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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‡ †

Department of Energy Engineering, Sharif University of Technology,Azadi Avenue, P.O. Box 113658639, 14588 Tehran, Iran 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 999077, Hong Kong ∥ Department of Physics, AlZahra University, Tehran 1993893973, Iran ‡

S Supporting Information *

ABSTRACT: Lead sulfide quantum dots (PbS QDs) were decorated onto a graphene surface in a semi-core−shell structure using supercritical ethanol. The 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 to be around 3.96 nm and by quantum models to be about 4.34 nm. PbS QDs prepared at 330 °C 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 a Pb−O−C bridge. UV and photoluminescence measurements along with theoretical considerations revealed that integration of PbS QDs with graphene results in efficient separation of the electron−hole, thus enhancing 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. Supercritical fluids (SCFs) are characterized by some unique properties such as gaslike diffusivity, liquidlike density, and an appreciably low viscosity.19 Organic SCFs also have a high capacity for exfoliation of the lamellar structures20 and reduction of oxygen functionalities,21,22the 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 direction, there is supercritical CO2 that principally has been used for fabrication of organic nanoparticles at comparatively low temperatures.23 In the other direction, 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 dates back to 1992 when Adschiri reported his original work on synthesis of inorganic nanoparticles by using supercritical water.25 At first, SST research was focused on synthesis of technically significant nanomaterials such as metal oxides, silicates, titanate, zeolites, and others for electronics, catalysis, and other applications.23,24,26 Systems for batch and continuous mode of preparation were developed, and the scale of work that was initially bench-size reached production scales as large as 1000 ton/year in South Korea.24 Attention was then shifted to new topics such as (1) synthesis of composite and hybrid nanoparticles, (2) surface modification

1. INTRODUCTION In the past decade, graphene as an emerging carbon material has attracted much attention owning to its unique electronic− optoelectronic characteristics, and an exceptionally large surface area.1,2 Graphene oxide (GO), is a single sheet of carbon atoms that hosts a great deal of oxygen functional groups such as epoxy (−O−) and hydroxyl (−OH) in its inner area3 and carboxylic groups4,5 in its edges. The presence of these functionalities makes the surface of GO an ideal template for nucleation and growth of useful nanoparticles including Au, Ag, TiO2, and Fe3O4.6−9 Such nanoparticle−graphene composites are particularly interesting because in addition to individual properties of the nanoparticles and graphene they presents some additional synergistic assets.10,11 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 targeted semiconductors.12−14 The synergism between the photoinduced charge separation of QDs and the electron transport properties of graphene makes their hybrids prominent factors for developing next-generation high-performance optoelectronic devices.15 Thus far, several metal sulfide (MS) QDs−graphene hybrids such as CdS+ZnS/G,16 ZnS/ G,17and CdS/G18 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 nanocomposites. Such a new method probably can be traced in the realm of supercritical technologies. © 2015 American Chemical Society

Received: Revised: Accepted: Published: 7382

January 1, 2015 July 1, 2015 July 6, 2015 July 6, 2015 DOI: 10.1021/acs.iecr.5b00008 Ind. Eng. Chem. Res. 2015, 54, 7382−7392

Article

Industrial & Engineering Chemistry Research

Because of the small size of the reactor and intensive rate of heating, the warm up time for surpassing the critical point was short, 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). The pressure of the reactor under 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. After removing the reaction vessel from the furnace, the reaction was rapidly quenched with cold water, and the obtained material was separated from the solution by a highspeed centrifuge. The PbS/G composite was transferred to Petri dishes and dried under argon atmosphere. Figure S2 in the Supporting Information presents a flowchart of the PbS/G fabrication process. Besides the PbS/G composite, pristine PbS quantum dots were also synthesized in the same manner as PbS/G but with GO solution eliminated from the fabrication mixture. Unless otherwise stated, PbS/G in the current study refers to the PbS/G composite that was synthesized under the supercritical condition (T = 330 °C). 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 thickness was several hundred nanometers. The films were dried in a nitrogen-filled glovebox, and gold contacts were sputtered on their top surfaces at rate of ∼0.4 Å/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 a frequency of 320 kHz and used 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 carried out at room temperature using a 785 nm Nd:YAG laser excitation source. As the second technique, X-ray photoelectron spectroscopy (XPS) revealed the chemical states of the GO nanosheets prior to and after their reduction to graphene. The XPS instrument was equipped with a hemispherical analyzer along with a data acquisition system and an Al Kα X-ray source (hν = 1486.6 eV) operating at pressure lower than 10−7 Pa. The XPS peaks were fitted by Gaussian components modeled after a Shirley background subtraction. Two other analysis techniques included X- ray diffractometry (Rigaku Miniflex XRD, Texas, USA) and Fourier transform infrared spectroscopy (FTIR, PerkinElmer, Spectrum RX, USA).

of the inorganic nanoparticles with functionalized organic, inorganic, and bio materials, and (3) application of other supercritical solvents for creating a tunable reducing or oxidizing medium.23,24 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 solvents.27−29 After emerging graphene technology in the 21st century, the 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 semiconductor quantum dots.32 One of the most famous quantum dots for fabrication of exciting-based optoelectronic devices is colloidal lead sulfide (PbS), which is particularly useful for manufacturing of lightemitting diodes and colloidal solar cells.33 Desirability of this material stems from its size-tunable optical properties and a suitable band gap.34,35 As for synthesis of the PbS QDs, so far a great deal of methods have been developed, including among others, liquid-phase synthesis36 and a gas-phase method.37,38 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, we address 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 those of 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 comprises three successive steps: (1) synthesis of graphene oxide, (2) preparation of lead ethyl xanthate, and (3) fabrication of PbS QDs. The two first steps are presented in the Supporting Information. The subsequent section describes the third stage. 2.3. Fabrication of PbS/G Composite and PbS QDs. About 0.1 g of graphene oxide (GO) was dispersed in 100 cm3 of ethanol, and the mixture 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 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 start PbS/G fabrication. The employed autoclave could endure a working temperature and pressure of 500 °C and 550 atm, respectively, but in order to keep an adequate safety margin, it was always loaded at 33% of its capacity (Figure S2 in the Supporting Information). 7383

DOI: 10.1021/acs.iecr.5b00008 Ind. Eng. Chem. Res. 2015, 54, 7382−7392

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

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 °C. These peaks that are possibly attributed to PbO2 and Pb2O3 do not exist in the XRD pattern of PbS/G because of the influence of one of the following factors: (1) intensifying of the reducing effect of ethanol in the presence of the graphene and (2) wrapping of the PbS QDs with graphene, which causes the intensity of the aforementioned peaks to decrease. To investigate the 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 of Figure S4a shows the diffraction spots and the resolved ring along the [001] zone axis of GO that confirm its crystalline structure. It should be noted in Figure S4b that increasing temperature up to 140 °C leads to disappearance of diffraction spots in the SAED and keeps only unsharp resolved rings that correspond to the mild crystalline structure of graphene. Figure S4c shows the TEM image of RGO after treatment with supercritical ethanol (T = 330 °C). 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 the XRD data of GO and RGO (Figure 1b). As for finding the size of the RGO sheets on which the PbS QDs were decorated, the dynamic laser scattering (DLS) method was employed. Figure S5 in the Supporting Information represents the size distribution of RGO plans on the basis of 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 imaging. Figure 2a shows the HRTEM image of PbS QDs 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 exhibits the HRTEM image of single PbS QDs in which the 0.3 nm interplanar spacing corresponds to the distance between two (200) planes. The

Optical characteristics of the diluted GO suspension (0.01 mg/cm3) were studied with a UV−visible spectrophotometer (PerkinElmer UV−vis-NIR model Lambda 950). The PL characteristics of the prepared materials were further investigated by using a PTI QuantaMaster 50 (USA), a spectrophotometer with a xenon lamp (75 V). The current−voltage (J−V) data were measured using a Keithley 2400 source meter (Keithley Instruments, Inc., Cleveland, OH, USA) under AM 1.5 G. The J−V sweeps were carried out between −1 and +1 V, with a 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 line, around 1 nm. Because of the presence of oxygencontaining groups on both sides of GO surface, it is broadly accepted that the thickness of a single GO layer is around 0.9 nm (i.e., 0.5 nm thicker than the typical thickness of the graphene, 0.4 nm thickness).39,40 Figure 1b shows XRD patterns of the GO that present a peak at 2θ = 11°, corresponding to 0.8 nm interlayer spacing. This is more than two times the 0.34 nm distance between graphite atomic layers (2θ = 26°).The XRD patterns of the reduced graphene oxide (RGO) presents a broad and low intensity peak at 2θ = 25°, 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, the XRD pattern of PbS/G composites produced at 140 °C shows a peak corresponding to PbO, indicating the presence of impurity under subcritical condition (Figure 1b). Figure S3 in the Supporting Information exhibits 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/G composites. Similar to the PbS/G, the PbO peak (at 2θ = 37°) of pristine PbS QDs disappeared in supercritical ethanol. Despite the disappearance 7384


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

inset in Figure 2b demonstrates the SAED pattern of the cubic single crystal of PbS QDs. The effect of the temperature of supercritical ethanol on size and agglomeration of the PbS nanoparticles is visualized in Figure 2d,c. The synthesized PbS QDs at 140 °C are agglomerated (Figure 2c), but when temperature of ethanol surpasses the critical point and reaches 330 °C, PbS QDs become homogeneously distributed on the surface with no agglomeration (Figure 2d). The size of the PbS QDs was obtained by several methods, the first of which we describe here and the others we discuss in section 3.3. The TEM images (Figure 2d), when analyzed by Microstructure Measurement Software, yield the histogram of Figure 2e, from which the average size of PbS QDs, 3.96 nm, was obtained. 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) the 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, and (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 in the supercritical region, (b) increasing the rate of nucleation of PbS QDs on the surface of GO (With regard to this effect, it is worth recalling that in supercritical fluids the rate of nucleation of solid particles is very high.),41 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 homogeneously distributed on the graphene surface and have a low aggregation tendency. Figure 2f shows that PbS QDs are wrapped by the graphene nanosheets. This figure is a clear indication of the semi-core− 7385


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Industrial & Engineering Chemistry Research ⎛ ΔAPbS/G(Core Shell) ⎞ ⎟ × 100 (Core Shell)% = ⎜ ΔAPbS/G ⎠ ⎝

shell structure of PbS/G composite as well as the hexagonal lattice of the graphene. The semi-core−shell structure means that PbS QDs deposited on the graphene sheets were later covered by the other graphene sheets in a blanketlike configuration. To calculate the percentage of PbS QDs wrapped by graphene (Figure 3), the PbS/G composite was exposed to a

=

⎛ 0.015 ⎞ ⎜ ⎟ × 100 ⎝ 0.036 ⎠

= 42%

(1)

where A shows the absorbance of the UV−visible beams. The UV−visible spectra of PbS/Gcore shell is shown in Figure 7a. 3.2. Chemical Composition and Structure. To investigate the yield of the synthesis process, the final concentration of Pb2+ in ethanol after finishing of the reaction was measured by atomic absorption spectrophotometry (Table S1 in the Supporting Information). The results indicated that with increasing temperature, the final concentration of Pb2+ decreases so that above critical point (T = 240 °C) 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 and 430 °C. For this reason, we selected 330 °C as the appropriate fabrication temperature to be assured of arriving at a complete reaction. Figure 4a shows the FTIR spectra of GO, RGO, PbS, and PbS/G composites. The peaks of the main oxygen functional groups of GO were removed or decreased in PbS/G after its reaction with supercritical ethanol. However, the ethanol treatment could not completely remove the characteristic peaks of 1620 and 3450 cm−1 that are ascribed to CC and OH bands, respectively.43 The peak at 625 cm−1 refers to PbO nanoparticles that are formed on the surface of PbS/G at 140 °C. This peak, along with epoxy peak, entirely vanished at temperatures above critical point (TC = 269 °C; Figure 4a). Figure 4b represents the Raman spectra of graphite, GO, and the pristine graphene that was obtained by reduction of GO in supercritical ethanol. Compared to the G band of the graphite (1578.5 cm−1), the G band of GO (1585 cm−1) is slightly higher. This is the 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 increase of the G band of RGO (1591 cm−1) compared to that

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

strong acid solution. Our reason for undertaking this procedure was as follows: If some of the PbS QDs are covered by the GO, then they must show some kind of resistance against chemical attacks by corrosive reagents. 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 crystals.42 After dissolution, the residual PbS QDs that 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 comparison of its intensity with the intensity of the intact PbS/G composite, the percentage of the covered PbS QDs was estimated as

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. 7386


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Figure 5. (a) Full-scan XPS survey of GO, RGO, and PbS/G. Curve fitting of C 1s core level of (b) GO, (c) RGO, and (d) PbS/G.

deconvoluted peaks located at 286.4, 287.1, 288.5, and 289.4 eV are attributed to C−OH, C−O−C, CO, and OC−OH bonds, respectively. Figure 5b shows considerable presence of oxygen functional groups on the surface of GO. Most of these oxygen groups were reduced after reduction of GO by supercritical ethanol (Figure 5c). In Figure 5a, in addition to two characteristic GO peaks at 285 eV (C 1s) and 532 eV (O 1s), 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 the Supporting Information). Supercritical ethanol plays a dual role as reaction-accelerating medium and as an efficient reductant in the synthesis of PbS/G composite. 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 are shown in Table S3 of the Supporting Information. It can be seen that the relative concentration of the oxygen functional groups on the surface of PbS/G was reduced to about 72% compared to that of pristine GO. The −COOH group was also removed completely, and the concentration of C−O−C and CO bonds were reduced noticeably. Nevertheless, the C−OH group grows in PbS/G compared to GO because of the alkoxylation property of supercritical ethanol.28 As it can be seen in the Figure S7, the O 1s 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 atoms.50 Indeed, because of some sort of chemical interaction (bonding) between lead or sulfur and the remaining oxygen atoms on the surface of PbS/G composite, the full width at half-maximum (fwhm) of O 1s in PbS/G (5.3 eV) increased compared to that in GO (3.6 eV).51 To prove the presence of new chemical bonds in PbS/G composites, O 1s spectra of GO and PbS/G were fitted and are shown in Figure

of 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 increase.45 The increased ID/IG ratio in graphene compared to that of 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 because of the surface phonon (SP mode), whose intensity greatly increased with decreasing 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 (LO = longitudinal optical), respectively.47 The strong peak at 966 cm−1 may be attributed to the photodegradation of PbS. These spectra show that PbS QDs have an octahedron structure.48 The XPS analysis was carried out 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 samples,49 but the intensity of the O 1s core level peak of RGO is notably lower, indicating deoxygenation of the GO by supercritical ethanol. For GO, the O/C ratio obtained was ∼0.41, whereas for RGO, it was found to be ∼0.17, again emphasizing the deoxygenation process. Energy dispersive X-ray analysis (EDX, Figure S6 in the Supporting Information) indicates reduction of GO by supercritical ethanol and existence of Pb and S in the PbS/G structure. To survey more completely the effect of the supercritical condition on the oxygen-containing groups of produced GO and PbS/G composites, a comparison was made between deconvoluted C 1s XPS spectra of GO, RGO, and PbS/G composite (Figure 5b−d), in which a binding energy of 285.0 eV is assigned to C−C and CC bonds. The other 7387


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evidence and address some general guidelines that are helpful for devising such a formation mechanism in future. 3.2.1.1. Evidence and General Guidelines. (1) Because 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 PbO. Consequently, formation of PbS QDs is considered to take place prior to or simultaneous to reduction of GO. (2) As discussed in section 3.1, treating of PbS/G even with strong acid cannot completely remove PbS QDs from the graphene surface. Preservation of 40% PbS QDs after such vigorous treatment is a clear sign of the formation of strong bonds between PbS and graphene. (3) The broader and less intensive XPS O 1s peak of PbS/G compared to that of GO suggests the formation of new chemical bonds between oxygen and other atoms in PbS/G hybrid.50 In addition, according to the previous studies, the positive shift of O 1s peak can be considered 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 the 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 (−CH 3 ) asymmetric and symmetric stretching vibrations becomes stronger. In addition, the intensity of a band around 1037 cm−1 assigned to the −C−O stretching increases. Both of these indicate formation of −O−CH3 or −O−C2H5 alkoxide groups on the surface of PbS QDs in supercritical alcohol, as was reported previously.27,28 It was demonstrated that at high temperature of supercritical alcohol, the percentage of OH− in the solution increases.55,56 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 because of the electrophilic properties of the GO, such electron transfer readily takes place, and its consequence is nothing but reduction of the GO. 3.3. 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 from nonuniform

6a,b. In Figure 6a, the O 1s peak of GO includes two spectral peaks appearing at 532 and 533.5 eV that are attributed to C

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

O (carbonyl and carboxyl) and C−O (epoxy and hydroxyl), respectively.51 The O 1s spectra of PbS/G are deconvoluted to four peaks at 532, 533.5, 535, and 537 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 537 eV are possibly attributed to bonding between PbS QDs and graphene nanosheets. In other words, the remaining oxygen on the surface of reduced GO bridges the PbS and graphene sheets through 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 the 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 of 139.4 eV corresponds to the Pb 4f7/2, and the other, located at 144.1 eV, is assigned to the Pb 4f5/2. Moreover, the peak appearing at binding energy of 164.6 eV is attributed to S 2p. 3.2.1. Search for a Plausible Formation Mechanism. Finding an appropriate mechanism that could account for simultaneous formation of PbS (from xanthate), reduction of GO by ethanol, and eventually formation of Pb−O−C or other cross-linking 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

Figure 7. (a) UV−visible and (b) PL spectra of PbS QDs and PbS/G composite. 7388


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

the theoretical values of Brus and Moreels equation is identified. Decreasing the QDs diameter increases the band gap because of the quantum confinement effect (Figure 8, section I). The band gap of the bulk (i.e., macrosized) 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 semiempirical equation that was adapted to the special case of the PbS QDs as follows:62

size distribution of PbS QDs on the surface of graphene. The absorption peaks of PbS/G were fitted by Gaussian functions, and the result were presented in Figure S10 in the Supporting Information. As 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: (1) from the optical band gap of PbS/G (1.3 eV) by the method of Hyun et al.,59 which was around 3.72 nm, and (2) 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 equation,60 which is used to evaluate the blueshift of the band gap of the QD semiconductors (see the Supporting Information). 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 = Eg0 + ⎜ 2 ⎟⎢ + mh* ⎦ 4πε0εQDR ⎝ 8R ⎠⎣ me*

−1.44 ⎧ ⎪ ECB = − 4.35 + 4.1D ⎨ ⎪ −0.9 ⎩ E VB = −4.76 − 0.64D

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 (O″I +S″I defect effect), then foregoing a 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 that connects PbS QDs to the graphene through Pb−O−C and S−O−C bridges. Our argument here is based on the Bai et al. findings, which reported that the existence of surface defects in a semiconductor hybridized with graphene can increase the valence band energy through the introduction of SI and OI defect states.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. Consequently, surface electrons of graphene transfer to PbS QDs and reduce the energy level of their conduction band by heterojunction.64 The PL spectra of the RGO, PbS QDs, and PbS/G are measured and depicted in Figures 7b and S11. The PL spectra of RGO represents a main peak centered at around 450 nm. This results is in 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 the near-infrared region (Figure 7b). The measurement indicated that coverage of the PbS QDs by the

(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 is the radius of the particle; and h is Planck’s constant. The particle size of PbS QDs was determined to be 4.34 nm by Brus’s equation. (The details of the calculation is embedded in the Supporting Information.) Although Brus’s equation overestimated the size of PbS QDs in comparison to the Hyun and TEM size distribution methods, the result is still in the same order. Furthermore, the calculated diameters comply well with Moreels et al. semiempirical equation, which was adapted for the especial case of the PbS QDs, and is represented as61 Eg = Eg0 +

1 0.0253d 2 + 0.283d

(4)

(3)

Table S4 in the Supporting Information summarized calculated diameters of PbS QDs via different methods. Evidently, a good agreement between our TEM and UV absorbance results with 7389


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the light-absorbing layer, devices with different thicknesses of 300, 400, and 600 nm were prepared. As can be seen in Table S5, when the thickness of such a layer is excessively low, the absorber may not harvest the light. In contrast, in a thick layer (600 nm) recombination of electron−hole reduces efficiency of the device compared to that of the 400 nm layer. Finally, as compared with the oleic acid capped PbS QDs, a solar cell with 400 nm (i.e., the optimum) thickness PbS/G hybrid shows the highest performance.

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, whereas dynamic quenching is arisen from energy transfer, electron transfer, or nonluminescence exciplex formation. To determine which of the static or dynamic mechanisms is responsible for the PL quenching, one needs to measure the fluorescence lifetime as a function of the GO concentration.66 Conducting 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: Although, the band gap of PbS/G is 0.2 eV less than that of pure PbS QDs, why is the rate of recombination of the electron−hole, or in other words the intensity of the PL emission of the former, slightly less than that of 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 3.4. Application. Decreasing the rate of electron−hole recombination in PbS/G allows the electrons to flow in an external circuit, thus enhancing the efficacy of photoelectric energy conversion. To take prominent advantage of this efficiency enhancement, a solar cell composed 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 because these data are to be expanded upon 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

4. CONCLUSIONS A green, rapid, and flexible method for simultaneous synthesis and decoration of PbS QDs onto the surface of graphene was reported. Supercritical ethanol, in whose medium such fabrication process was conducted, besides having a role as the reaction medium, acted as a reducing agent for reduction of GO to graphene and a surfactant for lowering 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. On the whole, presence of graphene improves the optoelectronic characteristics of the PbS QDs, and the prepared PbS/G composites show promising photovoltaic properties. Of the greatest importance among these properties is the 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.

■

ASSOCIATED CONTENT

* Supporting Information S

Synthesis of graphene oxide and lead ethyl xanthate, TEM images, EDX, XPS, and PL of RGO, and FE-SEM of device and tables. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.iecr.5b00008.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

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

ACKNOWLEDGMENTS We expresses their gratitude to Dr. Sodeh Sadjadi for her appreciable discussion on the mechanisms of formation. We are also thankful to Department of Energy Engineering of Sharif University of Technology for financial support of this project.

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 seen that in the PbS/G composite device, open voltage, current density, and, as a result, power conversion efficiency of the device were all improved. Figure S12 demonstrates a cross-sectional FESEM image of the solar cell device fabricated from the PbS/G hybrids colloid by spin coating method. To study the effect of the thickness of

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