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

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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*,†,‡ †

Notre Dame Radiation Laboratory and ‡Department of Chemistry & Biochemistry University of Notre Dame, Notre Dame, Indiana 46556, United States § Edgewood Chemical Biological Center, U.S. Army Research, Development and Engineering Command, 5183 Blackhawk Road, APG, Maryland 21010, United States S Supporting Information *

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.

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.

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 geometries 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 and electron and hole confinement as well as recombination dynamics.21−24 Some © 2015 American Chemical Society

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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, (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 Received: May 7, 2015 Revised: June 26, 2015 Published: June 29, 2015 5064

DOI: 10.1021/acs.chemmater.5b01689 Chem. Mater. 2015, 27, 5064−5071

Article

Chemistry of Materials

Scheme 1. Illustration of Electron Transfer from Excited CdSe/CdS NR with Rod-in-Rod Microstructure to Methyl Viologen (MV2+)

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.

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 2.53 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 rodlike particles, there is an introduction of a strong dipole moment along the c-axis effectively allowing for previously forbidden optical transitions of the lowest excitedstate 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%, and

(additional synthetic details in the SI). Figure 1 shows representative high resolution transmission electron microscopy (HRTEM) micrographs of the NR samples confirming their rodlike 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.

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CdSe/CdS EXCITED-STATE PROPERTIES Absorbance and emission spectra of the CdSe NR core exhibit characteristic absorption maxima at 590 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 5065


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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 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.

24% were determined for short, medium, and long NR, respectively (Table S1). The increase in emission yield upon growth of a CdS shell is generally attributed to passivation of core surface defects present, constricting radiative recombination in CdSe as well as electron transfer from the CdS shell to CdSe core.28 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 co-workers 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 NR with increasing CdS aspect ratios in the presence and absence of the electron acceptor MV2+. 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.32−34 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 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

Scheme 2. Quasi Type-II Band Alignment in CdSe/CdS Core/Shell NR

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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.

back electron transfer between MV•+ and trapped holes (reaction 4) competes with the forward electron transfer process (reaction 3).

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 1S transitions.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.

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NR + hν → NR(h + e)

(1)

NR(h + e) → NR + hν′

(2)

NR(h + e) + MV2 + → NR(h) + MV •+

(3)

NR(h) + MV •+ → NR + MV2 +

(4)

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+ solution 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

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 (reactions 1 and 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 (reactions 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

NR + MV2 + ⇌ [NR···MV2 +]

(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, ϕ0f , and complexed NR, ϕ′f with MV2+ molecules and their association constant Ka with expression 6.40 5067


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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 × 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.

φf0

1 1 1 + = 0 0 − φf (obs) φf − φf′ K a(φf − φf′)[MV2 +]

τ2, respectively. The average lifetime, ⟨τ⟩, 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-(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. Ultrafast charge transfer dynamics studies of semiconductor nanomaterials have shown that the time frame 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 is presented in Table 1. Because

(6)

Figure 4D shows the plot of 1/(ϕ0f −ϕf′) 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 −ϕf′) and the slope 1/(Ka(ϕ0f −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 exhibits the highest association constant at 7340 ± 290 M−1 could also exhibit greater photocatalytic activities. 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 5068


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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.

Table 1. Kinetic Fitting Parameters of Bleaching Recovery and the Rate Constants of Electron Transfer of CdSe Core and CdSe/CdS-(S), CdSe/CdS-(M), and CdSe/CdS-(L) Core/Shell at 488 and 483 nm, Respectivelya 30 μM MV2+

no MV2+

a

sample

τ1 (ps)

CdSe core CdSe/CdS-(S) CdSe/CdS-(M) CdSe/CdS (L)

219.1 ± 17.1 60.1 ± 6.42 80.1 ± 28.0 110.8 ± 14.4

τ2 (ps)

⟨τ0⟩ (ps)

τ1 (ps)

671.2 ± 62.5 706.9 ± 154.0 >1500

219.1 ± 17.1 454.6 ± 66.8 503.8 ± 84.9 >1500

142.0 ± 22.4 8.9 ± 1.1 20.1 ± 3.5 28.0 ± 2.5

τ2 (ps)

⟨τ′⟩ (ps)

ket (1010 s‑1)

Φet (%)

46.5 ± 7.3 105.0 ± 28.4 242.0 ± 20.5

142 ± 22.4 17.6 ± 8.8 39.5 ± 33.0 103.5 ± 24.5

0.25 5.45 2.33 1.0

2.0 4.0 10 11

ket was determined using expression 7; ϕet was determined from steady-state irradiation of NRs and ferrioxalate actinometry.

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 cutoff filter, we allowed for excitation of the CdSe core as well as CdS shell of the NR. Control experiments utilizing long wavelength (520 nm) cutoff 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 6B shows the evolution of MV•+ via the growth of absorbance at 608 nm for samples containing different CdSe/ CdS NR. The steady state is quickly achieved within 4−5 min 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 concentrations of ∼12, 21, and 22 μM with long exposure times (Figure 6 B) from short to long NR, 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 1 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 EF). 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

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 assumed that a decrease in lifetime is entirely associated with electron transfer at the CdS interface. The rate constant of electron transfer (ket) can then be estimated based on expression 7

ket = 1/ τ′ −1/ τ0

(7)

where ⟨τ0⟩ and ⟨τ′⟩ are the average bleaching recovery times in the absence and presence of MV2+, respectively. The values of apparent ket listed in Table 1 show an interesting trend. For the CdSe/CdS-(S) core/shell NR, ket is 2.9 × 1010 s−1, which is nearly an order of magnitude greater than the core alone (ket = 0.45 × 1010 s−1). This kinetic analysis shows that the MV2+ is effective in scavenging electrons from excited CdS before it can recombine with the holes that are localized in the core. Given the smaller CdS shell length, the electrons are able to interact with surface bound MV2+ quite effectively. With increasing thickness of the shell we see a decrease in the electron transfer rate constant for the CdSe/ CdS-(M) and -(L) NR as well as CdS NR. Although the association constant between the two is greater for CdSe/CdS(L) NR, we observe decreased ket. Because there is higher electron delocalization in longer CdSe/CdS NR, this leads to slower electron transfer rates as more nonradiative recombination pathways become available.

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STEADY-STATE PHOTOLYSIS Photochemical redox reactions of nanoscale semiconductors have been studied since the 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 excitedstate properties and advances in synthetic methods achieved throughout the last three decades provide new opportunities for the exploration of semiconductor quantum dots in 5069


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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 min.

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.

state MV2+ evolution for all samples using 390 and 520 nm long band-pass filters. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.chemmater.5b01689.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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 DEFC02-04ER15533.

CONCLUSIONS 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 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.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

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 steady5070


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