A reflection on ‘Using carbon nanodots as inexpensive and environmentally friendly sensitizers in mesoscopic solar cells’

Abstract

In this reflection article, we review our journey with carbon dots (CDs), which began with using them in dye-sensitised solar cells (J. T. Margraf, F. Lodermeyer, V. Strauss, P. Haines, J. Walter, W. Peukert, R. D. Costa, T. Clark and D. M. Guldi, Nanoscale Horiz., 2016, 1, 220–226, https://doi.org/10.1039/C6NH00010J) and ultimately led to their use in photocatalytic proton reduction. Key milestones along this journey included establishing the structure–function relationship and gaining a mechanistic understanding of how CDs participate in proton reduction.

Given recent advances in carbon-based nanoscience, it is conceivable that a significant portion of future everyday electronic components – such as computer chips, batteries, and solar cells – could be manufactured entirely from biodegradable materials. Among the most promising candidates for such applications are carbon dots (CDs), a new class of organic nanomaterials.¹ CDs offer several key advantages, including low-cost and environmentally friendly synthesis, high recyclability, and minimal toxicity.² CDs, characterized by their nanometer size, exhibit an astounding ability to manipulate light and charges, producing photons of several colors and forming long-lived, charge-separated states essential for the capture and conversion of solar energy.³ Furthermore, surface functionalization transforms them into versatile platforms for the detection of minimal amounts of environmentally and biologically relevant chemical species.⁴

Motivated by these considerations, we began exploring CDs as potential photosensitizers in mesoscopic solar cells (Fig. 1). The CDs were synthesized using a one-step, bottom-up microwave-assisted approach in aqueous media, employing citric acid, urea, and formic acid as precursors. We found that the assembly of CDs on TiO₂ electrodes was controlled via the pH of the solution, leading to twofold-improved solar-cell performance, compared to previous reports. In fact, through optimization of the solar cell architecture, we achieved promising power conversion efficiencies of up to 0.24% with these cost-effective and environmentally benign sensitizers (Fig. 2). Interestingly, extending light absorption into longer wavelengths, that is, the visible range, did not necessarily improve device performance, as these regions contributed minimally to the photoaction spectra and ultimately led to a decrease in overall efficiency. A comprehensive investigation indicated that this is because the extended absorption relates to trap states, which do not contribute to the photogenerated current. This decline was further corroborated by transient absorption spectroscopy and photovoltage decay analysis (https://doi.org/10.1039/C6NH00010J).⁵

Fig. 1 CDs as photosensitizers in mesoscopic solar cells.

Fig. 2 Overview of the evolution of CDs, from structural insights to applications as light harvesters, charge-transfer mediators, and photocatalysts for hydrogen evolution.

Based on our initial work, which highlighted the limitations of CDs in mesoscopic solar cells, synthetic snowflake CDs based on a carbon core featuring 19 benzene rings were developed. They gave rise to significant absorption cross section across the visible range of the solar spectrum and a power conversion efficiency of 10.3% when used as photoactive layer.⁶ Rather than using CDs as photosensitizers/photoactive material, CDs in the form of graphene quantum dots performed well as a superfast electron tunnel/electron-transport layer (ETL) material, that is, a layer enabling rapid electron extractions in perovskite solar cells, next to a hole-transport layer (HTL) material.⁷ Notable are the contributions of CDs in down-converting light to improve efficiency and stability.⁸ Likewise, amino groups carrying CDs served in the same capacity in inverted polymer solar cells.⁹ But, it was a decrease in the work function next to smoothing the surface of ZnO that made the difference.¹⁰

Alongside these DSSC advancements, we achieved a significant breakthrough in understanding the internal structure of bottom-up synthesized CDs (Fig. 2). Clear analysis of the internal composition of CDs is complicated by the presence of reaction products that typically include mixtures of various CD fractions, as well as molecular intermediates and side products. Purification and precise separation of the different CD fractions were necessary. In fact, a simple column chromatographic unit was used to systematically study the influence of the molar precursor ratios and temperature, reaction time, etc. on the CD solution composition. By investigating the structural and optical properties of the chromatographically separated fractions, three different fluorescent species were identified: free fluorophores, fluorophores attached to the surface of/embedded within CDs, and weakly fluorescent carbon particles lacking fluorophores. The results show that each fraction contained varying amounts of fluorescent species, depending on the retention time and the synthesis conditions applied. Additionally, we identified multiple fluorescent species within the middle CD fractions and propose that the fluorophores are inherently associated with the carbon cores. Time-correlated single-photon counting measurements confirmed the presence of molecular fluorophores in the solutions. A decline in fluorophore concentration was observed with increasing retention time. In short, we confirmed that the CD solution composition and the internal structure of the molecular fluorescent components depend strongly on the synthesis conditions and can be clarified via chromatographic separation.¹¹

In light of the aforementioned, we directed our attention to integrating CDs into covalent and non-covalent electron donor–acceptor systems involving, for example, pressure-synthesized CDs (Fig. 2). These studies shared the characteristics of being initial steps toward a comprehensive understanding of the (photo)catalytic performance of CDs in energy conversion systems such as (photo)catalytic hydrogen evolution reaction (HER) schemes – vide infra. In particular, CDs were linked to various electron donors and acceptors, giving evidence for unidirectional electron transfer either from, for example, electron-donating single-walled carbon nanotubes and porphyrins or to electron-accepting perylenediimides and tetracyanoquinodimethane.^12–15 Notably, the lifetimes of the charge-separated states – whether arising from (photo)reductive or -oxidative processes – were in the nanosecond range. At this point we reached the conclusion that charges, namely electrons or holes, are localized on the molecular fluorophore embedded with CDs rather than delocalized across the CD matrix.

Overall, the amphoteric behavior of CDs represents a valuable asset in photocatalytic applications.³

In the context of photocatalytic HER schemes, pressure-synthesized CDs performed effectively and were subsequently used as an internal benchmark (Fig. 2). In fact, optimal results were realized by using 0.015 mg of (photo)catalyst in 10 vol % solutions of TEOA and a pH of 8.5.¹⁶ However, their minimal absorption cross-section across the visible range of the solar spectrum raised concerns regarding their suitability as (photo)catalysts. The requirement for UV light irradiation is especially noteworthy.

In the next step, we modified the CDs through photochemical treatment in air-equilibrated aqueous solutions and enabled them to produce dihydrogen under visible light instead of UV light irradiation. Important is the fact that the resulting CD materials have a dual function. On one hand, they absorb light, and, on the other hand, they (photo)- and (electro)catalytically produce dihydrogen from water and seawater, without any external photosensitizer or co-catalyst. Record HER activities of 15.15 and 19.70 mmol(H₂) g(catalyst)⁻¹ h⁻¹ were obtained after 1 hour of 75 mW cm⁻² Xe lamp illumination, from water and sea water, respectively.¹⁶ At the time of these investigations, the impressive performance of the CD (photo)catalysts outweighed the remaining structural uncertainties. Our study concluded by pinpointing the molecular origins of post-synthetic transformations within the complex structure of CDs. We found that (photo)oxidation processes were clearly linked to citrazinic acid fluorophore fragments, which also served as the primary catalytic sites. It triggered a ring-closing reaction between amines and carboxylic acids during the solvothermal preparation and produced citrazinic acid and other molecular fluorophores. These structural features underpinned the outstanding HER performance of our materials – exceeding similar systems by three orders of magnitude under comparable conditions, and outperforming the previous benchmark by a factor of five under stronger illumination. Actually, CDs perform much better in seawater than in distilled water.¹⁶

Our photochemically altered CDs stand out through their broad-range visible-light absorbance and extraordinary photostability. Their potential remains largely untapped, as their use has been restricted to imaging research. This motivated us to look in complementary studies for a link between CDs’ photochemical features and their chemical structure. At the forefront were electron-rich CDs obtained with in situ addition of NaOH during the synthesis, whereas otherwise electron-poor CDs are obtained. These properties originate from the reduced and oxidized dimers of citrazinic acid within the matrix of the CDs, respectively. In line with this assumption, electron-rich CDs deposited on TiO₂ gave a 30% higher photocurrent density of 0.7 mA cm⁻² at +0.3 V vs. Ag/AgCl under Xe-lamp irradiation than electron-poor CDs. The difference in overall photoelectric performance was ascribed to fundamentally different charge-transfer mechanisms; the characteristic CD coloration was elucidated through chemical and (electro)chemical oxidation and reduction experiments. They yielded reddish and yellowish chromophores, spectroscopically similar to the electron-poor CDs and electron-rich CDs, respectively. Hence, NaOH added in situ mediates the reductive conditions during the solvothermal procedure. We continued our fundamental investigations with chronoamperometric assays with electron-poor CDs and electron-rich CDs immobilized on FTO/NiO and FTO/TiO₂ photoelectrodes, respectively. Interfacial electron injection from electron-poor CDs into TiO₂ is inefficient. In contrast, FTO/TiO₂/electron-rich CD photoelectrodes gave rise to a remarkable injection efficiency. A photocurrent flow suggests photoexcited electron-rich CDs that inject electrons into TiO₂ without needing to be reduced first by an electron donor. Relative to bare FTO/NiO photoelectrodes, FTO/NiO/electron-poor CDs produced an additional photocurrent upon photoirradiation. The ability to generate photocurrents underlines the electron-poor nature and, hence, their ability to transfer holes to NiO upon photoexcitation. In contrast, the chronoamperometric transients observed for FTO/NiO/electron-rich CDs are barely distinguishable from those seen for bare FTO/NiO photoelectrodes.¹⁷

Our CD voyage recently led us to decipher their HER mechanism.¹⁸ Instead of using solvothermally synthesized citrazinic acid, we focused on incorporating appropriate co-reactants in the form of in-situ-generated nitrogen-containing π-conjugated chromophores. Given the well-established photo- and electrochemical properties of phenazine (PNZ), we selected it as a molecular probe (Fig. 3). Therefore, we aimed to design a CD model that features phenazine (P-CD) to study how the CD-matrix affects its redox and photochemical behavior under HER conditions. P-CD together with nano-aggregated PNZ (PNZ_NA), used as a reference, were investigated using spectroscopic and electrochemical techniques, along with pulse radiolysis. In particular, both chemical and electrochemical reduction of PNZ_NA triggered a reaction cascade: the initial reduction of PNZ_NA is followed first by protonation, and subsequently disproportionation, yielding (PNZ-H)˙ and (PNZ-H₂). Disproportionation was, however, not observed when P-CD underwent reduction via either pulse radiolysis or reductive quenching with triethanolamine. Notably, in this case, PNZs are integrated into the polycitric acid matrix of the CD. Both approaches support the idea that the CD matrix inhibits the chemical disproportionation of two (PNZ-H)˙ into (PNZ-H₂). This, in turn, allows (PNZ-H)˙ to follow an alternative reaction pathway. Crucially, stabilizing (PNZ-H)˙ promotes hydrogen abstraction and ultimately facilitates H₂ evolution. Our experimental results underscore the key role of the CD matrix in enhancing the efficiency and versatility of CDs, particularly in photocatalytic fuel production.¹⁹

Fig. 3 CDs with embedded molecular fluorophores, including citrazinic acid derivatives and phenazine, drive photocatalytic hydrogen generation from water.

Conclusions

The field of CDs has undergone a significant transformation in recent years. The evolution is not just in the perception of their structures, which have shifted from being viewed as well-carbonized, carbon-based quantum-dot-like nanostructures to more dynamic, molecular entities containing carbon domains, but also in their application scope. Their straightforward design by means of an organic chemistry toolbox using various carbon and nitrogen containing precursors – including also waste – renders them promising candidates for the rational development of cost-effective, green, and recyclable nanomaterials. CDs possess not only unique optoelectronic properties but also potential to game-change solar energy conversion. Such advances are likely to evolve as cornerstones that will pave the road towards the realization of this complex goal, on one hand, and mark a step forward in the application of CDs in next-generation devices, on the other hand.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

L. Z. acknowledges 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, and D. M. G. acknowledges DFG 517/29.

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