Supercritical Synthesis and Characterization of GrapheneâPbS

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Supercritical Synthesis and Characterization of “Graphene-PbS Quantum Dots” Composite with Enhanced Photovoltaic Properties Ahmad Tayyebi, Mohammad Mahdi Tavakoli, Mohammad Outokesh, Azizollah Shafiekhani, and Abdolreza (Arash) Simchi Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 06 Jul 2015 Downloaded from http://pubs.acs.org on July 6, 2015

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Industrial & Engineering Chemistry Research

Supercritical Synthesis and Characterization of “Graphene-PbS Quantum Dots” Composite with Enhanced Photovoltaic Properties Ahmad Tayyebi1, Mohammad Mahdi Tavakoli2,3, Mohammad Outokesh1*, Azizollah Shafiekhani4, Abdolreza (Arash) Simchi 2 1

Department of Energy Engineering, Sharif University of Technology,Azadi Ave. P.O. Box: 113658639, Tehran, Iran. 2

3

Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran

Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 4

Departmentof Physics, AlZahra University, Tehran, 1993893973, Iran.

*Corresponding author's email address: [email protected]

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Abstract: Lead sulfide quantum dots (PbS QDs) were decorated onto graphene surface in a semi core shell structure using supercritical ethanol. Temperature of ethanol played significant role in controlling size and agglomeration of QDs, as well as the extent of reduction of graphene. Average size of the QDs was estimated by transmission electron microscopy around 3.96 nm, and by quantum models about 4.34 nm. The PbS QDs prepared at 330 oC were of high purity, and the yield was 99%. Instrumental and chemical analyses demonstrated formation of a strong bond between PbS QDs and graphene, through Pb-O-C bridge. The UV and photoluminescence measurements along with theoretical considerations revealed that integration of PbS QDs with graphene results in efficient separation of the electron-hole, and thus enhanced “photo→electric” energy conversion. This outcome was further evidenced by comparison of performance of PbS/G in a solar cell, with the performance of pristine PbS QDs.

Keywords: Graphene, PbS quantum dots, Supercritical ethanol, Photovoltaic solar cell, Electronhole separation, Brus equation.


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1. Introduction. In the last decade, graphene as an emerging carbon material has attracted much attention owning to its unique electronic–optoelectronic characteristics, and an exceptionally large surface area1, 2. Graphene Oxide (GO), is a single sheet of carbon atoms which hosts a great deal of oxygen functional groups such as epoxy (-O-) and hydroxyl (-OH) in its inner area 3, and carboxylic groups4, 5 in its edges. Presence of these functionalities makes the surface of GO an ideal template for nucleation and growth of useful nanoparticles including Au, Ag, TiO2, and Fe3O46-9. Such nanoparticle-graphene composites are particularly interesting because in addition to individual properties of the nanoparticles and graphene, they presents some additional synergistic assets10, 11. Recently, a considerable interest has been devoted to incorporation of semiconductor quantum dots (QDs) into carbon-based materials such as graphene for tuning of the band gaps of the aimed semiconductors12-14. The synergism between photo-induced charge separation of QDs and electron transport properties of graphene makes their hybrids prominent factors for developing nextgeneration high-performance optoelectronic devices15. Thus far, several metal sulfide (MS) QDs graphene hybrids such as CdS+ZnS/G 16, ZnS/G 17and CdS/G 18 have been prepared. However, the developed methods either involved complicated pathways, or consumed environmentally hazardous materials. Thus, there is still a need for a quick and green method to synthesize MS, and MS/G nano composites. Such a new method probably can be traced in the realm of supercritical technologies. Supercritical fluids (SCFs) are characterized with some unique properties such as gas like diffusivity, liquid like density, and an appreciably low viscosity 19. The organic SCFs have also high capacity for exfoliation of the lamellar structures20, and reduction of oxygen functionalities21, 22,the two properties that are essential in conversion of GO to graphene based products. Since its discovery, application of SCFs in synthesis of nanomaterials has evolved in two different pathways. In one of this directions, there is supercritical CO2 that principally has been used for fabrication of organic nanoparticles at comparatively low temperatures23. On the other side, 3 ACS Paragon Plus Environment


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supercritical water (SCW) and other high temperature solvents exist, which together have created a fertile branch of research, the so called supercritical solvothermal technology (SST)24. The first embarkation on SST is dated back to 1992 when Adschiri reported his original work on synthesis of inorganic nanoparticles by using supercritical water25. At first, SST research was focused on synthesis of technically significant nanomaterials such as metal oxides, silicates, titanate, zeolites, etc. for electronic, catalyst and other applications23, 24, 26. Systems for batch and continuous mode of preparation were developed, and the scale of work which was initially bench size reached to the plants as large as 1000 Ton /y in South Korea24. Attention, then was shifted to new topics as 1: Synthesis of composite and hybrid nanoparticles, 2: Surface modification of the inorganic nanoparticles with functionalized organic, inorganic and bio materials, 3: Application of other supercritical solvents for creating a tunable reducing, or oxidizing medium23, 24. Regarding the last topics, it is worthwhile to mention that a great number of important nanomaterials including some metal sulfides and telluride were successfully synthesized in supercritical ethanol, methanol or other organic solvents27-29. After emerging graphene technology in 21th century, advantages of SST for reducing of GO to graphene, and synthesis of graphene based materials was considered by several research groups. In this respect, the work of Nursanto et al.30, and Liu and co-workers31, are of the great importance. Recently, supercritical fluids have been considered for preparation of superfine nanoparticles and semiconductors quantum dots32. One of the most famous quantum dots for fabrication of exciting based optoelectronic devices is colloidal lead sulfide, that is particularly useful for manufacturing of light-emitting diode and colloidal solar cells33. Desirability of this material stems from its size-tunable optical properties, and a suitable band gap34, 35. As for synthesis of the PbS QDs, so far a great deal of methods has been developed, including among the others, liquid phase synthesis36, and gas phase method37,38. 4 ACS Paragon Plus Environment

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To the best of our knowledge, no investigation has been reported regarding synthesis of PbS-G Composite using supercritical ethanol. The current study is aimed at introducing such a method that utilizes GO as its raw material. After describing the synthesis process, the manuscript is to deal with physicochemical characterizations of the prepared material; but, in order to address the potentials of the PbS-G composite in fabrication of the optoelectronic devices, the last part of the study is devoted to elucidation of photoluminescence and photovoltaic behaviors of PbS-G, and their comparison with the plain PbS QDs, particularly in a real solar cell setup.

2. Experimental section 2.1. Chemicals. Natural ﬂake graphite, Sulfuric acid 98%, hydrochloric acid 37% (HCl), hydrogen peroxide 30% (H2O2), sodium nitrate, anhydrous ethanol, and potassium permanganate were purchased from Merck AG, Germany and used without purification. Lead acetate, and ethylxanthic acid were obtained from Sigma-Aldrich, USA. 2.2. Synthesis. The synthesis process of PbS QDs comprised of three successive steps as synthesis of graphene oxide, 2: Preparation of lead ethyl xanthate, and 3: Fabrication of PbS QDs. The two first steps are presented in supporting information. The next succeeding section describes the third stage. 2.3. Fabrication of PbS-G composite and PbS QDs. About 0.1 g graphene oxide (GO) was dispersed in 100 cm3 ethanol and it was sonicated by an ultrasonic bath (Elmasonic, S 30H ,Branson, CT, USA) until the solution became clear. Next, 10 cm3 of the 2.5 mol/ml of lead ethylxanthate dispersion was added into the GO solution and pH was adjusted to 5.7 by adding NaOH. Then, 10 cm3 of the foregoing mixture was poured into a 30 cm3 stainless steel autoclave, and the autoclave was inserted in a furnace to onset the PbS-G fabrication. The employed autoclave 5 ACS Paragon Plus Environment


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could endure working temperature and pressure of 500 °C and 550 atm, respectively; but in order to keep adequate safety margin, it was always loaded with 33% of its capacity (Figure S2 in supporting information). Due to the small size of the reactor, and intensive rate of heating, the warm up time for surpassing the critical point was short and around 10 min. Residence time which indicates “the heating period at the temperature set point”, was about 15 min. Two different temperatures of 140 and 330 °C were selected as the set points to cover both subcritical and supercritical states (TC=241°C). Pressure of the reactor at synthesis condition was estimated to be 7.53 and 165 bar at 140 and 330 °C, respectively. The first of these data was obtained from the saturated ethanol table, and the second was average of the values provided by “Redlich-Kwong”, and “Peng-Robinson” equations, as well as the “Lee-Kessler” generalized chart. The reaction vessel after removing from the furnace was rapidly quenched by cold water and the obtained material was separated from the solution by a high speed centrifuge. The PbS-G composite was transferred to Petri dishes and dried in argon atmosphere. Figure S2 in Supporting Information file presents flow chart of the PbS-G fabrication process. Besides the PbS-G composite, pristine PbS quantum dots were also synthesized by the same manner as PbS-G, but with eliminating GO solution from the fabrication mixture. Unless otherwise stated, the “PbS/G” symbol in the current study refers to the PbS-G composite that was synthesized at the supercritical condition (T=330oC).

2.4. Device fabrication. Schottky junction solar cells were prepared by deposition of PbS/G QDs and oleic acid-capped PbS nanocrystals onto glass/fluorine-doped tin oxide (FTO) substrates. A layer-by-layer deposition method by spin coating (2500 rpm) of the colloid was employed. The film


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thickness was several hundred nanometers. The films were dried in a nitrogen-filled glove box and gold contacts were sputtered on their top surfaces at rate of ~0.4 A°/s.

2.5. Characterization. Morphological microscopic pictures of the GO nanosheets and PbS-G composite were obtained using low and high-resolution transmission electron microscopy (TEM / HRTEM, JEOL, JEM-2100, Japan). The employed HRTEM mode also supported the selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) analysis of the samples. Evidence for the complete exfoliation of the graphene (i.e. formation of single layer product) was provided by atomic force microscopy (AFM, Park Scientific CP-Research model, VEECO), which worked in the tapping mode, with frequency of 320 kHz, and by using a 20 nm silicone tip. Samples for AFM imaging were prepared by drop casting of 0.01 mg/cm3 GO suspension onto a cleaned mica substrate. The current study exploited four different methods for identifying of the chemical and crystallographic structure of the prepared materials. The first method was Raman spectroscopy (SENTERRA, BRUKER, Germany) that was performed at room temperature using a 785 nm NdYAG laser excitation source. X-ray photoelectron spectroscopy as the second technique revealed the chemical states of the GO nanosheets prior and after their reduction to graphene. The XPS instrument was equipped with a hemispherical analyzer along with data acquisition system, and an AlKα X-ray source (hν=1486.6 eV) operating at pressure lower than 10-7 Pa. The XPS peaks were fitted by Gaussian components model after a Shirley background subtraction. Two other analysis techniques included X- ray diffractometry (Rigaku Miniflex XRD, Texas, USA) and Fourier transforms infrared spectroscopy (FTIR, Perkin-Elmer, Spectrum RX, USA). Optical characteristics of the diluted GO suspension (0.01 mg/cm3) were studied by a UVVisible spectrophotometer (Perkin Elmer UV-Vis-NIR model Lambda 950).



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The PL characteristics of the prepared materials were further investigated by using PTI QuantaMaster 50 (USA), a spectrophotometer with a Xenon lamp (75 V). The current-voltage (J-V) data were measured using Keithley 2400 source-meter (Keithley Instruments, Inc., Cleveland, OH, USA) under AM 1.5 G. The J-V sweeps were performed between -1 to +1 V, with the step rise of 0.02 V and a delay time of 200 ms at each point.

3. Results and discussions. 3.1. Morphology and crystalline structures. Figure 1a shows an AFM image of the synthesized GO, in which the thickness of the sheet corresponds to the height of the white-marked line, and it is around 1 nm. Due to the presence of oxygen containing groups on both sides of GO surface, it is broadly accepted that thickness of a single GO layer is around 0.9 nm (i.e., 0.5 nm thicker than the typical thickness of the graphene with 0.4 nm thickness)39, 40. Figure 1b shows XRD patterns of the GO that presents a peak at 2θ=11o, corresponding to 0.8 nm interlayer spacing. This is more than two folds of the 0.34 nm distance between graphite atomic layers (2θ=26o).The XRD patterns of the reduced graphene oxide (RGO) presents a broad and low intensity peak at 2θ=250, indicating the amorphous nature of RGO (Figure1b), which was likely obtained by the exfoliation action of high temperature supercritical ethanol. The XRD characteristic peaks of PbS/G composite confirm formation of the cubic crystalline PbS QDs in supercritical ethanol, which are apparently free of any PbO, and PbO2 impurities (Figure 1b). Wrapping of the PbS QDs with the graphene nanosheets (see discussion of Figure 2f) causes attenuation of intensity of the XRD peaks of the PbS/G. However, XRD pattern of PbS-Graphene composites produced at 140 0C shows a peak corresponding to PbO which indicates the presence of impurity at subcritical condition (Figure 1b).


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[Insert Figure 1 about Here] Figure S3 in supporting information exhibit the XRD patterns of two kinds of pristine PbS QDs that were prepared in subcritical and supercritical ethanol, by the same procedure that was used for synthesis of PbS-Graphene composites. Similar to the PbS/G, the PbO peak (at 2θ= 370) of pristine PbS QDs disappeared in supercritical ethanol. Despite disappearance of such large peaks, there were two extremely short peaks near the (111) and (200) peaks of pristine PbS QDs, that did not disappear even at 330 oC. These peaks that are possibly attributed to PbO2, Pb2O3, do not exist in XRD pattern of PbS/G due to the influence of one of the following factors. (1) Intensifying of the reducing effect of ethanol in presence of the graphene, (2) Wrapping of the PbS QDs with graphene, which causes the intensity of the aforementioned peaks to decrease. In order to investigate effect of temperature on the structure of GO and RGO nanosheets, the current study employed TEM images and SAED patterns, whose results are presented in Figure S4 of the supporting information. Figure S4a shows the TEM image of GO with low magnification. As can be seen, the most transparent regions indicated with arrows are likely monolayer. The SAED pattern of wrinkled GO sheets in the inset Figure S4a shows the diffraction spots and resolved ring along the [001] zone axis of GO that confirms its crystalline structure. It should be noticed in the Figure S4b that increasing of temperature up to 140 0C leads to disappearance of diffraction spots in the SAED, and keeping only un-sharp resolved rings that correspond to the mild crystalline structure of graphene. Figure S4c shows TEM image of RGO after treatment with supercritical ethanol (T=330 0C). Unresolved spots and rings in the SAED pattern of this material can be attributed to its fully amorphous nature. These results are in good agreement with XRD data of GO and RGO (Figure 1b). As for finding size of the RGO sheets, on which the PbS QDs were decorated, dynamic laser scattering (DLS) method was employed. Figure S5 in supporting information represents the size 9 ACS Paragon Plus Environment


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distribution of RGO plans based on the number and volume fractions of the scattered centers (i.e., those of the nanosheets), as well as the intensity of the scattered light. It is seen that majority of the RGO sheets possess sizes around 1400 nm. The interatomic picture of the PbS-G composite was obtained by HRTEM image. Figure 2a shows HRTEM image of PbS QDs, being decorated on the graphene nanosheets. Measured lattice spacing of d111=0.3 nm and d100 = 0.35 nm are consistent with the interplanar spacing of PbS and graphene, respectively. Figure 2b exhibited HRTEM image of single PbS QDs in which the 0.3 nm interplanar spacing corresponds to the distance between two (200) planes. Inset Figure 2b demonstrates SAED pattern of cubic single crystal of PbS QDs. [Insert Figure 2 about Here] The effect of temperature of supercritical ethanol on size and agglomeration of the PbS nanoparticles is visualized in Figures 2d, and 2c. The synthesized PbS QDs at 140 oC are agglomerated (Figure 2c). But, when temperature of ethanol surpasses the critical point and reaches 330oC, PbS QDs become homogenously distributed on the surface with no agglomeration (Figure 2d). Size of the PbS QDs was obtained by several methods, of which we describe the first one in here, and discuss the others in section 3.4. The TEM images (Figure 2d) when is analyzed by “Microstructure Measurement Software” can result in the histogram of Figure 2e, from which the average size of PbS QDs was obtained 3.96 nm. The smallness of the PbS QDs can be attributed to the following factors: 1- The enormous surface area of the GO sheets on which PbS QDs are deposited, 2-Presence of a great deal of active sites (oxygen functionalities) on the surface of GO, which act as nuclei for deposition and growth of the QDs. 3- The tuning effect of supercritical ethanol on the size of the nanoparticles which manifests itself through three different mechanisms (a): Complete exfoliation of graphene layers at 10 ACS Paragon Plus Environment

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supercritical region, (b): Increasing the rate of nucleation of PbS QDs on the surface of GO. Regarding to this effect, it is worthy to recall that in supercritical fluids, rate of nucleation of solid particles is very high41, and (c): Playing the role of a surfactant that hinders the agglomeration of the PbS QDs. The combined effect of the above three factors is simultaneous nucleation of a huge number of small QDs on the surface of GO, which are homogenously distributed on the graphene surface, and have a low aggregation tendency. Figures 2f shows that PbS QDs are wrapped by the graphene nanosheets. This figure is a clear indication of the semi core-shell structure of PbS/G composite, as well as the hexagonal lattice of the graphene. The semi core shell structure means that PbS QDs which had been deposited on the graphene sheets, were later covered by the other graphene sheets in blanket-like configuration. In order to calculate the percentage of PbS QDs that were wrapped by graphene (shown in Figure 3), the PbS/G composite was exposed to a strong acid solution. Our reason for undertaking this procedure was as follows: If some of the PbS QDs are covered by the GO, they must show some kind of resistance against chemical attacks by corrosive reagents. In order to check such supposition, we dissolved the PbS/G composite in a mixture of HCl/HNO3 which was considered as the standard solvent for PbS crystals42. After dissolution, the residual PbS QDs which were not dissolved in acid (i.e. were protected by the GO coating layer) were separated from the solution by dialysis filter. These particles then were tested by UV-visible spectrometer to examine the existence of the 920 nm PbS characteristic peak in their spectra (Figure 7a). Interestingly, such peaks existed, and by comparing of its intensity with the intensity of the intact PbS/G composite, the percentage of the covered PbS QDs was estimated, as:

 ∆APbS / G (Core (Core Shell )% =  ∆APbS / G 

Shell )

  0.015   ×100 =   ×100 = 42%  0.036   11

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(1)


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where “A” shows the absorbance of the UV-visible beams. The UV-visible spectra of PbS/Gcore shell is inserted in Figure 7a.

[Insert Figure 3 about Here] 3.3. Chemical composition & structure. In order to investigate the yield of synthesis process, final concentration of Pb2+ in ethanol after finishing of the reaction was measured by atomic absorption spectrophotometry (Table S1 in supporting information). The results indicated that with increasing of the temperature, final concentration of Pb2+ decreases so that above critical point (T=240 oC), most of the Pb2+ ions are converted to crystalline PbS QDs. As it can be noticed, there is just a small change between concentration of Pb2+ at 330 oC and 430 oC. For this reason we selected 330 oC as the appropriate fabrication temperature, to be assured about arriving at a complete reaction. Figure 4a shows the FTIR spectra of GO, RGO, PbS, and PbS-G composites. The peaks of main oxygen functional groups of GO were removed or decreased in the PbS/G after its reaction with supercritical ethanol. However, the ethanol treatment could not completely remove the characteristic peaks of 1620 cm-1 and 3450 cm-1 that are ascribed to C=C and OH bands 43. The peak at 625 cm-1 refers to PbO nanoparticles which are formed on the surface of PbS-Graphene at 140 0C. This peak along with epoxy peak entirely vanished at temperature above critical point (TC= 269 0C) (Figure 4a). Figure 4b represents Raman spectra of graphite, GO, and the pristine graphene that was obtained by reduction of GO in supercritical ethanol. Comparing to the G-band of the graphite (1578.5 cm-1), the G-band of the GO (1585 cm-1) is slightly upraised. This is result of presence of isolated double bonds (i.e. C=O) in the edges of GO planes, which resonate at frequencies higher than the graphite network 44. The similar upraising of the G-band of the RGO (1591 cm-1) compared to the GO may be attributed to the reducing effect of the supercritical ethanol on GO, which through elimination of the oxygen atoms makes the number of C=O double bands increased45. Increasing the 12 ACS Paragon Plus Environment

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ID/IG ratio in graphene compared to GO and graphite is a clear evidence for growing of the crystalline defects during reduction of GO by supercritical ethanol 46. Inset Figure 4b shows the Raman spectra of PbS/G composite. The peak at 271 cm-1 should be due to the surface phonon (SP mode), whose intensity greatly increased with decreasing of the crystal size. The weak and wide peak at 440 cm-1 and the strong and intense peak at 602 cm-1 are ascribed to 2LO and 3LO phonon modes (longitudinal optical phonons), respectively47. Strong peak at 966 cm-1 may be attributed to the photo degradation of PbS. These spectra show that PbS QDs have an octahedron structure 48. [Insert Figure 4 about Here] The XPS analysis was performed to find the compositions of the GO, RGO, and PbS/G samples. The XPS survey scans are shown in Figure 5a. As it can be seen, only two peaks at 285 and 532 eV (which are assigned to C(1s) and O(1s) core levels) are observed for both GO and RGO samples49. But, the intensity of O1(s) core level peak of RGO is notably lower, indicating deoxygenation of the GO by supercritical ethanol. For the GO, the O/C ratio was obtained ~0.41, while for the RGO, it was found ~0.17, again emphasizing on the deoxygenation process. Energy dispersive X-ray analysis (EDX) (Figure S6 in supporting information) indicates reduction of GO by supercritical ethanol, and existence of Pb and S in the PbS/G structure. To better survey the effect of the supercritical condition on the oxygen-containing groups of produced GO and PbS/G composites, a comparison was made between deconvoluted C1s XPS spectra of the GO, RGO and PbS/G composite (Figures 5b, c, and d). In those figures, binding energy of 285.0 eV is assigned to C-C and C=C bonds. The other deconvoluted peaks located at energies 286.4, 287.1, 288.5, and 289.4eV are attributed to C-OH, C-O-C, C=O, and O=C-OH bonds, respectively. Figure 5b show considerable presence of oxygen functional groups on the



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surface of GO. Most of these oxygen groups were reduced after reduction of GO by supercritical ethanol (Figure 5c). [Insert Figure 5 about Here] In Figure 5a, in addition to two GO characteristic peaks at 285 eV (C1s) and 532 eV (O1s), several other peaks represent formation of PbS QDs on the surface of graphene in supercritical medium. No discernible impurity is identified indicating that the as-prepared PbS/G composite is relatively pure (Table S2 in supporting information). Supercritical ethanol plays a dual role as reaction accelerating medium, and as an efficient reductant in the synthesis of PbS/G composite. In order to achieve a quantitative estimation about changes of the functional groups in supercritical ethanol, the peak area ratios of oxygen-containing bonds to C-C and C=C bonds were calculated and inserted in Table S3 of supporting information. It can be seen that the relative concentration of the oxygen functional groups on the surface of PbS/G was reduced about 72% compared to pristine GO. The -COOH group was also removed completely, and concentration of C-O-C and C=O bonds were reduced noticeably. Nevertheless, the C-OH group grows in PbS/G compared to the GO, due to alkoxylation property of supercritical ethanol28. As it can be seen in the Figure S7, the O1s peak in PbS/G composite is broader than that of the GO. This phenomenon is presumably attributed to the change in the number of chemical bonds of oxygen atoms50. Indeed, due to some sort of chemical interaction (bonding) between lead or sulfur and remaining oxygen atoms on the surface of PbS/G composite, the full width at half maximum (FWHM) of O1s in PbS/G (5.3 eV) increased compared to GO (3.6 eV)51. To prove the presence of new chemical bonds in PbS/G composites, O1s spectra of GO and PbS/G were fitted and shown in Figure 6a, and b. In Figure 6a, the O1s peak of GO includes two spectral peak appearing at 532 and 533.5 eV, that are attributed to C=O (carbonyl and carboxyl) and C-O (epoxy and hydroxyl), respectively51. The O1s spectra of PbS/G are deconvoluted to four peaks at 532, 533.5, 535, and 537 14 ACS Paragon Plus Environment

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eV (Figure 6b). The peaks at 532 and 533.5 eV should be referred to the remaining oxygen functionalities of PbS/G. In addition, the emerging peaks at 535.5 and 537eV are possibly attributed to bonding between PbS QDS and graphene nanosheets. In other words, remaining oxygen on the surface of reduced GO bridges the PbS and graphene sheets through the Pb-O-C bonds. The high resolution XPS peaks of Pb(4f) and S(2p) core levels of the PbS/G composite are displayed in Figure S8 of supporting information. Two peaks in the Pb(4f) and one in the S(2p) binding energy region were detected. The peak located at binding energy 139.4 eV corresponds to the Pb (4f 7/2), and the other located at 144.1 eV is assigned to the Pb(4f 5/2). Moreover, the peak appeared at binding energy of 164.6 eV is attributed to S (2p). [Insert Figure 6 about Here]

3.3.1. Search for a plausible formation mechanism. Finding an appropriate mechanism that could be accounted for simultaneous formation of PbS (from xanthate), reduction of GO by ethanol, and eventually formation of Pb-O-C or other crosslinking bonds between graphene and PbS QDs is a formidable task that is evidently out of the scope of the current study. Nevertheless, we tried to summarize the obtained evidence, and address some general guidelines that are helpful for devising such formation mechanism in future. Evidence and general guidelines. 1- Since both sulfur and lead are soft atoms, their tendency for reaction with each other is high. Thus, in supercritical ethanol, Pb prefers to form PbS rather than the PbO. Consequently, formation of PbS QDs is considered to takes place prior or simultaneous to reduction of GO.



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2- As was discussed in section 3.1. treating of PbS/G even with strong acid cannot completely remove the PbS QDs from the graphene surface. Preservation of 40% PbS QDs after such vigorous treatment is a clear sign for formation of strong bonds between PbS and graphene. 3- The broader and less intensive XPS O1s peak of PbS/G compared to GO suggests formation of new chemical bonds between oxygen and other atoms in PbS/G hybrid50. In addition, according to the previous studies, the positive shift of O1s peak can be considered as a clue for formation of the “metal-O-C” bonds such as Cu-O-C, Ag-O-C, and Zr-O-C.52-54 4- Our synthesis solvent was comprised of ethanol and water, which both affect GO and PbS at elevated temperatures. Evidence for ethanol effects are reviewed in following paragraphs: •

According to Figure S9, in the FTIR spectra of pristine PbS QDs that were subjected to supercritical ethanol, the 2900 cm-1 doublet peak corresponding to methylene (-CH2-) and methyl (-CH3) asymmetric and symmetric stretching vibrations becomes stronger. In addition, intensity of a band around 1037 cm-1 assigning to the –C-O stretching increases. Both of these evidence indicate formation of –O-CH3 or -O-C2H5 alkoxide groups on the surface of PbS QDs in supercritical alcohol, as was reported previously27, 28.

• It was demonstrated that at high temperature of supercritical alcohol, the percentage of OHin the solution increases55,

56

. Furthermore, it was shown that the reducing power of

supercritical ethanol shall be primarily attributed to electron transfer from the OH- ion to GO.30, 57 According to Zhang58, due to the electrophilic properties of the GO, such electron transfer readily takes place and its consequence is nothing but reduction of the GO.

3.4. Optical and photovoltaic characteristics. Figure 7a depicts UV-visible absorption spectra of alcoholic dispersions of PbS QDs and PbS/G with two asymmetric excitonic peaks at 920 and 960 nm. The asymmetry is possibly arisen from non-uniform size distribution of PbS QDs on the surface


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of graphene. The absorption peaks of PbS/G were fitted by Gaussian functions and the result were presented in Figure S10 in supporting information. As it can be seen, four distinct peaks at 918, 926, 954, and 963 nm are identified, which indicate existence of four different size domains for PbS QDs. The acuity of the emerging peaks indicates the narrowness of the aforementioned size domains. The average size of the PbS QDs was obtained by two different methods; first from optical band gap of PbS/G (1.3 eV), by the method of Hyun et. al.

59

, which was around 3.72 nm. Second, by

using TEM image described in section 3.1 which was 3.96 nm. In addition, in a completely different approach, the average particle radius of the PbS QDs was estimated by Brus’s equation60, which is used to evaluate the blue shift of the band gap of the quantum dot semiconductors (see appendix). The Brus’s equation makes a relation between radius of the semiconductors and the energy band gap (i.e. quantum size effect)

 h2   1 1  1.786e 2 Eg = Eg 0 +  2   * + *  −  8R   me mh  4πε 0ε QD R

(2)

Where Eg and Eg0 are the band gaps of the nanoparticles, and the bulk material, respectively; me, mh, and m0 denote the effective masses of electrons, holes and mass of electron, respectively. ε0 is the permittivity of free space, εQD is the optical dielectric constant of the QD core material, R shows radius of the particle, and “h” is the Plank’s constant. The particle size of PbS QDs was calculated 4.34 nm by Brus’s equation (The detail of calculation is embedded in the appendix).Although, Brus’s equation overestimated the size of PbS QDs in comparison to Hyun method and TEM size distribution; but the result is still in the same order. Furthermore, the calculated diameters comply well with Moreels et al. semi-empirical equation which was adapted for the especial case of the PbS QDs as follows61: Eg = Eg 0 +

1 0.0253d + 0.283d 2


(3)


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Table S4 in supporting information summarized calculated diameters PbS QDs via different methods. Evidently, a well agreement between our TEM and UV absorbance results with the theoretical values of Brus and Moreels equation is identified. [Insert Figure 7 about Here] Decreasing the QDs diameter increases the band gap due to the quantum confinement effect (Figure 8-Section (I)). The band gap of the bulk (i.e. macro sized) PbS is around 0.41 eV 62. When size of the PbS crystals is decreased to 3.96 nm, the band gap should increase to 1.5 eV according to the Jasiniak’s semi-empirical equation which was adapted to the especial case of the PbS QDs as follows62: −1.44  ECB = −4.35 + 4.1D  −0.9  EVB = −4.76 − 0.64 D

(4)

where ECB and EVB denote valence and conduction Band-Edge energies for PbS QDs, respectively. However, the optical band gap that is estimated from absorption UV peak (Figure 7a) is around 1.3 eV, or 0.2 eV lesser than the theoretical value. If, the energy level of the conduction band of PbS/G is reduced by some mechanism (Heterojunction), and the energy level of valence band increases by some other factors (Oi”+Si” defect effect), foregoing 0.2 eV difference between Jasiniak’s equation and the optical band gaps can be justified (Figure 8- Section(II)). The energy rise of the valence band may be related to existence of the defects in the deposited PbS QDs, and the remaining oxygen in the structure of PbS/G which connects PbS QDs to the graphene through Pb-OC and S-O-C bridges. Our argument here is based on the Bai et al. findings who reported that existence of surface defects in a semiconductor hybridized with graphene can increase the valence band energy through introducing of SI and OI defect state 63. The energy level of conduction band is reduced as a result of heterojunction between graphene and PbS QDs as shown in Figures 6c and 2d. 18 ACS Paragon Plus Environment

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Consequently, surface electrons of graphene transfer to PbS QDs, and reduces the energy level of their conduction band by heterojunction64. The PL spectra of the RGO, PbS QDs and PbS/G are measured and depicted in Figure 7b and Figure S11. The PL spectra of RGO represents a main peak centered at around 450 nm. This results is in a good agreement with the previously reported PL emission of RGO by Chien et al.65 that exhibited a blue line color emission. For the pristine PbS QDs, however, the PL spectra emerge in near infrared region (Figure 7b).The measurement indicated that coverage of the PbS QDs by the graphene sheets as depicted in the Figure 3, causes the PL peak of the PbS QDs to be greatly quenched (Figure 7b). The PL quenching is a sign of strong interaction between PbS QDs and the graphene. This interaction can have either a static or dynamic nature, where formation of nonluminescence complexes leads to static quenching, while dynamic quenching is arisen from energy transfer, electron transfer, or non-luminescence exciplex formation. In order to determine which of the static or dynamic mechanisms is responsible for the PL quenching, one needs to measure the fluorescence life time as a function of the GO concentration66. Conducting of such experiments was out of the scope of the current study; hence, it was not attempted. Instead, we tried to address a more essential question as follows: Although, the band gap of PbS/G is 0.2 eV less than the pure PbS QDs, why the rate of recombination of the electron-hole or in other word intensity of the PL emission of the former is slighter than the latter? The plausible answer to this question seems to be related to the electron transfer form conduction band of the PbS QDs to the excited LUMO orbit of the graphene. Such electron transfer increases the lifetime of the free electron, and reduces their recombination rate with the holes (Figure 8-section (III))67. [Insert Figure 8 about Here]



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3.5. Application. Decreasing the rate of electron-hole recombination in PbS/G, allows the electrons to flow in an external circuit, and thus enhances the efficacy of photo-electric energy conversion. In order to take prominent advantage of this efficiency enhancement, a solar cell comprising of the PbS/G composite was fabricated and tested. Before we discuss the results obtained by this test, it is worthwhile to mention that the application part of the current manuscript is rather brief, as these data are to be expanded and embedded in the next communication. The current-voltage (J-V) curve of this device under simulated 1.5 AMG sun spectra is illustrated in Figure 9. For the sake of comparison, an oleic acid-capped PbS QD solar cell was also fabricated and its data were included in the same graph. It can be noticed that in the PbS/G composite device, open voltage, current density and as a result power conversion efficiency of device were all improved. Figure S12 demonstrates a cross-sectional FESEM image of the solar cell device fabricated from the PbS-Graphene hybrids colloid by spin coating method. To study the effect of thickness of the light absorbing layer, devices with different thicknesses of 300, 400, and 600 nm were prepared. As it can be seen in Table S5, when thickness of such layer is excessively low, the absorber may not harvest the light. On the other hand, in thick layer (600 nm) recombination of electron-hole reduces efficiency of the device compared to 400 nm layer. Finally, as compared with the oleic acid-capped PbS QDs, solar cell with 400 nm (i.e. the optimum) thickness, PbS/G hybrid shows the highest performance. [Insert Figure 9 about Here] 4. Conclusions: A green, rapid, and flexible method for simultaneous synthesis and decoration of PbS QDs onto surface of graphene was reported. Supercritical ethanol in whose medium such fabrication process was conducted, besides the role of the reaction medium, acted as a reducing 20 ACS Paragon Plus Environment

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agent for reduction of GO to graphene, and a surfactant for lowering of the size of the PbS QDs. Results showed that size and distribution of the PbS QDs, and consequently most of its physiochemical characteristics can be tuned by changing the temperature of the reaction medium. Integration of the PbS QDs with graphene brings about two significant outcomes:1- Graphene acts as a substrate for nucleation and growth of the PbS QDs, and at the same time hinders their aggregation. In the absence of such substrate, PbS QDs tend to agglomerate and produce larger particles. 2: The electron–hole pairs in the excited PbS QDs could be efficiently separated through the transport of electrons from the PbS QDs to the graphene. In the whole, existence of graphene improves the optoelectronic characteristics of the PbS QDs, and the prepared PbS/G composites shows promising photovoltaic properties. Of the greatest importance among these properties, is high power conversion efficiency ratio for application in the solar cells. The current study primarily dealt with synthesis and characterization of the PbS/G composite. More information about photovoltaic behavior of the PbS/G solar cell is to be included in the next report.

Notes: The authors declare no competing financial interest. Acknowledgments: The authors would like to expresses their gratitude to Dr. Sodeh Sadjadi for her appreciable discussion on the mechanisms of formation. They are also thankful to Department of Energy Engineering of Sharif University of Technology for financial supports of this project. *Supporting Information Available file is available: Synthesis of graphene oxide and lead ethyl xanthate, TEM images, EDX, XPS, PL of RGO and FE-SEM of device and tables. This material is available free of charge via the Internet at http://pubs.acs.org.



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Figure 1: (a) Typical AFM image of GO sheet on mica and its corresponding height analysis, and (b) XRD patterns of as-prepared materials.

Figure 2: HRTEM images of (a) PbS/G composite; (b) PbS QDs, inset corresponding SAED pattern. The lattice spacing of the PbS QDs crystal in (b) is measured 0.3 nm corresponding to Pb(111) crystal plane, (c) PbS-G composites synthesized at subcritical ethanol (T=140oC); (d) Low magnification TEM image of PbS QDs; (e) Size distribution of PbS QDs related to Figure 2d , and (f) HRTEM image of PbS/G, Hexagonal lattice of graphene indicates by red color.

Figure 3: Schematic illustration of surface passivation of PbS QDs by graphene sheets.

Figure 4: (a) FTIR spectra of synthesized products, (b) Raman spectra of graphite, GO, and RGO, inset Figure Raman spectra of PbS/G composite.

Figure 5: (a) The full scan XPS survey of GO, RGO and PbS/G, and curve fitting of C1s core level of (b) GO (c) RGO, and (d) PbS/G.

Figure 6: Curve fitting of (a) O1s spectra of GO; and (b) O 1s spectra of PbS/G composite synthesized at supercritical ethanol condition ( T=330o C).

Figure 7: (a) UV-Visible and (b) PL spectra of PbS QDs and PbS/G composite.

Figure 8: Schematic drawing demonstrating the size-dependent band-edge shift and PL quenching mechanism.

Figure 9: Photovoltaic measurement under 1.5 AMG for the prepared Schottky devices.


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Figure 1: (a) Typical AFM image of GO sheet on mica and its corresponding height analysis, and (b) XRD patterns of as-prepared materials.


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Figure 2: HRTEM images of (a) PbS/G composite; (b) PbS QDs, inset corresponding SAED pattern. The lattice spacing of the PbS QDs crystal in (b) is measured 0.3 nm corresponding to Pb(111) crystal plane, (c) PbS-G composites synthesized at subcritical ethanol (T=140oC); (d) Low magnification TEM image of PbS QDs; (e) Size distribution of PbS QDs related to Figure 2d , and (f) HRTEM image of PbS/G, Hexagonal lattice of graphene indicates by red color. 31 ACS Paragon Plus Environment


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Figure 3: Schematic illustration of surface passivation of PbS QDs by graphene sheets.


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Figure 4: (a) FTIR spectra of synthesized products, (b) Raman spectra of graphite, GO, and RGO, inset figure shows Raman spectra of PbS/G composite.



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Figure 5: (a) The full scan XPS survey of GO, RGO and PbS/G; and curve fitting of C1s core level of (b) GO (c) RGO, and (d) PbS/G.


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Figure 6: Curve fitting of (a) O1s spectra of GO; and (b) O 1s spectra of PbS/G composite synthesized at supercritical ethanol condition ( T=330o C).



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(a)

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(b)

Figure 7: (a) UV-Visible and (b) PL spectra of PbS QDs and PbS/G composite.


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Figure 8: Schematic drawing demonstrating the size-dependent band-edge shift and PL quenching mechanism.



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Figure 9: Photovoltaic measurement under 1.5 AMG for the prepared Schottky devices.


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Supercritical Synthesis and Characterization of GrapheneâPbS

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