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Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles – Key Evidence from Action Spectrum of the Reaction Sarina Sarina, Esa A Jaatinen, Qi Xiao, Yiming Huang, Phillip Christopher, Jincai Zhao, and Huaiyong Zhu J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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The Journal of Physical Chemistry Letters

Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles –Key Evidence from Action Spectrum of the Reaction Sarina Sarina,a Esa Jaatinen,a Qi Xiao,a,b Yi Ming Huang,a Philip Christopher,c Jin Cai Zhao,d Huai Yong Zhua,* a

School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Brisbane, QLD 4001, Australia b

CSIRO Manufacturing, Bayview Avenue, Clayton, VIC 3168, Australia

c

Department of Chemical & Environmental Engineering, University of California, Riverside,

Riverside, California 92521, United States d

Key Laboratory of Photochemistry, Institute of Chemistry, the Chinese Academy of Sciences,

Beijing 100190, China.

AUTHOR INFORMATION Corresponding Author *Prof. Dr. Huai Yong Zhu. Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT. By investigating the action spectra - relationship between irradiation wavelength and apparent quantum efficiency of reactions under constant irradiance - of a number of reactions catalyzed by nanoparticles including plasmonic metals, non-plasmonic metals and their alloys at near ambient temperatures, we found that a photon energy threshold exists in each photocatalytic reaction: only photons with sufficient energy (e.g. higher than the energy level of the lowest unoccupied molecular orbitals) can initiate the reactions. This energy alignment (and the photon energy threshold) is determined by various factors, including wavelength and intensity of irradiation, molecule structure, reaction temperature etc. Hence, distinct action spectra were observed in the same type of reaction catalyzed by the same catalyst, due to a different substituent group, or a slightly changed reaction temperature etc. These results indicate that photon-electron excitations, instead of photo-thermal effect, play a dominant role in direct photocatalysis of metal nanoparticles for many reactions.

TOC GRAPHICS

KEYWORDS Wavelength dependent photocatalytic activity, Apparent quantum efficiency, Plasmonic metals, Driving force of photocatalyst, Energy alignment


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Nanoparticles (NPs) of transition metals, such as Au, Ag, Cu;1-7 Pd, Pt, Rh, Ir, Ru and their alloys,8-10 dispersed on optically and catalytically inert materials (ZrO2, Al2O3 etc.) have been found as efficient photocatalysts. The NPs exhibit strong optical absorption over the entire solar spectrum, and efficiently channel the photon energy into molecules that are adsorbed on their surfaces and initiate chemical transformations.4,11 This process can significantly promote catalytic activity at near ambient temperature and pressure.5 Direct photocatalysis by metal NPs has inspired the rapid expansion of the field of green photocatalysis, which is a promising alternative to the many heat driven reactions currently using thermal catalysts.1-18 As a result, it is of great interest to fully understand the mechanisms underpinning direct photocatalysis on metal NP surfaces. When the incident photon energy is less than the work function of metal, Φ, the incident light excites conduction electrons in metal NPs to higher energy levels by two pathways depending on the wavelength. Single-photon excitation (one incident photon excites one metal electron-hole pair) occurs in all metal particles. The excited metal electrons will rapidly thermalize by successive electron-electron scattering with the high density of free electrons in the metal.9,19 This results in an initial distribution of hot electrons with energies in the range EFermi < Ee < EFermi + Φ, where EFermi is the Fermi level of the metal. The single-photon excitation and subsequent electron-electron scattering occurs regardless of the metal NP’s physical properties (particle size, morphology etc.) due to the near-continuum of electron energy levels possessed by the metal. The maximum energy reached by the hot electrons and the rate at which the Fermi-Dirac electron distribution cools via electron-electron scattering, however, is determined by the incident photon energy or wavelength. Irradiation can also induce a collective oscillation of the conduction electrons in some metal NPs, when the light frequency is resonant


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with the metal’s plasma oscillation frequency, with the electrons gaining the light energy through the localized surface plasmon resonance (LSPR) effect.1-7 The LSPR absorption depends strongly on the properties of metal NPs, such as the dielectric coefficient and particle morphology.20 Nanoparticles made from Au, Ag, Cu and Al strongly absorb visible light through the LSPR effect, characterized by a strong absorption peak, and as a result, these metals are often referred to as plasmonic metals.18,20-27 Following a very short coherency lifetime of the LSPR, plasmons will decay resulting in electron excitations, similar to the single photon excitation process described above, albeit with much higher cross-sections. Over time, the hot free electron gas relaxes to lower energy levels through electronphonon scattering that distributes the absorbed energy to the larger thermal mass of the NP lattice. When high intensity light is applied, plasmonic metal NPs absorb a significant amount of light energy and the resulting hot NP has advantageous uses in some applications such as the photo-thermal effect, plasmonic photo-thermal therapy and plasmonic autoclaves.22,27-29 Initially, the photo-thermal effect was considered as the responsible or sole driving mechanism for direct photocatalysis involving metal NPs.30 In our earlier study, we also assumed that reactions on gold NPs could have been driven predominantly by a photo-thermal effect.1 However, further analysis reveals that when a metal NP of 5 nm in diameter is exposed to moderate light intensities (~ 0.5 W·cm-2, about five times of mean irradiance of sunlight, which is usually applied to photocatalytic reactions), and assuming that the whole photon energy absorbed by this NP converted to heat, the resulting temperature increase is about 1 K.31,32 This suggests that the photo-thermal effect plays a minimal role in direct photocatalysis of metal NPs when using low intensity continuous wave excitation sources.


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It is often noted that the photocatalytic performance of metal NPs depends exponentially on reaction temperature,4,5,8,15,31 while semiconductor photocatalysts generally do not show this dependence. Temperature can positively influence catalysis by providing increased driving force to overcome activation barriers by populating higher lying vibrational states along the reaction coordinate. For semiconductors, the minimal influence of temperature is because increasing temperatures also increases the rate of electron-hole pair recombination.33 The direct photocatalysis by supported metal NPs is only influenced by temperature through increased population of high lying vibrational states, because higher temperature doesn’t significantly modify hot electron and hole lifetimes, thus minimizing the required photocatalytically mediated non-adiabatic energy gain required to drive the reaction.15 Nevertheless, the temperature dependence of photocatalytic performance of metal NPs is often confused with photo-thermal effect, causing difficulty in understanding the mechanism of photocatalysis of the metal NPs. While it is known that direct photocatalysis of metal NPs is influenced by multiple effects,31 the underlying mechanisms are not fully understood and significant confusion exists. Experimentally distinguishing the role of photo-thermal effect, hot electron transfer and photonelectron excitation of hybridized metal-molecule states, will help shed light on the reaction mechanisms, but is a challenging undertaking. It is known that the hot electrons at higher energy levels of the Fermi-Dirac distribution, or non-thermal distribution at very short time scales after photoexcitation of the metal, could transfer the necessary energy to the adsorbed species on metal NPs and initiate reactions of the species.2,4,8,12,15 We note that an energy alignment is required for the reactions mediated by the hot electrons. Such energy alignment requirement is different from reaction to reaction and depends closely on reaction conditions that influence the rate limiting step of the reaction, for


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which the energy state population will drive the system along the reaction coordinate. Since the energy state of the photoexcited electrons is mainly determined by irradiation wavelength, it is possible to gain insight into the mechanism of direct photocatalysis of metal NPs from analysis of the impact of irradiation wavelength on the photocatalytic reactions catalyzed by metal nanoparticles. In this study, we attempt to clarify the situation by investigating the dependence of photocatalytic activity on the irradiation wavelength for many reactions directly photocatalyzed by a variety of metal NP photocatalysts strictly under the regime of low intensity continuous wave excitation. We analyze the dependence of photocatalytic activity in many photocatalytic reaction systems under illumination of different wavelengths, using seven catalysts in total (NPs made from six different metals, including Au, Ag, Pd, Pt, Rh, Ir and Au-Pd alloy). The performances and structure information of some of the catalysts were reported in our previous studies.8,34,35 To avoid the interference from light absorption by the support solids, metal NPs were supported on photocatalytically inert support materials (such as ZrO2, Al2O3 etc.) that have negligible light absorption at the wavelengths used. The metal NP catalysts were prepared by reducing the corresponding metal salt with NaBH4 in the presence of the support solids, and abbreviated as M@support. Light emitting diodes (LED) with narrow emission bands (365±5 nm, 400±5 nm, 470±5 nm, 530±5 nm, 590±5 nm, 620±5 nm) are used as the light sources for all reactions. The reaction temperature and irradiation intensities are kept constant for the reactions to ensure the impact of the external heating on each reaction remains identical.


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We use action spectra (the wavelength dependence of photocatalytic apparent quantum efficiency, AQE) in the present study to analyze the contribution of the different effects. By measuring the wavelength dependence of AQE we reveal the photon energy threshold; above which significant photocatalytic activity is observed. Analysis of the action spectra, the shape of the spectral dependence, allows us to distinguish reactions mediated by energetic electrons and those mediated by plasmon-induced heating, as the spectral dependence of heating induced catalytic processes should be identical regardless of reaction or conditions. The action spectra shapes also shed light on the nature of how energy is transferred to molecular species. The photocatalytic reaction rates under illumination are converted into AQE. AQE is the number of reactant molecules converted by each photon absorbed by the metal NPs, expressed as a percentage at each wavelength:

AQE% =

Y − Y × 100 the number of incident photons

where Ylight and Ydark are the number of reactants converted with and without light irradiation respectively. This metric directly measures the influence of the light only, as the number of reactants converted in the dark at the same temperature is deducted. Action spectra of plasmonic metal nanoparticles It is generally expected that the photocatalytic performance of a metal NP catalyst will follow its light absorption spectrum: the greater the light absorption, the higher the photocatalytic activity. However, we found that the action spectra of one reaction can differ due to small changes in moieties on the reactants. Figure 1 shows the action spectra of Sonogashira crosscoupling reactions of phenylacetylene and iodobenzene with different substituent groups.


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Figure 1. Different action spectra shown on Au-Pd alloy NP photocatalysts. The dash line is the light absorption of Au-Pd alloy NP photocatalyst and solid line is the absorption of Au NPs dispersed on the same ZrO2 support. Left axis are normalized absorption intensity of metal NP photocatalysts (lines) and right axis are normalized AQE of reactions (symbols). a: Sonogashira coupling of phenylacetylene with 1-iodo-4-methoxybenzene (4-OCH3-PhI) catalyzed by Au-Pd alloy NPs, show an action spectrum follows the light absorption of Au-Pd alloy NPs; b: Sonogashira coupling of phenylacetylene with 1-iodo-4-nitrobenzene (4-NO2-PhI) catalyzed by Au-Pd alloy NPs exhibits an action spectrum does not follow the light absorption of Au-Pd alloy NPs but follows absorption of Au NP. Reaction conditions are listed in Experimental section.

In these two reactions, light absorption by the catalysts - Au-Pd alloy NPs - is same (the dashed line) and all other reaction conditions are identical, but the reactant aryl iodides possess different substituent groups. When we used 1-iodo-4-methoxybenzene the action spectrum matches to the light absorption of Au-Pd alloy NPs (dash line, Figure 1a). Nonetheless, when 1iodo-4-nitrobenzene instead of 1-iodo-4-methoxybenzene, was the reactant, the action spectrum does not follow the light absorption spectrum of the Au-Pd NPs (dash line in Figure 1b). The highest AQE was observed at 530 nm, even though light absorption of Au-Pd alloy NP (dash


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line) at this wavelength was weaker than that at shorter wavelengths. We found that this action spectrum matches the light absorption of Au NPs instead of Au-Pd alloy, which shows an intensive LSPR absorption peak at about 530 nm. Evidently, in the Sonogashira coupling of 1iodo-4-nitrobenzene (Figure 1b), the LSPR absorption of the catalyst plays a more dominant role than the absorption at shorter wavelengths (365 nm and 400 nm). In contrast, the shorter wavelength photons more effectively drive the reaction when 1-iodo-4-methoxybenzene used as the reactant (Figure 1a). The mismatch of AQE spectral dependences with the most intensive light absorption of metal NP catalysts demonstrates directly that the major driving force of direct photocatalysis in these cases is not a simple photo-thermal effect. The –NO2 group is an electron-withdrawing group that increases the activity of iodobenzene in reaction with the nucleophilic reagent alkyne, and –OCH3 is an electron-donating group that decreases the activity. We deduce that the different observed action spectrum behaviors are a result of the existence of an energy threshold for inducing the carbon−iodine bond cleavage, and the particular substituent group of the reactants influences the threshold. Scheme 1 illustrates reactant molecules for the Sonogashira coupling with reactants of 1-iodo-4-nitrobenzene and 1-iodo-4-methoxybenzene, respectively. The energy of the lowest unoccupied molecular orbital (LUMO) of 1-iodo-4methoxybenzene is ~2.1 eV higher than that of 1-iodo-4-nitrobenzene, and thus light with shorter wavelengths are required to efficiently drive the reaction with 1-iodo-4-methoxybenzene. This indicates that shorter wavelengths are required to generate a significant population of hot electrons that have sufficient energy to transfer to the LUMO of 1-iodo-4-methoxybenzene. The result shown in Figure 1 demonstrates that the rate of direct photocatalytic conversions at each different wavelength is determined not only by the absorption spectrum of the photocatalyst, but


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also by the energy level of relevant molecular orbital of the reactants, which in turn, are dependent on the chemical bonds being activated. The photon energy threshold in reactions involving hot electron transfer depends on energy level alignment between hot electron energy (and therefore photon energy) and molecular orbitals of reactants.

Scheme 1. Energy levels of hot electrons generated by a short wavelength and a long wavelength excitation of the Au-Pd alloy NP, LUMO and HOMO orbitals of 1-iodo-4methoxybenzene (on the left hand side) and 1-iodo-4-nitrobenzene (on the right hand side) molecules. Longer wavelength irradiation (at LSPR absorption range, blue area) can drive hot electron transfer to the LUMO orbitals of 1-iodo-4-nitrobenzene only, while shorter wavelengths are required to produce hot electrons with sufficient energy to transfer to the LUMO orbitals of 1-iodo-4-methoxybenzene. As the adsorption of reactant molecules reduces the energy gap between their LUMO and HOMO orbitals,22 the relative positions of LUMO and HOMO with respect to the Fermi level are only qualitative and schematically show the relative positions for the reactants.

The plasmonic metal NPs catalyzed photocatalytic reactions include two light absorption mechanisms: 1) short wavelength (e.g. 400 nm) absorption via single electron excitation (also


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called interband excitation); and 2) LSPR absorption (e.g. the adsorption at 530 nm for Au NPs).36 Reactant molecules on the metal NP surface with different LUMO energy levels are indicated in the Scheme 2: high energy LUMO (a and b) and low energy LUMO (c and d). When irradiated with shorter wavelength (e. g. 400 nm), single-photon excitation generates hot electrons. Nonetheless, the number of the hot electrons with sufficient energy to inject into the higher LUMO (Scheme 2a) is much less than that able to inject into lower LUMO (Scheme 2c). This is the simplest situation involving only light absorption of short wavelength by single electron excitation.

Scheme 2. Hot electron distribution of a plasmonic metal or its alloy NPs under irradiation of different wavelengths and their contribution to AQE in different reactions. a: irradiation


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with light of 400 nm wavelength, only hot electrons located above the LUMO level (area between two red dash line) are able to contribute to AQE. b: irradiation with light of 530 nm wavelength (plasmonic wavelength of Au NPs in this study), hot electron distribution area above LUMO level is much smaller than a. c: irradiated with 400 nm wavelength on the NP and a reactant molecule with lower LUMO, more hot electrons can contribute to AQE. d: irradiated with 530 nm wavelength on the NP and a reactant molecule with lower LUMO,, the hot electrons able to contribute to AQE is the area above LUMO, involving hot electrons excited by both wavelengths.

When irradiated with longer wavelengths that drive LSPR excitation in the metal NP, the population of hot electrons is significantly increased, as shown in Scheme 2b and 2d, compared with that produced from absorption of non-LSPR wavelength light (even at short wavelengths) (Scheme 2a and 2c). The LSPR excitation is the collective excitation of the conduction electrons of metal NPs by the resonant incident light. The number of the hot electrons generated by the LSPR can be very different from the number of photons of the incident light. It is rational that more hot electrons generated by the same quantity of light energy absorbed by LSPR effect than that generated by single-photon excitation but the hot electrons generated by LSPR effect are at lower energy levels and can only induce the reactions with lower energy thresholds. Hence, in a photocatalytic reaction with lower LUMO (Scheme 2d), hot electrons produced from LSPR decay can effectively drive the reaction and in this case, the action spectra of plasmonic metal NPs are expected to align well with the light absorption spectra, with higher AQE values observed at LSPR wavelengths, such as Figure 1b. If the reactant molecule has a relatively higher energy LUMO, as shown in Scheme 2b, most of the hot electrons excited by LSPR absorption are unable to transfer to LUMO as they have insufficient energy. As a result, we


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observe a lower AQE at the long wavelength, as Figure 1a. Therefore, for reactions with high photon energy threshold, light at shorter wavelengths are more effective in driving the reactions than light at LSPR peak wavelength, as most of the hot electrons yielded though LSPR absorption have energies below the threshold. For reactions with a low photon energy threshold, the observed AQE at the LSPR peak wavelength dominates because the strong absorption yields a large number of hot electrons that have sufficient energy for transfer to the reactant. Consistent with this, Toker et al. reported that excitation with LSPR wavelength on Ag NP cannot increase photocatalytic activity to activation of C-Cl bond in ethyl chloride, while the short wavelength (266 nm for example) can.37 This is due to that the ethyl chloride LUMO level is relatively high and able to be activated by the high energetic electrons (excited by short wavelength) only. We achieved much lower conversion rate of bromobenzene and chlorobenzene on Au-Pd alloy NP photocatalyst, due to the significant higher bond energies of C-Cl and C-Br bonds than C-I bond. To investigate the impact of irradiation wavelength to the reaction clearly, considerable conversion rate of reactant is necessary; thus we chose iodobenzene as model reactant. In addition to the reactions shown in Figure 1, another pair of the comparable reactions is shown in Figure 2. The action spectra for Au-Pd alloy NP@ZrO2 catalyzed Sonogashira coupling and Suzuki coupling (coupling reactions of iodobenzene and phenylboronic acid) are distinctly different even though the reactant is 3-methyl-iodobenzene for both reactions. Sonogashira coupling has lower photon energy threshold for activation of the reaction. This is attributed to the other reactant – phenylacetylene – having a strong affinity with Au.13 Thus, both reactants have an easier activation on Au-Pd alloy NPs. In comparison, the other reactant of Suzuki reaction, phenylboronic acid, has a relatively weak affinity to the NPs, and thus a higher energy threshold for activating this reaction is expected. The results indicate that the threshold


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may depend on multiple factors, which are directly related to the energetic position of the adsorbate orbital involved in charge transfer.

Figure 2. Different action spectra shown on Au-Pd alloy NP photocatalysts. a: Sonogashira coupling show an action spectrum that follows plasmonic absorption of Au NPs (solid line). b: Suzuki coupling show an action spectrum that follows light absorption of Au-Pd alloy NPs (dash line). Left axis are normalized absorption intensity of metal NP photocatalysts (lines) and right axis are normalized AQE of reactions (symbols). Reaction conditions are listed in Experimental section.

In a recent study of Au-Cu alloy NP mediated photocatalytic reduction of nitrobenzene and 4-CH2OH substituted nitrobenzene (4-nitrobenzyl alcohol),35 we also found that the action spectra can be distinctly different even though the same catalyst was used. The above argument on energy alignment requirement is applicable to these cases. The LUMO of 4-CH2OH substituted nitrobenzene is higher than that of nitrobenzene, resulting in higher threshold for activating the molecules. For reactions driven by photothermal effect, there should be not such a threshold because metal NPs have continuous electronic energy levels and can absorb irradiation of all wavelengths to be heated.


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We found a photon energy threshold exits in many reactions. Data summarized in Table 1 show the wavelength dependence of AQE for several other reactions catalyzed by Au NPs and Au-Pd alloy NP. Irradiation with 590 nm wavelength is not able to activate Cl-substituted benzylamine for oxidation (entry 1) but able to activate benzylamine (entry 2); and benzyl alcohol can be activated by the longest considered wavelength, 620 nm (entry 3) while benzylamine cannot (entry 2). Furthermore, the highest AQE of benzyl alcohol oxidation (entry 3) and benzylamine oxidation (entry 2) is found at LSPR wavelength (530 – 590 nm) while that of Cl-substituted benzylamine is found at short wavelength only (365 nm, see entry 1). Table 1 Apparent quantum efficiency (AQE) at different wavelengths in LSPR based photocatalytic reactions catalyzed by Au-Pd alloy and Au NPs.

Wavelength (nm)

365±5

Entry

Photocatalysts and reactions

AQE at different wavelength

1

Au-Pd alloy @ ZrO2 Benzylamine oxidative coupling (-Cl)

2

Au-Pd alloy @ ZrO2 Benzylamine oxidative coupling

3

Au-Pd alloy @ ZrO2 Benzyl alcohol dehydrogenation

4

Au NP @ CeO2 Acetophenone hydrogenation

5

Au NP @ CeO2 Styrene hydrogenation

400±5

470±5

530±5

590±5

620±5

0.02%

0.02%

0.01%

0.01%

0

0

0.02%

0.01%

0.02%

0.03%

0.03%

0

1.12%

0.56%

0.98%

1.00%

1.05%

0.26%

0.21%

0.12%

0.09%

0.16%

0.07%

0

0.15%

0.11%

0.05%

0.17%

0

0

Reactions that have a low photon energy threshold and catalyzed by Au NPs, as shown in entry 4-5, the reaction rates under short wavelengths (365 nm and 400 nm) are relatively high (entry 4), but comparable to that under the LSPR absorption wavelength, 530 nm (entry 5). This series of results is a clear demonstration that a complete understanding of the energy of targeted


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molecular orbitals associated with driving rate-limiting steps in the reaction must be achieved before photocatalytically enhance the processes. Christopher et al. theoretically predicted similar phenomenon in small gas molecule – metal nanoparticle surface adsorbate system.38 These cases demonstrate convincingly that catalytic reactions are not driven by photothermal effect of the catalyst as the effect depends on the light absorption of photocatalysts. If the pair of reactions was driven by the photothermal effect, the action spectra should have been the same. Similarly, these cases directly show that the shape of action spectra reflects the photocatalytic light absorption and the energetic position of the molecular orbital that is accepting the hot electron. Action spectra of non-plasmonic metal nanoparticles Visible light irradiation also significantly enhances the catalytic performance of nonplasmonic metal NPs,8 and the wavelength dependence of AQE for reactions catalyzed by nonplasmonic metal NPs (Table 2) share a common feature, which differentiates them from the plasmonic NP’s: higher AQEs are always achieved with shorter wavelengths. Light absorption cross-sections of non-plasmonic metal NPs varies only slightly in the visible light range, as the LSPR absorption for these metal NPs is in the UV region.39 This implies that when exposed to low intensity visible light, reactions on non-plasmonic NPs are neither driven by LSPR light absorption, nor by the photo-thermal effect. As shown in Table 2 (entries 1-4), the AQE values when irradiated with wavelengths of 530 nm or longer are negligible. Appreciable reactions only take place when the irradiation wavelengths are shorter than 530 nm. There exists a photon energy threshold for each photocatalytic reaction, for example, the AQE values for reactions


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given by entries 2-4 at a wavelength of 470 nm are two to seven times greater than that observed at 530 nm. Table 2 AQE at different wavelengths in various photocatalytic reactions catalyzed by nonplasmonic metal nanoparticles, reproduced with permission of Ref. 8.

Wavelength (nm)

365±5

Entry

Photocatalysts and reactions

AQE at different wavelength

1

Pd@ZrO2 Benzyl alcohol dehydrogenation

2

Pt@ZrO2 Benzyl amine oxidative coupling

3

Rh@ZrO2 Benzyl amine oxidative coupling

4

Ir@ZrO2 Benzyl alcohol dehydrogenation

5

Pd@ZrO2 Heck coupling

400±5

470±5

530±5

590±5

620±5

0.17%

0.08%

0.07%

0.03%

0.02%

0

0.39%

0.28%

0.19%

0.04%

0.03%

0

0.50%

0.15%

0.14%

0.02%

0

0

0.25%

0.18%

0.12%

0.04%

0

0

0.15%

0.09%

0.07%

0.06%

0.04%

0.03%

Consider a reaction driven by hot electron transfer, which occurs when the interaction between the reactant and metal NPs is relatively weak. The electron energy distribution of the hot electrons of non-plasmonic metal NPs soon after photon absorption schematically depicted in Scheme 3 for two different photon energies. Hot electron transfer can be very rapid and can take place in tens of femtoseconds (fs),9,19,27,40-43 which is much shorter than the length of time that the population of electrons remains ‘hot’. Whether an electron transfer occurs will depend on whether there are enough electrons with sufficiently high energies to make the transition possible. As previously discussed, absorption of higher energy incident photons will increase the number of electrons that have energies aligned to the molecular orbitals during the time that the electrons remain ‘hot’. Thereby increases the probability of an electron transfer.44-45 Only hot


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electrons with energies higher than the LUMO of reactant molecule (area between two red dash line) are able to induce a reaction and contribute to the AQE. For equivalent light intensities, shorter wavelength (400 nm for example, as shown in Scheme 3a) is able to excite more hot electrons to higher energy levels than longer wavelength (530 nm, Scheme 3b). Hence, when the LUMO energy of the reactant molecule is relatively high, the hot electrons excited by shorter wavelength photons are more effective to activate the reactant.

Scheme 3. Hot electron distribution of non-plasmonic metal NPs under different wavelength irradiation and their contribution to AQE. a: irradiation with light of 400 nm wavelength, only the hot electrons located above LUMO of reactant molecule (area between two red dash line) are able to contribute to AQE; b: irradiation with light of 530 nm wavelength, hot electron distribution area above LUMO is much smaller than that in panel a.

This phenomenon occurs in all metal NPs, plasmonic and non-plasmonics, as all metals have conduction electrons (including hot electrons) distributing over continuous electron energy levels. Combining with the information given in Table 2, irradiation with longer wavelengths does not induce measurable reaction activity although the irradiation should cause similar photo-


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thermal effect on the metal NPs to with shorter wavelengths. Evidently, the photo-thermal effect is not driving force for these reactions. It is also important to note that the relative distribution of hot electrons is likely very different comparing plasmonic (Ag, Au, Cu and Al) metals and non-plasmonic metals. Plasmonic metals have filled d-bands that sit well below the EFermi and most of the time below the LSPR energy. This suggests that hot electron production from plasmon decay will occur by intraband transitions within the sp band of the metal, producing a flat probability distribution in the range EFermi < Ee < EFermi + Φ. On the other hand, non-plasmonic metals are characterized by unfilled d-bands, meaning that the EFermi cuts through the d-band. This suggests that photoexcitation of non-plasmonic metals will mostly produce hot holes and electrons with energies only slightly above EFermi, where the empty d-band density is positioned. This may explain why long wavelength excitation produces so little photocatalytic activity, because the hot electrons are not very hot and situated only just above EFermi. Other factors influencing action spectra We also noted that photon energy threshold in the reactions are influenced not only by the molecular structure of the reactants but also by other factors such as temperatures. Figure 3 shows one case where the highest reaction rate was observed at short wavelengths even if the light absorption at the short wavelength was not as intense as at LSPR absorption wavelength of the Au NPs. When the reaction temperature was raised from 30°C to 45°C, the highest AQE of nitrobenzene (Ph-NO2) reduction shifted from 365 nm (in the ultraviolet, UV, range) to 530 nm (typical LSPR absorption peak of the supported Au NPs). The difference in reaction temperature of this reaction system is sufficient to cause profound changes in the action spectrum. At lower


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temperature (30°C), the AQE does not match to the light absorption spectrum of Au NPs while matches at higher temperatures (≥45°C). This means that at 30°C the light energy absorbed by the Au NPs through LSPR absorption (peaked at 530 nm) is insufficient to surmount the activation barrier of the reaction. As afore mentioned that the changes in molecule vibration states resulted from a rise of reaction temperature, contribute to reduce the photon energy threshold. This inference can explain spectra in Figure 3a and 3b. As temperature rises, the energy threshold is reduced; at 45°C the hot electrons generated by LSPR absorption are able to induce this reaction.

Figure 3. Action spectra of nitrobenzene reductive coupling catalysed by Au NPs and Ag NPs. a: Catalyzed by Au NPs at 30°C exhibits an action spectrum that does not follow plasmonic absorption of Au NPs; b: the reaction catalyzed by Au NPs at 45°C, show an action spectrum following plasmonic absorption of Au NPs,

In summary, the analysis of wavelength dependence of AQE indicates that there is a photon energy threshold for many reactions photocatalyzed directly by metal NPs. The threshold is a feature of the photo-induced electron excitation driven reactions. The reactions are predominantly driven through interaction between the hot metal electrons with sufficient energy


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and reactant molecules adsorbed on the metal NPs or by exciting the hybridized states of metal electron states and orbitals of the reactant molecules strongly adsorbed on the metal NPs. By changing the position of the orbital that must be populated to drive the chemical reaction, the threshold varies and the action spectra are significantly modified. Thus, one can see a molecular imprint on the action spectra. The contribution to the observed photocatalysis from the photoinduced heating is not dominant in most cases when using low intensity light sources. The photon-electron excitation mechanisms are important as they can efficiently channel the photon energy into the chemical bonds to be activated. They also reveal opportunities to achieve efficient chemical transformations at near ambient conditions by tuning the wavelength and intensity of the irradiation as well as the reaction temperature. Such photocatalysis may lead to discoveries in the selective organic synthesis reactions driven by irradiation of solar spectrum. ASSOCIATED CONTENT Supporting Information.The following files are available free of charge. Experimental Methods, TEM images of photocatalysts (Figure S1) and photograph of the LED lamps setup (Figure S2) (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Huai Yong Zhu: 0000-0002-1790-1599 Notes


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The authors declare no competing financial interests. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the Australian Research Council (ARC DP110104990 and DP150102110). QX acknowledges an Office of the Chief Executive (OCE) Postdoctoral Fellowship from CSIRO. PC acknowledges funding from the US Army Research Office through grant no. W911NF-14-1-0347. The electron microscopy analysis was performed through a user project supported by the Central Analytical Research Facility (CARF), Queensland University of Technology.


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Updated Scheme 2 112x118mm (150 x 150 DPI)


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Photon Energy Threshold in Direct Photocatalysis ... - ACS Publications

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