Direct Conversion of Graphene Aerogel into Low-Density Diamond

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C: Physical Processes in Nanomaterials and Nanostructures

Direct Conversion of Graphene Aerogel into Low-Density Diamond Aerogel Composed of Ultra-Small Nanocrystals Luyao Zhu, Bingbing Liu, Mingguang Yao, Jiajun Dong, Kuo Hu, Ran Liu, Chen Gong, and Yan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03809 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Direct Conversion of Graphene Aerogel into Low-density Diamond Aerogel Composed of Ultra-small Nanocrystals Luyao Zhua, Mingguang Yaoa,b,∗, Jiajun Donga, Kuo Hub, Ran Liua, Chen Gongc, Yan Wanga and Bingbing Liua, ∗ a

State Key Laboratory of Superhard Materials, Jilin University, No. 2699 Qianjin

Street, Changchun 130012, People’s Republic of China b

College of Physics, Jilin University, No. 2699 Qianjin Street, Changchun 130012,

People’s Republic of China c

Department of Science, Jilin Jianzhu University, No. 5088 Xincheng Street,

Changchun 130118, People’s Republic of China

*

Corresponding author. E-mail addresses: [email protected] (M. Y)

*

Corresponding author. E-mail addresses: [email protected] (B. L)

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Abstract Diamond aerogel, a special kind of carbon aerogel made by sp3 carbon atoms, has been attracting intensive research interest due to its potential applications since it is firstly synthesized by the conversion of amorphous carbon. Despite of many expectations in diamond aerogel, the study on its synthesis is still not adequate compared with other carbon aerogel. Here we report the synthesis of diamond aerogel by laser heating graphene aerogel (GA) under high pressure in a diamond anvil cell. The results suggest that the density and microstructure of GA, as well as the heating duration obviously affect the diamond aerogel growth. When heating GA with lower laser power we also observe a transparent carbon phase in experiment, which transforms into graphite and amorphous carbon upon decompression. These results present new insights into our understanding on the transformation from ultra-low density carbon to sp3 carbon under high pressure and high temperature. It is possible to tune the microstructures of diamond aerogel by controlling the synthesis of GA precursors.


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1. Introduction Aerogel, dried gel with a very high relative porous volume 1, has received more and more attention. Due to its high surface area, high porous and low density, a large number of scientific and commercial applications have been developed

1,2

. Aerogel

can be classified by its composition, such as silica aerogel, carbon aerogel, and chalcogenide aerogel and so on 3. Among these aerogels, carbon aerogel has received a lot of interest in recent years for its unique properties like high electron conductivity and low-density. Carbon aerogel includes carbonized Resorcinol–Formaldehyde (CRF) aerogel, graphene aerogel, carbon nanotube aerogel and diamond aerogel 3. CRF aerogel is the first kind of carbon aerogel, which is stacked by carbon nanograins 4. Thanks to the electron conductivity of these carbon nanograins, CRF aerogel shows different electrical properties compared to traditional aerogel materials, like silica aerogel 5. Graphene aerogel, which is made by flexible graphene layers, has high electrical conductivity as well as high mechanical strength, which can be attributed to the elasticity and strength of graphene layers 6,7. Diamond aerogel is a full sp3 carbon system, maintains the excellent properties of diamond like super hardness, highest thermal conductivity, high refractive index and high dispersion, and shows potential in optical device, drug delivery, and enhanced thermal conduction 8,9. The synthesis of diamond aerogel can be seen as a fundamental improvement of synthesizing carbon aerogel, because it is a new way to synthesize carbon aerogel beyond sol-gel method 3. Diamond aerogel is firstly synthesized by the conversion of RF aerogel at 1580 K



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under 21 GPa in a diamond anvil cell (DAC) 9. In this case, realizing the conversion from sp2 carbon precursor to nanosized sp3 diamond (building blocks) while keeping the porosity and low density of the product are critical for the preparation of diamond aerogel. Later on, diamond aerogel has been synthesized by sol–gel method 8, but nanodiamond should be prepared at first and then is used as precursor for further process in this method. Despite of many advantages of diamond aerogel, the study on this topic is still not adequate compared with other carbon aerogel. In addition, in previous study only liquid neon is employed as pressure transmitting medium (PTM), and the melting of liquid neon under high pressure and high temperature (HPHT) is proposed to be important for diamond aerogel formation. However, for large scale preparation of diamond product, such as artificial diamond, multianvil apparatus is usually used and solid PTM is required for HPHT conditions. Therefore, to explore different carbon precursors, as well as suitable growth conditions towards diamond aerogel is very important for the synthesis and its potential applications. Graphene

aerogels

(GAs)

have

special

3D-network

structure

with

ultra-low-density, high electrical conductivity and large surface areas, which may become tunable by using different synthesis processes

7, 10, 11

. It is stacked by small

graphene/graphite layers with large amount of defects and stacking faults. In this study, we study the possible way for the conversion of GAs into diamond aerogel by laser heating under high pressure. After treating GAs with laser heating at suitable conditions, diamond aerogel is found. The effect of temperature and PTM is


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investigated. The correlation of microstructure between GAs and nano diamond is discussed. 2. Materials and methods GA is synthesized by a freeze-drying process, using graphene oxide (GO) as precursor

7,12

. The graphene oxide is bought from XianFeng Nano Material

Technology Inc. (Nanjing, China) and used as received, which is synthesized by a modified Hummer's method. First we make a colloidal dispersion of GO with 4.0 mg/mL GO by ultrasonication. Then the dark brown colloidal dispersion is heated to 180 oC for 12 h in a sealed reactor. The resulted product is freeze-dried for 72 h and heated to 250 oC for 2 h in atmosphere. The GO is partly reduced to graphene after the colloidal dispersion is heated 13. Then the GA is used as starting material. We pick one piece of GA with 120 µm diameter and 20 µm thick and load it into symmetric DAC with a culet size of 500 µm. Then a rhenium gasket (160 µm in diameter hole, 60 µm in thickness) is prepared for sample chambers. We try KBr, NaCl or liquid argon as PTM in several separated experiments. For the case of liquid argon as PTM, a ruby ball (20 µm in diameter) is put on the center of each diamond anvil and then four other ruby balls are put 50 µm away from the first one in four directions with angle of 90 degrees. Then a piece of sample is put on the top of the ruby balls anchored on diamond anvil. Liquid argon is filled as PTM, which can also act as thermal insulator when it solidifies at high pressure. Note that ruby ball is also used for pressure calibration. In the case of KBr



or NaCl as PTM, the sample is loaded between two pieces of dried KBr or NaCl platelets in the sample chamber (sandwich like configuration). Before laser heating, the sample is compressed up to 20 GPa at room temperature. A laser beam (λ=1064 nm, maximum power is 100 W) is focused on the sample in DAC, with 5 µm in diameter, to give a homogeneous heating by moving the DAC step by step. Then the power of laser is increased gradually to what we need for sample heating. Meanwhile temperature is calibrated by blackbody radiation, which is collected by Princeton Instruments Acton SP-2360 Spectrometer with Princeton Instruments PyLoN ‐ IR:1024 InGaAs Camera. Raman measurement is carried out by using a Renishaw inVia spectrometer equipped with a 514.5 nm laser or a LabRam HR Evolution spectrometer equipped with 473, 532, and 785 nm lasers. The samples after HPHT have been characterized by electronic microscopy. A JEOL (JEM- 2200FS) instrument with an accelerating voltage of 200 kV is employed for transmission electron microscopy (TEM), selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) analysis. Scanning electron microscope (SEM, FEI Magellan 400) is used to characterize the 3D network structure of GA. 3. Results and discussion SEM image (Figure 1a) shows that our as-synthesized GA has a three-dimension (3D) network structure. HRTEM (Figure 1d) observation shows that the 3D GA is constructed by nanosized wavy graphene layers. Only two diffracted rings, which can


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be indexed as (100) and (110) of hexagonal graphite phase, in the SAED pattern (Figure 1c) shows the graphene layers are randomly oriented. The intensive D-band of GA (Figure 2d, black line) indicates a large amount of defects in GA 14,15. The high pressure Raman spectra of GA up to 20 GPa are shown in figure 2a. It can be seen that no new peak appears during the compression process, no matter in the experiments by using liquid argon, KBr or NaCl as PTM. Due to the strong overlapping of Raman signals from the diamond anvil, it is difficult to observe the evolution of D-band upon compression. The frequency evolution of G-band (Figure 2b) under pressure also shows similar behavior to that of graphene nanoplate, but slightly different from that of graphite upon compression. We notice no visible difference in the pressure evolution of Raman shift of G-band in the experiments with different PTM. The GA is still black and opaque, as well as the appearance of G-band even at the highest pressure reached here indicates that the sp2 carbon component is still preserved

16

. The results thus show that no phase transition happens to the GA

under room temperature compression.



Figure 1. SEM (a), TEM (b), SAED (c) and HRTEM (d) of as-prepared GA.

Figure 2. High pressure Raman spectra (a) and the corresponding frequency of G-band (b) as functions of pressure for GA upon compression before laser heating. A 532 nm laser has been

used to excite the Raman signal. The Raman data of graphene nanoplate, graphite and micro


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graphite for comparison are from reference

17

. Raman spectra of GA samples after laser heating

with different power at 20 GPa (c), Raman spectra of laser heated samples decompressed to ambient pressure, pristine GA and a synthetic diamond 18 for comparison (d).

In the experiments with KBr as PTM, laser heating with different powers of 2 W, 3 W and 4 W have been carried out in three separated experiments. Blackbody radiation is collected to calibrate temperature by Wien displacement law. When a 2 W laser heats the sample, the temperature is about 2200 K. After laser heating we find that the sample turns transparent (the edge of the sample is not heated to avoid any damage to the diamond anvil) (Figure S1). Then Raman spectra are measured before released to atmosphere (Figure 2c, black line) and we find that the G-band disappears in the collected Raman spectra after this heating treatment. In the experiment with a laser power of 3 W or 4 W, which both give quite similar results, when the laser is focused on the sample, the temperature drops to about 2050 K. After heating we find that most of the sample turns transparent while some areas still look non-transparent. The Raman spectra from the transparent and nontransparent (Figure 2c) areas are different. The transparent part has a weak peak at 1100 cm-1, while the spectra of opaque area show a broad band ranging from 1500 cm-1 to 1600 cm-1. Note that the weak peak at 1100 cm-1 has also been observed in the Raman spectra of nanodiamond 19

. The broad band from 1500 cm-1 to 1600 cm-1 appears in the Raman spectra of

some sp3 rich carbon phases under high pressure 20. After the Raman measurement, all samples are released to atmospheric pressure.



The released samples are further studied by Raman spectrum with removing the diamond anvils (Figure 2d). The collected Raman spectra of the 2 W-treated samples exhibit some features similar to the pristine GA, while differences can also be observed. Although D-band and G-band still exist, the G-band shifts to 1586 cm-1, which is closer to the G-band of graphite (1580 cm-1) rather than the 1608 cm-1 of the pristine GA 21. Such down-shift in G-band frequency indicates its structure gets close to graphite22. In contrast, a sharp peak at 1332 cm-1 appears in the Raman spectra of the 3 W- and 4 W- treated samples, which is from cubic diamond. SEM, TEM, SAED and EELS are employed to analyze the microstructure and the bonding states of the released samples after HPHT. SEM images of the products after 3 W laser heating by using KBr (Figure 3a) as PTM show typical aerogel morphology. TEM observations show that the products after 3 W treatments are mainly constructed by nanosized particles, about 50 to 150 nm (Figure 3b, c), which are stacked and connected to form a porous structure. Further HRTEM (Figure 3e, f) shows that these diamond nanoparticles are made by ultra-small grains with diameters less than 10 nm. The grains are randomly oriented in the nanosized particles. The corresponding SAED patterns (Figure 3d) show three diffracted rings, which can be indexed as (111), (220) and (311) of cubic diamond. These SAED patterns show similar crystalline quality with diamond aerogel synthesized by RF aerogel 9. The interplanar spacing along (111) lattice is about 0.202 nm, which is close to the cubic diamond (0.206 nm). In contrast, HRTEM observations on the 2 W sample show that the sample mainly contains nano


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grains of graphite and amorphous carbon. The grain size is larger than the pristine GA. This indicates that graphitization should occur in the sample after such a HPHT treatment.

Figure 3. SEM (a), TEM (b, c), SAED (d), HRTEM (e, f) images of the samples of GA after 3 W laser heating using KBr as PTM.

EELS is a useful tool to analyze sample, giving information on the bonding and structure

23,24

. The EELS of 2 W sample (Figure 4a) shows a slight pre-peak before

the carbon K-edge that corresponds to π bonding. It is a proof of sp2 carbon. In contrast, the EELS of 3 W samples have no π*-peak, suggesting that the sample is made by sp3 carbon. EELS can also be used to estimate the density of sample according to the density of valence electron given by the plasmon energy. The plasmon energy is defined as 25

∗ In this equation, is Planck's constant divided by 2π, nF is the density of ACS Paragon Plus Environment


valence electron, e is the electron charge, is permittivity of vacuum and m* is the electron effective mass. The relationship between electron effective mass and the free-electron mass m0 is varied and the value of m*/m0 is ranging from 0.85 to 1 reported in different literatures

25–32

. So the volume of each carbon atoms can be

calculated. We measure the plasmon energy to be of 30 eV (Figure 4b), which is about 80% of diamond (33.8 eV, Figure 4b, pink line) and also lower than amorphous diamond (31.8 eV, Figure 4b, blue line). The density of nanodiamond we synthesize is calculated to be ~2.766 g/cm3 to 3.255 g/cm3, when considering different value of

m*/m0. Considering the lattice of nanodiamond is similar to the bulk diamond, the lower density of our samples proves the porosity of the synthesized products. Note that similar nanodiamond products can be found in the experiments with liquid argon (Figure S2) or NaCl (Figure S3) as PTM, while the threshold for the laser heating power is higher than 3.5 W and 7 W, respectively. We can also see that the obtained products consist of numerous diamond nano grains with round shape (liquid argon as PTM, figure S2(a)) or plate like shape (KBr as PTM, figure 3(a)), which are connected and interlaced with each other and thus all form aerogel morphology with similar spectroscopic features.


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Figure 4. (a) EELS of pristine GA and the GA samples after different power laser heating. (b) The

plasmon energy of GA and diamond aerogel after 3 W laser heating. The EELS and plasmon energy data of nanocrystalline diamond (ND) and amorphous diamond (AD) are from reference 26

The diamond products synthesized from GA are quite different from that synthesized by crystallized graphite, due to its ultra-low-density and randomly orientated nanosized graphene layers

18

. Although there are many curved graphene

layers in GA, no lamellar texture of diamond appears in the product after laser heating under high pressure. Note that the GA after 2 W laser heating loses the starting three dimension network structure and transforms into nano graphene/graphite species. We also note that when the sample is heated by 2 W laser, the temperature on the sample is higher than those heated by 3 W or 4 W laser. This should be due to the fact that when the laser power is increased to 3 W or higher, nanocrystal diamond formed quickly, and its poor laser absorption leads to the temperature drop 26. So it is believed that the transformation from GA to nanocrystal diamond in HPHT process takes place in a very short time scale. In addition, the growth of nanodiamond grains should be restricted by the loose structure of GA (i.e., carbon source supplied for the growth of



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diamond nanocrystals), leading to the ultra-small diamond nanocrystals. Therefore, it is concluded that the low density, the randomly oriented graphene layers and short growth time lead to the current diamond aerogel constructed by ultra-small nanocrystals. In addition, it is worth mentioning that the GA sample after 2 W laser heating turns transparent and exhibits no G-band in the corresponding Raman spectra under high pressure, but no diamond is found and only graphite and amorphous carbon are observed when the sample is released to atmospheric pressure (i.e., graphitization takes place). According to the P-T phase diagram of graphite-diamond transition from multi-anvil press, such HPHT condition (2200 K and 20 GPa) is enough to produce diamond 33,34. This result also indicates that a “post-graphite phase” may form at such conditions while it is reversible upon decompression

35–37

, and during this

transformation process, carbon atoms rearrange and graphitization takes place. Thus graphitization in nanoscale may also be the intermediate process during GA-diamond transition in our experiments. Transparent carbon phases have been observed in previous studies by cold compressing various carbon precursors such as glassy carbon 38

, graphite

35 36

, fullerene

39

and so on, which all can not be quenched to ambient

conditions. Graphitization of the current transparent phase upon decompression should be related to the different thermodynamic conditions by laser heating under pressure.

4. Conclusion


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In summary, we realize the direct conversion from GA to diamond aerogel by laser heating under high pressure for first time. The diamond aerogel obtained is constructed by ultra-small nanocrystals. It is found that the microstructure of GA, as well as the short heating time by laser favors the ultra-small diamond nanocrystal growth, and even the solid PTM can be used for diamond aerogel synthesis. We also observe a transparent carbon phase in our experiment at 20 GPa and 2200 K, which turns to graphite and amorphous carbon upon decompression. This result also indicates that during the GA-diamond transition graphitization may take place in nanoscale in the experiments. Since GA with tunable microstructures, such as density and porosity, can be synthesized in a controllable way, it is thus possible to tune the formation of light porous diamond aerogel by designing suitable GA precursors. This is important for the synthesis of diamond aerogel.

Supplementary data See supplementary data for Optical photograph of GA samples before and after laser heating and the TEM, SAED of GA after laser heating using NaCl or liquid argon as PTM.

Acknowledgements This work was supported financially by the National Natural Science Foundation of China (51320105007, 11634004, 11474121), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1132).

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Direct Conversion of Graphene Aerogel into Low-Density Diamond

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