One-Pot Fabrication of High Coverage PbS Quantum Dot Nanocrystal

Apr 6, 2018 - Lead sulfide quantum dot nanocrystal (QDNC) sensitized TiO2 nanotubes have been fabricated using a simple, wet chemical method that is ...
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Cite This: J. Phys. Chem. C 2018, 122, 13659−13668

One-Pot Fabrication of High Coverage PbS Quantum Dot Nanocrystal-Sensitized Titania Nanotubes for Photoelectrochemical Processes Pawan Pathak,† Mateusz Podzorski,† Detlef Bahnemann,‡ and Vaidyanathan Ravi Subramanian*,† †

University of Nevada, Chemical & Materials Engineering Department, Reno, Nevada 89557, United States. Leibniz Universität Hannover, Institute of Technical Chemistry, 30167 Hannover, Germany

‡

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S Supporting Information *

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, indicating the formation of PbS quantum dots along the nanotube walls and intertubular spacing. The optoelectronic, 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 24-fold increase in the photocurrent of TiO2−PbS heterostructure over bare TiO2 nanotube has been observed. Electrochemical impedance measurements of the TiO2 nanotube sample indicate donor density of ∼4.5 × 1019 cm−3 while TiO2/PbS heterostructure shows an 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 enhancement toward value-added product synthesis with PbS inclusion are 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 infrared 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 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) © 2018 American Chemical Society

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 nanoparticulate 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 chemical32 and chamber-based processes.33 The chalcogenides can either be prefabricated and linked to the surface of an oxide or assembled from precursors Special Issue: Prashant V. Kamat Festschrift Received: January 4, 2018 Revised: April 6, 2018 Published: April 6, 2018 13659

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The Journal of Physical Chemistry C 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 hydrothermal deposition. The SILAR approach is the most popular, costeffective, 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 timeintensive, multistep, and repetitive process that requires at least two different precursor sources. 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.

substrate was submerged within it. The T_NT was soaked in oleylamine for 5 min, after which the lead dithiocarbamate precursor was introduced. Varying concentrations of the precursor (0.05 mM, 0.1 mM, 0.5 and 1 mM) were 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 min. 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 h. 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). 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 Autolab PGSTAT 30 electrochemical analyzer was used to obtain the chronoamperometry (J/t) and linear sweep voltammetry (J/V) measurements. Electrochemical impedance measurements were also performed by autolab PGSTAT 30 equipped with a 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.

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 an ultrasonification bath for 5 min 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 h. The anodized samples thus prepared were annealed in air at 450°C for 2 h. Further details of the anodization process and chemistry are discussed in detail elsewhere.40 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 h. Precipitation occurred immediately upon mixing. After stirring, white precipitate was washed five times with DI water to remove uncomplexed salts and dried in an oven at a preset temperature of 50 °C for 6 h 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)

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 indicate decomposition of the 13660

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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 the 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 well-aligned arrays of nanotubes. The inset of Figure 2A shows the cross-sectional view of the nanotube surface indicating a nanotube length of ∼2 um. The nanotubes are 120 ± 6 nm in diameter and show 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 45 min 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 a reproducible image due to streaking effects and significant topographical changes. Multilayer deposition only resulted in stable imaging probably because the surface was completely covered. Subsequently, the topography mostly represents PbS nanoparticles and appears to uniformly coat the nanotube surface. For a 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 is shown in Figure 2C. The multiple peaks in the nanotube samples are identified as either elemental titanium (JCPDS # 44-1294) or

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

organic portion of the precursor as indicated with similar chalcogenides.43 The removal of the organic portion is usually accompanied by 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 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) planes of the cubic phase of PbS (JCPDS # 78-1055). The inset of Figure 1 is the model of the one-step synthesis. The black coloration of the precursor is consistent with the observed coloration of PbS.44 Based on thermal analysis and the XRD results, the PbS formation from the precursor can be written as Pb[(C2H5)2 NCS2 ]2 T = 220 to 350 °C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ PbS ↓ +C3H5NCS ↑ +S2 CN(C3H5)2 ↑ +C3H5↑

As indicated in the aforementioned reaction, organic components constitute 54% of the mass of the products,

Figure 2. 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. 13661

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Figure 3. (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. 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. (F) The EDX spectrum of the sample showing quantitative elemental distribution is also presented.

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 Xray 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 ranges from 6 to 10 nm. A section of the heterocomposite was examined under high resolution (HRTEM analysis). The lattice spacing of the FFT pattern presented in Figure 3C with a d-spacing of 0.34 nm matches with the (111) plane of the 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 the spatial uniformity of the nanoparticles and their building block elements in the heterocomposite 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 4B−E) and is a convincing crossverification 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 heterostructures 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 (ΔA) spectra, spread over 0.6 to 0.2 units, of the T_NT/PbS deposits formed under different precursor concentrations. 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 it 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 13662

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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. 3.4.3. Cyclic Voltammetry and BET Analysis. Figure S4 shows cyclic voltammetry on T_NT and T_NT/PbS samples in 0.1 M Na2S solution at 50 mV S1−. The electrochemically active surface areas calculated from the CV curves for the T_NT and T_NT/PbS samples are 6.5 mC/cm2 and 33.5 mC/ cm2, respectively.49 Further analysis on the surface area of the samples was performed using the BET technique. The application of the BET approach to estimate surface area for similar systems is reported in the literature.50,51 The BET surface area of T_NT and T_NT/PbS samples calculated for 1 g of material is 65 m2 and 80 m2, respectively. The area obtained for the nanotube is consistent with the numbers found in the literature. 3.4.4. Intrinsic Solar to Chemical (ISTC) Conversion Efficiency. Intrinsic solar to chemical conversion efficiency is given by52

Figure 5. Differential absorbance spectra of the PbS quantum dots deposited on T_NT at various initial concentrations.

heterostructured film was examined in a photoelectrochemical cell as a multifunctional electrode for energy conversion and environmental remediation. 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 on−off 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 0.5 mM 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 3.4.2. 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

ISTC =

2 ⎡ ⎤ 1.23(VRHE) ⎢ Jphoto (mA /cm ) × Vphoto(V ) ⎥ ≈ ⎥ Udark(VRHE) ⎢⎣ P(mW /cm2) ⎦

at AM 1.5 G

where Udark is the potential that must be applied in order to reach the respective photocurrent in the 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. The ISTC conversion efficiency values for the T_NT and T_NT/ PbS samples are 0.05% and 1.9%, respectively, indicating a 40fold enhancement with PbS addition. This demonstrates the importance of T_NT/PbS sample toward 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 is reported in the literature.53,54

Figure 6. Photoelectrochemical (PEC) responses of PbS deposited T_NTs shown are (A) J−t characteristics at multiple on−off cycles and (B) J−V characteristics [PEC conditions: CE = Pt; RE = Ag/AgCl; electrolyte = 0.1 N Na2S]. 13663

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results are attributable to the increase in the PbS loading with respect to the increase in the tube length. The photocurrent increases up to 4.8 mA cm−2 for the 8 h anodized sample, and further increases in the anodization time 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.

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. Electrochemical 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 the 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 (τ) of the charges. The results presented in Figure 9B are used to calculate the values of τ using the following equation: 1 τ= 2πf p

Figure 7. ISTC efficiency of the T_NT/PbS photanode as a function of photocurrent density. Inset: ISTC of T_NT sample.

3.4.5. Optimization of Photocurrent with Respect to the Tube Length. The length of the TiO2 nanotube can be increased with an increase in the anodization time. TiO2 nanotubes were anodized for 2 h, 4 h, 8 h, and 12 h. The J/t characteristics of the 0.5 mM PbS precursor sensitized T_NT with respect to the anodization are presented in Figure 8. These

where f p 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

Figure 8. Current−time or J−t characteristics of PbS deposited T_NTs at various anodization times.

Figure 9. (A) Nyquist plot of T_NT and T_NT/PbS hetero-composite; (B) Bode plot representing change in the impedance with respect to frequency; (C) Mott−Schottky analysis are presented. 13664

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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 indicated that the emission of PbS is diminished because of photogenerated charge transfer to the oxide. The absorbance of infrared energy produces electron−hole pairs in the PbS. These electrons travel to the conduction band of the TiO2 ensuring effective separation of the charges. Scheme I shows the synergy created

and T_NT/PbS system presented in Figure 9C was performed using 0.5 M Na2SO4 solution in a 3-electrode setup. The relation between the capacitance behavior and applied potential of a semiconductor is expressed by the Mott−Schottky (MS) equation and is given by59

(

2 V − Vfb − 1 = C2 εr ε0NDA2

kT e

)

where C, Vfb, ε0, εr, and ND are capacitance, flatband potential, permittivity in free space, the relative dielectric constant of the semiconductor, and donor density, respectively. The 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 The carrier density for the T_NT sample is ∼4.5 × 1019 cm−3, and for the T_NT/PbS sample ∼2.3 × 1020 cm−3 respectively is calculated from the graph presented in Figure 9C. The flat band potentials of the T_NT and T_NT/PbS samples are −0.05 V vs RHE and 0.16 V vs RHE, respectively. Thus, n-type PbS is deposited on the T_NT surface using a one-pot process, which supports its use in the enhancement of photoelectrochemical responses. 4.2. IPCE Analysis. The incident 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 expression61−64 IPCE (%) =

Scheme I. Charge Transfer Mechanism in OD/1D PbST_NT Heterostructure in the Presence of the Photoilluminationa

1240 × ISC(A) × 100 P(W ) × λ(nm)

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

a

The separated charges will trigger redox processes at the surface of the heterostructure.

by the process of a seamless integration between the 1D−0D semiconductors of two classes. It remains to be seen how the heterostructure can 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 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 the 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 h of continuous illumination. This observation indicates that the synergism between the oxide-chalcogenide may be leveraged for pursuing oxidative processes.

Figure 10. Incident photon to current conversion efficiencies or IPCE of the T_NT and T_NT/PbS heterocomposite measured as a function of wavelength.

has nearly zero IPCE. However, the 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 13665

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00120. XRD of lead dithiocarbamate, images of TiO2/PbS, absorbance spectra, cyclic voltammogram for electrochemical area, infrared spectroscopy result, and photocatalytic degradation plots for dye (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Detlef Bahnemann: 0000-0001-6064-6653 Vaidyanathan Ravi Subramanian: 0000-0001-9523-3074 Figure 11. Plot representing the photocatalytic degradation of methylene blue in the presence of light for the T_NT and T_NT/ PbS heterostructures.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS RSV gratefully acknowledges NSF funding (CBET 1337050) for partially supporting the initial part of this project. RSV would like to thank the the Alexander von Humboldt Foundation for supporting him to collaborate with Prof. Bahnemann’s Laboratory. RSV thanks Dr. Mo Ahmedian for HRTEM studies. He also acknowledges NSF - EPSCOR UG support for MP.

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 include FRET analysis and room temperature photoluminescence decay studies are required for a complete understanding of the charge transport and recombination mechanism. Specific to this heterostructure, the contribution of temperature during synthesis, solvent effects, and knowledge of trap sites and its distribution, alongside simultaneous interface characterization using XPS, are 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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5. 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. 13666

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