Nanodiamonds Produced from Low-Grade Indian Coals - ACS

Oct 4, 2017 - Coal is considered to be an abundantly available cheap feedstock for the fabrication of carbon nanomaterials. In this Letter, a report o...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9619-9624

Nanodiamonds Produced from Low-Grade Indian Coals Tonkeswar Das and Binoy K. Saikia*

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Polymer Petroleum and Coal Chemistry Group, Materials Science and Technology Division, CSIR-North East Institute of Science & Technology, Jorhat-785006, India ABSTRACT: Coal is considered to be an abundantly available cheap feedstock for the fabrication of carbon nanomaterials. In this Letter, a report on the formation of nanodiamond from low-grade coals during low-power ultrasonic-assisted stimulation in hydrogen peroxide (H2O2) followed by dialysis in 1 kDa is given. High resolution-transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), Raman spectroscopy, ultraviolet− visible spectroscopy (UV−vis), fluorescence (FL), and Fourier transform infrared (FT-IR) spectroscopy analyses revealed the formation of carbon nanocrystals of monocrystalline and polycrystalline form with multiple plans. The nominal size of the carbon nanocrystals are found to be in the range of 4−15 nm. The planar spacing of the crystal lattice fingers is in the range of 2.0−2.3 Å and are in good agreement with the lattice planes of various diamond phases including cubic diamond and lonsdaleite. The present work significantly contributes an additional synthetic methodology of nanodiamond production by using the cheap low-grade coal feedstock. The nanodiamond formed shows bright blue fluorescence under UV-light with excitation dependent and holding promising application in bioimaging engineering, photovoltaics, and optoelectronics. KEYWORDS: Low-grade coals, Ultrasonic-assisted stimulation, Nanodiamonds, Blue fluorescence, Value addition to low-grade coal

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INTRODUCTION After the first man-made bulk synthetic method discovered in 1955,1,2 nanodiamonds have re-emerged as an intensive research interest in recent years due to their combination of outstanding mechanical performance, chemical resistance, versatile surface chemistry, biocompatibility, and unique optical and electric properties.3 Nanodiamonds are reported to be less toxic than the other nanomaterials4−7 and have emerged as a key platform for nanoscience and nanotechnology developments.8 It has been found to possess a wide range of application in the field of microelectronics, optoelectronics, and biosensing.4 In addition, nanodiamonds are also used in novel wearresistant polymers, metal coatings,8,9 and lubricant additives10 due to their superhardness, exceptional chemical resistivity, and abrasive nature, respectively. Nanodiamonds have been widely used in biomedical imaging, drug delivery, and other areas of medicine.4−7 Although a number of different methods exist for the synthesis of nanodiamonds particles,11−17 only high-pressure and high-temperature (top-down) and detonation (bottom-up) methods are available on an industrial scale. However, there is always a sustainable demand for a “golden standard” to produce a universal nanodiamond for its versatile applications in industrial, commercial, or academic purposes.3 In the “highpressure high-temperature (HPHT)”and detonation methods, high pressure (tens of thousands of atmospheres) and high temperature (more than 2000 K) with the aid of explosive materials (TNT, RDX, etc.) have been used for the production of nanodiamonds. However, highly sophisticated equipment is needed and it is not economically viable worldwide to supply industrial diamonds via this route. Khachatryan et al.18 reported © 2017 American Chemical Society

that the use of high-power ultrasound is an innovative alternative for the synthesis of micro- and nanodiamonds and very efficient instead of using HPHT and detonation methods. In their method, however, they used pure graphite as a carbon source and expensive organic solvent as a liquid media for the synthesis of nanodiamonds. Recently, Lueking et al.19 reported the formation of nanocrystalline diamond (NCD) as a byproduct during the hydrogenative ball milling of anthracite coal with cyclohexene for the production and storage of hydrogen. Additionally, in the year of 2007, Lueking and her co-workers20 reported the formation of nanocrystalline diamond (NCD) after reactive ball milling of anthracite coal with cyclohexene, a high-temperature (1400 °C) thermal anneal, and 4 M HCl treatment followed by 10 M NaOH treatment. Sun et al.21 reported the recrystallization of the carbon network into diamonds when the anthracite coal functionalized with dodecyl groups was irradiated with electron beam. In an another work, Xiao et al.22 reported the synthesis of nanodiamonds from anthracite (Vietnam), bitumen (Indonesia), and coke (China) by laser ablation technique in a liquid at atmospheric pressure and temperature. However, the productions of nanodiamonds with purity, size selectivity, deaggregation, surface functionality, and photoluminescence have remained a challenging task.22 The coal is being considered as an abundantly available cheap feedstock for the fabrication of carbon nanomaterials and several researchers reported the same, including for the production of carbon Received: July 24, 2017 Revised: October 3, 2017 Published: October 4, 2017 9619

DOI: 10.1021/acssuschemeng.7b02500 ACS Sustainable Chem. Eng. 2017, 5, 9619−9624

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nanotubes,23−39 microballs,40 carbon nanodots,41−46 onion-like fullerenes,47−49 and graphene/graphene oxide.50−55 In some of the previous studies, the formation of carbon nanotubes, nanoballs,56 onion-like fullerenes, and chemically converted graphene-like carbon nanosheets from Northeast Indian lowgrade coal has been reported.49 The Northeast region (NER) of India has around 1.496 Gt of low-grade coal reserves. These tertiary coals have high-sulfur contents (2−8%), where 75−90% is organically bound, while the rest is in inorganic forms viz. sulfate sulfur and pyritic sulfur.49,56,57 Thus, the NER coals need beneficiation as well as value addition for further gainful utilization. In this Letter, the discovery of the formation of some typical nanodiamond suspensions, formed during low-power ultrasound-assisted exfoliation of Northeast Indian low-grade coals is reported. In our investigation, we used low-power ultrasound and low-grade coals as carbon source instead of high-power ultrasound and graphite as carbon source as reported by Khachatryan et al.18 We had also not used any high grade coal (i.e., anthracite coal) as reported by Lueking et al.19,20 and Sun et al.21 Although, several researchers58−61 along with our previous studies,62,63 reported the sulfur and mineral matter removal of different types of coals by using ultrasound-assisted methods, but the formation of nanodiamond was not reported in those experiments. The main novelty of our investigation is that the carbon source is from low-grade coal feedstock, which is typically different and cheap from other carbon sources. As compared to the pure sp2-carbon such as graphite and high grade coal, the exfoliation of the small graphite-like crystalline domains that are inherent in low-grade coal is reported to be easy.41−43

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RESULTS AND DISCUSSION During the detailed electron beam analysis (TEM/HR-TEM), unagglomerated, uniformly sized carbon nanoparticle with a crystalline phase (Figure 1a,b), as confirmed by the selected

EXPERIMENTAL SECTION

20 g of low-grade coal sample was mixed with 100 mL of H2O2 in a Teflon beaker and subsequently exposed to an ultrasound treatment at a frequency of 20 kHz in a Ultrasonic Processor (Sonapros; Model: PR-1000 M) at atmospheric pressure for 3 h. A sensor was placed in the reaction mixture to measure the reaction temperature. Then, the mixture of coal and H2O2 was filtered. The detailed methodology being adopted was well-described in our previous study.57 The H2O2 was used to remove the sulfur components as well as the mineral matters from the coals. The chemical characteristics of the feed as well as the residual coals were reported in our previous study.57 The filtrate part obtained after ultrasonication was poured into a beaker containing 500 mL of crushed ice and then neutralized. The neutral mixture was then filtered through a 0.22 μm polytetrafluoroethylene membrane and dialyzed in a 1 kDa dialysis bag for 5 days. The dialyzed solution was concentrated using rotary evaporation and collected. One drop of the solution was pipetted onto a carbon support film on a copper grid and characterized by transmission electron microscopy (TEM/HRTEM) (HR-TEM; Joel JEM-2100, resolution: 1.9 to 1.4 Å, accelerating voltage: 60−200 kV in 50 V steps). The TEM images were further developed by using the “ImageJ” program (software version 1.47). An X-ray diffraction spectrum was obtained using an Xray powder diffractometer (type: JDX-11P3A, JEOL, Japan). X-ray diffraction data was obtained with the starting angle of 2.00°, a stoping angle of 75.00°, and a step size of 0.05° with a scanning rate of 1° per minute with a Co (l 1/4 1.7902 Å) beam. The Raman analysis was performed on a Laser micro-Raman system (Make: Horiba JobinVyon; Model: LabRam HR). Ultraviolet−visible (UV−vis) and Fluorescence (FL) spectra were recorded in an UV−visible spectrophotometer (Analytikjena, SPECORD-200, Germany) and F-2700 FL spectrophotometer (2423−008), respectively. The Fourier transform infrared (FT-IR) spectrum was recorded in transmittance mode with 4 cm−1 spectral resolution using FT-IR spectrophotometer (IR Affinity-1, Shimadzu, Japan) and IR solution software.

Figure 1. TEM/HR-TEM images of the nanocrystal formed from coal; (a,b) unagglomerated carbon nanoparticles (c) energy-dispersive spectroscopy of carbon nanoparticle, mainly consist of carbon; (d) size distribution of the carbon nanoparticles; (e−h) HR-TEM images of some large-size carbon nanocrystal having the size of 4−15 nm.

area electron diffraction (SAED) pattern (inset Figure 1b), were observed. The chemical composition was assessed by energy-dispersive spectroscopy (ΣIGMA-Field Emission Scanning Microscope, Carl Zeiss Microscopy) as depicted in Figure 1c, which shows that the particles mainly consist of carbon and free from impurities. The nominal size of the carbon nanocrystal was found to be between 2.5 and 5.5 nm. Figure 1e−h shows the typical HR-TEM images of some large-size carbon nanocrystal. The nanocrystal particles are found to be monocrystalline and polycrystalline in nature with multiple twins. The nominal sizes of the carbon nanocrystals 9620

DOI: 10.1021/acssuschemeng.7b02500 ACS Sustainable Chem. Eng. 2017, 5, 9619−9624

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ACS Sustainable Chemistry & Engineering were found to be 15 nm (Figure 1e), 6 nm (Figure 1f), 10 nm (Figure 1g), and 4 nm (Figure 1h). The planner spacing of the crystal lattice fingers was measured to be in the range of 2.0− 2.3 Å (Figure 2a−d), which is in good agreement with the

Figure 2. (a) HR-TEM images of the nanocrystal with polycrystalline and monocrystalline domains. Insets are FFT patterns showing the crystalline hexagonal patterns. Planner spacing of the crystal lattice fingers was measured to be 0.2−0.21 nm. (b) HR-TEM images of the nanocrystal with polycrystalline and monocrystalline domains as highlighted by arrow and circles, respectively. Insets are FFT patterns showing crystalline hexagonal patterns. Planar spacing of the crystal lattice fingers was measured to be 0.203 nm. (c) HR-TEM images of the nanocrystal with monocrystalline domains as highlighted by hexagon. Insets are FFT patterns of highlighted domains. Planar spacing of the crystal lattice fingers was measured to be 0.201 nm. (d) HR-TEM images of the nanocrystal with monocrystalline domains as highlighted by circle. Insets are FFT patterns of highlighted area. Planar spacing of the crystal lattice fingers was measured to be 0.230 nm.

Figure 3. (a) XRD pattern; (b) Raman analysis; (a) blue fluorescence under UV-lamp (at 365 nm); (b) UV−visible spectra of the nanodiamond; (c) FL spectra of the nanodiamonds showing excitation dependence; (d) FT-IR spectra of the nanodiamonds showing CO, CO, and OH vibration modes.

lattice of graphite cluster and lattice defect including the sp3 hybridized carbon. Diamond exhibits well-known Raman scattering peak at 1333 cm−1.19−22 However, such a peak was not observed in the Raman spectra nanodiamond produced, which might be because of the way that the sp3 hybridized carbon such as diamond has a much smaller intensity (∼1/ 50th) in contrast with sp2 hybridized carbons in visible Raman excitation.19−22 Thus, the presence of sp2 carbons is higher in the product as observed in the Raman analysis and expected to shield the observation of any characteristic sp 3 peaks (diamonds). In addition, if the diamond content is less than ∼25%, it will not be evident in UV-Raman until graphite is removed by oxidation.19−22 Thus, the removal of graphite is warranted in the future research plan to get the characteristic Raman scattering peak of the as-synthesized nanodiamonds phase. The upshifting of G-band appearing at 1600 cm−1 corresponds to the presence of OH surface functional groups.22 Ultrasonic energy is a promising tool to produce nanodiamonds.18,64 It is also a very effective tool for the ultrasonic dispersion, deagglomerating, and functionalizing of the synthesized nanodiamonds. The formation mechanism of the nanodiamonds was hypothesized to be the mechanism as described elsewhere.49,56,62,65 However, in the present case, ultrasound energy is used, which locally creates very extreme effects and ultrasonic cavitation. During the formation of ultrasonic cavitation, very high temperatures (approximately 5000 K) and pressures (approximately 2000 atm) are reached locally, which may be the initiation point for the formation of nanodiamonds.18,64 The low-grade Northeast Indian coals were

lattice planes of various diamond phases including cubic diamond (111) (2.06 Å), lonsdaleite (002) (2.06 Å), and lonsdaleite (100) (2.18 Å) as reported elsewhere.35 These measurements indicate that the carbon nanocrystals are considered to be nanodiamonds, rather than common graphite quantum dots. Some amorphous carbons also exist as nanocrystals form. The nanocrystal particles also tend to form agglomerates and formed multiple and twin plans (Figure 2a). From the fast Fourier transform images (FFT; see insets of Figure 2a−d), it is observed that the particles are hexagonal (Figure 2a,b) and have an ordered (Figure 2c,d) crystalline structure. The nanodiamonds structure obtained in our study is similar to the nanodiamonds structure reported elsewhere.19−22 Figure 3a,b demonstrates the XRD and Raman spectra of the nanodiamond suspension produced. The XRD pattern demonstrates a broad peak based at 26.7°, which indicates that the nanodiamonds embedded within an amorphous carbon matrix and the introduction of abundant oxygen containing functional groups.19−22 The Raman spectra (Figure 3b) shows mainly two characteristic bands appearing at 1600 and 1350 cm−1. These two peaks are near the position of G-band and Dband for sp2 hybridized carbon framework in 2D hexagonal 9621

DOI: 10.1021/acssuschemeng.7b02500 ACS Sustainable Chem. Eng. 2017, 5, 9619−9624

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ACS Sustainable Chemistry & Engineering reported to have graphite-like polyaromatic structures,46 and the irregular and polyaromatic hydrocarbon in the coal are joined by very weak links and separated during the ultrasonication. The polyaromatic hydrocarbon fragments were then further broken into C2 carbon units and may finally lead to the formation of the nanodiamonds via polymorphic reaction. The formation of the nanodiamonds was also hypothesized based on the nanothermodynamic theory of metastable phase nucleation and growth materials at the nanometer size as reported by Wang and Yang.66 The hydrogen present in the feed coals also play a significant role in the formation of nanodiamonds as reported elsewhere.19−21 From the electron beam studies, interestingly, it was observed that the ultrasonicated filtrate contains some typical nanocrystals with nanodiamond phases. Another interesting property of the filtrate is its bright blue fluorescence under UVlight (at 365 nm), which can easily be seen, even in a diluted colloidal solution (Figure 3c). Thus, the nanodiamonds are thought to be more suitable for in vivo bioimaging applications due to the less toxic effect than the other carbon nanoparticles.4−6 Therefore, the special feature of fluorescence behaviors of the observed typical nanodiamonds present in the filtrate is an important goal to understand the presence of diamond cores and optically active defect centers for practical applications, especially in the field of biomedical imaging. Figure 3d shows the UV−visible absorption spectra of the filtrate containing nanodiamonds. The bands appear at around 250−350 nm are due to the excitation of π-electrons (π→π*) of the aromatic π system, while a shoulder at 300 nm attributes to the n-π* transition of CO bonds or other connected group. The broad absorption with a gradual change up to long wavelength indicates the existence of band tails due to defect states. Figure 3e shows the FL spectra of the filtrate containing typical nanodiamonds. The FL properties of the filtrate are found to be excitation dependent, which is similar to the reports on the fluorescence of nanodiamonds.65 The maximum intensity of the FL emission wavelength was found to be in the blue regions and red-shifted to green and yellow regions with increasing excitation wavelength, which is a special feature of nanodiamonds. It is also observed that the Stokes shift trends linearly to zero with increasing the excitation wavelength, which suggests that the FL properties of the filtrate containing nanodiamonds is driven by the mechanism of size-induced quantum-confinement effect. It is also believed that the redshift phenomena occurred due the presence of a multiple chromophore/fluorophore system with aromatic and oxidation groups. To know the various types of surface functional groups that affect the FL properties of observed nanodiamonds, the FT-IR spectroscopic analysis of the filtrate part containing nanodiamonds and unconverted carbon was examined (see Figure 3f). A broad absorption peak was observed at around 3400 cm−1 and is due to the stretching vibrations of OH bonds. The sharp absorption peaks observed at around 1600 and 1725 cm−1 are due to the CC and CO groups, respectively. The intensities of the peaks for OH groups are found to be predominant over CO groups. Therefore, the emission exhibits blue fluorescence, as reported elsewhere.65

the presence of H2O2. The nanodiamond particles formed show stable and bright blue fluorescence, which holds promise for application in bioimaging engineering, photovoltaics, and optoelectronics. The process might also be an alternative process for large-scale production of nanodiamonds from abundantly available cheap coal feedstock at a lower cost instead of the drastic and expensive methods available. However, the detail analysis of the structural features and relevant characteristic that opens diverse applications will be a subject of our future studies.

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AUTHOR INFORMATION

Corresponding Author

*B. K. Saikia. E-mail: [email protected]; [email protected]. in. ORCID

Binoy K. Saikia: 0000-0002-3382-6218 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Director, CSIR-NEIST for his constant encouragement during the research work. We express our special thanks to Prof. Michael Hochella, Dr. Jim Hower, Prof. Frans Waanders, and Prof. Birinchi Kumar Das (Department of Chemistry, Gauhati University) for their valuable comments, suggestion, and encouragements to our research work. The constructive comments received from the anonymous reviewers are highly acknowledged. The financial assistance from CSIR-New Delhi is duly acknowledged (OLP2003).

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REFERENCES

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CONCLUSIONS In summary, the observations suggest that the high-value nanodiamonds could be easily prepared from low-value coal feedstock by using ultrasonic-assisted wet-chemical method in 9622

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DOI: 10.1021/acssuschemeng.7b02500 ACS Sustainable Chem. Eng. 2017, 5, 9619−9624