Self-assembly of 2D MnO2 nanosheets into high-purity aerogels with ultralow density

Organic-free MnO2 aerogels with ultralow density have been achieved by self-assembly of two dimensional MnO2 nanosheets via an ice-templating approach.

2500 spectrophotometer (200-800 nm). X-ray diffraction (XRD) was performed on a Rigaku X-ray diffractometer (D/max rA, using Cu Kɑ radiation at a wavelength of 1.542 Å), and the data were collected from 5 ° to 80 °. Fourier transform infrared (FT-IR) spectra were collected on an FT-IR spectrometer (SHIMADZU, IRAffinity-1) using KBr pellets (32 scans), and the spectra were recorded at a resolution of 4 cm -1 . Freeze-drying was carried in a freeze-dryer (Beijing Boyikang, FD-1C-50) at -50 °C. Elemental analysis for K, Na and Mn was carried on an ICP spectrometer (iCAP6300, Thermo Scientific) and elemental analysis for C, H, N and S on another elemental analyzer (Vario EI Cube, Elementar). A nitrogen adsorption isotherm at -196 °C was performed in an apparatus of Micromeritics ASAP 2020. The samples were degassed overnight at 300 °C before adsorption measurements and the specific surface area was calculated by using the BET equation.

S2.1 Synthesis of the layered MnO 2 nanosheets
Layered MnO 2 nanosheets were synthesized through a slow redox reaction between KMnO 4 and SDS as we previously described with minor modification. First, SDS solution (100 mM, 32 mL), HCl solution (100 mM, 3.2 mL), KMnO 4 solution (50 mM, 3.2 mL) and 281.6 mL ultrapure water were mixed together and heated at 95 °C for 3 hours. After heating, the solution became colorless (Fig. S1) and the MnO 2 nanosheets formed had precipitated.
Then the precipitates were washed repeatedly with ultrapure water and alcohol to obtain pure MnO 2 nanosheets. The purified MnO 2 nanosheets were dried on corresponding substrates overnight at room temperature for characterizations of TEM, SEM and XRD.

S2.2 Preparation of the monodipersed nanosheet colloid
The layered MnO 2 nanosheets, which have been formed by weak van der Waals interaction and stabilized by balancing cations (e.g. H 3 O + , Na + and K + ) intercalated between the MnO 2 layers, can be easily dispersed by ultrasound treatment. Experimentally, MnO 2 precipitates were sonicated at 100 W for a certain period of time to yield homogeneous colloidal solutions of various concentrations (Fig. S3).
Elemental analysis of MnO 2 nanosheets shows that the material is indeed organic-free, exhibiting no potential organic elements, such as C and S, whereas only trace amount (less than 1.0 wt% in total) of Na and K, representing inherent balancing cations, has been detected in addition to Mn and H. Base on the elemental analysis, it is more reasonable to name our 2D MnO 2 nanosheets as Na 0.027 K 0.01 MnO 2 0.95 H 2 O. However, since such materials are often referred to as MnO 2 in the related literatures (Chem. Sci., 2012, 3, 433; J. Am. Chem. Soc., 2008, 130, 15938;Angew. Chem. Int. Ed., 2007, 46, 4951;Chem. Commun., 2014, 50, 7885), we followed the tradition to call it MnO 2 in the manuscript for simplification.

Fig. S6
Chemical structure of the δ-MnO 2 nanosheet. The δ-MnO 2 contains multiple twodimensional sheets of edge-shared [MnO 6 ] octahedra. Oxygen atoms are aligned on both sides of the sheets, whereas the layer of Mn atoms is sandwiched between two layers of oxygen atoms. As a result, the negative charge of oxygen atom is exposed on the surface of MnO 2 nanosheets, with the positive charge of Mn being shielded.

S2.3 Preparation of MnO 2 aerogels and density measurement of the aerogel
Colloids of MnO 2 nanosheets (at different concentrations) were poured into a glass mold, which were then frozen at -20 °C for 12 hours to form icy chunks. Subsequently, the obtained frozen samples were directly transferred to a freeze-drier and freeze-dried at -50 °C for 24 hours to prepare free-standing MnO 2 aerogels.
The density (ρ) of the aerogel was determined by dividing the mass weight (m) by the volume (V) (i.e. ρ = m/V) as done by other researchers (Adv. Mater., 2013, 25, 2554Carbon, 2014, 80, 174;Adv. Mater., 2012, 24, 3486;Angew. Chem. Int. Ed., 2012, 51, 11371). The lowest density was calculated for the aerogel shown in the inset of Fig. 3c, which maintained a relatively good cylindrical shape after aerogel formation. The minor deformation in the middle of the aerogel, as shown in the picture, was caused by the forceps used to handle the aerogel in weight measurement. The diameters for the widest and narrowest parts of the deformed aerogel were measured as 1.626 cm and 1.398 cm respectively, and averaged as 1.512 cm. Meanwhile, the height of aerogel (h) was measured as 1.784 cm. The diameters and height were measured using a Vernier caliper (specific to the dividing of 0.02 mm).
Subsequently, the volume of the aerogel monolith was calculated based on the geometric equation (V = π (d/2) 2 h) for cylinder by using the average diameter (d) of 1.512 cm and h of 1.784 cm, which yielded 3.203 cm 3 . The weight of the aerogel was measured by an electronic scale (Mettler Toledo, specific to the dividing of 0.0001 g, Fig. S12), which was 1.7 mg. Thus the density (ρ) was determined as 0.53 mg/cm 3 .

S3. Freeze-thawed experiment
Aqueous MnO 2 nanosheet colloid (1.0 mg mL -1 ) was cultivated at -20 °C until all liquid had solidified into a brown ice-containing chunk and then thawed at room temperature. The freeze-thawed sample shows colorless liquid with a large amount of brown sediments at the bottom (Fig. S9). The sediments were dried on silica substrates overnight at room temperature for SEM characterization (Fig. S10). When treated with ultrasound (100 W) for 30 minutes, the freeze-thawed sediments cannot be redispersed into a colloid (Fig. S9), indicating the formation of a stable assembly.

Fig. S14
Photographs of the freeze-thawed process and the ultrasound treatment. Aqueous MnO 2 nanosheet colloid (1.0 mg mL -1 ) was cultivated at -20 °C to form a brown icy chunk (left picture) and then thawed to yield a colorless liquid with brown sediments (middle picture). After the ultrasound treatment of 100 W for 30 minutes, the majority of the sediments remain undispersed (right picture).

S4.1 Preparation of homogeneous MnO 2 nanosphere colloid
Homogeneous colloid of MnO 2 nanospheres was synthesized by following the procedure described by Xiaoliu Huangfu et al. by mixing the appropriate amounts of KMnO 4 and Na 2 S 2 O 3 stock solutions (Environ. Sci. Technol. 2013, 47, 10285), according to the following reaction stoichiometry: 3S 2 O 3 2-+8MnO 4 -+2H + →8MnO 2 +6SO 4 2-+H 2 O. Briefly, KMnO 4 solution was rapidly stirred with a magnetic stir bar, and purged with N 2 . Then, the stoichiometric amount of Na 2 S 2 O 3 solution was added drop-wise. The brown MnO 2 nanosphere colloid was formed immediately but stirred for 12 hours to achieve a stable colloidal solution. The stock colloid was stored in the dark at 4°C.

S4.2 Construction of aerogels using MnO 2 nanospheres as the building blocks
The control experiment was carried out by using MnO 2 nanospheres instead of 2D MnO 2 nanosheets as the building blocks to construct aerogels under otherwise identical conditions. In details, the colloid of MnO 2 nanospheres (1.0 mg mL -1 ) was poured into a glass mold, which was then frozen at -20 °C for 12 hours to form icy chunks. Subsequently, the obtained frozen samples were directly transferred to a freeze-drier and freeze-dried at -50 °C for 24 hours.  (b)) were frozen at -20 °C to form icy chunks (middle pictures in (a) and (b)) and then freeze-dried to remove ice. The obtained results are shown in the right pictures in (a) and (b).

S5. MnO 2 aerogels as the absorbents for hydrazine
To produce hydrazine vapor, a small droplet (1 μL) of N 2 H 4 ·H 2 O was dropped at the bottom of a conical flask and heated at 60 °C for 10 minutes to evaporate. For the experiment in the absence of absorbents, the sealed N 2 H 4 vapor was collected by a syringe and injected into the N 2 H 4 -sensitive colorimetric probe (5 μg/mL colloidal suspension of MnO 2 nanosheets). For the experiment in the presence of absorbents, two pieces of MnO 2 aerogels (3.0 mg for each) or commercial MnO 2 powders (6.0 mg) were used as the absorbents in the conical flask as N 2 H 4 vapor being produced, and the residual N 2 H 4 vapor was collected by a syringe and injected into the colorimetric probe for detection.