## 1 Introduction  
Over the past few decades, water pollutants in the environment, such as polluted water from dye-stuffs and the textile industry, have become a serious environmental problem. The release of excess dyes into water sources affects both human life and the biosphere. Photocatalytic degradation is one of the most common techniques to effectively solve this problem due to its advantages (e.g., it is a simple, green technology using only sunlight and is effective for dye decontamination without adding a secondary pollutant). Wide-band-gap ZnO (3.37 eV [1]) metal oxide semiconductors have been widely applied for various applications, such as gas detectors [2; 3; 4], UV sensors [5; 6; 7], dye-sensitized solar cells [8; 9], and photocatalysts [10; 11], due to their low cost, environmental friendliness, excellent optical properties, good chemical stability, and catalytic activity [12; 13; 14; 15]. However, pristine ZnO photocatalysts can absorb ultraviolet light, which accounts for only three percent of sunlight, andtheyit show fast charge recombination. These are major obstacles to increasing the photocatalytic activity of ZnO [16]. Noticeably, photocatalytic reaction is dependent on the optical band-gap energy, charge carrier concentration, and charge separation [16]. Many researchers have studied metal doping/co-doping with ZnO through different preparation techniques [17; 18; 19] to improve the photocatalytic degradation performance. [PERSON] et al. synthesized ZnO nanoparticles with copper doping by using a sol-gel technique for photocatalytic applications, and these showed higher methyl blue degradation (88%, 6 h) compared with undoped ZnO [20].  
In recent years, rare-earth (RE) metals have been used as dopants in RE/ZnO; these include Er [21; 22], Eu [23; 24; 25], Nd [26; 27], Tm [28], and La [29; 30; 31]. [PERSON] et al. synthesized La-doped zinc oxide nanoparticles (La/ZnO NPs) using a precipitation and mechanical milling technique, and they investigated the blue-light-fluorescent-lamp-driven photodegradation activity of methyl blue. Photocatalysts based on La/ZnO NPs showed that the photocatalytic degradation performance improved with an increase in La doping, and the methyl orange (MO) degradation reached 93% after 120 min under light illumination [32]. [PERSON] et al. reported the enhanced photocatalytic decomposition of Rhodamine B dye by chemically synthesized Nd-doped ZnO powder. Introducing Nd into ZnO can induce defect formation (like antisite oxygen and interstitial oxygen) in Nd-doped ZnO powder, which would play an important role in trapping electron carriers and charge separation [33]. Presently, there are few research reports on Ce-doped ZnO nanocomposites for photocatalytic applications [34; 35]. [PERSON] et al. prepared the 3D flower-like mesoporous cerium-zinc oxide for sunlight-driven photodegradation of Rhodamine B (RhB), which showed 85.1% degradation in 125 min [36]. Overall, researchers reported that RE doping can generate different defects in RE/ZnO, which can tune the band gap to lower energy in order to use visible light as a light source instead of UV light. Additionally, RE ion dopants can prevent electron recombination by trapping electrons, thus improving photocatalytic activity.  
In this manuscript, we report cerium-oxide-nanoparticle-decorated zinc oxide by using a simple one-pot hydrothermal process. The effects of Ce ion doping on surface morphology and composition and on optical properties were systematically studied. As-prepared cerium-oxide-nanoparticle-decorated zinc oxide was manipulated to evaluate the photodegradation activity of methyl orange (MO) under fluorescent lamp illumination.