Controlled Photocatalytic Deposition of CdS Nanoparticles on Poly(3

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Controlled Photocatalytic Deposition of CdS Nanoparticles on Poly(3-hexylthiophene) Nanofibers: a Versatile Approach to Obtain Organic/inorganic Hybrid Semiconductor Assemblies Andras Varga, Balázs Endr#di, Viktoria Hornok, Csaba Visy, and Csaba Janáky J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09029 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 18, 2015

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The Journal of Physical Chemistry

Controlled Photocatalytic Deposition of CdS Nanoparticles on Poly(3-hexylthiophene) Nanofibers: A Versatile Approach to Obtain Organic/Inorganic Hybrid Semiconductor Assemblies A. Varga,†,‡ B. Endrődi, †,‡ V. Hornok,‡ C. Visy,‡ and C. Janáky †,‡,* †

MTA-SZTE, Lendület Photoelectrochemistry Research Group, Rerrich Square 1, Szeged, H-6720, Hungary

‡

Department of Physical Chemistry and Materials Science, University of Szeged, Reirrich Square 1, Szeged, H-6720, Hungary

*corresponding author, E-mail: [email protected] (C. Janáky) Fax: +36 62 546-482. Tel: +36 62 546-393

ACS Paragon Plus Environment


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ABSTRACT: To efficiently harness the possible synergies, stemming from the combination of organic conducting polymers and inorganic semiconductors, sophisticated assembling methods are required to control the composition and morphology at the nanoscale. In this proof-of-concept study, we demonstrate the in situ photocatalytic deposition of CdS nanoparticles on poly(3-hexylthiophene) (P3HT) nanofibers, exploiting the semiconducting nature of this polymer. The formation of the hybrid assembly was monitored by UV-Vis and Raman spectroscopy, Energy dispersive X-ray microanalysis, and X-ray powder diffraction (XRD). Transmission electron microscopic studies and AFM images confirmed that both the particle size and the loading can be tuned by the deposition time. Photoelectrochemical studies revealed the facile transfer of photogenerated electrons from P3HT to CdS, as well as that of the holes from CdS to P3HT. It is believed that ensuring intimate contact between the components in these nanohybrids will open new avenues in various application schemes, e.g., solar energy conversion.

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INTRODUCTION Nanostructured hybrid assemblies of conducting polymers (CPs) and inorganic semiconductors (SCs) are among the most widely studied functional materials these days.1 Solar energy conversion,2 charge storage,3 and electrochemical sensing4 are only selected examples where the complementary properties of the constituents can be harvested. The success of the hybridization however, strongly depends on the control achieved over the composition and nanoscale morphology of the hybrid material: and most importantly, the properties of the interface between the components. Ex situ methods (e.g., mechanical mixing or solution blending) generally fail to provide simultaneous control over the morphology and the interfaces in the hybrid. In most of these cases surfactants are employed to avoid extensive aggregation of the inorganic particles. Such coatings however, may hamper electronic communication (i.e., charge transfer) between the two semiconducting components. Consequently, in situ methods, where one component is obtained in the presence of the other one, are favored. The in situ generated component can be either the CP or the inorganic SC.5,6 In situ chemical and electrochemical polymerization have been widely employed to synthesize CPs on inorganic SC surfaces, however, the limited electrical conductivity of these materials often hampered controlled electropolymerization.6,7 Considering the photoactivity of inorganic SCs, it is not surprising that feasibility of different light-assisted synthetic methods was demonstrated in prior studies. For example photocatalytic8 and photoelectrochemical7,9 methods were employed to deposit CPs (such as polypyrrole, polyaniline, and poly(3,4-ethylenedioxythiophene)) on inorganic SC nanostructures. As we summarized in a recent review article,6 these methods elaborate on the oxidation of the monomer molecules by the holes, photogenerated in the inorganic SC upon photoexcitation. The such created radical cations form a dimer, which can be


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oxidized again, and thus the polymerization proceeds. As a result, thin CP coverage is obtained on the SC surface. On the other hand, much less is known about the in situ formation of inorganic SCs on organic CPs. Both polypyrrole/CdS10 and poly(3,4-ethylenedioxythiophene)/CdS11 hybrids were obtained via cathodic electrodeposition, where the CdS particles were obtained in the CP film. The size of the CdS particles in both cases, however, were out of the regime where quantum size effects can be witnessed. Cathodic electrodeposition of Cu2O and SnO2 was also performed on polyaniline12 and polypyrrole13 films, respectively. Direct precedence of this work was the photocatalytic deposition of different metal chalcogenides on the surface of TiO2 nanoparticles. Titania-based composites, containing PbSe,14 CdSe,15,16 CdS,17,18 PbS,19 and Sb2S320 have been realized through this method. This approach relies on the photoexcitation of TiO2, followed by the reduction of the respective metal cation in the presence of elemental S or Se. Mechanistic investigations in prior studies proved that the atomic route (M2+ + 2e- →M; M + S →MS) is dominantly responsible for the compound formation.21 It has also been shown that the conduction band edge position of the photoexcited SC particle (TiO2 in these studies), together with the reduction potential of the metal cation determines the feasibility, as well as the rate of the photodeposition reaction (i.e., the larger the difference the faster the reaction is).15 In this article, we demonstrate a simple and versatile method, exploiting the intrinsic semiconducting nature of CPs to photodeposit metal-chalcogenide nanoparticles on their surface under visible light irradiation. The photocatalytic behavior of CPs has already been demonstrated and utilized in organic synthesis,22 dye-degradation,23 as well as noble-metal deposition.24,25 In the presented study we exploit the photoelectrons generated in P3HT nanofibers to reduce Cd2+


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ions, in the presence of elemental S, thus CdS nanoparticles are formed (Scheme 1). Our choice on the P3HT/CdS as a model system was deliberate, since we wanted to demonstrate the feasibility of this approach on a composite, which may have practical significance. Composites of poly(3-alkylthiophenes) and metal-chalcogenides are considered as benchmark materials of organic/inorganic hybrid solar cells.26,27 1D nanostructures of CPs have attracted significant attention recently,28 because of their enhanced electronic properties compared to their bulk counterparts. For example, self-assembled highly crystalline nanofibers of P3HT showed a 6-fold improvement in the electrical conductivity, rooted in the enhanced charge carrier mobility.29,30 Such beneficial features were exploited even in hybrid configurations,31 for example P3HT nanofibers were decorated with CdSe32 and CdS33 (via a sophisticated chemical grafting procedure), and showed promising performance in solar cell studies.

EXPERIMENTAL METHODS Materials. All chemicals used were of analytical grade. 3-hexylthiophene (≥99%), lithium perchlorate (≥98%) and absolute ethanol was purchased from Sigma-Aldrich, L-ascorbic acid, cadmium nitrate tetrahydrate and sulfur from Reanal, acetonitrile from Carlo Erba, while tetrahydrofuran, chloroform, and anisole from VWR International. Water content of both anisole and chloroform was kept below 50 ppm (monitored by coulometric Karl-Fischer titration) by storing it over 3A zeolites. Preparation Methods. Poly(3-hexylthiophene) (P3HT) was prepared by oxidative chemical polymerization. Chloroform based solutions of 3-hexylthiophene monomer and FeCl3 were prepared and mixed at final reagent concentrations of 0.1 and 0.25 M, respectively. The


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continuously stirred reaction mixture was kept in a closed vessel on ice bath for 6 hours. The formed polymer was filtered on a filter paper (12-15 µm pore size), and washed repeatedly with absolute ethanol to remove the traces of the oxidant. The final product was dried in air, under infrared lamp. Whisker method was employed to form nanofibers from the bulk polymer. As the first step, a larger molecular weight fraction of P3HT was extracted by tetrahydrofuran (THF). Subsequently, the polymer was re-dissolved in a 9:1 ratio anisole/chloroform mixture to get a final polymer concentration of 1.0 g dm-3. Finally, the solution was warmed up to 70 °C and then instantly cooled down to room temperature on ice bath. P3HT nanofiber network thin layers were formed by drop-casting the P3HT solution on indium-tin-oxide (ITO) covered glass substrates. The P3HT loading was about 100 µg cm-2. Synthesis of P3HT/CdS Hybrids.

P3HT thin layers were placed in a closed cell and

immersed in an ethanol based solution, containing the precursors of the CdS synthesis ([Cd2+] = 1.38×10-2 mol dm-3, [S8] = 1.72×10-3 mol dm-3) and ascorbic acid (c = 0.1 mol dm-3), as sacrificial hole scavenger (Figure S1.). The solution was degassed by bubbling N2 through it for 30 minutes before the synthesis. Photocatalytic synthesis of the P3HT/CdS hybrids was then implemented by using a Fiber Lite A3000 type tungsten halogen lamp (150 W maximum power output), operating in the visible range, between λ=380-800 nm. After the illumination, the hybrids were carefully washed with distillated water and absolute ethanol, to remove the adsorbed precursors on the surface. Solution blending was also employed to synthesize randomly distributed P3HT/CdS hybrid samples with the same composition, for comparative purposes.


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Physical Characterization. Ex situ UV–vis–NIR spectroscopic measurements were carried out using an Agilent 8453 UV–visible diode array spectrophotometer in the range of 300–1100 nm. To follow the gradual formation of the hybrids, spectrum of the P3HT thin layer was recorded before the synthesis, and was later used as background. Raman spectroscopic studies were performed on a DXR Raman Microscope using a green laser (λ = 532 nm), operating at 1 mW laser power. P3HT solution was drop-cast on copper mesh grids covered by carbon film for transmission electron microscopic (TEM) investigations. Formation of the hybrids occurred in situ on the P3HT coated grids, according to the above described method. Note however, that during longer synthesis times (i.e., over 30 min) the copper grid corroded through its reaction with sulfur present in the solution. A FEI Tecnai G2 20 X-Twin type instrument, operating at an acceleration voltage of 200 kV was used. XRD spectra were recorded between 2Θ = 3-60° at 2° minute-1 scan rate by a Rigaku Miniflex II instrument, operating with a Cu Kα,1 radiation source (λ = 0.1541 nm). Electrochemical measurements were performed in a three electrode cell. A platinum sheet was used as counter electrode, while an AgCl-coated Ag-wire (having a potential of E = -420 mV, compared to the Fe3+/Fe2+ redox transformation in ferrocene in acetonitrile), was used as reference electrode. For the photovoltammetric measurements (sweep rate 5 mV s-1), the hybrids were periodically illuminated (0.1 Hz) with the above mentioned light source. All the measurements were performed by using a PGSTAT 302 (Autolab) potentiostat/galvanostat. Scanning electron microscopic (SEM) images were recorded by a Hitachi S-4700 field emission scanning electron microscope (coupled with a Röntec EDX detector), operating at an acceleration voltage of 10 kV.


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AFM images were recorded on a Nanoscope III type atomic force microscope (Digital Instruments) in tapping mode. Silicon cantilevers (Veeco Nanoprobe Tips RTESP model, 125 µm length, 300 kHz) were used. The scanning rate was 1 Hz.

RESULTS AND DISCUSSION To monitor the deposition process, the P3HT/ITO electrode was placed into a closed vessel containing stoichiometric amounts of the CdS precursors (Cd2+ and S) dissolved in ethanol, and UV-Vis spectra were recorded after selected irradiation times (Figure 1a). The pattern of the curves is very similar in each case, exhibiting a broad absorption related to the π−π* transition of the reduced P3HT.29 A fine vibronic structure is also visible (with absorption bands centered at 550 and 600 nm) which is a solid indication of the presence of the nanofibers.28,30

Careful

comparison of the curves however, reveals a continuous absorbance increase between 300-550 nm. To visualize these changes directly, difference spectra were obtained by subtracting the spectrum of P3HT recorded before irradiation (Figure 1b). The gradually developing difference spectrum indicates the steady formation of CdS nanoparticles. The bandgap energy of the formed material was calculated using Tauc-plot (Figure 1b), and the values were in the range of 2.5-3.0 eV. In accordance with the small, but significant gradual redshift of the absorption maximum (Figure 1b), a monotonous decrease in the bandgap-energy was observed (from 3.01 (after 15 min) to 2.51 (after 120 min)). These tendencies indicate that initially very small nanoparticles (quantum dots) are formed, which later grow with the deposition time. After 120 minutes, however, the bandgap of CdS reached the value generally reported for the bulk material (2.5 eV).34,35


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Figure 1. (a) Absorbance spectra recorded during the P3HT/CdS composite formation; (b) difference spectra from Figure 1a. The inset in Figure 1b shows the Tauc-plot derived from Figure 1b, for the sample obtained by 2 h photodeposition.

The chemical identity of the formed P3HT/CdS hybrid was confirmed by X-ray diffraction (XRD), Raman-spectroscopy, and energy dispersive X-ray microanalysis (EDX). On the EDX spectrum, both Cd and S peaks were observed (Figure 2c). Semi-quantitative analysis estimates about 10 wt% CdS content in the hybrid obtained after 2 h. Figure 2a depicts the Raman spectra of a P3HT nanonet and a P3HT/CdS hybrid (2h photodeposition). On the P3HT spectrum, the Cα‒S‒Cα' deformation at 684 cm-1, the symmetric Cα=Cβ stretching (the most intense peak) at 1455 cm-1, together with the asymmetric Cα'=Cβ' stretching at 1518 cm-1 are characteristic for the reduced form of P3HT.36 As for the hybrid material, the most important alteration is the appearance of a new band at 303 cm-1, which is distinctive for the Cd-S stretching in CdS.37 On the XRD patterns presented in Figure 2b, both P3HT and CdS related diffractions are present. Clearly, P3HT is completely crystalline: the most intensive sharp reflection at 2Θ = 5.2o (100), together with the two smaller one ((200) and (300)), is attributed to the lamellar ordering of the


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polymer chains, facilitated by the overlap of the aromatic rings (π-stacking) and the zipper-type connection of the hexyl side chains. The appearance of CdS-related broad diffractions, superimposed to the P3HT peaks, in the hybrid assembly points toward the formation of a nanocrystalline material. The diffraction pattern is consistent with the cubic CdS structure (hawleyite), as confirmed by literature data (JCPDS# J 10-0454). Scherrer equation was employed to determine the average crystallite size of the CdS particles, and d=5.9 nm was obtained. The broadening of the Raman bands, as well as the shift of the polymer related diffraction peaks (in the XRD pattern) to smaller 2Θ values is an indication of the partial

P3HT/CdS

800

1200

1600 -1

Raman shift (cm )

(c)

Cd

P3HT

P3HT

400

S

O

Log (intensity (cps))

CdS (311)

P3HT (300)

(b)

CdS (220)

CdS (111)

C

P3HT (200)

P3HT (100)

P3HT/CdS

Intenisity (a.u.)

Cα'=Cβ' stretching

Cβ-Cβ' stretching

Cα-S-Cα' deformation

Cd-S stretching

(a)

Cβ=Cβ' stretching

oxidation of P3HT during the hybrid formation (see details below, as well as in Scheme 1).29

Intenisity (a.u.)

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10

20

30

40

50

2θ (°)

60

2

4

E (keV)

Figure 2. (a) Raman spectrum; (b) XRD diffractogram of the bare P3HT nanofibers and P3HT/CdS composite (2 h synthesis); (c) EDX spectrum of a P3HT/CdS composite (2 h synthesis).

Development of the morphological features of the hybrid was monitored by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Figure 3a-d shows the TEM images of the bare PH3T nanofiber network and three P3HT/CdS hybrids, obtained within 5, 15 and 30 min, respectively. We note here that in the preparation of these samples, the polymer nanofibers were directly casted on the TEM grid, and the CdS nanoparticles were in situ


10

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deposited. During longer synthesis times (i.e., over 30 min) however, the copper grid corroded through its reaction with elemental sulfur present in the solution. Beyond the obvious appearance of the nanofibers, both individual CdS nanoparticles (dark spots) and some of their aggregates are seen in the case of the hybrids (Figure 3b, c, and d). As seen in the histograms (see Figure 3e), the average size of the primary particles was 6.8, 9.3 and 9.6 nm, after 5, 15, and 30 min respectively (in agreement with the slight decrease in the bandgap, shown above). At this juncture we emphasize again that the histograms depict the size-distribution of the primary nanoparticles (since that is the factor dictating the optical properties), and not that of the aggregated ones, which are visible in Fig. 3d. Another important lesson learned from TEM images was that the CdS nanoparticles are always attached to the P3HT nanofibers, they never show up among them, separately. This observation suggests that the CdS nanoparticles are formed at the nanofiber/solution interface and not in the bulk solution.


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Figure 3. TEM pictures of P3HT (a); and three P3HT/CdS hybrids after 5 min (b), 15 min (c), and 30 min photodeposition (d). Particle size distribution of the CdS particles for the samples shown in Figures 3b-c.


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AFM images, shown in Figure 4, depict very similar trends to those derived from TEM analysis. Most importantly, the CdS nanoparticles never grow individually at the supporting electrode surface, in contrast, they are evenly distributed along the P3HT nanofibers. In addition, the number of aggregated particles increases with the time, and after 60 min they constitute a significant portion.

Figure 4. AFM images of P3HT (a); P3HT/CdS 30 min photodeposition (b); P3HT/CdS after 60 min photodeposition (c).

Considering all the above data together as a whole, we propose the following mechanism illustrated in Scheme 1. Upon visible light irradiation, electron-hole pairs (excitons) are formed in P3HT. Some of these excitons reach the solid/electrolyte interface (before they recombine), and the photoelectrons then reduce the Cd2+ ions adsorbed on the P3HT surface or present in the solution. Note that the CB edge (or LUMO level) of P3HT lies more negative than the reduction potential of Cd2+, therefore this process is thermodynamically feasible (see band edge positions in Figure 5). Because of the presence of dissolved sulphur, however, the formation of CdS occurs on the surface, instead of depositing metallic Cd particles. Ascorbic acid acts as a sacrificial electron donor, thus boosting the rate of the photocatalytic reaction (by scavenging the


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photogenerated holes). At this juncture we turn back to the continuous increase in the particle size with time. Once CdS nanoparticles are formed, further CdS deposition is favored on the nanoparticles surface, since the photogenerated electrons in P3HT are rapidly transferred to CdS (see Figure 5, and the later discussion on the photoelectrochemistry data in Figure 6 and 7).

Scheme 1. Illustration of the photocatalytic synthesis of P3HT/CdS.

To validate the proposed mechanism, a set of control experiments was performed via the systematic variation of the synthesis parameters: (i) without ascorbic acid, (ii) in the presence of O2, (iii) no S in the solution, (iv) no Cd2+ in the solution, (v) no light irradiation. As expected, a much slower formation of CdS was witnessed in the first two cases. In the first case extensive recombination and the lack of facile oxidative conjugate reaction (e.g., ascorbate oxidation) are both responsible for the decreased reaction rate. When O2 is present, there is competition between the photoreduction of dissolved O2 38 and Cd2+, which clearly hampers CdS formation. Formation of metallic Cd deposits was experienced in the absence of sulphur, however, the


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amount of Cd nanoparticles was considerably smaller than that of CdS. Finally, no reaction was observed in the last two cases.

Figure 5. Comparison of the reduction potentials of different metal cations and the band positions of TiO2, CdS, and P3HT.

Photoelectrochemical methods were employed to study the photoactivity of the components in the hybrid configuration. Linear sweep voltammograms (together with stationary photocurrent data, see Figure 7) were recorded for the P3HT/CdS hybrid, as well as its constituents separately in 0.1 M LiClO4/acetonitrile (Figure 6). This voltammetry technique consists of a slow scan of the potential while the film irradiation is periodically interrupted. In this way, both the “dark” and the light-induced (photo)response of the samples can be evaluated in a single experiment. Note that the photovoltammograms are scaled differently to highlight the features of each individual curve and the arrows indicate the type of the photocurrents. As for the bare P3HT


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nanofibers cathodic photocurrents can be detected at negative potentials, related to the p-type behavior of the polymer. This cathodic photocurrent flow is sustained by the reduction of dissolved O2. On the other hand, in the case of CdS the photocurrents are anodic in polarity, consistent with its n-type semiconductor behavior. The photocurrents arise mainly from the photooxidation of adsorbed water traces, or acetonitrile molecules. Note that no hole-scavenger is added to the solution (unlike when photoelectrochemical studies are typically performed in Na2S containing media). The photovoltammogram of the hybrid material qualitatively shows the features of both of its components: while cathodic photocurrents below E = 0.1 V proves the p-type photoactivity of P3HT in the hybrid configuration, the n-type behavior with anodic photocurrents at higher potentials

is

related

to

CdS.

This

behavior

was

also

confirmed

by

stationary

photoelectrochemical experiments, where the photocurrents were registered at constant potentials (Figure 7). However, the absolute values of these photocurrents during the photovoltammetric scans is very interesting, while the cathodic photocurrents are five-times higher in the hybrid, the anodic photocurrents show a 15-20 fold improvement compared to the bare materials. This striking effect can be interpreted by the good electrical contact between the components in the hybrid material. Upon photoexcitation under cathodic polarization, CdS is able to receive photoelectrons from P3HT (the CB edge alignment favors this transition), thus boosting cathodic photocurrents by reducing the probability of recombination. As for CdS, the photogenerated holes can oxidize P3HT (which acts a hole-scavenger in this case) thus much higher photoanodic currents can be measured in the hybrid configuration. The spectacular enhancement of the photocurrents in our photoelectrochemical measurements proves that there is a rapid and facile electron transport through the P3HT/CdS interface.


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We further studied the effect of our in situ procedure (and the resultant nanoscale architecture) by comparing and contrasting these data with those obtained for a randomly blended P3HT/CdS sample (not shown here). Importantly, almost no enhancement of the CdSrelated anodic photocurrents were detected in this case. Finally, we emphasize that these charge transfer processes are exactly the same which occur in a P3HT/CdS hybrid solar cell, therefore this synthetic tool may be useful in that application avenue.

Figure 6. Photovoltamograms of P3HT, CdS, and P3HT/CdS (immobilized on an ITO electrode) in 0.1 M LiClO4/acetonitrile solution under visible light irradiation (λ>380 nm) at 5 mV s-1 scan rate.


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Figure 7 Chronoamperometric photoelectrochemical measurements of a P3HT/CdS composite (2 h) at different potentials, recorded in 0.1 M LiClO4/acetonitrile solution under periodic (tperiod = 30 s) visible light irradiation (λ>380 nm).

CONCLUSIONS This study adds to the library of synthetic procedures to obtain hybrid organic/inorganic

semiconductor

assemblies.

Photocatalytic

deposition

of

CdS

nanoparticles was achieved for the first time through exploiting the intrinsic semiconducting nature of P3HT. By a set of carefully designed control experiments, we proved that upon illumination, photogenerated electrons in P3HT reduce Cd2+ ions, which then react with sulfur dissolved in the solution, to form CdS on the P3HT nanofiber surface. By simply controlling the photodeposition time, both the amount and size (between 3-12 nm) of CdS nanoparticles can be tuned. We note here that this approach is


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very versatile (similar data were gathered in our laboratory for P3HT/Sb2S3) its applicability is only limited by the conduction band edge of the polymer and the reduction potential of the metal cation. Comparing and contrasting the presented method to conventional electrodeposition, one has to consider that, without being illuminated, most conducting polymers (including P3HT) have very limited electrical conductivity in their reduced form. Consequently, direct cathodic electrodeposition of metal-chalcogenides is hampered by the low conductivity of the polymer. Controlled photocatalytic deposition circumvents the above issues and also allows the deposition of metal-chalcogenides on CPs immobilized on insulating surfaces. Finally, we hope that the intimate physical and electronic contact between the two components (as diagnosed by photoelectrochemical measurements) may enable practical application of these hybrids in energy conversion and storage.

Author information Corresponding author Csaba Janáky* Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Square 1., Szeged, H-6720, Hungary E-mail: [email protected] Fax: +36 62 546-482 Tel: +36 62 546-393 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Acknowledgement This research was realized in the frames of TÁMOP 4.2.4. A/2-11-1-2012-0001 „National Excellence Program – Elaborating and operating an inland student and researcher personal support system” The project was subsidized by the European Union and cofinanced by the European Social Fund. The authors thank the two anonymous reviewers for their insightful comments on an earlier version of this manuscript.

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Controlled Photocatalytic Deposition of CdS Nanoparticles on Poly(3

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