Effect of Surface Termination on Diamond (100) Surface

Article pubs.acs.org/JPCC

Effect of Surface Termination on Diamond (100) Surface Electrochemistry Mir M. Hassan and Karin Larsson* Department of Chemistry−Ångström Laboratory, Uppsala University Box 538, 751 21 Uppsala, Sweden ABSTRACT: The combined effect of water adlayer composition and surface termination on diamond surface electrochemistry, has been studied theoretically using Density Functional Theory (DFT) calculations. The terminating species included H, O(ontop), O(bridge), OH and NH2. The chemical composition of the water adlayer was altered by using a very thin layer of water only, or by introducing oxygen, ozone or hydroxonium ions (H3O+) into the adlayer. A partial electron transfer toward the atmospheric adlayer was observed for the situation with either an H- or NH2-terminated diamond surface. Corresponding calculations for oxygen-termination (O(ontop) or O(bridge)), did not render any significant amount of electron transfer. The situation was completely different for the situation with OH-termination. The degree of electron transfer was approximately of the same order as for H- and NH2-terminations. The presence of oxidative species like oxygen, ozone, and H30+ (or combinations thereof) were observed to significantly increase the degree of electron transfer for the situation with either NH2-, OH-, or H-terminated diamond (100)-2 × 1 surfaces. Adsorption energy calculations revealed, with some exceptions, a quite good correlation between diamond//adlayer adhesion strength and degree of interfacial electron transfer. The electron transfer process were further verified and analyzed by performing partial density of state (pDOS) calculations for some selected diamond//adlayer systems. possible to use diamond thin films in high-power and highfrequency field-effect transistor (FET) applications.5 The induced surface conductivity, by specific surface termination of the diamond film, is basically an induced surface doping phenomenon. Landstrass and Ravi showed that both natural diamond and CVD grown diamond films show an increase in charge conductivity of several orders of magnitude when subjected to a hydrogen plasma.6 This observation has later on been verified by other groups.7 The explanation to this enhanced conductivity was not clear in the beginning. A hypothetical acceptor-like surface state was proposed by Kawarada et al.,8 whereas Hayasi et al.9 proposed very shallow hydrogen induced subsurface acceptor state inside diamond. Later, Takeuchi et al.10 and Ristein et al.11 continued to investigate the location of the acceptors. Though their result could be explained by assuming acceptor-states on the surface, their results showed a very unrealistic high surface defect density and a very sharp subsurface acceptor level. Another mechanism, including an atmospheric environment, was proposed by Maier et al.12 Supporting result were shown by Ri et al.,13 which were later confirmed by Foord et al.14 According to this proposed mechanism, an H-terminated diamond surface requires an atmospheric adlayer for p-type surface conductivity to occur. Ristein et al.11 proposed that electron transfer takes place from the upper part of the valence band to an oxidative species in the physisorbed adsorbate. This

1. INTRODUCTION Diamond is nowadays intensively studied because of its huge potential for different applications (i.e., microelectronics, renewable energy, and biotechnology). Some underlying causes to this renewed interest for diamond are the possibility to grow polycrystalline diamond with nanodimensions (so-called nanocrystalline diamond − NCD), and the possibility to functionalize the surface for a desired surface reactivity and property.1 However, what make diamond so promising are the combinations of these new skills with its unique material properties (e.g., wide band gap, large electrochemical potential window, high thermal conductivity, low frictional coefficient, extreme hardness, biocompatible, and high carrier mobility).1 Even though diamond is a wide band gap material, the material can have different energy states by selective doping and/or by modifying the surface (e.g., by surface termination). Thin film diamond has shown to have prominent possibilities as an electrode material for environmental and energy applications, as well as for sensor application and for quantitative analysis of various compounds.2,3 The high sensitivity of diamond toward red/ox reactions, together with its exceptional large potential window, will increase its potential for pH sensor applications.4 Intrinsic diamond is an insulator, whereas B-doped (of ptype) and N-doped (of n-type) diamond gives surface states in the band gap which increases the electronic conductivity within the bulk material. Different types of surface termination species have been shown to change the properties of the diamond surface region (e.g., by inducing surface electronic conductivity and interfacial charge transfer properties). It has already shown © 2014 American Chemical Society

Received: January 20, 2014 Revised: September 10, 2014 Published: September 11, 2014 22995

dx.doi.org/10.1021/jp500685q | J. Phys. Chem. C 2014, 118, 22995−23002

The Journal of Physical Chemistry C

Article

numbers of C atomic layers are adequate to use in studying surface processes in general. Here the lower surface C atoms were saturated by hydrogen atoms, with the purpose of maintaining the proper sp3 hybridization. The top layer of the surfaces was 100% terminated with (i) Oontop, (ii) Obridge, (iii) H, (iv) NH2, or (v) OH species. The bottom C layer, with its binding H species, was fixed to simulate a continuation of bulk diamond. All other layers were allowed to freely relax. In the geometry optimization process, the tolerances for total energy, force, stress and displacement were set to 2.0 e−005 eV/atom, 0.05 eV/ Å, 0.10 GPa, and 0.002 Å, respectively. 2.3. Water Adlayer. Within the supercell, the attached water adlayer was either genuine (i), or it also contained (ii) one oxygen molecule (O2), (iii) one hydroxonium ion (H3O+) plus one O2, (iv) one ozone molecule (O3), or (v) one hydroxonium ion plus one O3. All of these added species are oxidative ones of different strengths. Each of these oxidative species (O2, O3, and H3O+) was initially, and individually, positioned in the vicinity to the diamond surface. The example with O3 added to a thin water adlayer, can be seen in Figure 1.

type of charge transfer can be regarded as an electrochemical process where the upper diamond will be oxidized and the adlayer species reduced. This mechanism has recently been supported and further explained in some theoretical investigations by Larsson et al.15 The effect of both various surface termination species (H, Oontop, Obridge, and NH2), as well as of various oxidative species in the adlayer (water, water and oxygen, water + H30+ + oxygen, water + ozone, water + H3O+ + ozone), were then focused upon. The main goal with the present theoretical (using DFT) study is to continue from the earlier investigations15 by studying electron transfer between a terminated diamond surface and an attached water-based adlayer. In the present study, the supercell contains 4 C layers with 16 carbon atoms in each layer. The adlayer contains nine water molecules, and at most two additional types of species: I (H2O), II (O2, H2O), III (O2, H3O+, H2O), IV (O3, H2O), and V (O3, H3O+, H2O). The main purpose with the present study was to determine which type of surface termination species that will give the best electron transfer to an atmospheric adlayer.

2. THEORETICAL APPROACH 2.1. Methods. First-principle DFT calculations were performed under periodic boundary conditions, using the Cambridge Sequential Total Energy Package (CASTEP) program that is included within the Material Studio program package.16 The calculations were based on ultrasoft pseudo potentials (describing the inner atomic cores), and with a plane wave description for the outer valence electrons (with an energy cut off value of 300 eV, which has been proven appropriate to use by checking total energy convergence). The generalized gradient approximation (GGA) was used for the exchange and correlation effect in the geometry optimization calculations. Broyden−Fletcher−Goldfarb−Sharmo (BFGS) method (algorithm) was used to progress the geometry optimization.17 The GGA method is a DFT method where the electrons are situated in a gradient electric field caused by the other electrons in the system. The GGA-PBE (Perdew− Burke−Ernzerhof) version was used in the present study for further single point calculations.18 Previous test-calculations showed that an increase in number of k-points to a 3 × 4 × 2 mesh (yielding 12 symmetry unique k-points) yields no significantly different result to the electron transfer (∼2%).19 Hence, the 2 × 2 × 1 k-point mesh was taken as optimal to use for all calculations in the present study. The atomic charges, within both the surface and the adlayer, were calculated using Mulliken20,21 population analysis, which was performed using a projection of the plane wave states onto the localized basis. 2.2. Models. The supercell used for modeling the diamond (100)-2 × 1 surface was constructed by four layers of carbon, where each layer contained 16 C atoms. To suppress the artificial charge transfer between the two polar ends of the resulting slabs in the z-direction, the dangling bonds on the lower surfaces of the slabs were saturated with H atoms, and a large vacuum distance between the slabs was used (∼12 Å). Thorough test-calculations have previously been performed for the present system, in which (i) the number of geometry optimized C layers, (ii) vacuum depth, and (iii) supercell edges x and y were allowed to vary.15,19 The effect of these parameters on the geometric structures and atomic charges (describing the degree of electron transfer) were then especially studied. Other publications by the present authors have shown that these

Figure 1. Periodic model of an H-terminated diamond (100)-2×1 surface with an attached atmospheric adlayer containing nine water molecules and one ozone molecule: (a) side view, and (b) top view.

As presented above, all adlayer atoms were allowed to move freely in order to find a minimum energy on the potential energy curve, where the individual molecules experience the strongest interaction with the surface. An ozone molecule can have two different structural forms: a bent, or a cyclic one. The energy that has to be overcome when going from the bent to the cyclic structure is 35.8 kcal/mol.22 The bent structure is, hence, energetically the most favorable one, and was therefore chosen to be used in the present study. 2.4. Adhesion Energy Calculation. The adhesion energy for the attached atmospheric adlayer to the diamond surface will generally give information about adhesion strength and type of bond formations within the interface. The diamond// adlayer systems that show a significant amount of electron transfer are moreover expected to have a stronger interaction of an electrostatic type. The adhesion energy for an attached adlayer onto the terminated diamond (100)-2×1 surface can be calculated using ΔEadhesion = Esurface//adlayer − Esurface − Eadlayer

(1)

where Esuface//adlayer, Esurface, and Eadlayer are the total energies for the whole interface, the diamond surface only, and the adlayer, respectively. After the geometry optimization calculation, the final structures were used for fine single point calculations in 22996



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estimating the energy of the whole system, diamond film, and adlayer, respectively. 2.5. Electronic Transfer over the Diamond//Atmospheric Adlayer Interface. The combination of (i) diamond surface termination, (ii) atmospheric adlayer chemical composition, and (iii) geometrical structures are decisive for the surface electronic states, as well as for the molecular orbitals states of the adlayer. In turn, these parameters will strongly affect the tendency for chemical interactions and electron transfer across the diamond//adlayer interface. The degree of electron transfer over the interface can be identified by performing various theoretical analysis calculations. In the present study, electron population analyses have been used by projecting the electron density on atomic orbitals in a Mulliken atomic approach.23 The individual atomic charges could thereby be estimated from which information about eventual partial electron transfer over the interfaces could be obtained. The charge transfer has been estimated by making a sum of the atomic charges within the adlayer, and relating this value to the value that we knew we started with. For instance, a neutral water adlayer has a zero total charge. If we add hydroxonium, it will be +1.

different ways: either as on top of each individual surface C atom, or by forming a bridge between two surface C atoms. Moreover, these bridge formations can be formed by either (i) connecting the carbon atoms within, and between, the C−C pair (from here on called the OB direction), or by (ii) connecting the C−C pairs along the carbon dimer row (from here on called the OA direction). Since all of these three different surface arrangements show the same number of atoms of each kind, a picture of relative structural stability of the surface can be found by just calculating the total energy of the systems. As can be seen in Table 1, the most stable oxygen Table 1. Total Energies for Diamond Surfaces Being Terminated with Oontop, Obridge, NH2, and H, Respectively

3. RESULTS AND DISCUSSION 3.1. General. The ordinary atmosphere most usually contains species like water and oxygen. A diamond surface put in close contact with the atmosphere is expected to be covered by a very thin water adlayer containing these types of oxidative species. The chemical composition of this water adlayer will in turn most probably affect the diamond surface properties. In a previous study, when investigating the Hterminated diamond (100) surface with an attached water adlayer, a supercell was used containing water molecules in addition to at most two other oxidative species I (H2O), II (H2O, H3O+), III (O2, H2O), IV (O2, H3O+, H2O), V (O3, H2O), and VI (O3, H3O+, H2O).16 The present study includes the following adlayer combinations: I (H2O), II (O2, H2O), III (O2, H3O+, H2O), IV (O3, H2O), and V (O3, H3O+, H2O). In the earlier study, the diamond 100 surface was only Hterminated. In the present investigation, the diamond (100) surface has been terminated with H, Oontop, Obridge, NH2, and OH, respectively. Earlier experimental and theoretical investigations have shown that H-terminated diamond surfaces will not undergo any electron transfer toward a neutral water adlayer.15 However, the presence of oxidizing species like O2, O3, and H3O+ (in the water adlayer), will make an electron transfer possible.15 An electron transfer is generally expected to take place from the highest occupied molecular orbital (HOMO) level of one molecule, to the lowest unoccupied molecular orbital (LUMO) level of another molecule. The prerequisite is that (i) the energy levels of the HOMO and LUMO levels are similar, and (ii) there are overlapping HOMO and LUMO orbitals. Some exceptions to this trend may occur when studying the possibility for electron transfer between a surface and an adsorbed molecule (where the position of Fermi level will be decisive). A proposed theory states that the diamond surface will behave as the donor substrate (of p-type), which means that an electron from the upper valence edge of the diamond surface can move to the LUMO level of the adlayer (i.e., atmospheric adsorbates).15,24−28 3.2. Structural Geometry and Energetic Stability. The diamond surface was terminated with oxygen species in two

termination type

total energy (eV)

Oontop Obridge(OA) Obridge(OB) (NH2)ontop (NH2)bridge(OB) H OH

−17315.4 −17332.2 −17350.3 −15168.8 −15165.5 −10618.1 −17608.0

arrangement was observed for the OB bridge situation (i.e., with the oxygen atom connecting the carbon atoms within, and between, the C−C pairs perpendicular to the carbon dimer row), with a total energy of −17350.3 eV). The corresponding total energy value for the OA bridge situation was found to be −17332.2 eV. Moreover, the Oontop-terminated diamond (100) surface showed the least stable O termination scenario, with the highest total energy value of −17315.4 eV. For an oxygen atom in bridge formation along the OA direction, the O binding carbon atoms have to change their hybridization from the initial sp3 one. This surface restructuring will induce geometrical constraints and, hence, energetically destabilize the formation of this specific bridge formation (see Figure 2a). On the other hand, for an oxygen atom in bridge formation along the OB direction, no change in surface C sp3 hybridization was observed (see Figure 2b).

Figure 2. Diamond (100) surfaces terminated with (a) O in bridge position along a C−C dimer row (of type OA), and with (b) O in bridge position connecting the C atoms perpendicular to the carbon dimer row (of type OB).

When the geometry of the initially 100% Oontop-terminated diamond (100) surface was optimized, 50% of the oxygen atoms were observed to move their position to the energetically more favorable Obridge positions of type OB (see Figure 3). It should here be stressed that when initially positioning oxygen in the bridging OB position, all atoms in these positions during the geometry optimization. Hence, these two surface structures represent two different potential energy minima (the lower one 22997



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made by projecting the electron density on atomic orbitals in a Mulliken atomic approach.20,21 The individual atomic charges could thereby be estimated, from which information about eventual partial electron transfer over the interfaces was obtained. The degree of electron transfer over the diamond (100)// atmospheric adlayer interface has in the present investigation been calculated for both (initially) Oontop- and Obridgeterminated diamond (100)-2×1 surfaces (see Table 2). These

Figure 3. (a) An initial 100% Oontop-termination situation, and (b) the resulting geometry optimized termination structure.

Table 2. Partial Electron Transfer from the Terminated Diamond (100)-2×1 Surface, to Various Water-Based Adlayers (The Unit Is e)

for the 100% OB bridging, and the higher on for 50% Oontop+ 50% OB bridging), with an energy barrier separating them. A similar study was performed for NH2-termination of the diamond surface (Table 1). The ontop position of NH2 was found to give a somewhat more stable surface structure (with a total energy of −15168.8 eV), compared to the adsorption of NH2 in bridge positions within, and in between, the C−C dimer rows (with a total energy of −15165.5 eV). So, NH2 species chemisorbed in on top positions are about 0.2 eV more stable per NH2 adsorbate, compared with the situation of a diamond surface that has initially been terminated with NH2 groups in OB bridge positions. However, here it is quite obvious that this initial 100% termination with NH2 in OB positions will not stay like that in the course of the geometry optimization calculations. As can be seen in Figure 4a, the NH2 adsorbates will partially move over to ontop positions, or become desorbed from the

adlayer surface termination

water

water + O2

water + O2 + H3O+

water + O3

water + O3 + H3O+

Oontop Obridge (OB) H NH2(ontop) OH

0.19 0.11 0.08 0.03 0.10

−0.67 −0.07 0.21 1.21 1.15

0.22 −0.02 1.70 1.76 1.44

−0.21 0.06 0.81 1.04 0.69

0.17 0.40 0.92 1.72 1.22

terminated surfaces were found to show very similar results in that an insignificant degree of electron transfer occurs between the surface and the adlayer. For an adlayer containing only water (type (i)), the two different O-terminating systems show an electron transfer of 0.19 (Oontop) and 0.11 (Obridge), respectively, from the diamond surface to the water adlayer. These values correspond to 0.01 and 0.03 electrons per surface C, and can both be regarded as within the error of limit for the method used. For an adlayer containing O2 (type (ii)), an electron transfer of −0.67e (Oontop-termination) and −0.07e (Obridge-termination) was observed (the negative sign indicates that the transfer takes place from the attached adlayer to the Odiamond surface). The value for the Obridge-termination is also here within the error of limit for the method used. When adding H3O+ to the type (ii) systemending up in type (iii) (i.e., in the presence of both O2 and H3O+ in the water adlayer)a similar pattern was observed. The electron transfer for the Oontop-termination was observed to take place in the direction from the surface to the adlayer (0.22 vs 0.02). As compared with the situation of only O2 in the water adlayer, is obvious that the coexistence of H3O+ in the adlayer will induce an extra electron transfer in the opposite direction (compared to O2), with a numerical value of about 0.89 e. This is a result that is perfectly supported by earlier theoretical calculations.15,19 The water adlayers of type (iv) (i.e., O3 in the water adlayer), and type (v) (i.e., O3 and H3O+ in the water adlayer) show a similar pattern to the corresponding O2 situations (of type (ii) and (iii)). The numerical values, without any H3O+ species in the adlayers, are for Oontop-termination: type (ii) −0.67 vs type (iv) −0.21. For Obridge-termination, it is type (ii) −0.07 vs type (iv) 0.06. The numerical values with H3O+ species in the adlayer are for Oontop-termination: type (ii) 0.22 vs type (iv) −0.02. For Obridge-termination, it is type (ii) 0.17 vs type (iv) 0.40. There is, hence, a discrepancy found when comparing the situations with hydroxonium in the adlayer, but with different oxygen species. Ozone and hydroxonium will induce a larger electron transfer (compared to O2) from the diamond surface to the adlayer (0.40 vs −0.02). This is, however, expected since

Figure 4. Diamond (100)-2×1 reconstructed surfaces being initially 100% terminated with NH2 in (a) bridge (of type OB), and (b) on top positions.

surface in the form of NH2 molecules or H atoms. However, since NH2-termination in the on top position (Figure 4b) was found to be the energetically more stable one, it was also selected as a model in the remainder of the study. 3.3. Electronic Transfer over the Diamond//Atmospheric Adlayer Interface. Initially, all molecules within the atmospheric adlayer, with an exception for the hydroxonium ion (H3O+), were modeled strictly neutral. As a result of the geometry optimization procedure, the adlayer geometrical structure was changed toward a more realistic situation for a diamond surface in close contact with a very thin atmospheric adlayer. The combination of (i) diamond surface termination, (ii) adlayer chemical composition, and (iii) geometrical structures are decisive for the surface electronic states, as well as for the molecular orbitals states of the adlayer. In turn, these parameters will strongly affect the tendency for chemical interactions and electron transfer across the diamond//adlayer interface. The degree of electron transfer over the interface can be identified by performing various theoretical analysis calculations. Electron population analyses have here been 22998



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transfer from the diamond (100) surface to the water-based adlayer. This is also the situation for the neutral water adlayers being attached to H-, OH-, or NH2-terminated diamond (100) surfaces. However, the presence of the oxidative species O2, O3, and H3O+, respectively, are proven to induce measurable partial electron transfer from the diamond surface to the atmospheric adlayer. As has earlier been shown in several experimental and theoretical investigations, the presence of H3O+ in the adlayer will improve this electron transfer. 3.4. Adhesion Energies for the Attachment of Atmospheric Adlayers. The calculations of the adhesion processes for the attached atmospheric adlayer to the diamond surfaces will generally give an indication of the bond strengths, and eventually also of the type of interactions between the surface and the adlayer. Those systems that show a significant amount of electron transfer are expected to experience a stronger interaction of an electrostatic type. On the other hand, a smaller (or insignificant) degree of electron transfer over the interface must have a weak electrostatic binding across the interface. The adhesion energies between the surface and the respective adlayers have been calculated as the difference of the total energy of the interfacial system and the individual energies of the adlayer and the surface, respectively (see eq 1). The water-based adlayer adhesion energies for various diamond (100) surface termination types are shown in Table 3. It is clear that only H- and NH2-termination will render a

ozone is a much stronger oxidative species compared to the oxygen molecule. For the H- and NH2-terminated diamond surfaces, the degree of electron transfer from the diamond (100) surface to the adlayer was, with an exception for a neutral water as an adlayer, found to be much more pronounced (see Table 2). The presence of adlayer oxidizing agents, like O2 and O3, was found to increase the degree of electron transfer (from the surface to the adlayer). As was also the situation with Oontopand Obridge-termination, the addition of H3O+ ions to the adlayer was found to enhance the electron transfer even more. The degree of electron transfer between the H-terminated diamond surface and the neutral water adlayer (type (i)) was, however, found to be negligible (only 0.08). The degree of electron transfer increased to 0.21 in the presence of O2 (type (ii)) within the adlayer. Addition of H3O+ (reaching type (iii) was found to enhance the charge transfer even more (1.70). As was the situation with O2 within the adlayer, the addition of O3 (type (iv) was found to result in a partial electron transfer of 0.81, which was further enhanced to 0.92 when adding H3O+ to the system (type (v)). The NH2-terminated diamond (100) surfaces showed a similar trend. However, the degree of electron transfer over the surface//adlayer interface was found to be more pronounced (as compared with H-termination). The only exception is for a neutral water adlayer, which still would not induce any electron transfer from the surface: 0.03 (NH3) vs 0.08 (H). The presence of O2 in the water adlayer (type (ii)) was, however, found to increase the charge transfer to 1.21 (NH3) vs 0.21 (H). The relative increase with O3 in the adlayer (type (iv)), though, was not found to be that large: 1.04 (NH3) vs 0.81 (H). When adding hydroxonium to the systems of type (iii) and ((v), the following results were obtained: type (iii): 1.76 (NH3) vs 1.70 (H); type (v): 1.72 (NH3) vs 0.92 (H). For the situation with an OH-terminated diamond (100) surface, all adlayer containing species other than water were found to give a significant degree of electron transfer from the surface to the adlayer (see Table 2). However, there is an extra feature observed for this type of termination (relative the other termination types in the present investigation). In the presence of H3O+ ions within the adlayer, a transfer of H+ from the surface OH groups to the adlayer hydronium ions was observed, forming water and hydrogen molecules. As can be seen in Table 2, for the situation with type (iii) (i.e., O2 and H3O+ in the water adlayer), the numerical value of the partial electron transfer was 1.44. However, as there will also be a proton transfer occurring in the same direction, the resulting negative charge transfer will be 0.90 e from the diamond surface to the water-based adlayer. The situation is identical for type (v) (i.e., O3 and H3O+ in the water adlayer). The numerical value of the partial electron transfer was found to be 1.22, while the overall charge transfer was 0.22 e. The result for the OH-termination was otherwise very similar to the NH2 scenario. There was a minor partial electron transfer for neutral water adlayer (0.10 vs 0.30). Moreover, O2 (or O3) in the water adlayer was found to induce a larger degree of electron transfer, being unexpectedly somewhat lower for the more oxidative O3 species: 1.15 vs 1.21 (O2), and 1.04 vs 0.69 (O3). However, as expected, H3O+ ions in the adlayer will increase the degree of electron transfer: O2 from 1.15 to 1.44, and O3 from 0.69 to 1.22. In summary, is it apparent from Table 2 that Oontop- and Obridge-termination will not render any measurable electron

Table 3. Adsorption Energies for Various Water-Based Adlayers and Surface Termination Scenariosa adlayer termination

water

water + O2

water + O2 + H3O+

water + O3

water + O3 + H3O+

Oontop Obridge H NH2,ontop OH

8.0 8.5 −95.6 −102.2 −3.7

−19.9 −0.8 −191.8 −106.3 -----

−50.9 −62.2 −215.9 −197.6 -----

7.1 −7.0 −175.6 −120.4 −77.6

−39.5 −42.5 −138.8 −178.3 -----

a

The unit is kJ/mol.

more pronounced interfacial adhesion energy, and, hence, a larger degree of hydrophilicity. The smaller adhesion energies for the Oontop- and Obridge-terminations are well correlated with the minor degree of electron transfer over the interfaces (see Table 2). For the situation with pure water adhered to an Hterminated diamond surface, the adhesion energy is −95.6 kJ/ mol (all adhesion energies are here presented per surface carbon atom). The adhesion energies for the O2- versus O2+ H3O+-containing water adlayers onto the H-terminated diamond surfaces were found to be exothermic, with the numerical values −138.8 and −215.9 kJ/mol, respectively. These larger adhesion energies are very well correlated with the larger degrees of electron transfer: 0.81 vs 1.70 electrons over the super cell (see Table 2). As can be seen in Tables 2 and 3, the above presented electron transfer//adhesion energy correlation for the Hterminated surface has in the present work been shown to be similar to the situation for an NH2-terminated diamond surface (i.e., for water-based adlayers containing O2, O2 + H3O+, O3 and O3 + H3O+ species, respectively). As a matter of fact, the NH2-terminated diamond surfaces were observed to be involved in rather strong bonding situations over the diamond//adlayer interface, and for all investigated adlayer 22999



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types. As was the situation with the H-terminated surface, there is a clear correlation with the adhesion energies and the degree of electron transfer for all adlayer situations, except for pure water. The exothermicity for this type of surface termination is most probably caused by the probability to form hydrogen bonds between the NH2-adsorbates and the adlayer molecules (e.g., for the H2O molecules). The OH-terminated diamond surface, however, did not show the same features as the H- and NH3-terminated diamond surfaces (see Table 3). The adhesion of pure water to this type of surface was found to be very weak, with a good correlation to the degree of electron transfer over the interface. This is a result that shows a big resemblance with the corresponding results for the O-termination scenarios. In addition, by replacing O2 with O3 in the water-based adlayer, a hydrophilic surface was obtained with the tendency to undergo red/ox reactions with the atmospheric adlayer (i.e., with an apparent degree of electron transfer). The adhesion energies for the situations with hydroxonium ions in the adlayer were not calculated since the OH adsorbate will react with these ions, thereby donating hydrogen to the adlayer with the formation of H2. In summary, a more general correlation between the type of surface termination and degree of electron transfer has been observed in the present study. However, there are also some exceptions to this correlation, which is most probably due to the possibility to form hydrogen bonds over the diamond// adlayer interface. 3.5. Analysis of the Electron Transfer Process. Partial density of states (DOS) calculations have in the present work been performed with the purpose of analyzing in more detail the electron transfer processes between the terminated diamond (100) surfaces and the thin atmospheric water adlayers. Four different situations were thereby studied, out of which two represented a larger degree of electron transfer (an O2 + H3O+ containing atmospheric water adlayer attached to an H- or NH2-terminated diamond surface), and two represented an, in principle, zero electron transfer (a pure water adlayer attached to an H- or NH2-terminated diamond surface). For the former situation (i.e., with oxidative species within the water adlayer), it is apparent that the LUMO level of the adlayer will be partially filled when the adlayer is attached to either the H- or NH2- terminated diamond surface (see Figures 5 and 6).

Figure 6. Partial density of states (pDOS) for the isolated watercontaining adlayer (upper), and for this adlayer when attached to an NH2-terminated diamond (100)-2×1 surface (lower). The adlayer contains oxidative species like O2 and H3O+.

On the other hand, when there are no oxidative species in the adlayer, there are no tendency for an electron transfer to take place (see Figures 7 and 8). Even though the adlayer is

Figure 7. Partial density of states (pDOS) for the isolated pure water adlayer (upper), and this water adlayer attached to an H-terminated diamond (100)-2×1 surface (lower).

attracted to either the H- or NH2-terminated diamond surfaces (see Table 3), no tendency can be observed for an electron transfer to take place from the surface to the LUMO of the adlayer. As can be seen in Table 4, the diamond//adlayer interfaces that show an appreciable electron transfer from the diamond surface to the adlayer, do also show a clear tendency for an electron hole formation within the upper diamond surface. This is especially the situation for the H-terminated surface with an attached water adlayer containing hydroxonium and oxygen species. All C atomic layers show a positive charge, which is evenly distributed over all atoms within each atomic layer. On the contrary, this is not the situation for an H-terminated surface in contact with water only. Compared to the Htermination situation, the same tendency can be observed for the NH2-terminated diamond surface. For the more oxidative adlayer (containing water in addition to hydroxonium and oxygen species), there is a clear indication of holes (shown by positive C charges) within the various C atomic layers. The

Figure 5. Partial density of states (pDOS) for the isolated watercontaining adlayer (upper), and for this adlayer when attached to an H-terminated diamond (100)-2×1 surface (lower). The adlayer contains the oxidative species O2 and H3O+. 23000



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concentration in the atmosphere (about 20%) and generally serves as an oxidizing agent. One of the allotropes of oxygen, ozone, is a strong oxidizing agent that is present at small concentrations in the atmosphere (

Effect of Surface Termination on Diamond (100) Surface

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