Controlling the crystal phase of a material using solution-based method is a challenging task and has significant consequence to the material’s properties. Herein we report the phase and size-controlled synthesis of tungsten oxide hydrates at room temperature via a simple precipitation method. In the absence and presence of oxalic acid, orthorhombic WO₃·H₂O and monoclinic WO₃·2H₂O nanoplates of size in the range of 200–600 (thickness <50 nm) and 40–200 nm (thickness <20 nm) were respectively synthesized. Oxalic acid is found to play the central role in the phase transition due to its chelating nature that facilitates bonding of oxalate ions to tungsten cations leading to formation of WO₃·2H₂O. Upon annealing at 400 °C for 2 h under air, both WO₃·H₂O and WO₃·2H₂O nanoplates were converted to monoclinic WO₃ nanoplates. These nanoplates were demonstrated to be highly efficient for the photocatalytic detoxification of toxic Cr(VI) in the acidic pH under the visible light irradiation. The best Cr(VI) reduction performance was obtained with WO₃·2H₂O nanoplates due to its smaller band gap and larger effective surface area. In addition, a lower pH value is found to facilitates the Cr(VI) reduction. Furthermore, highly concentrated methylene blue was efficiently removed (>95%) by adsorption on the nanoplates within a minute, suggesting the importance and potential of a material that can be synthesized at room temperature.

中文翻译：

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室温下水合三氧化钨纳米板的晶相和尺寸控制合成：增强的Cr（VI）光还原和亚甲基蓝吸附性能

使用基于溶液的方法控制材料的晶相是一项艰巨的任务，并且对材料的性能产生重大影响。本文中，我们通过简单的沉淀方法报告了室温下氧化钨水合物的相和尺寸控制合成。在不存在草酸的情况下，斜方晶WO ₃ ·H ₂ O和单斜晶WO ₃ ·2H ₂ O纳米板的大小在200–600（厚度<50 nm）和40–200 nm（厚度<20 nm）范围内）分别合成。草酸由于其螯合性质而被发现在相变中起主要作用，螯合性质促进草酸离子与钨阳离子的键合，从而形成WO ₃ ·2H _2。O.在空气中在400°C下退火2小时后，WO ₃ ·H ₂ O和WO ₃ ·2H ₂ O纳米板均转化为单斜晶WO ₃纳米板。这些纳米板被证明对可见光照射下酸性pH值下的有毒Cr（VI）的光催化解毒非常有效。WO ₃ ·2H ₂获得最佳的Cr（VI）还原性能O纳米板由于其较小的带隙和较大的有效表面积。另外，发现较低的pH值有助于Cr（VI）的还原。此外，通过在一分钟内吸附到纳米板上，高浓度的亚甲蓝被有效去除（> 95％），这表明了可以在室温下合成的材料的重要性和潜力。