Fabrication of free-standing graphene paper decorated with flower-like PbSe_0.5S_0.5 structures

Abstract

We report a simple preparation procedure for free-standing and flexible rGO/PbSe_0.5S_0.5 paper. The fabrication process includes two steps: first, graphene oxide (GO) paper was obtained by vacuum-filtration of a GO dispersion through a membrane and then the GO paper was transformed to rGO paper by chemical reduction with HI. The second step includes formation of a rGO/PbSe_0.5S_0.5 hybrid paper electrode by one-pot electrodeposition of PbSe_0.5S_0.5 nanoparticles onto an rGO paper electrode from a solution containing saturated PbS and PbSe. The electrodeposition process is based on underpotential deposition of Pb²⁺, Se⁴⁺, and S²⁻ electroactive species which are produced by the dissolution of both PbS and PbSe in the solution at 70 °C. For this purpose, the electrodeposition step was initially tested on Au(111) electrodes and then applied on an rGO paper electrode. Suitable constant electrodeposition potentials were determined as −300 and −450 mV on Au(111) and rGO paper electrodes, respectively. Characterization of the prepared samples has been performed using scanning tunneling microscopy, scanning electron microscopy, powder X-ray diffraction, electron dispersive spectroscopy, Fourier transform-infrared spectroscopy, UV-vis-NIR, chronoamperometry, and cyclic voltammetry techniques.

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1. Introduction

Lead chalcogenides e.g. PbS and PbSe have very narrow bandgap values of 0.41 and 0.28 eV and large bulk exciton Bohr radii of 18 and 26 nm, respectively. Due to these optical properties, they are preferred for technological applications, such as infrared detectors,¹ solar cells,² Pb²⁺ ion selective sensors,³ and thermoelectric devices.⁴ In order to be utilized in the above technologies, the optical, electrical, physical, and chemical properties of lead chalcogenides are very important and they are required to have very high crystallinity. Metal chalcogenides can be prepared by using different techniques, for instance successive ionic layer adsorption and reaction,^5,6 solvothermal methods,⁷ atomic layer epitaxy,⁸ chemical bath deposition,⁹ molecular beam epitaxy,¹⁰ microwave-assisted preparation,¹¹ and electrodeposition.^12–22 Among these techniques, the electrodeposition technique has some advantages. It is a simple, quick, non-destructive, environmentally friendly, and economical technique. The electrodeposition technique is operated under ambient conditions and it provides thickness control of the deposited films by optimizing the electrodeposition potential and time. Optical properties of semiconductive materials can be tuned by adjusting their sizes at the nanometer scale as a result of the quantum-confinement effect.^15–18 Ternary lead chalcogenides have an extra advantage due to the ability to tune their optical properties by changing their chemical composition.^23–25 For an example, the bandgap value of bulk PbSe_xS_(1−x) can be varied between the bulk bandgap values of PbS and PbSe (0.41 and 0.28 eV), by altering only the x value in the chemical composition.^23,24

In recent years, free-standing graphene papers (GPs) have been fabricated due to their high mechanical flexibility, electrochemical performance and chemical stability in various potential applications as membranes,²⁶ fuel cells,²⁷ Li-ion batteries,²⁸ electrochemical sensors,^29–31 electrochemical capacitors,³² and energy storage devices.³³ Doping GPs with different amounts of metal or semiconductor nanoparticles can provide a tuning opportunity of the physical properties of GPs as desired.^27–33 GPs can be made of either graphene oxide (GO) or reduced graphene oxide (rGO) by a simple vacuum filtration of a graphene-containing suspension.^27,28,33 To date, semiconductive thin films have been prepared on metal electrodes. Decorating free-standing GPs with semiconductive thin films can provide portable electronics and lab-on-a-chip applications with a flexible platform. To our best knowledge, a PbSe_xS_(1−x) thin film structure on rGO paper has not been prepared using electrochemical techniques until now.

In this paper, we have prepared for the first time a flexible and free-standing rGO/PbSe_0.5S_0.5 hybrid paper by using the following steps: vacuum-filtration of a GO suspension through a membrane, followed by peeling it off, reduction of the free-standing GO paper, and then electrodeposition of PbSe_0.5S_0.5 on the rGO paper electrode. Before electrodeposition of PbSe_0.5S_0.5 thin films on rGO paper, PbSe_xS_(1−x) thin films (x = 0.0, 0.5, 1.0) were studied on a Au(111) single crystal electrode as a model platform in order to optimize the electrodeposition conditions. Morphological characterization of the prepared samples has been performed using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The crystal structure has been determined using a powder X-ray diffraction (XRD) system. Electron dispersive spectroscopy (EDS) and UV-vis-NIR experiments have been carried out for stoichiometry and optical characterization of the thin films, respectively.

2. Experimental section

2.1. Synthesis of GO

GO was synthesized from graphite powder (Sigma Aldrich, particle size < 150 μm) by using the modified Hummers method as originally presented by Kovtyukhova et al.³⁴ Concentrated H₂SO₄ (12.5 mL) was heated to 90 °C in a 500 mL beaker, then 2.5 g of K₂S₂O₈ and 2.5 g of P₂O₅ were added with stirring until all the reactants were completely dissolved. The reaction temperature was decreased to 80 °C. Then, 3 g of graphite powder was added slowly to the H₂SO₄ solution. This resulted in bubbling that subsided within 30 min. This reaction mixture was kept at 80 °C for 5 h. After 5 h, the heating was stopped and the mixture was diluted with 500 mL of distilled water and left overnight. The mixture was then filtered and washed to remove all traces of acid. The resulting solid was dried in air overnight.

Concentrated H₂SO₄ (115 mL) was kept in an ice bath maintained at 0 °C. The pre-oxidized graphite was then added to the acid and stirred. KMnO₄ (Sigma Aldrich, 15 g) was added slowly and allowed to dissolve. The addition was done carefully so that the temperature of the reaction mixture never went beyond 10 °C. This mixture was allowed to react at 35 °C for 2 h. After 2 h, 250 mL of distilled water was added. The water addition was done in such a way that the temperature did not exceed 50 °C. The mixture was stirred for an additional 2 h. Then, 750 mL of distilled water and 12.5 mL of 30% H₂O₂ were added. The mixture was kept undisturbed for a day and then the supernatant was decanted. The remaining solution was washed with 10% HCl solution several times, followed by washing with distilled water. The resulting solid was dried in air and diluted to make a 2% (w/w) dispersion. It was then put through dialysis for 3 weeks to remove any remaining metal contamination. Then, the graphite oxide solution after dialysis was filtered and 0.5 g of the dried graphite oxide was added to 500 mL of deionized water. The mixture was sonicated for 5 h in order to obtain GO nanosheets dispersed in water (1.0 mg mL⁻¹).

2.2. Preparation of PbSe and PbS

An aqueous 11.2 M, 50 mL NaOH solution and 50 mL of distilled water were separately prepared for the precipitation of PbSe and PbS, respectively. Nitrogen was continuously purged from these solutions and the temperature of these solutions was kept at 90 °C by a thermostat system. SeO₂ (2.5 mmol) and Na₂S·9H₂O (2.5 mmol) were added to the 11.2 M, 50 mL NaOH solution and 50 mL of distilled water, while stirring at 1250 rpm to achieve Se⁻² and S⁻² forms in the solutions, respectively. A 2.5 mmol, 50 mL Pb(CH₃COO)₂·3H₂O solution was then gradually added to the related solutions and PbSe and PbS precipitates with a black color were observed. After filtration, the precipitates were washed with diluted HNO₃ and ultrapure (Milli-Q) water, respectively. The precipitates were dried at 50 °C for 4 h.³⁵

2.3. Preparation of rGO paper

We prepared 80 mL of GO dispersion (1.0 mg mL⁻¹) by treatment for 1 h in an ultrasonic bath (Bandelin Sonorex, digitec, 160 W) so that a total dispersion of GO sheets was provided. Flexible GO paper was fabricated by vacuum filtration of this dispersion through a nuclepore polycarbonate membrane (Whatman, Ø = 47 mm, pore size: 0.2 μm) using an ultrafiltration vacuum cell (EZ-Stream). After washing with distilled water several times, air-drying, and peeling it off from the membrane, flexible GO paper was obtained. Chemical reduction of the GO paper was performed to increase its electrical conductivity by immersing the GO paper into a HI solution at room temperature (∼25 °C) for 1 h.³⁶ The paper was washed with ethanol and distilled water several times. After being dried at room temperature, free-standing and flexible rGO paper was obtained (Fig. 1).

Fig. 1 Experimental procedure for the fabrication of rGO/PbSe_0.5S_0.5 hybrid paper.

2.4. Preparation of PbSe_xS_(1−x) thin films on Au(111)

In order to optimize the electrodeposition conditions, electrodeposition of PbSe_xS_(1−x) thin films was initially studied on an Au(111) electrode by changing the x value (x = 0.0, 0.5, and 1.0). Electrodeposition of PbSe_0.5S_0.5 was applied from 10 mL of 0.1 M acetate (CH₃COOH/CH₃COONa) buffer solution (ABS) with a pH value of 5.9 including both 10 mg of PbSe and 10 mg of PbS. Electrodeposition of PbS and PbSe was carried out from 0.1 M ABS including 20 mg PbS or 20 mg PbSe, respectively. Pb⁺², S⁻², and Se⁺⁴ species were produced by dissolution of PbS and PbSe precipitates in the solution. PbS and PbSe are hardly soluble salts and the solubility product constant values for PbS and PbSe are 3.1 × 10⁻²⁸ and 1.0 × 10⁻³⁸, respectively.³⁷ In order to increase the dissolution of PbS and PbSe precipitates, the temperature of the electrochemical cell was fixed at 70 °C using a thermostat system.

The pH of the solution in an electrochemical cell is a very critical issue. If the electrochemical studies are applied at pH values lower than 5.9, both sulfur species turn immediately from S⁻² or HS⁻ to the H₂S(g) form³⁸ and the underpotential deposition (UPD) of selenium shifts to positive potentials, which does not fit into our one-pot electrodeposition procedure depending on the UPD of each element.³⁹ In cases where the pH value of the solution is kept higher than 5.9, soluble Pb²⁺ species start to precipitate as Pb(OH)₂(s). So, the buffer solution used for investigation of the electrochemical properties of PbSe_xS_(1−x) thin films was 0.1 M ABS with a pH of 5.9.

A suitable electrodeposition potential value taking into account the UPD of each element was determined using a potentiodynamic (cyclic voltammetry) method. Current–time (i–t) transients were obtained from potentiostatic application during the electrodeposition process. The solutions were deoxygenated by passing dry nitrogen through the electrochemical cell for at least 15 min prior to each electrochemical study. All electrochemical studies were carried out with an Epsilon (BASi) potentiostat system connected to a three electrode cell by using different working electrodes of single crystal Au(111), polycrystalline Au, indium tin oxide coated glass (ITO), and rGO paper.

Single crystal Au(111) electrodes were prepared as previously described by Hamelin.^17,40 Au(111) single crystal substrates with a surface area around 5 mm² were prepared especially for powder XRD characterization of the prepared thin films by polishing them parallel to the (111) direction.⁴¹ The polycrystalline regions of the prepared substrates were covered with an electrochemically inert epoxy (Epoxy-Patch) to perform electrochemical measurements on the Au(111) electrodes. Optical studies were applied on optically transparent ITO electrodes and they were cleaned by sonication in a detergent solution for 5 min and then rinsed with a large amount of doubly distilled water. Further sonication in ethanol for 5 min was applied before being blown dry with an argon stream. In all cases, an Ag/AgCl (3 M NaCl) (BASi) electrode served as the reference electrode and a Pt wire was used as the counter electrode.

2.5. Fabrication of rGO/PbSe_0.5S_0.5 hybrid paper

An rGO paper electrode was attached to a three-electrode cell as the working electrode in 0.1 M ABS solution (pH: 5.9) containing 10 mg of PbS and 10 mg of PbSe at 70 °C. In this electrochemical setup, an Ag/AgCl (3.0 M KCl, BASi) and a Pt wire (Alfa Aesar, 99.999% purity) were used as reference electrode and counter electrode, respectively. The rGO paper electrode was coated with PbSe_0.5S_0.5 thin films by applying a suitable constant potential (−450 mV) for 4 h. The prepared free-standing and flexible rGO/PbSe_0.5S_0.5 hybrid paper electrode was washed with distilled water and dried in air. As can be seen in Fig. 1, the one-pot electrodeposition process results in a thin film formation with gray color on the black-colored rGO paper surface.

2.6. Instrumentation

Morphological investigation of the prepared samples was performed under ambient conditions using both STM and SEM techniques. The STM investigation was performed with a Molecular Imaging model PicoScan instrument. STM tips were made by cutting a Pt–Ir wire with surgical scissors. The SEM analysis and elemental composition determination of the thin films were collected by EDS with a ZEISS system coupled to the SEM instrument. Optical absorption spectra of the films were collected on ITO substrates with a Shimadzu UV-3101PC UV-vis-NIR spectrophotometer. The powder XRD patterns were collected using a Rigaku brand Miniflex model powder X-ray diffractometer using Cu Kα radiation (λ = 1.54050 Å). Fourier transform infrared (FTIR) spectra were collected with a Spectrum One model Perkin-Elmer spectrophotometer using a specular reflectance attachment.

3. Results and discussion

3.1. Electrodeposition of PbSe_xS_(1−x) thin films on Au(111)

The main aim of this work is to prepare free-standing rGO/PbSe_0.5S_0.5 hybrid paper. For this purpose, optimum electrodeposition parameters were firstly obtained by applying one-pot electrodeposition of PbSe_xS_(1−x) thin films (x = 0.0, 0.5, and 1.0) on an Au(111) electrode as a model electrode. The one-pot electrodeposition process is performed at a constant potential, which is suitable for the UPD of each element (Pb⁺², S⁻², and Se⁺⁴) in the solution including PbS and PbSe precipitates. Based on this opinion, we initially performed cyclic voltammetry experiments to determine the UPD regions of each element and −300 mV was determined as a suitable constant potential value for electrodeposition of PbSe_xS_(1−x) thin films on the Au(111) electrode (Fig. S1†).

Fig. 2 exhibits potentiostatic i–t transients for 2 h electrodeposition of PbS, PbSe, and PbSe_0.5S_0.5 thin films on the Au electrode at an electrodeposition potential of −300 mV. In general view of all the i–t curves in Fig. 2, the current value rises initially and then declines exponentially after a maximum following a certain amount of time. This type of i–t transient is quite common for two-dimensional nucleation and growth phenomena, which possibly results in regular and highly-oriented crystalline thin film formation.^38,42

Fig. 2 Potentiostatic i–t transients for electrodeposition of PbS (a), PbSe (b) and PbSe_0.5S_0.5 (c) thin films on a polycrystalline Au electrode from 0.1 M ABS (pH: 5.9) including 20 mg PbS (a), 20 mg PbSe (b), and 10 mg PbS + 10 mg PbSe (c). Electrodeposition potential: −300 mV. Cell temperature: 70 °C.

Morphological investigation of PbSe, PbS, and PbSe_0.5S_0.5 thin films on an Au(111) electrode prepared at −300 mV was carried out using STM (Fig. 3) and SEM (Fig. 4) techniques. As shown in Fig. 3a, the PbSe thin film surface includes both randomly dispersed nanoparticles with an average diameter of around 40 nm and regular chain-like nanowires when compared to the naked Au(111) surface.⁴³ The SEM image of the PbSe thin film (Fig. 4a) exhibits that these nanoparticles coalesce and the surface is coated with large PbSe particles with an average diameter of about 500 nm. In the case of the PbS thin films, the surface is characterized by cubic-like nanoparticles with an average diameter of around 80 nm (Fig. 3b). The SEM image of the PbS thin film in Fig. 4b also shows that the Au(111) surface is almost totally covered with a very smooth PbS thin film, which includes nanoparticles.

Fig. 3 STM images (2 μm × 2 μm) of PbSe (a), PbS (b), and PbSe_0.5S_0.5 (c) thin films on an Au(111) electrode. The samples were prepared using the same conditions as those for Fig. 2.

Fig. 4 SEM micrographs of the electrodeposited PbSe (a), PbS (b), and PbSe_0.5S_0.5 (c) thin films on an Au(111) electrode. The samples were prepared using the same conditions as those for Fig. 2.

As can be seen in Fig. 3c and 4c, PbSe_0.5S_0.5 thin films on Au(111) demonstrate a different structure when compared to PbSe and PbS thin films. The PbSe_0.5S_0.5 surface is supposed to be a mixture of PbSe and PbS surface structures. Z-height profiles presented under the STM images in Fig. 3 indicate that the roughness of the PbSe thin film (∼50 nm) is higher than that of the PbS thin film (∼15 nm) and the roughness of PbSe_0.5S_0.5 thin film (∼50 nm) is very close to that of the PbSe thin film. This aspect shows that the growth mechanism of the ternary film is strongly dominated by PbSe thin films on the Au electrode.

The chemical composition of the thin film is calculated using EDS as 30.10% Se, 28.93% S, and 40.97% Pb, which indicates that the atomic ratio of Se : S is almost 1 : 1 (Fig. S2†). However S and Se species are higher and Pb is lower than the expected ratio for PbSe_0.5S_0.5 (25% for S and Se, 50% for Pb). This chemical composition is very close to the previous result, which was published by Onicha et al. They prepared ternary PbSe_xS_(1−x) nanowires with different x values by using a precipitation method and they reported that the stoichiometry deviates slightly from the expected metal/chalcogen concentrations with a maximum variation of 14% and average variation of 6.83% of the expected stoichiometry.²⁴

Fig. 5 demonstrates the powder XRD patterns of PbSe, PbS, and PbSe_0.5S_0.5 thin films on an Au(111) electrode prepared at −300 mV. It is clear in Fig. 5 that all the diffraction patterns contain an intensive peak at 2θ = 38.2°, which is due to the Au(111) electrode and is attributed to the (111) crystalline face of Au (JCPDS card no: 4-784). Except for the Au(111) diffraction, the PbS thin film includes only one diffraction peak at 2θ = 30.1° that corresponds to the cubic (200) crystalline face of PbS (JCPDS card no: 5-592). A diffraction peak at 2θ = 29.1° is observed for the PbSe thin film, which is due to the cubic (200) crystalline face of PbSe (JCPDS card no: 20-0494). The PbSe_0.5S_0.5 thin film contains a diffraction peak at 2θ = 29.7°, which is almost in the middle of the PbS and PbSe crystalline faces. The position of this peak strongly depends on the stoichiometry of lead chalcogenide (the x value of the PbSe_xS_(1−x) structure). If the x value increases, this peak shifts to 29.1°, while the peak moves to 30.1° when decreasing the x value.²⁴ Diffraction patterns for the PbSe_xS_(1−x) thin films (x = 0.0, 0.5, and 1.0) on the Au(111) electrodes indicate that the one-pot electrodeposition process based on the UPD of each element of the semiconductive material yields thin films with single crystal orientation. Similar results have been obtained by using one-pot electrodeposition methods based on UPD for binary semiconductive materials e.g. PbS,^17,19 PbTe,¹⁸ and CdS.⁴⁴

Fig. 5 Powder XRD spectra of Au(111) electrodes modified with PbS (a), PbSe_0.5S_0.5 (b), and PbSe (c) thin films. The samples were prepared using the same conditions as those for Fig. 2.

We have also investigated the optical properties of PbSe_0.5S_0.5 thin films on ITO electrodes for 15, 30, 60, and 120 minute electrodeposition times (Fig. S3†) and bandgap values have been calculated as 0.56, 0.52, 0.49, and 0.48 eV, respectively. The bulk bandgap value for PbS is 0.41 eV, while it is 0.28 eV for PbSe.²⁴ It is also determined that the color of the PbSe_0.5S_0.5 thin film, which is prepared at −300 mV, changes when increasing the electrodeposition time (Fig. S4†). The color of the PbSe_0.5S_0.5 thin films is observed as brown for 30 min electrodeposition time when compared to the naked Au electrode. The thin film is observed as purple, blue, light blue, gray, and dark gray for 1 h, 2 h, 3 h, 4 h, and 5 h electrodeposition time, respectively. Both the bandgap shift to lower values and the color variation with increasing the film thickness may be attributed to the quantum-confinement effect of the synthesized ternary thin film.^15,17,18

3.2. Preparation of free-standing rGO/PbSe_0.5S_0.5 hybrid paper

We have determined a suitable electrodeposition potential value to produce free-standing rGO/PbSe_0.5S_0.5 hybrid paper. For this purpose, a cyclic voltammetry experiment was performed using both Au and rGO paper as working electrodes in a solution containing 0.01 M K₃Fe(CN)₆ and 0.10 M KNO₃ (Fig. S5†). When compared to the Au electrode, reduction of Fe(CN)₆³⁻ to Fe(CN)₆⁴⁻ on the rGO paper electrode takes place with a shift of about 150 mV to the negative potential. Depending on this data, we fabricated rGO/PbSe_0.5S_0.5 hybrid paper by applying a −450 mV constant electrodeposition potential for 4 h in 0.1 M ABS containing 10 mg of PbSe and 10 mg of PbS (pH: 5.9).

While electrodeposition of PbSe_0.5S_0.5 is applied on rGO paper at −450 mV, current–time transients were recorded to evaluate the nucleation and growth mechanism on the rGO paper. Current–time data (Fig. S6†) exhibit a transient starting from a maximum current value which is followed by an exponential fall after a certain amount of time, which is due to random adsorption. By increasing the time, a broad curve at about 60 min is observed, which may be attributed to the nucleation and growth process for the formation of the PbSe_0.5S_0.5 thin film.^38,42 Depending on this i–t transient, it may be suggested that PbSe_0.5S_0.5 nanoparticles are adsorbed randomly onto rGO paper and they re-organize and grow regularly on the surface over time.

Typical XRD patterns of GO, rGO, and rGO/PbSe_0.5S_0.5 papers are presented in Fig. 6. As shown in this figure, the XRD pattern of GO paper exhibits only one peak centered at 11.2°, corresponding to the (002) diffraction of stacked GO sheets with a layer-to-layer distance (d-spacing) of 0.8 nm, due to the presence of hydroxyl, epoxy and carboxyl groups. rGO paper displays a broader (002) diffraction peak at 24.0° with a d-spacing of 0.4 nm, which is due to elimination of the oxygen-containing groups after the chemical reduction process is applied.^26,27 It is clear that rGO/PbSe_0.5S_0.5 paper prepared with a 2 h electrodeposition process includes two main peaks at 23.9° for rGO and 29.6° for PbSe_0.5S_0.5 thin film, respectively. This situation indicates that one-pot electrodeposition of PbSe_0.5S_0.5 onto rGO paper was successfully achieved at −450 mV. The particle size of PbSe_0.5S_0.5 was calculated as 87 nm from the peak at 2θ = 29.6° in Fig. 6 by using the Scherrer equation (ESI†), indicating the formation of nanoparticles on the rGO paper.

Fig. 6 Powder XRD spectra of GO, rGO, and rGO/PbSe_0.5S_0.5 paper prepared by 2 h electrodeposition at −450 mV.

Fig. 7a, c and e demonstrates large and close-view SEM images of rGO/PbSe_0.5S_0.5 prepared with a 1 h electrodeposition process at −450 mV. The surface structure of rGO paper contains characteristic wrinkles, arising from the flexibility of graphene (Fig. S7†). It is obvious from Fig. 7e that the rGO paper surface is covered with PbSe_0.5S_0.5 particles with an average size of 40 nm, while the wrinkled character of the rGO paper is still observed. Some of these nanoparticles are arranged to form worm-like wires on the rGO paper surface (Fig. 7e). A cross-sectional analysis of the rGO/PbSe_0.5S_0.5 paper shows layer-by-layer assembly of graphene sheets (Fig. 7g). If the electrodeposition process is extended to 2 h, flower-like large crystallites of around 7 μm in dimension arise on the rGO paper, which is probably due to growth of worm-like wires over time (Fig. 7b and d). A close-view of this surface exhibits regularly-oriented particles with a size of about 90 nm, besides flower-like large crystallites on the rGO paper (Fig. 7f). In this figure, the wrinkled graphene substrate is hardly observed, indicating that the rGO surface is almost covered with PbSe_0.5S_0.5 film after the 2 h electrodeposition process. The successive growth of worm-like and of flower-like crystallites can be assigned to re-organization of PbSe_0.5S_0.5 nanoparticles on the rGO paper over time as mentioned above. This case indicates that PbSe_0.5S_0.5 particle size and shape can be adjusted by controlling the electrodeposition time. Cross-sectional SEM analysis of rGO/PbSe_0.5S_0.5 paper after the 2 h electrodeposition process reveals flower-like vertically aligned PbSe_0.5S_0.5 crystallites on the layer-by-layer assembled rGO paper (Fig. 7h). However, flower-like electrochemical crystallization on rGO paper is not similar to that obtained on Au substrates. It may be suggested that the wrinkled structure of the rGO surface plays a template role for the formation of PbSe_0.5S_0.5 crystals and causes the re-organization of first worm-like and then flower-like crystals besides nanoparticles. The presence of flower-like PbSe_0.5S_0.5 crystals enlarges the electrode surface area and it can be proposed that this hybrid paper can be favorable for different electrochemical applications e.g. flexible electronics, electrocatalysis, and electrochemical capacitors with high performance.

Fig. 7 Top view (a–f) and cross-sectional view (g and h) SEM images of rGO/PbSe_0.5S_0.5 paper prepared by 1 h (a, c, e, g) and 2 h (b, d, f, h) electrodeposition at −450 mV.

FTIR characterization was carried out to verify the reduction of GO paper and formation of PbSe_0.5S_0.5 crystals on the rGO paper (Fig. 8). GO paper demonstrates a large band at 3470 cm⁻¹, which corresponds to the –OH functional group on the GO paper. The FTIR spectrum of GO paper contains other peaks at 2970, 1740, and 1650 cm⁻¹ due to C–H, C=O, and C=C functional groups, respectively. After treatment with HI and formation of rGO paper, the intensities of all these peaks decrease, indicating removal of oxygen-containing groups. The loss of oxygen-containing groups results in the formation of residual holes on the rGO paper, where PbSe_0.5S_0.5 nanoparticles can be easily anchored to form worm-like and flower-like large crystallites because the defects serve as active adsorption sites and can be useful places to start the nucleation and growth.⁴⁵ The rGO/PbSe_0.5S_0.5 paper includes almost the same FTIR spectrum as that of rGO paper. When compared to rGO paper, only one difference is observed for rGO/PbSe_0.5S_0.5 paper with an additional band at 2850 cm⁻¹, which may be attributed to PbSe_0.5S_0.5 crystals and this wavenumber corresponds to the bandgap value of 0.35 eV. The bulk bandgap values of PbS and PbSe are 0.41 and 0.28 eV, respectively, and the calculated bandgap value of PbSe_0.5S_0.5 crystals on rGO paper is between the bandgap values of PbS and PbSe as expected.

Fig. 8 FTIR spectra of GO, rGO and rGO/PbSe_0.5S_0.5 paper.

4. Conclusions

Free-standing and flexible rGO/PbSe_0.5S_0.5 hybrid paper was successfully fabricated using a one-pot electrodeposition process on rGO paper, which is produced by vacuum filtration of a GO dispersion and a further chemical reduction approach. Based on the UPD of each element of the semiconductive material, suitable electrodeposition potential values were determined as −300 and −450 mV vs. Ag/AgCl on Au and rGO paper electrodes, respectively. Morphological investigations exhibited that the one-pot electrodeposition process yielded flower-like PbSe_0.5S_0.5 crystals with about 7 μm size besides formation of ordered nanoparticles on rGO paper, which was assigned to a template role of the wrinkled graphene surface. However, the PbSe_0.5S_0.5 thin film on Au electrodes revealed the formation of only nanoparticles. It is supposed that the formation of flower-like large crystallites on rGO paper increases the effective electrode area and this case provides an extra sensitivity advantage for possible electrochemical sensor applications. Additionally, rGO/PbSe_0.5S_0.5 paper is very flexible, which enables modular approaches and lab-on-a-chip applications. EDS measurements indicated that the atomic ratio of Se : S of PbSe_0.5S_0.5 thin film composition is almost 1 : 1. UV-vis-NIR experiments revealed a bandgap shift with increasing film thickness, which is attributed to the quantum-confinement effect of prepared ternary thin films. The bandgap value of electrodeposited PbSe_0.5S_0.5 crystals on rGO paper was estimated as 0.35 eV, which is between the bulk bandgap values of PbS and PbSe. XRD data exhibited that the one-pot electrodeposition process resulted in single crystal PbSe_0.5S_0.5 thin films on both Au(111) and rGO paper.

Acknowledgements

This work has been supported by Atatürk University (Project no: 2014/59).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26803f

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