The development of green, sustainable, and economical chemical processes represents a cornerstone challenge within chemistry today. Semiconductor heterogeneous photocatalysis is currently utilized within a wide variety of societally impactful processes, spanning reactions such as hydrogen production and CO2 conversion, to the organic transformation of raw materials for value-added chemicals. Metal halide perovskites (MHPs) have recently emerged as a new promising class of cheap and easy to make photocatalytic semiconductors, though their unstable ionically bound crystal structure has thus far restricted widespread application. Recently years, we examine the issues hampering MHP-based photocatalysis and proposal a general approach being taken to achieve promising and stable photocatalytic reaction environments. Specifically, we utilize relatively low-polarity solvents such toluene and trifluorotoluene for CO2 photoreduction and organic transformations over MHPs. However, the effectiveness of single MHP photocatalysts is relatively poor. We further introduction of a second component to form a heterojunction, including Schottky, type II, and Z-scheme heterojunction, to accelerate carrier migration and boost reaction rates, thus increasing the photoactivity.

Lead halide perovskites (LHPs) have demonstrated enormous potentials in a wide range of application including optoelectronic devices and photocatalysis due to their exceptional optical and electronic properties. However, long-term stability and toxicity of LHPs are the main limiting factor towards their practical application. In this view, lead-free halide double perovskites (DPs)_, are under spotlight due to their superior material stability and attractive optoelectronic properties. Nevertheless, relatively larger band gap (> 4eV) of Cs₂AInCl₆ (A = Ag Na) DPs materials are the main bottlenecks towards their light harvesting application including photocatalysis.^[1,2] Moreover, the severe charge recombination and presence of fewer catalytic active sites are considered as main limiting factor towards efficient photocatalytic CO₂ reduction.^[3] The doping and compositional engineering of Cs₂AInCl₆ (A = Ag, Na) DPs are under spotlight due to their dopant-induced extended light harvesting abilities and enhanced self-trapped excitons (STEs) emission for a wide range of applications including optoelectronic devices and photocatalysis.^[1,2,4,5] Furthermore, the catalytic performances of lead-free DP NCs can be further improve by coupling them with two-dimensional (2D) nanosheet (NSs) with desirable energy offset. The  type II heterojunction between DP NCs and 2D NSs can enhance the charge separation and inhibit the charge recombination. It is well established that 2D ultrathin g-C₃N₄ NSs possess large surface area, rich density of catalytic active sites, and superior charge transport. All these features are advantageous for photocatalytic CO₂ reduction. In particular, the interface between Cs₂AgBiBr₆ DP NCs/ g-C₃N₄ 2D NSs would significantly facilitate the charge transfer and separation of photogenerated excitons.

In this talk, I will present the colloidal synthesis of iron-doped Cs₂AInCl₆ (A = Ag, Na) DP NCs, which resulted in a significant extension of absorption edge into the visible part of spectrum. Furthermore, I will discuss how iron doping allows precise tuning of the optical band gap and electronic band structures of the resulting DP NCs and their application in photocatalytic CO₂ reduction. Next, I will also explore our recent results on multifacet spheroidal Cs₂AgBiBr₆ DP NCs/g-C₃N₄ NSs heterostructure, including colloidal synthesis, optical properties and their application in visible-light driven photocatalytic CO₂ reduction.

Lead halide perovskite nanocrystals (LHP NCs) are a state-of-the-art light-harvesting material, prized for their outstanding optoelectronic properties—high absorption coefficient, extensive charge carrier diffusion lengths, and low exciton binding energy. Despite achieving remarkable photovoltaic performance with a photoconversion efficiency exceeding 25% within a decade, the transition of LHP to industrial-scale development encounters obstacles due to moisture intolerance and ambient instability [1].

The inorganic/hybrid core of LHP NCs contributes to their unique photophysical properties, while surface ligands play a crucial role in stabilizing and influencing charge transfer dynamics in diverse environments [2,3]. Therefore, understanding the complex NC-ligand interfacial chemistry is vital for achieving colloidal stability in aqueous and polar media.

Our solution involves optimizing hydrophilic and hydrophobic interactions on the LHP NC surface using engineered multidentate ligands. We developed NKE-12, a bolaamphiphilic ligand derived from lysine (K) and glutamic acid (E), featuring multiple cationic (NH₃⁺) and anionic (COO^-) anchoring groups. Applied in a ligand exchange treatment on CsPbBr₃ NCs, NKE-12 achieved effective surface passivation and water molecule localization away from the inorganic core through hydrophilic interactions [4]. Confirmation of successful ligand exchange was obtained through Fourier transform infrared and X-ray photoelectron spectroscopy. Structural integrity was preserved and validated by transmission electron microscopy and X-ray diffraction patterns. NKE-12 binding interactions on the NC surface were elucidated through multidimensional nuclear magnetic resonance ligand spectroscopy [5].

Deconstructing NKE-12 into NK-12 and NE-12 fragments provided insights into the individual roles of cationic and anionic terminal ends in passivation and solvent phase transfer. The synergistic effect of robust ligand binding and increased hydrophilicity resulted in significant two-week-long water stability for CsPbBr₃/NKE-12 NCs compared to oleylamine/oleic acid capped NCs. Enhanced charge separation and transport capabilities observed through electrochemical measurements prompted the exploration of photocatalytic attributes. Water-stabilized CsPbBr₃/NKE-12 NCs exhibited excellent photocatalytic activity for acrylamide polymerization, showcasing their potential for diverse applications.

Presently, we are expanding ligand engineering, exploring different multi-ionic ligands to further extend water stability and deepen our understanding of photogenerated charge extraction processes. Our findings on ligand design principles for developing moisture-resistant perovskite NCs hold significant promise for applications in photovoltaics and photocatalysis.

Over the past decade, metal halide perovskites (MHP, CsPbX₃: X = Cl, Br, I) have been widely investigated as promising materials for optoelectronics, achieving a record-breaking efficiency in solar cells.^[1] From a fundamental point of view, MHP could be excellent candidates for photocatalysis due to their high photogenerated charge-carrier production and mobility as well as their narrow and tunable bandgap energy.^[2] MHP with tuneable bang-gap energy could be obtained through fast substitution of bromide by iodine or chloride (CsPbBr_3-yX_y : X = Cl, Br, I).^[3]

In this presentation, the electronic properties of MHPs were encapsulated by TiO_2, and their electronic properties were tuned by anion substitution at room temperature. Then, charge-carrier lifetime and dynamics were investigated in CsPbBr_3-yX_y along with interfacial electron transfer from CsPbBr_3-yX_y to TiO₂ by means of time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS). The results show that the bandgap engineering and the position of the conduction band and valence band level in both materials are detrimental for optimal interfacial charge transfer. In addition, the optimal bandgap configuration for the most efficient charge injection positively affects the photocatalytic activity. The encapsulation of MHPs by TiO₂ did improve the stability for hydrogen generation from aqueous solution.

Metal halide perovskites (MHPs) are extremely appealing emerging materials for photocatalytically active heterojunctions, thanks to their ability to promote light-driven reactions. However, a comprehensive understanding of photoexcitation and charge transfer mechanisms, as well as predictive models for photocatalytic performances are still missing. We present here a robust method to investigate photocatalytic activity of MHP heterojunctions in a variety of reactions, that combines in synergy a wide array of experimental techniques with advanced computational modelling. For all the studied compounds, we were able to assess a clear link between the photocatalytic activity and optoelectronic properties of the materials, in particular by exploiting ultrafast spectroscopy techniques to study available transitions and carrier dynamics as a function of the perovskite loading and finding a connection with the performance of the material.

More in detail, we studied heterojunctions obtained by combining graphitic carbon nitride (g-C3N4) with different kinds of lead-free perovskites to enhance selected features. We focused first on double perovskite Cs3Bi2Br9, where hydrogen photogeneration rate was successfully controlled by tuning the perovskite loading in the Cs3Bi2Br9/g-C3N4 mixture.[1] We also studied Ge-based layered 2D perovskite and g-C3N4 for light-induced hydrogen evolution: we showed how, thanks to organic cation engineering, perovskites based on 4-phenylbenzilammonium (PhBz), such as PhBz2GeBr4 and PhBz2GeI4, result in hydrogen production with promising air and water stability.[2] Recently, we have extended our study to a double perovskite mixture, namely Cs2AgBiCl6/g-C3N4, used both for solar-driven hydrogen generation and nitrogen reduction, with an activity strongly depending on the perovskite loading.[3] Through advanced spectroscopic investigation and density function theory (DFT) modelling we have identified the perovskite loading that allows the best performance of the heterojunction and, most importantly, accounted for the microscopic processes responsible for the photocatalytic performance. Our results are showing a systematic approach of MHP-based heterojunctions that is of crucial importance to get the ability to engineer and optimize novel materials for photocatalysis.
 

Metal halide perovskites are arising a great interest of the scientific community because of their opto-electronic properties, as the high extinction coefficient, the low exciton binding energy, the excellent charge transport properties and optimal band gap in the visible range that make them good candidates for photocatalytic applications. Among the different perovskite materials, Cs₃Bi₂Br₉ (CBB) is a particularly interesting lead-free halide perovskite that shows excellent stability and has already been used as a visible light-driven photocatalyst in dye degradation experiments. However, the understanding of the interaction between dyes and the CBB and the mechanism underlying the behavior of the CBB in the photocatalytic system still needs to be fully elucidated [1]. Here, the chemical co-precipitation method was successfully applied to synthesize CBB powder that was then used as photocatalyst for the dye degradation. XRD investigation showed that the prepared Cs₃Bi₂Br₉ powder materials undergoes to structural evolution when exposed to different environment (water, isopropanol, air). Namely, we detected a systematic structural evolution of Cs₃Bi₂Br₉ to the kinetically stable non-perovskite BiOBr phase. In addition, such a transformation was found faster in water, where the conversion in BiOBr was already complete within 24 hours, than in isopropanol and in ambient air [2]. On the basis of the proved CBB higher stability in isopropanol, such a solvent has been selected as reaction medium for the heterogenous photocatalytic experiments carried out monitoring the decoloration of structurally different azodyes, cationic Methylene Blue (MB) and anionic Methyl Orange (MO), at different concentration. Preliminarily, the adsorption of each dye on the CBB under dark condition has been spectroscopically monitored to clarify the interaction between dyes and CBB powder samples. Finally, the photocatalytic experiments have been performed monitoring the dyes decoloration as a function of the catalyst conversion of the Cs₃Bi₂Br₉ in BiOBr, finally proving the beneficial effect of BiOBr in the degradation of both types of cationic MB and anionic MO dyes [3].

Photocatalytically active heterojunctions based on metal halide perovskites (MHPs) are drawing significant interest for their chameleon ability to foster several redox reactions. The lack of mechanistic insights into their performance, however, limits the ability of engineering novel and optimized materials. Herein, we report on a composite system including a double perovskite, Cs2AgBiCl6/g-C3N4, used in parallel for solar-driven hydrogen generation and nitrogen reduction. The composite efficiently promotes the two reactions, but its activity strongly depends on the perovskite/carbon nitride relative amounts. Through advanced spectroscopic investigation and density function theory modelling we studied the H2 and NH3 production reaction mechanisms, finding perovskite halide vacancies as the primary reactive sites for hydrogen generation, withstanding a positive contribution of low loaded g-C3N4, in reducing carrier recombination. For nitrogen reduction, instead, the active sites are g-C3N4 nitrogen vacancies, and the heterojunction best performs at low perovskites loadings, as the composites maximizes light absorption and reduced carrier losses. We believe these insights are important add-ons towards universal exploitation of MHPs in contemporary photocatalysis.

Photocatalytic conversion of water and carbon dioxide using solar energy offers a clean solution to the world energy requirements of a sustainable future. Achieving its full potential depends on developing inexpensive photocatalysts that can efficiently absorb solar light and drive separated photoinduced charges to react with water and carbon dioxide. In this talk, I will present recent developments we have achieved in the preparation of inexpensive photocatalyst composites with enhanced charge transfer separation and photocatalytic performance. For example, I will present fast and convenient mechanochemical syntheses of halide perovskite nanocrystals of CsPbBr₃ and Cs₂AgBiBr₆, together with copper-loaded reduced graphene oxide. I will also present our developed antisolvent crystallizations to prepare composites such as Cs₂AgBiBr₃/bismuthene and Cs₃Bi₂Br₉/g-C₃N₄, all of which are active in the reduction of CO₂ to CO and CH₄. Extensive characterizations of these materials have enabled us to gain insights into their physical and charge-transfer properties and relate them to their photocatalytic activity. These characterizations underline the importance of surface area, reactant adsorption, and charge separation to achieve best performances. Some of these composites such as Cs₃Bi₂Br₉/g-C₃N₄ separate charges via a direct Z-scheme heterojunction, boosting charge separation and photocatalytic conversions.  Altogether, these results will contribute to the rational design and application of halide perovskites for solar fuels and chemicals.

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. Nevertheless, the two main drawbacks of this system, the use of hazardous Pb and the long term stability, still to be open questions that have not been fully addressed. Beyond perovskite solar cells halide perovskite are outstanding for the development of other optoelectronic device, causing that currently the research with these materials widespread to different optoelectronic fields and photocatalytic applications, where again stability and reduction Pb content still at the center of research effort. In the present talk, we will describe our efforts towards the application of these materials for solar-driven hydrogen production coupled to other processes like organic transformations and waste valorization. We will discuss on the rational design of halide perovskites containing non-critical raw materials towards photoelectrochemical processes, and the importance of extracting basic electronic and optical information to understand the carrier dynamics to maximize the performance and stability of these materials. Moreover, proper interrogation tools are needed to validate their photo(electro)catalytic activity and selectivity. The need for integration of the developed materials into tailored photoelectrochemical devices highlight the urgent need for stabilization strategies to move beyond the proof-of-concept stage to relevant technological developments.

Metal halide perovskites nanocrystas (NCs) have promising applications as heterogeneous photocatalysts due to their strong visible-light absorption, band-gap engineering, charge transfer ability and easy recovery. There are several factors that influence the photocatalytic performance, such as NCs sizes, co-catalyst nature, solvent, and atmosphere. Although there are several examples in their use as photocatalyst for the CO₂ reduction and H₂ generation a more limited information is available for organic chemical transformations. Here, the photocatalytic activity of colloidal CsPbBr₃ NCs and Cs₃Sb₂Br₉ NCs for the photoreduction of p-substituted benzyl bromides under visible-light excitation, using a sacrificial electron donor (N,N-diisopropylethylamine or methanol), will be discussed. Interesting the electron donating and electron withdrawing ability of the substituent determines the selectivity of the reaction towards C-C formation or reductive dehalogenation. The role of the organic ligands to drive forward the C-C coupling and the affinity of the benzyl radicals to the metal halide NC surface will be illustrated.

The chemical instability of halide perovskites (HaP) in protic solvents remains the main obstacle to their utilization in photoelectrochemical devices. Despite the implementation of protective strategies for PEC applications[1], [2], systematic studies of (photo)electrochemical processes and the potential corrosion of perovskite/electrolyte interfaces and their impact on energy conversion and stability are still lacking [3].

In this study, we examined the photoelectrochemical behavior of 2D di-phenylethylammonium lead tetra iodide (PEAH₂PbI₄) thin films as a model system to study fundamental photoelectrochemical processes of metal halide perovskites. We found the use of excess PEAI in solution necessary to stabilize the thin films like by chemical equilibrium and a surfactant effect. Introducing the Fe(CN)₆ ^3-/4- redox couple electroactive species and considering PEAI a supporting electrolyte and stabilization agent [4],[5], we studied photo-electrochemical processes at the semiconductor/electrolyte interface by monitoring the open-circuit potential (OCP). Under illumination, a potential inversion was observed for different equilibrium concentrations of the redox probe. In-situ UV-Vis and XRD analyses provided compelling evidence indicating that the potential inversion, indicating either electron or hole accumulation on the PEAH₂PbI₄, leads to different photo-corrosion phenomena. Lead iodide, PbI₂, as well as a phenylethylamine-lead iodide intercalation compound, (PEA)-PbI₂ , are observed as degradation products. Surface photovoltage (SPV) studies revealed that the redox probe ratio determines carrier separation dynamics at the 2D perovskite / electrolyte interface. Based on the SPV results, we conclude that electron transfer to the redox couple is a critical step in suppressing photodegradation of the material in aqueous solutions. These findings shed light on the intricate interplay between carrier dynamics and chemical changes in the context of semiconductor/electrolyte interfaces, providing valuable insights for the development of design strategies to effectively utilize halide perovskites as photoelectrodes. In addition, our findings provide fundamental insight into the photo-corrosion processes of HaPs, which are currently of high interest for many applications foremost solar cells.

Hybrid AMX₃ perovskites (A=Cs, CH₃NH₃; M=Sn, Pb; X=halide) have in the last years revolutionized the scenario of photovoltaic technologies. Despite the extremely fast progress, the materials electronic properties which are key to the performance are relatively little understood. We developed an effective GW method incorporating spin-orbit coupling [1] which allows us to accurately model the electronic, optical and transport properties of halide perovskites, opening the way to new materials design. In parallel, a series of computational simulation carried out using Car-Parrinello molecular dynamics have been performed investigating the nature of the perovskites/TiO₂ interface, the role of moisture in the perovskite degradation process and the effect of the defect on the device working mechanism. Finally, a series of different strategies will be reported to increase the device stability and efficiency.[2] While instability in aqueous environment has long impeded employment of metal halide perovskites for heterogeneous photocatalysis, recent reports have shown that some particular tin halide perovskites (THPs) can be water-stable and active in photocatalytic hydrogen production. To unravel the mechanistic details underlying the photocatalytic activity of THPs, we compare the reactivity of the water-stable and active DMASnBr₃ (DMA = dimethylammonium) perovskite against prototypical MASnI₃ and MASnBr₃ compounds (MA = methylammonium), employing advanced electronic–structure calculations. We find that the binding energy of electron polarons at the surface of THPs, driven by the conduction band energetics, is cardinal for photocatalytic hydrogen reduction.[3] In this framework, the interplay between the A-site cation and halogen is found to play a key role in defining the photoreactivity of the material by tuning the perovskite electronic energy levels. Our study, by elucidating the key steps of the reaction, may assist in development of more stable and efficient materials for photocatalytic hydrogen reduction. The overall picture of our theoretical investigations underlines a crucial role of computational investigation, casting the possibility of performing predictive modeling simulations, in which the properties of a given system are simulated even before the materials laboratory synthesis and characterization. At the same time, computer simulations are shown to offer the required atomistic insight into hitherto inaccessible experimental observables.