# Fluorination of Diamond Nanoparticles in Slow Neutron Reflectors Does Not Destroy Their Crystalline Cores and Clustering While Decreasing Neutron Losses

^{1}

^{2}

^{3}

^{4}

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^{6}

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^{*}

## Abstract

**:**

^{2}amorphous shells of nanoparticles via the fluorination process. In this paper, we study the mechanism of this improvement using a set of complementary experimental techniques. To analyze the data on a small-angle scattering of neutrons and X-rays in powders of diamond nanoparticles, we have developed a model of discrete-size diamond nanospheres. Our results show that fluorination does not destroy either the crystalline cores of nanoparticles or their clustering in the scale range of 0.6–200 nm. This observation implies that it does not significantly affect the neutron scattering properties of the powder. We conclude that the overall increase in reflectivity from the fluorinated nanodiamond powder is primarily due to the large reduction of neutron losses in the powder caused by the removal of hydrogen contaminations.

## 1. Introduction

^{−1}nm. As soon as the neutron wavelength reaches this value, neutrons penetrate through the reflector and are lost. For example, filters made of pyrolytic graphite pass neutrons with the wavelength of 2.4 Å but intensively scatter neutrons with the wavelength of 1.2 Å. The present work is a part of a broader scientific program, and we pursue to overcome this limitation by developing a novel type of neutron reflectors. It is based on the coherent enhancement of elastic scattering of slow neutrons in nanostructured media.

^{C}

_{c.sc.}= 6.65 fm, the corresponding coherent scattering cross section is σ

^{C}

_{c.sc.}= 5.55 b), the high volume density of diamond (ρ

^{C}= 3.5 g/cm

^{3}), the low neutron losses (the absorption cross section of σ

^{C}

_{abs}= 3.5 mb, and low inelastic scattering cross sections depending on the temperature). Nanodiamonds of similar size and properties, but with a smaller amount of impurities, can also be produced through laser synthesis [15]. The geometric sizes and shapes [16] of DNDs are important for optimizing the specific properties of such reflectors, which in turn depend on applications [17,18]. Scattering cross sections have been studied in detail in references [19,20,21], which in particular underlined the importance of clustering/agglomeration of DND powder samples to allow for a proper interpretation of neutron scattering experiments, and the virtual absence of inelastic scattering. Significant efforts have been devoted to including the diffusion of slow neutrons in the DND powders to neutron transport simulations [21,22,23].

^{H}

_{abs}= 0.33 b) and an exceptionally large incoherent scattering cross section (σ

^{H}

_{in.sc.}= 108 ± 2 b at room temperature) that consists of inelastic and elastic contributions depending on the temperature [24]. In DNDs, H atoms are involved in C-OH, C-H, CH

_{2}, and COOH groups [25,26].

_{2})-solid fluorination [27,28,29] which reduced the quantity of hydrogen by the factor of ~30 (on average only one H atom remains per 430 ± 30 C atoms in freshly fluorinated DNDs) and provided a significantly higher efficiency of quasi-specular reflection of neutrons [30]. In this paper, we report results of our study of the mechanism of this improvement (using small quantities of samples <50 mg). In the future, we are going to use the knowledge gained from this study to produce much larger quantities of DNDs with designed parameters, needed for full-scale reflectors. Alternative approaches to produce DNDs with a reduced content of H are its deuteration or modifications of the production conditions of DNDs in order to avoid large hydrogen quantities. Deuterated DNDs seem to be unstable relative to the substitution of D by H. Modifications of the production procedure have been studied in reference [31].

## 2. Materials and Methods

#### 2.1. Samples

^{3}sample of the powder was added to 1 mL of distilled water, and the container with the mixture was placed in an ultrasonic bath filled with water. It was sonicated for 15 min. The resulting suspension (2–3 drops) was applied to a carbon replica placed on the copper grid. After drying, the replica was examined via TEM.

^{2}to sp

^{3}through the formation of covalent C–F bonds. The higher the crystalline order for graphitic carbons, the higher the fluorination temperature is. Since the sp

^{2}C shell on diamond cores is disordered, it is decomposed with F

_{2}gas at a high temperature (>450 °C). CF

_{4}and C

_{2}F

_{6}gases are then formed. On the contrary, diamond cores do not react with F

_{2}molecules because the C atoms are stabilized both in a sp

^{2}hybridization and in the stable (crystalline) C lattice. Moreover, F forming covalent C–F bonds may replace H atoms bonded to sp

^{3}C in CH, CH

_{2}, or C–OH groups.

^{3}hybridization) in the form of a polyhedron [37] surrounded by a non-crystalline C shell (with sp

^{3}-sp

^{2}hybridization of C atoms). The shell contains C–H

_{x}, C–C–H, C–OH, C–O, C=C [26]. The shell thickness is 0.4–1.0 nm [38,39,40]. The fraction of H atoms chemically bound to the surface is ~5%, which corresponds to the results of references [24,27]. At the same time, it should be emphasized that DNDs supplied by different manufactures differ significantly in the structure and chemical composition of the shell that covers the crystalline diamond core, and in particle sizes.

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

^{2}shells and clusters. XRD results are presented in Section 3.3.

^{2}shells) and the sizes and structure of clusters. However, the accurate determination of size distributions is more difficult than it is in the case of neutron and X-ray scattering because it requires the analysis of a large number of images. Examples of TEM and SEM images can be found in Figure 1 and Figure 6.

## 3. Experimental Results

#### 3.1. Small-Angle Neutron Scattering

^{−2}nm

^{−1}< Q < 10

^{0}nm

^{−1}. The neutron wavelengths at YuMO were 0.7–5.0 Å and the range of transferred momenta of 7 × 10

^{−2}nm

^{−1}< Q < 10

^{1}nm

^{−1}. The measured scattering curves for the YuMO spectrometer were corrected for the background scattering from the empty cuvette. The absolute calibration of the scattered intensity was made using the vanadium standard in the SAS program [55].

^{3}, at D11, and it was equal to 0.33 ± 0.01 and 0.36 ± 0.01 g/cm

^{3}at YuMO for DNDs and F-DNDs, respectively. The powder compaction by tapping explains its higher density for the use at YuMO.

^{0}nm

^{−1}. This observation means that fluorination does not significantly affect the primary clusters in the radii range of 0.6–200 nm, as well as the neutron scattering of individual DNDs. This result is important for two reasons. First, fluorination removes sp

^{2}C, and we are interested in how it affects clustering. Second, fluorination replaces at least 8% [27] of atoms in the powder with atoms which the nuclei have significantly different scattering lengths (−3.74 and 5.64 fm, for

^{1}H and

^{19}F, respectively).

#### 3.2. A Model of Discrete-Size Diamond Nanospheres

^{2}), non-C elements, scattering by pores, the difference between the shape of diamond cores of DNDs and ideal nanospheres, interference of scattering on neighbor DNDs, etc. In a sense, this simple model of clustering is analogous to expanding of a mathematical function in a series into simple basis functions. The amplitude of each term of the decomposition does not directly reflect the physical reality but allows analyzing the overall behavior of the investigated function. The effective mass of the powder, evaluated this way, is its useful characteristic. It yields the mass fraction of powder that is effectively involved in neutron scattering. The remaining mass is effectively involved in neutron loss, but does not participate in the scattering. The larger the effective mass, the greater the efficiency of the DND powder for neutron reflection is. For this particular F-DND sample and its conditioning used, the effective mass is ~63%; for DNDs, it is by the way nearly the same.

#### 3.3. SEM

#### 3.4. X-ray Powder Diffraction

^{2}C atoms onto DND cores do not contribute to the diffraction signal. Thus, we performed such a measurement to verify the effect of fluorination on diamond cores and estimate the mean size of the cores.

^{−3}. The four hundred and twenty-two powder diffraction line indicates the smallest spacing and allows the best sensitivity to the particle size effect to be achieved. The comparison of the lines for DNDs and F-DNDs shows that the coherent scattering regions, presumably coinciding with the DND cores, remain intact upon the fluorination (Figure 7).

## 4. Discussion

^{2}shell of the DND were almost completely eliminated [27]. The ratio of F and C atoms in F-DND is F/C = 0.09 [29]. The fluorination at least partially removes one more major impurity, and namely oxygen, in which the fraction can reach 10% [59]. The fraction of N may decrease, but it has not been studied quantitatively. The neutron activation analysis showed that the composition and amount of metallic impurities in powders did not change noticeably [27].

^{2}C, H, O, N, and other elements. The coherent scattering cross section of the DND is proportional to the square of the sum of corresponding scattering lengths of all nuclei in the DND. The scattering lengths of these elements are 5.65 fm on one side and 6.65, −3.74, 5.80, and 9.36 fm on the other one, respectively. The fractions of elements required for such a compensation are compatible with the existing experimental data. However, this compensation is not sufficient. The angular characteristics of neutron scattering are determined by the shape and size of the scatterers in the DND. The XRD data show that diamond cores do not change upon the fluorination. The SANS data show that the fluorination does not change the shape and characteristic sizes of DNDs as well taking into account the impurity layer on their surface.

^{H}

_{in.sc.}= 108 ± 2 b. This effect has led to a major reduction in neutron losses during neutron transport in powder, but to a much smaller effect on the results of SANS and a negligible effect on the results of XRD. The SANS data for DND and F-DND shown in Figure 3 are compatible with this statement. Incoherent scattering of neutrons by H explains the high scattering intensity I(Q) only at Q > 7·× 10

^{0}nm. This difference of the scattering curves for DND and F-DND is compatible with the reduction of H upon fluorination observed in reference [27].

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**(

**a**) A TEM image of the fluorinated nanodiamonds (F-DND) sample, (

**b**) a corresponding diameter distribution of the DNDs. The red dashed line corresponds to the lognormal distribution. The black solid line indicates a data fitting.

**Figure 2.**The probability density as a function of radius (nm) evaluated using neutron small-angle scattering (SANS) (red thick dashed line), X-ray small-angle scattering (SAXS) (black thin dash-double-dotted line), TEM (black thin dash-dotted line), and dynamic light scattering (DLS) (black thin solid line). The vertical black dotted line indicates a mean radius measured with XRD. All measurements are performed with F-DND samples, except for SAXS, which is performed with a DND sample. The details of the different measurements are given below in the text.

**Figure 3.**Measured intensity I (cm

^{−1}) of scattered neutrons as a function of the transferred momentum Q (cm

^{−1}) for DND and F-DND samples shown with black squares and red circles, respectively. For the convenience of comparing the results, the data are normalized to the equal sample mass.

**Figure 4.**Comparison of measured and simulated intensity I (cm

^{−1}) of neutron scattered as a function of the transferred momentum Q (cm

^{−1}) for the F-DND sample. Black squares denote the experimental data. The thin blue line shows the results of the simulation within the model of discrete-size diamond nanospheres, and the thick red line contains in addition the background intensity of 9·10

^{−3}nm

^{−1}.

**Figure 5.**The probability density as a function of radius (in nm) evaluated within the model of discrete-size diamond nanospheres for the F-DND sample. Red circles show the simulation results. The red solid line interpolates the simulation results.

**Figure 7.**Powder diffraction for DND (black lower line) and F-DND (red upper line, shifted in intensity for better visibility) samples in the proximity of a 422 Debye-Scherrer ring. Full width at half maximum (FWHM) from the Lorentz line shape fit were evaluated to 1.343 ± 0.018 and 1.354 ± 0.022 for DND and F-DND samples, respectively, therefore coinciding. The mean size of DND cores is 4.3 nm.

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Bosak, A.; Dideikin, A.; Dubois, M.; Ivankov, O.; Lychagin, E.; Muzychka, A.; Nekhaev, G.; Nesvizhevsky, V.; Nezvanov, A.; Schweins, R.;
et al. Fluorination of Diamond Nanoparticles in Slow Neutron Reflectors Does Not Destroy Their Crystalline Cores and Clustering While Decreasing Neutron Losses. *Materials* **2020**, *13*, 3337.
https://doi.org/10.3390/ma13153337

**AMA Style**

Bosak A, Dideikin A, Dubois M, Ivankov O, Lychagin E, Muzychka A, Nekhaev G, Nesvizhevsky V, Nezvanov A, Schweins R,
et al. Fluorination of Diamond Nanoparticles in Slow Neutron Reflectors Does Not Destroy Their Crystalline Cores and Clustering While Decreasing Neutron Losses. *Materials*. 2020; 13(15):3337.
https://doi.org/10.3390/ma13153337

**Chicago/Turabian Style**

Bosak, Alexei, Artur Dideikin, Marc Dubois, Oleksandr Ivankov, Egor Lychagin, Alexei Muzychka, Grigory Nekhaev, Valery Nesvizhevsky, Alexander Nezvanov, Ralf Schweins,
and et al. 2020. "Fluorination of Diamond Nanoparticles in Slow Neutron Reflectors Does Not Destroy Their Crystalline Cores and Clustering While Decreasing Neutron Losses" *Materials* 13, no. 15: 3337.
https://doi.org/10.3390/ma13153337