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Surface structure and properties of functionalized nanodiamonds: a first-principles study

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 22 (2011) 065706 (6pp)

doi:10.1088/0957-4484/22/6/065706

Surface structure and properties of functionalized nanodiamonds: a first-principles study Aditi Datta, Mesut Kirca, Yao Fu and Albert C To1 Department of Mechanical Engineering and Materials Science and Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA E-mail: [email protected]

Received 18 June 2010, in final form 15 December 2010 Published 7 January 2011 Online at stacks.iop.org/Nano/22/065706 Abstract The goal of this work is to gain fundamental understanding of the surface and internal structure of functionalized detonation nanodiamonds (NDs) using quantum mechanics based density functional theory (DFT) calculations. The unique structure of ND assists in the binding of different functional groups to its surface which in turn facilitates binding with drug molecules. The ability to comprehensively model the surface properties, as well as drug–ND interactions during functionalization, is a challenge and is the problem of our interest. First, the structure of NDs of technologically relevant size (∼5 nm) was optimized using classical mechanics based molecular mechanics simulations. Quantum mechanics based density functional theory (DFT) was then employed to analyse the properties of smaller relevant parts of the optimized cluster further to address the effect of functionalization on the stability of the cluster and reactivity at its surface. It is found that functionalization is preferred over reconstruction at the (100) surface and promotes graphitization in the (111) surface for NDs functionalized with the carbonyl oxygen (C=O) group. It is also seen that the edges of ND are the preferred sites for functionalization with the carboxyl group (–COOH) vis-`a-vis the corners of ND.

issue related to NDs as drug carriers is how the surface functionalization of NDs with different chemical groups affects the ND–drug interaction during the self-assembly process. On the other hand, surface functionalization may affect the structure and stability of NDs, which is controversial in itself. To date, there has been no conclusive evidence from experiments regarding the exact surface and overall structure of NDs. In the past, it was assumed that diamond nanoparticles were quasi-spherical in shape, but recent high resolution transmission electron microscope (HRTEM) images of a single nanodiamond cluster on the surface of a molybdenum tip have confirmed that nanodiamonds are polyhedral with distinct faceting [10, 11]. First-principles calculations by various groups have shown that the shape of the particles affects the stability and delamination (graphitization) of the surface [12–16]. Several likely candidates have been proposed which include cuboctahedron, truncated octahedron and octahedron [17–19]. The relaxed surface of each proposed shape is

1. Introduction Monodisperse nanometre-sized diamond particles or nanodiamonds (NDs) have recently attracted much interest from both experimentalists and theorists due to the wide spectrum of potential applications. One of the most recently recognized biomedical applications of NDs is in drug delivery devices [1–8]. The unique structure of ND assists in binding different functional groups to its surface which in turn facilitates binding with drug molecules. NDs also possess biocompatibility, very high surface-to-volume ratios, and stability both in vitro and in vivo [1, 9]. These novel properties contribute towards the multifunctional capabilities of NDs that are required for the efficient targeting of multiple disorders including cancer and inflammation. There have been several experimental efforts to functionalize NDs with different chemical groups [2, 3, 6, 7]. The existing unrequited 1 Author to whom any correspondence should be addressed.

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Nanotechnology 22 (2011) 065706

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largely covered by graphitic patches; for instance, an ND can have a fullerene-like surface and a diamond core as illustrated in figure 1. This proposed structure is consistent with several experimental findings and is called the bucky-diamond [4, 9]. Several oxygen containing chemical functional groups like hydroxyl, carboxylic, lactones, ketones and ethers are also found to be present at the surface of NDs [7–9] produced by detonation and post-synthetic treatments. The surface groups have been altered using various reaction chemistries reviewed in [5–8] including wet chemistry approaches as well as gas phase methods and atmospheric plasma treatment. But the exact binding energies of different functional groups and how the structure of NDs could be affected by adding the functional groups has not been addressed experimentally or theoretically. Another kind of ND considered in the literature is hydrogenated ND, whose surface is fully hydrogenated [20]. Theoretical studies on the stability of NDs show that dehydrogenated NDs become more stable than hydrogenated NDs when the size of the particle is larger than about 2.5 nm [13, 21, 22]. For dehydrogenated NDs, the hybridizations were identified as sp2 (graphite), sp2+x (where 0 < x < 1), and sp3 (diamond), whose spatial distribution is illustrated schematically in figure 1. It was also found that the sp2 (and sp2+x ) bonded atoms are localized on the (111) surfaces, but the method of determining the hybridization is empirical as it is based on the bond lengths [13]. A more refined approach is described in [23] which uses electronic charge density (ECD) as the input. But the ECD profile is difficult to obtain for large system sizes. Calculations on the electronic structure [24] of dehydrogenated NDs exhibit many states near the Fermi level, most of which come from the defects of the surface and interface. These localized defect states in the gap are believed to be originated not only due to quantum confinement but also surface effects. This is different from hydrogenated nanodiamond, where the size-dependent feature completely comes from the quantum confinement effects. The different surface molecular structures on a dehydrogenated and non-functionalized ND give rise to the surface electrostatics. Another first-principles calculation of dehydrogenated ND predicts that the (100) surfaces and the (100)/(111) edges exhibit a strong positive potential, whereas some of the graphitized (111) surfaces exhibit a strongly negative potential [17, 18]. The surface properties of NDs functionalized with chemical functional groups have not been investigated theoretically or experimentally yet, but they may affect the surface electrostatics significantly. Of particular interest are the carbonyl oxygen (=O) and carboxyl acid (–COOH) functional groups inherent in the ND primary particle production process. Although there have been calculations on nonfunctionalized NDs in the literature, the relaxed structure of NDs functionalized with different functional groups and their binding energies have not been predicted yet; this is investigated in detail in this paper using a quantum mechanics (QM) based first-principles multiscale approach. First, the structure of NDs of experimentally realistic size (∼4 nm) is optimized using classical mechanics based molecular mechanics (MM) simulations. QM based density functional

Figure 1. Schematic representation of the possible hybridizations in ND [16]. (This figure is in colour only in the electronic version)

theory (DFT) is then employed to simulate the structure and analyse the properties of relevant parts of the optimized cluster further to address the issues of stability of ND, hybridization, and reactivity of the surface. This work is extended to NDs functionalized with carboxylic acid (–COOH) and carbonyl oxygen (=O), their binding energies, and the effects they have on the stability of NDs.

2. Materials and methods A model of ND with truncated octahedral geometry having eight (111) faces and six (100) faces is constructed, since this shape has the lowest energy known to date [17]. It is anticipated that DFT calculations are unable to reach the realistic size of NDs (∼4 nm), and hence molecular mechanics (MM) calculation utilizing the empirical second generation adaptive intermolecular reactive empirical bond order (AIREBO) potential [25] is employed to optimize the geometry of the large ND particle (∼4 nm). The optimization of the truncated octahedral shaped ND by MM is performed using the widely used code LAMMPS (large-scale atomistic/molecular massively parallel simulator) developed by the DOE Sandia National Laboratories [26], and the MM optimized ND is shown in figure 2. After MM optimization, small clusters of carbon atoms (∼50 carbon atoms) are extracted from the MM optimized structures at a corner (see figure 2), which is surrounded by the largest number of dangling bonds in ND and thus is the preferred site for functionalization. The number of atoms is kept small enough to be handled by DFT. The small clusters are employed to study functionalization with the carbonyl group using one of the most accurate DFT functional and basis sets, B3LYP/6−311++G(d, p), available in GAMESS [27, 28]. GAMESS is well tested and benchmarked to run in parallel for a number of systems. The code uses a linear combination of atomic orbitals (LCAO) approach in the DFT routine, which makes it capable of predicting and rationalizing properties of highly anisotropic systems such as the NDs under consideration in this work. 2


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minimize the effect of bond termination artificially generated by this truncation. DFT optimization is carried out to study the structural evolution and the results are shown in figure 3. It can be seen from figures 3(a) and (b) that the (100) surface is heavily reconstructed. The atoms having unsaturated double bonds in the (100) face of the cluster were reduced from six to two in the optimized structure due to reconstruction. The only atoms to remain unchanged in terms of bond saturation are #42 (the apex of the tetrahedron/corner of the ND) and #45 (at the edge). It seems that the bonds of atom #45 would also have been saturated if the entire ND without the truncation was optimized. The atoms involved in the reconstruction are highlighted by blue circles in figure 3. It can be seen that the pair of atoms #38 and #40 as well as the pair #41 and #44 are re-bonded during the course of the optimization process and their dangling bonds reduce from two to one per atom. It can also be seen that the distance between the pair of atoms ˚ before optimization reduces to #38 and #40 which was 2.53 A ˚ after the relaxation. A similar outcome is obtained with 1.57 A atoms #41 and #44. The distances between the atoms before ˚ and 1.404 A, ˚ respectively. and after relaxation are 2.62 A The existing bonds between atom #38 and atoms #28 and #29 shrink slightly and the bond angles go up by another degree or two (more in the case of atom #44) to accommodate this re-bonding. On the other hand, minimal reconstruction occurs in the (111) plane. Therefore, it is concluded from this optimization that in a continuous structure/full cluster of non-functionalized ND, reconstruction in the (100) surface will occur, but, even after the reconstruction, the corners remain the active sites for functionalization since the bonds remain unsaturated after relaxation (atom #42 in figure 3). The graphitization/delamination process that might be happening in the (111) surface is not observed. The surface functionalization process is studied next by adding carbonyl oxygen to the dangling double bonds of the (100) face (see figure 4) and the binding energy of oxygen is calculated. The binding energy (BE) is defined as BE = |E pyramid+6O − E pyramid − 6 E O |, where E X is the total energy of the structure X obtained by DFT calculations. The reference energy of oxygen (O) is taken as 1/2 of the energy of the O2 molecule in its most stable triplet state. The BE for carbonyl oxygen is found to be 3.199 eV/atom. It is also found that

Figure 2. Schematic of the entire ND cluster of 4.8 nm size. The pyramid cluster is cut above the yellow plane for first-principles DFT analysis. The result is shown in figure 3.

3. Results and discussion The fabrication process of the NDs leads to the introduction of different functional groups on their surface. These functional groups, which likely include carboxyl acid and carbonyl oxygen, are responsible for the surface interactions of the NDs. The aim is to predict the surface structure of NDs whose surface is functionalized with carbonyl oxygen groups (=O) and to determine the energetics of the functionalization. In other words, the energy (binding energy) necessary to attach functional groups to the ND structure and their preference (if any) to attach to the ND surface are determined by DFT calculations. The irregular tetrahedral structure containing 46 carbon atoms cut from the corner of a 4.8 nm ND is shown in figure 3 in detail. The vertex of the pyramid is a corner of ND and the side faces contain one (100) and two (111) surfaces of an ND with the geometry shown in figure 3. The dangling bonds created by the cut at the base of the tetrahedron are saturated with hydrogen atoms. These H atoms as well as the C atoms at the base are held fixed during the structural optimization to

Figure 3. Schematic of the pyramid cluster (a) before optimization, and (b) after optimization, showing the reconstructed atoms (circled in blue) in the (100) plane. Colour scheme: green = carbon, grey = hydrogen.

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Figure 4. Schematic of the pyramid with carbonyl oxygen atoms: (a) before optimization and (b) after optimization, showing that no reconstruction happened in the (100) due to the saturation of dangling bonds with binding to oxygen atoms. Delamination (shown by the blue circle) due to graphitization occurs at (111). Colour scheme: green = carbon, red = oxygen.

the total energy of the ND cluster with oxygen is ∼6.4 eV lower than that without the oxygen. This result indicates that functionalization is energetically preferred in NDs. This seems understandable since all dangling bonds are saturated with binding to O atoms. For an analogous reason, there is no reconstruction in the (100) oxygen atoms figure 4(b). The atoms which were involved in the reconstruction for a nonfunctionalized ND move in the opposite direction (the effect is most prominent in the case of atom #44) to that in the case of figure 3(b). On the other hand, the (111) surface becomes flat (graphite-like). If atoms #20 and #13 (shown by blue circles in figures 4(a) and (b)) are observed more closely, it can be seen that the bond between them is broken after the optimization. This is an indication of the onset of the graphitization process on the (111) surface as the surface atom (#20) gets delaminated from the core atom (#13). It is expected from the DFT calculations of these ND clusters that the carboxyl groups (–COOH) are likely to attach to the edges and vertices/corners of the NDs since free dangling bonds are more likely to exist and react with these groups. On the other hand, carbonyl oxygens (=O) are likely to attach to the (100) faces of the ND because each carbon atom has two free bonds. A truncated octahedral structure of an ND particle considered here has a total of 24 corners and 36 edges altogether. Due to symmetry, this particular structure would have a unique corner bounded by two (111) and one (100) faces and have two different kinds of edges, (100)/(111) and (111)/(111). The (100)/(111) edge is the most relevant to study since we can not only study both the (100) and (111) faces but also the (100)/(111) edge accumulates electrostatic charge as shown by Barnard et al [18]. The edge geometry shown in figure 5 will be considered in order to study the BE of (=O) and (–COOH) at this edge. The result of the edge relaxation is shown in figure 6(b). It can be noted that the (100) surface is functionalized with carbonyl oxygen (=O) atoms since it yields better stability (the energy difference is ∼6.4 eV) to ND as shown in previous calculations. The edge atom #18 has relaxed the most and moves towards the surface atoms in the (100) plane. This edge geometry is then functionalized with carboxyl (–COOH) and carbonyl

Figure 5. The ND cluster near the (100)/(111) edge cut above the yellow plane is employed for first-principles DFT analysis. The result is shown in figure 6.

oxygen (=O) groups as shown in figure 7(a). The relaxed geometry is shown in figure 7(b). As explained before the (100) face is capped with oxygen atoms. The two (–COOH) groups reorient considerably during the relaxation and the carbon atom containing (=O) orients towards the (100) face as seen in 7(b). The BE of the carboxyl group as calculated from the optimized structure of figure 7(b) is found to be 3.15 eV. The magnitude of the BE is slightly lower than for the (=O) at the corner (3.199 eV/atom). On the other hand the carboxyl atom at the corner is found not to be stable in the ND cluster since the optimized structure could not be obtained for that geometry. The results obtained by means of this calculation (e.g., reconstruction of the (100) face and delamination of the (111) face) are consistent with the available theoretical and experimental results [6, 17, 18]. The binding energy calculations confirm that (=O) and (–COOH) addition stabilizes the ND structure and explains the hydrophilic nature of the ND surface which encourages the functional groups to adhere. Our results thus provide assistance on further control of surface properties of NDs. 4


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Figure 6. Schematic of the (100)/(111) edge with the (100) face capped with carbonyl oxygen atoms: (a) before relaxation and (b) after relaxation. Colour scheme: green = carbon, red = oxygen.

4. Conclusions In summary, the structure and stability of truncated octahedral nanodiamond (ND) clusters and the effect of functionalization are studied using a quantum mechanics based multiscale method. The following attributes have been found.

Figure 7. Schematic of the (100)/(111) edge with the (100) face capped with carbonyl oxygen atoms and two carboxyl group attached at the edge: (a) before relaxation and (b) after relaxation. Colour scheme: green = carbon, red = oxygen, grey = hydrogen.

(1) The (100) surface of an ND experiences (2 × 1) reconstruction to minimize the energy. (2) Binding energy calculations show that it is more energetically favourable for a pyramid like cluster containing a corner of ND to be in a state where carbonyl oxygen (=O) is bonded to the surface carbon atoms. (3) Functionalization with carbonyl groups is preferred over reconstruction in the (100) surface and promotes graphitization on the (111) surface since the (111) surface atoms are delaminated from the core atoms. (4) The edges of ND are the preferred sites for functionalization with carboxyl groups (–COOH) vis-`a-vis the corners of ND.

Acknowledgment The financial support from the Central Research Development Fund (CRDF) at the University of Pittsburgh is gratefully acknowledged.

References [1] Huang H, Pierstorff E, Osawa E and Ho D 2007 Nano Lett. 7 3305 [2] Shenderova O A, Zhirnov V V and Brenner D W 2002 Crit. Rev. Solid State Mater. Sci. 27 227 [3] Shenderova O and Gruen D (ed) 2006 Ultrananocrystalline Diamond (Norwich, NY: William-Andrew) [4] Baidakova M and Vul’ A 2007 J. Phys. D: Appl. Phys. 40 1 [5] Krueger A 2008 Chem.—Eur. J. 14 1382 [6] Schrand A M, Ciftan Hens S A and Shenderova O A 2009 Crit. Rev. Solid State Mater. Sci. 34 18 [7] Ho D (ed) 2010 Nanodiamonds: Applications in Biology and Nanoscale Medicine (Berlin: Springer) [8] Vul’ A et al 2009 Detonation nanodiamonds: technology, properties and applications in nanosciences and nanotechnologies EOLSS: Encyclopedia of Life Support Systems ed V N Kharkin, C Bai and S-C Kim, Oxford, UK http://www.eolss.net ¯ [9] Osawa E 2008 Pure Appl. Chem. 80 1365 [10] Tyler T, Zhirnov V, Kvit A V, Kang D and Hren J J 2003 Appl. Phys. Lett. 82 2904 [11] Kuznetsov V L et al 2004 Carbon 42 1057 [12] Raty J Y, Galli G, Bostedt C, Van Buuren T and Terminello L 2003 Phys. Rev. Lett. 90 037401

This work helps in understanding the functionalization process of an ND and the possible sites on the ND surface to be functionalized with carbonyl oxygen (=O) and carboxyl (–COOH) groups, which in turn help to attach drug molecules. However, the work carried out here corresponds to the situation occurring in the gas phase under low pressure (vacuum) whereas experimentally a majority of chemical reaction is carried out in solution. Since the unique structure and bonding (graphitization) of the NDs at the surface generate an electrostatic potential by accumulation of charges and the functional groups should alter the electrostatics of nonfunctionalized NDs (at least in a local neighbourhood), it is important to examine the surface electrostatics of ND in solution. The next step towards the successful design of a drug delivery device is to study the interaction between the ND and the solution molecules in different pH environments to determine the pH effects on the surface electrostatics of NDs. 5


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[23] Barnard A S, Russo S P and Snook I K 2005 J. Comput. Theor. Nanosci. 2 68 [24] Wang C, Zheng B, Zheng W T and Jiang Q 2008 Diamond Relat. Mater. 17 204 [25] Stuart S J, Tutein A B and Harrison J A 2000 J. Chem. Phys. 112 6472 [26] Plimpton S J 1995 J. Comput. Phys. 117 1 http://lammps. sandia.gov [27] Schmidt M W et al 1993 J. Comput. Chem. 14 1347 [28] Gordon M S and Schmidt M W 2005 Advances in electronic structure theory: gamess a decade later Theory and Applications of Computational Chemistry: The First Forty Years ed C E Dykstra, G Frenking, K S Kim and G E Scuseria (Amsterdam: Elsevier) p 1167 http://www. msg.chem.iastate.edu/GAMESS/GAMESS.html

[13] Barnard A S, Russo S P and Snook I K 2003 Diamond Relat. Mater. 12 1867 [14] Park N, Park S, Hwang N M, Ihm J, Tejima S and Nakamura H 2004 Phys. Rev. B 69 195411 [15] Kwon S J and Park J-G 2007 J. Phys.: Condens. Matter 19 386215 [16] Barnard A S, Russo S P and Snook I K 2003 Phil. Mag. Lett. 83 39 [17] Barnard A S and Sternberg M 2007 J. Mater. Chem. 17 4811 [18] Barnard A S 2008 J. Mater. Chem. 18 4038 [19] Barnard A S and Zapol P 2004 J. Chem. Phys. 121 4276 [20] Raty J Y and Galli G 2005 Comput. Phys. Commun. 169 14 [21] Raty J Y and Galli G 2003 Nat. Mater. 2 792 ¯ [22] Osawa E, Ho D, Huang H, Korobov M V and Rozhkova N N 2009 Diamond Relat. Mater. 18 904

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