Journal of NanomaterialsVolume 2012 (2012), Article ID 794625, 6 pagesdoi:10.1155/2012/794625

Research Article

Preparation and Photocatalytic Properties of Coated on Nitrogen-Doped Carbon Nanotubes

Lingling Wang,¹ Long Shen,² Luping Zhu,¹ Haiying Jin,¹ Naici Bing,¹ and Lijun Wang¹

¹School of Urban Development and Environment Engineering, Shanghai Second Polytechnic University, 2360 Jinhai Road, Shanghai 201209, China²Research and Development Department, Shanghai Shanshan Technology Co. Ltd., 3158 Jinhai Road, Shanghai 201209, China

Received 18 October 2011; Revised 15 December 2011; Accepted 29 December 2011

Academic Editor: Suprakas Sinha Ray

Copyright © 2012 Lingling Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

SnO₂ nanoparticles coated on nitrogen-doped carbon nanotubes were prepared successfully via a simple wet-chemical route. The as-obtained SnO₂/CNx composites were characterized using X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. The photocatalytic activity of as-prepared SnO₂/CNx for degradation Rhodamine B under UV light irradiation was investigated. The results show that SnO₂/CNx nanocomposites have a higher photocatalytic activity than pure SnO₂ and SnO₂/CNTs nanocomposites. This enhanced photoresponse indicates that the photoinduced electrons in the SnO₂ prefer separately transferring to the CNx, which has a high degree of defects. As a consequence, the radiative recombination of the electron-hole pairs is hampered and the photocatalytic activity is significantly enhanced for the SnO₂/CNx photocatalysts.

1. Introduction

Carbon nanotubes (CNTs) have been the focus of intensive study due to their unique structural, electrical, and mechanical properties since their discovery by Iijima [1]. Their potential applications include nanodevices, quantum wires, ultrahigh-strength engineering fibers, sensors, and catalyst supports [2–4]. Recently, the study of CNT-based composites is becoming a promising and challenging area of research owing to their improved properties [5–7]. Of them, CNT-based metal oxide hybrids represent an important class and are very promising as functional materials in lithium-ion batteries, supercapacitors, and photocatalysts [8].

However, to fabricate CNT-based composites, the CNTs required a pretreatment process to modify their inert surface nature via harsh processes for activation by refluxing in concentrated acids, which destroys the π conjugation and reduces the conductance of the CNTs base [9]. Unfavorably, the harsh process would risk CNTs to some damages in their inherent properties. To bypass the drawbacks suffered by CNTs, employing nitrogen-doped carbon nanotubes (CNx) without requiring any pretreatment to composite with the functional materials directly is a promising method, because the nitrogen atoms on the surface of the CNTs modifies the adsorption strength of the nanotubes towards foreign elements which can influence the dispersion of active phase. Moreover, nitrogen atoms in the framework of CNx will form chemically active points which are available for metal or metallic oxide nanoparticles anchoring. Ghosh prepared ZnO/CNx composites via a simple wet-chemical method and studied their field emission performance [9]. Yue and coworkers obtained Pt/CNx composites which presented obvious catalytic activity for methanol oxidation [10]. CNx decorated with CeO₂ and SnO₂ nanoparticles showed greater activity and sensitivity than the conventional CNT-based composites for NO electrooxidation [11].

SnO₂, a stable and large n-type bandgap ( = 3.6 eV) semiconductor, has the ability to degrade dyes under UV light [12]. Recently, a nanocomposite of SnO₂ nanoparticles coated on CNTs was synthesized and exhibited excellent photocatalytic activity due to the electron transfer between SnO₂ and CNTs [13]. However, the photocatalytic properties of SnO₂ nanoparticles coated on CNx (SnO₂/CNx) have been scarcely investigated.

In this study, according to the unique properties of CNx-based metal oxide, we have synthesized SnO₂/CNx composites via a simple and efficient room-temperature chemical route. The as-synthesized nanocomposites were well characterized and their photocatalytic activities were evaluated by the photodegradation of RhB.

2. Experimental

2.1. Synthesis of SnO₂/CNx

Following the procedures reported previously [14], CNx was synthesized using ethylene diamine and diethylamine as the carbon and nitrogen source.

Synthesis of SnO₂/CNx nanocomposites was according to the following procedures, which were modified from the reference [15]. 50 g of tin (II) chloride (anhydrous, Alfa, 99%) was put into 500 mL of distilled H₂O inside a glass flask, and then 70 mL of HCl (38%) was added. We then immersed 1 g of CNx in the above solution. This solution was sonicated for 10 min and then stirred for 60 min at room temperature. SnO₂/CNx nanocomposites were then obtained after centrifuging, washing with ethanol and deionized water for several times, and drying at 80°C for 8 h. SnO₂/CNTs composites were synthesized using the similar procedures besides CNTs pretreated in concentrated HNO₃ in 140°C for 14 h, and pure SnO₂ sample was synthesized without adding CNTs.

2.2. Characterization

The bare CNx and the composites were characterized by a range of analytical techniques. The degree of crystallinity of the SnO₂/CNTs composites was characterized by powder X-ray diffraction (XRD). The XRD patterns with diffraction intensity versus were recorded in a Bruker D8 ADVANCE instrument with Cu-Ka radiation ( = 1.5418 Å) from 10° to 80° at a scanning speed of 0.02 o/s. X-ray tube voltage and current were set at 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) was carried out on Hitachi S-4800 with an acceleration voltage of 5 kV. Transmission electron microscopy (TEM) was carried out on JEOL-JEM-1005 at 200 kV. The specimens for SEM and TEM imaging were prepared by suspending solid samples in ethanol with 15 min ultrasonication and placing a drop of this mixture on a 3.05 mm diameter copper mesh (300 lines/in. mesh), which was then dried in air.

2.3. Photodegradation of RhB

The photoreactor was designed with a cylindrical quartz cell configuration and an internal light source surrounded by a quartz jacket, where RhB aqueous solution completely surrounded the light source. An external cycled cooling flow of water was used to maintain the reaction temperature constant.

Photocatalytic experiments were carried out by adding 0.02 g SnO₂ or SnO₂/CNTs composites or SnO₂/CNx composites into photoreactor containing 30 mL RhB solution with an initial concentration of 10 mg/L. The RhB solution with catalyst was stirred in the darkroom for 10 h to reach the equilibrium of surface adsorption. Then the stirred suspensions were illuminated with a 300 W high-pressure mercury lamp 10 cm high over the solution. The solution was stirred continuously during the photocatalytic reaction. The change of RhB concentration with the illumination time was monitored by determining the absorbance at 555 nm [16], using Shimadzu UV-2550 spectrophotometer.

3. Results and Discussion

The XRD patterns of pure SnO₂, pure CNx, and SnO₂/CNx composites are displayed in Figure 1. The main peaks at 26.1° and 46.7° for CNx were similar to those of graphite, which indicated that the employed CNx were highly graphitized. The typical XRD patterns of the pure SnO₂ are illuminated in Figure 1(a). All the diffraction peaks are indexed to the tetragonal rutile structure of SnO₂ (JCPDS card no. 41–1445). No diffraction peaks from impurities are observed. Different amine for synthesis CNx has no obvious effect on the formation of SnO₂/CNx composites. The diffraction angle for SnO₂/CNx composites at = 33.6°, 51.8° can be assigned to 101 and 211 planes of the cassiterite SnO₂, respectively. The peak at 26.3° attributed to the peak of the 110 planes of SnO₂ overlapped with the peak of graphitic reflection from CNx. Average crystalline size can be estimated by Scherrer’s formula: where is the grain diameter, (0.89) is the shape factor, is the X-ray wavelength of Cu K radiation (0.154 nm), is the Bragg angle, and is the experimental full-width half-maximum (fwhm) of the respective diffraction peak. The calculated mean particle size of SnO₂ nanoparticles was calculated to be about 4 nm.

Figure 1: XRD patterns of (a) pure SnO₂, (b) pure CNx and SnO₂/CNx composites prepared with CNx obtained from (c) ethylene diamine and (d) diethylamine.

Figures 2 and 3 show the SEM and TEM images of pure CNx and SnO₂/CNx composites. CNx with relatively homogeneous (external) diameter (30 ~ 60 nm) were obtained, and the nanomaterial was composed of individual nanobamboo stacked together to form the long nanofibers. However, after coating with SnO₂, all CNx are uniformly coated with a layer of SnO₂ nanoparticles, and no free nanoparticles were found. The SnO₂ nanoparticles deposited on the surface of CNx are separated and the particle size is uniform. Figure 3(c) is TEM image of an individual CNx fully coated with SnO₂. The bamboo-like morphology of CNx can be also clearly observed, and its surface is entirely and homogeneously covered by SnO₂ nanoparticles. The intimate contact between CNx and SnO₂ favors the formation of junctions between the two materials, as a result, being helpful for improving the charge separation and thus the photocatalytic activity.

(a)

(b)

Figure 2: SEM image of (a) pure CNx and (b) SnO₂/CNx composites.

Figure 3: (a) TEM image of pure CNx. (b) Low-resolution TEM image of CNx bundles fully coated with SnO₂. (c) Low-resolution TEM image of an individual CNx fully coated with SnO₂. (d) High-resolution TEM image in selected location.

The size of SnO₂ nanoparticles on the surface of CNx is about 4 nm, which is consistent with the XRD result. Chemical analysis using EDS further confirms the presence of Sn, O, C, and Cu in the SnO₂/CNx composites (Figure 4). The C signal originates from the supporting CNx.

Figure 4: Typical EDS spectrum of SnO₂/CNx composites.

The photocatalytic activity of SnO₂/CNx composites was evaluated for the degradation of RhB in aqueous solution. Figure 5 shows that the spectrum changes during RhB (10 mg/L) photodegradation by SnO₂/CNx photocatalysts with reaction time. With the increase of irradiation time, the intensity of the maximum adsorption peak located near 555 nm decreased gradually, indicating the degradation and mineralization of the RhB dye solutions.

Figure 5: Temporal UV absorption spectral changes of RhB solution as a function of irradiation time (RhB = 10 mg/L).

Figure 6 presents the degradation profiles of RhB in the presence of SnO₂/CNx composites under UV light irradiation, together with CNx, SnO₂, and SnO₂/CNTs composites for comparison. Almost no degradation was detected when CNx alone was used as the catalyst. The photocatalytic activity of SnO₂/CNTs and SnO₂/CNx composites is much higher than that of the sole SnO₂. SnO₂/CNTs with higher photocatalytic activity than pure SnO₂ has been also observed in the literature [13]. It is noteworthy that SnO₂/CNx composites show superior activity to SnO₂/CNTs composites. With the reaction time at 50 min, the RhB degradation efficiencies of pure SnO₂ and SnO₂/CNx composites catalysts are about 70% and 80%, respectively. However, the value of SnO₂/CNx is 89%. Hence, SnO₂/CNx is an excellent photocatalyst in our experiment.

Figure 6: Comparison of photocatalytic degradation rate between pure SnO₂, SnO₂/CNx composites, and SnO₂/CNTs composites.

On the basis of the literature [13], a band configuration at the interface of the SnO₂/CNx composites is proposed, as shown in Figure 7. Under UV irradiation, the valence band electrons of SnO₂ can be excited to its conduction bands, giving rise to the formation of high energy electron-hole pairs. Heterojunction promoting the photocatalytic activity has been reported in several cases of semiconductor oxides and carbon nanotubes [17, 18]. They attributed the improvement of photocatalytic performance to the promotion of separation rate of photogenerated electron and hole by the formation of heterostructure.

Figure 7: Schematic diagram showing band configuration and electron-hole separation at interface of SnO₂/CNx nanocomposites under UV irradiation (CB—the bottom of conduction band, VB—the top of valence band).

As regards to CNx, N not only acts as a direct substitute impurity, but also affects the CNTs properties in distinctive manners. In our work, the enhanced photocatalytic activity of CNx may attribute to the following two possible reasons. Firstly, a high degree of defects were introduced by nitrogen doping [19], the electrons generated by SnO₂ can transfer into CNx, which could be taken for a larger capacity container of electron in comparison with the usual CNTs. The high separation efficiency of electron-hole pairs lead to the dramatically enhanced photoactivity. Secondly, compared to carbon, nitrogen has an extra electron, and from an electronic point of view it is natural to expect an excess of donors in the N-rich areas of the CNTs upon doping [20, 21]. This in fact occurs if N substitutes for a C atom generating an n-type semiconductor with localized states above the Fermi level. That is to say, impurities significantly enhanced the CNx metallic/conductive character [22]. Hence, the photoinduced electrons transport to the surface of the composites much easily to prevent the recombination between photogenerated electrons and holes, leading to enhanced photoactivity of SnO₂/CNx, as also found for N-graphene/CdS nanocomposites [23]. Further investigation on the mechanism of enhanced photocatalytic properties is underway.

4. Conclusions

SnO₂ nanoparticles were successfully coated on CNx via a simple wet-chemical method. The composites showed excellent photocatalytic activity compared with pure SnO₂ and SnO₂/CNTs. The rapid transferring of electron and high separation efficiency of electron-hole pairs lead to the dramatically enhanced photoactivity. The SnO₂/CNx composites would be promising for practical use in pollutant decomposition as effective photocatalysts.

Acknowledgments

This work is supported by Leading Academic Discipline Project of Shanghai Municipal Education Commission (J51803), the National Science Foundation of China (NSFC, nos. 21101105 and 51174274), Innovation Program supported by Shanghai Municipal Education Commission (12ZZ195), and “Shu Guang” Project supported by Shanghai Municipal Education Commission (09SG54).

References

S. Iijima, “Helical microtubules of graphitic carbon,” Nature, vol. 354, no. 6348, pp. 56–58, 1991.

A. Vijayaraghavan, K. Kanzaki, S. Suzuki et al., “Metal—semiconductor transition in single-walled carbon nanotubes induced by low-energy electron irradiation,” Nano Letters, vol. 5, no. 8, pp. 1575–1579, 2005.

S. F. Wang, L. Shen, W. D. Zhang, and Y. J. Tong, “Preparation and mechanical properties of chitosan/carbon nanotubes composites,” Biomacromolecules, vol. 6, no. 6, pp. 3067–3072, 2005.

X. Gui, J. Wei, K. Wang et al., “Carbon nanotube sponges,” Advanced Materials, vol. 22, no. 5, pp. 617–621, 2010.

X. Hu, T. Wang, X. Qu, and S. Dong, “In situ synthesis and characterization of multiwalled carbon nanotube/Au nanoparticle composite materials,” Journal of Physical Chemistry B, vol. 110, no. 2, pp. 853–857, 2006.

D. Eder, “Carbon nanotube-inorganic hybrids,” Chemical Reviews, vol. 110, no. 3, pp. 1348–1385, 2010.

D. Abdula and M. Shim, “Performance and photovoltaic response of polymer-doped carbon nanotube p-n diodes,” ACS Nano, vol. 2, no. 10, pp. 2154–2159, 2008.

H. Zhang, G. Cao, and Y. Yang, “Carbon nanotube arrays and their composites for electrochemical capacitors and lithium-ion batteries,” Energy and Environmental Science, vol. 2, no. 9, pp. 932–943, 2009.

K. Ghosh, M. Kumar, H. Wang, T. Maruyama, and Y. Ando, “Nitrogen-mediated wet-chemical formation of carbon nitride/ZnO heterojunctions for enhanced field emission,” Langmuir, vol. 26, no. 8, pp. 5527–5533, 2010.

B. Yue, Y. Ma, H. Tao et al., “CNx nanotubes as catalyst support to immobilize platinum nanoparticles for methanol oxidation,” Journal of Materials Chemistry, vol. 18, no. 15, pp. 1747–1750, 2008.

R. Zhang, L. Li, L. Chen, G. Zhang, and K. Shi, “N-doped carbon nanotubes synthesized in high yield and decorated with CeO₂ and SnO₂ nanoparticles,” Journal of Alloys and Compounds, vol. 509, no. 35, pp. 8620–8624, 2011.

A. L. Linsebigler, G. Lu, and J. T. Yates, “Photocatalysis on TiO₂ surfaces: principles, mechanisms, and selected results,” Chemical Reviews, vol. 95, no. 3, pp. 735–758, 1995.

N. Wang, J. Xu, and L. Guan, “Synthesis and enhanced photocatalytic activity of tin oxide nanoparticles coated on multi-walled carbon nanotube,” Materials Research Bulletin, vol. 46, no. 9, pp. 1372–1376, 2011.

L. Wang, L. Wang, H. Jin, and N. Bing, “Nitrogen-doped carbon nanotubes with variable basicity: preparation and catalytic properties,” Catalysis Communications, vol. 15, no. 1, pp. 78–81, 2011.

W. Q. Han and A. Zettl, “Coating single-walled carbon nanotubes with tin oxide,” Nano Letters, vol. 3, no. 5, pp. 681–683, 2003.

T. Watanabe, T. Takizawa, and K. Honda, “Photocatalysis through excitation of adsorbates. 1. Highly efficient N-deethylation of rhodamine B adsorbed to CdS,” Journal of Physical Chemistry, vol. 81, no. 19, pp. 1845–1851, 1977.

M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, “Environmental applications of semiconductor photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995.

W. Wang, P. Serp, P. Kalck, and J. L. Faria, “Visible light photodegradation of phenol on MWNT-TiO₂ composite catalysts prepared by a modified sol-gel method,” Journal of Molecular Catalysis A, vol. 235, no. 1-2, pp. 194–199, 2005.

Y. T. Lee, N. S. Kim, S. Y. Bae et al., “Growth of vertically aligned nitrogen-doped carbon nanotubes: control of the nitrogen content over the temperature range 900–1100 °C,” Journal of Physical Chemistry B, vol. 107, no. 47, pp. 12958–12963, 2003.

H. S. Kang and S. Jeong, “Nitrogen doping and chirality of carbon nanotubes,” Physical Review B, vol. 70, article 233411-1, 2004.

A. H. Nevidomskyy, G. Csányi, and M. C. Payne, “Chemically Active Substitutional Nitrogen Impurity in Carbon Nanotubes,” Physical Review Letters, vol. 91, no. 10, pp. 1055021–1055024, 2003.

M. Terrones, P. M. Ajayan, F. Banhart et al., “N-doping and coalescence of carbon nanotubes: synthesis and electronic properties,” Applied Physics A, vol. 74, no. 3, pp. 355–361, 2002.

L. Jia, D.-H. Wang, Y.-X. Huang, A.-W. Xu, and H.-Q. Yu, “Highly durable N-doped graphene/CdS nanocomposites with enhanced photocatalytic hydrogen evolution from water under visible light irradiation,” Journal of Physical Chemistry C, vol. 115, no. 23, pp. 11466–11473, 2011.