Elemental Quantification and Residues Characterization of Wet

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Elemental Quantification and Residues Characterization of Wet Digested Certified and Commercial Carbon Materials Filipa R. F. Simoes,† Nitin M. Batra,† Bashir H. Warsama,‡ Christian G. Canlas,§ Shashikant Patole,† Tahir F. Yapici,‡ and Pedro M. F. J. Costa*,† †

Physical Science and Engineering Division, ‡Analytical Core Laboratory, and §Imaging and Characterization Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia S Supporting Information *

ABSTRACT: Inductively coupled plasma optical emission spectroscopy (ICP-OES) is a common, relatively low cost, and straightforward analytical technique for the study of trace quantities of metals in solid materials, but its applicability to nanocarbons (e.g., graphene and nanotubes) has suffered from the lack of efficient digestion steps and certified reference materials (CRM). Here, various commercial and certified graphitic carbon materials were subjected to a “two-step” microwave-assisted acid digestion procedure, and the concentrations of up to 18 elements were analyzed by ICP-OES. With one exception (Sm), successful quantification of all certified elements in the two reference nanocarbons studied was achieved, hence validating the sample preparation approach used. The applicability of our “two-step” protocol was further confirmed for a commercial single-walled carbon nanotube sample. However, the digestion was markedly incomplete for all other commercial materials tested. Where possible, the digestion residues of the carbon materials analyzed (CRM included) were characterized to understand the structural changes that take place and how this may explain the challenge of disintegrating graphitic carbon. In this respect, it was found that solid state nuclear magnetic resonance holds considerable promise as a nonlocalized, easily interpretable, and reliable tool to access the efficient disintegration of these materials.

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sensitivities down to ppb), and the sample preparation is relatively straightforward.5,6 The latter is a particularly attractive feature for materials that are difficult to bring into solution such as nanocarbons. Although superior, NAA is not a routine technique for elemental quantification as it requires a large infrastructure with an integrated neutron source. Alternatively, analytical chemistry laboratories often resort to other techniques such as those based on inductively coupled plasmas (ICP). Whether hyphenated to optical emission (OES) or mass (MS) spectrometers, the ICP methods are the “de facto” staple in bulk quantitative elemental analysis of nanocarbons worldwide. However, one major roadblock persists, the sample preparation step. Generally, the approaches used can fit into two main types (or combination of these): wet digestion and dry ashing.2,7 Recently, some researchers have used microwaveactivated wet digestion to disintegrate carbon nanotubes and graphene nanoplatelets for ICP-OES measurements.4 Microwave-activated digestion has been employed for decades in chemical analysis8−10 because it offers increased safety and smaller loss/evaporation of sample/analyte, surpassing therefore the use of hot plates or furnaces. Despite our best efforts in a previous study,4 the wet digestion approach employed was

s estimates predict that nanocarbons will be widely incorporated into our daily lives in approximately three decades, there is a pressing need to develop viable strategies for industrial-scale standardization and quality control protocols for these materials.1,2 Part of this effort refers to ensuring low-cost and reliable detection and quantification of noncarbon elements in sample batches of nanocarbons. Transition metals, for instance, are one of the most common impurities as they are often used to catalyze the growth of carbonaceous materials. However, in the absence of certified reference materials (CRM), quantitative chemical analysis of nanocarbon batches may become costly and complex. That is justified not only by the sample heterogeneity, which is still common in these materials (with regard to purity, structure, orientation, etc.), but also by the chemical resilience and varied intercalation locations found in graphitic structures. In the case of carbon nanotubes (CNT), it is not uncommon that minute amounts of catalyst metals end up trapped within sturdy graphitic shells (i.e., encapsulated), which may considerably defy reliable elemental quantification.3,4 The complexity of batch-scale elemental quantification for nanocarbons has resulted in neutron activation analysis (NAA) becoming the “gold standard” of analytical methods to address this issue. NAA is nondestructive, accurate, and most importantly does not require the use of standards. It also covers a broad range of elements and concentrations (with © XXXX American Chemical Society

Received: August 30, 2016 Accepted: October 25, 2016 Published: October 25, 2016 A

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1D directly observed 13C SS-NMR spectra were obtained on a Bruker Avance III 600 MHz wide bore NMR spectrometer operating at a frequency of 150.94 MHz for 13C and 600.23 MHz for 1H using an H/X/Y cross-polarization magic angle spinning probe with 3.2 mm rotor diameter. For the microwave-assisted acid digestion, an ETHOS 1 Advanced Microwave Digestion Labstation (Milestone S.R.1, Italy) system was used. With this setup, parameters such as power, temperature, and time of the digestion were easily controlled. Note that, in contrast to other protocols in the literature,11 only two microwave-assisted digestion cycles were carried out (which we denominated the “two-step” approach). For the ICP-OES analysis, a Varian 720-ES spectrometer bearing a dual detector assembly and covering a wavelength window between 165 and 782 nm was employed. Further details on the experimental conditions used, namely for the acquisition of the NMR spectra, the digestion runs, and ICP-OES analysis, can be found in the Supporting Information.

not sufficient to entirely disintegrate the samples of graphitic carbon analyzed. Unstable suspensions were obtained instead of clear solutions. This may have influenced the reliability of the ICP-OES data and the subsequent comparison with the novel fusion sample preparation approach proposed in that work. Consequently, we started looking for ways to validate the wet digestion results and enquired on the possibility of employing standards of nanocarbons. In this respect, the Canadian National Research Council (NRC) and the National Institute for Standards and Technology (NIST) in the United States have each created certified reference materials (CRM) for single-walled carbon nanotubes (SWCNT). Designated SWCNT-1 (from NRC)11 and SRM2483 (from NIST),12 they constitute, to our knowledge, the only available elemental composition CRMs for the entire family of graphite-like carbon materials (a third CRM exists, RM8281, but this is certified by NIST for length not composition). The commercialization of these CRMs is expected to assist significantly in the field of metrology and, consequently, the much needed development of industrial quality control protocols for nanocarbon production/ application. Here, microwave-assisted acid digestion has been applied to the two existing SWCNT CRMs and assorted commercial graphitic carbon materials. The resulting analytes were studied with ICP-OES, and where available, particulate residues were characterized.

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RESULTS This section has been subdivided according to the sets of samples studied: the CRMs (SWCNT-1 and SRM2483) and the commercial carbon materials (SWCNT, DWCNT, MWCNT, graphite, and graphene). For each case, considerations on sample digestion are made first, followed by the characterization of the initial and (when available) postdigestion state of the materials and, finally, presentation of the ICPOES results. Certified Reference Materials. The outcome of the acid digestion procedure applied to the SWCNT-1 and SRM2483 is shown in Figure 1. In both cases, almost colorless and fairly

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EXPERIMENTAL SECTION Reagents and Solutions. SWCNT, multi-walled carbon nanotubes (MWCNT), and graphite were purchased from Sigma-Aldrich; the double-walled carbon nanotubes (DWCNT) powder was acquired from Tokyo Chemical Industries Co. Ltd., Tokyo, Japan, and the graphene nanoplatelets were acquired from STREM Chemicals, Newburyport, MA, USA. The two CRMs, designated SWCNT-1 and SRM2483, were procured from NRC and NIST, respectively. For the acid digestion procedure, nitric acid (HNO3, Aristar Ultra, BDH, 70%, Canada) and hydrogen peroxide (H2O2, Fisher Scientific, Certified ACS 30%, USA) were selected. Deionized water (DIW) was produced with a Milli Q system (Millipore, UK) and had a resistivity of 18 MΩ cm. Standard stock solutions of single elements of Al, Ca, Ce, Co, Cr, Dy, Eu, Fe, Gd, K, Mg, Mn, Mo, Na, Ni, Sm, Ti, and V were supplied by Inorganic Ventures, USA, and PerkinElmer, USA, in 2% (v/ v) HNO3. General Characterization. All materials were characterized as-received. For those analytes that presented residues, postdigestion characterization of the solid deposits was carried out. The Raman analysis was done in a Witec Alpha 300RA system fitted with 488 and 532 nm lasers and a UHT300 spectrometer. To account for the heterogeneity of the materials, at least three Raman spectra were collected at different locations for each sample. Bright-field transmission electron microscopy (TEM) imaging was performed on a FEI TECNAI G2 Spirit TWIN at 120 kV. EDS analysis was done at 300 kV in an FEI Titan SuperTWIN incorporating an EDAX octane silicon drift detector. For the samples to be prepared for TEM, some drops of a suspension in ethanol were placed on Holey carbon Au or Cu grids and dried in a vacuum oven at 70 °C. Solid state nuclear magnetic resonance (SS-NMR) was used to analyze the SRM2483 and respective residues. To prepare the sample for SS-NMR, the SRM2483 precipitate was evaporated in a Petri dish by blowing a stream of dry air. The

Figure 1. Postdigestion samples of SWCNT-1 and SRM2483.

transparent solutions were obtained. After a few hours, we could not identify with the naked eye the precipitation or dispersion of particles in the aqueous medium. Commonly, the lack of visible carbon residues is interpreted as evidence of an efficient digestion process.13 It was thus surprising that, upon overnight evaporation of the liquid in our control experiments, minute quantities of solid residues were extracted. These, along with the corresponding as-received materials, were characterized with Raman spectroscopy, SS-NMR, and TEM. Figure 2 shows the Raman spectra for the as-received CRMs and respective postdigestion residues. The latter were labeled SWCNT-1R and SRM2483R (where R stands for residue). The characteristic peaks of SWCNTs, namely the radial breathing mode (RBM), typically in the range of 100−300 cm−1, the D (∼1350 cm−1), G (1550−1600 cm−1), and 2D (∼2600 cm−1) bands were easily identified in most cases. The exception were the residues of SRM2483R, where the RBM and 2D bands were absent (Figure 2d). The spectrum of the as-received SWCNT-1 (Figure 2a) demonstrated that the nanotubes were of excellent structural quality and exhibited a narrow diameter distribution. Using the formula below (eq 1, where c1 and c2 are constants, see ref 14), the analysis of the RBM band showed that the nanotubes had B

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Figure 2. Raman spectra of the CRMs: (a) as-received SWCNT-1, (b) postdigestion residues of SWCNT-1, (c) as-received SRM2483, and (d) postdigestion residues of SRM2483. The peaks labeled with * are artifacts due to the high noise level.

Figure 3. 13C SS-NMR spectra for (a) as-received SRM2483 (the smaller peaks at ∼25 and ∼225 ppm are spinning side bands due to the sample spinning at 8770 Hz) and (b) SRM2483R, where the spectral window marked with * confirms the presence of sp3 carbon obtained from the disintegration of the nanotube.

predominantly a diameter (d) of 1.23(±0.05) nm (peak fitting and table of diameters in Figure S2 and Table SI3, respectively). c ωRBM = 1 + c 2 (1) d

clear that our two-steps protocol did not result in the complete dissolution of the SWCNT-1. Furthermore, the residues collected showed a fair degree of structural integrity retention. Concerning the as-received SRM2483, the Raman spectrum in Figure 2c shows that its nanotubes had lower structural quality than those of SWCNT-1. In addition, the peak analysis in the RBM region showed that the average diameter was 0.83(±0.14) nm, which is therefore narrower and with a wider size distribution (peak fitting and table of diameters in Figure S2 and Table SI3, respectively). Overall, the digestion was more complete, and fewer residual particles were collected, which explains the noisier spectrum for SRM2483R (Figure 2d). Interestingly, no clear RBM or 2D bands were identified in this sample, as opposed to the broad bands of the D and G signals. From the shape and intensity ratio of the latter (ID/IG), it is

When comparing this to the postdigestion sample (Figure 2b), the main difference derives from the intensity increase of the D band, which is evident in SWCNT-1R. This increment can be assigned to a higher density of lattice defects in the nanotubes, an expected result given the harsh oxidation process carried out. In relation to the RBM and G bands, there is no significant change after acid digestion in both wavenumber and relative intensity (the RBM/G intensity ratio is retained). Despite the absence of a visible postdigestion precipitate (Figure 1), it is C

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Figure 4. TEM images of the as-received CRMs: (a, c) low and (b, d) high resolution images of SWCNT-1 and SRM2483, respectively.

SRM2483 are constituted mostly by nongraphitic carbonaceous species. In Figure 4, representative TEM micrographs of the asreceived CRMs are shown. In the case of SWCNT-1 (Figure 4a and b), the nanotubes are structurally well-defined and commonly packed in bundles. The average diameter measured from high-resolution TEM micrographs was 1.2(±0.1) nm, matching the Raman results. Byproducts are also present, such as disordered carbon agglomerates and nanoparticles with various sizes. The EDS characterization (Figure S3) confirmed these to be composed of Fe, Co, Ni, and Mo, transition metals that are commonly used in the synthesis of SWCNTs. These metal particles were encapsulated by a graphitic shell. Taken together, these observations concur with the Raman results (sample containing high-quality SWCNTs with little amorphous carbon). With regard to as-received SRM2483 (Figure 4c and d), the SWCNTs look more disordered and generally have a smaller diameter (0.8 ± 0.1 nm) than those of the SWCNT-1 sample, which is also in agreement with the Raman analysis. Notably, this CRM presents a higher concentration of byproducts. Nanoparticles were observed and identified by EDS (Figure S4) as alloys of Co and Mo. A similar TEM analysis was attempted for the digested residues of SWCNT-1 and SRM2483. For both cases, the data was inconclusive because, along with the minute amounts of residues available, the strong acid medium used in the wet digestion caused partial dissolution of the supporting TEM metal grids (Cu and Au).

plausible to infer the presence of carbonaceous species other than SWCNTs in the residues. Such an observation is consistent with the known degradation path of nanotubes subjected to harsh chemical conditions: disruption of the nanotube’s rope network followed by gradual increase of sp3type carbon in the material.15,16 In light of the Raman results, a complementary spectroscopy technique was used to further compare the as-received SRM2483 and respective postdigestion residues. Although 13 C SS-NMR is not the most popular tool to characterize carbon nanotubes, a range of crucial information can be extracted from it, including the type of functional groups, chemical bonds, and structural integrity of these nanostructures.17 In Figure 3a, the spectrum of the as-received SRM2483 shows the presence of an intense, narrow peak at 126 ppm. This chemical shift (with reference to adamantane) is characteristic of the CC bonds in CNTs (i.e., sp2-type carbon hybridization). The spectrum changed drastically after acid digestion (Figure 3b). For SRM2483R, at least five peaks are seen in the 0−50 ppm range, a region commonly identified with sp 3 -type carbon hybridization (CH 3 R, CH 2 RR′, CHRR′R″, or CRR′R″R‴). Furthermore, two minor peaks can be identified at 126 ppm (as per previous due to the CC bonds) and 175 ppm (likely CO or CN). The NMR analysis confirms the results from Raman spectroscopy in that most of the SWCNTs in the SRM2483 were disintegrated. This adds to the evidence that the postdigestion residues of D

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Analytical Chemistry Table 1. ICP-OES Measurements for SWCNT-1 and SRM2483 After the Two-Step Acid Digestion Process SWCNT-1 (NRC) element Co Ni Mo Fe Ce Dy Eu Gd Sm Al Ca Cr K Mg Mn Ti Na V a

certificate11 (mg/L) 15900 14400 7300 2200

494 2650 285 3220 4180 136 193 167 4.4

b

(±100) (±800)b (±100)b (±200)b

(±94) (±300) (±26) (±200) (±380) (±2) (±22) (±7) (±0.3)

ICP-OESa (mg/L) 13695 12341 7392 2138

407 2150 205 2170 123 128 146 176

(±312) (±455) (±338) (±103)

(±35) (±135) (±29) (±34) (±28) (±3) (±6) (±7)

SRM2483 (NIST) recovery (%) 86 86 101 97

82 81 72 67 3 94 76 106

certificate12 (mg/L) 9630 (±17)

ICP-OESa (mg/L)

recovery (%)

7964 (±422)

83

29691 (±1215)

87

c

34060 (±29)c 193 8.4 2.3 11 13 723

(±7)c (±0.2)c (±0.1)c (±1)c (±1)c (±19)

170 8.0 2.1 10 6 858

(±7) (±0.1) (±0.2) (±1) (±1) (±62)

88 96 91 94 45 119

1150 (±11) 4.5 (±0.0)

988 (±46) 4.2 (±0.4)

86 93

6.9 (±0.1)

6.1 (±0.3)

88

Mean and standard deviation (±) values obtained with N = 3. bNRC-certified value. cNIST-certified value.

Figure 5. Postdigestion samples of commercial SWCNT, DWCNT, MWCNT, graphite, and graphene nanoplatelets. The SWCNT vial is the only one where residues are not visible.

not be appropriately recovered with the present two-step protocol. This exception might be due to background interference of Ce on the Sm emission line or the selection of the acid reagent (for instance, it is known that adding HCl to HNO3 can increase the recovery of rare earth elements).20 Concerning the noncertified elements listed in the NIST certificate, all were within the accepted recovery interval with a concentration that was close to the expected value. Overall, the agreement of the concentrations provided in the NIST certificate and our results is remarkable. The higher disintegration rate of SRM2483 upon two digestion runs could explain this result (less-ordered nanotubes are expected to be digested faster). Taken together, it is possible to state that a two-steps acid digestion approach such as the one employed is sufficient to provide satisfactory recovery and concentration readings for the two available nanocarbon CRMs. Commercial Samples. The two-step acid digestion protocol described above and successfully used for the CRMs was applied to five different commercial samples of graphitic carbon, namely, SWCNT, DWCNT, MWCNT, graphite, and graphene nanoplatelets. In some cases, the resulting liquid phase had a yellowish color, which is common in HNO3digested carbon materials (Figure S6). As shown in Figure 5, only the SWCNT sample resulted in a clear and stable liquid where, after a few hours, no residues could be identified. However, just like for the CRMs, after overnight evaporation of the liquid, it was possible to carefully collect enough residues for structural characterization. For all other sp2-type carbon materials, the precipitates were extracted by filtration and dully

This grossly contaminated the samples, rendering the TEM study unfeasible (Figure S5). Following the structural characterization of the as-received and postdigestion CRMs, the quantitative chemical analysis of these samples was carried out with ICP-OES. Table 1 lists the concentration and corresponding recovery of 18 elements, including those certified by the NRC for SWCNT-1 (i.e., Co, Ni, Mo, and Fe) and NIST for SRM2483 (i.e., Co, Mo, Ce, Dy, Eu, Gd, and Sm). The concentrations of the certified elements in SWCNT-1 were in accordance with those provided in the NRC certificate11 with the corresponding recoveries being within 86−101%. According to Rudel et al.,18 a recovery range of 80−120% for elements with certified concentration is acceptable. The highest concentrations measured in SWCNT-1 were those of Co and Ni. This is expected as the nanotubes were produced by the laser ablation method, which uses these two elements as growth catalysts.11 With respect to the other noncertified elemental concentrations, in most cases we could match them to those in the SWCNT-1 certificate. The recoveries of Al, Ca, Mn, and Na fell well within the range mentioned above in contrast to those of Cr, K, Mg, and Ti. In particular, for Mg, the disparity in the recovery values was extreme, attesting that the acid digestion may not be an appropriate method to measure this element in nanocarbons. Likewise, no reliable values were extracted for V. For Al, Ca, Cr, K, and Ti, it is probable that carrying out additional digestion cycles could have resulted in more acceptable readings. In fact, multistep preparations for microwave-assisted wet-digested analytes are often used to maximize recovery.2,19 As for SRM2483, from the list of certified elements, only Sm could E

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Figure 6. Raman spectra for (a) as-received graphene, graphite, MWCNT, DWCNT, and SWCNT and (b) postdigestion residues of the samples in (a).

Table 2. ICP-OES Measurements for SWCNT, DWCNT, MWCNT, Graphite, and Graphene SWCNT

a

a

element

ICP-OES (mg/L)

recovery (mg/L)

Co Ni Mo Fe Ce Dy Eu Gd Sm Al Ca Cr K Mg Mn Ti Na V

6298 61 38900 251 26 26 26 28 18 661 186 723 76 801 21 34 532 14

91 86 129 92 88 88 88 90 90 94 90 91 83 80 84 86 94 87

DWCNT

MWCNT

Graphite

Graphene

b

b

b

ICP-OESb (mg/L)

ICP-OES (mg/L) 302 53 12195 5089 1.88 0.50 0.13 1.37 19 302 62 5.78 50 124 39 0.60 110 2.30

(±2) (±7) (±50) (±583) (±0.58) (±0.26) (±0.04) (±0.21) (±0.2) (±2) (±2) (±0.11) (±1) (±0.4) (±0.7) (±0.10) (±4) (±0.25)

ICP-OES (mg/L) 639 81 194 1687 0.80

(±9) (±1) (±11) (±17) (±0.08)

0.18 2.39 23 1080 1662 26 57 1111 3.21 2.03 719 2.81

(±0.09) (±0.06) (±0.3) (±73) (±35) (±0.2) (±3) (±20) (±0.11) (±0.45) (±14) (±0.43)

ICP-OES (mg/L) 22 65 12 5064

(±1) (±11) (±1) (±60)

0.26 0.22 1.02 24 2321 883 5.01 280 2373 65 116 113 13

(±0.06) (±0.02) (±0.58) (±0.2) (±31) (±36) (±0.69) (±60) (±134) (±3) (±22) (±30) (±1)

19 273 15 2060 0.69 0.50 0.18 1.20 23 51 52 420 63 13 67 58 54 4.60

(±1) (±25) (±1) (±26) (±0.30) (±0.07) (±0.04) (±0.14) (±0.8) (±1) (±2) (±2) (±1) (±3) (±0.7) (±0.3) (±1) (±0.46)

The recovery for SWCNT was calculated through a spiked sample. bMean and standard deviation (±) values obtained with N = 3.

0.96(±0.29) nm. Remarkably, the D-band varies greatly from nonexistent in graphite to being the most prominent peak in MWCNT. These nanotubes are pyrolytic-type structures that lack the structural orientation degree (i.e., graphitization) seen in the other samples (Figure S8). Concerning the G-band, this is clearly broader for the nanotube samples with SWCNT showing signs of band splitting (into G− and G+).23 The second order 2D band was identified in all samples. When compared to the graphite sample, the nanoplatelets show a more symmetric peak while having a similar G/2D intensity ratio. We infer

analyzed. The limited applicability of the two-step protocol to DWCNT, MWCNT, graphene, and graphite is not surprising. An increasing number of stacked graphene layers/tubes will logically render the digestion by acids more challenging.21,22 The Raman spectra for the as-received commercial samples and respective postdigestion residues are presented in Figure 6. Expectedly, the as-received spectra show an RBM signature for SWCNT and DWCNT, which is absent in the other three materials. From the RBM analysis (Figure S7 and Table SI4), the average diameter of the commercial SWCNT was F

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Analytical Chemistry therefore that the flakes of the graphene sample are considerably thinner, at most a few tens of stacked graphene layers.24 In the spectra of the residues, the most notable difference was the intensity of the D-band. For both graphene and graphite, the peak increased slightly postdigestion, with the spectra otherwise remaining unchanged. Variations in the nanotube samples were more pronounced with MWCNT and SWCNT showing higher ID/IG ratios (Table SI5), particularly for the second. Furthermore, the shape of the SWCNT G-band reflected a different relation of the G− and G+ features, possibly a sign that other carbon structures than SWCNT coexisted in the residues (leading to peak superposition at the G− wavenumber).23 Interestingly, the RBM frequencies are no longer visible, possibly due to symmetry breakup and defects on the remaining nanotubes’ walls. Finally, the spectra for DWCNT seem to be counterintuitive. This is because the asreceived DWCNT material has some degree of amorphous carbon, seen by the defect peak at 1334 cm−1. This was confirmed by the TEM analysis of the sample (Figure S9). After the two-step digestion, the D-peak intensity was substantially reduced, but all other bands were retained. We assume this is the result of the oxidative disintegration of carbonaceous byproducts, whereas the DWCNTs were mostly preserved. From the Raman analysis, it appears that the residues of the commercial SWCNT are mostly composed of nanotubes that were not entirely digested. These show a degree of structural integrity that is between that of the nanotubes in SWCNT-1R (which remain mostly intact) and SRM2483R (which disintegrate entirely). To assert this, additional characterization with TEM was carried out for the as-received and residues of the commercial SWCNT. The as-received SWCNT micrographs are shown in Figure S10. As per the corresponding EDS analysis, it is clear that the frequently observed catalyst particles are alloys of Co and Mo (Figure S11). The postdigestion residue imaging for this sample is shown in Figure S12. Despite the issues stated above (upon TEM observation of residues), a few nanotubes were identified in addition to Au particles (these were present due to the leaching effect of the nitric acid). We confirmed therefore that a proportion of the nanostructures survived the digestion protocol. Following the structural characterization of the commercial samples, quantitative chemical analysis was carried out with ICP-OES. Table 2 lists the concentrations of the same 18 elements that were analyzed for the CRMs. In addition, to further validate the microwave digestion method used (i.e., on top of the work done with the CRMs), spike experiments were performed for the SWCNT sample to access the recovery yield. This step followed the EPA method 200.7, which states that a recovery between 70 and 130% for spiked analytes is acceptable.25 As per the table, all of the elements are within this range. With regard to the other commercial samples, the incomplete digestion of these means that the concentration values are merely indicative. It is very likely that a considerable amount of metals have not leached into the acid solution. However, one may infer which elements are present in higher concentrations. As such, Fe is one that consistently shows up as a dominant component (see, for instance, a corroborating EDS spectrum for the commercial MWCNT in Figure S13). This is logical because, aside from being a common catalyst for the growth of carbon materials, it is also present as an impurity in graphite

extracted from geological reserves.26 This same natural graphite may be the source of the graphene nanoplatelets used. Co and Mo are two other elements commonly used as catalysts for nanotube growth but would not be expected to be present in the layered materials, i.e., graphene and graphite (particularly if derived from mined resources). Ni and Cr are exclusively high in the graphene sample, which could again relate to the synthesis conditions, e.g., liquid-phase exfoliation from graphite. On the other hand, Cr, Fe, and Ni could also have a mineralogical origin. In a study performed by Ambrosi et al.,26 the concentration of Fe and Ni in graphite (natural and synthetic) was higher than those of other metallic impurities. Remarkably, these remained present after the exfoliation steps undertaken to produce graphene materials.26 Other elements of note are Al, Ca, and Mg. These are present in not just the graphite but also the MWCNT samples. Whereas for graphite these may again be seen to be of mineral origin, for the MWCNTs, it is possible that they are derived from the material used to support the catalyst for the pyrolytic growth of the nanotubes (metal oxide supports are common in floatingcatalyst CVD synthesis of nanotubes27).

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DISCUSSION Our two-step digestion protocol was not effective for disintegrating all of the graphitic carbon materials analyzed. This inconsistency in acid digestion yield is expected and represents the key conundrum for elemental analysis in the field of nanocarbons, forcing the arduous, costly customization of ICP-OES sample preparation for each carbon material batch received.19 However, it is interesting that despite the diversity of SWCNT samples tested (with different diameters, degrees of purity, and synthesis procedure) the protocol could be applied across this class of nanocarbons. In this respect, SRM2483 showed the highest digestion yield together with more consistent recoveries and better agreement to the certified concentrations. Thinner bundles added to higher structural disorder of the nanotubes could well explain this result. Also, it is of note that the certified and reference concentrations for both CRMs (SWCNT-1 and SRM2483) were appropriately determined well using ICP-OES. It looks therefore that more convoluted analytical methods such as NAA can henceforth be overlooked for the elemental analysis of unknown SWCNT samples (at least, as long as one of the above reference materials is available for validation). In contrast to the view promoted in the literature, it was interesting to realize that visual inspection of a digested sample may not be enough to assert full digestion of carbon materials. To validate the complete disintegration of nanocarbons, it is advisible to carry out the evaporation of the liquid medium in control experiments. Otherwise, the presence of suspended micrometer-sized particles, not visible by the naked eye, may go unnoticed. Nonetheless, although complete disintegration was never achieved (even for the SRM2483R, sp3 carbon particles were identified), the yield achieved for SWCNT provided accurate ICP-OES results. A key parameter in our approach was the digestion temperature used. Different from other reported microwave-assisted digestion procedures,19 we used a relatively high temperature (220 °C) for 20 min during the second stage of the cycle. We ran similar experiments entirely at 200 °C, but these did not result in successful digestions. Here, the sample mass used is also noteworthy. Because of the high cost of CRMs, relatively small samples of 10 mg were analyzed. However, for each material (CRM and commercial), three G

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Article

Analytical Chemistry

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different samples from the same batch were probed, totalling 30 mg for any given carbon in this study. Although not ideal, this strategy is feasible as the results shown above confirm. Possibly, the most interesting point of the work was to realize the importance of evaluating the acid-digested residues by NMR. To the best of our knowledge, this has never been shown. These results demonstrate that SS-NMR can provide clear, nonlocalized evidence for the disintegration of sp2 carbon materials. Accordingly, this technique is complementary, and even more reliable, than other popular characterization options such as localized analysis by electron microscope and the often challenging interpretation of Raman spectra. In the set of samples studied, only SRM2483R had a degree of disintegration compatible with the use of the NMR technique. For all other materials, the presence of nondigested graphitic carbon results in the spectral dominance of the sp2 signal, preventing the identification of the sp3 signal. In other words, it is necessary that most of the structural integrity of graphitic carbon is lost to see the sp3 peaks, thereby providing the best litmus test for the (almost) complete digestion of these materials.

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CONCLUSIONS A two-step microwave-assisted digestion protocol has been developed to prepare graphitic carbon materials for ICP-OES analysis. Besides the reduced number of steps, an additional advantage of the method reported is the lower amount of material required to run the analysis (validated by the experiments performed with two different CRMs). Furthermore, it was demonstrated that visual inspection of wetdigested samples may not be sufficient to assert complete disintegration of graphitic materials. For this purpose, it is advisible to entirely evaporate the liquid and investigate any remaining residues. Here, the use of SS-NMR is advantageous as it provides a nonlocalized and easily interpretable fingerprint of the effective change from sp2- to sp3-type carbon.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b03407. Experimental details for NMR and ICP-OES experiments and additional characterization of the CRM and commercial samples (Raman, TEM, and EDX) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for financial support from KAUST (BAS/1/1346-01-01). Technical guidance and support from the KAUST Core Laboratories is appreciated.

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REFERENCES

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DOI: 10.1021/acs.analchem.6b03407 Anal. Chem. XXXX, XXX, XXX−XXX