CdS Nanorod Photocatalysts: Tuning the ... - ACS Publications

Subscriber access provided by NEW YORK UNIV

Article

CdSe/CdS Nanorod Photocatalysts: Tuning the Interfacial Charge Transfer Process through Shell Length Victoria L Bridewell, Rabeka Alam, Christopher J. Karwacki, and Prashant V. Kamat Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01689 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 4, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18

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

Chemistry of Materials

CdSe/CdS Nanorod Photocatalysts: Tuning the Interfacial Charge Transfer Process through Shell Length Victoria L. Bridewell1, 2, Rabeka Alam1, Christopher J. Karwacki3 and Prashant V. Kamat1, 2* Notre Dame Radiation Laboratory and Department of Chemistry & Biochemistry University of Notre Dame, Notre Dame, Indiana 46556, United States 3

Edgewood Chemical Biological Center, U.S. Army Research, Development and Engineering Command, 5183 Blackhawk Road, APG, MD 21010

1

Notre Dame Radiation Laboratory

2

Department of Chemistry & Biochemistry

3

Edgewood Chemical Biological Center

*Address correspondence to [email protected]

ACS Paragon Plus Environment

1


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

Page 2 of 18

Abstract

CdSe/CdS core/shell semiconductor nanorods (NR) with rod-in-rod morphology offer new strategies for designing highly emissive nanostructures. The interplay between energetically matched semiconductors results in enhanced emission from the CdSe core. In order to further evaluate the cooperative role of these two semiconductors in a core/shell geometry, we have probed the photoinduced charge transfer between CdSe/CdS core/shell semiconductor NR and methyl viologen (MV2+). The quenching of the emission by the electron acceptor, MV2+, as well as the production of electron transfer product MV+• depends on the aspect ratio (l/w) of the NR thus pointing out the role of CdS shell in determining the overall photocatalytic efficiency. Transient absorption measurements show that the presence of MV2+ influences only the bleaching recovery of the CdS shell and not of the CdSe core recovery. Thus, optimization of shell aspect ratio plays a crucial role in maximizing the efficiency of this photocatalytic system. TOC:


2

Page 3 of 18

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


Interfacial electron transfer, particularly in semiconductor nanoparticles such as CdSe quantum dots (QD) and nanorods (NR), has drawn significant interest due to their applications in solar cells,1-2 photocatalysis,3-7 and solar hydrogen production.8-11 QD and NR are of particular interest as they offer size quantization in zero and one dimensions. These nanomaterials offer new opportunities in designing and optimizing hybrid photocatalysts due to their broad absorption profiles, size tunable emission, long lived excited-state lifetimes, and enhanced photostability. By coupling various metal oxides like TiO2 or ZnO with CdS and CdSe QD and NR (e.g., TiO2/ CdS and ZnO/CdX (X= Se or S), the selectivity and charge separation efficiency can be altered.12-16 Such hybrid structures in the coupled and core/shell geometry have been shown to enhance the efficiency of photocatalytic reduction and oxidation efficiencies,7, 17 providing a new platform. Visible light photocatalysis, in particular, offers an exciting and promising route for converting solar energy to chemical energy for the photoreduction of organic contaminants. Classically TiO2 nanoparticles have been employed and studied for such photocatalytic endeavors; however, CdS and CdSe nanoparticles absorb more of the visible spectrum making them promising materials for photocatalysis over TiO2. 18-20 CdSe/CdS NR have been investigated extensively and by various techniques to determine band alignments, electron and hole confinement as well as recombination dynamics.21-24 Some major advantages of using CdSe/CdS NR are that they are highly emissive and undergo light induced charge separation due to close conduction band alignment. The obvious questions are how the individual charge separation processes in the core and shell influence the photophysics of its partner and how the overall photocatalytic efficiencies are modulated. Herein we discuss the electron transfer and photocatalytic capabilities of CdSe/CdS NR with varying CdS shell lengths by preforming photoinduced reduction of methyl viologen (MV2+) to reduced methyl viologen (MV+•) as depicted in Scheme 1.


3


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

Page 4 of 18

Scheme 1. Illustration of electron transfer from excited CdSe/CdS NR with rod-in-rod microstructure to methyl viologen (MV2+).

CdSe/CdS Core/Shell Nanorods. With the goal of studying electron transfer kinetics between CdSe/CdS and MV2+, we first designed and synthesized CdSe NR with an aspect ratio (l/w) of 2.0 ± 0.2 nm, (l = 8.0 ± 1.0, w = 4.0 ± 0.4 nm) as shown in Figure 1A. Once synthesized, these particles were utilized for CdSe seeded growth of CdS shells thus resulting in particles with rod-in-rod (RR) morphologies.25-27 By controlling the CdSe NR core concentration we were able to tune the seeded growth of the CdS shell. Employing this approach, CdSe/CdS NRs with three different aspect ratios were prepared with varying CdS shell lengths (additional synthetic details in SI). Figure 1 shows representative high resolution transmission electron microscopy (HRTEM) micrographs of the NR samples confirming their rod like morphologies, as well as, schemes detailing their dimensions. The resulting particles will be referred to as CdSe, CdSe/CdS-(S), CdSe/CdS-(M), and CdSe/CdS-(L) based on their average shell lengths from here on. Analysis of TEM micrographs reveals l/w of 5.2 ± 0.8 (l = 25 ± 1.7, w = 4.8 ± 0.6 nm) for CdSe/CdS-(S), 5.7 ± 1.0 (l = 32 ± 2.7, w = 5.6 ± 0.9 nm) for CdSe/CdS-(M), and 8.3 ± 0.9 (l = 54 ± 6.6, w = 6.5 ± 2.0 nm) for CdSe/CdS-(L), which supports a uniform CdS shell growth over the CdSe core allowing one-dimensional confinement effects.


4

Page 5 of 18

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


Figure 1. HRTEM micrographs and schematic representations of (A) CdSe NR core l/w = 2.0 ± 0.2, l = 8.0 ± 1.0, w = 4.0 ± 0.4 nm, (B) CdSe/CdS-(S) l/w = 5.2 ± 0.8, l = 25 ± 1.7, w = 4.8 ± 0.6 nm, (C) CdSe/CdS-(M) l/w = 5.7 ± 1.0, l = 32 ± 2.7, w = 5.6 ± 0.9 nm, (D) CdSe/CdS-(L) l/w = 8.3 ± 0.9, l = 54 ± 6.6, w = 6.5 ± 2.0 nm RR NR structures.

CdSe/CdS Excited-State Properties. Absorbance and emission spectra of the CdSe NR core exhibit characteristic absorption maxima at 590 nm and 500 nm, indicating an effective bandgap of 2.10 eV, and an emission maximum at 600 nm (Figure 2A). CdS shell growth over CdSe NR results in a red-shift and increase in the characteristic absorbance below 510 nm from CdS and a decrease of the CdSe excitonic absorbance feature at 590 nm. The absorption spectra of the three different core/shell structures of varying CdS length are shown in Figure 2B-D. Interestingly, this feature also confirms the deposition of CdS shell, which is observed in the absorption spectra of all three CdSe/CdS NR where a broad shoulder around 490 nm indicates the presence of CdS with a bandgap of 2.82 eV. Despite this CdS absorption dominance in the NR, emission only arises from the CdSe core. CdSe/CdS NR particles exhibit a shape-dependent Stokes shift unlike their spherical core/shell counterparts. In the transition from spherical to rod-like particles, there is an introduction of a strong dipole moment along the c-axis effectively allowing


5


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

Page 6 of 18

for previously forbidden optical transitions of the lowest excited and subsequent hole confinement within the core. This swap-over of valence bands can be attributed to larger crystal field perturbation as well as slight lattice mismatches furthering hole confinement in the core and subsequent recombination.20 Growth of CdS shell greatly improved the emission quantum yields for CdSe/CdS NR, quantum yields of 41 %, 37 %, 24 % were determined for short, medium and long NR respectively (Table S1). The increase in emission yield with a CdS shell is generally attributed to passivation of core surface defects present, confining radiative recombination in CdSe as well as electron transfer from the CdS shell to CdSe core.28

Figure 2. (Top panel, A-D) Photographs of CdSe NR core, CdSe/CdS –(S), CdSe/CdS-(M) and CdSe/CdS-(L) particles under room (Top Panel, left) and ultraviolet (Top panel, right) light. Absorption (solid) and emission spectra of (A) CdSe NR cores, (B) CdSe/CdS-S, (C) CdSe/CdS-(M), (D) CdSe/CdS-(L) NR in toluene. Emission spectra were obtained after 420 nm excitation.


6

Page 7 of 18

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


Scheme 2. Quasi type-II band alignment in CdSe/CdS core/shell NR.

The composition and size of semiconductors determine whether core/shell NR exhibits type-I, type-II, or quasi type-II band structure, which dictates the electron-hole confinement properties.28-30 Rabani and coworkers showed strong localization of the hole in the core and delocalization of the electron in CdSe/CdS NR (Scheme 2) based on molecular dynamics and electronic structure simulation techniques.

31

Because quasi type-II band structures allow for

delocalization of the electron density along both the CdSe core and CdS shell conduction bands, improved electron transfer processes can potentially be realized. With this in mind, we strove to study the recombination pathways CdSe/CdS core/shell NRs with increasing CdS aspect ratios in the presence and absence of the electron acceptor MV2+.


7


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

Page 8 of 18

Figure 3. Femtosecond transient absorption spectra following a 0.3 mW 387 nm laser pulse excitation of a colloidal suspension of 5.3 nM (A) CdSe NR core, (B) CdSe/CdS-(S), (C) CdSe/CdS-(M), and (D) CdSe/CdS-(L) rod-in-rod NR in toluene.

We further elucidated the excited state behavior of the CdSe/CdS core/shell NR using femtosecond transient absorption spectroscopy. As shown earlier, the excitation of semiconductor nanocrystals results in charge separation, which causes the bleaching of the exciton absorption. 3234

The recovery of the bleaching represents charge carrier recombination, which can also be used

to probe the transfer of electrons to an acceptor molecule.35 Figure 3 shows time-resolved difference absorption spectra recorded following a 387 nm laser pulse excitation of CdSe and CdSe/CdS NR. In the case of pristine CdSe NR, we observe a transient bleaching at 490 and 590 nm. However, when CdSe/CdS NRs are excited with a 387 nm laser pulse, we see large contribution from the CdS shell around 483 nm and little influence from the CdSe core in the transient absorption spectra recorded (Figure 3 B-D). The holes are confined to the CdSe core in the CdSe/CdS NR while the electrons are delocalized over both conduction bands of CdSe and CdS. Therefore the recombination of charge carriers are dictated by the energetics of the CdSe core.24, 30 In fact the emission characteristic of the CdSe core seen in core/shell NR (Figure 2 B-D) irrespective of excitation wavelength support


8

Page 9 of 18

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


this phenomenon. From the femtosecond transient absorption spectra, it is obvious that RR CdSe/CdS NR do indeed exhibit quasi type-II band alignment. Within the first picosecond, the CdS bleach at 483 nm reaches its maximum absorbance and begins a quick decay and eventual long-lived state. This phenomenon occurs as the CdS shell undergoes hole transfer with the CdSe core, Figure 3B-D, followed by the delayed growth of the bleach at 635 nm corresponding to the highest predicted state of s- (1Se - 2S3/2) transitions within the core.30, 36 With increasing time, the bleaching recovers as the electrons recombine with the holes within CdSe. Incorporation of CdS shell on the CdSe core further increases the lifetime of the emission proportional to shell length suggesting improved charge separation via electron delocalization in these systems.

Excited-state interaction of CdSe/CdS NR and Methyl Viologen. Upon excitation of CdSe/CdS NRs, they can undergo a series of pathways to return to the ground state. In the absence of an electron acceptor, the NR undergo their natural recombination processes, in which they radiatively recombine within the CdSe core (reaction 1-2). In the presence of an electron acceptor such as MV2+, the NR can now undergo a separate, more complex and competitive recombination process. In order to probe electron transfer from CdSe and CdSe/CdS NR to MV2+, known concentrations of MV2+ were introduced into NR solutions in 5/95 (v/v) ethanol/toluene. Methyl viologen, in its stable state, MV2+, is colorless; however, upon reduction to its radical state, MV+•, the solution becomes blue with absorption maximum at 396 nm and a broad absorption maximum at 608 nm.37-38 In the presence of MV2+, the excited semiconductor NR undergoes electron transfer to MV2+, a competing process with electron–hole recombination (reaction (1-3)). The reduced MV+• can then undergo back electron transfer to trapped holes (reaction 4). By tracking the MV+• absorption growth, we can thus monitor the charge transfer events between our NR to MV2+. In the steady-state photolysis, this back electron transfer between MV+• and trapped holes (reaction 4) competes with the forward electron transfer process (reaction 3). NR + hν → NR (h + e)

(1)

NR (h + e) + MV 2+ → NR (h) + MV +•

(3)

NR (h + e) → NR + hν′

NR (h) + MV +• → NR + MV 2+


(2)

(4)

9


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

Page 10 of 18

Both emission quenching as well as femtosecond transient absorption experiments were carried out to probe the interfacial charge transfer process between our NRs and MV2+. We first investigated these interactions through emission quenching experiments where known amounts of a stock 1 mM MV2+ were added to an oxygen free toluene solution containing 5.3 nM CdSe/CdS NR. At MV2+ concentrations as low as 1.25 μM, significant emission quenching of the CdSe/CdS NR samples was observed suggesting static quenching interactions as shown in Figure 4. The slight blue-shift in the NR emission after MV2+ addition can be attributed to a change in solvent polarity.39 The extent of MV2+ quenching participation has been characterized by considering the equilibrium of the adsorbed and unadsorbed molecules regulated by the association constant, Ka, forming a complex with the ground state semiconductors.39 NR + MV 2+ ⇌ [NR ⋯ MV 2+ ]

(5)

As shown earlier, the observed quantum yield, ϕf (obs), of the NR in a solution of MV2+ can be related to the fluorescence yields of the uncomplexed NR, ϕf0, and complexed NR, ϕf′ with MV2+ molecules and their association constant Ka with the expression 6.40 1

ϕ0f −ϕf (obs)

Figure 4D shows the plot of

1

ϕ0f −ϕ′f

=

1

ϕ0f −ϕ′f

+

1

Ka �ϕ0f −ϕ′f �[MV+2 ]

(6)

versus 1/[MV2+]. The linear dependence of the double

reciprocal plots confirms the validity of expression 6 and the observed emission quenching by MV2+ arises from its association or complexation with NR in the ground state. From the intercept 1

ϕ0f −ϕ′f

and the slope K

1

0 ′ a �ϕf −ϕf �

of this plot we determined the apparent association constant, Ka, for

the equilibrium 5. Ka, values measured for CdSe/CdS-(S), CdSe/CdS-(M) and CdSe/CdS-(L) were

1200 ± 150, 3020 ± 430, and 7340 ± 290 M-1 respectively. These values indicate a strong interaction between the NRs and MV2+. It is interesting to know whether CdSe/CdS-(L) which exhibit the highest association constant at 7340 ± 290 M-1 could also exhibit greater photocatalytic activities.


10

Page 11 of 18

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


Figure 4. Normalized emission quenching of 5.3 nM (A) CdSe/CdS-(S), (B) CdSe/CdS-(M), and (C) CdSe/CdS-(L) NR in toluene by microliter additions of 1 mM methyl viologen (MV2+) in ethanol. (D) Double reciprocal plot analysis of emission quenching of 5.3 nM Short (A x 10), Medium (B), and Long (C) NR in toluene samples for determination of association constants, Ka, 1200 ± 150, 3020 ± 430, and 7340 ± 290 M-1 respectively.

Femtosecond transient absorption spectroscopy was employed to interpret electron transfer processes with and without MV2+ to better understand the influence of the quasi type-II band energy alignment on interfacial electron transfer processes. In the absence of any electron accepting molecules, electrons and holes from CdS can be transferred to the CdSe core; however, in the presence MV2+, electrons are transferred from CdS to MV2+ at a much higher rate than to CdSe. Figure 5 presents the kinetic traces for bleaching recovery of two representative samples following a 387 nm laser pulse excitation in the presence and absence of MV2+ (additional kinetic traces presented in Figure S1 of the SI). These traces exhibit biexponential decay kinetics with the results summarized in Table 1 and Table S2 in the supporting information. Parameters A1 and A2 are the relative amplitudes; the fast and slow components of the lifetimes, are represented by τ1 and τ2, respectively. The average lifetime, , was calculated by taking a weighted average. In the absence of MV2+, the recovery of the bleaching is dominated by hole transfer to the core and core/shell charge recombination. In the CdSe/CdS core/shell NRs, the excitation is mainly centered on CdS as is evident from the intense bleaching of the excitonic band at 483 nm. For CdSe/CdS-


11


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

Page 12 of 18

(S) the shell is relatively thin and it is expected that most of the recovery occurs within the core due to a slightly faster recovery time. This is further purported that with increasing shell lengths there is an analogous increase in the recovery time reflective of improved spatial charge separation in the core/shell systems.

Figure 5. Kinetic traces following a 0.3 mV 387 nm laser pulse excitation of 5.3 nM of the CdS band edge bleach recovery monitored at 483 nm in the absence and increasing concentrations of methyl viologen for (A) CdSe core and (B) CdSe/CdS-(S) core/shell particles respectively.

Ultrafast charge transfer dynamics studies of semiconductor nanomaterials have shown that the timeframe of electron transfer from CdS to MV2+ is within the first few picoseconds, therefore competing with the charge transfer processes from CdS shell to the CdSe core.41-43 The recovery of the bleaching monitored at probe wavelength of 483 nm thus represents the disappearance of electrons as a result of charge recombination within the NR and electron transfer to MV2+ at the interface. Control experiments of CdS NR were conducted as a point of comparison for these quasi type-II band alignment nanomaterials with the results showcased in the supporting information. The faster recovery of the bleaching with increasing MV2+ concentration for the CdSe/CdS NR shows that the electron transfer to MV2+ in fact becomes the dominant pathway. The transient absorption spectra recorded in the presence of varying concentrations of MV2+, ranging from 0 – 30 µM, and the details of bleaching recovery kinetics are presented in the supporting information Figure S1 and Table S2 for CdSe/CdS NR and Figure S5 for CdS NR. The summary of bleaching recovery lifetimes determined from biexponential decay kinetics are presented in Table 1. Because there is minimal change in the decay kinetics of the 635 nm CdSe bleach at early times irrespective of the concentration of MV2+ present (SI Figure S2), it can be


12

Page 13 of 18


1 2 3 assumed that a decrease in lifetime is entirely associated with electron transfer at the CdS interface. 4 5 The rate constant of electron transfer (ket) can then be estimated based on the expression (7), 6 7 8 ket= 1/ -1/ (7) 9 10 11 where and are the average bleaching recovery times in the absence and presence 12 of MV2+ respectively. 13 14 15 The values of apparent ket listed in Table 1 show an interesting trend. For the CdSe/CdS16 17 (S) core/shell NR, ket is 2.9 × 1010 s-1, which is nearly an order of magnitude greater than the core 18 19 alone (ket = 0.45 × 1010s-1). This kinetic analysis shows that the MV2+ is effective in scavenging 20 21 electrons from excited CdS before it can recombine with the holes that are localized in the core. 22 Given the smaller CdS shell length, the electrons are able to interact with surface bound MV2+ 23 24 quite effectively. With increasing thickness of the shell we see a decrease in the electron transfer 25 26 rate constant for the CdSe/CdS-(M) and –(L) NR as well as CdS NR. Although the association 27 28 constant between the two is greater for CdSe/CdS-(L) NR, we observe decreased ket. Because there 29 30 is higher electron delocalization in longer CdSe/CdS NR, this leads to slower electron transfer 31 rates as more nonradiative recombination pathways become available. 32 33 34 Table 1. Kinetic fitting parameters of bleaching recovery and the rate constants of electron transfer of CdSe core and 35 CdSe/CdS-(S), CdSe/CdS-(M), and CdSe/CdS-(L) core/shell at 488 and 483 nm respectively. ket was determined using 36 expression 7; ϕet was determined from steady state irradiation of NRs and ferrioxalate actinometry. 37 38 39 40 No MV2+ 30 µM MV2+ 41 Sample τ1 (ps) τ2 (ps) (ps) τ1 (ps) τ2 (ps) (ps) ket 42 (1010 s-1) 43 219.1 142 CdSe Core 44 219.1 ± 142.0 ±17.1 ± 22.4 0.25 --45 17.1 ±22.4 46 454.6 CdSe/CdS-(S) 60.1 ± 671.2 8.9 17.6 47 ±66.8 46.5 ±7.3 5.45 6.42 ±62.5 ±1.1 ± 8.8 48 49 503.8 CdSe/CdS-(M) 80.1 ± 706.9 105.0 50 ±84.9 20.1 ±3.5 39.5 ±33.0 2.33 28.0 ±154.0 ±28.4 51 52 CdSe/CdS (L) 110.8 ± 242.0 >1500 >1500 28.0 ±2.5 103.5 ± 24.5 1.0 53 14.4 ±20.5 54 55 56 57 58 59 60 ACS Paragon Plus Environment

13

Φet (%)

2.0

4.0

10 11


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

Steady State Photolysis.

Page 14 of 18

Photochemical redox reactions of nanoscale semiconductors

have been studied since the early 1980s.44 Examples from that time period include photoreduction of well-characterized dyes such as methylene blue and various viologen dyes mediated by CdS nanoparticles.45 Improvements in the understanding of excited-state properties and advances in synthetic methods achieved throughout the last three decades provide new opportunities for the exploration of semiconductor quantum dots in photocatalysis. Steady state photolysis experiments in particular provide insight into the net photocoversion efficiencies as they weigh in both forward and back electron transfer processes. When the NR were subjected to steady-state illumination under deaerated conditions in the presence of MV2+, MV+• started accumulating in the sample (Figure 6A). By exposing samples to visible light using 390 nm cut-off filter, we allowed for excitation of the CdSe core as well as CdS shell of the NR. Control experiments utilizing long wavelength (520 nm) cut-off filter were also explored to probe the involvement of CdSe core alone. When CdS shell excitation is excluded, we observe significantly lower levels of formation of MV+• thus reflecting lower photoconversion efficiency (SI Figure S3).

Figure 6. (A) Production of methyl viologen radical by light induced electron transfer from 5.3 nM CdSe/CdS-(L) core/shell nanorods using a 390 nm long band pass filter under varying xenon light exposure times. (B) The concentration of MV+•, at 608 nm, generated by electron transfer from CdSe and CdSe/CdS NRs to MV2+ produced for each sample under steady-state illumination over 15 minutes.

Figure 6B shows the evolution of MV+• via the growth of absorbance at 608 nm for samples containing different CdSe/CdS NRs. The steady state is quickly achieved within 4-5 minutes following initial illumination. It can be seen that with increasing CdS shell lengths the steady state concentration of the MV+• increases. The photogenerated MV+• attained steady-state


14

Page 15 of 18

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


concentrations of ~12, 21, and 22 µM with long exposure times (Fig. 6 B) from short to long NRs, respectively. The steady state concentration of MV+• is dictated by both forward (reaction 3) and back electron transfer (reaction 4) rates. Although the forward electron transfer, ket, as included in Table 2 for CdSe/CdS-(L) is lower by a factor of 6 than the CdSe/CdS-(S), the observed steady state yield of MV+• is nearly double. This shows that the back electron transfer in the case of CdSe/CdS-(L) NR is suppressed by more than an order of magnitude. Interestingly, the CdS NR performed in line with CdSe core material producing ~3 µM of MV+• (SI Figure S5 E-F). This proposes that as the holes become confined within the CdSe core, it becomes less and less accessible for recombination with MV+• as the length of the CdS shell increases. These results show how shell length in CdSe/CdS core/shell NR can influence not only the initial charge separation, but also the back electron transfer process. These results were further supported by MV+• quantum yields of 11% and 5% for CdSe/CdS-(L) and -(S) respectively determined from ferrioxalate actinometry. Experimental details and additional MV+• efficiencies can be found in the supporting information. With this system we have been able to achieve electron transfer efficiencies from the core/shell NRs to MV+• on the same scale as CdSe QD experiments while utilizing an entire order of magnitude lower in concentration of the photocatalyst- operating in the nanomolar range as opposed to the micromolar.39 This perpetuates that careful control of CdS shell thickness is of high importance in tuning hole confinement and optimizing the net conversion yield of photocatalytic charge transfer process. In summary, we have shown that CdSe/CdS NR with a RR morphology and quasi type-II band alignments undergo interfacial electron transfer processes at the CdS interface with an electron acceptor, MV2+. The CdSe core on the other hand plays a supporting role in the electron transfer process proving critical in confining the generated holes and removing them from recombination within the shell. This cooperative effort boasts improved electron delocalization and spatial separation of the electrons in the shell from the holes confined to the core in turn allowing higher electron transfer and photocatalytic capabilities over nonhybrid heterostructures. As such, the excited state interaction, rate of electron transfer and photocatalytic efficiency of this interfacial charge exchange can be tuned through simplistic modifications of the CdS shell length. Moreover, optimal apparent excited state association constant of ~7300 M-1 and photocatalytic efficiencies of 11% turnover of MV+• was realized through increasing CdS shell lengths from ~25 nm to ~54 nm. Considering that such simple and controllable changes in morphologies result in


15


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

Page 16 of 18

considerable changes in the excited and steady state properties, application of this approach could prove useful in designing and optimizing a photocatalyst for solar energy conversion ranging from hydrogen production to target compound remediation at very low photocatalyst concentrations. Corresponding Author: *e-mail: [email protected]. Acknowledgments. The research described in this paper is supported by the Army Research Office through the award ARO 64011-CH. This is a document number 5070 from the Notre Dame Radiation Laboratory, which is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. Supporting Information Available:

Experimental details that include synthesis and

characterization of CdSe nanorod cores, seeded CdSe/CdS and CdS nanorods with rod-in rod morphology and varying shell lengths, femtosecond transient absorption spectra and kinetic analyses, Stern-Volmer quenching studies, as well as additional steady state MV2+ evolution for all samples using 390 and 520 nm long band pass filters are included in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

References 1. Kamat, P. V., Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834-2860. 2. Hillhouse, H. W.; Beard, M. C., Solar Cells from Colloidal Nanocrystals: Fundamentals, Materials, Devices and Economics. Curr. Opin. Colloid. In. 2009, 14 (4), 245-259. 3. Kamat, P. V., Photochemistry on Nonreactive and Reactive (Semiconductor) Surfaces. Chem. Rev. 1993, 93 (1), 267-300. 4. Thompson, T. L.; Yates, J. T., TiO2-Based Photocatalysis: Surface defects, Oxygen and Charge Transfer. Topics in Catalysis 2005, 35 (3-4), 197-210. 5. Tachikawa, T.; Fujitsuka, M.; Majima, T., Mechanistic Insight into the TiO2 Photocatalytic Reactions: Design of New Photocatalysts. J. Phys. Chem. C 2007, 111 (14), 5259-5275. 6. Nowotny, M. K.; Sheppard, L. R.; Bak, T.; Nowotny, J., Defect Chemistry of Titanium Dioxide. Application of Defect Engineering in Processing of TiO2-Based Photocatalysts. J. Phys. Chem. C 2008, 112 (14), 5275-5300. 7. Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A., Chemistry and Properties of Nanocrystals of Different Shapes. Chem. Rev. 2005, 105 (4), 1025-1102. 8. Chen, Y. S.; Kamat, P. V., Glutathione-Capped Gold Nanoclusters as Photosensitizers. Visible Light-Induced Hydrogen Generation in Neutral Water. J. Am. Chem. Soc. 2014, 136 (16), 6075-6082. 9. Amirav, L.; Alivisatos, A. P., Photocatalytic Hydrogen Production with Tunable Nanorod Heterostructures. J. Phys. Chem. Lett. 2010, 1 (7), 1051-1054.


16

Page 17 of 18

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


10. Wu, K.; Zhu, H.; Lian, T., Ultrafast Exciton Dynamics and Light-Driven H2 Evolution in Colloidal Semiconductor Nanorods and Pt-Tipped Nanorods. Acc. Chem.Res. 2015, 48(3), 851-859. 11. Zhu, H. M.; Song, N. H.; Lv, H. J.; Hill, C. L.; Lian, T. Q., Near Unity Quantum Yield of LightDriven Redox Mediator Reduction and Efficient H-2 Generation Using Colloidal Nanorod Heterostructures. J. Am. Chem. Soc. 2012, 134 (28), 11701-11708. 12. Kim, H.; Jeong, H.; An, T. K.; Park, C. E.; Yong, K., Hybrid-Type Quantum-Dot Cosensitized ZnO Nanowire Solar Cell with Enhanced Visible-Light Harvesting. ACS Appl. Mater. Interfaces 2013, 5 (2), 268-275. 13. Khanchandani, S.; Kundu, S.; Patra, A.; Ganguli, A. K., Shell Thickness Dependent Photocatalytic Properties of ZnO/CdS Core-Shell Nanorods. J. Phys. Chem. C 2012, 116 (44), 2365323662. 14. Lu, G. Q.; Linsebigler, A.; Yates, J. T., Ti3+ Defect Sites on TiO2(110) - Production and Chemical-Detection of Active-Sites. J. Phys. Chem-Us 1994, 98 (45), 11733-11738. 15. Xian, J.; Li, D.; Chen, J.; Li, X.; He, M.; Shao, Y.; Yu, L.; Fang, J., TiO2 Nanotube ArrayGraphene-CdS Quantum Dots Composite Film in Z-scheme with Enhanced Photoactivity and Photostability. ACS Appl. Mater. Interfaces 2014, 6 (15), 13157-66. 16. Kamat, P. V., Manipulation of Charge Transfer Across Semiconductor Interface. A Criterion that Cannot be Ignored in Photocatalyst Design. J.Phys. Chem. Lett. 2012, 3 (5), 663-672. 17. de Mello Donega, C., Synthesis and Properties of Colloidal Neteronanocrystals. Chem. Soc. Rev. 2011, 40 (3), 1512-46. 18. Kelestemur, Y.; Cihan, A. F.; Guzelturk, B.; Demir, H. V., Type-tunable Amplified Spontaneous Emission from Core-Seeded CdSe/CdS Nanorods Controlled by Exciton-Exciton Interaction. Nanoscale 2014, 6 (15), 8509-14. 19. Ihara, T.; Sato, R.; Teranishi, T.; Kanemitsu, Y., Delocalized and Localized Charged Excitons in Single CdSe/CdS Dot-in-Rods Revealed by Polarized Photoluminescence Blinking. Phys. Rev. B 2014, 90 (3). 20. Talapin, D. V.; Koeppe, R.; Gotzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H., Highly Emissive Colloidal CdSe/CdS Heterostructures of Mixed Dimensionality. Nano Lett. 2003, 3 (12), 1677-1681. 21. Muller, J.; Lupton, J. M.; Lagoudakis, P. G.; Schindler, F.; Koeppe, R.; Rogach, A. L.; Feldmann, J.; Talapin, D. V.; Weller, H., Wave Function Engineering in Elongated Semiconductor Nanocrystals with Heterogeneous Carrier Confinement. Nano Lett. 2005, 5 (10), 2044-9. 22. Kraus, R. M.; Lagoudakis, P. G.; Rogach, A. L.; Talapin, D. V.; Weller, H.; Lupton, J. M.; Feldmann, J., Room-Temperature Exciton Storage in Elongated Semiconductor Nanocrystals. Phys. Rev. Lett. 2007, 98 (1). 23. Raino, G.; Stoferle, T.; Moreels, I.; Gomes, R.; Kamal, J. S.; Hens, Z.; Mahrt, R. F., Probing the Wave Function Delocalization in CdSe/CdS Dot-in-Rod Nanocrystals by Time- and TemperatureResolved Spectroscopy. ACS Nano 2011, 5 (5), 4031-4036. 24. Lupo, M. G.; Della Sala, F.; Carbone, L.; Zavelani-Rossi, M.; Fiore, A.; Luer, L.; Polli, D.; Cingolani, R.; Manna, L.; Lanzani, G., Ultrafast Electron-Hole Dynamics in Core/Shell CdSe/CdS Dot/Rod Nanocrystals. Nano Lett. 2008, 8 (12), 4582-4587. 25. Alam, R.; Fontaine, D. M.; Branchini, B. R.; Maye, M. M., Designing Quantum Rods for Optimized Energy Transfer with Firefly Luciferase Enzymes. Nano Lett. 2012, 12 (6), 3251-3256. 26. Wang, W.; Banerjee, S.; Jia, S. G.; Steigerwald, M. L.; Herman, I. P., Ligand Control of Growth, Morphology and Capping Structure of Colloidal CdSe Nanorods. Chem. Mater. 2007, 19 (10), 25732580. 27. Carbone, L.; Nobile, C.; DeGiorgi, M.; Sala, F. D.; Morello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.; Nadasan, M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingolani, R.; Krahne, R.; Manna, L., Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach. Nano Lett. 2007, 7 (10), 2942-2950.


17


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

Page 18 of 18

28. Borys, N. J.; Walter, M. J.; Huang, J.; Talapin, D. V.; Lupton, J. M., The Role of Particle Morphology in Interfacial Energy Transfer in CdSe/CdS Heterostructure Nanocrystals. Science 2010, 330 (6009), 1371-1374. 29. Luo, Y.; Wang, L. W., Electronic Structures of the CdSe/CdS Core-Shell Nanorods. ACS Nano 2010, 4 (1), 91-8. 30. She, C. X.; Demortiere, A.; Shevchenko, E. V.; Pelton, M., Using Shape to Control Photoluminescence from CdSe/CdS Core/Shell Nanorods. J. Phys. Chem. Lett. 2011, 2 (12), 1469-1475. 31. Eshet, H.; Baer, R.; Neuhauser, D.; Rabani, E., Multiexciton Generation in Seeded Nanorods. J. Phys. Chem. Lett. 2014, 5 (15), 2580-2585. 32. Klimov, V. I.; McBranch, D. W., Femtosecond 1P-to-1S Electron Relaxation in Strongly Confined Semiconductor Nanocrystals. Phys. Rev. Lett. 1998, 80 (18), 4028-4031. 33. Burda, C.; Link, S.; Mohamed, M.; El-Sayed, M., The Relaxation Pathways of CdSe Nanoparticles Monitored with Femtosecond Time-Resolution from the Visible to the IR: Assignment of the Transient Features by Carrier Quenching. J. Phys. Chem. B 2001, 105 (49), 12286-12292. 34. Peng, P.; Milliron, D. J.; Hughes, S. M.; Johnson, J. C.; Alivisatos, A. P.; Saykally, R. J., Femtosecond Spectroscom of Carrier Relaxation Dynamics in Type II CdSe/CdTe Tetrapod Heteronanostructures. Nano Lett. 2005, 5 (9), 1809-1813. 35. Biadala, L.; Siebers, B.; Gomes, R.; Hens, Z.; Yakovev, D. R.; Bayer, M., Tuning Energy Splitting and Recombination Dynamics of Dark and Bright Excitons in CdSe/CdS Dot-in-Rod Colloidal Nanostructures. J. Phys. Chem. C 2014, 118 (38), 22309-22316. 36. Braun, M.; Burda, C.; Mohamed, M.; El-Sayed, M., Femtosecond Time-Resolved Electron-Hole Dynamics and Radiative Transitions in the Double-Layer Quantum Well of the CdS/(HgS)(2)/CdS Quantum-Dot-Quantum-Well Nanoparticle. Phys. Rev. B 2001, 6403 (3), art. no.-035317. 37. Watanabe, T.; Honda, K., Measurement of the Extinction Coefficient of the Methyl Viologen Cation Radical and The Efficiency of Its Formation by Semiconductor Photocatalysis. J. Phys. Chem. 1982, 86 (14), 2617-9. 38. Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C. G.; Luk, Y. F.; Kohler, B., Excited State Dynamics of Methyl Viologen. Ultrafast Photoreduction in Methanol and Fluorescence in Acetonitrile. J. Phys. Chem. A 2001, 105 (24), 5768-5777. 39. Lakowicz, J. R., Principles of Fluorescence Spectroscopy. 2nd ed.; Springer: New York, 2006. 40. Kamat, P. V., Photoelectrochemistry in particulate systems. 9. Photosensitized Reduction in a Colloidal TiO2 System using Anthracene-9-Carboxylic Acid as the Sensitizer. J. Phys. Chem. 1989, 93 (2), 859-64. 41. Logunov, S.; Green, T.; Marguet, S.; El-Sayed, M. A., Interfacial Carriers Dynamics of CdS Nanoparticles. J. Phys. Chem. A 1998, 102, 5652-5658. 42. Yang, Y.; Liu, Z.; Lian, T., Bulk Transport and Interfacial Transfer Dynamics of Photogenerated Carriers in CdSe Quantum Dot Solid Electrodes. Nano Lett. 2013, 13 (8), 3678-3683. 43. Harris, C. T.; Kamat, P. V., Photocatalysis with CdSe Nanoparticles in Confined Media: Mapping Charge Transfer Events in the Subpicosecond to Second Timescales. ACS Nano 2009, 3 (3), 682-690. 44. Henglein, A., Small-Particle Research: Physicochemical Properties of Extremely Small Colloidal Metal and Semiconductor Particles. Chem. Rev. 1989, 89 (8), 1861-1873. 45. Duonghong, D.; Ramsden, J.; Gratzel, M., Dynamics of Interfacial Electron-Transfer Processes in Colloidal Semiconductor Systems. J. Am. Chem. Soc. 1982, 104 (11), 2977-2985.


18

CdS Nanorod Photocatalysts: Tuning the ... - ACS Publications

Recommend Documents