Adsorption Kinetics of WS2 Quantum Dots onto a Polycrystalline Gold

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Adsorption kinetics of WS2 quantum dots onto a polycrystalline gold surface MANILA Ozhukil Valappil, Mekkat Roopesh, Subbiah Alwarappan, and Vijayamohanan K. Pillai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03321 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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Adsorption kinetics of WS2 quantum dots onto a polycrystalline gold surface Manila Ozhukil Valappil, †,# Mekkat Roopesh ‡, Subbiah Alwarappan †#*, and Vijayamohanan K Pillai†# †CSIR-Central Electrochemical Research Institute, Karaikudi 630003, Tamilnadu, India. ‡Indian Institute of Science Education and Research, Thiruvananthapuram, Kerala, India. #Academy for Scientific and Innovative Research, New Delhi, India

ABSTRACT: In this work, we report the adsorption kinetics of electrochemically synthesized WS2 quantum dots (ca. 3 nm) onto a polycrystalline gold electrode. Langmuir adsorption isotherm approach was employed to explore the temperature and adsorbate concentration dependence of experimentally calculated equilibrium constant of adsorption (Keq) and free energy for adsorption (ΔGads). Subsequently, we extract other thermodynamic parameters such as adsorption rate constant (Kads), desorption rate constant (Kd), the enthalpy of adsorption (ΔHads) and the entropy of adsorption (ΔSads). Our findings indicate that ΔGads is temperature dependent and ca. -7.64 ± 0.6 kJ/mol, ΔHads = -43.72 ± 1.7 kJ/mol and ΔSads = -0.126 ± 0.017 kJ/ (mol.K).These investigations on the contribution of the enthalpic and entropic forces to the total free energy of this system underscore the role of entropic forces on the stability of the WS2 QDs monolayer and provide new thermodynamic insights into other TMDQDs monolayers as well.

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1. INTRODUCTION Rational designs for organizing metal nanoparticles and semiconductor quantum dots have received immense scientific acclaim due to the flexibility in the particle size tailoring and tunability of the interparticle interaction. Such engineered architectures exhibit unique electronic, optic and magnetic behavior.1–5Adsorption chemistry of molecules is one of the promising strategy for patterning such well-defined monolayers on noble metal surfaces. Molecules with appropriate geometry capable of organizing in two dimensions and with ability to bond to a surface are potential candidates for adsorption. However, the most studied adsorption-driven self-assembly chemistry is the adsorption of alkanethiols, sulfides and disulfides monolayers onto gold surface to

form a self-assembled monolayer (SAM).6–20 Such architectures are

explicitly preferred for the development of biosensors and bioelectronics circuitries due to the ease of miniaturization, high degree of mimicry with cellular environment albeit the ordered structure, ease of immobilizing biological molecules and moreover the facile procedure for SAM formation.17,21,22 Nonetheless, this chemistry has been well exploited for the self-assembly of inorganic semiconducting quantum dots (QDs) and nanoparticle superlattices onto a noble metal (preferably gold) surface as well.19,20,23–25 Interestingly, all of these approaches prefer either a sulfur containing capping agent or a bi-functional sulfur-based linker.

26,27

Such linkers are

capable of providing a better control over interparticle interaction and act as a building block on the substrate while they can also disorder the lattice. Recently, zero dimensional quantum dots derived from two-dimensional layered materials such as graphene, transition metal dichalcogenides (TMDs,MoS2,WS2,WSe2,MoSe2 etc.), hexagonal boron nitride, antimonene etc. are of wide interest due to their

interesting

optical,chemical,catalytic ,electronic and electrochemical properties.28 Very recently, transition


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metal dichalcogenides quantum dots (TMDQDs) have gained critical interests, ranging from theoretical predictions to experimental observations.29–36 The first report on the synthesis of WS2 quantum dots (QDs) appeared only in 2013.37 In addition, efficient and large scale synthesis of TMDQDs and attempts to understand their properties (especially WS2 QDs) are yet not well understood. In this context, we have recently reported a one-step electrochemical synthesis of WS2 QDs at room temperature.38 Methods to immobilize such QDs on specifically designed areas of these devices are essential requisites in fabrication aspects. Furthermore, coverage of the QDs immobilized on the designed areas, is another key factor in assembling QD-based devices. For example, in a single-electron device seen in an integrated circuit, a single or a few QDs, which functions as a Coulomb island, must be fixed between two electrodes.39 Interestingly, amongst all the above mentioned QDs of 2D atomic layers, the TMDQDs possess an advantage of chalcogenide-rich edges, which can directly attach on to the gold electrode surface. Wang et al. synthesized molybdenum disulfide (MoS2) QDs with diameters of 1.47± 0.16 nm from bulk MoS2 by a combination of ultra-sonication and centrifugation.40 The QDs were then assembled on a gold surface to form a film which exhibited efficient electrocatalytic activity for hydrogen evolution reaction (HER).40 Semiconductor QDs derived from 2D materials are promising candidates for a wide range of applications such as electronic and optoelectronic devices ( for example, field electric transistors (FET)),

photovoltaics

light-emitting

diodes,

bio

imaging,

sensors,

cancer

therapy,

electrocatalysis etc.41 However, from an application perspective, the stability of these QDs especially when adsorbed or deposited onto a substrate is very critical, which remains inadequate. This requires more studies on the adsorption kinetics and reactivity, the nature of interaction between the QDs and the substrate and so on. Virtually, while critical investigations


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on the applications of such ordered systems in diversified fields have been endeavored, attempts for a direct or a sequential organization of such QDs on a metal substrate are unknown till date and notably there are no efforts to understand the adsorption kinetics of such TMDQDS onto a gold surface. WS2 QDs being an emerging class of zero dimensional material, we chose to probe their adsorption kinetics onto a polycrystalline gold electrode which can be extended to other TMDQDs as well. In this context, herein, we investigated the possibility of an adsorption strategy to attach WS2 QDs onto a gold electrode, understand their kinetics of adsorption and to provide a model so as to gain a thermodynamic insight of adsorption of such a relatively obscure system. Moreover, we realized that the chalcogenide-rich edges of the WS2 QDs negate the use of any linkers where the reported methods for other QDs such as ZnS, CdSe, and CdS etc. required a bi-functional linker for the assembly. Our experimental data exhibited a kinetic response which is in excellent agreement with the Langmuir adsorption isotherm. By modeling the experimental adsorption data using this isotherm, the adsorption and desorption rate constants, equilibrium constant and free energy change were elucidated. In addition, we investigated the temperature dependence of the equilibrium constant Keq to determine the enthalpy of adsorption. Further, from free energy of adsorption and enthalpy of adsorption, the entropy of adsorption, ∆Sads was determined. Finally, the mass change of the electrode during the adsorption process was evaluated using a quartz crystal microbalance (QCM).The mass change information obtained from QCM resulting from the adsorption of WS2 QDs onto the gold allows access to direct information on the formation of the monolayer. 2. EXPERIMENTAL SECTION 2.1 Materials


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Bulk Tungsten (IV) sulfide (WS2, 99.9 %) was purchased from Alfa Aesar. Lithium perchlorate (LiClO4, 99.9%) was obtained from Sigma Aldrich and Propylene Carbonate (C4H6O3, 99%) and Potassium Chloride (KCl,99%) were procured from Merck. All the chemicals were used as received. All aqueous solutions were prepared using deionized water (Merck millipore, Milli-Q, 18 MΩ cm). 2.2 Electrochemical synthesis of WS2 QDs WS2 QDs were synthesized according to a one-step top-down approach reported recently by our group.38 The method involves an electric field induced transformation of bulk WS2 powder into WS2 quantum dots. Briefly, the exfoliation was carried out using a three-electrode system. Bulk WS2 powder (Alfa Aesar) was made into 10 mm diameter pellets and used as working electrode. Pt mesh was used as counter electrode and Pt wire as quasi reference electrode. Deoxygenated propylene carbonate containing 0.1 wt % of LiClO4 was used as the electrolyte and a fixed potential of 2.0 V was applied to the system for 24 h. The QDs were collected from the electrolyte by purification to remove the lithium ions. The particle characterizations for WS2 QDs are discussed in the supplementary information. The transmission electron microscopic investigation showed that the QDs are of average 3 nm size (Figure S1, Supporting information). The crystallinity of the QDs was investigated using X-ray diffraction (Figure S2, Supporting information) and the optical properties were explored using UV-Visible and photoluminescence spectroscopy (Figure S3, Supporting information). 2.3 Adsorption of WS2 QDs Adsorption experiments were carried out on a 1.5 mm diameter polycrystalline gold electrode (CH Instruments). Prior to the experiments, the electrode was polished using alumina powder (0.1 micron) followed by electrochemical cycling between -0.5 and 1.5 V vs. Hg/HgSO4 in 0.5 M


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sulfuric acid until three distinct peaks of the gold oxide formation were displayed. The as cleaned electrode was incubated in WS2 QDs solution of various concentrations (C1=1.067 mol L-1, C2= 0.5337 mol L-1, C3=0.3557 mol L-1, C4= 0.2668 mol L-1 and C5 = 0.2134 mol L-1). The incubation time was varied from 5 minutes to overnight. The electrode was then taken out and rinsed with deionized water and dried for further electrochemical characterization. The same incubation protocol was followed for measuring the adsorption kinetics of 1-decanethiol (in ethanolic solution of 3 mM 1-decanethiol). 2.4 Open Circuit Voltage (OCV) and Quartz crystal microbalance (QCM) measurements. OCV measurements were performed in aqueous 0.1 M KCl solution with a three electrode configuration where the WS2 QDs adsorbed gold electrode is the working electrode, Saturated Calomel Electrode (SCE) is the reference electrode and Pt wire is the counter electrode. In order to quantify the mass change of the electrode, we performed in situ QCM measurements. A QCM fabricated with gold coated quartz crystal measures the mass changes of the gold electrode by virtue of the change in oscillation frequency of the quartz crystal.42 The decrease in oscillation frequency of the quartz crystal electrode will be attributed to the increase in the adsorbed mass and vice versa. This frequency change and mass change relationship is well established by a simple equation known as Sauerbrey equation shown below:42 ∆f = −Cf ∆m

(1)

where ∆f is the change in oscillation frequency of the quartz crystal (Hz), Cf is the proportionality constant for the 5MHz overtone polished crystal (56.6 Hz cm2 µg-1) and ∆m is the mass change (µg cm-2).This equation has been widely adopted for the calculations of mass changes corresponding to the monolayer formation of alkanethiols on gold electrode.6,43–45


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For the in situ QCM measurements, the gold coated quartz crystal was immersed in WS2 QDs solution. AT- cut quartz crystals with vapor deposited gold electrodes (1’’ diameter) were procured from Stanford Research Systems and employed for analysis using QC microbalance ( QCM 200, 5 MHz resonance frequency). Despite several material parameters that affect the QCM resonant frequency response, the sensitivity towards temperature fluctuations is rather high. In order to minimize such frequency drifts contributed thermal fluctuations, the temperature of the QDs solution and QCM were kept under control using a temperature controller at 298 K. Measurements were performed in a 200 mL double walled beaker connected to a flowing liquid temperature controller (JULABO FT402). The change in frequency of the crystal was monitored as a function of time. 3. RESULTS AND DISCUSSION The key parameter we measure here in elucidating the adsorption kinetics of WS2 QDs is the OCV measurements. OCV is the voltage of the working electrode (here gold electrode) measured with respect to a reference electrode when not connected to any load in a circuit. Conventionally, for a given electrode, the OCV should not change if there is no reaction taking place at the electrode surface. Interestingly, we observed a drastic change in the OCV when the electrode was immersed in WS2 QDs solutions (at various concentrations). The OCV increases with respect to immersion time and concentration of the WS2 QDs (Figure S4, Supporting information). Gradually, the OCV attained saturation over a particular period of time which indicated that an equilibrium is established. More significantly, our experimental data exhibited a kinetic response that enabled us to model and investigate this reaction using Langmuir adsorption isotherm over a range of concentration. In order to provide a frame of reference, we would like to briefly recap the main assumptions involved in Langmuir adsorption isotherm:46


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1. All the surface sites are equivalent 2. Adsorption at one site is independent of the adsorption at the adjacent site. 3. Adsorption is restricted to monolayer The first assumption is that the adsorbent is devoid of defects which is in general, physically unrealistic in most of the systems. Despite the fact that the presence of adsorbed adatoms, surface roughness, grain boundaries and polycrystallanity of the gold electrode cause modulation in surface energies, the resultant change in the surface energies are considered negligible in order to validate the Langmuir adsorption.47 The second assumption is significant only when there is a complete coverage and the QDs likely interact with each other. Notably, we assume that the time scale required to proceed the interaction between adjacent QDs exceed the time scale associated with the formation of monolayer.


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The OCV value allows direct access to the coverage of the WS2 QDs as it is a measure of extend of electrode exposure. Hence this enables us to normalize the OCV values (OCV/OCVmax) and the relative coverage (θ) of the WS2 QDs (Figure 1) were determined.

Figure 1. Variation of relative saturation coverage (θ) as a function of incubation time for different concentration of WS2 QDs at 298 K.

Conventionally, the coverage is expressed as a fraction of occupied adsorption site (unitless) (0≤ θ ≥1). The calculated coverage for different concentrations is represented in Table 1. A steep increase in coverage is witnessed during the initial period of adsorption which then attains saturation coverage. The incubation time and concentration dependence of this response indicates a monolayer formation. The steady state coverage attains unity at a higher


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concentration. From the values mentioned in Table 1 an important physical quantity, the equilibrium constant, Keq can be obtained by the following equation.

=

(1)

here is the steady-state coverage, C is the concentration and Keq is the equilibrium constant.6 Table 1 lists the equilibrium constants determined for various concentrations of WS2 QDs. The equilibrium constant data enables us to calculate the free energy change of adsorption, ΔGads according to the following expression which relates the free energy change and equilibrium constant.47 ∆G = −RT lnKeq

(2)

The calculated ΔGads is shown in Table 1 WS2 QDs Relative saturation Keq (M-1) concentration coverage(Ꮎ) (mol L-1)

∆Gads (kJ mol-1)

0.5337

0.9102

22.29 ± 5.81

-7.64 ± 0.6

0.3557

0.81538

12.42 ± 4

-6.1 ± 1.17

0.2668

0.7692

12.5 ± 5.1

-6.03 ± 1.48

0.2134

0.5179

5.04 ± 1.6

-3. 84 ± 1.09

Table 1. Calculated coverage, Keq and ΔGads for various concentrations of WS2 QDs at 298 K a a

(The uncertainty in ΔGads is determined by the uncertainty in the propagation of Keq)

As evident from Figure 1, after a long time interval, the system attains equilibrium and the rate of diffusion of WS2 QDs towards the gold electrode interface becomes sluggish. This results in the small rate of change of coverage with time and the experimental errors associated with the


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measurements will be high.48 It can be assumed in a simple Langmuir adsorption that, during the initial stage of adsorption, the rate of desorption is negligible.

49

Therefore, it is possible to

determine the adsorption rate constant, Kads based on the following rate equation.49 ln1-θ=K ads c t

(3)

where θ is the coverage of the adsorbate, c is the concentration of the adsorbate is the immersion time. Based on equation (3), during the initial adsorption step, the relationship between ln(1- θ) and t should be linear. Thus, it is possible to determine the Kads from the slope of the ln(1- θ) vs. t. Figure 2 shows the plot of ln(1- θ) vs. t for a typical concentration , c=0.5335 mol L-1 (For other concentration variations ,see Figure S5 in the Supporting information).

Figure 2. Relation between ln(1- θ) and incubation time t in the initial period of the adsorption of WS2 QDs onto a gold electrode at 298 K. ACS Paragon Plus Environment

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The Kads values thus calculated using the slope of ln(1- θ) vs t plot for various concentrations of WS2 QDs are listed in Table 2. The equilibrium constant data determined from Table 1 and the adsorption rate constant data listed in Table 2 enable us to calculate the desorption rate constant Kd for the system as per the following equation.47 K =

K

(4)

K

The calculated Kd values using Kads and Keq for different concentrations of WS2 QDs are listed in Table 2. Concentration Kads (mol L-1) M-1 s-1

Kd s-1

0.5337

1.74 ± 0.6 x 10-3

0.95 ± 0.24 x 10-4

0.3557

1.86 ± 0.73 x 10-3

1.49 ± 0.38 x 10-4

0.2668

1.5 ± 0.52 x 10-3

1.2 ± 0.3 x 10-4

0.2134

0.6 ± 0.2 x 10-3

1.21 ± 0.29 x 10-4

Table 2. Adsorption and desorption rate constants determined for various concentration of WS2 QDs adsorbed onto a gold electrode at 298 K The values of ∆Gads at 298 K listed in Table 1 indicates that these values are relatively small, approximately in the order of hydrogen bond interaction characteristics observed in alcohols (2-6 kcal mol-1).50 These values indirectly correspond to the labile nature of the monolayer; viz. they exhibit a dynamic behavior. The value of ∆Gads alone is inadequate to explain this labile nature of the monolayer as it can be induced by either the weak adsorbent-adsorbate interactions or the counterpoise between the enthalpic (∆Hads) and the entropic (∆Sads) forces. In order to determine the contribution of the enthalpic and the entropic forces to the free energy change, we performed temperature dependent adsorption experiments for the same system at certain temperatures in the


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range between 305 – 341 K. Upon increasing the temperature, the relative saturation coverage decreased significantly (See, Figure S6 in the supporting information). Further, we measured the temperature dependence of Keq and ∆Gads (for a fixed concentration= 0.5335 mol L-1) corresponding to the adsorption of WS2 QDs onto the Au-surface. Though, we have performed experiments for other concentrations, we noticed better results for the concentration 0.5335 mol L-1. Maximum coverage is also noticed at this concentration. However, ∆Sads can vary with WS2 QDs concentration. At equilibrium, we have shown that the coverage of WS2 QDs on Au - surface is concentration dependent. In addition, the adsorption enthalpy (∆Hads) measured was attributed to the change in enthalpy due to the monolayer formation (assuming a negligible contribution from solvation enthalpy). The enthalpy change due to the monolayer formation is dominated by the Au-S bond formation and this is a function of QDs concentration (i.e., number of QDs available to form a monolayer). Temperature (K)

Keq (M-1)

∆Gads (kJ mol-1)

305

7.25 ± 0.4

-5.02 ± 0.12

315

3.7 ± 0.97

-3.37 ± 0.70

328

2.37 ± 0.63

-2.3 ± 0.72

341

1.17 ± 0.11

-0.44 ± 0.28

Table 3. Temperature dependence of the equilibrium constant (Keq) and the Gibbs free energy of adsorption (∆Gads) of WS2 QDs on a gold electrode. The data presented in Figure 3a exhibit a decrease in Keq with increase in temperature with a concomitant increase in ∆Gads (Figure 3b). Based on the reason that a spontaneous reaction condition is always associated with - ∆Gads , the observed increase in ∆Gads here indicates that the adsorption process associated with our system is exothermic.6


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Figure 3. Temperature dependence of a) Keq b) ∆Gads for the adsorption of WS2 QDs onto a gold surface. From Figure 3b, the enthalpy of adsorption (∆Hads) and the entropy of adsorption (∆Sads ) have been determined. The slope of Figure 3b yields a slope of ∆Sads= -0.126 ± 0.017 kJ/ (mol.K) and an intercept of ∆Hads = -43.72 &1.7 kJ mol-1. The temperature dependence of Keq also can be employed to extract ∆Hads using Van’t Hoff equation.6 −∆H'() = lnK−1eq * T

(5)


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The plot of ln Keq vs. T-1 (Figure 4) yields a linear relationship with a slope

+∆H ,

. Here, the

∆H-./ was calculated to be -44.75 kJ mol -1

Figure 4. Van’t Hoff plot of ln Keq vs.T-1 for adsorption of WS2 QDs onto a gold electrode. From these data we determine ∆H-./ = - 10.67 kcal mol-1( -44.75 kJ mol -1).

This alternate strategy to determine ∆Hads has been proven to be useful to validate the selfconsistency of our experimental results. The entropy data confirms that it plays a major role to determine the fate of chemical reactions and in this context, the strength of the quantum dots


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assembly. The entropy of adsorption noticed in the present system is comparable to the alkyl thiol adsorption onto the Au-surface. This significant entropic contribution confirms the change in the disorder possibly due to the alignment of randomly oriented WS2 QDs in solution as an organized monolayer. Since OCV measurements can only provide information on the surface coverage and are inadequate in terms of quantitative information, the surface modification resulting in the mass change can therefore be complemented by Quartz Crystal Microbalance (QCM) measurements. However both are time domain measurements and are sensitive to the kinetics of monolayer formation. In order to quantify the rate of mass change at the electrode surface due to the WS2 QDs adsorption during monolayer formation, we have performed in situ QCM measurements (C=1.067 mol L-1 at T=298 K). The gold coated quartz crystal was suspended in WS2 QDs solution overnight and the change in oscillation frequency (Hz) of the crystal was monitored as a function of time (See Figure S7 in the Supporting Information). The frequency was observed to decrease by 300 Hz, which refers a significant increase in the mass adsorbed. The calculated mass change is 5.3 µg cm-2 which is determined using the Sauerbrey equation.51 However, the measurement is still semi quantitative as the absolute measurement is limited by damping in the electrical circuit, solvent viscosity parameters, uncertainty in contribution to the mass from solvent coupling, uncertainty in contact mechanics between particles and solid surfaces (For example, particles can undergo site-exchange or sliding during the oscillation, which will affect the hydrodynamic coupling of solvent trapped between the particles causing the contribution of solvent to ∆f) etc.52 Cyclic Voltammetry experiments performed during the adsorption process (both in aqueous and non-aqueous media) also revealed a decrease in the double layer capacitance which supports the adsorption process (See Figure S8 in the Supporting


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information). The adsorption of WS2 QDs caused a decrease in the double layer capacitance. Furthermore, the formation of layers by WS2 QDs adsorption is analyzed by Scanning Electron Microscopy (SEM). For the SEM analysis, we repeated the same experiment on a gold foil incubated in WS2 QDs solution (overnight) and analyzed the surface after washing and drying. As expected, it resulted in the formation of flakes, resembling a monolayer of WS2 QDs on the gold. SEM image of bare gold foil is also provided for comparison (Figure S9, Supporting Information). However, we also observed a few agglomerated portions which may be attributed to slow reorganization occurred (during overnight incubation). The primary tool used for the measurements is the method of Open Circuit Voltage which exhibited adequate stability of measurements to undertake the present studies. However, given that the method is relatively obscure, we measured in parallel the adsorption kinetics of an alkanethiol on gold electrode, which is a well-known system (1-decanethiol, 3 mM ethanolic solution), so that the results obtained for WS2 QDs in the present study will be more convincing. We noticed a similar trend in the OCV for this system too (Figure S10, Supporting information) and calculated the Keq=17397 M-1 and ∆Gads= -24.2 kJ/mol (-5.78 kcal/mol). This is in agreement with the ΔGads reported for other alkanethiols such as octadecanethiol (-23 kJ/ mol) and 1-octanethiol (-18.4 kJ/mol). It is well reported that the ΔGads for

Au-S bond is

approximately -5 kcal/mol.47,53 In summary, we have investigated and elucidated the kinetic parameters for the adsorption of WS2 QDs onto a polycrystalline gold electrode for a wide range of concentrations. Up to a particular range, the adsorption behavior follows Langmuir adsorption. Our data demonstrated the existence of an equilibrium between the free adsorption sites on the gold surface and the WS2 QDs. From the open circuit measurements, we calculated the equilibrium constant Keq, and


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subsequently the ∆G-./ . The relatively small values of the ∆G-./ indicates the labile nature of the monolayers. The temperature dependent adsorption studies resolved the enthalpic and entropic contributions towards the total free energy. From the QCM kinetic response, we also calculated a mass change, ∆m= 5.3 µg cm-2 for the gold electrode after the monolayer formation. The present study opens up a new avenue to predict the self-assembly of other 2D TMD QDs on gold, assuming that the QD-QD interaction do not hamper the monolayer formation. ASSOCIATED CONTENT

Supporting information Transmission Electron Microscopy (TEM),X-Ray Diffraction (XRD), UV-Visible spectrum and Photoluminescence spectrum of WS2 QDs, OCV vs time plot for WS2 QDs, ln(1-θ) vs time for different concentration of QDs,relative saturation coverage at different temperatures, Cyclic voltammograms of gold and WS2 QDs modified gold electrodes.QCM measurements for WS2 QDs modified gold electrode,SEM images of bare gold & WS2 QDs adsorbed gold and coverage vs time & variation of OCV vs time for 1-decanethiol. AUTHOR INFORMATION Corresponding Author *E-mail:[email protected]

Author contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.


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ACKNOWLEDGEMENTS Authors acknowledge Prof. S Ramanathan and M.S Amrutha (Research Scholar), Department of Chemical Engineering, Indian Institute of Technology, Chennai, India for permitting the authors to use their QCM facility. MOV acknowledges Council for Scientific and Industrial Research (CSIR) for Senior Research Fellowship. SA Acknowledges CSIR for the award of the start-up grant OLP 0088.

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Graphical abstract


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Adsorption Kinetics of WS2 Quantum Dots onto a Polycrystalline Gold

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