Loading Cd0.5Zn0.5S Quantum Dots onto Onion-Like Carbon

Jun 16, 2017 - Carbon dots (C dots, size < 10 nm) have been conventionally decorated onto semiconductor matrixes for photocatalytic H2 evolution, but ...
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Loading Cd0.5Zn0.5S Quantum Dots onto Onion-Like Carbon Nanoparticles to Boost Photocatalytic Hydrogen Generation Xiaolong Zhou,† Xina Wang,*,† Xi Feng,‡ Kun Zhang,§ Xiaoniu Peng,† Hanbin Wang,† Chunlei Liu,† Yibo Han,§ Hao Wang,† and Quan Li*,§ †

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, China ‡ Department of Physics, The Chinese University of Hong Kong, Hong Kong, China § Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology, Wuhan 430072, China S Supporting Information *

ABSTRACT: Carbon dots (C dots, size < 10 nm) have been conventionally decorated onto semiconductor matrixes for photocatalytic H2 evolution, but the efficiency is largely limited by the low loading ratio of the C dots on the photocatalyst. Here, we propose an inverse structure of Cd0.5Zn0.5S quantum dots (QDs) loaded onto the onionlike carbon (OLC) matrix for noble metal-free photocatalytic H2 evolution. Cd0.5Zn0.5S QDs (6.9 nm) were uniformly distributed on an OLC (30 nm) matrix with both upconverted and downconverted photoluminescence property. Such an inverse structure allows the full optimization of the QD/ OLC interfaces for effective energy transfer and charge separation, both of which contribute to efficient H2 generation. An optimized H2 generation rate of 2018 μmol/h/g (under the irradiation of visible light) and 58.6 μmol/h/g (under the irradiation of 550−900 nm light) was achieved in the Cd0.5Zn0.5S/OLC composite samples. The present work shows that using the OLC matrix in such a reverse construction is a promising strategy for noble metal-free solar hydrogen production. KEYWORDS: onionlike carbon, photocatalyst, H2 evolution, Cd0.5Zn0.5S, nanoparticles, quantum dots

1. INTRODUCTION Among the various photocatalysts for hydrogen generation, Cd0.5Zn0.5S has attracted extensive attention because of its tunable band gap (from 2.3 to 3.6 eV), high photochemical stability, and low cost.1−3 An optimal Zn content of ∼0.5 was determined for the optimum photocatalytic property.4,5 Unfortunately, the hydrogen production by employing Cd0.5Zn0.5S alone is not satisfactory, and cocatalysts such as Pt, CoPt, Au, and RuS2 are usually required for lowering the energy barrier of the water reduction half reaction,6−9 thus enhancing the hydrogen production. With Au as the cocatalyst, the highest photocatalytic H2 generation achieved is ∼6.3 mmol/h/g.8 However, the noble metal components in the cocatalyst make it expensive, limiting its potential large-scale applications in solar fuel generation. Much effort has been devoted to develop a noble metal-free cocatalyst, and carbon-based materials are proposed as promising candidates. Both reduced graphene oxide (rGO) and carbon nanotubes are found to function as effective reservoirs and transporters of the photoelectrons generated in the photocatalysts such as Cd0.5Zn0.5S and TiO2, which inhibits charge recombination and enhances the photocatalytic property.10−12 © 2017 American Chemical Society

Carbon dots (CDs) including nanodiamond (ND), graphene quantum dot (GQD), carbon quantum dot (CQD), and onionlike carbon (OLC) nanoparticles serve as another family of cocatalyst candidates for solar hydrogen production.13−18 Both down and upconverted photoluminescence (PL) have been demonstrated in CDs.19−23 The downconverted PL (DCPL) property of CDs makes it possible for photocatalytic applications if the photoexcited charge carrier can be effectively separated before their recombination, whereas the upconverted PL (UCPL) suggests the possibility of using CDs to absorb longer wavelength photons and excite a photocatalyst with the band gap energy of a shorter wavelength. In fact, increased light absorption in both the visible and near infrared (NIR) region has been found when CDs are coupled with photocatalysts such as TiO2,15,24,25 ZnIn2S4,26 WO3,27 Ag3PW12O40,28 C3N4,13,29 and Cu2O.30−32 In all reported works, small-sized CDs (usually ≤10 nm) are loaded onto a relatively large photocatalyst matrix. The low loading capacity (CD/photocatalyst volume ratio 18 MΩ·cm). Fluorine-doped tin oxide (FTO) coated glass ( 700 nm, the OLC will emit a short wavelength light of λem < 530 nm because of the UCPL property, which can be readily absorbed by the neighboring Cd0.5Zn0.5S QDs, then generating electrons and holes for the HER behavior upon the NIR region light. 3.5. Performance Dependence on the Ratio between the OLC and Cd0.5Zn0.5S. The effect of the OLC-to-Cd0.5Zn0.5S ratio on the photocatalytic H2 generation property was studied systematically. As shown in Figure 7a,b, the hydrogen evolution rate first increases, then decreases with the OLC content x increasing from 1 to 5, and achieves an optimum rate of 2018 μmol/h/g at x of 3. The trend remains the same as the transient current evolution presented in Figure 7c. At the same time, the EIS result (Figure 7d) suggests that the charge-transfer resistance (Rct) first decreases, then increases with the OLC, and reaches the lowest value at x = 3. Higher x value means more OLCs were introduced into the composites (Figure S8). Given that once the surface of the Cd0.5Zn0.5S QDs starts to be wrapped by the OLCs, both the photon energy transfer from the OLCs to the QDs and charge separation at the OLC/QD interface can be satisfied. This will increase the light utilization and decrease the carrier recombination and thus promote the photocatalytic H2 evolution and transient current accordingly. However, the defective nature of the OLCs, particularly those at the QD/OLC interfaces, may become nonradiative recombination centers for the photogenerated carriers in the QDs. The experimental results suggest that an optimized interface is formed between the QD and the OLC at x = 3.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected] (Q.L.). ORCID

Xina Wang: 0000-0003-3350-6043 Hao Wang: 0000-0002-4894-7653 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (no. 51472080, 11104097) and Open Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics (no. KF201411).

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4. CONCLUSIONS The OLC nanoparticles of ∼30 nm with both UCPL and DCPL characters were adopted as matrixes for loading 6.9 nm-sized Cd 0.5 Zn 0.5 S QDs for noble metal-free photocatalytic H2 generation. Besides the enhanced absorption over the visible and NIR range light (compared with pure Cd0.5Zn0.5S), the charge separation, transfer, and energy transfer mechanisms are disclosed for the Cd0.5Zn0.5S/OLC composites based on the type-II band alignment between the Cd0.5Zn0.5S and OLC. By tuning the ratio between the OLCs and Cd0.5Zn0.5S QDs, an optimum photon energy transfer from the OLCs to QDs and charge separation at the OLC/QD interface can be achieved, leading to a maximum H2 generation rate of 2018 μmol/h/g upon visible light and appreciable H2 evolution rate of 58.6 μmol/h/g upon the NIR light. The Cd0.5Zn0.5S QDs-on-OLC structure allows the manipulation of the QD/OLC interface for efficient light utilization and photon-generated charge-carrier separation, serving as a promising strategy for developing noble metal-free photocatalysts with a high-efficiency H2 evolution.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05592. The photocatalytic evaluation system; XRD patterns; FTIR spectra; Tauc plot; Mott−Schottky curves; VB XPS spectra; band diagram; XPS survey scans; and TEM pictures and particle size distribution (PDF) 22566

DOI: 10.1021/acsami.7b05592 ACS Appl. Mater. Interfaces 2017, 9, 22560−22567

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b05592 ACS Appl. Mater. Interfaces 2017, 9, 22560−22567