One Pot Fabrication of High Coverage PbS Quantum Dot Nanocrystal

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One Pot Fabrication of High Coverage PbS Quantum Dot Nanocrystal Sensitized Titania Nanotubes for Photo-Electrochemical Processes Pawan Pathak, Mateusz Podzorski, Detlef W. Bahnemann, and Vaidyanathan (Ravi) Subramanian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00120 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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The Journal of Physical Chemistry

One-pot Fabrication of High Coverage PbS Quantum Dot Nanocrystalsensitized Titania Nanotubes for Photo-electrochemical Processes

Pawan Pathak,a Mateusz Podzorskia Detlef Bahnemann*b And Vaidyanathan (Ravi) Subramaniana*

a

University of Nevada, Chemical & Materials Engineering Department, Reno– NV, 89557, USA. Leibniz Universität Hannover Institute of Technical Chemistry, Hannover, Germany

b

0 ACS Paragon Plus Environment

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ABSTRACT Lead sulfide quantum dot nanocrystal (QDNC) sensitized TiO2 nanotubes have been fabricated using a simple, wet chemical method that is both time and cost effective. A single precursor source containing both Pb and S has been employed, with oleylamine; as a linker molecule to synthesize the PbS under an ambient pressure based approach. This approach serves to assemble the QDNCs on a TiO2 nanotube surface. Surface characterization was performed using electron microscopy, X-ray diffraction, and elemental analysis indicates the formation of PbS quantum dots along the

nanotube

walls

and

inter-tubular

spacing.

The

opto-electronic,

photoelectrochemical, and photocatalytic properties of the composite heterostructure have been characterized using absorbance spectroscopy, electrochemical studies (including efficiency measurements), and methylene blue conversion as a probe. A 24fold increase in the photocurrent of TiO2-PbS heterostructure over bare TiO2 nanotube has been observed. Electrochemical impedance measurements of the TiO2 nanotube sample indicates donor density of ~4.5 × 1019 cm-3 while TiO2/PbS heterostructure shows n-n photoactive heterojunction with a donor density of ~2.3 × 1020 cm-3. A 12% increase in photocatalytic activity and theoretical estimates suggesting almost 40-fold enhancment towards value-added product synthesis with PbS inclusion, is presented.

1. INTRODUCTION II-VI, III-V, and IV-VI chalcogenides have attracted considerable attention as key materials for energy conversion and sensing.1-7 Chalcogenide – based compounds demonstrate: composition dependent absorbance in the visible and infra-red regions of the solar spectrum, compatibility with broad ranging organic and inorganic materials to form composites, size tunable (quantization effect-driven) absorbance properties, multiple excitation generation, and amenability to synthesis using different techniques.8-9 These properties make them indispensable building block elements for light-driven processes such as those used in photovoltaics and in photocatalysis.10 Particularly, lead based chalcogenide quantum dot nanocrystals (QDNCs) have a wide absorbance


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range, including and up to near infrared radiation, which makes them promising and effective materials for solar energy harvesters and IR detectors. The

assembly

of

Pb-

based

chalcogenide

for

photovoltaics

or

photoelectrochemiclal processes typically requires a large bandgap oxide as an underlying substrate.11-13 Underlying large bandgap oxide provides features such as i) high surface area, ii) strong immobilization of the overlying light absorber, iii) necessary porosity to facilitate and stabilize the distribution of the absorber, iv) maintaining the physical integrity of the absorber (reduced aggregation), v) establishing the driving force for the electron to cascade to the oxide, and vi) enhancement of the electron transport once they are transfer to the oxide.14 These aspects are critical to improving heterostructure performance and lead to efficient light energy conversion.TiO2, a wide bandgap UV responsive semiconductor, is coupled with small

bandgap

chalcogenides

and

other

materials

to

form

heterostructured

composites.15-19 1D architectures, such as TiO2 nanotubes or nanorods, are considerably more useful as nanostructures than their nano-particulate counterparts on account of their high surface area, amenability to electrolyte transport, integration with other materials, and enhanced electron hole pair separation and diffusion.20-23 The nanotubes – based composites may be used for solar fuels and electricity.24-27 Our group and others have recently documented the use of 1D TiO2 as a substrate for chalcogenides, particularly CdS and CdSe for application as a photoanode. 28-31 The synthesis of chalcogenide – oxide heterostructures has been performed using both wet chemical

32

and chamber-based processes.33 The chalcogenides can

either be pre-fabricated and linked to the surface of an oxide or assembled from precursors directly on an oxide surface.32, 34 The leading techniques currently employed include chamber-based approaches (e.g. chemical vapor deposition), wet chemical methods such as chemical bath deposition (Successive ionic layer adsorption reaction – SILAR), atomic layer deposition, and hydothermal deposition. The SILAR approach is the most popular, cost effective, and scalable solution-based method of coupling QDNCs to a range of wide bandgap semiconductors (ZnO, TiO2, Ta2O5, Fe2O3 etc.).35-39 However, this method is a time-intensive, multi-step, and repetitive process that require at least two different precursor sources. 2 ACS Paragon Plus Environment


Herein, we report a simple and effective wet chemical strategy to assemble PbS QDNCs to TiO2 nanotube using a facile, one-pot method. This method requires a single source precursor containing, both Pb and S, along with an organic ligand; oleylamine. The thermal behavior of the precursor and the mechanism of the formation of PbS have also been studied. Layering the nanotubes with a complete coat of PbS results in the sensitized TiO2 demonstrating superior photoelectrochemical and photocatalytic activity. It is shown that the decrease in charge transfer resistance coupled with the rise in absorbance of radiation has yielded up to a 24-fold increase in photocurrent when compared to bare TiO2 nanotubes. The catalytic activity is demonstrated by using methylene blue as a probe molecule in the presence of UV-Vis illumination.

2. EXPERMINENTAL SECTION 2.1 Materials and chemicals Sodium diethyldithiocarbamate [(C2H5)2NCS2Na)], ammonium fluoride (NH4F), lead sulfate (PbSO4), oleylamine (Across Organics), titanium foil (purity: 99.7%, 0.2 mm thick, Strem Chemicals Inc), dichloromethane (Sigma Aldrich), ethylene glycol (VWR Analytical, locally sourced), ethanol (Koptec, locally sourced), sodium hydroxide crystals (Alfa Aesar) and deionized (DI) water (Millipore lab water purification system) were used as received without any further purification. 2.2 Synthesis Procedure The nanotubes as well as the Pb- and S- containing precursor were synthesized separately as indicated below. 2.2.1 Oxide nanotubes The nanotubes were synthesized by anodic oxidation of titanium foil. A titanium substrate was first polished unidirectionally before being placed in ultrasonification bath for 5 minutes each in isopropanol and then acetone. The substrate was electrochemically anodized in an electrolyte consisting of DI water (10% w/w) and a fluorinated solution of ethylene glycol (0.5% w/w). A two electrode system with the Ti foil as anode and a Pt wire as the reference was operated under a 40 V DC power supply for 2 hours. The anodized samples thus prepared were annealed in air at 450°C for 2 hours. Further details of the anodization process and chemistry is discussed in detail elsewhere.40 3 ACS Paragon Plus Environment

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2.2.2

Pb-

and

S-containing

precursor

A

lead

dithiocarbamate

precursor,

Pb[(C2H5)2NCS2]2, was prepared by mixing a 0.1 M aqueous solution of PbSO4 with a 0.2 M aqueous solution of [(C2H5)2NCS2Na)]. The mixture was vigorously stirred for 3 hours. Precipitation occurred immediately upon mixing. After stirring, white precipitate was washed five times with DI water to remove un-complexed salts and dried in an oven at a preset temperature of 50°C for 6 hours in ambient atmosphere. This dried material was used as a precursor for the Pb- and S-. 2.2.3 Preparation of the chalcogenide deposits on the oxide In a separate procedure, 30 ml of oleylamine was heated to 220°C, whereupon the Titanium Nanotube (T_NT) substrate was submerged within it. The T_NT was soaked in oleylamine for 5 minutes, after which the lead dithiocarbamate precursor was introduced. Varying concentrations of the precursor (0.05 mM, 0.1 mM, 0.5 and 1 mM) was prepared in the oleylamine. The mixture in contact with the T_NT was kept at 220°C (a temperature identified from TGA analysis) for 40 minutes. Upon removal, the sample was cooled in ambient air. The sample

was

then

washed

by

dipping

separately

into

ethanol followed

by

dichloromethane to remove loosely adhered or excessive organics from the substrate. Lastly, thermal treatment of the sample was performed in a nitrogen atmosphere at 350°C for 3 hours. 2.3 Characterization Thermal analysis of the precursor was performed using a PerkinElmer TGA/ DSC thermo gravimetric analyzer at a heating rate of 10 °C/min under a nitrogen flow. UV-visible absorbance measurements were performed using a Shimadzu UV-2501PC spectrophotometer in absorbance mode, in the range of 300 to 900 nm. Imaging of the samples was performed using a Hitachi FESEM scanning electron microscope (SEM) equipped with an oxford EDS analyzer. The size of the chalcogenide deposits on the surface of the oxide nanotubes was determined using a JEOL 2100F high-resolution transmission electron microscope (HR-TEM). A Philips XRG 3100 X-ray diffractometer operating at 35 kV was used to identify the phase of the nanocomposite material. Brunauer Emmett- Teller specific surface areas (SBET) of the synthesized materials were determined using a micromeritics system (model flow sorb II – 2300) 4 ACS Paragon Plus Environment


Photoelectrochemical measurements were conducted using a three-electrode system, where a quartz cell with a Pt mesh served as the counter electrode and a leak free Ag/AgCl (in 3 M KCl) was used as the reference electrode. A solution of 0.1 M Na2S in water served as the electrolyte in the measurements.41-42 A 500 W Newport xenon lamp equipped with 0.5 M CuSO4 solution was used to irradiate the working electrode; this attenuated the light intensity to about AM 1.5 (90 mW cm-2). An Auto-lab PGSTAT 30 electrochemical analyzer was used to obtain the choronoamperometry (J/t) and linear sweep voltammetry (J/V) measurements. Electrochemical impedance measurements were also performed by auto-lab PGSTAT 30 equipped with FRA32 module with a corresponding ac amplitude of 10 mV to obtain Nyquist and Bode Plot results. A stock solution of methylene blue in DI water was irradiated with a Xe lamp in the presence of the photoactive films to study photocatalytic activity.

3. RESULTS 3.1 Thermal analysis of the precursor Dithiocarbamate complex is a single source precursor that can stabilize elements to produce a chalcogenide. The included organic moieties can dissociate under appropriate thermal conditions to lead to the formation of chalcogenide nanocrystals. To identify these optimal conditions, TGA / DSC (thermal gravimetric analysis / differential scanning calorimetry) was performed, with results presented in the XYY plot of Figure 1.The presence of multiple peaks in DSC data indicates the pyrolysis occurs first prior to the formation of the nanocrystals. Qualitatively, the pyrolysis is noted to trigger at 220 °C as evident from the peak onset in the DSC data. The pyrolysis could lead to the rearrangement of the Pb- and S just before the initiation of the crystallization process, as indicated with a familiar oxide in nickel and copper.43 At ~ 350°C the spikes in the DSC indicates, decomposition of the organic portion of the precursor as indicated with similar chalcogenides.43 The removal of the organic portion is usually accompanied with a crystallization process that helps to narrow down the conditions necessary for the formation of the QDNCs. At this time, a change in the color of the precursor from pale yellow to black is also observed, as shown in the inset of Figure 1. The complementary aspect of the DSC with the TGA 5 ACS Paragon Plus Environment

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result is evident in Figure 1. An XRD analysis of the annealed samples after formation of the deposits was performed to determine the identity of the crystals formed and the phase of the deposits. The results shown in Figure S1 indicate clearly the presence of deposits suggesting that thermally induced crystallization has occurred from the deposits prepared. The presence of distinct peaks at 2θ of ~ 25°, 29° and 43° can be attributed to the (111), (200), and (220) plane of the cubic phase of PbS (JCPDS # 781055). The insert of Figure 1 is the model of the one step synthesis. The black coloration of the precursor is consistent with observed coloration of PbS.44 Based on thermal analysis and the XRD results, the PbS formation from the precursor can be written as: 0

= 220 to 350 C Pb[(C2H5)2NCS2]2 T  → PbS↓ + C3H5NCS↑ + S2CN(C3H5)2↑ + C3H5↑

As indicated in the aforementioned reaction, organic components constitute 54% of the mass of the products, which decompose on account of their volatile nature. During analysis of the precursor, at a temperature of 350±10 °C, 55±2 % mass loss was observed. This measurement is in good agreement with the theoretical calculation of a mass loss of ~54 %. The mass of the resultant PbS constitutes 46% of the mass of the precursor. At this point, a linker molecule was necessary to integrate the PbS to the T_NT surface. Oleylamine is a widely used chemical in the synthesis of nanoparticles, used primarily to control the size of the particle.45 In this work, we employed Oleylamine to (1) synthesize the nanoparticle and (2) to link the nanoparticle to T_NT surface. 3.2 Surface, Optical, and Photoelectrochemical Characterization 3.2.1 SEM and XRD analysis: The scanning electron microscopy (SEM) image of the anodized Ti substrate shown in Figure 2A indicates geometrically identical and wellaligned arrays of nanotubes. The inset of Figure 2A shows the cross-sectional view of the nanotube surface indicating nanotube length of ~2 um. The nanotubes are 120 ± 6 nm in diameter and shows walls with thicknesses of 20 ± 4 nm with some intertubular spacing. While nanotubes can be grown longer (up to several microns in length,46



previously published reports indicate that shorter (~ 3000 – 5000 nm) nanotubes are optimal for preparing chalcogenide deposits that are sufficient for solar-driven applications. Shorter nanotubes are also less vulnerable to physical disintegration at the base and hence this length was used in the discussed work.47 Figure 2B shows the SEM image of the deposits formed on the nanotube surface. At the end of the 45minute process, the deposits appear as a continuum distributed over the entirety of the nanotube. Even the interstitial spaces between the nanotubes are mostly covered. Thus, such a densely packed coating can be achieved using a one –step and single-pot approach. It should be noted that a lower loading and discontinuous coating might also be achieved by reducing the synthesis duration. The microscopy image obtained using AFM is shown in Figure S2. Low loading of PbS did not allow for obtaining reproducible image due to streaking effects and significant topographical changes. Multilayer deposit only resulted in stable imaging probably because the surface was completely covered. Subsequently, the topography mostly represents PbS nanoparticles and appear to uniformly coat the nanotube surface. For more clear indication of the particle size and its distribution SEM and (HR) TEM measurements were also performed and these results are discussed later. The XRD pattern of the anodized oxide and the deposits are shown in Figure 2C. The multiple peaks in the nanotube samples are identified as either elemental titanium (JCPDS # 441294) or anatase TiO2 (JCPDS # 21-1272) as indicated in Figure 2C. The presence of distinct peaks at 2θ of ~ 25°, 29° and 43° can be attributed to the cubic phase of PbS (JCPDS # 78-1055). 3.2.2 (High Resolution) Transmission Electron Microscopy Analysis – HRTEM of the op was performed to validate the X-ray diffraction results and reveal the size of the PbS nanocrystal. Figure 3A shows the TEM image of the optimized sample, prepared using a 0.5 mM precursor solution. Several noticeably distinct islands are observed as identified in the figure indicating successful nucleation and anchorage on the surface of the T_NT at low concentrations of the precursor. Figure 3B presents a magnified image of the T_NT surface, indicating the size of the PbS nanocrystals range from 6 to 10 nm. A section of the hetero-composite was examined under high resolution (HRTEM analysis). The lattice spacing of the FFT pattern presented in Figure 3C with a d7 ACS Paragon Plus Environment

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spacing of 0.34 nm matches with the (111) plane of PbS nanocrystal and is consistent with the XRD results presented in Figure 2C. Thus the HRTEM analysis supports the conclusions drawn from the XRD analysis confirming crystalline PbS formation. 3.2.3 EDX and color mapping - Figure 4 shows the EDX elemental color mapping of the T_NT/PbS sample. This technique provides insights into the distribution and compositional homogeneity, and spatial uniformity of the nanoparticles and their building block elements in the hetero-composite using color contrasts. The presented images indicate that the one-pot approach leads to a homogeneous distribution of PbS. The color mapping of the participating elements Ti, O, Cd, and S is shown (Figure 4 BE) and is a convincing cross-verification of the microscopy analysis in which the PbS is found to be uniformly distributed along the cross-sectional length of nanotubes. Further, the energy dispersive X-ray spectroscopy (EDX) analysis shown in Figure 4F gives the quantitative distribution of Pb, S, Ti and O in the sample. Therefore, from these complementary measurements using XRD analysis, SEM, (HR)TEM, and EDX the formation of crystalline PbS with a dense distribution is confirmed. 3.3 Optical Characterization T_NT/PbS hetero-structures were further characterized using UV/Vis spectroscopy. T_NT demonstrates an onset absorbance in the UV region presented in Figure S3. Figure 5 shows the differential absorbance (∆A) spectra, spread over 0.6 to 0.2 units, of the T_NT/PbS deposits formed under different precursor concentration. The absorbance of T_NT has been subtracted from the samples with PbS deposits for clarity. Spectroscopy results indicate that the deposits start to form at a very low concentration of 0.1 mM of precursor. As the precursor concentration increases, the net absorbance also increases and maximizes at a concentration of 0.5 mM. Any further increases in precursor concentration fail to raise the absorbance, rendering further increases in concentration of the precursor irrelevant. 3.4 Evaluation of photoelectrochemical and photocatalytic properties The performance

of

the

T_NT/PbS

heterostructured

film

was

examined

in

a

photoelectrochemical cell as a multifunctional electrode for energy conversion and environmental remediation. 8 ACS Paragon Plus Environment


3.4.1 Chronoamperometry (J/t) measurements of the nanocomposite are presented in Figure 6A. A prompt response to illumination is noted in the form of an instantaneous photocurrent. This response is reproducible and stable as indicated by the multiple onoff cycles. The presence of PbS shows an increase in the photocurrent when compared to bare T_NT. At the lowest level of PbS deposition a photocurrent of ~ 1.5 mA cm-2 is noted indicating that the presence of PbS contributes to boosting the T_NT’s photoactivity. The highest obtained photocurrent of ~ 3 mA cm-2 is observed when a precursor concentration of 0.5mM is used. This represents a 15-fold increase over the bare T_NT. Any further increase in the PbS precursor loading leads to reduction in the photocurrent with this length of T_NT The sulfide redox couple assists higher open circuit voltage and increases the photoconversion efficiency.41, 48 3.4.3 Linear Sweep Voltammetry: Figure 6B shows the J/V characteristics of T_NT and T_NT with PbS deposits at various precursor concentrations. Just as in the chronoamperometry measurements, higher photocurrent is noted with all instances of PbS loading. Further, a negative shift in the zero current potential is also noted with the deposition of PbS at any concentration. As the precursor concentration increases, the photocurrent increases up to a maximum of ~ 3 mA cm-2 for PbS deposits on T_NT at 0.5 mM precursor concentration. The voltage where the current becomes zero is indicative of the apparent flat band (thus called because of the particulate nature of the film). The negative shift in this potential is indicative of the heterostructure junction promoting efficient charge separation, justifying it for further consideration in a solar cell configuration. 3.4.4 Cyclic Voltammetry and BET analysis: Figure S4 shows cyclic voltammetry on T_NT and T_NT /PbS sample in 0.1 M Na2S solution at 50 mV S-1. Electrochemical active surface area calculated from CV curve for T_NT and T_NT/PbS sample is 6.5 mC/cm2 and 33.5 mC/cm2 respectively.49 Further analysis on the surface area of the samples was performed using BET technique. The application of BET approach to estimate surface area for similar systems are reported in the literature.50-51 BET surface area of T_NT and T_NT /PbS samples calculated for 1 gram of material is 65 m2 and 80 m2 respectively. The area obtained for the nanotube is consistent with the numbers found in the literature. 9 ACS Paragon Plus Environment

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3.4.5 Intrinsic Solar to Chemical (ISTC) Conversion Efficiency: Intrinsic solar to chemical conversion efficiency is given by:52

Where, Udark is the potential that must be applied in order to reach respective photocurrent at dark. Jphoto and Vphoto are the values obtained from J/V curve. The Intrinsic solar to chemical conversion efficiency of the T_NT and T_NT/ PbS samples is presented in Figure 7. ISTC conversion efficiency for T_NT and T_NT/PbS sample is 0.05% and 1.9% respectively, indicating a 40-fold enhancement with PbS addition. This demonstrates the importance of T_NT/PbS sample towards its practical applications such as hydrogen or other chemical conversion. This analysis reveals that the TiO2 nanotube – PbS composite may be used for the production of solar fuels such as Hydrogen. The application of TiO2 nanotube with PbS nanoparticles and other quantum dots for hydrogen production are reported in the literature.53-54 3.4.6 Optimization of Photocurrent with Respect to the Tube Length: Length of the TiO2 nanotube can be increased with increase in the anodization time. TiO2 nanotubes were anodized for 2h, 4h, 8h, and 12 h. J/t characteristics of the 0.5 mM PbS precursor sensitized T_NT with respect to the anodization are presented in Figure 8. These results are attributable to the increase in the PbS loading with respect to the increase in the tube length. The photocurrent increases upto 4.8 mA cm-2 for the 8 h anodized sample, with further increases in the anodization time has decreased the photocurrent. 3.4.6 Evaluation of the T_NT-PbS nanocomposite after PEC studies Exposure of the film to the electrolyte and/or illumination can potentially lead to physicochemical changes. To examine this aspect, the film was recovered and tested using an array of tools. Specifically, surface stability of the electrode has been studied by analyzing SEM, XRD, and FTIR results of the electrode after the photoelectrochemical measurements. These results are presented in Figure S5 – S7. The analysis indicted no significant change in the results when compared to the unused pristine composite films. Indicating that the electrode maintains its stability for the duration of the study. 10 ACS Paragon Plus Environment


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4. DISCUSSION The following section provides insights into the mechanistic properties of the heterojunction between the oxide and the well-integrated chalcogenide layer. 4.1 EIS Analysis: Electro chemical impedance spectroscopy, or EIS measurements, provides insight into the charge transport resistance and estimates of the majority charge carrier density in the film. The EIS measurements were performed at an applied bias of 0.6 V in dark and under simulated solar radiation. Figure 9A represents the Nyquist plot of the T_NT and T_NT/PbS films. The charge transfer resistance can be correlated to the diameter of the arc in the Nyquist plot at constant applied bias.28,

35

After loading PbS on T_NT sample the arc of the Nyquist plot decreases indicating a lower charge transfer resistance in comparison to bare T_NTs. The complementary Bode plots can be used to estimate the average lifetime ( presented in Figure 9B is used to calculate the values of

τ=

of the charges. The results using the following equation:

1 2πf p

Where fp is the characteristic frequency of the samples.

The estimated values of

electron lifetimes are ~2.8 ms for T_NT and ~ 16 ms for the T_NT /PbS electrodes.55-57 This shows the effective coupling between PbS quantum dots and TiO2 nanotubes. Mott-Schottky (MS) analysis is effective to determine the charge carrier densities, built in voltages, and the nature of coupling between interfaces.58 The MS analysis of the T_NT and T_NT/PbS system presented in Figure 9C was performed using 0.5 M Na2SO4 solution in a 3-electrode set-up. The relation between the capacitance behavior and applied potential of a semiconductor is expressed by the Mott-Schottky (MS) equation and is given by:59

1 = C2

kT ) e ε r ε 0 N D A2

2(V −V fb−

Where C, Vfb, ε0, εr, ND are capacitance, flatband potential, permittivity in free space, relative dielectric constant of the semiconductor and donor density, respectively. The 11 ACS Paragon Plus Environment

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slope from the linear portion of the Mott-Schottky analysis (1/C2 vs. V plot) is used to determine p- and n-type characteristics along with donor density, while the intercept gives the flat band potential.60 Carrier density for T_NT sample is ~ 4.5×1019 cm-3 and T_NT/PbS sample is ~ 2.3×1020 cm-3 respectively is calculated from the graph presented in Figure 9C. The flat band potential of T_NT and T_NT/PbS sample is 0.05V vs RHE and 0.16 V vs RHE respectively. Thus, n- type PbS is deposited on T_NT surface using a one-pot process, which supports its use in the enhancement of photoelectrochemical responses. 4.2 IPCE Analysis The incidence photon to current conversion efficiency, or IPCE, is a measure of the effectiveness of the film components to convert the energy from each wavelength of light within the domain of interest to usable electrons. It is estimated using the expression:61-64 IPCE (%) =

1240 × I SC ( A) × 100 P (W ) × λ (nm)

Where, Isc is short circuit current, P is the power at the given wavelength and

is the

wavelength of the monochromatic light. The response of the T_NT and T_NT/PbS composite is reported in Figure 10. In the visible wavelength range T_NT has nearly zero IPCE. However, T_NT/PbS nanostructured composite shows higher IPCE responses that extend even into the visible spectrum, which is responsible for the higher net photocurrent produced by the composite.65-66 The deposition of PbS leads to an enhancement of 4 fold at a wavelength of 350. This also underscores the fact that holes are also available for promoting oxidative reactions as observed in the results of MB conversion. In similar systems, the reported enhancements in the visible range indicate that the inclusion of PbS is beneficial.67 Previous studies using photoluminescence tracking in TiO2 – PbS nanocomposites have indicted that the emission of PbS is diminished because of photogenerated charge transfer to the oxide. The absorbance of infrared energy produce electron – hole pairs in the PbS. These electrons travel to the conduction band of the TiO2 ensuring effective separation of the charges. Scheme 1 shows the synergy created by the process of a seamless integration between the 1D – 0D semiconductors of two classes. It remains to be seen how the heterostructure can



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be integrated as a part of a device for PV applications, and its possible role in solar fuel generation and environmental remediation processes. 4.3 Role of T_NT/PbS as an electrode in oxidative processes: Here we examine the synergy between the two materials in promoting hole-mediated oxidation. An aqueous solution of methylene blue at a concentration of 22.8 µM was used as a simple color tracking representative probe. The absorbance was checked at regular time intervals with illumination in the presence of bare T_NT and T_NT/PbS samples. A decrease in the peak absorbance at ∼650 nm was immediately evident with both samples. Control experiments (in the absence of T_NT or T_NT/PbS) indicated ~ 10% decrease in the by UV light. Though such a decrease can be accompanied by the formation of other peaks revealing an isopiestic point, such crossover points were not observed (Figure S8). The relative concentration of the dye in presence of the T_NT and T_NT/PbS sample in light with respect to the time is presented in Figure 11. The presence of PbS alongside T_NT was found to expedite the discoloration of the Methylene Blue by ~ 12% more than the bare T_NT sample, and at a faster rate, over 2 hours of continuous illumination. This observation indicates that the synergism between the oxidechalcogenide may be leveraged for pursuing oxidative processes. Multiple

aspects

should

be

considered

when

evaluating

electron-hole

recombination processes. Within the TiO2-PbS system itself, prior research has identified i) material synthesis factors, ii) compositional ratio, iii) electrolyte composition, and iv) heterostructure interface as some of the key factors that influence charge transfer mechanism. These factors also determine if the heterostructure is suitable for photovoltaic or photocatalytic processes.68-74 Systematic studies that includes FRET analysis, room temperature photoluminescence decay studies are required for a complete understanding of the charge transport and recombination mechanism. Specific to this heterostructure, contribution of temperature during synthesis, solvent effects and knowledge of trap sites and its distribution, alongside simultaneous interface characterization using XPS is critical to determine the extent of the electron-hole recombination process. These studies must be considered for a comprehensive understanding of the functioning of this heterostructure.


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Conclusions This study presents a unique one-pot, single precursor approach to synthesize PbS QDNCs on the surface of TiO2 nanotubes. XRD analysis, SEM, (HR)TEM, and EDX analyses indicate the formation of crystalline cubic phase 8-10 nm diameter PbS with a dense distribution. A precursor composition dependent control of coverage has been demonstrated leading to film with a very high density (continuum –like), coverage of the T_NT. The optical absorbance spectra, chronoamperometry, and J/V characteristics confirm that heterostructure contact is effectively established between the PbS quantum dot and the titania nanotubes. Mott-Schottky analysis confirms n-type PbS formation that increased the photocurrent by ~24 times compared to bare TiO2 nanotubes. EIS spectra and IPCE provide further insight into the charge transport resistance and energy conversion effectiveness of the material. The catalytic ability of the composite is also examined by studying the hole mediated oxidation of MB as a probe, which indicates that PbS deposits boost the activity by ~10%. ISTC analysis indicates a 4-fold increase in the potential for photocatalysis and suggests that the heterostructure can be used for more complex and challenging processes such as solar fuel production.

ASSOCITED CONTENT Supporting Information The XRD of lead dithiocarbaomate, images of TiO2/PbS, absorbance spectra, cyclic voltammogram for electrochemical area, infrared spectroscopy result, and photocatalytic degradation plots for dye. The Supporting Information is available free of charge on the ACS Publications website. Author Information Corresponding Author* E-mail [email protected] Orchid 0000-0001-9523-3074 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS RSV gratefully acknowledges NSF funding (CBET 1337050) for partially supporting the initial part of this project RSV would like to thank the Alexander von Humboldt Foundation for supporting him to collaborate with Prof. Bahnemann Laboratory. RSV thanks Dr. Mo Ahmedian for HRTEM studies. He also thanks NSF - EPSCOR UG support for MP. 14 ACS Paragon Plus Environment


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One-pot Fabrication of High Coverage PbS Quantum Dot Nanocrystalsensitized Titania Nanotubes for Photo-electrochemical Processes

Figure File


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Figure 1 The TGA/DSC thermograph of the lead-dithiocarbamate (Inset) The photograph of the sample taken before heat treatment and after 350°C heat treatment is also shown.

Figure 2 The SEM images of A) bare Titania nanotubes (T_NTs) grown over titanium foil by anodization B) PbS deposited on T_NTs and C) The XRD pattern of bare T_NTs grown over Ti foil and PbS deposited on T_NTs. 1 ACS Paragon Plus Environment


Figure 3 The A) TEM images of the T_NT/PbS nanocomposite, B) TEM image of PbS nanocrystal showing size distribution, and C) HRTEM image of PbS nano crystal with (inset) corresponding fast Fourier transform (FFT) diffraction pattern of PbS nanocrystals.

Figure 4 The EDX elemental color mapping showing uniform deposition of Pb and S along the wall of the T_NTs A) different elemental distribution, B) Sulfur, C) Ti, D) Pb, and E) O is shown. F) The EDX spectra of the sample showing quantitative elemental distribution is also presented. 2 ACS Paragon Plus Environment

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Differential Absorbance, au

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Figure 6 The photoelectrochemical (PEC) responses of PbS deposited T_NTs shown are (A) J-t characteristics at multiple onoff cycles and (B) J- V characteristics [PEC conditions: CE = Pt; RE= Ag/AgCl; électrolyte = 0.1 N Na 2S.]



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Figure 9 The A) Nyquist plot of T_NT and T_NT/PbS hetro-composite, the B) Bode plot representing change in the impedance with respect to frequency, and the C) MottSchottky analysis are presented.

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Figure 11 A plot representing the photo-catalytic degradation of methylene blue in presence of light for T_NT and T_NT/PbS heterostructure is shown.

Scheme I The charge transfer mechanism in OD / 1D PbS-T_NT hetro-structure in presence of the photoillumination is represented in the figure. The separated charges will trigger redox processes at the surface of the heterostructure. 6 ACS Paragon Plus Environment

One Pot Fabrication of High Coverage PbS Quantum Dot Nanocrystal

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