Role of Metal Exchange toward the Morphology and Photocatalytic

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Role of Metal Exchange toward the Morphology and Photocatalytic activity of Cu/Ag/Au-ZnO: A study with Zn-Na-acetate complex as precursor Kasturi Sarmah, Ujjal Kanti Roy, Tarun Kumar Maji, and Sanjay Pratihar ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00436 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 22, 2018

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ACS Applied Nano Materials

Role of Metal Exchange toward the Morphology and Photocatalytic Activity of Cu/Ag/Au-ZnO: A Study with Zn-Na-Acetate Complex as Precursor Kasturi Sarmah,a Ujjal Kanti Roy,b Tarun K. Maji,a and Sanjay Pratihar a,* a

Department of Chemical Sciences, Tezpur University, Assam-784028, India

b

Department of Chemistry, Kazi Nazrul University, Asansol, WB-713340, India Email: [email protected] or [email protected]

Abstract: The present work disclosed a ZnII-Na acetate complex (C32H48O34Na8Zn4, C1), in which weakly bound sodium (Na) has been utilized for selective metal exchange with guest metals (CuII, AgI) for the synthesis of Cu/Ag/Au loaded ZnO. The morphology, growth, and band gap of the materials are found to be highly dependent upon their metal exchange capacity. The presence of Cu/Ag/Au with ZnO improves the spectral response and effectively suppresses the hole−electron pairs recombination process to facilitate efficient H2O2 production from water and oxygen under visible light and thus utilized as efficient reusable photocatalyst for the degradation and complete remediation of various toxic phenols and dyes into CO2, NO3−, and SO42- in water.

Keywords: Metal Exchange, Photocatalysis, ZnO, dye degradation, phenol.

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In the past few decades, the impending global energy crisis has encouraged intense research into the development of artificial sustainable energy conversion systems that can mimic natural photosynthesis to directly harvest and convert solar energy into usable or storable energy resources for various types of applications including treatment of wastewater or environmental pollutants.12345 In this regard, nanomaterials composed of semiconductor metal oxide such as ZnO has received a great deal of interest as they can interact with light to store the energy via charge separation and create electron−hole pairs to trigger many of these applications.

6,7,8,9

However, faster recombination of charge

carriers in ZnO and its band gap in the UV region (λ < 387 nm) limits its application under solar irradiation, which comprises 43% visible light and 4% ultraviolet light.1011 To overcome such limitations, many efforts have been made to obtain visible-light photocatalyst consisting engineered ZnO. Amongst various strategies, doping of plasmonic metal nanostructures into the ZnO heterostructures to enhance the visible-light absorption along with the improvement of reactivity by restricting the recombination of charge carriers is one of the widely used strategies to get better photo catalytic activity.121314 On the other hand, photocatalytic reaction, which occurs mainly at the interface between the catalyst surfaces and substrate, is highly dependent upon the structural and morphological features of the nanomaterial.15161718 In this regard, ZnO records one of the maximum assortments of varied morphologies including 1D, 2D and 3D structures.19202122 In most of the cases the synthesis of ZnO or plasmonic metal doped ZnO were made from their commercially available salts2324 using variety of methods, such as vapour deposition, precipitation in water solution, hydrothermal synthesis, the sol-gel process, precipitation from microemulsions and mechanochemical processes.25


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The common problems associated with these synthetic methods are; (i) requirement of metal salts with high purity, (ii) uncontrollable growth, (iii) prerequisite of capping agents for controllable morphologies (Figure 1). To overcome such limitations, metal complexes will be an ideal precursor, in which the binding between metal and ligand should be utilized for the generation of hybrid smart material via controlled growth.27 However, the choice of ligand and its complex is important for its utilization as precursor. The presence of ligand in the final material are just like “organic armor” brings extra complexity to the system and will hamper its reactivity by blocking the reactants to approach the metal surface. Ideally the ligand in the metal complex should be removed after its utilization as precursor to promote the activity in the final material. In this regard, a systematic study on the control of the morphology, growth, and band gap by the judicious choice of zinc complex is still on search, which motivated us to undertake this work. Herein, we reports a polymeric Zn-Na-acetate complex (C32H48O34Na8Zn4, C-1) as precursor for the synthesis of Cu/Ag/Au doped ZnO, wherein the morphology, growth, and band gap is found to be dependent upon the selective exchange between weakly bound sodium and guest metals [Cu(II), Ag(I), and Au(III)] and provides four different nanomaterials under benign conditions. The materials were further applied as reusable photocatalyst for visible-light-driven degradation and complete remediation of various phenols and dyes utilizing oxygen as oxidant (Figure 1). The complex C-1 was prepared from the reaction between zinc acetate and sodium acetate (1:2 ratio) in methanol at room temperature. The X-ray single crystal structure of C-1 suggests four Zn(II) and eight Na(I) attaining tetrahedral and octahedral geometry, respectively with sixteen bridging acetate within an asymmetric unit.2829 The bond length



of bridging carboxylate in C-1 further confirms the differential binding nature of acetate with Zn(II) and Na(I). Initially, from a screening of different variables, the reaction between C-1 and hydrazine hydrate at 150 °C under hydrothermal condition was found to be optimized reaction condition for the synthesis of ZnO. The powder X-ray diffraction (PXRD) data confirmed that materials obtained from C-1 were crystalline and had the hexagonal wurtzite structure of ZnO with lattice parameters matching those in the literature (Figure 2 & Figure S1). The high resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HR-TEM) images in Figure 3 and 4 indeed shows the formation of flower like morphology consisting of several rods like ZnO with average rod length of 200-300 nm and diameter of 60-90 nm (Figure S2). Further, PXRD spectra obtained at different time interval implied the hydrolysis of C-1 to corresponding Zn(OH)2, which condensed into the flower hexagonal wurtzite structure of ZnO via controlled extended growth along the 101 plane. It is noteworthy to mention that the morphology, band gap, and growth pattern of ZnO obtained from C-1 is entirely different compare to ZnO obtained from Zn(OAc)2 under optimized reaction condition (Figure S3). Gratifyingly, the synthesis of ZnO could be successfully scale up to several grams without compromising the yield and its property. To check the specifity of C-1 as precursor for the synthesis of ZnO over other zinc complex, the hydrolysis reaction was performed with reported ZnII-Schiff base complex under optimized reaction condition.30 However, an unidentified solid mixture containing Zn(OH)n, ZnO and other species was isolated, which directly suggest the specifity of C-1 and need of metal exchange for the synthesis of ZnO with controllable growth.


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Figure 1. Synthetic route of Cu/Ag/Au-ZnO via metal exchange.

The band gap of ZnO is found to be 3.1 eV, which fall in the UV region, which limits its application under solar irradiation (Figure S14). So, to overcome such limitations, we thought to utilize C-1 as precursor for the doping of coinage metal to obtain visible-light photocatalyst consisting engineered ZnO. In this regard, the reactions



between C-1 and metal salts (CuSO4, AgNO3, and HAuCl4) were done under refluxing methanol, which afforded three different materials (C-1-Cu, C-1-Ag, C-1-Au). The FTIR spectra of C-1 shows characteristic peaks at 1590 and 1410 cm-1 due to asymmetrical and symmetrical vibrations of C-O of carboxylate coordinated with Na(I) and Zn(II), respectively.31 Interestingly, peaks at 1590 cm-1 of C-1 shifted to lower wave number by 20 and 25 cm-1 in C-1-Cu, C-1-Ag indicate the exchange of metal between Na(I) and corresponding Cu(II) and Ag(I). Further, atomic absorption spectroscopy (AAS) study directly revealed the absence of sodium and presence of copper and silver in C-1-Cu and C-1-Ag, which confirms the exchange process (Table S1). However, in case of C-1-Au, a positive shift in FT-IR analysis and presence of sodium by AAS analysis is further ruled out the exchange process between Na(I) and Au(III).32 Next, the hydrolysis reactions of C-1-Cu, C-1-Ag, and C-1-Au were done with hydrazine hydrate in methanol under hydrothermal condition at 150 °C and afforded three different material with brown (S-1 for Cu), silver (S-2 for Ag), and black color (S-3 for Au), respectively. All the materials were characterized by PXRD, FT-IR, HR-SEM, HR-TEM, X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma optical emission spectrometry (ICPOES) to determine the crystallinity, morphology, particle size, oxidation state of the metals, and the content of the metal present in the materials. The PXRD data confirms the crystalline nature of all the synthesized materials. Crystallite size of ZnO, S-1, S-2, S-3 are found to be 40, 21, 37 and 32 nm, respectively.


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Figure 2. PXRD pattern (a), comparative FT-IR spectra between C-1 and C-1-Cu/Ag/Au (b), and high resolution XPS spectra of all the synthesized materials (c).

The presence of hexagonal wurtzite ZnO along with Cu(0) in S-1 and Ag(0) in S-2 was also confirmed from PXRD as peaks are in good agreement with reported PXRD pattern (Figure S6 and S7). The XPS spectra confirmed the presence of ZnO in both S-1 and S-2 as two peaks observed in the range of 1016-1044 eV for Zn 2p3/2 and Zn 2p1/2 with ∆m ∼23eV. Further, the presence Cu(0) and Ag(0) was also observed in S-1 and S-2, respectively. It is noteworthy to mention that the presence of other oxides of copper such as Cu2O and CuO may be ruled out in S-1, as no satellite peak was observed between 940-945 eV in high resolution XPS spectra of S-1 (Figure 2).33 However, the



transformation of reduced copper species into CuO was observed after the S-1 exposed to air. The HR-SEM (Figure 3) and HR-TEM images (Figure 4) indicated that the initial flower like morphology has been transformed into flakes in S-1 consisting of several agglomerated particles with average particle size of 33 nm and aspect ratio of 83 (Figure S9). The morphology of S-2 was found to be bundles of rods consisting silver in between the rods (Figure S10). Further, the growth in S-1 and S-2 was observed along 111 plane of Cu/Ag and 100 plane of ZnO and their SAED pattern matches well with PXRD data (Figure S6-S10). The metal loadings in S-1 (Zn and Cu) and S-2 (Zn and Ag) were found to be 10.7 and 11.3 wt %, and 7.5 wt % and 7.4 wt %, respectively (Table S1). On the other hand, the material S-3 is found to be polycrystalline in nature consisting ZnO nano-rods of average length of 200-300 nm and diameter of 60-90 nm, in which several Au(0) with average particle size of 50 nm deposited in each nano rod (Figure S12). The growth of the material was observed along 111 planes for Au and 100 plane for ZnO and matches well with PXRD pattern of S-3 (Figure S11). The oxidation state obtained from XPS data further indicates the presence of both ZnO and Au(0) in S-3 (Figure 2 & Figure S13(d)). The major advantage of this synthetic route utilizing complex C-1 as precursor is that it does not require any stabilizer or foreign agent to arrest particle growth and thus maximum utilization of available nanoparticle surface could be possible. Most interestingly, the calculated band gap from the plot of (αEp)2 versus Ep based on the direct transition and the extrapolated value of Ep at α=0 of the materials are found to be 1.7, 2.7, and 2.8 eV for S-1, S-2, and S-3, respectively (Figure S14).34 The red shifted of band in presence of plasmonic nanoparticles (PN) was due to the creation of space-charge due to


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the electron flow from n-type semiconductor to PN. However, the seminal work by But Bardeen35 and Tung36 said that metal-induced gap states or polarization of the bond during the chemical bonding between PN and semiconductor (ZnO) will generates a new band level to the semiconductor due to the density of the surface states and pins the Fermi level, responsible for reduction of band gap.

Figure 3. HR-SEM of ZnO (a-b), S-1 (c-d), S-2 (e-f), and S-3 (g-h)

To check the importance of Zn-Na complex (C-1) over other metal salts as precursor, the reaction between 1:1 mixture of Zn(OAc)2. 2H2O and metal salts (CuSO4, AgNO3, and HAuCl4) followed by hydrolysis with hydrazine hydrate was done under optimized reaction condition and isolated three different materials. The morphology, band gap, composition, and metal content in the materials were found to be entirely different as compared to synthesized materials mentioned here, which directly suggest the importance of C-1 as precursor for controlling the growth of the material through metal exchange.



Figure 4. TEM, and SAED pattern of ZnO (a-d), S-1 (e-h), S-2 (i-l), and S-3 (m-p)

Next, in order to check the photocatalytic activity of the synthesized materials, 2,4-dinitro phenol (DNP) was chosen as model substrate. The degradation of DNP under white LED (intensity, 298 mW/m2) light was monitored from the change in UV-vis absorbance of the reaction mixture with time. The reactivity in terms of % of degradation versus time of the synthesized materials were found to be in the order of S-1 > S-3 > S-2 (Figure S21). The activity of the synthesized materials (S-1 to S-3) were compared with the materials (S-4


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to S-6) obtained after the reaction between Zn(OAc)2. 2H2O and metal salts (CuSO4, AgNO3, and HAuCl4). Interestingly, no such photo catalytic activity for the degradation of DNP was observed with other three materials (S-4 to S-6). Further, degradation of DNP was monitored under different light sources (White, Yellow, Red LED, and UVLight) with a fixed intensity of 298 mW/m2. The similar reactivity order was also achieved in Yellow LED and UV light. Notably, degradation of DNP was also achieved with ZnO under UV light. Whereas, the inactivity of all the material except S-1 in red LED light source indicates the requirement of higher energy light source compare to their band gap. To understand the dependency of both the photo catalyst and substrate, the reaction order was determined by the initial rate method for the degradation of 2,4dintrophenol under white LED light. For all the materials (S-1 to S-3), the kinetics data showed first-order rate dependency on photo catalyst as well as DNP. In terms of rate constant, superior activity of S-1 in comparison to other materials was achieved for the degradation of DNP or phenol. So, to gain mechanistic insights, various control experiments were carried out with S-1 using DNP as substrate. It could be clearly shown that no degradation of DNP was determined in the absence of S-1 or light. Further plottime−conversion studies were performed under both “light on/off” and oxygen/nitrogen conditions to gain further insight into the role of oxygen and light (Figure 5). However, a distinct difference of % of degradation of DNP was achieved with freshly prepared S-1 (10%) and aerial oxidized S-1 material (32%) in presence of oxygen.37 Notably, a reaction-induction period with “light and oxygen on” of 25 min and 08 min was observed with freshly prepared and oxidized S-1 material, respectively.38 Whereas, light exposed oxidized S-1 in presence of oxygen does not show any induction period.39 If the light is



turned off, the % of degradation are reduced, suggesting that transient Cu(I/II) needs continued irradiation for its activity level to be maintained. Further, examination of samples (freshly prepared S-1, aerial oxidized S-1, and light exposed S-1) through PXRD and XPS analysis revealed that Cu(0) present in S-1 slowly oxidized into CuO via Cu2O, which was again reduced to Cu2O and Cu(0) during the light irradiation period. To unveil the mechanistic insight further, the degradation of DNP was monitored in presence of various scavenger molecules. In presence of radical scavenger such as DPPH, methanol, and tert-butyl alcohol, the rate of the reaction significantly decreased, which clearly suggests the involvement of radical species in the reaction (Figure S29). The addition of benzoquinone does not show any significant change in the degradation rate, which ruled out the formation of superoxide radical anions.40 The addition of NaN3 as a 1O2 scavenger resulted no change in the degradation rate, which ruled out its involvement in the reaction. The addition of hole scavenger (ammonium oxalate or EDTA) resulted significant reduction in degradation rate, confirming that the holes were the dominant active species. Based on the above mentioned study, the catalytic mechanism can be explained as shown in Figure 5. Irradiation of light to S-1 creates sets of e− and h+ pairs. The presence of CuO not only improve the spectral response of ZnO to visible range but also effectively suppressed the e−−h+ recombination process by utilizing the e− for its self reduction to Cu(I/0). The h+ of ZnO oxidizes water and produce O2, which could be transformed into H2O2 as determined by UV-vis absorption monitoring of triiodide in aqueous solution (Figure S25). Further by using titrimetric method, we found that S-1 promotes selective two electron reduction of O2 and produces H2O2 with a rate of 0.92 µmolh-1 in a water/O2


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system under white LED light irradiation. The rate of the O2 reduction reaction in presence of electron donor such as 2-PrOH increases to 23 µmolh-1, which suggest that water oxidation is the rate-determining step for H2O2 formation (Figure S32). However a decrease in degradation rate of DNP was observed in presence of OH° radical scavengers such as methanol or 2-PrOH, suggest possible involvement of OH° radical in the reaction, which was confirmed from both the benzoic acid hydroxylation method and the GC-MS detection of hydroquinone during the degradation of phenol (Figure S31). Further to check the source of OH° radical, additional control experiment with catalase as a H2O2 scavenger was conducted.414243 Under the standard condition, reaction failed to produce any degradation of DNP which clearly indicated that the formed H2O2 is playing a crucial role for the generation of OH° radical. The presence of CO2, H2O, Cl-, NO3−, and SO42- in the solution of DNP, phenol, and methylene blue after the degradation by ion chromatography and titrimetric analysis suggest the complete mineralization of dyes and phenols (Figure S33).



Figure 5. Proposed Mechanism (a) % of degradation versus time under oxygen/nitrogen (b) light on/off (c) and reusability plot for S-1 promoted photo degradation of DNP (d).

Table 1 shows a rate study of the scope and versatility of the reaction with respect to various phenols and dyes. All three catalysts showed great versatility towards various substrates. However, in terms of second order rate constant data and irrespective of substrate, the reactivity of the materials are in the order of S-1 > S-3 > S-2, which is also consistent with their half-cell potential. The M(I)/M(0) half-cell potential of Cu (0.52 V) versus Ag (0.80 V) and Au (1.69 V), which implied that Cu(0) is more air sensitive and could easily transformed into corresponding Cu2O or CuO and make the overall process faster as compared to other two members in the group. Finally, as shown in Figure 5(d), the reusability of catalyst


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S-1 was tested for the reaction of DNP under white LED irradiation for 6 h. In terms of rate constant, a slight decrease in its rate was observed after each cycle.

Table 1. Substrate scope #

Rate/rate activity parameter (k×10-3 min-1/k′(k/m)×10-3 min-1) S-1 S-2 S-3 Phenol 37/7.4 19/3.8 22/4.4 4-nitrophenol 23/4.6 14/2.8 15/3 2,4-dinitro phenol 45/9 32/6.4 37/7.4 2,4,6-trinitro phenol 39/7.8 21/4.2 26/5.2 Methylene Blue 67/13.4 49/9.8 54/10.8 Auramine 42/8.4 27/5.4 34/6.8 Methyl Orange 32/6.4 24/4.8 29/5.8 Crystal Violet 72/14.4 47/9.4 59/11.8 Rose Bengal 31/6.2 16/3.2 20/4 Rhodamine B 63/12.6 48/9.6 51/10.2 Melachite Green 91/18.2 59/11.8 71/14.2 Methyl Red 88/17.6 65/13 80/16 Reaction condition: For the degradation of substrate, 20m L of 1×10-4M solution was taken with 5mg of catalyst in presence of White LED.

However, S-1 showed great stability and no appreciable reduction in its degradation % were observed even after 10th cycle. ICP-OES analysis of the crude reaction mixture after 5th and 10th cycle showed barely 0.03% and 0.05% of the available copper. After comparing the metal contents between used material (after 5th & 10th cycle) and S-1, we did not observe any leaching of copper, which is in good agreement with its excellent performance during several catalytic cycles (Table S5). In summary, an easy, sustainable, scalable, and environmentally benign chemical method has been developed to synthesize Cu/Ag/Au-ZnO utilizing a polymeric Zn-Na-acetate complex (C32H48O34Na8Zn4, C-1) as precursor. The differential exchange capability of sodium and zinc in C-1 with guest metal (CuII, AgI, and AuIII) was further utilized to control the morphology, growth, and band gap of the synthesized materials. The materials were



further applied as a reusable photocatalyst for visible-light-driven degradation and complete remediation of various phenols and dyes via efficient H2O2 production from water and oxygen.

Associated Content The Supporting Information is available free of charge on the ACS Publications website at DOI: Procedural, spectral, and reaction kinetics data (PDF). The CCDC number of C-1 is 1589093. AUTHOR INFORMATION Corresponding Author*E-mail: [email protected] or [email protected]. ORCID Sanjay Pratihar: 0000-0002-0229-735X Notes: The authors declare no competing financial interest. Acknowledgements Financial support of this work by DST and DBT, New Delhi (for grant no: IFA/12-CH-39 & BMB/2015-42) and DST-Inspire for the fellowship to KS is gratefully acknowledged. The INUP, IITB and IITBNF, IITB (sponsored by DeitY, MCIT, Government of India) is gratefully acknowledged for giving us the access of XPS and HR-TEM facility.


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28

For similar structure: Zhang, L. Z.; Cheng, P.; Shi, W.; Liao, D. Z.; Xiong, Y.; Tang, G.-Q. [Na8Zn4(CH3CO2)16.2H2O]n: two-dimensional sheet-like coordination polymer with strong blue emission. Inorg. Chem. Commun. 2002, 5, 361-365. 29

For similar structure: Molaeia, F.; Bigdelia, F.; Morsali, A. Sodium(I)zinc(II) heteropolynuclear complex, crystal structure of a novel 2-D polymer, precursor for preparation of ZnO nanoparticles. J. Ind. Eng. Chem. 2015, 24, 229-232. 30

Goswami, L.; Pratihar, S.; Dasgupta, S.; Bhattacharyya, P.; Mudoi, P.; Bora, J.; Bhattacharya, S. S.; Kim, K-H. Exploring metal detoxification and accumulation potential during vermicomposting of Tea factory coal ash: sequential extraction and fluorescence probe analysis. Sci. Rep., 2016, 6:30402, 1-13.

31

The assignments of peaks were based on observed bond distance from crystal structure and FT-IR

spectra. 32

The exchange between Na(I) and HAuCl4 failed because of the following reason; (i) the expected reaction between HAuCl4 and Na(I) of complex C-1 will produce Na[AuCl4], (ii) the formed salt, Na[AuCl4] or HAuCl4 does not have vacant coordination site for bridging acetate of C-1, (iii) bigger size of gold will not allow to fit into the cavity generated after the exchange with Na(I). 33

Poulston, S.; Parlett, P. M.; Stone, P.; Bowker, M. Surface Oxidation and Reduction of CuO and CuzO Studied Using XPS and XAES. Surf. Interface Anal. 1996, 24, 811-820.

34

Further no appreciable change in the band gap of S-1 and S-2 was observed, when synthesized from different stotiometric ratio of C-1 and corresponding metal salts (Fig. S14-S15).

35

Bardeen, J. Surface States and Rectification at a Metal Semi- Conductor Contact. Phys. Rev. 1947, 71, 717−727. 36 Tung, R. T. Formation of an electric dipole at metalsemiconductor interfaces. Phys. Rev. B. 2001, 64, 205310. 37

No degradation was observed with freshly prepared S-1 under continuous flow of nitrogen.

38

Zhao, J.; Nguyen, S.C.; Ye, R.; Ye, B.; Weller, H.; Somorjai, G.A.; Alivisatos, A.P.; Dean Toste, F. A Comparison of Photocatalytic Activities of Gold Nanoparticles Following Plasmonic and Interband Excitation and a Strategy for Harnessing Interband Hot Carriers for Solution Phase Photocatalysis. ACS Cent. Sci. 2017, 3, 482–488.

39

S-1 was exposed in light with O2 for 2h prior to utilize for the degradation of DNP.

40

We did not observe the formation of formazine after the addition of NBT to the reaction mixture, which confirms the We did not observe NBT to conversion directly reveals the presence of O2− radical in the solution 41

Huang, W.; Ma, B.C.; Lu, H.; Li, R.; Wang, L.; Landfester, K.; Zhang, K. A. I. Visible-LightPromoted Selective Oxidation of Alcohols Using a Covalent Triazine Framework. ACS Catal. 2017, 7, 5438–5442. 42

Nosaka, Y.; Nosaka, A. Understanding Hydroxyl Radical (•OH) Generation Processes in Photocatalysis. ACS Energy Lett. 2016, 1, 356–359.



43

Ye, L.; Liu, J.; Gong, C.; Tian, L.; Peng, T.; Zan, L. Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge. ACS Catal. 2012, 2, 1677−1683.

TOC-Graphic


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TOC-Graphic 254x190mm (96 x 96 DPI)


Role of Metal Exchange toward the Morphology and Photocatalytic

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