n-ZnO Toward


Jun 14, 2015 - (4-7). A major factor in controlling photocatalytic efficiencies is their .... SEM and TEM mages of (a) Cu2O cubes, Cu2O/ZnO core/shell...
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Controlling Core/Shell formation of nano-cubic p-CuO/ n-ZnO toward enhanced photocatalytic performance Ahmad Esmaielzadeh Kandjani, Ylias Mohammad Sabri, Selvakannan R. Periasamy, Nafisa Zohora, Mohamad Hassan Amin, Ayman Nafady, and Suresh Kumar Bhargava Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01019 • Publication Date (Web): 14 Jun 2015 Downloaded from http://pubs.acs.org on June 21, 2015

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Controlling Core/Shell Formation of Nano-cubic p-Cu2O/n-ZnO Toward Enhanced Photocatalytic Performance Ahmad Esmaielzadeh Kandjani1, Ylias Mohammad Sabri1*, Selvakannan R. Periasamy1, Nafisa Zohora1, Mohamad Hassan Amin1, Ayman Nafady2,3and Suresh Kumar Bhargava1* 1

Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, GPO Box 2476 V, Melbourne, Victoria 3001, Australia. 2

3

Department of Chemistry, Faculty of Science, Sohag University, Sohag, Egypt

Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia

*Email: [email protected] , [email protected], Phone: +61 3 99252330 KEYWORDS: Monodispersed nanoparticles, nanocubes, core/shell, p-n junction, photocatalysis.

ABSTRACT: p-type Cu2O/n-type ZnO core/shell photo-catalysts has been demonstrated to be an efficient photocatalyst as a result of their interfacial structure tendency to reduce the recombination rate of photo-generated electron-hole pairs. Monodispersed Cu2O nanocubes were synthesized and functioned as the core, on which ZnO nanoparticles were coated as the shells having varying morphologies. The evenly distributed ZnO decoration as well as assembled nano-

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spheres of ZnO were carried out by changing the molar concentration ratio of Zn/Cu. The results indicate that the photocatalytic performance is initially increased owing to formation of small ZnO nanoparticles and production of efficient p-n junction hetero-structures. However, with increasing Zn concentration, the decorated ZnO nanoparticles tend to form large spherical assemblies resulting in decreased photocatalytic activity due to the inter-particle recombination between the agglomerated ZnO nanoparticles. Therefore, photocatalytic activity of Cu2O/ZnO hetero-structures can be optimized by controlling the assembly and morphology of the ZnO shell. 1. Introduction Photo-catalytic efficiency of a semi-conductor material highly depends on the photogenerated electron-hole separation processes. Therefore, forming hetero-structures between the semiconductor and other materials can reduce the electron-hole recombination rate, thereby increasing the photocatalytic efficiency. Introducing metal/semiconductor hetero-structures in the form of Ohmic or Schottky junctions enables the separation between electron and hole due to the role of metal as an electron sink in the former case or the formation of electric field between the metal and semiconductor in the latter case

1, 2, 3

. An alternative strategy for increasing the

electron-hole life-time is the formation of hetero-structures between the semiconductors with different majority carrier type. By forming junction between p-type and n-type semiconductors a depletion layer at the p-n interfacial region can be formed that induces an electric field, which results in decreased electron-hole pair recombination rate4, 5, 6, 7 A major factor in controlling photocatalytic efficiencies is their optical properties, which highly depends on size, dopants, impurities, and morphology of the materials8, 9. In addition to the optoelectronic properties, the catalytic performance of semiconductor materials also relies highly on the surface area as well as the number of active sites present on the catalyst surface10, 11. Thus, to

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obtain efficient and reproducible photocatalytic properties, the morphology and size of the photocatalyst need to be controlled. Nanoparticles having uniform size and morphology are particularly desirable in semiconductor material production, as the synthesis of uniform surface area, morphology and active sites ensure reproducibility in photo-catalytic properties 16, 17

12, 13, 14, 15,

. Cuprous oxide (Cu2O) is a p-type direct band gap semiconductor material (2.17 eV in bulk

form), which can be formed in a wide range of morphologies such as nanocubes, nanospheres, nanorods and nanooctahedron18, 19, 20, 21. As this material can be synthesized with well-defined morphologies and size, it has been used for studying different facet-depended application like catalysis and photocatalysis reactions11, 14. In this work, uniform size nanocubes of Cu2O were decorated with well controlled n-type zinc oxide (ZnO) nanoparticles using seed mediated approach. In these p-n type core/shells, the dependence of photo-catalytic activities upon the coverage and morphology of the ZnO shell in Cu2O/ZnO hetero-structures have been thoroughly studied. 2. Experimental Section 2.1.

Chemicals

The main reagents such as copper sulfate (CuSO4.5H2O), sodium hydroxide (NaOH), zinc acetate (Zn(Ac)2•2H2O), ethanol and sodium dodecyl sulfate (SDS) were purchased from Sigma Aldrich as laboratory grade reagents and used without any further purification treatments. Deionized water (18.2 MΩ.cm) was used throughout all the experiments. 2.2.

Cuporous oxide (Cu2O) nanocubes synthesis

Cubic Cu2O was first synthesized using seed mediated growth method.22 Cubic Cu2O was first synthesized using seed mediated growth method.22 Briefly, a 10 mL aqueous solution containing

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1 mM CuSO4.5H2O and 33 mM SDS was prepared and transferred to a bottle labelled I. To four other bottles labelled II to V were added 9 mL of the same solution. Then a 250 µL solution containing 200 mM sodium ascorbate was added into bottle I. This bottle was shaken immediately for a period of 5 s. A 500 µL of 1 M NaOH was then added to the same bottle and shaken for another 5 s. A 1 mL volume of solution from bottle I was then transferred to bottle II. Bottle I (now containing 9 mL of mixed solutions) was then left standing at room temperature for a period of 1 h at a temperature of 35°C, following which the solution displayed a brownish yellow colour that was due to the formation of Cu2O nanoparticles (NPs). Just 10 s after transferring 1 mL of solution from bottle I to bottle II, a 250 µL volume of 200 mM sodium ascorbate was added and bottle II shaken for a period of 5 s. Thereafter a 500 µL of 1 M NaOH was added to bottle II and was shaken for another 5 s. Again a 1 mL volume of the solution in bottle II was transferred to bottle III. The solution in bottle II was left standing for a period of 1 h at a temperature of 35°C after which a bright yellow solution was obtained. The same procedure was followed to prepare the solutions in bottles III to V. In order to characterize the prepared samples, the solutions were concentrated by centrifugation in ethanol for 10 min and repeated three times per sample. Each sample underwent rotation speeds of 3000 rpm. The precipitates collected following centrifugation were then dispersed in 1 mL of ethanol. The cubic Cu2O NPs from bottle V were used throughout this study therefore the cubic NPs prepared in bottles I to IV were only made in order to synthesise the nanocube NP size obtained from bottle V. The copper content of bottle V was estimated using microwave plasma atomic emission spectrometer (MPAES) system (4200 MP-AES, Agilent). It was found that the copper content of the suspended nanocubes in bottle V was 382 ppm. 2.3.

Decoration of Cu2O nanocubes with ZnO NPs

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In order to decorate the Cu2O nanocubes synthesised in bottle V (see 2.2) with different surface coverage of semi-spherical ZnO NPs, different volumes (see Table 1) of 360 mM stock solution of Zn(Ac)2•2H2O in ethanol was added to 1 mL batches of nanocube Cu2O NPs by fixing the final volume to 10000 µL using ethanol to balance. This mixture was stirred (310 rpm) at 60 °C for a period of 1 h. Then a certain volume (see Table 1) of 200 mM stock solution of NaOH in ethanol was added into the mixture before continuing the stirring at 60 °C for an additional 1 h period. Semi-spherical ZnO NPs directly on the cubic Cu2O NPs was obtained. The variation of the concentrations of both the Zn(Ac)2•2H2O and NaOH ethanol for each ZnO NP coverage density are shown in Table 1. Once synthesized, each batch of Cu2O nanocubes having different amount of ZnO semi-spherical NP coverage density was washed with ethanol and centrifuged (3000 rpm) over 3 times before being suspended in ethanol for further use. The concentration of the zinc precursor was the key parameter used to control the extent of ZnO decoration on the Cu2O nanocubes as shown schematically in Figure 1. The molar concentration ratio of the Zn precursor and the Cu is referred to as the Zn/Cu ratio. The molar concentration of Cu in the Cu2O is determined by first undergoing acid digesting of the Cu2O nanocubes or Cu2O/ZnO core/shell nanoparticles in aqua regia followed by quantitative analysis using microwave plasma atomic emission spectrometer (MP-AES) system as described in the section 2.2. 2.4.

Substrate preparation for photocatalysis experiments

For the investigation of the photocatalytic activity of Cu2O nanocubes decorated with ZnO semispherical NPs, each batch of samples synthesised (Table 2) were diluted in ethanol such that the copper content in each batch was the same as that of the CuZn-1 batch (i.e. 60.9 ppm) which had the lowest copper content (Table 2). Thereafter 10 µL of one solution was drop-cast on a 1.6 cm

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x 1.6 cm glass slide substrates and dried at 100°C. This step was repeated 5 times on the same substrate using the same sample so that the number of NPs is similar between the samples as well as sufficient to perform photocatalysis experiments. Testing the performance of Photocatalytic activity In order to test the synthesised Cu2O nanocubes decorated with ZnO semi-spherical NPs, 2 mL of 2 x 10-5 M rhodamine B (RB) solution was placed in a 10 mL beaker along with a prepared substrate and placed in the dark for a period of 15 min. A blank sample was also prepared using this solution however without the addition of any prepared substrate. Thereon the samples were irradiated with light from a 200 mW/cm2 UV (370 nm) lamp (Edmund Optics) with 2.54 cm optic cell placed 2 cm away from the samples. UV-Vis absorption spectra of these samples were taken before and after every 15 min of irradiation for up to 180 min. Following UV-vis absorption, the 700 µL solution from the UV-vis quartz cell was returned back in the beaker before the lamp was turned on for a further 15 min. 2.5.

Characterisation

Scanning electron microscopy (SEM) imaging were performed using a Nano-SEM instrument operating at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images were obtained using a JEOL 1010 operating at 100 kV accelerating voltage. X-ray diffraction (XRD) experiments were carried out using a Bruker D8 Discover micro diffraction system housing a Cu-Kα radiation source and having a general area detector diffraction system (GADDS) operating at a voltage of 40 kV and a current of 40 mA. X-ray photoelectron spectroscopy (XPS) characterization of the synthesised NPs was performed on a Thermo KAlpha instrument at a pressure of ~10-9 Torr. The core level binding energies (BEs) were aligned at 285 eV which is the binding energy for adventitious C 1s. The UV–vis absorption spectra

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were taken using a Cary 60 UV-Vis (Agilent Technologies) spectrophotometer. High resolution –TEM (HR-TEM) analysis was carried out using a JEOL JEM-2100F operating at an accelerating voltage of 80 kV. HR-TEM was collected in TEM mode using an Orius SC1000 CCD camera. EDS mapping was performed in STEM mode using an Oxford XMax-80T EDS detector. 3. Results and Discussion The two step synthesis procedure of Cu2O/ZnO core/shell is shown schematically in Figure 1. In step 1, the controlled seed method22 synthesis procedure results in the formation of Cu2O nanocubes with narrow particle size distribution. These synthesized cubic Cu2O nanoparticles were used as core particles and coated with different amount of ZnO nanoparticles using zinc acetate and NaOH as the precursors while employing the sol-condense method23 (steps 2). The ratio of the concentration between the zinc precursor and Cu2O nanocubes were varied to obtain different ZnO nanoparticle sizes with different assembly as shown in the schematic. The morphology of ZnO shell was highly dependent upon the ratio of cubes and zinc acetate precursor. In the case of low Zn/Cu ratio, the shell constituted an even distribution of small ZnO nanoparticles on the Cu2O nanocubes. However, increasing the concentration of the zinc acetate resulted in the ZnO nanoparticles agglomerating into larger spherical shapes that covered the total surface of the Cu2O nanocubes. After a certain threshold Zn/Cu ratio (>0.5), the ZnO nanoparticles tend to agglomerate to the extent where the Cu2O nanocubes are completely buried in the ZnO structures (see supporting information, Figure S1) with the excess ZnO nanoparticles being freely suspended in the synthesis solution. Due to the formation of free standing ZnO nanoparticles as well as the loss of homogeneity in morphology of the Cu2O nanocubes, these

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samples were deemed impractical to be used as p-n junction photo-catalysts, thus were not considered for any further study. Figure 2 shows the SEM and TEM images of the Cu2O nanocubes and Cu2O/ZnO core/shell nanoparticles. Figure 2a shows the Cu2O particles formed in bottle V during the synthesis process which produced monodispersed nanocubes with size of ~450 nm. These nanocubes were used to form Cu2O/ZnO core/shell nanoparticles by using Zn/Cu ratios of 0.05 (Figure 2b), 0.15 (Figure 2c) and 0.26 (Figure 2d). It can be observed that the ZnO nanoparticles distribute uniformly on the surfaces of Cu2O nanocubes for all Zn/Cu ratios shown. The higher magnification images for the lowest Zn/Cu ratio of 0.05 shows that the decorated ZnO nanoparticle size ranges between 2-4 nm (Figure 2-b3). As the Zn/Cu ratio is increased during the synthesis process, these small nanoparticles are transformed into larger spherical shaped aggregates with average diameter reaching up to 50 nm. Moreover, the number of the formed ZnO spherical structures on the Cu2O nanocubes was noted to vary with the Zn/Cu ratio (shown in Table 2). It was found that as the Zn/Cu ratio increased, the number of larger ZnO on the Cu2O nanotubes also increased. In order to study the Cu2O/ZnO core/shell interface, HR-TEM were performed for pure Cu2O nanocubes, Cu2O nanocubes decorated with both ZnO nanoparticles and agglomerated ZnO nanospheres. Figure 3 shows the HR-TEM images for pure Cu2O, Cu2O/ZnO core/shells with Zn/Cu=0.05 and Zn/Cu=0.15. As shown in Figure 3a1 and 3a2, the copper oxide cubes make right angle edges with an interplane distance of ~0.25 nm relating to cubic Cu2O (111) face24. It can also be clearly observed that by increasing the Zn concentration, initially the ZnO nanoparticles are formed on the Cu2O nanocubes (Figure 3b1 and 3b2) before agglomerating into larger ZnO nanospheres (Figure 3c1 and 3c2). In HR-TEM imaging, by focusing on zinc

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oxide lattice, only a dark shadow of the Cu2O cubes can be observed due to the larger size of the Cu2O nanocubes relative to the ZnO nanoparticles/spheres thereby enabling to observe the interface of the Cu2O/ZnO core/shell, as is represented by the red dashed lines. The indicated interplane distances of 0.26 nm and 0.28 nm are related to (01ത1) and (001) planes of ZnO, respectively25. The formation of ZnO nanospheres on Cu2O nanocubes were also studied by energy dispersive X-ray (EDX) spectroscopy. The STEM (Figure 3d) and EDX (Figure 3e and 3f) of ZnO on Cu2O nanocubes for Zn/Cu=0.15 clearly shows that ZnO is formed as spherical agglomerates on the Cu2O nanocubes surface and does not diffuse into the bulk. From individual K shell EDX mapping of Cu, Zn and O (see supporting information, Figure S2) it could be seen that Cu and Zn are only restricted to the core and shell structures respectively while in the O map shows that oxygen exists in both core and shell areas, as expected. Following their synthesis, XPS analysis was performed in order to better determine the content and state of the decorated ZnO nanoparticles on the 450 nm Cu2O nanocubes. Figure 4a shows the survey spectra of Cu2O nanocubes and Cu2O/ZnO core/shell (CuZn-3) nanoparticles. In the core/shell sample, the appearance of the zinc peaks indicates the formation of the ZnO on the surface of the Cu2O nanocubes. The high resolution XPS spectrum (Figure 4b) shows that the Cu 2p3/2 core level has a binding energy of 932.4 eV, which can be assigned to Cu+, thereby indicating that the synthesized nanocubes were in fact Cu2O. The presence and purity of Cu+ is also confirmed by the absence of satellite peaks (which would otherwise correspond to Cu2+) in the spectrum26, 27, 28. In addition, the Zn 2p3/2 core level spectra (Figure 4c) shows the formation of Zn2+ (Zn 2p3/2 component at ~ 1021.7 eV) on the surface of the Cu2O nanocubes, consistent with the conclusion that the decorated nanoparticles were in fact ZnO29. The Zn/Cu ratio within the 2-3 nm X-ray penetration depth30,

31

are calculated from the Zn 2p and Cu 2p spectra and

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compared with Zn/Cu ion concentration ratio deduced from MP-AES analysis, both of which are shown in Figure 4d. It is observed that the surface concentration of zinc (XPS data) increases drastically as compared to the gradual increase observed from the Zn concentration in the bulk of the material (MP-AES data). This trend is observed mainly as a result of complete coverage of Cu2O nanocube surfaces by ZnO nanoparticles. Although the total amount of the ZnO is far less than that of Cu2O, the formation of ZnO shell outside of the Cu2O nanocubes results in the drastic increase of the Zn/Cu ratio observed from the XPS data compared to the MP-AES. The synthesised Cu2O nanocubes and Cu2O/ZnO core/shells were subjected to XRD analysis in order to determine their crystallographic orientation, the data of which is shown in Figure 5a. The XRD patterns of the samples reveal the formation of the cubic-phase Cu2O (JCPDF No. 782076). This pattern was also observed for the Cu2O/ZnO core/shell samples with varying ZnO shell thickness thus indicating that there was no structural damage to the Cu2O nanocubes following their decoration with ZnO nanoparticles. It can also be observed that the ZnO peaks do not appear until the Zn/Cu ratio reaches a value of 0.42 (CuZn-5). This is due to the amount of ZnO decorated being relatively much lower than that of Cu2O as was previously mentioned (MPAES data). In order to confirm that the synthesised ZnO nanoparticles were in-fact crystalline, a large amount (100 mL) of ZnO nanoparticles (same concentrations used for CuZn-5 formation) were synthesized, centrifuged, dried and analysed by XRD (referred to as the ZnO spectrum). It can be observed that the produced ZnO was in-fact crystalline nanoparticles (JCPDF No. 361451). In order to study the optical properties of the samples, the prepared Cu2O nanocubes and Cu2O/ZnO core/shells were analysed with UV-Vis absorption spectroscopy as in Figure 5b. The Cu2O nanocubes showed an absorption band between 485-505 nm (pink shade in the figure)

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which is related to the bandgap of the material. This absorption band was used to calculate the bandgap of the Cu2O nanocubes which was estimated at 2.53eV and in good agreement with previous studies22,

32

. This is calculated considering the optical absorption coefficient of the

colloidal solution sample as is explained in our previous work33. The blue shift observed in the bandgap of the Cu2O nanocubes relative to bulk Cu2O is also attributed to the quantum confinement effects based on the numerous reports that have thoroughly studied and discussed the optical properties of Cu2O nanocubes22, 24. Furthermore, the cubic morphology of the Cu2O nanoparticles has resulted in another absorption tail in the red and near IR regions which can also be attributed to the light scattering of the cubic Cu2O materials based on previous studies22, 32. By increasing the ZnO coverage on the Cu2O nanoparticles an additional absorption band is observed to appear between 324-360 nm22, 34. This peak is attributed to the formation of ZnO nanoparticles on the surface of Cu2O nanocubes where increasing the Zn/Cu ratio increases the ZnO nanoparticle peak intensity. The produced core/shell nanoparticles were tested for their photocatalytic efficiency using Rohdamine B (RB) as a model organic dye, the data of which is presented in Figure 6. In the event where no photocatalyst material is present, the amount of radicals formed by UV light irradiation was insufficient to significantly degrade RB in the solution, even after 180 min irradiation period (Figure 6a). However, when the photocatalyst material was added to the solution, the degradation rate was observed to increase significantly (Figure 6b) due to role of the photocatalytic particles in the formation of free radicals in solution. The photocatalytic efficiency of the produced materials is shown in Figure 6c. It is clearly established that by increasing the Zn/Cu ratio (or amount of the ZnO decorated on the Cu2O nanocubes), the degradation efficiency increases and reaches its maximum value when CuZn-3 sample is

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employed (Zn/Cu ratio = 0.15). A further increase in the Zn/Cu ratio (CuZn-4 where Zn/Cu ratio = 0.26) is observed to decrease the photocatalytic activity of the core/shell nanoparticles. The degradation kinetics (shown in Figure 6d) indicates that the RB degradation follows a pseudofirst order kinetics. By plotting –Ln(C/C0) vs. degradation time, the degradation rate constant can be calculated. The coefficient of determination (R2) of the linear fits in all cases were found to be better than 0.95. The results indicate that the CuZn-3 sample had the highest degradation rate constant (kobs~ 6.88×10-3 1/min) among all other synthesized photocatalytic materials. The other materials’ rate constants were 2.23×10-3, 2.24×10-3, 3.48×10-3 and 4.56×10-3 for Cu2O, CuZn-1, CuZn-2 and CuZn-4, respectively. The results clearly show that the formation of the n-type semiconductor decorated on p-type Cu2O nanoparticles can enhance the electron/hole charge separations. Since ZnO has a bandgap of 3.37 eV while the bandgap of the Cu2O nanocubes were calculated to be 2.53 eV, there exists an obvious band energy offset between the two materials4. When Cu2O nanocubes is coated with ZnO nanoparticles, the Fermi levels tends to align in order to attain equilibrium. In order to reach this equilibrium state, charge carriers are diffused and drifted between Cu2O and ZnO thus forming a depletion layer at the interface. In this depletion layer an electric field is formed due to the formation of the positive charge at the n-side and negative charge at the p-side which can sweep out any electron and holes entering the depletion layer. It should be noted that in heterostructure junctions, the electron density and the material type both change at the interface which results in the formation of discontinuity and step in the band structure. This phenomenon can further assist in the electron/hole separation processes. Under UV irradiation the system face a non-equilibrium state due to optical excitation in semiconductors and therefore Fermi level bends, similar to the forward bias observed in p-n diodes. During irradiation, the electrons

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formed in Cu2O drift through the depletion layer and in to the ZnO while the holes are transferred from ZnO and into Cu2O structure. This can further assist the separation between electrons and holes thereby increasing the electron/hole lifetime. The separated electrons and holes can be consumed to form •O2 and •OH radicals, respectively. These free radicals can attack organic pollutants (i.e. RhB in this study) and degrade their structures6,

35, 36, 37

. The

photocatalytic degradation of RhB using Cu2O/ZnO p/n junction can be summarized as5, 38: ௛ఔ

‫ݑܥ‬ଶ ܱ/ܼܱ݊ ሱሮ (‫ݑܥ‬ଶ ܱ + ℎା ) + (ܼܱ݊ + ݁ ି )

1

݁ ି + ܱଶ →• ܱଶି

2

ℎା + ܱ‫ܪܱ •→ ି ܪ‬

3

• ܱଶି + ‫ܪ‬ଶ ܱ → ‫ܪ‬ଶ ܱଶ +• ܱ‫ܪ‬

4

‫ܪ‬ଶ ܱଶ +• ܱଶି →• ܱ‫ ܪ‬+ ܱ‫ ି ܪ‬+ ܱଶ

5 ி௨௥௧௛௘௥ ௗ௘௚௥௔ௗ௔௧௜௢௡

RhB+•OH → ‫ ݏ݀݊ݑ݋݌݉݋ܿ ݏݏ݈݁ݎ݋݈݋ܿ ݁ݐܽ݅݀݁݉ݎ݁ݐ݊ܫ‬ሱۛۛۛۛۛۛۛۛۛۛۛۛۛۛۛሮ ‫ܱܥ‬ଶ + ‫ܪ‬ଶ ܱ

6

Thus, the formation of p-n junctions can increase the photocatalytic activities of pure Cu2O nanocubes. However, this enhancement as shown in Figures 6c and 6d is highly dependent on the Zn/Cu ratio during the synthesis process. The effect of ZnO coverage on the photocatalytic activities of Cu2O/ZnO can be explained by considering two competing phenomena, namely, (i) the surface coverage of the Cu2O nanocubes with ZnO nanoparticles which makes more active pn heterojunctions throughout the Cu2O surfaces and (ii) the agglomeration of ZnO nanoparticles which increases the number of recombination sites between the ZnO particles. When ZnO decoration increases on the cubic Cu2O nanoparticles up to the threshold limit (Zn/Cu ≈ 0.26), the majority of the excited electrons and holes in the interface can be separated due to the higher

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coverage of the Cu2O with ZnO nanoparticles. When the ZnO amount increases further with Zn/Cu ≥ 0.26, the result is an increase in the agglomeration of ZnO nanoparticles on the Cu2O nanocubes. As the formed charges can transfer between the junctions of the ZnO nanoparticles, the generated electron-hole pairs can recombine and thus result in the composite materials having decreased photocatalytic activity. 4. Conclusion To summarize, a facile synthesis method has been developed for the controlled decoration of Cu2O nanocubes with ZnO nanoparticles. It is shown that the photocatalytic efficiency of the produced core/shell nanoparticles not only depends on the formation of the p/n junction between ZnO and Cu2O nanoparticles but also on controlling the coverage and morphology of the shell formation. Significantly, photocatalytic activity of the core/shell materials increased by increasing the coverage of Cu2O nanocubes with ZnO nanoparticles. However, after a certain threshold limit, large aggregates of ZnO nanoparticles are formed on the Cu2O nanocubes resulting in decreased photocatalytic efficiency due to increase in the number of boundaries between the agglomerated ZnO nanoparticles that act as recombination sites. Thus, it is demonstrated that not only the formation of p/n junction is important when increasing the photocatalytic activity of the Cu2O/ZnO core/shell nanoparticles, but the amount and morphology of the decorated ZnO nanoparticles also play a crucial role. Acknowledgment The authors acknowledge RMIT Microscopy and Microanalysis Facility (RMMF) for the help received from their technical staff and for allowing the use of their comprehensive facilities and services. We specially thank Dr. Matthew Field and Dr Deepa Dumbre for their immense help for technical assistance in some characterization techniques. A.N. and S.K.B. would like to thank

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Deanship for Scientific Research, Visiting Professor Program at King Saud University, for partial support of this work. Supporting Information Available SEM images of the samples after a certain threshold Zn/Cu ratio of >0.5 (CuZn-4 and CuZn-5) in addition to EDX mapping of individual elements. This information is available free of charge via the Internet at http://pubs.acs.org/. Competing Financial Interest: The authors declare no competing financial interests Author Contribution: The paper was written through contributions of all authors. References 1. Kandjani, A. E.; Mohammadtaheri, M.; Thakkar, A.; Bhargava, S. K.; Bansal, V. Zinc oxide/silver nanoarrays as reusable SERS substrates with controllable ‘hot-spots’ for highly reproducible molecular sensing. J. Colloid Interface Sci. 2014, 436, 251-257. 2. Kandjani, A. E.; Sabri, Y. M.; Mohammad-Taheri, M.; Bansal, V.; Bhargava, S. K. Detect, Remove and Reuse: A New Paradigm in Sensing and Removal of Hg (II) from Wastewater via SERS-Active ZnO/Ag Nanoarrays. Environ. Sci. Technol. 2014. 3. Mahmoud, M. A.; Qian, W.; El-Sayed, M. A. Following Charge Separation on the Nanoscale in Cu2O–Au Nanoframe Hollow Nanoparticles. Nano Lett. 2011, 11, 3285-3289. 4. Wang, Y.; Wang, Q.; Zhan, X.; Wang, F.; Safdar, M.; He, J. Visible light driven type II heterostructures and their enhanced photocatalysis properties: a review. Nanoscale 2013, 5, 8326-8339. 5. Liu, Y.; Li, G.; Mi, R.; Deng, C.; Gao, P. An environment-benign method for the synthesis of p-NiO/n-ZnO heterostructure with excellent performance for gas sensing and photocatalysis. Sensor. Actuat. B-Chem. 2014, 191, 537-544. 6. Liang, N.; Wang, M.; Jin, L.; Huang, S.; Chen, W.; Xu, M.; He, Q.; Zai, J.; Fang, N.; Qian, X. Highly Efficient Ag2O/Bi2O2CO3 p-n Heterojunction Photocatalysts with Improved Visible-Light Responsive Activity. ACS Appl. Mater. Inter. 2014, 6, 11698-11705. 7. Wang, K.; Qian, X.; Zhang, L.; Li, Y.; Liu, H. Inorganic–Organic p-n Heterojunction Nanotree Arrays for a High-Sensitivity Diode Humidity Sensor. ACS Appl. Mater. Inter. 2013, 5, 5825-5831. 8. Kandjani, A. E.; Shokuhfar, A.; Tabriz, M. F.; Arefian, N. A.; Vaezi, M. R. Optical properties of Sol-Gel prepared nano ZnO. The effects of aging period and synthesis temperature. J Optoelectron. Adv. M. 2009, 11, 289-295. 9. Kandjani, A. E.; Tabriz, M. F.; Moradi, O. M.; Mehr, H. R. R.; Kandjani, S. A.; Vaezi, M. R. An investigation on linear optical properties of dilute Cr doped ZnO thin films synthesized via sol–gel process. J. Alloys Compd. 2011, 509, 7854-7860.

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10. Kandjani, A. E.; Tabriz, M. F.; Arefian, N. A.; Vaezi, M. R.; Halek, F.; Sadrnezhaad, S. K. Photocatalytic decoloration of acid red 27 in presence of SnO2 nanoparticles. Water Sci. Technol. 2010, 62, 1256-1264. 11. Zheng, Z.; Huang, B.; Wang, Z.; Guo, M.; Qin, X.; Zhang, X.; Wang, P.; Dai, Y. Crystal Faces of Cu2O and Their Stabilities in Photocatalytic Reactions. J. Phys. Chem. C 2009, 113, 14448-14453. 12. Chen, D.; Caruso, R. A. Recent Progress in the Synthesis of Spherical Titania Nanostructures and Their Applications. Adv. Funct. Mater. 2013, 23, 1356-1374. 13. Chen, J. S.; Archer, L. A.; Wen Lou, X. SnO2 hollow structures and TiO2 nanosheets for lithium-ion batteries. J. Mater. Chem. 2011, 21, 9912-9924. 14. Kuo, C.-H.; Huang, M. H. Morphologically controlled synthesis of Cu2O nanocrystals and their properties. Nano Today 2010, 5, 106-116. 15. Huang, F.; Chen, D.; Zhang, X. L.; Caruso, R. A.; Cheng, Y.-B. Dual-Function Scattering Layer of Submicrometer-Sized Mesoporous TiO2 Beads for High-Efficiency Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2010, 20, 1301-1305. 16. Jiang, H.; Hu, J.; Gu, F.; Li, C. Large-Scaled, Uniform, Monodispersed ZnO Colloidal Microspheres. J. Phys. Chem. C 2008, 112, 12138-12141. 17. Deng, Z.; Chen, M.; Gu, G.; Wu, L. A Facile Method to Fabricate ZnO Hollow Spheres and Their Photocatalytic Property. J. Phys. Chem. B 2007, 112, 16-22. 18. Zoolfakar, A. S.; Rani, R. A.; Morfa, A. J.; O'Mullane, A. P.; Kalantar-zadeh, K. Nanostructured copper oxide semiconductors: a perspective on materials, synthesis methods and applications. J. Mat. Chem. C 2014, 2, 5247-5270. 19. Lu, C.; Qi, L.; Yang, J.; Wang, X.; Zhang, D.; Xie, J.; Ma, J. One-Pot Synthesis of Octahedral Cu2O Nanocages via a Catalytic Solution Route. Adv. Mater. 2005, 17, 2562-2567. 20. Zhang, J.; Liu, J.; Peng, Q.; Wang, X.; Li, Y. Nearly Monodisperse Cu2O and CuO Nanospheres:  Preparation and Applications for Sensitive Gas Sensors. Chem. Mater. 2006, 18, 867-871. 21. Park, J. C.; Kim, J.; Kwon, H.; Song, H. Gram-Scale Synthesis of Cu2O Nanocubes and Subsequent Oxidation to CuO Hollow Nanostructures for Lithium-Ion Battery Anode Materials. Adv. Mater. 2009, 21, 803-807. 22. Kuo, C. H.; Chen, C. H.; Huang, M. H. Seed-Mediated Synthesis of Monodispersed Cu2O Nanocubes with Five Different Size Ranges from 40 to 420 nm. Adv. Funct. Mater. 2007, 17, 3773-3780. 23. Kandjani, A. E.; Amiri, S. E. H.; Vaezi, M. R.; Sadrnezhaad, S. K. Optical and magnetic properties of Co3O4/ZnO Core/Shell nanoparticles. J Optoelectron. Adv. M. 2010, 12, 20572062. 24. Chen, L.; Zhang, Y.; Zhu, P.; Zhou, F.; Zeng, W.; Lu, D. D.; Sun, R.; Wong, C. Copper Salts Mediated Morphological Transformation of Cu2O from Cubes to Hierarchical Flower-like or Microspheres and Their Supercapacitors Performances. Sci. Rep. 2015, 5. 25. Zhong, K.; Mao, Y.; Sun, X.; Liang, C.; Liu, P.; Tong, Y. Electrochemical and Optical Properties of ZnO Nanowires Modified with Ag Nanoparticles by Electrodeposition. J. Electrochem. Soc. 2012, 159, K161-K164. 26. Pauly, N.; Tougaard, S.; Yubero, F. Determination of the Cu 2p primary excitation spectra for Cu, Cu2O and CuO. Surf. Sci. 2014, 620, 17-22. 27. Zou, X.; Fan, H.; Tian, Y.; Zhang, M.; Yan, X. Microwave-assisted hydrothermal synthesis of Cu/Cu2O hollow spheres with enhanced photocatalytic and gas sensing activities at room temperature. Dalton Trans. 2015, 44, 7811-7821.

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28. Zou, X.; Fan, H.; Tian, Y.; Zhang, M.; Yan, X. Chemical bath deposition of Cu2O quantum dots onto ZnO nanorod arrays for application in photovoltaic devices. RSC Adv. 2015, 5, 23401-23409. 29. Herring, N. P.; Panchakarla, L. S.; El-Shall, M. S. P-Type Nitrogen-Doped ZnO Nanostructures with Controlled Shape and Doping Level by Facile Microwave Synthesis. Langmuir 2014, 30, 2230-2240. 30. Sabri, Y. M.; Ippolito, S. J.; Atanacio, A. J.; Bansal, V.; Bhargava, S. K. Mercury vapor sensor enhancement by nanostructured gold deposited on nickel surfaces using galvanic replacement reactions. J. Mater. Chem. 2012, 22, 21395-21404. 31. George, M. A.; Glaunsinger, W. S.; Thundat, T.; Lindsay, S. M. Investigation of mercury adsorption on gold films by STM. J. Microsc. 1988, 152, 703-713. 32. Tsai, Y.-H.; Chanda, K.; Chu, Y.-T.; Chiu, C.-Y.; Huang, M. H. Direct formation of small Cu2O nanocubes, octahedra, and octapods for efficient synthesis of triazoles. Nanoscale 2014, 6, 8704-8709. 33. Kandjani, A. E.; Shokuhfar, A.; Tabriz, M. F.; Arefian, N. A.; R., V. M. Optical properties of Sol-Gel prepared nano ZnO. The effects of aging period and synthesis temperature. J. Optoelectron. Adv. M. 2009, 11, 289-295. 34. Kuo, C.-H.; Hua, T.-E.; Huang, M. H. Au Nanocrystal-Directed Growth of Au−Cu2O Core−Shell Heterostructures with Precise Morphological Control. J. Am. Chem. Soc. 2009, 131, 17871-17878. 35. Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X. Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances. Chem. Soc. Rev. 2014, 43, 5234-5244. 36. Jang, J. S.; Kim, H. G.; Lee, J. S. Heterojunction semiconductors: A strategy to develop efficient photocatalytic materials for visible light water splitting. Catal. Today 2012, 185, 270277. 37. Zhou, P.; Yu, J.; Jaroniec, M. All-Solid-State Z-Scheme Photocatalytic Systems. Adv. Mater. 2014, 26, 4920-4935. 38. Jang, E. S.; Won, J. H.; Hwang, S. J.; Choy, J. H. Fine Tuning of the Face Orientation of ZnO Crystals to Optimize Their Photocatalytic Activity. Adv. Mater. 2006, 18, 3309-3312.

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Figures Caption Figure 1. Schematic showing the synthesis steps in forming Cu2O/ZnO core/shell nanoparticles. Figure 2. SEM and TEM mages of a) Cu2O cubes, Cu2O/ZnO core/shells with b) Zn/Cu=0.05, c) Zn/Cu=0.15 and d) Zn/Cu=0.26. Figure 3. TEM and HR-TEM images of a) pure Cu2O; and Cu2O/ZnO core/shells with b) Zn/Cu=0.05 and c) Zn/Cu=0.15; d) STEM image of Zn/Cu=0.15 and e) corresponding EDX layered map (Green and magenta areas represent Cu and Zn, respectively); f) EDX spectrum of Zn/Cu=0.15. Figure 4. XPS spectra for a) general area scan of Cu2O and Cu2O/ZnO (CuZn-3) core/shells; b) Cu 2p core level from Cu2O nanocubes; c) Zn 2p core level from CuZn-3 and d) Zn/Cu ratios determined from XPS and MP-AES data. Figure 5. a) XRD patterns and b) UV-Vis absorbance spectra obtained from Cu2O, ZnO and Cu2O/ZnO core/shell samples. Figure 6. UV-vis absorbance of RB in the a) absence and b) presence of CuZn-3 photocatalyst; c) photocatalytic RB degradation efficiency; d) photocatalytic RB degradation rate of Cu2O/ZnO core/shells and e) energy band diagrams for isolated ZnO and Cu2O and corresponding Cu2O/ZnO hetero-junction in quasi-equilibrium under UV irradiation. Table Caption Table 1. Variation of volumes and concentrations of both the Zn(Ac)2•2H2O and NaOH ethanol content for each ZnO NP coverage density of Cu2O nanocubes. Table 2. MP-AES results showing zinc and copper content of each batch of Cu2O nanocubes having varying amount of ZnO nanoparticle coverage.

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Figures

Figure 1

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Tables Table 1.

Batch Name CuZn-1 CuZn-2 CuZn-3 CuZn-4 CuZn-5

Volume (µL ) of 360 mM Zn(Ac)2•2H2O added 20 25 33 50 100

Volume (µL ) of 200 mM NaOH added 20 25 33 50 100

Final Volume (µL) 10020 10025 10033 10050 10100

Final Zn concentration (µM) 40 90 118 179 356

Final NaOH concentration (µM) 399 499 658 995 1987

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Table 2.

Batch Name

Copper Concentration Zinc Concentration

Zn/Cu

(ppm)

(ppm)

ratio

Cu2O

382

0

0

CuZn-1

76.3

4

0.05

CuZn-2

204

18

0.09

CuZn-3

236

25

0.15

CuZn-4

116

30

0.26

CuZn-5

207

86

0.42

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Table of Contents (TOC) Graphic

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