Plasmonic reactivity of halogen thiophenols on gold nanoparticles studied by SERS and XPS

Plasmonic nanoparticles (NPs) illuminated by visible light enhance the rate of chemical reactions on their surface and provide a high potential for solar energy conversion and could reduce the consumption of fossil energy required in chemical synthesis.^1,2 The driving forces of these plasmon mediated reactions are either enhanced electric fields in the vicinity of the surface, elevated temperatures, or charge carriers, which are generated in the decay of plasmon resonances and trigger reactions in molecules on the surface of the NPs.^3–6 In recent years, there was an intensive debate in the plasmon chemistry community about the role of photothermal and charge-transfer effects^7,8 and significant effort has been made to disentangle different contributions.^9,10 Interestingly, even for the same chemical reaction on similar plasmonic substrates, studies concluded different reaction mechanisms like in the heterogeneously catalyzed Suzuki–Miyaura coupling¹¹ of bromobenzene derivates on AuPd NPs. On the one hand, a hot electron induced reaction mechanism has been worked out for this coupling reaction on spherical core shell AuPd NPs,¹² and on the other hand, on Pd decorated Au nanorods, it was observed that either thermal enhancement is dominating the reaction¹³ or both mechanisms are contributing to the catalytical activity.¹⁴ In addition, a strong dependency on the adsorption site of the molecule on the metal substrate has been observed for the Suzuki–Miyaura coupling of halogenated thiophenol derivates (4-XTP) on AuPd NPs revealing a high reactivity on edges of the NPs and a low reactivity on terraces, which indicates the significant role of the nanoscopic electronic properties.¹⁵ Recently, it has been shown that accessible initial and final states involved in the transfer of hot-charge-carriers and the effective work-function depend significantly on the metal-molecular complex and can affect the reactivity.^16–19 By energy matching of the initial and final states, reaction rates in plasmon mediated reactions are boosted;¹ in this way, Liu and co-workers have demonstrated that the dehalogenation kinetics of brominated nucleobases can be significantly tuned by applying a potential to the NP substrate and consequently changing the availability of accessible electronic states.²⁰ In addition to their role in heterogeneous catalysis, brominated molecules are a model-system for electron induced reactions as the attachment of an electron with an energy close to 0 eV can already trigger the cleavage of the carbon halogen bond.^21,22 On plasmonic substrates, the transfer of hot-electrons to ligand molecules leads to the formation of a transient negative ion, which can either decay by the cleavage of molecular bonds or further react with neighboring molecules.^23–25 Surface enhanced Raman scattering (SERS) is a widely used technique to study such plasmon mediated reactions on noble metal substrates since the reaction can be triggered and tracked by the incident laser light and products can be identified by their vibrational fingerprint.²⁶ The reactions of brominated molecules monitored by SERS revealed either the formation of dehalogenated species or intermolecular coupled reaction products.^20,27–30 For the hot-electron induced dehalogenation of 4-XTP on AgNPs and the Suzuki–Miyaura coupling, a strong dependency of the halogen group on the formation of 4,4′-biphenyldithiol (BPDT) and thiophenol (TP) has been observed with the highest reactivity for iodinated and brominated and decreasing for chlorinated and fluorinated species.^28,31 Halogenated molecules are typically used in Suzuki–Miyaura and other cross-couplings with brominated ones being the most popular due to their high reactivity. However, an activation of chlorinated molecules is strongly desired because chlorinated starting materials can be produced at higher quantities with much lower costs. Consequently, we raise the question whether a plasmonic activation of chlorothiophenol is feasible. To approach this question, we have systematically studied the properties of halothiophenols 4-XTP with X = F, Cl, and Br using SERS and x-ray photoelectron spectroscopy (XPS).

Within the present study, the dehalogenation kinetics of halogenated thiophenol derivates (4-XTP), namely, 4-fluorothiophenol (4-FTP), 4-chlorothiophenol (4-ClTP), and 4-bromothiophenol (4-BrTP), on the surface of laser illuminated AuNPs has been studied. Localized surface plasmon resonances (LSPRs) are excited under the laser illumination and can decay non-radiatively by forming electron–hole pairs,

The generated electrons can either be transferred to adsorbed molecules and trigger reactions therein or distribute their energy by electron–electron scattering and subsequent electron-vibrational scattering and heat up the AuNPs.⁶ In Scheme 1, the reaction pathways are sketched showing that both the formation of a transient 4-XTP⁻ anion and the elevated temperatures of the AuNPs may lead to a cleavage of the halogen–carbon bond. To elucidate the reaction mechanism, the reaction kinetics have been monitored by SERS, and in addition, insights in the electronic properties of the metal–molecular interface have been obtained by XPS of isolated 4-XTP coated AuNPs.

In Fig. 1, SERS spectra of 4-FTP, 4-ClTP, and 4-BrTP are presented, showing unique vibrational fingerprints of the molecules. Even though the molecules have a very similar structure, the halogenated thiophenols can be distinguished in SERS by their C–X stretch and deformation, ring deformation, and C–C stretch vibrational modes based on their wavenumber, as well as on their characteristic ring stretching modes (see marked areas in Fig. 1).^32,33 The C–X stretch and deformation and ring deformation vibrations are located at 292 and 500 cm⁻¹ for 4-BrTP, at 350 and 545 cm⁻¹ for 4-ClTP, and at 392 and 630 cm⁻¹ for 4-FTP, respectively.³³ The ring stretch vibration exhibits a single mode at 1080 cm⁻¹ for 4-FTP. For 4-BrTP coated AuNPs, the peak is located at 1071 cm⁻¹ with a shoulder at 1079 cm⁻¹, whereas 4-ClTP has a characteristic triple peak with contributions at 1067, 1086, and 1100 cm⁻¹. The C–C stretching mode at 1585 cm⁻¹ for 4-FTP, at 1573 cm⁻¹ for 4-ClTP, and at 1564 cm⁻¹ for 4-BrTP has been previously used to identify the formation of reaction products in plasmon mediated reactions of 4-XTP capped noble metal NPs.²⁸ 

In Figs. 2(a) and 2(b), SERS spectra of 4-BrTP are presented after different illumination times with 633 nm light. After 5 s of illumination, the SERS spectrum mainly shows the vibrational modes of 4-BrTP. With ongoing illumination time, the characteristic bands of 4-BrTP are decreasing; most prominently, the broad ring stretching mode located at 1071 cm⁻¹ and, in addition, new bands are arising at 690, 1000, and 1021 cm⁻¹, which can be clearly assigned to the formation of thiophenol (TP) as a consequence of the cleavage of the carbon halogen bond.^28,34 In addition, the C–C stretching vibration at around 1564 cm⁻¹ is significantly broadening due to the overlap with newly arising signals [see Fig. SI3(c)]. On the one hand, the signal of TP is contributing to the broadening; however, also further contributions of interlinked molecular species such as 4,4′-biphenyldithiol (BPDT) are possible, as observed previously in the plasmon mediated reactions of 4-BrTP;²⁸ however, a clear assignment of the products is not possible. In addition, besides the formation of the dehalogenated molecules and possible dimers, the formation of amorphous carbon networks is highly likely. Previously, Szczerbiński and co-workers observed the interlinking of thiolated phenol derivates on illuminated plasmonic nanostructures leading to the formation of amorphous carbon membranes and suggested a hot-electron driven reaction mechanism.³⁵ In SERS, the formation of the interlinked carbon networks is characterized by a broadening and increase of the relative intensity of the C–C stretching mode between 1550 and 1600 cm^−1 35,36 as it is observed in the present study. Broadening of the bands is observed as well in the thermal decomposition of organic molecules on laser illuminated plasmonic nanostructures due to the formation of amorphous carbon layers.³⁷ In addition to the intermolecular reactions, also thermal or electron-induced desorption of the ligand molecules is possible. Therefore, the intensities of the 1071 cm⁻¹ band and the broadened band around 1560 cm⁻¹ have been plotted in Fig. SI3(b), revealing a relative increase of the 1560 cm⁻¹ signal intensity compared to the 1071 cm⁻¹ band, which is accompanied by a slight decrease in the absolute peak intensity. In Fig. 2(c), SERS spectra of 4-ClTP coated AuNPs at different laser illumination times are presented. Similarly to 4-BrTP, the decrease of the 1080 cm⁻¹ signals and the increase and broadening of the 1560 cm⁻¹ peak are observed. However, at 1000 cm⁻¹, the intensity of the peak only slightly increases, which might indicate the formation of TP, but in a much lower amount than in the 4-BrTP reaction. Moreover, at illuminated 4-FTP coated AuNPs, shown in Fig. 2(d), no changes in characteristic peaks originating from TP could be observed, even though a small signal is present at 1000 cm⁻¹, which might originate from 4-FTP or sample impurities. The broadening of the 1560 cm⁻¹ signal is observable as well in this system indicating the formation of amorphous carbon networks. The formation kinetics of the 1000 cm⁻¹ peak is plotted in Fig. SI3(a), whereas differences in the formation of TP of the 4-halogented thiophenols might be assigned to the binding strength of the halogen group, as discussed more in detail below. Recently, a strong dependency of the hot-electron induced carbon-halogen cleavage reaction rates on the halogen species already has been revealed for halothiophenol coated AgNPs.²⁸ 

In order to determine the reaction kinetics of the plasmon mediated decomposition of the 4-XTP molecules, the intensity of the ring-stretching mode at 1071 cm⁻¹ has been monitored as a function of the illumination time for 532 nm (2.33 eV photon energy), 633 nm (2.0 eV photon energy), and 785 nm (1.6 eV photon energy) laser light [see Fig. 3(a)]. For 633 and 785 nm light, a comparable decomposition has been observed, and a significantly higher apparent decomposition rate for 532 nm with the same incident laser power. For the same laser wavelength and power, the decomposition kinetics of the 4-XTP molecules is presented in Fig. 3(b) and SI2 and show a comparable decline of the signal intensity for 4-FTP and 4-ClTP but a significantly stronger decline for 4-BrTP. In Fig. 3(c), the apparent reaction rates k for the 4-XTP molecules are presented for illumination with different laser wavelengths. k has been determined by fitting the ring stretching mode intensity with an integrated first order rate law with an additional offset $It=I0⋅e−k⋅t+I∞$⁠, where I₀ is the initial intensity and I_∞ is the residual intensity after long illumination times. There are no significant differences of the apparent reaction rates k observable [Fig. 3(c)] between the 4-XTP molecules for all laser wavelengths. This is remarkable, as the conversion $1−I∞I(t=0)$ is higher for 4-BrTP. In all cases, the apparent reaction rates are almost two orders of magnitude higher for illumination with 532 nm light compared to 633 and 785 nm light. The dependency of the apparent reaction rate of the 4-XTP decomposition on AuNPs is in contrast to the observations on AgNPs, where a significant dependency on the halogen group has been observed.²⁸ This indicates that the reaction kinetics of 4-XTP molecules on AuNPs is rather determined by the properties of the substrate than on the properties of the molecular system.

In order to elucidate the reaction mechanism, the reaction kinetics of the dehalogenation of 4-BrTP has been determined as a function of the incident laser power [see Fig. 4(a)]. With increasing laser power, the decay of 4-BrTP is significantly accelerated. The reaction rates have been determined from the fit with a first order integrated rate law with an additional offset, since the apparent reaction rates are proportional to the underlying reaction rates.³⁸ The apparent reaction rates k are plotted in Fig. 4(b) in a double logarithmic scale as a function of the incident laser power P and fitted with a power law function,

where A is a constant and N is the exponent, given by the number of photons involved in the process. In a purely hot-electron induced reaction, the reaction rate is proportional to the number of incident photons and, therefore, to the incident laser power.¹⁰ Thus, N would be equal to one,

This linear dependency of the reaction rate on the laser power has been observed previously for the plasmon mediated debromination of 8-bromoadenine on AgNPs.²⁷ On the other hand, in a purely thermally driven reaction, the reaction rate is exponentially dependent on the temperature and consequently on the incident laser power. It needs to be mentioned that it is often difficult to clearly distinguish between a linear and an exponential dependency on the incident laser power.¹⁰ In the present dataset, N = 1.5, revealing a super-linear dependency of the reaction rate k on the laser power P. This indicates that the plasmon mediated decomposition of 4-BrTP on AuNPs is most-likely not purely hot electron driven.

In order to elucidate the electronic properties of the AuNP-ligand interface, XPS measurements of isolated 4-XTP capped AuNPs have been performed in the gas-phase. A beam of 4-XTP coated AuNPs is generated and introduced to the vacuum, where it is crossed with the soft x-ray beam and the generated photoelectrons are detected. In this manner, on the one hand, radiation damage to the molecules is prevented due to the short interaction time with the beam, and on the other hand, the AuNPs are decoupled from a substrate and information about the effective work-function can be obtained. In standard laboratory XPS setups, the reference of the binding energy (BE) is referenced to the Fermi level of the system, since AuNPs and detector are in electric contact; however, in the present experimental setup, the AuNPs are decoupled from the detector and their Fermi energy may differ.¹⁸ Therefore, the reference of the BE is given by the vacuum energy determined by the ionization edges of Ar gas,

KE is the kinetic energy of the photoelectrons, PE is the photon energy, and Φ_eff is the effective work function given by the energy difference of the Fermi energy of the AuNP ligand system and the vacuum energy far away from the surface, where the action of surface dipoles can be neglected. In Fig. 5(a), the C1s states of 4-XTP AuNPs are presented showing slight spectral differences for the halogen species. The signal from this carbon atom at the four-position will significantly differ depending on the halogen group, as the mean chemical shifts for a carbon halogen bond are 0.7, 2.0, and 2.9 eV for Br, Cl, and F, respectively.³⁹ Furthermore, collective dipole moments of the molecular monolayer can induce shifts of the C1s states of the individual carbon atoms, which could exhibit significant differences for different halogen groups.^18,40 In Fig. 5(b), the high resolution XPS spectra of the Au4f and Br3d states of 4-BrTP coated AuNPs are presented. The Au4f_7/2 and Au4f_5/2 are well separated and located at 88.3 and 91.9 eV, respectively, which refers to a shift of 4.3 eV compared to the binding energy of bulk gold at 84.0 eV with respect to the Fermi level.⁴¹ It needs to be mentioned that the binding energy of surface Au atoms is typically slightly lower than the binding energy of bulk gold, due to the lower coordination number.⁴² However, since the binding energy of surface atoms of ligand free AuNPs is not accessible in the present experiment, the well-known XPS reference energy of bulk Au has been used as a reference for the effective work function.⁴¹ Close to the Au4f states, the Br3d states of the 4-BrTP ligands are observable in the spectrum around 75 eV, which are slightly overlapped with Auger electrons from residual N₂ with a kinetic energy of around 340 eV.⁴³ The contribution of the N₂ Auger electrons to the signal has been determined by separate measurements of N₂ under the same conditions. The corrected Br3d spectrum is presented in Fig. 5(c) revealing binding energies of Br3d_5/2 and Br3d_3/2 components of 74.2 and 75.1 eV, respectively.

The effective work-function has been determined for all 4-XTP coated AuNPs from the Au4f spectra (see Fig. SI4–6), as shown in Fig. 6(a) revealing significant differences depending on the halogen atom. With an increasing electronegativity of the halogen, the effective work-function is increasing from 4.3 eV for 4-BrTP, over 5.1 eV for 4-ClTP to 5.2 eV for 4-FTP. Previously, it has been shown that the dipole moments of a molecular monolayer, which depends on the functional groups, can significantly modify the work function of the substrate.⁴⁴ However, also a different molecular coverage might cause the differences in the work function.⁴⁵ We also present data for citrate capped AuNPs representing the precursors for 4-XTP capped NPs, which are prepared by the ligand exchange. The effective work-function for citrate capped AuNPs is 4.6 eV. With the knowledge of the effective work function, the BE energies can be referred to the Fermi energy as well. In Fig. 6(b), the valence band states of the ligand coated AuNPs are plotted with respect to the Fermi level, revealing the density of states that is accessible in light induced electron transfer reactions. For 4-FTP and 4-ClTP, the density of states is increasing approximately linearly with an onset at the Fermi energy. However, the 4-BrTP capped AuNPs exhibit a very low density of states between 0 and 2.5 eV with a slight increase starting around 1.1 eV and a sharp onset around 2.7 eV. In this way, the valence band electrons of 4-BrTP capped AuNPs are poorly accessible for the excitation with visible light.

In plasmon mediated reactions of halogenated thiophenols on 80 nm AgNPs, this efficient cleavage of the carbon–halogen bond has been monitored previously by SERS, and a strong dependency on the halogen species has been observed, which clearly indicates a hot-electron driven reaction mechanism.²⁸ The C–Br bond of 4-BrTP is very reactive toward low energy electrons and can be cleaved following the attachment of electrons with a kinetic energy close to 0 eV, as shown by dissociative electron attachment (DEA) experiments with 4-BrTP in the gas-phase.²² The electron affinity of Br is 3.36 eV⁴⁶ (see Table I), i.e., it is the energy that is released when Br- is formed by electron attachment to Br. Since the bond dissociation energy of the C–Br is close to or lower than 3.36 eV,⁴⁶ an electron of near zero eV can be attached to 4-BrTP followed by bond cleavage. Thermodynamically, this process is driven by the electron affinity of Br. In addition, the effective work function is lowest for 4-BrTP capped AuNPs compared to other halothiophenol derivates, which should generally facilitate the transfer of electrons toward the adsorbed 4-BrTP molecules [see Fig. 6(a)]. However, for an efficient electron transfer, a high density of occupied electronic states (DOS) close to the Fermi-energy is required.¹⁹ For 4-BrTP AuNPs, the accessible initial states are very limited in the relevant energy range [see Fig. 6(b)], and consequently, the electron transfer from the AuNPs to the adsorbed molecules is hampered. 4-FTP and 4-ClTP, on the contrary, have an onset of the DOS at 0 eV, which facilitates the electron transfer toward the molecules; however, the effective work function is significantly higher for the molecules compared to 4-BrTP, which on the other hand limits the electron transfer rate. Hence, for all molecules, certain unfavorable factors for an efficient electron transfer are present. In Table I, the relevant parameters determining the electron transfer toward the ligand molecules and the subsequent DEA are summarized, showing only minor differences in the electron affinity of the C–X bond for different halide groups, but clear differences in the bond dissociation energies with the highest value for C–F and the lowest for C–Br. Thus, the formation of TP, with the highest yield for 4-BrTP, and no observable formation from 4-FTP can be explained by differences in the bond dissociation energies, as the C–Br bond is typically the predetermined breaking point in electron induced reactions and upon heating. Furthermore, gas-phase DEA experiments of comparable molecules have revealed a poor cleavage of the C–F bond following an electron attachment to 2-FTP.⁴⁷ For the DEA to 4-chlorophenol in the gas-phase, the C–Cl cleavage is one of the main decay channels with a strong resonance between 0 and 1 eV; however, the H loss leaving the residual anion intact is formed with two orders of magnitude higher yield.⁴⁸ Only for 4-BrTP, the C–Br is the most abundant reaction channel.²² These observation of the dependency of the C–X bond cleavage on the halide group does not necessarily mean that electron induced reaction rates need to be tuned by the choice of the halide, since for halogenated nucleobases, very efficient electron induced cleavages of the C–Cl bond have been observed⁴⁹ and by additional sub-groups on the molecules, the reaction channels can be significantly influenced.⁴⁷ 

For the plasmon mediated transformation of 4-nitrothiophenol (4-NTP) to DMAB, it has been suggested that the reaction requires both an electron transfer generating a stable anion and thermal energy for the intermolecular coupling in a tandem reaction.⁵⁰ It has been shown previously that depending on the chemical environment beside the C–Br bond cleavage, also stable 4-BrTP⁻ anions can be formed.²² For 4-NTP, it is well known that even though the electron induced reaction channel leading to the cleavage of the nitro group exists, the formation of stable anionic intermediate states is far more relevant for the reactions on plasmonic NPs leading to intermolecular coupling.^23,51 Hence, it is possible that on the surface of AuNPs, the purely electron-based reaction pathways might be less relevant than on AgNPs and thermal contributions have a relevant role in the formation of amorphous networks. The heating of the AuNPs mainly depends on the absorption of light, which is the highest for blue and green light and low for red and near-infrared and which is generally independent on the functional groups of the ligand molecules.⁵² Therefore, a strong influence of the NP-heating on the reaction kinetics could explain, why the reaction rates are quite independent on the halogen group of the ligand molecules and furthermore, the high reaction rates under illumination with 532 nm light and the comparably low rates at 633 and 785 nm illumination. The possible reaction pathways for the electron transfer and the subsequent dissociation are sketched in Scheme 2. The comparable reaction rates for all studied 4-XTP molecules and the significantly higher conversion for 4-BrTP indicate on the one hand that the rate determining reaction step is based on the (plasmonic) properties of the AuNP, e.g., the electron–hole pair formation rate or the temperature of the illuminated AuNPs. On the other hand, the number of reactive sites is probably higher for 4-BrTP, e.g., due to a higher packing density of the molecules enabling inter-molecular reactions or due to the properties of the metal–molecular interface.

Plasmon mediated reactions of 4-XTP on the surface of AuNPs lead to the formation of interlinked molecular networks and dehalogenation for 4-BrTP and 4-ClTP causing the formation of TP. The reaction kinetics are highly wavelength-dependent, with comparably high reaction rates under illumination with green light and lower rates with red and near-infrared light. However, the reaction rates are relatively independent on the halogen atom of the adsorbed 4-XTP molecules, as most-likely the optical-properties of the AuNPs are more relevant for the reaction kinetics than the properties of the ligand molecules, even though a high reactivity on hot-electrons would be expected for 4-BrTP based on the comparatively low bond dissociation energy. Power dependent kinetic measurements exhibit a super-linear dependency of the reaction rate on the laser power, which can be explained by an at least partly thermally driven reaction. XPS measurements of isolated ligand capped AuNPs reveal a low density of accessible electronic states for electron transfer reactions of 4-BrTP capped AuNPs, even though the reduced work function would facilitate such an electron transfer. This indicates that the electron mediated reaction channels are hampered and thermally driven less specific reaction channels are favored. For 4-ClTP and 4-FTP capped AuNPs on the contrary, the work functions are relatively high, which is unfavorable for an electron transfer even though the DOS close to the Fermi level is higher compared to 4-BrTP. Moreover, the C–X bond dissociation energies are high; thus, for all studied molecules, certain parameters are limiting their reactivity. Consequently, a well-suited interplay of the electronic properties of the NP-ligand system at the interface together with the specific molecular reactivity is essential for the efficient design of plasmonic system for applications in chemical synthesis.

HAuCl₄, trisodium citrate (TSC), 4-FTP, and 4-ClTP have been purchased from Sigma-Aldrich and 4-BrTP has been purchased from Alfa Aesar. All chemicals have been used without further purification.

AuNPs with an average diameter of 10 nm have been synthesized by a slightly modified Turkevich method, described previously.^18,19,53 500 ml HAuCl₄ has been heated up to a rolling boil in a three neck round bottom flask under reflux. Subsequently, 4 ml trisodium citrate (500 mM) has been added to the stirred solution and kept boiling for one more hour, before cooling down to room temperature. The maximum of the LSPR is located at λ_max = 519 nm (see Fig. SI1).

500 µl citrate capped AuNPs have been centrifuged twice through a 100 kDa millipore centrifugal filter at 3000 g for 10 min and refilled with MilliQ filtered water to remove access citrate on the AuNP surface. In a third centrifugation step, the AuNP concentration was increased and a 10 µl drop of the concentrated AuNP solution has been placed on a clean Si wafer and dried to form a dense AuNP multilayer film. The AuNPs substrates have been incubated in an ethanolic 0.3 mM XTP solution overnight, subsequently rinsed with ethanol to remove unbound molecules, and dried with compressed air. The Raman measurements have been performed with a Witec Alpha300 confocal Raman microscope equipped with different lasers (488, 532, 633, 785 nm) with a 50× Nikon Objective (NA = 0.75). In order to avoid influences by possible modifications of the substrate, the Raman intensities have been normalized: $Icorr=Ipeak−IdarkIbackground−Idark$⁠.

XPS measurements of isolated 4-XTP capped AuNPs have been performed at the PLEIADES beamline at the Synchrotron SOLEIL using the multi-purpose-source chamber.^54,55 Prior to the measurement, the AuNP dispersion has been incubated for 1 h in 100 µM XTP-solution with 0.02% sodium dodecyl sulfate (SDS) to stabilize the AuNPs during the ligand exchange. In two centrifugation steps at 3000 g using Amicon 15 centrifugation filters, the surplus molecules have been removed and refilled with MilliQ H₂O. In a final centrifugation step, the AuNP concentration has been increased to around 3 mM Au per ml. A AuNP aerosol has been generated using a TSI 3076 atomizer with Ar (or N₂ in some measurements) as a carrier gas. The AuNP aerosol was passing a desiccator in order to remove the solvent and introduced to the vacuum through a 230 µm orifice. The AuNP beam was focused by a set of aerodynamic lenses and crossed with the soft x-ray beam of the PLEIADES beamline at the entrance of the SCIENTA4000 electron energy analyzer. For measurements of the core-level states, the KE of the photoelectrons has been calibrated by measuring the 2s, 2p, 3s, and 3p states of Ar⁵⁶ with PE = 400.865 eV calibrated by the N1s-π* transition of N₂ gas⁵⁷ (see Fig. SI7), as described elsewhere in detail.⁵⁸ In addition, the 12.6 eV signal of water has been used to reference the BE of the valence band states.⁵⁹ 

See the supplementary material for scanning electron microscopy and UV–Vis-spectroscopy data of the AuNPs. Moreover, additional Raman kinetics data, high resolution XPS-spectra of the 4-XTP coated AuNPs, and the electron energy calibration using the ionization edges of Ar gas are presented.

This research was supported by the European Research Council (ERC; consolidator Grant No. 772752), and we acknowledge beamtime at the synchrotron SOLEIL at the beamline PLEIADES through Project No. 20181557. Furthermore, the authors would like to thank Sergio Kogikoski Jr. for the scanning electron microscopy imaging of the AuNPs.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding authors upon reasonable request.