Reversible Nanodiamond-Carbon Onion Phase Transformations

Letter pubs.acs.org/NanoLett

Reversible Nanodiamond-Carbon Onion Phase Transformations J. Xiao,† G. Ouyang,‡ P. Liu,† C. X. Wang,† and G. W. Yang*,† †

State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Nanotechnology Research Center, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong People’s Republic of China ‡ Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of the Ministry of Education, Department of Physics, Hunan Normal University, Changsha 410081, Hunan People’s Republic of China ABSTRACT: Because of their considerable science and technical interest, nanodiamonds (3−5 nm) are often used as a model to study the phase transformation between graphite and diamond. Here we demonstrated that a reversible nanodiamond-carbon onion phase transformation can become true when laser irradiates colloidal suspensions of nanodiamonds at the ambient temperature and pressure. Nanodiamonds are first transformed to carbon onions driven by the laser-induced high temperature in which an intermediary bucky diamond phase is observed. Sequentially, carbon onions are transformed back to nanodiamonds driven by the laserinduced high temperature and high pressure from carbon onions as nanoscaled temperature and pressure cell upon the laser irradiation process in liquid. Similarly, the same bucky diamond phase serving as an intermediate phase is found during the carbon onion-to-nanodiamond transition. To have a clear insight into the unique phase transformation the thermodynamic approaches on the nanoscale were proposed to elucidate the reversible phase transformation of nanodiamond-to-carbon onionto-nanodiamond via an intermediary bucky diamond phase upon the laser irradiation in liquid. This reversible transition reveals a series of phase transformations between diamond and carbon allotropes, such as carbon onion and bucky diamond, having a general insight into the basic physics involved in these phase transformations. These results give a clue to the root of meteoritic nanodiamonds that are commonly found in primitive meteorites but their origin is puzzling and offers one suitable approach for breaking controllable pathways between diamond and carbon allotropes. KEYWORDS: Nanodiamond, bucky diamond, carbon onion, phase transformation, laser ablation in liquid

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transformation of nanodiamond-to-carbon onion-to-nanodiamond via an intermediary bucky diamond phase is achieved by laser-irradiating colloidal suspensions of nanodiamonds at the ambient pressure and room temperature. To gain a better understanding of the novel reversible phase transformation above, we establish a thermodynamic model to pursue the issue and have a clear and general insight into the basic physics involved in the conversions between diamond and carbon allotropes. Therefore, these results provide one suitable route for breaking controllable pathways between diamond and carbon allotropes such as carbon onion. In our study, raw detonation nanodiamonds (Aldrich, ≥97% trace metals basis, nanopowder) and absolute alcohol (Guangzhou chemical reagent) are used without further purification. The experiments are carried out in laser ablation in liquid.25 In this case, about 3 mg raw detonation nanodiamonds are dropped into 10 mL bottle filled with absolute alcohol to form the suspension. Then, a second harmonic

he phase transformation between graphite and diamond has been of intense interest from a point of view of science and technology for a century, and breaking controllable pathways between graphite and diamond such as being able to transform graphite to diamond is a subject that has fascinated scientists and engineers for several decades.1−10 Nanodiamonds (3−5 nm) have been found in meteorites, protoplanetary nebulae, and interstellar dusts; meanwhile, they can be produced on the Earth by some techniques.11−17 Very similarly, carbon onions are an extremely common form of nanoscaled carbon, being found in space and produced on the Earth.18−24 Therefore, there is a growing interest in pursuing the phase transformation between nanodiamonds and carbon onions in the laboratory recently, not only for the understanding to the origin of the meteoritic nanodiamonds,14 but also for breaking controllable pathways between diamond and carbon allotropes3,7 and industrial applications of nanodiamonds.9 Carbon onions can be transformed to nanodiamonds by an intense electron beam irradiating at high temperature (800 °C) and ultrahigh vacuum (10−6 Pa),3 while nanodiamonds can be transformed to carbon onions by annealing at high temperature (1000−1500 °C) and high vacuum (10−5 Pa).24 Here, we report that a reversible phase © XXXX American Chemical Society

Received: April 16, 2014 Revised: May 11, 2014

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dx.doi.org/10.1021/nl5014234 | Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. The reversible transitions of raw detonation nanodiamond-carbon onion-well-dispersed nanodiamond. (a) TEM image of raw detonation nanodiamonds, which obviously show that these particles are agglomerating. The distribution histogram of raw material and its Gaussian fitting curve (inset) shows that the mean size is 5.26 nm. (b) The corresponding selected area electron defraction pattern of raw nanodiamonds. (c) HRTEM image of raw materials, which prove that the abundant amorphous carbon is around crystal cubic nanodiamonds. (d,e) Two views of spherical carbon onion aggregated, which contain elongated particles with linked external graphite-like layers and closed quasi-spherical internal shells. These carbon onions are obtained by irradiated the raw materials for 20 min (taken from the 5th bottle of panel i). (f) Well-dispersed nanodiamonds regain after irradiation carbon onions for 40 min. Size distribution shows the size is 5.35 nm. (g) The corresponding selected-area electron diffraction pattern of well-dispersed nanodiamonds. (h) HRTEM of individual nanodiamond in final products. Note that the nanodiamond keeps fresh surface without amorphous carbon around it. (i) Color change with the increase of laser irradiation time. Clearly, the color of the solution changes from opaque greyish white to dark black and finally to transparent yellow.

produced by a Q-switched Nd:YAG laser device with a wavelength of 532 nm, pulse width of 10 ns, repeating frequency

of 10 Hz, and laser pulse power of 150 mJ is focused into the middle of the bottle with 1 mm beam size. During the laser B


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Figure 2. Nanodiamond with 5-fold twinning and bucky diamonds. (a) HRTEM of abundant nanodiamonds. Marked lattice spacing is measured to be 0.206 nm, which coincides with the d value of the (111) lattice plane of diamond with cubic structure. (b) Individual 5-fold twinning of nanodiamonds. (c) HRTEM image of the products taken from the 4th bottle of Figure 1i. Spherical particle consisting of onion shells with a diamond core (pointed by the while arrows). The core shows the (111) lattice fringes of diamond with interplanar spacing of 0.206 nm. (d) HRTEM image of the products taken from the 6th bottle of Figure 1i, the diamond cores within carbon onion regenerate after laser irradiating carbon onions.

irradiation, the solution is kept stirring with a magnetic stirrer. The irradiation time of nine bottles of Figure 1i is 0, 5, 8, 12, 20, 30, 45, 50, and 60 min, respectively. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images are acquired by using an FEI Tecnai G2 F30 transition electron microscope equipped with a field-emission gun. Raman spectra are recorded using an inVia Confocal Raman Microspectrometer (Renishaw, U.K.) utilizing a 325 nm HeCd laser excitation. The starting materials are raw detonation nanodiamonds as shown in Figure 1a−c, which shows that the raw detonation nanodiamonds are agglomerating consisting of well-defined crystalline cores and amorphous outer shells. The color of the solution begins to change when the laser irradiates the suspension as shown in Figure 1i. After 20 min irradiation, the solution becomes dark black (see fifth bottle of Figure 1i). Therefore, this color change can be ascribed to the generation of carbon onions based on the TEM observations, and different morphologies of carbon onions are found including quasispherical particles with closed concentric graphite shells, elongated particles with linked external graphite-like layers, and closed quasi-spherical internal shells as shown in Figure 1d,e. Clearly, these spherical carbon onions are 5−10 nm in size and the spacing of the lattice fringes is about 0.34 nm, being close to that of the (002) planes of graphite. Surprisingly, when the laser

irradiation time is extended up to 60 min, the dark black solution gradually becomes transparent yellow and keeps unchanged (see the last bottle in Figure 1i). On the basis of the TEM analysis, the final products turn out to be the nanodiamonds with a significant improvement in dispersity and purity as shown in Figure 1f. A statistical analysis (Figure 1g) on more than 200 nanoparticles in the TEM image demonstrates that the as-synthesized nanocrystals with diameters of 5.35 nm. Note that the amorphous carbon shell no longer exists based on the HRTEM image (Figures 1h and 2a). Especially, the 5-fold twinning structure is found in the nanodiamonds as shown in Figure 2b, which is often observed in cubic diamond.1 These results therefore show that a reversible nanodiamond-carbon onion phase transformation can be come true driven by laser in liquid. During the laser irradiating, we can clearly see a series of significant color changes of the colloid from opaque grayish white to dark black and finally to a transparent yellow as shown Figure 1i, which indicates the formation of various kinds of the products. We carefully check the products at the different laser irradiation stage. Figure 2c shows an intermediate product during the nanodiamond converting carbon onion process (see fourth bottle in Figure 1i). It is clear that these nanoparticles exhibit well-ordered onion-like structure in the outer shell and the remainder of cores composed of diamond (111) planes, which is different from carbon onions (see fifth bottle in Figure 1i). This C


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intermediate product is thus the typical bucky diamond.5 Additionally, the similar morphology is found during the carbon onion converting nanodiamond process (see sixth bottle in Figure 1i) as shown in Figure 2d. Based on the systemic experimental data analysis above, we find out that there is the same bucky diamond phase with diamondlike core and graphitelike shell serving as an intermediate phase in the two phase transformations, that is, from nanodiamond to carbon onion and from carbon to nanodiamond. Bucky diamond is classified as an intermediate phase and coexists with carbon onions and nanodiamonds.5,26−28 Accordingly, these experimental observations show that, upon the laser irradiating in liquid, the starting nanodiamonds (Figure 1c) are first converted to the bucky diamonds (Figure 2c) and then the bucky diamonds are transformed to carbon onions (Figure 1d). Sequentially, the carbon onions are first converted to the bucky diamonds (Figure 2d) and then the bucky diamonds are converted to the final nanodiamonds (Figure 1h). Therefore, this reversible phase transformation of nanodiamond-to-carbon onion-to-nanodiamond includes a series of phase transitions between diamond and carbon allotropes such as carbon onion and bucky diamond, which has a clear and general insight into the basic physics, involved in the conversion between diamond and carbon allotropes. In order to elucidate the phase transformation above more clearly, the Raman spectroscopy is employed in our study. As we know, UV Raman (325 nm) is suitable for nanodiamonds characterization due to fulfillment of the resonance condition, suppression of the D band of graphite, and avoiding luminescence of nanodiamonds.29 The raw detonation nanodiamonds (the curve i in Figure 3a) exhibit two feature as follows. One diamond peak is at ∼1325 cm−1 and another broadened peak is around ∼1600 cm−1, which are often simply referred to as “G band” in analogy to sp2- containing (graphitic) carbon. After the ablation for ∼10 min, the diamond peak shifts to ∼1360 cm−1. This peak can be ascribed to the appearance of bucky diamonds (the curve ii in Figure 3a). To elaborate this phase, we fit this feature to the Gaussian−Lorentzian curves as shown in Figure 3b. Clearly, the spectrum of bucky diamonds consists of the peak at 1324 cm−1 (the green curve) that can be assigned to that of nanodiamonds. The peak at 1324 cm−1 of nanodiamonds is lower than that of bulk diamonds (1332 cm−1), which is due to the phonon confinement effect.9 Sequentially, the peak continually shifts to around 1400 cm−1 after the laser ablation for ∼20 min, which is ascribed to the disorder sp2-bonded carbon phase (D band).29 Meanwhile, the G band also makes a noticeable downshift to 1584 cm−1, which is originated from the influence of the shell curvature of carbon onions (the curve iii in Figure 3a).30 Later, the D band downshifts while the G band shifts up to higher wavenumber (the curve iv in Figure 3a, which is similar to the curve ii in Figure 3a), which means forming bucky diamonds again. Finally, the diamond peak regenerates and the G band shifts to higher wavenumber (the curve v in Figure 3a) compared to that of the raw nanodiamonds, which is due to the presence of O−H functional groups adsorbed on the surface of the final nanodiamonds as shown in Figure 3c.9 Therefore, the Raman scattering measurements of the reversible phase transformation of nanodiamond-to-carbon onion-tonanodiamond provide the detailed evolution of diamond, graphite, and carbon onion phases upon the laser ablation in liquid process, which confirm the results from the TEM analysis shown in Figures 1 and 2.

Figure 3. Raman spectra analysis of the phase transformation from the raw nanodiamonds to the treated nanodiamonds. (a) Raw detonation nanodiamonds (curve i), a diamond peak at ∼1325 cm−1 and a broadened peak around ∼1600 cm−1; bucky diamonds (curve ii), the peak ∼1360 cm−1; carbon onions (the curve iii), the diamond peak continually shifting to around 1400 cm−1 (the D band), at the same time, the G band also making a noticeable downshift to 1584 cm−1; bucky diamonds (curve iv), the D band downshift while the G band shift up to higher wavenumber; and the final nanodiamonds (curve v), the diamond peak (1325 cm−1) regenerating and the G band shifting to 1650 cm−1, higher than that of raw nanodiamonds. (b) Detailed fitting of bucky diamonds (curve ii in panel a). The green fitting curve locates in 1324 cm−1 that can be assigned to nanodiamonds. (c) FTIR spectrum of the final as-synthesized nanodiamonds, showing the evidence of OH functional groups.

To gain a quantitative understanding of the unique reversible phase transformation of nanodiamond-carbon onion, we D


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propose the thermodynamic analysis to elucidate our findings on the basis of the thermodynamic theory at the nanometer scale.6−8 It is known that carbon onion is the stable form of carbon at ambient temperatures and pressures, 24 and diamond is metastable.7 Although the energy difference between the two phases is only about 0.02 eV per atom,31 they are separated by a high energy barrier (about 0.5 eV per atom similar to that of graphite and diamond.28,31,32), so high temperatures and pressures are needed to interconvert carbon onion to diamond.8 During the laser irradiating the starting nanodiamonds with thin amorphous carbon shells, the laser can induce a high temperature in the amorphous carbon shell by the amorphous carbon absorbing laser energy.33 Therefore, we first establish a heating modal of nanoparticles upon the laser irradiating in liquid on the basis that amorphous carbon and carbon onion can absorb light and be heated; in contrast, nanodiamonds have a large band gap of 5.5 eV and are transparent to visible light used in our case. The schematic diagram of various materials interacting with light is shown in Figure 4a. The energy absorbed by a nanoparticle from the laser pulse is spent in the heating process, which is described as following the equation34 λ Jσabs =m

∫T

T

Cp(T )dT

0

(1)

where m = ρ[(πd )/6] is the particle mass, d is the diameter of particle, ρ is the density, J is laser fluence, T0 is fixed to 300 K, σλabs is the particle absorption cross section that strongly depends on the laser wavelength, and Cp(T) is the heat capacity. It is known that the heat capacity is the function of temperature35 in which the exponential function is used for fitting these data points as Cp = Ai exp(−T/t) + Co. Thus, the relationship between the heat capacity and temperature can be described as Cp = −30.7 exp(−T/486.7) + 25.4. For a spherical particle, the absorption cross section can be calculated as36 3

λ σabs =

λ πd 2Q abs

4

(2)

Figure 4. Schematic diagram of interactions between various materials and laser. (a) Nanodiamond covered by amorphous carbon (left) and carbon onion (middle) irradiated by the visible light can be heated due to absence of band gap. However, the pure nanodiamond (right) without any covers is transparent to the used light owing to a large band gap of 5.5 eV. (b) Relationship between laser fluence and temperature rising result from the nanodiamonds covered by amorphous carbon absorbing the power of laser. (c) Relationship between laser fluence and temperature rising result from carbon onion absorbing the power of laser.

Qλabs

where denotes absorption efficiency, and the index λ indicates that both Q and σ depend on laser wavelength. In fact, Qλabs can be expressed as the function of relative refractive index m35 ⎧ m2 − 1 ⎫⎡ 4x 3 ⎧ m2 − 1 ⎫⎤ λ ⎬⎢1 + ⎬⎥ Q abs = 4x Im⎨ 2 Im⎨ 2 3 ⎩ m + 2 ⎭⎣ ⎩ m + 2 ⎭⎦

x = ka =

ñ m= n1̃

2πn1̃ a λ

(3) (4)

if (4x3/3)Im{(m2 − 1)/(m2 + 2)} ≪ 1, a condition that will be satisfied for sufficiently small d (d = 5 nm in this case). Thus, the absorption efficiency is expressed

(5)

⎧ m2 − 1 ⎫ ⎬ Q abs = 4x Im⎨ 2 ⎩m + 2⎭

x is size parameter, a is radius of particle, ñ and ñ1 are the refractive indices of particle and medium (in this case, alcohol, ∼1.36), respectively. In order to calculate the absorption efficiency of a spherical particle, we need to know two optical characteristics of the particle materials: refractive index n(λ) and extinction coefficient k(λ), or the real and imaginary parts of the complex refractive indices: ñ(λ) = n(λ) + ik(λ). Their values as the function of the wavelength n(λ) and k(λ) for many materials can be found in the reference book.37 In our case, we take amorphous carbon and 532 nm as the reference material and wavelength: ñ(532 nm) = 2.32626 + 0.85359i. Note that, for eq 3,

(6)

In addition, the density difference should be taken into account. The density of amorphous carbon is a relatively stable value of ∼2.1 g/cm3.38 Combining these equations above, the rising temperature of amorphous carbon is calculated to be a nearly linear increase with the laser fluence as shown in Figure 4b. In detail, in our case the temperature in the amorphous carbon induced by the laser irradiating in liquid can reach to 2500 K when the laser fluence of E


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70 mJ is used. Therefore, from the heating modal of nanoparticles upon the laser irradiating in liquid we can clearly see that the laser can induce enough of a high temperature in the amorphous carbon shell by the amorphous carbon absorbing laser energy to drive the nanodiamond core (Figure 1c) to transform into the carbon onion (Figure 1d) in our case.24 Similarly, laser can induce a high temperature in carbon onion by carbon onion absorbing laser energy as shown in Figure 4c on the basis of the proposed theoretical model. In detail, in our case the temperature in the carbon onion induced by the laser irradiating in liquid can reach to 2000 K when the laser fluence of 70 mJ is used. Then, the laser-induced high temperature will compress the interlayer distance of carbon onion. Finally, the interlayer distance reduction will result in a high pressure inside the carbon onion. We have developed a theoretical model for this issue as follows. Taking into account a carbon onion containing N carbon cages with different radii, each carbon cage (CC) under a high temperature will expand and the volume thermal coefficient (αV) is related with the Debye temperature based on the Lindemann’s equation39 and Gründisen’s theory of the solid state,40 that is, αV = c/(θ2D V2/3 Ar), where c, V, and Ar denote a constant, the mole volume, and the atomic weight compared to C12, respectively. According to this relationship, one may calculate the αV(∞) of each carbon cage from the Debye temperature data. On the basis of the proportional relationship of αV(∞) ∝ θD(∞) and assuming the size in this equation can be extended to nanoscale regime, we have αV(CC) θ 2(G) = 2D αV(G) θD(CC)

(7)

here αV(G) is the volume thermal coefficient of graphite. We know that θ2D ∝ Ec ∝ γ,41,42 where Ec and γ are the cohesive energy and the surface energy. Thus, we have αV(CC) γ(G) = αV(G) γ(CC)

(8)

As an approximation, the size-dependent surface energy of nanosolids can be expressed as γ(G)/γ(CC) = 1/[(1 + 2h)/R], where h and R represent the size of a carbon atom and the radius of carbon cage. Combining eqs 7 and 8 yields ⎛ 1 − 2h ⎞ ⎟ αV(CC) = αV(G)⎜ ⎝ R ⎠

(9)

Figure 5. Carbon onion served as a nanoscaled temperature and pressure cell. (a) Size and temperature dependence of volume thermal expansion coefficient in carbon cages. The inset is the heat capacity of graphite in c-axial and a-axial directions. (b) Temperature-dependent interlayer distance of C60 in C180. (c) Dependence of pressure (pinner) on interlayer distance in a double-layer carbon onion. Note that the inset is a double-layer carbon onion (C60 in C180) considered in our case.

Furthermore, based on the relation of thermal volumetric expansion coefficient αV = (1/V)(dV/dT),43 we can obtain the size and temperature dependence of volume of CC in carbon onion, that is ⎛ ⎛ 1 − 2h ⎞ ⎞ ⎟T ⎟ V (CC) = V0⎜1 + αV(G)⎜ ⎝ R ⎠ ⎠ ⎝

(10)

pouter =

Apparently, the volume change of each CC in carbon onion is nonuniform and will lead to the variation of interlayer distance and further result in the change of van der Waals (vdW) interaction between carbon layers. Considering a double-layer carbon onion consisting of an inner cage with radius R1 and a outer cage with radius R2 (see the inset of Figure 5c), the pressures (p) acting on the inner and the outer carbon cages are44 pinner = −

ρ∞2 4πR12

∫S

(0) inner

(

∫S

(0) outer

(0) (0) FdS 1 outer)dSinner

ρ∞2 4πR 22

∫S (∫S (0) outer

(0) inner

(0) (0) F2dSinner )dSouter

(12)

(0) where ρ∞ is the atom density of plane graphene, S(0) inner and Souter denote the areas of inner and outer carbon cages in the undeformed state that is used as a reference configuration, F1 and F2 are the vdW forces acting on each of the atoms (positive for the attraction), which can be gained from the first derivative of the Lennard-Jones (LJ) potential (V(d) = 4ε[(σ/d)12 (σ/d)6], with ε and σ being the LJ parameters) with respect to d.

(11) F


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high temperature will destroy the amorphous carbon shell.33 Meanwhile, the bucky diamond phase serving as an intermediate phase between nanodiamond and carbon onion5,26−28 forms upon the nanodiamond to carbon onion transition. Sequentially, the laser irradiates the carbon onions and then drives them into a state of a high temperature by carbon onions absorbing laser energy as shown Figure 4c. Further, the high temperature can induce a high pressure inside the carbon onions by compressing the interlayer distance of the carbon onions (Figure 5c). Therefore, carbon onion can be served as a nanoscaled temperature and pressure cell for generation of high temperature and high pressure inside carbon onion upon laser irradiation confined by liquid. Finally, the laser-induced high temperature and high pressure can drive the carbon onions to convert to the nanodiamonds. Similarly, an intermediary bucky diamond phase appears during the carbon onion to nanodiamond phase transformation. Because the primary nanodiamonds are transparent to the used laser due to the band gap of 5.5 eV, the final nanodiamonds are very stable upon the laser irradiation in liquid as shown Figure 1i. Astronomical observations have demonstrated that nanodiamonds are present in the primitive chondritic meteorites, but the origins of these cosmic sources are still under investigation.11−13 A number of different explanations for the origin of meteoritic nanodiamonds have been proposed, although each has major problems.14 For example, chemical vapor deposition (CVD) is the most widely accepted mechanism,13 but the laboratory conditions for CVD do not correspond to typical astrophysical conditions.14 The abundance of carbon onions in space22 might originate from isolated carbon fragments spontaneously rearranging under the high-temperature conditions that are easily met in space, both in the warm interstellar medium and in the vicinity of dying stars.14 Therefore, this reversible nanodiamond-carbon onion phase transformation driven by energetic irradiation (electron, photon, and other energetic beam irradiations) may give a clue to the root of meteoritic nanodiamonds. Our results suggest that nanodiamonds in meteorites are from carbon onions in space. In other words, where there are carbon onions, where there are nanodiamonds in the cosmic sources. In summary, we have demonstrated a reversible phase transformation between nanodiamond and carbon onion driven by the laser irradiation in liquid and established the corresponding thermodynamic analysis to elucidate the physical mechanism behind the experimental observations. Our results showed that the laser-induced high temperature in the amorphous carbon shell of the starting nanodiamonds result in the nanodiamond core-to-carbon onion phase transformation via an intermediary bucky diamond phase, and then the carbon onion is transformed back to the nanodiamond driven by the laser-induced high temperature and high pressure from the carbon onion as a nanoscaled temperature and pressure cell upon the laser irradiation in liquid. Similarly, an intermediary bucky diamond phase appears during the carbon onion-to-nanodiamond phase transformation. Therefore, these findings of the reversible nanodiamond-carbon onion phase transformation offer one suitable approach for breaking controllable pathways between diamond and carbon allotropes and give a clue to the origin of meteoritic nanodiamonds.

Noticeably, the carbon onions in our case are perfectly crystalline without considering any defect effect. From eq 9, the size- and temperature−volume thermal expansion coefficients of carbon cages have been calculated, as shown in Figure 5a. Note that the volume thermal expansion coefficient of graphite can be depicted as αV(G) = 2αa + αc, with αa and αc being the thermal expansion coefficients of graphite in a-axial and c-axial directions, which are related with the heat capacity and the Debye functions. The inset in Figure 5a is the heat capacity of graphite in a-axial and c-axial directions. It is shown that the volume thermal expansion coefficient of different carbon cages increases with decreasing size and increasing temperature, implying that the smallest carbon cage within carbon onion has the largest expansion velocity at the fixed temperature. Considering a double carbon onion containing two perfectly spherical layers such as C60 in C180, we can predict the temperature-dependent interlayer distance based on eq 10. In Figure 5b, the interlayer distance between C60 and C180 becomes smaller with increasing temperature. Therefore, it is concluded that the interlayer pressure will be altered due to the changes of carbon cages’ radii. Figure 4c shows pinner as a function of the interlayer distance. Evidently, the smaller interlayer distance leads to the larger pressure. According to these results as shown in Figure 5, we can conclude that in our case a high-temperature of 2000 K in the carbon onion induced by the laser will lead to a high pressure more than 10 GPa inside the carbon onion. Therefore, such a high-temperature and high-pressure environment inside the carbon onion driven by the laser irradiation in liquid is very suitable for the phase transformation from graphite to diamond based on the carbon thermodynamic equilibrium phase diagram.6 On the basis of the experimental observations and the corresponding thermodynamic analysis above, the reversible phase transformation of nanodiamond-to-carbon onion-tonanodiamond is ascribed as follows (Figure 6). In the

Figure 6. Schematic diagram of the physical mechanism of the reversible nanodiamonds-carbon onion phase transition. ΔG means the energy difference between diamond and carbon onion and ΔE means the activation energy that is necessary for the phase transition from diamond to carbon onion.

nanodiamond-to carbon-onion phase transformation upon the laser irradiation in liquid, the laser can induce a high temperature in the amorphous carbon shell by the amorphous carbon absorbing laser energy during the laser irradiating the starting nanodiamonds with thin amorphous carbon shells as shown Figure 4b. Then, the laser-induced high temperature drives the nanodiamond core to transform into the carbon onion, while the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G


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Author Contributions

(28) Barnard, A. S.; Russo, S. P.; Snook, I. K. Phys. Rev. B 2003, 68, 073406. (29) Cebik, J.; McDonough, J. K.; Peerally, F.; Medrano, R.; Neitzel, I.; Gogotsi, Y.; Osswald, S. Nanotechnology 2013, 24, 205703. (30) Obraztsova, E. D.; Fujii, M.; Hayashi, S.; Kuznetsov, V. L.; Butenko, Y. V.; Chuvilin, A. L. Carbon 1998, 36, 821−826. (31) Zhao, D. S.; Zhao, M.; Jiang, Q. Diamond Relat. Mater. 2002, 11, 234−236. (32) Wang, J. B.; Yang, G. W. J. Phys.: Condens. Matter 1999, 11, 7089− 7094. (33) Niu, K. Y.; Zheng, H. M.; Li, Z. Q.; Yang, J.; Sun, J.; Du, X. W. Angew. Chem., Int. Ed. 2011, 50, 4099−4102. (34) Wang, H.; Pyatenko, A.; Kawaguchi, K.; Li, X.; Swiatkowska Warkocka, Z.; Koshizaki, N. Angew. Chem., Int. Ed. 2010, 122, 6505− 6508. (35) Barin, I.; Sauert, F.; Schultze-Rhonhof, E.; Sheng, W. S. Thermochemical data of pure substances, 2nd ed.; VCH: Weinheim, 1993. (36) Bohren, C. F.; Huffman, D. R. Absorption and scattering of light by small particles; Wiley: New York, 2008. (37) Palik, E. D. Handbook of Optical Constants of Solids II; Academic Press: New York, 1991. (38) CRC Handbook of Chemistry and Physics, 86th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, 2005. (39) Swalin, R. A. Thermodynamics of Solids; Wiley: New York, 1972. (40) Zhao, Y. H.; Lu, K. Phys. Rev. B 1997, 56, 14330. (41) Ouyang, G.; Zhu, Z. M.; Zhu, W. G.; Sun, C. Q. J. Phys. Chem. C 2010, 114, 1805−1808. (42) Ouyang, G.; Tan, X.; Yang, G. W. Phys. Rev. B 2006, 74, 195408. (43) Kwon, Y. K.; Berber, S.; Tománek, D. Phys. Rev. Lett. 2004, 92, 015901. (44) Todt, M.; Rammerstorfer, F. G.; Fischer, F. D.; Mayrhofer, P. H.; Holec, D. Carbon 2011, 49, 1620−1627.

J.X. and G.O. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Basic Research Program of China (2014CB931700) and the State Key Laboratory of Optoelectronic Materials and Technologies of Sun Yat-sen University supported this work.

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