Catalysis Communications 159 (2021) 106352 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom Short communication Construction of 0D/2D composites heterostructured of CdTe QDs/ZnO hybrid layers to improve environmental remediation by a direct Z-scheme M. Alegría a, J. Aliaga a, P. Jofré a, L. Ballesteros b, D. Guzmán c, g, C. Sotomayor-Torres d, e, G. González f, **, E. Benavente a, g, * a Departamento de Química, Facultad de Ciencias Naturales, Matemática y Medio Ambiente, Universidad Tecnológica Metropolitana, Santiago, Chile b Instituto de Ciencias Químicas Aplicadas, Inorganic Chemistry and Molecular Material Center, Facultad de Ingeniería, Universidad Autónoma de Chile, El Llano, Subercaseaux, 2801 San Miguel, Santiago, Chile c Centro de Nanotecnología Aplicada, Universidad Mayor, Santiago, Chile d Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, 08193, Bellaterra, Barcelona, Spain e Institució Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, Spain f Departamento de Química, Facultad de Ciencias, Universidad de Chile, Santiago, Chile g Programa Institucional de Fomento a la Investigación, Desarrollo e Innovación (PIDi), Universidad Tecnológica Metropolitana, Santiago, Chile A R T I C L E I N F O A B S T R A C T Keywords: Layered hybrid ZnO (2D) intercalated by myristic acid (MA) with (0D) CdTe quantum dots (QDs) was designed to Photocatalysis increase the conversion efficiency of photochemical energy. The results showed that the introduction of CdTe Semiconductors QDs in ZnO(MA) layered with more active sites available enhanced the photocatalytic efficiency. The optimal Layered ZnO composite sample ZnO(MA)/CdTe (1:0.02) showed excellent dye removal efficiency under simulated solar light CdTe quantun dots Nanostructures irradiation, above 96% after three cyclic experiments. The correlation coefficients possessed the highest reaction Zscheme rate. This study offers an efficient research approach and vision to support the development of other photo­ catalytic systems featuring a direct Z scheme. 1. Introduction considered a potential method that could pass the obstacles of single semiconductors and greatly improve the photocatalytic performance in Semiconductor-based photocatalytic performance under solar light various solar-driven reactions, mainly in pollutant degradation [6,7]. It is believed to be an efficient and ecofriendly system for removing is well recognized that a mediator-free direct Z-scheme photocatalytic numerous hazardous pollutants from wastewater [1,2]. Many semi­ system only including two semiconductor components possesses sepa­ conductor materials have been applied as photocatalysts; among them, ration efficiency of electrons and holes on distinct semiconductors as ZnO has been generally employed due to their stability, non-toxicity, well as great redox ability [8,9]. Compared to bulk materials, zero- safety, low cost, high activity, and morphologies, such as 0D, 1D, 2D, dimensional (0D) quantum dots (QDs) have more prospective in pho­ and hierarchical nanostructures [3]. Compared to nanostructures, tocatalytic applications expected to their quantum confinement effect nanosheets (2D) are more suitable for creating photocatalysts with well and sizeable surface to volume ratios [10,11]. CdTe QDs are direct band performance as they have an elevated specific surface area, plenty active gap semiconductor (1.5–2.3 eV) nanoparticles with strong visible-light sites, and fast mass diffusion lengths to increase the efficiency of electron absorption ability, which have been utilized in fluorescent imaging of transport [4,5]. However, the single component photocatalysis, it very cells [12], solar cells [13,14], and in the construction of a direct Z- difficult because of the fast recombination of photogenerated electrons scheme [15,16]. Recently, excellent developments have been achieved and holes. Because of this, in the last few years, the coupling of semi­ mixed with materials such as 0D/2D type composite photocatalysts that conductors has received increased interest expected to the ability to have a direct Z scheme mechanism; for example, g-C3N4 QDs/ZnO integrate several semiconductors into one system. Binary coupling is nanosheets [17] and MoS2 QDs/C3N4 nanosheets [18] have been * Corresponding author at: Departamento de Química, Facultad de Ciencias Naturales, Matemática y Medio Ambiente, Universidad Tecnológica Metropolitana, Santiago, Chile. ** Corresponding author. E-mail addresses: ggonzale@uchile.cl (G. González), ebenaven@utem.cl (E. Benavente). https://doi.org/10.1016/j.catcom.2021.106352 Received 2 June 2021; Received in revised form 23 July 2021; Accepted 23 August 2021 Available online 25 August 2021 1566-7367/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). M. Alegría et al. Catalysis Communications 159 (2021) 106352 Fig. 1. (a) TEM images of CdTe QDs, (b) Size distribution of CdTe QDs, (c) UV–vis absorption and emission spectra of CdTe QDs, (d) XRD patterns of ZnO(MA) and ZnO(MA)/CdTe (e) SEM and (f) TEM images of ZnO(MA)/CdTe. constructed and utilized in pollutants degradation. They effectively characteristic of quantum dots, and the fluorescence spectrum of the enhance the charge transport capability and photoinduced electron-hole CdTe QDs displays a narrow and clear-cut band representative of the separation efficiency of the composites, thus considerably increasing the creation of fine crystals [19]. Changes in the absorption and emission photocatalytic performance. Motivated by the reports, an effective spectra towards the red are related to the characteristics of nanocrystals approach of coupling CdTe (0D) QDs with ZnO (2D) layered materials of semiconductor materials and the Stokes changes [20]. was developed for the advancement of additional visible-light-driven photocatalysts for pollution of the environment. We report a heterostructured nanocomposite formed by a layered 2.2. Characterization of ZnO(MA)/CdTe-QDs hybrid of ZnO combined with CdTe QD nanoparticles. The nano­ composite structure consists of ZnO, a self-assembled arrangement of Fig. 1d shows the XRD patterns of the preparated ZnO(MA) and ZnO intercalated sheets flanked by myristic acid (MA), which provides (MA)/CdTe nanocomposites. The pattern of ZnO(MA) shows a sequence properties of a two-dimensional semiconductor, combined with a nar­ of Bragg reflections at low 2-theta angles, indicating an ordered layered row band semiconductor such as CdTe QD with mercaptosuccinic acid arrangement. Related interlayer distances, Δd, determined from the (MSA) as a capping agent, which can expand the absorption range, positions of the 00 l reflections, of 39.5 Å for ZnO(MA) and ZnO(MA)/ achieving optimal performance in the degradation of congo red (CR) as a CdTe associate well with the molecular lengths of the myristic acid [21]. pollutant example under simulated solar light irradiation. The synergy In the insert, the pattern shows peaks that may be the hexagonal ZnO between both semiconductors is corroborated for light absorption and a wurtzite (JCPDS 36–1451), where the main facets (100), (101) and Z scheme mechanism is proposed to produce the separation of photo­ (002) appeared at 2θ = 31.7◦ , 34.4◦ , and 36.5◦ , and the CdTe cubic zinc- generated electrons and holes and preserve relatively great redox po­ blend phase shows characteristic peaks corresponding to the (111) and tentials. In addition, the stability and catalytic activity are maintained (220) at 2θ = 23.7◦ and 39.0◦ , respectively. The XRD patterns of the for three cycles. nanocomposites exhibit peaks from the CdTe QDs and ZnO(MA), in that way confirming the formation of the required ZnO(MA)/CdTe structure. 2. Results and discussion The reduce diffraction intensity of CdTe could be assigned to the low content of CdTe in the sample. The estimated average crystallite size of 2.1. Characterization of CdTe-QDs material of ~20 nm, as calculated by the Debye Scherer equation, cor­ responds to nearly 5 single hybrid nanosheets per particle [22]. Fig. 1e TEM was used to determine the size of the nanocrystals and their shows SEM image of aggregates of lamellar particles produced by distribution (Fig. 1a–b). The average particle size of the synthetized QDs assembled nanosheets. EDX elemental mapping of ZnO(MA)/CdTe were was around 3.64 ± 0.84 nm. The UV–vis absorption and emission completed, clearly indicating that Zn, Cd and Te are distributed in the spectra showed that the absorption peak was at 492 nm and the emission sample (Fig. SI1). The TEM image of Fig. 1f shows the morphology of the peak was at 528 nm (Fig. 1c). The form of the absorption spectra is nanosheets/nanoparticles in the ZnO(MA)/CdTe compound. In this figure, the CdTe QD of quite small size, they are highlighted to better 2 M. Alegría et al. Catalysis Communications 159 (2021) 106352 Fig. 2. (a) FTIR spectra, (b) UV–Vis spectra, in the insert plots of (FKM*hν)2 versus (hν) for ZnO(MA) and ZnO(MA)/CdTe, (c) Photocatalytic performance of samples under simulated solar light irradiation (d) UV–vis spectra of the degradation of CR of ZnO(MA)/CdTe (1:0.02) sample, (e) Photodegradation kinetics and (f) Pho­ todegradation of ZnO(MA)/CdTe (1:0.02) for radical scavengers solution. appreciate their distribution upon the sheets. value of 2.89 eV. Furthermore, the spectrum displayed a weak peak that Fig. 2a shows the FTIR spectra of MA, ZnO(MA), and ZnO(MA)/CdTe appeared approximately at 500 nm, which is the characteristic surface in the range 4000–500 cm− 1. A comparison of the spectra shows that the plasma absorption corresponding to the CdTe nanoparticles in the main spectral characteristics of myristic acid are present in the nano­ sample. The results show that the ZnO(MA)/CdTe gives a broadened composite and several structural and chemical differences between both light absorption profile in the UV and visible scale, indicating that the compounds are also observed. By contrast of the band at 1700 cm− 1 composite ZnO(MA)/CdTe could allow more visible light absorption, related to the ν(C=O) mode of MA, in the ZnO(MA) and ZnO(MA)/CdTe therefore increasing its photocatalytic comportment [25,26]. spectrum, we observe the dimeric MA band at 1592 cm− 1 and 1409 cm− 1, which corresponds to the modes of vibration νas(C=O) and 2.3. Photocatalytic activity νs(C=O) of the carboxylate salt, respectively. The values of the separa­ tion between the carboxylate stretches among the asymmetric and The photocatalytic properties were determined by analyzing the symmetric stretching modes (Δν = 183 cm− 1) indicate that the MA is degradation of CR as a pollutant under simulated sunlight irradiation. incorporated into ZnO(MA) with a monodentate myristate ligand [23]. Fig. 2c shows the degradation of the dye for the ZnO (MA) / CdTe The peak at 1500 cm− 1 observed for the ZnO(MA)/CdTe sample corre­ compounds with molar ratios of 1: 0.005, 1: 0.01, 1: 0.02 and 1: 0.03 and sponds to the presence of the COO- group of mercaptosuccinic acid. comparing them with CdTe and ZnO (MA) separately. The congo red dye Also, the lack of the − SH peak in the range of 2500–2600 cm− 1 indicates sample does not show photocatalytic activity under irradiation and, the the formation of the S − Cd bond in the QD. The asymmetric and sym­ CdTe QDs and ZnO(MA) composite showed a lower photocatalytic metric CH2 absorptions of the carboxylic acid at 2908 cm− 1 and 2839 behavior compared to the ZnO(MA)/CdTe composites. The photo­ cm− 1 continue unchanged in the samples. catalytic activity of the ZnO(MA)/CdTe (1:0.02) compound did not DRS was utilized to characterize the samples, the absorbed light show a substantial alteration with the performance of the ZnO(MA)/ wavelength dispersal is visibly affected by their electronic bandgap CdTe (1:0.03); this was probably because excessive CdTe QDs produced structure, which defines the photocatalytic activity. Fig. 2b shows the a light “shielding effect”, which could affect the ZnO(MA) absorbed UV–vis DRS of ZnO(MA) and ZnO(MA)/CdTe. A sharp fundamental visible light, as observed in Fig. 2.c [15]. Therefore, the optimal ratio for absorbance edge for ZnO(MA) was observed at 378 nm, assigned to the the ZnO(MA)/CdTe (1:0.02) was selected. Certainly, as observed in band edge emission or the exciton transition, and a bandgap of 3.26 eV Fig. 2c, the photocatalytic of ZnO(MA)/CdTe (1:0.02) in 120 min of was exhibited. The band gap of the samples was analyzed using the Tauc irradiation was about 100%, which is considerably superior than that of method. The value of the indirect optical energy gap (Eg), was estimated ZnO(MA) of 50%, whereas only 4% degradation was observed not from a plot of (αhʋ)2 versus the photon energy (hυ) and the intercept of a including the catalyst. Fig. 2d shows the variations in the UV–visible tangent to the x-axis (in the insert) [24]. In the composite ZnO(MA)/ absorption spectra of CR at distinct times. CR has a typical absorption CdTe, a shift to superior energy is seen in the absorbance maxima band at a wavelength of 497 nm. As the reaction progresses, the char­ because of contact between CdTe QDs with ZnO, yielding a band gap acteristic peaks slowly decrease as the reaction is irradiated. The insert 3 M. Alegría et al. Catalysis Communications 159 (2021) 106352 Fig. 3. (a) PL of ZnO(MA)/CdTe (1:0.02) in a solution of TA, (b) The recycling experiments for CR degradation of ZnO(MA)/CdTe (1:0.02) (c) XRD patterns of samples before and after reuse in photocatalytic degradation and (d) The proposed construction and photocatalytic reaction mechanism of the heterojunction ZnO (MA)/CdTe. displays the color change of the CR solution in the time. To understand appreciably inhibited when BQ and IPA are introduced, which indicates well the photocatalytic efficiency of the samples, they were investigated the following order h + < O2•− < •OH for the active species participating by pseudo first-order model, with ln(C0/C) = kt, where k is the kinetic in the photocatalytic degradation of the dye. constant and t is the irradiation time. Fig. 2e illustrates the linear fitting The hypothesis that •OH is equally an active species in the degra­ equation of the kinetics model. According to the correlation coefficients, dation was tested with terephthalic acid (TA) by the PL studies. We the ZnO(MA)/CdTe (1:0.02) composite possessed the highest reaction evaluated the rates of •OH formation in aqueous solution under irradi­ rate, and the apparent rate constant (kapp, min− 1) is about 3 times faster ation. TA can react with the •OH radical to generate highly fluorescent 2- than that of ZnO(MA) and 11 times faster than that of ZnO pristine. hydroxyterephthalic acid (TAOH). The peak intensity is proportional to These results suggest that a coupling of 0D/2D shows excellent photo­ the amount of produced •OH radicals. In Fig. 3a, the production rate of catalytic performance, which is assigned to the QDs that can act as radical •OH can be identified, showing that the surface of the sample electron traps to aid the separation of photoinduced electron-holes and facilitates the production of ‧OH radicals as photocatalytic intermediates ZnO (MA) layered with high surface exposure, with plus active sites to increase the photocatalytic activity in the degradation of the dye [28]. accessible to expand matching with the surfaces of the CdTe in the The electronic band alignment between CdTe and ZnO(MA) is done photocatalytic activity [27]. Moreover, the existence of a hydrophobic to recognize the charge transfer process and, thus, the probable mech­ organic moiety in ZnO(MA) promotes appreciably to the improved anisms. The band gap values (Eg) of CdTe (2.33 eV) was estimated using photocatalytic behavior, as observed in the aforementioned photo­ the Tauc approach, cyclic voltammetry, and the UV–vis absorption catalytic studies of hybrids of TiO2 and ZnO [16, 37]. spectra [29]. The band gap of ZnO(MA) (3.26 eV) was estimated from measured DRS using the Tauc plot. The conduction band (ECB) and 2.4. Proposed degradation mechanism valence band (EVB) of a semiconductor can be calculated by the equa­ tion: EVB = χ − Ee + Eg/2; ECB = EVB − Eg [30]. In the Fig. 3d shows the To additional investigate the probable mechanistic effects, we per­ band edge positions and band gaps. formed experiments using radical scavengers aiming to detect the spe­ Under simulated solar light irradiation, both ZnO(MA) and CdTe will cies involved in photodegradation. Ammonium oxalate (AO) as the h+ be excited and generate photoinduced electrons and holes [31], the scavenger, benzoquinone (BQ) as the super oxide anion (O2•− ) scav­ electrons on the CB of ZnO(MA) can move rapidly to recombine and enger and isopropyl alcohol (IPA) as the hydroxyl radicals (•OH) scav­ eliminate with holes on the VB of CdTe at the interface due to the enger. As shown in Fig. 2f, the photodegradation activities are presence of the strong electrostatic attraction. The VB potential value of 4 M. Alegría et al. Catalysis Communications 159 (2021) 106352 Fig. 4. (a) Photocurrent transients for electrodes (—) CdTe, (—)ZnO(MA), (—) ZnO(MA)/CdTe (1:0.02), (b) EIS and (c) PEIS spectra for electrodes (□) CdTe, (○) ZnO (MA), (Δ) ZnO(MA)/CdTe (1:0.02). ZnO(MA) (2.92 V) is more active than •OH/H2O (1.99 eV vs NHE, pH = coupling of 0D/2D showed excellent photocatalytic performance, 7) and the CB potential of CdTe (− 1.24 V) is more passive than O2/O2-• attributed to QDs that promote electron-hole separation and ZnO(MA) (− 0.33 eV vs NHE, pH = 7). The charge separation and transfer of 0D/ layered with high surface exposure, with plus active sites accessible to 2D heterojunction follows the Z-scheme heterojunction, leaving photo­ improve matching with the CdTe quantum dots in the photocatalytic induced electrons and photoinduced holes with stronger redox capac­ activity. The synergy between the semiconductors generated the charge ities on the CB of CdTe and CV of ZnO(MA) respectively, favoring the transfer and exhibited an improved solar light absorption, attributed to a formation of O2-• and •OH radicals, leading to more efficient pollutant Z-scheme mechanism demonstrating the clear contribution of CdTe and degradation. the ZnO(MA) in the photocatalytic degradation of Congo Red. The The photocatalytic efficiency remained constant until the third cycle photocatalytic efficiency remained constant until the third cycle. above 96% for 120 min, as shown in Fig. 3b. Furthermore, we have observed, by XRD, the stability of the sample after recycling in Fig. 3c, indicating the effective reuse of the sample without significant loss in the Declaration of Competing Interest degradation efficiency. The authors declare that they have no known competing financial 2.5. Electrochemical measurement interests or personal relationships that could have appeared to influence the work reported in this paper. The photoelectrochemical properties of the CdTe, ZnO(MA), and ZnO(MA)/CdTe (1:0.02) samples were analyzed. Fig. 4a shows the Acknowledgements photocurrent response under the light on and light off cycles [32]. It is observed that The ZnO (MA)/CdTe (1: 0.02) electrode shows a higher This work was supported by Project funded by the Research Conti­ photocurrent under illumination, than the CdTe and ZnO (MA) elec­ nuity Project Fund, year 2019, code LPC19-01, Project Scientific Tech­ trodes separately. These results are correlated with those obtained in the nological Equipment, L318-03 Universidad Tecnologica Metropolitana, photocatalytic tests, demonstrating that the combined semiconductors Universidad de Chile, FONDECYT 1171803, CONICYT PAI N◦ 77190086 lead to greater efficiency of charge separation and carrier availability and FONDEQUP SEM EQM 150101. due to the built-in electric field of heterojunction [33]. Electrochemical impedance spectroscopy (EIS) and photoelectrochemical impedance Appendix A. Supplementary data spectroscopy (PEIS) measurements were performed. Fig. 4b–c shows the Nyquist diagrams for the CdTe, ZnO (MA), and ZnO (MA) / CdTe (1: Supplementary data to this article can be found online at https://doi. 0.02) samples. EIS measurements show an abrupt rise without defining org/10.1016/j.catcom.2021.106352. semicircles throughout the sweep frequency spectrum due to absence charge carriers in the dark [34]. On the other hand, the Nyquist dia­ References grams for the PEIS measurements show a pronounced decrease in the Z´ and Z´values, accompanied by the definition of at least one semicircle [1] J. Luo, S. Zhang, M. Sun, L. Yang, S. Luo, J.C. Crittenden, A critical review on associated with charge transfer at semiconductor/electrode interface energy conversion and environmental remediation of Photocatalysts with [35]. The decrease in diameter of the semicircles in the Nyquist plots remodeling crystal lattice, surface, and Interface, ACS Nano 13 (2019) 9811–9840, https://doi.org/10.1021/acsnano.9b03649. (CdTe