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Evolution of photovoltaic and photocatalytic activity in anatase-TiO2 under visible light via simplistic deposition of CdS and PbS quantum-dots
Accepted Manuscript Evolution of photovoltaic and photocatalytic activity in anatase-TiO2 under visible light via simplistic deposition of CdS and PbS quantum-dots Mansoor Irfan Aziz, Faryal Mughal, Hafiz Muhammad Naeem, Alam Zeb, Muhammad Asif Tahir, Muhammad Abdul Basit PII:
S0254-0584(19)30242-1
DOI:
https://doi.org/10.1016/j.matchemphys.2019.03.042
Reference:
MAC 21481
To appear in:
Materials Chemistry and Physics
Received Date: 8 March 2019 Accepted Date: 13 March 2019
Please cite this article as: M.I. Aziz, F. Mughal, H.M. Naeem, A. Zeb, M.A. Tahir, M.A. Basit, Evolution of photovoltaic and photocatalytic activity in anatase-TiO2 under visible light via simplistic deposition of CdS and PbS quantum-dots, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.03.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Evolution of photovoltaic and photocatalytic activity in anatase-TiO2 under visible light via simplistic deposition of CdS and PbS quantum-dots
and Muhammad Abdul Basit1* 1
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Mansoor Irfan Aziz,1 Faryal Mughal,1 Hafiz Muhammad Naeem,1 Alam Zeb,2 Muhammad Asif Tahir3
Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, Pakistan 2
Pakistan 3
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Riphah Institute of Pharmaceutical Sciences, Riphah International University, Islamabad,
Department of Chemistry, University of Agriculture Faisalabad, Pakistan
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Abstract
A simple approach to deposit metal chalcogenide-based quantum-dots on TiO2 (anatase) nanoparticles via pseudo-successive ionic layer adsorption and reaction (p-SILAR) was opted to design and investigate the corresponding photoactivity under visible light. We used TiO2/PbS and TiO2/CdS nanocomposites for photocatalysis after which we prepared solar paints from them to put side by side the resultant
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photovoltaic performance. We developed solar paint type quantum-dot-sensitized solar cells (QDSCs) from TiO2/PbS and TiO2/CdS nanocomposites to divulge electron-hole generation, electron-hole recombination, and resistance against photo-corrosion. TiO2/PbS exhibited higher electron-hole generation than TiO2/CdS owing to its efficient visible light harvesting capability which resulted in higher
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photocurrent density (JSC) and thus higher power conversion efficiency (PCE) for respective QDSC; however, it had inferior resistance against photo-corrosion. Additionally, the degradation of an azo-based dye (acid orange-56) reflected an effectual increase in the photocatalytic activity of anatase-TiO2 as a
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result of deposition of PbS and CdS QDs. TiO2/PbS performed better than TiO2/CdS as a photocatalyst under visible light irradiation.
Keywords: Quantum-dots; Photocatalysts; Solar cell; Photostability; SILAR
*Corresponding author: [email protected] & [email protected]
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1. INTRODUCTION Rapid consumption of fossil fuels as the main energy source and exhaust from various industries is the reason why the modern urban world is facing environmental tribulations concerning air and water pollution. Therefore, it is important to explore newer, cheaper and environment friendly energy
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technologies [1, 2] while it is equally critical to devise strategies for the destruction of aqueous pollutants [3-5]. The fear of extinction of fossil fuels is touching its limits as the energy consumption per person has increased to more than 300% since 1850 [1, 6]. Such exponential increase in energy consumption plus a rapidly increasing world population has emerged as a challenge to fulfill energy demands; more importantly in a cost-effective and environment friendly way. This situation was previously summarized
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as “energy-economy-environment dilemma” by J.P. Holdren in his presidential address to the American Association for the Advancement of Science in 2007 [6]. In the same context, energy efficient
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technologies, such as photovoltaics, water splitting, hydrogen fuel generation, magnetic refrigeration, and thermoelectricity etc. have been extensively investigated for the development of alternative and/or renewable energy technologies [7]. Among various technologies, photovoltaics has gained an emerging status owing to its direct role in the energy conversion process, diversity of materials and/or designs for devices [8-10]. The modern concept of solar paints and its revival is actually a glimpse from photovoltaic applications of a future world which are realized because of the development of smarter and efficient photovoltaic materials [11, 13]. Concurrently, the development of photocatalysts for the destruction of
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pollutant dyes mainly produced by the paper and textile industries has also been an area of interest for researchers [14-16]. Recently, photocatalysts for decontamination of water were applied in a pilot plant [17].
In this work, we report the photovoltaic as well as the photocatalytic activity of anatase-TiO2
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having PbS and CdS QDs deposited via newly reported wet chemical route (i.e. pseudo successive ionic layer adsorption and reaction; p-SILAR). TiO2 is a versatile material and has its applications in photocatalysis, electrochemical solar cells, etc. [18-22], whilst anatase-TiO2 is commercially available
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and an unexplored form of TiO2 as far as solar paint is concerned. Herein, we affirm that the deposition of PbS and CdS QDs on anatase-TiO2 (resulting TiO2/PbS and TiO2/CdS nanocomposites, respectively) using p-SILAR evolves considerable visible light absorbance and consequent electron-hole generation. When TiO2/PbS and TiO2/CdS were applied in QDSCs as solar paints, TiO2/PbS yielded superior photocurrent density (JSC) and thus higher power conversion efficiency (PCE). However, TiO2/CdS exhibited lower electron-hole recombination and higher photostability. Moreover, TiO2/PbS and TiO2/CdS showed superior degradation of an azo-based dye (acid orange-56) under visible irradiation. Detailed and schematic interpretation of the evolution of photovoltaic and photocatalytic behavior in 2
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anatase-TiO2 is discussed to relate the results obtained from various qualitative and quantitative analysis, mainly, scanning and transmission electron microscopy (SEM/TEM), J-V characteristics measurements, open-circuit voltage decay (OCVD), normalized current stability and dye degradation.
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2. Methods 2.1 Deposition of Quantum Dots
PbS and CdS QDs were deposited on anatase-TiO2 using p-SILAR method [23, 24]. Briefly, we prepared cationic precursors for 0.02M Pb and 0.05M Cd by dissolving Pb(NO3)2 and Cd(NO3)2 in methanol while
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the anionic precursors were prepared by dissolving Na2S.5H2O in methanol-water (1:1, v/v) in corresponding molar concentrations. For PbS QDs deposition, commercially available anatase-TiO2 powder (Sigma-Aldrich) was added in a centrifuge tube in an adequate quantity and then 20ml of Pb-ions
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precursor was added. Subsequent to vigorous stirring for 1min, the mixture was centrifuged for 4min at 6000rpm. After the first centrifuging cycle, the supernatant was removed simply through decantation. For rinsing, 20ml of methanol was added to the mixture which was again centrifuged for 4min at 6000rpm. In the third centrifuging cycle 0.02M S-ion precursor was introduced and the same procedure was applied till decantation. Lastly, centrifuging was carried out once again to remove the unreacted S-ions and hence complete one p-SILAR cycle. The p-SILAR cycles were performed three times in order to obtain an
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optimum PbS QDs deposition on anatase-TiO2. For CdS QDs deposition, we used 10 p-SILAR cycles, however, the concentration of corresponding precursors was kept 0.05M. Both PbS QDs and CdS QDs sensitized anatase-TiO2 were lastly passivated through 2 cycles of ZnS deposition using 0.02M Zn(CH3COO)2 in methanol and Na2S.5H2O in methanol-water (1:1, v/v). QDs sensitized anatase-TiO2
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nanoparticles were then dried by heating at 60ᴼC for 48h. All the chemicals used in experiments were of analytical grade and purchased from Sigma-Aldrich.
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2.2 Characterization
Scanning electron microscope (SEM, TESCAN MIRA3) was used to study the morphological aspects and particle size analyzer (Matersizer 3000, Malvern) was utilized to re-affirm the size and range of anataseTiO2 nanoparticles. The effectual deposition of PbS and CdS QDs was qualitatively analyzed by transmission electron microscopy (HRTEM, JEOL) and XRD (Explorer, GNR) while energy dispersive spectroscopy (EDS, EMAX-Horbia) was carried out to divulge quantitative details. The light harvesting tendency was determined through UV-Vis spectrophotometer (S-3100, SCINCO) and a coupled diffuse reflector.
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2.3 Evaluation of Photovoltaic Activity Following the previously reported procedures [23], solar paints were prepared from TiO2/PbS and TiO2/CdS nanocomposites. In brief, we added 0.25gm of ethyl cellulose in toluene and ethanol (4:1, v/v), and 0.5gm of nanocomposites to a mortar grinder. After appropriate dissolution, we added 1.7ml of α-
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terpineol and carried on further mixing for 15min until a viscous paste was obtained. The paste was applied on fluorine-doped tin oxide (FTO) glass simply using doctor blade technique and then dried for 250ᴼC for 10mins in air to remove the binding materials and to evolve adequate porosity for infiltration of polysulfide electrolyte having 0.125M S/1M Na2S in methanol and deionized water (3:2, v/v). CuS film deposited on FTO by chemical bath deposition (CBD) was used as counter electrode [25] to assemble the
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sandwich-type solar cell. The photovoltaic response in terms of current-voltage (J-V) characteristics, charge carrier recombination, and photostability was measured using Keithley 2400 source meter coupled
2.4 Evaluation of Photocatalytic Activity
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with a solar simulator.
Degradation of an azo-based dye (acid orange-56) was adopted as a probe reaction to evaluate the photocatalytic activity of PbS and CdS QDs-sensitized anatase-TiO2 [26]. 0.10gm of each catalyst was dispersed in 50ml of dye-solution to have 50ppm initial dye concentration while the pH was kept at 4. Before visible light irradiation via 300W Xe arc lamp equipped with an ultraviolet cut off filter, each
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suspension was kept under magnetic stirring for 10min under dark condition. Then, each suspension was exposed to visible light radiation to determine the reaction progress. 4ml aliquot was removed from the reaction suspension after 15, 30, 45, 60, 90, 120 and 180min each, and the absorbance was measured using UV-vis spectrophotometer to calculate the %degradation of the dye using the following equation:
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%
=
(
)
× 100
(1)
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Where A0 and At denote the initial absorbance and absorbance after an interval of time ‘t’.
3. RESULTS AND DISCUSSION 3.1 Structural Analysis
Scanning electron microscopy (SEM) images (Fig. 1a, b) revealed the morphological details of anataseTiO2 used as reference materials for QDs deposition, confirming the presence of nanoparticles from ~50nm to ~300nm. It was found that the nanoparticles were not of the same size and were mixed uniformly and agglomerated. Such a mixture of nanoparticles is attractive for investigation because smaller size particles (A and B in Fig. 1b) provide a greater area for QDs-sensitization while larger size 4
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particles (C) act as light scattering centers in photoanodes for solar cell applications [27, 28]. Variance in the size and distribution was further affirmed by particle size analysis (Fig. 1c) which shows the presence of nanoparticles having a diameter of <100nm in minor portion and it was >100nm in the major portion. Corresponding EDS showed the purity of nanoparticles qualitatively (Fig. S1). As a result of 3 and 10
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cycles of QDs deposition using p-SILAR for PbS and CdS respectively [23, 24], explicit elemental presence of Pb (4.09%) and S, as well as Cd (1.11%) and S projected the deposition of PbS and CdS QDs (Fig. S2 and S3). It will be worth investigation for further studies and to enhance the efficacy of nanocomposites if the respective %age of PbS and CdS deposited over anatase-TiO2 are increased. Since it is not likely to show the presence of QDs using SEM directly, therefore high-resolution transmission
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electron microscope (TEM) images were obtained to affirm the manifestation of PbS and CdS QDs on anatase-TiO2 nanoparticles (Fig. 2a and 2c). Furthermore, TEM images at higher magnification revealed
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the distinct lattice fringes of 0.27nm and 0.336nm corresponding to the (200) plane of PbS QDs and (220) plane of CdS QDs (Fig. 2b and 2d). Fig. 2e shows the XRD spectra of anatase-TiO2 without and with QDs. A substantial peak was obtained at 30.1ᴼ corresponding to (200) plane of PbS QDs [23], whereas a weak XRD signal credited to lower weight percentage of CdS QDs was obtained at 25.1° and 43.9° corresponding to (100) and (110) plane respectively [24]. Anatase-TiO2 has a band gap (Eg) value of ~3.2eV and hence it is not active under visible spectrum of light, however the deposition of PbS and CdS QDs having lower band gap values (i.e. ~1eV and ~2.5eV, respectively for PbS and CdS QDs), evolves
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proficient light harvesting [18, 23, 24] in anatase-TiO2. Fig. S4 shows significantly increased visible light absorbance in TiO2/PbS and TiO2/CdS. TiO2/PbS showed more absorbance in the visible range of light than TiO2/CdS which is an obvious outcome of higher quantitative presence of PbS as revealed by EDS, and lower band gap value for PbS QDs. The evolution of light absorption in anatase-TiO2 from ~400nm to
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~700nm advocate that TiO2/PbS and TiO2/CdS can suitably be employed for photovoltaic as well as dye degradation application under visible light irradiation.
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3.2 Photovoltaic Performance
Fig. 3 exemplifies the charge carrier transfer mechanism for the photovoltaic performance of TiO2/PbS and TiO2/CdS. Although the size of QDs defines their band gap value, however it is previously affirmed that the position of conduction band (ECB) for PbS and CdS QDs remains reasonably higher than that of TiO2 for 3 and 10 cycles of QDs deposition using p-SILAR for PbS and CdS respectively [20, 23, 24]. Herein, it is depicted that electrons generated in PbS QDs in TiO2/PbS due to incidence of light, have better tendency to move to the conduction band of TiO2, ensuring higher contribution in photocurrent density (JSC) than CdS QDs. To discuss the electron-hole (e--h+) generation and its transfer mechanism in detail, TiO2/PbS and TiO2/CdS were transformed to solar paints for fabricating quantum-dot-sensitized 5
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solar cells (QDSCs) as per previously reported methods [23, 24] and their current-voltage (J-V) characteristics were measured. The photovoltaic performance of a photoactive material is determined through various imperative J-V parameters shown in the equation (2); =
(
×
×
)
(2)
!"
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Where PCE, VOC, FF, and Pin represent the power-conversion efficiency, open-circuit voltage, fill factor, and intensity of light of incidence, respectively. Fig. 4 shows that TiO2/PbS exhibited better PCE which is credited to its superior JSC value (~2.85mA/cm2) than TiO2/CdS (~2.15 mA/cm2), while VOC and FF remained quite similar. Ultra-fast transfer of electrons from PbS QDs to TiO2 was previously reported
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[29] by Lian et al. which is equally a significant feature of better photovoltaic performance of TiO2/PbS than TiO2/CdS in addition to higher absorbance as discussed in section 3.1. Another important feature associated with photovoltaic materials is e--h+ recombination that may occur within QDs or at
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TiO2/electrolyte or/and QDs/electrolyte interfaces [18, 30]. It is generally estimated via open-circuit voltage decay (OCVD) which is a simple but valuable technique to determine the rate of decay for VOC and electron lifetime (τ). Fig. 4c reflects that CdS QDs are less prone to undergo e--h+ recombination than PbS QDs in a poly-sulfide environment which is credited to their passive nature. This is in resemblance with the previous reports of CdS deposition over PbS for the enhancement of photostability of PbS QDs [13, 31]. OCVD data was further employed to calculate electron life time (τ) using the following
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equation;
#$ = −
Where *
+
+,
&' ( + * +, )
.
- (2)
- represents the rate of decay for VOC, T represents temperature, e represents unit charge, and
/0 represents Boltzmann constant. Inset Fig. 4c revealed that TiO2/CdS exhibited higher τ than TiO2/PbS,
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affirming the higher extent of e--h+ recombination in TiO2/PbS. Passive nature of CdS also exhibited resistance against photo-corrosion and hence only 20% of normalized photo-current (I/Io) was lost for
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TiO2/CdS, whereas TiO2/PbS lost ~35% of I/Io in the same interval of time [Fig. 4d]. It is worth mentioning that the decrease in I/Io with time reflects the deterioration of CdS and PbS QDs under solar light irradiation in polysulfide electrolyte which is a typical environment for QDSCs. It is therefore deduced that TiO2/CdS is more photostable than TiO2/PbS. 3.3 Photocatalytic Dye Degradation Considering the peculiarity in e--h+ generation, e--h+ recombination and photo-corrosion of PbS and CdS QDs, it becomes attractive to study their respective influence on the photocatalytic activity of anataseTiO2. Anatase-TiO2 has the least power for photocatalytic reactions under visible light compared to 6
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ultraviolet irradiation conditions [14-16, 32]. Previously, various heterostructures have been designed for enhanced photocatalytic activity under visible or/and ultraviolet irradiation condition. For example, the conjunctions of TiO2 and Ag2O, as well as CuO@ZnO core-shell nanocomposites, were reported for enhancement in the photocatalytic activity [33, 34]. Recently, TiO2/CdSe films fabricated by Lu et al.
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while ZnO/ZnSe microspheres synthesized by Liu et al. exhibited more than 80% and 90% photocatalytic activity, respectively [35, 36]. PbS and CdS QDs-sensitized anatase-TiO2 synthesized by us through simple and novel wet chemistry technique (i.e. p-SILAR) proved to be equally efficient as photocatalysts. Acid orange-56, as shown in Fig. 5, degraded much rapidly when TiO2/PbS and TiO2/CdS catalysts were incorporated, while anatase-TiO2 showed no photocatalytic activity in the same interval of time. Fig. 5a
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illustrates the proposed route for the degradation of azo-type dye. As a consequence of visible light absorbance in TiO2/PbS and TiO2/CdS, e--h+ generation takes place much more extensively compared to
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bare anatase-TiO2, which excites the electrons to the conduction band (CB) of TiO2 as well as QDs (whether PbS or CdS). Since the CB of TiO2 is located at a position lower than that of PbS and CdS QDs, it is anticipated that all the electrons generated in PbS or CdS QDs transfer to the CB of TiO2 where superoxide radicals are formed due to the reaction with dissolved O2-molecules. Subsequently, hydroxyl radicals are formed via protonation of water [37]. Being the strong oxidizing agent, the hydroxyl radicals decompose acid orange-56 dye. The rate of dye-degradation is directly dependent on the obtainability of electrons in the conduction band of TiO2 for TiO2/PbS and TiO2/CdS. It is worth mentioning that neither
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PbS nor CdS cover the anatase-TiO2 surface entirely, therefore it is more probable that the reaction takes place on the anatase-TiO2/dye-solution interface in all cases. Superior e--h+ generation in TiO2/PbS under visible light irradiation yields higher photodegradation of dye than TiO2/CdS, while TiO2-anatase remains least effectual owing to its least absorbance in the visible range of light. Fig. 5b and 5c show nearly ~80%
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dye degradation by TiO2-PbS in 3 hrs. While TiO2-PbS exhibited nearly 3 times higher dye degradation compared to TiO2-CdS. We further evaluated the kinetics of dye degradation by applying first-order kinetics model [38-41]. Fig. 5d shows a linear relationship between the logarithm of ratio of initial
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concentration to concentration at any time ‘t’ and irradiation time and irradiation time (i.e. log Co/Ct Vs t) to deduce imperative parameters including correlation coefficient (R2) and first-order rates constant (k). TiO2/PbS and TiO2/CdS have 0.204min-1 and 0.055min-1 value for k, respectively. A relatively higher value for TiO2/PbS affirmed its propensity as a better photocatalyst under visible light irradiation than TiO2/CdS. Comprehensive performance of TiO2/PbS and TiO2/CdS with reference to TiO2-anatase is exclusively depicted in Fig. 6 which reiterates that the evolution of photoactive behavior in anatase-TiO2 is credited to the deposition of PbS and CdS QDs via p-SILAR. PbS QDs, when deposited on TiO2anatase yield higher e--h+ pairs than CdS QDs for photovoltaic and photocatalytic utility, however, the 7
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latter exhibits better photostability and resistance against electron hole-recombination under similar irradiation and electrolytic environment.
4. CONCLUSIONS
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We demonstrated effectual deposition of PbS and CdS quantum-dots on anatase-TiO2 using p-SILAR for evolving significantly visible light absorbance. Using the photovoltaic cell performance and azo-based dye degradation investigations, it was shown that TiO2/PbS results in superior electron-hole pair generation and thus results in better dye degradation as well as photoconversion efficiency than TiO2/CdS nanocomposites. On the contrary, TiO2/CdS showed better resistance against photo-corrosion and
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electron-hole recombination, ensuring better normalized current features but reduced photocatalytic activity compared to TiO2/PbS. Overall, it is concluded that the conjunction of QDs through p-SILAR,
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whether PbS or CdS, uplifts the photoactivity of anatase-TiO2 and can be regulated simply by varying the type of metal-chalcogenide QDs. However, it is still challenging to increase the deposition of PbS and
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CdS on anatase-TiO2 to enhance the performance of nanocomposites introduced in this study.
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Figure Captions Figure 1. High magnification (a, b) scanning electron microscope images of anatase-TiO2 nano-powder used for PbS and CdS QDs deposition by p-SILAR along with corresponding (c) particle size analysis.
nanocomposites and their resultant (e) X-ray diffraction spectroscopy
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Figure 2. Transmission electron microscopy images of (a, b) TiO2/PbS and (c,d) TiO2/CdS Figure 3. Schematic illustration of photovoltaic activity in (a) TiO2/PbS and (b) TiO2/CdS nanocomposites when they were employed as solar paints to fabricate quantum-dot-sensitized solar cells. Figure 4. Photovoltaic characteristics of TiO2/PbS QDs (blue) and TiO2/CdS (red) nanocomposites. (a)
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Current density-voltage curves, (b) imperative parameters defining photovoltaic performance i.e. shortcircuit current density ( JSC), open circuit voltage ( VOC), fill factor (FF) and power conversion efficiency (PCE). (c) Open-circuit voltage decay and (d) normalized current stability.
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Figure 5. (a,b) Photocatalytic behavior of TiO2/PbS (blue) and TiO2/CdS (red) nanocomposites reference to anatase-TiO2 and corresponding first-order kinetics of reactions.
Figure 6. Schematic summary of photovoltaic and photocatalytic characteristics of (a) anatase-TiO2, (b) TiO2/PbS QDs and (c) TiO2/CdS nanocomposites. Figure S1.SEM-EDS analysis of TiO2-anatase
Figure S2.SEM-EDS analysis of TiO2/PbS QDs nanocomposite (a) before and (b) after heat-treatment at
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250ᴼC for 30min.
Figure S3. SEM-EDS analysis of TiO2/CdS QDs nanocomposite (a) before and (b) after heat-treatment at 250ᴼC for 30min.
Figure S4. Incident light absorbance characteristics of reference anatase-TiO2 nano-powder in
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comparison with TiO2/PbS and TiO2/CdS nanocomposites Figure S5. Calibration curve for Azo-dye degradation experiment employed for the determination of
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photocatalytic activity of anatase-TiO2, TiO2/PbS and TiO2/CdS nanocomposites
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Figures:
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Figure 1. High magnification (a, b) SEM images of anatase-TiO2 nano-powder used for PbS and CdS QDs deposition by p-SILAR along with corresponding (c) particle size analysis.
Figure 2. TEM images of (a, b) TiO2/PbS and (c, d) TiO2/CdS nanocomposites and their resultant (e) Xray diffraction spectroscopy 13
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Figure 3. Schematic illustration of photovoltaic activity in (a) TiO2/PbS and (b) TiO2/CdS nanocomposites when they were employed as solar paints to fabricate quantum-dot-sensitized solar cells.
Figure 4. Photovoltaic characteristics of TiO2/PbS QDs (blue) and TiO2/CdS (red) nanocomposites. (a) Current density-voltage curves, (b) imperative parameters defining photovoltaic performance i.e. shortcircuit current density ( JSC), open circuit voltage ( VOC), fill factor (FF) and power conversion efficiency (PCE). (c) Open-circuit voltage decay and (d) normalized current stability. 14
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Figure 5. (a) Schematic illustration of azo-dye degradation mechanism. (b, c) Photocatalytic behavior of TiO2/PbS (blue) and TiO2/CdS (red) nanocomposites reference to anatase-TiO2 and (d) corresponding first-order kinetics of reactions.
Figure 6. Schematic summary of photovoltaic and photocatalytic characteristics of (a) anatase-TiO2, (b) TiO2/PbS QDs and (c) TiO2/CdS nanocomposites. 15
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Figure S1.SEM-EDS analysis of TiO2-anatase
Figure S2.SEM-EDS analysis of TiO2/PbS QDs nanocomposite (a) before and (b) after heat-treatment at 250ᴼC for 30min. 16
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Figure S3. SEM-EDS analysis of TiO2/CdS QDs nanocomposite (a) before and (b) after heat-treatment at 250ᴼC for 30min.
Figure S4. Incident light absorbance characteristics of reference anatase-TiO2 nano-powder in comparison with TiO2/PbS and TiO2/CdS nanocomposites 17
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Figure S5. Calibration curve for Azo-dye degradation experiment employed for the determination of photocatalytic activity of anatase-TiO2, TiO2/PbS and TiO2/CdS nanocomposites.
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Graphical Abstract
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Highlights PbS and CdS quantum-dots were successfully deposited on anatase-TiO2 via p-SILAR.
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TiO2/PbS exhibited higher visible light absorbance than TiO2/CdS. TiO2/PbS performed better as a photovoltaic material as well as a photocatalyst.
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However, TiO2/CdS showed superior photostability than TiO2/PbS.
S0254-0584(19)30242-1
DOI:
https://doi.org/10.1016/j.matchemphys.2019.03.042
Reference:
MAC 21481
To appear in:
Materials Chemistry and Physics
Received Date: 8 March 2019 Accepted Date: 13 March 2019
Please cite this article as: M.I. Aziz, F. Mughal, H.M. Naeem, A. Zeb, M.A. Tahir, M.A. Basit, Evolution of photovoltaic and photocatalytic activity in anatase-TiO2 under visible light via simplistic deposition of CdS and PbS quantum-dots, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.03.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Evolution of photovoltaic and photocatalytic activity in anatase-TiO2 under visible light via simplistic deposition of CdS and PbS quantum-dots
and Muhammad Abdul Basit1* 1
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Mansoor Irfan Aziz,1 Faryal Mughal,1 Hafiz Muhammad Naeem,1 Alam Zeb,2 Muhammad Asif Tahir3
Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, Pakistan 2
Pakistan 3
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Riphah Institute of Pharmaceutical Sciences, Riphah International University, Islamabad,
Department of Chemistry, University of Agriculture Faisalabad, Pakistan
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Abstract
A simple approach to deposit metal chalcogenide-based quantum-dots on TiO2 (anatase) nanoparticles via pseudo-successive ionic layer adsorption and reaction (p-SILAR) was opted to design and investigate the corresponding photoactivity under visible light. We used TiO2/PbS and TiO2/CdS nanocomposites for photocatalysis after which we prepared solar paints from them to put side by side the resultant
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photovoltaic performance. We developed solar paint type quantum-dot-sensitized solar cells (QDSCs) from TiO2/PbS and TiO2/CdS nanocomposites to divulge electron-hole generation, electron-hole recombination, and resistance against photo-corrosion. TiO2/PbS exhibited higher electron-hole generation than TiO2/CdS owing to its efficient visible light harvesting capability which resulted in higher
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photocurrent density (JSC) and thus higher power conversion efficiency (PCE) for respective QDSC; however, it had inferior resistance against photo-corrosion. Additionally, the degradation of an azo-based dye (acid orange-56) reflected an effectual increase in the photocatalytic activity of anatase-TiO2 as a
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result of deposition of PbS and CdS QDs. TiO2/PbS performed better than TiO2/CdS as a photocatalyst under visible light irradiation.
Keywords: Quantum-dots; Photocatalysts; Solar cell; Photostability; SILAR
*Corresponding author: [email protected] & [email protected]
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1. INTRODUCTION Rapid consumption of fossil fuels as the main energy source and exhaust from various industries is the reason why the modern urban world is facing environmental tribulations concerning air and water pollution. Therefore, it is important to explore newer, cheaper and environment friendly energy
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technologies [1, 2] while it is equally critical to devise strategies for the destruction of aqueous pollutants [3-5]. The fear of extinction of fossil fuels is touching its limits as the energy consumption per person has increased to more than 300% since 1850 [1, 6]. Such exponential increase in energy consumption plus a rapidly increasing world population has emerged as a challenge to fulfill energy demands; more importantly in a cost-effective and environment friendly way. This situation was previously summarized
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as “energy-economy-environment dilemma” by J.P. Holdren in his presidential address to the American Association for the Advancement of Science in 2007 [6]. In the same context, energy efficient
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technologies, such as photovoltaics, water splitting, hydrogen fuel generation, magnetic refrigeration, and thermoelectricity etc. have been extensively investigated for the development of alternative and/or renewable energy technologies [7]. Among various technologies, photovoltaics has gained an emerging status owing to its direct role in the energy conversion process, diversity of materials and/or designs for devices [8-10]. The modern concept of solar paints and its revival is actually a glimpse from photovoltaic applications of a future world which are realized because of the development of smarter and efficient photovoltaic materials [11, 13]. Concurrently, the development of photocatalysts for the destruction of
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pollutant dyes mainly produced by the paper and textile industries has also been an area of interest for researchers [14-16]. Recently, photocatalysts for decontamination of water were applied in a pilot plant [17].
In this work, we report the photovoltaic as well as the photocatalytic activity of anatase-TiO2
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having PbS and CdS QDs deposited via newly reported wet chemical route (i.e. pseudo successive ionic layer adsorption and reaction; p-SILAR). TiO2 is a versatile material and has its applications in photocatalysis, electrochemical solar cells, etc. [18-22], whilst anatase-TiO2 is commercially available
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and an unexplored form of TiO2 as far as solar paint is concerned. Herein, we affirm that the deposition of PbS and CdS QDs on anatase-TiO2 (resulting TiO2/PbS and TiO2/CdS nanocomposites, respectively) using p-SILAR evolves considerable visible light absorbance and consequent electron-hole generation. When TiO2/PbS and TiO2/CdS were applied in QDSCs as solar paints, TiO2/PbS yielded superior photocurrent density (JSC) and thus higher power conversion efficiency (PCE). However, TiO2/CdS exhibited lower electron-hole recombination and higher photostability. Moreover, TiO2/PbS and TiO2/CdS showed superior degradation of an azo-based dye (acid orange-56) under visible irradiation. Detailed and schematic interpretation of the evolution of photovoltaic and photocatalytic behavior in 2
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anatase-TiO2 is discussed to relate the results obtained from various qualitative and quantitative analysis, mainly, scanning and transmission electron microscopy (SEM/TEM), J-V characteristics measurements, open-circuit voltage decay (OCVD), normalized current stability and dye degradation.
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2. Methods 2.1 Deposition of Quantum Dots
PbS and CdS QDs were deposited on anatase-TiO2 using p-SILAR method [23, 24]. Briefly, we prepared cationic precursors for 0.02M Pb and 0.05M Cd by dissolving Pb(NO3)2 and Cd(NO3)2 in methanol while
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the anionic precursors were prepared by dissolving Na2S.5H2O in methanol-water (1:1, v/v) in corresponding molar concentrations. For PbS QDs deposition, commercially available anatase-TiO2 powder (Sigma-Aldrich) was added in a centrifuge tube in an adequate quantity and then 20ml of Pb-ions
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precursor was added. Subsequent to vigorous stirring for 1min, the mixture was centrifuged for 4min at 6000rpm. After the first centrifuging cycle, the supernatant was removed simply through decantation. For rinsing, 20ml of methanol was added to the mixture which was again centrifuged for 4min at 6000rpm. In the third centrifuging cycle 0.02M S-ion precursor was introduced and the same procedure was applied till decantation. Lastly, centrifuging was carried out once again to remove the unreacted S-ions and hence complete one p-SILAR cycle. The p-SILAR cycles were performed three times in order to obtain an
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optimum PbS QDs deposition on anatase-TiO2. For CdS QDs deposition, we used 10 p-SILAR cycles, however, the concentration of corresponding precursors was kept 0.05M. Both PbS QDs and CdS QDs sensitized anatase-TiO2 were lastly passivated through 2 cycles of ZnS deposition using 0.02M Zn(CH3COO)2 in methanol and Na2S.5H2O in methanol-water (1:1, v/v). QDs sensitized anatase-TiO2
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nanoparticles were then dried by heating at 60ᴼC for 48h. All the chemicals used in experiments were of analytical grade and purchased from Sigma-Aldrich.
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2.2 Characterization
Scanning electron microscope (SEM, TESCAN MIRA3) was used to study the morphological aspects and particle size analyzer (Matersizer 3000, Malvern) was utilized to re-affirm the size and range of anataseTiO2 nanoparticles. The effectual deposition of PbS and CdS QDs was qualitatively analyzed by transmission electron microscopy (HRTEM, JEOL) and XRD (Explorer, GNR) while energy dispersive spectroscopy (EDS, EMAX-Horbia) was carried out to divulge quantitative details. The light harvesting tendency was determined through UV-Vis spectrophotometer (S-3100, SCINCO) and a coupled diffuse reflector.
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2.3 Evaluation of Photovoltaic Activity Following the previously reported procedures [23], solar paints were prepared from TiO2/PbS and TiO2/CdS nanocomposites. In brief, we added 0.25gm of ethyl cellulose in toluene and ethanol (4:1, v/v), and 0.5gm of nanocomposites to a mortar grinder. After appropriate dissolution, we added 1.7ml of α-
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terpineol and carried on further mixing for 15min until a viscous paste was obtained. The paste was applied on fluorine-doped tin oxide (FTO) glass simply using doctor blade technique and then dried for 250ᴼC for 10mins in air to remove the binding materials and to evolve adequate porosity for infiltration of polysulfide electrolyte having 0.125M S/1M Na2S in methanol and deionized water (3:2, v/v). CuS film deposited on FTO by chemical bath deposition (CBD) was used as counter electrode [25] to assemble the
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sandwich-type solar cell. The photovoltaic response in terms of current-voltage (J-V) characteristics, charge carrier recombination, and photostability was measured using Keithley 2400 source meter coupled
2.4 Evaluation of Photocatalytic Activity
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with a solar simulator.
Degradation of an azo-based dye (acid orange-56) was adopted as a probe reaction to evaluate the photocatalytic activity of PbS and CdS QDs-sensitized anatase-TiO2 [26]. 0.10gm of each catalyst was dispersed in 50ml of dye-solution to have 50ppm initial dye concentration while the pH was kept at 4. Before visible light irradiation via 300W Xe arc lamp equipped with an ultraviolet cut off filter, each
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suspension was kept under magnetic stirring for 10min under dark condition. Then, each suspension was exposed to visible light radiation to determine the reaction progress. 4ml aliquot was removed from the reaction suspension after 15, 30, 45, 60, 90, 120 and 180min each, and the absorbance was measured using UV-vis spectrophotometer to calculate the %degradation of the dye using the following equation:
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(1)
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Where A0 and At denote the initial absorbance and absorbance after an interval of time ‘t’.
3. RESULTS AND DISCUSSION 3.1 Structural Analysis
Scanning electron microscopy (SEM) images (Fig. 1a, b) revealed the morphological details of anataseTiO2 used as reference materials for QDs deposition, confirming the presence of nanoparticles from ~50nm to ~300nm. It was found that the nanoparticles were not of the same size and were mixed uniformly and agglomerated. Such a mixture of nanoparticles is attractive for investigation because smaller size particles (A and B in Fig. 1b) provide a greater area for QDs-sensitization while larger size 4
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particles (C) act as light scattering centers in photoanodes for solar cell applications [27, 28]. Variance in the size and distribution was further affirmed by particle size analysis (Fig. 1c) which shows the presence of nanoparticles having a diameter of <100nm in minor portion and it was >100nm in the major portion. Corresponding EDS showed the purity of nanoparticles qualitatively (Fig. S1). As a result of 3 and 10
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cycles of QDs deposition using p-SILAR for PbS and CdS respectively [23, 24], explicit elemental presence of Pb (4.09%) and S, as well as Cd (1.11%) and S projected the deposition of PbS and CdS QDs (Fig. S2 and S3). It will be worth investigation for further studies and to enhance the efficacy of nanocomposites if the respective %age of PbS and CdS deposited over anatase-TiO2 are increased. Since it is not likely to show the presence of QDs using SEM directly, therefore high-resolution transmission
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electron microscope (TEM) images were obtained to affirm the manifestation of PbS and CdS QDs on anatase-TiO2 nanoparticles (Fig. 2a and 2c). Furthermore, TEM images at higher magnification revealed
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the distinct lattice fringes of 0.27nm and 0.336nm corresponding to the (200) plane of PbS QDs and (220) plane of CdS QDs (Fig. 2b and 2d). Fig. 2e shows the XRD spectra of anatase-TiO2 without and with QDs. A substantial peak was obtained at 30.1ᴼ corresponding to (200) plane of PbS QDs [23], whereas a weak XRD signal credited to lower weight percentage of CdS QDs was obtained at 25.1° and 43.9° corresponding to (100) and (110) plane respectively [24]. Anatase-TiO2 has a band gap (Eg) value of ~3.2eV and hence it is not active under visible spectrum of light, however the deposition of PbS and CdS QDs having lower band gap values (i.e. ~1eV and ~2.5eV, respectively for PbS and CdS QDs), evolves
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proficient light harvesting [18, 23, 24] in anatase-TiO2. Fig. S4 shows significantly increased visible light absorbance in TiO2/PbS and TiO2/CdS. TiO2/PbS showed more absorbance in the visible range of light than TiO2/CdS which is an obvious outcome of higher quantitative presence of PbS as revealed by EDS, and lower band gap value for PbS QDs. The evolution of light absorption in anatase-TiO2 from ~400nm to
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~700nm advocate that TiO2/PbS and TiO2/CdS can suitably be employed for photovoltaic as well as dye degradation application under visible light irradiation.
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3.2 Photovoltaic Performance
Fig. 3 exemplifies the charge carrier transfer mechanism for the photovoltaic performance of TiO2/PbS and TiO2/CdS. Although the size of QDs defines their band gap value, however it is previously affirmed that the position of conduction band (ECB) for PbS and CdS QDs remains reasonably higher than that of TiO2 for 3 and 10 cycles of QDs deposition using p-SILAR for PbS and CdS respectively [20, 23, 24]. Herein, it is depicted that electrons generated in PbS QDs in TiO2/PbS due to incidence of light, have better tendency to move to the conduction band of TiO2, ensuring higher contribution in photocurrent density (JSC) than CdS QDs. To discuss the electron-hole (e--h+) generation and its transfer mechanism in detail, TiO2/PbS and TiO2/CdS were transformed to solar paints for fabricating quantum-dot-sensitized 5
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solar cells (QDSCs) as per previously reported methods [23, 24] and their current-voltage (J-V) characteristics were measured. The photovoltaic performance of a photoactive material is determined through various imperative J-V parameters shown in the equation (2); =
(
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×
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(2)
!"
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Where PCE, VOC, FF, and Pin represent the power-conversion efficiency, open-circuit voltage, fill factor, and intensity of light of incidence, respectively. Fig. 4 shows that TiO2/PbS exhibited better PCE which is credited to its superior JSC value (~2.85mA/cm2) than TiO2/CdS (~2.15 mA/cm2), while VOC and FF remained quite similar. Ultra-fast transfer of electrons from PbS QDs to TiO2 was previously reported
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[29] by Lian et al. which is equally a significant feature of better photovoltaic performance of TiO2/PbS than TiO2/CdS in addition to higher absorbance as discussed in section 3.1. Another important feature associated with photovoltaic materials is e--h+ recombination that may occur within QDs or at
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TiO2/electrolyte or/and QDs/electrolyte interfaces [18, 30]. It is generally estimated via open-circuit voltage decay (OCVD) which is a simple but valuable technique to determine the rate of decay for VOC and electron lifetime (τ). Fig. 4c reflects that CdS QDs are less prone to undergo e--h+ recombination than PbS QDs in a poly-sulfide environment which is credited to their passive nature. This is in resemblance with the previous reports of CdS deposition over PbS for the enhancement of photostability of PbS QDs [13, 31]. OCVD data was further employed to calculate electron life time (τ) using the following
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#$ = −
Where *
+
+,
&' ( + * +, )
.
- (2)
- represents the rate of decay for VOC, T represents temperature, e represents unit charge, and
/0 represents Boltzmann constant. Inset Fig. 4c revealed that TiO2/CdS exhibited higher τ than TiO2/PbS,
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affirming the higher extent of e--h+ recombination in TiO2/PbS. Passive nature of CdS also exhibited resistance against photo-corrosion and hence only 20% of normalized photo-current (I/Io) was lost for
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TiO2/CdS, whereas TiO2/PbS lost ~35% of I/Io in the same interval of time [Fig. 4d]. It is worth mentioning that the decrease in I/Io with time reflects the deterioration of CdS and PbS QDs under solar light irradiation in polysulfide electrolyte which is a typical environment for QDSCs. It is therefore deduced that TiO2/CdS is more photostable than TiO2/PbS. 3.3 Photocatalytic Dye Degradation Considering the peculiarity in e--h+ generation, e--h+ recombination and photo-corrosion of PbS and CdS QDs, it becomes attractive to study their respective influence on the photocatalytic activity of anataseTiO2. Anatase-TiO2 has the least power for photocatalytic reactions under visible light compared to 6
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ultraviolet irradiation conditions [14-16, 32]. Previously, various heterostructures have been designed for enhanced photocatalytic activity under visible or/and ultraviolet irradiation condition. For example, the conjunctions of TiO2 and Ag2O, as well as CuO@ZnO core-shell nanocomposites, were reported for enhancement in the photocatalytic activity [33, 34]. Recently, TiO2/CdSe films fabricated by Lu et al.
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while ZnO/ZnSe microspheres synthesized by Liu et al. exhibited more than 80% and 90% photocatalytic activity, respectively [35, 36]. PbS and CdS QDs-sensitized anatase-TiO2 synthesized by us through simple and novel wet chemistry technique (i.e. p-SILAR) proved to be equally efficient as photocatalysts. Acid orange-56, as shown in Fig. 5, degraded much rapidly when TiO2/PbS and TiO2/CdS catalysts were incorporated, while anatase-TiO2 showed no photocatalytic activity in the same interval of time. Fig. 5a
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illustrates the proposed route for the degradation of azo-type dye. As a consequence of visible light absorbance in TiO2/PbS and TiO2/CdS, e--h+ generation takes place much more extensively compared to
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bare anatase-TiO2, which excites the electrons to the conduction band (CB) of TiO2 as well as QDs (whether PbS or CdS). Since the CB of TiO2 is located at a position lower than that of PbS and CdS QDs, it is anticipated that all the electrons generated in PbS or CdS QDs transfer to the CB of TiO2 where superoxide radicals are formed due to the reaction with dissolved O2-molecules. Subsequently, hydroxyl radicals are formed via protonation of water [37]. Being the strong oxidizing agent, the hydroxyl radicals decompose acid orange-56 dye. The rate of dye-degradation is directly dependent on the obtainability of electrons in the conduction band of TiO2 for TiO2/PbS and TiO2/CdS. It is worth mentioning that neither
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PbS nor CdS cover the anatase-TiO2 surface entirely, therefore it is more probable that the reaction takes place on the anatase-TiO2/dye-solution interface in all cases. Superior e--h+ generation in TiO2/PbS under visible light irradiation yields higher photodegradation of dye than TiO2/CdS, while TiO2-anatase remains least effectual owing to its least absorbance in the visible range of light. Fig. 5b and 5c show nearly ~80%
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dye degradation by TiO2-PbS in 3 hrs. While TiO2-PbS exhibited nearly 3 times higher dye degradation compared to TiO2-CdS. We further evaluated the kinetics of dye degradation by applying first-order kinetics model [38-41]. Fig. 5d shows a linear relationship between the logarithm of ratio of initial
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concentration to concentration at any time ‘t’ and irradiation time and irradiation time (i.e. log Co/Ct Vs t) to deduce imperative parameters including correlation coefficient (R2) and first-order rates constant (k). TiO2/PbS and TiO2/CdS have 0.204min-1 and 0.055min-1 value for k, respectively. A relatively higher value for TiO2/PbS affirmed its propensity as a better photocatalyst under visible light irradiation than TiO2/CdS. Comprehensive performance of TiO2/PbS and TiO2/CdS with reference to TiO2-anatase is exclusively depicted in Fig. 6 which reiterates that the evolution of photoactive behavior in anatase-TiO2 is credited to the deposition of PbS and CdS QDs via p-SILAR. PbS QDs, when deposited on TiO2anatase yield higher e--h+ pairs than CdS QDs for photovoltaic and photocatalytic utility, however, the 7
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latter exhibits better photostability and resistance against electron hole-recombination under similar irradiation and electrolytic environment.
4. CONCLUSIONS
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We demonstrated effectual deposition of PbS and CdS quantum-dots on anatase-TiO2 using p-SILAR for evolving significantly visible light absorbance. Using the photovoltaic cell performance and azo-based dye degradation investigations, it was shown that TiO2/PbS results in superior electron-hole pair generation and thus results in better dye degradation as well as photoconversion efficiency than TiO2/CdS nanocomposites. On the contrary, TiO2/CdS showed better resistance against photo-corrosion and
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electron-hole recombination, ensuring better normalized current features but reduced photocatalytic activity compared to TiO2/PbS. Overall, it is concluded that the conjunction of QDs through p-SILAR,
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whether PbS or CdS, uplifts the photoactivity of anatase-TiO2 and can be regulated simply by varying the type of metal-chalcogenide QDs. However, it is still challenging to increase the deposition of PbS and
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CdS on anatase-TiO2 to enhance the performance of nanocomposites introduced in this study.
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Figure Captions Figure 1. High magnification (a, b) scanning electron microscope images of anatase-TiO2 nano-powder used for PbS and CdS QDs deposition by p-SILAR along with corresponding (c) particle size analysis.
nanocomposites and their resultant (e) X-ray diffraction spectroscopy
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Figure 2. Transmission electron microscopy images of (a, b) TiO2/PbS and (c,d) TiO2/CdS Figure 3. Schematic illustration of photovoltaic activity in (a) TiO2/PbS and (b) TiO2/CdS nanocomposites when they were employed as solar paints to fabricate quantum-dot-sensitized solar cells. Figure 4. Photovoltaic characteristics of TiO2/PbS QDs (blue) and TiO2/CdS (red) nanocomposites. (a)
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Current density-voltage curves, (b) imperative parameters defining photovoltaic performance i.e. shortcircuit current density ( JSC), open circuit voltage ( VOC), fill factor (FF) and power conversion efficiency (PCE). (c) Open-circuit voltage decay and (d) normalized current stability.
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Figure 5. (a,b) Photocatalytic behavior of TiO2/PbS (blue) and TiO2/CdS (red) nanocomposites reference to anatase-TiO2 and corresponding first-order kinetics of reactions.
Figure 6. Schematic summary of photovoltaic and photocatalytic characteristics of (a) anatase-TiO2, (b) TiO2/PbS QDs and (c) TiO2/CdS nanocomposites. Figure S1.SEM-EDS analysis of TiO2-anatase
Figure S2.SEM-EDS analysis of TiO2/PbS QDs nanocomposite (a) before and (b) after heat-treatment at
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250ᴼC for 30min.
Figure S3. SEM-EDS analysis of TiO2/CdS QDs nanocomposite (a) before and (b) after heat-treatment at 250ᴼC for 30min.
Figure S4. Incident light absorbance characteristics of reference anatase-TiO2 nano-powder in
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comparison with TiO2/PbS and TiO2/CdS nanocomposites Figure S5. Calibration curve for Azo-dye degradation experiment employed for the determination of
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photocatalytic activity of anatase-TiO2, TiO2/PbS and TiO2/CdS nanocomposites
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Figures:
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Figure 1. High magnification (a, b) SEM images of anatase-TiO2 nano-powder used for PbS and CdS QDs deposition by p-SILAR along with corresponding (c) particle size analysis.
Figure 2. TEM images of (a, b) TiO2/PbS and (c, d) TiO2/CdS nanocomposites and their resultant (e) Xray diffraction spectroscopy 13
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Figure 3. Schematic illustration of photovoltaic activity in (a) TiO2/PbS and (b) TiO2/CdS nanocomposites when they were employed as solar paints to fabricate quantum-dot-sensitized solar cells.
Figure 4. Photovoltaic characteristics of TiO2/PbS QDs (blue) and TiO2/CdS (red) nanocomposites. (a) Current density-voltage curves, (b) imperative parameters defining photovoltaic performance i.e. shortcircuit current density ( JSC), open circuit voltage ( VOC), fill factor (FF) and power conversion efficiency (PCE). (c) Open-circuit voltage decay and (d) normalized current stability. 14
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Figure 5. (a) Schematic illustration of azo-dye degradation mechanism. (b, c) Photocatalytic behavior of TiO2/PbS (blue) and TiO2/CdS (red) nanocomposites reference to anatase-TiO2 and (d) corresponding first-order kinetics of reactions.
Figure 6. Schematic summary of photovoltaic and photocatalytic characteristics of (a) anatase-TiO2, (b) TiO2/PbS QDs and (c) TiO2/CdS nanocomposites. 15
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Figure S1.SEM-EDS analysis of TiO2-anatase
Figure S2.SEM-EDS analysis of TiO2/PbS QDs nanocomposite (a) before and (b) after heat-treatment at 250ᴼC for 30min. 16
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Figure S3. SEM-EDS analysis of TiO2/CdS QDs nanocomposite (a) before and (b) after heat-treatment at 250ᴼC for 30min.
Figure S4. Incident light absorbance characteristics of reference anatase-TiO2 nano-powder in comparison with TiO2/PbS and TiO2/CdS nanocomposites 17
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Figure S5. Calibration curve for Azo-dye degradation experiment employed for the determination of photocatalytic activity of anatase-TiO2, TiO2/PbS and TiO2/CdS nanocomposites.
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Graphical Abstract
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Highlights PbS and CdS quantum-dots were successfully deposited on anatase-TiO2 via p-SILAR.
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TiO2/PbS exhibited higher visible light absorbance than TiO2/CdS. TiO2/PbS performed better as a photovoltaic material as well as a photocatalyst.
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However, TiO2/CdS showed superior photostability than TiO2/PbS.