Colloidal carbon quantum dots as light absorber for efficient and stable ecofriendly photoelectrochemical hydrogen generation
Graphical Abstract
Introduction
With the rapid growth of global population, the increasing use of energy through fossil fuels has caused severe environmental pollution and climate change due to the over release of carbon emission [1], [2], [3], [4]. It is highly important to produce clean energy using sustainable natural resources [1], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Directly converting solar light into hydrogen through photoelectrochemical (PEC) or photochemical devices has attracted a lot of attention because hydrogen is a clean and sustainable fuel with high energy density [17], [18], [19], [20]. Up to now, many types of semiconductors have been used as efficient absorbers for optoelectronic devices [2], [3], [4], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Typically, in a PEC system, semiconductors absorb sunlight and convert it into electron-hole pairs; subsequently, the electron-hole pair separates and migrates to the corresponding electrodes for water reduction or oxidation reaction [2]. As electron-hole pair can easily recombine or be quenched because of the surface defects/traps on semiconductors, the solar-to-hydrogen conversion efficiency is still low in most semiconductor-based PEC devices [2].
Among various types of semiconductors, quantum dots (QDs) have been used as a new type of absorbers for efficient hydrogen production due to their size-dependent excellent optical properties, such as broad absorption, high absorption coefficient, high quantum yield (QY) and size-tunable band energy levels. Inorganic QDs have been widely studied for PEC devices [17], [18], [33]. For example, by optimizing the core/shell structure in CdSe/CdS QDs, the saturated current density (J) in PEC devices based on QDs/TiO2 increases up 10–17 mA/cm2 [18], [34], [35]. However, there are still several serious issues present in the inorganic QDs based PEC devices: (i) most of used QDs (PbS, CdSe/CdS, CdS, CuInSe, ZnAgInSx, Ag2S) contain either toxic elements (Cd, Pb) or expensive non-earth-abundant elements (e.g. Ag, In) [33], [36], [37], [38], [39], [40], [41]. Compared to widely used metal oxides (e.g. TiO2, and ZnO) [42], [43], [44], [45], these elements prohibit these types of QDs for practical use in PEC hydrogen production; (ii) the electrolyte used in most of the PEC devices contains chemically-active Na2S/Na2SO3, which serves as efficient hole scavenger [17], [19], [20]. However, this type of electrolyte (pH of 13) is very corrosive and may release H2S during chemical storage and handling. In addition, the QDs become unstable during cell operation in such electrolyte due to both the chemical- and photo- corrosion [17], [19], [20]; (iii) Pt plate/wire or commercial Pt/C is very efficient for water reduction, but Pt is very expensive and not an earth-abundant element. Even though the transition metal oxides/sulfides are found to be efficient for PEC devices as counter electrode, their long-term stability is still a challenge compared to Pt [25]. It is highly desirable to produce efficient and stable PEC devices based on environmentally-friendly and cost-effective materials or reducing the used amount of Pt by increasing the active surface of Pt material.
Recently, carbon quantum dots (C-dots) have been used for optoelectronic devices, such as solar cells, luminescent solar concentrators, photoredox catalysis, battery and bio-sensors due to their excellent optical and electrical properties [1], [8], [46], [47], [48]. The C-dots contain earth-abundant elements, such as C, N and O; they can be easily prepared by using wet-chemistry approaches (microwave reaction, vacuum-heating and hydro/solvo-thermal reaction) using earth-abundant and renewable precursors, such as citric acid and glucose [8], [49], [50], [51], [52], [53]. The absorption of C-dots covers from 300 to 900 nm and can be controlled by their size, composition and surface functional groups [8], [49], [50], [51], [52], [53]. The band energy levels of C-dots can be adjusted to be suitable for the water splitting. Last but not least, the colloidal stability and photostability of C-dots are superior compared to inorganic QDs (e.g. Cd/Pb based QDs, In/Ag based QDs, Sn based QDs and perovskite QDs) [8], [43], [44], [49], [50], [54], [55], [56], [57], [58], [59], [60], [61], [62]. Recently, C-dots have been used as light absorber for hydrogen production by sensitizing metal oxides [42], [43], [45], [50], [52], [55], [56], [62], [63]. Benefiting from either the enhancement of the sunlight absorption or improved charge separation/transport, the solar-to-hydrogen conversion efficiency of the modified devices can be dramatically enhanced. For example, Ye et al. reported that C-dots could enhance the sunlight absorption of the NiOOH/FeOOH/C-dots/BiVO4 photoanode, achieving a high J of 3.91 and 5.99 mA/cm2 at 0.6 V and 1.23 V vs. reversible hydrogen electrode (RHE) under AM 1.5 G in KH2PO4 aqueous solution [8]. Li et al. reported C-dots decorated Cu2S nanowire array-based PEC device with a J of 1.05 mA/cm2 at 0 V vs. RHE under AM 1.5 G in 1.0 M KCl aqueous solution [64]. Zhou et al. used the C-dots modified anatase/rutile TiO2 nanorods photoanode, generating a J of 2.76 mA/cm2 at 1.23 V vs. RHE under AM 1.5 G in 0.2 M Na2SO4 and 1 M Na2SO3 aqueous electrolyte [44]. Although C-dots based PEC devices have shown a great potential for hydrogen production, the presently obtained current density J is still lower than other PEC devices based on Cd/Pb QDs, which have a typical J between 10 and 20 mA/cm2 [18], [35]. It is still a challenge to obtain highly efficient PEC devices using colloidal C-dots. Considering the reported C-dots currently used for PEC devices, (i) the C-dots have a limited absorption, mainly in the ultraviolet range (300–450 nm), which matches only a small portion of Sun’s spectrum; (ii) the C-dots have excitation-dependent photoluminescent properties, making the band energy levels of C-dots tunable, leading to slow charge injection; (iii) the C-dots do not have a well-defined crystalline structure, holding molecular-like structure inside or on the surface of C-dots. This behavior usually leads to slow charge mobility.
In order to solve the above-mentioned problems, here we demonstrated a highly efficient and stable PEC device using red C-dots sensitized TiO2 as photoanode, glucose aqueous solution (pH = 7) as electrolyte and Pt loaded on carbon nanofibers (Pt/CNFs) with low Pt loading amount as counter electrode. The C-dots were prepared using a solvothermal reaction, with the absorption spectrum ranging from 300 to 600 nm and a quantum yield (QY) of 50%. The C-dots have excitation independent emission peak positions and a highly crystalline structure with well-defined energy levels, which result in discrete de-excitation spectra, avoiding the quasi-continuum energy spectrum, which is typical of the surface/defect states. The hydroxyl group on the C-dots can strongly interact with the TiO2, forming a very stable complex compared to carboxyl capped C-dots. Benefiting from these features, the PEC devices based on red C-dots using glucose solution as electrolyte exhibit a saturated J as high as ~4 mA/cm2 and an outstanding stability. The J of the device can maintain 95% of its initial value after 10 h illumination (100 mW/cm2). Furthermore, the Pt plate can be replaced by Pt/CNFs with Pt loading ten times lower than commercial Pt/C catalysts. This work indicates that the red C-dots/TiO2-Glucose-Pt/CNFs system potentially applies to the fields of ecofriendly optoelectronic and catalytic devices.
Section snippets
Materials
Phloroglucinol, sulfuric acid, methanol, glucose, Na2SO4, ethanol, and dimethyl formamide (DMF), dichloromethane (99.9%) were purchased from Aladdin. All chemicals were used as-received.
Synthesis of C-dots
C-QDs were prepared following the reported reference using a hydrothermal approach. [64] Typically, five hundred milligrams phloroglucinol was mixed with 10 mL of DMF with 2 mL of concentrated H2SO4 as catalyst. The solution was heated at 200 °C for 3 or 12 h, then the autoclave was naturally cooled down to
Structure and optical properties of C-dots
In order to obtain C-dots with a highly crystalline structure and broad absorption, we synthesized C-dots via a solvothermal approach using phloroglucinol as precursor [64]. With all other identical reaction parameters, the yellow and red color C-dots were obtained by heating the reaction for 3 h and 12 h, respectively (Fig. S1a). Fig. 1 shows the typical transmission electron microscopy (TEM) images of yellow and red C-dots and their corresponding C-dots/TiO2 composites. It can be seen from
Conclusions and Perspectives
In summary, we designed and demonstrated an ecofriendly PEC device using R C-dots/TiO2 as an efficient and stable anode, glucose as electrolyte and Pt/CNFs as cost-effective and efficient counter electrode for hydrogen production. The C-dots prepared using a solvothermal reaction have an absorption spectrum ranging from 300 to 600 nm with a QY of 50%. Compared to carboxyl capped C-dots, the hydroxyl group on the C-dots can strongly interact with the TiO2, forming a very stable complex.
CRediT authorship contribution statement
Xiaohan Wang: Methodology, Writing - original draft. Maorong Wang: Methodology, Writing - original draft. Guiju Liu: Investigation, Supervision, Conceptualization. Yuanming Zhang: Investigation, Supervision. Guangting Han: Conceptualization, Supervision, Project administration, Funding acquisition, Writing - review & editing. Alberto Vomiero: Supervision, Project administration, Funding acquisition, Writing - review & editing. Haiguang Zhao: Conceptualization, Supervision, Project
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
H.G. Zhao acknowledges support from Shandong Natural Science Funds for Distinguished Young Scholar (ZR2020JQ20). H.G. Zhao acknowledges support from the National Key Research and Development Program of China (Grant No. 2019YFE0121600). H.G. Zhao also acknowledges support from State Key Laboratory of Bio-Fibers and Eco-Textiles (Qingdao University), No. ZKT03, No. ZFZ201807, and No. GZRC202004. A. Vomiero acknowledges the Kempe Foundation, the Knut & Alice Wallenberg Foundation and the ÅFORSK
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