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Influence of graphene and copper on the photocatalytic
response of TiO2 nanotubes
E. Zghab, M. Hamandi, F. Dappozze, H. Kochkar, M. Zina, G. Berhault, C.
Guillard
To cite this version:
E. Zghab, M. Hamandi, F. Dappozze, H. Kochkar, M. Zina, et al.. Influence of graphene and copper
on the photocatalytic response of TiO2 nanotubes. Materials Science in Semiconductor Processing,
Elsevier, 2020, 107, �10.1016/j.mssp.2019.104847�. �hal-02499666�
Influence of graphene and copper on the photocatalytic response of
TiO
2
nanotubes
E. Zghab
a,c, M. Hamandi
a, F. Dappozze
a, H. Kochkar
b,**, M. Saïd Zina
c, C. Guillard
a,
G. Berhault
a,*aInstitut de Recherches sur la Catalyse et l’Environnement de Lyon, CNRS Universit�e Lyon I, 69100, Villeurbanne, France
bBasic & Applied Scientific Research Center, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, 31441, Dammam, Saudi Arabia cUniversit�e de Tunis El Manar, Facult�e des Sciences de Tunis, Laboratoire de Chimie des Mat�eriaux et Catalyse, 2092, Tunis, Tunisia
The photocatalytic properties of TiO2 nanotubes (NTs) modified by copper nanoparticles (NPs) and (reduced)
graphene oxide ((r)GO) are herein evaluated. The hybrid TiO2 nanomaterials were deeply characterized by
means of N2 adsorption-desorption measurements, X-ray diffraction, X-ray photoelectron and UV–vis diffuse
reflectance spectroscopies, and transmission electron microscopy. On Cu/TiO2 NTs, results show a stabilization
of Cu NPs at a þI oxidation state due to a strong interaction with TiO2 NTs leading to an 80% increase in activity
for formic acid (FA) degradation under UV irradiation. Contrary to GO/TiO2 NTs, in the presence of Cu, addition
of non-reduced GO leads to a 150% increase in activity compared to its free counterpart Cu/TiO2 NTs while the
use of partially reduced GO leads to a complete loss of any beneficial effect due to graphene oxide.
1. Introduction
Although TiO2 is the most used semiconductor in photocatalysis [1], its low quantum yield remains an obstacle to its widespread application in the photodegradation of pollutants. As a result, many studies have been carried out to improve the TiO2 photocatalytic activity. These methods can be divided into two categories. A first approach is to extend its photocatalytic activity to the field of visible light. Another approach is to prevent the recombination of photogenerated electron-hole pairs necessary to form reactive species such as hydroxyl radicals. Among the various candidates for TiO2 doping, copper is a promising choice. Cop-per has several oxidation states leading to different oxide species on TiO2 photocatalysts, Cu2O and CuO. This has led to controversial results about the real nature (CuO or Cu2O) of the copper oxide species involved in the enhancement of TiO2 photocatalytic properties [2–5]. Improved catalytic activities have also been reported by combination with gra-phene oxide, which can play both the roles of electron acceptor and photosensitizer [6]. Zeng et al. reported a graphene-CuO nanoflower composite for use as a high performance photocatalyst [7]. Tran et al. studied a copper oxide-reduced graphene oxide composite photocatalyst for hydrogen generation [8]. Lv et al. evidenced the synergetic effect
between copper and graphene as cocatalysts on TiO2 for enhanced photocatalytic hydrogen evolution for solar water splitting [9]. Using a one-pot approach, Liu et al. synthesized CuO nanoflower-decorated reduced graphene oxide for application in the photocatalytic degrada-tion of dyes [10].
Our objective is to understand the nature of the copper oxide species (Cu(II)O or Cu2(I)O) responsible for enhanced photocatalytic activity of copper-doped TiO2 photocatalysts. The first part of this work will be devoted to study the effect of the copper loading and of the calcination temperature (400–600 �C). The second part will be focused on exam-ining the effect of the addition of GO and of post-thermal treatments under different atmospheres (oxidizing or reducing). Physicochemical properties of Cu/TiO2 NTs and Cu/(r)GO/NTs nanomaterials will then be correlated with their photocatalytic behaviors.
2. Experimental
2.1. Elaboration of x wt % Cu/NTs and Cu (rGO)/TNs nanomaterials
Titanate nanotubes (NTs) were elaborated via an alkaline hydro-thermal method as described in our previous work [11]. The obtained
* Corresponding author. ** Corresponding author.
E-mail addresses: hbkochkar@iau.edu.sa (H. Kochkar), gilles.berhault@ircelyon.univ-lyon1.fr (G. Berhault).
NT materials were doped with different copper amounts (0.2, 0.5, 0.7 and 1.0 wt %) following an incipient wetness impregnation method using Cu(NO3)2.3H2O (Sigma Aldrich, 99%) as precursor. The obtained paste was dried in an oven at 80 �C for 24 h and then calcined under air at 400 �C for 2 h. The resulting nanomaterials are named x wt % Cu/NTs. Further post-thermal treatments were performed for 0.5 wt % Cu/NTs materials in the 400–600 �C temperature range. In order to study the effect of graphene oxide obtained following a modified Hummers’ method [11], an optimum copper loading (0.5 wt %) was added onto NTs initially wrapped by GO (1 wt% GO-NTs) using an incipient wetness impregnation method [11]. The obtained 0.5 wt% Cu/1 wt% GO/NTs materials can also be submitted to a reduction step under H2 at 200 �C to form 0.5 wt% Cu/1 wt% rGO/NTs.
2.2. Catalyst characterization
Textural properties of catalysts were determined by N2 adsorption- desorption isotherms at 77 K with the BET method using a Micro-meritics ASAP 2020 analyzer. The photocatalysts were structurally characterized by means of X-ray Diffraction, Raman, UV–visible diffuse reflectance and photoluminescence spectroscopies. Details about char-acterization methods can be found elsewhere [11]. The morphology of the different samples was analyzed by transmission electron microscopy (TEM) on a JEOL 2010 (200 kV) microscope. X-ray photoelectron spectroscopy (XPS) studies were carried out to identify the different copper species.
2.3. Photocatalytic experiments
Photocatalytic degradation tests of formic acid (FA) were performed using the same protocol as described in our previous work [11].
3. Results and discussion
Cu/NTs catalysts with different copper amount (0.2, 0.5, 0.7 and 1.0 wt %) were evaluated for the degradation of formic acid under UV (Fig. 1). Fig. 1A shows the evolution of the rate constant k as a function of Cu loading. The addition of copper onto NTs improves the catalytic activity with an optimum at 0.5 wt %. For higher loading, the activity decreased due to a loss in specific surface area (Table S1). The surface properties of Cu/NTs nanomaterials were investigated by XPS showing the formation of only Cu2O with a characteristic Cu 2p3/2 signal at 932.3 eV (Fig. S1). This observation is in a good agreement with optical properties determined by UV–vis spectroscopy showing that copper is reduced from Cu2þto Cuþ(Fig. S2) and characterized by the appearance of an absorption signal between 400 and 500 nm. In order to improve Cu (I) dispersion onto NTs, the optimum 0.5 wt % NT catalyst was calcined at different temperatures (400–600 �C) and examined for the FA pho-todegradation (Fig. 1B).
Fig. 1B shows that the post-thermal treatment at 400 �C applied to the 0.5 wt % Cu/NTs catalyst enhances the photocatalytic activity. This result can be explained by the formation of new Cu2O–TiO2 hetero-junctions. By contrast, the activity decreases dramatically when calcining at higher temperatures due to the anatase crystallite growth (e. g. 10 nm at 400 �C vs 21 nm at 600 �C) (Table S1) which leads to lower surface area. It can also be noticed by MET analysis that the nanotubular morphology of NTs was lost at 600 �C (Fig. S3). In the next section, we will examine the effect of addition of GO to the Cu2O–TiO2 system. Therefore, the optimum copper loading was added to 1 wt % GO-NTs material that undergoes an additional reduction step to transform GO into reduced graphene oxide (rGO) (identified by XPS in our previous study [11]). MET analysis of 0.5 wt % Cu-1wt% GO-NTs showed that Cu (I) oxide was dispersed onto GO/TiO2 nanotubes (Fig. S4).
The photocatalytic performances of NTs, GO/NTs, Cu/NTs, Cu/GO/ NTs and Cu/rGO/NTs nanomaterials were evaluated for the degradation of FA (Fig. 2). The kinetic of degradation can be fitted well using the
Figure 1. (A) rate constant vs copper loading and (B) rate constant and size crystal vs temperature of calcination.
Fig. 2. Left: rate constants (k) values obtained for the photocatalytic degradation of FA using NTs, GO/NTs, Cu/NTs and Cu/(r)GO/NTs hybrid nanomaterials. Right: HRTEM image of Cu-rGO/NTs.
normalized Langmuir-Hinshelwood formalism and can be used to describe the photodegradation of FA using a simplified pseudo-first order kinetics approach. The addition of 1 wt % GO onto NTs im-proves the degradation rate constant by 115% (Fig. 2A). However, this beneficial effect is less important than the one due to copper since in this latter case, the activity improved by 160% compared to NTs alone. Finally, the combined use of Cu and GO produced the most significant improvement in degradation efficiency (240%) due to a better charge separation. However, partially reduced GO leads to a similar photo-catalytic response compared to Cu/NTs. This effect is due to a partial dislocation of the graphene layers wrapped around TiO2 NTs leading to a preferential relocation of Cu NPs directly onto TiO2 NTs. Fig. 2B shows the loss of the nanotubular morphology and the direct contact between bigger Cu NPs (�10 nm) and TiO2. These results indicated that the op-timum Cu loading (0.5 wt %) should be deposited onto wrapped TiO2 nanotubes with graphene oxide (but non-reduced) to ensure significant degradation efficiency.
4. Conclusion
The present study focuses on the development of a new hybrid Cu-GO-TiO2 nanotubes nanomaterial. Results showed first undoubtedly how Cu(I) oxide species are responsible for enhancing photocatalytic properties in combination with TiO2 nanotubes. An optimum cupper loading (0.5 wt %) onto wrapped TiO2 with graphene oxide showed excellent photocatalytic efficiency. This finding opens new routes for energy applications such as H2 production and CO2 conversion into olefins.
Acknowledgement
The authors gratefully acknowledge the financial support by the Tunisian Ministry of Higher Education and Scientific Research and of the French Ministry of Foreign Affairs in the framework of the PHC-Utique 16G1202.
Appendix ASupplementary data
Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104847.
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