A major challenge in the development of electrocatalysts is to determine a detailed catalysis mechanism
on a molecular level for enhancing catalytic activity. Here, we present bottom-up studies for an electro-
catalytic hydrogen evolution reaction (HER) process through molecular activation to systematically
control surface catalytic activity corresponding to an interfacial charge transfer in a porphyrin monolayer
on inactive graphene. The two-dimensional (2D) assembly of porphyrins that create homogeneous active
sites (e.g., electronegative tetrapyrroles (N4)) on graphene showed structural stability against electro-
catalytic reactions and enhanced charge transfer at the graphene-liquid interface. Performance oper-
ations of the graphene field effect transistor (FET) were an effective method to analyse the interfacial
charge transfer process associated with information about the chemical nature of the catalytic com-
ponents. Electronegative pristine porphyrin or Pt-porphyrin networks, where intermolecular hydrogen
bonding functioned, showed larger interfacial charge transfers and higher HER performance than Ni-, or
Zn-porphyrin. A process to create surface electronegativity by either central N4 or metal (M)–N4 played
an important role in the electrocatalytic reaction. These findings will contribute to an in-depth under-
standing at the molecular level for the synergetic effects of molecular structures on the active sites of
electrocatalysts toward HER.
Introduction
Many of the effective electrocatalysts for the hydrogen evol-ution reaction (HER), including single crystal Pt surfaces,1,2
non-Pt-based metal compounds such as MoS2,3,4 metal-free
electrocatalysts,5,6 and carbon-based metal compositecatalysts,7–9 have been reported. In particular, conjugatedcarbon materials as an economical catalyst have recently been
shown to have good electrical properties and metal attractivedefects that allow them to catalyze electrochemical reactionsfor HER and oxygen reduction.10 Defects containing nitrogendopants on the conjugated carbon materials can activate theelectrocatalytic activity and increase the catalytic efficiencywith the assistance of metal nanoparticles.8,11 These nitrogen-doped and defected carbon materials containing pyrrolicN and pyridinic N can have irregular, porphyrin-like electro-catalytic molecular structures.8,12,13 Thus, porphyrins (withnatural tetrapyrroles (N4) at the central cavity), such as free-base porphyrins14–20 and metalloporphyrins,21,22 on conduc-tive carbon surfaces are promising catalytic molecules or activesites. Until now, however, studies on the enhancement of theelectrocatalytic activity of conjugated carbon materials havebeen conducted mainly at a bulk scale and not at a molecularscale. This is because of the difficulty of a molecular design tostructurally control surfaces, even though this should provideimportant catalysis information. For example, carbonmaterials that have defected frameworks containing atomicdopants, such as nitrogen atoms, showed enhanced catalyticactivity due to synergetic effects to increase the positive chargedensity on adjacent carbon atoms.23 However, this synergeticprocess on catalytic reactions cannot be examined in terms of
†Electronic supplementary information (ESI) available: Electrochemical scan-ning tunnelling microscopy (EC-STM) images; characterization of porphyrinadsorption and its metalation on graphene; changes in Raman spectra withregard to adsorption of 2H-TPyP and its metalation; changes in transfer charac-teristics of graphene field effect transistors (FETs) in response to porphyrinadsorption and metalation on graphene. See DOI: 10.1039/c6nr09428g
aCentre for Integrated Nanostructure Physics (CINAP), Institute of Basic Science
(IBS), 2066 Seoburo, Jangan-gu, Suwon 16419, Republic of Korea.
E-mail: [email protected] of Chemistry, Sungkyunkwan University, 2066 Seoburo, Jangan-gu,
Suwon 16419, Republic of KoreacDepartment of Energy Science, Sungkyunkwan University, 2066 Seoburo, Jangan-gu,
the electrochemical proton adsorption on the active sites and theelectron transfer for the electrochemical proton reduction separ-ately in bulky-structured or hetero-structured catalysts. Activesites, such as defects (or heteroatom-doped sites) on carbonframeworks, cannot be well controlled in top-down processedmaterials. Understanding of the electrocatalytic roles of activatedsurfaces on conjugated carbon materials at the molecular mono-layer level is therefore highly desirable. To simplify the system, thecarbon electrode itself should be nonvolatile to electrochemicalreactions over a wide potential window range in aqueous solution,and electrocatalytic active sites should be homogeneous and con-trollable. In that sense, the electrocatalytic activity of self-assembled porphyrins as functional molecular building blocksthat are surface-confined via strong electronic coupling with thesupporting carbon electrode is a topic of great interest.
Two dimensional (2D) molecular assembly can yield con-trollable surface systems on electrodes, and the interfacial fea-tures and responsive characteristics of the resulting electrodes
are dependent on intermolecular and molecule–electrodeinteractions.24 In particular, noncovalent assembly of mole-cules via supramolecular engineering is a nondestructive wayto arrange the chemical structures of molecules and sub-strates.25 For example, planar π-conjugated porphyrins facili-tate cofacial attachment on surfaces, where the molecular com-ponents lead to efficient electron transfer in devices due toπ-electron delocalization of the high π-conjugation system.26,27
Herein, we used a simplified two-dimensional electro-catalytic system consisting of catalytically active porphyrinmolecules and atomically thin graphene electrodes that rep-resent conjugated carbon catalysts to understand a governingmechanism or process at a molecular level in HER. Fig. 1adepicts the interface engineering to tailor the interfacialcharge transfer of the metal–organic complexation to the cata-lytically inactive graphene and shows the detailed mechanismsfor an enhancement of the electrocatalytic activity of the gra-phene. Measurements of changes in the interfacial electronic
Fig. 1 Schematics of the two-dimensional electrocatalytic model system representative of conjugated carbon catalysts to investigate interfacialelectronic effects on surface-electrocatalytic changes: (a) self-assembly of porphyrin and its metalation on graphene; proposed electronic structuresof graphene in response to surface modification with central tetrapyrroles (N4) and the meso-pyridines of porphyrin (i.e., 2H-TPyP).28
(b) Electrochemical catalytic activity toward the hydrogen evolution reaction (HER) versus charge transfer measurements between graphene andporphyrin layers. (c) Chemical structures of the three porphyrin molecules used in this work.
characteristics of the molecules/graphene provided an in-depth understanding of the electrocatalytic contributions ofeach component at the molecular monolayer level for all stepsranging from 2D assembly of catalytic active molecules to theirmetalation (which were conducted by the electrochemicaland electronic devices as shown in Fig. 1b). Porphyrin mole-cules (e.g., 5,10,15,20-tetraphenyl-21H,23H-porphine (2H-TPP),5,10,15,20-tetrakis-(4-pyridyl)-21H,23H-porphine (2H-TPyP),and 5,10,15,20-tetrakis-(4-aminophenyl)-21H,23H-porphine(2H-TAPP) in Fig. 1c) were well-organized into a monolayer ongraphene. In this work, an in situ observation of a porphyrinmonolayer on graphene was reported at the solid–liquid inter-face. Metalation of the porphyrins assembled on the graphenewas facilely achieved by metal insertion into the central N4
sites of the free-base porphyrins via cation complexation inaqueous solution, which is a new approach for porphyrinmetalation, especially with noble metals such as Pt, on sur-faces. An increase in the electronegativity of the central N4 ormetal (M)–N4 directly enhanced the electrocatalytic reaction.Also, the electronic properties of bare graphene in field effecttransistors (FETs) were modulated with respect to the electro-negativity of the central N4 and metal (M)–N4, which were indi-cators to the interfacial charge transfer toward HER.Intermolecular hydrogen bonding interactions significantlyimproved the electronic characteristics of graphene to enhancethe electrocatalytic activities of porphyrin–graphene systems.
These findings provide new insights into the modulation ofmolecular structures in conjugated carbon materials to createhighly active proton adsorption sites.
Results and discussion
In order to understand the improvement process of the electro-catalytic activities of the surface-immobilized catalysts towardHER, we focused on the formation of homogeneous activesites into a monolayer and their electrocatalytic behavioursrelated to the interfacial charge transfer characteristics.
Monolayer formation of porphyrin on graphene: in situelectrochemical scanning tunnelling microscopy (EC-STM)
Surface attachment of π-conjugated molecules ontoπ-conjugated surfaces lowers the surface free energy via π–πstacking without changes in the molecular composition.29
Thus, graphene, single-layer graphite with a π-conjugatedsystem, can function as a platform for noncovalent π–π stack-ing and allows the control of the organization of planarπ-conjugated porphyrins into long-range 2D architectures.30–32
As shown in the EC-STM images (Fig. 2), graphene provided agood platform for physisorption of a free-base porphyrinmonolayer (e.g., a 2H-TPyP monolayer) at the solid–liquidinterface. An Au(111) substrate was used to understand mole-
Fig. 2 In situ electrochemical scanning tunnelling microscopy (EC-STM) images and corresponding schematics: (a) single-layer graphene on thereconstructed Au(111) surface in 0.1 M H2SO4 under scanning conditions with a tunnelling current (It) of 0.1 nA and a tip bias (Vbias) of 0.1 V at asample potential of 0.0 VSCE. The black arrow indicates the boundary of a graphene sheet (the upper part) on Au(111). (b) Zoomed-in image of (a)(inset: zoomed-in image of the selected area). (c) Illustration of single-layer graphene on the reconstructed Au(111). (d) Single-layer graphene on thereconstructed Au(111) surface in 0.05 mM 2H-TPyP/0.1 M H2SO4 under scanning conditions with a tunnelling current (It) of 0.1 nA and tip bias (Vbias)of 0.1 V at a sample potential of 0.0 VSCE after holding the sample potential at 0.5 VSCE for 90 s. The line profile (black line) indicates single-layer gra-phene (∼3.5 Å thick). (e) Zoomed-in image of (d). The line profile (grey line) indicates 2H-TPyP molecules in a monolayer. (f ) Illustration of a mono-layer of 2H-TPyP on graphene/Au(111).
cular self-assembly of porphyrin/graphene at electrochemicalinterfaces.33–35 The reconstruction of the Au(111) surfaceresults in a unique structure, leading to a predictable fashionagainst applied potentials,36 which is distinguishable fromgraphene (or graphite). Chemical vapour deposition (CVD)-grown graphene was transferred onto an Au(111) substrate andthermally annealed as described in the methods section.Single-layer graphene on Au(111) was successfully observed insolution (Fig. 2a and b), where the honeycomb pattern of thehexagonal graphene lattice overlapped on the reconstructedstripes of Au(111). This was clearly distinguishable from thebare Au(111) and multilayer graphene on Au(111) (Fig. S1a andb in the ESI†), showing a superlattice pattern, which was con-sistent with a previous report.37 Moiré structures of grapheneon Au(111) due to the lattice mismatch of graphene andAu(111) varies with the lattice rotation.37 Fig. 2c shows theschematic structure of graphene deposited on the surface-reconstructed Au(111). To observe the adsorption behaviour of2H-TPyP molecules onto single-layer graphene on Au(111), weused several potentials within the range of −0.2 VSCE to0.65 VSCE. According to previous literature,33,34 2H-TPyP mole-cules can form a monolayer at a potential more negative thanthe potential at the point of zero charge (pzc) for a substrate inan electrolyte solution. For example, the pzc of Au(111) isaround 0.3 VSCE in a 0.1 M H2SO4 solution, which resulted in2H-TPyP molecules assembling into a monolayer at 0.0 VSCEand multilayers (or aggregated layers) at 0.5 VSCE.
33 When thesample potential was held at 0.0 VSCE during scanning afterapplication of a voltage pulse of 0.5 VSCE (Fig. 2d), the 2H-TPyPmolecules assembled into an ordered adlayer on graphene(marked with a white arrow), while they aggregated on Au(111)(marked with a black arrow). Fig. 2d shows the image of acertain region where graphene was torn to measure the thick-ness of the porphyrin-adsorbed graphene on Au(111) againstthe rest of the surface. In the line profile, the apparent thick-ness of the porphyrin-adsorbed graphene was 0.35 nm fromthe rest of the surface that could have porphyrin molecules.Considering the thickness of single-layer graphene (0.34 nm),the line profile indicated the formation of an ordered por-phyrin layer on graphene (Fig. S1c in the ESI†). The line profilefrom the zoomed-in image of the aggregated region (Fig. S1din the ESI†) was consistent with that in Fig. 2d. Meanwhile,the aggregated porphyrin molecules on bare Au(111) dis-appeared after application of a 0.65 VSCE pulse (Fig. S1e in theESI†), which can lift the reconstructed Au(111) and form Auislands (0.25 nm high indicating the step depth of Au(111)),34
whereas the graphene-covered Au(111) still showed the her-ringbone pattern. Two possibilities for the disappearance ofthe porphyrin molecules can be suggested: (1) the aggregated(disordered) porphyrin molecules may form clusters on Auislands after lifting the reconstructed Au(111); (2) the por-phyrin molecules may desorb from the surface after oxi-dation.33,34 In Fig. 2e, the 2H-TPyP molecules are clearlyobserved on graphene at 0.0 VSCE, and the line profile showingthe lateral size of 2H-TPyP (approximate 2 nm) suggested aflat-lying assembly,33 where the apparent height of a 2H-TPyP
molecule was approximately 0.08 nm. According to a previouspaper,33 the apparent height of a 2H-TPyP monolayer onAu(111) at the solid–liquid interface was measured to beapproximately 0.12–0.15 nm. Considering the experimentalconditions (e.g., tunnelling set point), the cross-sectional lineprofile through the porphyrin adsorbed single-layered gra-phene sheet indicated that porphyrin assembled in a mono-layer on graphene.33,34 The monolayer was maintained in thepotential window from −0.2 VSCE to 0.5 VSCE, which is animportant finding for porphyrin/graphene systems, showingthe structural stability against electrocatalytic reactions. Froma direct observation of the 2H-TPyP molecules adsorbed ongraphene at the solid–liquid interface (Fig. S2 in the ESI†), themolecular lines on graphene/Au(111) appeared at 0.2 VSCE anddisappeared at 0.55 VSCE, repeatedly. Consequently, graphene-2H-TPyP interactions can play a predominant role in the for-mation of molecular layers, and graphene can function as aplatform for the formation of a 2H-TPyP monolayer (consistingof approximate 2.5 × 1013 molecules cm−2) at the solid–liquidinterface.
Characterization for the metalation of porphyrin in amonolayer on graphene
Surface-mediated metalation of porphyrin with surface metalatoms such as iron and nickel has been achieved in avacuum.20,31–33 It would be a great challenge if at the solid–liquid interface, in situ surface metalation on porphyrin-assembled surfaces with powerful catalytic metals such as Ptcan be facilely achieved.38 From the STM observation, gra-phene can provide a platform to form a stable porphyrinmonolayer via strong π–π interactions, and intermolecularhydrogen bonding interactions ensure that the assembled por-phyrin molecules can act as a molecular template offeringactive sites (i.e., approximate 2.5 × 1013 sites cm−2) for embed-ment of metal atoms. Porphyrin-metal (ion) adducts can alsobe stabilized by π–π interactions of porphyrin with grapheneand electrostatic interactions between the π-electrons of N4
and metal ions. To investigate changes in the electronic pro-perties of graphene due to porphyrin adsorption and metala-tion, we conducted several ex situ spectroscopic analyses, suchas ultraviolet-visible (UV-Vis) spectroscopy, X-ray photoelectronspectroscopy (XPS), Raman spectroscopy, and ultravioletphotoelectron spectroscopy (UPS), at each step. Either thermalannealing or voltage application was used for porphyrin meta-lation after adsorption of metal ions on the porphyrin/graphene samples.
In the UV-Vis spectra of graphene, porphyrin adsorptiongenerated new absorption peaks at 417 ± 2 nm and 427 ±2 nm, corresponding to the 2H-TPyP and 2H-TAPP layers,respectively, which originate from the Soret band of the por-phyrins (Fig. 3a). Soret bands are caused by the π–π* tran-sitions in porphyrins.39 Subsequent insertion of metal cationsinto the active centre N4 of porphyrins was conducted bysimple metal cation complexation at the graphene–liquidinterface and accomplished by well drying at 80 °C for morethan 2 days and rapid thermal annealing at 200 °C for 30 s.
This induced a red-shift of the Soret bands to 427 ± 2 nm(2H-TPyP layer) and 437 ± 2 nm (2H-TAPP layer) (Fig. S3a inthe ESI†), indicating deprotonation of the free-base porphyrinsand the formation of metalloporphyrins.39 Also, porphyrinmetalation was achieved by applying a range of voltages(voltage sweeping from −2.5 V to +2.5 V) between the two elec-trode contacts on either end of the graphene at room tempera-ture (Fig. S3b and c†).
XPS measurements were used to characterize changes inthe N 1s peak shapes and binding energies due to porphyrinmetalation on graphene (Fig. 3b).40 After adsorption of the2H-TPyP molecules on graphene, the spectra of the N 1s corelevels showed two components assigned to the pyrrolic(–N(H)–) peak at 400.7 ± 0.26 eV and the iminic (vN–) peak at398.6 ± 0.21 eV, which is in agreement with the previousstudies22,40–42 even though they were not clearly split. Inaddition, a peak broadening of the C 1s peak was observed inthe XPS spectrum for porphyrin on graphene (Fig. S3d in theESI†), which is likely due to the influence of layer formationvia π–π stacking.43 As porphyrin metalation with Pt2+ cationsoccurred via a deprotonation process, the C 1s peak shiftedtoward a higher energy, and the N 1s peaks merged into onebroad peak at 399.6 ± 0.27 eV.20,36–38 The doublet peaks of thePt(II)4f7/2/Pt(II)4f5/2 core levels were observed at 72.3 ± 0.1 eVand 75.6 ± 0.1 eV, respectively, after the formation of Pt-TPyPon graphene (Pt-TPyP/G), which is characteristic of Pt(II) com-plexes (Fig. S3e in the ESI†).44 Table S1 (in the ESI†) shows thebinding energies of N 1s and metal for adsorbed porphyrin/graphene and metalated porphyrin/graphene, confirming theformation of metalloporphyrins on graphene.
In the Raman spectra (Fig. 3c and Fig. S4 in the ESI†), theinitial graphene structure had G and 2D peaks representative ofin-plane vibrational modes at 1593.1 ± 2.6 cm−1 and 2691.9 ±2.5 cm−1, respectively.45 After 2H-TPyP adsorption and Pt-meta-lation of 2H-TPyP on graphene, the G bands of 2H-TPyP/G andPt-TPyP/G compared to that of graphene become broader andshifted to 1595.4 ± 3.4 cm−1 and 1597.5 ± 3.9 cm−1, respectively.This indicated the strong charge transfer between graphene andboth 2H-TPyP and Pt-TPyP, while their 2D bands barely shiftedtoward a higher wavenumber. The full-range Raman spectraand Raman peak shifts corresponding to each step of the por-phyrin metalation are summarized in Table S2 in the ESI;†these indicated the contribution of the electron transfer fromgraphene to porphyrins and metalloporphyrins. Moreover, UPSconfirmed that due to the adsorption and metalation of por-phyrins on graphene, charge transfer between graphene andporphyrins or metalloporphyrins tuned the work function ofgraphene (Fig. 3d). The measured work function of pristinegraphene was an average of 4.60 eV in this work, which is areasonable value for single-layer graphene based on a previousmeasurement obtained using Kelvin probe force microscopy.46
Adsorption of porphyrins such as 2H-TPyP, 2H-TAPP, and2H-TPP increased the work function values to 4.65–4.76 eV. Inthe case of 2H-TPyP/graphene (4.76 eV), metalation with Ni(II),Zn(II), and Pt(II) resulted in work function values of Ni-TPyP/graphene, Zn-TPyP/graphene, and Pt-TPyP/graphene of 4.69,4.77, and 4.82 eV, respectively (Fig. S3f in the ESI†) Apparently,the work function changes regarding porphyrin metalation didnot follow the tendency of the work function values for thecenter metals. The measured work function of the modifiedgraphene deviated from that of the pristine graphene, where ashift in the Fermi level of graphene due to electron transferfrom graphene to 2H-TPyP and metal-TPyP is expected.47
Electrocatalytic activity of either porphyrin or metal-porphyrinmonolayers toward HER
In the porphyrin/graphene systems (surface-immobilized electro-catalytic systems), the formation of a 2H-TPyP monolayer createdthe active proton adsorption sites (i.e., N4 sites of approximate2.5 × 1013 cm−2 according to EC-STM measurements) on electro-catalytically inactive graphene. Either chemical or electro-chemical adsorption of a proton can occur under acidicmedium: porphyrin(0)H2 or porphyrin(0)H4
2+ can undergo areduction process (porphyrin(-I)H4 or porphyrin(-II)H4).
34,48 Also,for the electrocatalytic proton reduction, the electron transferbetween the active sites and the supporting graphene can playan important role. Thus, we hypothesized that the N4 sites ofthe porphyrin monolayers strongly stacked on the graphenewould have catalytic activity due to the larger charge separationat the interface, which results in the better catalytic activity.
For electrochemical measurements of porphyrin- (or metal-porphyrin-) immobilized graphene, a graphene working elec-trode was fabricated by transferring single-layer graphene ontoa poly(dimethylsiloxane) (PDMS) spin-coated polyethylene tere-phthalate (PET) substrate.45 Metalation of the porphyrin/gra-phene was performed by voltage sweeping from −2 V to +2 V
Fig. 3 Characterization of porphyrin adsorption and its metalation ongraphene: (a) changes in ultraviolet-visible (UV-Vis) spectra from pristinegraphene (G) to 2H-TPyP/G and metalated Ni-, Zn-, and Pt-TPyP/G onquartz substrates. (b) X-ray photoelectron spectra (XPS) of N 1s and (c)Raman spectra of the G-mode of pristine G, 2H-TPyP/G, and metalatedPt-TPyP/G on SiO2 substrates. (d) Ultraviolet photoelectron spectra(UPS) of pristine graphene, 2H-TPyP/G, and metalated Pt-TPyP/G onSiO2 substrates.
between two gold pads, and the active electrode area (e.g.,0.25 cm2) was determined by the other PDMS/PET substrate,as shown in Fig. S5 (in the ESI†). Normally, polarization curvesfor electrocatalytic HER are scanned from 0 VSCE to −0.8 VSCEat 10 mV s−1 for 100 cycles to pick up the stabilized 45th cycle(after an hour scan). Surface-electrocatalytic changes towardHER (2H+ + 2e− → H2) were verified by a change in the surfacepolarization of graphene electrodes in an aqueous acid electro-lyte (e.g., 0.5 M H2SO4). Surface polarization curves of the por-phyrin/graphene electrodes showed a significant enhancementin the electrocatalytic HER relative to graphene (Fig. 4). In par-ticular, 2H-TAPP/graphene and 2H-TPyP/graphene electrodescontaining intermolecular hydrogen bonds had better electro-catalytic effects than 2H-TPP/graphene electrodes, indicatingthat surface hydration via intermolecular hydrogen bondingcan play an important role in HER.49 Furthermore, the effectof porphyrin adsorption on the electrocatalytic HER of highlyoriented pyrolytic graphite (HOPG) was conducted (Fig. S6 inthe ESI†). The surface polarization curves showed that either2H-TAPP or 2H-TPyP molecules adsorbed HOPG and enhancedthe electrocatalytic HER activity of HOPG. However, the overpo-tential of porphyrin/HOPG is much higher than the values ofporphyrin/graphene, indicating that single-layer graphene isquite unique for HER applications.
After metalation of porphyrins on graphene electrodes, theelectrocatalytic activity of the M–N4 sites depends on porphyr-ins and central metal atoms. All M-porphyrins/graphenesystems showed better catalytic activity than pristine graphene.Electrocatalytic activity toward HER clearly indicated that thecatalytic activity increased in the order of Zn-porphyrins/
graphene < Ni-porphyrins/graphene < Pt-porphyrins/graphene.Considering the local d-band states of the transition metal sur-faces, the ability to adsorb protons corresponds to the positionof the d-band relative to the Fermi level.50 A d-band centercloser to a metal’s Fermi level allows for a stronger H+ adsorp-tion, which determines the degree of filling of the antibondingadsorbate-metal d states (e.g., Zn < Ni < Pt).50 The excellentcatalytic activity of the Pt-porphyrin/graphene and porphyrin/graphene systems was associated with the adsorption of H+ toform H+
ads-Pt-N4/graphene or H+ads-N4/graphene due to the
extended 5d states of the central Pt atoms and the largeelectronegativity of the Pt-N4 or N4 sites. Each metal-free por-phyrin molecule or their metal-porphyrin complex as an inde-pendent active site is supposed to be isolated in a monolayer,indicating that the electrochemical proton adsorption and theproton attack to evolve H2 could occur consecutively, such asin a heterolytic H2-evolving process.9,51 Overall, the results inour system showed that the HER activity increases as thestrength of the M–H bond increases,52 corresponding to theelectronegativity of metal atoms (Ni = 1.9, Zn = 1.6, Pt = 2.3 forPauling electronegativity53).49 Pt-porphyrin/graphene showedthe best performance towards HER due to its high electro-negativity that stabilizes the network structures of the activesites. The repeated cycles in a potential range revealed theelectrocatalytic stability of a surface-immobilized Pt-porphyrin/graphene system (Fig. S7 in the ESI†).
Interfacial charge transfer related to HER activity
Surface charge separation due to electronegativity differencesbetween nitrogen and carbon atoms at π–π stacked interfaces
Fig. 4 Surface-electrocatalytic activities of the porphyrin–graphene systems for the hydrogen evolution reaction in 0.5 M H2SO4: (a) changes in thepolarization curves of graphene (G) and porphyrins (2H-TPP, 2H-TAPP, and 2H-TPyP) on graphene. (b)−(d) Changes in the polarization curves due toporphyrin metalation to M-TPP, M-TAPP, and M-TPyP (M = Ni, Zn, Pt) on graphene, respectively. (e) Illustrations of active sites in the porphyrin-graphene systems for HER.
can influence the electrocatalytic activity of graphene electrodesurfaces. The electrocatalytic activity of M-porphyrin/grapheneelectrodes can be attributed to changes in the charge densityon adjacent atoms due to the formation of M–N4. Thus,investigations on the charge transfer between porphyrin andgraphene (or M-porphyrin and graphene) are needed. Tounderstand the interactive charge transfer at the interfacebetween the adsorbed porphyrins and the graphene substrate,the electronic characteristics of graphene were systematicallyinvestigated using field effect transistor (FET) devices. Fig. 5a–c
shows the typical changes in the transfer characteristics ofgraphene FETs for each step of the sequence process from theannealing of pristine graphene, porphyrin (e.g., 2H-TPyP,2H-TAPP, and 2H-TPP) adsorption on graphene, and metala-tion of porphyrins with Ni, Zn, and Pt on graphene. All chargetransfer measurements were conducted under a vacuum aftera certain stabilized condition, such as a long evacuated timeover several hours. In the first step, pristine graphene sampleswere treated by rapid thermal annealing at 200 °C for 30 s. Fora reliable comparison, we chose bare graphene samples exhi-
Fig. 5 Changes in transfer characteristics of graphene field effect transistors (FETs) in response to porphyrin adsorption and metalation on gra-phene: (a) 2H-TPyP adsorption on graphene-1 (G1), -2 (G2), and -3 (G3) and voltage-applied metalation to Ni-TPyP, Zn-TPyP, and Pt-TPyP, respect-ively, (b) 2H-TAPP adsorption on graphene-4 (G4), -5 (G5), and -6 (G6) and voltage-applied metalation to Ni-TAPP, Zn-TAPP, and Pt-TAPP, respect-ively, (c) 2H-TPP adsorption on graphene-7 (G7), -8 (G8), and -9 (G9) and voltage-applied metalation to Ni-TPP, Zn-TPP, and Pt-TPP, respectively.(d) Illustrations showing the interfaces of 2H-TPyP/graphene and M-TPyP/graphene based on the relative electronegativity of the atoms.
biting minimum current densities flowing between the drainand source electrodes at a similar gate voltage (i.e., the chargeneutrality point or Dirac point), which are slightly located atpositive gate voltages due to the electropositive (electron accep-tor) doping effect in graphene during the transfer of the CVDgraphene.54 The value of the charge neutrality point is nor-mally very sensitive to experimental environments, such asadsorption of water or oxygen, so the same batch of grapheneFETs was used and characterized in a vacuum after being in avacuum condition for several hours. In the second step, theadsorption of porphyrin molecules led to electropositivedoping of graphene; 2H-TPyP and 2H-TAPP induced largedrifts in the charge neutrality point to positive voltages, while2H-TPP induced relatively small drifts. We believe that thereare three factors that contributed to the charge transferbetween graphene and the adsorbed porphyrin layer: (1) inter-molecular hydrogen bonding between the substitutes (e.g., pyr-idine groups in 2H-TPyP) of neighbouring porphyrins, (2)lone-pair electrons in the central N4 units, and (3) electro-negativity of the central N4 units.
Based on the adsorption of 2H-TPyP and 2H-TAPP, theinfluence of intermolecular hydrogen bonding (involving watermolecules) on the transfer characteristics of graphene wasevident. With regard to the adsorption of 2H-TPyP, the lone-pair electrons of the pyridine groups at the meso-positions didnot participate in the π-system, while the lone-pair electrons ofthe central N4 sites contributed to the π-system, making thepyridine groups a weak base and allowing the formation ofhydrogen bonds with water molecules.55,56 Intermolecularhydrogen bonding via water molecules between the protonatedpyridine substituents of the neighbouring porphyrins inducedelectron transfer from graphene to the 2H-TPyP layer, indicat-ing electropositive doping, and the charge neutrality pointsshifted in the positive direction. Even though the 2H-TPyP/graphene samples were thoroughly dried under vacuum at 80 °Cfor several days, water molecules trapped by intermolecularhydrogen bonding were confined between the pyridine term-inals in the 2H-TPyP layer. However, when the applied temp-erature was high enough (i.e., 250 °C) to disturb (or destroy)the intermolecular hydrogen bonding, the electropositivedoping effects were significantly reduced (Fig. S8a in the ESI†).The adsorbed 2H-TAPP molecules, which have four terminalamine groups that form intermolecular hydrogen bondsbetween substituents, had the same effects on the electronicproperties of graphene as the 2H-TPyP molecules. Thus, as theintermolecular hydrogen bonding interactions decreased, theelectropositive doping effects of graphene decreased. In con-trast, the adsorbed 2H-TPP molecules, which have no substitu-ents such as pyridine or aminophenyl groups that can formintermolecular hydrogen bonds in solution, led to a slightshift in the charge neutrality point to positive voltages, but theinfluence of thermal annealing was negligible (Fig. S8b in theESI†). Consequently, the ability of porphyrins to form inter-molecular hydrogen bonds has a large effect on the chargetransfer between the active molecular monolayer andgraphene.
Differences in the atomic electronegativity can be a factorthat influences charge transfer. The electronegativity of nitro-gen atoms (N = 3.0 for Pauling electronegativity)53 in centralN4 sites is larger than that of the carbon atoms (C = 2.5 forPauling electronegativity) in graphene. This induced positivecharges (electropositive doping effect) on the graphenesurface, resulting in the drift of the charge neutrality points topositive voltages, which clearly appeared in Fig. 5c of 2H-TPP,excluding the effects of intermolecular hydrogen bonding.
In the third step, in order not to severely disturb the hydro-gen bonding in the 2H-TPyP and 2H-TAPP layers by thermalannealing, metalation of 2H-TPyP/graphene and 2H-TAPP/graphene was performed by voltage sweeping from −2 V to +2 Vbetween the drain and source electrodes. Electron-richmetalloporphyrins with electropositive metal atoms preferentiallyenhance the electrophilicity at the meso-positions (i.e., pyridineand aminophenyl substituents of 2H-TPyP and 2H-TAPP,respectively), which can help sustain the intermolecular hydro-gen bonding interactions at the meso-positions, despite geo-metric changes in the porphyrin layer. As a result, metalationwith Ni2+ and Zn2+ induced negative shifts in the chargeneutrality point of 2H-TPyP/graphene and 2H-TAPP/graphene,while metalation of the porphyrins with Pt2+ induced morepositive shifts in the charge neutrality point than those of theporphyrin-modified graphene. Thus, charge transfer aftermetalation showed the quantity of the atomic electronegativityof the center metals. The tendency of the charge transfer quan-tity was in the order of Zn-TPyP < Ni-TPyP < 2H-TPyP < Pt-TPyP. On the other hand, this charge transfer did not show aquantitative correlation to the work function values measuredin Fig. 3d and Fig. S3f in the ESI,† indicating that the workfunction changes were not governed by the quantity of chargetransfer induced by the adsorbates.57 When excluding theintermolecular hydrogen bonding effects, the correlation ofthe charge transfer to the electronegativity of the central metalwas revealed by 2H-TPP, which had no intermolecular hydro-gen bonding interactions (Fig. 5c). Thus, the electronegativityof both M and N4 had a strong influence on the electron trans-fer at the interface between the porphyrin layer and graphene.The charge neutrality points for porphyrin/graphene andM-porphyrin/graphene were located at positive voltages com-pared to that of graphene, indicating that electron transferoccurred from graphene to the M-porphyrins. Consequently,the larger electron affinity of the nitrogen atoms in the M–N4
sites than that of the carbon atoms of graphene induced elec-tropositive doping on the graphene.
In control experiments, (1) the adsorption effects of metalions on graphene and (2) intermolecular hydrogen bondingeffects (or thermal effects) on M-porphyrin/graphene wereexamined. Without porphyrins, the electronic characteristicsof the graphene samples were not influenced by metal ions,which were prepared by immersion of graphene into solutionscontaining metal ions and voltage application/thermal anneal-ing, indicating that the metal ions cannot adsorb onto gra-phene (Fig. S9a and b in the ESI†), which was also confirmedby field emission scanning electron microscope (FE-SEM)
images (Fig. S9c in the ESI†) showing no metal nanoparticleson the surface. All M-TPyP/graphene (or M-TAPP/graphene,M = Ni, Zn, and Pt) prepared by voltage-applied metalationshowed negative shifts in the charge neutrality points afterthermal annealing at 250 °C (Fig. S10 in the ESI†), indicating aweak network of intermolecular hydrogen bonds. Also, thisindicates that the voltage-applied metalation sustained theintermolecular hydrogen bonding networks in M-TPyP/graphene (or M-TAPP/graphene), which was attributed to thecharge transfers at the interfaces of M-TPyP (or M-TAPP)/graphene. Even though the surface structures networked byintermolecular hydrogen bonds could be disturbed by thermalannealing, moreover, all the M-porphyrin/graphene showedelectropositive doping effects on graphene compared with pris-tine graphene as shown in the case of M-TPP/graphene, indi-cating the M–N4 effect on the electronic change of the gra-phene surface.
Conclusions
In summary, we designed porphyrin/graphene systems to sys-tematically verify charge transfer effects on changes in thecatalytic activity of the conjugated carbon materials. Planarπ-conjugated porphyrins could attach co-facially on graphenesurfaces. The strong π-electron delocalization through the highπ-conjugation system of the central N4 atoms of porphyrinallowed for efficient charge transfer to the graphene. Inaddition, the high electronegativity of the central N4 and theintermolecular hydrogen bonding in the assembled porphyr-ins enhanced the charge transfer from the porphyrins tographene.
Metalation of the porphyrins assembled on graphene wasfacilely achieved by metal insertion via cation complexation offree-base porphyrins in solution. The interfacial charge trans-fer from M–N4 to graphene increased as the electronegativityof the central metal atom increased. Consequently, the activeproton adsorption sites of N4 and M–N4 on graphene werecreated by the formation of an active molecular monolayer thatenhanced the electrocatalytic HER of graphene by chargetransfer from either N4 or M–N4. Due to the relatively highelectronegativity of Pt, Pt-porphyrin showed the best electro-catalytic activity for the reduction of H+, while Ni- and Zn-porphyrins were less active than the free-base porphyrincontaining functional moieties to make the intermolecularhydrogen bonds. Our bottom-up approach can provide a goodplatform to clearly understand the electrocatalytic contributionof the catalytically active molecules in stabilized networks tocarbon materials at the molecular monolayer level.
Experimental sectionMaterials
Three free-base porphyrin molecules were purchased fromSigma-Aldrich and used without further purification. Nickel
sulfate hexahydrate (Ni(II)SO4·6H2O), zinc acetate dehydrate(Zn(II)(CH3COO)2·2H2O), and potassium tetrachloroplatinate(K2Pt(II)Cl4) were purchased from Sigma-Aldrich and usedwithout further purification. Single-layer graphene grown bychemical vapour deposition (CVD) on Cu foil was supplied bythe graphene centre of Sungkyunkwan University.
Sample preparation
CVD graphene was transferred to the substrates by the poly(methyl methacrylate) (PMMA)-mediated method as describedpreviously45 and thoroughly dried in a vacuum oven at 80 °Cfor several days. Graphene samples were immersed in0.05 mM 2H-TPyP and 2H-TAPP in 0.1 M H2SO4 aqueous solu-tion and 0.05 mM of 2H-TPP in anhydrous DMF solution for24 h. The porphyrin-modified graphene samples werethoroughly washed with each solvent several times and thendried for >24 h in a vacuum oven at 80 °C. Ni2+, Zn2+, and Pt2+
cations were prepared in a 0.1 M H2SO4 aqueous solution. Formetalation, porphyrin/graphene samples were immersed intoeach metal cation solution for 24 h, were thoroughly washedwith DI water several times, and then dried for >24 h in avacuum oven at 80 °C. Metalation of metal cations-porphyrin/graphene samples was completed by rapid thermal annealingat 200 °C or voltage application under a vacuum.
Measurements
Electrochemical scanning tunnelling microscopy (EC-STM).CVD graphene was transferred to Au(111) (a single crystal disc,MaTeck) that was pre-annealed by H2 flame after chemicalcleaning with piranha solution (H2SO4 : H2O2 = 3 : 1 (v/v)) andthen post-annealed at 950–980 °C for 60 s with a rapid thermalannealing system under a vacuum. The graphene/Au(111)sample was transferred into a electrochemical cell of an EC-scanning tunnelling microscope (Agilent 5100 AFM/SPMsystem) with a Pt counter and Pt reference electrodes andimmersed under potential control (0.2 VSCE) in a 0.1 M H2SO4
solution (Fisher Scientific Co., trace metal grade). The electro-chemically etched tungsten tip in 6 M KOH was used aftercoating with paraffin to minimize leakage currents. Afterimaging the bare graphene surface in a neat 0.1 M H2SO4 solu-tion, a drop of the 2H-TPyP solution was added to produce afinal concentration of about 0.05 mM 2H-TPyP. EC-STM wasperformed under nitrogen atmosphere.
Surface characterization. Samples were prepared on SiO2/Sior quartz substrates that were pre-cleaned by sonication in DIwater (18 MΩ cm), acetone (HPLC grade), and ethanol (HPLCgrade) for 10 min followed by immersion into piranha solution(H2SO4 : H2O2 = 3 : 1 (v/v)) for 10 min. They were thenthoroughly washed with DI water and ethanol (HPLC grade)and dried with nitrogen gas. Substrates were treated with O2
plasma to improve the surface wettability just before CVD gra-phene was transferred. All graphene samples were thermallyannealed by rapid thermal annealing at 200–250 °C. Ramanspectroscopy (Witec Alpha300), XPS (VG microtech ESCA2000), UV-Vis spectroscopy (Shimadzu UV-3600 UV-Vis-NIR),and UPS (ESCALAB250, Thermo) were used for surface charac-
terization. Raman spectroscopy (with 514 nm) and UV-vis spec-troscopy were performed under ambient conditions.
Electronic characterization. Field effect transistor deviceswere composed of channels (5–10 μm long and 25–250 μmwide) and pre-patterned Au source and drain electrodes(60 nm thick) on SiO2 (300 nm thick)/Si substrates. Thesedevices were sonicated in DI water, acetone, and ethanol for10 min, immersed in piranha solution for 5 min, washed withDI water several times, and then dried with N2. Source–draincurrent versus back gate voltage for FET devices was measuredby a 4200 Keithley semiconductor characterization system atroom temperature in a vacuum of 1 × 10−4–1 × 10−5 torr.
Electrochemical characterization. Electrocatalytic activitywas measured using a CHI electrochemical system composedof three electrodes (Ag|AgCl reference, Pt counter, and gra-phene working electrodes) in 0.5 M H2SO4 under a gentlepurge of nitrogen gas after vigorous purging of nitrogen gasfor 1 h without a working electrode and additionally for30 minutes with a working electrode.
Adsorption test of porphyrin on reduced graphene oxide. Asan example for a bulky catalyst system based on conjugatedcarbon materials, we tested porphyrin adsorption on reducedgraphene oxide (rGO, defective graphene) that was preparedaccording to a previous paper.58 Optical images of a mixtureaqueous solution of 2H-TPyP molecules and rGO sheets exhibi-ted a color change from deep green to transparent regarding2H-TPyP adsorption on rGO (Fig. S11 in the ESI†), indicatingthat rGO also can act as a platform for porphyrin adsorptionto lead to a synergetic effect in HER.
Acknowledgements
This work was supported by IBS-R011-D1.
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