Interface engineering of colloidal CdSe quantum dots thin films as acid

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Interface engineering of colloidal CdSe quantum dots thin films as acid-stable photocathodes for solar-driven hydrogen evolution Hui Li, Peng Wen, Adam Hoxie, Chaochao Dun, Shiba Adhikari, Qi Li, Chang Lu, Dominique Itanze, Lin Jiang, David L. Carroll, Abdou Lachgar, Yejun Qiu, and Scott Geyer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19229 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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ACS Applied Materials & Interfaces

Interface engineering of colloidal CdSe quantum dots thin films as acid-stable photocathodes for solar-driven hydrogen evolution Hui Li,†,⊥ Peng Wen,‡, ⊥ Adam Hoxie,† Chaochao Dun,§ Shiba Adhikari,# Qi Li,|| Chang Lu,† Dominique Itanze,† Lin Jiang,∆ David Carroll,§ Abdou Lachgar,† Yejun Qiu,‡ Scott M. Geyer*† †

Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina

27109, USA ‡

Shenzhen Engineering Lab of Flexible Transparent Conductive Films, Department of Materials

Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China §

Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest

University, Winston-Salem, North Carolina 27109, USA #

Material Science and Technology Division (MSTD), Oak Ridge National Laboratory (ORNL),

Oak Ridge, TN, 37831, USA ||

Physical Science Division, IBM TJ Watson Research Center, Yorktown Heights, NY 10598,

USA ∆

Institute of Functional Nano and Soft Materials (FUNSON), Soochow University, Suzhou,

Jiangsu 215123, China *Corresponding author. E-mail: [email protected] ⊥These

authors contributed equally to this work

Abstract: Colloidal semiconductor quantum dots-based (CQD) photocathodes for solar-driven hydrogen evolution have attracted significant attention due to their tunable size, nanostructured morphology, crystalline orientation, and band-gap. Here, we report a thin film heterojunction photocathode composed of organic PEDOT:PSS as a hole transport layer, CdSe CQDs as a 1 ACS Paragon Plus Environment

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semiconductor light absorber, and conformal Pt layer deposited by atomic layer deposition (ALD) serving as both a passivation layer and cocatalyst for hydrogen evolution. In neutral aqueous solution, a PEDOT:PSS/CdSe/Pt heterogeneous photocathode with 200 cycles of ALD Pt produces a photocurrent density of -1.08 mA/cm2 (AM-1.5G, 100 mW/cm2) at a potential of 0 V vs. RHE (j0) in neutral aqueous solution, which is nearly 12 times that of the pristine CdSe photocathode. This composite photocathode shows an onset potential for water reduction at +0.46 V vs. RHE and long-term stability with negligible degradation. In acidic electrolyte (pH = 1), where the hydrogen evolution reaction is more favorable but stability is limited due to photocorrosion, a thicker Pt film (300 cycles) is shown to greatly improve the device stability and a j0 of -2.14 mA/cm2 is obtained with only 8.3% activity degradation after 6 h, compared to 80% degradation under the same conditions when the less conformal electrodeposition method is used to deposit the Pt layer. Electrochemical impedance spectroscopy and time-resolved photoluminescence results indicate that these enhancements stem from a lower bulk charge recombination rate, higher interfacial charge transfer rate, and faster reaction kinetics. We believe that these interface engineering strategies can be extended to other colloidal semiconductors to construct more efficient and stable heterogeneous photoelectrodes for solar fuel production. Keywords: interface engineering, colloidal quantum dots, atomic layer deposition, photocathode, hydrogen evolution

Introduction Photoelectrochemical (PEC) water splitting, a process that mimics natural photosynthesis for conversion of solar energy to chemical fuels, provides a potential route to simultaneously address current energy challenges and environmental concerns.1-4 To maximize the solar-to-hydrogen

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(STH) efficiency, semiconductor photocathodes with an appropriate band-gap and conduction band level must be selected for efficient light absorption, charge generation, charge separation, and charge transfer for the hydrogen evolution reaction (HER).5-15 Additionally, photocathodes should possess long-term stability under operation conditions.16-18 The unique set of properties of colloidal quantum dots (CQDs), including solution processability, size-, shape-, and composition-tunability, and the ability to control the band edge and work function via quantum confinement, have led to the use of CQDs as alternative sensitizers to molecular complexes and organic dyes for solar energy conversion.19-24 To date, CQDs have been used mainly as sensitizers in photoanodes, where photogenerated electrons transfer from the CQDs to the conduction band of an n-type semiconductor with a high injection rate.25-28 However, the slow hole-transport rate in the CQDs in sensitized photocathodes is considered to be a bottleneck for PEC water reduction.29-31 Ultrafast charge dynamic studies of CQD-molecular systems have demonstrated that the electron-transfer rate is several orders higher than the hole-transfer rate and the later is the limiting factor for enhancing photoconversion efficiency of CQDs based photocathodes.32-34 Recently, hole-transport layers (HTLs), such as fullerene,35 NiO,36,37 graphdiyne,38 and hole-accepting ligands,39 were incorporated into CQDs based photocathodes to boost the hole-transfer rate and improve their PEC activity. Several issues still need to be addressed. First, long-chain organic ligands (eg. oleylamine, oleic acid, trioctylphosphine) capping the CQDs have a deleterious effect on charge transport properties of photocathodes.40-42 While improved charge transport can be obtained by removal of ligands using ozone, thermal annealing treatments, or exchange with shorter organic and inorganic ligands, this process can generate abundant surface defects on the surface of CQDs based photocathodes and increase the recombination rate of photogenerated electron-hole pairs.43-46 Second, the limited number of



catalytically active surface sites on CQD-sensitized photocathodes leads to slow HER kinetics and less-favorable onset potential. Furthermore, having surface states that are active in the catalytic process can lead to instability, as for example, when surface metal species (eg. Cd2+/Cd0) serve as a “cocatalyst” for hydrogen production.36,39 Finally, CQD-sensitized photocathodes are commonly investigated in neutral solution, where they show good stability and decent PEC activity, but are rarely tested in acidic media because of poor stability even though the high proton concentration in low pH electrolyte favors ion diffusion and the reactions kinetics for HER.36,47,48 As a consequence, the photocurrent density at 0 V vs. RHE (j0) under simulated solar illumination or visible light irradiation is limited to 0.2 mA/cm2 or below. An important approach to addressing some of these challenges is the addition of surface passivation and cocatalyst layers. Tremendous efforts have been made to modify the surface of semiconductor photoelectrodes to reduce the charge recombination and accelerate the reaction kinetics while ensuring minimal light blocking.49,50 Atomic layer deposition (ALD) is a promising tool to controllably deposit conformal metal oxides (eg. Al2O3, TiO2, and ZnO) as ultrathin layers on the semiconductor surface to protect photoelectrodes from electrolyte corrosion and passivate the surface defects to decrease charge recombination.51-53 However, an additional cocatalyst layer is still required to increase the catalytic activity. Alternatively, proper choice of the ALD overlayer allows it to serve the dual roles of defect passivation and cocatalyst on the surface of photoelectrodes. Bifunctional cobalt oxide (CoOx) films deposited by ALD have been reported to enhance the PEC activity of photoanodes in alkaline solution,54,55 however, only few ALD materials possess the necessary bifunctionality for photocathode applications. ALD platinum nanoparticles and ultrathin conformal films have been well-studied as catalysts for various heterogeneous reactions, such as oxygen reduction,56,57 CO oxidation,58


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dehydrogenation,59 and tandem hydrogenation,60 yet have rarely been used as a simultaneous passivating layer and cocatalyst on photocathodes for HER.61,62 Moreover, Pt films deposited by ALD are highly stable and efficient in acidic electrolyte, indicating the potential of enabling Ptpassivated colloidal semiconductor NCs to serve as efficient and acid-stable photocathodes. Given the low-efficiency of charge transfer and HER kinetics at the interface of unmodified CQDs-based photocathodes, comprehensive interface engineering strategies are required to enhance the hole-transfer efficiency, passivate the surface defects without blocking light penetration, and accelerate the HER rate. Herein, we report a heterojunction photocathode composed of organic PEDOT:PSS as hole-transport layer, CdSe CQDs as light-absorber, and conformal Pt-layer deposited by ALD that provides both defect-passivation and as a cocatalyst for HER. A heterogeneous photocathode composed of PEDOT:PSS/CdSe/Pt with 200 cycles of ALD Pt produces a photocurrent density of 1.08 mA/cm2 (AM-1.5G, 100 mW/cm2) at a potential of 0 V vs. RHE (j0) in neutral aqueous solution, which is among the highest values reported for colloidal CdSe photocathodes. Furthermore, by increasing the number of ALD cycles, we can significantly enhance the CdSe layer chemical stability in acidic environment. A PEDOT:PSS/CdSe/Pt with 300 cycles of ALD Pt achieves an impressive j0 of -2.14 mA/cm2 in acidic electrolyte (pH = 1) with only 8.3% activity degradation after 6 h, compared to 80% degradation when traditional electrodeposition is used instead of ALD to deposit the Pt.

Results and Discussion The monodisperse CdSe CQDs were synthesized by the well-reported “hot-injection” method (details in Supporting Information).63 The CdSe CQD solution exhibits an absorption edge at 608 nm corresponding to a bandgap of 2.08 eV (Figure S1). Figure 1a shows that the X-ray diffraction (XRD) pattern of the prepared CQDs matches well with the hexagonal crystal



structure of CdSe (ICDD PDF: 00-002-0330). The transmission electron microscope (TEM) image of CdSe NCs shows that the monodisperse CQDs have a spherical shape with uniform diameter of about 8 nm (Figure 1b). The high-resolution TEM (HRTEM) image of a single CdSe CQD is shown in Figure 1c. Two sets of lattice fringes with d-spacing of 0.33 nm and 0.35 nm in the central region correspond respectively to (101) and (002) planes of wurtzite CdSe phase. The observed angle between the (101) and (002) planes is 62°, close to the theoretical value of 65°. The selected-area electron-diffraction (SAED) image confirms the exposure of (002), (101), and (112) planes (Figure 1d). The high-angle annular dark-field scanning TEM (HAADF-STEM) image further demonstrates that the monodisperse CdSe CQDs have a spherical shape and uniform size distribution (Figure 1e). Figure 1f and g show the electron energy loss spectroscopy (EELS)-elemental mapping of an individual CdSe QD, where Cd and Se elements are homogeneously distributed throughout the QD.


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Figure 1. Characterization of CdSe CQDs. (a) XRD pattern, (b) TEM, (c) HRTEM, (d) SAED, (e) HAADF-STEM, and (f, g) EELS-elemental mapping images.

Figure 2a shows a schematic diagram of the device structure and an energy level diagram of the CdSe CQDs-based photocathode. The valence and conduction band levels of the CdSe CQDs were determined by cyclic voltammetry to be -6.38 and -4.31 eV (Figure S2), which is consistent with the values in literature.64 The photogenerated holes are capable of injecting into the PEDOT:PSS due to favorable energy level alignment with the CdSe valence band, and the electrons can inject into Pt from the CdSe conduction band. The band-gap of CdSe CQDs determined by cyclic voltammetry is 2.07 eV, in good agreement with the band-gap obtained from the absorption spectrum (2.08 eV). The hybrid photocathode device was fabricated on a fluorine-doped tin oxide (FTO) coated glass substrate, and a detailed description is available in the Supplementary Information. PEDOT:PSS was deposited by successive layer-by-layer spincoating, with annealing at 150 °C for 20 min between layers. CdSe CQDs were deposited by spin coating, followed by annealing at 300 °C for 20 min between successive layer deposition. The conformal Pt layer was deposited by gas-phase ALD.65 The optical photographs of PEDOT:PSS, CdSe, PEDOT:PSS/CdSe, and PEDOT:PSS/CdSe/Pt[200] are shown in Figure 2b. The top-view SEM image of the PEDOT:PSS/CdSe device shows the nanostructured porous nature of the film (Figure 2a), presumably due to the removal of the ligands during the sintering process. This provides a high surface area for photoelectrocatalysis. After coating with Pt by ALD, the surface becomes smoother and less-porous (Figure 2d), which is important in reducing corrosion and promoting long-term photocathode stability by limiting the direct contact of CdSe CQDs and electrolyte. Figure 2e shows the cross-sectional scanning electron microscope (SEM) image of the device structure which is glass/FTO/PEDOT:PSS (160 nm)/CdSe CQDs (200 nm)/Pt. The thickness of the Pt film is approximately 10 nm based on a growth rate of 0.05 nm per cycle for 7 ACS Paragon Plus Environment


the ALD chamber used (Figure S3). The corresponding line-scan energy-dispersive X-ray spectroscopy (EDS) is shown in Figure 2f. S element is mainly found in PEDOT:PSS layer due to its abundant thiophene skeletons and sulfonic acid groups. Cd and Se elements with similar content exist in CdSe light absorber layer. Notably, the Pt element is mainly located on the top layer, while the depth of penetration for Pt in CdSe layer is about 30-40 nm. Moreover, Fig. g and h further show the corresponding elements distribution on the interfaces.

Figure 2. (a) Schematic structure of the PEDOT:PSS/CdSe/Pt[200] photocathode showing the layers for light absorption, charge transfer, and hydrogen evolution. (b) Optical images of the as-prepared photocathodes. Top view of SEM images of (c) PEDOT:PSS/CdSe and (d) PEDOT:PSS/CdSe/Pt[200]. (e) Cross-sectional SEM image of PEDOT:PSS/CdSe/Pt[200] and (f) the corresponding EDS spectrum. (g, h) EDS spectra took from the interfacial regions in (e).


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The PEC hydrogen evolution performance of the PEDOT:PSS/CdSe/Pt[200] photocathode was evaluated with a three-electrode configuration in a neutral electrolyte (0.1 M NaSO4, pH = 7). The optical image of the PEC cell under operation is shown in Figure S4. Figure 3a shows current density vs. applied potential (j-E) curves for CdSe, PEDOT:PSS/CdSe, and PEDOT:PSS/CdSe/Pt[200] photocathodes under simulated solar illumination (AM-1.5G, 100 mW/cm2). The pure CdSe CQDs photocathode obtained at 10 spin cycles exhibits a small but optimized photocurrent (j0 = 0.09 mA/cm2) for HER until the applied potential reaches 0.3 V vs. RHE (Figure S5a), indicating only a small fraction of the photogenerated electrons are involved in the hydrogen evolution process. The addition of a PEDOT:PSS layer before deposition of CdSe (PEDOT:PSS/CdSe) leads to a 7-fold higher photocurrent (j0 = -0.63 mA/cm2), and the onset potential positively shifts approximately 110 mV, compared to the CdSe photocathode, to 0.41 V vs. RHE. The best PEC performance was obtained with a PEDOT:PSS film thickness of 160 nm (10 spin cycles), (Figure S5b). The ALD deposition of 200 cycles of a Pt overlayer leads to j0 =-1.08 mA/cm2, which is the highest j0 ever reported for colloidal CdSe CQDs based photocathodes in neutral solution.35-39 It should be noted that the choose of 200 cycles for ALD Pt is for the sake of ensuring sufficient active sites for HER and minimizing incident light blocking. The onset potential is positively shifted to 0.46 V which is slightly superior to the wellstudied Cu2O66, p-Si,67 and WSe268. Transient photocurrent measurements under chopped illumination were performed (Figure S6), and a rapid photoresponse upon illumination was observed for all photocathodes. Stable steadystate photocurrent is observed for PEDOT:PSS/CdSe/Pt[200] while a small amount of transient current decay occurs for CdSe and PEDOT:PSS/CdSe samples, indicating that the ALD Pt layer reduces charge recombination rate due to defect passivation and/or an increase in the charge



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transfer rate at the electrode-electrolyte interface. The open-circuit photovoltage (OCP) versus time measurement

is

shown

in

Figure

3b,

and

the

generated

photovoltage

for

PEDOT:PSS/CdSe/Pt[200], PEDOT:PSS/CdSe, and CdSe is 0.45, 0.41, and 0.29 V vs. RHE, close to the onset potential value at which the photocurrent is generated. The OCP as a function of illumination intensity were measured to investigate the quasi-static equilibrium of the PEDOT:PSS/CdSe/Pt[200] photocathode (Figure S7). The OCP initially increases with increasing illumination intensity until 120 mW/cm2, after which it remains at 0.48 V vs. RHE, indicating that the quasi-Fermi level splitting is near the maximum and approaches the true flat band potential. The flat-band potential of PEDOT:PSS/CdSe/Pt[200] calculated from its MottSchottky plot is 0.51 V vs. RHE (Figure S8), slightly higher than the above measured onset potential. The stability of the PEDOT:PSS/CdSe/Pt[200] photocathode was also evaluated during 24 h of operation. The photocurrent measured at 0 V vs. RHE is shown in Figure 3c. The pure CdSe photocathode exhibits poor stability, with the photocurrent decaying by 80% after 6 h of measurement. To explore the poor stability of the CdSe film, we observed that the resistance of the CdSe photocathode increased after 6 h of PEC measurement (Figure S9), which we ascribe to photocorrosion occurring at the interface of CdSe and the FTO substrate due to the strong oxidation ability of accumulated photogenerated holes. The PEDOT:PSS/CdSe photocathode shows significantly improved stability with only a 10% decay over the 24 h testing period. The enhanced stability due to the PEDOT:PSS underlayer is attributed to the heterojunction which effectively traps the photogenerated holes from the CdSe bulk film and reduces the photocorrosion. The addition of a Pt layer makes an equally significant contribution to stability, and the PEDOT:PSS/CdSe/Pt[200] photocathode exhibits excellent stability in neutral solution,


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with negligible activity degradation during the 24 h measurement. The ALD Pt overlayer protects the CdSe film from being corrosively attacked by the electrolyte ions.

Figure 3. (a) j-E curves of CdSe, PEDOT:PSS/CdSe, and PEDOT:PSS/CdSe/Pt[200] photocathodes under simulated solar illumination (AM-1.5G, 100 mW/cm2) in 0.5 M Na2SO4 neutral solution. Scan rate is 5 mV/s in the cathodic direction. (b) Open-circuit potential measurements, (c) stability tests, and (d) IPCE spectra of the as-obtained photocathodes.

The CdSe CQDs-based photocathodes were further characterized by measuring the incident photon-to-charge conversion efficiency (IPCE) at 0 V vs. RHE (Figure 3d). The wavelength 11 ACS Paragon Plus Environment


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dependence of the IPCE for all samples is consistent with their optical absorbance spectrum (Figure S10) and the onset wavelength absorption for IPCE is found at ~600 nm corresponding to the band-gap of the CdSe CQDs. The PEDOT:PSS/CdSe/Pt[200] photocathode shows the highest IPCE among all samples over the entire spectrum (400-650 nm), and achieves the maximum IPCE of 14.8% at 520 nm, which is higher than that of many reported CdSe based photocathodes, such as CdSe QD-C60 (8.2% at 470 nm),35 CdSe QDs/NiO (4.3% at 430 nm),36 and PTZ-CdSe QDs (10.8% at 430 nm).39 There was no significant change in the light absorption capability of CdSe modified by the PEDOT:PSS underlayer and ALD Pt overlayer (Figure S10), indicating that the enhanced IPCE is due to improved charge transfer and reaction kinetics. Moreover,

the

applied

bias

photon-to-current

efficiency

(ABPE)

for

the

PEDOT:PSS/CdSe/Pt[200] is 0.215% at 0.30 V vs. RHE, 15 times better than that found for the CdSe-only photocathode (0.014% at 0.19 V vs. RHE) and ~2 times greater than the PEDOT:PSS/CdSe photocathode (0.127% at 0.27 V vs. RHE) (Figure S11). The photoexcited charge kinetics of interfacial properties of PEDOT:PSS/CdSe/Pt[200] photocathode, were investigated using time-resolved photoluminescence (TRPL), and the results were fit with a bi-exponential decay model (Figure 4). The decay curve for all samples exhibits a fast component on the order of a nanosecond. Processes that typically occur on this timescale in nanocrystal films include non-radiative recombination and the separation of the exciton into free carriers.[69,70] However, the long decay component is significantly shorter for the heterojunction films, with an average lifetime (τaverage) of 44.2, 23.1, and 18.6 ns for CdSe, PEDOT:PSS/CdSe, and PEDOT:PSS/CdS/Pt[200], respectively. The insertion of the PEDOT:PSS underlayer results in a shorter average lifetime for PEDOT:PSS/CdSe film compared to the CdSe film. For the CdSe-based hybrid films, we attribute the decrease in lifetime to either better charge separation


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occuring at the interface of the type-II heterojunction or physical separation of holes from electrons due to hole injection into the PED:PSS layer, which could decrease free-carrier recombination. [69,70] The lifetime is further decreased with the ALD Pt overlayer, because of accelerated electron transfer into the Pt film. The bifunctional role of the ALD deposited Pt film was further investigated by measuring the j-E curves of PEDOT:PSS/CdSe in neutral solution with and without the addition of 10 mM KIO3 for photogenerated electron consumption (Figure S12). KIO3 acts as an electron scavenger for photogenerated electrons in the CdSe film, creating an additional route for electron transfer to the electrolyte that does not require a cocatalyst. In the presence of KIO3, the photocurrent is greatly increased and close to that of the photocathode with electrodeposited Pt, but still lower than that of PEDOT:PSS/CdS/Pt[200]. This indicates that the ALD Pt ultrathin film is acting as both surface defect passivating layer and a cocatalyst for HER.



Figure 4. Time-resolved photoluminescence decay curves fit with a bi-exponential decay model (excitation: 420 nm, pulse length: 1 ns).

Electrochemical impedance spectroscopy (EIS) measurements were conducted under simulated solar illumination (AM-1.5G, 100 mW/cm2) or dark condition and the corresponding Nyquist plots were fitted with a simplified circuit model (Figure 5a, and Figure S13). In this model, the Rs is the series resistance, including the solution resistance, bulk resistance, and ohmic contact resistance. The Rtrap and Csc account for bulk trapping resistance and space-charge layer capacitance, respectively. Rct and Cct are related to charge transfer resistance across the solid/solution interface and the solid surface states capacitance. The parameters of Rs, Rtrap, Rct, Csc, and Cct extracted from Nyquist plots under dark and light are included in Figure 5b. Under dark condition, the interfacial charge transfer resistance is increased with the loading of ALD Pt, indicating the catalytic role of Pt for promoting electron transfer from solid catalyst surface to electrolyte (Figure S13). Under irradiation, the total resistance to charge transfer (Rtrap,1 + Rtrap,2 + Rtrap,3) of PEDOT:PSS/CdS/Pt[200], is smaller than that of CdSe and PEDOT:PSS/CdSe, indicating that both the PEDOT:PSS underlayer and ALD Pt overlayer allow for better charge transfer in the hybrid photocathode. The Rct for CdSe and PEDOT:PSS/CdSe are similar but much higher than that of PEDOT:PSS/CdS/Pt[200], consistent with the role of the ALD Pt as a cocatalyst that facilitates the interfacial electron transfer from the photocathode to the electrolyte. The PEDOT:PSS/CdS/Pt[200] shows the highest Csc compared to CdSe and PEDOT:PSS/CdSe, which is attributed to the formation of appropriate heterojunctions at PEDOT:PSS/CdSe and CdSe/Pt[200] interfaces. The surface state capacitance (Cct), is proportional to the surface density of states (Nct) and provides an indication of the quantity of surface defect states under dark condition. The Cct of PEDOT:PSS/CdS/Pt[200] is significantly smaller than that of CdSe and


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PEDOT:PSS/CdSe (Figure S13), further confirming that the ALD Pt overlayer plays a significant role in suppressing surface defects and reducing the charge recombination rate. However, under light illumination, more photogenerated holes with positive charges accumulated on the interface between PEDOT:PSS/CdSe/Pt and aqueous solution. This leads to increased Helmholtz capacitance which becomes a main contributor for the extracted Cct value. Therefore, an inverse trend on Cct is observed between dark and light condition.



Figure 5. (a) EIS Nyquist plots and (b) corresponding parameters of resistance and capacitance of CdSe, PEDOT:PSS/CdSe, and PEDOT:PSS/CdSe/Pt[200] photocathodes measured under AM-1.5G irradiation and at 0 V vs. RHE in neutral solution.

Using metal chalcogenides as acid-stable photocathodes is still a significant challenge, and thus the working conditions for most of the reported chalcogenide-based photocathodes is in neutral solution, even though the HER kinetics are inherently more favorable in an acidic medium. To investigate the ability of the ALD Pt film to protect the CdSe surface, photocathodes were made with varying number of ALD cycles (Figure 6a). The effect of cycle number on j0 and its retention (j/j0) after 10 min of measurement are shown, and the optimal j0 is achieved after 300 deposition cycles, while its retention increases with Pt thickness and attains 97.6% at 400 cycles. These results are consistent with the fact that Pt films grown by ALD exhibit a transition from Pt islands to a continuous, conformal film between 200 and 400 cycles which improves stability.68 At high cycle numbers, the catalytic activity improves but the overall PEC performance is reduced under front side illumination due to the presence of a thick, reflective Pt film. Considering the balance of activity and stability, we chose the 300-cycle sample for further study. Figure 6b shows the current-potential curves of PEDOT:PSS/CdSe/Pt[300] obtained at various electrolyte pH under simulated solar illumination (AM-1.5G, 100 mW/cm2). The photocurrent clearly increases with decreasing solution pH and j0 is as high as -2.14 mA/cm2 at pH = 1. Notably, the PEDOT:PSS/CdSe/Pt[300] achieves the highest j0 among all the colloidal CdSe-based photocathodes for hydrogen production (Table S1). The onset potential of the photocurrent is 0.52 V vs. RHE, which reflects the high photovoltage that originates from the efficient junctions between the PEDOT:PSS/CdSe and CdSe/Pt[300] layers. A long-term PEC measurement was conducted in 0.1 M H2SO4 (pH = 1) as shown in Figure 6c. The 16 ACS Paragon Plus Environment

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PEDOT:PSS/CdSe/Pt[300] shows relatively stable photocurrent, with only 8.3% degradation after 6 h of measurement.

Figure 6. (a) Photocurrent density at 0 V vs. RHE and its retention as a function of ALD Pt cycles in acidic solution (0.5 M H2SO4). (b) j-E curves of PEDOT:PSS/CdSe/Pt[300] measured with a scan rate of 5 mV/s at different pH electrolytes. (c) Photocathodic stability at 0 V vs. RHE for PEDOT:PSS/CdSe/Pt[ED] and PEDOT:PSS/CdSe/Pt[300] under AM-1.5G irradiation at pH = 1. (d) The theoretical and experimental hydrogen gas amount generated by PEDOT:PSS/CdSe/Pt[300] at 0 V vs. RHE in 0.5 M H2SO4 for the Faradaic efficiency determination.

The use of ALD as deposition method is important in achieving stability in an acidic environment. When the Pt film is deposited by electrodeposition at -0.4 V vs. Ag/AgCl for 1 min 17 ACS Paragon Plus Environment


(denoted as PEDOT:PSS/CdSe/Pt[ED]), the resulting photocathode loses nearly 80% activity after 2 h. It should be noted that the deposited amount of Pt (1.9 mg) by ED is much larger than the estimated weight of Pt by ALD (0.032 mg), but the surface morphology of Pt film by ED is rough and discontinuous (Figure S14), thus unable to sufficiently protect the CdSe CQDs-based photocathode in acidic solution. These results indicate the superiority of the ALD technique for depositing a thin conformal Pt catalyst layer for a protective coating on the photocathode without light blocking. Moreover, the initial j0 for PEDOT:PSS/CdSe/Pt[300] is -2.14 mA/cm2, which is about 1.85 times that of PEDOT:PSS/CdSe/Pt[ED] (-1.16 mA/cm2). To confirm that the impressive photocurrent from the PEDOT:PSS/CdSe/Pt[300] photocathode originates from PEC hydrogen production, we measured the amount of hydrogen evolved from the constant potential (0 V vs. RHE) photoelectrolysis and compared it with the value calculated from the measured current (Figure 6d). The Faradaic efficiency of H2 on the PEDOT:PSS/CdSe/Pt[300] photocathode is 91%, slightly below unit efficiency, which may be ascribed to a small amount of H2 oxidation due to gas product crossover in the single compartment PEC cell.

Conclusion In summary, we designed, fabricated and studieda hybrid photocathode using a PEDOT:PSS underlayer, photoactive CdSe CQDs film, and

ultrathin ALD Pt overlayer. This structure

benefits from strong, tunable light absorption in the CdSe CQDs, band alignment appropriate for efficient interfacial charge transfer at both interfaces, and abundant active catalytic sites on Pt for a fast reaction rate. The PEDOT:PSS/CdSe/Pt[200] photocathode was found to exhibit significantly improved PEC HER performance. A photocurrent density of -1.08 mA/cm2 (AM-


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1.5G, 100 mW/cm2) at a potential of 0 V vs. RHE (j0) was achieved in neutral aqueous solution, which is 12 times that of pristine CdSe photocathode. By increasing the thickness of the ALD Pt layer, the reaction can be done in the more favorable acidic electrolyte, and the PEDOT:PSS/CdSe/Pt[300] photocatode achieves an improved j0 of -2.14 mA/cm2 (at pH = 1) with only 8.3% activity degradation after 6 h, compared to 80% degradation when the Pt layer is deposited by electrodeposition. Both the photoelectrochemical impedance spectroscopy and time-resolved photoluminescence are consistent with more efficient extraction of holes due to the addition of the PEDOT:PSS underlayer and the bifunctional role of the ultrathin ALD Pt as both a surface defect passivation layer and cocatalyst for HER. The reported PEDOT:PSS/CdSe/ALD Pt provides a new interface engineering strategy for designing more efficient and acid-stable photocathodes for solar-driven hydrogen evolution.

Methods Preparation of colloidal CdSe quantum dots (CQDs) To a three neck, round-bottom flask, 3.125 g of purified trioctylphosphine oxide (TOPO), 2.875 g hexadecylamine, and a magnetic stir bar were added. To remove air, the flask was placed under vacuum and refilled with argon gas, after which 1.7 mL of trioctylphosphine (TOP) was injected. This solution was placed under vacuum at 110 °C for 1 hour, and then placed under argon and heated to 360 °C. Meanwhile, a second precursor solution comprising of 1 mmol (0.3106 g) of cadmium acetylacetonate (Cd(acac)2) and 2.2 mmol (0.3727 g) of dodecanol in 3 mL TOP was degassed at 100 °C for 1 hour in a 40 mL vial with a septum cap in an oil bath. This solution was cooled to room temperature, and 2 mL of 1.5 M trioctylphosphine selenide was rapidly injected into the bath at 360 °C. Following the initial temperature drop, the solution 19 ACS Paragon Plus Environment


was maintained at ~280 °C for 5 min then cooled to room temperature. The reaction mixture was centrifuged at 6000 rpm for 5 min. The isolated solid was suspended in hexane, then acetone and methanol were added to create a 1: 6: 1 (v: v: v) hexane: acetone: methanol solution to cause aggregation of the CQDs. The resulting cloudy solution was centrifuged. The isolated solid was redissolved and the process repeated twice to achieve increased purity.

Preparation of hybrid photocathode The well-cleaned FTO substrates were coated with a thin film of PEDOT:PSS deposited by layer-by-layer (10 cycles) spin coating using a 25:75 vol% solution of PEDOT:PSS in isopropanol, which was sonicated for 10 min and filtered with a 2.7 µm glass fiber filter before use. The spin coating was done at 2000 rpm for 30 s and subsequently annealed at 150 °C for 20 min on a hotplate. The thickness of PEDOT:PSS was controlled by varying the number of layerby-layer cycles. After cooling to ambient temperature, 100 µL of the CdSe CQDs solution (50 mg/mL) in hexane was spin-coated onto the PEDOT:PSS at 2500 rpm for 30 s, followed by annealing at 300 °C for 20 min in a tube furnace under a flow of 5 % H2/N2 gas to partially remove surface organic ligands. This step was repeated for 10 cycles. Finally, the Pt overlayer was deposited on the PEDOT:PSS/CdSe by a GEMSTAR-6 atomic layer deposition (ALD) system. The reaction took place at 275 °C using trimethyl(methylcyclopentadienyl) platinum (MeCpPtMe3) in a steel bubbler heated to 65 °C and O2 as precursors. Each ALD cycle used three sequential pulses of MeCpPtMe3 to increase exposure and ensure complete surface coverage, followed by one pulse of O2. The growth rate of the Pt on a reference silicon substrate with a native oxide was 0.05 nm/cycle as determined by x-ray reflectivity. As the initial Pt islands merge into a continuous film in the range of 200-300 cycles, this was the thickness regime studied. For comparison, a Pt overlayer was also deposited through conventional 20 ACS Paragon Plus Environment

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electrochemical deposition method in a solution containing 2 mM H2PtCl6 and 0.1 M HClO4 by applying a constant potential of -0.4 V vs. Ag/AgCl for 1 min. The deposited amount of Pt by electrodeposition is 1.9 mg which is much larger than the estimated amount of Pt by ALD for 300 cycles (0.032 mg). It should be noted that the pure CdSe CQDs photocathode is prepared by successive spin-coating of CdSe CQDs solution and post-annealing treatment on FTO substrate.

Materials Characterizations Powder X-ray diffraction (PXRD) patterns were obtained on an X-ray diffractometer (Bruker D2 Phaser) using Cu Kα radiation. Transmission electron microscopy (TEM) and HRTEM were carried out on a Tecnai G2 F20 microscope at 200 kV. The elemental mappings were performed on a STEM unit with HAADF detector (FEI Technai G2 F30, 200 kV). All samples for TEM measurements were prepared by ultrasonic dispersion in ethanol and were dropcast onto copper grids covered with a carbon film. The morphologies of the samples were characterized by fieldemission scanning electron microscopy (FE-SEM, Zeiss Supra 55) with EDX mapping (Oxford) at an acceleration voltage of 15 kV. X-ray photoelectron spectrometry (XPS, Kratos Axis Ultra DLD, UK) was used to detect the chemical composition of the as-prepared samples. The binding energy was calibrated based on the C 1s peak at 284.8 eV. UV-Vis absorption spectra were recorded on a UV-Vis spectrophotometer and were collected over the range of 400-800 nm. The UV-Vis diffuse reflectance spectra (DRS) were recorded by a Shimadzu 3600 UV−Vis−NIR spectro photometer with fine BaSO4 powder as reference. For the time-resolved photoluminescence measurements, the specimens were optically excited by a 420 nm SHG (second harmonic generation) signal from a femtosecond Ti: sapphire laser system, with a repetition rate of 80 MHz. The photoluminescence signal from the sample was collected by a NA = 0.42 lens and sent to a 0.75 m focal length spectrometer. Time-integrated



photoluminescence

was

detected

by

a

liquid-nitrogen

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cooled

Si-CCD,

while

the

photoluminescence lifetime was analyzed by an avalanche photodiode together with a timecorrelated single-photon counting module.

Electrochemical and Photoelectrochemical measurements For determining the energy level of the CdSe CQD electrode, we used an electrochemical cell setup with a three-electrode configuration. A L-type glassy carbon disk (0.2 cm2) coated with the CdSe CQDs film was the working electrode, a Pt wire was the counter electrode, and a silver wire was the pseudo-reference electrode. The electrolyte consisted of acetonitrile with 0.1 M tetrabutylammoniumhexafluorophosphate (TBAPF6). The ferrocenium/ferrocene (Fc/Fc+) redox potential was measured at the end of each experiment in order to calibrate the pseudo-reference electrode as recommended by the International Union of Pure and Applied Chemistry (IUPAC). The cyclic voltammograms were measured from -1.4 to 2.6 V vs. Ag (identical to -2.0 to 2.0 V vs. Fc/Fc+ because the calibrated potential for Fc/Fc+ is 0.6 V vs. Ag) at a scan rate of 20 mV/s. The energetic levels of CdSe CQDs were determined as follows: EVB (eV) = -4.8 - Eox-onset and ECB (eV) = -4.8 - Ered-onset. The photoelectrochemical activity of the photocathodes were also measured in a threeelectrode configuration using a Reference 600 potentiostat (Gamry Instrument Inc). The prepared PEDOT:PSS/CdSe/Pt served as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. The electrolyte used were a 0.1 M Na2SO4 neutral aqueous solution (pH = 7) and a 0.1 M H2SO4 acidic solution (pH = 1). A 300 W Xenon arc lamp (Newport) equipped with an AM-1.5G filter was used to simulate the solar spectrum. The light intensity was calibrated to 100 mW/cm2 through a Newport 843-R power meter. The measured potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE)


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scale according to the Nernst equation: ERHE = EAg/AgCl + 0.059 × pH + 0.1976. The scan rate for the linear sweep voltammetry was 5 mV/s in the cathodic direction. Time dependent photoresponse tests were carried out by measuring the photocurrent under chopped light irradiation (light/dark cycles of 10 s) at 0 V vs. RHE. Photocurrent stability measurements were also carried out at a fixed electrode potential of 0 V vs. RHE. The incident photoelectron conversion efficiency was measured by a specially designed IPCE system (Zolix Solar Cell Scan 100) with a 150 W Xe lamp and a monochromator (Oriel Cornerstone 130). Faradaic efficiency for hydrogen production measurements were taken in a manner similar to the photocurrent tests at a constant potential of 0 V vs. RHE under simulated solar illumination (AM-1.5G, 100 mW/cm2). The photocathode was PEDOT:PSS/CdSe/Pt[300] and the electrolyte used was a 0.1 M H2SO4 acidic solution (pH = 1). The generated gas was sampled using a gas-tight syringe (50 µL) and analyzed by gas chromatography (GC, Agilent 6890) with a thermal conductivity detector and argon carrier gas. The Faradaic efficiency can be calculated using the equation: FE% =

௡ಹమ ொൗ ଶி

, where F is the Faradaic constant, Q is the total charges passed through the cell,

and nH2 is the total amount of hydrogen produced. Electrochemical impedance spectroscopy was performed at 0 V vs. RHE under dark or AM-1.5G illumination over a frequency range of 105 to 0.1 Hz with an AC voltage of 10 mV. The raw data was fitted with a suitable circuit model in the software of EIS Spectrum Analyser. The Mott-Schottky plot of PEDOT:PSS/CdSe/Pt electrode was obtained at a fixed frequency of 5 kHz to determine the flat band potential.

Associated Contents The authors declare no competing financial interest.

Supporting Information 23 ACS Paragon Plus Environment


The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional experimental results are included in the support information; UV-visible absorption spectrum of CdSe nanocrystals solution, CV for CdSe band level determination, XRR of ALD Pt film deposited at different cycles, optical image of PEC cell under measurement, photocurrent varies with spin cycles of PEDOT:PSS, transient photocurrent response in neutral solution, opencircuit potential determined illumination intensity, Mott-Schottky and Nyquist plots, DRS of different photocathodes, ABPE curves, and etc.

Author Information Corresponding Author *Email: [email protected]

Acknowledgements This work was financially supported by Wake Forest University, and Shenzhen Bureau of Science, Technology and Innovation Commission (JCYJ 20160525163956782).

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Table of Contents



A thin film heterojunction photocathode, composed of organic PEDOT:PSS, CdSe CQDs, and conformal Pt layer deposited by atomic layer deposition (ALD), shows notable PEC HER performance and high stability in acidic solution.


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709x763mm (72 x 72 DPI)

ACS Paragon Plus Environment


193x120mm (300 x 300 DPI)


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709x567mm (72 x 72 DPI)



270x217mm (150 x 150 DPI)


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202x154mm (300 x 300 DPI)


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72x30mm (300 x 300 DPI)


Interface engineering of colloidal CdSe quantum dots thin films as acid

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