Hierarchical Fe3O4-reduced graphene oxide nanocomposite grown on NaCl crystals for triiodide reduction in dye-sensitized solar cells

Cost-effective reduced graphene oxide sheets decorated with magnetite (Fe3O4) nanoparticles (Fe3O4-rGO) are successfully fabricated via a chemical vapor deposition (CVD) technique using iron (III) nitrate as an iron precursor, with glucose and CH4 as carbon sources, and NaCl as a supporting material. TEM analysis and Raman spectroscopy reveal hierarchical nanostructures of reduced graphene oxide (rGO) decorated with Fe3O4 nanoparticles. Fe K-edge x-ray absorption near edge structure (XANES) spectra confirm that the nanoparticles are Fe3O4 with a slight shift of the pre-edge peak position toward higher energy suggesting that the fabricated Fe3O4 nanoparticles have a higher average oxidation state than that of a standard Fe3O4 compound. The hierarchical Fe3O4-rGO is found to exhibit an excellent catalytic activity toward the reduction of triiodide to iodide in a dye-sensitized solar cell (DSSC) and can deliver a solar cell efficiency of 6.65%, which is superior to a Pt-based DSSC (6.37%).

Many studies of graphene-based Fe 3 O 4 nanocomposites have been previously reported. Most of them used reduced graphene oxide-Fe 3 O 4 (rGO-Fe 3 O 4 ) that exhibited outstanding electrochemical properties and were employed as supercapacitor electrode materials [23][24][25] . These imply the feasibility of the composites to promote electrocatalytic activities toward triiodide reduction in the case of DSSCs. Recently, highly dispersed Fe 3 O 4 nanoparticles on reduced graphene oxide (rGO) have shown superior catalytic activity in reducing triiodide to iodide in DSSCs. This resulted in the highest ever reported efficiency of 9% (Pt ~ 9.46%) 26 .
rGO is a chemically derived graphene obtained from reducing graphene oxide (GO). It is considered a cost-effective graphene-based material that offers many advantages including good dispersibility in water enabling solution processability and versatile properties via specialized chemical functionalization 27 . Fe 3 O 4 -rGO composite materials were fabricated using various methods and typically a GO solution was employed as a precursor, which was generally prepared by a modified Hummers' method 24,25,28-30 . In this work, an excellent counter electrode material made of a Fe 3 O 4 -rGO nanocomposite was fabricated via a CVD based technique using glucose and CH 4 as carbon sources and NaCl crystal as a supporting material. The as-synthesized Fe 3 O 4 -GO nanocomposites were achieved by dissolving the CVD product in water to remove NaCl crystals. It was then thermally reduced to form Fe 3 O 4 -rGO nanocomposites. Unlike previous fabrication methods of rGO composites, GO precursor is not required. The obtained Fe 3 O 4 -rGO exhibited excellent electrocatalytic activity and power conversion efficiency that was superior to the Pt DSSC. The fabrication and characterization of the Fe 3 O 4 -rGO nanocomposite are presented in this paper. The photovoltaic performance and electrochemical properties of the Fe 3 O 4 -rGO DSSC were investigated.

Results and Discussion
Fabrication of Fe 3 o 4 -rGO nanocomposite. The Fe 3 O 4 -rGO nanocomposite was synthesized by a CVD technique as described by the schematic diagram in Fig. 1(a). The CVD system consists of a tube furnace with a quartz tube of 2 cm diameter and 100 cm long, vacuum pump, and flow system for CH 4 and Ar buffer gas. This CVD system was employed to prepare all the samples in this work. The precursors of the CVD reaction were prepared by dissolving Fe(NO 3 ) 3 ·9H 2 O (0.50 g), glucose (0.50 g) and NaCl (9 g) in 10 ml DI water. The mixture was dried in an oven at 80 °C for 24 h. Then the products were ground to obtain very fine powders that were used as precursors for the CVD process (step i). After placing an alumina combustion boat containing the precursor powders in a tube furnace, the system was purged with Ar for 30 min before the furnace was heated. Then the furnace temperature was raised from room temperature to 800 °C with a flow of Ar to maintain the constant pressure of 1 torr. When the temperature reached 800 °C, Ar gas was stopped. The furnace was then maintained at this reaction temperature with a flow of CH 4 at constant flow rate of 2 sccm for 30 min (step ii). After the completion of reaction, the flow of CH 4 was stopped and the furnace was cooled down to room temperature under the same Ar flow condition. This sample was referred to as "FGC", where F, G, and C represent Fe(NO 3 ) 3 ·9H 2 O, glucose and CH 4 , respectively.
Additionally, other composites were synthesized using the precursors prepared from the following mixtures. These include (i) glucose and NaCl with CH 4 (without Fe(NO 3 ) 3 ·9H 2 O) referred to as "GC", (ii) Fe(NO 3 ) 3 ·9H 2 O, glucose and NaCl (without CH 4 ) called as "FG", (iii) Fe(NO 3 ) 3 ·9H 2 O, NaCl and CH 4 (without glucose) referred to as "FC", and, (iv) Fe(NO 3 ) 3 ·9H 2 O and NaCl (without glucose and CH 4 ) called as "F". After the products were cooled to room temperature, they were washed with DI water for several times to remove NaCl (step iii). The products then were centrifuged and dried at 80 °C to obtain the nanocomposite powders (step iv).
Nanostructural characterizations. The SEM images of the NaCl-Glucose-Fe precursor powders, the as-synthesized powder (CVD resultant powders) before and after removing NaCl are shown in Fig. 1(b-d), respectively.
The TEM images of the annealed FGC, FG and GC samples are presented in Fig. 2. Sheet structures of carbon films decorated with nanoparticles were observed in FGC and FG samples, as shown in Fig. 2(a,b), respectively. Nanoparticles with sizes ranging from 10-30 nm in the FGC sample and those with a slightly larger size distribution (from 10-50 nm) in the FG sample were Fe 3 O 4 as indexed from the SAED patterns in Fig. 2(d,e). The measured d-spacings from the patterns correspond to (111), (220), (311), (400), (422), and (511) planes of a cubic  The observed sheet structures in FGC, FG and GC samples were amorphous carbon since there was no diffraction from a graphite structure detected in the SAED pattern ( Fig. 2(d-f)). To confirm the crystal structure observed in TEM, XRD analysis of the FGC, FG and GC samples was carried out which showed a consistent result with the TEM as presented in Fig. S1 of the supplement information. The FGC features are distinct in the FG sample in that thin carbon films in the FGC sample had hierarchical structures, whereas those found in the FG sample were relatively flat. This difference in carbon film morphology can be ascribed to the use of CH 4 for the CVD reaction in the FGC sample. In the case of the GC sample, which was prepared to investigate the effects of Fe in the composite, carbon films similar to those in the FG sample were observed as shown in Fig. 2(c). For the FC and F samples, Fe 3 O 4 nanoparticles were detected with relatively large particle sizes of 50-80 nm, as shown in Fig. S2 of the supplementary information.
The role of NaCl crystals on the fabrication of the nanocomposites was also investigated by using the precursor powers (only glucose powders and a mixture of glucose and FeNO 3 ) prepared without NaCl crystals for CVD reaction. It was found that without NaCl supporting material the nanocomposite was not deposited; the decomposed carbon molecules were carried away by pressure gradient. After several attempts, we failed to produce the sample without the presence of NaCl supporting material and hence the results of those samples were not included in this report.
XANES spectroscopy is a powerful technique to investigate the local environment of the Fe atom. Fe K-edge XANES spectra of the FGC and FG samples, as well as the standard samples of FeO, Fe 2 O 3 , and Fe 3 O 4 are shown in Fig. 3(a). The XANES spectra of the FGC and FG samples had very similar shapes to that of Fe 3 O 4 standard. The pre-edge features of the K-edge of transition metal compounds have been found to be affected by the oxidation state and coordination environment of the atom of interest [31][32][33] , which can be determined from the pre-edge position and the height of the peak 34 . The pre-edge position can thus be used to probe the average Fe-redox state 31 . The pre-edge peak at ~7114 eV is related to the transition from 1s to 3d 33 . Its shift toward a higher energy indicates an increased oxidation state 34 . The pre-edge peak of the FGC sample was at a higher energy than that of the Fe 3 O 4 standard, but lower than that of the FG sample, as shown in the first derivative intensity plot of Fig. 3(b). The shift toward a higher energy in the FG sample suggests a larger numbers of Fe 3+ ions present in the sample than those of the FGC sample and the Fe 3 O 4 standard. Fe 3 O 4 has a cubic inverse spinel structure, where Fe 3+ ions occupy tetrahedral sites and equal numbers of Fe 3+ and Fe 2+ ions occupy the octahedral sites. Its pre-edge peak intensity is approximately a weighted average of tetrahedral and octahedral intensities 35 . Since the nanoparticle size controls the local order, the pre-edge intensity can also be used to determine the crystallite size of the sample 32,34 . A lower pre-edge intensity indicates a smaller average particle size of the Fe 3 O 4 in the FGC sample than those in the reference and the FG samples, respectively. This is consistent with the TEM results shown in Fig. 2. Raman spectroscopy is an excellent technique for characterization of carbon nanomaterials. The Raman spectra in Fig. 4 are from the as-synthesized and as-annealed (at 480 °C for 1 h) FGC, FG and GC samples are denoted as "unannealed" and "annealed", respectively ( Fig. 4(a-c)). Two characteristic peaks of GO are visible in all samples at wavenumbers of ~1,340 and ~1,580 cm −1 , which are defined as the D and G peaks, respectively. The D peak originates from out of plane vibrations due to the presence of structural disorders (structural defects, edge effects, and dangling sp 2 carbon bonds) whereas the G peak arises from the in-plane vibrations of sp 2 carbon atoms, which are common in graphitic materials 36 . The higher disorder in graphitic plane leads to a broader G band and a broad D band with higher relative intensity compared to that of the G band 37 . The broad D and G bands can be deconvoluted into four components: D*, D, D** and G centered at 1,200, 1,340, 1,530, and 1,585 cm −1 , respectively. (The deconvoluted Raman spectra's details are shown in Table S1 in the Supplementary Information.) D* and D** are, respectively, the sum and difference of carbon double bond stretching and hydrogen-carbon wagging modes, suggesting that the structures contained a reasonable number of defects 38,39 . Additionally, another signature band of graphitic sp 2 material was observed at 2700-2800 cm −1 , called the 2D-band. The intensity ratio; I D /I G , can be used as a measure of defect density which can indicate the quality of GO 39 . The lower the I D /I G ratio the higher quality of GO 40 . The rGO is therefore expected to have lower I D /I G since the reduction process could remove the oxygen functional groups from GO and the repair of defects by recovery of hexagonal network 40 . Therefore, the electrical conductivity can be improved. The I D /I G ratios of all the samples were found to decrease after annealing, i.e., from 1.96 to 1.48 in FGC, from 1.74 to 1.55 in FG and from 1.82 to 1.38 in GC samples. This suggests that upon annealing a considerable number of defects such as oxygen functional groups were removed from the GO sheets, which then became reduced GO (rGO) 37 .
Moreover, the sharper G peaks in all samples indicate the formation of sp 2 hexagonal networks due to the self-recovery of carbon atoms 41 . It is notable that the 2D band became small modulated bumps that appeared in all the as-annealed samples, suggesting the samples formed wrinkles or corrugated structures which could be the consequence of the reduction process 38 . Post-annealing is, therefore, regarded as a crucial reduction method for removing defects and facilitating the restoration of sp 2 carbon networks.
From these results, we propose a growth mechanism of the Fe 3 O 4 -rGO in the FGC sample as shown in the schematic presentation of Fig. 5. During the CVD process, glucose (C 6 H 12 O 6 ) is primarily reduced (oxygen and hydrogen atoms are removed) and this carbon source would form a thin carbon layer covering the NaCl crystal surface. NaCl crystals act as seeding templates for carbon (reduced glucose) to transform it into a hexagonal GO network at an elevated temperature (800 °C). Simultaneously, nucleation of Fe 3 O 4 on the GO network took place. The formation of GO on both NaCl and Fe 3 O 4 is possible due to the d-spacing matches of the three materials as listed in Table 1. A hexagonal network of carbon atoms can form on the {100} planes of NaCl as the d-spacing matches between the (100) planes of graphite and the (220) planes of NaCl, as well as those of the (400) planes of Fe 3 O 4 . In the presence of CH 4 , another carbon source, the disassociated carbon atoms continue to grow from the GO sheet edges or nucleate on Fe 3 O 4 nanoparticles generating a hierarchical-like structure as shown in Fig. 2(a). This was detected in the FGC sample but not in the GC sample. Thus, it can be deduced that Fe 3 O 4 nanoparticles can act as another nucleation site for GO formation.
DSSCs performance. The posted annealed FGC, FG, GC, FC, F electrodes and a Pt electrode were used as counter electrodes in dye-sensitized solar cell devices. The preparation of counter electrode, working electrode, DSSC assembly and cell characteristic measurements are described in the method section. The photocurrent density-voltage (J-V) curves and photovoltaic characteristics of the DSSCs are presented and summarized in Fig. 6 and Table 2, respectively. It was found that the DSSCs with an FGC CE exhibited excellent performance, superior to the Pt CE. The highest achieved power conversion efficiency (η) from FGC DSSCs was 6.65%, which surpassed that of Pt DSSCs (6.37%). The DSSCs with GC and FG CEs also showed promising solar cell efficiencies of 5.99% and 5.41%, respectively. Alternatively, the photovoltaic efficiencies were very low in the FC and F DSSCs. The maximum power conversion efficiency of the FGC DSSC can be deduced from the highest J sc of 13.74 mA cm −2 , which was greater than that of Pt DSSC (13.16 mA cm −2 ). The V oc values of all DSSCs were comparable and in the range of 0.76-0.77 V.
The highest achieved photovoltaic performance was from the hierarchical rGO with Fe 3 O 4 nanoparticles in the FGC sample. In order to investigate the role of each component in the FGC nanocomposite, the correlation of solar cell efficiencies and microstructural characteristics of the three counter electrode materials including FGC, GC and FG samples are considered. In this respect, it is found that the rGO in the GC sample gave the efficiency of upto 5.99%, thus it can be deduced that rGO contribute mainly to the DSSC performance, since rGO is one of components in the FGC CE. The relatively high performance of the produced rGO counter electrode is due to its high electrical conductivity and high specific surface area which are good for electrocatalytic activity toward the reduction of triiodide. Our result showed a consistent trend with many previous studies on using graphene and graphene-related materials as counter electrode 7,42-48 . However, in the rGO with Fe 3 O 4 in the FG sample, the solar cell efficiency dropped to 5.41%. Please note that for the counter electrode preparation the amount of counter electrode materials were controlled by weight. Therefore, GC counter electrode would contain higher rGO content than the FG one that contained rGO plus Fe 3 O 4 nanoparticles, and hence giving rise to the higher J sc and solar cell efficiency.
Please also note that rGO sheets both in the GC and FG samples were not in hierarchical structure like those in the FGC sample. Therefore, the surplus efficiency of the FGC counter electrode is therefore attributed to the increased specific surface area of hierarchical rGO and the enhanced redox activity by Fe 3 O 4 catalyst.
EIS measurement was also carried out to investigate electrochemical properties of the FGC, GC and FG CEs using symmetrical cells (CE/electrolyte/CE). The Nyquist plots of the fabricated CEs and Pt with their equivalent circuit are shown in Fig. 7. It sees that the FGC, FG and Pt CEs exhibit single semicircle curve. The series resistance (R s ) correlates with FTO resistance and the contact resistance of CE material and FTO surface, which can be determined from the intercept on the real axis. The R s values of the FGC, FG, GC and Pt CEs were 7.22, 9.45, 8.50 and 7.23 Ω, respectively. The resistance-capacitance (RC) network of the electrode/electrolyte interface includes the charge-transfer resistance (R ct ) and the corresponding capacitance (C ct ). The lower R ct value the better charge-transfer between CE and electrolyte, and hence the more effective catalytic activity for triiodide Scientific RepoRts | (2019) 9:1494 | https://doi.org/10.1038/s41598-018-38050-z reduction. Among the three CE materials, the FGC electrode had the lowest R ct of 5.3 Ω, lower than that of Pt CE which was 5.4 Ω, and lower than those of GC and FG CEs (~6.5 and 24 Ω, respectively). The R s and R ct derived from EIS data are summarized in Table 2 and the fitted EIS curves are presented in Fig. S3 of the supplementary information.
The catalytic activities of the CE materials toward the triiodide reduction in DSSC were investigated by cyclic voltammetry (CV) to understand the variation in cell performance. The CV measurements were performed at  a scan rate of 20 mV s −1 using the FGC, GC, FG and Pt samples as working electrodes. The results are shown in Fig. 8. For the Pt CE, two pairs of redox peaks were typically visible in the CV. The left and the right pairs corresponded to the redox reactions represented in Equations (1) and (2), respectively.   The role of a CE is to catalyze the reduction of − I 3 to I − , and this corresponds to the left reduction peak. The low value of the peak to peak separation (E pp ), which is inversely correlated with the standard electrochemical rate constant of the redox reaction, and high cathodic peak current density are required for the excellent electrocatalytic performance of a CE. The GC CE had the highest cathodic peak current with a large E pp value of 680 mV, whereas the FGC and FG CEs had smaller E pp values of 324 and 274 mV, respectively. The E pp values of FGC and FG CEs were smaller than that of Pt CE which was 453 mV, indicating that the Fe 3 O 4 can provide high redox reaction rate.
The role of Fe 3 O 4 nanoparticles in counter electrode is to enhance the kinetic of the triiodide reduction process in electrolyte solution by the redox reaction in Equation (3)  Working electrode preparation. The TiO 2 anode was prepared using a previously reported screen printing method 17 . Briefly, the TiO 2 films were fabricated using commercial TiO 2 pastes, PST-18NR and PST-400C (JGC Catalysts and Chemicals Company, Japan) on FTO substrates. Then, the FTO/TiO 2 samples were annealed at 500 °C for 1 h, and treated with UV radiation for 10 min. Next, the working electrodes were immersed in a dye solution consisting of 0.3 mM cis-bis-(isothiocyanato) bis (2, 2-bipyridyl-4, 4-dicarboxylato)-ruthenium(II) -bis-tetrabutylammonium (N-719, Solaronix), for 24 h to achieve a dye-sensitized electrode.
DSSC assembly. Semi-closed DSSCs were assembled using TiO 2 coated dye-sensitizer films as the work- Film and cell characteristics. The film morphologies and crystal structures were characterized using scanning electron microscopy (SEM, LEO 1450 VP, Germany), transmission electron microscopy (TEM, FEI, TECNAI G 2 , the Netherlands), respectively. Raman spectroscopy was employed to investigate the carbon nanostructures using a triple-monochromator JOBIN YVON HORIBA T64000 spectrometer with a 532 nm laser excitation line. X-ray absorption near-edge structure (XANES) spectra for FGC and FC samples were acquired in transmission mode at beamline 1.1 W of the Synchrotron Light Research Institute (SLRI), Nakhon Ratchasima, Thailand. The photovoltaic performance of the DSSCs was measured using a solar simulator (PEC-L11, Japan) under a light intensity of 100 mW cm −2 . Their electrocatalytic activity was analyzed using a cyclic voltammogram (CV, CS150 Electrochemical Workstation, Wuhan Corrtest Instrument Co., Ltd) with a three-electrode system, i.e., an Ag/AgCl electrode as the reference electrode, Pt film as the counter electrode and the fabricated materials (FGC, GC and FG) as the working electrodes at a scan rate of 20 mV s −1 in 10 mM LiI, 1 mM I 2 , and 0.1 M LiClO 4 in an acetonitrile solution.