Preparation and Photocatalytic Properties of SnO 2 Coated on Nitrogen-Doped Carbon Nanotubes

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.


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][3][4].Recently, the study of CNT-based composites is becoming a promising and challenging area of research owing to their improved properties [5][6][7].Of them, CNTbased 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 CeO 2 and SnO 2 nanoparticles showed greater activity and sensitivity than the conventional CNT-based composites for NO electrooxidation [11].
SnO 2 , a stable and large n-type bandgap (E g = 3.6 eV) semiconductor, has the ability to degrade dyes under UV light [12].Recently, a nanocomposite of SnO 2 nanoparticles coated on CNTs was synthesized and exhibited excellent photocatalytic activity due to the electron transfer between SnO 2 and CNTs [13].However, the photocatalytic properties of SnO 2 nanoparticles coated on CNx (SnO 2 /CNx) have been scarcely investigated.
In this study, according to the unique properties of CNx-based metal oxide, we have synthesized SnO 2 /CNx

Experimental
2.1.Synthesis of SnO 2 /CNx.Following the procedures reported previously [14], CNx was synthesized using ethylene diamine and diethylamine as the carbon and nitrogen source.
Synthesis of SnO 2 /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 H 2 O 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.SnO 2 /CNx nanocomposites were then obtained after centrifuging, washing with ethanol and deionized water for several times, and drying at 80 • C for 8 h.SnO 2 /CNTs composites were synthesized using the similar procedures besides CNTs pretreated in concentrated HNO 3 in 140 • C for 14 h, and pure SnO 2 sample was synthesized without adding CNTs.

Characterization.
The bare CNx and the composites were characterized by a range of analytical techniques.The degree of crystallinity of the SnO 2 /CNTs composites was characterized by powder X-ray diffraction (XRD).The XRD patterns with diffraction intensity versus 2θ were recorded in a Bruker D8 ADVANCE instrument with Cu-Ka radiation (λ = 1.5418Å) 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.

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 SnO 2 or SnO 2 /CNTs composites or SnO 2 /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.

Results and Discussion
The XRD patterns of pure SnO 2 , pure CNx, and SnO 2 /CNx composites are displayed in Figure 1.The main peaks at 26.
where D is the grain diameter, K (0.89) is the shape factor, λ is the X-ray wavelength of Cu Kα radiation (0.154 nm), θ is the Bragg angle, and β is the experimental full-width half-maximum (fwhm) of the respective diffraction peak.The calculated mean particle size of SnO 2 nanoparticles was calculated to be about 4 nm.Figures 2 and 3 show the SEM and TEM images of pure CNx and SnO 2 /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 SnO 2 , all CNx are uniformly coated with a layer of SnO 2 nanoparticles, and no free nanoparticles were found.The SnO 2 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 SnO 2 .The bamboo-like morphology of CNx can be also clearly observed, and its surface is entirely and homogeneously covered by SnO 2 nanoparticles.The intimate contact between CNx and SnO 2 favors the formation of junctions between the two materials, as a result, being helpful for improving the charge separation and thus the photocatalytic activity.The size of SnO 2 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 SnO 2 /CNx composites (Figure 4).The C signal originates from the supporting CNx.
The photocatalytic activity of SnO 2 /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 SnO 2 /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 6 presents the degradation profiles of RhB in the presence of SnO 2 /CNx composites under UV light irradiation, together with CNx, SnO 2 , and SnO 2 /CNTs composites for comparison.Almost no degradation was detected when CNx alone was used as the catalyst.The photocatalytic activity of SnO 2 /CNTs and SnO 2 /CNx composites is much higher than that of the sole SnO 2 .SnO 2 /CNTs with higher photocatalytic activity than pure SnO 2 has been also observed in the literature [13].It is noteworthy that SnO 2 /CNx composites show superior activity to SnO 2 /CNTs composites.With the reaction time at 50 min, the RhB degradation efficiencies of pure SnO 2 and SnO 2 /CNx composites catalysts are about 70% and 80%, respectively.However, the value of SnO 2 /CNx is 89%.Hence, SnO 2 /CNx is an excellent photocatalyst in our experiment.On the basis of the literature [13], a band configuration at the interface of the SnO 2 /CNx composites is proposed, as shown in Figure 7.Under UV irradiation, the valence band electrons of SnO 2 can be excited to its conduction bands, giving rise to the formation of high energy electronhole 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.
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 SnO 2 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 SnO 2 /CNx, as also found for N-graphene/CdS nanocomposites [23].Further investigation on the mechanism of enhanced photocatalytic properties is underway.

Conclusions
SnO 2 nanoparticles were successfully coated on CNx via a simple wet-chemical method.The composites showed excellent photocatalytic activity compared with pure SnO 2 and SnO 2 /CNTs.The rapid transferring of electron and high separation efficiency of electron-hole pairs lead to the dramatically enhanced photoactivity.The SnO 2 /CNx composites would be promising for practical use in pollutant decomposition as effective photocatalysts.

Figure 5 :Figure 6 :
Figure 5: Temporal UV absorption spectral changes of RhB solution as a function of irradiation time (RhB = 10 mg/L).

Figure 7 :
Figure 7: Schematic diagram showing band configuration and electron-hole separation at interface of SnO 2 /CNx nanocomposites under UV irradiation (CB-the bottom of conduction band, VBthe top of valence band).
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 SnO 2 are illuminated in Figure 1(a).All the diffraction peaks are indexed to the tetragonal rutile structure of SnO 2 (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 SnO 2 /CNx composites.The diffraction angle for SnO 2 /CNx composites at 2θ = 33.6 • , 51.8 • can be assigned to 101 and 211 planes of the cassiterite SnO 2 , respectively.The peak at 26.3 • attributed to the peak of the 110 planes of SnO 2 overlapped with the peak of graphitic reflection from CNx.Average crystalline size can be estimated by Scherrer's formula: