Multiwalled carbon nanotubes in titania based

Materials Research Bulletin 106 (2018) 271–275

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Short communication

Multiwalled carbon nanotubes in titania based nanocomposite as trap for photoexcitons for enhanced photocatalytic hydrogen production under solar light irradiation

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N. Ramesh Reddy, U. Bhargav, B. Chandra Mohan, M. Mamatha Kumari , M.V. Shankar Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science & Nanotechnology, Yogi Vemana University, Kadapa, 516 005, Andhra Pradesh, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon nanotubes Photocatalysis Glycerol Hydrogen Production Solar light

The impact of functionalized multiwalled carbon nanotubes in titania based nanocomposite for photocatalytic hydrogen production was investigated. Founctionalization experiments were carried out to remove amorphous carbon atttached to carbon nanotubes by strong acid treatment followed by calcination at desired temperature and time intervals. Materials characterization revealed that anatase TiO 2 nanoparticles randomly dispersed onto the walls of one dimensional multiwalled carbon nanotubes with surface hydroxyl and carbonyl groups besides significant change in optical absorption. Photocatalytic activity was performed in aqueous glycerol solution under solar light and volume of hydrogen production was quantified using gas chromatography. Highest hydrogen production rate of 13,107 μmol·h −1 ·g−1 of catalyst was recorded under optimized conditions due to cocatalyst role of functionalized carbon nanotubes. Here, photo electrons were trapped by carbon nanotubes for continuous hydrogen production through reduction reaction.

1. Introduction

carrier recombination [7]. CNTs used in this present work were prepared from Arc discharge method. Our earlier research work on Arc discharge synthesized random mixture of carbonaceous structures (MWCNTs, CNHs etc.) showed better photocatalytic H2 production in carbonaceous structures/TiO2 nanohybrids than CVD grown CNTTiO2 composite [8] due to straight walls, defect-free structures which are beneficial for recombination free transport of photogenerated charge carriers onto the surface active sites [9]. This motivated us to explore the beneficial effects of purified arc discharge grown CNTs alone on TiO2 for possible superior photocatalytic hydrogen production. Previous literature regarding CNT-TiO2 composites synthesized through different methods such as hydrothermal, sol-gel and wet impregnation etc. All the synthesized samples were tested for H2 production either under UV or visible light irradiation and that too using harmful sacrificial reagents such as IPA, MeOH, TEOA, and Na2SO3 etc. to enhance the hydrogen production [10,11]. So, there is a need to explore by-products of biomass industry such as glycerol, which is abundantly available, low cost and eco-friendly [12] which can be utilized as sacrificial reagent effectively to produce H2. Further

TiO 2 absorbs ultraviolet radiation that covers a small portion of the solar spectrum and suffers from fast recombination of photogenerated charge carriers. So, in order to absorb the larger portion of solar light and to restrain fast recombination of electron-hole pairs, TiO 2 has been modified with several materials including carbonaceous materials which have been reported extensively [1,2]. Carbon nanotubes (CNTs) became an eye-catching material due to high strength, thermodynamic stability, high surface area, good electrical and tunable optical properties [3]. It was previously reported that CNTs are one of the major materials which offer less agglomeration at nanoscale level [4]. In Pt@MWCNT/TiO2 ternary composite, CNTs are helpful to improve the light absorption capacity for efficient hydrogen (H2) production [5], since H2 is the efficient source for future energy needs [6]. Addition of CNTs increases the absorption edge of TiO2 to visible region, accessing to a bigger share of the solar spectrum and also retard the photo-generated charge

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Corresponding author. E-mail address: [email protected] (M.K. M.).

https://doi.org/10.1016/j.materresbull.2018.06.009 Received 28 March 2018; Received in revised form 18 May 2018; Accepted 5 June 2018 Available online 07 June 2018 0025-5408/ © 2018 Elsevier Ltd. All rights reserved.


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Scheme 1. Synthesis of FCNTs-TiO2 nanocomposite.

temperature such as 1 h, 2 h and 3 h heated at 300 °C, 350 °C, 400 °C, 450 °C respectively as shown in the Scheme 1. Nomenclature of the synthesized nanocomposites by this method was given in Table 1.

Table 1 Nomenclature of Photocatalysts. Photocatalyst

Nomenclature

Functionalized Carbon Nanotubes TiO2 (P-25) FCNTs-TiO2 FCNTs-TiO2-350 °C-1h FCNTs-TiO2-350 °C-2h FCNTs-TiO2-350 °C-3 h

FCNTs TiO2 FCT FCT-1 FCT-2 FCT-3

3. Results and discussion The crystal phase analysis of all the prepared catalysts were explained using X-ray diffraction and Raman spectra as shown dipalyin Figure S1 and S2 respectively. In order to understand the optical properties of FCNTs/TiO2 nanocomposite, UV–vis diffuse reflectance spectra of TiO2, FCT-1, 2, 3 & FCNTs are as depicted in Fig. 1(a). Observed absorption edges of TiO2, FCT-1, 2, & 3 photocatalysts are 373, 394, 408, and 404 nm respectively. The absorption edge of TiO2 is lower than synthesized nanocomposites which is due to the bonding to anti-bonding π electrons and n(nonbonding) to π* anti-bonding transitions between carbon and n-orbit of oxygen species of TiO2 in the nanocomposite. Kubelka–Munk plot of FCNTs, FCT-1, FCT-2, FCT-3, TiO2 is in Fig. 1(b). Average energy bandgaps of FCNTs, FCT-1, FCT-2, FCT-3 & TiO2 are in the order of 1.62, 3.11, 3.0, 3.16 & 3.23 eV respectively. Optimized photocatalyst band gap is reduced to 3.0 eV from 3.23 eV of TiO2, and this depletion in the band gap of the nanocomposite may be attributed to the surface interaction between TiO2 & FCNTs [17]. In order to understand the role of functional groups in the photocatalysts-FCNTs, TiO2 and FCT-2, FTIR analysis had been carried out and as shown in Fig. 1(c). A broad vibration band was associated at 3400 cm−1 with hydroxyl groups on the MWCNTs sidewall surface. The bands at 2919, 2849 cm-1 corresponds to the CeH stretching vibration. The bands at 1728, 1128 and 606 cm-1 were assigned to the C]O, CeO, and CeH stretching vibrations respectively. Intense peak at 616 cm-1 is observed in FCT-2, which is more intensive than that of FCNTs, this may be ascribed to the overlap of CeH, TieOeTi stretching vibrations. A similar type of FTIR peaks in the CNTs-TiO2 composite confirmed the presence of TieOeC bond in the composite which is also the reason for the extension of the absorption edge in the solar spectrum [18]. To know more about the morphology of the synthesized samples, Transmission electron microscopy (TEM) characterization performed. TEM morphology of FCT-2 photocatalyst is as shown in Fig. 2. Observed dark spots in the TEM image shows the TiO2 overlapping with FCNTs

research work using natural sunlight for photocatalytic water splitting is scarce [13]. So, the present work demonstrates the eco-friendly approach for improved photocatalytic H 2 production of FCNTs/TiO2 nanocomposite. In most of the previous reports FCNTs used, were consists of defective walls as CNTs reported in the present work are defect free and straight walls synthesized by arc discharge method. Many related works proposed on utilizing the UV, visible and UV–vis lamps as a light source [14,15], whereas choosing sunlight as the energy source makes the present work more novel and affordable too. Glycerol used in the present work is bio-derived which haven’t impacted the environment negatively and reports containing the glycerol as a sacrificial agent in the combination of FCNTs-TiO2 are limited. 2. Material synthesis Detailed experimental procedure on functionalization of carbon nanotubes, materials characterization and photocatalytic hydrogen production are discussed in supplementary information (S). On the basis of our earlier report [16],wet impregnation method is chosen for the preparation of FCNTs-TiO 2 nanocomposite.1 g of TiO 2 and 0.003 g of FCNTs (S) [16] of each dispersed in ethanol separately and subjected to sonication for 1 h individually. Both solution mixed and kept under sonication for 1 more hour. After completion of sonication, this solution was kept for evaporation at 80 °C. The formed amorphous compound further dried overnight, optimized for various calcination time interval and calcinations

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and intimate mixing of FCNTs and TiO2. It also indicates the uniform dispersion of FCNTs in TiO2. In addition, the presence of TiO2 onto the FCNTs represents the formation of a good surface interaction between the FCNTs and TiO2 by forming TieOeC bond as mentioned in FTIR spectra. TEM image of Fig. 2(a and b) gives the magnified view of the different CNTs surrounded by TiO2 nanoparticles of the FCT-2 sample whereas Fig. 2(c) and (d) shows the high content of TiO2 nanoparticles compared to FCNTs in concurrence with its composition in the nanocomposite. It shows that TiO2 nanoparticles were mainly associated at the surface and sidewalls of FCNTs where the diameter of FCNTs is from 10 to 30 nm (Fig. 2(a and b)) and lengths greater than 100 nm (Fig. 2(b–d)). Hung et al. [19] confirmed the presence of highly loaded TiO2 on CNTs at after calcinations and hence confirmed good surface interaction/bonding between TiO2 and CNTs during calcination. The efficiency of FCNTs/TiO2 photocatalyst for releasing H2 is evaluated under natural sunlight irradiation with glycerol as a sacrificial agent (hole scavenger). Our previous report [20] explained that glycerol acts as the efficient hole scavenger as compared to ethanol and methanol due to the presence of α-hydrogen and the polarity of the aqueous glycerol solution encourage proton generation [21]. The present work uses 5 mg of a photocatalyst for H2 evolution (based on our previous reports and literature survey). Since 5 mg photocatalyst shows the efficient H2 production due to several factors such as good catalyst dispersion in the glycerol-water mixture and also effective in reducing the particle-particle agglomeration [22]. The details of H2 production rates for optimization of calcination temperature (300–450 °C) and time (1–3 h) are as shown in Fig. 3(a) and (b) respectively. As the time of photocatalytic irradiation increases, the effective H2 production also increases because of the progressive increase in the solar light absorption with time. The photocatalytic behavior of the calcined catalyst shows an increase in the volume of H2 production for all the catalysts calcined at 300 °C–450 °C and at 350 °C highest value is recorded. The significant improvement of photocatalytic activity at optimal condition is attributed to better utilization of electron-hole pairs for the splitting of water and H2 generation. At 450 °C, photocatalytic activity depletion is may be due to aggregation of the nanocomposite. Optimized calcination time for highest H2 production of 13107 μmol h−1g−1 h-1 is observed at 2 h in FCT-2 nanocomposite due to the formation of bonding between titanium oxide and carbon (TieOeC). This bond formation leads to the effective charge transfer between the bonded molecules. Production of H2 is also strongly improved because of increase in the crystallization of oxide shell, forming nanocrystalline anatase with the high surface area, good accessibility of the CNTs active phase [23]. Li et al. [24], reported high H2 generation rate of 16.1 μmol h−1 g−1 for CNTs/TiO2 nanocomposite within 2 h under UV–vis light illumination with Na 2S and Na2SO3 as a sacrificial agent. This work reports highest H2 production rate of 13107 μmol h−1 g−1, which is about eight times greater than the previously reported value 1333 μmol h−1 g−1 [25]. Hydrogen production values of other similar literature reports were given in Table 2. This high photocatalytic efficacy of FCNTs-TiO2 nanocomposite is attributed to (i) At optimized conditions the synthesized compound offers effective light absorption and unidirectional flow of electrons along the tube to suppress recombination, which results in

Fig. 1. (a) UV–vis Spectra of TiO2,FCT-1,2,3 h and FCNTs, (b) Kubelka–Munk plot of TiO2, FCT-1, 2,3 h and FCNTs, (c) FTIR of TiO2, FCT-2 and FCNTs.

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Fig. 2. TEM images of catalyst at different magnifications.

higher H2 production values (ii) FCNTs which allow flow of electrons in one direction results reduced electron-hole recombination (iii) Glycerol played role of good hole scavenger [28]. FCNTs has the unique property of unidirectional electron flow (1D-nanostructures) which means the generated charge carriers freely move along tubular nanostructure of FCNTs. In the FCNTs-TiO2 photocatalyst under full spectra, FCNTs enhanced the light absorption ability of TiO2 generating photo charge carriers and migrate those onto the nanocomposite thereby suppress the photogenerated electron-hole recombination to yield high H2 production by the reduction reaction on the surface of the nanocomposite.

4. Conclusions In this work, FCNTs-TiO2 nanocomposites were synthesized by wet impregnation method. In order to achieve better hydrogen production efficiencies optimization has been done by varying the calcinations time and temperature. Finally, the composite calcinated at 350 °C for 2 h showed the enhanced H2 production. TiO2 nanoparticles were deposited on the surface of FCNTs and gave good surface interaction by forming Ti-O-C bond formation as confirmed by TEM and FTIR respectively. This TieOeC bond formation played a key role in enhancing the photocatalytic H2 production. Addition of FCNTs in the nanocomposite extended the absorption into the longer wavelengths of solar light and efficiently transferred the photogenerated charge carriers onto the surface for efficient redox reactions. At the optimized calcination time and temperature, we have explored the highest H2 production of 13,107 μmol h−1 g−1 under solar light irradiation with 5 vol.% aqueous glycerol sacrificial reagent.

Fig. 3. (a) Effect of calcination temperature on FCNTs-TiO2 composite for 2 h, (b) Effect of calcination time interval at 350 °C.

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Table 2 Comparison of H2 Production values with previous reports. S.No

Photocatalyst

Light Source

H2 Generation value (μmol h−1)

Ref.

1 2 3 4 5

TiO2/MoS2/Graphene Pt/TiO2-ZnO Pt/MWCNT/TiO2 CNTs-CNHs/TiO2 FCNTs/TiO2

UV light Mercury Vapor lamp UV-Vis light Solar light Solar light

165.3 203 398.2 2134 13107

[26] [27] [5] [16] Present work

Acknowledgements

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Financial Support from University Grants commission (UGC), New Delhi, India (F.No. 43-405/2014), is gratefully acknowledged. We are thankful to Dr.K.S.V. Krishna Rao, Yogi Vemana University, AP, India, for extending their support to carryout FTIR analysis. We are also grateful to Dr. Venkata Krishnan and Mr.Suneel Kumar from IIT Mandi, India for their constant support and extension of characterization facilities. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2018.06. 009. References [1] M. Hakamizadeh, S. Afshar, A. Tadjarodi, R. Khajavian, M.R. Fadaie, B. Bozorgi, Improving hydrogen production via water splitting over Pt/TiO2/activated carbon nanocomposite, Int. J. Hydrogen Energy 39 (2014) 7262–7269, http://dx.doi.org/ 10.1016/j.ijhydene.2014.03.048. [2] V.N. Rao, N.L. Reddy, M.M. Kumari, P. Ravi, M. Sathish, B. Neppolian, M.V. Shankar, Synthesis of titania wrapped cadmium sulfide nanorods for photocatalytic hydrogen generation, Mater. Res. Bull. 103 (2018) 122–132, http://dx.doi. org/10.1016/j.materresbull.2018.03.030. [3] K.C. Nguyen, M.P. Ngoc, M. Van Nguyen, Enhanced photocatalytic activity of nanohybrids TiO2/CNTs materials, Mater. Lett. 165 (2016) 247–251, http://dx.doi. org/10.1016/j.matlet.2015.12.004. [4] X. Wang, Y. Chen, B. Zheng, F. Qi, J. He, B. Yu, W. Zhang, Significant enhancement of photocatalytic activity of multi-walled carbon nanotubes modified WSe2 composite, Mater. Lett. 197 (2017) 67–70, http://dx.doi.org/10.1016/j.matlet.2017.03. 150. [5] Y. Haldorai, A. Rengaraj, J.B. Lee, Y.S. Huh, Y.K. Han, Highly efficient hydrogen production via water splitting using Pt@MWNT/TiO2 ternary hybrid composite as a catalyst under UV-visible light, Synth. Met. 199 (2015) 345–352, http://dx.doi.org/ 10.1016/j.synthmet.2014.12.014. [6] N. Lakshmana Reddy, V.N. Rao, M.M. Kumari, P. Ravi, M. Sathish, M.V. Shankar, Effective shuttling of photoexcitons on CdS/NiO core/shell photocatalysts for enhanced photocatalytic hydrogen production, Mater. Res. Bull. 101 (2018) 223–231, http://dx.doi.org/10.1016/j.materresbull.2018.01.043. [7] Y. Yao, G. Li, S. Ciston, R.M. Lueptow, K.A. Gray, Photoreactive TiO2 /Carbon nanotube composites: synthesis and reactivity, Environ. Sci. Technol. 42 (2008) 4952–4957, http://dx.doi.org/10.1021/es800191n. [8] M.M. Kumari, A. Priyanka, B. Marenna, P. Haridoss, D.P. Kumar, M.V. Shankar, Benefits of tubular morphologies on electron transfer properties in CNT/TiNT nanohybrid photocatalyst for enhanced H2 production, RSC Adv. 7 (2017) 7203–7209, http://dx.doi.org/10.1039/C6RA26693B. [9] J. Prasek, J. Drbohlavova, J. Chomoucka, J. Hubalek, O. Jasek, V. Adam, R. Kizek, Methods for carbon nanotubes synthesis—review, J. Mater. Chem. 21 (2011) 15872, http://dx.doi.org/10.1039/c1jm12254a. [10] C.R. López, E.P. Melián, J.A. Ortega Méndez, D.E. Santiago, J.M. Doña Rodríguez, O. González Díaz, Comparative study of alcohols as sacrificial agents in H2 production by heterogeneous photocatalysis using Pt/TiO2 catalysts, J. Photochem. Photobiol. A Chem. 312 (2015) 45–54, http://dx.doi.org/10.1016/j.jphotochem. 2015.07.005. [11] M. Wang, S. Shen, L. Li, Z. Tang, J. Yang, Effects of sacrificial reagents on photocatalytic hydrogen evolution over different photocatalysts, J. Mater. Sci. 52 (2017)

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