Effect of titanium nitride coating on physical properties of three-dimensional graphene

Applied Surface Science 356 (2015) 399–407

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Effect of titanium nitride coating on physical properties of three-dimensional graphene Fatemeh Dabir a , Rasoul Sarraf-Mamoory a,∗ , Manuela Loeblein b,c , Siu Hon Tsang c , Edwin Hang Tong Teo b a

Materials Eng. Department, Tarbiat Modares University, 1411713116, Tehran, Iran School of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore c CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, 637553, Singapore b

a r t i c l e

i n f o

Article history: Received 10 May 2015 Received in revised form 15 July 2015 Accepted 11 August 2015 Available online 13 August 2015 Keywords: Three-dimensional graphene Titanium nitride Ammonia Physical properties

a b s t r a c t In this paper, titanium nitride (TiN) was applied on the surface and into the porous structure of threedimensional graphene (3DG) by chemical method. This method consists of immersing 3DG into a solution containing Ti ions and annealing under ammonia atmosphere at 850 ◦ C. The effects of TiN coating and high temperature annealing under NH3 on the physical properties of 3DG were investigated. For this purpose, the 3DG samples, with and without TiN coating, were characterized via XRD, SEM, XPS, and Raman spectroscopy. Then, the electrical resistivity, work function, and wettability of samples were determined by Van der Pauw method, contact angle meter, and UV photoelectron spectroscopy (UPS), respectively. The results showed that an almost pure and very crystalline TiN phase with titanium/nitrogen atomic ratio of 1.09 was formed on the 3DG network. Annealing of 3DG under NH3 resulted in locally doping of graphene with nitrogen and generation of defects in its structure. After TiN coating, the work function value of 3DG (5 eV) was reduced to 4.68 eV, while its initial water contact angle decreased from 127◦ to 83◦ . © 2015 Published by Elsevier B.V.

1. Introduction Graphene (G), two-dimensional hexagonal arrangements of carbon atoms, is at the forefront of materials research due to its unique physical, chemical, and mechanical properties [1]. The outstanding properties of graphene provide a wide range of potential applications for graphene-based functional materials [2]. However, it is worth pointing out that the performance of these functional materials is still lower than the expected data because of the restacking or aggregation of 2D graphene sheets owing to the strong van der Waals interactions between them [3]. Recently, a three-dimensional form of graphene (3DG) has been introduced as an ultra-light, very porous, conductive, and flexible interconnected network [4]. This unique structure of 3DG, while maintaining outstanding properties of two-dimensional graphene, has opened many applications for it such as in sensors [5], fuel cells

∗ Corresponding author. Tel.: +98 9121334979; fax: +98 2182884390. E-mail addresses: [email protected] (F. Dabir), [email protected] (R. Sarraf-Mamoory), [email protected] (M. Loeblein), [email protected] (S.H. Tsang), [email protected] (E.H.T. Teo). http://dx.doi.org/10.1016/j.apsusc.2015.08.086 0169-4332/© 2015 Published by Elsevier B.V.

[6], Li-batteries [7], and supercapacitors [8]. Moreover, the 3DG can be utilized as a scaffold for the growth of various nanomaterials in order to improve its own surface properties and expand the range of applications. There are some reports about the growth of different nanomaterials on 3DG, such as metal oxides [9,10], hydroxides [11], sulfides [12], noble metals [13], and polymers [14]. Nevertheless, to the best of our knowledge, there are too few studies on decoration of 3DG with transition metal nitrides. Transition metal nitrides, such as TiN, are important functional materials in research and industrial fields because of their superior electrical conductivity and high chemical stability [15]. In reports on TiN/graphene composites, so far the two-dimensional form of graphene has been used [16–19]. However, to this date, there is no study focused on the growth of TiN on the CVD-grown three-dimensional graphene networks. We expect that the TiN will increase the surface wettability of 3DG, while retaining its high electrical conductivity and chemical stability. Additionally, we also predict TiN to reduce the work function of 3DG which affects the charge transport processes. Therefore, it is expected that the TiN/3DG composite creates new applications in graphene field, because it combines superior properties of both 3DG and TiN. As a result, it is highly beneficial to evolve an

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efficient method for integrating 3DG with TiN into threedimensional networks and also to discover their functional properties and possible applications. One of the most common and cost-effective routes for 3DG synthesis is chemical vapor deposition (CVD) using a porous transition metal foam as a template with subsequent removal of metal by chemical etching [4]. In this study, the 3DG foam was synthesized via thermal CVD process using Ni template. Then, the TiN was coated on porous CVD-grown three-dimensional graphene using a chemical method, followed by annealing under NH3 atmosphere. Finally, the effects of TiN coating and high temperature annealing under NH3 atmosphere on physical properties of 3DG were investigated by comparing the 3DG properties before and after applying TiN. 2. Materials and methods 2.1. Preparation of 3DG on quartz glass The nickel foam (Latech Scientific Supply Pte. Ltd.) with thickness of 0.5 mm was used as a template and heated under Ar and H2 flow (200:50 sccm). Graphene was grown on Ni foam using an ethanol bubbler at 1000 ◦ C for 10 min. After growth, the 3DG-coated Ni foam was dip-coated with poly(methyl methacrylate) (PMMA) (950A4) to protect the structure and then immersed into diluted HCl with temperature of 80 ◦ C for 3 h to remove the Ni template. After rinsing with DI water, the PMMA-covered 3DG was transferred on quartz slide and annealed under Ar and H2 flow at 700 ◦ C for 1 h. 2.2. Coating the TiN film on 3DG At the first, a solution containing Ti ions was prepared. For this purpose, 0.5 ml of titanium tetra isopropoxide (TTIP, Aldrich), serving as the titanium source, was dissolved in 5 ml of absolute ethanol (Merck). Then, 3DG-coated quartz glass was immersed in the prepared solution with temperature of 50 ◦ C for 2 h to adsorb effectively the titanium complexes. Finally, the samples were annealed at 850 ◦ C for 3 h under NH3 flow (50 sccm) in a tube furnace. 2.3. Characterization Phase analysis of pristine 3DG and TiN/3DG was performed using low angle XRD (Panalytical) with Cu-K␣ radiation ( = 0.154056 nm). Surface morphology of samples was evaluated by SEM analysis (LEO 1550 Gemini). The TEM analysis (JEOL 2100) was used for investigation of 3DG cross section. The chemical composition of pristine 3DG and TiN/3DG was characterized via XPS (ESCALab 250i-XL & Thetaprobe A1333). The details of 3DG structure before and after TiN coating were identified by Raman spectroscopy (WITec CRM200 Raman) using an Ar ion laser ( = 532 nm). The work function of pristine 3DG and TiN/3DG was determined by UPS using a helium lamp radiation at 21.2 eV. The wettability of 3DG before and after applying TiN was measured by contact angle meter (VCA Optima-AST products, Billerica, MA) through dropping DI water on their surfaces under ambient environment. The contact angle data for three different places of each sample were averaged for more accuracy. 3. Results and discussion 3.1. Structure and phase analysis Fig. 1 represents the low angle XRD patterns of pristine 3DG and TiN/3DG. In both patterns, the peaks placed at about 26◦ , 44.4◦ , and

Fig. 1. XRD patterns of (a) pristine 3DG and (b) TiN/3DG.

54.5◦ are related to the (0 0 2), (1 0 1), and (4 0 0) planes of graphene, respectively (JCPDS 41-1487). In Fig. 1b, the other four peaks at about 36.8◦ , 43◦ , 62◦ , and 74.5◦ belong to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of rock-salt structure of TiN, respectively (JCPDS 870633). These results are indicative of formation of crystalline TiN on 3DG. There are no other peaks in the diffraction patterns of TiN/3DG, representating that an almost pure TiN phase has been grown on the graphene surface. This result has been induced from XRD analysis and should be clarified by another analytical technique such as XPS. Additional information such as number of stacked layers of 3DG samples with and without TiN coating was extracted from XRD data. The average number of the stacked graphene layers (n) can be obtained using the Scherrer equation (Eq. (1)) and by fitting the (0 0 2) peak of graphene [20]. L002 =

K ˇ002 cos 002

(1)

where, L002 is the crystallite size perpendicular to the (0 0 2) plane, K is the shape factor (0.89),  is the X-ray wavelength (0.154056 nm), ˇ is the full width at half maximum (FWHM), and  is the peak position. Fig. 2 shows the Lorentzian fits for (0 0 2) reflection of graphene in 3DG with and without TiN coating. The average number of graphene layers (n) was estimated from Eq. (2) after calculation of L002 using Eq. (1). n=

(L002 + d002 ) d002

(2)

where, d002 is the interlayer space [21]. Table 1 shows the extracted data from (0 0 2) reflection of graphene for both pristine 3DG and TiN/3DG. The (0 0 2) reflection of pristine 3DG (Fig. 2a) shows an asymmetry in line shape and can be fitted with three different peaks which means that there is an inhomogeneity in structure of synthesized 3DG. 71% of 3DG structure has a number of graphene layers of 31. 21% of 3DG is comprised by 54 layers and the rest, which is 8%, has 47 layers. Therefore, the average number of stacked graphene layers in pristine 3DG is about 37 layers. It is clear from Table 1 that the decoration of 3DG with TiN and annealing under NH3 have led to a change in the number of graphene layers. Moreover, the (0 0 2) reflection of graphene for TiN/3DG (Fig. 2b) is symmetric and can be fitted with one peak.

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Table 1 Extracted data from XRD for 3DG before and after TiN coating. Sample

 (◦ )

ˇ (◦ )

d002 (nm)

L002 (nm)

L/L002 (%)

n

3DG with TiN coating Pristine 3DG

13.26 13.05 (71%) 13.26 (21%) 12.72 (8%)

0.61 0.80 0.46 0.50

0.3359 0.3414 0.3360 0.3502

13.24 10.08 17.55 16.11

7.68 5.75 6.02 9.53

41 31 54 47

Fig. 3. TEM image from the edge of CVD-grown 3DG.

The increase in average number of stacked graphene layers after applying TiN from 37 to 41 arises due to the high temperature annealing under NH3 atmosphere. During annealing process, the active radicals originated from NH3 decomposition and the remained oxygen atoms due to the TTIP precursor may etch away some parts of 3DG structure with lower number of layers and also lower chemical stability in the form of gases. As a result, the average number of graphene layers increases. It should be noted that the number of layers obtained from Scherer equation is an approximate result due to the uncertainty in calculating the L002 by Eq. (1). Assuming the variables in Eq. (1) are uncorrelated, the squared uncertainty level in determining the L002 by Scherrer equation can be achieved using Eq. (4) obtained by differentiating Eq. (1) individually with respect to K, , ˇ, and . The K value usually is estimated 0.89 for 0 0 2 line of graphene and graphite. However, it depends on the crystallite size and varies between 0.85 and 0.9 for AB-stacking type carbon-based materials with crystallite size larger than 6 nm [23]. Therefore, the K value was chosen 0.05. The  value was chosen 4.04 × 10−5 nm with respect to Cu k␣1 radiation width of 2.11 eV. The step size in XRD analysis was 0.02◦ . Therefore, the  and ˇ were chosen 0.01◦ and 0.02◦ , respectively. The calculated percentage of uncertainty level (%L/L002 ) for each diffraction angle has been given in Table 1.

Fig. 2. Fitted (0 0 2) reflection of graphene in (a) pristine 3DG and (b) TiN/3DG.

This is indicative of more homogeneity in 3DG structure compared to its structure before applying TiN. There are two phenomena in TiN/3DG which can affect the 3DG structure: first, applying TiN coating on 3DG and second, high temperature annealing of 3DG under ammonia atmosphere. The TiN coating induces the strain on 3DG structure due to its interaction with graphene. This strain originates from the difference in supercell lattice parameters and thermal expansion coefficients between TiN and graphene. According to the upward shift of (0 0 2) peak of graphene in TiN/3DG compared to the pristine 3DG, it can be concluded that the outoff plane strain is compressive in nature and results in a decrease in both graphene interlayer space (d) and lattice constant in C direction. The strain in 3DG lattice can be computed quantitatively using Eq. (3) obtained by differentiating Bragg’s law [22]; where, ε is the strain and  is the shift in peak position resulted from strain. The calculated out-of plane strain in 3DG lattice is −1.04%.  = −εtan 

(3)

2



(L) =

2

(L) =



∂L ∂K

2

 K 2 +

 K ˇcos 

∂L ∂

2  +

2

 2 +

∂L ∂ˇ

2

2 

K  ˇcos 

+

 2 ˇ2 +

∂L ∂

−K ˇ ˇ2 cos 

 2

2  +

Ksin   ˇcos2 

2 (4)

The accurate number of stacked layers of 3DG can be identified by TEM analysis. Fig. 3 shows the TEM image from the edge of CVDgrown 3DG. According to this image, the number of layers of CVDgrown 3DG is about 29 layers, which is in good agreement with the result extracted from the XRD data (71% with 31 layers). 3.2. Surface morphology Fig. 4 shows the SEM images of CVD-grown 3DG with and without TiN coating. It is clear that the CVD-grown 3DG (Fig. 4a) follows

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Fig. 6. XPS spectra of (a) pristine 3DG and (b) TiN/3DG.

annealing process. In fact, during the first step, complexes containing Ti ions inside solution have been deposited on the 3DG surface as discrete films due to the non-flat and hydrophobic surface of 3DG. Equation (5) gives the formation of Ti complexes (Ti(C2 H5 O)4 ) inside ethanolic solution of TTIP. These complexes are soluble in organic solvents.



Ti OCH(CH3 )2

Fig. 4. SEM images of CVD-grown 3DG, (a) before, and (b) after TiN coating.



4

+ 4C2 H5 OH = Ti(C2 H5 O)4 + 4C3 H8 O

(5)

Over time, Ti complexes have been preferably nucleated on the discrete films instead of the 3DG surface. In fact, thermodynamically, the required energy for nucleation of particles on the initial discrete films, with similar structure, is much lower than the one required for their nucleation on the 3DG surface. After taking out the samples from solution, the deposited Ti complexes on 3DG react with humidity in air according to Eq. (6). In these equations R stands for C2 H5 . Ti(RO)4 + H2 O = Ti(RO)3 (OH) + ROH

(6)

In Eq. (6), if the molar ratio of H2 O to Ti(RO)4 is 4, the Ti hydroxide with chemical formula of Ti(OH)4 will form. Finally, the Ti complexes react with atomic N originated from NH3 decomposition and convert to TiN. 3.3. Chemical composition

Fig. 5. SEM image of TiN/3DG coating before annealing in NH3 .

the three dimensional interconnected porous structure of the Ni foam. In inset of Fig. 4a, some wrinkles can be seen on 3DG surface which have emerged during the thermal CVD process owing to the different thermal expansion coefficients between graphene and nickel [24,25]. These wrinkles enhance the surface area and improve the mechanical properties of 3DG [26]. Regarding the SEM image of TiN/3DG coating (Fig. 4b), the TiN phase has been deposited as a discrete film on 3DG surface. Moreover, there are some particles deposited on the discrete TiN film which can be seen in the inset of Fig. 4b which is a higher magnification image. The surface morphology of TiN/3DG coating before annealing in NH3 is given in Fig. 5. Here it can be seen that the discontinuity of TiN thin film on 3DG surface arises before the

Fig. 6 shows the XPS spectra of pristine 3DG and TiN/3DG coating annealed at 850 ◦ C. In both spectra, the peaks located at about 284.5 and 531 eV are assigned to the C1s and O1s, respectively. In Fig. 6b, there are two other peaks of Ti2p and N1s at about 456 and 397 eV due to the presence of TiN coating on 3DG surface [27]. In order to explain why 850 ◦ C was selected for final annealing, it needs to say that the amount of oxygen in TiN/3DG structure decreases with an increase in annealing temperature. However, at annealing temperatures higher than 850 ◦ C, peeling off TiN film from 3DG was occurred owing to the increase of thermal mismatch between TiN and graphene with temperature. Therefore, 850 ◦ C was chosen as optimum annealing temperature. The calculated Ti/N atomic ratio of TiN/3DG coating, annealed at 850 ◦ C, is 1.09. For further investigation, high-resolution XPS spectra of the C1s and O1s peaks for two different samples are shown in Figs. 7 and 8, respectively. The C1s spectrum of pristine 3DG (Fig. 7a) has six peaks of 284.40, 284.72, 285.31, 285.71, 285.99, and 286.50 eV. The peaks at 284.40 and 284.72 eV are assigned to the sp2 and sp3 hybridization of carbon–carbon bonds, respectively. The peaks

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Fig. 7. High-resolution XPS spectra of the C1s peaks for (a) pristine 3DG, and (b) TiN/3DG.

located at 285.31, 285.99, and 286.50 are attributed to the oxygen containing groups of C OH, C O, and C O, respectively [28]. The peak at 285.71 eV can be related to the airborne hydrocarbon contaminants adsorbed onto the 3DG surface. These contaminants are desorbed by thermal annealing [29]. In the C1s spectrum of TiN/3DG (Fig. 7b), the peaks assigned to the C C, C C, C OH, C O, and C O bonds have been located at 284.44, 284.83, 285.39, 285.89, and 286.84 eV, respectively. In this spectrum (Fig. 7b), the peak placed at 286.29 eV is ascribed to the sp2 hybridization of carbon–nitrogen bonds [30]. The emersion of C–N peak in the C1s spectrum of TiN/3DG is indicating that graphene has been doped with nitrogen during annealing process under ammonia. The O1s spectrum of pristine 3DG (Fig. 8a) has one peak at about 531.9 eV that is due to the C–O bond. However, in the O1s spectrum of TiN/3DG (Fig. 8b), there are two peaks of O–Ti and O–C bonds at about 530 and 532 eV, respectively [31]. The O–Ti peak is related to titanium oxide phases formed on the coating surface due to the surface oxidation process. In XRD pattern of TiN/3DG (Fig. 1b), there were no characteristic peaks corresponding to titanium oxide phases. The thickness of oxide layer on the surface of coating, originated from exposure of sample to air, is very thin (below 10 nm). Therefore, XPS as a surface sensitive analysis can detect the surface oxide layer, while XRD is not able to characterize it because of high X-ray penetration depth. The high-resolution XPS spectra of the Ti2p and N1s peaks of TiN/3DG are shown in Figs. 9 and 10, respectively. In the Ti2p spectrum (Fig. 9), there are two low and high binding energy regions, corresponding to Ti2p3/2 and Ti2p1/2 , respectively. In both regions,

Fig. 8. High-resolution XPS spectra of the O1s peaks for (a) pristine 3DG, and (b) TiN/3DG.

Fig. 9. High-resolution XPS spectrum of Ti2p peak.

the peak with the lowest binding energy belongs to TiN phase and the peaks with higher binding energy are attributed to TiNx Oy , and TiO2 phases [32]. The N1s peak (Fig. 10) contains three peaks at about 396.52, 397.25 and 398.55 eV which are ascribed to N–Ti, N–Ti–O, and N–C

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Fig. 10. High-resolution XPS spectrum of N1s peak.

bonds, respectively [32,33]. The N–C bond originates from pyridinelike nitrogen atoms located at the edge of graphene or adjacent to carbon vacancies inside graphene network. In this configuration, each nitrogen atom has sp2 hybridized bonding with two neighboring carbon atoms [33]. The appearance of sp2 hybridized N–C peak in the N1s spectrum is in correspondence with the C–N peak in the C1s spectrum of TiN/3DG (Fig. 7b). High temperature annealing of graphene at NH3 atmosphere can result in the formation of N-doped graphene. However, it should be pointed out that the doping level is low by this method. The quaternary graphitic N (401.1–402.7 eV), representative of real nitrogen doping, was not observed in this process which is in agreement with literature [34]. 3.4. Raman spectroscopy Fig. 11. Raman spectra of (a) pristine 3DG and (b) TiN/3DG.

Fig. 11 shows the Raman spectra of pristine 3DG and TiN/3DG. In the Raman spectrum of 3DG (Fig. 11a), there are two main peaks at 1590 and 2700 cm−1 , assigned to the G and 2D bands of graphene, respectively. The G-peak has higher intensity than the 2D-peak that is representative of multi-layer graphene [35]. There is no Dpeak at about 1350 cm−1 in Fig. 11a, indicating that the CVD-grown 3DG, synthesized in this work, has good quality without presence of defects in its structure. In the Raman spectrum of TiN/3DG coating (Fig. 11b), there is a weak D peak of graphene at about 1350 cm−1 , indicating the formation of defects in 3DG structure during high temperature annealing under ammonia. This result is in accordance with the presence of N–C peak in high resolution XPS spectra of the C1s and N1s for TiN/3DG (Figs. 7b and 10). As stated before, high temperature annealing of 3DG under NH3 induces locally doping of graphene with nitrogen atoms which is one kind of point defects. Another reason for the appearance of D-peak in the Raman spectrum of TiN/3DG is related to the strain effect on atomic position of carbon atoms resulting from the applied TiN coating on 3DG. The creation of strain on 3DG structure by applying TiN coating was proved before in XRD section (Section 3.1). Fig. 12 shows the G-peak and 2D-peak of pristine 3DG and TiN/3DG. In the Raman spectrum of TiN/3DG, the G band (1579 cm−1 ) has shifted down and the 2D band (2718 cm−1 ) has shifted up compared to the G (1583 cm−1 ) and 2D (2705 cm−1 ) bands of pristine 3DG. The positions of G and 2D bands of graphene are sensitive to several factors such as number of graphene layers, doping, and residual strain. The increase in average number of graphene layers leads to both red-shift of the G-peak and blue-shift of the 2D-peak [36]. Electron or hole doping of graphene results in an upward shift of

the G-peak, while the 2D peak shows an upward shift with hole doping and a downward shift with electron doping [37]. The strain on graphene also causes the shifting of both G and 2D bands because of the change in the length of carbon–carbon bonds which affects the bonds strength and thus their vibrational frequency [38]. Both the G and 2D peaks of graphene show a downward shift under tensile strain and an upward shift upon compression [22]. As noted before, applying the TiN coating on 3DG and high temperature annealing of 3DG under ammonia have resulted in three different phenomena: the change in number of graphene layers, creation of strain on 3DG, and locally N-doping of graphene, which are affecting the position of Raman peaks. However, according to the red shift of G-peak and the blue shift of 2D-peak of graphene in the TiN/3DG compared to the pristine 3DG, it can be concluded that the change in number of graphene layers is the most influential factor in the Raman signature. In the Raman spectrum of TiN/3DG (Fig. 11b), there are three additional peaks located at about 200, 300, and 550 cm−1 . These peaks are ascribed to the nonstoichiometric TiN Raman spectrum. The perfect stoichiometric TiN has symmetric cubic structure. Therefore, the first-order Raman scattering is prohibited for it. However, the nonstoichiometric TiN is specified by the Raman modes due to the defects such as vacancies and interstitials [39]. In the Raman spectrum of nonstoichiometric TiN, the low frequency peaks at around 200 and 300 cm−1 are assigned to the first order transverse and longitudinal acoustic (TA and LA) phonons, respectively. These acoustic phonons originate from vibrations of heavy Ti ions near N vacancies. The high frequency peak at about

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550 cm−1 is ascribed to the transverse optical (TO) phonons due to the vibrations of light N ions near Ti vacancies [40,41]. 3.5. Work function Fig. 13 shows the UPS spectra of pristine 3DG and TiN/3DG. The work functions of pristine 3DG and TiN/3DG were obtained from their UPS spectra around the secondary electrons threshold regions which are represented in Fig. 13a. Eq. (7) was used to calculate the work function value (˚) of coatings [42]: ˚ = h − Eth

Fig. 12. (a) G-peak and (b) 2D-peak of pristine 3DG and TiN/3DG.

(7)

where, h is the photon energy of the excitation light (21.2 eV) and Eth is the secondary electron threshold energy. The Eth was obtained by linear extrapolation of the background and the straight intercept of the secondary electrons threshold region in the UPS spectra. The Eth for 3DG coating was achieved at 16.2 eV, suggesting that the work function of CVD-grown 3DG is 5.0 eV. Applying the TiN film has reduced the work function of 3DG to 4.68 eV. The titanium nitride behaves like metals, and according to theory [43], metal adatoms reduce the work function of carbon-based nanostructures. The decrease in work function of the TiN-coated 3DG can be attributed to the creation of a surface dipole moment due to the electron transfer from TiN to 3DG. Another probable reason for lower work function of TiN/3DG coating compared to pristine 3DG is the nitrogen doping of graphene during high temperature annealing of 3DG at NH3 . The change of graphene work function through nitrogen doping depends on the nature of C–N bonding configuration. The pyridinicN results in p-type doping of graphene. However, the existence of bare pyridinic-N may be possible only in high vacuum because of its high reactivity. Under real conditions, the pyridinic-N is stabilized by a hydrogen atom [44]. The hydrogenation of pyridinic-N transforms its effect from p to n-type doping [45,46]. The n-type doping of graphene leads to an upward shift in its Fermi level [47], thus, a decrease in the energy difference between the vacuum and Fermi levels which is defined as work function. Fig. 13b shows the UPS spectra of pristine 3DG and TiN/3DG coating in the vicinity of the Fermi level (EF ). As can be seen, there is a main peak at the binding energy of about 13–13.5 eV for the both pristine 3DG and TiN/3DG which is attributed to the emitted photoelectrons from the mixed 2s–2p hybridized state of graphene. In inset of the Fig. 13b, a shoulder at about 2.25 eV can be seen

Fig. 13. UPS spectra of pristine 3DG and TiN/3DG at (a) the secondary electrons threshold regions and (b) the vicinity of Fermi edge.

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for both samples. This shoulder is ascribed to the emitted photoelectrons from the 2p-␲ state of graphene [48]. The presence of these peaks in the UPS spectrum of TiN/3DG declares that the sp2 hybridized carbon skeleton of graphene has been retained after applying TiN coating. However, the 2s–2p hybridized state in the UPS spectrum of TiN/3DG (13.3 eV) has a blue shift compared to pristine 3DG (13.1 eV). This shift may be related to the increase of graphene layers in TiN/3DG compared to pristine 3DG. According to Fig. 13b, there is an additional shoulder at about 6 eV in the UPS spectrum of TiN/3DG which is not observed for pristine 3DG. This shoulder originates from the TiN coating and can be attributed to the hybridized N 2p-Ti 3d states of TiN which is responsible for the covalent character of the Ti–N bonding and gives the high chemical stability to TiN [49]. The other difference between UPS spectra of TiN/3DG and pristine 3DG is in the region near the Fermi edge which can be seen in the inset of Fig. 13b. The appearance of new shoulder at about 3.3 eV for TiN/3DG may be related to the metal Ti 3d state of TiN which gives relatively high electrical conductivity to it. The other probable reason for this phenomenon is the existence of vacancies in nonstoichiometric titanium nitride [49]. 3.6. Wettability The initial contact angle of DI water on 3DG surface was obtained at 127◦ , representative of hydrophobic properties. The wettability, as a surface property, is extremely sensitive to surface contaminations. According to the C1s spectrum of pristine 3DG (Fig. 7a), the adsorption of airborne hydrocarbon contaminants on 3DG surface was proved. These hydrocarbon groups decrease the surface energy of the 3DG and increase its water contact angle [29]. The initial water contact angle of TiN/3DG was measured to be 83◦ , indicating that the wettability of 3DG sample increased by applying the TiN coating on its surface. The TiN coating increased surface roughness of the 3DG which improves the wettability. Moreover, the creation of defects into 3DG structure, after high temperature annealing under NH3 atmosphere, increased the polarity of the surface, and so, the surface energy increased. 4. Conclusions In this study, titanium nitride was applied on 3DG by immersing the 3DG into a solution containing Ti complexes and then annealing under NH3 atmosphere. The results showed the formation of crystalline TiN phase on 3DG with Ti/N atomic ratio of 1.09 and presence of titanium oxides phases due to surface oxidation. The high temperature annealing of 3DG under NH3 led to the partially etching the regions with low graphene layers and thus an increase in average number of stacked graphene layers. With regard to SEM images, TiN was deposited on 3DG surface as a discrete film with some particles on it. The discontinuity of TiN film emerged before annealing process. Annealing of 3DG at high temperature under NH3 atmosphere resulted in locally doping of graphene with nitrogen at the edge of graphene. Applying the TiN coating reduced the work function of 3DG from 5 eV to 4.68 eV and increased its wettability. The sp2 hybridized carbon skeleton of graphene was preserved after applying TiN coating. These results show that the decoration of 3DG with TiN can open new applications for it in advanced devices because of modifying its surface properties, while retaining its sp2 carbon skeleton. Acknowledgments The authors would like to acknowledge the funding support from NTU-A*STAR Silicon Technologies Centre of Excellence under

the program grant No. 1123510003 and Singapore Ministry of Education Academic Research Fund Tier 2 No. MOE2013-T2-2-050.

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