Crumpled Graphene pân

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Fast One-Pot Synthesis of MoS/Crumpled Graphene p-n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production Francesco Carraro, Laura Calvillo, Mattia Cattelan, Marco Favaro, Marcello Righetto, Silvia Nappini, Igor Píš, Veronica Celorrio, David J. Fermin, Alessandro Martucci, Stefano Agnoli, and Gaetano Granozzi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06668 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on November 2, 2015

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ACS Applied Materials & Interfaces

Fast One-Pot Synthesis of MoS2/Crumpled Graphene pn Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production Francesco Carraro,† Laura Calvillo,† Mattia Cattelan,† Marco Favaro,†, § Marcello Righetto, † Silvia Nappini, ‡ Igor Píš, ‡,# Verónica Celorrio,¶ David J. Fermín, ¶ Alessandro Martucci,◊ Stefano Agnoli,*,† and Gaetano Granozzi† † Department of Chemical Sciences, University of Padova, via Marzolo 1, Padua 35131, Italy § Advanced Light Source (ALS) Joint Center for Artificial Photosynthesis (JCAP), Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., M/S 6R2100 Berkeley, CA 94720 ‡ Istituto Officina dei Materiali (IOM)-CNR, Laboratorio TASC, Area Science Park-Basovizza, Strada Statale 14, Km.163.5, I-34149 Trieste, Italy # Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, Km.163.5, I-34149 Trieste, Italy ¶ School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, U.K. ◊ Industrial Engineering Department and INSTM, University of Padova, Padova 35131, Italy

KEYWORDS: graphene oxide, hydrogen evolution reaction, transition metal dichalcogenides, photocatalysis, nanohybrids. 1

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ABSTRACT: Aerosol processing allows preparing in high yield and short time hierarchical graphene nanocomposites with special crumpled morphology. By modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene based materials ranging from heteroatoms doped graphene nanoballs, to hierarchical nanohybrids made up by nitrogen doped crumpled graphene nanosacks that wrap finely dispersed MoS2 nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), grazing incidence X-ray diffraction (GIXRD) and spectroscopic (high resolution photoemission, Raman and UV-visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS2 nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photo-electrochemical production of hydrogen, obtaining that N-doped graphene/MoS2 nanohybrids are seven times more efficient with respect to single MoS2 because of the formation of local p-n MoS2/N-doped graphene nanojunctions, which allow an efficient charge carrier separation.

INTRODUCTION Besides being a material with exceptional electrical conductivity, outstanding mechanical toughness and remarkable optical properties, graphene is, at its very nature, a perfect and soft 2D atomic sheet. This combination of confinement to nanodimension and easy mechanical pliability makes it an incredibly versatile material that can be shaped in almost any form. This great potential for manipulation paves the way toward the development of advanced architectures that combine graphene intrinsic properties with nanodesign optimized for specific functions.1 Interestingly, the two-dimensional nature of graphene implies the ability to undergo easy folding and bending, just like macroscopic objects, thus graphene sheets can be opportunely crushed to form crumpled nanoballs.2,3,45 This special conformation 2


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is quite intriguing since it prevents the re-stacking of single sheets, allowing the preparation of high surface area systems.6 Moreover, mechanical strain induced in the material by wrinkles and folds can promote unexpected chemical reactivity and better electrochemical performances.7,8,9,10 Crumpling of graphene sheets can be conveniently carried out by using aerosol processing and employing aqueous suspensions of graphene oxide (GO).3 The capillary forces that develop during the drying of aerosol microdroplets, induce the collapse of the flat geometry of graphene sheets that wrinkle and eventually are compressed into a spherical shape.11,12 But not only: every aerosol droplet can be seen as a microreactor and, if proper co-reactants are provided, different reactions (i.e. doping and functionalization of GO sheets, precipitation of solid particles) can take place during water evaporation, thus highly complex functional nanoarchitectures can be synthetized through a simple one-pot procedure.13,14,15 Therefore, the combination of aerosol process and GO represents a quite powerful synthetic strategy that can open the way toward the preparation of hybrid nanosystems with tailored morphology and chemical functionalization.16,17 In the present work, aerosol processing was used as a fast and simple method to modify graphene sheets by introducing heteroatoms and forming chemically modified graphenes (CMGs).1,18,19,20 Moreover, by adding a soluble salt in the precursor solution, which undergoes aerosolization, we were able to prepare nanohybrids made up by N-doped crumpled graphene (N-cGO) filled with a second component, in this case MoS2 (hereafter N-cGO/ MoS2). Differently from other techniques that are generally adopted for the preparation of similar nanocomposites, e.g. solvothermal synthesis21,27 or solid-solid or solid/gas reactions22, the aerosol processing allows a surgical design of morphology and chemical composition, but at the same time, it maintains high yield and exceptionally short synthesis time. MoS2 is one of the most studied candidates for the substitution of Pt as electroactive catalyst in the hydrogen evolution reaction (HER). It is a nontoxic, environmentally friendly and abundant 3



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semiconductor that exhibits all the preconditions to build a sustainable technology. However, bulk MoS2 at the present time is not yet a valid competitor of commercial catalysts based on Pt due to poor performances.23,24 In fact, one of the main limitations of MoS2 as electroactive catalyst toward HER stems from its poor electron conductivity. Therefore the combination of MoS2 and graphene, which exhibits exceptional electron transport properties, can be quite advantageous in order to overcome these problems.

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Moreover, the stabilization of small MoS2 nanoparticles (NPs) on graphene surface

offers a simple way to maximize the number of edge sites, which are known to be the active phase for the HER.28,29 On the other hand, the use of CMGs opens the way towards new concepts for the fabrication of advanced materials. Actually, by aerosol process, p-type MoS2 NPs can be easily prepared and efficiently dispersed on nitrogen doped graphene forming localized p-n nanojunctions26 which are quite efficient for the separation of photogenerated charge carriers. Actually, N-cGO/MoS2 nanohybrids result to be extremely active in the photoelectrochemical hydrogen generation.

EXPERIMENTAL DETAILS Synthetic procedures. In order to reach the final goal of this work, i.e the synthesis of N-cGO/MoS2 nanohybrids, we investigated the synthesis by means of aerosol process of progressively complex nanosystems, namely cGO, nitrogen doped cGO, MoS2, and doped cGO/MoS2. All synthesis protocols are based on the formation of an aerosol operated by a Sonaer ultrasonic 2.4 MHz nebulizer (particle size about 1.7±1 µm) starting from an aqueous precursor solution. Particles generated by the ultrasonic nebulizer are forced to pass through the furnace, set at 900°C, by using a N2/H2 (9:1, v/v) carrier gas. Crumpling process takes place during the whole time-of-flight of the aerosol droplets, but mainly during the passage through the hot furnace, where water evaporation is very fast. Si(100) wafers, gold coated silica filters (EPM2000), and Toray paper were used as substrates for collecting the particles. Typical collection time was 30 min. 4


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The precursor solutions for the different materials were prepared as follows: c-GO: GO suspension (0.5 mg/ml) N doped c-GO: GO suspension (0.5 mg/ml) in 5M NH3 (Sigma-Aldrich) solution. MoS2 NPs: 1 mM (NH4)2MoS4 aqueous solution (Sigma-Aldrich) N-cGO/MoS2: GO suspension (0.5 mg/ml) in 5M NH3 (Sigma-Aldrich) and 1 mM (NH4)2MoS4 (Sigma-Aldrich). Structural characterization tools. High-resolution X-ray photoemission (HR-XPS) spectra were acquired at the BACH beamline at ELETTRA synchrotron facility (Trieste, Italy) using 1000 eV photons and a total energy resolution of 0.27 eV. The measurements were taken after a short annealing at 300°C in UHV. The calibration of the binding energy (BE) scale was determined using the Au 4f7/2 photoemission line as reference. The XPS peaks were separated into individual components (after Shirley background removal) using symmetrical Voigt functions and non-linear least squares routines for the χ2 minimization. The nano- and micro-scale morphology of materials was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images were acquired using a field emission source equipped with a GEMINI column (Zeiss Supra VP35), micrographs were obtained with an acceleration voltage of 5 or 10 kV using in-lens high-resolution detection. TEM images were acquired using a FEI Tecnai 12 microscope with an acceleration voltage of 100 kV. STEM and HRTEM micrographs were acquired using a JEM-2100F Field Emission Electron Microscope operating at 200 kV. Energy Dispersive X-ray Spectroscopy (EDS) elemental maps were acquired with the same instrument. Grazing incidence X-ray diffraction (GIXRD) patterns were collected using a Philips PW 1710 diffractometer equipped with a Cu Kα radiation tube operating at 40 kV and 40 mA and with an incidence angle of 1°. Characterization by Raman spectroscopy was performed using a ThermoFisher DXR Raman microscope. The spectra were recorded using a laser with an excitation wavelength of 532 5



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nm (1 mW), focused on the sample with a 50× objective (Olympus). Raman maps were recorder using the same parameters, but with the 100× objective. UV−visible-NIR absorption spectroscopy was performed using a Cary 5000 spectrometer (Varian), in the 500 −1200 nm range. Powder samples were dispersed in ethanol, forming a stable colloidal dispersion. All spectra were recorded using 1 mm optical path quartz cells. The concentration and the experimental parameters were tuned to minimize scattering effects. Electrochemical Characterization. Electrochemical measurements were carried out in a conventional three-electrode cell, using an Ar-saturated 0.5 M H2SO4 solution as electrolyte. An Ag/AgCl (KClsat) electrode (calibrated as 0.217 V vs RHE) and a glassy carbon rod were used as reference and counter electrodes, respectively. Working electrodes were prepared by collecting the MoS2 NPs and the NcGO/MoS2 nanohybrids in Toray paper (TGP-H-60), as described above, and adding 2.5 µL of Nafion solution (8 vol. %, Sigma Aldrich). Commercial MoS2 (Sigma-Aldrich) was drop-casted on Toray paper and used as reference. Materials activity toward HER was evaluated by recording polarization curves from a certain potential to -0.4 V vs RHE using a scan rate of 2 mVs-1. For each material the starting potential was set at 50 mV more positive than its open circuit potential (OCP). To investigate the possible activity enhancement due to the p-n nanojunction formed between the smallest MoS2 NPs and the N-cGO, i-t curves were recorded at different potentials (-0.1, -0.15, -0.25 V vs RHE) in absence and presence of light, using a white LED lamp (OSRAM - 9 LED Star Oslon 150 white 1170 lm/cm2@350 mA) as light source. The current densities were normalized to the electrode geometric area (in cm2) and the amount of Mo (in mg) for a better comparison among different materials. Impedance spectroscopy was performed in the frequency range between 100 and 0.1 KHz on different materials drop-casted on glassy carbon electrodes. All the experiments were performed at room temperature. RESULTS AND DISCUSSION

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In this work, we investigated the preparation by aerosol technique of progressively more complex nanosystems, namely pure and doped cGO, MoS2 NPs, MoS2/cGO composites and eventually NcGO/MoS2 nanohybrids. Notably, all materials were prepared using the same synthesis protocol (e.g. forming an aerosol transported by a H2/N2 gas carrier through a furnace kept at 900°C), simply by adding a further component (i.e. a NH4OH to induce nitrogen doping, and (NH4)2MoS4 for the formation of MoS2 NPs) in solution that undergoes aerosolization (see scheme 1 and Supplementary Information for more details). The structural and chemical properties of these systems were thoroughly investigated by spectroscopic and microscopic techniques, and the knowledge acquired on the single component materials, allowed us to rationally design a new nanohybrid based on a p-n junction between MoS2 and N-doped graphene that exhibits excellent catalytic activity for photoelectrochemical hydrogen generation.

Scheme 1: schematic representation of the rationale for materials Physico-chemical characterization. The chemical composition of the different materials was studied by HR-XPS employing synchrotron radiation. Figures 1a and 1b shows the C 1s and N 1s photoemission spectra of N-cGO. The C 1s photoemission line (Figure 1a) was separated into six chemically shifted components according to the standard procedure reported in literature:30,31,32 the most intense peak, centered at 284.4 eV, is associated with sp2 hybridized carbon atoms, whereas the peak at 285.4 eV can be related either to sp3 hybridized carbon component, which is connected to the crumpling of the sheets, or to C-N bonds. The features centered at 286.2, 286.8 and 287.9 eV are associated with tertiary alcohols33, epoxy34 and carbonyl groups33, respectively. These three components are strongly reduced with respect to pristine GO (see the C 1s spectra reported in Figure S2). A minority component 7



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associated with defective carbon atoms is found at 283.7 eV.35,36 The successful nitrogen doping of graphene by aerosol processing, as well as the nature of the N functional groups formed, was determined by separating the N 1s photoemission line into three chemically shifted components (Figure 1b). The peak centered at 398.5 eV is associated with pyridinic defects, whereas the components at 400 eV and 401.3 eV are related to pyrrolic and N substitutional defects, respectively.32,37 Elemental analysis, carried out by considering the area of the C 1s and N 1s peaks and their corresponding sensitivity factors, indicates a nitrogen content of about 4% (at %). To the best of our knowledge, this is the first example where the aerosol technique is used for preparing CMGs. A detailed investigation of the doping process is beyond the scope of the present paper; however we want to mention that by using other simple water soluble precursors, it is possible to introduce into GO a full range of dopants (N, S) in different combinations. In the case of the N-cGO/MoS2 nanohybrids, the C 1s photoemission line was separated into five components (Figure 1c). The only significant difference with respect to N-cGO is the absence of the minority component related to defective carbon at 283.7 eV. In N-cGO/MoS2, the N 1s and Mo 3p3/2 photoemission lines overlap, making difficult the analysis of the N-doping. We noticed an anomalous intensity ratio between Mo 3p3/2 and Mo 3p1/2 peaks that can be associated with the presence of the N 1s photoemission line. In order to separate both contributions, the Mo 3p spectral region was carefully separated into two single chemically shifted components with a fixed spin orbit split of 17.6 eV, as shown in Figure 1d

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Figure 1: (a) C 1s and (b) N 1s photoemission lines of N-cGO; (c) C 1s and (d) Mo 3p and N 1s photoemission lines of N-cGO/MoS2 nanohybrids.

The Mo 3p3/2 component related to MoS2 is centered at 395 eV, whereas the Mo 3p3/2 components centered at 398.3 eV is related to MoOx oxidized species.38 The presence of MoOx species is not unexpected since the XPS data were collected on ex-situ prepared samples and MoS2 edges can partially be oxidized in air.39 To increase the accuracy of the fitting procedure, the ratio between these Mo species was based on the deconvolution of the Mo 3d photoemission line (see below). Subsequently, two additional components related to the N 1s region were added; one at 397.4 eV attributed to C-N-Mo species,40 and another one centered at 399.2 eV associated with C-N bonds, which includes the contribution of pyridinic and pyrrolic groups and of N substitutional defects.37,32 Quite interestingly, the 9



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presence of C-N-Mo bonds indicates that N heteroatoms in doped cGO act as nucleation centers for the MoS2 NPs. This hypothesis is also supported by the fact that the component at 397.5 eV cannot be related to N doping of MoS2, or to the formation of MoN, because this component is not present in the spectrum of bare MoS2 NPs synthesized in presence of ammonia (Figure S3). The Mo 3d photoemission line was taken into consideration and fitted by three components with a fixed spin orbit separation of 3.13 eV, one related to MoS2 (229.2 eV) and the others related to MoO3 and MoOx (233.0 and 231.6 eV respectively) species, whereas the S 2s photoemission line was fitted by a single component centered at 226.4 eV (Figure S4).41,42 To quantify experimentally the MoSx composition, we considered the intensity ratio between the S 2s and Mo 3d peaks and compared it with the value measured on commercial MoS2 powder as reference for a 2:1 stoichiometry. By using this procedure, we verified that MoSx NPs are slightly under-stoichiometric in sulfur content (x=1.7). Moreover, by considering the area of the C 1s, Mo 3d and S 2s peaks and their corresponding sensitivity factors, we determined a MoS2/C ratio of about 26% (at %). SEM and TEM investigations were carried out to study the microscopic structure of the prepared materials. SEM micrographs clearly document the crumpling of GO sheets and the formation of deeply wrinkled and globular structures (Figure 2a, b), whose size ranges from 100 to 500 nm. No morphological differences can be observed between N-doped and undoped cGO materials (Figure 2c) indicating that the presence of dopants plays only a minor role in the physical process of crumpling. SEM images of the N-cGO/MoS2 nanohybrids (Figure 2e, f) show that the surface of graphene is decorated with MoS2 NPs characterized by bimodal size distribution: large aggregates ranging from 50 to 100 nm, and NPs smaller than 10 nm, which are scattered on the GO surface.

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Figure 2: SEM micrographs of (a, b) pristine cGO, (c) N-cGO, (d) MoS2 NPs obtained in absence of GO sheets, and (e, f) N-cGO/MoS2 nanohybrids. TEM micrographs (g, h) of NcGO/MoS2 nanohybrids. The larger structures present morphological features very similar to those obtained by direct aerosol synthesis of MoS2 NPs starting from (NH4)2MoS4 in the absence of GO sheets (see Figure 2d). In this case spherical particles with deep crevices and rough surfaces are formed, whose dimensions (diameter ranging from 200 to 1000 nm) correlate well with the dimension of aerosol droplets. Actually, each aggregate consists of smaller units quite homogeneous in size (about 50 nm) that are densely assembled into globular superstructures, and are due to precipitation and aggregation processes driven by water evaporation from every aerosol microdroplet. Therefore, SEM data indicate that the presence of GO sheets in microdroplet reactors drastically modifies the final structure of MoS2 particles since it prevents NP aggregation.

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TEM micrographs of N-cGO/MoS2 nanohybrids (Figure 2g, h) confirm the bimodal growth of MoS2 in presence of GO sheets, as evidenced by the presence of large globular aggregates of MoS2 with a diameter of about 50 nm that can be either attached to the GO surface or more frequently enclosed inside the c-GO, forming nanosacks as reported in previous works for metal and oxide NPs.2 Moreover, very small NPs, smaller than 10 nm, can be easily detected on less crumpled sheets (see inset in Fig 2g).

Figure 3: HRTEM micrograph of N-cGO/MoS2 nanohybrids presenting MoS2 NPs highlighted by red squares. HR-TEM analysis (Figure 3) shows that the smallest NPs have a diameter of ~ 3 nm and that nanostructured MoS2 grown on graphene surface is crystalline as determined by the lattice spacing corresponding to the (002) facet (0.62 Å)43. The GIXRD patterns (Figure S5b) confirm that MoS2 NPs grown on graphene as well as synthesized by aerosol method, are crystalline with a hexagonal structure (2H phase).44,45 A sharp (002) peak is observed, along with peaks from other reflection groups. The position of the (002) diffraction peak is slightly shifted to lower angle (14.02° for the nanohybrids and 13.75° for MoS2 NPs) compared to what is reported in [AMCSD 0009788], indicating a lattice 12


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expansion as already reported in the literature.46 An estimation of the mean crystallite size determined by the line broadening of the (002) diffraction peak yields about 4 nm diameter. The peak at 24°, typical of reduced GO, cannot be identified in cGO, N-cGO and N-cGO/MoS2 nanohybrids (Figure S5a, S5b).26 This is correlated to the special synthesis method: the crumpling of the graphenic structure avoids the restacking of the graphene sheets and prevents to measure a clear diffraction peak.27 In conclusion, the mean size of MoS2 aggregates in the composite with graphene is one order of magnitude smaller with respect to materials obtained without GO (60 nm vs 600 nm). The presence of GO sheets in the aerosol droplets therefore has a twofold effects: it favors the nucleation of small MoS2 NPs on the GO surface, and prevents aggregation of the NPs because GO crumpling can encapsulate and keep separated different aggregates. The uniform dispersion of MoS2 on the crumpled surface of graphene is confirmed also by C Kα, Mo Kα and S Kα elemental maps obtained by EDS (Figure S6). Raman spectroscopy is very useful to characterize the composition of the product, since GO and MoS2 show characteristic Raman fingerprints. Raman spectra (Figure S7a) of N-cGO/MoS2 nanohybrid present the characteristic D (~1350 cm-1), G (~1590 cm-1), 2D (~2700 cm-1) and D+D’(~2900 cm-1) bands of graphene47 and the A1g (~410 cm-1) and E12g (~385 cm-1) mode of MoS2.48 The D/G band intensity ratio is higher with respect to pure cGO synthesized at the same temperature (not reported here). This could be attributed to the presence of N defects that locally break the carbon sp2 lattice symmetry.49 The Raman frequencies of E12g and A1g peaks related to MoS2 increase monotonously with the number of layers in MoS2 flakes, while the intensity and width of the peaks vary arbitrarily. This behavior has been related to increased electronic transition energies or elongated interlayer atomic bonds in ultrathin MoS2.50 In our N-cGO/MoS2 nanohybrids, the wavenumber separation between A1g and E12g mode of MoS2 in different points of the same sample varies from 25 to 27 cm-1, this means that MoS2 NPs are mainly composed by multiply stacked layers as in bulk MoS2.50

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Conversely, UV-visible spectra (Figure S8) of the N-cGO/MoS2 nanohybrids provide another confirmation of the bimodal growth of MoS2 NPs. The spectrum of N-cGO/MoS2 exhibits three characteristic absorption peaks: the peak centered at 975 nm evidences the presence of bulk-like MoS2 and is the only peak observed in the UV-visible-NIR spectrum of solitary MoS2 synthesized by aerosol methods. 51 The peaks at around 640 and 680 nm are related to the smaller MoS2 NPs and is consistent with quantum size effects on this material, as previously reported for nanosized MoS2. 26,51 With a view to the application of the N-cGO/MoS2 nanohybrids as photocatalyst, the presence of these absorptions peaks in the visible range is remarkable due to the correspondence with the maximum of the solar emission spectrum. This also indicates an increase on the band gap of MoS2 up to 1.8-2-2 eV, which makes it very appealing for water splitting applications. Photo-electrochemical characterization. The photo-electrochemical catalytic behavior toward HER of the N-cGO/MoS2 nanohybrids and of bare MoS2 NPs obtained by aerosol processing without GO sheets were investigated. In all cases the Toray paper was used as support. The results obtained for the commercial MoS2 on Toray paper and bare Toray paper are also presented as reference. Figure 3a shows the comparison of the polarization curves of the whole series. Bare MoS2 NPs exhibit an overpotential for HER around 180 mV vs RHE, which is very similar to that of commercial MoS2, and fully consistent with values previously reported in literature.7,52,53 However, MoS2 NPs prepared by the aerosol method exhibit higher current densities than commercial MoS2 powders. This result can be attributed to the increase of active sites due to the small size of the NPs obtained by the aerosol method. The N-cGO/MoS2 nanohybrids, however, show an improved HER activity as evidence by lower overpotential (~100 mV vs RHE) and significant higher current densities. This latter reaches in nanohybrids a value two times higher than in pure MoS2 NPs at -250 mV vs RHE (94 mA cm-2mgMo-1 and 39 mA cm-2mgMo-1, respectively). The low overpotential of the nanohybrids is comparable with the

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lowest overpotentials for the electrochemical HER reported in literature for graphene/MoS2 based materials. 21,25,26,27 To obtain a further insight into HER activity of the different samples, Tafel plots were determined (Figure 3b). The resulting Tafel slope for bulk commercial MoS2 is 122 mV/dec, indicating that hydrogen adsorption (Volmer step) is the rate-limiting step of HER.25,54 Tafel slope is lower for the samples synthesized by aerosol method: 93 mV/dec and 94 mV/dec for N-cGO/MoS2 nanohybrids and MoS2 NPs, respectively. This indicates that the HER occurs through several reaction pathways.25,54 Tafel slopes around 120mV/dec have been reported for MoS2 grown on Toray paper55 (120 mV/dec), for multi walled MoS2 nanotubes grown on multi walled carbon nanotubes (CNT)52 (109 mV/dec) and for MoS2/CNT/graphene (100 mV/dec)56. On the other hand, a Tafel slope of ~41 mV/dec has been reported for ultrathin MoS2 nanosheets grown on graphene.25 Our higher slopes could be influenced by two factors: (i) charge transport limitations through the fibrous, porous network characteristic of Toray carbon paper used as substrate,55 and (ii) the influence of the bigger MoS2 NPs with a bulk-like structure (as previously demonstrated by Raman and UV-visible spectroscopy) that compromises the conduction through MoS2 inner layers.52,57 However, the Tafel slope and the small overpotential of N-cGO/MoS2 nanohybrids highlight the role of N-cGO in the enhancement of the activity of the MoS2 NPs. This effect can be attributed to the improved electronic contact between the active MoS2 NPs and the conductive graphene through the N-groups, as seen by XPS. Besides, UV-VIS and microscopy investigations indicate that N-cGO/MoS2 nanohybrids present the smallest NPs, and therefore a larger number per mass of highly active edges, which are the main catalytic sites of MoS2.24

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Figure 4: (a) Polarization curves (not iR corrected) in dark (d) and under illumination (l) for N-cGO/MoS2, bare MoS2 NPs obtained by the aerosol method, commercial MoS2 (SigmaAldrich) and Toray paper in 0.5 M H2SO4 at room temperature; (b) Corresponding Tafel plots for N-cGO/MoS2, MoS2 NPs obtained by the aerosol method and commercial MoS2 (SigmaAldrich); (c) J-t curves under light (ON) and dark (OFF) conditions at -0.1 V and -0.25 V vs RHE and room temperature for N-cGO/MoS2; (d) Mott-Schottky plot at 1 kHz of the NcGO/MoS2 in dark.

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It is known that the N-doped graphene forms p-n nanojunctions with MoS2 NPs, enhancing the charge separation.26 For this reason, we investigated if light could further enhance the electrocatalytic activity of the N-cGO/MoS2 nanohybrids toward HER. Figure 4c reports the j-t curves obtained at -0.1 V and 0.25 V vs RHE under light and dark conditions. Under illumination, the sample shows a percentage increase of 5 and 7 % in the current density, whereas the relative increase of bare MoS2 NPs is only of 1 % (Figure S9), at -0.1 V and -0.25 V, respectively. Moreover, under illumination the onset potential slightly shifts to a higher value (~ -75 mV vs RHE, Figure 4a), which is somewhat lower than the one obtained by Meng et al.26 (~ +50 mV vs RHE). Such difference can be traced back to the contribution of bigger MoS2 NPs, as mentioned above. The better performance under illumination therefore is not simply due to the semiconductive character of MoS2 alone, but it is connected to the presence of doped graphene oxide. Actually as previously reported26 a p-n nanojunction is formed between the two materials, which has the effect of helping the charge separation thus boosting photoelectrochemical activity. In fact, MoS2 NPs and N-cGO show respectively positive (p-type behavior) and negative (ntype behavior) slope for the Mott-Schottky plots (Figure S10). When coupled in the MoS2/N-cGO nanohybrids, the formation of p-n nanojunctions is confirmed as indicated by the characteristic “inverted V-shape” of the Mott-Schottky plot (Figure 3d).

CONCLUSIONS The present work fully demonstrates the huge potential of the aerosol processing applied to the synthesis of complex multicomponent systems. Actually, simply by adding additional components in the precursor solution we prepared in a simple and fast way, hierarchical graphene based materials with increasing level of complexity. For the first time we were able to prepare by aerosol, doped materials without the need for long processing time or the use of toxic gases. Not only, since during aerosol processing graphene sheets 17



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heavily bend and wrinkle, and it is possible to avoid the restacking of graphene sheets and to combine specific electronic properties induced by doping with the enhanced reactivity of surface sites induced by high local curvature.7 Therefore, by following this general one pot aerosol route, we demonstrate that it is possible to realize nanocomposites characterized by specific interaction between the single components. Such strategy is exemplified by N-cGO/MoS2 nanohybrids: the rational assembly of their single constituents has allowed improving the performance in the electrochemical generation of hydrogen, which under illumination shows an increase in the current density 7 times higher with respect to simple MoS2 NPs. This can be traced back to the formation of p-n nanojunctions between N-doped graphene and MoS2 and to a better dispersion and smaller size distribution of catalyst NPs.

ASSOCIATED CONTENT Supporting information: Experimental details; photoemission spectra of GO and MoS2 NPs; XRD patterns of GO, N-cGO, MoS2 NPs, commercial MoS2 and N-cGO/MoS2; EDS elemental maps of NcGO/MoS2 ; Raman spectra of N-cGO and of N-cGO/MoS2; UV-visible spectra of GO, N-cGO, MoS2 NPs and N-cGO/MoS2; J-t curves of MoS2 NPs. AUTHOR INFORMATION Corresponding Author * Tel.: +39 049 8275122. E-mail: [email protected] . ACKNOWLEDGEMENTS This work was partially supported by the Italian MIUR through the national grant Futuro in Ricerca 2012 RBFR128BEC “Beyond graphene: tailored C-layers for novel catalytic materials and green chemistry and by the University of Padova funded project: CPDA128318/12 “Study of the catalytic activity of complex graphene nanoarchitectures from ideal to real conditions” and through the grant "Attrezzature scientifiche finalizzate alla ricerca ‐ Bando 2012". 18


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