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Nanomaterials 2020, 10, 454 2 of 13 of GO can be achieved by chemical exfoliation of graphite directly [21]. GO membranes present oxygen containing, negatively charged functional groups, enabling them to reject negatively charged organic dyes [22]. Joshi et al. reported that ions smaller in size than nanochannels can easily pass through GO membranes, and that size exclusion is the dominant sieving mechanism [23]. Tuning the interlayer spacing of GO sheets is a promising strategy for improving the water flux of GO membranes [24]. Interlayer spacing can be altered via the incorporation of silver nanoparticles (AgNPs), to make GO/AgNP composite membranes [25,26]. Sun et al. [27] investigated the influence of the thickness of GO/AgNPs membrane to permeability. The result showed that the water flux increased with reduction of the membrane thickness. They also investigated the water contact angle of GO and GO/AgNPs membrane. The result showed that the water contact angle of GO/AgNPs membrane was larger than that of GO membrane, which indicated GO/AgNPs membrane is more hydrophobic. Liu et al. investigated influence of membrane thickness to rejection to RhB. The result showed membrane thickness positively correlated to rejection [28]. These studies indicate membrane thickness is critical to nanofiltration performance. Chen et al. [29] found a negative correlation between the size of AgNPs in GO@PEG composite membranes and antibacterial activities. However, there are few studies about how the size of AgNPs on GO membranes effects the permeability and rejection of organic dye. In our previous work, graphene-based hybrid materials, such as GO/rare-earth materials [14], graphene/silver hybrid membranes [28], GO/polyacrylamide composite membranes [7], and graphene- gold nanoparticles membranes [30], have been reported. We have investigated the influence of the amount of AgNPs on membrane nanofiltration performance [28]. The fabrication method of different-sized AgNPs is mature [29]. Therefore, in this work, we chose AgNPs as spacer, and successfully fabricated three types of GO/AgNP composite membranes. The size of AgNPs loaded on GO membrane was 8 nm, 20 nm, and 33 nm respectively. The water flux and rhodamine B (RhB) rejection of three types of GO/AgNP composite membranes and GO membrane and their relationship with membrane thickness was investigated. We also investigated the relationship of these features with the size of AgNPs and proposed the separation mechanism. This work is helpful for understanding the separation mechanism of GO/AgNPs composite membranes, and reveals significant research questions for future study. 2. Experimental Section 2.1. Materials Natural flake graphite (99%) with average particle sizes of 300 mesh was purchased from Qingdao Tianheda Graphite Co., Ltd (Qingdao, China). H2SO4 (98 wt%) and H2O2 (30 wt%) were purchased from Zhiyuan (Tianjin, China) Chemical Reagent Co., Ltd. Silver nitrate (AgNO3), sodium citrate dihydrate (C6H5Na3O7·2H2O), ethanol (C2H5OH), methanol (CH3OH) and ethylene glycol ((CH2OH)2) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Rhodamine B (RhB) and methylene blue (MB) were purchased from Macklin Reagent Co., Ltd.,Shanghai, China. Poly(vinyl pyrrolidone) (PVP, (C6H9NO)n) with average molecular weight 2.4 × 104 g/mol was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, China. 2.2. Preparation of GO/AgNPs Composite Membrane GO was prepared by the improved Hummers’ method from natural flake graphite mentioned above. 25 mg GO was dissolved in 50 mL ultra pure water followed by sonication in Ultrasonic cell grinder for 4 min. 25 mg AgNO3 was dissolved in 2 mL ultra pure water and then added into GO solution. 4.2 g PVP was dissolved in 20 mL ultra pure water. Then, 1 mL PVP solution was added into the reaction solution to prevent the aggregation of silver nanoparticles. After sonication for 20 min, the mixture was heated to boiling, and then 150 mg sodium citrate dissolved in 10 mL ultra pure water was added to the solution. After boiling under reflux for 40 min, protected from light, GO/AgNPPDF Image | Graphene Oxide Nanofiltration Membranes Silver Nanoparticles
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