Schematic illustration of modified g-C3N4for visible-light photocatalytic water splitting to hydrogen. The CN-DPT shows remarkably enhanced hydrogen evolution performance.

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comparison with their structures, it can be found that the [CuSe 4 ] units are connected together to form a two-dimensional (2D) layer structure in BaCu 2 SnSe 4 , which is different from the 3D framework structure formed by the interlinked [CuQ 4 ] (Q ¼ S and Se) units in other four title compounds. In addition, BaCu 2 SnSe 4 exhibits only one type of tunnel structure with the isolated [BaSe 8 ] units existing in each tunnel, which is also different from the other title compounds (two types of tunnels with the isolated [BaSe 8 ] units and [BaSe 6 ] n chains located). The interesting structural changes also indicate that slight change of cation size would result in different structure features, and future structure prediction should devote considerable attention to the different chalcogen atoms. Moreover, important optical properties (optical bandgap, infrared (IR) absorption edge, second harmonic generation (SHG) response)of the title compounds were systematically investigated. Among them, IR and Raman spectra indicate that all of them exhibit the wide IR absorption edges ($22 mm). Powder SHG measurement shows that BaCu 2 SnS 4 possesses good SHG response about 1.6 times that of benchmark AgGaS 2 (AGS) at the particle size 55-88 mm. All results indicate that BaCu 2 SnS 4 can be expected as a potential IR nonlinear optical (NLO) candidate. Theoretical calculation was also used to analyze the structure-property relationship and their electronic structures and origin of NLO effect were studied in detail.coordinates, isotropic displacement parameters; selected bond lengths and angles; powder XRD patterns and IR spectra; particle size versus SHG intensity for BaCu 2 SnS 4 and AgGaS 2 ; absorption spectra of title compounds; calculated electronic structures; projected density of states; calculated SHG density. CCDC 1540053, 1540054, 1540055, 1540056 and 1540057 for