CdSe Heterostructure for Efficient H2 Generation and

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C: Energy Conversion and Storage; Energy and Charge Transport

Design of CdS/CdSe Heterostructure for Efficient H Generation and Photovoltaic Applications 2

Rajesh Bera, Avisek Dutta, Simanta Kundu, Vivek Polshettiwar, and Amitava Patra J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry

Design of CdS/CdSe Heterostructure for Efficient H2 Generation and Photovoltaic Applications

Rajesh Beraa, Avisek Duttaa, Simanta Kundub#, Vivek Polshettiwarb, Amitava Patraa*

a

Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata

700032,India b

Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Mumbai 400005, India

#

Shibpur Dinobundhoo Institution (College), Department of Chemistry, Howrah-711102

*To whom correspondence should be addressed. E-mail: [email protected] Phone: (91)-33-2473-4971, Fax: (91)-33-2473-2805

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The design of nano-heterostructures for light harvesting systems for the photocatalysis and photovoltaic applications is an emerging area of research. Here, we report the synthesis of one dimensional quasi type-II CdS/CdSe heterostructure where holes are confined in CdSe nanoparticle and electrons can delocalize throughout the conduction band of both CdS nanorod and CdSe nanoparticle due to smaller conduction band offset. By controlling the oxidation and reduction sites of CdS/CdSe heterostructure, we achieved maximum H2 generation of 5125 µmol/g/h for 27.5 wt% CdSe loaded CdS heterostructure which is found to be 44 times higher than bare CdS nanorod and 22 times higher than CdSe nanoparticle. Furthermore, this heterostructure exhibits photovoltaic effect (Voc =0.8 V, Jsc =0.56 mA/cm2, FF= 40 %, η = 0.18) which could be useful for solar cell application. The bleaching recovery kinetics and hot electron cooling dynamics have been studied by using femtosecond transient spectroscopy which confirms the efficient charge separation and long excited state lifetime of 27.5% CdSe loaded CdS heterostructure. Thus, the slow recombination process is the reason for efficient H2 generation and photovoltaic properties.


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INTRODUCTION The design of efficient photocatalytic systems based on semiconducting nanocrystals is an active area of research for hydrogen generation by water splitting.1-8The semiconductor nanocrystals are now well recognized for photovoltaics and photocatalysis applications using solar energy because of their high visible light extinction coefficient, tunable excited state energy, and exciton generation.9-14 The photo-generated electron and hole of semiconductor nanocrystals are being used for reduction and oxidation reactions in photocatalysis. The negative potential of the conduction band than the redox potential of H+/H2 (0 V vs NHE) and the positive potential of the valence band than the redox potential of O2/H2O (1.23 V) of semiconductor nanocrystals are required for water splitting.15 On the other hand, the charge migration of semiconductor nanocrystals towards opposite electrodes would be required in photovoltaic purposes. Therefore, several strategies are undertaken for efficient charge separation and suppress the charge recombination for getting better result in photoinduced applications.16-17 Fujishima and Honda et al. have first demonstrated the semiconductor based photocatalytic system for hydrogen generation by water splitting.18 The poor efficiency in this process was due to less availability of photo-excited electron and hole. Therefore, the attention

has

been

given

to

metal-semiconductor,

semiconductor-semiconductor

heterostructures where CdS based materials have taken into account such as CdS/Pt,19 CdS/Pd,20 CdSe/Au,21 CdS/Ni,22 CdSe/CdS, CdS/ZnS,23 Cu1.94SZnxCd1-xS,24 CdS/CdTe,25 for better efficiency by efficient charge separation. For efficient photocatalytic hydrogen generation, Domen et al. have designed porous CdS nanostructures loaded with Pt nanocrystals.26 Larsen et al. have reported 10 fold increment of H2 evolution from CdSe/CdS core shell structures than CdSe quantum dots, although the CdS shell hinders the flow of electrons from CdSe core to the reacting sites. Recently, significant attention has been paid on designing one dimensional (1D) nanostructures for enhancing charge separation and high absorption co-efficient.27 Kuno et al. have designed 1D heterostructures based on CdSe nanowires, specifically CdSe/CdS core-shell along with Au NPs decorated core and coreshell nanostructures for efficient charge separation.28 The efficient hydrogen generation was observed by Li et al. without using co-catalysts in 1D alloy hetero-nanorods due to high interfacial charge separation.24 A system consists of Pt-tipped 1D CdS/CdSe nanorod has been found efficient photocatalyst because holes are confined to CdSe and electrons are transferred to the Pt tip which facilitates charge separation.29 Recently, Feldman and his coworkers have designed Ni decorated CdS nanorods for hydrogen generation without using



noble metal and co-catalysts where hole transfer process takes a major role for enhancing hydrogen generation.22 Detailed analysis of these reports reveals that that H2 generation depends on various factors such as light absorption co-efficient of nanomaterials, efficiency of charge separation, pH of the medium, type of hole scavenger and also co-catalyst. Indeed, the charge separation process is facilitated in type-II system rather than type-I system.30 From the ultrafast dynamics of electron and hole transfer processes in different semiconductor heterostructures, Lian and his co-workers31 have observed that the effective charge separation occurs in quasi type-II heterostructure. Here, the conduction band offset of both component are very small which enables delocalization of electron throughout the shell and holes (h) are localized in the core.30 Therefore, emphasis has been given on designing of quasi type-II heterostructure for efficient charge separation. Zamkov and his co-worker have designed quasi type-II 1D CdSe/CdS heterostructure where the holes are confined in CdSe core.32 They have reported that the H2 generation is reduced if the CdSe core is fully covered by thick CdS shell. However, they observed 3-4 times higher hydrogen generation after etching the CdS shell in CdSe/CdS heterostructure. In the present work, we have designed quasi type-II nano heterostructure of 1D CdS nanorod (NR) covered with CdSe nanoparticles (NP). The surface of CdS NR is not fully covered by CdSe NP in order to enhance the accessibility of both hole and electron to environment by which carrier can easily participate redox reactions. Here, we have investigated the photocatalytic H2 generation efficiency of these heterostructures and their applicability for photovoltaic application. Again, the ultrafast charge carrier dynamic of these heterostructures have been carried out by using femtosecond transient absorption to understand the influence of loading of CdSe nanoparticles on the CdS nanorods. EXPERIMENTAL SECTION MATERIALS 1-octadecene (ODE), tri octylphosphine (TOP), CdO, tellurium, selenium, sulphur powder (Sigma Aldrich), glutathione (GSH), tetramethylamonium hydroxide, toluene, oleic acid Diethylenetriamine (DETA), (Merck India) were used as received. Synthesis of CdS/CdSe Heterostructure First we have synthesized CdS NR followed by our previously reported method11. Briefly 0.073 g of cadmium diethyl dithiocarbamate was added to a 20 ml Teflon lined stainless steel autoclave. Then, 13 ml of diethylenetriamine (DETA) was added and the 4 ACS Paragon Plus Environment

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temperature was maintained 180o C for 24 hours. After that, the autoclave was cooled at room temperature. The yellowish product was obtained and centrifuged with water-ethanol mixture for several times and dried at 60o C in vacuum oven. Then we have followed the methods as described below for the synthesis of CdS/CdSe heterostructure. 0.018 g of CdO and 0.1 ml oleic acid were added to 1 ml ODE and heated at 1500 C under inert atmosphere until a clear solution of Cd-oleate appeared. 0.01 g Se powder was added to 0.5 ml TOP in a one neck RB under inert atmosphere and heated to 600C until a clear solution of TOP-Se appeared. The two precursors were kept in inert condition. As prepared CdS NR (0.045 g ) was dispersed in 5 ml 1-octadecene (ODE) and 2 ml oleic acid by ultra sonication and stirring process repeatedly. The dispersed CdS NR was degassed at 1500 C with continuously argon flow for 10 minutes. Then temperature is increased to 2000 C and TOP-Se was added drop wise at this temperature and kept it at this temperature for 10 minutes. The as prepared Cd-oleate then drop wise added to this reaction mixture and kept it for another 10 minutes. For better growth of CdSe NP on CdS surface, we have increased temperature to 250oC at 10oC/min rate and kept at this temperature for 20 minutes. During the course of synthesis inert atmosphere was maintained all time. The growth of CdSe NP was monitored by using UV-visible spectroscopy at different time intervals (Figure S1). The surface coverage of CdSe NPs on CdS NR surface is controlled by changing the concentration of Se and Cd precursor. The product was cooled down to room temperature after 20 minutes and washed (2 times) with dry ethanol. The product was dried at 60oC under vacuum and stored it for further experiment. To transfer this heterostructure in aqueous medium, we have prepared a stock solution of GTMA solution by dissolving 0.210 g of glutathione (GSH) and 0.378 g of tetramethylamonium hydroxide (TMAH) in 3 ml methanol solution. From this stock solution we have taken 0.75 ml salt solution and were added drop wise to 5 ml heterostructure (2 mg/ml) of toluene. After 10 minutes stirring it was precipitated and centrifuged to wash out unwanted by product. For further purification, we dispersed it by adding small amount water and again centrifuged at 12000 rpm for 20 minutes. The final product was collected and kept for further study. CHARACTERIZATION The crystalline phase and corresponding planes of CdS/CdSe heterostructure have been identified by X-ray diffraction (XRD) study by using a Cu Kα source (1.5418 Å) radiation (Siemens model D500). Transmission electron microscopy using JEOL, JEM5 ACS Paragon Plus Environment


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2100F at an operating voltage of 200 kV is used for morphological study. The XPS measurements were carried out by using an Omicron Nanotechnology instrument. Room temperature optical absorption spectra were taken by an UV−vis spectrophotometer (Shimadzu). Room temperature photoluminescence study was carried out by using a Fluoro Max-P (Horiba Jobin Yvon) luminescence spectrophotometer. For the time correlated single photon counting (TCSPC) measurements, the samples were excited at 371 nm using a pulse diode Nano LED (IBH Nanoled-07) in an IBH Fluorocube apparatus. The repetition rate was 1 MHz. The fluorescence decays were analyzed using IBH DAS6 software. The following equation was used to analyze the experimental time resolved fluorescence decays, P(t ) : n

P (t ) = b + ∑ α i exp( − i

t

τi

)

(1)

Here, n is the number of discrete emissive species, b is a baseline correction (“dc” offset), and αi and τi are the pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. For multi-exponential decays the average lifetime,〈τ〉, was calculated from the following equation: n

< τ >= ∑ β iτ i

(2)

i =1

Where β i = α i / ∑ α i and β i is contribution of the decay component. Cyclic volatammetric measurements were done under nitrogen atmosphere using Ag/Ag+ reference electrode, with a Pt disk working electrode and a Pt wire auxiliary electrode, in DCM containing 0.1 M TBAP in a PC-controlled PAR model 273A electrochemistry system. Gas chromatography (GC) connected with thermal conductivity detector (TCD) was used to quantify hydrogen evolution with time by injecting 1 mL of gas from the RB headspace into the GC. Newport solar simulator equipped with filters (400–800 nm; light intensity = 140 mW/cm2) was used for solar light source. Photocurrent was measured using a Newport solar simulator of 300 W xenon lamp (power supply Newport model no. 69911) equipped with 1 sun (AM 1.5 G) illumination at 100 mW cm−2. In the femtosecond transient absorption spectrophotometer (TAS) setup, a mode-locked Ti:sapphire oscillator (Seed laser, Mai-Tai SP, Spectra Physics) generates pulses of ~ 80 fs duration with a wavelength of 800 nm, at a repetition rate of 80 MHz. The energy required for amplifying the seed pulse is supplied by a separate pump laser (Nd:YLF laser, 527 nm, ASCEND EX, Spectra-Physics). The Spitfire Ace amplifier system (consist of optical strectcher, regenerative amplifier and optical


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compressor) amplify low-energy laser pulses to mJ energy level. The output from the amplifier was (800 nm,

CdSe Heterostructure for Efficient H2 Generation and

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