Volume 9 Number 24 28 June 2017 Pages 8035–8510 Nanoscale rsc.li/nanoscale ISSN 2040-3372 COMMUNICATION Han Zhang et al. Graphene oxide/black phosphorus nanofl ake aerogels with robust thermo-stability and signifi cantly enhanced photothermal properties in air Nanoscale View Article Online COMMUNICATION View Journal | View Issue Graphene oxide/black phosphorus nanoflake aerogels with robust thermo-stability and Cite this: Nanoscale, 2017, 9, 8096 significantly enhanced photothermal Received 27th January 2017, properties in air† Accepted 21st March 2017 DOI: 10.1039/c7nr00663b Chenyang Xing,a Guanghui Jing,a,b Xin Liang,a,c Meng Qiu,a Zhongjun Li,a,d a,d e a a rsc.li/nanoscale Rui Cao, Xiaojing Li, Dianyuan Fan and Han Zhang * Here we report a new kind of three-dimensional (3D) hybrid egies including chemical reduction self-assembly and hydro- aerogels, based on graphene oxide (GO) and black phosphorus thermal reduction of GO solution, both followed by freeze- nanoflakes (BPNFs), for the first time. Our results demonstrate that drying, can readily realize the construction of 3D graphene the as-prepared GO/BPNF hybrid aerogels exhibited significantly aerogels.13,17,19,21 enhanced photothermal as well as electrical properties of GO Stimulated by successful development of graphene, investi- aerogels due to the addition of BP. Moreover, they also possessed gations into other 2D materials have been explored in recent – excellent photothermal stability under ambient conditions without years.22 30 Among those, so much attention has been paid to – any protection, which can be ascribed to the coverage of BPNFs black phosphorus (BP),31 33 also called “phosphorene”, due to with GO nanosheets in these aerogels. This exceptional photo- its thickness-tuneable bandgap (0.3 to 2.0 eV),34,35 biocompat- thermal property along with robust stability renders GO/BPNF ibility, degradability, excellent photothermal properties,36,37 aerogels with promising bio-related applications, such as photo- etc. BP, with similar but quite unique features in comparison thermal therapy for cancer treatment. with graphene, is expected to be charting a similar course to get application development. In view of the advancement of Graphene has been a two-dimensional (2D) star material since 3D graphene structures, 3D BP materials may also possess a 1 it was first mechanically exfoliated by Novoselov and Geim. larger specific surface area and porous structures, which Published on 12 April 2017. Downloaded 29/06/2017 01:52:59. The last two decades have seen tremendous achievements of – would be promising in many fields, compared with 2D BP graphene in electrics,2 6 biological engineering,7,8 environ- – materials that usually exist in the form of BP nanoflakes 9 12 – mental and energy fields. It makes still further progress (BPNFs),38 42 BP nanoparticles (BPNPs)43 and BP quantum – when it comes to 3D graphene-based materials, such as the dots (BPQDs),44 46 just like that of graphene.47 However, 13–17 known graphene aerogel and graphene fibrous unlike graphene whose oxidation state (i.e. GO) can be reduced 18–20 aerogel, with macroscopic size, microscopic opening to graphene (i.e. reduced GO) so as to build 3D graphene in holes, significantly enhanced specific surface area, excellent water, the oxidation state of BP (i.e. PxOy) is uncrystallised and electricity as well as flexibility. In conversion of 2D graphene can be directly dissolved in water into phosphoric acid, which into 3D graphene, graphene oxide (GO), also a 2D material, may be impossible to be reduced to crystalline BP either by – plays a critical role as an important precursor. Typical strat- reductive agents or by hydrothermal reduction in water.13 20 Although there is a possible difficulty to synthesize a pure BP based 3D aerogel, BP may attach itself to a 3D support/ aShenzhen Engineering Laboratory of Phosphorene and Optoelectronics, stand to form a composite 3D structure. In this regard, 3D gra- Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and phene or 3D GO may be the best choice since it has some Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, similar aspects to BP in many fields. Recently, Wan et al.48 first Shenzhen 518060, P.R. China. E-mail: [email protected] reported ultrafast gelation (1 min) of GO with poly(oxypropyl- bInstitute for Advance Study, Shenzhen University, Shenzhen 518060, P.R. China cCollege of Materials Science and Engineering, Shenzhen University, Shenzhen ene) diamine (D400) by means of covalently cross-linking to 518060, P.R. China form a 3D GO aerogel. In view of BP degradation in air, it is dFaculty of Information Technology, Macau University of Science and Technology, believed that such an ultrafast gelation of GO in water is Taipa, Macau 519020, P.R. China favourable to make BP materials attach themselves to the 3D eGraduate school at Shenzhen, Tsinghua University materials and devices testing GO support and therefore overcome the oxidation and insta- center, Shenzhen 518055, P.R. China †Electronic supplementary information (ESI) available. See DOI: 10.1039/ bility issue of BP. Inspired by these, we fabricated 3D GO/ c7nr00663b BPNF aerogels in the present study and demonstrated the first 8096 | Nanoscale,2017,9,8096–8101 This journal is © The Royal Society of Chemistry 2017 View Article Online Nanoscale Communication prototypic example of BP-related aerogels, which may arouse broader interest among other related research fields, including clean energy, optics, biological engineering, etc. GO and BPNFs were firstly prepared by the modified Hummers method and liquid exfoliation (see the Experimental section in the ESI†), respectively. Typically, the as-prepared BPNFs are of several hundred nanometers in lateral size (Fig. 1a). The clear lattice fringes of 2.23 Å assigned to the (014) plane41 and selected-area electron diffraction (SAED) (Fig. 1b) suggest the maintenance of BP crystal struc- tures during preparation. Atomic force microscopy (AFM) for BPNFs in Fig. 1c and e shows a thickness range of 10–30 nm for BPNFs. In contrast, the as-prepared GO nanosheets have a larger size with several micrometers and a smaller thickness of ca. 1.5 nm in Fig. 1d and f. In a typical 3D GO/BPNF aerogel preparation, a homo- geneous aqueous solution, based on GO nanosheets, BPNFs and a cross-linking agent, poly(oxypropylene) diamine (D400), was first obtained (Fig. 2a) and then covered with aluminium foil thoroughly (Fig. 2b) in view of the degradation of BP trig- gered by light,49 followed by immersion in an oil bath at 90 °C for 1 min. After gelation of GO (Fig. 2c), a freeze-drying pro- Fig. 2 Characterization of 3D GO/BPNF aerogel with 13.4 wt% BP cedure was performed to obtain 3D GO/BPNF aerogels (Fig. 2d). content: (a) homogeneous aqueous solution containing BPNFs, GO nanosheet and D400. (b) Aluminium foil covered reaction vessel. (c) Gelation of GO and BPNFs forming a hydrogel. (d) Macroscopic view of GO/BPNF aerogel after freeze-drying treatment. Field emission scanning electron microscopy (FE-SEM) images of neat GO aerogel (e–f) and GO/ BPNF aerogel (g–h), respectively. X-ray photoelectron spectroscopy (XPS) curve of bulk BP (i), neat GO aerogel and GO/BPNF aerogel ( j), respectively. The macroscopic shape and size of GO/BPNF aerogels are dependent on the original shape of the vessel. The mass Published on 12 April 2017. Downloaded 29/06/2017 01:52:59. content of BP in GO/BPNF aerogels was evaluated by using inductively coupled plasma-atomic emission spectroscopy (see the Experimental section in the ESI†). Neat GO aerogel without BPNFs was used as a control sample in this study. It was found that such a 3D GO/BPNFs aerogel with 13.4 wt% BP content exhibited macroporous structures at low magnification (Fig. S1, ESI†) and typical layer-stacked structures (Fig. 2g and h), which is similar to those from neat GO aerogel (Fig. 2e and f). In addition, BPNFs were not readily observed in the GO/BPNF aerogel because of their full coverage with large-sized GO nanosheets. The corresponding P element analysis by using an energy dispersive spectrometer (EDS) was also performed (Fig. S2, ESI†). In comparison with 3D graphene aerogels,16,50 such a 3D GO/BPNF aerogel has a relatively large density (11.7 ± − 0.62 mg cm 3) and weak mechanical strength (Fig. S3, ESI†). Zhou et al.49 reported the degradation principle of BP under light, oxygen and water. In this study, attention has Fig. 1 Characterization of BPNFs and GO nanosheets used in 3D GO/ been paid to the stability of BP in GO/BPNF aerogels in air. In BPNF aerogels: (a) transmission electron microscopy (TEM) image illus- the preparation procedure, GO gelation in the presence of trating the morphologies of BPNFs. (b) High-resolution TEM (HR-TEM) BPNFs was conducted in water at 90 °C, it is thus important to image showing the lattice fringes of BPNFs with an insert image of selected-area electron diffraction (SAED). (c) and (d) Atomic force determine whether BP was degraded in this gelation. Fig. 2i microscopy (AFM) for BPNFs and GO nanosheets, respectively. (e) and and j show the X-ray photoelectron spectra (XPS) of bulk BP (f) Height profiles along the lines in (c) and (d). and GO/BPNF aerogels, respectively. The bulk BP shows 2p3/2 This journal is © The Royal Society of Chemistry 2017 Nanoscale,2017,9,8096–8101 | 8097 View Article Online Communication Nanoscale 5+ at 129.8 eV and p at 134.2 eV. The former is attributed to the cross-linked agent D400 (detailed structural formula: H2N–[CH – – – – – – 48 zero-valent crystalline BP and the latter is assigned to oxidized (CH3) CH2 O ]6.1 CH2 CH(CH3) NH2) with and without 44,45 phosphorus (i.e.