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

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 cheapfeedstock for the fabrication of carbon nanomaterials. In this Letter, a reporton the formation of nanodiamond from low-grade coals during low-powerultrasonic-assisted stimulation in hydrogen peroxide (H2O2) followed bydialysis 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 transforminfrared (FT-IR) spectroscopy analyses revealed the formation of carbonnanocrystals of monocrystalline and polycrystalline form with multiple plans.The nominal size of the carbon nanocrystals are found to be in the range of4−15 nm. The planar spacing of the crystal lattice fingers is in the range of2.0−2.3 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 cheaplow-grade coal feedstock. The nanodiamond formed shows bright blue fluorescence under UV-light with excitation dependentand 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

■ INTRODUCTION

After the first man-made bulk synthetic method discovered in1955,1,2 nanodiamonds have re-emerged as an intensiveresearch interest in recent years due to their combination ofoutstanding mechanical performance, chemical resistance,versatile surface chemistry, biocompatibility, and unique opticaland electric properties.3 Nanodiamonds are reported to be lesstoxic than the other nanomaterials4−7 and have emerged as akey platform for nanoscience and nanotechnology develop-ments.8 It has been found to possess a wide range of applicationin the field of microelectronics, optoelectronics, and biosens-ing.4 In addition, nanodiamonds are also used in novel wear-resistant polymers, metal coatings,8,9 and lubricant additives10

due to their superhardness, exceptional chemical resistivity, andabrasive nature, respectively. Nanodiamonds have been widelyused in biomedical imaging, drug delivery, and other areas ofmedicine.4−7

Although a number of different methods exist for thesynthesis of nanodiamonds particles,11−17 only high-pressureand high-temperature (top-down) and detonation (bottom-up)methods are available on an industrial scale. However, there isalways a sustainable demand for a “golden standard” to producea universal nanodiamond for its versatile applications inindustrial, commercial, or academic purposes.3 In the “high-pressure high-temperature (HPHT)”and detonation methods,high pressure (tens of thousands of atmospheres) and hightemperature (more than 2000 K) with the aid of explosivematerials (TNT, RDX, etc.) have been used for the productionof nanodiamonds. However, highly sophisticated equipment isneeded and it is not economically viable worldwide to supplyindustrial diamonds via this route. Khachatryan et al.18 reported

that the use of high-power ultrasound is an innovativealternative for the synthesis of micro- and nanodiamonds andvery efficient instead of using HPHT and detonation methods.In their method, however, they used pure graphite as a carbonsource and expensive organic solvent as a liquid media for thesynthesis of nanodiamonds.Recently, Lueking et al.19 reported the formation of

nanocrystalline diamond (NCD) as a byproduct during thehydrogenative ball milling of anthracite coal with cyclohexenefor the production and storage of hydrogen. Additionally, in theyear of 2007, Lueking and her co-workers20 reported theformation of nanocrystalline diamond (NCD) after reactive ballmilling of anthracite coal with cyclohexene, a high-temperature(1400 °C) thermal anneal, and 4 M HCl treatment followed by10 M NaOH treatment. Sun et al.21 reported the recrystalliza-tion of the carbon network into diamonds when the anthracitecoal functionalized with dodecyl groups was irradiated withelectron beam. In an another work, Xiao et al.22 reported thesynthesis of nanodiamonds from anthracite (Vietnam), bitumen(Indonesia), and coke (China) by laser ablation technique in aliquid at atmospheric pressure and temperature. However, theproductions of nanodiamonds with purity, size selectivity,deaggregation, surface functionality, and photoluminescencehave remained a challenging task.22 The coal is beingconsidered as an abundantly available cheap feedstock for thefabrication of carbon nanomaterials and several researchersreported the same, including for the production of carbon

Received: July 24, 2017Revised: October 3, 2017Published: October 4, 2017

Letter

pubs.acs.org/journal/ascecg

© XXXX American Chemical Society A DOI: 10.1021/acssuschemeng.7b02500ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

nanotubes,23−39 microballs,40 carbon nanodots,41−46 onion-likefullerenes,47−49 and graphene/graphene oxide.50−55 In some ofthe previous studies, the formation of carbon nanotubes,nanoballs,56 onion-like fullerenes, and chemically convertedgraphene-like carbon nanosheets from Northeast Indian low-grade coal has been reported.49

The Northeast region (NER) of India has around 1.496 Gtof low-grade coal reserves. These tertiary coals have high-sulfurcontents (2−8%), where 75−90% is organically bound, whilethe rest is in inorganic forms viz. sulfate sulfur and pyriticsulfur.49,56,57 Thus, the NER coals need beneficiation as well asvalue addition for further gainful utilization.In this Letter, the discovery of the formation of some typical

nanodiamond suspensions, formed during low-power ultra-sound-assisted exfoliation of Northeast Indian low-grade coalsis reported. In our investigation, we used low-power ultrasoundand low-grade coals as carbon source instead of high-powerultrasound and graphite as carbon source as reported byKhachatryan 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 Sunet al.21 Although, several researchers58−61 along with ourprevious studies,62,63 reported the sulfur and mineral matterremoval of different types of coals by using ultrasound-assistedmethods, but the formation of nanodiamond was not reportedin those experiments. The main novelty of our investigation isthat the carbon source is from low-grade coal feedstock, whichis typically different and cheap from other carbon sources. Ascompared to the pure sp2-carbon such as graphite and highgrade coal, the exfoliation of the small graphite-like crystallinedomains that are inherent in low-grade coal is reported to beeasy.41−43

■ EXPERIMENTAL SECTION20 g of low-grade coal sample was mixed with 100 mL of H2O2 in aTeflon beaker and subsequently exposed to an ultrasound treatment ata frequency of 20 kHz in a Ultrasonic Processor (Sonapros; Model:PR-1000 M) at atmospheric pressure for 3 h. A sensor was placed inthe reaction mixture to measure the reaction temperature. Then, themixture of coal and H2O2 was filtered. The detailed methodologybeing adopted was well-described in our previous study.57 The H2O2was used to remove the sulfur components as well as the mineralmatters from the coals. The chemical characteristics of the feed as wellas the residual coals were reported in our previous study.57 The filtratepart obtained after ultrasonication was poured into a beaker containing500 mL of crushed ice and then neutralized. The neutral mixture wasthen filtered through a 0.22 μm polytetrafluoroethylene membraneand dialyzed in a 1 kDa dialysis bag for 5 days. The dialyzed solutionwas concentrated using rotary evaporation and collected. One drop ofthe solution was pipetted onto a carbon support film on a copper gridand characterized by transmission electron microscopy (TEM/HR-TEM) (HR-TEM; Joel JEM-2100, resolution: 1.9 to 1.4 Å,accelerating voltage: 60−200 kV in 50 V steps). The TEM imageswere further developed by using the “ImageJ” program (softwareversion 1.47). An X-ray diffraction spectrum was obtained using an X-ray powder diffractometer (type: JDX-11P3A, JEOL, Japan). X-raydiffraction data was obtained with the starting angle of 2.00°, a stopingangle of 75.00°, and a step size of 0.05° with a scanning rate of 1° perminute with a Co (l 1/4 1.7902 Å) beam. The Raman analysis wasperformed 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 spectropho-tometer (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.

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

area electron diffraction (SAED) pattern (inset Figure 1b),were observed. The chemical composition was assessed byenergy-dispersive spectroscopy (ΣIGMA-Field Emission Scan-ning Microscope, Carl Zeiss Microscopy) as depicted in Figure1c, which shows that the particles mainly consist of carbon andfree from impurities. The nominal size of the carbonnanocrystal 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 arefound to be monocrystalline and polycrystalline in nature withmultiple twins. The nominal sizes of the carbon nanocrystals

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

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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 thecrystal 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

lattice planes of various diamond phases including cubicdiamond (111) (2.06 Å), lonsdaleite (002) (2.06 Å), andlonsdaleite (100) (2.18 Å) as reported elsewhere.35

These measurements indicate that the carbon nanocrystalsare considered to be nanodiamonds, rather than commongraphite quantum dots. Some amorphous carbons also exist asnanocrystals form. The nanocrystal particles also tend to formagglomerates and formed multiple and twin plans (Figure 2a).From the fast Fourier transform images (FFT; see insets ofFigure 2a−d), it is observed that the particles are hexagonal(Figure 2a,b) and have an ordered (Figure 2c,d) crystallinestructure. The nanodiamonds structure obtained in our study issimilar to the nanodiamonds structure reported elsewhere.19−22

Figure 3a,b demonstrates the XRD and Raman spectra of thenanodiamond suspension produced. The XRD patterndemonstrates a broad peak based at 26.7°, which indicatesthat the nanodiamonds embedded within an amorphous carbonmatrix and the introduction of abundant oxygen containingfunctional groups.19−22 The Raman spectra (Figure 3b) showsmainly two characteristic bands appearing at 1600 and 1350cm−1. These two peaks are near the position of G-band and D-band for sp2 hybridized carbon framework in 2D hexagonal

lattice of graphite cluster and lattice defect including the sp3

hybridized carbon. Diamond exhibits well-known Ramanscattering peak at 1333 cm−1.19−22 However, such a peak wasnot observed in the Raman spectra nanodiamond produced,which might be because of the way that the sp3 hybridizedcarbon such as diamond has a much smaller intensity (∼1/50th) in contrast with sp2 hybridized carbons in visible Ramanexcitation.19−22 Thus, the presence of sp2 carbons is higher inthe product as observed in the Raman analysis and expected toshield the observation of any characteristic sp3 peaks(diamonds). In addition, if the diamond content is less than∼25%, it will not be evident in UV-Raman until graphite isremoved by oxidation.19−22 Thus, the removal of graphite iswarranted in the future research plan to get the characteristicRaman scattering peak of the as-synthesized nanodiamondsphase. The upshifting of G-band appearing at 1600 cm−1

corresponds to the presence of OH surface functionalgroups.22

Ultrasonic energy is a promising tool to produce nano-diamonds.18,64 It is also a very effective tool for the ultrasonicdispersion, deagglomerating, and functionalizing of thesynthesized nanodiamonds. The formation mechanism of thenanodiamonds was hypothesized to be the mechanism asdescribed elsewhere.49,56,62,65 However, in the present case,ultrasound energy is used, which locally creates very extremeeffects and ultrasonic cavitation. During the formation ofultrasonic cavitation, very high temperatures (approximately5000 K) and pressures (approximately 2000 atm) are reachedlocally, which may be the initiation point for the formation ofnanodiamonds.18,64 The low-grade Northeast Indian coals were

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

Figure 3. (a) XRD pattern; (b) Raman analysis; (a) blue fluorescenceunder UV-lamp (at 365 nm); (b) UV−visible spectra of thenanodiamond; (c) FL spectra of the nanodiamonds showing excitationdependence; (d) FT-IR spectra of the nanodiamonds showing CO,CO, and OH vibration modes.

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reported to have graphite-like polyaromatic structures,46 andthe irregular and polyaromatic hydrocarbon in the coal arejoined by very weak links and separated during the ultra-sonication. The polyaromatic hydrocarbon fragments were thenfurther broken into C2 carbon units and may finally lead to theformation of the nanodiamonds via polymorphic reaction. Theformation of the nanodiamonds was also hypothesized basedon the nanothermodynamic theory of metastable phasenucleation and growth materials at the nanometer size asreported by Wang and Yang.66 The hydrogen present in thefeed coals also play a significant role in the formation ofnanodiamonds as reported elsewhere.19−21

From the electron beam studies, interestingly, it wasobserved that the ultrasonicated filtrate contains some typicalnanocrystals with nanodiamond phases. Another interestingproperty of the filtrate is its bright blue fluorescence under UV-light (at 365 nm), which can easily be seen, even in a dilutedcolloidal solution (Figure 3c). Thus, the nanodiamonds arethought to be more suitable for in vivo bioimaging applicationsdue to the less toxic effect than the other carbon nano-particles.4−6 Therefore, the special feature of fluorescencebehaviors of the observed typical nanodiamonds present in thefiltrate is an important goal to understand the presence ofdiamond cores and optically active defect centers for practicalapplications, especially in the field of biomedical imaging.Figure 3d shows the UV−visible absorption spectra of the

filtrate containing nanodiamonds. The bands appear at around250−350 nm are due to the excitation of π-electrons (π→π*)of the aromatic π system, while a shoulder at 300 nm attributesto the n-π* transition of CO bonds or other connectedgroup. The broad absorption with a gradual change up to longwavelength indicates the existence of band tails due to defectstates. Figure 3e shows the FL spectra of the filtrate containingtypical nanodiamonds. The FL properties of the filtrate arefound to be excitation dependent, which is similar to thereports on the fluorescence of nanodiamonds.65 The maximumintensity of the FL emission wavelength was found to be in theblue regions and red-shifted to green and yellow regions withincreasing excitation wavelength, which is a special feature ofnanodiamonds. It is also observed that the Stokes shift trendslinearly to zero with increasing the excitation wavelength, whichsuggests that the FL properties of the filtrate containingnanodiamonds is driven by the mechanism of size-inducedquantum-confinement effect. It is also believed that the red-shift phenomena occurred due the presence of a multiplechromophore/fluorophore system with aromatic and oxidationgroups.To know the various types of surface functional groups that

affect the FL properties of observed nanodiamonds, the FT-IRspectroscopic analysis of the filtrate part containing nano-diamonds and unconverted carbon was examined (see Figure3f). A broad absorption peak was observed at around 3400cm−1 and is due to the stretching vibrations of OH bonds.The sharp absorption peaks observed at around 1600 and 1725cm−1 are due to the CC and CO groups, respectively. Theintensities of the peaks for OH groups are found to bepredominant over CO groups. Therefore, the emissionexhibits blue fluorescence, as reported elsewhere.65

■ CONCLUSIONSIn summary, the observations suggest that the high-valuenanodiamonds could be easily prepared from low-value coalfeedstock by using ultrasonic-assisted wet-chemical method in

the presence of H2O2. The nanodiamond particles formed showstable and bright blue fluorescence, which holds promise forapplication in bioimaging engineering, photovoltaics, andoptoelectronics. The process might also be an alternativeprocess for large-scale production of nanodiamonds fromabundantly available cheap coal feedstock at a lower costinstead of the drastic and expensive methods available.However, the detail analysis of the structural features andrelevant characteristic that opens diverse applications will be asubject of our future studies.

■ AUTHOR INFORMATIONCorresponding Author*B. K. Saikia. E-mail: bksaikia@gmail.com; bksaikia@neist.res.in.

ORCIDBinoy K. Saikia: 0000-0002-3382-6218NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to the Director, CSIR-NEIST for hisconstant encouragement during the research work. We expressour special thanks to Prof. Michael Hochella, Dr. Jim Hower,Prof. Frans Waanders, and Prof. Birinchi Kumar Das (Depart-ment of Chemistry, Gauhati University) for their valuablecomments, suggestion, and encouragements to our researchwork. The constructive comments received from theanonymous reviewers are highly acknowledged. The financialassistance from CSIR-New Delhi is duly acknowledged (OLP-2003).

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