm/h). The synergistic effect within MWCNTs and MMT promotes visible

Acknowledgements

The authors would like to extend their deepest appreciation to Ministry of Education (MOE) Malaysia for financial support of this research under FRGS (Fundamental Research Grant Scheme, Vot 4F876) Malaysia, and University Research Grant (GUP, Vot 17H6).

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