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Remarkable oxyg
State Key Laboratory of Chemical Resource E
Technology, Beijing, 100029, China. E-mai
64425385; Tel: +86-10-64412131
† Electronic supplementary information (reconstructed H-LDH powders (Fig. S1(H-LDH/CTS)10 lms (Fig. S2); XRD patteUV-vis transmittance spectrum of the (H-of (H-LDH/CTS)n lms (Fig. S5); SEM imscratching its surface (Fig. S6); o-P(P-LDH/CTS)10 and (H-LDH/CTS)10 lms(H-LDH/CTS)10 lms (Fig. S8); schematicoxygen barrier mechanism (Fig. S9); OT(Fig. S10); and the thermogravimetric(Fig. S11). See DOI: 10.1039/c5ta02520f
Cite this: J. Mater. Chem. A, 2015, 3,12350
Received 8th April 2015Accepted 1st May 2015
DOI: 10.1039/c5ta02520f
www.rsc.org/MaterialsA
12350 | J. Mater. Chem. A, 2015, 3, 123
en barrier films based on a layereddouble hydroxide/chitosan hierarchical structure†
Ting Pan, Simin Xu, Yibo Dou, Xiaoxi Liu, Zhongze Li, Jingbin Han,* Hong Yanand Min Wei
A high performance gas barrier film was fabricated via alternate spin-coating of chitosan (CTS) and
hierarchical layered double hydroxide (H-LDH). The H-LDH synthesized by a calcination–rehydration
method shows a hierarchical structure with nanowalls aligned vertically on the LDH platelets, which
were subsequently assembled in the CTS matrix, generating a hybrid film with excellent gas barrier
properties. Compared with the (P-LDH/CTS)10 barrier film based on plate-like LDH (P-LDH), the (H-LDH/
CTS)10 film exhibits significantly enhanced oxygen barrier properties with an oxygen transmission rate
(OTR) below the detection limit of commercial instruments (<0.005 cm3 m�2 day�1 atm�1). The greatly
improved performance of the (H-LDH/CTS)10 film is attributed to the tortuous diffusion path in
hierarchical architecture space. Moreover, experimental results and theoretical calculations reveal the
existence of the adhesive force between oxygen and H-LDH (adsorption energy ¼ �2.46 eV), which
further reduces the oxygen diffusion rate and thus promotes oxygen barrier properties. Therefore, this
work provides a facile and cost-effective strategy to fabricate high gas barrier materials, which can serve
as a promising candidate for food/pharmaceutical packaging and encapsulation of electronic devices.
1. Introduction
Polymer lms have been applied as packaging materials due totheir light-weight, low-cost and good processability. However,their application as gas barrier materials is restricted because oftheir relatively high gas permeability.1–3 In order to meet theextreme requirements for gas barrier materials in food pack-aging, membrane separation and encapsulation of exibleelectronic devices,4–6 a wide variety of hybrid materials con-taining polymers and two-dimensional (2D) inorganic plateletshave been designed and fabricated.7–10 For instance, clay andgraphene were incorporated into a polymer matrix to achievehigh-performance gas barrier materials with a low oxygentransmission rate and high transparency.11,12 Since the inllingof high-oriented platelets into polymer matrices would induce a
ngineering, Beijing University of Chemical
l: [email protected]; Fax: +86-10-
ESI) available: Powder XRD patterns of); XPS spectra of H-LDH, CTS andrn of the (H-LDH/CTS)10 lm (Fig. S3);LDH/CTS)10 lm (Fig. S4); AFM imagesage of the (H-LDH/CTS)10 lm aers lifetime and intensity of PET,(Fig. S7); OTR values for PET, CTS andrepresentation showing the possibleR values for the (H-LDH/CTS)10 lmcurve of the (H-LDH/CTS)10 lm
50–12356
long diffusion length and strong resistance to the permeatinggas, considerable efforts have been devoted to the synthesis oforganic–inorganic composites as gas barrier materials. Never-theless, for such a brick-and-mortar structure, the diffusion ofgas molecules is only suppressed in the normal direction of thelms,13,14 i.e., the gas diffusion parallel to the inorganic plateletsis non-restricted. As a consequence, the mass loading of inor-ganic platelets would be very high to meet the requirement oflow gas permeability, which nally results in poor exibility andinferior toughness of the barrier materials. Furthermore, theaggregation of polymers is normally inevitable due to thedisordered structure and intertwist of cross-linkers, leading to atremendous free volume for gas molecule diffusion, which isunfavorable to the suppression of gas permeability.15 Theseissues are serious problems in the enhancement of gas barrierproperties for practical applications. Therefore, how to developsuperb barrier lms that restrain gas migration effectively isstill a challenging goal.
Layered double hydroxides (LDHs) are a kind of important2D layered material, whose structure can be generally expressedas [M1�x
IIMxIII(OH)2](A
n�)x/n$mH2O (MII and MIII are divalentand trivalent metals, respectively, An� is the interlayeranion).16–19 Recently, various LDHs with a hierarchical archi-tecture have been widely investigated in gas adsorption andseparation,20 energy storage,21 catalysis22 and pharmaceuticalelds23 due to their large surface area and high exposure ofactive sites. Our previous work reported that the plate-like LDHassembled with a polymer matrix dramatically inhibits the
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Scheme 1 Schematic representation for the fabrication of (H-LDH/CTS)n films with excellent oxygen barrier properties.
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diffusion of oxygenmolecules across the hybrid lm.24However,the gas barrier properties were not satisfactory in the case ofvery thin lms (for instance, <5 mm). If a LDHwith a hierarchicalarchitecture is incorporated with polymers, the resulting hybridlms as barrier materials would possess the following advan-tages even with less thickness: rstly, hierarchical LDH iscapable of restraining the migration of oxygen from multipledirections, inducing a more tortuous pathway for oxygendiffusion; secondly, a large amount of oxygen molecules couldbe adsorbed on the surface of hierarchical LDH due to its highsurface area, which may ll in the free volume between LDHand the polymer and consequently produce a low gas diffusionrate.
In this work, hierarchical LDH (H-LDH) was synthesized by acontinuous calcination–rehydration treatment of plate-likeLDH (P-LDH), which was then used as a building block tofabricate multilayer lms with chitosan (CTS) by an alternatespin-coating technique (Scheme 1). The resulting (H-LDH/CTS)nlm exhibits ultrahigh gas barrier behavior with an oxygentransmission rate (OTR) below the detection limit of commer-cial instruments (<0.005 cm3 m�2 day�1 atm�1). In addition, the(H-LDH/CTS)n lm displays excellent storage and thermalstabilities, which would guarantee its practical applications infood/pharmaceutical packing and electronic encapsulation.
2. Experimental section2.1 Materials
Chitosan (CTS) was purchased from Sigma-Aldrich Co. Ltd.Analytical grade chemicals including Mg(NO3)2$6H2O,Al(NO3)3$9H2O, urea, ethanol and acetone were used withoutfurther purication. Deionized water was used in all theexperimental processes.
2.2 Synthesis of plate-like LDH
Plate-like LDH (P-LDH) was prepared by a hydrothermalmethod, which was reported in our previous work.25 Typically,1 mmol Mg(NO3)2$6H2O, 0.5 mmol Al(NO3)3$9H2O, and 5mmolurea were dissolved in 100 ml of water and transferred into a
This journal is © The Royal Society of Chemistry 2015
Teon lined stainless steel autoclave. Subsequently, the auto-clave was sealed and hydrothermally treated at 110 �C for 24 h.The powdered P-LDH sample was obtained via centrifugation,washing thoroughly and drying at 60 �C for 12 h.
2.3 Synthesis of hierarchical LDH
The obtained P-LDH powdered sample was calcined in a mufflefurnace at 450 �C for 3 h, with a heating rate of 2 �Cmin�1. Aercalcination, the LDH phase was transformed into mixed metaloxides (MMOs) with a porous structure. The MMO sample(0.2 g) was then placed into 1 M NaOH solution followed byvigorous stirring for 120 min at room temperature. Then ahierarchical LDH (H-LDH) paste was obtained aer washingtwice with deionized water, followed by centrifugation. Subse-quently, the H-LDH paste was dispersed in 400 ml of deionizedwater for the fabrication of (H-LDH/CTS)n lms.
2.4 Fabrication of (H-LDH/CTS)n hybrid lms
The (H-LDH/CTS)n multilayer lms were constructed byapplying the spin-coating technique. Prior to deposition, thepoly(ethylene terephthalate) (PET) substrates were treated in anultrasonic bath with deionized water, ethanol, and acetone for10 min, respectively. Chitosan (3 g) was dissolved in deionizedwater (100 ml) containing 2 wt% acetic acid. Aer vigorousstirring for 24 h, it was expected that the amine groups of chi-tosan were fully protonated by the acetic acid.
The H-LDH suspension and CTS solution were spin-coatedonto a pre-cleaned PET substrate with 2500 rpm for 60 s and4500 rpm for 60 s alternately. The lm was dried at ambienttemperature in air for 20 min aer each spin-coating process.Subsequently, a series of these operations for H-LDH and CTSwere repeated n times to obtain the (H-LDH/CTS)n multilayerlms. As comparison samples, the (P-LDH/CTS)n and pure CTSlms were fabricated by a similar spin-coating process.
2.5 Characterization techniques
X-ray diffraction (XRD) patterns were recorded on a RigakuXRD-6000 diffractometer, using Cu Ka radiation (l ¼ 0.1542nm) at 40 kV, 30 mA. Fourier transform infrared (FT-IR) spectrawere obtained using a Vector 22 (Bruker) spectrophotometerwith 2 cm�1 resolution. UV-vis absorption spectra werecollected in the range of 200�800 nm on a Shimadzu U-3000spectrophotometer. X-ray photoelectron spectra (XPS) wererecorded on a Thermo VG Escalab 250 X-ray photoelectronspectrometer at a pressure of about 2 � 10�9 Pa with Al KaX-rays as the excitation source. The morphology was investi-gated using a scanning electron microscope (SEM; Zeiss SUPRA55) with an accelerating voltage of 20 kV, a FEI Cs-correctedTitan 80-300 high resolution transmission electron microscope(HRTEM) operated at 300 kV and a NanoScope IIIa atomic forcemicroscope (AFM) from Veeco Instruments. The specic surfacearea was obtained by the Brunauer–Emmett–Teller (BET)method with the use of a Quantachrome Autosorb-IC-VPanalyzer. Thermogravimetry (TG) curves were obtained on aBeifen PCT-IA instrument in the temperature range of30�650 �C. Temperature programmed desorption (TPD)
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Fig. 2 SEM images of the (A) P-LDH (inset: a single P-LDH micro-crystal), (B) MMO (inset: a single MMO microcrystal) and (C) H-LDH(inset: a single H-LDH microcrystal). (D) EDX mapping results for asingle microcrystal of H-LDH.
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measurements were carried out on a Micromeritics AutoChemII 2920 automated characterization system. Positron annihila-tion measurements were taken with a fast-slow coincidenceORTEC system with a time resolution of 187 ps for the full widthat half-maximum. Oxygen transmission rates (OTRs) weremeasured using VAC-V2 gas transmission rate testing equip-ment. All permeability coefficient values were averaged from atleast ve separate lms.
3. Results and discussion3.1 Preparation and characterization of H-LDH
The H-LDH sample with a hierarchical structure was synthe-sized by the following three-step route: hydrothermal synthesisof P-LDH, calcination of P-LDH to MMO and subsequent rehy-dration treatment. The pristine P-LDH, calcined sample andreconstructed H-LDH were rst characterized by powder X-raydiffraction (XRD) measurements (Fig. 1). For the pristine P-LDHsample (curve a), the peaks at 2q¼ 12.18�, 24.31�, 35.96�, 39.15�,47.22�, 61.22� and 62.31� are attributed to the (003), (006), (009),(015), (018), (110) and (113) facets of the LDH phase. Thecharacteristic reections (200) and (220) ascribed to the crys-talline MgO phase were detected aer calcination (curve b),indicating the occurrence of phase transition from LDH toMMO. Aer rehydration, the reappearance of the original LDHpeaks (curve c) indicates the back-transformation from theMMO to LDH phase, owing to the well-recognized memoryeffect.26 Moreover, the XRD patterns of the samples with variousrehydration times are shown in Fig. S1 (see ESI†). The intensityincreases gradually upon prolonging the rehydration time from1 h to 6 h, indicating the gradual growth of reconstructedH-LDH.
The structure and morphology details of the as-preparedLDH, MMO and reconstructed H-LDH samples were investi-gated by SEM and TEM. The top-view SEM image of the pristineLDH shows hexagonal plate-like microcrystals with a diameterof 2.0–2.5 mm (Fig. 2A). Calcination of the P-LDH leads to its
Fig. 1 XRD patterns of the (a) as-prepared P-LDH, (b) MMO, and(c) reconstructed H-LDH sample.
12352 | J. Mater. Chem. A, 2015, 3, 12350–12356
transformation to MMO, whose SEM image shows that MMOinherits the original hexagonal morphology of the P-LDHprecursor (Fig. 2B). It is worth noting that a large number ofsmall nanowalls vertically aligned on the original large plate ofLDH were observed aer reconstruction (Fig. 2C), giving rise toa hierarchical architecture. Meanwhile, the correspondingenergy dispersive X-ray spectroscopy (EDX) mapping revealsthat the magnesium, aluminium and oxygen elements arehomogeneously distributed throughout the H-LDH (Fig. 2D).Moreover, the TEM images of P-LDH, MMO and H-LDH(Fig. 3A–C) further illustrate the phase transition process, inaccordance with the XRD and SEM results. In addition, the
Fig. 3 TEM images of the (A) P-LDH, (B) MMO and (C) H-LDH. (D) Thelattice fringe of H-LDH (inset: the fast Fourier transform pattern).
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HR-TEM image shows a lattice fringe of 0.26 nm (Fig. 3D),which is attributed to the (012) plane of the LDH phase.
The N2 adsorption–desorption measurements were carriedout to study the surface area of the original P-LDH and thereconstructed H-LDH sample (Fig. 4A). Both the samplesexhibit a typical IV isotherm with a H3-type hysteresis loop(P/P0 > 0.4), indicating the presence of mesopores. The H-LDHexhibits a much larger specic surface area (154.3 m2 g�1) thanthat of the P-LDH sample (57.8 m2 g�1), due to the existence ofabundant nanowalls on the surface of the microcrystal. Sincethe interaction between the barrier lm and permeating gas isa crucial key for barrier properties, O2 temperature pro-grammed desorption (TPD) was performed to detect thebinding capacity for P-LDH and H-LDH to O2 molecules. Asshown in Fig. 4B, the O2 TPD prole of the H-LDH sample(curve b) displays a desorption peak located at 164 �C. Incontrast to that of H-LDH, no apparent peak is observed for theP-LDH sample (curve a). This result illustrates the existence ofstrong chemisorption between H-LDH and oxygen moleculesdue to the emergence of oxygen vacancies accompanied by thereconstruction of LDH. The oxygen vacancies serving ascapture traps would adsorb the permeating oxygen andconsequently inhibit the gas diffusion. Therefore, the H-LDHcould be an ideal building block for the fabrication of oxygenbarrier lms.
Fig. 4 (A) The N2 adsorption–desorption isotherms and (B) O2 TPDprofiles for (a) P-LDH and (b) H-LDH samples.
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3.2 Fabrication, structural and morphological study of (H-LDH/CTS)n lms
The H-LDH sample was used as building blocks to fabricate(H-LDH/CTS)n lms by the alternate spin-coating technique.UV-vis absorption spectra of the (H-LDH/CTS)n lms withvarious bilayer numbers are shown in Fig. 5. The characteristicabsorption intensity of CTS at 200 nm enhances linearly withthe increase of bilayer number n (Fig. 5, inset), indicating astepwise and regular growth process for the (H-LDH/CTS)nlms. X-ray photoelectron spectroscopy (XPS) spectra wereinvestigated to illustrate the interactions between H-LDH andCTS (Fig. S2, ESI†). For pure H-LDH and CTS polymers, thebinding energies of Mg 1s, Al 2p and C 1s are located at 1304.4,74.1 and 285.7 eV, respectively. Aer the combination of H-LDHwith CTS, the binding energies of Mg 1s and Al 2p increase to1305.2 and 74.8 eV. Concomitantly, a decreased binding energyof C 1s (C–O–H in CTS) from 285.7 to 284.4 eV is observed. Thebinding energy shi is indicative of an increased oxidation stateof Mg and Al in H-LDH and a decreased binding energy of C inthe CTS polymer, possibly due to the formation of a hydrogenbond between H-LDH and CTS.
The XRD pattern (Fig. S3, ESI†) of the obtained (H-LDH/CTS)10 lm displays a peak at 2q¼ 12.18�, which is attributed tothe (003) reection of the H-LDH structure. Moreover, this lmexhibits remarkable transparency with an average light trans-mittance of 80% across the visible-light spectrum (400–800 nm,Fig. S4, ESI†). The AFM topographical image shown in Fig. 6Ashows the morphology and roughness information of the(H-LDH/CTS)10 lm, from which a root-mean square roughnessof �15.9 nm is observed. Additionally, the surface roughness ofthe as-prepared (H-LDH/CTS)n lms (Fig. S5, ESI†) increases to�6.3, �10.6 and �15.9 nm upon increasing the assembledbilayer to 3, 6 and 10. The typical top-view SEM image of the(H-LDH/CTS)10 lm (Fig. 6B) shows that the small platelets arealigned vertically in the CTSmatrix; a lm thickness of�360 nmis observed from its side-view SEM image (Fig. 6B, inset). Theelement line scanning prole (Fig. 6C) along the cross-section
Fig. 5 UV-vis absorption spectra of the (H-LDH/CTS)n (n¼ 2–10) filmson quartz glass substrates (inset: the linear relationship between theabsorbance at 200 nm and the bilayer number n).
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Fig. 6 (A) AFM image, (B) top-view SEM image (inset: side-view SEMimage), (C) elements line scanning along the cross-section of the(H-LDH/CTS)10 film, and (D) free volume fraction of the pristine PETsubstrate, (P-LDH/CTS)10 film and (H-LDH/CTS)10 film.
Fig. 7 Oxygen transmission rate values for pristine PET, (P-LDH/CTS)nand (H-LDH/CTS)n films on the PET substrate (n ¼ 3, 6, 10).
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further veries the homogeneous distribution of magnesium,aluminium, oxygen and carbon throughout the (H-LDH/CTS)10lm. Furthermore, no delamination or peeling occurred uponscratching of the lm surface, indicating a strong adhesion ofthe lm to the substrate (Fig. S6, ESI†).
It is well known that the free volume of barrier materials isclosely related to their gas permeability.27 Thus, positron anni-hilation lifetime spectroscopy (PALS) was applied as a quanti-tative probe to explore the fraction of free volume (Fv). Fig. 6Dshows the Fv values for pristine PET, (P-LDH/CTS)10 and (H-LDH/CTS)10 lms, respectively, which are obtained based on thepositron lifetime component (s3) as well as the relative intensity(I3) (Fig. S7A and S7B, see details in the ESI†). For the pristinePET substrate, the free volume is 14.41%. Aer combinationwith P-LDH, the Fv decreases to 10.64%, illustrating that theincorporated LDH platelets reduce the free volume fraction.Moreover, further reduction of Fv (8.88%) is observed for the(H-LDH/CTS)10 lm, which is attributed to the suppression ofpolymer mobility by vertically aligned nanowalls in the H-LDH.The decreased Fv value together with strong chemisorptionbetween H-LDH and oxygen molecules (from TPD result, Fig. 4B)plays an important role in the promotion of gas barrier proper-ties, which will be discussed in the next section.
3.3 The investigation of oxygen barrier properties for the (H-LDH/CTS)n lms
The oxygen transmission rate (OTR) was measured to investi-gate the barrier properties of (P-LDH/CTS)n and (H-LDH/CTS)nlms (Fig. 7). The pristine PET substrate displays an OTR of�8.431 cm3 m�2 day�1 atm�1. The coating of (P-LDH/CTS)n and(H-LDH/CTS)n lms leads to a dramatic decrease in the OTR.Fig. 7 shows that the OTR values for (P-LDH/CTS)n lmsdecrease from �6.71 to �0.5 cm3 m�2 day�1 atm�1 withincreasing bilayer number n from 3 to 10. In contrast to the
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(P-LDH/CTS)10 lm, the pure CTS lm with the same thicknessdisplays relatively weak barrier ability with an OTR of�3.824 cm3 m�2 day�1 atm�1 (Fig. S8, ESI†). These resultsillustrate that the incorporation of highly oriented P-LDHplatelets suppresses the diffusion of oxygen, as a result of theincreased oxygen diffusion length in the vertical direction. Inaddition, the (H-LDH/CTS)n lms display superior oxygenbarrier properties to (P-LDH/CTS)n lms. The OTR of the(H-LDH/CTS)10 lm decreases dramatically with a value belowthe detection limit of commercial instruments (<0.005 cm3 m�2
day�1 atm�1). The improved oxygen barrier properties aremainly attributed to the following two factors: rstly, H-LDH iscapable of restraining the migration of oxygen both in thevertical and parallel directions (Fig. S9, ESI†) owing to itsunique hierarchical structure; secondly, the oxygen vacancieson the H-LDH surface serve as capture traps to absorb perme-ating oxygen, which results in a decreased free volume andfurther enhancement of the barrier properties.
Taking into account the potential applications of oxygenbarrier lms, the stability of the (H-LDH/CTS)10 lm was furtherstudied. The storage stability test was carried out aer sixmonths of storage (Fig. S10, ESI†). No obvious change in theOTR value is observed, which illustrates that the (H-LDH/CTS)10lm possesses excellent storage stability. Moreover, the weightof the (H-LDH/CTS)10 lm does not show obvious loss until thetemperature increases to�200 �C (Fig. S11, ESI†), indicating thehigh thermal stability of the (H-LDH/CTS)10 lm. Therefore, theultrahigh oxygen barrier properties, together with the satisfac-tory storage and thermal stabilities, would hold promise for itsprospective applications.
3.4 Computational simulation
Molecular dynamics (MD) simulations were further carried outto provide deep insight into the thermal motion of oxygen inH-LDH/CTS lms (see details in the ESI†). Aer moleculedynamics simulation of 1000 ps, the geometry of the H-LDH/CTS system is displayed in Fig. 8A. The binding energy betweenH-LDH and CTS is calculated to be as high as �0.83 eV per CTS
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Fig. 8 (A) The geometry of the H-LDH/CTS system after moleculardynamics simulation of 1000 ps; (B) the amplified picture of the redzone in (A), the blue dashed line in (A) and (B) represents the hydrogenbond; the optimized geometries of oxygen adsorbing on (C) H-LDHand (D) P-LDH.
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monomer based on rst-principles. The strong interactionindicates the existence of a hydrogen bonding network (bluedashed line in Fig. 8B) in this system, which was also conrmedby XPS results (Fig. S2, ESI†). Moreover, the interactionsbetween oxygen and the H-LDH surface were also investigatedbased on density functional theory calculations. The adsorptionenergy of oxygen on the surface of H-LDH is �2.46 eV, which ismuch higher than that on P-LDH (�0.03 eV), indicating thestronger chemical interaction between oxygen and H-LDH. Thegeometries of oxygen interacting with H-LDH/CTS and P-LDH/CTS (as control sample) systems are shown in Fig. 8C and D,respectively. The results show that the oxygen molecule isadsorbed on the H-LDH surface at the oxygen vacancy (Fig. 8C),while the oxygen molecule is located on the surface of P-LDHatly (Fig. 8D). Therefore, the result illustrates that the oxygen istrapped on the surface of H-LDH by oxygen vacancies, whichdepresses the oxygen diffusion rate.
4. Conclusions
In summary, we demonstrate the fabrication of (H-LDH/CTS)nmultilayer lms with remarkable gas barrier properties througha cost-effective spin-coating technique. The H-LDH with ahierarchical architecture plays a key role in the improvement ofoxygen barrier properties: (1) the H-LDH could inhibit thediffusion of oxygen molecules in both vertical and paralleldirections, which introduces a longer pathway for oxygenpermeation in comparison with P-LDH; (2) the oxygen diffusionrate is depressed by strong interfacial chemisorption betweenoxygen and (H-LDH/CTS)n lms due to the presence of oxygenvacancies on the H-LDH surface. In addition, the (H-LDH/CTS)nlm exhibits high stability as well as long-term maintenance.Therefore, this work offers a promising strategy for the devel-opment of novel gas barrier materials, which are needed for avariety of applications, including food/pharmaceutical pack-aging, electronics encapsulation and related areas.
Acknowledgements
This work was supported by the 863 Program (Grant no.2013AA032501), the National Natural Science Foundation of
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China (NSFC), the Doctoral Fund of Ministry of Education ofChina (20120010120010), and the Fundamental ResearchFunds for the Central Universities (YS1406). M. Wei particularlyappreciates the nancial aid from the China National Funds forDistinguished Young Scientists of the NSFC. We acknowledgeNational Supercomputing Center in Shenzhen for providing thecomputational resources and materials studio (version 6.1,CASTEP module).
Notes and references
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