Highly Enhanced Photoelectrochemical Water Oxidation Efficiency

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Article

Highly Enhanced Photoelectrochemical Water Oxidation Efficiency Based on Triadic Quantum Dot/Layered Double Hydroxide/BiVO Photoanodes 4

Yanqun Tang, Ruirui Wang, Ye Yang, Dongpeng Yan, and Xu Xiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04937 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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ACS Applied Materials & Interfaces

Highly Enhanced Photoelectrochemical Water Oxidation Efficiency Based on Triadic Quantum Dot/Layered Double Hydroxide/BiVO4 Photoanodes Yanqun Tang a, b, Ruirui Wang a, Ye Yang c, Dongpeng Yan a,b* and Xu Xiang a* a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029 (P.R. China) E-mail: [email protected]

b

Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875 (P.R. China) E-mail: [email protected]

c

Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401 (USA).

KEYWORDS. water oxidation; photoanodes; layered double hydroxides; quantum dots; photoelectrochemistry

ACS Paragon Plus Environment

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ABSTRACT

The water oxidation half-reaction is considered to be a bottleneck for achieving highly efficient solar driven water splitting due to its multi-proton-coupled four-electron process and sluggish kinetics. Herein, a triadic photoanode consisting of dual-sized CdTe quantum dots (QDs), Cobased layered double hydroxide (LDH) nanosheets and BiVO4 particles i.e., QD@LDH@BiVO4 was designed. Two sets of consecutive Type-II band alignments were constructed to improve photo-generated electron-hole separation in the triadic structure. The efficient charge separation resulted in a 2-fold enhancement of the photocurrent of the QD@LDH@BiVO4 photoanode. A significantly enhanced oxidation efficiency reaching above 90% in the low bias region (i.e., E < 0.8 V vs. RHE) could be critical in determining the overall performance of a complete photoelectrochemical cell. The faradaic efficiency for water oxidation was almost 90%. The conduction band energy of QDs is ~1.0 V more negative than that of LDH, favorable for the electron injection to LDH and enabling a more efficient hole separation. The enhanced photonto-current conversion efficiency and improved water oxidation efficiency of the triadic structure may result from the non-negligible contribution of hot electrons or holes generated in QDs. Such a band-matching and multi-dimensional triadic architecture could be a promising strategy for achieving high-efficiency photoanodes by sufficiently utilizing and maximizing the functionalities of QDs.

Introduction Development of new catalytic and energy materials enabling solar-to-fuel conversion is highly desirable because it provides a clean and sustainable way to balance our long-term


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dependence on fossil-based fuels.1 Photoelectrochemical (PEC) water splitting or carbon dioxide reduction can convert and store solar energy to chemical fuels in the form of either H2 or hydrocarbon compounds.2-6 The water oxidation half-reaction is recognized as a speed-limiting step in the overall water splitting reaction because it involves a sequential multi-proton-coupled four-electron process.7-9 Hence, great effort has been devoted to the search for high-performance photoanodes capable of oxidizing water efficiently and persistently.10,

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For instance, the

photoanodes based on TiO2,12-14 ZnO,15-17 hematite,18-20 BiVO4, 21-24 WO325, 26 and Ta3N527, 28 have been extensively studied. Despite the intense efforts, enhancement of the water-oxidation efficiency remains challenging due to the combination of its unfavorable thermodynamic and kinetic characteristics. Modification of the semiconductor photoanodes with water oxidation catalysts (WOCs) for expediting the hole-involved oxidation reactions on the electrode/electrolyte interface is an effective approach for improving the PEC performance.29-31 Catalysts made of earth-abundant elements (Co, Ni, Fe) have been proven to be effective WOCs for water splitting.29 For example, the well-known solution-synthesized Co-Pi WOC was widely utilized in a variety of photoanodes.12, 32, 33 Another strategy is to construct a p-n heterojunction or to combine narrow and wide band-gap semiconductors for improving photo-generated charge separation.34-38 Inspired by the two approaches described above, it is highly desirable to design catalytic and semiconductor bi-functional materials because they are capable of simultaneously accelerating the oxidation by photo-excited holes and facilitating electron-hole separation. The Co-based layered double hydroxide (LDH), a graphene-like layered compound, is a possible candidate because the octahedron-coordinated Co(II)/Co(III) species are distributed atomically within the LDH ultrathin layers, which is beneficial for the exposure of highly active sites toward water


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oxidation.39,40 Additionally, the colored Co-containing LDH can serve as a narrow band-gap semiconductor for the capture of visible light.41-44 Furthermore, the quasi-two-dimensional (2D) LDH micro/nanosheets usually exhibit a large radial/c-axial ratio (usually > 50) that is favorable for the charge transfer along the c direction. Due to their excellent light-harvest and charge-transfer characteristics, semiconductor quantum dots (QDs) have been utilized as photo-sensitizers or charge transfer promoters in PEC photoelectrodes.45-47 For instance, Liu and co-workers reported that in CdTe QD-sensitized ZnO nanowire arrays, QDs efficiently harvest solar light to yield separated electrons and holes for water splitting.45 The enhancement in the efficiency is mainly ascribed to the improved light absorption and favorable valence band edge of CdTe that is more positive than the water oxidation potential (H2O/O2 1.23 V vs. RHE). However, the construction of suitable QDs toward functionalities of both charge separation and hole oxidation on the photoanode has rarely been explored. Herein, we have designed a triadic photoanode consisting of dual-sized CdTe QDs, CoAlLDH and BiVO4 i.e., QD@LDH@BiVO4. The photoanode possesses two sets of consecutive Type-II band alignments of QD@LDH and LDH@BiVO4, greatly improving electron transfer to the counter electrode and hole diffusion to the surface during water oxidation. The photocurrent and incident-photon-to-current conversion efficiency (IPCE) of QD@LDH@BiVO4 is twice higher than that of dyadic LDH@BiVO4 at 1.23 V vs. RHE. Moreover, water oxidation efficiency reaches above 90% even at a low potential of 0.5 V vs. RHE higher than most of the values reported to date for the state-of-the-art BiVO4-based photoanodes. Furthermore, the effects of hot electrons or holes on the enhanced efficiency were discussed. Therefore, the energy-matching triadic micro/nanostructure designed in this work is proven to represent a


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highly effective strategy for improving the PEC efficiency of photoanodes and for highlighting the role of QDs in charge separation and transfer.

Results and discussion The triadic QD@LDH@BiVO4 photoanode was constructed in three main consecutive steps (Scheme 1). The details were provided in the experimental section.

Scheme 1. Illustration of the synthesis pathway for a triadic QD@LDH@BiVO4 photoanode. First, the BiVO4 particles were prepared on FTO electrodes using a Bi precursor method. Second, the LDH nanosheets were vertically grown on BiVO4 as an overlayer via a hydrothermal precipitation approach. Third, the dual-sized CdTe QDs were anchored onto LDH nanosheets through a sequential deposition of red- and green-emission QDs from an aqueous solution. Co(II) species on LDH are catalytically active sites for photoelectrochemical water oxidation. The phase structure of the samples grown on the FTO electrodes was confirmed by XRD. The reflections at 18.8 and 28.9 degrees are assigned to monoclinic scheelite BiVO4 (JCPDS no.75-1866) in the pristine BiVO4 photoanode (Figure 1A). New reflections appear after the growth of LDH on the BiVO4 that are indexed to the (003), (006), (009), (015) and (018) planes


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of a hydrotalcite-like structure (Figure 1B).48 The lower I(003)/I(009) intensity ratio suggested that the LDH units formed in a preferred orientation relative to the FTO substrate.43 It is known that the hexagonal LDH tends to grow vertically or at a certain angle relative to the substrate due to its 2D rigid feature i.e., large radial/c-axial ratio.49 After further growth of CdTe quantum dots (QDs) by an aqueous solution synthesis, no new reflections associated with CdTe were detected (Figure 1C). This could be due to high dispersion of QDs and/or low concentration. The relative intensity of the (003) peak increased, which could be ascribed to the improved crystallinity of the

+

*

+

20

#(018) #(018)

*

* FTO + BiVO4 # LDH C *

30

*

B

*

A

* *

*

10

*

#(015)

* +

#(009)

#(006)

+

#(015)

+

*

#(009)

#(003)

+

#(006)

#(003)

2D LDH.50

Intensity (a.u.)

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40

*

50

60

2 Theta (degree) Figure 1. XRD patterns of (A) BiVO4, (B) LDH@BiVO4 and (C) QD@LDH@BiVO4 photoanodes. Reflections arising from FTO are labelled with stars.

The morphological and structural evolution of the photoanodes was characterized by SEM and TEM. BiVO4 shows a particulate and irregular morphology with rough surfaces (Figures 2AB). The particles have sizes of 200~300 nm and are distributed on the FTO substrate. After the hydrothermal growth of LDH, interconnected nanosheets appear, thoroughly covering the


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underlying BiVO4 (Figures 2C-D). The thickness of isolated LDH nanosheets is estimated as 15~20 nm, and the radial/c-axial ratio is larger than 50, indicating a 2D structure. Such structure facilitates the carrier separation and transfer along the c-axial direction of LDH. The subsequent growth of QDs hardly changes the overall structure of LDH while making the surface of the nanosheets rougher (Figures 2E-F). This could be due to the deposition and anchoring of the hydrophilic QDs on the hydroxyl-rich LDH surface. The cross-sectional SEM images show that the thickness of the deposited LDH layer onto BiVO4 is approximately 2 µm (Figure S1). The thick covering layer could increase the light absorption of the photoanode because of the visiblelight responsive behavior of CoAl-LDH.40 High-resolution TEM (HRTEM) observations show that QDs are highly dispersed onto the LDH nanosheet surfaces (Figure 2G). The average sizes of QDs are 2.2 and 3.6 nm, corresponding to the green- and red-emission QDs. The dual-sized QDs were obtained through the sequential growth controlled by the aging time.51 The lattice image of a single QD clearly displays the d-spacing of 0.37 nm, corresponding to the plane (111) of CdTe (Figure 2I). The EDS mappings of QD@LDH@BiVO4 verified the existing elements from the triadic components (Figure 2J).


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Figure 2. SEM images of (A) and (B) BiVO4, (C) and (D) LDH@BiVO4, (E) and (F) QD@LDH@BiVO4. HRTEM images of (G) QD@LDH@BiVO4, (H) size distribution of QD, (I) lattice image of QD and (J) EDS mappings of QD@LDH@BiVO4 (scale bar 25 nm).

The chemical compositions and valence states were analyzed by the XPS technique. The surface oxygen species can be studied using the O1s spectra. Three deconvoluted peaks can be fitted using the O1s spectra of BiVO4 and QD@LDH@BiVO4 (Figure 3A) that originate from the O2- (531.0 eV), OH- species (531.6 eV), and adsorbed water (532.3 eV).52 The peaks have the same binding energy (B.E.) values in both samples. The OH-/O2- species ratio increases from 0.9


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in BiVO4 to 1.5 in QD@LDH@BiVO4, suggesting an increase of hydroxyl groups in the surface species. This is also consistent with the characteristic abundance of hydroxyl on LDH. The Co2p spectra can be deconvoluted into four contributions in the Co2p3/2 region (Figure 3B). The peaks at B.E. 780.9 and 781.9 eV are assigned to the Co3+ and Co2+ species, respectively.43, 53 The contribution of the Co2+ species has a higher intensity, indicating that Co2+is the dominant species, whereas the Co3+ species co-exist in the LDH. The Bi and V elements from BiVO4 cannot be detected due to the thickness and complete coverage of the LDH layer on the surface. The Cd3d spectra shows two peaks at 411.7 and 404.9 eV in the Cd3d3/2 and Cd3d5/2 regions, respectively (Figure 3C). The Te3d spectra also show two main peaks at 582.6 and 572.2 eV in the Te3d3/2 and Te3d5/2 regions, respectively (Figure 3D). These B.E. values agree well with those in the CdTe compound.54

Figure 3. (A) XPS O1s spectra of BiVO4 and QD@LDH@BiVO4, (B) Co2p spectra, (C) Cd3d spectra and (D) Te3d spectra of the QD@LDH@BiVO4.


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The optical properties of the photoanodes were characterized using UV-vis absorption spectrometry. The BiVO4 photoanode shows intense absorption below 450 nm and with the absorption edge extending to ~490 nm (Figure 4A). The bandgap was estimated as 2.47 eV by Kubelka−Munk plot converted from the reflectance spectra (Figure S2), consistent with that of the monoclinic phase of BiVO4.34 The absorption of LDH@BiVO4 increases throughout the UV to visible region (350~700 nm). The CoAl-LDH has been recognized as a colored semiconductor with visible light absorption capability.41 After anchoring the QDs onto the LDH nanosheets, the absorption of QD@LDH@BiVO4 further increases below 460 nm. The as-prepared QDs in an aqueous solution show strong absorption below 460 nm, especially for red-emission QDs (Figure S3). This is consistent with the improved absorption of QD@LDH@BiVO4, where QDs are immobilized onto the surface of the 2D LDH. However, the exciton absorption peaks of QDs are not clearly present in the solid electrodes because of spectral overlapping with the spectrum of the intrinsic LDH. Fluorescent emission is one of the possible charge recombination channels, and fast emission can lead to a low charge separation yield. BiVO4 and LDH@BiVO4 show no fluorescence in the 450~700 nm region under excitation at 365 nm (Figure 4B), suggesting that the fluorescence-associated recombination by LDH is greatly suppressed.42 In contrast, QD@LDH@BiVO4 displays a weak emission band centered at 602 nm, in agreement with that of the free red-emission QDs in an aqueous solution (Figure S4),51 whereas an emission band associated with the integrated green-emission QDs (peak @556 nm) is not observed. The fluorescence dynamics were revealed by time-resolved fluorescence decay measurements (Figure 4C). The measured fluorescence lifetime for the QD@LDH@BiVO4 is 3.7 ns (Table S1), much shorter than that for the free QDs (38.1 ns). The remarkably shortened lifetime reflects the


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fast fluorescent quenching that is probably caused by the efficient electron-hole separation.55 The observed optical properties suggested that the triadic structure is favorable for light absorption and charge separation.

Figure 4. (A) UV-vis absorption spectra of the photoanodes, (B) Fluorescence spectra of the photoanodes, λex = 365 nm. (C) Time-resolved fluorescence decay curves. λex = 365 nm, λprobe =602 nm, and instrument response function (IRF) = 0.3 ns.

PEC measurements were carried out to characterize the performance of the triadic structure. The current-potential curves were measured under chopped light illumination from the backside of photoanodes in 0.1 M PBS, pH = 7 (Figure 5A). The dark current-potential curves were first detected to compare the electrochemical water oxidation capability, verifying the catalytic role of the LDH (Figure S5). The modification of LDH on BiVO4 has two advantages. First, LDH could act as a water oxidation catalyst. LDH@BiVO4 showed obvious dark (electrochemical) current compared to the BiVO4 itself (Figure S5). The catalytic role of Co-based LDH has also been reported in other works. For instance, ZnCo-LDH exhibited excellent catalytic activity for electrochemical water oxidation.43 The CoAl-LDH was proved to be an efficient oxygen evolution photocatalyst by combined theoretical and experimental methods.56 The photocurrent


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density of LDH@BiVO4 shows a little increase compared to that of the pristine BiVO4, indicating that the modification of BiVO4 by only the LDH overlayer is insufficient. In contrast, the photocurrent density of QD@LDH@BiVO4 shows a remarkable increase and reaches 2.23 mA/cm2 (at 1.23 V vs. RHE), twice as high as that of LDH@BiVO4. This value is close to the record current value (2.71 mA/cm2 at 1.23 V) obtained for Co-catalyzed BiVO4 photoanodes.34 It is comparable to or higher than that of the W-doped BiVO4.32, 57, 58 The photocurrent in this particular case was limited by both the photon collection and charge transfer efficiencies. Even though LDH did not significantly enhance the photon-to-current efficiency for bare BiVO4 due in part to the poor light harvesting, it helps to collect the photocarriers from QDs. In other words, the light-harvesting rate, rather than charge transfer rate, is increased by applying QDs on LDH. Furthermore, the onset potential of QD@LDH@BiVO4 is as low as 0.3 V vs. RHE, comparable to the dual-layer catalyst (FeOOH/NiOOH)-modified BiVO4 (~0.3 V vs. RHE).22 The photocurrent measurements highlighted the role of QDs in the QD@LDH@BiVO4 photoanode because the photocurrent is a key indication of charge separation. The differences between the QDs with different emissions in the photoelectrochemical properties

were

investigated.

QD(green)@LDH@BiVO4,

We

respectively.

synthesized The

QD(red)@LDH@BiVO4

QD(red)@LDH@BiVO4

showed

and higher

photocurrent density than QD(green)@LDH@BiVO4 (Figure S6), which could be caused by a wider light absorption region of QDs(red) extending to 650 nm. In this work, the dual-emission QDs were utilized to further enhance the photocurrent. The effect of QD and LDH content on the photocurrent was also studied. The QD and LDH content was adjusted by the growth time. The growth time was changed from 10 to 20 hours to obtain different LDH samples with adjustable thickness. The QD@LDH(15h)@BiVO4 in this work showed the higher photocurrent density


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than

those

with

larger

or

shorter

growth

time

(QD@LDH(20h)@BiVO4

and

QD@LDH(10h)@BiVO4, Figure S7). This behavior can be attributed to that the thicker LDH nanosheets with longer growth time are unfavorable to the charge transfer to the surface (Figure S8), while the shorter growth time led to lower crystallinity of LDH, which degrades the PEC performance. The QD content was changed by adjusting the initial content of NaTeO3 (Cd2+ was always excessive) from 0.8 mmol/L (QD-1), 1 mmol/L (QD-2) to 1.2 mmol/L (QD-3). The QD content was compared by UV-vis absorption spectra (Figure S9). The QD-2@LDH@BiVO4 in this work showed the highest photocurrent density (Figure S10). The less QD content led to the reduced photocurrent. The more QD content did not cause the enhancement in photocurrent. Therefore, the QD content within the triadic system was optimized. To confirm little QDs attached onto BiVO4, we deposited QDs directly onto BiVO4 and measured the PEC performance of the QD@BiVO4 photoanode (Figure S11). The QD@BiVO4 has a much lower photocurrent density than QD@LDH@BiVO4. And the photocurrent is not stable. Besides, we scraped off the LDH layer on QD@LDH@BiVO4 photoanode. The remaining electrode was conducted the elemental analyses by EDS. No Cd and Te elements were detected (Figure S12). It suggests that the QDs are hardly deposited onto BiVO4. To reveal the mechanism of charge transfer at the electrode/electrolyte interface, the electrochemical impedance spectra (EIS) measurements were determined.59 The charge transfer resistances (Rct) were calculated and compared according to the corresponding equivalent circuit from the Nyquist plots (Figure 5B). The Rct value of LDH@BiVO4 (353 Ω) is smaller than that of BiVO4 (767 Ω), and the Rct of QD@LDH@BiVO4 is further decreased to 120 Ω. Smaller Rct values correspond to more favorable charge transport across the electrode/electrolyte interface.


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Therefore, this is a clear indication that QD@LDH@BiVO4 exhibits superior charge transport property. However, the EIS result is not directly associated with PEC water oxidation performance.32 Hence, the oxidation efficiency (ηox) of the photoanodes by photo-generated holes was studied. The efficiency of substrate oxidation by surface-reaching holes is closely related to carrier recombination on the surface.60 The ηox was calculated by comparing the photocurrent for H2O2 oxidation and water oxidation (see the Experimental section and Figure S13) because the ηox can be assumed to be 100% in the presence of the easily-oxidized species e.g., H2O2 acting as a hole scavenger. It is impressive that the ηox of QD@LDH@BiVO4 is as high as 90% throughout the measured potential window (0.5~1.6 V vs. RHE) and reaches almost 100% (98%) at 1.23 V vs. RHE, much higher than the corresponding values for LDH@BiVO4 or BiVO4 (Figure 5C). It is believed that the outstanding oxidation performance in the low bias region (i.e., E < 0.8 V vs. RHE) is critical for determining the overall photocurrent density of a complete p-n photoelectrochemical diode cell.22 The much higher ηox indicates that more than 90% of the population are the separated holes that are transferred to the electrode/electrolyte interface and are involved in the oxidation of water under illumination. The light harvesting, charge separation and charge injection are considered to be the three main factors in the PEC performance, with charge injection (i.e., hole oxidation) believed to be a more limiting factor for the overall water splitting.9, 10 Herein, the ηox beyond 90% even in the low bias window verifies the remarkable PEC performance of the triadic photoanode. Additionally, the surface charge separation efficiency (ηsep) of QD@LDH@BiVO4 is highly improved (Figure S14). For example, ηsep reaches 50% at 1.23 V (vs. RHE) and shows a 2-fold enhancement that is comparable to that of the Co3O4/BiVO4 p-n heterojunction photoanode.34


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Figure 5. (A) Current-potential curves of the photoanodes under chopped AM1.5G illumination. (B) Nyquist plots of the photoanodes under illumination. Inset: corresponding equivalent circuit. Rct is the charge transfer resistance across the electrode/electrolyte interface, Rs is the solution resistance, and CPE is the constant phase component. (C) Oxidation efficiency of the surfacereaching holes injected into the solution species. (D) IPCE measured by monochromatic light irradiation. (E) O2 produced (blue dots) on the QD@LDH@BiVO4 photoanode under illumination at 0.8 V vs. RHE. Dashed line is the theoretical result based on the number of transferred electrons. (F) Faradaic efficiency of the triadic photoanode for water oxidation. Symbols represent the data calculated from the experimental results, and the dotted line is a fitted trace. Solution: 0.1 M PBS (pH 7); scan rate: 10 mV·s-1; and back illumination (intensity: 100 mW·cm-2).

To examine the possible degradation of CdTe QDs by the photo-generated holes,61 photocurrent stability measurements were conducted. The photocurrent responses of the


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QD@LDH@BiVO4 for the 1st and 16th scans were almost identical (~40 min period) and thus verified the stability of the photoanode (Figure S15). Additionally, the amperometric (I-t) measurement (0.8 V vs. RHE) showed that the triadic photoanode was stable during the PEC water oxidation (Figure S16). Furthermore, the structural stability of QDs was verified by HRTEM after PEC reactions (Figure S17). The stability studies provide evidence that the remarkable improvement of the photocurrent arises from water oxidation rather than from the anodic decomposition/corrosion of QDs. The incident photon-to-current efficiency (IPCE) was measured to study the solar conversion efficiency (Figure 5D). The QD@LDH@BiVO4 shows a higher IPCE than LDH@BiVO4 and BiVO4 from the UV to the visible region (up to 520 nm). For example, the IPCE of the QD@LDH@BiVO4 at 400 nm is 43%, much higher than that of LDH@BiVO4 and BiVO4 (23% and 17%, respectively). The enhanced IPCE is partially associated with the improved light absorption of QD@LDH@BiVO4, where the triadic photoanode has more intensive absorption up to ~460 nm (Figure 4A). To obtain the absorbed-photon-to-current efficiency (APCE), the light harvesting efficiency (LHE) of photoanodes was first calculated using the formula LHE = 1-10-A(λ) (A: absorbance at wavelength λ). Both QD@LDH@BiVO4 and LDH@BiVO4 have higher LHE than the pristine BiVO4 from UV to the visible region (Figure S18). The triadic photoanode has a similar LHE to the dyadic one from ~470 nm to 700 nm whereas the former shows higher APCE than the latter in the same wavelength region (Figure S19). These observations verify that the triadic structure is more favorable for charge separation. The quantity of evolved O2 gas and the faradaic efficiency are shown in Figures 5E and 5F, respectively. The amount of evolved O2 was measured using an O2 fluorescent probe in a gastight PEC cell at 0.8 V vs. RHE. The theoretical O2 amount was calculated according to the


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number of the transferred electrons (assuming 100% faradaic efficiency). In turn, the faradaic efficiency was calculated by comparing the experimentally observed O2 amount with that obtained by the theoretical results. The faradaic efficiency for water oxidation reached nearly 90%, in agreement with the oxidation efficiency measurement (0.8 V vs. RHE) (Figure 5C). This finding suggested that the separated holes were almost completely utilized for water oxidation. To obtain the insight into charge transfer pathways, we also measured the energy levels between the triadic components. The flatband potentials (Efb) of the photoanodes were estimated by electrochemical Mott-Schottky measurements. The Mott-Schottky plots of all samples exhibit the positive slopes, suggesting characteristic of n-type semiconductors (Figure S20). The Efb values are estimated by extrapolating the plots to the bias axis to read the intercept and are obtained as 0.22, 0.04 and -0.96 V (vs. RHE) for BiVO4, LDH and the triadic, respectively. For n-type semiconductors, the Efb value is 0~0.1 V more positive than the conduction band potential.2 Consequently, the conduction band minimum (CBM) of the three samples lies at approximately 0.12, -0.06 and -1.06 V. According to the XPS valence band (VB) spectra, the VB of LDH and QDs was at 1.78 eV and 1.25 eV, respectively (Figure S21). The energy levels between BiVO4 and LDH correspond to Type-II band alignment (Figure 6) analogous to the QDs system,62, 63 facilitating the electron-hole separation under illumination. The Type-II energy band alignment can result in fast photoinduced charge separation,64 which is consistent with the fast fluorescence quenching. The VB of QDs is slightly more positive than the water oxidation potential (H2O/O2 1.23 V). This is consistent with the reported VB level of the CdTe QD-sensitized ZnO photoanode.45 In that case, it was claimed that CdTe exhibits a conduction band energy (ECB) that was 0.4 V more negative than that of CdSe and thus may inject electrons into ZnO more efficiently. The


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rapid electron transfer from CdTe to the ZnO makes hole extraction more feasible due to a weaker Coulomb attraction and is thus beneficial for the hole involved water oxidation.45 In the QD@LDH@BiVO4 micro/nanostructure, the ECB of QDs is -1.0 V more negative than that of LDH (Figure 6). The photo-excited electrons in QDs can be transferred to LDH nanosheets in an energetically favorable manner. Furthermore, the monolayer deposition of QDs (from the HRTEM of Figure 2G) can promote electron transfer to LDH and improve the stability of QDs, thus avoiding the anodic decomposition/corrosion of QDs.

Figure 6. Scheme of energy levels and charge transfer pathways. Photo-excited electrons in QDs are transferred to the conduction band of LDH and then to BiVO4, and the holes can be transferred to the trapping sites of the LDH surface states for water oxidation. Proton reduction (H+/H2) and water oxidation (H2O/O2) potentials were also indicated. hν: light energy irradiated; CB: conduction band; VB: valence band; Es: energy levels associated with surface states on LDH. Examination of the UV-vis absorption spectra shows that the QD@LDH@BiVO4 has stronger absorption than LDH@BiVO4 up to ~460 nm (Figure 4A), and the QDs show little


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contribution to the longer wavelength region in the QD@LDH@BiVO4. In contrast, the triadic structure has higher APCE than the dyadic from 460 nm to 600 nm (Figure S19). The triad photoanode showed stronger absorption up to ~460 nm than the dyad one (Figure S18). Consequently, it could be an indication that the hot electrons and holes in QDs created by absorbing high-energy photons (2% Efficient Water Splitting. Adv. Energy Mater. 2016, 6, 1501645. (25) Fuku, K.; Wang, N.; Miseki, Y.; Funaki, T.; Sayama, K. Photoelectrochemical Reaction for the Efficient Production of Hydrogen and High-Value-Added Oxidation Reagents. ChemSusChem 2015, 8, 1593-1600. (26) Zhang, J.; Liu, Z.; Liu, Z. Novel WO3/Sb2S3 Heterojunction Photocatalyst Based on WO3 of Different Morphologies for Enhanced Efficiency in Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2016, 8, 9684-9691. (27) Liu, G.; Shi, J.; Zhang, F.; Chen, Z.; Han, J.; Ding, C.; Chen, S.; Wang, Z.; Han, H.; Li, C. A Tantalum Nitride Photoanode Modified with a Hole-Storage Layer for Highly Stable Solar Water Splitting. Angew. Chem. Int. Ed. 2014, 53, 7295-7299.


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Table of Contents Graphic and Synopsis

A triadic photoanode integrating both the functionalities of QDs and the unique structure of twodimensional nanosheets was constructed, and exhibited significantly enhanced photocurrent, photon-to-current conversion and water oxidation efficiency (ηox > 90%) even in the low bias region.


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Highly Enhanced Photoelectrochemical Water Oxidation Efficiency

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