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Synthesis of Few-Atomic-Layer BN Hollow Nanospheres and Their Applications as Nanocontainers and Catalyst Support Materials Haibin Si,∥,‡ Gang Lian,*,∥,‡,⊥ Jun Wang,§ Liyi Li,⊥ Qilong Wang,§ Deliang Cui,*,‡ and Ching-Ping Wong*,⊥ ‡
State Key Lab of Crystal Materials, and §Key Lab for Special Functional Aggregated Materials of Education Ministry, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, P. R. China ⊥ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: In this work, few-atomic-layer boron nitride (BN) hollow nanospheres were directly synthesized via a modified CVD method followed by subsequent high-temperature degassing treatment. The encapsulated impurities in the hollow nanospheres were effectively removed during the reaction process. The BN shells of most nanospheres consisted of 2−6 atomic layers. Because of the low thickness, the obtained BN hollow nanospheres presented excellent performance in many aspects. For instance, they were demonstrated as useful nanocontainers for controllable multistep release of iodine, which could diffuse and be encapsulated into the few-layer BN hollow nanospheres when heating. They were also promising support materials that could markedly increase the photocatalytic activity of TiO2 nanocrystals. KEYWORDS: boron nitride, hollow nanospheres, few layers, nanocontainers, catalysis many fields, such as hydrogen storage, organic pollutants adsorption, drug delivery and catalysis in harsh environments. Recently, we developed a template-free solid state method to prepare BN hollow spheres with thin shell.23 However, they still encountered several problems: nonuniformity, serious aggregates and low SSA (Figure S1). Sometimes, there were impurities encapsulated inside the hollow spheres, which were hard to be removed by dissolution and annealing. Thus, it is necessary to explore a new strategy to overcome abovementioned issues. In this work, few-atomic-layer (2−6 layers) BN hollow nanospheres with high purity, uniformity and low degree of aggregation were synthesized by a modified chemical vapor deposition (CVD) method followed by subsequent hightemperature degassing treatment. The BN hollow nanospheres possess the thinnest walls reported up to now. It could also achieve large-scale production. They exhibited high surface area (318.7 m2 g−1) and large pore volume (1.41 cm3 g−1), compared to other reported values (Table 1). Furthermore, the few-layer BN hollow nanospheres presented excellent performance in controllable encapsulation-release of iodine and increasing TiO2 photocatalysis as support materials. In the synthesis of BN sample, high-pressure N2, large reaction space and high-temperature degassing treatment were brought in the CVD process (Scheme S1). Then, pure well-
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ew-atomic-layer hollow nanospheres have drawn substantial attention because of their high surface-to-volume ratio, hollow inner space, high specific surface area (SSA), and good penetration of shell, so they are promising materials in the fields of energy storage, catalysis, adsorption, and drug delivery.1−3 For example, Xia and co-workers fabricated platinum nanospheres with subnanometer-thick walls, which exhibited distinctive catalytic activities toward oxygen reduction.1 Teng et al. synthesized thin-walled carbon nanospheres as useful nanocontainers for encapsulating iodine.3 For preparation of ultrathin-walled hollow nanospheres, nowadays the most popular strategy involves the controlled coating of desired materials on templates, which are then removed via either chemical etching or thermal sintering means.1−7 However, the process is complicated and may damage the ultrathin walls.8 In view of the wide applications, developing new template-free methods to prepare few-atomic-layer hollow nanospheres is essential. Hexagonal boron nitride (h-BN), analogous to graphite, displays superior thermal and chemical stability, high oxidation resistance, and high thermal conductivity.9 Following the fast development of single- or few-layer carbon nanotubes and graphene, many efforts have been made to synthesize few-layer BN nanostructures, such as BN nanotubes and nanosheets, which present unique physical and chemical properties.9−17 Compared to the well-studied BN nanotubes and nanosheets, few work was focused on the synthesis of BN hollow nanospheres/cages.18−22 However, they generally present thick-layer structures, nonuniform morphologies, low SSA, etc., which severely limited the potential applications of them in © XXXX American Chemical Society
Received: November 13, 2015 Accepted: January 11, 2016
A
DOI: 10.1021/acsami.5b10978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
developing new containers and achieving controllable release of iodine is in high demand. For the few-atomic-layer BN hollow nanospheres, the iodine may not only be adsorbed on the outer surface, but also diffuse and be enclosed into the hollow space, which could avoid undesirable release and increase its stability at ambient conditions. Herein, the few-layer BN hollow nanospheres sample and iodine powders with a mass ratio of 1:10 were sealed in a Teflon-lined autoclave and heated under 150 °C for 10 h. After that, the mixtures were washed in ethanol and dried in air at 80 °C to remove the unloaded iodine. As shown in Figure 2a, iodine was successfully encapsulated inside the hollow nanospheres. The magnified TEM image showed the typical iodine-filled BN hollow nanospheres (Figure 2b). Remarkable cores were observed in the hollow nanospheres, in contrast to the unfilled BN samples (Figure 2c). Because the graphite-like shells were only a few layers and contained plenty of micropores/mesopores, iodine could diffuse into the nanospheres and occupy the hollow space. EDS analysis indicated that the cores consisted of iodine with a content of ∼15.47 wt %. It should note that, because high-energy electrons beam easily volatilized iodine during the acquisition of TEM image, the iodine content evaluated from EDS was lower than that calculated by TGA. As shown in Figure 2d, pristine iodine completely vaporized as low as 160 °C, but BN displayed well thermal and chemical stability before 600 °C in air. The weight loss of iodine-filled BN sample nearly resulted from the release of iodine, which showed a content of iodine loaded as 26 wt %. More importantly, the thermogram presented three stage plateaus and could be divided into several regions, which indicated several states of iodine loaded in the BN hollow nanospheres and variable sensitivity for different temperatures. It also indicated that iodine could be controllably released from BN hollow nanospheres, which was further verified by a programmed heat treatment. As shown in Figure 2e, when the iodine-filled BN hollow nanospheres were quickly heated to 100 °C and kept for 1.5 h, one portion of iodine was slowly released and the first weight-loss plateau appeared. It possibly resulted from the evaporation of iodine adsorbed on the outer surface (i). With sequentially raising temperature to 200 °C at the same rate and also holding for 1.5 h, the largest ratio of iodine encapsulated in the hollow space was released
Table 1. Comparison of Specific Surface Area and Pore Volume Based on BN Hollow Spheres/Cages materials hollow spheres hollow spheres hollow nanocages few-layer BN hollow nanospheres
specific surface area (m2g−1)
pore volume (cm3g−1)
133-215
0.62−1.19
318.7
1.41
ref 23 24 18−22 this work
defined BN hollow nanospheres with low contrast brim and an external diameter of 50−200 nm were obtained after reaction without any washing or annealing treatment (Figure 1a, b), which agreed with the XRD and FTIR results (Figures S2 and S3). Furthermore, the elemental composition and purity of the BN hollow nanospheres were also studied and confirmed by EELS and XPS (Figure S4). Hence, the high-pressure degassing was an effective way to remove the volatile impurities encapsulated in the BN hollow nanospheres (Figure S5). More interestingly, most of the graphite-like walls of hollow nanospheres were very thin (
