Since the electrocatalytic activity of layered molybdenum disulfide (MoS2) for hydrogen evolution reaction (HER) closely depends on its exposed edges, the morphology and size of the material are critically important. Herein, we introduce a novel solvent-evaporation-assisted intercalation method to fabricate the hybrid of alternating MoS2 sheets and reduced graphene oxide layers, in which the nanosize of the MoS2 nanosheets can be effectively controlled by leveraging the confinement effect within the two-dimensional graphene layers. Significantly, the resulting MoS2/reduced graphene oxide (RGO) composite shows excellent catalytic activity for HER characterized by higher current densities and lower onset potentials than the conventional pre-exfoliated RGO supported MoS2 nanosheets. Further experiments on the effect of oxidation degree of graphene, the crystallinity of MoS2, and the exposed active site density on the HER performance of the MoS2/RGO composites show that there is an optimum condition for the catalytic activity of HER due to a balance between the numbers of exposed active sites of MoS2 and the internal conductive channels provided by graphene.

hole transport materials (HTMs), and metal contact electrode. Discouragingly, the conventional organic HTMs (e.g., spiro-OMeTAD) are usually expensive and unstable. The noble metal electrode is also costly and requires vacuum thermal evaporation device. These problems will hinder the commercialization of PSCs in the future. Fortunately, it was found that HTM-free PSCs could also operate efficiently since the unique ambipolar property of the perovskite allows it to serve not only as a light harvester but also as a hole conductor. [12][13][14][15][16][17] In particular, carbon-based HTM-free PSCs show much promise to solve the above-mentioned problems because carbon materials are earth abundant, low-cost, and environmentally stable. [ 11,[18][19][20][21][22][23][24][25][26] To date, the most effi cient (PCE ≈ 13%) carbon-based PSCs were reported by Han and colleagues. [ 18,19,21,27 ] In these devices, a TiO 2 scaffold layer, a porous ZrO 2 insulating layer, and a porous carbon electrode were sequentially deposited, followed by infi ltration of a perovskite precursor solution. In our previous works, we have simplifi ed the architecture and the fabrication process of the carbon-based PSCs by eliminating the ZrO 2 insulating layer and depositing the perovskite layer by the two-step method, [ 20,[28][29][30] demonstrating the PCE at ≈11%. [ 20,29,30 ] Very recently, a paintable carbon-based PSC was reported by several groups, [31][32][33][34][35] which was fabricated by fi rst depositing the perovskite layer and then printing the carbon electrode with a pre-prepared carbon paste, as illustrated in Figure 1 and Figure S1 (Supporting Information). The whole process is simple, at low temperature and low cost, apparently compatible with roll-to-roll production. However, the PCE of the paintable carbon-based PSCs is relatively low, viz., ≤10%. [31][32][33][34][35][36] Plausibly, the most important limiting factor for achieving high-effi ciency of the paintable carbon-based PSCs lies in the poor contact at perovskite/carbon interface because of the postdeposition process of carbon electrode, which seriously decreases fi ll factor (FF). [31][32][33][34] In order to tackle such a contact problem, it is necessary to produce an even perovskite layer to afford intimate contact at the perovskite/carbon interface. Onestep method has been successfully applied to prepare an even perovskite layer on planer PSCs. [ 8,[37][38][39] However, this method is unsuitable for depositing a good pore-fi lling perovskite layer in a relatively thick mesoscopic TiO 2 scaffold required for the Carbon-based hole transport material (HTM)-free perovskite solar cells (PSCs) have shown much promise for practical applications because of their high stability and low cost. However, the effi ciencies of this kind of PSCs are still relatively low, especially for the simplest paintable carbon-based PSCs, in comparison with the organic HTM-based PSCs. This can be imputed to the perovskite deposition methods that are not very suitable for this kind of devices. A...

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