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Tailoring the surface chemical bond states of the NbN films by doping Ag: Achieving hard hydrophobic surface
Accepted Manuscript Title: Tailoring the surface chemical bond states of the NbN films by doping Ag: Achieving hard hydrophobic surface Authors: Ren Ping, Zhang Kan, Du Suxuan, Meng Qingnan, He Xin, Wang Shuo, Wen Mao, Zheng Weitao PII: DOI: Reference:
S0169-4332(17)30572-X http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.199 APSUSC 35308
To appear in:
APSUSC
Received date: Revised date: Accepted date:
24-11-2016 13-1-2017 22-2-2017
Please cite this article as: Ren Ping, Zhang Kan, Du Suxuan, Meng Qingnan, He Xin, Wang Shuo, Wen Mao, Zheng Weitao, Tailoring the surface chemical bond states of the NbN films by doping Ag: Achieving hard hydrophobic surface, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.199 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tailoring the surface chemical bond states of the NbN films by doping Ag: achieving hard hydrophobic surface
Ren Ping1, Zhang Kan1, Du Suxuan1, Meng Qingnan2, He Xin1, Wang Shuo3, Wen Mao1*, Zheng Weitao1*
1
Department of Materials Science, State Key Laboratory of Superhard Materials, and
Key Laboratory of Automobile Materials, MOE, Jilin University, Changchun 130012, People's Republic of China. College of Construction Engineering, Jilin University, Changchun 130026, People’s
2
Republic of China. 3
Department of Materials Science and Engineering, College of Engineering, Peking
University, Beijing 100871, China.
*
Corresponding author, Tel. /Fax: 86 431 85168246.
E-mail address: [email protected] (Wen Mao), [email protected] (Zheng Weitao).
Highlights
The factor dominating the transition of wettability from hydrophilic to hydrophobic in Nb-Ag-N films is that the surface chemical bond, especially the
1
hydrophobic Ag2O groups, can be tailored through self-oxidation by controlling the solute Ag atoms.
The theoretical calculation further confirmed that the substitution of Nb atom by Ag atom could break the symmetry in the electronic structure of nearby N atoms and simultaneously increase the instability of nearby N atoms caused by higher repulsive energy between N and Ag atom, which promoted that the hydrophobic Ag2O groups formed on the Nb-Ag-N film surface through self-oxidation.
The present work may provide a straightforward approach for the production of robust hydrophobic ceramic surfaces through tailoring the surface chemistry by doping Ag into transition metal nitrides.
Abstract Robust hydrophobic surfaces based on ceramics capable of withstanding harsh conditions such as abrasion, erosion and high temperature, are required in a broad range of applications. The metal cations with coordinative saturation or low electronegativity are commonly chosen to achieve the intrinsically hydrophobic ceramic by reducing Lewis acidity, and thus the ceramic systems are limited. In this work, we present a different picture that robust hydrophobic surface with high hardness (20 GPa) can be fabricated through doping Ag atoms into intrinsically hydrophilic ceramic film NbN by reactive co-sputtering. The transition of wettability from hydrophilic to hydrophobic of Nb-Ag-N films induced by Ag doping results from the appearance of Ag2O groups on the films surfaces through self-oxidation, 2
because Ag cations Ag in Ag2O are the filled-shell (4d105S0) electronic structure with coordinative saturation that have no tendency to interact with water. The results show that surface Ag2O benefited for hydrophobicity comes from the solute Ag atoms rather than precipitate metal Ag, in which the more Ag atoms incorporated into Nb-sublattice are able to further improve the hydrophobicity, whereas the precipitation of Ag nanoclusters would worsen it. The present work opens a window for fabricating robust hydrophobic surface through tailoring surface chemical bond states by doping Ag into transition metal nitrides.
Keywords: Nb-Ag-N films; Surface chemical bond states; Ag2O; Hydrophobicity; Hardness
1. Introduction Robust hydrophobic surfaces based on durable metal or ceramic[1-13], which are capable of withstanding harsh conditions such as erosion, abrasion and high temperature, have attracted significant attentions in recent years owing to their high hardness, wear resistance and excellent thermal stability[14-17]. Controlling surface roughness[8,9] and developing intrinsically hydrophobic ceramics[10-13] have become two most important approaches to achieve hydrophobic surfaces, instead of coating hydrophilic metal and ceramic with polymeric modifiers[18] because these are not robust to harsh environments[19]. Zhang etc.[8] have fabricated highly hydrophobic metallic coatings by thermal spraying of Fe-based amorphous powders 3
through controlling surface roughness. However, the surface roughness control is usually more complicated, thus more attentions have been paid on intrinsically hydrophobic ceramics. Varanasi and colleagues show that the entire lanthanide oxide series are intrinsically hydrophobic as a result of their particular electronic structure10, in which the unfilled 4f orbitals are shielded by the full octet of electrons in the 5s2p6 outer shell[20,21] giving these atoms a low tendency to form hydrogen bonds with nearby water molecules. Zenkin etc. 12,13 further extended the
intrinsically
hydrophobic
ceramics
to
oxides
and
nitrides
of
other
low-electronegativity metals including zirconium, yttrium, or hafnium. Although their surface metal cations are coordinately unsaturated, the hydrophobicity can also be achieved due to low Lewis acidity of the low-electronegativity cations giving rise to a low ability of the cations on the surface to form coordinate bonds with water oxygen anions[22,23]. Consequently, the intrinsically hydrophobic ceramic systems are limited by the coordinative saturation or electronegativity of metal cations. Obviously, there is still a strong requirement for exploiting new robust hydrophobic surfaces for a broad range of applications. It is known that surface energy dominated by surface chemical bond states is a crucial factor determining the surface wettability[24], which may be tailored by doping[25]. In this paper, we report that intrinsically hydrophilic NbN films can transfer to hydrophobic Nb-Ag-N films by doping Ag atoms into NbN sublattice, in which formation of NbOx and Ag2O on the surface through self-oxidation is caused by solute Ag atoms with less valence electrons. Appearance of Ag2O on Nb-Ag-N film surface 4
can destroy the hydrogen-bonding network of water molecules next to hydrophilic niobium oxide surface because coordinative saturation of Ag has no tendency to interact with water molecules, thereby obtaining robust hydrophobic surfaces. Moreover, surface Ag2O would raise with increasing of solute Ag atoms content, resulting in the increment of water contact angle (WCA), whereas further increase of Ag content in Nb-Ag-N film reaching oversaturation can induce the precipitation of hydrophilic Ag nanoclusters, giving rise to the drop of water contact angle. 2. Experimental 2.1 Sample preparation The Nb-Ag-N films with a thickness of about 800nm were fabricated onto Si 100) substrates by reactive co-sputtering. Elemental targets of Nb (purity 99.95%) and Ag (purity 99.95%) were used as the source material in the mixed discharge gases of Ar (99.99%) and N2 (99.99%). The Si (100) substrates were cleaned respectively with acetone, alcohol and distilled water in an ultrasonic bath, and then were dried with nitrogen before introducing them into the vacuum chamber. Prior to deposition, the chamber was evacuated to a base pressure of 3×10-4 Pa by turbomolecular pump. During the deposition, the flow rates of argon Ar and nitrogen N2 were controlled at 60 and 20 sccm, respectively, maintaining the deposition pressure at 0.8 Pa by regulating gate valve. In order to be benefit for forming solution structure, no additional bias and temperature were applied to substrate. Nb and Ag targets were powered by direct current (DC) and radio frequency (RF), respectively, and various powers ranging from 20W to 80 W were applied to Ag target to realize the increase of 5
Ag/Nb atomic ratio from 0.03 to 0.26 (listed in Table ), while sputtering current on Nb target was kept constant at 0.4 A. 2.2 Characterization The surface morphology and roughness of Nb-Ag-N films were examined by atomic force microscope AFM, Dimension Icon, Veeco Instrumentsbruker, German. The structure of the all films was determined by X-ray diffraction (XRD) using a Bragg-Brentano diffractometer (D8_tools) in θ-2θ configuration with Cu Kα radiation and selected area electron diffraction (SAED, field emission JEOL 2100F). Occupied positions of Ag atoms in Nb-Ag-N films were further confirmed by transmission electron microscopeenergy dispersive spectra (TEMEDS, field emission JEOL 2100F). The hardness of all films was tested by the nanoindentation with Berkovich type pyramidal diamond indenter using continuous stiffness measurements (CSM) mode. The water contact angle (WCA) of the Nb-Ag-N films was evaluated using sessile-drop method with a Krus-DSA30 device by averaging the values of each sample at least six tests. The surface chemical bond states of Nb-Ag-N films were further analyzed by X-ray photoelectron spectrum (XPS, ESCALAB-250) using Al Kα as X-ray source with energy of 1 keV. Furthermore, in order to elucidate the contribution of solute Ag atoms to surface chemistry, electron localization function (ELF) and displacement-dependence energy of atomic movement for both NbN and Ag substituted Nb-Ag-N structure were calculated by the density functional theory DFT formalism. 3．Results and discussion 6
3.1 The wettability and hardness of Nb-Ag-N films The Ag/Nb atomic ratio (R), water contact angle (WCA) and hardness H of all films are listed in Table . The NbN film exhibits hydrophilic with WCA of about 81°, which is in close proximity to the result of 79° reported by Ramírez et al[26], meanwhile it has a relatively low hardness of about 17GPa. Surprisingly, a small amount of Ag atoms doped into NbN film (R=0.03) leads to the increase of WCA to 103°, transforming into hydrophobic behavior, whilst accompanying by a great improvement in hardness reaching to ~28 GPa. The water contact angle (WCA) firstly increases, and then reduces with further increasing of the Ag/Nb atomic ration (R), and reaches the maximum value of WCA ~118° at R=0.15, which is even higher than that of smooth Teflon (103°)[27]. Accordingly, Ag doping seems to be an effective method to achieve the hydrophobic hard surfaces. 3.2The existing formation of Ag and structure of Nb-Ag-N films In fact, the wettability of Nb-Ag-N films is greatly affected by Ag doping, which should strongly depend on the existing formation of Ag and structure of films. Thus, we carried out the XRD, SAED, and TEM/EDS to clarify the occupied position of doped Ag atoms in Nb-Ag-N films and its effect on the structure. It can be seen from XRD patterns displayed in Fig. 1(a) that a small cubic (111) peak and a strong cubic (200) peak appear in pure NbN film, exhibiting a (200) preferred orientation. When Ag atoms are added into NbN films, only a strong (111) diffraction peak remains, meaning a strong (111) texture. At R≤0.15, with increasing the Ag content the (111) peak shifts towards the higher angle, remarkable higher than the standard peak 7
position of ICDD PDF card (No. 38-1155), which just can be caused by two following reasons: (1) Continuous increase of tensile stress, (2) The substitution of Nb atoms by the smaller Ag atoms in NbN lattice. We can first eliminate the contribution of residual stress because all films at R≤0.15 exhibit the compressive stress state and their variations is nonlinear relation with Ag content within a very narrow range Table . We further calculated the lattice constant from the (111) interplanar spacings, which is displayed in Fig.1(b), presenting a good linear relation with Ag content and following the Vegard’s rule[28]. Such similar shift of the diffraction peaks for TiN/Fe films was reported by Zerkout et al, resulting from the substitution of Ti atoms by the smaller Fe atoms in TiN lattice[29]. It means that Ag atoms should replace partial Nb atoms sites forming solution structure at R≤0.15, which is further confirmed by the result of TEM-EDS mapping (Fig.1(c)) that the three elements corresponding to Nb, Ag and N are uniformly distributed and there are no obvious Ag aggregation occurring at R=0.15. However, further increasing R to 0.26 results in the reversal of (111) peak towards the lower angle and remarkable peak broadening because Ag nanoclusters begin to segregate from supersaturated Nb-Ag-N grains, which is in agreement with the result of TEM-EDS mapping that Ag nanoclusters appear at R=0.26. Furthermore, the SAED patterns for Nb-Ag-N films with R=0.15 and 0.26 were performed to support the above results in Fig.1(d). Diffraction rings corresponding to Ag phase appear at R=0.26, but can’t be detected at R=0.15. Although most recent reports focus on TMN/Ag nanocomposite composed of Ag phase and transition metal nitride TMN phase, less attention has been paid on 8
TM-Ag-N solid solution. In fact, Ni, Cu and Ag can substitute the metal position forming solution structure, which have been reported in other metal nitrides systems achieving the substituted nitridometalates, such as Al-Cu-N30, Li-Cu-N31, Li-Ni-N 31 and Al-Ag-N32. However, we find that the TM-Ag-N solid solution is sensitive to deposit parameter (mainly substrate temperature) and Ag content. Sample S3 R=0.15 exhibits the maximum of WCA (118°), and WCA changes to ~80° becoming hydrophilic by only increasing substrate temperature to 400 C while the other parameters remain same as S3, wherein the Nb-Ag-N solution structure is destroyed and transfers to mixed Ag phase and NbN phase at high substrate temperature. Consequently, in our case, the Nb-Ag-N solutions at R≤0.15 have been achieved through doping Ag atoms into NbN sublattice at room temperature and floating bias, meanwhile the solute Ag atoms play an important role in obtaining hydrophobic surface. Combining the result of water contact angles (WCA) with the structure of Nb-Ag-N films, it is found that the hydrophobic behavior is strongly dependent on the solute Ag atoms content, in which more Ag atoms incorporated into Nb-sublattice are able to further improve the hydrophobicity, whereas the precipitation of Ag nanoclusters would worsen the hydrophobic behavior. 3.3 The surface roughness of Nb-Ag-N films Generally, the wettability of a solid surface can be controlled by a combination of surface energy, which depends on the surface chemical characteristics, and surface roughness, which can enhance both hydrophilicity and hydrophobicity[33-37]. In order to determine the factor dominating the transition of wettability from hydrophilic 9
to hydrophobic in Nb-Ag-N films, we firstly compare the results of WCA with surface roughness, which is shown in Fig.2. All Nb-Ag-N films are smooth and with homogeneous surface topography. The pure NbN film exhibits very small surface roughness with Rq=6.2 nm. When a small amount of Ag atoms are doped into films (R≤0.15), the surface roughness just slightly drops. This small value of roughness gives a negligible effect on the value of the water contact angle (WCA) for Nb-Ag-N films[38]. Conversely, the film surface at higher Ag content (R=0.26) becomes rougher, and Rq increases to 14.5 nm, which may result from the precipitation of Ag nanoclusters revealed by the following SAED result. It is known that the effects of roughness on wettability can be explained by Wenzel's equation[39]. If the WCA on a smooth surface is lower than 90°, larger roughness will reduce the WCA, but if it is higher than 90º, larger roughness will further increase the WCA. Noting that the sample with larger roughness at R=0.26 shows a lower WCA (still higher than 90º) compared with that of film at R=0.15. Therefore, the surface roughness neither contribute to the transition from hydrophilicity to hydrophobicity of Nb-Ag-N films nor conduce to the evolution of WCA with variation of Ag content. 3.3 The surface chemical bond states of Nb-Ag-N films After ruling out the effect of surface roughness on the wettability of Nb-Ag-N films, the surface chemical bond states determining the surface energy should be primarily considered. XPS experiments have been carried out to investigate the surface chemistry, which is displayed in Fig.3a). For pure NbN film, Nb 3d peaks can be deconvoluted into two different doublet peaks corresponding to the Nb 3d 5/2 and Nb 10
3d3/2 electrons. The first doublet peaks located at 207.2eV and 209.8eV can be ascribed to Nb-O bond; the second ones located at 205.8eV and 208.6eV are from Nb-O-N[40]. However, only very few Ag atoms added into NbN lattice (R=0.03), the peaks corresponding to Nb-O-N bond almost completely disappear but that for Nb-O bond remain. As a result, Ag doping can promote the surface self-oxidation of Nb-Ag-N films exposed to air. According to the Zenkin’s report, for niobium oxide or nitride, surface Nb cations are coordinately unsaturated and have high electronegativity[12], acting as efficient Lewis acids, meanwhile, oxygen or nitrogen anions can act as efficient Lewis bases[22,41], exhibiting intrinsic hydrophilicity. In addition, the niobium oxide surface should have higher hydrophilicity compared with the corresponding nitride, resulting from the fact that nitrogen is a poorer Lewis base than oxygen due to a reduced number of lone pairs of electrons12. Obviously, the conversion of surface chemistry from Nb-O-N to Nb-O bond of Nb-Ag-N films caused by Ag doping could not be responsible for the observed high hydrophobic surfaces. The Ag 3d XPS patterns are also concerned, and the surface chemical bond states of Ag are closely related to existing form of Ag atoms in Nb-Ag-N films. When Ag atoms occupy the sites of Nb atoms (R0.15), only Ag-O bond with the binding energy (3d5/2) approximating 367.7eV occurs on the surface[42], indicating that Ag exists in the form of Ag2O, and its content is directly proportional to the concentration of solute Ag atoms. As Ag nanoclusters precipitating at higher Ag content (R=0.26), the Ag-Ag bond also emerges besides the Ag-O bond. It is clear that the appearance 11
and increment of surface Ag2O is remarkably associated with the transition of wettability from hydrophilicity to hydrophobicity and improvement in the hydrophobicity of Nb-Ag-N films. Noting that the metal silver cation Ag in Ag2O owning a special electronic structure with the filled-shell (4d105S0) achieve the coordinative saturation[43], the similar electronic structure also has been reported in the lanthanide oxide system10,11, impeding the formation of hydrogen bonds with nearby water molecules. It is expected that the Ag in Ag2O has no tendency to interact with water molecules, consequently surface Ag2O would destroy the hydrogen-bonding network of water molecules formed on the surface of hydrophilic niobium oxide, obtaining a high hydrophobic surface of Nb-Ag-N films (R0.15), which is illustrated in the schematic diagram in Fig.3c). Nevertheless, the metal Ag nanoclusters with high polarity[44] precipitated in film (R=0.26) can eliminate partially the contribution of Ag2O, thereby reducing the hydrophobicity. To a certain extent, the hydrophobic behavior can be also reflected by non-lattice oxygen content shown in O1s spectrum impacted by Ag2O content, in which non-lattice oxygen content is inversely proportional to surface Ag2O at R ≤0.15, whereas the maximum value of non-lattice oxygen content is observed when metal Ag precipitates at R=0.26. Apparently, the surface Ag2O benefited for hydrophobicity of Nb-Ag-N films originates from the solute Ag rather than precipitate metal Ag. Solute Ag atoms can promote surface self-oxidation, forming the surface of NbOxAg2O. In order to further analysis why does solute Ag atoms contribute to surface chemistry, electron 12
localization function ELF and displacement-dependence energy of atomic movement of for both NbN and Ag-substituted Nb-Ag-N structures were calculated by the density functional theory DFT code VASP[45,46] for both NbN and Ag-substituted Nb-Ag-N structures (2×2×2 supercell) are presented in Fig.3b). The ELF patterns show that the Nb atoms and N atoms act as electron donors and acceptors respectively, and the charges around each atom distribute uniformly. When Ag atom with less valence electrons substitute the site of Nb atom, Ag atom is in the charge depletion state, simultaneously breaking the symmetry in the electronic structure of nearby N atoms and increasing the charge depletion of nearby Nb atoms, which can promote the self-oxidation of metal atoms in Nb-Ag-N films exposed to air. The
results
of displacement-dependence
energy
indicate
that
the
repulsive energy between N and Ag atom is greater than that between N and Nb atom, implying that the N atom escaping from Nb-Ag-N structure tends to be more flexible than that from NbN structure, which is in accordance with the result of the ELF. Therefore, the surface chemical bond, especially the hydrophobic Ag2O groups, can be tailored through self-oxidation by controlling the solute Ag atoms, taking place in this case. 4. Conclusions In summary, the robust hydrophobic Nb-Ag-N films with high hardness (20 GPa) were directly fabricated through doping Ag into intrinsically hydrophilic NbN films by reactive co-sputtering. We show that surface Ag2O groups came from the solute Ag atoms and played a dominant role in achieving hydrophobic surface, because the 13
silver cation Ag+ in Ag2O with coordinative saturation has no tendency to interact with water molecules, and thus the appearance and increment of surface Ag2O would destroy the hydrogen-bonding network of water molecules next to hydrophilic niobium oxide surface. More broadly, water contact angle WCA was direct proportional to surface Ag2O content and raised with increasing the concentration of solute Ag atoms (R≤0.15), obtaining maximum value of WCA ~118° at R=0.15, whereas WCA began to drop (~96°) at R=0.26 when the hydrophilic Ag nanoclusters were precipitated. The theoretical calculation further confirmed that the substitution of Nb atom by Ag atom could break the symmetry in the electronic structure of nearby N atoms and simultaneously increase the instability of nearby N atoms caused by higher repulsive energy between N and Ag atom, which promoted that the hydrophobic Ag2O groups formed on the Nb-Ag-N film surface through self-oxidation. The present work may provide a straightforward approach for the production of robust hydrophobic ceramic surfaces through tailoring the surface chemistry by doping Ag into transition metal nitrides.
Acknowledgements The support from National Natural Science Foundation of China (Grant Nos. 51672101, 51602122, 51102111 and 51572104), the National Key Research and Development Program of China (2016YFA0200400), the NSF of Jilin Province (No.20160520010JH) China, China postdoctoral Science Foundation (Grant No. 2016M600229), is highly appreciated. 14
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cycloaddition on boron doped silicon surfaces: a predictive DFT-D study, Phys. Chem. Chem. Phys., 16 (2014) 12164-12171. [46] K. Liu, Q. Yan, M. Chen, W. Fan, Y. Sun, J. Suh, D. Fu, S. Lee, J. Zhou, S. Tongay, Elastic properties of chemical-vapor-deposited monolayer MoS2, WS2, and their bilayer heterostructures, Nano Lett., 14 (2014) 5097-5103.
20
*Figure Captions Fig. 1.(a)XRD patterns of the Nb-Ag-N films with various Ag/Nb atomic ratio. (b)The variation of lattice parameters with Ag/Nb atomic ratio of Nb-Ag-N films. (c)TEM-EDS mapping images of the Nb-Ag-N films with R=0.15 and R=0.26. (c) SAED images of the Nb-Ag-N films with R=0.15 and 0.26. Fig. 2. The water contact angles WCA and AFM images of Nb-Ag-N films with various Ag/Nb atomic ratio. Fig. 3. (a) XPS core level spectra of Nb 3d, Ag 3d, and O 1s of the Nb-Ag-N films. b ELF diagrams corresponding to the plane (001) and the displacement-dependence energy of atomic movements for NbN and Ag-substituted Nb-Ag-N structures. cThe schematic diagram for the hydrophobic mechanism on the surface of Nb-Ag-N films.
21
Fig. 1.
22
Fig. 2.
23
Fig. 3.
24
Table . Comparison of the Ag/Nb atomic ratio (R), water contact angle (WCA), hardness (H), compressive stress y, surface roughness (Rq) of Nb-Ag-N films. Samp
R
WCA
H (GPa)
y GPa
Rq nm
le S0
0
81.25±0.55
17.27±0.21
1.00
6.2
S1
0.03
103.68±0.91
28.22±0.42
0.27
4.8
S2
0.08
107.60±1.00
24.44±0.52
0.49
5.7
S3
0.15
118.80±0.30
22.44±1.10
0.26
5.3
S4
0.26
96.48±0.83
19.08±1.21
0.16
14.5
25
S0169-4332(17)30572-X http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.199 APSUSC 35308
To appear in:
APSUSC
Received date: Revised date: Accepted date:
24-11-2016 13-1-2017 22-2-2017
Please cite this article as: Ren Ping, Zhang Kan, Du Suxuan, Meng Qingnan, He Xin, Wang Shuo, Wen Mao, Zheng Weitao, Tailoring the surface chemical bond states of the NbN films by doping Ag: Achieving hard hydrophobic surface, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.199 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tailoring the surface chemical bond states of the NbN films by doping Ag: achieving hard hydrophobic surface
Ren Ping1, Zhang Kan1, Du Suxuan1, Meng Qingnan2, He Xin1, Wang Shuo3, Wen Mao1*, Zheng Weitao1*
1
Department of Materials Science, State Key Laboratory of Superhard Materials, and
Key Laboratory of Automobile Materials, MOE, Jilin University, Changchun 130012, People's Republic of China. College of Construction Engineering, Jilin University, Changchun 130026, People’s
2
Republic of China. 3
Department of Materials Science and Engineering, College of Engineering, Peking
University, Beijing 100871, China.
*
Corresponding author, Tel. /Fax: 86 431 85168246.
E-mail address: [email protected] (Wen Mao), [email protected] (Zheng Weitao).
Highlights
The factor dominating the transition of wettability from hydrophilic to hydrophobic in Nb-Ag-N films is that the surface chemical bond, especially the
1
hydrophobic Ag2O groups, can be tailored through self-oxidation by controlling the solute Ag atoms.
The theoretical calculation further confirmed that the substitution of Nb atom by Ag atom could break the symmetry in the electronic structure of nearby N atoms and simultaneously increase the instability of nearby N atoms caused by higher repulsive energy between N and Ag atom, which promoted that the hydrophobic Ag2O groups formed on the Nb-Ag-N film surface through self-oxidation.
The present work may provide a straightforward approach for the production of robust hydrophobic ceramic surfaces through tailoring the surface chemistry by doping Ag into transition metal nitrides.
Abstract Robust hydrophobic surfaces based on ceramics capable of withstanding harsh conditions such as abrasion, erosion and high temperature, are required in a broad range of applications. The metal cations with coordinative saturation or low electronegativity are commonly chosen to achieve the intrinsically hydrophobic ceramic by reducing Lewis acidity, and thus the ceramic systems are limited. In this work, we present a different picture that robust hydrophobic surface with high hardness (20 GPa) can be fabricated through doping Ag atoms into intrinsically hydrophilic ceramic film NbN by reactive co-sputtering. The transition of wettability from hydrophilic to hydrophobic of Nb-Ag-N films induced by Ag doping results from the appearance of Ag2O groups on the films surfaces through self-oxidation, 2
because Ag cations Ag in Ag2O are the filled-shell (4d105S0) electronic structure with coordinative saturation that have no tendency to interact with water. The results show that surface Ag2O benefited for hydrophobicity comes from the solute Ag atoms rather than precipitate metal Ag, in which the more Ag atoms incorporated into Nb-sublattice are able to further improve the hydrophobicity, whereas the precipitation of Ag nanoclusters would worsen it. The present work opens a window for fabricating robust hydrophobic surface through tailoring surface chemical bond states by doping Ag into transition metal nitrides.
Keywords: Nb-Ag-N films; Surface chemical bond states; Ag2O; Hydrophobicity; Hardness
1. Introduction Robust hydrophobic surfaces based on durable metal or ceramic[1-13], which are capable of withstanding harsh conditions such as erosion, abrasion and high temperature, have attracted significant attentions in recent years owing to their high hardness, wear resistance and excellent thermal stability[14-17]. Controlling surface roughness[8,9] and developing intrinsically hydrophobic ceramics[10-13] have become two most important approaches to achieve hydrophobic surfaces, instead of coating hydrophilic metal and ceramic with polymeric modifiers[18] because these are not robust to harsh environments[19]. Zhang etc.[8] have fabricated highly hydrophobic metallic coatings by thermal spraying of Fe-based amorphous powders 3
through controlling surface roughness. However, the surface roughness control is usually more complicated, thus more attentions have been paid on intrinsically hydrophobic ceramics. Varanasi and colleagues show that the entire lanthanide oxide series are intrinsically hydrophobic as a result of their particular electronic structure10, in which the unfilled 4f orbitals are shielded by the full octet of electrons in the 5s2p6 outer shell[20,21] giving these atoms a low tendency to form hydrogen bonds with nearby water molecules. Zenkin etc. 12,13 further extended the
intrinsically
hydrophobic
ceramics
to
oxides
and
nitrides
of
other
low-electronegativity metals including zirconium, yttrium, or hafnium. Although their surface metal cations are coordinately unsaturated, the hydrophobicity can also be achieved due to low Lewis acidity of the low-electronegativity cations giving rise to a low ability of the cations on the surface to form coordinate bonds with water oxygen anions[22,23]. Consequently, the intrinsically hydrophobic ceramic systems are limited by the coordinative saturation or electronegativity of metal cations. Obviously, there is still a strong requirement for exploiting new robust hydrophobic surfaces for a broad range of applications. It is known that surface energy dominated by surface chemical bond states is a crucial factor determining the surface wettability[24], which may be tailored by doping[25]. In this paper, we report that intrinsically hydrophilic NbN films can transfer to hydrophobic Nb-Ag-N films by doping Ag atoms into NbN sublattice, in which formation of NbOx and Ag2O on the surface through self-oxidation is caused by solute Ag atoms with less valence electrons. Appearance of Ag2O on Nb-Ag-N film surface 4
can destroy the hydrogen-bonding network of water molecules next to hydrophilic niobium oxide surface because coordinative saturation of Ag has no tendency to interact with water molecules, thereby obtaining robust hydrophobic surfaces. Moreover, surface Ag2O would raise with increasing of solute Ag atoms content, resulting in the increment of water contact angle (WCA), whereas further increase of Ag content in Nb-Ag-N film reaching oversaturation can induce the precipitation of hydrophilic Ag nanoclusters, giving rise to the drop of water contact angle. 2. Experimental 2.1 Sample preparation The Nb-Ag-N films with a thickness of about 800nm were fabricated onto Si 100) substrates by reactive co-sputtering. Elemental targets of Nb (purity 99.95%) and Ag (purity 99.95%) were used as the source material in the mixed discharge gases of Ar (99.99%) and N2 (99.99%). The Si (100) substrates were cleaned respectively with acetone, alcohol and distilled water in an ultrasonic bath, and then were dried with nitrogen before introducing them into the vacuum chamber. Prior to deposition, the chamber was evacuated to a base pressure of 3×10-4 Pa by turbomolecular pump. During the deposition, the flow rates of argon Ar and nitrogen N2 were controlled at 60 and 20 sccm, respectively, maintaining the deposition pressure at 0.8 Pa by regulating gate valve. In order to be benefit for forming solution structure, no additional bias and temperature were applied to substrate. Nb and Ag targets were powered by direct current (DC) and radio frequency (RF), respectively, and various powers ranging from 20W to 80 W were applied to Ag target to realize the increase of 5
Ag/Nb atomic ratio from 0.03 to 0.26 (listed in Table ), while sputtering current on Nb target was kept constant at 0.4 A. 2.2 Characterization The surface morphology and roughness of Nb-Ag-N films were examined by atomic force microscope AFM, Dimension Icon, Veeco Instrumentsbruker, German. The structure of the all films was determined by X-ray diffraction (XRD) using a Bragg-Brentano diffractometer (D8_tools) in θ-2θ configuration with Cu Kα radiation and selected area electron diffraction (SAED, field emission JEOL 2100F). Occupied positions of Ag atoms in Nb-Ag-N films were further confirmed by transmission electron microscopeenergy dispersive spectra (TEMEDS, field emission JEOL 2100F). The hardness of all films was tested by the nanoindentation with Berkovich type pyramidal diamond indenter using continuous stiffness measurements (CSM) mode. The water contact angle (WCA) of the Nb-Ag-N films was evaluated using sessile-drop method with a Krus-DSA30 device by averaging the values of each sample at least six tests. The surface chemical bond states of Nb-Ag-N films were further analyzed by X-ray photoelectron spectrum (XPS, ESCALAB-250) using Al Kα as X-ray source with energy of 1 keV. Furthermore, in order to elucidate the contribution of solute Ag atoms to surface chemistry, electron localization function (ELF) and displacement-dependence energy of atomic movement for both NbN and Ag substituted Nb-Ag-N structure were calculated by the density functional theory DFT formalism. 3．Results and discussion 6
3.1 The wettability and hardness of Nb-Ag-N films The Ag/Nb atomic ratio (R), water contact angle (WCA) and hardness H of all films are listed in Table . The NbN film exhibits hydrophilic with WCA of about 81°, which is in close proximity to the result of 79° reported by Ramírez et al[26], meanwhile it has a relatively low hardness of about 17GPa. Surprisingly, a small amount of Ag atoms doped into NbN film (R=0.03) leads to the increase of WCA to 103°, transforming into hydrophobic behavior, whilst accompanying by a great improvement in hardness reaching to ~28 GPa. The water contact angle (WCA) firstly increases, and then reduces with further increasing of the Ag/Nb atomic ration (R), and reaches the maximum value of WCA ~118° at R=0.15, which is even higher than that of smooth Teflon (103°)[27]. Accordingly, Ag doping seems to be an effective method to achieve the hydrophobic hard surfaces. 3.2The existing formation of Ag and structure of Nb-Ag-N films In fact, the wettability of Nb-Ag-N films is greatly affected by Ag doping, which should strongly depend on the existing formation of Ag and structure of films. Thus, we carried out the XRD, SAED, and TEM/EDS to clarify the occupied position of doped Ag atoms in Nb-Ag-N films and its effect on the structure. It can be seen from XRD patterns displayed in Fig. 1(a) that a small cubic (111) peak and a strong cubic (200) peak appear in pure NbN film, exhibiting a (200) preferred orientation. When Ag atoms are added into NbN films, only a strong (111) diffraction peak remains, meaning a strong (111) texture. At R≤0.15, with increasing the Ag content the (111) peak shifts towards the higher angle, remarkable higher than the standard peak 7
position of ICDD PDF card (No. 38-1155), which just can be caused by two following reasons: (1) Continuous increase of tensile stress, (2) The substitution of Nb atoms by the smaller Ag atoms in NbN lattice. We can first eliminate the contribution of residual stress because all films at R≤0.15 exhibit the compressive stress state and their variations is nonlinear relation with Ag content within a very narrow range Table . We further calculated the lattice constant from the (111) interplanar spacings, which is displayed in Fig.1(b), presenting a good linear relation with Ag content and following the Vegard’s rule[28]. Such similar shift of the diffraction peaks for TiN/Fe films was reported by Zerkout et al, resulting from the substitution of Ti atoms by the smaller Fe atoms in TiN lattice[29]. It means that Ag atoms should replace partial Nb atoms sites forming solution structure at R≤0.15, which is further confirmed by the result of TEM-EDS mapping (Fig.1(c)) that the three elements corresponding to Nb, Ag and N are uniformly distributed and there are no obvious Ag aggregation occurring at R=0.15. However, further increasing R to 0.26 results in the reversal of (111) peak towards the lower angle and remarkable peak broadening because Ag nanoclusters begin to segregate from supersaturated Nb-Ag-N grains, which is in agreement with the result of TEM-EDS mapping that Ag nanoclusters appear at R=0.26. Furthermore, the SAED patterns for Nb-Ag-N films with R=0.15 and 0.26 were performed to support the above results in Fig.1(d). Diffraction rings corresponding to Ag phase appear at R=0.26, but can’t be detected at R=0.15. Although most recent reports focus on TMN/Ag nanocomposite composed of Ag phase and transition metal nitride TMN phase, less attention has been paid on 8
TM-Ag-N solid solution. In fact, Ni, Cu and Ag can substitute the metal position forming solution structure, which have been reported in other metal nitrides systems achieving the substituted nitridometalates, such as Al-Cu-N30, Li-Cu-N31, Li-Ni-N 31 and Al-Ag-N32. However, we find that the TM-Ag-N solid solution is sensitive to deposit parameter (mainly substrate temperature) and Ag content. Sample S3 R=0.15 exhibits the maximum of WCA (118°), and WCA changes to ~80° becoming hydrophilic by only increasing substrate temperature to 400 C while the other parameters remain same as S3, wherein the Nb-Ag-N solution structure is destroyed and transfers to mixed Ag phase and NbN phase at high substrate temperature. Consequently, in our case, the Nb-Ag-N solutions at R≤0.15 have been achieved through doping Ag atoms into NbN sublattice at room temperature and floating bias, meanwhile the solute Ag atoms play an important role in obtaining hydrophobic surface. Combining the result of water contact angles (WCA) with the structure of Nb-Ag-N films, it is found that the hydrophobic behavior is strongly dependent on the solute Ag atoms content, in which more Ag atoms incorporated into Nb-sublattice are able to further improve the hydrophobicity, whereas the precipitation of Ag nanoclusters would worsen the hydrophobic behavior. 3.3 The surface roughness of Nb-Ag-N films Generally, the wettability of a solid surface can be controlled by a combination of surface energy, which depends on the surface chemical characteristics, and surface roughness, which can enhance both hydrophilicity and hydrophobicity[33-37]. In order to determine the factor dominating the transition of wettability from hydrophilic 9
to hydrophobic in Nb-Ag-N films, we firstly compare the results of WCA with surface roughness, which is shown in Fig.2. All Nb-Ag-N films are smooth and with homogeneous surface topography. The pure NbN film exhibits very small surface roughness with Rq=6.2 nm. When a small amount of Ag atoms are doped into films (R≤0.15), the surface roughness just slightly drops. This small value of roughness gives a negligible effect on the value of the water contact angle (WCA) for Nb-Ag-N films[38]. Conversely, the film surface at higher Ag content (R=0.26) becomes rougher, and Rq increases to 14.5 nm, which may result from the precipitation of Ag nanoclusters revealed by the following SAED result. It is known that the effects of roughness on wettability can be explained by Wenzel's equation[39]. If the WCA on a smooth surface is lower than 90°, larger roughness will reduce the WCA, but if it is higher than 90º, larger roughness will further increase the WCA. Noting that the sample with larger roughness at R=0.26 shows a lower WCA (still higher than 90º) compared with that of film at R=0.15. Therefore, the surface roughness neither contribute to the transition from hydrophilicity to hydrophobicity of Nb-Ag-N films nor conduce to the evolution of WCA with variation of Ag content. 3.3 The surface chemical bond states of Nb-Ag-N films After ruling out the effect of surface roughness on the wettability of Nb-Ag-N films, the surface chemical bond states determining the surface energy should be primarily considered. XPS experiments have been carried out to investigate the surface chemistry, which is displayed in Fig.3a). For pure NbN film, Nb 3d peaks can be deconvoluted into two different doublet peaks corresponding to the Nb 3d 5/2 and Nb 10
3d3/2 electrons. The first doublet peaks located at 207.2eV and 209.8eV can be ascribed to Nb-O bond; the second ones located at 205.8eV and 208.6eV are from Nb-O-N[40]. However, only very few Ag atoms added into NbN lattice (R=0.03), the peaks corresponding to Nb-O-N bond almost completely disappear but that for Nb-O bond remain. As a result, Ag doping can promote the surface self-oxidation of Nb-Ag-N films exposed to air. According to the Zenkin’s report, for niobium oxide or nitride, surface Nb cations are coordinately unsaturated and have high electronegativity[12], acting as efficient Lewis acids, meanwhile, oxygen or nitrogen anions can act as efficient Lewis bases[22,41], exhibiting intrinsic hydrophilicity. In addition, the niobium oxide surface should have higher hydrophilicity compared with the corresponding nitride, resulting from the fact that nitrogen is a poorer Lewis base than oxygen due to a reduced number of lone pairs of electrons12. Obviously, the conversion of surface chemistry from Nb-O-N to Nb-O bond of Nb-Ag-N films caused by Ag doping could not be responsible for the observed high hydrophobic surfaces. The Ag 3d XPS patterns are also concerned, and the surface chemical bond states of Ag are closely related to existing form of Ag atoms in Nb-Ag-N films. When Ag atoms occupy the sites of Nb atoms (R0.15), only Ag-O bond with the binding energy (3d5/2) approximating 367.7eV occurs on the surface[42], indicating that Ag exists in the form of Ag2O, and its content is directly proportional to the concentration of solute Ag atoms. As Ag nanoclusters precipitating at higher Ag content (R=0.26), the Ag-Ag bond also emerges besides the Ag-O bond. It is clear that the appearance 11
and increment of surface Ag2O is remarkably associated with the transition of wettability from hydrophilicity to hydrophobicity and improvement in the hydrophobicity of Nb-Ag-N films. Noting that the metal silver cation Ag in Ag2O owning a special electronic structure with the filled-shell (4d105S0) achieve the coordinative saturation[43], the similar electronic structure also has been reported in the lanthanide oxide system10,11, impeding the formation of hydrogen bonds with nearby water molecules. It is expected that the Ag in Ag2O has no tendency to interact with water molecules, consequently surface Ag2O would destroy the hydrogen-bonding network of water molecules formed on the surface of hydrophilic niobium oxide, obtaining a high hydrophobic surface of Nb-Ag-N films (R0.15), which is illustrated in the schematic diagram in Fig.3c). Nevertheless, the metal Ag nanoclusters with high polarity[44] precipitated in film (R=0.26) can eliminate partially the contribution of Ag2O, thereby reducing the hydrophobicity. To a certain extent, the hydrophobic behavior can be also reflected by non-lattice oxygen content shown in O1s spectrum impacted by Ag2O content, in which non-lattice oxygen content is inversely proportional to surface Ag2O at R ≤0.15, whereas the maximum value of non-lattice oxygen content is observed when metal Ag precipitates at R=0.26. Apparently, the surface Ag2O benefited for hydrophobicity of Nb-Ag-N films originates from the solute Ag rather than precipitate metal Ag. Solute Ag atoms can promote surface self-oxidation, forming the surface of NbOxAg2O. In order to further analysis why does solute Ag atoms contribute to surface chemistry, electron 12
localization function ELF and displacement-dependence energy of atomic movement of for both NbN and Ag-substituted Nb-Ag-N structures were calculated by the density functional theory DFT code VASP[45,46] for both NbN and Ag-substituted Nb-Ag-N structures (2×2×2 supercell) are presented in Fig.3b). The ELF patterns show that the Nb atoms and N atoms act as electron donors and acceptors respectively, and the charges around each atom distribute uniformly. When Ag atom with less valence electrons substitute the site of Nb atom, Ag atom is in the charge depletion state, simultaneously breaking the symmetry in the electronic structure of nearby N atoms and increasing the charge depletion of nearby Nb atoms, which can promote the self-oxidation of metal atoms in Nb-Ag-N films exposed to air. The
results
of displacement-dependence
energy
indicate
that
the
repulsive energy between N and Ag atom is greater than that between N and Nb atom, implying that the N atom escaping from Nb-Ag-N structure tends to be more flexible than that from NbN structure, which is in accordance with the result of the ELF. Therefore, the surface chemical bond, especially the hydrophobic Ag2O groups, can be tailored through self-oxidation by controlling the solute Ag atoms, taking place in this case. 4. Conclusions In summary, the robust hydrophobic Nb-Ag-N films with high hardness (20 GPa) were directly fabricated through doping Ag into intrinsically hydrophilic NbN films by reactive co-sputtering. We show that surface Ag2O groups came from the solute Ag atoms and played a dominant role in achieving hydrophobic surface, because the 13
silver cation Ag+ in Ag2O with coordinative saturation has no tendency to interact with water molecules, and thus the appearance and increment of surface Ag2O would destroy the hydrogen-bonding network of water molecules next to hydrophilic niobium oxide surface. More broadly, water contact angle WCA was direct proportional to surface Ag2O content and raised with increasing the concentration of solute Ag atoms (R≤0.15), obtaining maximum value of WCA ~118° at R=0.15, whereas WCA began to drop (~96°) at R=0.26 when the hydrophilic Ag nanoclusters were precipitated. The theoretical calculation further confirmed that the substitution of Nb atom by Ag atom could break the symmetry in the electronic structure of nearby N atoms and simultaneously increase the instability of nearby N atoms caused by higher repulsive energy between N and Ag atom, which promoted that the hydrophobic Ag2O groups formed on the Nb-Ag-N film surface through self-oxidation. The present work may provide a straightforward approach for the production of robust hydrophobic ceramic surfaces through tailoring the surface chemistry by doping Ag into transition metal nitrides.
Acknowledgements The support from National Natural Science Foundation of China (Grant Nos. 51672101, 51602122, 51102111 and 51572104), the National Key Research and Development Program of China (2016YFA0200400), the NSF of Jilin Province (No.20160520010JH) China, China postdoctoral Science Foundation (Grant No. 2016M600229), is highly appreciated. 14
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*Figure Captions Fig. 1.(a)XRD patterns of the Nb-Ag-N films with various Ag/Nb atomic ratio. (b)The variation of lattice parameters with Ag/Nb atomic ratio of Nb-Ag-N films. (c)TEM-EDS mapping images of the Nb-Ag-N films with R=0.15 and R=0.26. (c) SAED images of the Nb-Ag-N films with R=0.15 and 0.26. Fig. 2. The water contact angles WCA and AFM images of Nb-Ag-N films with various Ag/Nb atomic ratio. Fig. 3. (a) XPS core level spectra of Nb 3d, Ag 3d, and O 1s of the Nb-Ag-N films. b ELF diagrams corresponding to the plane (001) and the displacement-dependence energy of atomic movements for NbN and Ag-substituted Nb-Ag-N structures. cThe schematic diagram for the hydrophobic mechanism on the surface of Nb-Ag-N films.
21
Fig. 1.
22
Fig. 2.
23
Fig. 3.
24
Table . Comparison of the Ag/Nb atomic ratio (R), water contact angle (WCA), hardness (H), compressive stress y, surface roughness (Rq) of Nb-Ag-N films. Samp
R
WCA
H (GPa)
y GPa
Rq nm
le S0
0
81.25±0.55
17.27±0.21
1.00
6.2
S1
0.03
103.68±0.91
28.22±0.42
0.27
4.8
S2
0.08
107.60±1.00
24.44±0.52
0.49
5.7
S3
0.15
118.80±0.30
22.44±1.10
0.26
5.3
S4
0.26
96.48±0.83
19.08±1.21
0.16
14.5
25