Synthesis and utilization of platinum(II) dialkyldithiocarbamate precursors in aerosol assisted chemical vapor deposition of platinum thin films as counter electrodes for dye-sensitized solar cells

Polyhedron 166 (2019) 186–195

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Synthesis and utilization of platinum(II) dialkyldithiocarbamate precursors in aerosol assisted chemical vapor deposition of platinum thin films as counter electrodes for dye-sensitized solar cells Muhammad Ali Ehsan a, Muhammad Younas a, Abdul Rehman b,⇑, Muhammad Altaf e, Mohd. Yusuf Khan a, Amir Al-Ahmed c, Saeed Ahmad d, Anvarhusein A. Isab b,⇑ a

Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia c Center of Research Excellence in Renewable Energy (CoRERE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia d Department of Chemistry, College of Sciences and Humanities, Prince Sattam bin Abdulaziz University, Al-Kharj 11942, Saudi Arabia e Department of Chemistry, GC University, Lahore 54000, Pakistan b

a r t i c l e

i n f o

Article history: Received 1 February 2019 Accepted 26 March 2019 Available online 6 April 2019 Keywords: Platinum(II) dithiocarbamate Single crystal X-ray Aerosol assisted chemical vapor deposition Thin film Dye sensitized solar cell

a b s t r a c t A facile and cost effective AACVD procedure for the synthesis of platinum thin films has been reported using newly synthesized Pt-dialkyldithiocarbamate complexes, [Pt(S2CNR2)] (where R = isobutyl, iBu (1); benzyl, Bz (2)), as single source precursors. The structural characterization of the dithiocarbamate complexes has been performed by single crystal X-ray diffraction, 1H and 13C NMR, FT-IR and thermogravimetric analysis. The structure, composition, and morphology of the resulting Pt-films is established through XRD, XPS, EDX and FE-SEM analyses. The catalytic performance of the as-synthesized Pt-films is evaluated by using them as counter electrodes in dye sensitized solar cells as an example application. The efficiency of the AACVD produced electrodes is found to be better than conventionally used Pt-counter electrodes made from the doctor blade’s method. It is also demonstrated that films having a well-connected and defect free surface topography show better catalytic performance, which is due to their high conductivity and reflectivity. Thus a simple and low cost method employing dithiocarbamate precursors is manifested to be immensely efficient in generating Pt-films and electrodes of tremendous applicability. Ó 2019 Published by Elsevier Ltd.

1. Introduction Facile fabrication of thin metal films has always been an important topic of research because of their wide range of applications [1,2]. Platinum (Pt), for example, due to its electrical resistivity of
10.5 mX cm at ambient temperatures, work function of 5.7 eV, melting point of 1772 °C, high catalytic activity and excellent chemical stability both in oxidizing and corrosive media, has many interesting applications in the form of thin film electrodes [3,4]. This includes electrodes in micro- and nanoelectronic devices [5], electrochemical sensors and biosensors [6–8], photo or electrochemical water splitting reactions [9], fuel cells based on proton exchange membranes [10], as well as solid oxides [11] and, most importantly, dye sensitized solar cells (DSSC) [4,12–14]. For all these electrode systems, Pt has to be coated on a variety of sub-

⇑ Corresponding authors. E-mail addresses: [email protected] (A. Rehman), [email protected] (A.A. Isab). https://doi.org/10.1016/j.poly.2019.03.058 0277-5387/Ó 2019 Published by Elsevier Ltd.

strates utilizing different deposition techniques, during which the morphology, microstructure and interface of Pt with the substrate must be optimized [3,13]. The tool box for scientists to achieve these goals includes two main strategies. One of these is the employment of physical vapor deposition (PVD) using sources of pure metal to form thin films by magnetron sputtering [15,16] or e-beam evaporation [17]. The other is the use of metal precursors for their high temperature pyrolysis [18], electrochemical [19], electroless [20] and chemical vapor depositions [1,21]. However, there are certain limitations for each deposition methodology which need to be surpassed in order to achieve an optimized performance of the resulting electrodes. Generally, the morphological stability of the grown films is quite a significant issue [3,13,15]. Although, the bulk material has a high melting point, the morphological stability of nano-sized Pt-films at elevated temperatures is often poor. Consequently, solid-state dewetting and agglomeration in thin-films start to occur, leading to higher resistivity and lower catalytic activity [11,22,23]. Furthermore, some of the deposition processes can

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cause this agglomeration by themselves. Inter-atomic interactions between individual metal atoms in this case are higher than the interactions between the metal and the substrate. This initiates the phenomena of 3-D island formation [1]. In addition to that, a high vacuum and high power is needed for the deposition from pure metal, using either magnetron sputtering or e-beam evaporation, owing to the high melting point of Pt, increasing the cost substantially. Radiation damage of the substrates often results in such depositions as well. Electrochemical methods, contrarily, require a conductive, non-corrosive substrate in the electrolytic environment. For electroless depositions, a layer of active metal needs to be coated on the substrate before the actual deposition. These electrodes demonstrate inferior photovoltaic signals as compared to electrodes made from chemical vapor deposition or pyrolysis. Even in the case of CVD, tuning of thin films in relation to the particle size, crystal structure and orientation, porosity and stress must be done to obtain an optimized performance. Therefore, new and simple approaches of metal deposition which can overcome these problems can have a strong impact both on the applicability of the metal films and the processing costs of the deposition. One such approach is the use of aerosol assisted chemical vapor deposition (AACVD) to grow thin films of Pt. AACVD is a versatile technique conventionally used to generate metal oxide films [24,25]. However, it can also be applied to grow noble metal films due to their low reactivity, if an appropriate precursor can be designed and utilized as an aerosol. The AACVD process occurs at moderately high temperatures of 300–600 °C, under ambient pressure conditions, making it industrially viable and scalable [6,26]. Further, it generates coherent thin films even up to mm thicknesses without agglomeration. This is the result of low sticking coefficients due to higher kinetic energies and higher densities of the Pt nuclei. As only a solution of the precursor is required to be transported to the reaction zone in the case of AACVD, the selection of the precursor is more flexible with regards to its vapor pressure and thermal stability [27,28]. In order to control the morphology of the resulting films, only the homogeneity and the size of the aerosol droplets has to be tuned. This can be done quite easily by adjusting two factors: the first is the frequency of the aerosol generator and the second is the viscosity of the precursor solutions, and so films comprised of a controlled structure can be obtained [29]. The particle growth and the sintering processes simultaneously occur directly on the surface of the substrate, instigating interconnected morphological features and adhesive films of the metal. These interconnecting features are characterized by a particle-particle or particle-substrate connection, enhancing the conductivity of the thin films. The payoff is the enhanced photocatalytic or catalytic performance by improved charge transport properties of the resulting electrodes. The only important demand to accomplish all this via AACVD is an adequately soluble metal-organic precursor in organic solvents. Consequently, that gives an intriguing opportunity to design and implement novel precursors with relatively low temperature decomposition profiles which otherwise are deemed unsuitable for conventionally used deposition procedures. In this work, we report the synthesis and characterization of platinum dialkylthiocarbamate [Pt(S2CNR2)] (where R = isobutyl, i Bu (1); benzyl, Bz (2)) complexes and demonstrate their suitability as AACVD precursors to fabricate pure platinum thin films. This is in continuation of our previous work where we used analogous dithiocarbamate complexes of palladium for the synthesis of palladium sulfide thin films [30]. However, due to the lower reactivity of platinum as compared to palladium, we hypothesized that the films generated from the platinum complexes would more likely be only composed of platinum rather than its sulfide and can be further used in platinum thin film applications. The resultant films were extensively characterized by FESEM, XRD, EDX and XPS anal-

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yses, validating our hypothesis that the product was fully composed of pure platinum and not its sulfide. Finally, the thickness controlled performance of these films was evaluated by replacing them as platinized counter electrodes in DSSCs instead of commercially available Pt-paste electrodes; the comparative data is reported to demonstrate a high suitability. 2. Experimental 2.1. Materials and methods Platinum(II) chloride (PtCl2) and sodium dibenzyldithiocarbamate (NaS2CNBz2) were purchased from Sigma Aldrich and used as received, while sodium diisobutyldithiocarbamate (NaS2CNiBu2) was prepared according to the typical procedure reported in the literature [31]. For DSSC fabrications, the ruthenizer (N719 ID: Ruthenizer 535-bis TBA), fluorine doped tin oxide (2.2 mm, 7 X/ Seq FTO ID: TCO22-7/LI), electrolyte (I/I 3 ID: Iodolyte Z-50), titanium paste (TiO2 ID: Ti-Nanoxide T/SP) and platinum paste (Pt ID: Platisol T) were purchased from Solaronix, Switzerland. A Series 11 (CHNS/O), Analyzer 2400 was used to perform elemental analyses of the complexes. A Perkin Elmer FTIR 180 spectrophotometer or NICOLET 6700 FTIR was used to record solid state FTIR spectra of the free ligands and their corresponding platinum complexes over the range 4000–400 cm1. A JEOL JNM-LA 500 NMR spectrometer was used to record the 1H and 13C NMR spectra in DMSO at operating frequencies of 500.00 and 125.65 MHz, respectively. The chemical shifts were measured relative to tetramethylsilane (TMS). The current–voltage (J–V) characteristics for the prepared DSSCs were measured using a Keithley 2400 source meter and a 1.5G (100 mW/cm2) IV-5 solar simulator (Sr.# 83, PV measurements Incorporation). 2.2. General procedure for the synthesis of the [Pt(dithiocarbamate)2] complexes (1 and 2) Sodium diisobutyldithiocarbamate (500 mg, 2.20 mmol) was reacted with platinum(II) chloride (300 mg, 1.10 mmol) in acetone as the solvent (30 mL) in a two-neck flask. The resultant egg yolklike solution was stirred for 20 min with the subsequent addition of 30 mL pyridine, which resulted in a transparent yellow solution that was kept under stirring for a further 30 min. The solution was filtered and kept at room temperature for crystallization. Yellow crystals were obtained on slow evaporation of the solvent. [Pt(S2CNiBu2)2] (1). Yield: 0.85 g, 75%. M.p.: 230–235 °C. Elemental analysis, C18H36N2S4Pt, Calc: C, 35.80; H, 6.01; N, 4.64; S, 21.24%; Found: C, 35.21; H, 6.05; N, 4.55; S, 21.35%. FTIR (m/ cm1): 3853w, 3742w, 3438br, 2957s, 2865w, 2356s, 1641w, 1510s, 1430m, 1354m, 1249s, 1150s, 1091m, 969w, 921w, 869w, 808w, 691w, 614s. 1H NMR (500 MHz, DMSO) d, ppm: 3.27–3.39 (8H, m, 4(CH2CH(CH3)2), 2.2–2.5 (4H, m, 4(CH2CH(CH3)2), 0.87– 0.92 (24H, m, 4(CH2CH(CH3)2). 13C NMR (125.65 MHz, DMSO) d, ppm: 19.62 (CH2CH(CH3)2), 28.23 (CH2CH(CH3)2), 55.85 (CH2CH (CH3)2), 208.1 (CS2). The complex [Pt(S2CNBz2)2]py (2) was prepared by following a similar procedure as for complex 1, and the resulting analytical data are as follows: [Pt(S2CNBz2)2]py (2). Yield: 1.24 g, 81%. M.p.: 250–265 °C (decomposition). Elemental analysis, C35H33N3S4Pt, Calc: C, 51.33; H, 4.06; N 5.13; S, 15.66%; Found: C, 50.78; H, 4.04; N 4.99; S, 15.77%. IR (mmax/cm1): 3742w, 3433br, 3022w, 2359w, 2921w, 1591w, 1503s, 1438s, 1350m, 1223s, 1143m, 1070w, 1029w, 981m, 923w, 881w, 812w, 741s, 694s, 627w, 556w, 514m. 1H NMR (500 MHz, DMSO) d, ppm: 7.63–9.29 (5H, m, (NC5H5)), 7.27–7.38 (20H, m, 4(C6H5)), 4.72 (8H, s, 4(CH2)). 13C NMR

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(125.65 MHz, DMSO) d, ppm: 52.0 (CH2), 123.8–149.6 (NC5H5)), 125.7–138.1 (C6H5), 211.9 (CS2). 2.3. X-ray crystallography A Stoe Mark II-Image Plate Diffraction System [32] equipped with a two-circle goniometer was used to collect the intensity data for complexes 1 and 2 at 173 K (100 °C) using Mo Ka graphite monochromated radiation (k = 0.71073 Å). Direct methods with SHELXS-97

were used to solve the structures. The refining of the structures and all further calculations were carried out using SHELXL-2014.

The C-bound H atoms were treated as riding atoms and included in calculated positions: C–H = 0.97–0.99 Å with Uiso(H) = 1.5Ueq(C) for methyl H atoms and = 1.2Ueq(C) for other H atoms. The non–H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. A MULABS routine in PLAwas used to apply a semi-empirical absorption correction. Table 1 summarizes the crystal data and refinement details for complexes 1 and 2.

TON

2.4. Thin film fabrication by AACVD Both complexes [Pt(S2CNiBu2)2] (1) and [Pt(S2CNBz2)2]py (2) were employed as precursors in AACVD. The design and infrastructure of AACVD is already known in the literature [1,24,25,33]. For thin film deposition experiments, 100 mg of each precursor dissolved in 10 mL of pyridine was used in AACVD. Prior to the deposition, the glass substrates (i.e. FTO glass) with dimensions 1.0  2.0 cm2 (W  L) were washed with soapy water, acetone and isopropanol and were then kept to air dry. For each of the deposition experiments, the substrate was loaded horizontally inside the reactor tube, heated up to a deposition temperature of 500 °C, left for 10 min to equilibrate the temperature, and then the deposition process was started. The aerosol mist from each precursor solution was generated using a piezoelectric ultrasonic humidifier and the aerosol was carried to the reactor tube by a stream of N2 gas at a rate of 120 cm3/min. The deposition experiments were continued for 30 min. The waste exhaust of the precursor mist was completely vented into a fume hood. After the depositions, the films were allowed to cool to room temperature

Table 1 Crystal data and refinement details for complexes 1 and 2. Parameter

Complex 1

Complex 2

Formula Formula weight Crystal system Space group Unit cell dimension a, b, c (Å)

C18H36N2PtS4 603.82 monoclinic P21/c

C30H28N2PtS4C5H5N 818.97 monoclinic C2/c

11.8928(5), 13.0577(5), 16.4868(7) 98.84(3) 2529.84(18) 4 5.88 0.40  0.40  0.40 203 0.71073 hmax = 25.6, hmin = 1.7 0.631, 1.000 36252, 5087, 4352

20.8211(14), 6.4105 (2), 25.3665(17) 98.766(5) 3346.2(3) 4 4.47 0.45  0.17  0.13 203 0.71073 hmax = 25.7, hmin = 2.2 0.722, 1.000 23293, 3367, 2538

0.059 0.022, 0.046, 0.95 0.79, 0.86

0.050 0.017, 0.032, 0.87 0.69, 0.59

b (°) V (Å3) Z l (mm1) Crystal size (mm) Temperature (K) Wavelength (Å) h value (°) Tmin, Tmax No. measured, independent and observed [I > 2r(I)] reflections Rint R[F2 > 2r(F2)], wR(F2), S Largest diff. peak, hole (e Å3)

under a continuous flow of N2 gas. The resultant coatings were uniform, metallic in color and reflective, like a mirror. The adhesion properties of the platinum thin films were verified by the ‘‘Scotch tape test’’ and layers were found to strongly intact with the FTO substrate. Multiple deposition experiments were performed for each sample film in order to determine the repeatability of the process, however the data presented is typical for each of the thin films synthesized. 2.5. Characterization of the thin films A Rigaku MiniFlex X-ray diffractometer (Japan) with Cu Ka1 radiation (c = 0.15416 nm) was used to record XRD patterns of the platinum thin film electrodes at a tube current of 10 mA and an accelerating voltage of 30 kV. A field emission scanning electron microscope (FESEM, Lyra3, Tescan, Czech Republic) was used to record and analyze topographical and cross-sectional images of the film electrodes at an accelerating voltage of 20 kV. The elemental stoichiometry and composition of the film electrodes were investigated by Energy dispersive X-ray spectroscopy (EDX, INCA Energy 200, Oxford Inst.). A Thermos Scientific Escalab 250Xi spectrometer equipped with a monochromatic Al Ka (1486.6 eV) X-ray source, having a resolution of 0.5 eV, was used to perform X-ray photoelectron spectroscopy (XPS) experiments. During the XPS characterization, ambient conditions of temperature were maintained, while the pressure was controlled at 5  1010 mbar. The spectra were referenced with the adventitious C 1s peak at 284.5 eV. 2.6. Fabrication of the DSSC The doctor blade method was used to coat titanium dioxide (TiO2) paste on a specifically marked area of the cleaned FTO. The TiO2 coated substrates were then calcined at 200 °C for 10 min and 455 °C for 25 min. Finally, the FTO conductive glasses, coated with TiO2, were soaked in the dye solution (N719 0.5 mM in ethanol) for 24 h. Afterwards, these photoanodes were extracted from the dye solution and rinsed with ethanol to removed unanchored dye. The electrodes prepared by the AACVD deposition of platinum using precursors 1 and 2, as well as their samples generated as function of time, were used as the counter electrodes in different measurements. Both the photoanode and counter electrode were joined together with superglue (the superglue corporation) and the iodide based electrolyte (I/I 3 ) was poured between the two joined substrates. The active area of the fabricated DSSCs was calculated to be 0.25 cm2. For comparison, similar cells were also prepared, with different platinum counter electrodes made from Pt-paste by employing doctor’s blade method. 3. Results and discussions 3.1. Pt-dithiocarbamates as single source precursors; synthesis and characterization The selection of a precursor for any metal deposition can be an important criterion in order to control the properties and performance of the resulting thin films. Metal dithiocarbamates have been extensively applied in the deposition of various metal sulfide nanostructures and provide a diversity due to a readily alterable ligand structure in the complex [29,34–36]. This allows various functionalities to be imparted upon the resultant films or materials just by the modification of the substituents in the ligand (S2CNRR0 ), thereby influencing the thermal decomposition of the precursor and hence affecting the growth patterns of the deposited materials. For instance, precursors containing heterocyclic, alipha-

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tic or aromatic substituents generate different material properties. However, if these precursors are made up of Pt metal, there is a high probability that the resulting films are composed of pure metal because of the inertness of this element. For demonstration of this idea, two different mononuclear platinum dialkylthiocarbamates, one with aliphatic substituents [Pt(S2CNiBu2)2] (1) and other with aromatic substituents [Pt(S2CNBz2)2] (2), were prepared. This was done by treating platinum(II) chloride with the sodium salts of the dithiocarbamates in a stoichiometric ratio of 1:2 in acetone-pyridine solution, as shown by Eq. (1). Acetone

PtCl2 þ 2NaðS2 CNR2 Þ !

Pyridine



PtðS2 CNR2 Þ2 þ 2NaCl



ð1Þ

where R ¼ i Buð1Þ; R ¼ Bz. The resulting complexes 1 and 2 were isolated as dry crystalline solids, which are readily soluble in dichloromethane, chloroform, DMSO, pyridine and other organic solvents. The stoichiometry of both complexes was formulated on the basis of single crystal XRD analysis and was further verified by 1H NMR, CHN and FTIR spectroscopy. The IR spectra of complexes 1 and 2 show typical absorptions in the 1650–1420 cm1 region, which are associated primarily to the stretching vibration of the C–N group present in the N–CSS moiety. The bands in the region 1030–960 cm1 represent m(CSS)sym and m(CSS)asym. The m(N–CSS) band defines that the carbon–nitrogen bond order is intermediate between a single bond (i.e., m = 1350–1250 cm1) and a double bond (i.e., m = 1690– 1640 cm1) [35]. The m(N–CSS) mode of the dithiocarbamate ligands is shifted to a higher frequency upon coordination, which is consistent with an increase in the double bond character of the carbon–nitrogen bond, thereby supporting the bidentate coordination of the S atoms of dithiocarbamate moieties with the central metal atom. The m(N–CSS) value of these synthesized complexes is comparable to other dithiocarbamates [37–39]. The usual splitting patterns for protons attached to the corresponding R groups were also observed in the 1H NMR spectra of both dithiocarbamate complexes, as expected. The spectrum for complex 2 showed multiple peaks centered between d 7.31 and 8.70 ppm, which are due to the presence of the protons of the pyridine ring. Such multiple peaks were absent in the spectrum of complex 1, which suggest the absence of pyridine in its molecular structure.

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respectively, whereas Table 2 indicates selected bond lengths and bond angles. The structure of complex 1 consists of neutral molecules, having the platinum ion coordinated by two structurally equivalent diisobutyldithiocarbamate ligands. The structure of complex 2 is also composed of a platinum ion and dibenzyldithiocarbamate ligands, but with a pyridine solvate molecule in the crystal lattice. In both complexes, the platinum atom lies on a center of inversion and adopts a distorted square planar geometry, having cis bond angles of around 75 and 105°. The trans bond angles in 1 are about 178°, while in 2, they are 180°. The dithiocarbamates bind as symmetrical bidentate ligands. The bidentate binding of two sulfur atoms of the dithiocarbamates to the platinum ion makes the S1–Pt–S2 angle significantly smaller (
75°). The average bond length of
2.32 Å for the Pt–S bonds is comparable with values found in analogous Pt-dithiocarbamate complexes [40,41], however, it is somewhat longer than observed in a Pt5 metallacycle, [PtCl(pyrrolidinedithiocarbamate)]5 [42]. The C–S distances in complexes 1 and 2 are almost identical. The shorter bond lengths for N–C(S2) as compared to N–C(C) in the dithiocarbamates correspond to a bond order that is intermediate between a single and double bond. There are no significant interactions present between the molecules in the crystal packing.

3.3. Thermogravimetric (TG) analysis The pyrolysis characteristics of both complexes were examined by thermogravimetric analysis (TGA). Fig. 3 compares the TG curves of both complexes recorded in the temperature range 35– 700 °C, under a continuous flow of N2 gas (20 mL/min) and at a heating rate of 10 °C/min. Fig. 3 reveals that the decomposition of complex 1 occurs in a single step (black curve), while degradation of complex 2 is accomplished in multi-steps (green line), until both convert into the final products. Complex 1 remains stable up to 270 °C and the diisobutyl dithiocarbamate moiety is lost in the discrete temperature range 275–450 °C, leaving a residue of 32.5% at 500 °C. This residual weight (32.5%) matches well with the theoretical weight percentage of 32.3% calculated for pure platinum metal from complex 1. Contrary to complex 1, the thermal degradation of complex 2 starts very early and the first weight loss

3.2. X-ray structures description The molecular structures of the so formed complexes [Pt(S2CNiBu2)2] (1) and [Pt(S2CNBz2)2]py (2) are provided in Figs. 1 and 2,

Fig. 1. The molecular structure of the complex [Pt(S2CNiBu2)2] (1) with the atom labeling. The displacement ellipsoids are drawn at the 50% probability level.

Fig. 2. The molecular structure of the complex [Pt(S2CNBz2)2]py (2) with the atom labeling. The displacement ellipsoids are drawn at the 50% probability level.

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Table 2 Selected bond lengths (Å) and bond angles (°) of complexes 1 and 2. Bond Lengths

(Å)

Bond Angles

(°)

Precursor 1 Pt1–S1 Pt1–S2 Pt1–S3 Pt1–S4 C1–S1 C1–S2 N1–C1 N1–C2

2.3290(8) 2.3099(9) 2.3255(8) 2.3172(8) 1.727(3) 1.729(3) 1.317(4) 1.481(4)

S1–Pt1–S2 S1–Pt1–S3 S1–Pt1–S4 S2–Pt1–S3 S2–Pt1–S4 S3–Pt1–S4 N1–C1–S1 N1–C1–S2

75.06(3) 106.22(3) 178.67(3) 178.72(3) 103.68(3) 75.04(3) 123.93(12) 128.61(11)

Precursor 2 Pt1–S1 Pt1–S2 C1–S1 C1–S2 N1–C1 N1–C2

2.3283(7) 2.3159(6) 1.723(2) 1.720(3) 1.319(4) 1.480(3)

S1–Pt1–S2 S1–Pt1–S2i S2–Pt1–S2i N1–C1–S1 N1–C1–S2

74.95(2) 105.05(2) 180.0 123.93(12) 128.61(11)

Fig. 4. XRD patterns of cubic-platinum films deposited from the precursors [Pt (S2CNiBu2)2] (1) (black line) and [Pt(S2CNBz2)2]py (2) (green line) on a plain glass substrate at 500 °C via AACVD.

Fig. 3. Thermogravimetric (TG) curves of complexes 1 and 2 recorded in flowing N2 gas (20 ml/min) at heating rate of 10 °C/min.

step falls in the temperature range 70–115 °C and is attributed to the removal of the solvent molecules (pyridine) present in the crystal lattice of complex 2. The multi-decomposition steps in the temperature range 115–500 °C are attributed to the complete elimination of the dibenzyl dithiocarbamate group, producing a sustainable residue mass of 24.8% which matches well with the theoretical mass (
24.0%) estimated for pure platinum from complex 2. Further annealing beyond 500 °C did not produce any significant change in the residue weight, suggesting the end of the weight loss features and formation of a product that remains stable even at the higher temperature of 700 °C.

3.4. XRD studies Both the molecular precursors 1 and 2 in solution form were applied in AACVD and the thin film growth was first examined on plain glass substrates at 500 °C in a N2 atmosphere. The obtained films were investigated by powder X-ray diffraction and the results are shown in Fig. 4. Apparently, the XRD patterns of both films look similar in terms of peak positions and reveal the formation of similar products. The diffraction peaks originating at 2h = 40.0, 46.5, 68.0, 81.0 and 86.4° corresponds to the reflection planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) respectively. The peak positions, peak intensities and d-spacing values of XRD patterns matched well with those of the standard cubic platinum pat-

tern (01-087-0647), suggesting the synthesis of pure platinum thin films from both precursors. In both cases, the product is highly crystalline and all the peaks are well resolved. Any kind of crystalline impurities, such as oxide or sulfide formation or other crystalline phases of platinum, were not identified from these XRD patterns. Another feature that is visible in this XRD analysis is the strong preferential growth of the crystalline Pt films in the direction of the (1 1 1) plane; this is also true for a previous description of the growth of similar platinum films. Schlupp et al. demonstrated the AACVD generation of Pt-films on Si/SiOx as well as polycrystalline YSZ substrates using Pt(acac)2 which show the same pattern of the preferred orientation as the films grown herein on plane glass substrates [1]. Therefore, it can be concluded that the growth of Pt is not dependent upon the nature or orientation of the substrate, rather it is controlled by the closely packed (1 1 1) planes having a low surface energy. The effect of the precursor is also shown not to influence the crystalline nature of the deposited Pt films, however, it might have a role in the growth patterns due to differences in the decomposition process in the aerosol. Moreover, the thickness of the film is a representation of the crystalline size in the preferred orientation. 3.5. Surface morphology and compositional investigations The surface morphologies of the platinum thin films developed on FTO glass substrates using both precursors 1 and 2 were investigated by SEM and the recorded micrographs are displayed in Fig. 5. The low resolution images (a and b) show the growth of uniform films, with the complete coverage of the substrate surface in a time of 30 min. Even these low resolution images show that the nature of the precursor has affected the film growth pattern in terms of film microstructure. The surface coverage of the film deposited from 1 is
85%, with parts of the underlying substrate clearly visible. On the other hand, the surface coverage in the case of deposition from 2 is more than 98%, with only very thin fault lines in the structure. In comparison to a previously reported deposition on silicon based substrates, the rate of deposition is quite high and comparable to the YSZ substrates [1]. The high resolution images of the same films were taken in order to more deeply explore the microstructures, which further demonstrates the effect of the precursor. For precursor 1, the film is composed of a network of interconnected nanorods, with empty spaces in between. However, for precursor 2, the film developed into a network of islands with small fractures in between. Such a film can have a high perco-

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Fig. 5. SEM images of platinum thin films. The low (a and b) and high (a1 and b1) resolution surface images deposited from the precursors [Pt(S2CNiBu2)2] (1) and [Pt (S2CNBz2)2]py (2), respectively, on FTO glass substrates. The corresponding crossectional SEM images (a2 and b2) of the films.

lation affect, leading to lesser sheet resistance that is moving towards a value of the bulk material, and thus, it can be more applicable in various electrode systems. However, there is no clear agglomeration of the Pt nuclei to form large particles, rather there is a homogeneous growth of the film, which is most often seen in AACVD depositions. Further, the grain boundaries of the crystallites from precursor 1 are in the size range of 50–100 nm and can be clearly marked. Unlikely, the grain boundaries of the platinum film produced from precursor 2 lie in the size range of 0– 20 nm and diffuse into each other. The Fig. 5a2 and b2 display cross sectional views of the platinum films, which reveal that both films are compact in nature and uniform in thickness. Thicknesses of around 550 and 600 nm have been estimated. The elemental stoichiometry of the thin films was observed through energy dispersive X-ray analysis. Fig. 6 shows the EDX spectra of the films and the platinum element is clearly detected under the microscope. Elements from the FTO substrate, such as Sn, Si and O, were not excluded from both spectra. Remarkably, there is no sulfur contamination indicated, as is usually the case in depositions from dithiocarbamate ligand-based precursors. On most occasions, sulfur is a part of the crystal lattice, present as the metal sulfide. This shows that the dithiocarbamate moiety is completely removed upon thermal decomposition of both precursors under the AACVD course of reaction, and only platinum is deposited as the final product from both complex precursors. The visual appearance of the platinum films seemed to be silverish-white and reflective like a mirror. The Pt films were adhesively bonded with the FTO substrate and adequately qualify the scotch tape test. The grown films were not spoiled upon exposure to air, moisture and even dipping in the liquid electrolytes, which reveal the good quality of the Pt films prepared through AACVD. In AACVD, the solvent plays a critical role in building the appropriate nano/microstructure of the thin films by controlling the homogeneous and heterogeneous CVD reactions [29,43]. The phys-

Fig. 6. EDX spectra of platinum thin films deposited from the precursors (a) [Pt (S2CNiBu2)2] (1) and (b) [Pt(S2CNBz2)2]py (2) on FTO glass substrates.

ical properties of a solvent, such as boiling point, density and heat of evaporation, can significantly alter the homogeneous and heterogeneous gas phase reactions and thus have a strong impact

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on the properties (adhesion and growth pattern) of the resultant thin films [29,43]. The platinum complexes synthesized in this work are soluble in many other solvents, such as dichloromethane, chloroform and DMSO, along with pyridine. However, the boiling points of dichloromethane and chloroform are 39.6 and 61.5 °C respectively, which are considered as low boiling solvents for AACVD work. These low boiling solvents generally evaporate at a faster rate and precursor precipitates accumulate on the surface of the heated substrate through homogeneous CVD gas phase reactions. On the other hand DMSO has a relatively high boiling point (189 °C) and would lead towards heterogeneous CVD reactions. Completely homogeneous or completely heterogeneous reactions are not favorite for the production of good quality thin films [43]. Contrarily, pyridine, with a boiling point of 115 °C, is an intermediately boiling solvent as compared to DMSO and chloroform type solvents and can create a balance between homogeneous and heterogeneous gas phase reactions which is essential to achieve good quality adhesion of contamination free thin films. Further, the purity and chemical composition of the platinum thin films were studied by XPS spectroscopy. The high resolution XPS spectra of the Pt films surface are presented in Fig. 7. Pt doublet peaks were positioned at 71.1 (4f 7/2) and 74.4 eV (4f 5/2), that concur with the literature values for pure Pt metal [44,45]. 3.6. Pt-film electrodes as counter electrodes in DSSC After complete characterization of the Pt-films deposited via AACVD of the prepared precursors, both films were employed as counter electrodes in self-fabricated DSSCs and the resulting performance of the solar cells was evaluated in comparison with counter electrodes prepared by the doctor’s blade method. The DSSCs were prepared using a TiO2 coated anode and different kinds of Pt coated counter electrodes. The fabricated solar cells were tested under simulated solar light of 100 mW/cm2. For sustaining devices with high sensitizer loadings, the role of the counter electrodes is highly significant as reduction of triiodide ion takes place at the counter electrode and electrolyte interface, which in turn provides sufficient iodide ions for the regeneration of the dye [4]. For this purpose, the electrocatalytic activity as well as the conductivity of the films in the counter electrodes has to be optimized so as to obtain a low charge transfer resistance and a low overpotential for the redox species to be regenerated. To attain these features, the morphology of the films and the microstructure has to be free from agglomerations and structural defects in connectivity,

even at high metal loadings. SEM data has already shown that both films generated from the two different metal complexes have very similar metal loadings, which is probably a time dependent attribute. However, the nature of the precursor dictates the decomposition behavior, leading to different types of connectivity between the deposited metal nuclei. Consequently, the film made up of complex 2 has a higher connectivity, as indicated by SEM. When both of these films were applied as counter electrodes in DSSC, the resulting photovoltaic performances of the cells were different from each other too. As the current-voltage characteristics of these films in Fig. 8 indicates, the current density of the film made up from precursor 2 is much better than that from precursor 1, as its film has a higher coverage as well as a higher conductivity. The corresponding photovoltaic parameters, along with the data from a commercial configuration Pt-metal based solar cell, are summarized in Table 3. As indicated, the photovoltaic performance can be attributed to a few critical factors, such as short circuit current density Jsc (mW/ cm2), open circuit voltage Voc (mV), the fill factor FF and the energy conversion efficiency g (%). In addition, the values of the series resistance (Rs) and shunt resistance (Rsh) are also reported. It is noteworthy that the fill factor was almost unaffected on changing the type of dithiocarbamate precursor in making the counter electrode, whereas the open circuit potential is higher in case of precursor 2. The higher Voc value means a decreased possibility of recombination at the photoanode and electrolyte interface. As a result, the short circuit current as well as the efficiency of the resulting cells is higher for the films made from precursor 2. Moreover, these performance values from both electrodes are comparable to the values obtained from a homemade cell fabricated in a commercial configuration. The efficiencies in these cases are calculated from an active electrode area of 0.25 cm2, providing efficiencies in the same range as per the earlier reports. On the other hand, the FF value is dependent upon various factors, including two very important parameters, Rs and Rsh. As shown in the table, lower resistance values force the FF value of the commercial configuration to be lower than AACVD-made electrodes, and so the efficiency as well as the current density of this cell is slightly higher as compared to the cell made of the counter electrode using precursor 2. Thus, it can be concluded that the conductivity of the films can control the catalytic efficiency. In order to further understand the effect of the metal loadings, reflectivity, connectivity and the resulting conductivity of the films on the performance, we performed a time dependent study of the

14.00

Current density (mA/cm2)

Pt Complex 1 Pt Complex 2

12.00 10.00 8.00 6.00 4.00 2.00 0.00 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Voltage (V) Fig. 7. High resolution XPS spectra of platinum thin films deposited from the precursors [Pt(S2CNiBu2)2] (1) (black line) and [Pt(S2CNBz2)2]py (2) (green line). (Color online.)

Fig. 8. Current–voltage characteristics of fabricated DSSC with films formed from the two different Pt-thiocarbamate complexes with a deposition time of 30 min.

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M.A. Ehsan et al. / Polyhedron 166 (2019) 186–195 Table 3 Photovoltaic properties of fabricated DSSCs for both Pt complexes. Cell Structure

Jsc (mA/cm2)

Voc (mV)

FF

g (%)

Rs

Rsh

TiO2/N719/Pt complex 1 TiO2/N719/Pt complex 2 TiO2/N719/Pt commercial

11.40 13.34 14.62

751 765 771

0.432 0.423 0.389

3.69 4.32 4.40

169 168 153

4463 3485 1106

a thickness of 600 nm, however, that also results in some hairline cracks appearing in the thick metal film. As a result, the connectivity and conductivity start to diminish. When all three of the time controlled films were applied as counter electrodes in DSSCs, we obtained coherent data on the photovoltaic performance, as shown in Fig. 10 and Table 4.

18.00 10 mints

Current density (mA/cm2)

formation of the films from precursor 2 as it showed a better performance than the other film. For this purpose, the films were prepared from precursor 2 with deposition times of 10 and 20 min, in addition to a 30 min deposition, and the SEM micrographs of these films are shown in Fig. 9. If we compare them with the precursor 2 film microstructure in Fig. 5, it becomes evident that a regular growth pattern of the metal films is being followed. The deposition pattern of the metal films follow the Volmer-Weber growth mode in the form of an island [46], which significantly affect the connectivity and the resultant conductivity of the films. This phenomenon is particularly expressive when the film thickness is in the range of the electron mean free path (MFP), even changing the film behavior to an insulator when the thickness reaches the percolation length. According to this growth pattern, the films start to form spherical nanosized entities from the original metal nuclei, which then coalesce together to form networks or films. A 10 min deposition, shown in Fig. 9, indicates that the film has already attained a thickness of 140 nm, in which a closely wound network has already been formed. However, this network has nano-spaces in between the agglomerated particles over the surface, as shown by the low resolution image. After a deposition time of 20 min, the coalescence of the materials is quite enhanced, leading to a completely homogeneous metal film with no agglomeration or empty spaces. The film thickness reached 200 nm with no visible cracks. If this is compared to the 30 min deposition, it is clear that the growth rate of the films is very high in this period because of the increased concentration of possible atomic nuclei, reaching to

16.00

20 mints 30 mints

14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Voltage (V) Fig. 10. Current–voltage characteristics of fabricated DSSCs with Pt complex 2 having various deposition times.

Fig. 9. SEM images (low resolution, high resolution, and cross section) of platinum thin films deposited from precursor [Pt(S2CNBz2)2]py (2) with 10 min and 20 min deposition times.

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Table 4 Photovoltaic properties of fabricated DSSCs for Pt complex 2 for various thicknesses. Cell Structure

Jsc (mA/cm2)

Voc (mV)

FF

g (%)

Rs

Rsh

TiO2/N719/Pt (10 min) TiO2/N719/Pt (20 min) TiO2/N719/Pt (30 min)

12.90 17.55 13.34

764 764 765

0.425 0.371 0.423

4.18 4.97 4.32

167 170 168

9812 855 3485

This table indicates that the open circuit potential for all three films remain the same and looks like it is a material property. However, the Rsh values for 10 and 30 min deposition are very high as compared to the 20 min deposition. As a result the value of the fill factor is also smaller in comparison to the other two films, providing the highest efficiency of 4.97% with a current density of 17.55. These values are even higher than the commercial configuration we used in this study. These results quite correctly correspond to the surface characteristics of the film. The higher the surface connectivity and homogeneity, the higher is the conductivity, as well as the catalytic efficiency, of the resulting film. A visual inspection of the films also proves this coherency as the films deposited for 20 min shows a mirror-like surface with high reflectivity, whereas the 10 min deposition provides a bit of a dull surface and the 30 min deposition generates a slightly rough surface. The enhancement in the short circuit current and efficiency can therefore be attributed to the following two factors: (1) the suppression of charge recombination due to better catalytic activity and better morphology of the counter electrode surface [47,48] and (2) enhancement due to the more reflective property of the counter electrode film. When the counter electrode is more reflective, more photons will be available for excitation of the dye, hence more photoelectrons will be generated, which ultimately can be collected by the outer circuit. The optimized performance of a novel Pt-dithiocarbamate precursor is thus shown to have better and promising efficiency for a fabricated DSSC using the AACVD method to prepare platinum counter electrodes with a very low thickness and an ideal morphology. This gives an insight for future utilization of these counter electrodes for DSSC fabrication on a large scale as well as for other applications using Pt-film electrodes.

4. Conclusions Two dithiocarbamate precursors of Pt with different alkyl moieties were crystallized and fully characterized in this work in order to be effectively applied in AACVD depositions of platinum films and for their further use as counter electrodes in DSSCs. As a single source precursor of Pt metal, the studied metal complexes exhibited a high growth rate (i.e.,
600 nm in 30 min) and highly pure metal films were obtained. However, the growth structure was different for the precursor containing the aromatic substituents, following a well-defined Volmer-Weber model, thereby forming mirror-like, well-connected and highly conductive films of
200 nm thickness in 20 min. A further increase in time lead to an island growth pattern, making some hairline cracks appear, and the conductivity as well as the catalytic performance consequently were diminished. All the performance parameters, including current density, open circuit potential, fill factor and the efficiency, were compared with cells made up from commercially usable Pt-paste electrodes using the doctor’s blade method and the performance of the AACVD based films were found to be slightly better. This is in fact remarkable, keeping in view the speed, flexibility, simplicity and controllable growth of the films as a function of time using this versatile method. Further work is underway in our lab to utilize similar Pt films in various other applications, including electrochemical sensors and energy harvesting.

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