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3DG composite
Materials and Design 90 (2016) 524–531
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Effect of annealing temperature on physical properties of nanostructured TiN/3DG composite Fatemeh Dabir a, Rasoul Sarraf-Mamoory a,⁎, Manuela Loeblein b,c, Siu Hon Tsang c, Edwin Hang Tong Teo b a b c
Materials Eng. Department, Tarbiat Modares University, 1411713116 Tehran, Iran School of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, 637553, Singapore
a r t i c l e
i n f o
Article history: Received 12 January 2015 Received in revised form 30 September 2015 Accepted 28 October 2015 Available online 29 October 2015 Keywords: Titanium nitride Three-dimensional graphene Chemical method Annealing temperature Physical properties
a b s t r a c t Recently, three-dimensional graphene (3DG) has attracted much attention in many research fields due to its unique structure and considerable properties. In order to expand the range of applications of 3DG, the suitable nanomaterials can be grown on its surface. In this study, titanium nitride (TiN) phase was deposited on 3DG porous structure by chemical method. This method contains two steps of immersing 3DG into a solution containing Ti ions and then annealing under ammonia atmosphere. The effect of annealing temperature on type of synthesized phases, their morphology, and stoichiometry was investigated. For this purpose, the samples were annealed at different temperatures (750–900 °C) and analyzed via various techniques. The results showed that increasing annealing temperature results in increased crystallite size and lattice constant, while decreased oxygen content in TiN structure. Annealing at 850 °C resulted in the most stoichiometric composition with titanium/nitrogen atomic ratio of 1.09, which had the lowest electrical resistivity of 0.41 Ω cm and lowest work function of 4.68 eV. After applying TiN, the water contact angle of 3DG (127°) was reduced to lower than 90°. Such TiN/ 3DG composite can be a promising candidate as an electrode in solar cells. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Graphene, a two-dimensional (2D) nanomaterial, is now at the forefront of materials research owing to its exceptional physical, chemical, and mechanical properties such as excellent electrical and thermal conductivity, high chemical stability, and outstanding mechanical strength [1]. One of the challenges in this field is utilization of the 2D carbonbased nanomaterials in 3D bulk networks, while retaining their outstanding properties. Composite networks based on graphene sheets have limited its physical properties because of introducing structural defects within the blocks [2]. In the last few years, three-dimensional graphene has been proposed as an ultralight new material with a flexible, highly porous, and electrically conductive interconnected network [3,4]. This 3DG has found some applications in sensors [5], Li-batteries [6], fuel cells [7], and electrochemical cells [8] due to its unique properties. The 3DG also can be used as a scaffold for adsorbing and keeping molecules or as a fast transport channel for charge carriers [9]. The hydrophobic surface of 3DG has limited its application as an adsorbent scaffold [10]. The other limitation of 3DG is its high work function (~5 eV) which affects the charge transport processes [11]. In order to extend the range of applications, various nanomaterials have been coated on 3DG such as polymers [12], noble metals [13], metal oxides [14], ⁎ Corresponding author. E-mail address: [email protected] (R. Sarraf-Mamoory).
http://dx.doi.org/10.1016/j.matdes.2015.10.152 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
sulfides [15], and hydroxides [16]. However, to the best of our knowledge, to this date there is no report on the synthesis of transition metal nitride/3DG composites. Transition metal nitrides are important functional materials in many applications because of their superior electrical conductivity and high chemical stability [17,18]. Among them, TiN is the most appealing compound, because it has one of the lowest electrical resistivity. In reports on decoration of graphene with TiN, the twodimensional form of graphene has been used [19–22]. In these reports, the TiN/2DG has been utilized as an electrode in energy conversion and storage systems such as solar cells [19], fuel cells [20], electrochemical capacitors [21], and Li-batteries [22]. However, there is no research focused on the growth of titanium nitride on the porous threedimensional graphene networks. It is expected that the TiN/3DG composites have applications in energy conversion field such as photovoltaic cells because it integrates unique properties of both 3DG and TiN. For example, these composites may be utilized in dye sensitized solar cells (DSSCs), simultaneously as dye adsorbent framework and charge transport channel, because TiN improves surface wettability of 3DG and has no detrimental effect on its high electrical conductivity. Meanwhile, TiN can reduce the work function of 3DG which facilitates the electron injection from excited dye molecules to the 3DG. In this paper, the 3DG was synthesized by chemical vapor deposition (CVD) method using nickel foam template with subsequent chemical etching of Ni. Then, TiN was applied on CVD-grown 3DG via chemical method, followed by high temperature annealing at NH3. Finally, the
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effect of annealing temperature on physical properties of TiN/3DG composite was analyzed. The TiN microstructure is one of the important parameters influencing the TiN/3DG properties and the annealing temperature has the strong effect on it. 2. Materials and methods 2.1. Synthesis of 3DG on quartz glass Nickel foam template with thickness of 0.5 mm was heated under Ar and H2 flow until 1000 °C inside quartz tube. The soaking time at 1000 °C was 5 min. Then, ethanol vapors were entered into the quartz tube for 10 min via an ethanol bubbler. After graphene growth on Ni template, the graphene/Ni foam was dip-coated with poly(methyl methacrylate) (PMMA) solution for structure protection. In order to remove the Ni template, the PMMA-covered graphene/Ni foam was immersed in HCl solution. Finally, the PMMA/3DG was transferred on quartz substrate and annealed under Ar and H2 flow at 700 °C for 1 h. 2.2. Synthesis of TiN on 3DG For the synthesis of TiN, 0.5 ml of titanium isopropoxide (Aldrich), as titanium source, was dissolved in 5 ml of absolute ethanol (Merck). Then, the 3DG-transferred on quartz glass was immersed in the prepared solution for effective adsorption of titanium ions. The immersion time was 2 h at 50 °C. After drying, the coatings were annealed at various temperatures (750–900) °C for 3 h under NH3 flow (50 sccm). 2.3. Characterization Phase analyses of TiN/3DG coatings were identified using XRD (Bruker) with Cu–Kα radiation (λ = 0.154060 nm). Investigation of surface morphology and chemical composition analysis of the coatings were conducted by scanning electron microscopy (SEM) and energydispersive X-ray (EDX) spectroscopy, respectively. In order to identify the TiN stoichiometry, X-ray photoelectron spectroscopy (XPS) was performed with ThetaProbe A1333 system using monochromic Al–Kα radiation (hυ = 1486.7 eV). Raman spectroscopy (WITec) was conducted on coatings by using an Ar ion laser with wavelength of 532 nm. The Van der Pauw method was used to determine electrical resistivity of samples. Work function of the coatings was measured by UPS using a helium lamp radiation at 21.2 eV. The wettability of 3DG samples before and after TiN coatings was identified by contact angle meter (VCA Optima-AST products, Billerica, MA) using dropping DI water on their surfaces at room temperature. The wettability experiments were repeated in three different sites of each sample for more accuracy.
Fig. 1. XRD patterns of TiN/3DG coatings annealed at various temperatures.
where, β is the FWHM (full width of half maximum) of the prominent peak of TiN phase in the XRD patterns (200) and θ is the peak position in radian. Fig. 2 shows the relation between TiN crystallite size and annealing temperature. It should be pointed out that the uncertainty level in calculating the crystallite size (ΔD) was accounted as error bars in Fig. 2. It can be observed that the TiN crystallite size increases with annealing temperature which is due to the increase of the mobility of atoms and acceleration of the diffusion with temperature. The other information estimated from XRD data is the lattice constant. The lattice constant for cubic structure (a) is calculated using Eq. (2), where d is the distance between (hkl) planes and is measured from Bragg's law (Eq. (3)) [24]. The uncertainty level in calculating the a value approaches zero as θ = 90° (i.e., 2θ = 180°). Therefore, for achieving the true value for lattice constant, the a value for each diffraction angle was calculated from Eq. (2), then the measured a values was plotted vs. cos2θ for each annealing temperature. After extrapolation to
3. Results and discussions 3.1. XRD analysis Fig. 1 shows the XRD patterns of TiN/3DG coatings annealed at various temperatures. In diffraction patterns of all samples, the peak of 26° is related to the (002) plane of graphene (JCPDS 41-1487). The other three peaks at 36°, 43° and 62° belong to (111), (200) and (220) planes of FCC structure of TiN, respectively (JCPDS 87-0632). The hump between 20° and 25° in XRD patterns originated from quartz substrate. These results represent that an almost pure and crystalline TiN phase has been formed on the graphene surface. The crystallite size (D) of TiN phase at various annealing temperatures can be obtained using Scherrer's equation [23]:
D¼
0:9λ βcosθ
ð1Þ
Fig. 2. Crystallite size of TiN vs. annealing temperature.
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cos2θ = 0 (i.e. 2θ = 180°), the precise value of lattice constant was obtained. 1 2
d
¼
2
2
h þk þl
2
ð2Þ
a2
λ ¼ 2dsinθ
ð3Þ
Table 1 represents the crystallite size and lattice constant of different samples annealed at various temperatures. In all the samples, the calculated lattice constant is different from stoichiometric pure TiN phase (a = 4.241 Å), which is probably due to the presence of residual compressive stresses, nonstoichiometric TiN, and impurities such as oxygen inside TiN structure. Oxygen atoms may substitute nitrogen atoms in TiN structure resulting in the formation of titanium oxynitride (TiNxOy). Indeed, TiNxOy is a solid solution of TiN and TiO, which are crystallized together in a FCC structure. Therefore, according to Vegard's law, the lattice constant of TiNxOy phase lies between the lattice constants of TiN and TiO (a = 4.173–4.185 Å) phases (Eq. (4)). However, it should be noted that the Vegard's law is not accurate for nanocrystals and defective structures [25]. aTiNxOy ¼ xaTiN þ yaTiO
ð4Þ
3.2. SEM/EDX analysis Fig. 3 shows the SEM image of CVD-grown 3DG. With regard to this figure, the CVD-grown 3DG has three-dimensional interconnected porous structure, which has taken it from the nickel template. Fig. 4 shows the SEM images of TiN/3DG coatings annealed at different temperatures. In all images, it can be seen that the TiN phase has been deposited on 3DG surface as a discontinuous film with some particles on it. The discontinuity of TiN film on 3DG surface has been emerged before annealing process and is because of nonflat and hydrophobic surface of 3DG. The presence of particles on discrete TiN film is related to the deposition of remained Ti complexes heterogeneously on the initial deposited Ti-complexes instead of on the 3DG surface, because the required energy for nucleation of particles on 3DG is much higher than the one for their nucleation on discrete film with similar structure. The deposited Ti-complexes have been converted to titanium nitride after high temperature annealing under NH3 atmosphere. At annealing temperature of 900 °C (Fig. 4d), peeling off the TiN film from 3DG substrate is observed, which is due to the increase of thermal mismatch between TiN and graphene with temperature. Fig. 5 shows the SEM images of TiN/3DG coatings, focused on discrete TiN film, annealed at various temperatures. According to this figure, the morphology of TiN film has not been changed with an increase in annealing temperature from 750 to 800 °C. However, with further increase up to 850 °C, very small holes have been created on TiN film. At annealing temperature of 900 °C, the morphology of TiN film has been changed remarkably and its surface is uneven. The reason for creation of holes and unevenness on TiN surface at annealing temperatures higher than 800 °C can be related to the desorption of oxygen and nitrogen atoms from TiN/3DG coating, because the increase in annealing temperature provides the required energy for desorption phenomenon.
Fig. 3. SEM image of CVD-grown 3DG.
Meanwhile, according to Fig. 5, as the annealing temperature increases, the average size of TiN particles deposited on discrete TiN film increases. By using ImageJ analyzer, the average particle sizes at various annealing temperatures of 750, 800, 850, and 900 °C were obtained 447, 521, 557, and 839 nm, respectively. Fig. 6 shows the EDX analyses performed on three different regions of TiN/3DG coating, annealed at 850 °C. According to this figure, all three regions consist of C, Ti, N, O, and Si elements. However, the amounts of these elements are not equal in different regions. It should be mentioned that the result of EDX analysis for each region is impressed by composition of other regions near it because of high diffusion depth of X-ray. The C and Si peaks in EDX spectra are attributed to 3DG and quartz substrate, respectively. The oxygen peak can be related to the existence of oxygen-containing groups onto 3DG network, surface oxides formed on TiN film, Ti oxynitride phases, and SiO2 substrate. The Ti and N peaks can be attributed to the presence of TiN and TiNxOy phases. The other reason for the presence of N peaks in EDX spectra can be related to the incorporation of N atoms inside 3DG network. In region 1 (3DG surface), the amount of C is maximum. In both regions 2 (discrete TiN film) and 3 (TiN particle), the amounts of Ti, N, and O elements have increased, while the amount of C has decreased compared to region 1. The higher amount of C element and Ti/N atomic ratio in region 2 than them in region 3 may indicate that in region 2, some C atoms from 3DG have diffused into TiN discrete film.
Table 1 The crystallite size and lattice constant of TiN phase at various annealing temperatures. Temperature (°C)
Crystallite Size (nm)
Lattice constant (nm)
750 800 850 900
11.53 ± 0.70 12.93 ± 0.80 15.24 ± 1.00 17.06 ± 1.10
0.4206 0.4208 0.4225 0.4228
Fig. 4. SEM images of TiN/3DG coatings annealed at temperatures of (a) 750, (b) 800, (c) 850, and (d) 900 °C.
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Fig. 5. SEM images of TiN/3DG coatings focused on discrete TiN film, annealed at temperatures of (a) 750, (b) 800, (c) 850, and (d) 900 °C.
3.3. XPS analysis To evaluate the effect of annealing temperature on TiN stoichiometry, XPS characterization was carried out on different samples annealed at various temperatures (750–900) °C. As shown in Fig. 7, all the XPS spectra for different temperatures contain four peaks located at about 284.5, 397, 456, and 531 eV, related to C1s, N1s, Ti2p, and O1s, respectively [26]. The quantities of these elements extracted from XPS results are given in Table 2 for different temperatures. According to Table 2, the increase of annealing temperature resulted in decrease of oxygen and nitrogen contents, while increase of N/O atomic ratio that is indicative of higher decrement rate of oxygen impurity in TiN structure with temperature. This is because the higher temperature gives sufficient energy
527
Fig. 7. XPS spectra of TiN/3DG coatings annealed at different temperatures.
to oxygen and nitrogen adatoms to desorb from TiN/3DG surface and return into the vapor phase [27]. The increase of annealing temperature affects slightly the TiN lattice constant, as previously mentioned in XRD analysis (Table 1). With regard to XPS results, it may be due to the decrease of oxygen impurity in TiN structure with temperature. As can be seen from EDX (Fig. 6) and XPS (Fig. 7) results, there is a certain amount of oxygen atoms in TiN/3DG coatings which affects their physical properties. These oxygen atoms originate from different sources such as titanium precursor, reaction chamber, and environmental exposure of samples. Some of the mentioned factors are inevitable, but maybe by using Ti precursors without oxygen atoms such as TDMT (tetrakis(dimethyamido)titanium) or high vacuum techniques, it will be possible to reduce more the amount
Fig. 6. EDX analyses of three different regions of TiN/3DG coating, annealed at 850 °C.
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Table 2 Amount of elements and their ratios calculated from XPS data. Annealing temperature (°C)
Ti
O
N
Ti/O
N/O
Ti/N
750 800 850 900
9.20 8.10 9.15 7.40
6.64 4.70 4.10 4.00
11.1 9.20 8.40 8.40
1.38 1.72 2.23 1.85
1.67 1.95 2.04 2.10
0.82 0.88 1.09 0.88
of oxygen. However, TDMT is a dangerous highly flammable compound and high vacuum techniques are expensive. The other important parameter affecting the physical properties of titanium nitride is Ti/N atomic ratio. The stoichiometric TiN, with Ti/N atomic ratio of 1, has the lowest electrical resistivity [28]. Generally, increase in stoichiometry of hard metals like TiN improves their properties [29]. According to Table 2, the Ti/N atomic ratio of TiN annealed at 850 °C (1.09) is the closest to 1. However, the Ti/N atomic ratio is lower than 1 for other annealing temperatures, indicative of nitrogenrich TiN formation. This phenomenon may be due to the existence of titanium vacancies in TiN structure [30]. For further investigation, high-resolution XPS spectra of the Ti2p, O1s, and N1s peaks at 850 °C are shown in Fig. 8. There are two regions in the Ti2p spectrum (Fig. 8a), corresponding to Ti2p1/2 and Ti2p3/2. In both regions, there are three peaks attributed to TiN, TiNxOy, and TiO2, respectively [31]. The O1s spectrum (Fig. 8b) contains two peaks at about 529.9 and 531.7 eV related to O–Ti and O–C bonds, respectively [32]. The O–C bond originated from oxygen-containing groups formed on 3DG surface due to adsorption of species such as CO2 and H2O from environment. There are three peaks at about 396.4, 397.2 and 398.5 eV in the N1s peak (Fig. 8c). The peak at 396.1 eV is ascribed to titanium oxynitride phase. The peak at 397.1 eV originated from titanium nitride phase [33,34]. The third peak at 398.4 eV originated from incorporation of N atoms, generated from decomposition of NH3 at high temperature, into 3DG network. There are several possible configurations for incorporation of nitrogen into graphene-based materials. The C–N bond at 398.5 eV is related to pyridine-like nitrogen atoms. In this configuration, each of nitrogen atoms is linked to two neighboring carbon atoms at defect sites [35]. According to DFT studies, the pyridine-like nitrogen induces p-type doping in graphene structure, if it is not hydrogenated [36]. However, because of existence of hydrogen atoms due to decomposited NH3 in reaction chamber, and also high reactivity of unstable pyridine-like nitrogen, it can be concluded that the configuration of N atoms in 3DG is hydrogenated pyridine-like nitrogen. Hydrogenation of pyridine-like nitrogen converts its effect from p-type to n-type doping [36]. By investigation of high resolution N1s spectra at various annealing temperatures, it was observed that the increase in annealing temperature from 750 to 850 °C resulted in the increase in amount of C– N bond, while with further increase up to 900 °C, its amount decreased. It can be stated that at temperatures higher than 850 °C, the required energy for breaking the C–N bonds is provided [37]. 3.4. Raman spectroscopy
Fig. 8. High resolution XPS spectra of the (a) Ti2p, (b) O1s, and (c) N1s peaks at 850 °C.
Fig. 9 shows the Raman spectra of TiN/3DG coatings at various temperatures in the range of (100–1000) cm−1. In all coatings, there are several peaks located at around 200, 300, and 550 cm−1 which are assigned to the transverse and longitudinal acoustical (TA and LA) and transverse optical (TO) phonons of TiN [38]. The presence of firstorder Raman scattering modes for cubic TiN is representative of the existence of defects inside TiN structure such as nitrogen or titanium vacancies [39]. The TA and LA modes are related to vibrations of Ti atoms near nitrogen vacancies, while the TO mode is attributed to vibrations of N atoms near titanium vacancies [27]. According to Fig. 9, the ratio of the acoustical to optical peaks intensity, which is in proportion with TiN stoichiometry, has increased from
800 to 850 °C and then decreased from 850 to 900 °C. This is in agreement with the XPS results (Table 2) which states that the TiN coating annealed at 850 °C has the most stoichiometric composition. Meanwhile, in all coatings, the intensity of the nitrogen-related TO peak is higher than the titanium-related acoustical peaks, indicating that titanium vacancies are dominant defects inside TiN/3DG coatings compared with nitrogen deficiencies [27,40]. For further investigation, the Raman spectrum of TiN/3DG coating annealed at 850 °C in a wider range (100–3500 cm−1) is shown in the inset of Fig. 9. The graphene Raman bands of D, G, and 2D are observed at about 1350, 1590 and 2700 cm− 1, respectively [41]. The higher
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Fig. 9. Raman spectra of TiN/3DG coatings at various temperatures (inset is the Raman spectrum of TiN/3DG coating annealed at 850 °C in a wider range).
intensity of G peak than the 2D peak is indicative of multi-layer graphene [41]. The presence of weak D peak in the Raman spectrum is due to the generation of defects into 3DG network after high temperature annealing under NH3 atmosphere. This result is in agreement with the presence of N–C peak in high resolution XPS spectrum of N1s (Fig. 8c).
which is proportional with charge carrier density, in 3DG network. As stated before in Section 3.3, increasing the annealing temperature from 750 to 850 °C increased the doping level due to higher amount of nitrogen incorporation into 3DG structure, while further increasing the temperature from 850 to 900 °C decreased it.
3.5. Electrical resistivity Fig. 10 shows the electrical resistivity of TiN/3DG coatings at various annealing temperatures. According to this figure, increasing the annealing temperature from 750 to 850 °C has resulted in the decrease of electrical resistivity of coatings. This is because of decreasing the oxygen content inside TiN structure and improving TiN stoichiometry. The lower oxygen impurity leads to the lower electrical resistivity of TiN phase [42]. The stoichiometric TiN, with Ti/N atomic ratio of 1, has been reported to have the lowest electrical resistivity [31]. Therefore, the TiN/3DG coating annealed at 850 °C had the minimum electrical resistivity. The increase of electrical resistivity from 850 to 900 °C is because of disturbing the TiN stoichiometry. The other probable reason for trend of electrical resistivity changes with annealing temperature is the changes of nitrogen doping level,
Fig. 10. Electrical resistivity of TiN/3DG coatings vs annealing temperature.
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3.6. Work function
Table 3 Work function of 3DG and TiN/3DG coatings at various temperatures.
Fig. 11 represents the UPS spectra around the secondary electrons threshold for 3DG and TiN/3DG coatings prepared at various temperatures. Table 3 gives the calculated work function (Φ) of coatings using Eq. (5), where, hν is the exciting photon energy (21.2 eV). The Eth is the secondary electrons threshold, obtained by intersection between linear extrapolation of the secondary electrons region and the background in the UPS spectra [43]. ϕ¼ hν‐Eth
ð5Þ
According to Table 3, the work function of CVD-grown 3DG was obtained 5 eV. The work functions of all four TiN/3DG coatings annealed at various temperatures are lower than the one for 3DG. The reason of this phenomenon originated from two factors; first is the existence of TiN coating on 3DG and second is the nitrogen incorporation into 3DG network. The TiN behaves like transition metals electrically because of the presence of metallic bonds in its network, and according to density functional theory studies [44], adsorbed transition metal atoms on graphene-based nanostructures usually reduce their work function. By applying TiN on 3DG, electron transfer occurs from TiN to 3DG and as a result, a surface dipole moment creates on 3DG surface. Meanwhile, as stated before in Section 3.3, hydrogenated pyridine-like nitrogen, which was formed into 3DG network during annealing in NH3, exerts n-type doping into 3DG structure. The n-type doping of 3DG results in the upward shift of its Fermi level [45]. Therefore, n-type nitrogen doping of graphene leads to the decrease of its work function value. According to Table 3, the work function value for TiN/3DG coatings varies with annealing temperature. The TiN/3DG annealed at 850 °C has the lowest work function of 4.68 eV. The decrease in work function of TiN/3DG coatings with increasing the annealing temperature from 750 to 850 °C may be because of decreasing the oxygen content inside TiN/3DG structure (Table 2). Electronegative impurities such as oxygen usually increase the work function [46]. The increase of work function from 850 to 900 °C may be because of decrement of nitrogen doping level due to breaking the C–N bonds at high temperatures [37,47]. 3.7. Water contact angle Table 4 represents the water contact angle (WCA) of pristine 3DG and TiN/3DG coatings annealed at various temperatures. According to this table, the WCA of pristine 3DG was obtained 127°, indicative of hydrophobic surface. After applying TiN, the WCA of coatings have
Sample
Work Function (eV)
3DG TiN/3DG (750 °C) TiN/3DG (800 °C) TiN/3DG (850 °C) TiN/3DG (900 °C)
5.00 4.90 4.83 4.68 4.80
decreased to lower than 90°, representing the hydrophilic surface. Therefore, it can be stated that the wettability of 3DG has been enhanced by TiN coating. The TiN coating has increased the surface roughness and surface free energy of the 3DG which improves its surface wettability. This phenomenon is in agreement with the result reported by Z. J. Dong et al. [48]. They stated that the wettability of carbon fibers improves remarkably by applying TiC coating due to the enhanced surface free energy of carbon fibers after coating with a TiC layer. The TiC and TiN belong to the same type of materials with name of hard metals which have similar structure and properties. Meanwhile, the generation of defects into 3DG structure in TiN/3DG coatings, due to high temperature annealing at ammonia, has increased the surface polarity and surface free energy of coatings, therefore the WCA has decreased compared to pristine 3DG [49]. With regard to Table 4, the WCA of TiN/3DG coatings has increased with the increase in annealing temperature. According to Table 2, this trend can be attributed to the decrease in amount of oxygen atoms in the surface of coatings by increasing annealing temperature, because the existence of oxygen-containing groups on surface increases the surface polarity, and as a result the surface free energy of coatings increases. 4. Conclusions In this study, nanostructured TiN/3DG composite was synthesized by immersing the 3DG into a solution containing Ti ions and then annealing at ammonia. The XRD results showed the formation of crystalline TiN phase on 3DG, without impurities such as metallic titanium or titanium dioxide. According to SEM images, the morphology of deposited-TiN on 3DG surface was discontinuous film with some spherical particles on it. The Raman results verified that titanium vacancies were dominant defects inside TiN coatings compared to nitrogen deficiencies. The various annealing temperatures affected the TiN structure such as its crystallite size, lattice constant, and stoichiometry which resulted in changing its electrical resistivity and work function. Annealing at 850 °C resulted in the most stoichiometric composition with titanium/nitrogen atomic ratio of 1.09, which had the lowest electrical resistivity of 0.41 Ω cm and the lowest work function of 4.68 eV. The annealing temperature impressed also the 3DG structure. Annealing of 3DG at high temperatures under NH3 atmosphere resulted in locally doping of graphene with nitrogen at the edge of graphene or adjacent to carbon vacancies inside graphene. However, changing the annealing temperature has no significant effects on 3DG structure compared to TiN structure. Generally, TiN/3DG composite may be as a good candidate for electrode in DSSCs because of high electrical conductivity, Table 4 Water contact angles of 3DG and TiN/3DG coatings at various annealing temperatures.
Fig. 11. UPS spectra of 3DG and TiN/3DG coatings annealed at various temperatures.
Sample
WCA (°)
3DG TiN/3DG (750 °C) TiN/3DG (800 °C) TiN/3DG (850 °C) TiN/3DG (900 °C)
127.0 80.9 82.4 83.0 85.1
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increased hydrophilicity for efficient dye adsorption, and decreased work function for effective charge transfer between dye and 3DG. 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. References [1] M.I. Katsnelson, Mater. Today 10 (2007) 20. [2] L. Zhang, F. Zhang, X. Yang, G. Long, Y. Wu, T. Zhang, K. Leng, Y. Huang, Y. Ma, A. Yu, Y. Chen, Sci. Rep. 3 (2013) 1408. [3] X. Cao, Z. Yin, H. Zhang, Energy Environ. Sci. 7 (2014) 1850. [4] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.M. Cheng, Nat. Mater. 10 (2011) 424. [5] F. Yavari, Z. Chen, A.V. Thomas, W. Ren, H.M. Cheng, N. Koratkar, Sci. Rep. 1 (2011) 166. [6] H.X. Ji, L.L. Zhang, M.T. Pettes, H.F. Li, S.S. Chen, L. Shi, R. Piner, R.S. Ruoff, Nano Lett. 12 (2012) 2446. [7] T. Maiyalagan, X. Dong, P. Chen, X. Wang, J. Mater. Chem. 2 (2012) 5286. [8] Z.S. Wu, Y. Sun, Y.Z. Tan, S. Yang, X. Feng, K. Müllen, J. Am. Chem. Soc. 134 (2012) 19532. [9] H. Wang, K. Sun, F. Tao, D.J. Stacchiola, Y.H. Hu, Angew. Chem. 125 (2013) 9380. [10] J. Rafiee, M.A. Rafiee, Z.Z. Yu, N. Koratkar, Adv. Mater. 22 (2010) 2151. [11] A. Misra, M. Waikar, A. Gour, H. Kalita, M. Khare, M. Aslam, A. Kottantharayil, Appl. Phys. Lett. 100 (2012) 233506. [12] J. Jia, X. Sun, X. Lin, X. Shen, Y.W. Mai, J.K. Kim, ACS Nano 8 (2014) 5774. [13] S. Sattayasamitsathit, Y. Gu, K. Kaufmann, W. Jia, X. Xiao, M. Rodriguez, S. Minteer, J. Cha, D.B. Burckel, C. Wang, R. Polsky, J. Wang, J. Mater. Chem. A 1 (2013) 1639. [14] Y.H. Chang, C.T. Lin, T.Y. Chen, C.L. Hsu, Y.H. Lee, W. Zhang, K.H. Wei, L.J. Li, Adv. Mater. 25 (2013) 756. [15] X. Cao, B. Zheng, X. Rui, W. Shi, Q. Yan, H. Zhang, Angew. Chem. Int. Ed. 53 (2014) 1404. [16] F. Zhang, D. Zhu, X. Chen, X. Xu, Z. Yang, C. Zou, K. Yanga, S. Huang, Phys. Chem. Chem. Phys. 16 (2014) 4186. [17] Y.H. Yue, P.X. Han, S.M. Dong, K.J. Zhang, C.J. Zhang, C.Q. Shang, G.L. Cui, Chin. Sci. Bull. 57 (2012) 4111. [18] J.P. Coad, P. Warrington, R.B. Newbery, M.H. Jacobs, Mater. Des. 6 (1985) 190. [19] Z. Wen, S. Cui, H. Pu, S. Mao, K. Yu, X. Feng, J. Chen, Adv. Mater. 23 (2011) 5445. [20] M. Liu, Y. Dong, Y. Wu, H. Feng, J. Li, Chem. Eur. J. 19 (2013) 14781. [21] P. Han, Y. Yue, X. Wang, W. Ma, S. Dong, K. Zhang, C. Zhang, G. Cui, J. Mater. Chem. 22 (2012) 24918.
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Effect of annealing temperature on physical properties of nanostructured TiN/3DG composite Fatemeh Dabir a, Rasoul Sarraf-Mamoory a,⁎, Manuela Loeblein b,c, Siu Hon Tsang c, Edwin Hang Tong Teo b a b c
Materials Eng. Department, Tarbiat Modares University, 1411713116 Tehran, Iran School of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, 637553, Singapore
a r t i c l e
i n f o
Article history: Received 12 January 2015 Received in revised form 30 September 2015 Accepted 28 October 2015 Available online 29 October 2015 Keywords: Titanium nitride Three-dimensional graphene Chemical method Annealing temperature Physical properties
a b s t r a c t Recently, three-dimensional graphene (3DG) has attracted much attention in many research fields due to its unique structure and considerable properties. In order to expand the range of applications of 3DG, the suitable nanomaterials can be grown on its surface. In this study, titanium nitride (TiN) phase was deposited on 3DG porous structure by chemical method. This method contains two steps of immersing 3DG into a solution containing Ti ions and then annealing under ammonia atmosphere. The effect of annealing temperature on type of synthesized phases, their morphology, and stoichiometry was investigated. For this purpose, the samples were annealed at different temperatures (750–900 °C) and analyzed via various techniques. The results showed that increasing annealing temperature results in increased crystallite size and lattice constant, while decreased oxygen content in TiN structure. Annealing at 850 °C resulted in the most stoichiometric composition with titanium/nitrogen atomic ratio of 1.09, which had the lowest electrical resistivity of 0.41 Ω cm and lowest work function of 4.68 eV. After applying TiN, the water contact angle of 3DG (127°) was reduced to lower than 90°. Such TiN/ 3DG composite can be a promising candidate as an electrode in solar cells. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Graphene, a two-dimensional (2D) nanomaterial, is now at the forefront of materials research owing to its exceptional physical, chemical, and mechanical properties such as excellent electrical and thermal conductivity, high chemical stability, and outstanding mechanical strength [1]. One of the challenges in this field is utilization of the 2D carbonbased nanomaterials in 3D bulk networks, while retaining their outstanding properties. Composite networks based on graphene sheets have limited its physical properties because of introducing structural defects within the blocks [2]. In the last few years, three-dimensional graphene has been proposed as an ultralight new material with a flexible, highly porous, and electrically conductive interconnected network [3,4]. This 3DG has found some applications in sensors [5], Li-batteries [6], fuel cells [7], and electrochemical cells [8] due to its unique properties. The 3DG also can be used as a scaffold for adsorbing and keeping molecules or as a fast transport channel for charge carriers [9]. The hydrophobic surface of 3DG has limited its application as an adsorbent scaffold [10]. The other limitation of 3DG is its high work function (~5 eV) which affects the charge transport processes [11]. In order to extend the range of applications, various nanomaterials have been coated on 3DG such as polymers [12], noble metals [13], metal oxides [14], ⁎ Corresponding author. E-mail address: [email protected] (R. Sarraf-Mamoory).
http://dx.doi.org/10.1016/j.matdes.2015.10.152 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
sulfides [15], and hydroxides [16]. However, to the best of our knowledge, to this date there is no report on the synthesis of transition metal nitride/3DG composites. Transition metal nitrides are important functional materials in many applications because of their superior electrical conductivity and high chemical stability [17,18]. Among them, TiN is the most appealing compound, because it has one of the lowest electrical resistivity. In reports on decoration of graphene with TiN, the twodimensional form of graphene has been used [19–22]. In these reports, the TiN/2DG has been utilized as an electrode in energy conversion and storage systems such as solar cells [19], fuel cells [20], electrochemical capacitors [21], and Li-batteries [22]. However, there is no research focused on the growth of titanium nitride on the porous threedimensional graphene networks. It is expected that the TiN/3DG composites have applications in energy conversion field such as photovoltaic cells because it integrates unique properties of both 3DG and TiN. For example, these composites may be utilized in dye sensitized solar cells (DSSCs), simultaneously as dye adsorbent framework and charge transport channel, because TiN improves surface wettability of 3DG and has no detrimental effect on its high electrical conductivity. Meanwhile, TiN can reduce the work function of 3DG which facilitates the electron injection from excited dye molecules to the 3DG. In this paper, the 3DG was synthesized by chemical vapor deposition (CVD) method using nickel foam template with subsequent chemical etching of Ni. Then, TiN was applied on CVD-grown 3DG via chemical method, followed by high temperature annealing at NH3. Finally, the
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effect of annealing temperature on physical properties of TiN/3DG composite was analyzed. The TiN microstructure is one of the important parameters influencing the TiN/3DG properties and the annealing temperature has the strong effect on it. 2. Materials and methods 2.1. Synthesis of 3DG on quartz glass Nickel foam template with thickness of 0.5 mm was heated under Ar and H2 flow until 1000 °C inside quartz tube. The soaking time at 1000 °C was 5 min. Then, ethanol vapors were entered into the quartz tube for 10 min via an ethanol bubbler. After graphene growth on Ni template, the graphene/Ni foam was dip-coated with poly(methyl methacrylate) (PMMA) solution for structure protection. In order to remove the Ni template, the PMMA-covered graphene/Ni foam was immersed in HCl solution. Finally, the PMMA/3DG was transferred on quartz substrate and annealed under Ar and H2 flow at 700 °C for 1 h. 2.2. Synthesis of TiN on 3DG For the synthesis of TiN, 0.5 ml of titanium isopropoxide (Aldrich), as titanium source, was dissolved in 5 ml of absolute ethanol (Merck). Then, the 3DG-transferred on quartz glass was immersed in the prepared solution for effective adsorption of titanium ions. The immersion time was 2 h at 50 °C. After drying, the coatings were annealed at various temperatures (750–900) °C for 3 h under NH3 flow (50 sccm). 2.3. Characterization Phase analyses of TiN/3DG coatings were identified using XRD (Bruker) with Cu–Kα radiation (λ = 0.154060 nm). Investigation of surface morphology and chemical composition analysis of the coatings were conducted by scanning electron microscopy (SEM) and energydispersive X-ray (EDX) spectroscopy, respectively. In order to identify the TiN stoichiometry, X-ray photoelectron spectroscopy (XPS) was performed with ThetaProbe A1333 system using monochromic Al–Kα radiation (hυ = 1486.7 eV). Raman spectroscopy (WITec) was conducted on coatings by using an Ar ion laser with wavelength of 532 nm. The Van der Pauw method was used to determine electrical resistivity of samples. Work function of the coatings was measured by UPS using a helium lamp radiation at 21.2 eV. The wettability of 3DG samples before and after TiN coatings was identified by contact angle meter (VCA Optima-AST products, Billerica, MA) using dropping DI water on their surfaces at room temperature. The wettability experiments were repeated in three different sites of each sample for more accuracy.
Fig. 1. XRD patterns of TiN/3DG coatings annealed at various temperatures.
where, β is the FWHM (full width of half maximum) of the prominent peak of TiN phase in the XRD patterns (200) and θ is the peak position in radian. Fig. 2 shows the relation between TiN crystallite size and annealing temperature. It should be pointed out that the uncertainty level in calculating the crystallite size (ΔD) was accounted as error bars in Fig. 2. It can be observed that the TiN crystallite size increases with annealing temperature which is due to the increase of the mobility of atoms and acceleration of the diffusion with temperature. The other information estimated from XRD data is the lattice constant. The lattice constant for cubic structure (a) is calculated using Eq. (2), where d is the distance between (hkl) planes and is measured from Bragg's law (Eq. (3)) [24]. The uncertainty level in calculating the a value approaches zero as θ = 90° (i.e., 2θ = 180°). Therefore, for achieving the true value for lattice constant, the a value for each diffraction angle was calculated from Eq. (2), then the measured a values was plotted vs. cos2θ for each annealing temperature. After extrapolation to
3. Results and discussions 3.1. XRD analysis Fig. 1 shows the XRD patterns of TiN/3DG coatings annealed at various temperatures. In diffraction patterns of all samples, the peak of 26° is related to the (002) plane of graphene (JCPDS 41-1487). The other three peaks at 36°, 43° and 62° belong to (111), (200) and (220) planes of FCC structure of TiN, respectively (JCPDS 87-0632). The hump between 20° and 25° in XRD patterns originated from quartz substrate. These results represent that an almost pure and crystalline TiN phase has been formed on the graphene surface. The crystallite size (D) of TiN phase at various annealing temperatures can be obtained using Scherrer's equation [23]:
D¼
0:9λ βcosθ
ð1Þ
Fig. 2. Crystallite size of TiN vs. annealing temperature.
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cos2θ = 0 (i.e. 2θ = 180°), the precise value of lattice constant was obtained. 1 2
d
¼
2
2
h þk þl
2
ð2Þ
a2
λ ¼ 2dsinθ
ð3Þ
Table 1 represents the crystallite size and lattice constant of different samples annealed at various temperatures. In all the samples, the calculated lattice constant is different from stoichiometric pure TiN phase (a = 4.241 Å), which is probably due to the presence of residual compressive stresses, nonstoichiometric TiN, and impurities such as oxygen inside TiN structure. Oxygen atoms may substitute nitrogen atoms in TiN structure resulting in the formation of titanium oxynitride (TiNxOy). Indeed, TiNxOy is a solid solution of TiN and TiO, which are crystallized together in a FCC structure. Therefore, according to Vegard's law, the lattice constant of TiNxOy phase lies between the lattice constants of TiN and TiO (a = 4.173–4.185 Å) phases (Eq. (4)). However, it should be noted that the Vegard's law is not accurate for nanocrystals and defective structures [25]. aTiNxOy ¼ xaTiN þ yaTiO
ð4Þ
3.2. SEM/EDX analysis Fig. 3 shows the SEM image of CVD-grown 3DG. With regard to this figure, the CVD-grown 3DG has three-dimensional interconnected porous structure, which has taken it from the nickel template. Fig. 4 shows the SEM images of TiN/3DG coatings annealed at different temperatures. In all images, it can be seen that the TiN phase has been deposited on 3DG surface as a discontinuous film with some particles on it. The discontinuity of TiN film on 3DG surface has been emerged before annealing process and is because of nonflat and hydrophobic surface of 3DG. The presence of particles on discrete TiN film is related to the deposition of remained Ti complexes heterogeneously on the initial deposited Ti-complexes instead of on the 3DG surface, because the required energy for nucleation of particles on 3DG is much higher than the one for their nucleation on discrete film with similar structure. The deposited Ti-complexes have been converted to titanium nitride after high temperature annealing under NH3 atmosphere. At annealing temperature of 900 °C (Fig. 4d), peeling off the TiN film from 3DG substrate is observed, which is due to the increase of thermal mismatch between TiN and graphene with temperature. Fig. 5 shows the SEM images of TiN/3DG coatings, focused on discrete TiN film, annealed at various temperatures. According to this figure, the morphology of TiN film has not been changed with an increase in annealing temperature from 750 to 800 °C. However, with further increase up to 850 °C, very small holes have been created on TiN film. At annealing temperature of 900 °C, the morphology of TiN film has been changed remarkably and its surface is uneven. The reason for creation of holes and unevenness on TiN surface at annealing temperatures higher than 800 °C can be related to the desorption of oxygen and nitrogen atoms from TiN/3DG coating, because the increase in annealing temperature provides the required energy for desorption phenomenon.
Fig. 3. SEM image of CVD-grown 3DG.
Meanwhile, according to Fig. 5, as the annealing temperature increases, the average size of TiN particles deposited on discrete TiN film increases. By using ImageJ analyzer, the average particle sizes at various annealing temperatures of 750, 800, 850, and 900 °C were obtained 447, 521, 557, and 839 nm, respectively. Fig. 6 shows the EDX analyses performed on three different regions of TiN/3DG coating, annealed at 850 °C. According to this figure, all three regions consist of C, Ti, N, O, and Si elements. However, the amounts of these elements are not equal in different regions. It should be mentioned that the result of EDX analysis for each region is impressed by composition of other regions near it because of high diffusion depth of X-ray. The C and Si peaks in EDX spectra are attributed to 3DG and quartz substrate, respectively. The oxygen peak can be related to the existence of oxygen-containing groups onto 3DG network, surface oxides formed on TiN film, Ti oxynitride phases, and SiO2 substrate. The Ti and N peaks can be attributed to the presence of TiN and TiNxOy phases. The other reason for the presence of N peaks in EDX spectra can be related to the incorporation of N atoms inside 3DG network. In region 1 (3DG surface), the amount of C is maximum. In both regions 2 (discrete TiN film) and 3 (TiN particle), the amounts of Ti, N, and O elements have increased, while the amount of C has decreased compared to region 1. The higher amount of C element and Ti/N atomic ratio in region 2 than them in region 3 may indicate that in region 2, some C atoms from 3DG have diffused into TiN discrete film.
Table 1 The crystallite size and lattice constant of TiN phase at various annealing temperatures. Temperature (°C)
Crystallite Size (nm)
Lattice constant (nm)
750 800 850 900
11.53 ± 0.70 12.93 ± 0.80 15.24 ± 1.00 17.06 ± 1.10
0.4206 0.4208 0.4225 0.4228
Fig. 4. SEM images of TiN/3DG coatings annealed at temperatures of (a) 750, (b) 800, (c) 850, and (d) 900 °C.
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Fig. 5. SEM images of TiN/3DG coatings focused on discrete TiN film, annealed at temperatures of (a) 750, (b) 800, (c) 850, and (d) 900 °C.
3.3. XPS analysis To evaluate the effect of annealing temperature on TiN stoichiometry, XPS characterization was carried out on different samples annealed at various temperatures (750–900) °C. As shown in Fig. 7, all the XPS spectra for different temperatures contain four peaks located at about 284.5, 397, 456, and 531 eV, related to C1s, N1s, Ti2p, and O1s, respectively [26]. The quantities of these elements extracted from XPS results are given in Table 2 for different temperatures. According to Table 2, the increase of annealing temperature resulted in decrease of oxygen and nitrogen contents, while increase of N/O atomic ratio that is indicative of higher decrement rate of oxygen impurity in TiN structure with temperature. This is because the higher temperature gives sufficient energy
527
Fig. 7. XPS spectra of TiN/3DG coatings annealed at different temperatures.
to oxygen and nitrogen adatoms to desorb from TiN/3DG surface and return into the vapor phase [27]. The increase of annealing temperature affects slightly the TiN lattice constant, as previously mentioned in XRD analysis (Table 1). With regard to XPS results, it may be due to the decrease of oxygen impurity in TiN structure with temperature. As can be seen from EDX (Fig. 6) and XPS (Fig. 7) results, there is a certain amount of oxygen atoms in TiN/3DG coatings which affects their physical properties. These oxygen atoms originate from different sources such as titanium precursor, reaction chamber, and environmental exposure of samples. Some of the mentioned factors are inevitable, but maybe by using Ti precursors without oxygen atoms such as TDMT (tetrakis(dimethyamido)titanium) or high vacuum techniques, it will be possible to reduce more the amount
Fig. 6. EDX analyses of three different regions of TiN/3DG coating, annealed at 850 °C.
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Table 2 Amount of elements and their ratios calculated from XPS data. Annealing temperature (°C)
Ti
O
N
Ti/O
N/O
Ti/N
750 800 850 900
9.20 8.10 9.15 7.40
6.64 4.70 4.10 4.00
11.1 9.20 8.40 8.40
1.38 1.72 2.23 1.85
1.67 1.95 2.04 2.10
0.82 0.88 1.09 0.88
of oxygen. However, TDMT is a dangerous highly flammable compound and high vacuum techniques are expensive. The other important parameter affecting the physical properties of titanium nitride is Ti/N atomic ratio. The stoichiometric TiN, with Ti/N atomic ratio of 1, has the lowest electrical resistivity [28]. Generally, increase in stoichiometry of hard metals like TiN improves their properties [29]. According to Table 2, the Ti/N atomic ratio of TiN annealed at 850 °C (1.09) is the closest to 1. However, the Ti/N atomic ratio is lower than 1 for other annealing temperatures, indicative of nitrogenrich TiN formation. This phenomenon may be due to the existence of titanium vacancies in TiN structure [30]. For further investigation, high-resolution XPS spectra of the Ti2p, O1s, and N1s peaks at 850 °C are shown in Fig. 8. There are two regions in the Ti2p spectrum (Fig. 8a), corresponding to Ti2p1/2 and Ti2p3/2. In both regions, there are three peaks attributed to TiN, TiNxOy, and TiO2, respectively [31]. The O1s spectrum (Fig. 8b) contains two peaks at about 529.9 and 531.7 eV related to O–Ti and O–C bonds, respectively [32]. The O–C bond originated from oxygen-containing groups formed on 3DG surface due to adsorption of species such as CO2 and H2O from environment. There are three peaks at about 396.4, 397.2 and 398.5 eV in the N1s peak (Fig. 8c). The peak at 396.1 eV is ascribed to titanium oxynitride phase. The peak at 397.1 eV originated from titanium nitride phase [33,34]. The third peak at 398.4 eV originated from incorporation of N atoms, generated from decomposition of NH3 at high temperature, into 3DG network. There are several possible configurations for incorporation of nitrogen into graphene-based materials. The C–N bond at 398.5 eV is related to pyridine-like nitrogen atoms. In this configuration, each of nitrogen atoms is linked to two neighboring carbon atoms at defect sites [35]. According to DFT studies, the pyridine-like nitrogen induces p-type doping in graphene structure, if it is not hydrogenated [36]. However, because of existence of hydrogen atoms due to decomposited NH3 in reaction chamber, and also high reactivity of unstable pyridine-like nitrogen, it can be concluded that the configuration of N atoms in 3DG is hydrogenated pyridine-like nitrogen. Hydrogenation of pyridine-like nitrogen converts its effect from p-type to n-type doping [36]. By investigation of high resolution N1s spectra at various annealing temperatures, it was observed that the increase in annealing temperature from 750 to 850 °C resulted in the increase in amount of C– N bond, while with further increase up to 900 °C, its amount decreased. It can be stated that at temperatures higher than 850 °C, the required energy for breaking the C–N bonds is provided [37]. 3.4. Raman spectroscopy
Fig. 8. High resolution XPS spectra of the (a) Ti2p, (b) O1s, and (c) N1s peaks at 850 °C.
Fig. 9 shows the Raman spectra of TiN/3DG coatings at various temperatures in the range of (100–1000) cm−1. In all coatings, there are several peaks located at around 200, 300, and 550 cm−1 which are assigned to the transverse and longitudinal acoustical (TA and LA) and transverse optical (TO) phonons of TiN [38]. The presence of firstorder Raman scattering modes for cubic TiN is representative of the existence of defects inside TiN structure such as nitrogen or titanium vacancies [39]. The TA and LA modes are related to vibrations of Ti atoms near nitrogen vacancies, while the TO mode is attributed to vibrations of N atoms near titanium vacancies [27]. According to Fig. 9, the ratio of the acoustical to optical peaks intensity, which is in proportion with TiN stoichiometry, has increased from
800 to 850 °C and then decreased from 850 to 900 °C. This is in agreement with the XPS results (Table 2) which states that the TiN coating annealed at 850 °C has the most stoichiometric composition. Meanwhile, in all coatings, the intensity of the nitrogen-related TO peak is higher than the titanium-related acoustical peaks, indicating that titanium vacancies are dominant defects inside TiN/3DG coatings compared with nitrogen deficiencies [27,40]. For further investigation, the Raman spectrum of TiN/3DG coating annealed at 850 °C in a wider range (100–3500 cm−1) is shown in the inset of Fig. 9. The graphene Raman bands of D, G, and 2D are observed at about 1350, 1590 and 2700 cm− 1, respectively [41]. The higher
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Fig. 9. Raman spectra of TiN/3DG coatings at various temperatures (inset is the Raman spectrum of TiN/3DG coating annealed at 850 °C in a wider range).
intensity of G peak than the 2D peak is indicative of multi-layer graphene [41]. The presence of weak D peak in the Raman spectrum is due to the generation of defects into 3DG network after high temperature annealing under NH3 atmosphere. This result is in agreement with the presence of N–C peak in high resolution XPS spectrum of N1s (Fig. 8c).
which is proportional with charge carrier density, in 3DG network. As stated before in Section 3.3, increasing the annealing temperature from 750 to 850 °C increased the doping level due to higher amount of nitrogen incorporation into 3DG structure, while further increasing the temperature from 850 to 900 °C decreased it.
3.5. Electrical resistivity Fig. 10 shows the electrical resistivity of TiN/3DG coatings at various annealing temperatures. According to this figure, increasing the annealing temperature from 750 to 850 °C has resulted in the decrease of electrical resistivity of coatings. This is because of decreasing the oxygen content inside TiN structure and improving TiN stoichiometry. The lower oxygen impurity leads to the lower electrical resistivity of TiN phase [42]. The stoichiometric TiN, with Ti/N atomic ratio of 1, has been reported to have the lowest electrical resistivity [31]. Therefore, the TiN/3DG coating annealed at 850 °C had the minimum electrical resistivity. The increase of electrical resistivity from 850 to 900 °C is because of disturbing the TiN stoichiometry. The other probable reason for trend of electrical resistivity changes with annealing temperature is the changes of nitrogen doping level,
Fig. 10. Electrical resistivity of TiN/3DG coatings vs annealing temperature.
530
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3.6. Work function
Table 3 Work function of 3DG and TiN/3DG coatings at various temperatures.
Fig. 11 represents the UPS spectra around the secondary electrons threshold for 3DG and TiN/3DG coatings prepared at various temperatures. Table 3 gives the calculated work function (Φ) of coatings using Eq. (5), where, hν is the exciting photon energy (21.2 eV). The Eth is the secondary electrons threshold, obtained by intersection between linear extrapolation of the secondary electrons region and the background in the UPS spectra [43]. ϕ¼ hν‐Eth
ð5Þ
According to Table 3, the work function of CVD-grown 3DG was obtained 5 eV. The work functions of all four TiN/3DG coatings annealed at various temperatures are lower than the one for 3DG. The reason of this phenomenon originated from two factors; first is the existence of TiN coating on 3DG and second is the nitrogen incorporation into 3DG network. The TiN behaves like transition metals electrically because of the presence of metallic bonds in its network, and according to density functional theory studies [44], adsorbed transition metal atoms on graphene-based nanostructures usually reduce their work function. By applying TiN on 3DG, electron transfer occurs from TiN to 3DG and as a result, a surface dipole moment creates on 3DG surface. Meanwhile, as stated before in Section 3.3, hydrogenated pyridine-like nitrogen, which was formed into 3DG network during annealing in NH3, exerts n-type doping into 3DG structure. The n-type doping of 3DG results in the upward shift of its Fermi level [45]. Therefore, n-type nitrogen doping of graphene leads to the decrease of its work function value. According to Table 3, the work function value for TiN/3DG coatings varies with annealing temperature. The TiN/3DG annealed at 850 °C has the lowest work function of 4.68 eV. The decrease in work function of TiN/3DG coatings with increasing the annealing temperature from 750 to 850 °C may be because of decreasing the oxygen content inside TiN/3DG structure (Table 2). Electronegative impurities such as oxygen usually increase the work function [46]. The increase of work function from 850 to 900 °C may be because of decrement of nitrogen doping level due to breaking the C–N bonds at high temperatures [37,47]. 3.7. Water contact angle Table 4 represents the water contact angle (WCA) of pristine 3DG and TiN/3DG coatings annealed at various temperatures. According to this table, the WCA of pristine 3DG was obtained 127°, indicative of hydrophobic surface. After applying TiN, the WCA of coatings have
Sample
Work Function (eV)
3DG TiN/3DG (750 °C) TiN/3DG (800 °C) TiN/3DG (850 °C) TiN/3DG (900 °C)
5.00 4.90 4.83 4.68 4.80
decreased to lower than 90°, representing the hydrophilic surface. Therefore, it can be stated that the wettability of 3DG has been enhanced by TiN coating. The TiN coating has increased the surface roughness and surface free energy of the 3DG which improves its surface wettability. This phenomenon is in agreement with the result reported by Z. J. Dong et al. [48]. They stated that the wettability of carbon fibers improves remarkably by applying TiC coating due to the enhanced surface free energy of carbon fibers after coating with a TiC layer. The TiC and TiN belong to the same type of materials with name of hard metals which have similar structure and properties. Meanwhile, the generation of defects into 3DG structure in TiN/3DG coatings, due to high temperature annealing at ammonia, has increased the surface polarity and surface free energy of coatings, therefore the WCA has decreased compared to pristine 3DG [49]. With regard to Table 4, the WCA of TiN/3DG coatings has increased with the increase in annealing temperature. According to Table 2, this trend can be attributed to the decrease in amount of oxygen atoms in the surface of coatings by increasing annealing temperature, because the existence of oxygen-containing groups on surface increases the surface polarity, and as a result the surface free energy of coatings increases. 4. Conclusions In this study, nanostructured TiN/3DG composite was synthesized by immersing the 3DG into a solution containing Ti ions and then annealing at ammonia. The XRD results showed the formation of crystalline TiN phase on 3DG, without impurities such as metallic titanium or titanium dioxide. According to SEM images, the morphology of deposited-TiN on 3DG surface was discontinuous film with some spherical particles on it. The Raman results verified that titanium vacancies were dominant defects inside TiN coatings compared to nitrogen deficiencies. The various annealing temperatures affected the TiN structure such as its crystallite size, lattice constant, and stoichiometry which resulted in changing its electrical resistivity and work function. Annealing at 850 °C resulted in the most stoichiometric composition with titanium/nitrogen atomic ratio of 1.09, which had the lowest electrical resistivity of 0.41 Ω cm and the lowest work function of 4.68 eV. The annealing temperature impressed also the 3DG structure. Annealing of 3DG at high temperatures under NH3 atmosphere resulted in locally doping of graphene with nitrogen at the edge of graphene or adjacent to carbon vacancies inside graphene. However, changing the annealing temperature has no significant effects on 3DG structure compared to TiN structure. Generally, TiN/3DG composite may be as a good candidate for electrode in DSSCs because of high electrical conductivity, Table 4 Water contact angles of 3DG and TiN/3DG coatings at various annealing temperatures.
Fig. 11. UPS spectra of 3DG and TiN/3DG coatings annealed at various temperatures.
Sample
WCA (°)
3DG TiN/3DG (750 °C) TiN/3DG (800 °C) TiN/3DG (850 °C) TiN/3DG (900 °C)
127.0 80.9 82.4 83.0 85.1
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