Structure and surface characterization of co-adsorbed layer of oleic acid and octadecylamine on detonation nanodiamond

Diamond & Related Materials 60 (2015) 50–59

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Structure and surface characterization of co-adsorbed layer of oleic acid and octadecylamine on detonation nanodiamond Xiangyang Xu a,⁎, Xiaofeng Wang a, Lin Yang a,b, Hanping Yu a, Hao Chang a a b

School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, PR China Changsha Research Institute of Mining and Metallurgy Co. Ltd., Changsha 410012, PR China

a r t i c l e

i n f o

Article history: Received 8 March 2015 Received in revised form 12 October 2015 Accepted 17 October 2015 Available online 20 October 2015 Keywords: Diamond particle Nanocrystalline Colloids in apolar liquids Steric stabilization Alkyl-functionalization

a b s t r a c t Aiming to form an alkyl-functionalized surface and to realize the particle dispersibility in apolar solvents, oleic acid (OA) and octadecylamine (ODA) were introduced to modify detonation nanodiamond (DND). Compared with their single addition, the combined use of OA and ODA resulted in better particle dispersion and suspension stability. The co-adsorbed layer containing both OA and ODA is of a crystal structure, while the single-added OA and ODA form only amorphous structures on DND. The acid–base interaction originated from the electrostatic attraction between the amino groups on ODA and the carboxyl groups on OA may contribute to the assembly of both surfactants. With the significant increase of adsorption concentration and layer thickness, the steric repulsion of DND particles can be strengthened. By taking this approach, the alkyl-functionalization effect and the dispersion of DND particles in organic solvents or in polymer composites can be improved. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Possessing excellent properties of superhardness, biocompatibility, thermal conductivity and chemical stability, nanoscaled diamond powder is one of the hot topics in the field of superhard materials [1–3]. Nanodiamond powders synthesized by different commercial approaches, such as milled HPHT monocrystal diamond, dynamic highpressure shock-compacted polycrystalline diamond and detonation nanodiamond (DND), have varied morphology, size and surface properties, and can be employed for different applications. Among them, DND has the smallest crystalline size (around 2–6 nm), and the morphology and diameter of DND crystallines are quite homogeneous [4]. As DND monocrystallines always form aggregates of micron scale or submicron scale because of the high surface free energy for nanoparticles, it is crucial to actualize particle dispersion for better utilization [5]. Over the past decade, many encouraging achievements have been made in disaggregation and surface modification technique of DND, especially for its application in aqueous systems [6–8]. Thereafter, a lot of progresses have been made in its application trials in CVD seeding and nucleation [9,10], drug/gene transfection [11–14], and as a biomaterial to accelerate osteogenesis [15]. The diamond structure in DND was encircled by a large quantity of functional groups, among which, the majority groups for the welloxidized DND are of high polarity [16–18]. The polar surface ensures the active property as it provides abundant anchoring points for surface

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.diamond.2015.10.014 0925-9635/© 2015 Elsevier B.V. All rights reserved.

functionalization. Meanwhile, its hydrophilic property makes it difficult to attain a good dispersion and distribution in oil-based matrixes [19]. Unlike in aqueous system, where the electrostatic repulsion mechanism can be utilized to realize the dispersion of particles and the surface electrical potential (ζ-potential) can be modified by introducing some electrolytes or ionic surfactants, the steric hindrance turns to be the main effect to ensure the dispersion of DND in organic systems, especially in apolar matrixes. As the effect of surface charge is not so prominent, it is usually essential to introduce some polymer dispersants to achieve steric repulsion of particles [20]. Researches on the application of DND particle in polymers as filler have started for decades, and the results show that DND can bring excellent advantage by improving the performance of composites [21]. DND ensures uniform thermal stability and excellent compatibility between polyurethane (PU) and epoxy (EP), and it can thus be used as a reinforcing additive in PU/EP interpenetrating nanocomposites [22]. With small content of DND particles, the tribological and mechanical properties of epoxy-based nanocomposites can be obviously reinforced [23]. Significant increase in scratch resistance and thermal conductivity for the composites of epoxy polymer-binded DND can be observed [24]. When DND was introduced to form composite with the polyvinyl alcohol (PVA) matrix, the thermal conductivity can be increased drastically and it is superior to other carbon-based nanofillers as the high optical transparency of PVA can be guaranteed [25]. Composites prepared by DND particles and PVA aqueous solutions can benefit the seeding in chemical vapor deposition diamond growth [26]. Owing to the enhanced intrinsic photoluminescence (PL) within DND particles due to ion-implantation generated defects and by PL originating from

X. Xu et al. / Diamond & Related Materials 60 (2015) 50–59

structural transformations produced by protons at the DND/matrix interface, prominent PL enhancement was observed for the protonirradiated poly(dimethylsiloxane) (PDMS) incorporated with DND [27]. Containing DND functionalized by poly L-lactic acid (PLLA) and octadecylamine (ODA), fluorescent composite bone scaffold material shows properties close to that of the human cortical bone [28]. To enhance adhesive interactions with polymer matrix, a common measure is to introduce long-chain alkyl groups to modify particle surface [29]. Aiming to intensify alkyl groups on DND which can improve the thermo-mechanical properties of nanocomposites, an alkylfunctionalization approach was introduced through which alkylated DND compounds can be generated after an esterification of a dried DND-COOH sample prepared by surface oxidation in aqueous solution with excess alcohol [30]. To increase alkyl groups on DND, this esterification approach has as well be conducted by DND hydroxylation with carboxylic acid chlorides, and the modified particles exhibit a significantly improved dispersibility in organic solvents such as tetrahydrofuran (THF) and dichloromethane [31]. Oleic acid (OA) was used by Peng et al. to modify diamond and SiO2 nanoparticles in liquid paraffin, and the tribological property for liquid paraffin containing modified particles is better than that of pure liquid paraffin [32]. Before dispersed in mineral oil, the deagglomeration of DND can be actualized in OA/octane system, and after the evaporative removal of octane, the thermal conductivity enhancement of the remaining fluids exceeds 11% [33]. ODA was as well be introduced to modify DND in dichloromethane, and blue fluorescent DND particles can be obtained in hydrophobic solvent [34]. Oleylamine (OLA) can be used to disperse DND in THF and these as-received suspensions can enhance the diamond nucleation [35]. Long chain fatty acids can adsorb onto DND coated by a water nanolayer, and organo-modification of DND can be actualized, resulting in DND dispersion in general organic solvents as a mimic of solvency [36]. Similarly, in order to create well-dispersed DND suspensions in apolar solvents, it is always a primary solution to introduce surfactants with long carbon chain. As the adsorption mechanism and behavior for different surfactants varies, and there may form interaction between different types of reagents amid surface modification, it is occasionally more efficient when surfactants are used combinedly. In this case, it is necessary to study the interrelationship between the introduced dispersants. Fatty acids and amines, including stearic acid (SA), OA, ODA and OLA, are commonly used surfactants for modification and dispersion of particles and the synthesis of particles including nano-gold particles, magnetic Fe3O4 nanoparticles, ZnO nanorod arrays, and so forth [37–42]. When used as dispersant, OA can improve the fluidity of the delivery system of magnetic lipid nanoparticles [43] and the dispersibility and hydrophobicity of magnetic strontium hexaferrite particles [44]. Using OA as a modifier for surface coating, Cu nanoparticles smaller than 15 nm can be prepared from Cu wire by electrical explosion in nhexane, an apolar solvent [45]. ODA can be used to stabilize gold colloidal particles in toluene and ODA-capped particles can be readily dissolved in different organic solvents [46]. By the Langmuir–Blodgett method, well-ordered multilayer structure of ODA-coated nanogolds with the mean diameter of 6.8 nm was created [47]. ODA has been introduced to improve the dispersion of thermal oxidized DND in THF, methyl ethyl ketone or acetone, organic solvents of medium polarity, and its performance is better than OA, yet, both dispersants are inferior to OLA, as colloidal solutions can be prepared after bead-milling in the presence of OLA [48]. Combinations of OA and OLA play an important role for magnetic iron oxide nanoparticles synthesizing, as a perfect fit can be created between the particle surface charge, free proton concentration in the dispersion medium, and ζ-potential [49]. With the aim to attain an alkyl-functionalization of DND surface in apolar solvents, OA and ODA were introduced to evaluate the dispersion behavior of DND in petroleum ether, and the mechanisms for the reagents-DND interaction and the synergy effect of reagents were studied.

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2. Experimental 2.1. Materials Raw DND powder was supplied by Chengdu Fortune Myriad Diamond Nano-Tech Co., Ltd. This gray DND powder is synthesized with 2, 4, 6-trinitrotoluene as the carbon source and with other explosives such as cyclotrimethylenetrinitramine to increase the detonation pressure. After the detonation process in the combustion container, the black detonation soot was collected and purified with chlorine acid, sulfuric acid and potassium permanganate. OA (Analytical reagent, AR) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. and ODA of AR-grade was synthesized by Tianjin Guangfu Fine Chemical Research Institute. An apolar solvent, petroleum ether (AR) with the boiling range of 60–90 °C, was purchased from Tianjin Hengxing Chemical Reagent Co. Ltd. It was used as the reaction media for dispersion, adsorption and milling processes, and for sample cleaning in the infrared sample preparation. 2.2. Sample preparation To enhance the surface modification in the presence of adsorbates, sonication approach was introduced to treat DND powder in apolar solvent. Two ultrasonic apparatuses with the same frequency of 53 kHz, SK5200HP and SK2200HP, were purchased from Shanghai Kudos Ultrasonic Instrument Co., Ltd. The output powers for these two apparatuses are 200 W and 100 W respectively. The liquid–solid separation was accomplished by centrifuge produced by Changsha Xiangzhi Centrifuge Instrument Co. Ltd. DND suspensions in petroleum ether were prepared by adding solvent, DND sample and reagent(s) to each dried glass beaker in sequence as 200 mL petroleum ether, 1 g DND sample and 2 g OA (2 g ODA, or, 1 g OA and 1 g ODA simultaneously), then the beakers were put into the ultrasonic apparatus and sonicated for 30 min. Then, samples were collected individually and transferred into glass cuvettes for size measurement, and 40 mL solution for each sample was transferred to the glass bottle and numbered for further observation of DND suspension stability. Using solvent, powder and reagents with correspondingly doubled quantities, samples for structure and surface properties study were prepared following a similar procedure. To remove the unadsorbed reagents, the suspensions were transferred into centrifugal bottles and centrifuged 7 times at a speed of 2000 r/min for the initial 2 times and 4000 r/min for the following 5 times. The supernatants were removed and pure petroleum ether was replenished for each washing. The washed samples were air-dried for further characterization. The milling process was conducted with a planetary ball milling apparatus QM-3SP4 from Nanjing NanDa Instrument Plant, the rotation speed was 350 r/min, and the revolution speed of 175 r/min, 304 stainless steel jar of 1 L each was used and stainless steel beads of 1 mm was used as milling media. In each stainless steel jar, 1400 g stainless steel beads of 1 mm were added in advance. Mixed with 150 mL of petroleum ether, 2 g DND, 1 g OA and 1 g ODA were added to the jar and were milled for 8 h. The milled sample was washed with pure petroleum ether while the solid phase was separated out by centrifuge and airdried, and the structure, surface properties and thermal behavior were investigated. The impurities are not removed by sedimentation or magnetic separation approaches in this study. 2.3. Characterization The particle size distribution (PSD) for DND aggregates modified by OA, ODA and their combination was measured with photon correlation spectrum (PCS) method with Zetasizer 3000HS from Malvern Instruments Ltd. The analyzer contains a power source of 633 nm He–Ne laser and the scattering angle is fixed at 90°. After sonication treatments,

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the suspensions of DND in petroleum ether were transferred timely into the 12 mm square quartz glass cell for PSD measurement. X-ray diffraction (XRD) analysis was conducted with a diffractometer DX-2700 from Dandong Haoyuan Instrument. CuKα radiation (40 kV, 40 mA) was used as the X-ray source, and the diffraction intensity dependence on the diffraction angle was recorded within 2θ angle scan range from 10° to 110° with the step size of 0.02° and the scan speed of 4°/min. Fourier transform infrared (FTIR) spectroscopy was used to study the surface functional groups on pristine and modified DNDs with spectrometer Nicolet 6700 from Thermo Electron Scientific. The midinfrared light ranging from 4000 cm−1 to 400 cm−1 was used for illumination. The modified samples and milled powders was washed with petroleum ether and then air-dried before measurement. KBr pelleting method was adopted for test sample preparation. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were conducted using a simultaneous TGA–DTA thermal analyzer SDT2960 from TA Instruments. The apparatus was operated under air from ambient temperature up to 900 °C at a heating rate of 10 °C/min and with the carrier gas flux as 50 mL/min. X-ray photoelectron spectroscopy (XPS) was adopted to investigate the chemical composition and environments of elements on DND surface. The data were obtained with an ESCA Lab220i-XL from VG Scientific using 300 W AlKα radiation under the base pressure of around 3 × 10−9 mbar, and the binding energy (BE) data were referenced to the C1s line at 284.8 eV from adventitious carbon.

3. Results and discussion 3.1. Influence of co-absorption on DND dispersion When DND powder was added directly into petroleum ether, serious agglomeration and sedimentation occurred even in the presence of sonication. Dynamic light scattering (DLS) measurement on aggregate size was tried with Zetasizer 3000HS, which shows that a large quantity of particles (36%) are around 3.7 μm. Nonetheless, the result is not reliable as the size exceeds the optimal measure range of the analyzer. According to the direct and microscopy observation, the aggregates are in a diameter of micron scale. The graphic PSD data collected by the analyzer and a photo of the instantly prepared suspension were given as supplementary materials.

With the addition of different long chain dispersants, the dispersion property of DND in petroleum ether varies. The left graph in Fig. 1 shows the size distribution (volume fraction) of DND aggregates modified by OA (curve a), OA–ODA combination (curve b) and ODA (curve c), while the corresponding actual systems were shown sequentially in the picture on the right. When OA and ODA was introduced in single addition to modify DND in petroleum ether, obvious flocculation forms for both cases, and an instantaneous sedimentation occurred if there is no ultrasonic or mechanic stirring to generate a turbulent flow. When measured by Zetasizer3000HS, the ODA-coated DND particles are ranging from around 700 nm to over 2000 nm, while the OA-coated particles are distributed from ~400 nm to ~1200 nm. Yet, when OA and ODA were added simultaneously, PSD curve of DND agglomerates shifts towards small size section. There appeared some small agglomerates of less than 100 nm, including some with a diameter of only 20–50 nm (~10%), while the majority of the solid is of submicron scale, ranging from ~200 to ~700 nm. The mean size of the aggregates is comparatively smaller than those modified by singleadded reagents. It can be observed that, suspension of OA & ODA coated DND in sample bottle b (Fig.1) is more homogeneous and its sedimentation is not so serious as the systems containing respectively OAmodified and ODA-modified particles (in bottle a and c) for the same duration. 3.2. Crystal structure of adsorbed layers XRD patterns of pristine DND and DND coated by OA, ODA and OA & ODA were shown in Fig.2. For pristine DND (curve (a)), three characteristic diffraction peaks at 2θ angle of 43.8°, 74.6° and 91.6° can be well indexed to the PDF card 06–0675 as diamond planes of (111), (220) and (311) for cubic diamond. The broad knoll centered at ~26° indicates that there is still amorphous non-diamond carbon residue on DND. For DND samples covered separately by OA (curve (b)) and ODA (curve (c)), besides the diamond facets, there formed broad bulges centered at ~21°. It supposes that, amorphous layers were formed on both the OA-coated and ODA-coated DND and the long carbon chains wherein are of a random configuration. When OA and ODA co-adsorbed onto DND (curve (d)), the layer turns to be more orderly and there appeared two sharp diffraction peaks at 21.44° and 24.44°. In the inset graph (band A) representing a magnified and fitted section of curve d) (r2 = 0.9763) with 2θ angle ranging from 15° to 30°, two distinct peaks (peak 2 and peak 3) raised

Fig. 1. Left: size distribution of DND modified by OA (a), OA & ODA combination (b) and ODA (c); right: actual systems of modified DND in petroleum ether corresponding to the PSD curves.

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Fig. 2. XRD patterns of pristine DND (a) and DND modified by OA(b), ODA(c) and OA–ODA(d), and the band A presents the peak fitting for a diffraction section on curve d) (2θ = 15°–30°), while band B illustrates the difference of intensity of (111) plane diffraction of cubic diamond for pristine DNA and DND coated by OA, ODA and OA & ODA.

up from a broad hummock (peak 1) centered at 21.10°, showing that there formed an order structure of hydrocarbon bonds after the coadsorption. Taking (111) plane of cubic diamond as an illustration(Fig 2, band B), for pristine DND and DND modified by single-added OA and ODA, the intensity of diffraction peaks corresponding to diamond is almost identical, while, for the OA and ODA co-adsorbed particles, the intensity of (111) diffraction is obviously reduced. This reason probably lies to the difference of surface layers. The single-added reagents may be distributed and piled randomly on DND, which brings little influence on layer thickness, yet, the conjugated layer is much more orderly and is comparatively thicker, which has apparent effect on the intensity of diamond diffractions. Without adsorbent, OA and ODA molecules can form some orderly structures which can be observed and analyzed by XRD techniques (Table 1). Nevertheless, when OA and ODA were used in single addition for DND modification, the reagents tended to adsorb preferentially onto particle surface and form an amorphous layer of disorderly piled molecules (curve b and c in Fig. 2). When OA and ODA reacted together with DND surface, the adsorbates turn to be well-arrayed. Compared with the diffraction peaks of OA and ODA, the corresponding sharp peaks at ~21° and 23° shifted to 21.44° and 24.44° (Table 1), respectively, and the

crystal plane spacing (d-value) decreased, indicating that, after the coadsorption, the molecules in the newly-created crystal structure are tightly arrayed.

3.3. FTIR study on surface adsorption To clarify the reaction mechanism of OA/ODA with DND, FTIR was employed to investigate functional groups on pristine DND and DND modified by reagents (Fig. 3). For pristine DND (spectrum a), the band

Table 1 XRD peaks (within 2θ range of 15°–30°) for OA, ODA and DND modified by both adsorbates. Sample OA ODA OA–ODA coated DND

2θ/°

d/Å

21.136 22.723 20.63 23.453 21.44 24.44

4.2 3.91 4.3 3.79 4.14 3.64

Reference PDF#11–0802 [50,51], Supplementary data

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Fig. 3. FTIR spectra of pristine DND (a) and DND modified by OA (b), ODA (c) and their combination (d).

at 3426 cm−1 could be ascribed to the stretching vibration of bonded – OH. The weak peaks located at 2922 cm−1 and the adjacent band may be attributed to the asymmetrical and symmetrical stretching vibration of hydrocarbon groups (νas–CH2 and νs–CH2). The band at 1630 cm−1 can be assigned to the C_C stretching and O–H or N–H bending vibrations [25,52,53]. And the adsorption vibration bands of surface bound water at ~ 3450 cm−1 and ~ 1630 cm−1 may be overlapped by the broad peaks of the aforementioned groups. The peak located at 1712 cm− 1 can be attributed to the stretching vibration of carbonyl groups [37,54]. It can be concluded that the intensity for carbonyl and hydroxyl groups are comparatively strong, which may contribute to the surface polarity of DND particles and they can serve as active points for interaction between reagents and DND. For OA or ODA modified DND (spectrum b and c), a commonality is the dramatic increase of the intensity of hydrocarbon groups, including the asymmetrical (νas–CH2, ~ 2920 cm−1) and symmetrical (νs–CH2, ~2850 cm−1) stretching vibrations, the C–H scissor bending vibrations for methyl and methylene (δas–CH3 and δs–CH2 at ~ 1460 cm− 1; δs– CH3 at ~ 1375 cm− 1), and the skeleton vibration band of − (CH2)n–(~727 cm−1). These strong peaks of hydrocarbon groups on IR spectra verified that OA and ODA formed stable adsorbed layers on DND. In addition, the specific peaks for the adsorption of reagents are quite obvious: for OA modified DND (spectrum b), there appeared a C-H stretching vibration of alkene double bond (cis-double bond) at ~3007 cm−1, while the carbonyl vibration peak at ~1712 cm−1 is significantly intensified, indicating that OA has chemisorbed on DND; for ODA coated DND (spectrum c), there appeared a scissor bending vibrations (δ–NH2, 1570 cm−1), a rocking vibration (ρ–NH2, 816 cm−1) and a C–N stretching vibration(ν C–N, 1125 cm− 1)of amino group, which verifies the chemisorption of ODA. In spectrum b, the intensity of ν–OH vibration (~ 3421 cm−1) decreased, while the relative intensity of νC–O(H) vibration (1113 cm−1) increased and shift towards the low-frequency region, indicating that there forms hydrogen bonding between OA and DND surface amid the surface adsorption process. The adsorption peak at 3426 cm− 1 shift to 3416 cm−1(spectrum c), which may attribute to the adsorption of amino groups, as the asymmetrical and symmetrical stretching vibrations are of lower wavenumber, and the total broad

peak for –OH shifts to lower wavenumber owing to the overlap of amino groups. Compared with the single addition, when OA and ODA were introduced together to the system (spectrum d), the adsorption peaks for hydrocarbon groups are extremely strong, as the peaks for stretching vibration of C–H of alkene bond (cis = CH stretch, 3008 cm−1), methylene group (νas–CH2 at ~2919 cm−1 and νs–CH2 at ~2850 cm−1), C–H scissor bending vibrations for methylene and methyl (δas–CH3 and δs– CH2 at ~ 1466 cm− 1) and skeleton vibration band of − (CH2)n–(~723 cm−1) are all enhanced, which means that the concentration of the adsorbed reagents intensified significantly when OA and ODA were introduced simultaneously. Meanwhile, the adsorption peaks for amino groups, including the scissor bending vibration (1557 cm− 1) and the C–N stretching vibration adsorption (1097 cm− 1), are comparatively stronger than the single usage of ODA(spectrum c), and there is remarkable red-shifts for these peaks, indicating that there is some interaction between ODA and OA in the coadsorbed structure. Compared with OA-coated sample (spectrum b), another remarkable change for the conjugated layer (spectrum d) lies in the distinct weakening of the carbonyl adsorption, which may attribute to the electrostatic interaction of OA and ODA, as there may form a bonding between polar groups of both reagents, as a consequence of charge-clouds homogenization and ionization of a majority of carbonyl groups, the adsorption peak for carbonyl groups weakened and two strong peaks could be ascribed as the asymmetrical (νasCOO−) and symmetrical(νsCOO−) stretching vibration of carboxyl ions appeared at 1620 cm−1 and 1400 cm−1, respectively. Carboxylate forms always appear when fatty acid reacts with particle surface, especially in aqueous surroundings where a chelating reaction of carboxyl group and metal ions occurs [39,45,55]. It can be concluded that, the electrostatic interaction of OA and ODA may be the synergistic effect for both reagents on DND surface.

Fig. 4. Schematic representation of co-adsorption of OA and ODA on DND.

X. Xu et al. / Diamond & Related Materials 60 (2015) 50–59

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Fig. 5. XRD pattern of DND milled in petroleum ether in the presence of OA and ODA. Fig. 7. FTIR spectrum of DND milled in petroleum ether with the presence of OA and ODA.

3.4. Co-adsorption model of OA and ODA According to the aforementioned discussion, OA and ODA can adsorb in random distribution on DND, and hydrogen bonding is a possible interrelationship between particle surface and both reagents. Taking hydroxyl groups to represent the anchoring points on DND, the interactions for OA, ODA, and OA & ODA with DND are schematically illustrated in Fig.4. The characterization and analysis are in good accordance with the research of Niu et al. in synthesizing fluoride upconversion nanoparticles with amidation reaction of OA and ODA, which resulted in the formation of an acid–base complex and then its transformation to Noctadecyloleamide [56]. Because of the difference in reaction surroundings, OA & ODA layers on DND coated in petroleum ether are of a structure of acid–base complex (C17H33CO−+ NH3C18H37) bonded with the 2 anchoring points on DND surface. Similar mechanism for the rigid characteristic of SA/ODA mixed monolayer at the air/water interface was analyzed by Lee et al., which showed that, owing to the electrostatic adsorption of headgroups of SA and ODA, “catanionic surfactant”

formed and the acid–base reaction can eventually form a salt structure [57]. As it shows, the conjugated layer is much more orderly, and the layer thickness can therefore be increased. The association of long alkyl chain and the electrostatic adhesion between polar groups of OA and ODA may serve as the main mechanisms for molecules' assembly and orderly conjugation. Because of the entropy effect [58], the repulsion energy between OA & ODA-capped DND particles can be increased owing to the increase of both the adsorption concentration and the layer thickness, and a strengthening in steric stabilization can be achieved.

3.5. Influence of milling process on OA & ODA coating As shown in Fig. 5, besides the cubic diamond diffraction peaks, the diffraction of iron oxide can be detected, which indicates that a considerable quantity of iron oxide impurities derived in the powder amid milling with stainless steel beads. A broad diffused peak appeared in

Fig. 6. TGA–DTA curves of pristine DND (a) and DND milled in petroleum ether at the presence of OA & ODA (b).

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Table 2 FTIR wavenumbers and band assignments*. Wavenumber/cm−1

Assignment

3445

Stretch. of O–H and N–H cis-double bond Asym. stretch. of –CH3 Asym. stretch. of –CH2 Sym. stretch. of –CH2 Stretch. of −C_O Asym. stretch. of COO− Stretch. of C_C Bend. of O–H and N–H Scissor bend. of −NH2 Asym. bend. of –CH3 and −CH2 Sym. stretch. of COO− Stretch. of C–N Stretch. of C–N Skeleton vibration of −(CH2)n −

3008 2925 2854 1712 1632 1559 1460 1402 1265 1101 725

*: Asym. : asymmetric; sym. :symmetric; Stretch. : stretching vibration; bend. : bending vibration.

band A, namely, the 2θ range of 15°–30°, and the magnified image of band A was shown in the insert graph. This image is similar to band A of OA & ODA modified sample without milling (Fig. 2). Though these peaks are not as distinct as those of the unground sample, they could still be ascribed as the diffraction of co-adsorbed reagents. Thermal behavior of pristine DND and DND with co-adsorbed layer was studied by TGA–DTA method. On DTA curve of pristine DND (Fig. 6a), a small peak at around 274.7 °C may attribute to the oxidation and volatiling of organic adsorbates, and the total weight loss for demoisturing and the decomposition of organic impurities is 8.3%. Then, a dramatic loss zone appeared from ~ 450 °C to ~ 600 °C with a weight loss ratio of 91.6%, which can attribute to the oxidation of DND particles For DND with OA–ODA layer (Fig. 6b), there formed a broad exothermal peak (with the initial point at ~ 100 °C and the final point of ~ 400 °C) centered at 278.05 °C. The accumulative weight loss by ~ 400 °C, including loss amid demoisturing and layer oxidation, is 19.73%, indicating that the co-adsorbed layer weighed more than 10% of the coated sample. The oxidation for non-diamond carbon and diamond phase occurred at ~ 450 °C to ~ 600 °C, with a weight loss of 55.6%. After the oxidation of diamond, there is still ~ 25% of residues left even after being heated to 900 °C, which can be ascribed to impurities derived amid milling such as iron oxide, as verified by XRD analysis (Fig. 5). Bead milling and beads-assisted sonication (BASD) techniques have been introduced for deagglomeration of pristine DND aggregates in water or other solvents such as THF, and primary grains can be prepared and particle surface modification can be optimized [6,59]. Contamination

caused by milling and BASD is one of the drawbacks and it is similar in this study as some ferrous impurities were derived. When sedimentation or magnetic separation approaches were introduced to remove contaminants in sample milled in the presence of OA and ODA, a portion of small DND aggregates can be obtained. Still, for better deagglomeration and dispersion of DND in apolar solvents, an optimal selection on dispersants is essential. FTIR spectrum of DND milled with the presence of both OA and ODA is shown in Fig. 7, while the tentative assignments of the main adsorption peaks are listed in Table 2. The hydrocarbon adsorption peaks thereon are stronger than pristine DND, including peaks for alkene bond, methyl and methylene groups (cis-double bond, 3008 cm−1, νas–CH2 at ~2925 cm−1 and νs– CH2 at ~ 2854 cm− 1), C–H scissor bending vibrations for methylene and methyl (δas–CH3 and δs–CH2 at ~1460 cm−1) and skeleton vibration band of −(CH2)n–(~725 cm−1). Meanwhile, the adsorption peaks for amino groups, such as the scissor bending vibration δ–NH2 (1559 cm−1) and C–N stretching vibration adsorption ν C–N (1101 cm−1), are comparatively strong than pristine DND (Fig. 3, spectrum a). Remarkable red-shifts for these peaks indicate that there is some interaction between ODA and OA in the adsorbed layer. The transfer of carbonyl peak can as well be observed as the adsorption peak for carbonyl groups (1712 cm−1) is weakened, while asymmetrical (νasCOO−) and a symmetrical (νsCOO−) stretching vibrations which may attribute to the carboxyl ions appeared at 1632 cm−1 (intensified while overlapping the C_C stretching and O–H or N–H bending vibrations) and 1402 cm−1, respectively. It can be concluded that, amid the milling process, the electrostatic interaction of OA and ODA may be the main mechanism for the synergistic effect for the reagents' co-adsorption onto DND surface. 3.6. XPS study on the co-adsorbed layer In order to study the surface structure of DND before and after the adsorption of OA and ODA, the chemical environments of main elements, C, O and N, were studied by XPS. The assignments of C1s, O1s and N1s components of pristine DND and DND modified by OA & ODA co-adsorbed layer are shown in Table 3. As illustrated in Fig.8, the C1s narrow scan XPS spectrum for the pristine DND can be fitted into three characteristic peaks (Fig.8a). The peak centered at 284.05 eV (labeled as C1 in Fig.8a) may attribute to carbon in sp2 type chemical environments in the form of graphite, amorphous carbon phase or C–H (sp2), which indicates that there is some nondiamond carbon and hydrocarbon groups in DND aggregates. These results are in good accordance with the aforementioned data, including

Table 3 Assignment of deconvoluted C1s, O1s and N1s spectra of pristine and modified DND. Pristine DND

OA & ODA coated DND

Assignment

Spectrum

C1s

O1s

N1s

Component

BE/eV

Area/%

C1 C2 C3

284.05 285.2 286.15

55.7 32.9 11.4

O1 O2 O3 N1 N2

529.3 530.9 532.1 398.3 399.5

19.8 61.4 18.8 50.4 26.7

N4 N5

401.7 402.8

14.3 8.6

Component

BE/eV

Area/%

C1 C2 C3 C4 C5 C6 O1 O2 O3

284.6 285.4 286.2 286.9 287.5 288.3 529.5 530.9 532.3

40.1 27.8 13.3 12.4 5.7 0.7 17.8 52.2 30.7

N2 N3

399.5 400.9

56.4 31.9

N6

403.3

11.7

C–C of graphite; C–H (sp2) C–H (sp3); C–C of diamond C–OH or C–O–C C–N or C–O C_O O_C–O C–OH C–O C_O −C–NH2 (sp3C–N) C–CH2–NH2 (sp2C–N) −N–H−C–NO2 N–O C17H33CO−+ NH3C18H37 2

X. Xu et al. / Diamond & Related Materials 60 (2015) 50–59

curve a in Fig.2 and spectrum a in Fig.3, and it is similar to the band at 283.9 eV of polycrystalline diamond [60]. The peak at 285.2 eV (C2 in Fig.8a) may attribute to carbon in sp3 environments in the form of the diamond structure or C–H (sp3), while the peak at 286.15 eV (C3 in Fig.8a) may attribute to C–OH or C–O–C groups. However, the carbon environments became complicated after the adsorption of OA and ODA on DND (Fig.8b). Since the reagents can form a composite layer on DND, the hydrocarbon groups may become the main types of carbon species. Owing to the adsorption of OA and ODA, the fitted peak centered at 284.6 eV (labeled as C1 in Fig.8b) may be attributed mainly to the C–H (sp2) of alkyl group, the peak at 285.4 eV (C2 in Fig.8b) may be attributed to the C–H (sp3) and diamond phase, and the peak at 286.2 eV (C3) may be attributed to the C–OH or C–O–C groups [60]. The peaks at 286.9 eV (C4) and 287.5 eV (C5) which may be connected to C–N or C–O groups, and C_O structure, respectively, appeared after the coadsorption [61]. Areas for these newly appeared bonds, which may be attributed to the terminal groups on OA and ODA, are relatively high, showing the existence of OA and ODA on DND. The peak at 288.3 eV (C6) may be connected to carbonyl groups, O_C–O or the bonding between the polar groups of OA and ODA. Comparing with the C1s spectrum of pristine DND, spectrum of OA & ODA modified DND shifts towards the high BE region, and more carbon atoms are in the form oxygen-containing groups. It can be deduced that, exterior carbon brought by the chemisorption of OA and ODA changes greatly the chemical environment and intensity of carbon element in the whole system, and the increase of oxygen-containing species may originate mainly from OA in the co-absorbed layer. Fig. 8c and d shows the O1s XPS spectra of pristine and modified DND. For the pristine DND (Fig.8c), the curve can be fitted into three peaks. The peak centered at 529.3 eV (labeled as O1 in Fig.8c) may attribute to the C–OH environment, while peak at 530.9 eV (O2 in Fig.8c) and 532.1 eV (O3 in Fig.8c) may correspond to the C–O and C_O, respectively. According to the XPS study on carbon environments, there are carbonyl and carboxyl functional groups on DND surface, which is in consistence with the FTIR analysis. After surface coating(Fig.8d), there is as well three fitted peaks centered at 529.5 eV, 530.9 eV and 532.3 eV (labeled sequentially as O1, O2 and O3 in Fig.8d), which may contribute correspondingly to C–OH and iron oxide(derivatives of milling process), C–O, and C_O or interacted structure of reagents. The spectrum shift to higher BE region, and the peak at 532.3 eV increased remarkably. The increase of relative intensity of C_O groups may originate as well from the OA in the layer. As shown in Fig.8e, the chemical environments for nitrogen element in pristine DND are mainly −C–NH2 (located at 398.3 eV, labeled as N1), C–CH2–NH2 (399.5 eV, N2), −C–NO2 (401.7 eV, N4) and nitrogen doped in DND crystalline (402.8 eV, N5) [62]. It shows that the chemical form of nitrogen in DND is complicated. As some raw reactants for detonation synthesis contain varied nitrogen species, different reaction mechanism may contribute to the variety of nitrogen groups. After being milled in petroleum ether in the presence of both OA and ODA, the peak at 398.3 eV vanished, and the intensity for peak at 399.5 eV (labeled as N2 in Fig.8f) for C–CH2–NH2 and C–(CH2)n–NH2 increased, these changes may be connected to the overlapping of amino groups exist on pristine DND and the fragments of amino polar groups of ODA which are chemically reacted with the DND surface. There appeared a new peak at 400.9 eV (N3 in Fig.8f), which is possibly the newly created − N–H-groups. The significant increased intensity of − NH2 groups can attribute to ODA in the co-adsorbed layer. The peak at 403.3 eV (N 6 in Fig.8f) may attribute to the newly created chemical environment of nitrogen element because of the interaction of OA and ODA, and the mechanism for this reaction between amino head of OA and the carbonyl head of ODA may attribute to the transportation of charge clouds and the protonation of amino groups while the carbonyl heads turns correspondingly to an anionic form as –COO−.

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Fig. 8. C1s (a and b), O1s(c and d) and N1s (e and f) XPS spectra of pristine DND (a, c and e) and OA & ODA-coated DND (b, d and f).

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4. Conclusions When OA or ODA was added individually to disperse DND particles, the suspensions are unstable and sedimentation occurs instantaneously, while the system containing both OA and ODA is comparatively more stable and a portion of small aggregates with a diameter of 20–50 nm forms. According to XRD analysis, OA and ODA co-adsorbed layer on DND is of an orderly structure and exhibits good crystallization, while the single-added reagents distribute at random and form amorphous layers. FTIR study shows that the intensity of alkyl chain vibration are increased significantly owing to the adsorption of surfactants with long hydrocarbon chain such as OA and ODA, especially for their combined usage. Carboxylate adsorption vibration peaks appeared on FTIR spectrum, while, on XPS spectra, C_O binding energy peak was strengthened and the N–H peak was arisen. These changes verified that there is an obvious acid–base interaction between amino groups of ODA and carboxyl groups of OA in the co-adsorbed layer, which may attribute to the electrostatic adhesion. The interaction of terminal groups and the association of long alkyl chain induce an assembly of oriented surfactants and an increase in layer thickness. This alkyl-functionalization featured with synergy effect of OA and ODA on DND may benefit the dispersion and relevant applications of DND additives in oils and polymer composites. Prime novelty statement An improved dispersion behavior can be obtained by the coadsorption of oleic acid and octadecylamine on detonation nanodiamond, owing to the increase of adsorption concentration and layer thickness which can be attributed to the electrostatic adhesion of polar groups of both dispersants and the association of long alkyl chain. This alkyl-functionalization approach may benefit the modification of DND surface and enhances its properties in apolar solvents and polymer-based composites. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.diamond.2015.10.014. References [1] V.V. Danilenko, On the history of the discovery of nanodiamond synthesis, Phys. Solid State 46 (4) (2004) 595–599. [2] V.Y. Dolmatov, M.V. Veretennikova, V.A. Marchukov, et al., Currently available methods of industrial nanodiamond synthesis, Phys. Solid State 46 (4) (2004) 611–615. [3] V.Y. Dolmatov, Applications of detonation nanodiamond, In: O. Shenderova, D. Gruen (Eds.), Ultrananocrystalline Diamond: Synthesis, Properties and Applications, William-Andrew Publishing, Oxford, UK, 2006. [4] A.E. Aleksenskii, M.V. Baidakova, A.Y. Vul, et al., The structure of diamond nanoclusters, Phys. Solid State 41 (1999) 668–671. [5] X. Xu, Z. Yu, Y. Zhu, et al., Effect of sodium oleate adsorption on the colloidal stability and zeta potential of detonation synthesized diamond particles in aqueous solutions, Diam. Relat. Mater. 14 (2005) 206–212. [6] E. Osawa, A.Ya Vul', Disintegration and purification of crude aggregates of detonation nanodiamond : a few remarks on nano methodology, In: D. Gruen, O. Shenderova (Eds.), Synthesis, Properties and Applications of Ultrananocrystalline Diamond, Springer, Dordrecht, The Netherlands, 2005. [7] I. Cha, K. Hashimoto, T. Fujiki, Modification of dispersibility of nanodiamond by grafting of polyoxyethylene and by the introduction of ionic groups onto the surface via radical trapping, Mater. Chem. Phys. 143 (2014) 1131–1138. [8] N. Gibson, O. Shenderova, T.J.M. Luo, et al., Colloidal stability of modified nanodiamond particles, Diam. Relat. Mater. 18 (2009) 620–626. [9] O.A. Williams, O. Douheret, M. Daenen, et al., Enhanced diamond nucleation on monodispersed nanocrystalline diamond, Chem. Phys. Lett. 445 (4–6) (2007) 255–258. [10] J.C. Arnault, H.A. Girard, Diamond nucleation and seeding techniques: two complementary strategies for the growth of ultra-thin diamond films, In: O.A. Williams (Ed.), Nanodiamond, RSC Publishing, London, UK, 2014.

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