Correlating Carrier Dynamics and Photocatalytic Hydrogen

Article

Correlating Carrier Dynamics and Photocatalytic Hydrogen Generation in Pt Decorated CdSe Tetrapods as a Function of Cocatalyst Size Melike Karakus, Younghun Sung, Hai I. Wang, Zoltan Mics, Kookheon Char, Mischa Bonn, and Enrique Cánovas J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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The Journal of Physical Chemistry

Correlating Carrier Dynamics and Photocatalytic Hydrogen Generation in Pt Decorated CdSe Tetrapods as a Function of Cocatalyst Size Melike Karakus1, Younghun Sung2, Hai Wang1, Zoltán Mics1, Kookheon Char2, Mischa Bonn1 and Enrique Cánovas1* 1

2

Department of Molecular Spectroscopy, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

The National Creative Research Initiative Center for Intelligent Hybrids, Seoul National University, Seoul 08826, Republic of Korea.

ABSTRACT. We investigate photo-induced carrier dynamics by time-resolved terahertz spectroscopy (TRTS) in Pt nanoparticle decorated CdSe tetrapods as a function of Pt nanoparticles size (and identical areal density). We find that the collection efficiency of electrons photo-generated in the tetrapods by the Pt particle increases as a function of Pt nanoparticle size. However, the photocatalytic H2 generation efficiency is reduced for tetrapods decorated with larger Pt particles. Our results demonstrate a competition between electron capture efficiency at the semiconducting/metal interface, increasing with nanoparticles size, and electron release efficiency at the metal/water interface, decreasing with nanoparticles size. This trade-off defines an optimum for photocatalytic H2 generation in Pt co-catalyst decorated CdSe tetrapods as a function of Pt size.

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Introduction Photocatalysis represents an appealing approach for energy storage in chemical fuels such as hydrogen, methane or methanol.1,2 Semiconductor nanocrystals constitute a class of building block systems that are promising photocatalysts for H2 generation owing to the fact that their optical and electronic properties are tunable by size, composition and morphology.3–8 Wave function engineering in type II band-alignment core/shell QDs,9,10 dot-in-rod nanorods6,11 and tetrapods11,12 architectures allows for fine tuning electron and hole localization within the semiconducting hetero-nanostructures; this aspect has a direct impact on photo-induced charge carrier relaxation within the semiconducting nanostructure and hence on the photocatalytic H2 generation.6,10,11 Furthermore, the photocatalytic activity of semiconducting nanostructures can be boosted by decorating them with metal nanoparticles acting as charge scavengers; the socalled co-catalysts.5,6,13,14 Therefore, controlling the size, composition, coverage and specific location of the metal co-catalyst is an important factor for improving photocatalytic H2 generation efficiency.4,15–20 Specifically, the interplay between metal co-catalyst loading (size and coverage) and the photocatalytic H2 generation performance for metal decorated nanostructures has been interrogated by several groups.15,16,19,20 Berr et al. and Schweinberger et al. have previously reported photocatalytic H2 generation efficiency in Pt decorated CdS nanorods as a function of metal particle coverage and size (for Pt clusters between 8 atoms and 68 Pt atoms).15,16 They found that co-catalyst coverage improves photocatalytic H2 generation efficiency. On the other hand, for a fixed co-catalyst coverage, they reported that an intermediate Pt cluster size among those analyzed (specifically ~1 nm consisting of 46 Pt atoms) provided the best photocatalytic H2 generation efficiency.15 Analogously, Ben-Shahar et al.19 analyzed the effect of gold co-catalyst


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particle size in gold-tipped CdS rods. While increasing the Au particle size was found to speed up the rates of electron collection at the co-catalyst, an intermediate Au particle size provided the maximum photocatalytic H2 generation efficiency. All of these previous findings on metaldecorated semiconducting nanorods have been rationalized by assuming the work function of the co-catalyst to be size-dependent, directly affecting interfacial charge transfer rates at both the semiconducting/metal and metal/water interfaces. Similarly, some of us recently reported20 on the photocatalytic H2 generation performance of Pt-decorated CdSe tetrapods as a function of cocatalyst size (with identical coverages and variable Pt particle size). We found that samples containing the smallest Pt particles provided the highest photocatalytic H2 generation.20 Understanding whether metal decorated tetrapods follow similar photo-physics when compared to those reported in nanorods as a function of co-catalyst size requires establishing a clear correlation between co-catalyst Pt sizes, photocatalytic H2 generation and carrier dynamics. In this study, we interrogate photo-induced carrier dynamics by Time-Resolved Terahertz Spectroscopy (TRTS) in Pt decorated CdSe tetrapods showing reduced photocatalytic H2 production performances as a function of Pt size. From our analysis of the carrier dynamics, we conclude that electron collection efficiency from the semiconducting tetrapod towards the Pt particle improves from ~94% (for small co-catalyst size of ~0.6 nm radius) to unity quantum yield for higher loading (radius of Pt ~2.1 nm). However, the photocatalytic H2 generation efficiency of the Pt decorated CdSe tetrapods shows an opposite trend: the samples decorated with 0.6 nm radius Pt particles outperform the tetrapods decorated with larger Pt sizes (Pt ~2.1 nm) by ~35-fold. This demonstrates that the collection of photogenerated electrons at the larger Pt particles does not represent a kinetic bottleneck for photocatalytic H2 generation (where larger Pt co-catalyst particles capture electrons faster than smaller Pt particles). Rather, the


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recombination of the electron from the Pt particle with the hole in the tetrapod hybrid represents a kinetic bottleneck for larger Pt sizes. This relaxation channel, consistent with a Pt sizedependent work-function, competes with electron release from the Pt particle at the metal/water interface. A change in the work function would explain modified interfacial metal/semiconductor energetics within a Marcus–Gerischer21–23 electron transfer picture. Analogous to the case of metal decorated nanorods,15,16,19 our results demonstrate that electron capture efficiency at the semiconducting/metal interface and electron release efficiency at the metal/water interface follow competing trends as a function of co-catalyst particle size towards photocatalytic H2 generation in Pt decorated CdSe tetrapods. This trade-off sets an optimum for photocatalytic H2 generation as a function of Pt loading.

Experimental Section Chemicals Cadmium oxide (CdO, 99.95%) was purchased from Alfa Aesar. Selenium (99.99%, powder), ntriocytylphosphine (TOP, 90%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), 1,2hexadecanediol (90%), oleylamine (70%), platinum(II) acetylacetonate (Pt(acac)2, 97%), 1,2dicholrobenzene

(anhydrous,

99%),

11-mercaptoundecanoit

acid

(MUA,

95%),

tetramethylammonium hydroxidepentahydrate salt (> 97%) and cetyltrimethylammonium bromide (CTAB, 99+ %) were all purchased from Sigma Aldrich. Phenyl ether (90%) was purchased from TCI. Toluene, methanol and ethanol were purchased from Samchun Chemicals. Synthesis of Pt decorated CdSe Tetrapods For the preparation of Cadmium Oleate (Cd(OA)2) solution, 12 mmol CdO, 10.8 mL OA and 6 mL ODE were placed in a 100 mL 3-neck round flask equipped with a condenser. The reaction


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mixture was degassed under vacuum at 100 °C, and followed heating to 280 °C under Ar for 20 min till an optically clear solution was obtained, afterwards, the solution was cooled down to room temperature. In parallel, 12 mmol Se and 6 ml TOP were mixed in a 50 mL 3-neck round flask with a condenser and heated to 200 °C until powdered Se fully dissolved. Once the SeTOP reached room temperature, a solution containing 6 mL SeTOP

and 14 mL Cd(OA)2 was

prepared for the injection. Zincblende (ZB) CdSe QDs were grown in a 100 mL 3-neck round flask equipped with a condenser. A 1 mmol Se and 10 mL ODE were loaded and heated to 100 °C for degassing under vacuum. The reaction mixture (under Argon) was then heated to 300°C for the injection of asprepared Cd(OA)2. At 300 °C, 2.8 mL Cd(OA)2 and 7.2 mL ODE were injected to make the total volume of 20 mL and the reaction solution was reacted at 270 °C for 15 min. Spherical ZB CdSe quantum dot seeds with ~5 nm diameter (1st excitonic peak ~630 nm) were obtained, and this crude solution was used without further purification for the synthesis of tetrapods. For the latter, 5 mL ZB CdSe seed solution, 2.25 mL OA, 1.5 mL TOP, 21.25 mL ODE and 0.21 mmol CTAB were loaded in a 100 mL 3-neck round flask equipped with a condenser and heated to 100 °C under vacuum for degassing. Under Ar, the reaction mixture was heated to 270 °C for the injection of as-prepared Cd(OA)2 and SeTOP injection solution; a injection rate of 0.4 mL/min for 50 min by syringe pump followed. After the reaction, the sample was loaded to centrifuge tube and equal amount of toluene and ethanol were added repeatedly for the precipitation of the products by centrifugation at 4000 rpm. The decoration of Pt nanoparticles onto CdSe tetrapod side walls was realized as follows: a 100 mL 3-neck round flask equipped with a condenser, 43 mg 1,2-hexadecanediol, 0.2 mL OA, 0.2 mL oleylamine and 10 mL phenyl ether were loaded and heated to 80 °C under vacuum for


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degassing. Under Ar, the reaction mixture was heated to 225 °C for further injection. Meanwhile, 25 mg as-synthesized CdSe tetrapods along with a controlled amount of platinum precursors, which were platinum acetylacetonate, were dissolved in 1 mL 1,2-dichlorobenzene for the injection solution. At 225 °C, the injection solution was injected into the reaction mixture and reacted for 8 min, followed by cooling down to room temperature and 5 mL toluene were injected under 100 °C to prevent further solidification of the products. The crude product solution was then transferred to a centrifuge tube, and a relative amount of ethanol and toluene was added to selectively remove free Pt nanoparticles with centrifugation at 2500 rpm. Pt decorated CdSe tetrapods were precipitated by adding an excess amount of methanol. Next, 250

mg 11-mercaptoundecanoic

acid

(MUA) were dissolved

in

20

g methanol.

Tetramethylaamonium hydroxidepentahydrate salt was added until the solution pH of 11 was obtained. This solution was added to the precipitates of Pt decorated CdSe tetrapods and sonicated for a few seconds, followed by adding toluene and centrifugation at 4000 rpm for the precipitation of the products. Finally, Pt decorated CdSe tetrapods passivated with MUA were dispersed in water (for photocatalytic tests) and toluene (for THz characterization). Photocatalytic tests: For the photocatalytic H2 generation reaction, the as-prepared Pt decorated CdSe tetrapod photocatalyst solution was mixed with 0.35 M Na2SO3/0.25 M Na2S aqueous solution (10 mL). The amount of the photocatalysts was set at 2.5 mg considering the concentration of solution and head space. The reaction mixture was loaded to a homemade quartz tube with a total volume of 19 mL sealed with a rubber septum and purged with Ar for 30 min prior to the reaction, which was triggered by illumination with AM 1 SUN solar simulator (ABET Technologies). The


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aliquot of the reaction mixture was collected by a syringe from the head space every 30 min. The amount of H2 generated was measured by a gas chromatograph (GC, YL6100). Spectroscopic and morphological characterization of metal decorated tetrapods: The synthesized CdSe tetrapods are defined by a band gap offset of ~680 nm. The resulting Pt decorated CdSe tetrapods with different Pt nanoparticle sizes were characterized by highresolution transmission electron microscopy (HR-TEM), revealing large uniformity in the arm/core aspect ratios for each Pt decorated tetrapods. Furthermore, the HR-TEM analysis revealed CdSe arms consisting of wurtzite (WZ) structure and zincblende (ZB) tetrapod core.20 X-ray diffraction (XRD) analysis confirmed WZ arms, ZB core and face centered cubic (fcc) crystal lattice of Pt nanoparticles.20 The two distinct crystal structures of ZB core and WZ arms result in quasi type II band structure between core and arms in the CdSe tetrapods.7,8,24,25 Time resolved terahertz spectroscopy (TRTS): The system is driven by a Ti:sapphire amplified laser system delivering laser pulses with 100 fs temporal length, 800 nm mean wavelength and 1 kHz repetition. The mobile charge carriers are generated in the sample by a pump laser pulse with a wavelength of 400 nm generated using second harmonic generation in a BBO crystal. The probing THz pulses are generated by opticalrectification in a 1 mm ZnTe crystal, and detected by electro-optic sampling in a 1 mm thick ZnTe crystal.26 To monitor photo-induced carrier dynamics we follow pump induced changes at the peak of the THz probe pulse transmitted through the sample by varying the time delay between the pump and probe/sampling pulses. The monitored signal is proportional to the number of photogenerated carriers in the sample.26 By measuring the transmitted THz waveform at a fixed pumpprobe delay, we can determine the frequency-resolved complex conductivity ω of the


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photogenerated carriers.26 The frequency-resolved complex valued photoconductivity ∆σ(ν) can be expressed as a function of the differential transmissivity ∆T(ν) via the following formula:27 ∆ ∝

∆

− 1

(1)

where Z0 = 377 Ω is the impedance of free space, d is the thickness of the photo-excited region (thickness of the cuvette 1 mm); is the transmitted reference THz spectrum and n is the

refractive index of the cuvette behind the photo-excited region ( = 1.5). The frequency resolved complex conductivity spectra are described using the Drude –Smith Model as:28

σ

!"#$% ω

ω(& ' )

= !$ ω ) 1 + !$ ω )

(2)

where ωp, ε0, τ, and + are the plasma frequency, vacuum permittivity, scattering time, and scattering parameter, respectively. Here, the Drude-Smith Model describes photogenerated carriers as quasi-free experiencing back scattering described by τ and + parameter. In the model,

+ ranges between 0 and -1 indicating the degree of localization of the photogenerated charges.

The + parameter reflects the degree of charge localization in conf0ined nanostructures;26,29 e.g. may be able to discriminate between the carrier localization in 0D (+ = -1) and 1D nanostructures (0 > + > -1).30

Results and Discussion Figure 1 (left) shows transmission electron microscopy (TEM) images of bare CdSe tetrapods and Pt decorated CdSe tetrapods grown by employing 1 mg and 25 mg Pt precursors. Under these conditions, the cocatalyst decoration is defined by a constant ~20 Pt particles per tetrapod


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arm and Pt radius of ~0.6nm and ~2.1nm respectively (see below). Figure 1 (right) summarizes the photocatalytic hydrogen generation20 of the Pt decorated CdSe tetrapods as a function of Pt radius for bare CdSe, Pt(~0.6nm)/CdSe and Pt(~2.1nm)/CdSe. As shown in Figure 1 (right), for the samples analyzed in this work, we found photocatalytic H2 generation of ~3 nH2 µmol/gCdSe for bare CdSe tetrapods, ~250 nH2 µmol/gCdSe for Pt(~0.6nm)/CdSe tetrapods and ~7 nH2 µmol/gCdSe for Pt(~2.1nm)/CdSe tetrapods. Note that the photocatalytic H2 generation (per gram of CdSe) for tetrapods decorated with small Pt clusters represents a ~35-fold increase compared to samples loaded with larger Pt clusters (both with the same particle areal density as discussed later; 20 particles/arm). From these estimates we can infer turnover frequencies (TOF) of ~388 and ~10 H2_molecules/Pt_particle/s for samples decorated with small and big Pt particles respectively. The samples were stable under H2 evolution conditions over the probed 2 hours (displaying linear dependences over H2 generation, see inset figure 1 right).

Figure 1. (Left upper panel): TEM image of bare CdSe tetrapods (scale bar: 100 nm); (Left bottom panel) TEM for bare CdSe, Pt(~0.6nm)/CdSe and Pt(~2.1nm)/CdSe tetrapods. (Right panel): Photocatalytic hydrogen generation per gram of CdSe and Pt (blue and red bars) for the Pt decorated CdSe tetrapods versus cocatalyst Pt radius (and identical Pt areal density; ~20particles/arm). The inset shows the time evolution of H2 generation over 2 hours.


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In our previous contribution20 we discussed that, following our growth recipe, an increase in the amount of Pt precursors resulted in the formation of larger Pt nanoparticles, yet keeping the areal number density of Pt particles per CdSe tetrapod invariant. This was achieved following a continuous precursor injection (CPI) approach (previously reported by some of us31) where a specific amount of alkyl halides (e.g. cetyltrimethylammonium bromide, CTA) is employed during the synthetic procedure. We have previously shown31 that the introduction of alkyl halides enables the decoration of Pt nanoparticles throughout the entire surface of CdSe tetrapods; this can be attributed to the presence of Cd halide ligand-free sites on the surface sidewalls of wurzite CdSe tetrapod arms. Once nucleation Pt sites are “available” at the CdSe surface, further incorporation of Pt during the growth recipe is simply expected to increase the Pt size. In order to further support these claims, we characterize here our samples by HAADF-STEM. In Figure 2(a-b) we show high resolution HAADF-STEM images and intensity scan profiles (obtained using Gwyddion software) of 2 tetrapod arms for growth recipes using 1 mg and 25 mg Pt precursor solutions, respectively. The scan intensity profiles clearly reveal the presence of Pt particles onto the CdSe arm (cross-sectional profiles of non-Pt-covered parts of the CdSe arm are highlighted as grey areas in the figure). In Figure 2c, we present a histogram revealing the number of Pt particles per tetrapod arm (as identified from intensity profiles over the arm length); we find that the analyzed recipes, 1 mg (red bars) and 25 mg (black bars), provide similar areal density (~20 Pt particles per arm, as estimated from the top view characterization). In Figure 2d, we present histograms for the resolved Pt particle radius; the Pt particle radii were inferred from the FWHM of Gaussian fits of the intensity profiles obtained by subtracting crosssectional profiles representative of bare tetrapod arm from cross-sections containing Pt particles.


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Following this procedure, we obtain that the CdSe tetrapods prepared from precursor solutions containing 1 mg and 25 mg Pt, are characterized by Pt particles with an averaged radius of ~0.6 nm and ~2.1 nm, respectively. The apparent large broadening in Pt size inferred from intensity line scans from HAADF-STEM data could be affected by e.g. Pt particle orientation towards be probing beam (e.g. HAADF-STEM intensity depends on target orientation).

Figure 2. HAADF-STEM images of Pt-decorated CdSe tetrapods following growth recipes of a) 1 mg and b) 25 mg of Pt precursors and length profiles of HAADF-STEM intensities for both recipes. As described in the text, from the HAADF-STEM intensity length profiles we obtain c) a histogram for the number of Pt particles per tetrapod arm, and d) a histogram for Pt particle radius in nanometers for samples prepared from precursor solutions containing 1 mg (black bars) and 25 mg (red bars) Pt. The blue dotted lines in panel d) are estimates for the particle radius obtained from ICP-AES measurements as described in the text.


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To independently verify the results presented in Figure 2 (c-d), we performed inductively coupled plasma atomic emission spectroscopy (ICP-AES) on batches of 25 mg CdSe tetrapods prepared from precursor solutions containing 0 mg, 1 mg and 25 mg Pt. The Pt(mol.%) obtained for the analyzed samples was 0%, 1.61% and 41.62%, respectively. From these figures we can estimate the amount of Pt decoration per sample as: 0 mg of Pt for bare CdSe; msmall(~0.6nm) = 0.42 mg and mlarge(~2.1nm) = 18.2 mg of cocatalyst Pt loading. Assuming identical density for the grown Pt particles, we can infer the expected relationship between Pt particle radius (Rlarge and Rsmall) for both recipes as: Nlarge·Rlarge = ( mlarge / msmall)1/3 · Nsmall · Rsmall (where N represent the Pt areal density and we assume an spherical volume for the Pt particles). If Nlarge=Nsmall as revealed in Figure 2c; we obtain that Rlarge = 3.5 · Rsmall. This relationship (highlighted as blue dotted lines in Figure 2d) is in perfect agreement with the findings made from HAADF-STEM analysis. In this respect, under our growth recipe, we can conclude that the Pt areal density is kept constant in the tetrapods and larger Pt sizes are obtained as a function of Pt precursor concentrations.

THz carrier dynamics on Pt decorated CdSe tetrapods: In Figure 3, we present the time-dependent real conductivity following photoexcitation measured by TRTS of toluene solutions containing bare tetrapods (red squares) and tetrapods decorated with Pt particles with averaged diameters of ~0.6 nm and ~2.1 nm (blue and black squares, respectively). All the measurements were carried out in the regime of, on average, appreciably less than one electron-hole pair being generated per tetrapod (400 nm pump fluence ~10 µJ/cm2), and the optical density of the samples at 400 nm was kept the same, allowing for a direct comparison of the photoconductivity amplitudes measured on different samples. The time-


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dependent real photoconductivity in bare CdSe tetrapods (Figure 3 - red squares) reveals a subpicosecond rise followed by biphasic decay dynamics with characteristic lifetimes of 1.25 ± 0.05 ps and 500 ± 25 ps. A similar fast component (< 2 ps) has been resolved by transient absorption spectroscopy for CdSe tetrapods and was assigned to localization of electrons from the arm to the core.24,25 The localization of the electron at the core is a consequence of the type II band alignment between zincblende core and wurzite arm for CdSe tetrapods,7,8,20 and is consistent with the frequency resolved THz conductivity of our samples presented below. The long lived TRTS lifetime resolved for bare CdSe tetrapods can be assigned to the lifetime of quasi-free carriers populating tetrapod,,-!.,0-1 = 500 ± 25 ps.

Figure 3. Real conductivity (a.u.) (normalized to the optical density of samples) versus pump probe delay in CdSe and Pt decorated CdSe tetrapods in toluene. Solid lines show exponential fits providing lifetimes ,-,231425647538 = 1.25 ± 0.05 ps and ,-!.,0-1 = 500 ± 25 ps for bare CdSe, ,-,231425647538 = 1.30 ± 0.05 ps and ,70,- 97 = 26.9 ± 4.8 ps for ~0.6 nm Pt decorated CdSe, and


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,70,. = 0.53 ± 0.02 for ~2.1 nm Pt decorated CdSe tetrapods.

The blue trace in Figure 3 shows the effect of ~0.6 nm Pt decoration in CdSe tetrapods on the carrier dynamics. The real conductivity signal right after pump is comparable in magnitude to that obtained for bare tetrapods (red trace in Figure 3), revealing a similar yield of quasi-free carrier generation. The dynamics are still well modelled by a biphasic decay, where the fast component is assigned as in the case of bare tetrapods to the localization of electrons in the core (with the same lifetime that is observed for the bare tetrapods (,-,231425647538 = 1.25 ± 0.05 ps). The slow component for this sample is characterized by a lifetime of ~30 ps, and associated with the capture of electrons at the ~0.6 nm Pt particles ,70,- 97 . This assignment is consistent with the expected role of the metal co-catalyst as electron scavenger.5,32 The lifetime of electron capture at the metal Pt particle competes with the exciton recombination lifetime in the tetrapod (,-!.,0-1 = 500 ± 25 ps); this will co-determine the efficiency of the photocatalytic process as will be discussed later. The decoration of tetrapods with larger Pt co-catalyst sizes (~2.1 nm Pt, black trace in Figure 3), reduces the overall amplitude of the real conductivity when compared with bare and ~0.6 nm Pt decorated tetrapods. The reduced amplitude indicates that a substantial fraction of electrons populating the tetrapod are captured at the Pt particles faster than our experimental resolution of ~200 fs. The remaining signal is characterized by a lifetime of ~ 0.5 ps (Figure 3 - black squares); the nature of this signal is discussed below. To investigate the nature of the monitored photoconductivity in the samples, we determined the frequency-resolved complex conductivity (σ ω of the bare CdSe tetrapods (15 ps after photoexcitation) and of the ~2.1 nm Pt decorated CdSe tetrapods (0.9 ps after photoexcitation),


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respectively. The spectral response of bare tetrapods shows an increasing positive real conductivity Re[σ] and decreasing negative imaginary conductivity Im[σ] that can be well modelled by phenomenological Drude-Smith model (red traces in Figure 4a; see experimental section for fitting details); this response is characteristic for conductivity mediated by quasi-free carriers experiencing backscattering due to confinement effects.28 Using Drude-Smith fitting, we infer a backscattering parameter of c = -0.99 ± 0.01 (and scattering time of τ = 20 fs) which refers to the complete backscattering for highly localized charges (e,g, electrons). For ~2.1 nm Pt decorated CdSe tetrapods, fitting of Drude-Smith28 model to the data provides a value of + = 0.91 ± 0.01 (and scattering time of τ = 27 fs) (black line in Figure 4b). Note that, the degree of confinement can be inferred from the c parameter as described in the experimental section (with c = 0 and c = -1 representing a lack and complete backscattering experienced by photogenerated charges, respectively).26,30 Hence, these numbers imply increased delocalization for the photogenerated free charges in the sample loaded with larger Pt particles. We therefore assign the short-lived component resolved in the pump-probe traces to holes partially delocalized within the 1D tetrapod arms with a lifetime of ~ 0.5 ps (Figure 3 - black squares). The localized character of signal attributed to electrons (Drude-Smith model with c = -1) and quasi-delocalized nature of probed holes (Drude-Smith model with c = -0.92) is consistent with the expected type II band alignment7,8 induced by the two distinct crystal structures of zincblende core and wurzite arm in CdSe tetrapods (with electrons localized in the 0D core and holes partially delocalized in the 1D tetrapods arms). Within this scenario, the amplitude of the terahertz signal observed for the bare CdSe tetrapod is determined by the response of both electrons and holes, while that of the ~2.1 nm Pt decorated CdSe sample right after the pump (~0.9 ps pump-probe delay) is determined by holes only. As such, the ratio of the two signals should reflect the effective mass


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ratio of quasi-free holes versus electrons for CdSe (mh/me). This ratio amounts to ~4, which is within the range of the reported values in the literature for CdSe (mh/me = 3-6),10,24,33,34 supporting the assignment of the real conductivity signal monitored in ~2.1 nm Pt decorated CdSe tetrapods (Figure 3 - black squares) being associated with hole dynamics (with a lifetime of ,70,. = 0.5 ps). From our data, we cannot establish the nature of the hole trapping, e.g. a bulk or surface trap and/or eventually capture at the hole acceptor MUA ligands.32 Trapping of holes in dot-in-rod structures CdSe/CdS35 and CdSe nanorods11 has been resolved by transient absorption to occur in ~0.7 ps, and has been tentatively associated with unpassivated traps at the rod surface. The fast trapping of the hole has been suggested to be a kinetic bottleneck for photocatalytic H2 generation efficiency by other authors. 32,36

Figure 4. a) Frequency-resolved complex-valued photoconductivity for a) bare CdSe tetrapods after 15 ps pump-probe delay, b) ~2.1 nm Pt decorated CdSe tetrapods after 0.9 ps pump-probe delay. Red, black and blue lines represent fits using the Drude-Smith model providing different c parameters as shown.

Correlation between carrier dynamics and photocatalytic H2 generation efficiency:


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In Figure 5, we summarize our findings of electron and hole dynamics in Pt decorated CdSe tetrapods. The overall photocatalytic H2 generation efficiency is related to 2 kinetically relevant steps as:15,16,20,37 (i) electron capture from CdSe tetrapods to the Pt co-catalyst at CdSe/Pt interface (green line labelled as =70,- 97 in Figure 5), and (ii) electron release from Pt to water at

the Pt/water interface (blue line in Figure 5; =>? ). In this respect, the overall photocatalytic H2 generation efficiency (@>? ) measured in the Pt decorated tetrapods can be defined as: @>? = @ABC-→97 ∗ @97→>?F = {=70,- 97 /=70,- 97 + =!%, where =70,- 97 and =!%,

} ∗ {=>? /=>?

+ =70,-!. }

(1)

are, respectively, the competing rates of electron trapping at Pt co-

catalyst and exciton lifetime in the tetrapod (defining the electron capture efficiency at the Pt particle @ABC-→97 ); and =>? and =70,-!. are the rate of electron release from the Pt to water and competing relaxation back to the tetrapod (defining the electron release efficiency from the Pt particle; @ABC-→97 ). From Equation 1 and the inferred TRTS rates, the electron capture efficiency at the Pt particle for the ~0.6 nm Pt decorated CdSe tetrapods is estimated to be @ABM-→97 ~94%;

meaning that 94 out of 100 photogenerated electrons are efficiently collected at the Pt cocatalyst. The electron capture efficiency for the sample containing larger Pt particles (~2.1 nm) reaches unity quantum efficiency; @ABM-→97 ~100%. The faster collection of electrons revealed in tetrapods decorated with larger Pt particles (blue vs black trace in Figure 3) is consistent with previous reports,19 and might be directly related (for a fixed Pt particle areal density) to an increase in capture cross section (associated with donor-acceptor coupling) and/or with an increase in the excess free Gibbs energy between tetrapod LUMO and Pt Fermi level according to Marcus–Gerischer electron transfer reactions.21–23


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As mentioned above, for the samples analyzed in this work, we found that photocatalytic H2 generation (@>? in Equation 1) was ~7 nH2 µmol/gCdSe for ~2.1 nm Pt decorated CdSe tetrapods and ~250 nH2 µmol/gCdSe for ~0.6 nm Pt decorated CdSe tetrapods (see Figure 1).20 Given that the maximum photocatalytic H2 generation is observed for ~0.6 nm Pt decorated CdSe tetrapods (see Figure 1), and according to the trend in @ABM-→97 vs Pt size, we have to conclude that the

second term in Equation 1 (@97→>?F = {=>? /=>? + =70,-!. }) is necessarily larger for small Pt nanoparticles. Our findings thus indicate that, while electron capture at the Pt =70,- 97 will be

favored in larger Pt particles, the electron release rate from the Pt co-catalyst to water (=>? ) is reduced as Pt particles become larger. This trade-off sets an optimum for photocatalytic H2 generation as a function of Pt loading, as the two kinetically relevant steps for electrons at semiconductor/metal and at metal/water interfaces follow opposite trends as a function of the Pt co-catalyst size, analogous to what has previously been concluded for gold-tipped CdS nanorods19. Our findings are consistent with the Marcus–Gerischer theory21–23 where increasing the driving force for charge transfer between tetrapod LUMO and Pt Fermi level (∆G in Figure 5) would increase the =70,- 97 rate but at the same time reduce the =>? rate in proportion, thereby establishing a trade-off for the photocatalytic H2 generation as a function of Pt size.


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Figure 5. Summary for carrier dynamics taking place in Pt decorated CdSe tetrapods showing rates of electron trapping at Pt co-catalyst =70,- 97 ), localization of quasi-free electrons from wurtzite arms (WZ) towards zincblende core (ZB) (=-,231425647538 ), hole trapping (=70,. ), electronhole recombination in tetrapod =-!.,0-1 ) and electron-hole recombination from Pt to the tetrapod (=70,-!. ). We note that our estimates regarding the photocatalytic H2 generation efficiency refer to Pt decorated CdSe tetrapods dispersed in 0.35 M Na2SO3/0.25 M Na2S aqueous solution, where sodium sulfite acts as hole scavenger or reducing agent. In contrast, our THz measurements are performed in toluene and without reducing agents, as the THz probe will be quenched in polar solvents as water. Care should be taken to extrapolate the conclusions drawn from THz studies on test bed systems to in-operando devices. The conclusion that a kinetic competition exists between electron capture and release from the co-catalyst, and that this depends critically on Pt loading, will hold even if carrier dynamics are measured under the presence of a reducing agent. This claim is supported by a previous study that reported electron capture timescales versus Pt


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loading by transient absorption in Pt decorated CdS nanorods in the presence and absence of sodium sulfite.3 In this study, absolute electron lifetimes were shown to be affected by the presence of the hole scavenger, yet the dependence of the rates on Pt loading was very similar both in presence and absence of the hole scavenger. As such, while the presence of a hole scavenger will surely affect the lifetime of electrons populating the tetrapods, we expect it will not affect the increase in capture rate with Pt loading. This is consistent with the expectation that the electron transfer from CdSe to the Pt particles will be simply determined by the product of number of Pt particles and their capture cross section as Ktr,e (Pt) = NPt*; in our work NPt is constant and hence the variation in rates should refer to a size dependent change in the capture cross section term. The latter depends primarily on the CdSe-Pt wave function overlap (donoracceptor coupling) and CdSe-Pt interfacial energy offset (donor-acceptor free Gibbs energy).

Conclusions The interplay between carrier dynamics and photocatalytic H2 generation was analyzed in Pt decorated CdSe tetrapods as a function of Pt co-catalyst size (and fixed Pt number density). Our results demonstrate that a trade-off exist as a function of Pt size between the two kinetically relevant steps involved in photocatalytic H2 generation; (1) electron transfer from the tetrapods towards the Pt particle increases as a function of Pt co-catalyst size and (2) electron capture from the metal towards water is reduced as a function of Pt co-catalyst size. This trade-off set an optimum metal co-catalyst size for obtaining maximized photocatalytic H2 generation in metal decorated CdSe tetrapods.


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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work has been financially supported by the Max Planck Society. Melike Karakus acknowledges the fellowship of the International Max Planck Research School for Polymer Material Science (IMPRS-PMS) in Mainz and Soham Roy for proof reading of the manuscript. Enrique Cánovas acknowledges financial support from the Max Planck Graduate Center (MPGC).

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Correlating Carrier Dynamics and Photocatalytic Hydrogen

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