Full length articleLocal photocatalytic H2 generation on individual microscale two-dimensional CdSe nanosheets
Graphical abstract
Introduction
Since the discovery of graphene, two-dimensional (2D) nanomaterials have drawn great interests for their unique properties [1]. These 2D nanomaterials create one-dimensional (1D) quantum confinement and large surface areas. There are increasing efforts to synthesize various 2D systems, such as metal oxides and semiconductors [2]. Among them, CdSe is one of the most investigated 2D semiconductors due to its interesting optical properties [[3], [4], [5]]. CdSe nanoplatelets (NPLs) [6] and nanosheets (NSs) [7] with lateral dimensions up to a few hundreds nm have been successfully synthesized through colloidal chemistry. However, wet chemistry approach for synthesizing 2D CdSe with a lateral size of micrometers is still a challenge mainly due to the difficult control of growth in a specific direction [8], limiting their applications.
Cation exchange reaction involves replacing cations within performed ionic crystal by different metal cations [9]. Under suitable conditions, the resulted materials retain the size, shape and anion framework of parent crystals [10,11]. For instance, the cation exchange of Cu+ by Zn2+ within Cu2-xSe nanocrystals (NCs) yielded ZnSe NCs which preserved parent materials' morphologies and crystal structures [12]. These successful examples suggest that cation exchange may serve as a versatile method to produce large-area 2D materials that are difficult to synthesize directly [11,13]. Once the synthesis is achievable, varieties of fundamental and applied researches can be performed, such as photocatalytic hydrogen (H2) generation. H2 generation from solar energy has attracted tremendous attention since H2 is a promising alternative resource to fossil fuels [[14], [15], [16], [17], [18], [19], [20]]. Over the recent years, numerous low-dimensional semiconductor nanomaterials have been applied in photocatalytic H2 generation [[21], [22], [23], [24], [25], [26]], and CdSe systems [[27], [28], [29], [30], [31]] are notable examples for this application.
The H2 generation efficiencies are sensitive to the size and morphology of nanomaterials [32]. However, the microscopic nature of H2 generation in these systems is little known. In most studies, the H2 generation rate is determined by gas chromatography (GC), which is the average rate over time of the ensembles [27,33]. At this point, a measurement of local H2 generation rate is important. However, there are only a few reports in this field [34,35]. One reason is that Abbe's diffraction limit [36] for traditional microscopy needs large-area photocatalyst.
In this paper, we manifest the usage of cation exchange to synthesize micrometer-sized CdSe NSs using pre-synthesized microscale CuSe NSs as a starting material. Cation exchange was performed sequentially starting with the transformation of CuSe to Cu2-xSe NSs followed by replacing Cu+ with Cd2+ within the Cu2-xSe NSs that serve as a template. Having established the synthesis, we have conducted the measurements of photocatalytic H2 generation on individual CdSe NS specimens, and in turn, the local H2 evolution rate was determined.
Section snippets
Materials
Hydroxylamine hydrochloride (NH2OH·HCl, 99%) was purchased from Alfa Aesar. 1-octadecene (ODE, 90%), cadmium chloride (CdCl2, >99.0%), 1-dodecanethiol (DDT, 98 + %), copper (II) acetate monohydrate [Cu(CH3COO)2·H2O, 98 + %], methanol (anhydrous, 99.8%), oleylamine (OLA, technical grade, 70%), copper(I) chloride (CuCl, 99.99%), platinum (II) acetylacetonate [Pt(acac)2], trioctylphosphine (TOP, 90%), sodium sulfide (Na2S), selenium powder (100 mesh, 99.5%), and sodium sulfite (Na2SO3) were
Results and discussion
Two-step synthesis process of 2D CdSe NSs is depicted in Fig. 1. In this manner, CdSe NSs with microscale lateral dimensions have been synthesized in the following steps. The synthesis of micrometer-sized CuSe NSs was performed first [37]. In brief, a mixture solution of selenium powder dissolved in DDT and OLA was injected into OLA containing copper acetate at 120 °C under an inert atmosphere. The CuSe NSs were then transformed to comparably sized Cu2-xSe NSs [37]. Cation exchange of Cu+ ion
Conclusion
To summarize, we illustrate the use of cation exchange reaction employing Cu2-xSe NSs as a template to produce micrometer-sized CdSe NSs. The edge length of the obtained CdSe NSs with well-defined morphologies reaches lateral dimensions of up to 6 μm and approximately 10 nm in thickness. The local photocatalytic H2 generation on individual CdSe NSs was successfully detected, for the first time, and the local H2 generation rates can be determined on individual photocatalysts. Our demonstration
Acknowledgments
We gratefully acknowledge the financial support from the Center for Sustainable Energy at Notre Dame. We appreciate the ND Energy Materials Characterization Facility, the Notre Dame Integrated Imaging Facility, and the Notre Dame Radiation Laboratory for usage of their facilities and resources. The authors thank Dr. Masaru Kuno for his helpful guidance and input.
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