Novel Synthesis of N-Doped Graphene As an Efficient Electrocatalyst Towards Oxygen Reduction

Nano Research

DOI 10.1007/s12274-015-0960-2

Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction

Ruguang Ma1,2, Xiaodong Ren1,2, Bao Yu Xia3, Yao Zhou1,2, Chi Sun1,2, Qian Liu1,2 (*), Jianjun Liu1,2 (*), and Jiacheng Wang1,2 (*)

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0960-2 http://www.thenanoresearch.com on November. 30, 2015

Just Accepted

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TABLE OF CONTENTS (TOC)

Novel Synthesis of N-doped Porous Graphene as An Efficient Electrocatalyst toward Oxygen Reduction Reaction Ruguang Ma,1,2 Xiaodong Ren,1,2 Bao Yu Xia,3 Yao Zhou,1,2 Chi Sun,1,2 Qian Liu, *1,2 Jianjun Liu, *1,2 and Jiacheng Wang*1,2

1 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 200050, China 2 Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai, 200444, China 3 School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore Nitrogen doping into graphene is achieved via a one-pot solvothermal method under a mild reaction condition. The following annealing at 600 oC initiates a conversion of N-containing species in the graphene, i.e. from pyrrolic N to pyridinic N, thus boosting the catalytic efficiency toward ORR with a good selectivity of four-electron pathway in alkaline and acidic electrolytes, respectively.

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DOI (automatically inserted by the publisher) Research Article Please choose one

Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction

1,2 1,2 3 1,2 1,2 1,2 1,2 Ruguang Ma , Xiaodong Ren , Bao Yu Xia , Yao Zhou , Chi Sun , Qian Liu (*), Jianjun Liu (*), 1,2 and Jiacheng Wang (*)

Received: day month year ABSTRACT Revised: day month year Nitrogen-doped graphene (NG) is successfully synthesized by a novel, facile Accepted: day month year and scalable bottom-up method. The annealed NG (NG-A), possessing high (automatically inserted by specific surface and hierarchical porous texture, exhibits remarkably improved the publisher) electrocatalytic activity towards ORR in both alkaline and acidic media, respectively. Ab initio molecular dynamic (MD) simulation indicates that fast H-transfer and thermodynamic stability of six-membered N structure promote

the transformation of N-containing species from pyrrolic N to pyridinic N at 600 oC. In O2-staturated 0.1 M KOH solution, the half-wave potential (E1/2) of © Tsinghua University Press NG-A is only 62 mV lower than that of commercial Pt/C catalyst and the and Springer-Verlag Berlin limiting current density of NG-A is 0.5 mA cm-2 larger than that of Pt/C. Both Heidelberg 2014 the Koutecky-Levich (K-L) plots and rotating ring-disk electrode (RRDE) measurement reveal the four-electron-transfer pathway of NG-A, which could be ascribed to the large percentage of pyridinic N with higher catalytic activity

than that of pyrrolic N. KEYWORDS Nitrogen doping, graphene, molecular dynamic simulation, oxygen reduction reaction

2Nano Res.

1 Introduction improved understanding on mechanism of N-doped graphene, the mentioned synthesis Nowadays, various clean and renewable energy approaches are still not satisfactory enough, either technologies, such as fuel cells and metal-air employing complicated steps (e.g. graphite→GO→ batteries, have stepped into the spotlight in the reduced GO (rGO)) or involving in contamination arena responding to environmental issues and of transition metal catalysts (e. g. Co, Ni or Mn) energy crisis [1, 2]. However, the sluggish oxygen [21]. Moreover, the catalytically active site in reduction reaction (ORR) at the cathode, arising N-doped graphene remains a controversial topic in from the strong chemical bond of oxygen in both the literature. The role of N species, such as alkali and acidic electrolytes, notoriously hinders pyrrolic N, pyridinic N and graphitic N, needs to the widespread implementation and be further clarified in the preparation process as commercialization of these technologies [3]. well as catalysis process. Although precious metals, e.g. platinum-based Herein we report a versatile and scalable strategy electrocatalysts, possess excellent catalysis to to synthesize porous NG via a facile solvothermal overcome the high overpotential of ORR, the method followed by a mild heat treatment scarceness along with high cost is an unavoidable (denoted as NG-A), as shown in Scheme 1. In both limitation [4, 5]. As an alternative, transition alkaline and acidic environments, the as-prepared metal-based electrocatalysts, such as metal N-doped graphene demonstrates excellent oxides/sulfides, have been demonstrated promising electrocatalytic activity toward ORR, in terms of electrochemical activities for ORR, however, low half-wave potential, electron-transfer pathway, electrical conductivity and poor stability in acid kinetic current density, long-term stability, and and alkaline electrolytes are primary problems to tolerance to crossover effect. The transformation be solved [6-9]. Therefore, it is imperative to mechanism of N-containing species from pyrrolic develop efficient and cost-effective electrocatalysts N to pyridinic N is elucidated by Ab initio with good activity and durability for ORR. molecular dynamic (MD) simulation. Recently, graphene (G) has initiated an enormous interest in the field of electrochemistry as a highly promising electrode candidate because of many advantages including immense specific area, remarkable mechanical flexibility, and good electrical conductivity [10-12]. In particular, N-doped graphene (NG) sheets have shown high catalytic activity toward the reduction of oxygen, resulting from the altered charge distribution on the carbon atoms [13-15]. For example, N-doped graphene was prepared by either chemical vapor deposition (CVD) of methane in the presence of ammonia (NH3) or thermal annealing graphene oxide (GO) using melamine or polypyrrole as nitrogen source, exhibiting excellent electrocatalysis for ORR [16-18]. Furthermore, N-doped or N, B-doped graphene quantum dots were synthesized by electrochemically reducing GO, electrocatalysis of which towards ORR was Scheme 1 Schematic illustration of fabrication procedure of N-doped graphene and the oxygen reduction reaction investigated experimentally and theoretically.[19] catalysed by the annealed N-doped graphene. Very recently, freestanding 3D N-doped graphene with high catalytic efficiency has been reported by Moreover, the density functional theory (DFT) Ito et al., which was fabricated by a nanoporous calculations and experimental results indicate that Ni-based chemical vapor deposition (CVD) method the pyridinic N plays a more important role in [20]. improving catalytic efficiency than pyrrolic N and Despite the harvest of good catalytic efficiency and

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 3 graphitic N. The pyridinic N not only facilitates size analyzer at liquid nitrogen temperature reductive O2 adsorption, but also eliminates H2O2 (-196 °C). Prior to measurement, the powders were formation by raising the density of p states near the dehydrated under vacuum at 120 °C overnight. The Fermi level and lowering the work function [22-24]. specific surface areas were calculated by the In addition, the hierarchically porous structure is Brunauer-Emmett-Teller (BET) method. The pore also favourable for the increase of catalytically size distribution curves were calculated based on active sites and the exchange of reactants, due to the analysis of the desorption branch of the the enhanced interactions of oxygen molecule with isotherm using the Barrett-Joyner-Halenda (BJH) catalytically active sites at a low overpotential [20]. model. Raman spectra were recorded on a DXR Raman Microscope (Thermal Scientific Co., USA) 2 Experimental with 532 nm excitation length. 2.3 Electrochemical Measurements 2.1 Synthesis The N-doped graphene (5 mg) was dispersed in the In a typical synthesis of N-doped graphene mixture of water (0.5 mL) and ethanol (0.5 mL) nanosheets, 5 mL of tetrachloromethane (CCl4) and containing 25 µL Nafion solution (5 wt.%) under 112 µL (56 or 74 µL) of pyrrole as nitrogen source ultrasonic irradiation for ca. 2 h until a were placed into a 100 mL Teflon-lined stainless homogeneous ink was formed. Then, 20 µL of ink steel autoclave. Then, 1.0 g of metallic potassium (K) containing 62.5 µg catalyst was transferred onto the was rapidly added to the autoclave in a plastic glassy rotating disk electrode with a diameter of 5 glove box purged with Ar gas. The autoclave mm, yielding a catalyst level of 0.32 mg cm-2. The sealed in the glove box was heated at 200 oC in an electrode with the catalyst was dried at 50 °C, oven for 6 h, and then naturally cooled down to which was used as the working electrode for room temperature. The resultant product was further electrochemical measurements. For 20 wt.% dispersed in a mixed solution of ethanol and Pt/C commercial catalyst (Johnson Matthey), the distilled water (1:1 vol.%) under magnetic stirring electrode was prepared following the same for 2 h to remove the residual CCl4. After it was procedure. filtered and washed several times with ethanol and An aqueous solution of 0.1 M KOH (or 0.5 M water, the product was dried in a vacuum oven at H2SO4) was used as the electrolyte for the 80 oC for 12 h. For comparison, pristine graphene electrochemical studies. Electrochemical activity of was also prepared following the procedure the working electrode was studied by cyclic mentioned above, except that no pyrrole was voltammetry (CV), rotating disk electrode (RDE), added. All the samples were annealed at 600 oC for and rotating ring disk electrode (RRDE) 2 h under Ar atmosphere. measurements using a standard three-electrode cell 2.2 Physical Characterization with a Pt plate as the counter electrode and a The samples were characterized by X-ray powder saturated calomel electrode (SCE) in 0.1 M KOH diffraction (XRD) on a Rigaku D/MAX-2250 V solution. The measured potentials vs. SCE were diffractometer with Cu Kα radiation. Scanning converted to a reversible hydrogen electrode (RHE) electron microscopy (SEM) images were recorded scale. In 0.1 M KOH solution (pH=13.8), ERHE = ESCE using a JEOL JSM-6700F field emission scanning + 0.2412 + 0.059 × pH, while in 0.5 M H2SO4 solution electron microscopy. Transmission electron the pH value is equal to 0.2. The electrochemical microscopy (TEM) images were observed using a cell was controlled by a bipotentiostat (Pine JEOL 2010F electron microscope operating at 200 Instrument Co.). Prior to measurement, the kV. The as-prepared powders were glued onto electrolyte was saturated by bubbling O2 (or N2) for indium (In) metal particles by pressing for X-ray 20 min. The working electrode was cycled at least photoelectron spectroscopy (XPS) measurements fifty times before the CV data were recorded at a with an ESCALAB 250 X-ray photoelectron scan rate of 50 mV s-1. The RDE measurements spectrometer with Al Kα (hυ = 1486.6 eV) radiation. were performed at different rotating rates varying All spectra were calibrated using 284.5 eV as the from 400 to 2025 rpm with a scan rate of 10 mV s-1. line position of adventitious carbon. Nitrogen The number of electrons transferred (n) is sorption isotherms were measured using a calculated based on the Koutecky-Levich equation Micromeritics ASAP 2010 surface area and pore as follows:

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---1 1 1/2 1 JJ(B)=k + w fluctuation of the thermostat variable is controlled 2/3- 1/6 by coupling it with another thermostat variable [27]. B= 0.2nFDOOn C 2 2 The time step is 1 fs and the total simulation time is where J and Jk are the measured current density 2 ps. and kinetic-limiting current density, respectively, n is the electron transfer number, F is the Faraday Results and discussion constant (96485 F g-1), υ is the viscosity of the electrolyte (0.01 cm2 S-1), CO2 is the concentration of O2 (1.2×10-6 mol cm-3 in 0.1 M O2-saturated KOH a b solution), and DO2 is the diffusion coefficient (1.9×10-5 cm2 s-1 in 0.1 M O2-saturated KOH solution, 1.8×10-5 cm2 s-1 in 0.5 M O2-saturated H2SO4 solution). The coefficient 0.2 is adopted when the 100nm 500nm rotating speed is expressed in rpm. c d In the case of RRDE measurement, ring current (IR) and disk current (ID) were collected using a Pt ring-disk electrode in O2-saturated 0.1 M KOH d=0.34nm solution at a rotating speed of 1600 rpm with a sweep rate of 10 mV s-1. The Pt ring electrode was 100nm polarized at 1.2 V (vs. RHE). The peroxide (H2O2) e f yield and the electron transfer number (n) were determined by the following equations: G I/N %(H O )= 200 ´ R NG 2 2 II/N+ DR NG-A I n= 4 ´ D II/N+ DR where N is the current collection efficiency of the Pt Figure 1 (a) SEM, (b) TEM, (c) enlarged TEM, and (d) HRTEM images of NG-A. ring electrode (N = 0.37, from the reduction of

K3Fe[CN]6). The morphology of the as-prepared NG-A is first 2.4 Computational Methods investigated by scanning electron microscopy The first-principles density-functional theory (DFT) (SEM), as presented in Figure 1. SEM image (Figure calculations were conducted with the Vienna Ab 1a) reveals a typical crumpled laminar structure of initio simulation package (VASP) [25, 26]. The graphene sheets, which indicates a successful exchange-correlation energy has been calculated assembly of –C=C– into graphene after the with the PBE functional. The structures have been dechlorination of CCl4 by metallic K [28, 29]. The relaxed until a maximum force of <0.02 eV Å-1 and elemental mapping shows that nitrogen is the energy convergence criterion is 10−6 eV. The homogenously distributed on the corrugated electron-ion interactions were described by graphene, accompanying with partial oxidization projector augmented wave (PAW) potential and the (Figure S1 in the Electronic Supplementary valence electrons were treated explicitly with a Material (ESM)). The microstructure is further plane-wave basis set at a cutoff energy of 400 eV. investigated by transmission electron microscopy Brilliouin zone was sampled by K-points with 0.04 (TEM). Figure 1b presents many pores in the large Å-1 spacing using the Monkhorst-Pack scheme. NG-A layers, signifying a porous structure. TEM Ab initio molecular dynamic (MD) simulation in image with high magnification (Figure 1c) clearly DFT scheme was carried out at a relatively high reveals that a large amount of pores, looking like temperature (1000 K) in order to effectively some bubbles, locate in the NG-A sheets. High describe structural transformation in a reasonable resolution TEM (HRTEM) image in Figure 1d time-scale. The constant temperature and volume further indicates that the as-synthesized NG-A ensemble (NVT) was adopted in our simulation. consists of different numbers of layer and the The temperature was controlled by the lattice spacing is 0.34 nm, which is a little larger Nosé-Hoove method in which the kinetic energy than the interlayer distance of graphite (0.335 nm),

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 5 suggesting the intercalation of N atoms into its 37]. Upon N doping, the G band visibly shifts up to graphitic plans [16, 30]. 1580 cm-1 for NG-A, indicating a successful doping This unique architecture with a high portion of of N atoms [29]. Another prominent feature of meso- and macro-pores is further confirmed by a graphene is the second-order two phonon mode 2D type-IV nitrogen adsorption/desorption isotherm band at about 2662 cm-1 as shown in the spectrum (Figure 1e). Brunauer–Emmett-Teller (BET) surface of pristine graphene, which corresponds to the area of NG-A is measured to be 413.6 m2 g-1, which graphitic graphene and is sensitive to the layers of is higher than those of NG (363.7 m2 g-1, Figure S2a graphene [38]. The 2D bands show a blue-shift, 3 in the ESM) and G (225.3 m2 g-1, Figure S2b in the cm-1 for NG and 10 cm-1 for NG-A in the spectra of ESM) as well as the previously reported values [17, NG and NG-A, respectively, indicating a few-layer 18, 31]. Correspondingly, inset of Figure 1e presents graphene. The defects caused by N doping would the pore-size distribution curve of NG-A, where lead to a significant improvement in the the peak at 4 nm is probably attributable to the electrocatalysis. inner cavity of graphene sheets and the peak located at 100 nm is assigned to the macropores. In contrast to the pore size distribution of pristine G (inset of Figure S2b in the ESM), an extra peak at 30 nm appears on the curve of NG (inset of Figure S2a in the ESM) and NG-A (inset of Figure 1e), which could be an important reason why the specific surface areas of NG and NG-A are so higher than that of pristine G. This indicates that pyrrole plays a significant role in modulating the pore structure, resulting in the formation of abundant meso- and macro-pores [32]. A large amount of gas, e.g. HCN and C2H2, will release during the decomposition of pyrrole at high temperature [33]. On the other hand, many functional groups absorbed on the graphene sheets also will be evacuated by forming gas, such as NOx. The resultant porosity coupled with high specific surface area would favour the access to reactants and the formation of gas-electrode-electrolyte triple-phase boundary, thus promoting the electrochemical activity of NG Figure 2 (a) XPS survey spectra of G, NG, and NG-A, catalysts [34]. Raman spectroscopy provides and (inset) N content in NG and NG-A; high-resolution further insights into the structural and electronic XPS spectra of (b) C 1s for G, NG, and NG-A; properties of the as-prepared graphene. As high-resolution XPS spectra of (c) N 1s for NG, and (d) N displayed in Figure 1f, the characteristic D and G 1s for NG-A; (e) content variation of N-containing bands of carbon materials locate at around 1332 species from NG to NG-A, (f) schematic illustration of evolution of N-containing species from NG to NG-A. and 1572 cm-1, respectively. The D band is The X-ray photoelectron spectroscopy (XPS) also associated with disordered samples or graphene confirms the successful doping of nitrogen into edges, while the G band is the result of the graphene, as shown in Figure 2. The survey spectra first-order scattering of the E2g mode of sp2 carbon in Figure 2a, reveals the presence of C, O, and N for domains [35]. Compared with the value of pristine NG and NG-A, whereas the N peak is absent in the G (ID/IG=0.93), the higher intensity of D band spectrum of pristine G. The residual Cl in the G (ID/IG=1.13 for NG and ID/IG=1.17 for NG-A) and NG disappears via heat treatment, indicating indicates the successful doping of the graphitic an improvement of purity. The content of various sheets [36]. Because only defects in the sp2 C atoms elements for NG and NG-A is shown in the inset of activate the D band via an intervalley Figure 2a, where the atomic percentage of N double-resonance Raman process, while N doping decreases slightly from 2.9 at.% for NG to 2.8 at.% introduces a large amount of topological defects [30,

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 6Nano Res. for NG-A. Figure 2b shows the high-resolution XPS spectra of C 1s of pristine G, NG, and NG-A, respectively. It is noted that the C 1s spectrum becomes broader after doping N into graphene, in particular the deconvoluted peaks corresponding to bonds of C-O&C=N and C=O&C-N, also signifying a successful N doping. The high-resolution N 1s spectra of NG (Figure 2c) and NG-A (Figure 2d) can be respectively deconvoluted into four components: pyridinic N (398.6 eV), pyrrolic N (399.6 or 400.1 eV), graphitic N (401.0 or 401.3 eV), and oxidized N (402.3 eV).18 In contrast to NG, the spectrum of NG-A splits into two Figure 3 Ab initio molecular dynamic simulation on independent peaks, one of which separately transformation of N-containing species from pyrrolic N corresponds to pyridinic N. This is a remarkable to pyridinic N. indication of a high amount of pyridinic N converting from pyrrolic N upon heat treatment. To elucidate the transformation mechanism of The quantitative analysis based on the N-containing species from pyrrolic N to pyridinic deconvolution presents the evolution of the N, Ab initio molecular dynamic (MD) simulation percentage of N functionality, as shown in Figure calculations are performed. Figure 3 displays free 2e. Initially, pyrrolic N accounts for a large energy change and some characteristic structures percentage of ~29.9% in NG, while the percentage with MD simulation time. The initial structure is of pyridinic N is only 14.7%. However, upon obtained by relaxing C-defected graphene which is annealing, the content of pyrrolic N drops sharply saturated by pyrrolic N, H, and O atoms. At 600 oC to 11.4% in NG-A, whereas the content of pyridinic temperature, the MD simulation surmounts three N increases to 30.9%. The percentage of graphitic N high-energy intermediate states which mainly and oxidized N correspondingly increases a little describe exchange of H and N. Such an exchange from 37.7% to 38.5%, and 17.7% to 19.2%, realizes a structural transformation from respectively. The large amount of pyridinic N as five-membered N to six-membered N, namely from well as graphitic N in the total atomic N content pyrrolic N to pyridinic N and graphitic N, which is has been demonstrated to be beneficial for the consistent with the previously proposed “ring electrocatalytic activity toward ORR [23, 39, 40]. It expansion” whereby the five-membered ring is is reasonable to conclude that the heat treatment opened [41]. In this process, fast mobility of light H boosts the transformation of N-containing species atom plays an important role in the structural from pyrrolic N to pyridinic N as illustrated in transformation. And as shown in Figure 3, the Figure 2f, accompanying with only a tiny variation structure containing pyridinic N is maintained in a of graphitic N. Such conversion probably improves relatively long simulation time, indicating a the electrocatalysis of N-doped graphene toward preferentially stable state. Therefore, it is ORR. It is worth pointing out that the N content in reasonable to speculate that fast H-transfer and the graphene can be easily controlled by varying thermodynamic stability of structure containing the ratio of pyrrole and CCl4, although Ruoff et. al pyridinic N promote the so-called “ring expansion”, reveals that the total atomic content of N is not a i.e. the transformation of N-containing species from pivot factor influencing the electrocatalysis in the pyrrolic N to pyridinic N. ORR process [23]. With lower pyrrole/CCl4 ratios of 74uL:5mL and 56µL: 5mL, the N content in the annealed samples is reduced to 1.91 and 1.32 at.%, respectively (Figure S3 in the ESM).

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there is only a small downshift of 62 mV in the half-wave potential at 1600 rpm, which is comparable to the excellent performance of NG/carbon nanotube hybrids synthesized by CVD (ΔE1/2=63 mV measured from Figure 3c in Ref.[43]) and much better than the reported values (ΔE1/2≥115 mV measured from Figure 4a in Ref.[23]) of N-doped graphene (such as polypyrrole/RGO, polyaniline/RGO, and NH3/RGO) produced by pyrolysis of various N precursors and GO [23, 44]. Moreover, the limiting current density of NG-A relative to that of Pt/C increases by 0.5 mA cm-2, indicating an excellent electrocatalytic activity for ORR. This could be caused by the faster reaction kinetics and more electrochemically active sites inside the porous NG-A with a large surface area[45]. Based on the slope of Koutecky-Levich (K-L) plots derived from LSV curves (Figure S5 in the ESM), the number of transferred electron of Figure 4 (a) CV curves of NG-A in N2/O2-saturated 0.1M KOH solution (b) LSV curves of G, NG, G-A, NG-A, and NG-A is calculated to be 3.8-3.92 between 0.2 and Pt/C at 1600 rpm (c) K-L plots of NG-A based on the LSV 0.6 V as seen in Figure 4c, suggesting a nearly curves at different rotation speeds, (d) Peroxide yield four-electron process of ORR on NG-A catalyst. The and number of transferred electron of NG-A, and Pt/C in selectivity of the four-electron reduction of oxygen O2-saturated 0.1M KOH solution; (e) LSV curves of for the catalyst is further confirmed by rotating NG-A and G-A at different rotation speeds, (f) Peroxide ring-disk electrode (RRDE) technique. On the basis yield and number of transferred electron of NG-A and of the RRDE curves (Figure S6 in the ESM), the Pt/C in O2-saturated 0.5 M H2SO4 solution. number of electron transfer is calculated to be

3.79-3.98 from 0.3 to 0.8 V (vs. RHE) as shown in The catalytic activity of NG-A is firstly examined in Figure 4d, which is comparable to the value of Pt/C a conventional three-electrode system in (3.98-3.99) and is consistent with the values N2/O2-saturated 0.1 M KOH electrolyte. In the calculated from the K-L plots. The amount of H2O2 O2-saturated alkaline environment, an obvious reached the ring electrode is about 10.5-1.22%, cathodic current with a peak at 0.79 V appears as which is a little higher than that of Pt/C shown in Figure 4a, whereas there is no cathodic (0.68-0.87%), but is comparable to the reported peak in N2-saturated electrolyte, indicating the value of 20 wt.% Pt/C in Ref.[46]. occurrence of ORR on the NG-A surface. Rotating Therefore, NG-A exhibits an intrinsic disk electrode (RDE) measurement was also four-electron-transfer process with a low peroxide performed to investigate the eletrocatalysis of all yield for ORR. This could be ascribed to the samples. For comparison, commercial 20 wt.% function of large percentage of pyridinic N, which platinum on carbon black (Pt/C) (Johnson Matthey) is proposed to eliminate H2O2 formation by raising was also measured (Figure S4 in the ESM). Figure the density of p states near the Fermi level and 4b shows the comparison of the LSV curves of lowering the work function [22]. The DFT-based pristine G, G-A, NG, NG-A and Pt/C at 1600 rpm in first-principles calculations reveal that according to 0.1 M KOH solution. Both of NG and pristine G the Sabatier principle, the adsorption energy of O2 exhibit inferior catalytic activity, which could be (AE: ΔE=EO2-NG–EO2-ENG) on pyridinic N is -1.41 eV, explained by the high content of Cl atoms in NG higher than that of pyrrolic N (-1.36 eV). And the and pristine G [42], as evidenced by XPS spectra in relaxed lowest energy adsorption structures were Figure 2a. It is noted that NG-A exhibits much shown in Figure S7 in the ESM. This indicates that better performance than pristine G, annealed the pyridinic N has higher catalytic activity than graphene (G-A), and NG, in terms of half-wave pyrrolic N. Given the XPS results that a large potential and current density. Relative to Pt/C, percentage of pyridinic N are converted from

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 8Nano Res. pyrrolic N and the amount of graphitic N only undergoes a negligible change, it is reasonable to conclude that that pyridinic N plays a pivotal role in the enhancement of catalytic activity. In contrast to graphene without doping in Figure 4e, the as-prepared NG-A also demonstrates improved electrocatalytic activity in the acidic media (0.5M H2SO4), where is a more harsh environment than alkaline condition for ORR. Although the activity of NG-A is inferior to Pt/C (Figure S8 in the ESM), the onset potential shifts positively by ca. 0.5 V after N doping, along with pronounced increase of limiting current density as displayed in Figure 4e. The onset potential in this work is comparable to the previously reported Figure 5 Chronoamperometric responses of NG-A and N-doped carbon and N-doped carbon nanotube, Pt/C electrodes at 0.30 V (vs. RHE) in O2-saturated (a) 0.1 M KOH solution (b) 0.5 M H2SO4 solution, (c) 0.1 M KOH but the limiting current density is much higher solution with 3 M methanol (d) 0.5 M H2SO4 with 3 M than those values [47, 48]. Based on the RRDE methanol solution added at about 5 min at a rotation rate measurement (Figure S9 in the ESM), the H2O2 of 1600 rpm, respectively. yield is 0.02-6.02% from 0 to 0.4 V and the electron transfer number correspondingly is 3.88-3.81 as When 3M methanol is introduced into the shown in Figure 4f, which are very close to the O2-saturated alkaline electrolyte at 5 min, no values of Pt/C (0.02% of H2O2 and n=4), suggesting noticeable degradation of current is observed for a closely four-electron pathway for ORR with high NG-A electrode, as shown in Figure 5c, whereas a current density in acidic media. This is in distinct drop of current appears for Pt/C electrode. accordance with the simulation results that the In the acidic media (Figure 5d), both NG-A and ORR is a four-electron process on the N-graphene Pt/C present a slight decrease of current when but pure graphene does not have such catalytic methanol is added at 5 min, but the dropping rate activities in acidic environment [49]. of NG-A is lower than that of Pt/C. After 30 min, The long-term durability and resistance to the NG-A still holds 79.6% of kinetic current crossover effect of the electrocatalysts for ORR are density, higher than Pt/C (73%). These results two important considerations for their practical signify that the NG-A with a better durability and application to fuel cells. Therefore, the current-time resistance to crossover effect than Pt/C, has (i-t) chronoamperometric responses were evaluated potential application in direct methanol and at 0.3 V in O2-saturated 0.1 M KOH and 0.5 M alkaline/acidic fuel cells. H2SO4 solutions, respectively, at a rotation rate of 1600 rpm. In alkaline electrolyte, NG-A and Pt/C 4 Conclusions show similar degradation rate before 5 h. Afterwards, NG-A exhibits better stability than In summary, N-doped porous graphene is Pt/C, remaining 87.4% of kinetic current density successfully synthesized by a simple and scalable after 10 h, higher than the value of Pt/C (80.6%) as solvothermal method. The following annealing shown in Figure 5a. Correspondingly, in the acidic step remarkably boosts the conversion of environment (Figure 5b), the superiority of NG-A N-containing species from pyrrolic N to pyridinic shows up against Pt/C after 4 h. 85.9% of current N. The theoretical simulation unravels that fast density is maintained over 10 h, which is higher H-transfer and thermodynamic stability of than that of Pt/C (78.1%). This could be explained six-membered N structure promote the by the robust morphology with no obvious damage transformation of N-containing species from (Figure S10a in the ESM) and stable composition pyrrolic N to pyridinic N. Such transformation (Figure S10b and Table S1 in the ESM) which is illustrates the important role of pyridinic N in from the inert nature of carbon materials as well as largely improving the catalytic activity of NG-A strong covalent C-N bonds [43].

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 9 towards ORR, because both K-L plots and RRDE electrocatalysts for water electrolyzers and reversible fuel measurement reveal the improved electrochemical cells: status and perspective. Energy Environ. Sci. 2012, 5, performance of NG-A, such as a more positive 9331-9344. half-wave potential, four-electron-transfer pathway, [4] Faber, M. S.; Jin, S., Earth-abundant inorganic and low H2O2 yields in the ORR process. This work electrocatalysts and their nanostructures for energy will pave the way to further understand the catalysis mechanism of N-doped graphene and to conversion applications. Energy Environ. Sci. 2014, 7, develop various metal-free ORR catalysts for fuel 3519-3542. cell applications. [5] Greeley J; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; RossmeislJ; Acknowledgements ChorkendorffI; Nørskov, J. K., Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat. This work is financially supported by Shanghai Chem. 2009, 1, 552-556. Institute of Ceramics, the One Hundred Talent [6] Lee, D. U.; Kim, B. J.; Chen, Z., One-pot synthesis Plan of Chinese Academy of Sciences, National of a mesoporous NiCo O nanoplatelet and graphene hybrid Natural Science Foundation of China (21307145), 2 4 Key Project for Young Researcher of State Key and its oxygen reduction and evolution activities as an Laboratory of High Performance Ceramics and efficient bi-functional electrocatalyst. J. Mater. Chem. A Superfine Microstructure, the Youth Science and 2013, 1, 4754-4762. Technology Talents “Sail” Program of Shanghai [7] Ge, X.; Liu, Y.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Municipal Science and Technology Commission Xiao, P.; Zhang, Z.; Lim, S. H.; Li, B.; Wang, X.; Liu, Z., (15YF1413800), and the research grant Dual-Phase Spinel MnCo O and Spinel (No.14DZ2261200) from Shanghai government. 2 4 MnCo2O4/Nanocarbon Hybrids for Electrocatalytic Oxygen Electronic Supplementary Material: Reduction and Evolution. ACS Appl. Mater. Interfaces 2014, Supplementary material (Elemental mapping of 6, 12684-12691. NG-A; Nitrogen adsorption–desorption isotherms [8] Liu, Q.; Jin, J.; Zhang, J., NiCo2S4@graphene as a of NG and G; High-resolution XPS spectra of N 1s bifunctional electrocatalyst for oxygen reduction and for NG-A with a low ratio (1.91 at.% and 1.32 at.%); evolution reactions. ACS Appl. Mater. Interfaces 2013, 5, LSV curves and K–L plots of commercial 20% Pt/C 5002-5008. in an O2-saturated 0.1 M solution of KOH; RRDE [9] Ganesan, P.; Prabu, M.; Sanetuntikul, J.; Shanmugam, voltammograms of NG-A and Pt/C; DFT-calculated S., Cobalt sulfide nanoparticles grown on nitrogen and sulfur O2 adsorption structures and energies on pyridinic codoped graphene oxide: an efficient electrocatalyst for N and pyrrolic N; TEM image and XPS spectrum of oxygen reduction and evolution reactions. ACS Catal. 2015, NG-A after stability test.) is available in the online version of this article at 5, 3625-3637. http://dx.doi.org/10.1007/s12274-***-****-* [10] Wang, X.; Sun, G.; Routh, P.; Kim, D.-H.; Huang, W.; Chen, P., Heteroatom-doped graphene materials: syntheses, References properties and applications. Chem. Soc. Rev. 2014, 43,

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Wu, Y. H.; Feng, Y. P.; Shen, Z. X., Graphene thickness R., Development of CaMn1-xRuxO3-y (x = 0 and 0.15) determination using reflection and contrast spectroscopy. oxygen reduction catalysts for use in low temperature Nano Lett. 2007, 7, 2758-2763. electrochemical devices containing alkaline electrolytes: ex [39] Saidi, W. A., Oxygen Reduction Electrocatalysis situ testing using the rotating ring-disk electrode Using N-Doped Graphene Quantum-Dots. J. Phys. Chem. voltammetry method. J. Mater. Chem. A 2014, 2, 3047-3056. Lett. 2013, 4, 4160-4165. [47] Chen, P.; Wang, L.-K.; Wang, G.; Gao, M.-R.; Ge, J.; [40] Bao, X.; Nie, X.; von Deak, D.; Biddinger, E.; Luo, Yuan, W.-J.; Shen, Y.-H.; Xie, A.-J.; Yu, S.-H., W.; Asthagiri, A.; Ozkan, U.; Hadad, C., A first-principles Nitrogen-doped nanoporous carbon nanosheets derived from study of the role of quaternary-N doping on the oxygen plant biomass: an efficient catalyst for oxygen reduction reduction reaction activity and selectivity of graphene edge reaction. Energy Environ. Sci. 2014, 7, 4095-4103. sites. Top Catal. 2013, 56, 1623-1633. [48] Yu, D.; Zhang, Q.; Dai, L., Highly efficient [41] Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; metal-free growth of nitrogen-doped single-walled carbon Thomas, K. M., Evolution of nitrogen functionalities in nanotubes on plasma-etched substrates for oxygen reduction. carbonaceous materials during pyrolysis. Carbon 1995, 33, J. Am. Chem. Soc. 2010, 132, 15127-15129. 1641-1653. [49] Zhang, L.; Xia, Z., Mechanisms of oxygen reduction [42] Jeon, I.-Y.; Choi, H.-J.; Choi, M.; Seo, J.-M.; Jung, reaction on nitrogen-doped graphene for fuel cells. J S.-M.; Kim, M.-J.; Zhang, S.; Zhang, L.; Xia, Z.; Dai, L.; Phys.Chem. C 2011, 115, 11170-11176. Park, N.; Baek, J.-B., Facile, scalable synthesis of

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Electronic Supplementary Material

Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction

Ruguang Ma1,2, Xiaodong Ren1,2, Bao Yu Xia3, Yao Zhou1,2, Chi Sun1,2, Qian Liu1,2 (*), Jianjun Liu1,2 (*), and Jiacheng Wang1,2 (*)

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Address correspondence to Q Liu, [email protected]; J Liu, [email protected]; J Wang, [email protected]

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Figure S1 Elemental mapping of NG-A.

Figure S2 Nitrogen adsorption–desorption isotherms of (a) NG and (b) G; inset is the corresponding pore size distribution calculated from the desorption branch on the basis of the Barrett–Joyner–Halenda (BJH) mode.

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Figure S3 High-resolution XPS spectra of N 1s for NG-A with a low ratio (a) 1.91 at.% (b) 1.32 at.%.

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Figure S4 (a) LSV of commercial 20% Pt/C on a RDE at different rotation speeds in an O2-saturated 0.1 M solution of KOH (scan rate: 10 mV s-1), and (b) K–L plots of Pt/C at different potentials on the basis of the RDE data.

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Figure S5 LSV curves of NG-A at different rotation speeds.

Figure S6 RRDE voltammograms of NG-A and Pt/C for the ORR in O2-saturated 0.1 M KOH solution.

Figure S7 DFT-calculated O2 adsorption structures and energies (AE: eV) on pyridinic N and pyrrolic N.

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Figure S8 Comparison of LSV curves of G-A, NG-A and Pt/C at 1600 rpm at a scan rate of 10 mV s-1 in

O2-saturated 0.5 M H2SO4 solution.

Figure S9 RRDE voltammograms of NG-A and Pt/C for the ORR in O2-saturated 0.5 M H2SO4 solution.

a b

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Figure S10 (a) TEM image and (b) High-resolution XPS spectra of N 1s for cycled NG-A for 10 h.

Table S1 Comparison of N content of NG-A before and after durability test.

N at.% Pyridinic N Pyrrolic N Graphitic N N-Oxides

NG-A 2.8 30.9% 11.4% 38.5% 19.2%

NG-A (after 10 h) 2.67 24.3% 12.9% 42.3% 20.5%

Silver Nanowires with Semiconducting Ligands for Low Temperature Transparent Conductors

Brion Bob,1 Ariella Machness,1 Tze-Bin Song,1 Huanping Zhou,1 Choong-Heui Chung,2 and Yang Yang1,* 1 Department of Materials Science and Engineering and California NanoSystems Institute,

University of California Los Angeles, Los Angeles, CA 90025 (USA)

2 Department of Materials Science and Engineering, Hanbat National University, Daejeon

305-719, Korea

Abstract Metal nanowire networks represent a promising candidate for the rapid fabrication of transparent electrodes with high transmission and low sheet resistance values at very low deposition temperatures. A commonly encountered obstacle in the formation of conductive nanowire electrodes is establishing high quality electronic contact between nanowires in order to facilitate long range current transport through the network. A new system of nanowire ligand removal and replacement with a semiconducting sol-gel tin oxide matrix has enabled the fabrication of high performance transparent electrodes at dramatically reduced temperatures with minimal need for post-deposition treatments of any kind.

Keywords: Silver Nanowires, Sol-Gel, Transparent Electrodes, Nanocomposites

1. Introduction. Silver nanowires (AgNWs) are long, thin, and possess conductivity values on the same order of magnitude as bulk silver (Ag) [1]. Networks of overlapping nanowires allow light to easily pass through the many gaps and spaces between nanowires, while transporting current through the metallic conduction pathways offered by the wires themselves. The high aspect ratios achievable for solution-grown AgNWs has allowed for the fabrication of transparent conductors with very promising sheet resistance and transmission values, often approaching or even surpassing the performance of vacuum-processed materials such as indium tin oxide (ITO) [2-6]. Significant electrical resistance within the metallic nanowire network is encountered only when current is required to pass between nanowires, often forcing it to pass through layers of stabilizing ligands and insulating materials that are typically used to assist with the synthesis and suspension of the nanowires [7, 8]. The resistance introduced by the insulating junctions between nanowires can be reduced through various physical and chemical means, including burning off ligands and partially melting the wires via thermal annealing [9, 10], depositing additional materials on top of the nanowire network [11-14], applying mechanical forces to enhance network morphology [15-17], or using various other post-treatments to improve the contact between adjacent wires [18-21]. Any attempt to remove insulating materials the network must be weighed against the risk of damaging the wires or blocking transmitted light, and so many such treatments must be reined in from their full effectiveness to avoid endangering the performance of the completed electrode. We report here a process for forming inks with dramatically enhanced electrical contact between AgNWs through the use of a semiconducting ligand system consisting of tin oxide (SnO2) nanoparticles. The polyvinylpyrrolidone (PVP) ligands introduced during AgNW synthesis in order to encourage one-dimensional growth are stripped from the wire surface using ammonium ions, and are replaced with substantially more conductive SnO2, which then fills the space between wires and enhances the contact geometry in the vicinity of wire/wire junctions. The resulting transparent electrodes are highly conductive immediately upon drying, and can be effectively processed in air at virtually any temperature below 300 °C. The capacity for producing high performance transparent electrodes at room temperature may be useful in the fabrication of devices that are damaged upon significant heating or upon the application of harsh chemical or mechanical post-treatments.

2. Results and Discussion

2.1. Ink Formulation and Characterization

Dispersed AgNWs synthesized using copper chloride seeds represent a particularly challenging material system for promoting wire/wire junction formation, and often require thermal annealing at temperatures near or above 200 °C to induce long range electrical conductivity within the deposited network [22, 23]. The difficulties that these wires present regarding junction formation is potentially due to their relatively large diameters compared to nanowires synthesized using other seeding materials, which has the capacity to enhance the thermal stability of individual wires according to the Gibbs-Thomson effect. We have chosen these wires as a demonstration of pre-deposition semiconducting ligand substitution in order to best illustrate the contrast between treated and untreated wires.

Completed nanocomposite inks are formed by mixing AgNWs with SnO2 nanoparticles in the presence of a compound capable of stripping the ligands from the AgNW surface. In this work, we have found that ammonia or ammonium salts act as effective stripping agents that are able to remove the PVP layer from the AgNW surface and allow for a new stabilizing matrix to take its place. Figure 1 shows a schematic of the process, starting from the precursors used in nanowire and nanoparticle synthesis and ending with the deposition of a completed film. The SnO2 nanoparticle solution naturally contains enough ammonium ions from its own synthesis to effectively peel the insulating ligands from the AgNWs and allow the nanoparticles to replace them as a stabilizing agent. If not enough SnO2 nanoparticles are used in the mixture, then the wires will rapidly agglomerate and settle to the bottom as large clusters. Large amounts of SnO2 in the mixture gradually begin to increase the sheet resistance of the nanowire network upon deposition, but greatly enhance the uniformity, durability, and wetting properties of the resulting films. We have found that AgNW:SnO2 weight ratios ranging between 2:1 and 1:1 produce well dispersed inks that are still highly conductive when deposited as films.

The nanowires were synthesized using a polyol method that has been adapted from the recipe described by Lee et al. [22, 23] Silver nitrate dissolved in ethylene glycol via ultrasonication was used as a precursor in the presence of copper chloride and PVP to provide seeds and produce anisotropic morphologies in the reaction products. Synthetic details can be found in the experimental section. Distinct from previous recipes, we have found that repeating the synthesis two times without cooling down the reaction mixture generally produces significantly longer nanowires than a single reaction step. The lengths of nanowires produced using this method fall over a wide range from 15 to 65 microns, with diameters between 125 and 250 nm. This range of diameters is common for wires grown using copper chloride seeds, although the double reaction produces a number of wires with roughly twice their usual diameter. The morphology of the as-deposited AgNWs as determined via SEM is shown in Figure 2(a), higher magnification images are also provided in Figures 2(c) and 2(d).

10 The SnO2 nanoparticles were synthesized using a sol-gel method typical for multivalent metal oxide gelation reactions. A large excess of deionized water was added to SnCl4·5H2O dissolved in ethylene glycol along with tetramethylammonium chloride and ammonium acetate to act as surfactants. The reaction was then allowed to progress for at least one hour at near reflux conditions, after which the resulting nanoparticle dispersion can be collected, washed, and dispersed in a polar solvent of choice. The material properties of SnO2 nanoparticles formed using a similar synthesis method have been reported previously [24], although the present recipe uses excess water to ensure that the hydrolysis reaction proceeds nearly to completion.

After mixing with SnO2 nanoparticles, films deposited from AgNW/SnO2 composite inks show a largely continuous nanoparticle layer on the substrate surface with some nanowires partially buried and some sitting more or less on top of the film. Representative scanning electron microscopy (SEM) images of nanocomposite films are shown in Figure 2(b). Regardless of their position relative to the SnO2 film, all nanowires show a distinct shell on their outer surface that gives them a soft and slightly rough appearance, as is visible in the higher magnification images shown in Figure 2(e) and 2(f). The SnO2 nanoparticles do a particularly good job coating the regions near and around junctions between wires, and frequently appear in the SEM images as bulges wrapped around the wire/wire contact points.

The precise morphology of the SnO2 shell that effectively surrounded each AgNW was analyzed in more detail using transmission electron microscopy (TEM) imaging. Figures 3(a) to 3(c) show individual nanowires in the presence of different ligand systems: as-synthesized PVP in Figure 3(a), inactive SnO2 in Figure 3(b), and SnO2 activated with trace amounts of ammonium ions in Figure 3(c). The as-synthesized nanowires show sharp edges, and few surface features. In the presence of inactive SnO2, which is formed by repeatedly washing the SnO2 nanoparticles in ethanol until all traces of ammonium ions are removed, the nanowires coexist with somewhat randomly distributed nanoparticles that deposit all over the surface of the TEM grid. When AgNWs are mixed with activated SnO2, a thick and continuous SnO2 shell is formed along the nanowire surface. In when sufficiently dilute SnO2 solutions are used to form the nanocomposite ink, nearly all of the nanoparticles are consumed during shell formation and effectively no nanoparticles are left to randomly populate the rest of the image.

As the AgNWs acquire their metal oxide coatings in solution, the properties of the mixture change dramatically. Freshly synthesized AgNWs coated with residual PVP ligands slowly settle to the bottom of their vial or flask over a time period of several hours to one day, forming a dense layer at the bottom. The AgNWs with SnO2 shells do not settle to the bottom, but remain partially suspended even after many weeks at concentrations that are dependent on the amount of SnO2 present in the solution.

A comparison of the settling behavior of various AgNW and SnO2 mixtures after 24 hours is shown in Figures 3(d) and 3(e). The ratios 8:4, 8:16, and 8:8 indicate the concentrations of AgNWs and SnO2 (in mg/mL) present in each solution. The 8:8 uncoupled solution, in which the PVP is not removed from the AgNW surface with ammonia, produces a situation in which the nanowires and nanoparticles do not interact with one another, and instead the nanowires settle as in the isolated nanowire solution while the nanoparticles remain well-dispersed as in the solution of pure SnO2. The mixtures of nanowires and nanoparticles in which trace amounts of ammonia are present do not settle to the bottom, but instead concentrate themselves until repulsion between the semiconducting SnO2 clusters is able to prevent further settling.

Our current explanation for the settling behavior of the wire/particle mixtures is that the PVP coating on the surface of the as-synthesized wires is sufficient to prevent interaction with the nanoparticle solution. The addition of ammonia into the solution quickly strips off the PVP surface coating and allowing the nanoparticles to coordinate directly with the nanowire surface. This explanation is in agreement with the effects of ammonia has on a solution of pure AgNWs, which rapidly begin to agglomerate into clusters and sink to the bottom as soon as any significant quantity of ammonia is added to the ink. We attribute the stripping ability of ammonia in these mixtures to the strong dative interactions that occur via the lone pair on the nitrogen atom interacting with the partially filled d-orbitals of the Ag atoms on the nanowire surface. These interactions are evidently strong enough to displace the existing coordination of the five-membered rings and carbonyl groups contained in the original PVP ligands and allow the ammonia to attach directly to the nanowire surface. Since ammonia is one of the original surfactants used to stabilize the surface of the SnO2 nanoparticles, we consider it reasonable that ammonia coordination on the nanowire surface would provide an appropriate environment for the nanoparticles to adhere to the AgNWs.

11 Scanning Energy Dispersive X-ray (EDX) Spectroscopy was also conducted on nanoparticle-coated AgNWs in order to image the presence of Sn and Ag in the nanowire and shell layer. The line scan results are shown in Figure 3(f), having been normalized to better compare the widths of the two signals. The visible broadening of the Sn lineshape compared to that of Ag is indicative of a Sn layer along the outside of the wire. The increasing strength of the Sn signal toward the center of the AgNW is likely due to the enhanced interaction between the TEM’s electron beam and the dense AgNW, which then improves the signal originating from the SnO2 shell as well. It is also possible that there is some intermixing between the Ag and Sn x-ray signals, but we consider this to be less likely as the distance between their characteristic peaks should be larger than the detection system’s energy resolution.

2.2. Network Deposition and Device Applications

For the deposition of transparent conducting films, a weight ratio of 2:1 of AgNWs to SnO2 nanoparticles was chosen in order to obtain a balance between the dispersibility of the nanowires, the uniformity of coated films, and the sheet resistance of the resulting conductive networks. Nanocomposite films were deposited on glass by blade coating from an ethanolic solution using a scotch tape spacer, with deposited networks then being allowed to dry naturally in air over several minutes.

The as-dried nanocomposite films are highly conductive, and require only minimal thermal treatment to dry and harden the film. Without the use of activated SnO2 ligands, deposited nanowire networks are highly insulating, and become conductive only after annealing at above 200 °C. The sheet resistance values of representative films are shown in Figure 4(a). The capability to form transparent conductive networks in a single deposition step that remain useful over a wide range of processing temperatures provides a high degree of versatility for designing thin film device fabrication procedures.

Figure 5(a) shows the sheet resistance and transmission of a number of nanocomposite films deposited from inks containing different nanowire concentrations. The deposited films show excellent conductivity at transmission values up to 85%, and then rapidly increase in sheet resistance as the network begins to reach its connectivity limit. The optimum performance of these networks at low to moderate transmission values is a consequence of the relatively large nanowire diameters, which scatter a noticeable amount of light even when the conditions required for current percolation are just barely met. Nonetheless, the sheet resistance and transmission of the completed nanocomposite networks place them within an acceptable range for applications in a variety of optoelectronic devices. Figure 5(b) shows the wavelength dependent transmission spectra of several nanowire networks, which transmit light well out into the infrared region. The presence of high transmission values out to wavelengths well above 1300 nm, where ITO or other conductive oxide layers would typically begin to show parasitic absorption, is due to the use of semiconducting SnO2 ligands, which is complimentary to the broad spectrum transmission of the silver nanowire network itself.

Avoiding the use of highly doped nanoparticles has the potential to provide optical advantages, but can create difficulties when attempting to make electrical contact to neighboring device layers. In order to investigate their functionality in thin film devices, we have incorporated AgNW/SnO2 nanocomposite films as electrodes in amorphous silicon (a-Si) solar cells. Two contact structures were used during fabrication: one with the nanocomposite film directly in contact with the p-i-n absorber structure and one with a 10 nm Al:ZnO (AZO) layer present to assist in forming Ohmic contact with the device. The I-V characteristics of the resulting devices are shown in Figure 6(a).

The thin AZO contact layers typically show sheet resistance values greater than 2.5 kΩ/⧠, and so cannot be responsible for long range lateral current transport within the electrode structure. However, their presence is clearly beneficial in improving contact between the nanocomposite electrode and the absorber material, as the SnO2 matrix material is evidently not conductive enough to form a high quality contact with the p-type side of the a-Si stack. We hope that future modifications to the AgNW/SnO2 composite, or perhaps the use of islands of high conductivity material such as a discontinuous layer of doped nanoparticles will allow for the deposition of completed electrode stacks that provide both rapid fabrication and good performance.

Figure 6(b) contains the top view image of a completed device. The enhanced viscosity of the nanowire/sol-gel composite inks allows for films to be blade coated onto substrates with a variety of surface properties without reductions in network uniformity. In contrast with traditional back electrodes deposited in vacuum environments, the nanocomposite can be blade coated into place in a single pass under atmospheric conditions and dried within moments. We anticipate that the use of sol-gel mixtures to enhance wetting and dispersibility may prove useful in the formulation of other varieties of semiconducting and metallic inks for deposition onto a variety of substrate structures.

3. Conclusions

In summary, we have successfully exchanged the insulating ligands that normally surround as-synthesized AgNWs with shells of substantially more conductive SnO2 nanoparticles. The exchange of one set of ligands for the other is mediated by

12 the presence of ammonia during the mixing process, which appears to be necessary for the effective removal of the PVP ligands that initially cover the nanowire surface. The resulting nanowire/nanoparticle mixtures allow for the deposition of nanocomposite films that require no annealing or other post-treatments to function as high quality transparent conductors with transmission and sheet resistance values of 85% and 10 Ω/ , respectively. Networks formed in this manner can be deposited quickly and easily in open air, and have been demonstrated as an effective n-type electrode in a-Si solar cells when a thin interfacial layer is deposited first to ensure good electronic contact⧠ with the rest of the device. The ligand management strategy described here could potentially be useful in any number of material systems that presently suffer from highly insulating materials that reside on the surface of otherwise high performance nano and microstructures.

4. Experimental Details Tin oxide nanoparticle synthesis. Tin chloride pentahydrate was dissolved in ethylene glycol by stirring for several hours at a concentration of 10 grams per 80 mL to serve as a stock solution. In a typical synthesis reaction, 10 mL of the SnCl4·5H2O stock solution is added to a 100 mL flask and stirred at room temperature. Still at room temperature, 250 mg ammonium acetate and 500 mg ammonium acetate were added in powder form to regulate the solution pH and to serve as coordinating agents for the growing oxide nanoparticles. 30 ml of water was then added, and the flask was heated to 90 °C for 1 to 2 hours in an oil bath, during which the solution took on a cloudy white color. The gelled nanoparticles were then washed twice in ethanol in order to keep trace amounts of ammonia present in the solution. Additional washing cycles would deactivate the SnO2, and then require the addition of ammonia to coordinate with as-synthesized AgNWs.

Silver nanowire synthesis. Copper(ii) chloride dihydrate was first dissolved in ethylene glycol at

1 mg/ml to serve as a stock solution for nanowire seed formation. 20 ml of ethylene glycol was then added into a 100 ml flask, along with 200 µL of copper chloride solution. the mixture was then heated to 150 °C while stirring at 325 rpm, and .35g of PVP (MW 55,000) was added. In a small separate flask, .25 grams of silver nitrate was dissolved in 10 ml ethylene glycol by sonicating for approximately 2 minutes, similar to the method described here.22 The silver nitrate solution was then injected into the larger flask over approximately 15 minutes, and the reaction was allowed to progress for 2 hours. After the reaction had reached completion, the various steps were repeated without cooling down. 200 µL of copper chloride solution and .35g PVP were added in a similar manner to the first reaction cycle, and another .25g silver nitrate were dissolved via ultrasonics and injected over 15 minutes. The second reaction cycle was allowed to progress for another 2 hours, before the flask was cooled and the reaction products were collected and washed three times in ethanol.

Nanocomposite ink formation. After the synthesis of the two types of nanostructures is complete,

13 the double washed SnO2 nanoparticles and triple-washed nanowires can be combined at a variety of weight ratios to form the completed nanocomposite ink. The dispersibility of the mixture is improved when more

SnO2 is used, although the sheet resistance of the final networks will begin to increase if they contain excessive SnO2. AgNW agglomeration during mixing is most easily avoided if the SnO2 and AgNW solutions are first diluted to the range of 10 to 20 mg/ml in ethanol, with the SnO2 solution being added first to an empty vial and the AgNW solution added afterwards. The dilute mixture was then be allowed to settle overnight, and the excess solvent removed to concentrate the wires to a concentration that is appropriate for blade coating.

Film and electrode deposition. The completed nanocomposite ink was deposited onto any desired substrates using a razor blade and scotch tape spacer. The majority of the substrates used in this study were

Corning soda lime glass, but the combined inks also deposited well on silicon, SiO2, and any other substrates tested. Electrode deposition onto a-Si substrates was accomplished by masking off the desired cell area with tape, and then depositing over the entire region. The p-i-n a-Si stacks and 10 nm AZO contact layers were deposited using PECVD and sputtering, respectively.

ACKNOWLEDGMENTS The authors would like to acknowledge the use of the Electron Imaging Center for Nanomachines

(EICN) located in the California NanoSystems Institute at UCLA.

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Figure 1. Process flow diagram showing the synthesis of AgNWs and SnO2 nanoparticles followed by stirring in the presence of ammonium salts to create the final nanocomposite ink. Transparent conducting films were produced by blade coating the completed inks onto the desired substrate.

Figure 2. (a,c,d) SEM images of as-synthesized AgNWs at various magnifications. (b,e,f) SEM images of nanocomposite films, showing the tendency of the SnO2 nanoparticles to coat the entire outer surface of the AgNWs, increasing their apparent diameter and giving them a soft appearance.

Figure 3. Schematic diagrams and TEM images of (a) a single untreated AgNW, (b) an AgNW in the presence of uncoupled SnO2 (all ammonium ions removed), and (c) an AgNW with a coordinating

SnO2 shell. Scale bars in images (a), (b), and (c) are 300 nm, 400 nm, and 600 nm, respectively. (d,e)

Optical images of AgNW and SnO2 nanoparticle dispersions mixed in varying amounts (d) before and

(e) after settling for 24 hours. The numbers associated with each solution represent the AgNW:SnO2 concentrations in mg/ml. The uncoupled solution contains AgNWs and non-coordinating SnO2 nanoparticles, and shows settling behavior similar to the pure AgNW and pure SnO2 solutions. (f)

Normalized Ag and Sn EDX signal mapped across the diameter of a single nanowire, with the inset showing the scanning path across an isolated wire.

Figure 4. Sheet resistance versus temperature for films deposited using (red) AgNWs that have been washed three times in ethanol and (blue) mixtures of AgNW and SnO2 with weight ratio of 2:1. The annealing time at each temperature value was approximately 10 minutes. The large sheet resistance values of the bare AgNWs when annealed below 200 °C is typical for nanowires fabricated using copper chloride seeds, which clearly illustrate the impact of SnO2 coordination at low treatment temperatures.

Figure 5. (a) Sheet resistance and transmission data for samples deposited from solutions of varying nanostructure concentration. Each of these samples were fabricated starting from the same nanocomposite ink, which was then diluted to a range of concentrations while maintaining the same

AgNW to SnO2 weight ratio. (b) Transmission spectra of several transparent conducting networks chosen from the plot in plot (a).

22 Figure 6. (a) I-V characteristics of devices made with AgNW/SnO2 rear electrodes with (blue) and without (red) a 10 nm AZO contact layer. The dramatic double diode effect is likely a result of a significant barrier to charge injection at the electrode/a-Si interface. (b) Top view SEM image of the

AgNW/SnO2 composite films on top of the textured a-Si absorber. (c) Schematic cross section of the a-Si device architecture used in solar cell fabrication. The thickness of the thin AZO contact layer is exaggerated for clarity.