UV and visible-light driven photocatalytic removal of caffeine using ZnO modified with different noble metals (Pt, Ag and Au)

doi:10.1016/j.materresbull.2018.12.034

Materials Research Bulletin

Volume 112, April 2019, Pages 251-260

https://doi.org/10.1016/j.materresbull.2018.12.034 Get rights and content

Highlights

•
ZnO photocatalyst was modified with different noble metals (Pt, Ag and Au).
•
Photocatalytic treatment of caffeine aqueous solutions under UV and visible light.
•
The optimal photocatalyst was ZnO modified with Ag.
•
Complete caffeine degradation also under visible light.

Abstract

In this work, ZnO photocatalyst was modified with different noble metals (Pt, Ag and Au) through photodeposition method and then characterized by different techniques (XRD, XRF, BET, UV–vis DRS, FESEM, and XPS). The addition of noble metals produces important changes in the light absorption properties with a significant absorbance in the visible region due to the existence of surface plasmon resonance (SPR) observed at about 450 nm and 550 nm for ZnO modified with Ag and Au, respectively. The morphology of the samples was studied by TEM and the size ranges of the different metals were estimated. Noble metal nanoparticles were in every case heterogeneously deposited on the larger ZnO particles. All the prepared photocatalysts were tested in the photocatalytic removal of caffeine (toxic and persistent emerging compound) under UV and visible light irradiation. It was observed an enhancement of photocatalytic caffeine removal from aqueous solutions under UV light irradiation with the increase of metal content (from 0.5 to 1 wt %) for ZnO modified with Ag and Au (Ag/ZnO and Au/ZnO). In particular, Ag/ZnO and Au/ZnO with higher Ag and Au content (1 wt %) allowed to achieve the almost complete caffeine degradation after only 30 min and a TOC removal higher than 90% after 4 h of UV light irradiation. These two photocatalysts were investigated also under visible light irradiation and it was found that their photocatalytic performances were strongly enhanced in presence of visible light compared to unmodified ZnO. In particular, Ag/ZnO photocatalyst was able to reach the complete caffeine degradation and a TOC removal of about 70% after 4 h of visible light irradiation.

Graphical abstract

Introduction

Emerging pollutants, such as pharmaceuticals, hormones, endocrine disruptors, are receiving increasing attention from scientific researchers and public administrations [1,2]. Although these contaminants are at low concentrations (typically a few μg L⁻¹ or even ng L⁻¹), they are persistent in the environmental with possible phenomena of bioaccumulation or synergetic toxicity [1,3,4]. Caffeine (C₈H₁₀N₄O₂) is an example of toxic and persistent emerging compound that is often revealed in surface waters [[5], [6], [7]]. Belonging to the methylxanthine family [8], caffeine is a psychoactive substance widely consumed either for beverages (coffee, cola drinks, tea, energy drinks) or for pharmaceutical purpose acting as a psychomotor stimulant for the human body [9]. Different processes were studied for the removal of emerging compounds from contaminated water, such as coagulation/flocculation [10], biological treatments [11], adsorption [12,13], Fenton process [14] and membrane process [15]. Because these conventional treatments are not effective to completely remove emerging contaminants [[16], [17], [18]], in recent years, there is an urgent need to develop efficient, clean and economic processes to treat water and wastewater containing pharmaceutical pollutants. For this purpose, the photocatalytic treatment has become an attractive field due to its great potential for environmental purification [19,20] and the ability to remove efficiently emerging compounds in water even at very low concentrations [21]. Specifically, several semiconductors were studied in photocatalytic reactions but among them, zinc oxide (ZnO) has demonstrated a strong oxidation capacity and good photocatalytic properties [[22], [23], [24]] compared to titania (TiO₂) [[25], [26], [27]]. Some papers report that the principal advantage of ZnO is that it absorbs a fraction of the solar spectrum larger than TiO₂ [28]. However ZnO exhibits a rapid recombination of photogenerated charge carriers due to its wide band gap value (3.3 eV) [29] with a consequently inefficient use of sunlight to hinder the photocatalytic efficiency [30]. In order to use ZnO catalyst under visible light, and because the latter covers about 43% of the solar spectrum, there is the necessity to modify ZnO catalyst [31,32]. There are different procedures to reduce the recombination of photogenerated electron-hole pairs in order to improve the use of ZnO under solar light, such as doping with metal and non-metal elements [22,[33], [34], [35]], coupling of ZnO with another semiconductor [36,37], hybridization with carbon materials [38] and metal deposition [[39], [40], [41]]. In the last case, deposition of noble metals such as gold [[42], [43], [44]], platinum [44,45], silver [40,[46], [47], [48], [49]] and palladium [50] on semiconductor oxides can act as sinks for the photogenerated electron and thus elongates the life time of electron–hole pairs [30]. In addition, noble metals, such as Au or Ag, possess the ability to absorb visible light due to the existence of surface plasmon resonance (SPR) [51] and so they could confer to ZnO the ability to work under visible light. It was shown that the noble metal nanoparticles have the property of the collective excitations of electrons in the visible region, known as the process of surface plasmon resonance (SPR), so such photocatalysts are called plasmonic photocatalysts [52,53]. In fact, in a plasmonic photocatalyst, the excited electrons have enough energy to move directly into the conduction band of the semiconductor. Most of these studies deal with photocatalytic activity of ZnO or TiO₂ modified with noble metals for the removal of dyes from water [[54], [55], [56], [57]], while papers dealing on the removal of emerging compounds are mainly focused on noble metals deposited on TiO₂ [58,59]. To our knowledge, it is still scarce the literature about the effect of ZnO modified with different noble metals on the photocatalytic degradation of pharmaceutical compounds under visible light irradiation. For this reason, the aim of this work is to investigate the influence of Au, Pt or Ag supported on ZnO surface both in the UV and in the visible light driven photocatalysis for the removal of caffeine from aqueous solutions. Photocatalytic activities of the unmodified ZnO and ZnO modified with noble metals (Ag, Pt and Au), at two different contents (0.5 and 1 wt %), were performed under UV light irradiation in order to find out the best photocatalysts. Then, photocatalytic tests under visible light were carried out to investigate the influence of presence of noble metals on ZnO catalyst, the effect of catalyst dosage and initial caffeine concentration.

Section snippets

Synthesis of ZnO and ZnO modified with noble metals

ZnO was synthesized according the procedure reported in literature [60]. Briefly, Zn(CH₃COO)₂·2H₂O was dissolved in bi-distilled water and a Na₂CO₃ solution was added to it. After vigorously stirring, the final pH was 6.8. The solution was aged for 24 h and a white precipitated was formed, centrifuged, washed and dried at 100 °C overnight. Finally, the powder was calcined at 400 °C for 2 h. Metal-ZnO materials were prepared by photodeposition of the metal (Pt, Ag or Au) on the calcined ZnO

Characterization of the photocatalysts

XRD patterns showed that the crystalline phase of ZnO was the hexagonal wurtzite for all the samples (JCPDS Card File Nº 079-2205). Strong intensity and sharpness of the peaks give account of a high crystallinity of the samples. For the samples with 1 wt% of Ag or Au loadings, an incipient peak at 2ϴ around 38° can be observed, attributed to metallic Ag and metallic Au respectively (marked in Fig. 1 with asterisks). No peaks for Pt could be found for any of the platinized samples. This is

Conclusions

The photocatalytic activity of ZnO photocatalyst modified with noble metals (Pt, Ag and Au) was investigated for the treatment of caffeine aqueous solutions under UV light and visible light irradiation. The chemical-physical characterization results showed that ZnO was in hexagonal wurtzite phase and that noble metal particles in 1%Au/ZnO and 1%Ag/ZnO samples are in their metallic state. Moreover, the presence of noble metals on ZnO surface induced the presence of surface plasmon resonance

Acknowledgements

This work was supported by research fund from Project Ref. CTQ2015-64664-C2-2-P (MINECO/FEDER UE). Research services of CITIUS University of Seville are also acknowledged. We thank the University of Tolima for economic support in the studies commission of César Augusto Jaramillo Páez.

References (75)

A. Bernabeu et al.
Catal. Today
(2011)
Ml. Farré et al.
TrAC – Trends Anal. Chem.
(2008)
T. Smital et al.
Mutat. Res. – Fundam. Mol. Mech. Mutagen.
(2004)
C.I. Kosma et al.
Sci. Total Environ.
(2014)
S. Zhang et al.
Chemosphere
(2007)
A. Elhalil et al.
J. Environ. Chem. Eng.
(2017)
F.F. Al-Qaim et al.
J. Hazard. Mater.
(2015)
T.K.F.S. Freitas et al.
Ind. Crops Prod.
(2015)
B. Bonakdarpour et al.
Int. Biodeterior. Biodegradation
(2011)
O. Sacco et al.
Sep. Purif. Technol.
(2018)

Y. Zhang et al.

Mater. Res. Bull.

(2016)

M. Plodinec et al.

J. Alloys Compd.

(2014)

C. Jaramillo-Páez et al.

Catal. Today

(2017)

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UV and visible-light driven photocatalytic removal of caffeine using ZnO modified with different noble metals (Pt, Ag and Au)

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of ZnO and ZnO modified with noble metals

Characterization of the photocatalysts

Conclusions

Acknowledgements

Catal. Today

TrAC – Trends Anal. Chem.

Mutat. Res. – Fundam. Mol. Mech. Mutagen.

Sci. Total Environ.

Chemosphere

J. Environ. Chem. Eng.

J. Hazard. Mater.

Ind. Crops Prod.

Int. Biodeterior. Biodegradation

Sep. Purif. Technol.

Water Resour. Ind.

Sci. Total Environ.

Desalination

J. Environ. Chem. Eng.

Chem. Eng. J.

J. Catal.

J. Saudi Chem. Soc.

Appl. Catal. B: Environ.

J. Hazard. Mater.

Mater. Res. Bull.

J. Photochem. Photobiol. A: Chem.

J. Hazard. Mater.

Appl. Catal. A: Gen.

J. Water Process Eng.

Optik

Appl. Catal. B: Environ.

J. Photochem. Photobiol. A: Chem.

J. Mol. Catal. A: Chem.

J. Colloid Interface Sci.

Appl. Catal. B: Environ.

Chemosphere

Mater. Lett.

Mater. Res. Bull.

Mater. Res. Bull.

Mater. Res. Bull.

J. Alloys Compd.

Catal. Today