Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities

Review Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities Luka ´ s Zdra zil, 1,2,3 Alejandro Cadranel, 1,4,5 Miroslav Medved‘, 3,6 Michal Otyepka, 3,7 Radek Zbo ril, 2,3, * and Dirk M. Guldi 1, * SUMMARY Carbon dots (CDs) are a fascinating class of nanomaterials with a straightforward design by means of an organic chemistry toolbox and an unsurmountable potential in the field of artificial photosynthesis. The vast structural diversity of CDs and the complex photophysics thereof impose, however, significant challenges on their full utilization. Gathering a profound understanding of the structure-activity relationship and precise identification of the photo-catalytically active sites within CDs is crucial. This review summarizes the current understanding of photo-catalytically active CDbased systems. First, we analyze the structural complexity of CDs in the context of hydrogen photo-production, addressing the different roles of CDs in photo-catalytic hydrogen evolution as photosensitizers, co-catalysts, and catalysts. Second, we present the most importantaspectstobeconsideredforthedesignofCDs-based photo-catalysts, focusing on the fine-tuning of optical properties and charge management and discussing the timescales of events in the photo-excited state. Both experimental and theoretical methods relevant to studying structurally complex CDs are outlined. Finally, we share our thoughts on the future opportunities in CDbased photo-catalysis. INTRODUCTION Addressing the dual challenge of meeting the world’s escalating energy needs while simultaneously decreasing carbon dioxide emissions has emerged as a pivotal challenge in the new millennium. The latest report by the U.S. Energy Information Administration reveals that global power consumption in 2023 was around 21.4 terawatts (TW), with projections estimating a surge to 30 TW by 2050. 1 In stark contrast, solar radiation delivers 120,000 TW to the Earth’s surface, a quantity 4,000 times greater than the projected energy consumption for 2050. 2 Efficient harnessing and storage of this immense, perpetual energy source is increasingly seen as a viable route toward sustainable energy management. However, the shift toward renewable sources like solar and wind power brings its own set of challenges, primarily due to their inherently variable nature of energy production. The intermittency, coupled with fluctuating electricity demands, implies that the current output from renewables cannot consistently meet societal electricity needs. 3 To bridge this gap, the development of systems capable of storing harvested energy is critical to offset significant time shifts in energy availability. In the past, several technologies for energy storage have been utilized, including pumped hydro, compressed air, superconducting magnets, capacitors, and batteries. 4 THE BIGGER PICTURE Pioneering nanomaterials that harness and convert light en route toward integrating solar harvesting technologies into our daily lives play a central role in advancing renewable energy solutions. Carbon dots are ubiquitous, biocompatible, and cost effective and offer extraordinary photo-physical properties, including the generation of long-lived chargeseparated states. These attributes render them ideal candidates for widespread application in artificial photosynthesis. However, a comprehensive understanding of their structure-activity relationships remains elusive due to the inherent diversity of photophysical states within carbon dot domains. Significant advancements in the field are currently starting to appear. Deeper atomistic insights into the photo-physical characteristics could steer the precise design of carbon-dot-based photocatalysts and, thereby, propel forward the vast capabilities of light-driven fuel synthesis in this appealing class of nanomaterials. Chem 10, 1–24, September 12, 2024 ª2024 Published by Elsevier Inc. 1 ll Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Artificial photosynthesis has emerged as a promising solution to overcome the challenges associated with the intermittent nature of renewable energy sources. This innovative process enables the conversion of solar energy into chemical forms, offering key advantages such as excellent portability and the high potential to supply high-energydensity materials. 5 Integral to this approach is the concept of solar-driven water-splitting, a process delivering hydrogen as a clean, sustainable, and abundant source of energy. 6,7 Given that seawater covers 71% of the Earth’s surface, its utilization offers a particularly appealing solution for an ample and sustainable source of hydrogen, without compromising the planet’s limited freshwater resources. Many research efforts have been dedicated to maximizing the efficiency of artificial photosynthesis while prioritizing the use of affordable, non-critical, and eco-friendly materials, marking a significant technological hurdle to overcome. Presently, most photo-catalysts are derived from metal binary compounds. These are characterized by defined band gaps that are determined by the respective semiconductor material. To accomplish better efficiencies, a need is seen to adjust the band gaps of these semiconductors. 8 Existing strategies, such as introducing defects and/or doping with different elements, fail to offer a straightforward and scalable way to modify photo-catalytic properties. 9 Additionally, the inflexible crystal structure of conventional photo-catalysts limits their use in (stereo-)specific catalytic reactions, like enzymatic or chiral reactions, due to their inability to render structurally precisely engineered catalytic sites. 10,11 Another challenge is the limited reusability and disposal of such photo-catalysts, primarily due to photo-bleaching, aggregation, and insufficient charge extraction. This prompts the development of new, environmentally friendly photo-catalysts with customizable properties, surpassing the limitations of current metal binary-compound-based systems. A promising alternative is carbon-based systems. Among others, carbon dots (CDs), an intriguing class of nanomaterials, show great potential in this field. 12 They are readily synthesized through scalable methods using various carbon and nitrogen containing precursors—also including waste—making them both cost-effective and recyclable. 13 CDs offer a variety of photo-physical properties, which range from tunable photoluminescence (PL) to the photo-generation of spin-active states (systems where S s0, e.g., S = 1/2 or S = 1), which are essential in photo-catalysis. 14 Furthermore, CDs are biocompatible, non-toxic, dispersible in water, and environmentally friendly. 15 Unlike traditional metal binary semiconductors, their photo-catalytic attributes are adjustable using a variety of organic chemistry functionalizations. 16 Although CDs have primarily been optimized for PL properties, 17 their application in metal-free photo-catalysis remains largely untapped. 18 Although some pioneering works have been conducted, a major obstacle is the large structural diversity and complex multicomponent arrangement of CDs. 19–21 This hinders the precise identification of the sources of photo-catalytic activity within CD domains. Per se, it prevents the rational design and optimization of CD-based photo-catalysts. Overcoming this bottleneck would enable the development of more effective and precisely engineered CDs for solar-driven water splitting. To achieve this, establishing a profound understanding of the complex structure-activity relationships of CDs is essential. Therefore, in this review, we delve into the current understanding of photo-catalytically active CD-based systems. The focus is set on CD systems without co-catalysts. We provide the overall picture of CD-based photo-catalysts, explore the role of CDs, and detail the possibilities of tailoring the pertinent photo-physical characteristics. Additionally, we share our ideas on the future opportunities in photo-catalysis that emerge from a deeper comprehension of the structure and photo-physical processes in CDs. 1 Friedrich-Alexander-Universita ¨t Erlangen-Nu ¨rnberg, FAU Profile Center Solar, Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Egerlandstraße 3, 91058 Erlangen, Germany 2 Nanotechnology Centre, Centre for Energy and Environmental Technologies, VSB – Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic 3 Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research Institute (CATRIN), Palacky University Olomouc, K rı ´ zkovske ´ho 511/8, 779 00 Olomouc, Czech Republic 4 Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Quı ´mica Inorga ´nica, Analı ´tica y Quı ´mica Fı ´sica, Pabello ´n 2, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina 5 CONICET Universidad de Buenos Aires, Instituto de Quı ´mica-Fı ´sica de Materiales, Medio Ambiente y Energı ´a (INQUIMAE), Pabello ´n2, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina 6 Department of Chemistry, Faculty of Natural Sciences, Matej Bel University, Tajovske ´ho 40, 974 01 Banska ´Bystrica, Slovak Republic 7 IT4Innovations, VSB – Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic *Correspondence: [email protected] (R.Z.), [email protected] (D.M.G.) https://doi.org/10.1016/j.chempr.2024.07.018 ll 2Chem 10, 1–24, September 12, 2024 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review STRUCTURAL COMPLEXITY OF CDs AND THE ROUTE TO EFFICIENT PHOTO-CATALYSIS Diverse structure of CDs Ever since their discovery, 22 CDs have captivated the attention of scientists owing to their intricate structure, which still remains a subject of vigorous debate. Originally emerging from the functionalization chemistry of pre-existing small carbon nanoparticles found in carbon nanopowders, 23 CDs were initially recognized as a distinct class of carbon-based quantum-dot-like nanostructures. The application of organic functionalization and surface defect engineering emerged as pivotal approaches to enhance the intrinsic properties of CDs. 20,24 With the advent of bottom-up syntheses using molecular precursors, the perception of CDs has transcended the original narrative rooted in solid-state physics into a broader concept. This shift in perspective has not only redefined CDs within the scientific community and expanded their possible application but has also introduced a transformative approach in their synthesis. 25 Specifically, the selection of precursors, along with the variation in reaction conditions, is intricately linked to the desired properties of the final product and has enabled the design of CDs for specific applications. 26 Apart from that, as-synthesized CDs undergo further modifications through, for example, post-synthetic treatments such as photo-reduction/photo-oxidation 27–29 or functionalization. 30,31 Recently, there has been a gradual shift in focus from the synthesis of individual CDs to their assembly into larger superstructures, a process that unlocks additional exploitable features, such as the communication between excited states of different counterparts of such assemblies. 32–34 The term CD refers to a diverse range of sub-categories, each containing unique atomic structures and degrees of structural disorder. These comprise carbonized polymer dots (CPDs) next to amorphous and minimally carbonized structures. As the degree of carbonization increases, molecular chromophores become embedded within the matrix of carbon nanodots (CNDs) (Figure 1A).Atlargerdegrees of carbonization, we refer to carbon quantum dots (CQDs), characterized by multi-layered graphitic sheets, and graphene quantum dots (GQDs), representing the most crystalline structures based on graphene fragments with up to three layers of thickness. 14 Overall, this multifaceted approach in the development of CDs unlocks a plethora of research directions leading to precisely targeted applications. Nevertheless, the complex nature and structural diversity of CDs renders it challenging to understand the relationships between their structure and their photophysical properties. A leading example is visible light absorption, which is also one of the crucial properties for solar energy harvesting technologies. 35 In previous research, the distinctive absorption of visible light by CDs has been linked to the small size of their p-conjugated sp 2 domains 36,37 and a given level of nitrogen doping. 38–40 These studies also suggested that quantum confinement and a graphitic core are integral. Notably, some controversy arose around these hypotheses. 20 More recent findings indicate that molecular chromophores play a significant role in the spectral characteristics of CDs. For example, experiments with citric acid and formamide at temperatures lower than what is typically needed for any carbonization have resulted in the formation of red-colored suspensions. 21 Implicit is that the visible light absorption of green-absorbing CDs might stem from N-heterop-conjugated molecular chromophores formed during the chemical reactions with the N-centers in amides. 21,41–43 The argument becomes even more compelling after the identification of visible-light-absorbing porphyrinoids in CDs synthesized from glutathione and formamide (Figure 1B). 44,45 Furthermore, the development of CDbased superstructures also enhances the light absorption across the visible and ll Chem 10, 1–24, September 12, 2024 3 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review near-infrared parts of the solar spectrum. 46 The aforementioned points reveal the intricate and multifaceted nature of visible light absorption of CDs, which is, moreover, a fundamental process that directly relates to their ability to function as efficient photo-sensitizers in photo-catalytic reactions or even to serve as photo-catalysts on their own. Role of CDs in photo-catalytic systems for HER Photo-production of high-energy molecules often depends on effectively linking the single-electron processes of the photo-sensitizer to a multi-electron catalytic reaction. 47 Increasing the efficiency of the photo-catalytic system requires both fine-tuning of the optical properties of the photo-sensitizer and its interaction with the active site. This is meant to inhibit competitive radiative relaxation channels. Specifically, as a complement to absorb light across a wide range of the solar spectrum, an essential characteristic of photo-sensitizers is their capacity to effectively transfer the photogenerated charges to catalytically active centers. 12 As already discussed, CDs absorb visible light, which underlines their significance in the field. Additionally, the surface of CDs is tailorable by means of organic chemistry to facilitate interactions with catalysts or redox mediators. 48 These benefits over traditional semiconductor photo-sensitizers have been widely recognized in the past and utilized for solar-driven hydrogen evolution reaction (HER), particularly in systems that combine CDs with metal complexes, 49–54 redox enzymes, 48,55 TiO 2 nanotubes, 56,57 or platinum (Pt) nanoparticles. 58,59 The next steps in advancing the development of CD-based photo-catalysts aim at limiting their complexity to CD particles. This was, for example, achieved by anchoring Pt single atoms onto CDs, 60,61 leading to an efficient HER. The holy grail, however, lies in the fabrication of a sole, metal-free CD photo-catalyst. Despite the rapidly increasing interest in this field, only a few cases that Figure 1. Structural diversity of CDs (A) Evolution of diverse structures of CDs during syntheses based on the use of molecular precursors. (B) Functional units associated with the visible light absorption of CDs. ll 4Chem 10, 1–24, September 12, 2024 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review unambiguously document CDs as the only photo-catalytically active species are known to date. 62–66 HER rates, nevertheless, were rather minor. Recently, a gamechanger was realized by using citric acid/urea-based CDs that reached record HER activities of 15 mmol(H 2 )g(catalyst) 1 h –1 in water (pH = 8) and 20 mmol(H 2 )g(catalyst) 1 h –1 in seawater (Figure 2). 29 The difference in activity was attributed to the varying ionic strength of the two media. 29 A similar enhancement in photo-catalytic activity was observed when CDs were used as co-catalysts for the hydrogen peroxide photo-production in seawater. 67 A possible mechanism was based on the ionization of the CD’s surface functional groups due to the presence of metal cations in seawater. Ionization was proposed to enhance the electron sink effect of CDs, to prolong the separation time of electrons and holes, and to improve the catalytic activity of the photo-catalyst. 67 In addition to this groundbreaking activity, control experiments revealed that citrazinic acid (CZA), a well-known molecular fluorophore (MF) that is formed during the synthesis, 68–71 is capable of HER by itself. 29 Interestingly, an analogous link between the photo-physical properties of CDs containing MFs and those of the standalone MFs was observed in water-to-ice-induced PL quenching 72 and in Hg 2+ sensing. 73 Although these are not directly correlated to HER, the fact that a similar phenomenon was observed across various application fields highlights the involvement of MFs in the photo-physical processes taking place in CDs and suggests MFs should be studied in further detail within the context of HER applications of CDs. It emphasizes the critical needs for a deeper understanding of the structure-activity relationship and the precise identification of photo-catalytically active components within the CD domains. These are crucial prerequisites for the knowledge-driven design of efficient CD-based HER catalytic platforms. Identification of the photo-catalytically active site within CDs and limitations of the current state of the art The exploration of active sites in CDs for HER has evolved significantly over time (Figure 3). Initial studies have indicated that CDs can effectively facilitate HER when exposed to UV-light irradiation and with methanol as a sacrificial electron donor (SED). 63,65 However, these studies have offered only limited insights into Figure 2. Champion HER photo-catalyst based on just CDs (A) Preparation of CDs from aqueous solution of citric acid and urea under pressure (for simplicity abbreviated as CD) as well as photo-oxidized oCD-1 and oCD-2. (B) Photo-catalytic HER activity as a function of catalyst identity (left, 2 mg catalyst, pH 8.5, 10 vol % TEOA, 6 h, distilled water), pH (center, 2 mg oCD-2, 10 vol % TEOA, 6 h, distilled water), and reaction medium (right, optimized conditions with 0.015 mg oCD-2, pH 8.5, 10 vol % TEOA, 1 h). 29 ll Chem 10, 1–24, September 12, 2024 5 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review the complex mechanism of HER. The photo-catalytic activity was tentatively attributed to the quantum confinement effect, with the CD’s conduction band reaching suitable reduction potentials for HER. Nonetheless, the authors did not explore the structure-activity relationships, nor did they provide any details on the dynamics within the photo-catalytic system. 63 In a related context, pristine CDs and nitrogendoped CDs were evaluated in photo-induced electron transfer reactions. 74 These reactions involved both amorphous as well as graphitized forms of the CDs on one hand, and methyl viologen dichloride (MV 2+ ) on the other hand, and were conducted in the presence of ethylenediaminetetraacetic acid (EDTA) as SED. None of the observed photo-catalytic activities correlated with the key photo-physical properties of CDs, such as PL quantum yield (PL QY), molar extinction coefficient, or excitedstate lifetime. In a related study, the trade-off between PL QY and photo-catalytic activity in HER from water was demonstrated. 62 ItwasfoundthatCDswithmoderate nitrogen-to-carbon (N/C) ratios feature predominantly graphitic nitrogen centers and exhibit high PL QY. In contrast, CDs produced with very high or very low N/C ratios predominantly incorporate nitrogen centers at the edges of the aromatic domains. Peripheral nitrogen sites are key in promoting charge trapping and facilitating charge separation. Both significantly enhance photo-catalytic HER. 62 This research provided a crucial perspective on the two-domain structural organization of CDs. Important features include aromatic domains and regions that are enriched with a higher concentration of partially charged heteroatoms. Such an arrangement is a well-known concept in the field. But, it also represents the first tool for engineering CDs to achieve a better charge separation by means of increasing Coulombic gradients between these two regions. 75,76 Although this concept provides some insights into the structure-activity relationships, its utility is limited due to the degree of randomization that is linked to the incomplete knowledge about the exact CD structure. Such limitations render it challenging to precisely tailor CD-based photo-catalysts. Furthermore, similar to the trend of visible-light absorption, a clear Figure 3. Role of CDs in photo-catalytic systems Schematic representations of photo-catalytic systems comprising (A) a CD as photo-sensitizer and a separate catalyst and (B) a CD as photo-sensitizer and catalyst. ll 6Chem 10, 1–24, September 12, 2024 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review correlation between the HER activity of CDs and the presence of MFs was drawn. 29 For example, the photo-oxidation of CDs derived from citric acid and urea, resulting in high HER activity, was rationalized by the presence of dimers and trimers of CZA within the flexible structure of CDs. Interestingly, standalone CZAs exhibit HER activity approximately half of that observed for the corresponding CDs, suggesting that CZA-based MFs primarily contribute to the photo-catalytic nature of CDs. 29 At first glance, these two perspectives appear disconnected. A closer look reveals a common denominator. Highly carbonized CDs, 62 as well as CZA-based CDs, 29 were found to contain nitrogen-rich sites. Next to boosting charge separation, the nitrogen sites within organic structures are photo-basic, 77 which eases proton abstraction. 78 It was documented in recent studies that the photo-basic pyridinic nitrogen sites in CDs, which were prepared from citric acid and urea, not only enable the rapid proton abstraction from water in the excited state but also enhance HER activity. 79 To some extent, this represents a pontoon bridge between the two main opposing views on CDs; that is, well-carbonized structures vs. molecular-like assembly (Table 1). Still, the precise role of the CD matrix and its interaction with the active sites remains rather unclear. 29 Thus, gaining deeper insights into the intricate electronic interactions within the CD domain is crucial for achieving a comprehensive understanding of the photo-physical properties leading to efficient HER. So far, we examined specifically the structure of CDs leading to their exclusive role in HER. Considerable progress has been seen in recent years, especially with respect to transcending the basic model of matching the valence/conduction bands to the thermodynamic requirements for HER redox potential. 63 This uncovered, at least in part, the interactions between the individual domains within the CDs and the nature of the catalytically active sites. A comprehensive understanding of their roles remains rather elusive. A key point is whether MFs are covalently or non-covalently embedded inside CDs, attached to the CD surface, or whether they are freely diffusing. These scenarios can be scrutinized using fluorescence correlation spectroscopy (FCS). 80 Theoretical calculations also provide important insights into the changes of optical properties of MFs due to structural integration into CDs. 81,82 For further elaboration, we revisit the significant difference in the HER activity between CZA materials and complex citric acid-/urea-based CDs. 29 Intuitively, several assumptions should be considered. First, many CZA-like active sites are associated to or trapped within the carbon backbone of a single CD. This assumption is supported by the fact that CZAs need to be assembled into dimers or trimers to exhibit efficient HER. Such a formation implies that different CZA MFs are in close contact with each other. 29 Furthermore, CZA dimerization/trimerization rates by means of photo-oxidation were significantly faster in CDs than in just CZA. This suggests that CZAs are not just existing as freely diffusing entities and aggregating but that these MFs are associated to CDs with electronic interactions between them. 29 Table 1. Comparison of CDs with various structures, active sites, and HER activity Precursors Synthesis Structure Active site HER activity (mmol g 1 h –1 ) Reference Carbon nanotubes hydrothermal crystalline whole CD 3,615 Yang et al. 63 Citric acid/BPEI a microwave crystalline peripheral N sites 19 Bhattacharyya et al. 62 Citric acid/urea microwave flexible CZA dimers/trimers 19,700 Jana et al. 29 PEG/acridine solvothermal crystalline acridine moieties 5.5 Fang et al. 78 Citric acid/urea hydrothermal flexible pyridinic N sites 39 Fang et al. 79 a BPEI, branched polyethyleneimine. ll Chem 10, 1–24, September 12, 2024 7 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review Independent theoretical studies on another prototypical MF, namely 5-oxo-1,2,3,5tetrahydroimidazo-[1,2-a]-pyridine-7-carboxylic acid (IPCA), have demonstrated that IPCAs are not only integrated into the CD structure but also determine the CD’s overall optical properties, indirectly supporting this molecular-based perspective. 81 Consequently, the CD matrix contains more active sites compared with just CZA. Such a ‘‘quantitative’’ perspective was further reinforced by a comparative study showing that CDs richer in pyridinic active sites exhibit higher HER activity. It is important to acknowledge that the overall picture of CDs appears quite different whenthepresenceofMFwithincitric acid-/urea-based CDs is omitted. 79 Apart from that, the carbon backbone is likely to increase the reductive capacity of the active sites. In this regard, an unambiguous assignment is precluded by the complexity of the overall system. In Stern-Volmer analysis, higher quenching constants are expected to correlate with enhanced interactions with SEDs and with a boost of the CD reduction potential. Stern-Volmer constants for CZA materials and CDs were, however, found to be very similar. In other words, no direct evidence for this effect was attained. 29 A similar mechanism was inferred when combining CDs with intrinsically photo-basic acridines, where the CDs act as electron reservoirs for the active sites. Just as in the case of CZA materials, acridine molecules themselves exhibited HER activity. A nearly 3-fold increase in HER was observed once acridines were incorporated into the CDs, suggesting an additional reducing force from the CD domain. Moreover, steady-state PL spectroscopy and cyclic voltammetry measurements indicated a suitable band alignment for electron transfer from CDs to acridines. However, once again, no direct evidence for any charge transfer (CT) between the different components was provided. 78 Overall, it is plausible to assume that the presence of multiple MF-like active sites per CD particle, and a positive influence of the carbon backbone on the electron transfer processes contribute to a complex photo-physical cascade leading to HER. Nevertheless, from current state-of-the-art, it is impossible to draw any definitive conclusions. Such a gap in the current knowledge represents a major obstacle hindering the rational design of CDs for HER applications. Future research efforts should be directed toward revealing the exact structure and role of each photo-active center and its contribution to the complex photo-catalytic cycle. Role of purification in synthesis of CD-based photo-catalysts Another challenge, particularly significant for CDs that are synthesized from molecular precursors via a solvothermal route, is the formation of by-products through competitive reaction pathways. Their presence not only affects the carbonization of the precursors to afford CDs but also influences the basic chemical transformations leading to the formation of MFs and their aggregates. 83,84 It is rather likely that by-products and impurities impact the characteristics of CD dispersions, including their optical properties and their photo-catalytic performance. Therefore, associating such characteristics with the intrinsic features of CDs should be backedup by measurements with carefully purified samples. In fact, an examination of over 550 publications on CDs indicates that in nearly half of them insufficient purification techniques were employed. 85 Also, it is crucial to recognize that microscopic techniques such as transmission electron microscopy (TEM) and atomic force microscopy (AFM) might lead to unreliable differentiations between CDs and aggregates of organic molecules. 86,87 Another controversy arises around the mismatch between the optical properties of solutions containing CDs and the structures observed in TEM. For example, the application of FCS for CDs solvothermally synthesized from phenylenediamines has demonstrated that it is not the carbon domains seen in the TEM images that cause PL but rather supramolecular structures of small ll 8Chem 10, 1–24, September 12, 2024 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review organics that were present in solution. 80,87 In addition, when polydisperse CDs are purified via, for example, size-exclusion chromatography, the optical properties of the different fractions vary considerably. 38,88 Given that the optical properties of CDs are usually associated with their HER activity, this phenomenon is fundamental. Indeed, a 2-fold increase in HER activity was observed with monodispersed CDs compared with the synthesized polydisperse CDs. This finding underscores the size-dependent impact on the photo-catalytic performance (Figure 4) 89 and the need for performing multi-level purification. Implicit is filtration/dialysis, followed by ion-exchange or size-exclusion chromatography, and rigorous characterization of CDs before attributing any of their photo-physical properties. DESIGN OF PHOTO-CATALYTICALLY ACTIVE CDs The successful design of CDs for artificial photosynthesis applications requires deep understanding of the relationships between their structure and photo-physical properties (Figure 5). As exemplified in previous studies, elucidating such links is anything but straightforward. On one hand, we should consider the limited knowledge about their structure and multifaceted character of the optically active components. On the other hand, it is the complexity of the photo-physical processes that take place in the photo-catalytic cycle that should be thought of. Starting at the initial photo-activation of CDs, various competitive radiative and non-radiative de-excitation processes, which are accompanied by structural relaxation, need to be considered before charge separation and charge trapping at specific sites commences. By virtue of their structural and electronic characteristics, trap states might be photo-catalytically active with respect to a targeted reaction. Needless to say, all these processes are affected by reaction conditions, presence of SEDs, electron acceptors, radical scavengers, etc., as well as environmental effects. Addressing any of these issues calls for interdisciplinary efforts between experimentalists and theoreticians. Tailoring the optical properties of CDs Until now, CD optimization has primarily been focused on PL characteristics, including PL QY. The emphasis on PL markers was driven by the demand for red and near-infrared emissive CDs in bioimaging applications. In these spectral regions, high tissue penetration is secured. 90 CDs have traditionally exhibited strong Figure 4. Purification of CDs A schematic diagram showing the purification of polydisperse CDs, using dialysis, gel filtration, and size exclusion chromatography, alongside their size-dependent HER activity. 89 ll Chem 10, 1–24, September 12, 2024 9 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review The predictive power of the multiscale approach was demonstrated by investigating the structural features and absorption/emission characteristics of several types of CDs in several combinations of classical MD simulations and QM methods (mostly TD-DFT). 81,82,136 These studied provided useful insights into the dynamics, structural organization, and interplay of PL centers in CDs. In the case of aspartic acidbased CDs, theoretical analysis of the singlet and triplet excited state of model CD fragments elucidated the nature of spin species in CDs photo-irradiated by UV light, in agreement with X-band LEPR measurements, and rationalized the generation of CS states, which played a key role in the photo-catalytic formation of hydrogen peroxide. 76 Such hand-in-hand combinations of state-of-the-art theoretical approaches and modern experimental techniques offer a very efficient strategy for studying CDs. Nevertheless, it should be highlighted that the optically active region(s) in CDs are large, multifaceted, and communicate with each other. Under such conditions, the size of QM regions in simulations might well become unfeasible even for TD-DFT. In addition, it would be of high interest to explore the dynamics of excited-state transformations in reasonably large representative models, e.g., by using non-adiabatic surface hopping (SH) dynamics simulations, which are even more computationally demanding. Therefore, computationally feasible approaches to predict the electronic structure of CDs will need to be used. However, such cheaper computational approaches rely on semi-empirical (SE) QM methods or introduce various approximations, and thus their performance has to be carefully assessed. Due to their modest computational costs, SE QM methods enable the study of large systems (containing thousands of atoms), and they have already been employed to study the electronic structure, optical properties, and/or chemical reactivity of CDs. 107,137 They have also been used for investigating the non-adiabatic excited-state dynamics of large systems, although not yet for CDs. In particular, the OM2/MRCI approach 138 was successfully applied in numerous on-the-fly non-adiabatic excited-state MD simulations Figure 8. Computational approaches used to describe the photo-physics in carbon dots (A) Cascade of electronic transitions within the vertical excitation approximation for a stacked complex of polycyclic aromatic hydrocarbons (PAHs) leading to the formation of a charge transfer state (D, electron donor; A, electron acceptor; LE, local excitation; ET, energy transfer; CT, charge transfer; insets: electron density difference plots with the blue/red color indicating decrease/increase of the electron density upon excitation). (B) Jablonski diagram as a base for application of static computational approaches. (C) Energy diagram for a selected nonadiabatic dynamics trajectory of excited stacked PAHs model evolving toward a CT state. ll 16 Chem 10, 1–24, September 12, 2024 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review for various sizeable organic systems, providing reasonable estimates of excitedstate lifetimes. 139 The density-functional-based tight binding (DFTB) method, 140 which is an SE method defined within the DFT framework, scales 2–3 times faster compared with DFT methods. Its time-dependent formulation (TD-DFTB) combined with SH appears to be a highly efficient alternative to reduce the computational cost of electronic structure calculations. 141 In particular, its TD-lc-DFTB version, including long-range exchange corrections, is a valuable tool to investigate charge and energy transfer processes in carbon-based systems. 142 Another promising alternative is to use simplified TDDFT (sTDDFT) and its Tamm-Dancoff approximations (sTDAs), 143 which also include long-range exchange corrections. 144 Their reliability was proven through a comparison of the experimental and calculated UV-visible absorption and circular dichroism spectra of various types of systems. By restricting the configuration space to a specified energy range of excitations, these methods can achieve up to a 2–3 orders of magnitude speedup in comparison with conventional TDDFT. When modeling the excited-state dynamics of larger CD models, the SH procedure is typically coupled with one of the cheaper methods mentioned. Alternatively, large structures might be treated fully at the QM level by employing the classical reaction path approximation 145 in which trajectories are propagated in the ground state while the hops are realized between excited states, assuming it is safe to neglect the nuclear distortions due to the excited-state dynamics. 146 To analyze the nature of critical excited states (including the electronic character of the catalytically active site), one can rely on methods that extract local and CT contributions in excited states by analyzing density difference and transition density matrices 147,148 and/or the multistate energy decomposition analysis. 149 In the meantime, we are witnessing rapid advancements in machine learning (ML) methods. ML, a subset of artificial intelligence, empowers computers to learn from data and make predictions or decisions. It holds significant potential in assisting the synthesis and design of CDs by leveraging existing information on synthesis techniques and known CD properties. ML can help identify the optimal combination of precursors, reaction conditions, and post-treatment methods to achieve desired optical properties in CDs. Evaluation of various regression ML models revealed that the XGBoost regressor outperformed others for ethylenediamine-based CDs, leadingtoenhancedPLQY. 150 Furthermore, regression-based models accurately predicted properties of red and NIR-emissive CDs from diverse sources, demonstrating the predictive capability of such models. 151 A deep convolution neural network (DCNN) model successfully predicted emission color and excitation dependence for CDs, particularly in lower optical spectra wavelengths, with an accuracy of up to 81%, 152 albeit less accurately in the red region. In efforts to enhance predictive power, artificial neural networks (ANNs) were utilized. Hybrid models combining color classification and ANN k-ensemble models achieved an impressive 94% accuracy. 153 Although ML-based methods offer valuable insights for optimizing CD properties, considerations beyond model accuracy, such as interpretability, are crucial. Many ML tools, especially ANNs, lack interpretability, prompting interest in explainable AI (EAI). EAI aims to provide transparent and interpretable explanations for AI system decisions and actions, although its application to CD properties remains unexplored. 154 Besides their high predictive power for properties and reactivity of different types materials, ML tools have been extensively employed to significantly speed up ground-state MD simulations via development of ML potentials, which enabled the exploration of various complex systems at large timescales, often approaching the accuracy of QM computations. 155 Importantly, significant efforts have also been made to apply ML to photo-dynamics simulations via combining the ML excited-state potentials trained on QM data with non-adiabatic excited-state ll Chem 10, 1–24, September 12, 2024 17 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review MD protocols, which promises to make the computational exploration of photophysical transformations in CDs feasible. 156,157 To sum up, ML presents a plethora of advanced tools that can drive the future development of photo-catalytically active CDs. CONCLUSIONS AND OUTLOOK The field of CDs has undergone a significant transformation in recent years. The evolution is not just in the perception of CD structures, which have shifted from being viewed as well-carbonized, carbon-based quantum-dot-like nanostructures to more dynamic, MF-containing carbon domains, but also in their application scope. 20 Nowadays, the applications of CDs are going beyond conventional light-emitting processes, with charge management emerging as a key driver, thereby paving the way for their use in photo-catalysis. Although significant progress has been made in this emerging field, akin to the situation with CD emission properties, the limited understanding of structural features remains one of the major unresolved issues. 14 Therefore, the first step in the rational development of an efficient CD-based photocatalyst is to accurately comprehend the structure-activity relationship and to understand not only the exact role of individual photo-active centers within the CD domain but also their mutual interactions. This task is challenging in its own right due to the inherent variability of CDs. It is, however, achievable through rigorous characterization in combination with computational chemistry. Such a deep understanding, combined with a plethora of precursors for synthesis of CD-based photo-catalysts reflecting particle properties at the molecular level, 19 will eventually enable the precise tailoring of adaptable structures, suitable not only for HER but also for other important photo-catalytic reactions such as CO 2 photo-reduction or H 2 O 2 photoproduction. Moreover, the efficiency of the underlying design procedure will be enhanced by implementing ML techniques. Moving forward, it will also be crucial to precisely dissect the photo-catalytic cycle and analyze each individual reaction step—a multidisciplinary task that continues to pose formidable challenges to the field. Thoroughly unraveling the kinetics of each step is essential, as it not only deepens the fundamental understanding of photo-catalytic mechanisms but also unlocks pathways en route toward higher energy conversion efficiencies. Consequently, a pivotal future task will be to compile these insights into a comprehensive picture, supported by evidence stemming from cutting-edge experimental techniques. The stability and recyclability of CD-based photo-catalysts presents another challenge. Although CDs show good stability across several photo-catalytic cycles, 158 they may experience photo-bleaching under light exposure, leading to catalyst deactivation. 18 It is also crucial to promptly extract the photo-induced charges to prevent material degradation. This is usually addressed by adding SEDs. However, a more effective strategy would involve using both photo-generated electrons and holes in multiple catalytic reactions without depending on SEDs. Such an intricate system, inspired by water splitting, would constitute yet another significant milestone. 18 Finally, devising a recycling strategy for CDs in which the recycled material serves as a precursor for another batch of synthesized CDs presents a sophisticated and sustainable approach at whose end is a truly circular system. In summary, understanding the photo-catalytic activity of CDs and the development of efficient, precisely tailored structures remains a significant scientific challenge. ll 18 Chem 10, 1–24, September 12, 2024 Please cite this article in press as: Zdra zil et al., Designing carbon dots for enhanced photo-catalysis: Challenges and opportunities, Chem (2024), https://doi.org/10.1016/j.chempr.2024.07.018 Review Overcoming these obstacles is crucial for advancing toward green and sustainable energy production, utilizing materials that are cost-effective, scalable, and environmentally friendly. In this review, we have outlined the key attributes of CDs that contribute to their photo-catalytic activity and highlighted several critical phenomena and strategies that are essential for the design of efficient CD-based photo-catalysts. Such advances are likely to evolve as cornerstones that will pave the road toward the realization of this complex goal, on one hand, and mark a step forward in the application of CDs in the field of artificial photosynthesis, on the other hand. ACKNOWLEDGMENTS The work was supported by the ERDF/ESF project TECHSCALE (no. CZ.02.01.01/00/ 22_008/0004587). This article has been produced with the financial support of the European Union under the REFRESH—Research Excellence For Region Sustainability and High-Tech Industries project number CZ.10.03.01/00/22_003/0000048 via the Operational Programme Just Transition. M.M. acknowledges the support of the Slovak Research and Development Agency (APVV-20-0098). The COST Action CA21101 is also acknowledged. This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic through the e-INFRA CZ (ID: 90254). A.C. is a member of the research staff of CONICET. D.M.G. acknowledges support from Solar Technologies go Hybrid, an initiative from the free state of Bavaria. L.Z. acknowledges support from the Horizon Europe project HORIZON-WIDERA-2022TALENTS ‘‘APPROACH’’ (no. 101120397). AUTHOR CONTRIBUTIONS The manuscript has been written with contributions from all authors. DECLARATION OF INTERESTS The authors declare no competing interests. REFERENCES 1. Eisenberg, R., and Nocera, D.G. (2005). Preface: overview of the forum on solar and renewable energy. Inorg. Chem. 44, 6799– 6801. https://doi.org/10.1021/ic058006i. 2. Armaroli, N., and Balzani, V. (2007). The future of energy supply: challenges and opportunities. Angew. Chem. Int. Ed. Engl. 46, 52–66. https://doi.org/10.1002/anie. 200602373. 3. Yin, J., Molini, A., and Porporato, A. (2020). 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