## 1 Introduction  
The relentless escalation in environmental pollution presents a critical and detrimental global problem. In particular, the release of large quantities of toxic dye in wastewater is responsible for the evident and severe destruction of our ecosystem [1]. Semiconductor nanomaterials promise to play a key role in wastewater purification, a prospect that has attracted major attention from the government, industries, and the public. In general, photocatalysis by semiconductors results from the interaction between photo-formed electron and hole pairs on semiconductor surfaces irradiated by UV light [2]. Electrons (e\({}^{-}\)) are able to reduce oxygen molecules (O\({}_{2}\)) to superoxide (O\({}_{2}\)\({}^{-}\)), or to hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), while some of the holes (h\({}^{+}\)) react with H\({}_{2}\)O and/or OH\({}^{-}\) to produce hydroxyl radicals (\(\bullet\)OH). Depending on the actual conditions, the holes, \(\bullet\)OH radicals, O\({}_{2}\)\({}^{-}\), H\({}_{2}\)O\({}_{2}\) and O\({}_{2}\) can all play important roles in photocatalytic reactions [1]. Compared with the traditional wastewater treatment process, semiconductor photocatalysis is a promising way to treat toxic, incuous, and non-biodegradable contamination in wastewater, at room temperature and atmospheric pressure [3; 4; 5]. While TiO\({}_{2}\) has been investigated extensively [6; 7], relatively few photocatalysis studies related to ZnO have been reported. ZnO, a direct wide-bandgap, semiconducting material, with an energy gap of 3.37 eV, and an exciton binding energy of 60 meV [8], has been harnessed as a multifunctional material to realize numerous technological advances in fields spanning piezoelectrics [9], optics [10], electronics [11], catalysis [12] and photocatalysis [13; 14; 15; 16; 17]. Moreover, ZnO may be more efficient than TiO\({}_{2}\) in photocatalytic degradation of some dyes, even in aqueous solutions [18; 19]. A lot of recent research has been focused on pure and modified ZnO photocatalytic nanomaterials, including noble metal loading [20], metal and/or nonmetal ion doping [21; 22], composite semiconductors [23; 24; 25], surface sensitization [26], high-energy ion implantation [27; 28], etc. While these methods can widen the spectrum of visible light response and/or lower the recombination rate of electrons/holes, to improve the photocatalytic efficiency, these techniques are not industrially viable as they require costly infrastructure and complex synthesis processes. Consequently, simpler and more effective treatment methods, such as vacuum deoxidation [29], hydrogen thermal treatment [30; 31], and chemical oxidation/reduction [32; 33], have been actively pursued for the practical application of ZnO as a photocatalyst. [PERSON] et al. [29; 30] synthesized photocatalysts with a high concentration of oxygen defects by annealing ZnO nanoparticles (NPs) in a He atmosphere at 240-260 \({}^{\circ}\)C, or in a H\({}_{2}\) atmosphere at 455-700 \({}^{\circ}\)C. These catalytic materials exhibited high activity during the degradation of methylene blue dye, under visible (or UV) light irradiation. By using H\({}_{2}\)O\({}_{2}\) aqueous solution to pretreat ZnO powder, [PERSON] et al. [32] obtained ZnO NPs with oxygen vacancies, which was found to be efficient for catalytic decomposition of 2.4-dichlorophenol, under visible light irradiation. However, the safety risk and high cost posed by the utilization of dangerous chemicals (H\({}_{2}\), H\({}_{2}\)O\({}_{2}\), etc.) is also bad for mass production of this kind of photocatalyst. Here, we report on a simple and effective approach to introduce oxygen defects into ZnO materials through a low-temperature, dry vacuum treatment, without any gas. Our ZnO photocatalyst exhibits high photocatalytic activity and good stability during aqueous environmental remediation, and its photocatalytic efficiency is competitive with other reports (Table 1).