Charge Transfer from Upconverting Nanocrystals to Semiconducting

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Charge Transfer from Upconverting Nanocrystals to Semiconducting Electrodes: Optimizing Thermodynamic Outputs by Electronic Energy Transfer Bing Shan, Ting-Ting Li, M. Kyle Brennaman, Animesh Nayak, Lei Wu, and Thomas J. Meyer J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11110 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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Journal of the American Chemical Society

Charge Transfer from Upconverting Nanocrystals to Semiconducting Electrodes: Optimizing Thermodynamic Outputs by Electronic Energy Transfer

Bing Shan1, Ting-Ting Li1,2, M. Kyle Brennaman1, Animesh Nayak1, Lei Wu1, and Thomas J. Meyer*,1 1

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States.

2

Research Center of Applied Solid State Chemistry, Ningbo University, Ningbo, Zhejiang 315211, China.

ABSTRACT Light-harvesting inorganic nanocrystals play an important role in emerging solar energy conversion and optoelectronic devices. We describe here a strategy for a new family of photoelectrodes with upconverting nanocrystal assemblies as the photosensitizer. The assemblies consist of oleic acidcapped cadmium selenide (CdSe) nanocrystals that coordinate directly onto a layer of surface-bound, carboxylic acid-derivatized anthracenes through displacement of the oleic acid capping ligands. Steady-state emission and transient absorption measurements show that the upconverting nanocrystal assemblies, selectively excited by green light, generate singlet excitons that enable efficient charge injection into both the conduction band of TiO 2 at the photoanode and the valence band of NiO at the photocathode. The singlet excitons form by sensitized triplet-triplet annihilation within the compact layer of anthracenes on the electrode surfaces. Density of state analysis reveals that the electronic coupling between the anthracene singlet excited states and the oxides provide a thermodynamic basis for light-induced charge transfer. The interplay between the excited state populations at the surface-bound molecules and the assembled nanocrystals, presents new design rules that can potentially overcome the limitations of previous dye-sensitized photoelectrochemical cells for catalytic applications.

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INTRODUCTION Solar energy conversion devices such as photoelectrocatalytic cells require light absorbers to photogenerate electron/hole pairs at energies sufficient to drive water splitting or CO2 reduction.1-5 The use of organometallic complexes as sensitizers has been limited by the need to match light absorptivity and energy requirements for the catalytic half-cell reactions.6-9 Alternatives to organometallic chromophores are expected to combine the electronic and optical properties of those complexes with small energy loss during rapid excited state spin transitions. Along those lines, semiconductor quantum dots (QDs) that are nanoscale crystals of analogous bulk inorganic semiconductors, provide a promising platform as photosensitizers for solar cell devices with benefits of unique atomic-like properties, photostability, small Stokes Shift and the use of cost-effective solution processing techniques.10-17 For dye-sensitized photoelectrocatalytic cells, QDbased chromophores can enable continuous spectral tunability over a wide energy range and enhance the overall light conversion efficiencies.18 QDs such as CdSe form stable bonds with carboxylic acid derivatized surface ligands,19, 20 allowing for bottom-up assembly into solid-state photoelectrodes. Their excited state energies can be tailored by coupling with molecules of diverse chemistry.21 For photoelectrodes with QD sensitizers that absorb low energy photons, their thermodynamic outputs can be enhanced by surface-functionalization with an optical upconversion layer that enables conversion of sub-bandgap and incoherent excitation photons.22 As an emerging wavelength-shifting technology, photon upconversion represents a viable route toward converting low-power photons into light with adequate energy to drive electron transfer in solutions and photovoltaics.23-28 The efficiencies of those single-junction devices can be increased to potentially exceed the Shockley–Queisser limit3 due to the distinct anti-Stokes character of upconversion process. Here, we investigate a series of oxide-based, dye-sensitized photoelectrodes where molecular chromophores are replaced with upconverting nanocrystal assemblies shown in Figure 1a. The assemblies are based on oleic acid (OA)-capped CdSe QDs with high molar absorptivity in the visible region. The QDs coordinate directly onto a layer of surface-bound, carboxylic acid-derivatized anthracenes through displacement19 of the OA capping ligands. Preparation of the photoelectrodes is illustrated in Figure S1. In the final structures, the intermediate anthracene layer acts as both a bridge for anchoring CdSe onto the oxide electrodes and as an energy transfer acceptor for the excited state of CdSe. The coordination of CdSe onto the anthracene layers enables facile Dexter-type, triplet-triplet energy transfer between them. As shown by the energy diagrams in Figures 1b and 1c, selective green light excitation of CdSe (extinction coefficients (ελ) in Figure S2) sensitizes formation of the excited state, CdSe*.19 The excitation is followed by energy transfer to the surface-bond anthracenes and generation of the anthracene triplet excited states (3An*, 3Ph2An*). The compact anthracene layer on the electrode surfaces allows for cross-surface triplet exciton migration, enabling encounter of two neighboring triplet excitons and formation of one singlet excited state (1An*, 1Ph2An*) through triplet-triplet annihilation (TTA) 23, 29, 30. The anthracene singlet excited states are sufficiently energetic to populate the valence band electrons at p-type NiO electrodes or the conduction band holes at n-type TiO2 electrodes. The reaction sequences at the two photoelectrodes are illustrated schematically in Figures 1b and 1c.


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c TiO2│-Ph2An-CdSe

b NiO│-Ph2An-CdSe

a

hv

E/V vs NHE

Ph2An0/-

-1.2

E/V (1Ph2An*)+/0 vs NHE

e-

CdSe0/-

-1.2

hv

-0.6

(3Ph2An*)0/-

hv

0.6

Ph2An

NiO 1.2

TTA

(CdSe*)0/-

EnT

1.2

e-

hv

eCdSe+/0

Ground /Excited State

Ground /Excited State Energy Transfer (EnT);

TTA

Ph2An+/0

(1Ph2An*)0/-

An

(CdSe*)+/0

e-

(3Ph2An*)+/0

EnT

e-

TiO2

Transfer;

Triplet-Triplet Annihilation (TTA)

Figure. 1. (a) Photoelectrode and assembly structures. The green light excitation, triplet exciton migration and TTAinduced blue light emission are represented by green, black and blue arrows, respectively. (b), (c) Schematic redox potential diagrams showing the photoinduced energy and electron transfer at the nanocrystal-sensitized NiO and TiO2, respectively.

RESULTS AND DISCUSSION CdSe-OA nanocrystals (~2.72 nm in Figure S3) dispersed in toluene were prepared as described in the experimental section. Loading of CdSe onto oxide films was achieved by using a bridging layer of 2aminoethyl-phosphonic acid 31 (abbreviated as -NH2-). The bridge links to oxide surfaces through oxidephosphonate binding and to CdSe through amino-CdSe binding31 (Figure S1). A toluene solution of CdSe (6.7 μM, UV-Vis absorption spectrum in Figure S4) was used for loading on Al2O3, TiO2 and NiO films modified with the -NH2- bridge. The CdSe adsorption isotherms in Figure S5 were measured by immersing the films in toluene solutions of CdSe at concentrations of 97 nM-6.7 μM for 24 hours each. After loading, the slides were removed, rinsed with toluene, and dried under a stream of nitrogen. Surface coverages (Γ in mol/cm2) of CdSe were estimated by Γ = Aλ/ελ/1000 32, 33. In these analyses, ε560nm(CdSe) in Figure S2 were used, and A560nm is the absorbance of CdSe at 560 nm of the sensitized slides. Surface binding constants (Kad in M-1) and maximum surface coverages (Γm) of CdSe were obtained by using the Langmuir equation: (Γ = Γm (Kad Conc(CdSe))/(1 + Kad Conc(CdSe))) 34, as listed in Table S1. The UV-Vis absorption spectra of the resulting films are shown in Figure 2a. For the samples with anthracenes, the oxide films were first modified with a layer of the carboxylic acid-derivatized anthracenes through oxide-carboxylate binding, followed by loading of CdSe via ligand displacement of CdSe-OA. The surface coverages of the anthracenes and the CdSe were estimated from the absorption difference spectra in Figures S6-S7 and the molar absorptivities in Figures S2, S8. As summarized in Table 1, this assembling method results in molar ratios of anthracene to CdSe in a range of 20:1 - 40:1. Figures 2b and 2c show absorption profiles for those samples. The efficiencies for selective green light absorption (ηA in Table 1), estimated from the absorbance at 560 nm (A560nm) by ηA=1-10−A560nm , range from 30% to 70%. Although the surface coverages of CdSe are small relative to the anthracenes, the fractions of light absorbed are high because of the high molar absorptivity of the CdSe QDs (Figure S2). Three nanocrystalline oxide films were investigated here: Al2O3, n-type TiO2 and p-type NiO with particle sizes 20-40 nm. The Al2O3 samples were used to investigate intra-assembly energy and electron 3 ACS Paragon Plus Environment


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transfer, given that the wide bandgap of Al2O3 prevents interfacial charge injection.35 Photoluminescence of CdSe* is shown by the emission spectra in Figure 2d for -NH2-CdSe on oxides. In a CdSe surface coverage range of 0.39-2.4 nmol/cm2, the emission integrals of CdSe* are linearly related to the absorption efficiencies of CdSe at 560 nm, as shown in Figure S9. For all samples in Figures 2d-2f, the emission intensities were normalized based on the light absorption efficiencies of CdSe listed in Table 1. Using the emission of -NH2-CdSe* on Al2O3 as a reference, the integrated emission for the NiO sample gives a quenching efficiency of 18% (Table S2). Comparison of the integrated emission for the TiO2 and Al2O3 samples gives no apparent evidence for electron injection into TiO2 by CdSe* with only an efficiency of 3.0% (Table S2). Thermodynamically, CdSe* should be able to inject into NiO and TiO2, as shown by the potential diagrams in Figures 1b and 1c. The low injection (quenching) yields observed here for the -NH2CdSe samples could result from dynamic reasons involving faster interfacial back electron transfer than the forward charge injection. Previous reports on electron transfer between CdSe* and TiO236 or NiO14, 37 focused on directly linked CdSe on oxides or CdSe linked by mercapto-based bridges. For those systems, the observed quenching efficiencies are a result of several effects including charge transfer distance and bridge trapping. The direct contact between TiO2 and CdSe enables much faster electron transfer quenching than the bridged connection.38 The mercapto group in the bridges has been observed to induce charge trapping that interferes with injection into the oxides.14, 37, 39 In the -NH2-CdSe assembly, replacing the bridge (2-aminoethyl-phosphonic acid) with 4-aminobenzoic acid (abbreviated as -PhNH2-) resulted in quenching (injection) efficiencies of 9.3% and 21% for the TiO2 and NiO samples, respectively, as shown in Figure S10. Those observations do not show significant difference between the quenching efficiencies obtained from -NH2- or -PhNH2- linked CdSe samples. By contrast, replacing the redox-inert bridge with the anthracenes results in greatly enhanced quenching efficiencies of 60% and 65% (Table S2) for -AnCdSe and -Ph2An-CdSe on Al2O3, respectively, due to the facile triplet-triplet energy transfer.

Table 1. Surface Coverages and Light Absorption Efficiencies. Surface Coverage (Γ / nmol cm-2)a

Light Absorption Efficiency (ηA) at 560 nm b

-An

-Ph2An

CdSe in -NH2-CdSe

CdSe in -An-CdSe

CdSe in -Ph2An-CdSe

-NH2-CdSe

-An-CdSe

-Ph2An-CdSe

Al2O3

46

39

2.4

1.8

1.2

0.68

0.59

0.43

TiO2

96

82

2.2

1.7

1.6

0.67

0.57

0.53

NiO

23

20

1.3

1.0

0.89

0.47

0.38

0.32

MOx

Γ was quantified with molar absorptivities of CdSe and An/Ph2An in Figures S2 and S8. absorbance of the CdSe samples at 560 nm (A560nm) by the equation, ηA=1-𝟏𝟎−𝑨𝟓𝟔𝟎𝒏𝒎 . a


b

ηA was calculated with

(1) Al2O3 TiO2 (2) (1) (3) NiO Al2O3| -NH2-CdSe TiO2| -NH2-CdSe NiO| -NH2-CdSe (4)

2.0 1.5

(4) (5) (6)

(2)

2.5

(3)

(5)

(6)

1.0 0.5

(4) (5) (6)

2.0 1.5

c (2)

(5)

(6)

1.0

450

500

550

600

650

700

Wavelength (nm) 1.0

Al2O3| -NH2-CdSe TiO2| -NH2-CdSe

0.8

NiO| -NH2-CdSe

0.6 0.4

-An-CdSe

0.2

-Ph2An-CdSe 580

590

Wavelength (nm)

450

600

610

500

550

600

650

1.5

(2)

(3)

(5)

(6)

1.0

0.6

TiO2| -An-CdSe hinj ~0.63

0.2

420

440

460

480

Wavelength (nm)

500

550

600

650

700

0.8 0.6

NiO| -Ph2An-CdSe hinj ~0.50

0.4

TiO2| -Ph2An-CdSe hinj ~0.58

0.2

NiO| -An-CdSe hinj ~0.72 400

500

Al2O3| -Ph2An-CdSe

1.0

0.8

0.4

450

Wavelength (nm)

f

Al2O3| -An-CdSe

1.0

400

700

Wavelength (nm)

0.0 570

(4) (5) (6)

0.0 400

e

d

2.0

0.5

0.0

400

(1) Al2O3 (2) TiO2 (1) (3) NiO Al2O3| -Ph2An-CdSe TiO2| -Ph2An-CdSe NiO| -Ph2An-CdSe (4)

2.5

(3)

0.5

Upconverted Emission

0.0 350

(1) Al2O3 (2) TiO2 (1) NiO (3) Al2O3| -An-CdSe TiO2| -An-CdSe NiO| -An-CdSe (4)

Upconverted Emission

Absorbance

2.5

b

Absorbance

a

Photoluminescence

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance

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400

420

440

460

480

500

Wavelength (nm)

Figure 2. Steady-state absorption and emission profiles. (a),(b),(c) UV-Vis absorption spectra for -NH2-CdSe, -AnCdSe and -Ph2An-CdSe, respectively, on Al2O3 (gray), TiO2 (blue) and NiO (red) films. Solid and dashed lines show the samples with and without the surface-bound assemblies, respectively. (d),(e),(f) Steady-state emission following excitation of -NH2-CdSe, -An-CdSe and -Ph2An-CdSe, respectively, at 560 nm (55 mW/cm2), on Al2O3 (gray), TiO2 (blue) and NiO (red). Dashed lines in (d) show emission from -An-CdSe (green) and -Ph2An-CdSe (pink) on Al2O3. All samples were kept in argon-degassed acetonitrile with 0.1 M LiClO4.

Upconverted emission from the singlets (1An* and 1Ph2An*) was observed following green light excitation of the assemblies, -An-CdSe and -Ph2An-CdSe, as shown by the emission spectra in Figures 2e and 2f. The excitation spectra in Figure S11 for Al2O3│-An-CdSe and Al2O3│-Ph2An-CdSe, obtained by controlling the emission at 410 nm, show a broad excitation range with peaks at 563 nm and 558 nm, respectively. The excitation spectra correlate with the absorption spectrum of CdSe in Al2O3│-NH2-CdSe, demonstrating the effective upconversion process by sensitized triplet-triplet annihilation. Since the emission intensities in Figures 2e and 2f were normalized based on light absorption efficiencies (Table 1), comparison of the emission integrals gives direct information about the charge injection efficiencies (ηinj) from 1An* and 1Ph2An* into the conduction band of TiO2 (blue curves in Figures 2e, 2f) or the valence band of NiO (red curves in Figures 2e, 2f). From the resulting ηinj in Table S3, 1An* and 1Ph2An* inject into TiO2 at comparable ηinj values (0.63 and 0.58), while 1An* injects into NiO at a higher ηinj (0.72) compared to ηinj (0.50) of 1Ph2An*. The upconversion quantum efficiencies of 1An* and 1Ph2An* in the assembles were estimated by using Ru(bpy)32+ in a poly(methyl methacrylate) matrix as the actinometer 40 (experimental section). The resulting upconversion yields of 1An* and 1Ph2An* following excitation of the assemblies at 560 nm (55 mW/cm2) are 0.021% and 0.28%, respectively. Taking into account the fractions of light absorbed by the Al2O3 samples (Table 1), the upconversion efficiencies (ηUC) based on absorbed photons are 0.036% and 0.65% for 1An* and 1Ph2An*, respectively. Reducing the surface coverages of An/Ph2An by co-loading with the -NH2- bridge results in decreased upconversion emission and smaller ηUC as shown in Figure S12. 5 ACS Paragon Plus Environment


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The results suggest high surface packing of the anthracene annihilators for effective upconversion of their triplet excited states. The quantum yields of the final redox-separated states (ηRS) can be estimated by the product of ηA, ηUC and ηinj listed in Table 2. For the TiO2 samples, ηRS of TiO2(e-)│-An-CdSe(h+) and TiO2(e-)│-Ph2An-CdSe(h+) are 0.013% and 0.20%, respectively. For the NiO samples, ηRS of NiO(h+)│An-CdSe(e-) and NiO(h+)│-Ph2An-CdSe(e-) are (9.8×10-3)% and 0.10%, respectively. Based on these results, Ph2An performs better than An as the energy transfer relay for the photoelectrodes under relatively low-intensity light excitation (560 nm, 55 mW/cm2), because of the much higher upconversion efficiency of 1Ph2An* 27. Table 2. Upconversion Quantum Efficiencies for the Photoelectrodes. ηA a

ηinj b

ηUC c

ηRS d

TiO2-An-CdSe

0.57

0.63

3.6×10-4

1.3×10-4

TiO2-Ph2An-CdSe

0.53

0.58

6.5×10-3

2.0×10-3

NiO-An-CdSe

0.38

0.72

3.6×10-4

9.8×10-5

NiO-Ph2An-CdSe

0.32

0.50

6.5×10-3

1.0×10-3

a Light

absorption efficiency (ηA) at 560 nm. b Injection efficiency (ηinj) by 1An* or 1Ph2An*. c Upconversion efficiency (ηUC) for -An-CdSe and -Ph2An-CdSe under 560 nm excitation (55 mW/cm2). d Quantum yield of redox-separated states (ηRS) from the equation, ηRS = ηA × ηinj × ηUC.

The nanosecond transient absorption (TA) spectra in Figure 3 provide direct evidence for formation of redox-separated states at the photoelectrodes. In the absence of the anthracenes, the TA in Figure 3a for -NH2-CdSe on TiO2 shows only features of CdSe* with a lifetime (τTA) corresponding to the emission lifetime (τE) monitored at 580 nm (59 ns in the inset of Figure 3a). The τE of CdSe* on TiO2 is close to that on Al2O3 (62 ns in Figure S13), which is consistent with the negligible injection observed in the steadystate emission measurements (Figure 2d and Table S2). Addition of the anthracenes between CdSe and TiO2 results in formation of the oxidized CdSe (CdSe+) following green light excitation of the assemblies, as shown by the TA spectra in Figures 3b and 3c. The TA data were analyzed with the spectroelectrochemical data in Figures S14-S16 for the involved redox species (CdSe+/0, CdSe0/-, An+/0, An0/-, Ph2An+/0 and Ph2An0/-). The observed absorptive bleach bands in Figure 3b and 3c are consistent with the spectroelectrochemistry data of CdSe+ in Figure S14a. The bleach may originate from the changes in the surface structures with the ligands (oleic acid and 2-aminoethylphosphonic acid) during one-electron oxidation of CdSe. The lifetime of CdSe+ was estimated to be 2.3 and 11 μs for the photoelectrodes, TiO2│An-CdSe (Figure 3b) and TiO2│-Ph2An-CdSe (Figure 3c), respectively, from fitting of the time-resolved TA traces at 560 nm (insets of the figures) based on the Kohlrausch-Williams-Watts (KWW) stretched exponential model41. Back electron transfer from TiO2(e-) to CdSe(h+) is slower for the sample with Ph2An, due to the longer spatial separation distance for Ph2An compared to An. Figures 3d-3f show TA spectra for the corresponding NiO samples. For -NH2-CdSe on NiO (inset of Figure 3d), the emission of CdSe* decays faster, 34 ns, compared to it on TiO2 or Al2O3, consistent with hole injection into NiO by CdSe*. The TA spectra in Figures 3e and 3f for the samples with the anthracenes show the formation and decay of the redox-separated states, NiO(h+)│-An-CdSe(e-) and NiO(h+)│-Ph2AnCdSe(e-), with lifetimes of 2.1 and 2.8 μs, respectively, as estimated by KWW fitting of the time-resolved traces in the insets. The photoelectrodes with the two anthracenes exhibit little difference in charge 6 ACS Paragon Plus Environment

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recombination lifetimes, perhaps due to the presence of surface trap states at polycrystalline NiO that are known to induce rapid interfacial back electron transfer 42, 43.

0.5

0.0

20

40

60

t (ns)

400

450

500

550

600

650

-2 0

-4

CdSe+ CdSe0 2.3 ms

-2 0

5

700

400

e

NiO I-NH2-CdSe

450

0.5

0.0

-8 400

Time Delay 30 ns 40 ns 50 ns 60 ns 0.1 ms

(3CdSe*) CdSe0 34 ns

20

40

60

t (ns)

450

500

550

600

Wavelength (nm)

500

550

600

650

-8

-2

CdSe+ CdSe0 11 ms 0

10

20

650

700

-2

450

-3 400

0

5

Time Delay 50 ns 0.1 ms 0.5 ms 1.0 ms 4.0 ms 9.0 ms

10

15

20

t (ms)

450

500

550

600

500

40

550

600

650

700

Wavelength (nm)

0

CdSe- CdSe0 2.1 ms

-2

30

t (ms)

400

f

0

Time Delay 50 ns 0.1 ms 0.5 ms 2.0 ms 5.0 ms 21 ms

0

-4

700

-1

DA

Emission

-6

1.0

20

NiO I-An-CdSe

0

-2 -4

15

-4

Wavelength (nm)

DAbsorbance /10-3

0

10

t (ms)

Wavelength (nm)

d

Time Delay 50 ns 0.1 ms 0.5 ms 1.0 ms 2.0 ms 5.0 ms

TiO2 I-Ph2An-CdSe

650

700

Wavelength (nm)

NiO I-Ph2An-CdSe

-2 Time Delay 50 ns 0.1 ms 0.6 ms 1.3 ms 3.0 ms 9.0 ms

0

-4

DA

(3CdSe*) CdSe0 59 ns

0

DAbsorbance /10-3

-6

1.0

Time Delay 30 ns 40 ns 50 ns 60 ns 0.1 ms

DAbsorbance /10-3

-4

Emission

DAbsorbance /10-3

-2

c

TiO2 I-An-CdSe

DA

0

DAbsorbance /10-3

b

TiO2 I-NH2-CdSe 0

DA

a

DAbsorbance /10-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CdSe- CdSe0 2.8 ms

-2

-6 400

0

5

10

15

20

t (ms)

450

500

550

600

650

700

Wavelength (nm)

Figure 3. Spectroscopic and kinetic evidence for charge transfer from upconverting assemblies to oxide films. (a),(b),(c) Transient absorption spectra following excitation of TiO2│-NH2-CdSe, TiO2│-An-CdSe and TiO2│Ph2An-CdSe, respectively, at 532 nm (2.6 mJ/cm2/pulse). The inset of (a) shows the time-resolved emission of CdSe* at 580 nm. The insets of (b),(c) show the time-resolved absorptive changes of CdSe+ at 560 nm. (d),(e),(f) Transient absorption spectra following excitation of NiO│-NH2-CdSe, NiO│-An-CdSe and NiO│-Ph2An-CdSe, respectively, at 532 nm. The inset of (d) shows the time-resolved emission of CdSe* at 580 nm. The insets of (e),(f) show the timeresolved absorptive changes of CdSe- at 560 nm.

To gain further perspectives into the key electron-transfer steps, density of states as a function of applied bias were investigated for the individual components. The integrated concentration changes of the redox species were measured spectroscopically44 after a potential step at 0.2 mV/s (details in the experimental section). The results are plotted in capacitance in Figure 4 for the ground states of the anthracenes, the CdSe and the valence/conduction bands of NiO and TiO2. The potentials at which equal concentrations of the reduced and oxidized forms were present were taken as the formal potentials of An+/0, An0/-, Ph2An+/0 and Ph2An0/- (Table S4). The excited state distributions of the anthracenes were determined from the ground state distributions and the excited state energies. The anthracene singlet and triplet excited state energies in Table S4 were obtained from the fluorescence spectra shown in Figures S17-S18 and from the reported literature values45-47, respectively. In Figure 4, all anthracene singlet excited states (shaded in green) have energies sufficiently high to overlay the distributions entirely within that of TiO2 conduction band and NiO valence band (shaded in gray). By contrast, the energies of the triplet excited states are 1.22-1.31 eV lower than the corresponding singlets, showing insignificant overlap with the band distributions of the oxides. The singlet excitons populate the holes at the conduction band of TiO2 (Figures 4a, 4b) or the electrons at the valence band of 7 ACS Paragon Plus Environment


NiO (Figures 4c, 4d), generating the oxidized or reduced anthracenes (shaded in blue in Figure 4), respectively. The redox active anthracenes, An+/Ph2An+ or An-/Ph2An-, are capable of transferring the charges into the valence band (shaded in orange in Figures 4a,4b) or the conduction band (shaded in orange in Figures 4c,4d) of CdSe.

Capacitance (mF cm-2)

a 40

20

0

20

40

60

-1

(3An*)+/0

0

c

-1

10

0

d

10

NiO

An0/-

VB(NiO)

ΔE = 1.29 V

40

TiO2 (1Ph2An*)+/0 CB(TiO2)

ΔE = 1.23 V

TiO2

Ph2An+/0 CdSe

Capacitance (mF cm-2) -2

CdSe

20

0

2

(3An*)0/-

2

0

(3Ph2An*)+/0

TiO2

CdSe

0

1

20

-1

1

An+/0

Capacitance (mF cm-2) -2

Potential(V vs NHE)

CB(TiO2)

Potential(V vs NHE)

Potential(V vs NHE)

ΔE = 1.28 V

2

40 -2

(1An*)+/0

-2

1

Capacitance (mF cm-2)

b TiO2

Potential(V vs NHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1

10

0

10

CdSe

Ph2An0/-

NiO

0

(3Ph2An*)0/1

VB(NiO)

ΔE = 1.23 V (1Ph2An*) 0/-

(1An*)0/NiO

2

NiO

Figure 4. Density of state profiles for the associated ground/excited states and valence/conduction bands. (a) TiO2│An-CdSe. (b) TiO2│-Ph2An-CdSe. (c) NiO│-An-CdSe. (d) NiO│-Ph2An-CdSe. The distributions shaded in blue, red and green correspond to the ground, triplet excited and singlet excited states of An/Ph2An, respectively. Valence/conduction band distributions are shaded in gray for NiO/TiO2, and in orange for CdSe.

CONCLUSION New dye-sensitized photoelectrodes with upconverting nanocrystals as the photosensitizers have been described here which feature rapid energy and electron transfer between surface-bound anthracenes and the nanocrystals. The assemblies were prepared based on ligand exchange of the oleic acid-capped CdSe quantum dots with the surface-bound, carboxylic acid-derivatized anthracenes. The photoelectrodes have been demonstrated to undergo sensitized triplet-triplet annihilation upon green light excitation, resulting in multiphoton energy transfer upconversion. The singlet excitons from upconversion are sufficiently energetic to inject electrons into the conduction band of n-type TiO2 or holes into the valence band of ptype NiO electrodes. Our results demonstrate that high-energy excitons can be extracted from surfacebound nanocrystals by low-energy photon excitation for sensitization of semiconducting oxides. The 8 ACS Paragon Plus Environment

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approach implemented here optimizes the thermodynamic outputs of oxide-based, dye-sensitized photoelectrodes, which can be used for driving the half-reactions of water splitting or CO2 reduction in photoelectrocatalytic cells. Future studies will include enhancing the efficiencies of the upconversion step by improving surface packing of the assemblies and using other annihilators with higher fluorescence quantum yields (e.g., rubrene48) and enhanced photostability toward O2 (e.g., 1-chlorobis(phenylethynyl)anthracene and 2-chloro-bis(phenylethynyl)anthracene49). Replacing CdSe with coreshell type QDs with longer excited state lifetimes and higher photoluminescence quantum yields will further improve the light conversion performances of the photoelectrodes for use in photoelectrocatalytic devices.

EXPERIMENTAL SECTION Synthesis of CdSe quantum dots (QDs). CdSe QDs were prepared according to a previously reported method50 by using cadmium myristate and SeO2 as the precursors. Preparation of cadmium myristate. Cadmium nitrate (5 mmol) in 50 mL anhydrous methanol was added dropwise into 500 mL methanol solution of sodium hydroxide (15 mmol) and myristic acid (15 mmol) under vigorous stirring. A white precipitate formed and was collected after centrifugation. The product, cadmium myristate, was obtained after washing the white solid with methanol and ether. The product was dried under vacuum overnight. Preparation of CdSe QDs with oleic acid capping ligand. Cadmium myristate (0.1 mmol), SeO2 (0.1 mmol) and 1-octadecene (6.3 mL) were added into a 25 mL three-neck flask. The mixture was degassed under vacuum and then heated to 320 ℃ under argon flow and vigorous stirring. After heating at 320 ℃ for 30 min, 0.1 mL oleic acid was added dropwise into the solution for stabilizing the resulting QDs. The reaction was terminated by cooling the mixture to room temperature. The product precipitated out upon addition of acetone. The mixture was centrifugated at 3.5 k rpm for 15 min. The resulting QDs were collected after washing with acetone and methanol for three times. The diameter of the QDs was estimated as ~2.72 nm by TEM and UV-Vis absorption51. Synthesis of 9,10-dicarboxylic acid-anthracene (An). An was prepared based on a reported procedure52. 9,10-dibromo anthracene (2.00 g, 5.95 mmol) was added dropwise into n-butyllithium in dry ether (8.1 mL, 1.6 M) at 0˚C. The mixture was stirred under 0˚C for 2 h. Dry ice (0.3 g) was added to the mixture under -78˚C. The entire reaction was allowed to proceed for about 10 h before adding water and hydrochloric acid. The product, An, was filtered out and dried under vacuum. Yield: 67%. 1H NMR (500 MHz) is shown in Figure S19 for An in DMSO-d6 with δ: 8.06 (dq, J = 7.0, 3.5 Hz, 4H) and 7.69 (dd, J = 6.8, 3.3 Hz, 4H). 13C NMR (126 MHz) is shown in Figure S20 for An in DMSO-d6 with δ: 169.94, 131.76, 127.30, 126.17, 125.37. Synthesis of 9,10-dicarboxylic acid-anthracene (Ph2An). Ph2An was prepared based on a reported method through Pd-catalyzed Suzuki coupling of 9,10-dibromo-anthracene and 4(methoxycarbonyl)phenylboronic acid, and a base-catalyzed hydrolysis reaction.53 9,10-dibromo anthracene (5.70 g, 17.0 mmol), 4-methoxycarbonylphenylboronic acid (7.63 g, 42.4 mmol), K2CO3 (8.29 g, 60.0 mmol) and palladium tetrakis(triphenylphosphine) (0.510 g, 0.440 mmol) were mixed in 140 mL of toluene, CH3OH and H2O (8:3:3). The mixture was heated at 100 °C for 48 h under argon and stirring. After cooling the mixture to room temperature, the product was extracted with CH2Cl2, washed with H2O, and filtered over anhydrous MgSO4. The solvent was removed under reduced pressure. The crude product was purified by a silica column with methylene chloride as the eluent. The precursor, dimethyl-4,4′-(anthracene9,10-diyl)-dibenzoate, was obtained after removal of the solvents. 5.00 g of the precursor was mixed with 9 ACS Paragon Plus Environment


20.0 g NaOH in 130 mL of dioxane and H2O (10:3). The mixture was heated at 90 °C for 24 h under stirring. After the reaction, the mixture was cooled to room temperature, followed by removal of the solvent under reduced pressure. The residue was mixed with H2O and stirred for 2 h. After adjusting the pH to 2 with concentrated HCl, a yellow solid precipitated out and was collected by filtration. The product, Ph2An, was washed with water, HCl (1 M) and diethyl ether, and dried under vacuum. Yield: 95%. 1H NMR (500 MHz) is shown in Figure S21 for Ph2An in DMSO-d6 with δ: 8.22 – 8.08 (m, 4H), 7.77 – 7.68 (m, 4H), 7.58 (d, J = 8.6 Hz, 4H), 7.39 (d, J = 6.5 Hz, 4H). 13C NMR (126 MHz) is shown in Figure S22 for Ph2An in DMSOd6 with δ: 168.67, 137.70, 130.16, 129.38, 128.56, 127.87, 127.21, 126.31, 121.64. Loading of the assemblies. The QD assembly was surface-synthesized on metal oxides through a layer-by-layer assembling method 54. Loading of the -NH2-CdSe assembly was carried out by immersing the oxides sequentially in 2-aminoethyl-phosphonic acid (5 mM in 0.1 M HClO4 (aq.)) and CdSe (6.74 μM in anhydrous toluene) solutions for 24 h each. The assemblies, -An (Ph2An)-CdSe, were prepared by immersing the oxides in An or Ph2An solutions (5 mM, DMSO/CH3OH) and CdSe (6.74 μM in anhydrous toluene) for 24 h each. The synthetic procedure is illustrated in Figure S1. Transient absorption measurements. Transient absorption measurements were performed with a nanosecond TA spectrometer as previously reported 55. Single wavelength TA changes of the samples upon excitation by 532 nm laser pulses (2.6 mJ/cm2/pulse) were monitored by using a digital oscilloscope. Film samples with an area of 1.0 cm2 were placed in a 1 cm pathlength quartz cuvette filled with 0.1 M LiClO4 in MeCN. The time resolved data in Figure 3 were fitted according to equations 1,2 by using the Kohlrausch-Williams-Watts (KWW) stretched exponential model 55. 𝑡

ΔAt = ΔA0 ×exp[- (𝜏0) β] 𝜏

1

1/τb = [( 𝛽0 ) ×Γ(𝛽)]-1

eq. 1 eq. 2

ΔAt and ΔA0 are the absorption changes at time t and 0, respectively, after the laser pulse. β is the stretching parameter. τ0 is the characteristic KWW relaxation time. τb is the observed recombination lifetime. Γ(x) is the Gamma function. Photoluminescence measurements. Steady-state emission spectra at room temperature were acquired with a PTI 4SE-NIR Quanta Master fluorimeter with light excitation at 560 nm (55 mW/cm2). Film samples with an area of 1.0 cm2 were prepared and placed in a 1 cm pathlength, quartz cuvette filled with 0.1 M LiClO4 in MeCN (anhydrous) in an argon-filled glovebox. The reaction cuvette was sealed with rubber septa before taking out from the glove-box. For the upconversion assemblies, the excitation spectra were obtained by controlling the emission at 410 nm. All data were acquired at an integration time of 3 s /1 nm under room temperature (20 °C). Injection yield (ηinj). Comparison of the upconversion emission integrals gives the charge injection efficiencies (ηinj) for 1An* and 1Ph2An* into the conduction band of TiO2 or the valence band of NiO. The ηinj was determined by the equation below. ηinj = 1 - (I1 × ηA0) / (I0 × ηA1) I0 is the upconversion emission integral for 1An* (or 1Ph2An*) in the assembly, -An-CdSe (or Ph2An-CdSe), on Al2O3. I1 is the emission integral for 1An* (or 1Ph2An*) in the assembly, -An-CdSe (or Ph2An-CdSe), on TiO2 or NiO. ηA0 and ηA1 are the light absorption efficiencies for the assemblies on Al2O3 are TiO2 (or NiO), respectively.


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Upconversion quantum yield (ηUC). The upconversion quantum efficiencies of 1An* and 1Ph2An* in the assembles were estimated by using Ru(bpy)32+ in a poly(methyl methacrylate) matrix as the actinometer 40, under the assumption that the quantum yield of the triplet excited state emission of 3 Ru(bpy)32+* is unity 56. The actinometer were prepared by drop-casting a solution of CH2Cl2-CH3CN (1:1) with Ru(bpy)32+ and 2 wt.% poly(methyl methacrylate) (m.w.~350,000) onto the Al2O3 film. ηUC was determined based on the equation below. ηUC = (I0 × ηAR) / (IR × ηA0) I0 is the upconversion emission integral for 1An* (or 1Ph2An*) in the assembly, -An-CdSe (or Ph2An-CdSe), on Al2O3. IR is the emission integral for the triplet excited state, 3Ru(bpy)32+*, monitored at 600 nm -800 nm. The emission spectra for both of the samples and the actinometer were acquired by excitation at 560 nm (55 mW/cm2). The absorbance of the actinometer film at 560 nm was controlled at 0.05, and the light absorption efficiency of Ru(bpy)32+, ηAR, was ~0.11. Spectroelectrochemical measurements. Spectroelectrochemistry measurements were carried out for An and Ph2An on TiO2 for oxidative spectra (spectral changes under positive applied bias) in acetonitrile with 0.1 M lithium perchlorate, and on NiO for reductive spectra (spectral changes under negative applied bias) in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate. For the spectroelectrochemistry of CdSe, the assembly, -NH2-CdSe, was surface-attached on mesoporous ITO electrode 57. All electrolyte solutions were degassed with argon for 40 min prior to experimentation. Spectral changes were monitored by a HP 8453 Photodiode array. A CHI 660E potentiostat was used to control the applied potential to the electrodes. The potential was stepped at every 40 mV for 200 seconds with UV-Vis absorption spectra taken at each step. The observed spectral changes were corrected by background subtraction with the corresponding absorptive changes from the bare TiO2, NiO or ITO electrodes. Chemical capacitance. The distributions of chemical capacitance for the ground-state An+/0, An0/-, Ph2An+/0, Ph2An0/-, CdSe+/0 and CdSe0/- were abstracted from spectroelectrochemical data 44 as described 𝐸1

above with the equation: capacitance = F × ∫𝐸0 ∂Γ/ ∂E, where F, ∂Γ and ∂E are the Faraday constant, the partial derivatives of the surface coverages of the redox species, and the derivatives of the applied bias. The distributions of capacitance for the singlet excited-states, (1An*)+/0, (1An*)0/-, (1Ph2An*)+/0 and (1Ph2An*)0/-, were obtained with the ground-state distributions and the excited state energies (ΔGES1, Table S4) of 1An* (ΔGES1 = 3.11 eV) and 1Ph2An* (ΔGES1 = 3.00 eV) based on the emission data shown in Figures S17-S18. The distributions of capacitance for the triplet excited-state (3An*)+/0, (3An*)0/-, (3Ph2An*)+/0 and (3Ph2An*)0/- were obtained with the ground-state distributions and the excited state energies (ΔGES3, Table S4) of 3An* (ΔGES3 = 1.80 eV) 45 and 3Ph2An* (ΔGES3 = 1.78 eV) 46, 47. The distributions of capacitance for the conduction and valance bands of TiO2 and NiO, respectively, were obtained from slow linear sweep voltammetry at a scan rate of 0.2 mV/s.58 ASSOCIATED CONTENT Supporting Information Supporting Information contains synthetic scheme, molar absorptivities, excitation spectra, additional emission spectra, NMR, additional tables of efficiencies and ground/excited state potentials.

AUTHOR INFORMATION



Corresponding Author * [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The research was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0015739. The experiments with nanosecond TA and fluorimeter were performed with the instruments within the AMPED EFRC Instrumentation Facility established by the Alliance for Molecular PhotoElectrode Design for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DESC0001011. The TEM experiment was performed at the Chapel Hill Analytical and Nanofabrication Laboratory, CHANL, a member of the North Carolina Research Triangle Nanotechnology Network, RTNN, supported by the National Science Foundation, Grant ECCS-1542015, as part of the National Nanotechnology Coordinated Infrastructure, NNCI. REFERENCES 1.

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Charge Transfer from Upconverting Nanocrystals to Semiconducting

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