Didier Stiévenard

David Guérin

Stéphane Lenfant

Gaëtan Lévêque

Christian A Nijhuis

Dominique Vuillaume
email: dominique.vuillaume@iemn.fr

& Dominiquevuillaume

Electrical detection of plasmon-induced isomerization in molecule-nanoparticle network devices

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Networks of metal nanoparticles linked by molecules (or for short NPSAN: nanoparticles self-assembled network) are very valuable and useful in molecular electronics to study fundamental electron transport mechanisms, as well as to assess their potential applications in nanoelectronics. [1][2] For instance, NPSANs with simple molecules (alkyl chains, short π-conjugated oligomers) have been used to gain new insights in metal-insulator transitions, [3][4] plasmon enhanced photoconductance, [5][6][7] and co-tunneling, [8][9][10][START_REF] Tran | Sequential tunneling and inelastic cotunneling in nanoparticle arrays[END_REF] while NPSANs with relative complex molecules give access to functional devices such as optically [START_REF] Liao | Reversible Formation of Molecular Junctions in 2D Nanoparticle Arrays[END_REF][START_REF] Matsuda | Conductance Photoswitching of Diarylethene-Gold Nanoparticle Network Induced by Photochromic Reaction[END_REF][START_REF] Van Der Molen | Light-controlled conductance switching of ordered metal-molecule-metal devices[END_REF][START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF] and redox [START_REF] Liao | Cyclic Conductance Switching in Networks of Redox-Active Molecular Junctions[END_REF] driven molecular switches, molecular memory and negative differential resistance, [START_REF] Mangold | Negative Differential Photoconductance in Gold Nanoparticle Arrays in the Coulomb Blockade Regime[END_REF][START_REF] Zhang | Negative Differential Resistance, Memory, and Reconfigurable Logic Functions Based on Monolayer Devices Derived from Gold Nanoparticles Functionalized with Electropolymerizable TEDOT Units[END_REF] and unconventional computing approaches. Bose et al. demonstrated the training of molecule/ nanoparticle networks via genetic algorithms in an alkylthiol capped Au NPs network dominated by Coulomb blockade below 5K. [START_REF] Bose | Evolution of a designless nanoparticle network into reconfigurable Boolean logic[END_REF] Viero et al. demonstrated that NPSANs of Au NPs functionalized by optically driven molecular switches exhibit both optically driven reconfigurable logic operations and reconfigurable strongly non-linear electron transport and dynamic behaviors (high-harmonic generation) required for reservoir computing. [START_REF] Viero | Light-stimulable molecules/nanoparticles multi-terminal networks for switchable logical function and reservoir computing[END_REF] In molecular electronic plasmonics (see Ref. [START_REF] Wang | Molecular electronic plasmonics[END_REF] for a review), molecules chemically grafted, or deposited as thin films, on metallic nanostructures are used to modify the local surface plasmon (LSP) frequency depending on their molecular states (i.e., redox state, conformation or configuration, dipole moment changing the dielectric constant around the metal nanostructures), which, in turn, can be controlled via an applied bias. [START_REF] Hsiao | Light-Driven Plasmonic Switches Based on Au Nanodisk Arrays and Photoresponsive Liquid Crystals[END_REF][START_REF] Leroux | Tunable Electrochemical Switch of the Optical Properties of Metallic Nanoparticles[END_REF][START_REF] Zheng | Active molecular plasmonics: controlling plasmon resonances with molecular switches[END_REF][START_REF] Stockhausen | Giant Plasmon Resonance Shift Using Poly(3,4ethylenedioxythiophene) Electrochemical Switching[END_REF][START_REF] Zheng | Incident-Angle-Modulated Molecular Plasmonic Switches: A Case of Weak Exciton-Plasmon Coupling[END_REF][START_REF] Müller | Localized Surface Plasmon Resonance Investigations of Photoswitching in Azobenzene-Functionalized Self-Assembled Monolayers on Au[END_REF][START_REF] Joshi | Ultrasensitive Photoreversible Molecular Sensors of Azobenzene-Functionalized Plasmonic Nanoantennas[END_REF][START_REF] Schaming | Tunable Plasmon Resonance of Gold Nanoparticles Functionalized by Electroactive Bisthienylbenzene Oligomers or Polythiophene[END_REF] Optical inputs can also be used, for example, the photoisomerization of photochromic molecules (azobenzene, diarylethene, or spiropyran derivatives) directly bound to metallic nanoparticles or nanostructures changed the frequency of the LSP resonance. [START_REF] Hsiao | Light-Driven Plasmonic Switches Based on Au Nanodisk Arrays and Photoresponsive Liquid Crystals[END_REF][START_REF] Zheng | Incident-Angle-Modulated Molecular Plasmonic Switches: A Case of Weak Exciton-Plasmon Coupling[END_REF][START_REF] Müller | Localized Surface Plasmon Resonance Investigations of Photoswitching in Azobenzene-Functionalized Self-Assembled Monolayers on Au[END_REF][START_REF] Joshi | Ultrasensitive Photoreversible Molecular Sensors of Azobenzene-Functionalized Plasmonic Nanoantennas[END_REF] This plasmonic detection of the isomer state of the photochrome molecules has been observed at a single nanoparticle level. [START_REF] Song | Reversible photoisomerization of azobenzene molecules on a single gold nanoparticle surface[END_REF] Plasmon resonances can be also detected by electric means with tunnel junctions via optical rectification where the surface plasmon modes modulate the tunneling current flowing across the junction. [START_REF] Du | Highly efficient on-chip direct electronic-plasmonic transducers[END_REF][START_REF] Ward | Optical rectification and field enhancement in a plasmonic nanogap[END_REF] Several authors reported an increase of the tunnel current measured through various metal/molecules/metal junctions when these devices are irradiated by light with the same frequency as the plasmon resonance frequency; [START_REF] Ward | Optical rectification and field enhancement in a plasmonic nanogap[END_REF][START_REF] Banerjee | Plasmon-induced electrical conduction in molecular devices[END_REF][START_REF] Noy | Response of Molecular Junctions to Surface Plasmon Polaritons[END_REF][START_REF] Arielly | Accurate Determination of Plasmonic Fields in Molecular Junctions by Current Rectification at Optical Frequencies[END_REF][START_REF] Zelinskyy | Photoinduced Switching of the Current through a Single Molecule: Effects of Surface Plasmon Excitations of the Leads[END_REF][START_REF] Vadai | Plasmon-Induced Conductance Enhancement in Single-Molecule Junctions[END_REF]7,[START_REF] Duché | Molecular diodes in optical rectennas[END_REF] this phenomenon (also called plasmon-assisted tunneling (PAT)) can be modeled based on a photon-assisted tunneling model initially introduced by Tien and Gordon. [START_REF] Tien | Multiphoton Process Observed in the Interaction of Microwave Fields with the Tunneling between Superconductor Films[END_REF] Plasmonic electronics is also interesting for applications in quantum plasmonics. [START_REF] Tame | Quantum plasmonics[END_REF][START_REF] Zhu | Quantum mechanical effects in plasmonic structures with subnanometre gaps[END_REF] For example, the conductivity of an organic semiconducting thin film is increased when strongly coupled to the plasmonic modes of a nanostructured metallic layer, [START_REF] Orgiu | Conductivity in organic semiconductors hybridized with the vacuum field[END_REF] and tunneling charge transfer plasmon (tCTP), resulting from quantum interactions between tunneling electrons and plasmonic nanostructures, was recently demonstrated in molecular tunnel junctions of various molecules connected between two NPs. [START_REF] Tan | Quantum Plasmon Resonances Controlled by Molecular Tunnel Junctions[END_REF][START_REF] Koya | Charge transfer plasmons: Recent theoretical and experimental developments[END_REF] Albeit the isomerization of molecules attached onto a NPs induced by LSP excitation (here referred to as PII: plasmon-induced isomerization) has been observed from purely optical methods (Raman or fluorescence spectroscopies), [START_REF] Mahmoud | Electromagnetic Field of Plasmonic Nanoparticles Extends the Photoisomerization Lifetime of Azobenzene[END_REF][START_REF] Tsuboi | Near-Infrared Continuous-Wave Light Driving a Two-Photon Photochromic Reaction with the Assistance of Localized Surface Plasmon[END_REF][START_REF] Garcia-Amorós | Plasmonic Activation of a Fluorescent Carbazole-Oxazine Switch[END_REF][START_REF] Impellizzeri | Two-Photon Excitation of a Plasmonic Nanoswitch Monitored by Single-Molecule Fluorescence Microscopy[END_REF] the modulation of the electrical conductance of a molecularly functionalized NP network induced by PII of the molecules decorating the NPs has not been reported. Here, we combine NPSANs and molecular plasmonics to demonstrate the electrical detection (conductance variation) of PII of azobenzene derivatives (azobenzene bithiophene: AzBT). These molecules are linked to gold nanoparticles and embedded in a planar nanodevice consisting of NPSANs contacted by nanoelectrodes separated by 100-200 nm inter-electrode gaps. We show that the PII effect is observed in the cluster-NPSANs due to an efficient enhancement of the LSP resonance through an antenna effect, while a weak effect is observed in 2D-NPSANs (i.e., a monolayer of AzBT functionalized Au NPs). The usual optical isomerization of AzBT (UV-visible light) is also electrically detected in both 2D-and cluster-NPSANs with similar kinetics for both types of devices. In sharp contrast, due to PII, the switching kinetics are faster by a factor of ∼10 .

In this study, we focus on AzBT (Fig. 1-a)-capped gold nanoparticles (AuNPs, Fig. 1b) that were synthesized by ligand exchange reactions from a suspension of citrate-capped AuNPs, 10 nm in diameter, in the presence of AzBT. The details of the synthesis of AzBT molecules, the preparation of the AzBT NPSANs, and the deposition of the NPSANs on two nanoelectrodes, have been reported by us before [START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF][START_REF] Karpe | Oligothiophene-derivatized azobenzene as immobilized photoswitchable conjugated systems[END_REF][START_REF] Smaali | High On-Off Conductance Switching Ratio in Optically-Driven Self-Assembled Conjugated Molecular Systems[END_REF] and are briefly described in the Methods. The choice of this molecule is motivated by the fact that high conductance ratios between the two photoisomerizable states (trans and cis) have been observed when these molecules are self-assembled as a monolayer on Au surfaces [START_REF] Smaali | High On-Off Conductance Switching Ratio in Optically-Driven Self-Assembled Conjugated Molecular Systems[END_REF] and embedded in NPSANs. [START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF] Since the plasmonic effect is very sensitive to the geometry of the NP assemblies, which act as antennas for the detection and the amplification of the incident electromagnetic waves, [START_REF] Johansson | Surface-enhanced Raman scattering and fluorescence near metal nanoparticles[END_REF][START_REF] Muhlschlegel | Resonant Optical Antennas[END_REF] we fabricated two types of devices: monolayer and multilayer NPSANs were deposited within the electrodes leading to the hereafter named 2D-NPSAN (Fig. 1-c) and cluster-NPSAN (Fig.

1-d).

"

Effect of the plasmonic excitation.

To study the effect of a plasmonic excitation, we preconfigured the NPSANs with the AzBT such that a maximum number of molecules were in their trans or cis state by illuminating the NPSANs at 365 nm and 470 nm (for 1-1.5 h), respectively. This preconfiguration is followed by a period in dark (typically 10-20 min) until a stable current was reached, then we illuminated devices at 590 nm to excite the LSP for 3-15 min until a stable current was observed (see Methods). We recorded the time evolution of the current through the NPSANs at a constant dc voltage of 2.5 V (see Methods). This voltage was chosen because the junctions remain stable and the currents albeit weak are above the noise levels.

For the 3D NPSANs preconfigured with the AzBT trans state (illumination at 470 nm for ca. 1h), Fig. 2-a shows the current vs. time, I(t), curves recorded under LSP excitation starting from devices in the dark followed by illumination at 590 nm and, subsequently, turning the light source off rendering the devices again in the dark. Fig. 2-b shows the results for the NPSANs preconfigured with AzBT in the cis isomer (illumination at 365 nm for ca. 1h). The current increases when the AzBT molecules are preconfigured in their trans state (Fig. 2-a) while it decreases for the cis state (Fig. 2-b). The time constants are 90 ± 9 s and 103 ± 10 s (i.e., not significantly different) for both types of devices, respectively. The value of the difference in current before and after illumination, ΔI, for the trans-AzBT devices is 4 pA, but the value of ΔI of -70 pA is more than one order of magnitude larger for the cis-AzBT devices. Here, the relative current amplitude variations are 10 to 20 % of the steady-state currents before illumination. Moreover, when the 590 nm light source is switched off, the current remains stable. NPSANs with the AzBT molecules preconfigured in their trans state. It was not possible to do the same experiment when the AzBT molecules are preconfigured in their cis state because the current immediately decreased when the UV light was switched off (Fig. S2, supporting information). We have not obtained a stable dark current (as in cluster-NPSANs, Fig. 2) before turning on the LSP excitation light at 590 nm. This is likely due to the metastable character of the cis state at room temperature in the dark, which spontaneously returns to the trans state. [START_REF] Feringa | Molecular Switches[END_REF] We rationalize the feature that a more stable cis state in the dark is observed for the cluster-NPSANs by rising the hypothesis that the NPs are more closely packed in the cluster than in the 2D layer, reducing the free space around the AzBT molecules and consequently reducing the cis-to-trans spontaneous back isomerization efficiency. For example, it was observed that the cis state, in dark at room temperature, can be stable up to several hours in densely packed azobenzene SAMs. [START_REF] Smaali | High On-Off Conductance Switching Ratio in Optically-Driven Self-Assembled Conjugated Molecular Systems[END_REF] Under this condition, when we apply the LSP excitation, a large fraction of the AzBT molecules have turned back to the trans state and we again observe a current increase (Fig. 3-b) as in the case with the preconfigured trans state (Fig. 3-a). Exponential fits give rising time constants of 22 ± 4 s and 63 ± 7 s in both cases, respectively. The value of ΔI is in the range 1 to 4 pA, i.e., relative current variations around 0.5 % of the steady-state currents, much lower than in the case of the cluster-NPSANs.

UV and visible light photoisomerizations.

For the sake of comparison, we also measured the kinetics of the AzBT photoisomerisation in the NPSANs under usual UV and visible light illuminations [START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF] and we examined the respective behaviors of the 2D-NPSANs and cluster-NPSANs. Figures 4-a and b give typical I(t) curves at 2.5 V for (a) the trans to cis (365 nm light excitation) and (b) the cis to trans (470 nm light excitation) AzBT photoisomerization in the 2D-NPSANs using preconfigured devices (Methods). Figures 4-c andd show that a similar behavior is observed for the cluster-NSANs but with higher currents (as expected due to a more important number of current pathways). However, in all these cases, we observed multi-exponential kinetics with characteristic time constants ranging from few tens to few 10 4 s (see details and values in the supporting information, Table S1). The main observation is that these kinetics are markedly different from those measured under excitation at 590 nm (Figs.

Discussion of mechanisms under plasmonic excitation.

Several hypotheses have to be considered to explain the current increase under light (590 nm) illumination. Since there is no optical absorbance of the AzBT molecule at 590 nm, [START_REF] Karpe | Oligothiophene-derivatized azobenzene as immobilized photoswitchable conjugated systems[END_REF] a direct trans-to-cis and cis-to-trans photoisomerization can be ruled out for the observed changes in current under illumination shown in Figs. 2 and3. Since the optical excitation energy at 590 nm (2.1 eV) is lower than the molecule HOMO-LUMO gap (3.4 eV 49 ), we do not believe that photocurrents, as reported for networks of NPs functionalized by conjugated molecules [START_REF] Mangold | Resonant Photoconductance of Molecular Junctions Formed in Gold Nanoparticle Arrays[END_REF] and molecular junctions, [START_REF] Zhou | Photoconductance from Exciton Binding in Molecular Junctions[END_REF] are important in our devices. We also dismiss thermal effects associated with the absorption of the light as it has been reported for NP arrays involving the LSP in Ref. 6 . In this latter case, an increase of the current of 5 pA was obtained under 4.1 kW/cm 2 illumination. 6 Considering that we use much lower power of 10 mW/cm 2 , we can safely dismiss thermal effects. For these reasons, we believe that excitation of the molecules is possible via the LSP resonance of the gold NPs.

It is well known that plasmons can increase tunneling currents flowing across tunneling junctions; this process is called plasmon-assisted tunneling (PAT). [START_REF] Ward | Optical rectification and field enhancement in a plasmonic nanogap[END_REF][START_REF] Banerjee | Plasmon-induced electrical conduction in molecular devices[END_REF][START_REF] Noy | Response of Molecular Junctions to Surface Plasmon Polaritons[END_REF][START_REF] Arielly | Accurate Determination of Plasmonic Fields in Molecular Junctions by Current Rectification at Optical Frequencies[END_REF][START_REF] Zelinskyy | Photoinduced Switching of the Current through a Single Molecule: Effects of Surface Plasmon Excitations of the Leads[END_REF][START_REF] Vadai | Plasmon-Induced Conductance Enhancement in Single-Molecule Junctions[END_REF] However, the slow rising response times (∼ 20-100 s, Figs. 2 and3) cannot be explained with PAT as tunneling events occur on 1fs time scales, plasmon lifetimes are on the order of tens of fs time scales. Consequently, when the light is switched off, the current must decrease almost immediately, which was not observed in our case. Moreover, we can estimate the PAT current using the same model as Vadai et al. [START_REF] Vadai | Plasmon-Induced Conductance Enhancement in Single-Molecule Junctions[END_REF] , based on Tien-Gordon model. [START_REF] Tien | Multiphoton Process Observed in the Interaction of Microwave Fields with the Tunneling between Superconductor Films[END_REF] The plasmon field is treated as a local potential Vω across the NP-molecules-NP junctions, which oscillates at the plasmon frequency ω. This potential is given by : 37

" ( 1
)
where Gdc is the single molecule conductance with (Gdc(ω)) and without (Gdc(0)) the light excitation, G0 is the quantum of conductance (7.75 x 10 -5 S), ħ the reduced Planck constant, e the electron charge and T(EF ± ħω) is the electron transmission probability through the junction for electrons at EF ± ħω. We calculated Vω, based on a classical electromagnetic model (see the supporting information) and estimated a typical value of 1.3x10 -7 V, leading to a PAT current to be around 10 -17 A at maximum (see supporting information) in the NPSANs (below the detection limit of our equipment, about 10 fA).

We propose that the observed variation of the current is due to a plasmon-induced isomerization (PII) of the AzBT molecules inserted in the NPSANs. This PII effect is observed for cluster-NPSANs devices with an enhanced effect compared to 2D NPSANS due to an antenna effect. We discuss several hypotheses to explain the AzBT isomerization by a LSP excitation.

(i) It was shown that the isomerization of azobenzene molecules can be triggered by an electric field instead of UV and visible light. [START_REF] Alemani | Electric Field-Induced Isomerization of Azobenzene by STM[END_REF] However, this mechanism requires electric fields in the range of few V/nm. Here the plasmonic field Eω is around 3x10 -8 V/nm (see above, Vω ∼1.3x10 -7 V divided by an average gap of 4.5 nm between NPs 15 ), well below the threshold required for an electric field-induced isomerization. Even if we assume a collective 3D antenna effect enhancing the local electric field by ca. 10 (the precise factor depending on the size of NP clusters and its organization, i.e. number of neighboring NPs), the field generated by the LSP excitation remains below this threshold. Moreover, the experimental conditions in Ref. [START_REF] Alemani | Electric Field-Induced Isomerization of Azobenzene by STM[END_REF] are very different from our case involving single molecule measured by STM in UHV (see details in the supplementary information).

(ii) A second hypothesis is a two-photon mechanism. This mechanism was observed by fluorescence experiments on Ag nanoparticles decorated with benzooxazine ring derivatives and diarylethene. [START_REF] Tsuboi | Near-Infrared Continuous-Wave Light Driving a Two-Photon Photochromic Reaction with the Assistance of Localized Surface Plasmon[END_REF][START_REF] Garcia-Amorós | Plasmonic Activation of a Fluorescent Carbazole-Oxazine Switch[END_REF][START_REF] Impellizzeri | Two-Photon Excitation of a Plasmonic Nanoswitch Monitored by Single-Molecule Fluorescence Microscopy[END_REF] Here, the LSP excitation at 590 nm requires a significant molecular absorption at around 295 nm. For the AzBT molecules, the absorption at this wavelength is lower (about 1/3) than at the peak of maximum absorption (at 336 nm) but not zero. [START_REF] Karpe | Oligothiophene-derivatized azobenzene as immobilized photoswitchable conjugated systems[END_REF] However, at low light power excitation as in the present case, the probability of a two-photon absorption is several orders of magnitude weaker than direct absorption.

(iii) Plasmon-induced resonance energy transfer (PIRET) [START_REF] Li | Plasmon-induced resonance energy transfer for solar energy conversion[END_REF] is another possible mechanism that enables energy transfer from the LSP resonance towards the shorter wavelength of the isomerization bands of AzBT. The required spectral overlap between the LSP band and the absorbance of the AzBT molecules exists around 400-500 nm in the AzBT-NPSANs (see Fig. S7 in the supporting information). Distinguishing PIRET and 2-photon process would require measurements varying the light incident power (PIRET scale linearly with light power, albeit 2-photon process scale quadratically), which are not possible with our apparatus. The twophoton absorption probability is 10 -4 -10 -5 smaller than the one predicted to PIRET, [START_REF] Li | Plasmon-induced resonance energy transfer for solar energy conversion[END_REF] thus we conclude that PIRET is the most likely mechanism to explain the PII in AzBT-NPSANs. A plasmon energy transfer process from the NPs to the molecules was also proposed, albeit without molecular isomerization, to explain a light-increase conductance of molecular junctions of hemicyanime dyes. 7,[START_REF] Gholamrezaie | Photovoltaic action in a self-assembled monolayer of hemicyanine dyes on gold from dissociation of surface plasmons[END_REF] Nevertheless, these hypotheses deserve more detailed theoretical calculations, especially to understand the difference between 2D-and cluster-NPSANs.

The time constants measured for the PII (typically from 20 to 100 s, see Figs. 2 and3) are lower compared to the UV-visible photoisomerization time constants (> 10 3 s, Fig. 4 and Table S1 in the supplementary information, if we except the fast one in the case of the trans to cis isomerization). The light powers are not rigorously the same at the various wavelengths (see Methods) but cannot explain such large differences. Following theoretical calculations by Hush et al., [START_REF] Hush | Electron and energy transfer through bridged systems. 6. Molecular switches: the critical field in electric field activated bistable molecules[END_REF] and experimentally reported by Mahmoud, [START_REF] Mahmoud | Electromagnetic Field of Plasmonic Nanoparticles Extends the Photoisomerization Lifetime of Azobenzene[END_REF] the photo-isomerization time constant decreases with increasing the local dynamic electric field induced by the LSP. The observed decrease of PII time constants compared to usual UV-visible isomerization is qualitatively consistent with the dynamic increase of the local electric field induced by the LSP excitation. We also note that PII time constants observed in our work (20-100 s with a light power of ca. 10 mW, see Methods) are qualitatively consistent with the ones reported in Ref. [START_REF] Mahmoud | Electromagnetic Field of Plasmonic Nanoparticles Extends the Photoisomerization Lifetime of Azobenzene[END_REF] (ca. 8 minutes but at a lower light power of 1 mW, thus at a lower plasmonic electric field). Since the plasmonic electric field depends on the inter-NP distance, a related question is whether or not this distance changes upon AzBT isomerization. It is known that trans-cis isomerization of azobenzene functionalized NP embedded in a polymer matrix can induce a large and reversible change of the NP aggregation, due to large change of the dipole moments. [START_REF] Klajn | Writing self-erasing images using metastable nanoparticle "inks&quot[END_REF] Atomic force microscopy images (supporting information, Fig. S9) of the AzBT-NPSANs with molecules in the trans and cis states show no measurable modification of the NP network morphology in our case of NPs deposited on a solid surface. Similarly, the modification of AzBT dipole upon trans-cis isomerization, interacting with the plasmonic dipole, can induce some change in the NP-molecule energy transfer. However, we have not seen any significant changes in the PII kinetics (Fig. 2 and Table 1 in the supplementary information). This is due to the fact that in our experiments, we detect the molecule isomerization kinetics, which is induced by the energy transfer, this latter being much faster (ps regime, see Ref. [START_REF] Li | Plasmon-induced resonance energy transfer for solar energy conversion[END_REF] ) than the AzBT isomerization process itself.

Conclusion

In summary, we demonstrate the electrical detection (change in conductance) of the plasmon-induced isomerization (PII) in a network of gold nanoparticles functionalized with azobenzene derivatives. We rule out several mechanisms, which can also induce conductance changes in molecular devices such as excitonic photocurrent, thermal photoconductance, and plasmon-assisted tunneling (PAT). We discuss several mechanisms for this plasmon-induced isomerization, among them plasmon-induced resonance energy transfer (PIRET) seems more likely than electric field-induced isomerization and two-phonon absorption. The plasmoninduced isomerization is faster (at least a factor 10) than the usual trans-cis isomerization triggered by UV-visible light, which may have practical outcomes for light-driven molecular memories and light reconfigurable molecular circuits.

Methods.

The coplanar titanium/gold (1nm/10nm thick) electrodes connecting the NPSANs were fabricated using standard electron-beam lithography and electron-beam metal deposition on silicon substrate covered by a 220 nm thick thermally grown silicon dioxide. [START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF] The synthesis of AzBT molecules and AzBT functionalized gold NPs (10 nm in diameter) were performed following protocols already used in our previous works. [START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF][START_REF] Karpe | Oligothiophene-derivatized azobenzene as immobilized photoswitchable conjugated systems[END_REF] The 2D-NPSANs were prepared according to already reported methods. [START_REF] Bourgoin | Langmuir-Blodgett films of thiol-capped gold nanoclusters: fabrication and electrical properties[END_REF][START_REF] Santhanam | Self-Assembly of Uniform Monolayer Arrays of Nanoparticles[END_REF] More precisely here, we followed the method of Santhanam et al., [START_REF] Santhanam | Self-Assembly of Uniform Monolayer Arrays of Nanoparticles[END_REF] The electrical measurements were performed with an Agilent4156C semiconductor parameter analyzer. To avoid any degradation of the organic molecules during the measurements, the nanoelectrodes were contacted with a probe station (Suss Microtec PM-5) inside a glove box (MBRAUN) under a atmosphere of nitrogen (less than 0.1 ppm of oxygen or water vapor). Before light illumination, current voltage (I-V) curves were recorded in dark to check the quality of the measured devices. Then, current-time I(t) were recorded, under a constant bias voltage (2.5 V) and under a specific sequence of light excitation (see below).

For the light exposures, an optical fiber, with a 400µm diameter, was brought near the samples inside the glovebox. We used three power LED from Thorslabs, with the following characteristics: i) 365 nm, bandwidth of 10 nm, for the trans-to-cis isomerization, ii) 470 nm, bandwidth of 20 nm, for the cis-to-trans isomerization and iii) 590 nm, bandwidth of 80 nm for the gold plasmonic excitation. The light power values were measured with a calibrated Optical Power and Energy meter PM200 (Thorlabs). They were 10.2 mW, 8.6 mW, and 11.5 mW, on the devices at the output of the optical fiber for the 365, 470 and 590 nm sources, respectively.

After the synthesis of the molecules, the AzBT molecules are mainly in the trans isomer. [START_REF] Monti | Features of the photochemically active state surfaces of azobenzene[END_REF] Before we started our experiments, we ensured all the devices were preconfigured in the either the cis or trans state as follows. Pre-configuration in trans (cis, respectively): the devices were illuminated with the 470 nm (365 nm, respectively) source for typically 1-1.5h.

Then we let the devices in the dark for 10-20 min, before we switched on the 595 nm source (LSP excitation) for 3-15 min till steady state currents were observed.

Associated content.

Supplementary information is available in the online version of the paper. The supplementary information includes: Drift base line corrections, kinetics of the UV-vis photoisomerizations, PAT current calculations, UV-visible absorption spectrum, plasmonic simulations, AFM images of the NPSANs.

Correspondence and request for materials should be addressed to D.V.

Author information.

Correspinding author

dominique.vuillaume@iemn.fr

2D-NPSAN : corrected current

For 2D NPSANs, it was not possible to obtain a stable current during the preconfiguration, the dark period and the LSP excitation. Figure S1 shows a typical current vs. time recorded for a sequence : pre-configuration in trans (470 nm light), dark, LSP excitation (590 nm) and dark. To extract the corrected current during PII shown in Fig. 3-a (main text), we subtracted a liner fit (blue line) from the raw data. Figure S2 shows the data when the AzBT are pre-configured in their cis state, and the fitted base line used to calculate the corrected current shown in Fig. 3-b. For the 2D-NPSANs, the trans to cis isomerization reveals several features, starting by a "fast" kinetics with a time constant of the order of τTC1 = 100 s, followed by slower kinetics characterized by multi-exponential behaviors with time constants larger than ∼10 3 s (Fig. S3). For the cis to trans isomerisation (Fig. S4) three typical time constants are extracted, with a first time constant τCT1 = 8900 s much slower than for the trans to cis case. The same behavior is observed for the cluster-NSANs (Figs. S5 andS6). In the cluster-NPSANs the current levels are higher (as expected due to a more important number of current pathways), again with several time constants. The extracted time constants for the trans-to-cis isomerisation are of the order of τTC1 = 30 s (Fig. S5), again followed by several time constants > 10 3 s. For the cis to trans isomerization, we have τCT1 = 1950 s (Figure S6) for the first time constant. We note that these multiexponential behaviors of the isomerization kinetics are quite similar for the 2Dand cluster-NPSANs, suggesting that the NP organizations are similar in the two types of devices, or that NP organization is of little importance to influence the AzBT isomerization. Table 1 summarizes all the measured time constants. We note that the multi-exponential behaviors of the UV-vis isomerization of AzBT in NPSANs differ from the previously observed behavior (single time constant) of the same molecules in a self-assembled monolayer, albeit an initial faster kinetics with a small amplitude is also observed for AzBT in SAMs. 1 This feature is likely related to a more disordered molecular structure in NPSANs.

PAT current.

The PAT current is calculated from the amplification factor M of the local electric field induced by the LSP between NPs for various NP sizes (10-60 nm) and gap distance between adjacent NPs (1-5 nm). These simulations show (next section 4, Fig. S8) that M does not overcome a value of ∼10 for NPs with a diameter of 10 nm, spaced by an average gap of 4.5 nm and up to ca. 80 for a gap of around 1 nm. With a light power of 11.5 mW at 590 nm, the incident electric field E0 is ∼ 2.9x10 -2 V/cm, as calculated from P=0.5(ε/µ) 0.5 E 2 with P the light power, ε and µ the permittivity and permeability in vacuum. Thus, we estimated Eω = ME0 = 2.9x10 -1 V/cm. Therefore, as the nano-gap between two NP is ∼ 4.5 nm, 2 the average LSP electric field Vω across the NP-molecule-NP gap is of the order of 1.3x10 -7 V. We calculated an upper limit of the PAT-induced change in the molecular conductance ΔGmol = Gdc(ω) -Gdc(0) = 8.25x10 -20 S using Eq. (1) in the main text, considering that the transmission coefficient though the molecule τ(EF between the electrodes and W (40 nm) the width (see Fig. 1-c). Thus, from ΔGmol = 8.25x10 -20 S, we estimated an upper limit for the PAT-induced current ΔI = 1.3x10 -17 A. 4 and size correction has been applied as the diameter of the nanospheres is comparable to the mean free path of the conduction electron in gold. 5 The inter-particle medium is modeled as homogeneous and isotropic, and its dielectric constant has been adjusted in order to match the resonance wavelength of 630 nm obtained in UV-vis measurements (Fig. S7), which led to a value of 3.8, in agreement with literature for organic materials based on azobenzene derivatives. 6 The obtained M-factor is about 6.5 at resonance.

UV-vis spectrum

LSP simulations

Then, we have estimated numerically the M-factor as a function of the gap and radius in a dimer of gold nanospheres embedded in an homogeneous medium of dielectric constant 3.8, at a fixed wavelength of 590 nm corresponding to the experimental LSP excitation. Simulations were done using a multi-particle Mie method. 7 The enhancement factor M reaches about 80 for a gap of 1 nm and radius of 15 nm, and drops to 11 for a gap of 4.5 nm and radius 5nm in our case (red cross). This value is slightly larger than in the modeled NPSAN due to increased confinement of light within the dimer.

Electrical switching of AzBT

In STM studies reported in Ref. 8, the molecules are lying flat on Au(111) surface, and the samples are in UHV and at 5K. Such configuration and conditions are quite different from our case. Moreover, during the STM measurements, the electric field is applied perpendicular to the surface/molecule whereas in our case, various angles exist between the plasmonic electric field and the molecules covering the Au NPs. This is of prime importance because the orientation of the molecule (i.e. its permanent dipole and the component of its polarizability tensor along the electric field direction) is a key factor for the isomerization. 9 We also note that for Au(100) or Cu(111) surfaces, 10 no switching effect was detected,

showing that these observations are surface dependent, and, to the best of our

Figure 1 .

 1 Figure 1. (a) Molecular structure of the azobenzene-dithiophene-alkylthiol (AzBT). (b) Schematic illustration of Au NPs functionalized with AzBT. (c) Scanning electron microscope images of 2D-NPSAN spanning the L ∼150 nm gap between two electrodes (W ∼40 nm) and (d) a cluster-NPSAN nanodevice with ∼ 3-5 layers of AzBT-NPs (inter-electrode distance of ∼

"

Figure 2 .

 2 Figure 2. Measured current vs. time traces under 590 nm excitation for cluster-NPSAN with the AzBT in (a) the trans and (b) the cis configurations measured at 2.5 V with an electrode spacing of ∼200 nm (see Fig. 1d). Blue lines are fits to single exponential functions (time constants of 90 ± 9 s and 103 ± 10 s, respectively). The vertical arrows indicate the change in the current ΔI.

Figure 3 .

 3 Figure 3. Current vs. time traces (corrected from drift) under 590 nm excitation for 2D-NPSAN with the AzBT in (a) the trans and (b) the unstable cis configurations measured 2.5 V

Figure 4 .

 4 Figure 4. Current vs. time traces recorded from 2D-NPSANs, (a) for the trans to cis isomerization (under 365 nm illumination), and (b) for the cis to trans isomerization (under 470 nm illumination). Current vs. time traces recorded from cluster-NPSANs, (c) for the trans to cis isomerization (under 365 nm illumination ), (d) for the cis to trans isomerization (under 470 nm illumination at 2.5 V electrode spacing of ∼150 nm and ∼ 200 nm, respectively).

  by forming a Langmuir film of AzBT-AuNPs floating on the water surface and transferring it by dip-coating onto the lithographed substrates with the electrodes.[START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF] During the formation of the Langmuir film by evaporation of a solution of AzBT-AuNPs in TCE (1,1,2,2-tetrachloroethane), the addition of an excess of hexane to the solvent at the water surface produced the aggregation and formation of multilayers of AzBT-AuNPs, which are transferred on the electrode to fabricate the cluster-NPSANs. Detailed characterizations of the NPSANs were reported in our previous work.[START_REF] Viero | High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices[END_REF] These included: AFM images to determine the average NP diameter (10.2 nm), inter-NP distances (with AzBT in the trans state, 4.5 nm), UV-visible absorption spectra of the AzBT-NPs in solutions and the AzBT-NPSANs on a substrate (trans and cis states) to check the reversible isomerization of the AzBT molecules, XPS analysis to check the chemical composition of the AzBT-NPs from which we deduced the density of ca. 3.2 molecules/nm 2 (i.e. about 2560 AzBT molecules per NP).

ORCIDD.

  Vuillaume: 0000-0002-3362-1669 D. Stievenard: 0000-0003-2748-0254 C. Nijhuis: 0000-0003-3435-4600

! 2 !Figure S1 .!Figure S2 .!Figure S3 .!Figure S4 . 4 !Figure S5 .!Figure S6 .

 2S1S2S3S44S5S6 Figure S1. Measured current for the 2D NSPSAN under a sequence : preconfiguration in trans (470 nm light), dark, LSP excitation (590 nm) and dark.The blue line is a linear function : i = 2.16x10 -10 -2.74x10 -15 t, fitted for 0<t<2500 and extrapolated to subtract the base line in Fig.3-a.

Figure S7 .

 S7 Figure S7. UV-vis absorbance measurement for the trans-AzBT NPSAN deposited on quartz following the same methods as for NSPANs deposited on SiO2 substrate used for electrical measurements (see Methods in the main paper). A transparent

Figure S8 :

 S8 Figure S8: (a) Extinction spectrum of the modeled three-layer cluster-NSPAN, with a dielectric constant of 3.8 for the interparticle medium. (b) M-factor as a function of gap and radius of two gold nanospheres in a homogeneous medium with refractive index 3.8. The field is computed 0.2nm away from one nanosphere surface, inside the gap. The red cross marks a radius of 5 nm and gap 4.5 nm.

! 10 knowledge

 10 , electrical switching of azobenzene has only been reported in Ref. 8.Finally, STM experiments used a static electric field compared to a dynamic one in our case.

7 .

 7 AFM images.

Figure S9 .

 S9 Figure S9. AFM images of the NPSANs with the AzBT in the trans isomer (left)and cis isomer (right). We do not observed a significant modification of the NP network morphology. We only note some random distance variations, which may be due fluctuations during the long time of experiments (light illumination and AFM images). Formation example, the point-to-point distances AB (421 nm), CD (305 nm) and EF (337 nm) vary by -4.9%, -0.1% and +4.1%, between the trans and cis isomers, respectively. This ±5% relative variation applied to the inter-NP distance correspond to ±0.22 nm, i.e below the statistical size distribution of ± 1 nm.2

Table 1 .

 1 Summary of the typical measured time constants under the 590 nm light irradiation for the cluster-NPSANs with the AzBT molecules in the trans state and

	cis state (Figs. 2-a and 2-b); similar data from Figs. 3-a and 3-b for the 2D-
	NPSANs with the AzBT molecules in the trans state only (see main text); and for
	both the 2D-NPSANs and cluster-NPSANs (data from Figs. 4 and S3-S6) under a
	365 nm light irradiation (trans to cis isomerisation) and 470 nm light irradiation
	(cis to trans isomerisation).

  ± ħω) is 1 at the maximum. We converted ΔGmol to ΔI, the PAT-induced current in the NPSAN, according to the following approach. Taking into account that about 200 molecules are inserted in the nanogap between two adjacent NPs (see Molecular Dynamics calculations in Ref. 2), we have ΔGJ = 200ΔGmol with ΔGJ the conductance of one NP-molecules-NP junction. Assuming a hexagonal compact organization of the NPs in the network, the sheet conductance in the NPSAN is ΔG ⬜ = (2/√3)ΔGJ, with ΔG ⬜ = ΔG*L/W; with the NPSAN conductance ΔG=ΔI/V (V the dc applied voltage, 2.5 V), L (150 nm) the length

		590 nm	365 nm trans to cis	470 nm cis to trans
		trans state (s)	cis state (s)	τTC1 (s)	τTCi (s), i≥2	τCTi (s), i≥1
	2D-NPSAN	22/63	n.a.	100	1360, 1650, 2.2x10 4	8900, 3x10 4 , 5.4x10 4
	cluster-NPSAN	90	103	30	1570, 2900, 1.35x10 4	1950, 4200, 1.2x10 4

Acknowledgements.

The French nanotechnology network RENATECH supported the nanotechnology work in clean room. We thank Philippe Blanchard (MOLTECH-Anjou, CNRS, Univ. Angers) for the synthesis of AzBT molecules. CAN acknowledges the Ministry of Education (MOE) of Singapore for supporting this research under award No. MOE2015-T2-2-134. We thank Y. Viero for the AFM images.

Notes

The authors declare no competing financial interests.

Supporting information

Electrical detection of plasmon-induced isomerization in molecule-nanoparticle network devices


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Hynd Remita

Conjugated Polymer Polypyrrole Nanostructures: Synthesis and Photocatalytic Applications

Conjugated polymers (CPs) have been recently widely investigated for their properties and their applications in different fields including photocatalysis. Among the family of CPs, polypyrrole (PPy) has been the most extensively studied owing to its good environmental stability, high electrical conductivity, superior redox properties and easy synthesis. Besides, nanostructured polypyrrole-based nanomaterials are type of active organic materials for photocatalysis, which is one of their emerging applications. Nanostructuration of polypyrrole can reduce the electron-hole recombination because of short charge transfer distances and reactant adsorption, and product desorption can be enhanced owing to the high surface area offered by nanostructures. This review summarizes synthesis of different nanostructures based on π-conjugated polymer polypyrrole, and the latest developments for photocatalytic applications, including degradation of organic pollutants and hydrogen generation.

Introduction

Solar to chemical energy conversion provides a promising way to solve environmental and energy problems. Although solar light has the advantages of being cheap, abundant and clean, it is difficult to use solar energy effectively due to its intermittent nature. Half of the energy that sunlight reaches the surface of the earth comes from visible light photons (400 ~ 750 nm). However, most of inorganic semiconductors (such as TiO2 or ZnO) are only active under UV light, which constitutes 4~5% of the solar spectrum. Therefore, effective use of visible light is one of the biggest challenges in the field of photocatalysis. π-conjugated polymers (such as poly(diphenylbutadiyne) (PDPB), poly (3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), poly(3-hexylthiophene) (P3HT)) can be considered as organic semiconductors, and have many inherent advantages, making them competitive alternatives to inorganic semiconductors for photocatalytic applications. [START_REF] Ghosh | Visible Light-Active Photocatalysis: Nanostructured Catalyst Design, Mechanisms, and Applications[END_REF][START_REF] Ghosh | Visible-light-active photocatalysis: nanostructured catalyst design, mechanisms, and applications[END_REF] First of all, organic semiconductors generally show very high absorption coefficients. In addition, narrow band gap and unique π-conjugated system of organic semiconductors make them easy for photogenerated charge carriers' separation, and the strong polarity can achieve effective charge transfers of both positive and negative charges, which is essential to initiate photoredox reactions.

An emerging and promising research topic consists in the development of organic semiconductors for photocatalytic applications and in particular for transformation of small molecules. Benefiting from the immense potentialities of solution processable organic semiconductors in the field of organic photovoltaics, [START_REF] Kippelen | [END_REF] a lot of attention is put to exploit the photophysical and electrochemical properties of these materials in the solar to fuel driven chemistry. In 1980, Shirakawa et al. introduced for the first time polyacetylene at a photoelectrode for solar energy conversion. 4 The conjugated polymer poly(ρphenylene) was first reported for photocatalytic application in 1985 (see the timeline in Fig. 1) 5 . In the presence of trimethylamine (TEA) or diethylamine, the radiation of an aqueous suspension of poly(ρphenylene) (λ > 290 nm) leads to the photocatalytic H2 generation. In 2009, Domen and Antonetti discovered that graphitic carbon nitride g-C3N4 (melon) acts as a photocatalyst for the hydrogen evolution reaction in the presence of a sacrificial electron donor, and without any co-catalyst. 6 This first reports inspired the research on polymeric photocatalysts in the past decade, including covalent organic frameworks (COF), [7][8][9] covalent triazine-based frameworks (CTFs) pyrene-based conjugated microporous polymers (CMPs) and conjugated poly(azomethine) networks. [10][11][12][13][14][15] It has been demonstrated that conjugated polymer nanostructures such as poly(diphenylbutadyine)(PDPB), 16 poly(3,4-ethylenedioxythiophene)(PEDOT), 17 poly(3hexylthiophene) (P3HT) 18 and polypyrrole (PPy) 19 (synthesized by controlled polymerization in soft templates made by hexagonal mesophases) are very active photocatalysts for water treatment under UV and visible light, with an activity under visible light overcoming that of a plasmonic Ag/TiO2 taking as a reference. 19 These organic semiconductors show very high absorption coefficients and narrow band gaps (with absorption in the visible or near IR range), high stability and high carrier mobility. Their unique π-conjugated systems help photogenerated charge carriers' separation, and the strong polarity can achieve effective charge transfers, which are essential for initiating photoredox reactions.

Despite the great progress made in the pursuit of highly efficient organic photocatalysts, there are still considerable efforts necessary to understand their photocatalytic properties, and important progresses to achieve to increase the photocatalytic yields. In addition, although typical conductive polymers have been widely investigated, there are a few systematic reports on the performance, synthesis and application of conjugated polymers.

PPy has been studied in great details because of its good electrical conductivity, redox properties and environmental stability. The monomer (pyrrole, Py) is easily oxidized, water soluble and commercially available. It has been synthesized for the first time in 1912, 4 and its conductive property was discovered in 1977. 4 PPy nanostructures (NSs) have a wide range of applications such as in batteries, 20 sensors, 21 photovoltaics, 22 self-cleaning surfaces, 23 etc. 24 However, it is only in 2019 that the first studies reported photocatalytic applications of bare PPy NSs. 19,25,26 In this review, we will introduce various synthesis strategies of PPy nanostructures (chemical polymerization, electrochemical polymerization and radiolytic polymerization) and PPy-based nanocomposites for photocatalytic applications (Fig. 2). More importantly, we will also focus on introducing the polymerization mechanisms, physical and chemical properties of PPy, and synthesis of different PPy nanostructures. Finally, the review concludes with the challenges and suggestions for improving the performance of nanostructured polypyrrole-based photocatalysts.

Fig. 1. Timeline showing the key developments in photocatalytic applications using conjugated polymers. 5,6,16,19,25,27 Fig. 2. Overview of polypyrrole nanostructures (PPy NSs) for various photocatalytic applications.

Chemical structures and the nature of electronical conductivity of PPy

π-conjugated polymers are considered as organic semiconductors and consist of a linear series of overlapping pz orbitals with sp 2 or sp hybridization, thereby forming a conjugated chain of delocalized electrons. 28 Polypyrrole is one of the easiest conducting polymers to synthesize and one of the most stable. One-dimensional or two-dimensional PPy structures can be obtained depending on the synthesis conditions. The one-dimensional structure mainly refers to polymerization at the α-α positions (low oxidant concentration). On the other hand, the two-dimensional structure is mainly polymerized at α-β position (high oxidant concentration) (Fig. 3). 29 Fig. 3. One-dimensional (α-α) and two-dimensional (α-β) polypyrrole structures. 29 Neutral (undoped) bulk polypyrrole is a dielectric material with a bandgap of 4 eV. After oxidation (doping), the π-electrons are removed from the HOMO (the highest occupied molecular orbital) to LUMO (the lowest unoccupied molecular orbital). The bandgap is reduced to less than 2.5 eV, and the polymer becomes a semiconductor. [START_REF] Mazumder | Functional Polymers[END_REF] PPy conduction is of particular interest for the mechanism of charge transfer. [START_REF] Tat'yana | [END_REF] The charge carriers are formed by polarons and bipolarons doping. Chemically speaking, the formation of polarons is equivalent to the formation of radical cations and of bipolarons, to dications (Fig. 4). The polarons and bipolarons in PPy are extended structures, distributed in three to four monomer units of the chain. Polymers doping is accomplished in a variety of ways: chemical, electrochemical or photochemical doping, as well as by charge injection at the metal-insulatorsemiconductor (MIS) interface. [START_REF] Nalwa | Handbook of surfaces and interfaces of materials, five-volume set[END_REF] The conductivity of PPy depends on the number of carriers and charge carriers' mobility, which varies greatly depending on its synthesis conditions. Higher mobilities will occur with higher crystallinity, better orientation, and defect-free materials. [START_REF] Ansari | [END_REF] The main changes in conductivity usually occurs in the early stages of doping. [START_REF] Tat'yana | [END_REF]34 Fig. 4. Proposed structures for polarons and bipolarons. 35

Synthesis of polypyrrole (PPy) nanostructures

PPy was for the first time synthesized in 1912. 25 Polymerization of pyrrole monomers (Py) into PPy can be achieved by traditional chemical oxidation (using for example K2S2O8, FeCl3, H2O2) 36 or electrochemical methods. 37 Chemical polymerization usually provides powdery PPy, and can be easily scaled up. Other polymerization methodologies have been also developed such as photo-induced synthesis 38 or radiolysis. 39 Although the polymerization technologies of chemical and electrochemical methods are quite different, the first step of polymerization corresponds to the preliminary oxidation of Py monomers into their ionic form.

In addition, nanostructured conjugated polymers are attractive as advanced nanomaterials due to their performance and versatile applications. By using chemical and electrochemical polymerization, 0D-1D or 3D-nanostructures (NSs) of PPy such as nanoballs, nanorods, nanotubes, nanobowls or nanoclips can be obtained using hard or soft templates or structure-guiding agents (such as methyl orange). 40

Chemical polymerization

Polymerization of Py occurs steadily in the presence of an oxidant (such as FeCl3, ammonium persulfate (APS) or H2O2). [START_REF] Ansari | [END_REF]41 Halogens and organic electron acceptors were also used as oxidants to synthesize PPy. 42,43 Besides, there are many factors such as the nature of the solvent and the oxidant, initial Py/oxidant ratio, reaction duration and temperature, which can affect the yield and the conductivity of PPy. 44 For example, the optimized Fe(III)/Py ratio is 2.4 for a yield in PPy of about 100%. Changing the initial ratio of the reactants affects the yield, but not the chemical component nor conductivity. 45 The shorter times of polymerization and lower temperatures, (0 o C ~ 5 o C), the better conductivity of PPy is obtained. 46 When the oxidant is FeCl3, the overall reaction is as follows (equation 1): One chlorine atom can accept one electron (e -) from three Py unites to be a doped anion.

PPy nanostructures of different shapes can be obtained using templates or surfactants as explained in the next paragraphs.

1.1 PPy nanospheres

The main disadvantages of templated and seeded growth synthetic routes are that the use of templates or seeds can add cost and complexity to the synthesis, and removing the template or the seeds can affect the physical properties of the nanostructures. Emulsion and dispersion polymerizations often need external emulsifiers or stabilizers, which can be difficult to remove. Hong et al. reported a facile way to synthesize uniform nanometer-sized PPy based on water-soluble polymers and metal cations in aqueous solutions without any surfactant nor template (Fig. 5a-c) 47 . The metal cation (Fe 3+ ) was used as an oxidizing agent to initiate the chemical oxidation polymerization of Py, and the facile process provides a great possibility for mass production of PPy NPs. Liao et al. synthesized waterdispersible PPy nanospheres with size less than 100 nm without any template, surfactant, nor functional dopant via introducing 2,4-diaminodiphenylamine as initiator into a reaction mixture containing Py monomer, an oxidant (such as FeCl3), and hydrochloric acid (HCl) (Fig. 5d). 48 The size and morphologies of PPy NPs are dependent on the initiator and oxidant concentrations. and cross-section SEM images of the PPy nanoparticles stacked on a substrate (scale bar: 100 nm). 47 Copyright 2010

Wiley-VCH. (d) Formation mechanism proposed for PPy and carbon nanospheres. 48 Copyright 2010, American Chemical Society.

PPy hollow spheres

PPy hollow spheres providing some immediate advantages over their solid counterparts were used for various applications because of their relatively low densities. During the past decades, different synthetic methods were developed for the PPy hollow spheres preparation, however, the most efficient procedure is based on template-assisted synthesis, e.g., polystyrene latex spheres (PS) 49,50 , SiO2 spheres 51 , metal and metal derivatives as templates (such as Au, Fe, AgCl) [52][53][54] , PPy hollow spheres could be subsequently achieved via removal of the templates. For example, D. Su et al. used poly(methyl methacrylate) (PMMA) nanoparticles (NPs) as templates to synthesize PPy hollow spheres 55 . Fig. 6a is a schematic diagram, which illustrates the synthesis of PPy hollow nanospheres.

The PMMA can be washed away with acetone. SEM and TEM images showed the as-prepared PPy retained the spherical shape of the templates with a homogeneous size distribution, without shrinkage nor structural deformation (Fig. 6b-c). X. Liu et al. demonstrated a simple in situ synthetic method to obtain SiO2/PPy core-shell particles (Fig. 6d). PPy hollow spheres were obtained after removing the SiO2 template 56 (Fig. 6e); Their size and thickness were dependent on the size of SiO2 and the added volume of Py monomer.

PPy nanowires and nanotubes

One dimensional nanowires and nanotubes have attracted much attention in recent years due to the high sensitivity to surface effects and a configured channel for carrying charge and exciton efficiently. 57 The unique geometrical advantages and properties promote the various applications of nanowires and nanotubes. Generally, synthesis of PPy nanotubes or nanowires is based on templates such as anodic aluminum oxide (AAO), 58 structure-guiding agent MO, 59 V2O5 nanofibers. 60 For example, Zhang et al. synthesized one-dimensional PPy nanotubes using FeCl3 as oxidant and V2O5 nanofibers as sacrificial hard templates (Fig. 7a-b ande-f) and the synthetic doped PPy nanotubes with hollow holes can spontaneously reduce the noble metal ions to nanoparticles (Ag, Au, Pd etc.) on the tubes' surface (Fig. 7c-d). 60,61 Methyl orange (MO) was for the first time reported by Yang et al. in 2005 as a simple and easily accessible structure-guiding agent to synthesize PPy nanotubes. 62 MO with a planar hydrophobic section and hydrophilic edge group (-SO - 3 ) is soluble in water, and possesses anionic and no surfactant characteristics in aqueous solution due to the absence of critical micelle concentration. At room temperature (25 o C), MO can dimerize and form oligomers at a concentration of 1 mM and 5~10 mM, respectively. 62 Complexation can be obtained when an organic compound (such as phenol or a dye) and a flocculant (Fe 3+ or Al 3+ ) are together. In the work reported by Yang et al., FeCl3 was used to suppress the electrostatic repulsions between MO aggregates and/or reacts with negatively charged aggregates of MO in solutions, which induces destabilization of the charged particles and formation of amorphous aggregates. 62 PPy hollow nanotubes were obtained in high yield using FeCl3-MO reactive self-degraded template, which the outer and inner diameters are TEM images of PPy-NTs obtained in the presence of MO. 63 Copyright 2017 ESG (Belgrade, Serbia).

Other nanostructures of PPy

Other two-dimensional PPy nanostructures have been developed in the recent years. PPy nanorings obtained by using a soft template ((CTA)2PdBr4 complex), while the Pd complex was synthesized via the reduction reaction between PdBr 2- 4 and the monomer (Py) and finally Pd nanoparticles supported on PPy (Fig. 8a). 64 Liu et al. described for the first time synthesis of 2-D nanoclip structures of PPy by using an oxidative template composed of cetrimonium cations and peroxydisulfate anions in the presence of CTAB (Fig. 8b). The diameter of the homogeneous nanoclips was in the 50~70 nm range. 65 Monodispersed nanobowl sheets of PPy were synthesized by using polystyrene spheres at the aqueous/air interface as templates via chemical polymerization (Fig. 8c). These PPy nanobowls can be easily lifted-off and deposited, in full size, on any flat substrate. 66 L.M. Santino reported on a modified vapor-phase synthesis of polypyrrole nanofibers (PPy), which can conformably coat 3D fibrillar substrates such as hard carbon papers. This facile polymerization method provides the opportunity to deposit PPy NSs onto many electrochemically active materials, but it may not be stable at high temperatures (Fig. 8d-e). 67 with permission from the Royal Society of Chemistry.

2 Electrochemical polymerizations

Electropolymerization limits the reactions on the surface of the electrodes, and the PPy (prepared by this method) normally grows on the surface of the electrode as a film. Typically, PPy NSs grow along with the orientation of the electric field to form special structures. 68 The mechanism of electropolymerized PPy is still controversial although different mechanisms have been proposed so far by Diaz, 69 Kim, 70 Pletcher 71 and Reynold. 72 Among them, Diaz's mechanism is the most frequently encountered in the literature. The reaction process of this mechanism is as follows: 69,73 In the first step, the oxidation of Py leads to a radical cation formation (Scheme 1)

and several resonance forms of this cation are expressed in Scheme 2:

Two unpaired electron density in the α-position of dimerizes by resonance to form a dihydromer dication (Scheme 3), and the two protons loss leads to formation of a dimer (Scheme 4).

The bipyrrole is more easily oxidized due to the its lower oxidation potential than the one of the monomer (Scheme 5).

The resonance cation form reacts with a monomer to form the trimer dication, and then deprotonates to obtain the neutral trimer (Scheme 6). Trimer α-and β-positions can be coupled with the following oligomers even if β-position could not be accessible sterically and α-coupling will predominate (Scheme 7), the longer the length of the chain, the higher number of β-bands formed.

The propagation continues in the same order: oxidation, coupling and deprotonation until the final polymer product is obtained (Scheme 8).

Indeed, Diaz et al. were the first to synthesize PPy by electropolymerization. The method features a combination of several successive reactions such as radical cation formation, radical coupling and deprotonation. However, some steps like propagation and termination are hard to investigate. Besides, many electropolymerization parameters also influence the property of polymerization. 9a-c), which shows that the polymerization process of pyrrole is very sensitive to the reactivity of radical cations. 74 J. Xing et al. showed a facile approach to construct antimicrobial peptide functionalized PPy nanowires array conductive electrodes for bacterial environment electrical signal detection application (Fig. 9d-e). They introduced an antimicrobial peptide (AMP) immobilized PPy nanowires array conductive electrode (PNW-AMP) using facile electrochemical deposition, and further molecular immobilizing process through the existing dopant (dopamine) as an anchor. 75 PPy nanotubes can be synthesized using sacrificial templates (α-Fe2O3 nanowires grown on the steel substrate), and then α-Fe2O3 nanowire template was removed by H3PO4 solution at constant current of 5 mA cm -2 (Fig. 9f). 76 PPy nanowire electrode (PNW-AMP) surface. Insets in (d) and (e) were high magnification images. 75 Copyright 2019

Wiley-VCH. (f) high-magnification SEM image of polypyrrole nanotubes. 76 Copyright 2011, American Chemical Society.

2. 2 PPy microsuckers and microcontainers

Patterned conducting polymers have attained promising applications in various fields. A general strategy for the fabrication of patterned PPy, with precisely controlled microstructures (e.g., polypyrrole microsuckers) was developed by regulating the solid/liquid/gas triphase interface and the electrochemical polymerization. The PPy microsuckers were successfully deposited on micropillars with different top shapes (circular pillars, spindle pillars and hexagonal pillars) (Fig. 10 a). 77 L. Qu et al. prepared PPy microcontainers with morphology like cups, bottles and bowls by electrochemical oxidation of Py in the aqueous solution of β-NSA (β-naphthalene sulfonic acid, an anionic surfactant), 78 camphorsulfonic acid, 79 or polyelectrolyte 80 (Fig. 10b-g). These surfactants coated on the surface of gas bubbles and resulted in forming "soap bubbles", which are negatively charged. They can assemble on the surface of working electrodes under a positive potential during the electropolymerization. These microcontainers with diameters of 20~100 nm and thickness of 10~150 nm stand upright on the working electrode surface. Then, Y. Gao et al. developed a microdroplet template for the electrochemical synthesis of PPy microcontainers by combining the layer-by-layer (LBL) technique (Fig. 10f). 81,82 In the synthesis process, Py was dispersed as microdroplets in aqueous LiClO4 solution by sonication following assembled on the ITO electrode decorated with multilayers of poly(diallyldimethylammoiun chloride) and poly(styrene sulfonic acid) at a positive potential.

Finally, the well-ordered PPy microcontainers formed by the polymerization of the outermost layers of the droplets with the diameter of 1~10 μm, which are smaller than those of the microcontainers prepared by the "soap bubble" template strategy. Electrochemical polymerization provides many advantages compared to chemical methods.

Firstly, the obtained product is an electroactive film, which attaches to the electrode surface and shows high conductivity. Secondly, it is easy to control the mass and the thickness of the film; Thirdly, the film can be controlled directly in the synthesis process. [START_REF] Tat'yana | [END_REF] Using electrochemical polymerization, various complex structures can be achieved.

3 Polymerization by radiolysis

Radiolysis is the interaction of high energy radiation (γ-rays, X-rays, electrons or ions beams) with matter. It is a powerful technique to synthesize metal nanoparticles, polymer nanostructures and composite nanomaterials. 83,84 The interaction of high-energy radiation (γ-rays, X-rays, accelerated electrons or ions beams) with polar solvents such as water generates free radicals (solvated electrons e -s, H • , HO • ) homogeneously in the solution, H3O + and molecular recombination products (H2 and H2O2). [START_REF] Remita | Recent Trends in Radiation Chemistry[END_REF][START_REF] Wishart | Recent trends in radiation chemistry[END_REF] The process of free radical formation in the case of radiolysis of water is as follows (equation radiolysis. [89][90][91][92][93][94] The oxidative or reducing radicals used for polymerization are induced by solvent radiolysis, therefore no external oxidative (or reducing) chemicals are needed to initiate the polymerization as in the case of chemical method. The team synthesized PEDOT polymers for the first time using γ radiolysis in water. 90 In this way, the synthetic procedure is easy and environmentally friendly. In other words, water is used as solvent without addition of chemical oxidants or reductive agents, the oxidative or deducing radicals are induced by water radiolysis.

PPy spherical NSs were synthesized by γ-ray irradiation in water under N2O atmosphere without any templates. While hydroxyl radical is a very oxidative species, N2O was not only used to scavenge solvated electrons, but also to produce more oxidative HO • radicals (e - aq + N2O + H2O

→ HO • + HO -+ N2). Py was polymerized into PPy due to the hydroxyl radicals produced by water radiolysis (Fig. 12a-b). 19,39 On another hand, spherical PPy polymer nanostructures were recently radiosynthesized in dichloromethane, the polymerization being initiated by the generated chloromethyl (CH2Cl˙) and dichloromethyl (CHCl2˙) radicals under N2 during DCM radiolysis, and a higher yield of monomer oxidation as well as a greater efficiency of polymerization can be obtained. Radiolysis provides a facile way to synthesize monodisperse PPy nanospheres. 93,[START_REF] Chouli | Radiation Physics and Chemistry[END_REF] Wang et al. reported synthesis of PPy nanospheres with uniform size about 30.5 nm through γ-ray irradiation in an acidic aqueous solution of Py, and the monodispersed PPy NPs showed highefficient NIR photothermal conversion for cancer therapy (Fig. 12c-d). 96 e) pH = 1; f) pH = 0.8. [START_REF] Wang | [END_REF] Copyright 2017 Wiley-VCH.

4 Photopolymerization

There are few reports about photopolymerization of Py. In photopolymerization process, photogenerated electron transfer plays an important role. Generally, metal complexes (such as ruthenium, cobalt and copper complexes) have been used as photosensitizers and electron acceptors. ions are reduced and incorporated to the PPy matrix (Fig. 13a). 98 Ag/PPy core/shell NPs with a diameter of 60 nm were obtained by one-step UV-induced polymerization in the presence of PVP (polyvinylpyrrolidone), which acts as stabilizer ((Fig. 13b). 99 PPy films can be deposited on silicon substrates by spin coating. These films were polymerized by UV light (172 nm wavelength), and the structures are affected by UV irradiation and exposure time. The UV-photo processing includes two stages: photopolymerization and then PPy film surface etching and modification.

Polypyrrole nanostructures for environment and energy applications

Conjugated polymers with their unique electron system (one-dimensional delocalized conjugated structures), excellent optical, electrical and electrochemical properties are used in various applications (such as actuators, supercapacitor electrodes, ablation of cancer cells, fuel cells and catalysis).

Especially, conjugated polymers have attracted much attention for photovoltaic cells and photocatalysis due to their high conductivity, stability, charge carriers' mobility, broad absorption in the visible light and excellent electrochemical performance. For example, thin films of conjugated polymer are one of the potential alternatives of crystalline silicon photovoltaic cells, which can provide electricity at a lower cost than crystalline silicon solar cells and could be achieved on a large scale. 100 In addition, conjugated polymer nanostructures also emerge as new photocatalysts very efficient under visible light. [16][17][18][19]101 X. Yuan et al. reported for the first-time the high photocatalytic activity of bare PPy NSs for water treatment under visible light and that the PPy NSs are stable with cycling (Fig. 14ac). The NSs were synthesized by chemical oxidation in soft templates (PPy-NSs-c), gamma irradiation (PPy-NSs-γ) and without any template by chemical oxidation (PPy-bulk). The authors demonstrated that nanostructuration is a key factor for the high photocatalytic activity of PPy-NSs. 19 Nanostructuration provides large surface area, which is beneficial for photocatalysis and less defects compared to the bulk counterpart, these defects acting as recombination sites decreasing the photocatalytic activity. Interesting, the HOMO, LUMO energy levels and the band gap can be tuned by the control of the polymerization degree of the conjugated polymer and its nanostructuration. 19,16,17 The proposed photocatalytic mechanism was shown in Fig. 14d. Separated photogenerated charge carries participate in redox reactions during the photocatalytic process and superoxide and hydroxyl radicals are very active species to oxidize organic pollutants into CO2 and H2O (complete mineralization of the pollutant is achieved).

It is worth noting that the photocatalytic activity of PPy NSs was shown to depend on the polymer morphology, particles size, as well as on optical and chemical states. 19 It has been shown recently that PPy polymers radiosynthesized in dichloromethane (DCM) solvent (PPyDCM) exhibit very high photocatalytic activity for water treatment compared to PPy synthesized by radiolysis in water (PPyH2O), and this activity is comparable to that of the best conjugated polymer based photocatalysts synthesized within hexagonal mesophases. [START_REF] Coletta | [END_REF] This activity was explained by longer chain length, the doping state of PPy synthesized in CH2Cl2, by its very small optical band gap (1.30 eV), and also by its large extended absorption band from the UV to the near infrared region. [START_REF] Coletta | [END_REF] These photocatalysts were found to be very stable with cycling. In addition, a great deal of efforts has been devoted to increase the photocatalytic efficiency and to extend the light absorption range of TiO2. Among these approaches, formation of semiconductor heterojunction is an efficient way to enhance the photocatalytic properties. According to the electronic affinity and bandgap of semiconductors, heterostructures can be divided into three types: type I (straddling gap), type II (staggered gap), and type III (broken gap) (Fig. 15). 102 Formation of type II heterostructures is the most investigated and efficient way to decrease the recombination of charge carriers for enhanced photocatalytic activity for degradation of organic pollutants and water splitting.

On the other hand, the formed heterostructures can effectively increase the utilization efficiency of solar energy owing to the synergetic absorption of different semiconductors.

PPy-based heterojunctions can play a pivotal way for photocatalytic applications owing to the narrower band gap and broadened light absorption in the visible region.

1 Polypyrrole-based composites for water treatment

PPy can be combined with inorganic semiconductor nanomaterials for application in photocatalysis. In organic-inorganic nanocomposites, organic materials are usually used to support and separate inorganic nanomaterials, and improve their properties (such as electronic and catalytic properties) by allowing a higher surface area to volume ratio. ) by sonication (5 min) and followed by 2 h stirring in ethanol for water and air treatment. 109 Compared with PPybulk-TiO2 (in this case PPy prepared without any template in water had larger size that PPyNS), the nanocomposite shows a significant improvement in photocatalytic performance under visible light for photodegradation of organic pollutants in water and in air (MO and phenol were taken as model water pollutants and toluene as air pollutant) (Fig. 16a-c). This enhancement of photocatalytic activity was attributed to the nanostructured PPy and to the formation of heterojunction at the interface between PPyNS and TiO2, which promotes the charge carriers' separation (Fig. 16d). Another study showed that PPy/TiO2 can also be used for photocatalytic degradation of polyethylene plastic under sunlight irradiation. 110 In the PPy/TiO2 system, PPy (narrow band gap) can harvest visible light matching the energy levels of the inorganic semiconductor, and subsequently inject photogenerated electrons into the CB of TiO2, which facilitates the electron transfer and decreases the recombination of charge carriers. Time resolved microwave conductivity (TRMC) technique was used to study the dynamics of charge carriers in the TiO2-based composite. Under visible excitation (450 nm), the TRMC signal of PPyNS-TiO2 was higher than that of bare TiO2, which indicated the longer life time of charge carriers due to less recombinations in the PPyNS-TiO2 composite (Fig. 16c). The photocatalytic activity can be improved by the formation of a heterojunction between PPy and TiO2 and this activity is stable with cycling. Bismuth oxyhalides BiOX (X = Cl, Br, I) with unique layered structure featuring (Bi2O2) 2-layers and inter-grown X -layers, which exhibit good photocatalytic performance for pollutant degradation, are believed as the promising photocatalysts. Composite materials based on BiOX with a heterojunction with another semiconductor such as TiO2 show improved photocatalytic activity than the bare materials. 115 In case of BiOX-PPy, computational calculations showed that oxygen vacancies (formed due to strong interactions between BiOCl and polypyrrole (PPy)) work as sites to activate O2 molecules, and therefore the relative barrier energies of NO oxidation were significantly reduced due to the O2 activation process. BiOCl/PPy was more efficient for photocatalytic NO removal than that BiOCl, and the activity increases with the amount of PPy, and BiOCl/PPy photocatalysts exhibited enhanced activity for NO oxidation (NO - 3 ) and inhibited toxic NO2 generation (Fig. 17a-b). 116 Likewise, X. Liu et al. fabricated a hybrid BiOBr-Ag-PPy (BAP) system, which showed superior photocatalytic activity in degradation of triphenylmethane dye (malachite green) and organic pollutant (phenol). In BAP system, PPy and BiOBr serve as electron and hole donors, and Ag NPs act as electron mediators, building a bridge for charge transfer and separation, which significantly enhances the photocatalytic performance (Fig. 17c). 117 Fe2O3 has a smaller band gap compared to TiO2 and ZnO, and F. A. Harraz reported mesoporous heterostructure α-Fe2O3/PPy for MB degradation at room temperature under UV irradiation. After 20 min, the MB was completely degraded in the presence of optimized 10% Py (volume ratio). 122 Very recently, heterojunction nanocomposite PPy@Fe2O3 showed high photocatalytic activity in photoreforming of micro plastic to hydrogen and other value-added organic products. 123 A visiblelight-driven three-dimensional PPy/Zn3In2S6 nanoflower photocatalyst was developed, which presents very high activity for photoreduction of Cr(VI), (100% Cr(VI) (50 mg/L) reduction in 24 min) and MO (10 mg/L) degradation (99.4% degradation in 25 min). 124 Another photocatalyst, Ag2MoO4/PPy nanocomposite, was in situ synthesized and utilized it as a photocatalyst for the degradation of MB (99.9%) and ciprofloxacin (CIP) drug (99.8%), and reduction of Cr (VI) (99%) within 10 min. The composite obtained a higher photocatalytic performance than PPy and Ag2MoO4 alone. 125 Non-metal oxides nanoparticles were also combined to PPy. Carbon dots (CDs) (prepared using waste watermelon seeds (Fig. 18a-b)) were deposited on polypyrrole grafted chitosan, and the composite material exhibits enhanced photocatalytic activity for degradation of toxic 2-chloro phenol (2-CP). 126 Polyoxometallates (POMs) (metal (Mo VI , Ta V , Mo V , W VI , V V , Mo V ) (which are oxide polyanion clusters linked together by shared oxygen atoms to form well-defined frameworks) can store and exchange a large amount of electrons, and have been widely investigated in photocatalysis. 127

1. 2 Polypyrrole-based ternary composites for water treatment

PPy-Ag-TiO2 ternary composite formed by a shell of PPy, a core of TiO2 decorated with Ag NPs showed an important increase of the photocatalytic activity compared with that of single-and twocomponent systems. The enhanced photocatalytic activity may be due to the synergistic effect of PPy, Ag and TiO2 nanostructures in the ternary system (Fig. 19a-b). 129 BiOBr-Ag-PPy photocatalyst shows excellent activity and stability for decomposition of malachite green. 117 Another example of composite photocatalyst is TiO2-polydopamine (PDA)/PPy/cotton, which presents an excellent activity with a ∼96% degradation of methyl orange (MO) under simulated solar irradiation over 3 h, and the degradation efficiency still maintained up to 60% after 10 cycling tests, which showed that the TiO2-PDA/PPy/cotton exhibited a good photocatalytic stability. 130 It has been shown that the PDA/PPy structure can enhance the photocatalytic performance of TiO2 by promoting the separation of photogenerated electron-hole pairs and decreasing charge recombination (Fig. 19c-e). Conductive polypyrrole-polyaniline/TiO2 nanocomposites (PPy-PANI/TiO2) prepared by in situ oxidative copolymerization of pyrrole and aniline monomers in the presence of TiO2 exhibit high visible-light photocatalytic activity for degradation of 4-nitrophenol. 131 The efficiency of PPy-PANI/TiO2 is owing to its conductivity, conjugated structure, as well as to the synergy between polypyrrole, polyaniline and TiO2. Ag3PO4-BiPO4-PPy heterostructures (by co-precipitation hydrothermal technique and oxidative polymerization method) showed significantly improved photocatalytic activity for malachite green degradation. 132 Recently, Y. Lin et al. demonstrated that a spatial separation system of photogenerated carriers in Ag3PO4@MWCNTs@PPy composite presented excellent photocatalytic activity for the degradation of phenol (100% in 20 min) and tetracycline hydrochloride (100% in 5 min). Importantly, the small-sized Ag3PO4 showed higher photocatalytic activity than large-sized. Copyright 2021, Elsevier.

Conclusions and perspectives

In this review, we have summarized various preparation methods (chemical, electrochemical, radiolytic and photo-polymerization) of conjugated polymer polypyrrole to obtain different nanostructures (such as nanoballs, nanotubes, nanoclips, etc.) with or without templates, and the application of polypyrrole-based nanomaterials in photocatalysis (especially for photocatalytic degradation of organic pollutants and H2 generation). Although PPy has been studied for a long time, there are still different questions to be addressed. Firstly, more research is needed to understand the chemical structures during the polymerization since PPy is easy to be overoxidized, and the relationship between changes of chemical structure (such as polymerization degree, doping) and the photocatalytic performance should be further studied. Secondly, it is important to develop simple and effective methods to synthesize large-scale, long-term stable, and more efficient PPy-based materials for practical applications. Properties of these materials can be optimized through rational syntheses.

Fig. 5 (

 5 Fig. 5 (a) Schematic diagram of synthesis process of water-soluble PPy. (b) SEM image of PPy nanoparticles. (c) Tilted

Fig. 6 .

 6 Fig. 6. (a) Schematic diagram of the synthesis of PPy hollow nanospheres. (b) FESEM images of as-prepared PPy hollow nanospheres. (c) TEM image of as-prepared PPy hollow nanospheres; 55 Reproduced from Ref. 49 with permission from the Royal Society of Chemistry. (d) The formation mechanism of SiO 2 /polypyrrole core-shell structure particles and PPy hollow spheres; (e) TEM image of PPy hollow spheres. 56 Copyright 2007 Elsevier B.V.

about 70 nm and 50

 50 nm, respectively. Notably, when the oxidant FeCl3 is replaced by ammonium peroxysulfate (APS), no tubular precipitates are observed after mixture with MO. Yan et al. reported synthesis of PPy nanotubes (PPy-NTs) with uniform diameters (~100 nm) by in situ chemical polymerization of Py with FeCl3 and MO as oxidant and dopant, respectively. They showed that the diameter of the PPy-NTs can be controlled by adjusting the concentration of the reactants, the types of solvents and the reaction temperature (Fig.7g-h). Granular PPy NPs were obtained in the absence of MO.63

Fig. 7 .

 7 Fig. 7. Polypyrrole•Cl nanotubes: (a) SEM, as synthesized. (b) TEM, as synthesized showing a hollow tube. Inset: TEM showing a pore filled with V 2 O 5 . (c) TEM, Ag nanocomposite. Insets: coaxial cable of Ag and particle size distribution (d) TEM, Au nanocomposite; 60 Copyright 2005 American Chemical Society. SEM images of polypyrrole•Cl nanofibers synthesized in ethanol/FeCl 3 using V 2 O 5 as the seed. Prior to the reaction, V 2 O 5 was stirred in ethanol for (e) 30 min and (f) 12 h; 61 Copyright 2004 American Chemical Society. (g) SEM images of PPy-NTs obtained in the presence of MO, (h)

Fig. 8 .

 8 Fig. 8. (a) Schematic diagram of the fabrication of PPy-Pd composite nanorings. 64 Reproduced from Ref. 58 with permission from the Royal Society of Chemistry. (b) SEM images of PPy•Cl nanoclips (scale bar, 1 μm; inset, digital picture of paper clips).65 Copyright 2010 American Chemical Society. (c) SEM images of the PPy-CSA nanobowl sheet.66 Copyright © 2006 WILEY-VCH. (d) SEM of unwashed PPy after vapor phase polymerization onto a hard carbon paper substrate, (e) Upon washing in 6 M HCl and methanol, the fibers relax yet remain separated.67 Reproduced from Ref.61

Fig. 9 (

 9 Fig. 9 (a) SEM images of the polypyrrole nanowires synthesized by cathodic electropolymerization at -0.6 V SCE for 10 min from an aqueous solution containing 0.2 M NaNO 3 and 0.8 M HNO 3 with concentrations of pyrrole 0.20 M, (b) at 0.25 M pyrrole and 0.8 M HNO 3 with 0.2 M NaNO 3 , (c) the changes in electrolyte color before and after cathodic electropolymerization at -0.6 V SCE for 10 min: (left) 0.025 M pyrrole, 0.2 M NaNO 3 and 0.8 M HNO 3 and (right) 0.2 M pyrrole, 0.2 M NaNO 3 and 0.8 M HNO 3 ; 74 Reproduced from Ref. 68 with permission from the Royal Society of Chemistry. (d) SEM image of constructed PPy nanowire electrode (PNW) surface. (e) SEM image of AMP immobilized

Fig. 10 (

 10 Fig. 10 (a) ESEM images of PPy microsuckers deposited on different micropillars, including circular pillars, spindle pillars and hexagonal pillars; 77 Copyright 2018 Wiley-VCH. SEM images of PPy microcontainers and microspheres synthesized by electrochemical polymerizations. (b) Microcontainers doped with camphorsulfonic acid; 78 (c) Microcontainers doped with poly (styrenesulfonic acid); Copyright 2003, American Chemical Society. (d) Microcontainers doped with camphorsulfonic acid and prepared by the CV technique; 79 Copyright 2004, Elsevier. (e) Arranged microcontainers on patterned electrodes; (f) Microspheres doped with naphthalene sulfonic acid; 80 Copyright 2004 Wiley. (g) Microcontainers prepared by electrochemical growth of PPy on layer by layer (LBL) film modified electrode. 82 Copyright 2006, Elsevier.

  , H3O •+ , H˙, HO˙, H2, H2O2 (equation 3) Solvated electrons and H • radicals are strong reducing species while HO • are strong oxidative radicals Eº (HO • /H2O) = 2.2 V (vs SHE).[START_REF] Coletta | [END_REF] These radicals can be used for reduction or oxidation reactions.S. Remita et al. have developed a methodology based on radiation chemistry to synthesizeconducting polymers in aqueous solutions and halo-methane solvents (such as DCM, CH2Cl2)

Fig. 12 (

 12 Fig.12 (a-b) SEM images of PPy-NSs-γ.19 Copyright 2019, Elsevier. Cryo-TEM images of radio synthesized PPy (γ-PPy) at 72 kGy. (c) Nanostructures of γ-PPy; (d) full view of chaplets of γ-PPy.39 Copyright 2014, American Chemical Society. TEM images of the products prepared from γ-ray radiation on an acidic aqueous solution of Py at different pH:

97

 97

C

  .R. Martins et al. prepared PPy/Ag composite via photopolymerization process. The strategy was to use the transition metals ions assisted by UV light to polymerize monomers, and in the same time Ag

38

 38

Fig. 13 (

 13 Fig. 13 (a) SEM of polypyrrole film on glass substrate for exposure of the monomer solution 72 h, 98 (b) TEM images of silver/polypyrrole core/shell particles centrifuged. 99 Copyright 2005, Elsevier.

Fig. 14

 14 Fig. 14 Degradation rates of phenol in the presence of as-prepared samples of PPy under UV-Vis (a) and visible light irradiation (b); (c) Photocatalytic activity of PPy-NS-c with cycling. (d) Possible photocatalytic mechanism with charge separation in PPy nanostructures with electron reducing oxygen and the hole oxidizing water. 19 Copyright 2019 Elsevier.

Fig. 15

 15 Fig. 15 Schematic energy band diagram of three types of semiconductor heterojunctions. 102

  103

4. 1 . 1

 11 Polypyrrole-based binary composites for water treatment a. PPy/TiO2 composites Organic-inorganic semiconductor nanocomposites exhibit both the performance of pure organic and inorganic semiconductor materials, and synergistic effects can be obtained for some applications.It has been shown that PPy decorated with inorganic metal oxides presents large visible light absorption and good photocatalytic activity. For example, PPy/TiO2 nanocomposite particles showed enhanced photocatalytic activity for degradation of pollutants in water ( MO and phenol were used as model pollutant) under visible light irradiation 104, 105 ; PPy-TiO2 composite films at the air-water interface presented higher photocatalytic activity than a suspension of PPy-TiO2 for MO and MB degradation.106 Surface molecularly imprinted polymers do not only show strong affinity towards target contaminants, but also facilitate mass transfer owing to the accessible sites and the PPy/TiO2 nanocomposites exhibit high efficiency for photodegradation of MO. 107 X. Yuan et al. modified commercial P25 TiO2 with nanostructured PPy (PPyNS) (prepared in soft templates formed by hexagonal mesophases108

Fig. 16 (

 16 Fig. 16 (a) Photocatalytic activity of PPy NS and PPy NS -TiO 2 for the toluene degradation under λ = 465 nm irradiation. (b) Photocatalytic degradation rate of phenol under visible light. (c) TRMC (time resolved microwave conductivity) signals of PPy NS -TiO 2 and TiO 2 at excitation wavelength 450 nm irradiation. (d) Proposed photocatalytic mechanism for the PPy NS -TiO 2 system and charge transfer mechanism under visible light. 109 Copyright 2020, Elsevier.

  photocatalytic activity of Bi2WO6 photocatalyst modified with different amounts of polypyrrole (PPy)

Fig. 17 .

 17 Fig. 17. NO removal (a) and NO 2 production (b) of BiOCl and BiOCl/PPy composites under visible light, 116 Reproduced from Ref. 109 with permission from the Royal Society of Chemistry. (c) Schematic of charge carriers transfer in BiOBr-PPy and BiOBr-Ag-PPy. 117

  However, investigations of PPy/POM-based photocatalysts for water depollution are rare. X. Xu et al. reported loading of PPy on a transition metal coordination polymer (TMCP)/POM via a facile in situ polymerization process, and the PPy/CuSiW12 composite shows enhanced photocatalytic activity for degradation of RhB under visible light (Fig. 18c-f).

  128

Fig. 18 (

 18 Fig. 18 (a) PPy/PEG-borate was electrodeposited onto the ZnO microrod arrays; (b) Degradation rates of methylene blue by photolysis under visible light. 126 Copyright 2019 American Chemical Society. (c) Fundamental unit of CuSiW 12 ; (d) 2D framework of CuSiW 12 ; (e) Photocatalytic efficiency of PPy/CuSiW 12 and (f) Diagram of the photocatalytic mechanism for PPy/CuSiW 12 under visible light. 128 Reproduced from Ref. 120 with permission from the Royal Society of Chemistry.

  133

Fig. 19 (

 19 Fig. 19 (a) The visible-induced photocatalytic activity of different samples; (b) Postulated mechanism of the visible light-induced photo-degradation of acetone with PPy-Ag-TiO 2 nanocomposites. 129 Copyright 2013, American Chemical Society. (c) Photodegradation of MO with different samples under simulated solar power for 180 min; (d) UV-vis adsorption spectra showing photo degradation of MO with TiO 2 -PDA/PPy/cotton under 1 kW m -2 illumination; (e) Pictures showing the color change of the MO solution containing TiO 2 -PDA/PPy/cotton. 130 Copyright 2018, American Chemical Society.

4. 2 Fig. 20 (

 220 photocatalytic activity (664 μmol/h/g) compared with a pure Pt loading (341 μmol/h/g) or a pure Ni

Fig. 21 (

 21 Fig. 21 (a) Electron transfer route of PPy-TiO 2 , TEM and SEM images of PPy-TiO 2 ; (b) Yield of H 2 production over time. Two solutions with different weight percent of Pt were tested. Both contained a total of 24 mg of TiO 2 /PPy-Pt nanocomposites and 6.4 mmol of TEA. 137 Reproduced from Ref. 129 with permission from the Royal Society of Chemistry. (c) H 2 evolution by using 0.5 g l -1 modified catalyst, 1.0 wt% Pt and polypyrrole, respectively, 75 ml aqueous

Furthermore, modification

  of PPy nanostructures with cocatalysts (such as Fe, Ni, Cu or NiO, CuO, Sn) and another semiconductor (TiO2, BiVO4 or C3N4) to form heterojunctions will change the molecular structure, band gap, charge carriers' mobility and active sites, which plays an important role in various photocatalytic applications such as water splitting, water oxidation and CO2 reduction. Their possible band gap engineering to optimize light absorption and the charge transfer pathways should be further studied. Finally, PPy nanostructures and PPy-based composite materials are very promising for application in photocatalysis. These materials can be very active for solar light harvesting, and can

  [START_REF] Tat'yana | [END_REF]

  69,71 3. 2.1 PPy nanowires and nanotubesThe electrochemical polymerization of Py favors formation of PPy bulk materials such as conductive films. D.-H. Nam successfully fabricated PPy nanowires with a one-step process by cathodic electropolymerization without templates nor chemical additives. The electrochemically generated NO + was utilized to oxidize neutral Py monomers, making it possible to use oxidable metal substrates such as Cu and Ni. The synthesized nanowires are directly deposited on the Cu substrate as

a thin film, and the kinetics of Py polymerization in cathodic electropolymerization is highly influenced by the radical cations' reactivity. The results showed that PPy nanospheres initially deposited on the Cu substrate gradually changed to nanowire structures in the form of a thin film. The growth kinetics of PPy nanowires was studied by analyzing the effect of the electrolyte and synthetic time (Fig.

have applications in air and water treatment, water splitting, hydrogen and solar fuels' generation, 145 water oxidation, 146,147 and self-cleaning surfaces, 148,149 and studies are conducted to understand the photocatalytic mechanisms involved in PPy-nanostructures and PPy-based nanocomposites. These materials are also promising for CO2 reduction 145,150 and valorization, methane conversion, 151 and NOx reduction. 152

Conflict of Interest Statement:

On behalf of all authors, the corresponding author states that there is no conflict of interest.