Elsevier

Nano Energy

Volume 86, August 2021, 106122
Nano Energy

Colloidal carbon quantum dots as light absorber for efficient and stable ecofriendly photoelectrochemical hydrogen generation

https://doi.org/10.1016/j.nanoen.2021.106122Get rights and content

Highlights

  • The red C-dots have an excitation independent emission peak and highly crystalline structure with dominant energy levels.

  • A PEC device consists of C-dots/TiO2 as photoanode, Pt/CNFs as CE, and glucose aqueous solution as electrolyte.

  • The PEC device exhibits saturated photocurrent density as high as ~ 4 mA/cm2 at 0.6 V vs. RHE and the device is very stable.

Abstract

Solar-driven hydrogen production is one of the most promising strategies for solar-to-hydrogen energy conversion. Compared to inorganic quantum dots (QDs), carbon quantum dots (C-dots) have attracted a lot of attention for optoelectronic devices due to their structure-dependent optical properties and green composition. However, the solar-to-hydrogen conversion efficiency of most of the photoelectrochemical (PEC) devices based on colloidal QDs is still low. Here we demonstrated a highly efficient and stable ecofriendly PEC device using C-dots sensitized TiO2 photoanode, Pt loaded on carbon nanofibers as counter electrode, and glucose aqueous solution as electrolyte. The red-color C-dots were prepared using a solvothermal reaction, with an absorption spectrum ranging from 300 to 600 nm and a quantum yield (QY) of 50%. The C-dots have excitation independent photoluminescence peak positions and highly crystalline structure. The hydroxyl group on the C-dots can strongly interact with the TiO2, forming a very stable complex. Benefiting from these features, the PEC devices based on C-dots exhibit a saturated photocurrent density as high as ~4 mA/cm2 at 0.6 V vs. RHE and the device is very stable (keeping 95% of its initial value after 10-hour illumination upon 100 mW/cm2). This work indicates the promising properties of the C-dots/TiO2 system, which holds huge potential for applications in the fields of optoelectronic and catalytic devices.

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

References (70)

  • M.L. Sun et al.

    Sn4+ doping combined with hydrogen treatment for CdS/TiO2 photoelectrodes: an efficient strategy to improve quantum dots loading and charge transport for high photoelectrochemical performance

    J. Power Sources

    (2019)
  • T. Zhou et al.

    Carbon quantum dots modified anatase/rutile TiO2 photoanode with dramatically enhanced photoelectrochemical performance

    Appl. Catal. B: Environ.

    (2020)
  • D. Benetti et al.

    Hole-extraction and photostability enhancement in highly efficient inverted perovskite solar cells through carbon dot-based hybrid material

    Nano Energy

    (2019)
  • S. Qiu et al.

    Carbon dots decorated ultrathin CdS nanosheets enabling in-situ anchored Pt single atoms: a highly efficient solar-driven photocatalyst for hydrogen evolution

    Appl. Catal. B: Environ.

    (2019)
  • L. Tian et al.

    Carbon-quantum-dots-embedded MnO2 nanoflower as an efficient electrocatalyst for oxygen evolution in alkaline media

    Carbon

    (2019)
  • I. Sargin et al.

    Green synthesized carbon quantum dots as TiO2 sensitizers for photocatalytic hydrogen evolution

    Int. J. Hydrog. Energy

    (2019)
  • X. Meng et al.

    Carbon quantum dots assisted strategy to synthesize Co@NC for boosting photocatalytic hydrogen evolution performance of CdS

    Chem. Eng. J.

    (2020)
  • Q. Wang et al.

    Silk fibroin-derived nitrogen-doped carbon quantum dots anchored on TiO2 nanotube arrays for heterogeneous photocatalytic degradation and water splitting

    Nano Energy

    (2020)
  • Y. Zhu et al.

    Red phosphorus decorated and doped TiO2 nanofibers for efficient photocatalytic hydrogen evolution from pure water

    Appl. Catal. B: Environ.

    (2019)
  • L. Cao et al.

    Carbon dots for energy conversion applications

    J. Appl. Phys.

    (2019)
  • F. Navarro-Pardo et al.

    Structure/property relations in “giant” semiconductor nanocrystals: opportunities in photonics and electronics

    Acc. Chem. Res.

    (2018)
  • Y. Zhou et al.

    Harnessing the properties of colloidal quantum dots in luminescent solar concentrators

    Chem. Soc. Rev.

    (2018)
  • G.S. Selopal et al.

    Core/shell quantum dots solar cells

    Adv. Funct. Mater.

    (2020)
  • S.P. Borderud et al.

    Electronic cigarette use among patients with cancer: characteristics of electronic cigarette users and their smoking cessation outcomes

    Cancer

    (2015)
  • Z. Zhang et al.

    Photoelectrochemical reforming of biomass for hydrogen generation

    RSC Adv.

    (2014)
  • Y. Zhang et al.

    Glucose oxidation over ultrathin carbon-coated perovskite modified TiO2 nanotube photonic crystals with high-efficiency electron generation and transfer for photoelectrocatalytic hydrogen production

    Green Chem.

    (2016)
  • K.-H. Ye et al.

    Carbon quantum dots as a visible light sensitizer to significantly increase the solar water splitting performance of bismuth vanadate photoanodes

    Energy Environ. Sci.

    (2017)
  • J. Liu et al.

    The dimensional distribution of kenaf and apocynum fibers

    J. Nat. Fibers

    (2020)
  • K. Nie et al.

    A Facile degumming method of kenaf fibers using deep eutectic solution

    J. Nat. Fibers

    (2020)
  • B. He et al.

    Low-cost counter electrodes from CoPt alloys for efficient dye-sensitized solar cells

    ACS Appl. Mater. Interfaces

    (2014)
  • Y. Li et al.

    N-doped carbon-dots for luminescent solar concentrators

    J. Mater. Chem. A

    (2017)
  • W. Ma et al.

    Carbon dots and AIE molecules for highly efficient tandem luminescent solar concentrators

    Chem. Commun.

    (2019)
  • X. Tong et al.

    Optoelectronic properties in near-infrared colloidal heterostructured pyramidal “giant” core/shell quantum dots

    Adv. Sci.

    (2018)
  • H. Zhao et al.

    Tailoring the interfacial structure of colloidal “giant” quantum dots for optoelectronic applications

    Nanoscale

    (2018)
  • G.S. Selopal et al.

    Synergistic effect of plasmonic gold nanoparticles decorated carbon nanotubes in quantum dots/TiO2 for optoelectronic devices

    Adv. Sci.

    (2020)
  • Cited by (61)

    View all citing articles on Scopus
    View full text