# Clustering of Diamond Nanoparticles, Fluorination and Efficiency of Slow Neutron Reflectors

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## Abstract

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## 1. Introduction

_{2}, and COOH [10,11]. In previous studies, we reduced, by the chemical treatment of DNDs in F

_{2}gas [12,13,14], the fraction of H by at least ~30 times. In freshly prepared F-DNDs, one H atom is present per 430 ± 30 C atoms. Alternative approaches would be to reduce the fraction of H by deuteration or modification of conditions of its production [15]. However, we focus on fluorination due to its efficiency and universality.

## 2. Materials and Methods

#### 2.1. Samples

^{3}hybridization, and it has the polyhedron shape [29]. A non-crystalline C shell has sp

^{3}-sp

^{2}hybridization of C atoms and surrounds the core. C-H

_{x}, C-C-H, C-OH, C-O, C=C are present in the shell [11]. The shell thickness is 0.4–1.0 nm [30,31,32]. Industrial DND powder contains strongly connected, most dense agglomerates (agglutinates) with sizes of 40–200 nm consisting of 4–5 nm individual particles. DND powders can be deagglomerated. One process is based on mechanical milling [33,34]. Another one uses annealing in different gases [35,36]. DNDs in commercially available powders have a hierarchical structure of agglomerates [34,37,38]. The sizes of larger agglomerates are above 1 μm; they are more easily destroyed [39].

^{3}diamond carbons (the DND cores) are non-reactive towards F

_{2}gas, two reactions compete for the sp

^{2}carbon shell at the reaction temperature of 450 °C in pure F

_{2}gas (pressure of 1 atm): fluorination and decomposition. During fluorination, the hybridization of C changes from sp

^{2}to sp

^{3}, and covalent C-F bonds are formed. We denote them C

_{ex}-

_{sp}

^{2}-F. During decomposition, CF

_{4}and C

_{2}F

_{6}gases are formed. The treatment conditions are chosen so as to favor the decomposition. Diamond cores are not affected by F

_{2}molecules because of the stabilization of C atoms in the sp

^{2}hybridization and in the crystalline C lattice. Besides, F from covalent C-F bonds can substitute H atoms bonded to sp

^{3}C in CH, CH

_{2}, or C-OH groups.

_{max}= 1.8 × 10

^{4}g for 40 min) in order to separate particles in ethanol by size. The supernatant containing deagglomerated F-DND particles with sizes of ~5 nm was carefully separated from the sediment. The yield of deagglomerated particles was 20%; it is estimated as the ratio of their mass after ethanol removal to the initial mass of the F-DND powder. Ethanol was removed in two stages: (1) drying of supernatant in a rotary evaporator and (2) heating of wet powder in air at the temperature of 300 °C for 1 h. We call the resulting powder DF-DND, where DF means that the sample was first fluorinated and then deagglomerated.

#### 2.2. Rationale for the Choice of Experimental Methods

^{2}shells; otherwise, there would be two contributions to the change in SANS results, and it would be more difficult to interpret the data. Results of XRD measurements are given in Section 3.4.

#### 2.3. The Model of Discrete-Sized Diamond Nanospheres

^{2}-phase, other than C elements, the presence of pores, the real shapes of DND cores, the interference effects, etc.

## 3. Results

#### 3.1. SEM and TEM

#### 3.2. DLS

#### 3.3. Chemical Composition by NAA and IR Spectroscopy

^{−1}using an Infralum FT-08 spectrometer with Fourier transform (CG “Lumex”, Russia).

_{x}groups in addition to the narrow band of C-C vibration for diamond core (1340 cm

^{−1}). Bands between 1090 and 1220 cm

^{−1}correspond to CF

_{2}and CF

_{3}symmetric stretch, CF

_{2}asymmetric stretch, as well as CF=CF

_{2}, CF

_{3}-CF

_{2}and CF stretch vibrations [48]. This latter contribution dominates according to solid-state NMR characterization [12]; CF

_{2}and CF

_{3}are not evidenced neither by

^{19}F nor

^{13}C measurements. C-F bonds result from both conversion of C-H and COH into C-F in the diamond core and fluorination of sp

^{2}C in the shell (C

_{ex}-

_{sp}

^{2}-F). O-H bending and stretching in molecules of adsorbed water corresponds to the weak absorption peak at 1627 cm

^{−1}and a wide band between 2750 and 3750 cm

^{−1}. The peak at 1798 cm

^{−1}is probably due to C=O stretching. Dangling bonds are formed during fluorination by cleavage of C-C, C-O, and C-H bonds. Some of them react with O

_{2}and moisture in air after exposure to air after fluorination. We can conclude that IR spectra stay virtually unchanged by the deagglomeration procedure.

#### 3.4. XRD

#### 3.5. SANS

^{−2}nm

^{−1}< Q < 10

^{0}nm

^{−1}. At YuMO, these were 0.7–5.0 Å and 7·10

^{−2}nm

^{−1}< Q < 10

^{1}nm

^{−1}. Both samples were studied at each of the instruments mentioned above. Two other instruments are NGB30 and NG7 at the NIST Center for Neutron Research [55]. F-DNDs were measured only at NGB30, and DF-DNDs were measured only at NG7. At NGB30, the neutron wavelength and the range of transferred momenta were 6 Å and 3.4·10

^{−2}nm

^{−1}< Q < 1.2·10

^{0}nm

^{−1}. At NG7, these were 6 Å and 3.5·10

^{−2}nm

^{−1}< Q < 6.0·10

^{0}nm

^{−1}. We calibrated the absolute SANS intensity for both samples using the transmission data obtained at NGB30 and NG7. This absolute calibration has a critical importance for the simulation of neutron transport in the powders. As usual, the correction for background scattering from empty cuvettes was applied. The SANS data obtained at NGB30 and NG7 were evaluated using the Igor macros [56].

^{3}at D11. It was 0.36 ± 0.01 g/cm

^{3}at YuMO and 0.19 ± 0.01 g/cm

^{3}at NGB30. The density of the DF-DND sample was equal to 0.56 ± 0.01 g/cm

^{3}at NG7 and varied from 0.43 ± 0.02 to 0.52 ± 0.02 and from 0.61 ± 0.02 to 0.65 ± 0.02 g/cm

^{3}at D11 and YuMO, respectively. These values are different because of different sample holders and different powder compaction by tapping. We tended to measure and analyze the samples with the lowest density in order to avoid a contribution of multiple scattering. For the comparison of results, the powders were brought to the same density; the renormalization can lead to slight distortion.

^{−1}nm

^{−1}. Thus, the fraction of the agglomerates in DF-DNDs is much lower than it is in F-DNDs. The number of individual DNDs (corresponding to the large Q-values Q > 8·10

^{−1}nm

^{−1}) increases by ~10% due to the destruction of agglomerates. Despite this slight difference in intensity, the shapes of two scattering curves are very similar. This observation is important for two reasons. First, it means that the size distribution of individual DNDs remains the same after the F-DNDs deagglomeration. Second, the incoherent scattering cross-section for both samples does not change, as evidenced by the equal intensity at Q > 7·10

^{0}nm

^{−1}. Since for DNDs, this part of the cross-section is determined mainly by H, it implies that the fraction of H does not change after the F-DNDs deagglomeration. Both conclusions agree with our expectations based on the knowledge of the deagglomeration procedure.

## 4. Discussion

#### 4.1. Size Distribution of Nanoparticles

_{min}. They differ due to the different values of Q

_{min}for the samples.

#### 4.2. Calculation of Albedo from a Flat Reflector

#### 4.3. Calculation Storage Times in Closed Nano-Diamond Traps

## 5. Conclusions

## 6. Patents

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**(

**a**,

**c**) TEM images of the F-DND and DF-DND samples; (

**b**,

**d**) respective diameter distributions of the DNDs. Black solid lines correspond to the lognormal distribution.

**Figure 5.**Profile of 111 diamond diffraction peak before and after the deagglomeration treatment. Graphs are shifted along the vertical axis for the visibility.

**Figure 6.**The intensity I (cm

^{−1}) of scattering versus the neutron-transferred momentum Q (cm

^{−1}) for the F-DNDs (solid circles) and DF-DNDs (open squares). Both curves are normalized to the equal sample density of 0.19 g/cm

^{3}.

**Figure 7.**Comparison of measured (open circles and squares) and simulated (solid lines) scattering intensity I (cm

^{−1}) versus the neutron-transferred momentum Q (nm

^{−1}): (

**a**) F-DND; (

**b**) DF-DND samples.

**Figure 8.**The probability density versus radius (in nm) calculated with the discrete-sized diamond nanospheres model for the F-DNDs (solid circles) and DF-DNDs (open squares). Points correspond to the results of calculation.

**Figure 9.**The log-linear volume distribution of scatterers evaluated using the discrete-sized diamond nanospheres model for the F-DNDs (solid circles) and DF-DNDs (open squares). Solid (DF-DNDs) and dashed (F-DNDs) lines interpolate the calculated results.

**Figure 10.**The probability of neutron reflection (black lines) and absorption (red lines) for F-DNDs (dashed lines) and DF-DNDs (solid lines) and DNDs (dash-dotted lines) versus neutron velocity. The incident neutrons are isotropic, the powder density is 0.19 g/cm

^{3}, and the powder thickness is infinite.

**Figure 11.**The probability of neutron reflection (black lines) and transmission (blue lines) for F-DNDs (dashed lines) and DF-DNDs (solid lines) and DNDs (dash-dotted lines) versus neutron velocity. The probability of neutron absorption is below 1% for F-DNDs and DF-DNDs at any velocity and 1–5% for DNDs. The incident neutrons are isotropic, the powder density is 0.19 g/cm

^{3}, and the powder layer thickness is 3 cm.

**Figure 12.**Neutron albedo for the velocities of 50 m/s (black lines), 100 m/s (red lines), and 150 m/s (blue lines) for F-DNDs (dashed lines) and DF-DNDs (solid lines) versus the cavity radius. The incident neutrons are isotropic, the powder thickness is infinite, and densities of F-DNDs and DF-DNDs are equal to 0.19 and 0.56 g/cm

^{3}, respectively.

**Figure 13.**Neutron albedo for the velocities of 50 m/s (black lines), 100 m/s (red lines), and 150 m/s (blue lines) for DF-DNDs (solid lines) and DNDs (dash-dotted lines) versus the cavity radius. The incident neutrons are isotropic; the powder thickness is infinite. DF-DNDs density is equal to 0.56 g/cm

^{3}. The DNDs density is the same as it is in Figure 12 and equal to 0.19 g/cm

^{3}.

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Aleksenskii, A.; Bleuel, M.; Bosak, A.; Chumakova, A.; Dideikin, A.; Dubois, M.; Korobkina, E.; Lychagin, E.; Muzychka, A.; Nekhaev, G.;
et al. Clustering of Diamond Nanoparticles, Fluorination and Efficiency of Slow Neutron Reflectors. *Nanomaterials* **2021**, *11*, 1945.
https://doi.org/10.3390/nano11081945

**AMA Style**

Aleksenskii A, Bleuel M, Bosak A, Chumakova A, Dideikin A, Dubois M, Korobkina E, Lychagin E, Muzychka A, Nekhaev G,
et al. Clustering of Diamond Nanoparticles, Fluorination and Efficiency of Slow Neutron Reflectors. *Nanomaterials*. 2021; 11(8):1945.
https://doi.org/10.3390/nano11081945

**Chicago/Turabian Style**

Aleksenskii, Aleksander, Markus Bleuel, Alexei Bosak, Alexandra Chumakova, Artur Dideikin, Marc Dubois, Ekaterina Korobkina, Egor Lychagin, Alexei Muzychka, Grigory Nekhaev,
and et al. 2021. "Clustering of Diamond Nanoparticles, Fluorination and Efficiency of Slow Neutron Reflectors" *Nanomaterials* 11, no. 8: 1945.
https://doi.org/10.3390/nano11081945