Multilayer boron nitride nanofilm as an effective barrier for atomic oxygen irradiation

Journal Pre-proofs Full Length Article Multilayer boron nitride nanofilm as an effective barrier for atomic oxygen irradiation Li Cheng, Yanbin Shi, Yu Hao, Wensheng Li, Siming Ren, Liping Wang PII: DOI: Reference:

S0169-4332(19)33210-6 https://doi.org/10.1016/j.apsusc.2019.144394 APSUSC 144394

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Applied Surface Science

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1 August 2019 8 October 2019 12 October 2019

Please cite this article as: L. Cheng, Y. Shi, Y. Hao, W. Li, S. Ren, L. Wang, Multilayer boron nitride nanofilm as an effective barrier for atomic oxygen irradiation, Applied Surface Science (2019), doi: https://doi.org/10.1016/ j.apsusc.2019.144394

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Multilayer boron nitride nanofilm as an effective barrier for atomic oxygen irradiation Li Chenga,b, Yanbin Shia,b, Yu Haoa,c, Wensheng Lid, Siming Rena,*, Liping Wanga,*

a

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of

Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b

University of Chinese Academy of Sciences, Beijing 100039, China

c

College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024,

China d

State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou

University of Technology, Lanzhou 730050, China

[email protected] [email protected] * Corresponding author.

Abstract Boron nitride (BN) nanofilm, as a structural analogue of graphene, is one of the most promising candidates for an effective anti-oxidation barrier to protect the underlying metal. Herein, the oxidation resistance of monolayer and multilayer BN nanofilms against space atomic oxygen (AO) irradiation is studied systematically. Experimental results reveal that multilayer BN coated Cu foils exhibit superior

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oxidation resistance against AO irradiation than that of monolayer BN coated ones and bare Cu, resulting from the high impermeability of multilayer films. Density functional theory calculations and molecular dynamics simulations indicate that AO can be absorbed spontaneously on the surface of BN plane and can’t move arbitrarily. However, AO can still damage monolayer BN film after long time irradiation, leading to the oxidation of underneath Cu. In comparison to monolayer BN, the multilayer designing of BN may adsorb AO on the top layer, and increases the diffusion difficulties of AO passing layers, which enhances the barrier performance against AO. These results prove that multilayer BN have an important promising implication as one oxidation-inhibiting barrier to anti-AO in space.

Keywords: Boron nitride nanofilm; Atomic oxygen; Density functional theory; Molecular dynamics simulation; Oxidation resistance;

1. Introduction Low earth orbit (LEO) is about 200-700 km away from earth surface [1-3], where many space vehicles are in motion at an orbit velocity of ~8 km/s [4]. However, it is worth noting that many factors in LEO can cause a negative effect to optoelectronics and mechanical devices on space vehicles such as vacuum ultraviolet, thermal cycling, charged particles, and high energy atomic oxygen (AO) [5-6]. AO with high kinetic energy (~5 eV) is one of the most destructive factors in LEO to damage the spacecraft materials [4,7]. Hence, to avoid AO damage, organic coatings have been widely

2

applied to aerospace as a kind of effective protective material [8,9]. However, the presence of shortcomings in organic coatings limits their further application and development in space vehicles, such as thicker thickness, bad effect on optical properties and toxic additives [10-14]. As a result, it’s still desired to explore innovative

anti-AO

protective

materials

with

excellent

barrier

and

environment-friendly properties. Inorganic two-dimensional materials such as graphene and boron nitride nanofilm exhibit excellent resistance to oxidation and corrosion [15-20]. Graphene coating materials may have potential to be applied in space vehicles due to thermal stability, great optical characteristic, electrical properties and impermeability [21]. However, high conductivity of graphene can enhance the electrochemical reaction in long-term corrosion process owing to the formation of a galvanic cell at the metal-graphene interface [22,23]. This limits the application of graphene into space since the satellites need to wait an extended period in natural environments before launching. Fortunately, boron nitride (BN) film is a type of layered materials which has an analogue structure of graphene [24], such as thin thickness, high heat conductivity, and outstanding impermeability [18,20,25-27]. More importantly, the insulating nature of BN renders it a promising candidate as nanometer-scale coating to provide a long-term protection for space materials [28]. Up to now, BN as a new anti-corrosion protective layer on copper in air or special solution have been investigated widely [25, 29-31]. However, BN nanofilms as protective barriers against AO in simulated space environment have not been investigated in experiment and theory. Hence, the investigation of oxidation

3

behaviors of BN nanofilms coated Cu foils in AO environment is necessary and significant. In the present work, the oxidation behaviors of monolayer and multilayer BN coated Cu foils (referred as Cu/1L BN and Cu/5L BN) were discussed by optical micrograph, scanning electron micrograph (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy after AO irradiation with different duration time, and found that Cu/5L BN showed better anti-oxidation performance than others. In addition, we used density functional theory (DFT) calculations and molecular dynamics (MD) simulations to investigate the total energy changes of AO on monolayer BN basal plane and simulate dynamic process of AO attacking BN film. Calculation results showed that AO was more likely chemisorbed on the BN surface. It was considered that this process prevented diffusion of AO at the BN-Cu interface. The experiments and calculations helped to prove effective barrier performance of multilayer BN for AO irradiation. The obtained results are expected to provide protective technology at atomic level to promote the development of space materials.

2. Experimental and calculated section The BN films were deposited on Cu foils via chemical vapor deposition (CVD) method [32], and different film thickness were obtained by controlling the deposition time. Prepared monolayer and multilayer BN films were transferred onto SiO2/Si substrates via Poly (methyl methacrylate) (PMMA) assisted method for further characterization [32,33]. AO irradiation experiments were carried out by an AO

4

simulator located at Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, which used electron cyclotron resonance to produce oxygen plasma and oxygen plasma was accelerated by Mo plant to be neutralized to generate a neutral oxygen beam. The flux of effective AO after irradiated for two hours could be calculated by following equation [34]:
  =

 −   × ×  × 

Where M1 and M2 were the mass of Kapton before and after AO irradiation, A and ρ were the exposure area and density of Kapton, t was irradiation time and AORC was the AO reacting coefficient of Kapton. The vacuum pressure was 1.5 × 10-3 Pa and the effective AO flux was calculated as 1.6 × 1016 atom·cm-2·s-1. The average kinetic energy of the AO simulator was slightly higher than the actual kinetic energy of AO in space. Bare Cu, Cu/1L BN and Cu/5L BN were exposed in AO simulation environment for 10 min, 20 min and 30 min, respectively. The surface morphologies of samples before and after AO irradiation were observed by optical microscope (LSM 700), SPM (Dimension 3100V, Veeco) and SEM (Hitachis-4800). In addition, the thickness of transferred films was measured via SPM. The oxidation degree of copper was analyzed by Raman spectroscopy which was excited by 532 nm wavelength at a power of 10 mW and each spectrum was obtained by exposing time of 10 s. XPS (Axis ultra DLD) using mono Al Kαradiation was carried to evaluate the oxide compositions of Cu surface with peaks of Cu 2p, O 1s, B 1s and N 1s calibrated through the binding energy of C 1s at 284.6 eV. DFT method in the Vienna Ab initio Simulation Package (VASP) [35-37] and MD

5

simulation in the LAMMPS [38] were carried out to calculate energy and dynamic process. In DFT method, electron exchange-correlation was described via the generalized gradient approximation in the Perdew-Burke-Ernzerhof (PBE) form. The cutoff energy was 450 eV, and the grid of the k-point is 3 × 3 × 1, building 4 × 4 supercell with a 15 Å vacuum layer to avoid the interference between layers. In addition, we also used ReaxFF force field parameters for B, N and O [39,40] in this MD simulation process. Initial model of system energy was minimized via conjugate gradient method, and Verlet algorithm with 0.2 fs time scale was used to integrate atomic trajectories. At last, dynamic process was viewed in Ovito.

3. Results and discussion The surface morphology and thickness of transferred monolayer and multilayer BN films were identified by optical microscope and SPM image. In general, nanofilms are more easily adsorbed impurities owing to the high surface energy. As a result, from optical images in Fig. 1a and 1b, many adsorbed impurities exist on the BN surface, which is beneficial to distinguish BN films and SiO2 substrate by color difference. Moreover, the color contrast of BN/SiO2 is more obvious with the increasing of film thickness. Fig. 1c-f show the SPM image and the corresponding height trace of monolayer and multilayer BN. The cracks, folds, wrinkles, and residual PMMA particles are observed clearly on the samples, which are attributed to the mechanical transferring process. The corresponding height profiles of the film thickness are presented in Fig. 1d and 1f, showing that the thickness of monolayer and

6

multilayer BN films are about 0.5736 nm and 1.6535 nm (the thickness of multilayer BN film is around five layers), respectively, similar to the previous reports [41]. These results indicate that BN nanofilms with expected thickness are successfully synthesized via traditional CVD method.

Fig. 1. (a,b) Optical images of monolayer and multilayer BN films transferred to SiO2/Si substrate with PMMA assistance. (c,e) SPM images of transferred monolayer

7

and multilayer BN films and corresponding (d,f) surface height difference between marked red dots, respectively.

To evaluate the barrier performance of monolayer and multilayer BN against AO, bare Cu, Cu/1L BN and Cu/5L BN were exposed to the simulate AO environment for 10, 20 and 30 min, respectively. Fig. 2a-c are representative optical images of three samples before AO irradiation, exhibiting clear metallic luster due to the high optical transmittance of BN thin films. After 10 min of AO irradiation, as shown in Fig. 2d-f, it can be seen that bare Cu appears some discolored spots due to the oxidation of copper whereas the surface of Cu/1L BN and Cu/5L BN samples is almost no changes in colors compared to the as-prepared ones, suggesting that BN can effectively block AO for a short-term irradiation. After 20 min of AO irradiation, the oxidized area of bare Cu is enlarged (Fig. 2g), evidencing that severe oxidation has been occurred on the surface. In contrast, the oxidized area is reduced for BN coated Cu samples, and the corresponding oxidation spot decreases with the increasing BN layer (Fig. 2h and 2i). With the further increasing of irradiation time (30 min, Fig. 2j-l), some areas of bare Cu are further damaged by high energy AO (Fig. 2j). The surface of Cu/1L BN and Cu/5L BN are also decorated with some deep yellow or dark color (oxidized region) since AO can pass through the broken areas of BN film to react with the underlying Cu after a long-term irradiation. However, Cu/5L BN retains much of its metallic luster by comparing with other samples, which indicates the multilayer BN films can effectively resist AO.

8

Fig. 2. Optical images of bare Cu, Cu/1L BN and Cu/5L BN before and after 10 min, 20 min and 30 min of AO irradiation.

Fig. 3 shows the typical SEM images of bare Cu, Cu/1L BN and Cu/5L BN after 30 min of AO irradiation. The grain boundaries of copper are clearly visible and the surface is smooth before AO irradiation (Fig. 3a-c). After AO irradiation, large-scale oxidized areas appear on the surface of bare Cu (Fig. 3d) due to the formation of loose and thick oxidation products, confirming the serious damage of bare Cu surface. For the Cu/1L BN case (Fig. 3e), the morphology becomes rougher than that of as-preparation and many white spots appear on its surface after AO irradiation, which

9

may be attributed to the aggregation of oxidized products on copper surface. However, for Cu/5L BN (Fig. 3f), the surface topography is relatively homogeneous with some white lines appearing since the oxidation is more frequently observed at the wrinkles of multilayer BN films [42], indicating that oxidation degree substantially reduced.

Fig. 3.

The typical SEM images of bare Cu, Cu/1L BN and Cu/5L BN before and

after 30 min of AO irradiation.

Raman spectroscopy was used to quantify and analyze the oxidation degree of copper. As shown in Fig. 4a, Raman curves of samples are relatively flat and there is no characteristic peaks of CuO or Cu2O before AO irradiation, indicating that the as-prepared samples are almost undamaged and in good consistency with optical and SEM characterization. After 10 min of AO irradiation, four distinct Raman peaks at 218 cm-1, 337 cm-1, 535 cm-1 and 635 cm-1 appear on the bare Cu; meanwhile, Raman peaks at 218 cm-1 and 635 cm-1 belong to Cu2O and the characteristic peaks at the 337 cm-1 and 535 cm-1 can be assigned to CuO [15,43], respectively. However, the Cu/1L BN only shows a small peak at 635 cm-1 and Cu/5L BN doesn’t show insignificant 10

changes at this stage (Fig. 4b), which indicates the good oxidation resistance of BN in a short-term AO irradiation. With the further increasing irradiation time (Fig. 4c and 4d), the peak intensities of copper oxides for the bare Cu and Cu/1L BN become much stronger gradually, suggesting that the surface of these samples experiences severe and large-scale oxidation. On the contrary, there is less oxidation for Cu/5L BN sample. After 30 min of AO irradiation, only two weak peaks of copper oxides at 535 cm-1 and 635 cm-1 appears on the Cu/5L BN, which supports that the excellent barrier performance against AO can be obtained by multilayer designing of BN.

Fig. 4. Raman spectra of bare Cu , Cu/1L BN and Cu/5L BN before and after 10 min, 20 min and 30 min of AO irradiation.

11

The oxidation kinetics and valence structure of BN coated Cu foils were analyzed by XPS. Fig. 5 shows the XPS spectra of bare Cu, Cu/1L BN and Cu/5L BN before and after 30 min of AO irradiation. It can be observed that two peaks appear at the binding energy of 932.6 and 952.5 eV for all samples, belonging to the Cu 2p3/2 and Cu 2p1/2 orbital electrons of metallic copper [15]. The signal of the Cu 2p peaks for Cu/1L BN and Cu/5L BN is weaker than that of bare Cu due to the BN films coating. What’s more, the satellite peaks located at 944.0 eV and 962.5 eV representing divalent Cu2+ ions appear on the bare Cu (Fig. 5a), while Cu/1L BN and Cu/5L BN have no similar shake-up line, indicating the surface of bare Cu has been oxidized slightly in atmosphere. It indirectly proves the impermeability of BN thin film to oxidizing medium in ambient air [44]. After 30 min of AO irradiation (Fig. 5b), a specific peak shape and the satellite peaks of Cu 2p1/2 and Cu 2p3/2, characteristic of CuO [45], can be observed clearly on the bare Cu sample, evidencing that the surface of bare Cu has been oxidized severely. The BN coated Cu samples also exhibit two satellite peaks after AO irradiation, suggesting that some areas of underlying copper are oxidized. However, the corresponding intensity of Cu oxides is lower than that of bare Cu, indicating a good barrier performance of BN thin film to AO. Fig. 5c and 5d show the fit results (non-linear least square fit program using Gauss-Lorentzian peak shapes) of Cu 2p2/3 peak to clarify the composition of oxidation products for copper. Three fit peaks located at 932.6 eV, 934.0 eV and 935.1 eV are assigned to the metal Cu/Cu2O (differ by only 0.1eV), CuO and Cu(OH)2 components, respectively [46,47]. The three components may be formed through three possible oxidation processes as

12

follows at the early oxidation stages: (a) 2 + →  

(b)  + 2  →  ( ) (c)  ( ) →  +  and   + → 

The stage (a) may start rapidly at the beginning of oxidation, mainly depending on the concentration of AO. And with the increase in the thickness of oxide layer, its growth rate gets slow down gradually, which is restricted by the microstructure and the content of lattice defects on surface. The stage (b) refers to copper ion migration from the interior to the surface and then reacts with hydroxide to form Cu(OH)2. This process relies on the concentration of hydroxyl adsorbed on the copper surface. At last, the stage (c) is Cu(OH)2 meta-stable phase transformed to the CuO stable phase, at the same time, Cu2O is further oxidized to CuO [46]. In the ordinary temperature, the decomposition of Cu(OH)2 is a slow process, which explains the reason why the of the CuO component is lower than other copper oxides in all samples.

13

Fig. 5. (a,c) XPS spectra of Cu 2p3/2 and Cu 2p1/2 for bare Cu, Cu/1L BN and Cu/5L BN before and after 30 min of AO irradiation. (b,d) The peak fit of Cu 2p3/2 peak of Cu/1L BN and Cu/5L BN after 30 min AO irradiation.

XPS spectra of B 1s, N 1s and relative atomic contents are shown in Fig. 6a and 6b, respectively. Two peaks with binding energy at 190.4 eV and 398.0 eV are corresponding to the B-N bonding. Before AO irradiation, the intensity of characteristic peaks for the BN is much higher, while the intensity of these peaks obviously declines after 30 min of AO irradiation. The scatter plots express the mutative trend of B and N atom relative contents more clearly (inset in Fig. 6a and b). After AO irradiation, the relative contents of B and N atoms decrease evidently, which 14

may be caused by destructiveness of BN nanofilms owing to continuous AO irradiation and accumulation of oxygen signals on sample surface leading to the B and N signals reducing relatively. The peak fit of O 1s XPS spectra for irradiated samples is shown in Fig. 6c. For all samples, there are two distinct peaks at 530.4 eV (Cu2O and CuO), 531.7 eV (Cu(OH)2 and OH¯) [44,47,48]. Before AO irradiation, the oxygen on Cu surface mainly exists as physical absorption oxygen with relatively low level content of Cu2O and CuO which is formed in air. Therefore, the intensity of OHˉ groups is higher than lattice oxygen. For Cu/5L BN, the content of lattice oxygen (Cu2O and CuO) is the least due to the great protecting of multilayer BN. After AO irradiation, the relative content of lattice oxygen becomes much higher for all samples. It can be found that the content of CuO and Cu2O takes a bigger proportion on the surface oxide layer for bare Cu, which is attributed to a lot of copper oxidation products. However, the relative amount of lattice oxygen for Cu/5L BN is reduced apparently in comparison to the bare Cu and Cu/1L BN, indicating the excellent barrier property of BN multilayer films. The XPS results convincingly confirm the excellent protective effect of multilayer BN nanofilm against AO.

15

Fig. 6. (a,b) The XPS spectra of B 1s and N 1s peaks. The inserts are the corresponding B and N atomic relative contents scatter plots. (c,d) The peak fit of the O 1s XPS spectra before and after AO irradiation, respectively.

In order to investigate the anti-oxidation mechanism of BN films, the DFT calculation was used to measure the total system energy of AO bonding with BN. Moreover, the dynamic process of AO bombardment on BN film was simulated by MD simulation. In this calculation, three common defect types on BN films were built, which included one B or N atom vacancy, three B and one N atom vacancies [49]. Fig. 7a-c exhibits the variation of total energy of AO gradually closing to BN monolayer film with different defect types. As shown in Fig. 7a, we put one O atom on the pristine BN plane with the distance ranging from 0.5 Å to 5 Å. The calculated results 16

show that the total energy increases with the decrease in the distance between O atom and BN plane. Energy difference curve that expresses total system energy changes exhibits that the value is high up to 15 eV when one O atom close to the 6-ring center of BN (i.e., the distance between O atom and BN plane is 0.5 Å), indicating that the system state is instable. In this case, the O atom is easy to bond with BN as the models showing. This result also illustrates O atom without kinetic energy is difficult to close to pristine BN film, which explains the impossibility of O atom spontaneously passing through pristine BN. However, for B or N vacancies, total energy of one O atom on 0 Å position is the lowest and gradually increases with longer distance between O and BN plane, revealing the O atom prefers to stay at vacancies to form B-O-N or N-O-N bonding. It supports that O atom can be fixed on the BN plane, implying its diffusion process is blocked. For multilayer films, O atom is possibly absorbed on the top BN layer with no contacting with Cu, which can effectively support excellent anti-oxidation performance of multilayer films as expected. In addition, the MD simulations are carried out with a larger flux than real flux in space. For pristine film (Fig. 7d and 7e), when AO with 5 eV kinetic energy collide with BN film, they form B-O or B-O-N bonding at first, and adding more AO will damage the plane to form more defects. However, it’s still difficult for AO to pass the BN plane even at large AO flux attacking. Movie S1 clearly shows the dynamic process of AO attacking pristine BN. For the model of three B atoms and one N atom defect (Fig. 7f and 7g), O atoms can be easily absorbed on defects, and finally more defective sites also appear on the BN plane with more O atoms attacking. But AO still

17

can’t pass the built plane due to the chemisorption. Our simulation clearly illustrated AO prefer to be absorbed on BN plane rather than directly pass through the plane. Based on the above results, AO may contact with copper substrate on broken areas of monolayer film for Cu/1L BN. However, AO is difficult to pass through multilayer films to contact with Cu, implying the good barrier performance of Cu/5L BN sample against AO.

Fig. 7. (a-c) Total energy, energy difference curves and corresponding atomic models with AO at different distances from (a) pristine BN plane, (b) BN plane with B vacancy and (c) BN plane with N vacancy. (d-g) MD simulations of AO attacking pristine film (d,e) and film losing three B atoms and one N atom (f,g) with attacking time prolonging.

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Fig. 8 shows the possible barrier mechanism of monolayer and multilayer BN against AO. When AO bombards directly on the bare Cu, large oxidized areas appear on its surface. Therefore, the oxidation degree of bare Cu increases tempestuously with the extension of irradiation time (Fig. 8a). For the monolayer BN coated Cu case (Fig. 8b), AO can easily bond to BN, which inhibits arbitrary diffusion of AO so that monolayer BN blocks the large flux AO at first. Then AO can gradually damage the monolayer BN film with irradiation time prolonging but they are still difficult to directly pass monolayer BN due to adsorption effect. However, AO can contact with the underlying Cu through the defects or damaged area of BN film, resulting in the attenuation of protection performance. For the multilayer BN coated ones (Fig. 8c), AO first react with top BN layer, which also fixes AO on BN surface and makes them difficult to move. With damage accumulation, AO can undermine the top layer but it’s too hard to pass multilayer films to contact with underlying Cu due to the firm chemical adsorption and the diffusion difficulties of AO between layers. As reported before, the diffusion of AO in the layers has to last several times in vertical and horizontal direction [42,50], which also needs much energy to drive. As a consequence, it’s considered that multilayer BN film can successfully prevent the contact between AO and Cu. Hence, it shows the outstanding resisting AO performance.

19

Fig. 8. The oxidation mechanism of AO on (a) bare Cu, (b) Cu/1L BN and (c) Cu/5L BN.

4. Conclusion In this paper, the better anti-AO performance of multilayer BN films was confirmed in experiments and theoretical calculation. First, the morphology and thickness of monolayer and multilayer BN nanofilms prepared via CVD were observed. After 10, 20 and 30 min of AO irradiation, bare Cu, Cu/1L BN and Cu/5L BN eroded gradually with irradiation prolonging. However, the oxidation degree was significantly reduced for Cu/5L BN. XPS and Raman observations also confirmed the oxidation situation of Cu/5L BN was obviously ameliorated. DFT calculations and MD simulations helped to reveal anti-AO mechanism of BN films. The results clearly illustrated that the formation of chemical bonds between BN and AO led to difficulties of AO diffusion. Based on calculation results, it’s believed AO were difficult to pass multilayer BN films arriving copper surface. Thus, multilayer BN nanofilm could effectively block AO and exhibited outstanding barrier performance. 20

Acknowledgement Authors are grateful for financial support from the Key Research Projects of Frontier Science, Chinese Academy of Sciences (QYZDY-SSW-JSC009), the National Science Fund for Distinguished Young Scholars of China (Grant No. 51825505), and the National Natural Science Foundation of China (No. 51905278).

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Conflicts of interest Manuscript ID: APSUSC-D-19-11006 Title of Paper: Multilayer boron nitride nanofilm as an effective barrier for atomic oxygen irradiation

There are no conflicts to declare.

Li Cheng, Yanbin Shi, Yu Hao, Wensheng Li, Siming Ren, Liping Wang E-mail: [email protected]

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