Accepted Manuscript Thiol functionalized reduced graphene oxide as a base material for novel gra‐ phene-nanoparticle hybrid composites Chuyen V. Pham, Michael Eck, Michael Krueger PII: DOI: Reference:
S1385-8947(13)00914-5 http://dx.doi.org/10.1016/j.cej.2013.07.007 CEJ 10986
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
11 February 2013 27 June 2013 3 July 2013
Please cite this article as: C.V. Pham, M. Eck, M. Krueger, Thiol functionalized reduced graphene oxide as a base material for novel graphene-nanoparticle hybrid composites, Chemical Engineering Journal (2013), doi: http:// dx.doi.org/10.1016/j.cej.2013.07.007
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Thiol functionalized reduced graphene oxide as a base material for novel graphene-nanoparticle hybrid composites Chuyen V. Phama,b, Michael Ecka,b and Michael Kruegera,b* a
Freiburg Materials Research Center (FMF), University of Freiburg, Stefan-Meier-Str. 21, 79104 Freiburg, Germany. b
Department of Microsystems Engineering (IMTEK), University of Freiburg, Georges-KöhlerAllee 103, 79110 Freiburg, Germany
ABSTRACT. In this work, the synthesis and characterization of thiol functionalized reduced graphene oxide (TrGO) as a novel base material for the generation of graphene-nanoparticle hybrid materials is reported. TrGO was directly synthesized by refluxing graphene oxide (GO) with phosphorus pentasulfide (P4S10). The introduction of sulfur containing chemical groups and the partial reduction of GO to TrGO were proven by UV-Vis, XPS and FTIR spectroscopy. CdSe nanocrystal-TrGO (CdSe-TrGO) as well as silver nanoparticle-TrGO (Ag-TrGO) hybrid materials are achieved by attachment of the respective nanoparticles to the thiol groups of the graphene using self-assembly decoration. The resulting hybrid materials were investigated by TEM and photoluminescence quenching experiments.
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KEYWORDS: reduced graphene oxide, thiol functionalized grapheme nanocomposite, semiconductor nanocrystals;
1. INTRODUCTION
Graphene as a single atomic layer of sp2 carbon atoms, has been recently attracting tremendous attention within the scientific community owing to its unique properties such as high conductivity, optical transparency and mechanical stability [1, 2]. So far, several methods have been utilized to synthesize graphene. Pristine graphene was firstly obtained by using Scotch tape to peel off a single or a few layers of graphene sheets from bulk graphite which was reported by Geim, Novoselov and co-workers [3]. Alternatively, the chemical reduction of graphene oxide (GO) is a method for obtaining graphene-like structures in high quantities [4], while chemical vapor deposition (CVD) is the method of choice to produce thin and continuous graphene films of larger size [5]. Already various applications utilizing graphene based materials have been reported such as transparent electrodes in solar cells [6, 7, 8], catalytic electrodes in fuel cells [9], supercapacitors [10,11] as well as electrodes in transistors [12] and sensors [13]. Due to its low catalytic activity, graphene is often decorated with catalytic nanopartices or quantum dots for applications in fuel cells [9] or optoelectronics [14] respectively, resulting in hybrid materials with both advantages of graphene and nanopartices often inducing novel properties. For example platinum or titanium dioxide nanoparticles have high specific surface area and catalytic activity; they have been proven for applications in catalysis [15,16] and for energy conversion [17,18]. Also, semiconducting CdSe nanocrystals (NCs) with easily tunable optical and electrical properties, have benn demonstrated successfully for application in hybrid photovoltaic devices14. However, the nanoparticles are often synthesized in solution with ligand shells to prevent aggregation; this in turn decreases the effectivity of catalysis or charge transfer in fuel cells and
2
solar cells [21]. Moreover, before nanoparticles are incorporated into devices ,the synthesis ligands have to be exchanged ideally by a monolayer of more conductive ligands with the disadvantage that the nanoparticles might agglomerate [22,23] leading to a significant reduction of their active surface area. This general problem can be overcome by using a framework to support the nanoparticles, keeping them separated and avoiding aggregation. Graphene is an excellent ´framework and support for nanoparticles [25,26]. Additionally it favors charge transfer processes at the nanoparticle-graphene interface [14] as well as charge transport processes making them ideal candidates for interlayers or electrode material in electrocatalysis or photovoltaics [27,28]. Taking advantages of the high electrical conductivity (electron mobility of 15,000 cm2 V-1s-1)2, the ultra-high specific area of graphene (theoretical value, 2630 m2/g) [24] and good catalytic or optoelectric properties of nanoparticles, the obtained nanoparticle-graphene hybrid materials have high potential in energy harvesting applications. So far, much effort has been paid to the synthesis and application of graphene-metal nanoparticle hybrid materials. Platinum or silver nanoparticle decorated graphene was successfully synthesized and exhibits highly catalytic behaviour towards methanol and ethanol oxidation and oxygen reduction as electrode material in fuel cells [25,26]. They have also been used as catalytic counter electrode for reduction of triiodide in dye sensitized solar cells [19, 20]. Another example is CdSe NCs decorated graphene with promising properties for applications in photovoltaics and OLEDs [14, 29-31]. Usually, graphene based hybrid materials are synthesized by in-situ growth of nanoparticles on graphene oxide sheets (GO). However, graphene might affect the formation of nanoparticles by hindering or influencing their growth, resulting in non uniform nanoparticle shapes. Moreover, during the nanoparticle formation, GO is simultaneously reduced to reduced graphene oxide (rGO) and tends to agglomerate. Therefore, it is difficult to control the nanoparticle quality (e.g. size distribution and uniformity) and at the same time the degree of GO
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reduction towards rGO which determines important properties such as electrical conductivity, work function and the solubility of the resulting hybrid material. Also, rGO is often insoluble in either aqueous media, used for the synthesis of metal nanoparticles out of e.g. silver [32] or platinum [26,27] or organic solvents such as hexadecylamine (HDA) and tri-n-octylphosphine oxide (TOPO) often used for the synthesis of semiconductor quantum dots [31,14]. A major drawback of these in-situ approaches is, that during the reduction of GO to rGO, graphene sheets are often irreversibly stacking together by van der Waals forces, which therefore hinders nanoparticles to reach the graphene surface leading to a less effective nanoparticle loading. Another approach to fabricate nanoparticle-graphene hybrid materials is the functionalization of graphene with specific functional chemical groups which have a high affinity towards nanoparticles.
CdSe
decorated
poly-
(diallyldimethylammonium
chloride)
(PDDA)-
functionalized graphene was synthesized by Lu et al. [29] using positively charged PDDA to attach negatively charged CdSe nanoparticles. The as-received hybrid material exhibits good solubility in polar solvents such as alcohol and water. Feng et al. [33] achieved cadmium sulfide (CdS) decorated graphene by using benzyl mercaptan as linker binding to CdS nanoparticles via thiol group (-SH) and attaching to graphene via non-covalent π-π stacking. Such nanoparticlegraphene hybrid materials obtained by binding nanoparticles to rGO through non-covalent linker molecules can overcome the above mentioned solubility problem of graphene. However, they might induce less effective charge transfer when applied in fuel cell or solar cell applications. Here, we introduce a simple synthesis method towards thiol-functionalized graphene directly from GO, and its effective decoration with transition metal nanoparticles by self-assembly at room-temperature. The self-assembly decoration is based on the fact that thiol groups strongly bind
to nanoparticles of transition metals such as e.g. silver,gold or metal chalcogenide
semiconductors, such as e.g. CdSe or CdS as reported by Mann et al. [34] and Colvin et al. [35].
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To the best of our knowledge, this is the first report on thiol-functionalization and simultaneous reduction of GO by direct thionation of GO with P4S10, which was demonstrated for carbon nanotubes in a previous report [36]. Our approach is a general scheme to obtain nanoparticlegraphene hybrid material with high nanoparticle loading. The respective hybrid materials have high potential to be utilized in optoelectronics, sensors and transistors, 2. EXPERIMENTAL 2.1. Synthesis of GO by modification of Hummers’s method [37] and an improved procedure of Marcano et al [38]. Materials: Graphite was purchased from Merck, NaNO3 (95%) and KMnO4 (99%) were obtained from Grussing GmbH (Germany). H2SO4 (95-98%) and H2O2 (30%) were obtained from Sigma-Aldrich. In a typical synthesis approach, 1 g graphite, 46 ml H2SO4 and 0.5 g NaNO3 were mixed together and stirred at 35°C for 2 minutes. Then, the solution was continuously stirred in an ice bath until the temperature reached 0°C, which usually took 15 min. After that, 3 g KMnO4 was added gradually so that the temperature was not allowed to exceed 20°C. Subsequently, the solution was held and stirred at 35°C for 6 h. In an additional step, another 3 g KMnO4 was added to the solution, and stirred again for 12 h at 35°C. Finally, 150 ml H2O containing 6 ml of 30% H2O2 was added slowly while keeping the temperature below 80°C. As a result, residual KMnO4 and MnO2 were reduced to Mn2+. The colour of the solution changed from dark brown to bright yellow. Post synthetic cleaning procedure for GO. Firstly, the obtained bright yellow solution was sonicated for 30 min and centrifuged at 1000 rpm for 20 min to remove bigger particles and aggregates which have not been fully oxidized. Then, the remaining solution is centrifuged at 4400 rpm for 3 h to collect the product from the solution. Subsequently, 80 ml of 37% HCl was
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added to the black powder which was dispersed in HCl by stirring for 2 min. By an additional centrifugation step at 4400 rpm for 3 h, remaining metal ions were removed. Then, the precipitate was washed two times in 150 ml of DI-H2O adding 50 ml absolute C2H5OH and centrifugated at 4400 rpm for 2.5 h to remove acid and Mn+2 in the sample. Hence, clean GO was obtained. Finally, the GO was dispersed in DI-water followed by sonication for 1 h to obtain a homogeneous aqueous dispersion of GO with a pH ranging between 6-7. 2.2. Thionation of GO to TrGO. Materials: HPLC reagent grade dimethylformaminde (DMF), phosphorus pentasulfide (P4S10) (99%) and 0.45 µm polyamide filter membrane were purchased from Scharlau Chemie S.A, Sigma Aldrich and Whatman Int.Ltd, respectively. 100 mg GO was dispersed in 100 ml DMF and sonicated for 1 h. After removing undispersed GO by centrifugation at 1000 rpm for 10 min, a homogeneous solution of GO in DMF was obtained. Then, 300 mg P4S10 was added to the solution and the reaction flask was evacuated to 5.10-3 mbar at 100°C for 2 min to remove traces of water in the flask. The thionation was performed for 12 h under vacuum and continuous stirring at 120°C. Finally, the reaction product was collected by filtering the solution through a 0.45 µm polyamide membrane filter and was extensively washed firstwith 100 ml of water, followed by 100ml ethanol and acetone, respectively. If the reaction was performed at the boiling point of DMF (152-154°C) under reflux for 36 h, TrGO was obtained with a higher degree of reduction, but less thiol groups. The reduction degree of TrGO can be determined by measuring the UV-Vis absorption as well as the sheet resistance. 2.3. Synthesis of CdSe NCs. In brief, CdSe NCs were synthesized by heating up 1449 mg of HDA (hexadecylamine, ≥95%, Merck Schuchardt), 1546 mg of TOPO (trioctylphosphine oxide, 99%, Sigma-Aldrich), and 111 mg of Cd-stearate (preparation described by Yuan et al. [39]) under nitrogen atmosphere inside a 25 ml three neck bottle to 300°C. Subsequently 100 µl of a
6
1M solution of Selenium in TOP (trioctylphosphine, 97%, ABCR) was rapidly injected. The synthesis was continued at 300°C and stopped after 30 min. The resulting CdSe NCs had an average diameter of about 6 nm. 2.4. Synthesis of CdSe NCs – TrGO hybrid materials by self-assembly decoration. Asreceived TOPO/HDA capped CdSe NCs were purified using our previously published post synthetic ligand reduction procedure [40], where the NCs were treated with hexanoic acid to remove excess of HDA followed by a subsequent addition of methanol and a final centrifugation step. By this method almost all organic shell ligands were removed. Then, 1 mg TrGO was dispersed in 10 ml DMF by sonication for 30 min. After that, the TrGO solution was centrifuged at 2000 rpm for 2 min to remove larger TrGO agglomerated particles which were not well dispersed. The resulting TrGO solution was centrifuged at 14500 rpm for 10 min, the supernatant was poured away, and the TrGO deposit at the bottom of the centrifugation tube was collected. Subsequently, 2 mg CdSe NCs and 1mg TrGO were both dispersed in 4 ml chlorobenzene (CB) in a 10 ml glass vial by sonication for 15 min. A homogeneous solution of TrGO and CdSe NCs was obtained and was held at 110°C under continuous stirring for 30 min. As a result, CdSe decorated TrGO was obtained. 2.5. Synthesis of Ag nanoparticles. Sodium borohydride (NaBH4) and silver nitrate (AgNO3) were purchased from Merck. The synthesis of Ag nanoparticles was performed based on a modified procedure reported by Mulfinger el al. [41]. Briefly, 30 ml of 0.002M NaBH4 aqueous solution was added to an Erlenmeyer flask. Then, a magnetic stirring bar was added and the flask was placed in an ice bath onto a stirring plate. The solution was stirred and cooled for about 20 minutes. Afterwards, 2 ml of 0.001M AgNO3 aqueous solution was dripped into the NaBH4 solution under continuous stirring with approximately 1 drop per second. The stirring was stopped as soon as all of the AgNO3 solution had been added.
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2.6. Characterization techniques. Spectroscopic characterization. UV-Vis absorption spectra were obtained by using a TIDAS 100 spectrophotometer (J&M, Germany). The spectra have been taken by measuring GO, TrGO and CdSe-TrGO dissolved in water, DMF and chlorobenzene, respectively with typically 0.02 weight %. AFM characterization. For the preparation of samples for AFM measurements, a 1x1 cm ITO/glass substrate was treated with O2 plasma for 2 min to form a hydrophilic surface. Then, 10 µl of an aqueous solution of exfoliated GO has been spin-coated onto the substrate at 2000 rpm during 30 s, followed by an additional spin coating step (3000 rpm for 30 s) to dry the sample. The AFM profile of GO was recorded in the tapping mode using a Multimode AFM (Fa. Veeco) at 1.00Hz scan rate, and was analyzed by a Nanoscope analysis program. Sheet resistance measurements. The sheet resistance measurements of TrGO films (usually ca. 8 µm thick and 0.5 cm in diameter) were performed using the van der Pauw method. A TrGO thin film was obtained by filtering TrGO dispersed in DMF through a 0.45 µm polyamide filter membrane (Sigma Aldrich and Whatman INT.Ltd). First, the thickness of the tested TrGO film was measured by a profilometer (Bruker Dektak 150). Then the film was fixed on a non conductive substrate by four small droplets of silver paste (Acheson Silver DAG 1415), which acted also as four contact pads with a diameter of about 2 mm for the following sheet resistance measurement. The four contact pads were contacted by four probeheads (Süss MicroTech PH100)
and
connected
to
a
source-meter
(Keithley
2602).
Electron microscopical investigation. SEM imaging of GO onto an Indium tin oxide (ITO) substrate was performed by using a Quanta 250 FEG (FEI, USA). For TEM measurements, the samples were prepared by dip-coating structured holey carbon coated-copper grid (Quantifoil
8
Micro Tools GmbH, Germany) into a solution of CdSe-TrGO in chlorobenzene. A LEO 912 OMEGA (Zeiss) transmission electron microscope was used for TEM characterization. XPS measurements. A Specs X-Ray Source XR50 with an Al-K anode (1486.6 eV) and source (12 kV, 20 mA) was used to excite the electronic states of atoms below the surface of the sample. Electrons ejected from the surface were energy filtered and detected via the Specs Hemispherical Energy Analyzer Phoibos 50 (SPECS Surface Nano Analysis GmbH). FTIR measurements. For preparing FTIR samples, GO and TrGO were grinded into a powder and mixed with KBr (KBr for IR spectroscopy, Merck) by taking 1 mg TrGO and GO respectively per 100 mg KBr. The FTIR spectra of TrGO and GO were recorded by a Nicolet/Thermo Magna-IR 760 spectrometer with a probe chamber using a diamond ATR-Unit (detection range: 400 cm–1 – 4000 cm–1). PL quenching experiments. Two samples were prepared following the procedure of the CdSe-TrGO hybrid synthesis as described above. One was made out of 1.0 mg CdSe NCs and 0.10 mg of TrGO dispersed in 4 ml chlorobenzene (CB) and the other was made by 1.0 mg CdSe NCs in 4 ml CB without TrGO. Their PL spectra were recorded by utilizing a J&M FL3095 spectrometer (J&M, Germany). 3. RESULTS AND DISCUSSION GO has been prepared by a modified Hummers method [37] which is described in detail in the experimental part. Figure 1 is an AFM image of GO deposited onto ITO/glass, indicating that GO single layer sheets are successfully exfoliated by the previously described modified Hummers method, with thicknesses of about 0.8 nm. The thicknesses are a little thinner than mentioned in previous reports [29,33-44] where single layer thicknesses of around 1 nm have been reported for GO. It should be considered that the GO thickness in AFM measurements is established by not only the van der Waals thickness of pristine graphene, which is 0.34 nm, but
9
also by functional groups above and below the graphene plane and the interaction between GO and
the
substrate.
Thus,
the
Figure 1. Height profile of a single GO sheet mesured by AFM across position 1. An average thickness of about 0.8 nm was detected from about 20 measured sheets. thinner thickness is attributed to the stronger interaction of GO with the ITO substrate used in this AFM measurement compared with silicon substrates in other reports. SEM investigation reveals that individual exfoliated GO platelets can be deposited onto the ITO substrate out of an aqueous solution (Fig. 2a). The size distribution of GO sheets varies from several hundreds of nanometers to less than 10 µm. UV-Vis spectra of an aqueous GO dispersion show a characteristic absorption peak at 231 nm which is consistent with previous reports [38, 42,44]. Figure 2b shows a red-shift in the UV-Vis absorption signal from 231 nm to 271 nm for GO and TrGO, respectively and the camera image inset reveals a color change from brown yellow of the GO aqueous solution to black of TrGO dispersed in DMF, which is a typical color for reduced GO. A higher reduction state leads to more red-shifted signals in the UV-Vis absorption spectrum. The absorption red-shift indicates the restoration of sp2 bonds enhancing the
10
conjugation within the graphene lattice. By this way, the electrical conductivity of rGO free standing paper is enhanced significantly from
Figure 2. (a) SEM image of GO sheets deposited onto a Si/SiO2 substrate and (b). UV-Vis absorption spectra of GO and TrGO. The GO spectrum has a typical peak at 231 nm, and the peak of TrGO red-shifts to 272 nm due to reduction of GO and a partial restoration of the π conjugated network. The inset image represents a photograph of GO in aqueous solution (yellow-brown solution) and TrGO in DMF (black-brown solution). electrically insulating GO to 5300 Sm-1 for TrGO measured by the van der Pauw method and can be controlled by adjusting the degree of the chemical reduction of TrGO. This conductivity is comparable with one of the best literature values of rGO free standing paper which is 7850 Sm-1 as reported by Moon et al. [4] where GO was reduced to rGO by HI as a strong reducing agent. GO is directly thionated by transforming hydoxyl (-OH) and carboxylic (-COOH) functional groups of GO into thiol (–SH) and thiocarboxylic (-COSH) or dithiocarbonxylic (–CSSH) groups, respectively [46] (schematically shown in Fig. 3) which was already demonstrated for multiwalled carbon nanotubes by ech et al. [37]. The chemical reaction between GO and
11
phosphorus pentasulfide (P4S10) takes place under refluxing condition at the boiling point of DMF (152-154°C). We suppose that the thionation reaction produces hydrosulfide byproduct (H2S) which in turn reduces
Figure 3. Schematic illustration of the thionation of GO and decoration of thionated rGO with CdSe nanoparticles. GO partially to rGO inducing a red-shift in the UV-Vis absorption. The thiol groups are also allowing the individual graphene sheets to keep distance, preventing them from aggregation and making the resulting thionated rGO well dispersible in various solvents. For the formation of silver nanoparticle decorated graphene, TrGO and silver nanoparticles were both dispersed in dimethylformamide (DMF) as common solvent and for the creation of CdSe NC decorated TrGO, chlorobenzen was utilized as common solvent. During simply stirring of the solutions for 30 min at 110°C, nanoparticles are self-anchoring to the TrGO nanosheets through the binding of the thiol groups to the respective nanoparticles as illustrated schematically in Figure 3. By this principle, it is possible to control the nanoparticle loading and the solubility of TrGO by tuning
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the degree of reduction via the adjustment of the temperature and reaction time ; the nanoparticles seem not to be affected by the decoration process since now change in size and shape was detected.. GO normally contains carboxylic, hydroxyl and epoxy groups and is therefore easily dissolvable in polar solvents including water and DMF, the latter one serving as a reaction medium for GO and P4S10. Based on a thionation mechanism reported by Ozturk et al. [45] and demonstrated for carbon nanotubes by ech et al. [36], we propose the thionation mechanism of GO
Figure 4. Schematic illustration of the proposed thionation mechanism of GO by P4S10 according to Ozturk et al. [45]. by P4S10 which is shown in Figure 4. In solution, P4S10 can be dissociated into P2S5 subunits with a pronounced polar character of the Pδ+ – Sδ- bond, which then alternatively binds to Cδ+ = Oδ- or
13
O δ- – H
δ+
groups of GO. Nucleophilic Sδ- will attack Cδ+ and electrophilic Pδ+ will, in contrast,
bind to Oδ- forming a four ring transition state, leading to a replacement of oxygen atoms from GO by sulphur atoms to form C = S and S–H groups in TrGO. During the thionation, GO is simultaneously partially reduced to rGO by hydrosulfide (H2S), a byproduct of this thionation reaction which can be detected by its characteristic smell. The resulting TrGO is soluble in DMF which prevents the aggregation of graphene sheets. Furtermore, the obtained TrGO is dispersible in various solvents such as chlobenzene or chloroform, depending on the degree of GO reduction. Controlling the solubility is very valuable for future applications, as well as for the formation of TrGO-nanoparticle hybrid materials. The presence of sulfur containing chemical groups in TrGO is confirmed by XPS and FTIR investigations. XPS spectra of pristine GO and of TrGO are shown in Figure 5a (b, c and d are high-resolution measurements). The black line shown in Fig. 5 c indicates a compound peak at a binding energy of 163.6 eV deriving from 3 peaks of sulfur 2p3/2 that is consistent with previous
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Figure 5. XPS spectra of TrGO and GO: (a). The whole spectrum with a carbon peak at 289 eV and oxygen peak at 535 eV; (b). A high resolution spectra showing a carbon 1s peak at 285 eV 2 for sp hybridized electron of C-C bond, as well as additional compound peak at 289 eV for C-
O(O) and C=O bonds and 286.5 eV for C-O bonds ; (c). The XPS spectra of TrGO and GO around sulfur 2p peak at 163.6 eV and silicon peak at 149 eV and (d). The XPS spectra of TrGO and GO around Sulfur 2s peak at 230 eV. Reports [46,47]. The intrinsic Si peak deriving from the Si-substrate at 151 eV [48] exhibits the accuracy of the measurement. Moreover, a sulfur 2s peak at 230 eV (Fig. 5d red line) with a high intensity is detected and confirms a significant amount of sulfur present in TrGO. The respective sulfur peaks are easily observable in comparison with the spectra of GO (red lines in Fig. 5 c and
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d). Before performing XPS measurements, the TrGO samples have been extensively washed by DI-water, ethanol, and acetone to ensure that no noncovalent sulfurs from residual P4S10 and byproducts are present in the samples. The whole XPS spectra (Fig. 5a) and XPS C1s spectra (Fig. 5b) of GO and TrGO reveal a significant reduction degree of TrGO after the thionation process. While the spectrum of GO (Fig. 5b red line) has a peak intensity ratio IO1s/IC1s - between the oxygen peak (O1s) at 535 eV and carbon 1s (C1s) peak at 289 eV - of about 1.80, the IO1s/IC1s ratio in TrGO (Fig. 5a black line) is about 0.51 and therefore much smaller. This shows that a considerable amount of oxygen is removed by the reduction from GO to TrGO. XPS C1s of TrGO (Fig. 5b black line) has a sharp peak at 285 eV assigned to a C peak which is composed out of a major C-C sp2 carbon peak (284.6 eV) and three minor peaks: C–O (286.5 eV); C = O (287.8 eV); C(O)O (289.1 eV) [39]. In contrast, the C1s of GO (Fig. 5b red line) has two major peaks at 289 eV of C=O, C(O)O, and 286.5 eV of C-O and a very small peak at 284.6 representing a C-C sp2 carbon peak.
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Figure 6. FTIR spectra of TrGO (black line) with vibration peaks of: C-S stretch at 671 cm-1, CSH bending at 804 cm-1, C=S stretch at 1095 cm-1 and C-H stretch at 2576 cm-1; and GO (blue line) with epoxy (C-O-C) stretching peak at 1047 cm-1, hydroxyl (O-H) stretching peak at 1230cm-1, C=O stretching peak at 1733 cm-1, C=C sp2 stretching peak at 1625 cm-.1 The occurrence of characteristic 2p and 2s peaks of sulfur in XPS spectra proves the presence of sulfur elements in TrGO. To demonstrate that the sulfur elements in the TrGO samples are covalently bonded to carbon within graphene, FTIR spectra were taken in addition. Figure 6 is a FTIR spectrum of TrGO in parallel with that of GO. The spectrum of GO (Fig. 6. blue line) shows three strong peak signals at 1625 cm-1, 1733 cm-1 and 1047 cm-1 assigned to C=C sp2, C=O and C-O-C bond stretching vibration, respectively and a weak peak signal of O-H at 1222 cm-1 which are consistent with previous reports [38,49]. The FTIR spectrum of TrGO (Fig. 6 black line) reveals not only peaks at 1625 cm-1 and 1733 cm-1 representing C=C sp2 and C=O bond stretching vibrations, respectively, but also peaks related to thiol groups. A strong band at 1095 cm-1 can be assigned to the presence of thiocarbonyl groups (C=S) that are formed by replacing the oxygen element (O) in carbonyl (C=O) or carboxylic group (HOC=O) by sulfur (S). The hydroxyl groups (OH) are exchanged into thiol groups (S-H) proven by the presence of the sharp C-S stretching peak at 671cm-1 and the C-SH bending peak at 804 cm-1 as well as a weak S-H stretching peak at 2576 cm-1 [50]. The combination between XPS and FTIR signals fully demonstrates the thionation of GO to TrGO while noncovalent attachment of sulfur containing compounds onto graphene was excluded. The CdSe-TrGO hybrid material was fabricated by self-assembly decoration of TrGO with CdSe NCs as described in the experimental part. TrGO and CdSe NCs were coincidentally stirred in chlobenzene solvent and the CdSe NCs self-bind to thiol groups of TrGO leading to the
17
formation of the respective hybrid material. Interestingly, we find that TrGO decorated with CdSe NCs becomes well soluble in chlorobenzene, resulting in a clear homogeneous dispersion.
Figure 7. (a). TEM image of TrGO decorated with CdSe NCs and (b). a zoom in TEM image of (a) and (c). TEM image of TrGO decorated with silver nanoparticles. This improved solubility is caused by the CdSe NCs attached to the TrGO. To investigate the morphology of the CdSe-TrGO hybrid material, a typical TEM image was taken, as shown in Figure 7, clearly revealing the high loading of TrGO with spherical CdSe NCs of 6 nm in diameter. The CdSe NCs were synthesized as described in the method section by using a standard procedure published in a previous report [39]. They were already utilized successfully in photovoltaic applications after integrating them into hybrid solar cells [51]. Because the TrGO-CdSe NC hybrid material is made by a mild self-assembly decoration process, the shape and size of CdSe NCs is not affected. As a result, a highly nanoparticle decorated CdSe-TrGO hybrid materials is obtained. Strong noncovalent bonds between CdSe NCs and thiol functional groups of TrGO are formed [34,35] improving a light-induced charge transfer from CdSe NCs to the graphene for potential applications such as photovoltaics. In Fig. 7b, wrinkles from graphene sheets and few places without CdSe NCs can be observed; showing that graphene in the CdSeTrGO hybrid consists out of a single layer or few layers and does not agglomerate during the
18
reduction and decoration process into macroscopic assemblies. Due to its good solubility, a homogeneous solution of the CdSe-TrGO hybrid in DMF was achieved and an UV-Vis absorption spectrum of the hzybrid material has been taken, (Fig. 8 b). The spectrum reveals a sharp signal at 250 nm deriving from TrGO and a typical peak deriving from CdSe NCs at 611 nm. We applied a similar self-assembly decoration approach to create silver nanoparticle decorated TrGO (Ag-TrGO) in order to show the generality of the method to achieve other functrional nanoparticle-TrGO hybrid materials. Ag-TrGO hybrid materials was achieved as proven by TEM imaging (Fig. 7c). The TEM image of the Ag-TrGO hybrid shows that Ag nanoparticles are present on both sides of the transparent graphene sheets and the nanoparticle loadingis rather high, proving an efficient and specific nanoparticle decoration process. This result provides additional evidence that TrGO might be used as a base material for the formation of various nanoparticle-graphene nanocomposites.
Figure 8. (a) Photoluminescence quenching of CdSe decorated on TrGO in comparison with that of CdSe without TrGO at the same QD concentration. (b) UV-Vis absorption spectrum of the CdSe-TrGO hybrid material with a TrGO peak at 250 nm and CdSe peak at 611 nm.
19
Photoluminescence (PL)-quenching experiments were performed to investigate whether charge transfer from CdSe to TrGO might occur. TrGO with suitable work function can be utilized as an acceptor [14,52] for extracting free electrons from CdSe NCs resulting in a PL-quenching of CdSe. In our experiment the photoluminescence of 1 mg CdSe in 4 ml chlorobenzene was compared to the PL of the same amount of CdSe while adding 0.1 mg of TrGO. As depicted in Fig. 8, the photoluminescence of CdSe in the hybrid material is dramatically quenched. This quenching thereby demonstrates a potential charge or energy transfer from CdSe to TrGO as reported in literature from similar hybrid systems [30,52], but here probably enhanced by the direct binding of CdSe NCs to TrGO. So, the PL-quenching is an indication of
efficient
photoelectron extraction in the hybrid material from CdSe to TrGO. This might be very promising for future application as energy harvesting material e.g. in hybrid solar cells or in transitors, photodetectors and sensors.
4. CONCLUSIONS In summary, we described a unique and easy approach for direct thionation and simultaneous reduction of GO into TrGO by refluxing GO with P4S10. The thiol containing functional groups were successfully introduced into GO via transformation of oxygen containing groups of GO into sulfur containing functional groups. In addition GO is partially reduced and restauration of the carbon sp2 framework is partially achieved. As a result, TrGO thin films exhibit good electrical conductivities and excellent solubilities in organic solvents. The degree of reduction can be tuned and adjusted by e.g. reaction time of the thionation process. TrGO was demonstrated as a good base material for obtaining highly loaded nanoparticle-graphene hybrid materials by self-assembly decoration, utilizing two examples such as CdSe-TrGO and Ag-
20
TrGO. Since nanoparticle-graphene hybrid materials become more and more important in fundamental as well as in applied research our general method leading to nanoparticle-graphene hybrid materials might stimulate research in this direction and might become important for future graphene-based applications.
AUTHOR INFORMATION Corresponding Author: Michael Krueger Postal Address: Freiburg Materials Research Center (FMF), University of Freiburg, StefanMeier-Str. 21, 79104 Freiburg, Germany, Email:
[email protected], Fax: +49 (0)761 203 4701, Tel: +49 (0)761 203 4755 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge Dr R. Thomann for TEM measurements, Ms F. Scholz for FTIR measurements and Ms C. Haas for XPS measurements. C. V. Pham thanks the Vietnam International Education Development (VIED) for financial support. M. Eck thanks the DFG Graduate School 1322 “Micro Energy Harvesting” for financial support. ABBREVIATIONS
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TrGO: thiol functionalized reduced graphene oxide; GO: graphene oxide; rGO: reduced graphene oxide; NCs: nanocrystals.
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We report the synthesis of thiol functionalized reduced graphene oxide (TrGO). TrGO was synthesized by refluxing graphene oxide with phosphorus pentasulfide. A novel base material for graphene-nanoparticle hybrid materials was created. The creation of TrGo from GO was proven by UV-Vis, XPS and FTIR spectroscopy. CdSe nanocrystal-TrGO and silver nanoparticle-TrGO hybrids were achieved.
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