Research Article: Steering Photoelectrons Excited in Carbon Dots into Platinum Cluster Catalyst for Solar‐Driven Hydrogen Production

Date Published: September 21, 2017

Publisher: John Wiley and Sons Inc.

Author(s): Xiaoyong Xu, Wenshuai Tang, Yiting Zhou, Zhijia Bao, Yuanchang Su, Jingguo Hu, Haibo Zeng.


In composite photosynthetic systems, one most primary promise is to pursue the effect coupling among light harvesting, charge transfer, and catalytic kinetics. Herein, this study designs the reduced carbon dots (r‐CDs) as both photon harvesters and photoelectron donors in combination with the platinum (Pt) clusters and fabricated the function‐integrated r‐CD/Pt photocatalyst through a photochemical route to control the anchoring of Pt clusters on r‐CDs’ surface for solar‐driven hydrogen (H2) generation. In the obtained r‐CD/Pt composite, the r‐CDs absorb solar photons and transform them into energetic electrons, which transfer to the Pt clusters with favorable charge separation for H2 evolution reaction (HER). As a result, the efficient coupling of respective natures from r‐CDs in photon harvesting and Pt in proton reduction is achieved through well‐steered photoelectron transfer in the r‐CD/Pt system to cultivate a remarkable and stable photocatalytic H2 evolution activity with an average rate of 681 µmol g−1 h−1. This work integrates two functional components into an effective HER photocatalyst and gains deep insights into the regulation of the function coupling in composite photosynthetic systems.

Partial Text

Hydrogen (H2) energy is clean and considered as one of the most promising alternatives for fossil‐based energy in the future.1, 2 Photocatalytic water splitting into H2 and oxygen (O2) using solar energy has attracted considerable attention as a green, low‐cost, and sustainable approach for large‐scale H2 production.3, 4, 5, 6 Photocatalytic H2 evolution reaction (HER) proceeds a consistent route that protons in solution are reduced by photoexcited electrons to hydrogen atoms chemisorbed on catalyst surface followed by their desorption into hydrogen gas.7, 8 Thus, the injection of thermodynamic photoelectrons and the Gibbs free energy of atomic H adsorption (ΔGH) are two critical factors in determining the photocatalytic HER activities of catalysts.9 Platinum (Pt) yields the most desirable ΔGH of near‐zero value (−0.09 eV),10, 11 while its photochemical inertness toward sunlight restricts the potential for direct photocatalysis. In recent years, the heterogeneous systems equipped with efficient coupling of solar photon harvesting and photoinduced charge transfer with catalysis kinetics have shown great success for solar photocatalytic applications.12, 13, 14, 15 In particular, the low‐cost and nontoxic carbon dots (CDs) have recently received significant interest as an effective function unit in designing composite photocatalysts.16, 17, 18, 19, 20 For example, Kang and co‐workers reported the loading of CDs on traditional semiconductor photocatalysts and demonstrated the promising results for the photocatalytic and photo‐electrochemical water splitting.21, 22, 23 Metal nanoparticles like gold (Au) and Pt have also been developed to integrate with CDs to form composite photocatalysts with high efficiency and high selectivity for the oxidation of cyclohexane24 and the reduction of carbon dioxide.25 Recently, Reisner and co‐workers reported a smart photocatalytic HER system using CDs as a photosensitizer in combination with a molecular Ni catalyst.26 Along with these pioneering results, CDs were demonstrated to possess the excellent photon harvesting and photoinduced electron transfer properties. However, to the best of our knowledge, CDs have been underexplored to combine with Pt catalyst for photocatalytic H2 production; moreover, there have been a few efforts devoted to control CDs on light absorption character and electron transfer mode (acceptor or donor) for specific photocatalysis.

The r‐CD/Pt composite was fabricated through the synthesis route shown in Scheme1 (see the “Experimental Section” for full details). In brief, 2–5 nm large CDs with rich surface groups were first prepared by the dehydrolysis reaction between urea and citric acid in dimethylformamide (DMF) solvent27 and used as precursor materials. Then, sodium borohydride (NaBH4) as a reducing agent was introduced into the CD solution to reduce the surface carbonyl/carboxyl species and to modify surface charge by adsorbing Na+ cations. The surface components of CDs before and after treated with NaBH4 were analyzed in Figure S1 (Supporting Information) in terms of the Fourier transform infrared (FT‐IR) and the X‐ray photoelectron spectroscopies (XPS). From FT‐IR spectra in Figure S1a (Supporting Information), the as‐synthesized CDs are found to be hybridized with abundant oxygenous groups, such as hydroxyl, carbonyl, and carboxyl groups, on their surfaces. Because oxygen atoms are more electronegative than carbon atoms,28 the negatively charged surface can form and then inhibit the electron output as well as the electrostatic adsorption of Pt anionic species.29 Hence, the NaBH4 was used as a reducing agent to reduce carbonyl and carboxyl groups, inducing an increased amount of hydroxyl groups on the surface. As expected, in the FT‐IR spectrum of r‐CDs, the absorption band of C=O stretching vibration at 1710 cm−1 almost quenches, whereas the absorption band at 3000–3600 cm−1 of O—H stretching vibration increases when compared to that of CDs.30, 31 Moreover, the XPS spectra of C 1s region (Figure S1c,d, Supporting Information) also display that the C=O peak at 288.9 eV obviously declines and the —OH peak at 286.0 eV raises after the reduction treatment, while the conjugated sp2 carbon peak at 284.6 eV is almost unchanged.32 In addition, the full‐range XPS spectra (Figure S1b, Supporting Information) reveal the presence of C, N, O, and Na in the r‐CDs, whereas no Na signal is detected in the pristine CDs. These analyses for chemical composition suggest the surface reduction and charge modification making r‐CDs release electron‐donating ability, which is crucial for anchoring Pt clusters and steering photoelectron transfer toward Pt clusters. Next, the purified r‐CDs were added to H2PtCl6 aqueous solution to allow the electrostatic adsorption of Pt anionic species on r‐CDs as supports. The Pt‐ion‐adsorbing r‐CDs were extracted by centrifugation and re‐dissolved in deionized water to remove the excess Pt species. Finally, the above solution of r‐CDs absorbed with Pt ions was irradiated under sunlight for 60 min, and the solution color varied from dark brown to light yellow, indicating the transformation of Pt ions to atomic clusters. In the contrast experiments for photochemical Pt deposition (Figure S2, Supporting Information), the unreduced CDs were found to fail to anchor Pt species, whereas the r‐CDs operated the continuous Pt deposition from the precursor H2PtCl6 solution under light irradiation, further confirming that only the r‐CDs can allow for Pt species anchoring and steer photoelectron donating for Pt nucleation growth. In previous reported works,26, 33 the CDs conjugated with carbonyl (C=O) and carboxyl (COOH) groups were also found to be unfavorable for the photochemical deposition of metallic Pt, probably because more negative surface restricts the electron overflowing. In addition, note that the photochemical Pt deposition in the H2PtCl6 solution was difficult to be controlled, so we implemented a two‐step route, that is, the separated chemical adsorption of Pt anionic species and photoreduction of metallic Pt, for the control of ultrafine Pt cluster anchoring on r‐CDs’ surface to avoid the Pt growth or aggregation.

In conclusion, we reported an effective r‐CD/Pt composite photocatalyst based on the combination of two superior function components for solar H2 production. In the r‐CD/Pt composite, the r‐CDs are capable of harvesting solar light, generating and transferring “hot” electrons to the Pt clusters in contact; the Pt is most favorable for proton reduction reaction because of its appropriate ΔGH. Thus, an excellent and stable photocatalytic HER activity was cultivated through the efficient coupling among three attractive properties of r‐CDs in harvesting photons and donating electrons, and Pt clusters in reducing protons. This work highlights the necessity of steering photoinduced charge transfer in composite photosynthetic systems to work the synergistic action.

Syntheses of CDs, r‐CDs, and r‐CD/Pt: The CDs hybridized with various surface groups were prepared via a solvothermal route from citric acid and urea using DMF as solvent. In a typical synthesis, 1 g of citric acid and 2 g of urea were reacted in 10 mL of DMF solvent at 160 °C for 6 h under solvothermal condition. After naturally cooled to room temperature, the obtained dark brown solution was mixed with ethyl alcohol (1:8), and then centrifuged at 12 000 rpm for 5 min. The precipitate was collected, dispersed in deionized water, and centrifuged (16 000 rpm, 5 min) twice to wash off residual salts and alkali, and then freeze‐dried to obtain the product of CDs. To prepare the surface‐reduced CDs (r‐CDs), 0.1 g of sodium borohydride (NaBH4) was introduced into the 10 mL of CDs aqueous solution (10 mg mL−1) and then stirred gently overnight at room temperature. The obtained solution was mixed with ethyl alcohol (1:8), and then centrifuged at 12 000 rpm for 5 min. The precipitate was collected, dissolved in deionized water, and centrifuged (16 000 rpm, 5 min) twice to wash off residual reducing agent, and then freeze‐dried to obtain the product of r‐CDs. Finally, a two‐step photochemical strategy was adopted to anchor the ultrafine Pt clusters on the surface of r‐CDs. As a typical procedure, 200 µL of H2PtCl6 aqueous solution (2 × 10−3m) and 10 mL of r‐CDs aqueous solution (10 mg mL−1) was mixed and then gently stirred for 24 h at room temperature. The precipitate was collected and washed repeatedly with ethyl alcohol by centrifugation at 16 000 rpm, and then dissolved in 50 mL of deionized water. The obtained solution was irradiated under sunlight for 60 min, and the color of this solution varied from dark brown to light yellow, indicating the reduction of absorbed Pt ions to metallic Pt. The r‐CD/Pt product was repeatedly washed with ethyl alcohol and deionized water by centrifugation at 16 000 rpm for 5 min. For comparison, pure Pt particles were also synthesized via the similar process without the presence of r‐CDs as supports.

The authors declare no conflict of interest.