Coalescence-Driven Simultaneous Enhancement and Quenching of

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Coalescence-Driven Simultaneous Enhancement and Quenching of the Excited States of Silver Nanoclusters Muhammad A. Abbas, Seog Joon Yoon, Rizwan Khan, Junghyun Lee, and Jin Ho Bang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02562 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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The Journal of Physical Chemistry

Coalescence-Driven Simultaneous Enhancement and Quenching of the Excited States of Silver Nanoclusters Muhammad A. Abbas,†,# Seog Joon Yoon,‡,ξ,# Rizwan Khan,§ Junghyun Lee,§ Jin Ho Bang*,†,§,ǁ †

Nanosensor Research Institute, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan,

Gyeonggi-do 15588, Republic of Korea ‡

Notre Dame Radiation Laboratory and Department of Chemistry and Biochemistry, University

of Notre Dame, Notre Dame, Indiana 46556, United States ξ

Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk-do 38541, Republic of

Korea §

Department of Bionano Technology, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu,

Ansan, Gyeonggi-do 15588, Republic of Korea ǁ

Department of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdaehak-

ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea AUTHOR INFORMATION #

These authors equally contributed to this work.

ACS Paragon Plus Environment

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Abstract

Despite the successful launch of Au nanoclusters (NCs) in light energy conversion applications, the utilization of Ag NCs has been limited by their instability under continuous illumination. The main cause of photo-induced degradation pathways remains elusive. Hence, understanding the underlying mechanism behind the low stability is an urgent task to provide a new impetus for the development of Ag NCs. The slow regeneration of Ag NCs by a redox couple leads to the build of holes in the NCs, which could result in either photoetching of NCs or transformation into plasmonic nanoparticles. Transient absorption spectroscopy reveals that Ag NCs coalesce into plasmonic nanoparticles and begin to experience two conflicting effects (plasmonic enhancement and quenching effect) during this in situ transformation. It also discloses that the quenching effect prevails over the plasmonic enhancement, which eventually leads to photocurrent loss under illumination.


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Introduction Nobel metal nanoclusters (NCs), whose size is smaller than the Fermi wavelength of an electron, possess a discrete electronic structure suitable for applications involving light energy conversion.17

An interesting application of NCs is sensitization of mesoporous TiO 2 electrodes to broaden their

absorption capabilities. These photoanodes can be used for photoelectrochemical water splitting and metal-cluster-sensitized solar cells (MCSSCs), for which Au NCs are the most widely studied NCs.3,6,8 An alternative contender, Ag NCs, has shown very limited progress for this purpose. The major problem with Ag NCs is their limited chemical and photochemical stability.4 Recently, Ag NC-based MCSSCs have been reported to achieve a power conversion efficiency (PCE) as high as 1.43%.9 Despite this advance, the best reported stability of Ag MCSSCs is still lacking compared to that of Au MCSSCs, and the underlying degradation mechanism remains largely unknown. Therefore, it is of utmost importance to identify the mechanism that causes degradation of photocurrent in Ag NC-sensitized photoelectrodes (denoted as Ag NC-TiO 2 hereafter), such that a strategy could be devised to remedy this issue. When Ag NC-TiO 2 is excited by irradiation, electrons in the NC move from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) (eq. 1). If the LUMO level is located above the conduction band of TiO 2 (the desired condition for light energy conversion applications), the electrons can be transferred to the conduction band of TiO 2 (eq. 2), which leaves the NC with a positive charge (i.e., hole). In a working MCSSC device, this charge is compensated by a redox couple (e.g., Co(II, III) complexes) to regenerate the NC. However, if regeneration is slow or does not take place, the positive charge will build up in the NC. This positive charge is detrimental to the NC because it begins to oxidize the metallic atoms, leading to photoetching of the Ag NC (eq. 3). This eventually results in reduction of NC size with a wider


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optical gap until the NCs are no longer able to absorb photons.10 If the photon energy is too large, it can lead to complete disappearance of NCs (Scheme 1).10 Otherwise, irradiation may cause photo-induced surface diffusion of the NCs, leading to coalescence of NCs to reduce surface energy and form nanoparticles (NPs).11,12 These NPs can dissipate further incoming photon energy by phonon interactions (Scheme 1, eq. 4). However, it is unclear which mechanism causing a reduction in photocurrent in Ag NC MCSSCs is dominant. Here, we studied the change in optical behavior of Ag NC-TiO 2 under continuous illumination in an attempt to pinpoint the cause of photocurrent loss under illumination. 𝐴𝐴𝐴𝐴𝑛𝑛 (𝑆𝑆𝑆𝑆)𝑚𝑚 + ℏ𝜈𝜈 ⇌ 𝐴𝐴𝑔𝑔𝑛𝑛 (𝑆𝑆𝑆𝑆)∗𝑚𝑚

(1)

∗ 𝐴𝐴𝐴𝐴𝑛𝑛 (𝑆𝑆𝑆𝑆)∗𝑚𝑚 + 𝑇𝑇𝑇𝑇𝑂𝑂2 ⇌ 𝐴𝐴𝑔𝑔𝑛𝑛 (𝑆𝑆𝑆𝑆)+ 𝑚𝑚 + 𝑇𝑇𝑇𝑇𝑂𝑂2 + 𝐴𝐴𝐴𝐴𝑛𝑛 (𝑆𝑆𝑆𝑆)+ 𝑚𝑚 → 𝐴𝐴𝑔𝑔𝑛𝑛−1 (𝑆𝑆𝑆𝑆)𝑚𝑚 + 𝐴𝐴𝑔𝑔

(2)

(3)

+ 𝐴𝐴𝐴𝐴𝑛𝑛 (𝑆𝑆𝑆𝑆)+ 𝑚𝑚 + 𝐴𝐴𝑔𝑔𝑛𝑛 (𝑆𝑆𝑆𝑆)𝑚𝑚 → 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 (4)


4

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Scheme 1. Schematic representation of photoetching (left) and coalescence (right) of Ag NCs that cause loss in photocurrent and the resulting changes in absorption spectra.

Experimental Section Synthesis of Ag NCs. Ag(0)/Ag(I)-thiolate Ag NCs were synthesized by a previously reported method.9 Briefly, 12.5 ml of 20 mM AgNO 3 solution and 7.5 ml of 50 mM reduced-glutathione solution were poured into a 500 ml flask containing 220 ml of deionized water. After 2 min of stirring, the pH was adjusted to 11 using 1 M NaOH solution. The reaction mixture was then heated to 70 °C for 1 h. After cooling to room temperature, the pH was adjusted to 3.75 with 1 M HCl, and the reaction mixture was aged for 24 h. To remove any precipitates formed, the mixture was centrifuged at 6000 rpm for 10 min. The final solution was concentrated to 50 mL by a rotary evaporator to prepare the sensitization mixture. Fabrication of Ag MCSSCs. A TiO 2 film with a 10-μm-thick mesoporous layer and a 10-μmthick scattering layer was prepared by a previously reported method.3,13 The TiO 2 film was dipped


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in Ag NC solution (at pH 3.75) for 48 h for sensitization. The Ag NCs adsorbed onto the TiO 2 surface via electrostatic interaction between the carboxylic groups in the glutathione and the TiO 2 surface. After sensitization, the Ag NC-sensitized film was sequentially rinsed with water and acetone. A working device was assembled with sputtered Pt as the counter electrode, a Co2+/Co3+ redox couple as the electrolyte, and Ag NC-sensitized TiO 2 as the photoelectrode. The electrolyte contained 0.1 M LiClO 4 , 0.5 M 4-tert-butylpyridine, 0.2 M Co[(phen) 3 (PF 6 ) 2 ]2+, and 0.03 M Co[(phen) 3 (PF 6 ) 3 ]3+ in anhydrous acetonitrile. The cobalt complexes with 1,10-phenanthroline were synthesized by a previously reported method.3,14 Characterization. UV-vis absorption spectra of Ag NCs were produced by a spectrometer (SCINCO S-3100). To measure the film absorbance spectra, the same equipment was assembled with

a

diffuse-reflector.

Photoluminescence

(PL)

spectrum

was

obtained

using

a

spectrofluorometer from Horiba Scientific (Nanolog), and PL lifetime measurements were carried out using an inverse time-resolved fluorescence microscope (PicoQuant, MicroTime-200). Current-voltage curves and photocurrent stability were measured by a source meter (Keithley 2400), while MCSSCs were illuminated using a solar simulator (HAL-320 by Asahi Spectra) that was calibrated to 1 sun using a standard silicon diode (CS-20, Asahi Spectra) prior to the experiments. Transmission electron microscopy (TEM) images were captured with a TITAN 80300 (FEI) electron microscope. Transient absorption spectroscopy (TAS). Ag NCs were adsorbed on a semi-transparent mesoporous TiO 2 film on fluorine-doped tin oxide glass to perform TAS. The samples were sealed in a vacuum cuvette to prevent interference from atmospheric species. A femtosecond laser system (Clark MXR CPA-2010) was used to generate 130 fs pulses (1 mJ/pulse with 1 kHz repetition rate), and a transient absorption spectrometer (Ultrafast Systems) was used to record the TAS


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spectra. The pump beam was 95% and the probe beam was 5% of the fundamental. The white light continuum for the probe beam was generated with a CaF 2 crystal, and the laser wavelength was 387 nm. The laser power was set to 30 mW∙cm-2.

Results and Discussion We prepared Ag NCs by the method described elsewhere.9 We employed aggregation-induced emission (AIE)-type Ag(0)/Ag(I) core/shell NCs in this work because they are stable for a fairly long time compared to other reported Ag NCs that decompose within seconds under illumination.4,15,16 These Ag NCs that can absorb photons up to 600 nm and show no plasmonic peak, indicating the presence of NCs with a discrete electronic structure (Figure S1). The featureless UV-vis absorption spectrum is a characteristic feature of AIE-type NCs.17 PL spectra with the lifetime of 111 ns (Figures S2,3 and Table S1) and TEM analysis (Figure S4) further affirmed the formation of NCs. The nature of Ag NCs employed in current work has also been characterized in details in our recent report.9 Ag NC were adsorbed onto the TiO 2 surface via electrostatic interaction, and the Ag NC-TiO 2 film showed the same absorption feature as NCs in a solution phase (inset of Figure 1A). When assembled as a working solar cell, the Ag NC-TiO 2 produced a J SC of 2.32 mA/cm2 and V OC of 790 mV with a PCE of 1.40% (Figure 1A). However, under constant illumination at 1-sun conditions, the photocurrent was reduced to 56% of the initial value after 30 min. There was no indication within the measurement time window that the current degradation would stop after a certain time (Figure 1B).


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3.0 2.5 2.0 1.5 10

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(B) 0.6

lized Current

(A)

Abs.

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

ent Density mA.cm-2)


1.0 0.8 0.6 04

Figure 1. (A) Current-voltage characteristics and (B) photocurrent stability of Ag NC-sensitized MCSSC under 1-sun (air mass 1.5 global) illumination. Inset of (A) shows the absorbance of Ag NC-TiO 2 film. The change in excited state behavior of Ag NC-TiO 2 provides important insights into the origin of the decrease in photocurrent. Therefore, we carried out TAS to observe the change in its excited state dynamics under illumination.18 To accelerate photo-degradation and eliminate any external factors, light irradiation was carried out in a vacuum in the absence of hole-scavenging media. A freshly prepared Ag NC-TiO 2 sample showed a positive transient signal after 500 fs, indicating formation of excited states due to intra-band transitions (Figure 2A).19-21 There was no indication of the presence of any negative bleaching signal corresponding to formation of plasmonic NPs.22,23 Therefore, it was ascertained that the freshly prepared photoelectrode only contained moleculelike Ag NCs. Analysis of the decay curve at 600 nm revealed the presence of two lifetime components (Figure 2B): a short lifetime of only 2.52 ps and a dominant long lifetime of 834 ps (Table 1). The short component is usually attributed to relaxation of higher energy states from the metallic core of the NC to long-lived energy states in the ligand shell.19,24 On the other hand, the long-lived component is comprised of radiative or non-radiative charge decay from the long-lived


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excited state to the ground state.19,24 In the current case, however, the NCs were adsorbed on the TiO 2 surface, and the excited electrons were transferred to the TiO 2 conduction band. This charge transfer event further shortened the lifetime of Ag NCs compared to their intrinsic lifetime.9,20,25 (A)

(B)

5

1.0

a

Normalized ∆A

∆A × 10-3

4 3 2 a b c d e f

1 f 0 400

500

500 fs 1 ps 10 ps 100 ps 500 ps 956 ps

600

0.8 0.6 1

0.4 0.2 0

0

700

0

0

100

300

600

W avelength (nm)

h 6 a b c d e f g h

-1

h 500

0 min 5 min 10 min

∆A × 10-3

0

400

1200

(D) 8 a

-2

900

Time (ps)

(C) 1

∆A × 10-2

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


20 min 30 min 60 min 120 min

600

4 2

700

0 400

W avelength (nm)

a a b c d

500


e f g h

600


700

W avelength (nm)

Figure 2. (A) Difference absorption (ΔA) spectra of Ag NC-TiO 2 film at various time scales. (B) Kinetic decay of ΔA in a fresh sample at 600 nm (inset shows the first 100 ps of the kinetics trace). (C) ΔA spectra of Ag NC-TiO 2 samples after various exposure times measured at 500 fs and (D) zoom-in spectra of (C) to show the positive transient signal.


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Table 1. Calculated Lifetimes (τ) from Transient Traces at 420 and 600 nm, Respectively. Exposure Kinetics at 420 nm Time τ avg (ps)

(min)

a

Kinetics at 600 nm A1

τ 1 (ps)

A2

τ 2 (ps)

τ avg (ps)a

0

-

0.37 ± 0.04

2.52 ± 0.29

0.63 ± 0.07

834.3 ± 147 527.2 ± 110.2

5

1.12 ± 0.04

0.72 ± 0.04

1.29 ± 0.06

0.28 ± 0.02

513.4 ± 63

145.9 ± 19.9

10

1.22 ± 0.03

0.81 ± 0.05

0.93 ± 0.04

0.19 ± 0.01

388.3 ± 48

73.5 ± 10.2

15

1.15 ± 0.03

0.83 ± 0.04

1.13 ± 0.04

0.17 ± 0.01

228.1 ± 26

39.3 ± 4.9

20

1.30 ± 0.02

0.87 ± 0.05

0.88 ± 0.04

0.13 ± 0.01

69.5 ± 10

10.1 ± 1.5

30

1.38 ± 0.02

0.89 ± 0.03

0.80 ± 0.02

0.11 ± 0.00

48.1 ± 6.7

5.8 ± 0.8

60

1.31 ± 0.02

0.91 ± 0.02

0.92 ± 0.03

0.09 ± 0.00

40.2 ± 5.3

4.14 ± 0.5

120

1.25 ± 0.02

0.92 ± 0.03

0.96 ± 0.03

0.08 ± 0.00

23.5 ± 3.4

2.9 ± 0.4

The average lifetimes were calculated using the following equation: ∑ 𝐴𝐴𝑖𝑖 𝜏𝜏𝑖𝑖 / ∑ 𝐴𝐴𝑖𝑖

Under continuous exposure to the laser source, a bleaching signal started to appear around 420 nm, which is a clear indication of formation of plasmonic Ag NPs (Figure 2C).26,27 This bleaching signal continued to gain strength with prolonged laser exposure, indicating that the number of NPs present on the TiO 2 surface was increasing (Figure 2C and Figure 3). It should be noted that this change is in sharp contrast to the observation of Kogo et al.,10 who showed that Ag NC-TiO 2 became photoetched to form smaller Ag NCs until their absorbance edge was lower than the energy of the excitation source. The higher energy excitation source caused the NCs on the TiO 2 surface to eventually disappear. However, our results showed the opposite behavior of Ag NCs. We speculated that this could be the result of the distances among the Ag NCs. Our experiment utilized a mesoporous TiO 2 film with a high NC loading where the average distance among NCs was ~10


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nm (Figure 4A and 4B). In contrast, the Tatsuma group used a single crystal TiO 2 with a very low NC loading with distances among Ag NCs of ~200 nm.10 Under this circumstance, the Ag NCs may not have been able to diffuse such a large distance and eventually decomposed to smaller NCs or Ag ions.

0.2 0

f

-0.2 -0.4 400

a b c d e f

500 fs 1 ps 2 ps 10 ps 100 ps 500 ps

500

600

0.8 0.6 0.4 0.2 0

700

1.0

1.0

Norm alized ∆A

a

0.4

Norm alized ∆A

∆A × 10-2

(A)

0

3

Wavelength (nm) a

Norm alized ∆A

∆A × 10-2

-0.3

a b c d e f

f

-0.6 -0.9 400

500 fs 1 ps 2 ps 10 ps 100 ps 500 ps

500

600

∆A × 10-2

-1.0 -1.5 400

500 fs 1 ps 2 ps 10 ps 100 ps 500 ps

500

600

700

0.6 0.4 0.2 0

3

∆A × 10-2

f

-1.0 -1.5 400

500 fs 1 ps 2 ps 10 ps 100 ps 500 ps

500

600

700

-0.5 -1.0 -1.5 400

500

500 fs 1 ps 2 ps 10 ps 100 ps 500 ps

600

700

0.4 0.2 0

3

0 f

-2.0 400

a b c d e f

500

500 fs 1 ps 2 ps 10 ps 100 ps 500 ps

600

700

-2.0 400

500

a b c d e f

500 fs 1 ps 2 ps 10 ps 100 ps 500 ps

600

Wavelength (nm)

12

0.4 0.2 0

3

6

9

12

0.8

0.4 0.2 0

700

300

600

900 1200

0.8 0.6 0.4 0.2 0

15

0

300

600

900 1200

Tim e (ps) 1.0

1.0 0.8 0.6 0.4 0.2 0

3

6

9

12

0.8 0.6 0.4 0.2 0

15

0

300

600

900 1200

Tim e (ps) 1.0

1.0 0.8 0.6 0.4 0.2 0

3

6

9

12

0.8 0.6 0.4 0.2 0

15

0

300

600

900 1200

Tim e (ps) 1.0

1.0 0.8 0.6 0.4 0.2 0

900 1200

1.0

0.6

0

600

Tim e (ps)

0.8

0

300

0.6

0

15

Norm alized ∆A

Norm alized ∆A

a

f

0

Tim e (ps)

0 -1.0

9

1.0

Wavelength (nm)

(G) 1.0

6

Norm alized ∆A

a

-1.0

0.2

Tim e (ps) Norm alized ∆A

∆A × 10-2

1.0

0.4

Tim e (ps)

0.6

Wavelength (nm)

(F)

0.6

0

15

Norm alized ∆A

∆A × 10-2

a b c d e f

Norm alized ∆A

a

0.5

900 1200

0.8

Tim e (ps)

(E) 1.0

f

12

0.8

0

600

1.0

Wavelength (nm)

0

9

Norm alized ∆A

Norm alized ∆A

a

0.5

-0.5

6

1.0

0

300

Tim e (ps)

Tim e (ps)

(D) 1.0

a b c d e f

0

1.0

Wavelength (nm)

0

0

Norm alized ∆A

Norm alized ∆A

a

a b c d e f

0.2

Tim e (ps)

0.5 f

0.4

15

0.8

Wavelength (nm)

-0.5

12

1.0

0

700

(C) 1.0 0

9

0.6

Tim e (ps)

0.3 0

6

0.8

Norm alized ∆A

(B) 0.6

∆A × 10-2

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


0

3

6

9

12

15

Tim e (ps)

0.8 0.6 0.4 0.2 0

0

300

600

900 1200

Tim e (ps)


11


Page 12 of 21

Figure 3. Difference absorbance spectra (1st column), kinetic traces at 420 nm (2nd column), and kinetic traces at 600 nm (3rd column) at (A) 5 min, (B) 10 min, (C) 15 min, (D) 20 min, (E) 30 min, (F) 60 min, and (G) 120 min of laser exposure time. Spectra at longer times than 500 ps are not shown for clarity.

Further confirmation of the nature of these new particles was obtained by observing the decay rate of the ΔA signal at 420 nm. The decay at 420 nm can be clearly fit to a mono-exponential function, which produced a lifetime of ~1 ps under all exposure times (Figure 3, Table 1). Therefore, the nature of the plasmonic NPs did not change with prolonged exposure. This lifetime is similar to the short lifetime component of plasmonic NPs, which has been assigned to the electron-phonon interaction in Ag NPs.22,23 Interestingly, we did not observe the relatively longer lifetime component of Ag NPs from the bleaching signal, which was likely the result of interference from the transient signal of Ag NCs because our system contained both NCs and NPs. Indeed, the Kamat group revealed recently that the presence of NCs and NPs in the same vicinity leads to a synergistic interaction that can enhance the excited-state signal of the photoelectrode.27 In our system, increased exposure time caused the formation of Ag NPs that would result in a decrease in the number of Ag NCs on the TiO 2 , which should decrease the positive transient signal in the wavelength range (>550 nm) where Ag NPs are optically inactive.27 However, we observed a continuous increase in the transient signal with the increase in exposure time even in that range (Figure 2D and Figure 3). Other materials are known to experience a similar excited state enhancement when in the vicinity of plasmonic NPs;28,29 this is known as the plasmonic enhancement effect.30 Therefore, the increase in positive ∆A signal in our experiment resulted from plasmon-induced enhancement of the excited states of Ag NCs.


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At a quick glance, this increased excited state signal should positively influence the photocurrent of Ag MCSSC; however, that was not the case in our study (Figure 1). The reason behind this discrepancy was revealed by analyzing the decay signals at 600 nm (Figure 3), which is distant from the transient signal of the plasmonic NPs.27 The freshly prepared Ag NC-TiO 2 sample showed two lifetime components of 2.52 and 834 ps with 0.37 and 0.63 contributions, respectively (Table 1). However, a 5 min exposure time caused the long lifetime component to decrease to 513 ps. The short lifetime component showed no significant change, but its contribution increased from 0.37 to 0.72. As evident from the average lifetime at 600 nm, the excited state lifetime of Ag NCs decreased dramatically within the 120 min exposure, with values ranging from 527.2 ps to 2.9 ps (Table 1, Figure S5). Given that direct excitation of Ag NCs usually yields a lifetime in the ns to µs range,9,20,21 this rapid decline in excited state lifetime of the NCs could be due to quenching by nearby Ag NPs since NPs are a good quencher of excited states.28,31,32 From this unfortunate consequence, any gain made by the excited states due to plasmonic enhancement was nullified, which led to swift deterioration of the excited state lifetime of the NC upon prolonged laser exposure. C)

60

Count

45 30 15

F)

40 30

Count

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


20 10

Figure 4. TEM images (A-B) before and (D-E) after irradiation. Particle size histograms (C) before and (F) after illumination.


13


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TEM analysis of Ag NC-TiO 2 performed before and after illumination further support our claim. The TEM images before irradiation showed the presence of particles with an average size of 2.22 nm on the TiO 2 surface (Figure 4A-C). After the TAS experiment, the Ag NC-TiO 2 film was scratched in the area where the laser was focused to observe the change in NC size and the formation of plasmonic NPs. After illumination, the average particle size increased to 2.77 nm with the distribution ranging from 1 to 8 nm (Figure 4D-F). The increased particle size observed by the TEM analysis was consistent with our assertions from the TAS analysis. Furthermore, the UV-vis spectra of the Ag NC-TiO 2 films before and after continuous illumination were also consistent with the absorption trend, as anticipated in Scheme 1 (Figure S6). The absorption of Ag NC-TiO 2 film increased substantially over the whole absorption range, indicating the formation of larger nanoparticles. However, when Ag NCs sensitized on ZrO 2 film were exposed to the same irradiation conditions, there was no significant change in the absorption of the sample (Figure S7), affirming our assertion that the hole in the HOMO level of the NCs may play a key role in the degradation of the Ag NCs.

Conclusions Stability is a prominent issue for utilization of Ag NCs in light energy-conversion applications. The continuous illumination of Ag NCs on TiO 2 cause them to combine via photo-induced surface diffusion to form plasmonic NPs in an irreversible manner. Although the plasmonic NPs do enhance the excited states of Ag NCs via the plasmonic enhancement effect, the in situ formed Ag NPs also act as recombination centers by quenching the excited states. Unfortunately, the kinetics of the transformation of NCs into NPs for Ag is so rapid that quenching becomes a dominant process. Therefore, plasmon-induced enhancement of the excited states can hardly be exploited in


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the current working device (due to slow hole scavenging), and the continual transformation of NCs to NPs eventually leads to photocurrent degradation. While the interaction between Ag NCs and the in situ transformed Ag NPs seems to be negative, we envision that the plasmonic enhancement effect could be harnessed in a positive way if the coalescence of Ag NCs to NPs could be stopped at an optimum point. Efforts (e.g., a surface treatment to stabilize Ag NCs) are currently underway to exploit the plasmonic enhancement effect to increase photocurrent.

ASSOCIATED CONTENT Supporting Information. UV-vis absorption spectrum of Ag NCs in solution, PL spectra of Ag NCs and PL lifetime decay results, TEM images of Ag NCs, average excited state as a function of laser exposure time, and UV-vis absorption spectra of Ag NC-sensitized TiO 2 and ZrO 2 films before and after 20 min of continuous illumination. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *[email protected] Author Contributions MAA and SJY contributed equally to this work. Notes The authors declare no competing financial interests.


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Acknowledgment This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2019R1A2C1003429 and NRF-2018R1E1A2A02086254) and by the Ministry of Education (NRF-2018R1A6A1A03024231). S. J. Yoon acknowledges the 2019 Yeungnam University Research Grant. This is contribution number NDRL No. 5237 from the Notre Dame Radiation Laboratory, which is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. We acknowledge Prof. P.V. Kamat for his helpful discussion.

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Coalescence-Driven Simultaneous Enhancement and Quenching of

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