Elsevier

Synthetic Metals

Volume 267, September 2020, 116472
Synthetic Metals

New carbon quantum dots nano-particles decorated zinc peroxide (Cdots/ZnO2) nano-composite with superior photocatalytic efficiency for removal of different dyes under UV-A light

https://doi.org/10.1016/j.synthmet.2020.116472Get rights and content

Highlights

  • New nanocomposite CDots/ZnO2 has been successfully synthesized for photocatalysis applications.

  • CDots/ZnO2 displays a superior photocatalytic activity and high stability in UV-A light.

  • CDots/ZnO2 showed photocatalysis efficiency for MO, MB and Rh.B dyes higher than CDots/TiO2.

Abstract

In this study, eco-friendly carbon quantum dots/zinc peroxide (CDots/ZnO2) nano-composite has been fabricated for photocatalytic applications. The structural and chemical compositions of (CDots/ZnO2) catalyst have been described using XRD, BET, SEM, and TEM. The decoration of CDots on ZnO2 surface is confirmed by XRD and TEM microscope. Then the photocatalysis activity of CDots/ZnO2 has studied for methyl orange, methylene blue and Rhodamine B dyes and compared with CDots/TiO2. CDots/ZnO2 shows higher photocatalytic activity than CDots/TiO2, due to its excellent charge separation, large total pore volume and/or size and higher photoresponsive behavior of CDots/ZnO2. Photocatalyst (CDots/ZnO2) nano-composite shows excellent photocatalytic activity and high stability in UV-A spectrum range thanks to the up-conversion luminescence and photoinduced electron transfer behavior of CDots. Moreover, the superior photocatalytic activity of CDots/ZnO2 is attributed to the synergetic effect and the design of charge-transfer channel. Also, the stability of CDots/ZnO2 stands for along five cycles without any change. This effort introduces an invaluable methodology aiming at eliminating the environmental pollution by designing new photocatalysts with brilliant photocatalytic activity and high operational stability.

Introduction

Undoubtedly, water is the main source of our life, and it is considered the main and only source that creates and keeps all the surviving beings in the universe. Recently, the quality of the water captured the attention of scientists, and the study of improving the quality of water is much concerned for them. In order to address this point, water research laboratories and institutes have been established. However, the problem is still remaining, and the quality of water decreases thanks to the unwanted reactions (physical, chemical and biochemical) that take place by the time in the distribution systems and the harmful bacteria, as well [1,2]. Drinking water faces a lot of physical and chemical parameters affecting its quality such as concentrations of chlorides and nitrates in water, water hardness, pH and turbidity [3]. In the last few years, water body surface has been polluted, owing to the ejections of wastewater that have many pollutants like organics, wastes of agriculture and the household sludge [4]. The existence of very small quantities of organic pollutants, like water‒coloring agents/dyes, in water surroundings have been proposed to realize clashing effects on human and ecological systems [5]. In fact, dyes are applied in many industrial applications, for example, leather tanning, fabric industry, food flavors, medical inspection, and also inserting in the industries of cosmetics. It is known that even low concentrations of dyes (<1 mg/L) in water are noticeable, unfavorable, not familiar and harmful [6]. Methyl orange (MO) is one of the most known dyes with the chemical formula (C14H14N3NaO3S), which is extensively used in the fabric industry. Also, methylene blue (MB) and Rhodamine B are very harmful dyes due to their negative effects on human health. Long time contact with methyl orange, methylene blue and Rhodamine B dyes will produce rising in the heart beats, cyanosis, tumours, liver diseases and human skin irritation [7]. In order to remove methyl orange, methylene blue and Rhodamine B from liquid solutions, many approaches were applied such as membrane separation, coagulation/flocculation, photocatalysis, biological treatment, electrochemical and adsorption technique [[5], [6], [7], [8]]. Particularly, photocatalysis approaches that used some metal oxide compounds like zinc oxide (ZnO) [9], titanium oxide (TiO2) [10] and others; have been demonstrated to be an effective procedure with enhanced efficiency and capability to remove dyes on a large scale besides having other benefits such as recovery, and recycling of adsorbents [11].

Starting from this point, the configuring and discovery of new photocatalytic materials are considered one of the most important fields today due to their applications in the removal of organic pollutants and solar energy conversion [12]. The innovation of a new photocatalyst with a superior catalytic performance and high stability is a great challenge and attracted the attention of researchers and academics. Recently, large amount of literatures based on metal oxide photocatalysts have been reported, for instance ZnO, TiO2, and SrTiO3 [[13], [14], [15], [16], [17]]. Among all the metal oxide structures, ZnO and TiO2 are considered the most using materials for photocatalysis applications and environmental sustainability [18,19], due to their ease of preparation in nano-scale or bulk scale, high photocatalytic activity, economical and their abundance. But the large energy gap (3.2 eV) of ZnO prevents its use as a photocatalyst in UV light. This is considered as a great limitation where 5 % of solar energy is unusable or wasted [20,21]. As well, many problems for these metal oxide materials were reported such as poor stability, using a small amount of solar energy and the low rate of electron-hole pairs recombination are still limiting their applications [[19], [20], [21], [22], [23], [24], [25]]. In fact, TiO2 nanoparticles share ZnO nanoparticles in the same problems such as the large band gap and the wasted solar energy. However, it is still a great challenge to obtain the photocatalyst with high catalytic activity that can be recycled. Therefore, the need for innovation and building novel photocatalysts to increase both the operational stability and photochemical activity is crucial and essential. To realize these aims, an ideal photocatalyst material should at least realize the following features: high stability, highly activation by low photon energy and efficient separation capacity of electron–hole pairs. To achieve these goals, carbon materials especially carbon quantum dots (CDots) are extensively applied in the photocatalysis uses due to their high electrical conductivity, harmlessness and up-conversion PL behavior. In addition, CDots warp the surface of metal oxide materials to protect them from the corrosion during the process of photocatalytic and increase the structural stability [26]. In fact the composite of CDots with metal oxides is not only increasing the photo-stability but also increasing the photocatalytic activity. Moreover, the unique photo-chemical and physical properties of CDots facilitate its usage as photosensitizers [27]. Particularly, CDots are used as photocatalysts for harvesting (N)IR light due to their PL properties that depend on the excitation wavelength and photo-induced electron-transfer [28].

Bearing in mind such significant properties, we offered an engineered and novel metal peroxide-CDots system, which will be a trail to enhance the photo-stability, high responsiveness to UV-spectrum area and balance charge separation. Lately, there are numerous reports on using the composite (CDots/Zinc oxide ZnO) and CDots/TiO2 in photocatalytic applications, for instance, Li et al. [29] reported on CDots/ZnO system for the removal of Rh.B dyes. They found that pure ZnO has photodegradation efficiency about 30 % and CDots/ZnO has photodegradation efficiency about 81 % within 2 h of UV irradiation. Bozetine et al. [30] reported on nano-composites CDots/ZnO for photocatalytic properties under visible light. They found that CDots/ZnO (at 80 °C) will be able to remove Rh.B dyes with an efficiency that may reach 83 % within 105 min. Also, the efficiency of this system increases to be 94 % after 105 min of irradiation when annealing the system at 200 °C. The authors also reported that the efficiency reduces to be 88 % for CDots/ZnO (200 °C) after four runs of the experiment stability. Ding et al. also studied the effect of ZnO foam/CDots on the photo-degradation of three different dyes Rh.B, MO and MB under visible light irradiation [31]. They studied the effect of 100 mg of ZnO foam decorated with (0.1, 0.2 and 0.3 g of CDots) on the photo-degradation of MO, MB and Rh.B dyes. They found that (100 mg of ZnO foam with 0.2 g of CDots) is able to remove Rh.B dyes completely within 4 h under visible light. Also, they found that at the same concentration of the photocatalyst, the system was able to remove 50 % of MO and 95 % of MB within 3 h of visible light irradiation. Also, Muthulingam et al. prepared (CDots/N-ZnO) photocatalyst for degradation of dyes such as MB, fluorescein dyes and malachite green dye under daylight [32]. They also stated that CDots/N-ZnO system can be able to remove MB completely with efficiency (100 %) and N-ZnO has the ability to remove 82 % of MB in 60 min of daylight irradiation. Also, they found that both N-ZnO and CDots/N-ZnO systems are stable for four recycles, where in the 4th cycle; CDots/N-ZnO required 90 min to remove the dyes completely and N-ZnO needed 4 h to remove 95 % of MB.

As seen, although these photocatalysts seems to be good catalysts due to their higher efficiency, but these catalysts are restricted by special conditions (such as they only work in alkaline media) and this in turn hinders their large scale industrial applications and they also have many shortcomings such as low specific surface area, fast electron-hole pairs recombination, low response to light and the requirement of a long time to reach the desired efficiency. Also, the stability and recovery of these materials is still problematic [1,33] and they must be functionalized by (N, S, and other) groups to break these restrictions. Thus to overcome these shortcomings we introduce cheap, rapid/fast, environment friendliness photocatalyst based on carbon dots decorated zinc peroxide (CDots/ZnO2) can be used for removing three different dyes with higher efficiency, high recovery, high photo-stability, high recycling and also the capability of operating under ambient conditions and also action in a natural pH medium. Also, it can be able to remove a huge volume of the dye molecules in many industries. Moreover, this (CDots/ZnO2) system has a high operational stability without any loss in its efficiency.

In this study, we are fascinated with zinc peroxide (ZnO2) particles in nano-scale, as their structure has two oxygen atoms linked by a single bond. Besides, ZnO2 particles have much attention from scientists and academics due to their unique properties and promising functional potential applications, which arise from the increase in the electronegativity (excess of oxygen) behavior and their higher oxidization potential behavior of these nano-particles. From this fact, the MgO2 nano-rods were used to eliminate the MO dye under the UV irradiation [33]. Also, the ZnO2 particles have been used in Rhodamine B (RB) removal under UV irradiation [34]. Also, ZnO2 particles have been used in detoxification of mustard gas [35]. Additionally, the ZnO2 powder is applied in many industrial fields, such as oral surgeries, wear resistance, cosmetic, and others [35]. However, the study of photocatalytic behavior of the metal peroxides and their composites with CDots are available until now. And the photocatalytic behavior of the metal peroxides and their composites materials still need more research.

In this work, we report a new photocatalyst based on ZnO2 and CDots/ZnO2 in which ultra-thin layers of insoluble CDots wrapping ZnO2 nano-structures are used as the synergetic catalysts. The photocatalyst CDots/ZnO2 shows an excellent charge separation and superior photocatalytic performance in removing methylene (MB) blue (99 ± 1 % in 50 min), methyl (MO) orange (91 ± 2 % in 60 min), and Rhodamine B dyes (99 ± 1 % in 80 min) under the UV-A light. Also, the main roles of CDots layers and ZnO2 nano-particles in photocatalysts were studied, respectively. CDots/ZnO2 has a photocatalytic activity higher than ZnO2, TiO2 and CDots/TiO2 due to the synergetic effects of the following: (1) ZnO2 and CDots nano-particles contributed in electron-hole pairs separation; (2) CDots have mutual-benefits as electron reservoir behavior and the brilliant up-converted PL; (3) a favorable electron pathway provided by CDots/ZnO2 architectures, CDots→ZnO2, for the electron-hole pairs separation. The engineering of CDots/ZnO2 nano-structure displayed a high catalytic performance and a large capacity of migrated electrons at UV-A light than CDots/TiO2 due to their high electronegativity (excess of oxygen/charges) behavior of ZnO2, which facilitates to produce more and more photoelectrons, by the way increasing the photo-current response. Also, the CDots/ZnO2 is still working for along five cycles, according to the recycling experiment. However, the photocatalyst CDots/TiO2 prepared in our experiment showed some higher photocatalytic efficiency higher than that reported in many previous literatures, this is may be due to the differences in the experimental conditions and environments during the photocatalytic process. Finally, this work introduces an invaluable methodology aiming at eliminating the environmental pollution by designing new photocatalysts with brilliant photocatalytic activity and high operational stability.

Section snippets

Experimental details

D-glucose (99.8 % purity), Zinc acetate dehydrate (Mw =142.394 g/mol, 99.9 % purity), Hydrogen peroxide (Mw = 34.01 g/mol, 99.8 % purity), Absolute ethanol (CH3CH2OH, 99.5 % purity) and Sodium hydroxide (99.9 % purity) were purchased from (Adwic, El-Nasr Chemical Co., Cairo, Egypt).

Characterization (TEM, SEM,XRD, XPS) of the samples

XRD peaks of all samples are depicted in Fig. 1. It can be seen that a single hump was shown at 2θ = 23° for C(002) plane of CDots, due to the hexagonal lattice structure of graphite CDots [1,[14], [15], [16], [17],[23], [24], [25], [26], [27],33] (Fig. 1a). Fig. (1b) shows the XRD patterns of ZnO2 nano-particles. The (hkl) planes are specified on top of each pattern. All the diffraction patterns of the obtained ZnO2 nano-particles have been attributed to a cubic system of ZnO2, which is

Conclusions

In conclusion, photocatalyst (CDots/ZnO2) with high performance is engineered by combining the tunable luminescence CDots and ZnO2. CDots/ZnO2 has a photocatalytic activity against MB, MO and Rh.B higher than ZnO2, TiO2 and CDots/TiO2 due to the synergetic effects of the following: (1) ZnO2 and CDots nano-particles contributed to electron-hole pairs separation; (2) CDots have mutual-benefits as electron reservoir behavior and the brilliant up-converted PL; (3) a favorable electron pathway

CRediT authorship contribution statement

Ahmed gamal El-Shamy: Conceptualization, Methodology, Software, Visualization, Investigation, Software, Validation, Data curation, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

There is no declaration of interest.

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