Understanding Interfaces in MetalâGraphitic Hybrid Nanostructures

Perspective pubs.acs.org/JPCL

Understanding Interfaces in Metal−Graphitic Hybrid Nanostructures Mengning Ding, Yifan Tang, and Alexander Star* National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, Pennsylvania 15236, United States Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States ABSTRACT: Metal−graphitic interfaces formed between metal nanoparticles (MNPs) and carbon nanotubes (CNTs) or graphene play an important role in the properties of such hybrid nanostructures. This Perspective summarizes different types of interfaces that exist within the metal−carbon nanoassemblies and discusses current efforts on understanding and modeling the interfacial conditions and interactions. Characterization of the metal−graphitic interfaces is described here, including microscopy, spectroscopy, electrochemical techniques, and electrical measurements. Recent studies on these nanohybrids have shown that the metal−graphitic interfaces play critical roles in both controlled assembly of nanoparticles and practical applications of nanohybrids in chemical sensors and fuel cells. Better understanding, design, and manipulation of metal−graphitic interfaces could therefore become the new frontier in the research of MNP/CNT or MNP/graphene hybrid systems.

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generates the metal−graphitic interface. There has been an increasing recognition of the fundamental importance of surface chemistry and interface details within these systems as they generally act as key factors to achieve the precise placement of metal nanocrystals with fine controlled morphology and advanced functions. The metal−graphitic interface is complicated because different synthetic approaches result in distinct surface functionalities on both metal and graphitic surfaces. However, it can be divided into four categories, based on different surface functionalities on graphitic surfaces (pristine or nonpristine), which appear to be most influential on the system, as demonstrated in Figure 1. The pristine interface is defined by the direct physical contact between pristine metal and graphitic surfaces, as illustrated in Figure 1a. Such a concise scenario has usually been achieved in the method where two criteria were matched, (i) MNPs were formed directly on the graphitic surfaces without the employment of any chemical reducing agent or other surfactant as those typically result in adsorbed molecules on nanoparticles surface for their stabilization and (ii) the graphitic template was used without any oxidative treatment or surfactant adsorption. Direct formation of MNPs on pristine CNT surfaces was accomplished by a variety of methods, which included direct reduction of metal ions by single-walled carbon nanotubes (SWNTs),10,11 galvanic reduction with a metallic surface,12,13 or electrodeposition.14−17 As no chemical treatment of CNTs was required for these strategies, noble MNPs could be formed all over the sidewalls of pristine CNTs and thus created multiple pristine metal−graphitic interfaces. Other solvent-free methods, such as thermal decomposition18 and H2 reductionyu of metal complexes, thermal and electron beam evaporation,14,20−23 or sputtering coating,24 have also been employed to generate MNPs

onstruction of complex nanoscale architectures using metal nanoparticles (MNPs) and graphitic carbon nanostructures including carbon nanotubes (CNTs) and graphene as building blocks has received considerable research attention. The resulting MNP/CNT and MNP/graphene hybrid materials, sometimes known as heterostructures, demonstrated combined or even synergistic material properties.1,2 For example, one-dimensional CNTs or two-dimensional graphene can provide an ideal graphitic carbon surface for the assembly, dispersion, and stabilization of zero-dimensional MNPs. The assembled functional hybrids have been employed in many applications such as (i) catalysis because of the high surface area of both building blocks and extremely reactive surface of metal nanocrystals3 and (ii) chemical sensing, where the high electrical conductivity of CNTs or graphene and the selective chemical activity of the MNPs play important roles.4 Synthetic methodologies that have been developed for these metal−graphitic nanohybrid assemblies can be divided into two categories depending on the MNP preparation. In the first category, MNPs were synthesized and characterized prior to their attachment to graphitic surfaces. The second category encompasses various synthetic methods where MNPs were formed in situ on graphitic surfaces, including aggregation of metal atoms and thermal, chemical, or electrochemical reduction of metal ions. Different synthetic routes, characterization methods, and general applications of metal−graphitic nanohybrid assemblies have been summarized in great detail in several recent review articles.2,5−9 In this Perspective, we will focus on recent progress made on the synthesis and characterization of metal−graphitic nanoscale heterostructures with an emphasis on the interfaces in these hybrid systems. The interactions between MNPs and graphitic nanomaterials will be specifically discussed as well as their influence on the assembly, functional properties, and applications of the hybrid systems in chemical sensors and fuel cells. Types of Metal−Graphitic Interfaces in the Hybrid Structures. Formation of any metal−graphitic hybrid nanostructure consequently © 2012 American Chemical Society

Received: October 23, 2012 Accepted: December 13, 2012 Published: December 13, 2012 147

dx.doi.org/10.1021/jz301711a | J. Phys. Chem. Lett. 2013, 4, 147−160

The Journal of Physical Chemistry Letters

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atoms tends to create natural defect sites in the graphitic structure while nitrogen atoms themselves can serve as active sites for further functionalization similar to oxygen-containing functional groups.55 Moreover, other than generating linkages from defected graphitic surfaces, a covalent interface was also accomplished though covalently attaching organic linkers to the pristine graphitic surfaces. Successful reactions included the Bingel reaction, photoinitiated carbine addition, and azide photochemistry.56−58 The covalent interfaces were also frequently reported in the graphene-based hybrid system59−66 as chemical exfoliation of graphite (the modified Hummer’s method) was a widely applied, simple, and scalable method to generate graphene materials. Graphite oxides (GO) and reduced graphite oxides (rGO) prepared by this method intrinsically possessed many defect sites and functional groups, and therefore, a covalent interface was formed after decoration with MNPs. Another type of interface reported in many metal−graphitic nanohyrbid materials has a “quasi-pristine” nature. In this system, organic linker molecules were also employed for assembly of MNPs onto carbon nanostructures, but the interactions between organic linkers and graphitic surfaces were noncovalent, as illustrated in Figure 1d. In this scenario, graphitic surfaces maintained their pristine structures while there was a molecular layer between the metal and graphitic surfaces. The metal−graphitic nanohybirds with a noncovalent interface have attracted increasing attention because of the nondestructive nature of the noncovalent functionalization. This approach has the advantage of preserving the unique optical, electrical, and other physical properties of carbon nanomaterials while resulting in higher density of functionalities which in turn led to higher loading of MNPs on the graphitic surfaces as compared with covalent functionalization. Because of the aromatic structure of graphitic surfaces, most of the applied noncovalent interactions included their hydrophobic interactions with molecular surfactants,67−72 wrapping by macromolecules73,74 or biomolecules,75−77 and π−π stacking with aromatic molecules.75,78−81 The relatively weaker interactions at the noncovalent metal− graphitic or linker−graphitic interfaces have also resulted in some unique surface phenomena that led to additional advances in the assembly of metal nanocrystals on graphitic surfaces. For example, the use of CNTs for templating the nanocrystal assembly and the bottom-up construction of complex metal nanostructures (such as nanowires) were successfully demonstrated in the presence of noncovalent interfaces,81,82 while similar covalent interfaces were proven to be less effective for this purpose.81

Figure 1. Schematic illustrations of four different types of metal− graphitic interfaces. (a) The pristine interface where MNPs and pristine graphitic surfaces are in direct contact. (b) Metal−defect interface where atoms of MNPs are bonded to the defect sites of the graphitic surfaces. (c) Covalent interface where covalent bonding between organic linkers and the graphitic surfaces is formed. (d) Noncovalent interface where organic linkers are attached to the graphitic surface through noncovalent interactions.

on CNTs with pristine interfaces. For graphene-based hybrid systems, fewer examples of pristine interfaces were reported as high-quality pristine graphene could only be prepared in limited synthetic approaches such as mechanical exfoliation of graphite,25 thermal annealing of SiC,26 or chemical vapor deposition (CVD) growth on metal substrates.27−31 As compared to the pristine metal−graphitic interface, the nonpristine interface was demonstrated more frequently in the literature where a covalent functionalization approach was involved for the generation of MNPs-CNT/graphene hybrids. Application of covalent functionalization has certain advantages: covalently bonded functional groups can provide linkage sites with intrinsically stronger bindings to the metal nanocrystals, and the presence of functional groups on CNTs/graphene surfaces could improve their solubility in many solvents for the solution-phase reactions. The solution-phase synthesis allowed the attachment of presynthesized MNPs. This approach could significantly benefit the control over the nanohybrid morphologies as the preparation of free metal nanocrystals in solution has progressed to the level of precisely controlled size, shape, and crystallinity.32,33 Additionally, the well-developed chemistry of carbon nanotubes34−36 and graphene37−39 provided a toolbox for the multiple possibilities of this synthetic approach and for the diverse structure design of the hybrid nanostructures. Covalent interfaces have been developed based on either graphitic defect sites, such as nonhexagonal rings, edges, and vacancies,40,41 or oxygenated defects sites (Figure 1b).22,42−48 Further chemical reactions with these functional groups were also applied to generate organic linkers to the MNPs (Figure 1c).49−51 In these cases, oxidative acid treatment52 was one of the most frequently employed methods for the production of oxygen-containing functional groups including hydroxyl, carbonyl, and carboxyl groups.42−46 It should also be mentioned that nitrogen doping, either post synthesis53 or during synthesis,54 is another emerging method of functionalizing the graphitic materials. The MNP decoration of nitrogen-doped carbon nanotubes (N-CNTs) results in interfaces of combined types as the incorporation of nitrogen

The interactions at metal−graphitic interfaces play essential roles in the synthesis (morphology control), properties (material performance), and applications of nanohybrids. Theoretical Modeling and Understanding of the Interactions at Metal−Graphitic Interfaces. The interactions at metal−graphitic interfaces play essential roles in the synthesis (morphology control), properties (material performance), and applications of nanohybrids. For example, because of the weak van der Waals interactions between metal and carbon, most in situ formed metal atoms (Au, Ag, Pt, Pd, Fe, Al, and Pb) nucleated and grew into nanoparticles instead of forming a continuous thin 148



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Figure 2. (a) Schematic illustration of the potentials considering the repulsive Coulombic interactions (dashed line) and attractive van der Waals interactions (dotted line) as a function of distance between negatively charged metal nanocrystals and negatively charged carbon nanotubes.83 (b) DFT simulation of the diffusion of a Au nanoparticle on a graphene surface with a pristine interface. The left image represents the illustration of two diffusing directions (P1 and P2), and the middle and right images are the calculated energy barriers for each diffusion pathway, respectively.81 (c) Bonding configuration (left) and charge variation maps (right) at a metal−graphitic interface where a “corner-on” adsorption of a Au20 nanocluster is bonded to a defect site of the (14,0) SWNT. (d) Charge difference plot across the metal−graphitic interface. (a) Reprinted with permission from ref 83. (b) Adapted with permission from ref 81. (c,d) Reprinted with permission from ref 85. Copyright American Chemical Society.

represents a typical Au20 cluster binding on a defect site of a SWNT, and Figure 2d demonstrates the average charge difference at the interface. A charge transfer of 0.050 e− from the (14,0) SWNT to Au20 was observed after the formation of a gold−graphitic interface, which was in accordance with the experimental results.85

layer as a result of the low binding energy (low coordination and interfacial energies) and low diffusion barriers on the pristine graphitic surfaces.15,20 As a comparison, metal nanowires were formed on carbon nanotubes with Ti, which has a strong interaction with carbon nanotubes. Therefore, the formation of Au, Pd, Fe, Al, and Pb nanowires could be achieved by using a Ti buffer layer, demonstrating the importance of interfacial conditions to the final morphology control.20 For the same reason, CNTs or graphene (most used as GO) were oxidized in many examples to introduce covalent functionalities and defect sites, for a better interaction between MNPs and carbon nanostructures. It has been generally recognized that van der Waals interactions between MNPs and graphitic nanostructures were the major force that regulated the adsorption83 and diffusion81 of nanoparticles on the graphitic surfaces, when pristine or noncovalent metal−graphitic interfaces were present. A thermodynamic model of such interactions was proposed (see Figure 2a for illustration), and diffusion energy barriers with pristine-type interfaces have also been studied recently by density functional theory (DFT),81 as shown in Figure 2b. This dispersive interaction and corresponding diffusion significantly contributed to the controlled assembly of metal nanocrystals on the graphitic surfaces.81−84 Theoretical modeling such as DFT calculations were also frequently employed to help understand the charge distribution across the interface between metal nanocrystals and graphitic surfaces, especially when covalent bonding was formed. Figure 2c

The effective characterization of metal−graphitic interfaces is relatively difficult compared with the more straightforward characterization of the overall hybrid morphology. Characterizations of the Metal−Graphitic Interface in the Hybrid Nanostructures. Obtaining accurate information about interfacial conditions is required in order to better understand the metal−graphitic interface. The effective characterization of metal−graphitic interfaces is relatively difficult compared with the more straightforward characterization of the overall hybrid morphology. Although electron microscopy techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been extensively used for the general visual identifications of nanomaterials (size, shape, morphology, etc.) including MNPs and graphitic nanostructures, they were 149



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AFM had been applied to determine the morphologies of metal−graphitic nanohybrids in most electrodeposition studies.14−17 However, this technique is based on the surface contact and cannot provide any further information about the interface. Successful application of AFM to characterize the metal−graphitic interface was demonstrated through indirect measurement. For example, it was demonstrated that MNPs first formed at the defect sites of CNTs at fine controlled deposition conditions as they were chemically more reactive and thus required less electrical potential for the reduction of metal ions.40,41 Therefore, AFM characterizations were applied to identify the defect sites in CNTs by the locations of MNPs, as illustrated in Figure 3c and d.40 Scanning tunneling microscopy (STM) is another surface characterization method that can provide resolution at the atomic level. STM characterization has recently been employed to obtain detailed visualization and useful chemical information of both covalent88 and noncovalent89 functionalities on the graphitic surfaces. Therefore, by further decoration of MNPs on a well-characterized graphitic surface, atomic information at the metal−graphitic interface could potentially be obtained for a hybrid system. As electrochemical properties of CNTs/graphene enabled the electrodeposition approach for the generation of metal− graphitic interfaces, their electrochemical characteristics could also be utilized as a characterization method for the metal− graphitic hybrid structures. The left panel of Figure 4a depicts a typical three-electrode cell for electrochemical measurements of the hybrid nanomaterials that were usually deposited on a glassy carbon (GC) working electrode. This electrode could be changed to a rotating ring-disk electrode to further study the detailed interfacial processes (Figure 4a right). Cyclic voltammetry (CV) is one of the most used electrochemical characterization techniques as it can be used to qualitatively evaluate oxygen-related functionalities on CNTs/graphene90 or surface reactions related to different metal−graphitic hybrid nanostructures.16,91−93 CV measurements of Pt−SWNTs and Pt−graphene for the hydrogen evolution reaction (HER) were used to characterize the active electrochemical surface area (ECSA) of PtNPs (as illustrated in Figure 4b),94 where Pt-SWNTs were determined to have a higher ECSA than Pt−graphene under the same synthetic conditions. Rotating ring-disk electrode voltammetry was also applied to study the electrochemical properties of metal−graphitic hybrid nanostructures (Figure 4c).95 Pt dispersed on poly(sodium 4-styrenesulfonate) (PSS)-wrapped SWNTs showed a 50 mV positive shift of the onset potential for the oxygen reduction reaction (ORR) compared to that of a traditional Pt/ carbon black electrode. Additionally, a lower ring current was also observed for the nanotube electrode, indicating a higher electron-transfer number per oxygen molecule. Such an improved electrochemical performance was not just related to an increase of surface area and stability of the supporting materials but possibly to some more complicated electron transfer/ interaction across the Pt−SWNT interface.95 Electrochemical impedance spectroscopy (EIS) is another useful technique that can provide more in-depth information about the electrode structure. Figure 4d depicts a typical EIS Nyquist plot for Pt− Ru nanoparticles on a CNT support with different surface functionalities.96 The smallest semicircle diameter indicated the smallest charge-transfer resistance for the methanol oxidation reaction (MOR) on graphitized CNTs, which could be explained by strong interaction between MNPs and a more pristine CNT surface after graphitization treatment. This result was also in

usually not sufficient enough to characterize the interfaces at the atomic scale. High-resolution TEM (HR-TEM) could achieve the atomic resolution and was thus used in many cases to reveal the interfacial structure. For example, in the electrodeposition of MNPs on CNTs, HR-TEM revealed that large nanoparticles were formed from multiple small clusters of 3−5 nm (shown in Figure 3a),

Figure 3. (a) High-resolution transmission electron microscopy (HRTEM) image of Pd nanoparticles electroplated on SWNTs. (The scale bar is 5 nm.) Nanotubes were deposited on a TEM grid prior to the electroplating step.14 (b) An 80 keV AC-HRTEM image of an individual Os nanocrystal inside of a SWNT.18 (c,d) Atomic force microscopy (AFM) characterizations of a CVD-grown SWNT circuit during an electrodeposition study.40 (c) A topographically clean SWNT circuit; (d) the SWNT’s most chemically reactive sites (defects) were decorated with Ni nanocrystals (14 nm in diameter) after electrochemical growth for 10 s. The scale bar (400 nm) in (c) also applies to (d). (a) Reprinted with permission from ref 14. Copyright 2006 American Chemical Society. (b) Reprinted with permission from ref 18. Copyright 2012 Royal Society of Chemistry. (c,d) Reprinted with permission from ref 40. Copyright 2005 Nature Publishing Group.

indicating multiple growth stages in this process.14 Such an aggregative growth mechanism that is different from those formed by evaporation was discussed in more detail in recent studies.17 Similarly, aberration-corrected HR-TEM (AC-HRTEM) offered effective characterization of the metal−graphitic interface even when MNPs were formed inside of CNTs.18,86 With highresolution images providing details at the atomic level, the pristine interface and the physisorption nature of MNPs on the graphitic surface were demonstrated (Figure 3b). Moreover, prolonged electron beam irradiation could generate defect sites in situ on the carbon nanotube surface; therefore, different interactions at the metal−pristine/defect graphitic interface were visually demonstrated by the different motion behavior of MNPs.18 In a similar example, the AC-HRTEM was also employed to monitor the motion of Co nanocrystals on a graphene surface.87 The sample preparation for the HR-TEM, however, required the placement of materials on a TEM grid. Therefore, this characterization could not be employed directly where CNTs or graphene were pregrown on other surfaces (such as SiO2) before the attachment of MNPs, as typically happened in the electrodeposition approach. In fact, for the electrodeposited MNP−CNT hybrid nanostructures, CNTs were deposited onto the TEM grid before the electrodeposition of MNPs specifically for the HRTEM characterization.14 Atomic force microscopy (AFM) is an alternative microscopic technique that could be applied for the visual identification of the hybrid nanostructure with nanoscale resolution. 150



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Figure 4. (a) Schematic illustration of a three-electrode cell with a ring-disk electrode. W.E.: working electrode; C.E.: counter electrode; R.E.: reference electrode; GC: glassy carbon. (b) Cyclic voltammograms (CVs) of Pt-SWNT (black solid line) and Pt-CCG (red dashed line) catalyst modified GC electrodes for the HER in a N2-saturated 0.5 M H2SO4 solution. (c) Rotating ring (upper panel) and disk (bottom panel) electrode voltammograms for a Pt-decorated polymer wrapped single-walled carbon nanotubes (PW-SWCNT, red solid line) and carbon black (CB, blue dot line) for the ORR at 24 °C in O2-saturated 0.1 M HClO4 and the corresponding Tafel plot (inset). Rotating rate = 1600 rpm, ERing = 1.2 V. (d) Impedance spectroscopy for MOR at 0 V versus Hg/Hg2SO4 for different carbon nanotube supports (NTs: CNTs from a CoxMg1−xO catalyst; NTs graph: graphitized CoxMg1−xO CNTs; NTs act.: KOH-activated CNTs; NTs tempt.: CNTs from an Al2O3 template). (b) Adapted from ref 94. Copyright 2011 Wiley-VCH. (c) Adapted from ref 95. Copyright 2006 American Chemical Society. (d) Reprinted from ref 96. Copyright 2006 Elsevier.

agreement with the reported observation on a heat-treated Pt− graphene system.97 For more detailed information about the interface including chemical bonding and charge distribution, spectroscopic characterization tools such as Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, or X-ray photoelectron spectroscopy (XPS) have been employed. However, these methods were effective only in few cases where the graphitic surface was oxidized in a mild and controlled manner51 or specific organic reactions were used to control the metal−graphitic interface56,57 due to the relatively strict requirements on the chemical purity of the sample. Even in these situations, the FTIR and NMR were only applied to characterize the organic linkers attached to either CNTs51,56 or gold nanoparticles (AuNPs),57 rather than directly to the final nanohybrids. The limitation of NMR characterization of hybrid systems could be explained by the well-known loss of 1H NMR signals (peak broadening and reduced intensity) in close proximity to MNPs57 and CNTs.98,99 Once organic linkers are immobilized at the metal−graphitic interface, this signal loss would be enhanced, and detection of their 1H NMR signals could become more difficult. Some successful examples of directly revealing the information of the metal−graphitic interface were conducted by XPS, which was able to demonstrate the chemical bonding at the CNT−CO−NH−AuNP interface (Figure 5a).51

Spectroscopy of the nanomaterial itself can provide an additional approach in characterization of a metal−graphitic hybrid system. For example, SWNTs possess unique electronic structure governed by the electron confinement in their radial direction, with several energy bands (known as van Hove singularities) in their density of states (DOS).100 Therefore, electron transitions between these singularities resulted in characteristic absorption spectra in the ultraviolet (UV), visible, and nearinfrared (NIR) regions, and UV−vis−NIR spectroscopy became a valuable tool for the characterization of SWNTs and related nanohybrid systems, where both suspended samples and solidstate thin films (Figure 5b) could be studied. Specifically, three transition bands were present for semiconducting SWNTs (S11− S33) and one for metallic SWNTs (M11), as demonstrated in Figure 5c (black curve, and the inset depicts the band diagram of semiconducting and metallic SWNTs). The S11 band in the NIR region corresponded to the transition between the HOMO and LUMO of SWNTs and was thus most sensitive toward any charge-transfer effect with SWNTs,101 while the influence on the M11 band was also observed in some studies.102 In fact, this technique has been successfully applied to study the electronic interactions within many CNT-based hybrid systems including small molecule−SWNTs complexes,103−105 polymer−SWNTs composites,106,107 and MNP−SWNTs hybrids.85,108 Therefore, monitoring the change in the S11 band could effectively characterize any charge transfer across the metal−graphitic interface 151



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Figure 5. Spectroscopic characterization of metal−graphitic interfaces. (a) XPS of C 1s, N 1s, and Au 4f core levels of Au nanoparticles (AuNPs) attached to irradiated carbon nanotubes through a covalent amide bonding.51 (b) A digital photograph (top) of a AuNP-decorated SWNTs film on a quartz plate, with a SEM image (bottom) of the hybrid film. (c) UV−vis−NIR absorption spectroscopic monitoring of a SWNT film during the electrodeposition of AuNPs; spectra were normalized at 1350 nm, the black curve is the spectrum of the bare SWNT film, and the red curves are after the Au deposition for 100, 150, and 200 s. The inset depicts a density of states (DOS) diagram of metallic and semiconducting SWNTs demonstrating their transition bands. (d) Raman spectra of a bare SWNT film (black) and Au-decorated SWNT film (red).85 (a Reprinted with permission from ref 51. Copyright 2006 Wiley-VCH. (b−d) Reprinted with permission from ref 85. Copyright 2010 American Chemical Society.

ratio between the disorder-induced D band and the tangential mode (G band), information on the electronic structure could also be revealed. As shown in Figure 5d, the radial breathing mode (RBM) of the SWNTs was broadened and shifted after the generation of the metal−graphitic interface, while the intenssity of the G band was also decreased.85 Such decreases in the intensity of the RBM and G band were correlated with the tranfer of electronic density from the SWNTs to the AuNPs.85,104,111 Similarly, Raman spectroscopic characterization has been employed to reveal the electronic interactions between MNPs and graphene nanostructures,25,112 and their dependence on the ionization energies of metals112 or the number of graphene layers25 was also demonstrated. Electrical measurements of CNTs or graphene can also be used to characterize the metal−graphitic interface in the related nanohybrid systems. For example, 1/3 of SWNTs are metallic, and 2/3 are semiconducting depending on their electronic structures. The semiconducting SWNTs are theoretically intrinsic but generally p-type under ambient conditions due to a partial electron depletion effect caused by adsorbed molecular oxygen.113 Such electrical sensitivity to the molecular or charge doping provides a valuable characterization tool in the studies of SWNTbased hybrid systems. Field-effect transistor (FET) devices based

in the MNP−SWNTs hybrid systems. Figure 5c shows typical UV−vis−NIR spectra of the SWNT network with electrodeposited AuNPs, where a decrease in the S11 absorption band clearly indicated the partial removal of electronic density from the valence band of SWNTs upon the decoration of AuNPs.85 AuNPs also have a surface plasmon resonance (SPR) peak in the visible region (Figure 5c), which was usually used to indicate the formation of nanoparticles and the generation of a metal−graphitic interface. Recent studies have shown that this SPR signal was also sensitively dependent on the distance at the spaced nanoparticle−graphene interface.109 This result indicated a potential use of nanoparticle SPR as a probe to reveal the metal−graphitic interfacial conditions. Overall, the ease of sample preparation, in situ investigation, and information about both interface generation and charge transfer through the interface are several advantages of UV−vis−NIR spectroscopy and the reason that this technique has been frequently employed in characterization of metal−graphitic interfaces. Raman spectroscopy is another technique utilizing the unique electronic and vibrational structure of the graphitic nanomaterials110 for the characterization of a MNP−graphitic nanohybrid system. Although it was mostly effective in revealing the level of disorder and defect sites on the graphitic surface through the 152



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to the metal−SWNT network, which resulted in a decrease in the charge carrier (hole) concentrations due to the electron− hole recombination (Figure 7a (right)). Moreover, such an electron-transfer effect (gate voltage shift) was found to be related to the work function of the metal rather than the particle size (Figure 7c). This was another clear indication that a nanoscale Schottky barrier was formed across the interface between MNPs and SWNTs. Such an energy barrier was dependent on the work function of the metal, and the charge transfer from NO molecules was only limited by this barrier. On the basis of the information collected from FET measurements, a band diagram with a potential barrier at the metal−graphitic interface could be determined (Figure 7d).108 Compared with UV−vis−NIR characterization, the FET method is intrinsically more sensitive toward the charge-transfer effect.115 Moreover, combination of the spectroscopic and electrical measurements could provide complementary information on metal−graphitic interfaces between MNPs and SWNTs.115,116

on either individual semiconducting nanotubes or SWNT networks were most frequently fabricated to evaluate any charge effect from the metal−graphitic interface. Figure 6a depicts a

Spectroscopic and electrical measurements could provide complementary information on metal−graphitic interfaces.

Figure 6. Electrical characterization of metal−graphitic interfaces. (a) Schematic illustration of a FET device composed of a network of SWNTs decorated with MNPs. (b) Conductance (GSD) of the SWNT network FET device as a function of gate voltages (Vg) before and after AuNP functionalization.14 Reprinted with permission from ref 14. Copyright 2006 American Chemical Society.

Applications in Chemical Sensing. The sensitive modulation of electrical properties upon exposure to specific molecules has also led to a wide-range application of MNP−CNT hybrids in chemical sensing. For example, the conductance of the AuNP− SWNTs film experienced a significant decrease when exposed to 2500 ppm of CO (Figure 8a) as a result of the charge-transfer effect that was also rationalized with theoretical simulations (Figure 8b).85 Such an electrical response could be used to detect different gas/vapor molecules such as NO2, NH3, H2, CO, CO2, O2, H2S, and volatile organic vapors (VOCs).4 Different MNPs were employed to introduce specific sensitivity and selectivity toward target analytes; some typical combinations are summarized in Table 1. Moreover, as multiple metal decorations could be conducted in a single sensor chip (Figure 8c), sensor arrays (also known as electronic noses) could be fabricated to determine multiple gas components in a dynamic environment (Figure 8d). The interface between MNPs and CNTs was also found to be a critical factor that determines the sensor performance. For example, when AuNP-decorated SWNTs were applied to detect H2S, defect sites on the SWNTs resulted in a reversible detection due to the localized phonon scattering and increased Joule heating at the gold−defect interface.47 Existence of defect sites was also discovered to increase the hydrogen sensitivity of the Pd-decorated SWNTs.117 Recently, we demonstrated another example of manipulating the gold−nanotube interface for improved H2S sensitivity.81 In this case, a noncovalent interface between AuNPs and SWNTs was generated with 1-pyrenesulfonic acid (PSA), which enabled a unique assembly of AuNPs that could not be achieved with other gold−graphitic interfaces. In such a gold nanowire (AuNW)-decorated SWNTs hybrid network, electricity passed through the AuNW−SWNT interface (as illustrated in Figure 9a); therefore, the modulation of the Schottky barrier across the interface led to a better sensitivity toward H2S compared with that for AuNP-decorated SWNTs, as demonstrated in Figure 9b.81 Compared with MNP−SWNTs, the chemical sensitivity of graphene-based nanohybrid systems is limited due to its 2-D

typical setup of the carbon nanotube field-effect transistor (NTFET) fabricated from CVD-grown SWNTs on a Si wafer with an oxide layer on the surface.14 Before the introduction of any MNPs, the SWNT network demonstrated a typical p-type FET characteristic, as shown in Figure 6b. After the deposition of AuNPs on the SWNTs, the resulting FET device showed a decreased modulation (loss of gate dependence), which was attributed to the screening effect from the AuNPs.14 The same FET characterization could also be applied to the electrodeposited hybrid system.108 While similar loss of gate dependence in the FET characteristics was present for the SWNT devices decorated with large MNPs, a suppression of the I−Vg curve at all gate voltages was also observed, especially for small particles. This suppression was most likely caused by a decrease in the charge carrier mobility from metal−graphitic interactions. A similar effect was previously demonstrated in NTFETs decorated with porphyrin molecules, which were claimed to act as scattering sites and thus decreased the hole mobility in the SWNT network.114 Therefore, it could be hypothesized that the decoration of SWNTs with MNPs resulted in charge redistribution across the metal−graphitic interface through partial electron transfer from SWNTs to MNPs, and a localized charge depletion region was formed at each nanoparticle−nanotube interface. These localized depletion regions served as scattering sites that consequently reduced hole mobility in SWNTs, as illustrated in Figure 7a (left panel).108 Small redox molecules could be subsequently introduced to FET device as a molecular gating to further probe the metal−graphitic interface. For example, the Au-decorated SWNT FET experienced a shift toward a more negative gate voltage after it was exposed to 10 ppm of NO in a dry N2 environment (Figure 7b). This result indicated an electrontransfer process from the NO molecule (a weak electron donor) 153



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Figure 7. Charge redistribution across the metal−graphitic interface. (a) The left panel depicts the formation of a localized electron depletion region (dashed line) at the metal−graphitic interface, which acted as a charge scattering site and reduced the charge carrier (hole) mobility. The right panel illustrates the electron transfer from the molecular probe (NO) to MNPs. The electron density was further transferred to the SWNTs after crossing a potential barrier at the metal−graphitic interface. The addition of electron density reduced the hole density of SWNTs through electron−hole recombination and therefore caused a negative shift in gate voltages. (b) G−Vg characteristics of a gold-nanocrystal-decorated SWNT FET device before and after exposure to 10 ppm NO in N2. (c) Dependence of the gate voltage shift in a MNP-decorated SWNT FET device (after NO exposure) to the work function of the functional metals. (d) Schematic illustration of the electronic interactions and potential barriers occurring at the metal−graphitic interface; the potential barriers could be understood as nanoscale Schottky barriers that were dependent on the work functions of the MNPs.108 Reprinted with permission from ref 108. Copyright 2007 American Chemical Society.

Figure 8. Chemical sensing with MNP-decorated SWNTs. (a) Relative conductance (G/G0) change of bare SWNTs and Au-decorated SWNT chemiresistive films during the exposure to the CO gas (2500 ppm in N2). The red dashed lines represent the Langmuir fitting of the experimental results. (b) Calculated electrical conductance of a Au20−(14, 0) SWNT as a function of the number of adsorbed CO molecules. The inset depicts the charge variation maps of nine carbon monoxide (CO) molecules attached to the Au20 nanocluster.85 (c) Optical microscope photograph (200×, top) and SEM images (bottom) of five separated SWNT devices after electrodeposition of different MNPs on four devices. The scale bar represents 200 nm. (d) The principle component analysis (PCA) of the chemical response forms a sensor array composed of SWNTs decorated with different metal nanocrystals. Each analyte has a specific pattern of sensor array response.14 (a,b) Reprinted with permission from ref 85. (c,d) Reprinted with permission from ref 14. Copyright American Chemical Society.

depletion region only contributes to a small portion of the conducting surface of 2-D graphene.118 To overcome such limitation, 1-D graphene nanoribbons (GNRs) or holey reduced graphene oxide (hRGO) that mimics the structure of a SWNT network

nature. Specifically, 1-D nanostructures such as SWNTs experience a significant change in their electrical conductivities as the charge depletion depth (Debye length) from an adsorbed molecule is comparable to their cross-sectional radius, while a similar 154



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ordered graphitic structure of CNTs and graphene provides higher resistance toward electrochemical oxidation/corrosion during the fuel cell working condition.124,125 Figure 10a shows a long-term (30 days) performance of a phosphoric acid fuel cell (PAFC) using Pt-CNTs as the cathode material. By replacing regular Pt/carbon black with a Pt-CNT catalyst, much higher stability (up to 240 days tested, operating in 190 °C, 85% H3PO4) was achieved using a thinner catalyst layer (Figure 10b) without sacrificing the power density.126 Moreover, such a hybrid structure also possessed some synergistic effect due to the interaction between the MNP and graphitic structure, exhibiting improved potential performance at both the cathode for the ORR (Figure 3c)95 and the anode for the MOR (Figure 10c).127 Nitrogen doping of CNTs and graphene can have additional advantages. The N-doped graphitic nanostructures demonstrated good ORR catalytic activity in basic media by themselves,54,128 and functionalization with MNPs could further improve the performance (Figure 10d),129 as result of not only the better dispersion of the MNP on the N-doped graphitic surface130 but also the intrinsic catalytic activity of N-doped structures131

Table 1. MNP-Decorated CNT-Based Gas Sensors with Their Detection Limits analytes CNT decorations

H2

CO

H2S

bare Au Pd Pt

40 ppm21 60 ppm119

2500 ppm85 20 ppm133 1 ppm134

3 ppb47 -

have been developed for chemical sensing applications. Figure 9c shows an example of fabricating PtNP-decorated hRGO devices. With a unique interface between PtNPs and the edge of hRGO, the resulting hybrid device demonstrated ultrahigh H2 sensitivity (Figure 9d).119 Applications for Fuel Cells. Metal−graphitic hybrid structures are also of great interest for fuel cell applications,120−123 where MNP (usually Pt or its alloys) serves as the catalyst and CNT/ graphene works as the catalyst support. Compared to conventional carbon-based catalyst supports such as carbon black, the

Figure 9. (a) Schematic illustration of a chemiresistive hydrogen sulfide (H2S) sensor that utilized metal−graphitic interfaces in the Au-nanowirefunctionalized SWNTs (AuNW−SWNTs) network. The inset depicts the schematic illustration of the interfacial conditions in the network. (b) Electrical response comparison of AuNW-SWNTs, AuNP−SWNTs, and bare SWNT devices to various concentrations of H2S.81 (c) Schematic illustration of a FET device fabricated from holey reduced graphite oxide (hRGO) decorated with MNPs. The inset depicts a SEM image of the device. (d) Calibration curve of Pt-hRGO devices to H2 gas; the insets depict the H2 sensitivity in a CO (0.25% in N2) environment (left) and crosssensitivity of Pt-hRGO to CO (0.05−0.25%) and CH4 (0.4−4%) gases in N2 (right). (a,b) Reprinted with permission from ref 81. (c,d) Reprinted with permission from ref 119. Copyright American Chemical Society. 155



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Figure 10. (a) Long-term performance of a phosphoric acid fuel cell (PAFC) using Pt-CNTs as cathode material. (b) Cross-sectional SEM image of the cathode showing the graphite paper, the carbon/Teflon wet-proofing layer, and the Pt-CNT network. (c) CV curves of 20 wt % Pt/carbon black (black), 20 wt % Pt/graphene (red), and 20 wt % Pt-10 wt % Ru/carbon black (blue) for the MOR in 1 mol/dm3 CH3OH + 0.05 mol/dm3 H2SO4 at room temperature. Scan rate: 5 mV/s. (d) Polarization graphs of a proton exchange membrane fuel cell (PEMFC) with Pt/nitrogen-doped graphene as the cathode catalyst and Pt/MWNT as the anode. (a,b) Reprinted from ref 126. (c) Reprinted from ref 127. Copyright American Chemical Society. (d) Reprinted from ref 129. Copyright 2010 Royal Society of Chemistry.

of controlled interfaces are important in the future research in this area. Control and understanding of the interface at the atomic level will greatly promote the development of advanced metal−graphitic nanohybrid materials that can further improve the material properties and their related applications.

and the modified interface between the MNP and graphitic nanomaterials.132

Control and understanding of the interface at the atomic level will greatly promote the development of advanced metal−graphitic nanohybrid materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Future Challenges and Outlook. Significant success has been achieved in the development of novel metal−graphitic nanohybrid materials over the past two decades, and their practical importance has been demonstrated in numerous applications including chemical sensing, electrocatalysis, and heterogeneous catalysis. For such a hybrid system that is composed of two types of nanostructures as building blocks, challenges remain in the development of synthetic methodology that allows manipulation and engineering of such nanoassemblies with atomic precision. Development of atomically controlled MNPs (or recently more referred to as metal nanocrystals) and SWNTs/ graphene will be critical as it will provide the basic building blocks for the further construction of the hybrid nanostructures. The progress in understanding and manipulation of surface functionalities on both metal and graphitic surfaces will be equally important. There is still much room for the systematic exploration of covalent and noncovalent functionalizations of both building blocks with traditional standards of chemical synthesis and purification. Furthermore, as discussed in this Perspective, the importance of the metal−graphitic interface has been gradually recognized in many recent studies and should be specifically emphasized in the future development of metal−graphitic nanohybrid systems. Better characterization, design, and fabrication

Notes

The authors declare no competing financial interest. Biographies Mengning Ding received his B.S. in Chemistry in 2007 from Nanjing University. He is currently a Ph.D. candidate in the Department of Chemistry at the University of Pittsburgh. His current research interests include synthetic and physical chemistry of carbon nanotube/graphenebased hybrid nanomaterials and their application in chemical sensing. Yifan Tang is a Ph.D. candidate in the Department of Chemistry at the University of Pittsburgh. He received his B.S. in Chemistry in 2007 from Nanjing University. His current research interests include the synthesis and characterization of carbon nanomaterials and their applications for sensors and sustainable energy. Alexander Star is an associate Professor of Chemistry at the University of Pittsburgh. His research focuses on synthesis and properties of carbon nanomaterials, nanotechnology-enabled chemical sensors, energy conversion devices, nanotoxicology, and nanotherapeutics. Prior to joining the Pitt faculty in 2005, Star developed carbon nanotube-based sensors at Nanomix, Inc. Additional information regarding Star’s research can be found at http://www.pitt.edu/~astar. 156



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ACKNOWLEDGMENTS We acknowledge the financial support from National Energy Technology Laboratory (NETL) under URS Contract DEFE0004000 and the National Science Foundation (NSF) under NSF CAREER Award No. 0954345.

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ABBREVIATIONS MNPs, metal nanoparticles; CNTs, carbon nanotubes; SWNTs, single-walled carbon nanotubes; CVD, chemical vapor deposition; FET, field-effect transistor; GC, glassy carbon; CV, cyclic voltammetry; HER, hydrogen evolution reaction; ORR, oxygen reduction reaction; MOR, methanol oxidation reaction; EIS, electrochemical impedance spectroscopy; ELSA, electrochemical surface area; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital

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