Rapid and low temperature synthesis of Ag nanoparticles on the ZnO nanorods for photocatalytic activity improvement

Results in Physics 13 (2019) 102209

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Rapid and low temperature synthesis of Ag nanoparticles on the ZnO nanorods for photocatalytic activity improvement

T

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Tio Mahardikaa, Nur Ajrina Putria, Anita Eka Putria, Vivi Fauziaa, , Liszulfah Rozab, Iwan Sugihartonoc, Yuliati Herbanid a

Departemen Fisika, Fakultas MIPA, Universitas Indonesia, Depok 16424, Indonesia Jurusan Pendidikan Fisika, Universitas Muhammadiyah Prof. Dr. Hamka, Jakarta Timur, Indonesia c Program Studi Fisika, FMIPA Universitas Negeri Jakarta, Jl. Rawamangun Muka No. 01, Rawamangun, Jakarta 13220, Indonesia d Research Center for Physics, LIPI, Serpong 15314, Kompleks Puspitek, Serpong 15314, Banten, Indonesia b

A R T I C LE I N FO

A B S T R A C T

Keywords: ZnO/Ag Nanohybrids Hydrothermal Photocatalytic Recombination

Zinc oxide (ZnO) is one of the potential semiconductors for photocatalytic applications. However, ZnO has a high recombination rate between electrons and holes, which reduces the efficiency of its photocatalytic activity. Thus, a nanohybrid structure between ZnO and a noble metal, such as Ag, has been proposed because it is cost effective, is chemically stable, and has enhanced photocatalytic activity. In general, ZnO/Ag nanohybrids are not easily synthesized due to the self-nucleation of Ag NPs during the deposition on ZnO. In this study, the Ag nanoparticles were deposited on the ZnO nanorods (NRs) prepared on glass substrate by using the facile and rapid hydrothermal method at low temperature 80 °C for 90 min. The result analysis shows that the Ag nanoparticles deposition process did not change the morphological and microstructural properties of the ZnO NRs. The Ag NPs with the diameter range of 10–20 nm spread uniformly on the surface of the ZnO NRs. The photodegradation efficiency of methyl blue using the ZnO/Ag nanohybrids was higher than pure ZnO NRs. The ease of electron transfer between the ZnO and the Ag NPs was a major cause of the increased photocatalytic activity in both UV and visible-light irradiation.

Introduction The contamination of aquatic life ecosystems by organic pollutants has become a serious worldwide problem. Heterogeneous photocatalysis is a promising process for the remediation of wastewater from contaminants utilizing solar light [1]. For example, semiconductorbased photocatalytic technology is an effective, economical, and environmentally friendly method that has attracted great interest. In addition to titanium dioxide (TiO2), ZnO is an ideal photocatalyst because it is a group II–VI semiconductor with a wide band gap of 3.37 eV that environmentally friendly and chemically stable [1,2]. However, pristine ZnO nanoparticles (NPs) have limited photocatalytic efficiency due to their rapid electron–hole recombination properties. To overcome this drawback, nanohybrid structures of ZnO with noble metal NPs, graphene oxide [3] and two-dimensional materials such as MoS2 [4–6] have been proposed as photocatalysts for many chemical reactions. Various noble metal NPs have been deposited on the surface of ZnO, including ZnO/Au [7,8], ZnO/Pt [9,10], ZnO/Pd [11], and ZnO/Ag [12–15]. Of the noble metals, Ag has attracted the

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most attention for further scientific investigations because of its low cost, chemical stability, and high efficiency [16]. However, the ZnO/Ag nanohybrids have been generally fabricated in a powder form to produce micro-sized particles [17–23], nanoparticles [12,19,24–28], nanoflowers [29–32], or nanorods (NRs) and nanoneedles [16,33–35]. The use of a ZnO photocatalyst in a powder form is time consuming because the ZnO powder needs to be separated from the contaminated water, which can lead to wastage of most of the photocatalyst. Therefore, researchers have prepared ZnO nanostructures on substrates to improve the efficiency of the photocatalyst. However, few researchers have investigated the ZnO/Ag photocatalyst grown on the surface of a substrate such as Zn foil substrate [36–38], polyethylene terephthalate (PET) flexible substrates and silicon wafers [39]. Recently, Gabriel et al. electrodeposited Ag NPs on ZnO NRs on fluor doped tin oxide (FTO) coated glass substrates [40]. Moreover, the deposition of Ag NPs is not easy due to the self-nucleation of Ag NPs during the deposition on ZnO [16]. Several research groups have tried to deposit Ag NPs by using AgNO3 as a silver precursor with a long time deposition time, such as by stirring for 24 h at room temperature [35], stirring at 90 °C for 12 h

Corresponding author. E-mail address: [email protected] (V. Fauzia).

https://doi.org/10.1016/j.rinp.2019.102209 Received 1 January 2019; Received in revised form 15 March 2019; Accepted 18 March 2019 Available online 21 March 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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Fig. 1. FESEM images of (a) the surface and (b) the cross-section of the ZnO NRs; (c) HRTEM image and (d) SAED pattern of the ZnO NRs.

1 h and at 400 °C for 30 min.

[22], immersing at 50 °C for 8 h [15], stirring at 95 °C for 6 h [18] and using the solvothermal reactor at 120 °C for 3 h and 6 h [41]. In this research, the Ag NPs were deposited on the ZnO NRs using the hydrothermal method at a low temperature (80 °C) for a relatively short time (90 min). Previously, ZnO NRs were synthesized on glass substrates using the simple and inexpensive seed-mediated growth. The ultrasonic spray pyrolysis method was used for seeding process to produce ZnO NRs that were strongly attached to the substrate. This study provides an in-depth and comprehensive analysis of the effect of Ag NPs on the physical properties of ZnO to obtain a ZnO/Ag nanohybrid with high photocatalytic activity.

Characterizations The morphological properties of the samples were observed using a field emission scanning electron microscope (FESEM; Zeiss Supra 55VP) and a transmission electron microscope (TEM; FEI 200 kV Tecnai G20 S Twin). The microstructural properties were examined using X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer. An optical Perkin-Elmer UV–Vis Lambda 900 spectrophotometer, an UV–Vis diffuse reflectance spectrophotometer (U-3900H), and an FLS920 photoluminescence spectrometer (Edinburgh Instruments) were employed to study the optical properties of the ZnO/Ag samples. X-ray photoelectron spectroscopy (XPS) measurements were performed using the Ulvac-PHI Quantera II with Al Kα X-ray beam at 1486.6 eV. Raman spectra were recorded using a WITec Raman microscope equipped with a 532-nm laser source. All the characterizations were performed at room temperature.

Experimental method Synthesis of ZnO/Ag The ZnO NRs were prepared directly on a glass substrate using ultrasonic spray pyrolysis and hydrothermal methods, as described in our previously published paper [42]. First, 0.2 M zinc acetate dehydrates dissolved in deionized water was used as the seed solution. The seed solution was sprayed onto a heated glass substrate at 450 °C for 15 min by using a commercial ultrasonic nebulizer (1.7 MHz). Then, the samples were annealed at 450 °C for 1 h. The glass substrate covered with the ZnO seed layers was subsequently immersed in 20 ml of an equimolar solution of 0.05 M zinc nitrate tetrahydrate and hexamethylenetetramine and heated at 95 °C for 6 h. The deposition of Ag NPs was started with immersing the ZnO NRs in a 0.1% poly-L-lysine solution for 15 min. Then, the ZnO NRs were immersed for 30 min in the Ag precursor solution, which consisted of 0.5 ml of 0.1 M sodium citrate tribasic dihydrate, 0.5 ml of 0.01 M silver nitrate, and 18.5 ml deionized water. Then 0.5 ml of 0.1 M ice-cooled NaBH4 was added and the samples were left undisturbed at 80 °C for 90 min inside an oven. The samples were then annealed at 200 °C for

Photocatalytic study The photocatalytic activity of the ZnO/Ag nanohybrids was evaluated through the photodegradation of methyl blue (MB) dye in an aqueous medium. The samples were immersed in 20 ml of 10 mM MB solution and irradiated with UV dan visible light (40 W). At certain time intervals, the samples were removed from the solution and the optical absorption spectra of the MB solution was recorded using a UV–Vis spectrophotometer at the characteristic absorption peak wavelength of the MB dye at 596 nm. The stability performance of ZnO and ZnO/Ag nanohybrids was also evaluated by using the same above procedure in five successive cycles. 2

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Fig. 2. (a, b) HRTEM image of the ZnO/Ag nanohybrid; (c) SAED pattern of the Ag NPs; (d) ZnO/Ag nanohybrid and (e) Ag element distribution on ZnO/Ag nanohybrid.

Results and discussions

positively charged poly-L-Lysine coated ZnO and negatively charged Ag3(C6H5O7)− resulted in a low density of Ag NPs on ZnO surface. Moreover, Coulomb repulsion between Ag+ ion and ZnO were also exist hence a low density of Ag NPs is formed [29]. The XRD spectra of the ZnO NRs and ZnO/Ag nanohybrids are shown in Fig. 3. The diffraction peaks are all very sharp, which reflect a polycrystalline with good crystallinity. According to the JCPDS (file no. 98-005-7478), the ZnO NRs were formed in a hexagonal wurtzite structure. The seven diffraction peaks at 2θ of 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.8°, and 67.9° correspond to the crystal planes of (1 0 0),

Morphological and structural analysis The morphology of pristine ZnO NRs is shown in the FESEM and TEM images in Fig. 1. The high-density hexagonal ZnO NRs successfully grew perpendicularly on the glass substrate; the diameter varied from 90 nm to 168 nm and the average length was 1.6 μm. The high-resolution TEM (HRTEM) image in Fig. 1(c) displays the clear lattice fringes of 2.51 Å, which correspond to the d-spacing of the (0 0 2) crystal planes of hexagonal ZnO. The selected area electron diffraction (SAED) pattern shown in Fig. 1(d) confirms the polycrystalline structure of the ZnO NRs. Fig. 2 shows the morphological images of the ZnO/Ag nanohybrid. The Ag NPs formed on the surface of the Zn NRs and can be seen as dark circles with a diameter of 10–20 nm. Fig. 2(b) shows the HRTEM image of an Ag nanosphere with lattice fringes of 2.17 Å. Fig. 2(c) shows the SAED pattern of the Ag NPs in which the bright dots reveal the single crystalline structure of the Ag NPs. The element-mapping images shown in Fig. 2(d) and (e) show the Ag elements as red dots spread uniformly on the surface of the ZnO NRs with an atomic percentage of 0.2%, which confirms the presence of Ag NPs. The low density of deposited Ag NPs is similar to the results obtained with a longer deposition time [18,22,35,41]. In our previous study, we have deposited the Ag NPs in 30–120 min on ZnO NRs and the result showed that the optimum deposition time is 90 min because it could produce the Ag NPs with a relatively large size (∼20 nm). At lower deposition time, only fewer amounts of Ag nuclei nucleate and grow into nanoparticles with the diameter range of 5–10 nm. The thermal energy used to drive the silver ions (Ag+) to the surface of the ZnO nanorods is not enough to produce a large number of Ag NPs [16]. The other report stated that the low density of deposited Ag NPs is probably due to lower affinity of Ag ions towards ZnO surface. The weak electrostatic interaction between the

Fig. 3. The XRD spectra of the ZnO NRs and ZnO/Ag nanohybrids. 3

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Table 1 Texture coefficient values of the ZnO NRs and ZnO/Ag nanohybrids. Sample

Table 3 Microstructure data of the Ag nanoparticles.

Crystal plane

ZnO ZnO/Ag

Crystal plane

a = b = c (Å)

d-spacing (Å)

(1 0 0)

(0 0 2)

(1 0 1)

(1 0 2)

(1 0 3)

(1 1 2)

111

Standard Sample

38.12 38.18

4.09 4.09

2.36 2.36

0.237 0.387

2.490 2.735

0.403 0.430

1.475 0.773

0.812 1.199

0.582 0.477

002

Standard Sample

44.30 44.27

4.09 4.09

2.04 2.04

202

Standard Sample

64.45 64.41

4.09 4.09

1.44 1.44

(0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), and (1 1 2), respectively. Based on the ICDD standard reference no. 98-060-4631, the ZnO/Ag nanohybrid also showed 3 other peaks at 2θ of 38.11°, 44.30° and 36.23°, which correspond to the face-centered-cubic Ag structure with the crystal planes of (1 1 1), (0 0 2), and (2 0 2), respectively. Fig. 3 reveals that the (0 0 2) crystal plane has the highest intensity. This result indicated that the preferred orientation of the ZnO NRs is perpendicular to the substrate. The preferred orientation was then quantitatively measured as the texture coefficient (TC) parameter using the following equation [43]:

TChkl =

2θ

and composed of three peaks of oxygen species. The first peak with a binding energy centered at 529.9 eV is associated with the presence of bound oxygen in a hexagonal wurtzite structure of ZnO [19,21]. The second peak of the CeO/C]O bond is generally associated with the presence of oxygen vacancies [17,23]. The binding energy of the CeO/ C]O bond in ZnO/Ag at 531.1 eV shifts from those of pure ZnO at 530.9 eV. The third peak at 532.7 eV and 531.8 eV relates to the oxygen (O2) or hydroxyl ions (OH−) adsorbed on the surface [21,46]. The hydroxyl ions adsorbed on the surface are closely related to the ZnO photocatalytic process. The crystal defects on the ZnO surface become the active sites for capturing holes to form the hydroxyl radicals that are active in the photocatalytic process. The high photocatalytic activity of ZnO is closely related to the surface defects at a high OH– concentration [46]. In the structure of the ZnO/Ag nanohybrid, the second and third peaks were also slightly shifted, which could be tentatively attributed to the bound OH− and the chemisorbed O2 on the surface of the Ag in the form of the oxidized sub-monolayer Ag2O [19,46]. The existence of Ag NPs was also confirmed by the presence of two separated peaks of Ag3d5/2 and Ag3d3/2 with a binding energy centered at 367.2 eV and 373.2 eV, respectively [16]. The binding energy of the Ag3d spectrum slightly shifted compared with the Ag bulk binding energy (368.2 eV and 374.2 eV). This shift may be due to the electron transfer from Ag to ZnO, which initiates the decrease in the binding energy of Ag [22,25,28,30]. Fig. 5 depicts the Raman spectra at room temperature of the ZnO NRs and ZnO/Ag nanohybrids. The Raman spectra of the ZnO NRs show that the 5 vibration modes E2 (low), E2 (high)–E2 (low), A1 tranversal optical (TO), E2 (high), and A1 longitudinal optical (LO) are centered at 96.9 cm−1, 334.68 cm−1, 376.8 cm−1, 436 cm−1, and 579.76 cm−1, respectively [47]. The high intensities at 96.9 cm−1 (E2 low), and 436 cm−1 (E2 high) correlate with the vibration of the O and Zn atoms in the ZnO wurtzite lattice structure, respectively, and indicate the high crystalline ZnO [41,46]. The peak centered at 334.68 cm−1 (E2 high–E2 low) corresponds to the second-order Raman vibration of the acoustic phonons, which indicates the presence of heavy Zn sublattice and oxygen atoms [23]. The peak centered at 579.76 cm−1 relates to the defect in the ZnO crystal, especially in the oxygen vacancies and zinc interstitials [23,44]. It can be clearly observed that the intensity of all Raman peaks were damped in the ZnO/Ag nanohybrids probably because the Ag NPs partly covered the surface of the ZnO and thus, the Raman vibration spectrum was not clearly detected.

Im (hkl) Io (hkl) n I (hkl) 1 ⎡∑ m ⎤ n ⎣ 1 Io (hkl) ⎦

(1)

where Im(hkl) and Io(hkl) represent the measured relative intensity and the standard intensity of the (hkl) plane, respectively, and n represents the number of peaks. Table 1 shows that the (0 0 2) crystal plane has the highest TC value, which indicates that the crystal growth direction of ZnO is the c-axis on both samples. This result is consistent with the FESEM images in Figs. 1 and 2. Based on the TC values, the lattice parameters, d-spacing, crystallite size, and full width at half maximum (FWHM) were calculated only from the (0 0 2) crystal plane; the results are presented in Table 2. Generally, the lattice parameters, volume, and d-spacing were relatively unchanged for the different samples, which clearly confirms that the Ag atoms are not doped in the ZnO structure. This may be because the ionic radius of the silver ions (1.26 Å) is greater than the ionic radius of the zinc ions (0.75 Å) [19,27]. The crystallite size of the ZnO/Ag nanohybrid was slightly smaller than the pristine ZnO. The decrease in crystallite size of ZnO was also found in a previous study that used the hydrothermal method at 220 °C [16]. The decrease in crystallite size of ZnO may be attributed to the recrystallization process during the deposit of Ag NPs on the surface of ZnO [44]. The 2θ peak position, lattice parameters, and d-spacing of the Ag NPs are presented in Table 3. Generally, all parameters except the 2θ peak position are in a good agreement with the standard value. The slight shift from the standard value of the 2θ peak position may be due to the binding with the ZnO NRs. The d-spacing of the (1 1 1) crystal plane from the XRD spectrum is slightly greater than that shown in the HRTEM image in Fig. 2(b). The XPS spectra of the ZnO NRs and ZnO/Ag nanohybrids are shown in Fig. 4. Fig. 4(a) and (b) show that the element Zn in both samples has the highest intensity for Zn2p3/2 with a binding energy centered at 1021.1 eV and the lowest intensity for Zn2p1/2 with a binding energy centered at 1044.2 eV. These results confirmed that the Zn on the surface presents in the form Zn2+ in the ZnO structure [30,36,45]. Fig. 4(c) and (d) show that the O1s spectrum is asymmetric

Optical properties Fig. 6(a) shows the UV–Vis absorption spectra of the ZnO NRs and ZnO/Ag nanohybrids. The strong absorption peak in the UV region at wavelengths of 300–400 nm is associated with the excitonic transition of the electrons from the valence band (VB) to the conduction band (CB) of the ZnO NRs. The sharp near-band edge at 385 nm correlates with the band gap value of ZnO [14]. The ZnO/Ag nanohybrid exhibits a lower intensity of absorbance in the UV region and a higher intensity of absorbance in the visible region compared with the ZnO NRs. The decreased UV absorbance may be related to the Ag NPs partly covering the ZnO NRs, while the enhanced absorbance in the visible region may

Table 2 Microstructure data of the (0 0 2) crystal plane of the ZnO NRs. Samples

2θ

a = b (Å)

c (Å)

Volume (Å3)

Crystallite size (Å)

d-spacing (Å)

Standard ZnO ZnO/Ag

34.42 34.51 34.41

3.25 3.25 3.25

5.21 5.20 5.20

47.72 47.61 47.58

– 82.74 82.41

2.60 2.60 2.60

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Fig. 4. XPS spectra of the ZnO NRs and ZnO/Ag nanohybrids.

value of the absorption coefficient (α) in the following Tauc plot equation [15]:

α hv = A(α hv − Eg) n/2

where Eg is the band-gap energy, h is Plank’s constant, ν is the frequency of light, λ is the wavelength, and α is the absorption coefficient. When a graph is plotted between αhν and hν, the resulting intercept represents the band-gap energy of the material. Using this method, the estimated optical band gap for the ZnO NRs and ZnO/Ag nanohybrids is 3.25 and 3.22 eV, respectively. The reason for the decrease in optical band gap of the ZnO/Ag nanohybrids is not yet clearly understood, although this phenomenon has occurred in previous studies [15,16,30,39,46,50]. Sarma et al. reported that a decrease in band gap could be related to the increase of oxygen vacancies in the crystal structure of the ZnO/Ag nanohybrid [39]. This defect can prevent the recombination of the charge carrier in the ZnO nanocatalyst, which promotes the separation of photo-induced electrons and holes at the interface of the ZnO and Ag NPs. Fig. 6(b) clearly shows that the reflectance intensity in the visible region decreases dramatically for the ZnO/Ag nanohybrid compared with the ZnO NRs. This may be due to the reflectance of visible light by the ZnO NRs is well absorbed by the Ag NPs, as indicated in Fig. 6(a). Fig. 7 shows the photoluminescence (PL) spectra at room temperature of the ZnO NRs and ZnO/Ag nanohybrids. The PL spectrum of the ZnO NRs shows a broad emission band in the 375–425 nm region, multiple peaks in the 450–500 nm region, and a very wide peak centered at 622 nm. However, the highest peak at 440 nm comes from the glass substrate [42]. The broad emission band in the 375–425 nm region can be recognized as the recombination of free electrons from the CB to the VB of ZnO, which was commonly called the near band edge emission (NBE) [16]. The multiple emission peaks in the visible light region (450–500 nm) is generally due to electron–hole recombination at the intrinsic defects of ZnO, such as Zn vacancies (VZn), Zn interstitials (Zni), oxygen vacancies, and structural defects in different shallow levels inside the band gap [16,29]. The emissions in the visible

Fig. 5. Raman spectra of the ZnO and ZnO/Ag nanohybrids.

be due to the localized surface plasmon resonance (LSPR) effect of the Ag NPs, which have a resonance peak in the range of 400–450 nm [48]. Fig. 6(b) shows the diffuse reflectance spectrum (DRS) of the ZnO NRs and ZnO/Ag nanohybrids. The DRS spectra also revealed a sharp band edge at 385 nm, which was similar to the sharp near-band edge in the optical absorbance spectrum. The DRS spectra were used to calculate the band-gap energy of ZnO using the following Kubelka–Munk equation [49]:

F(R) =

(1 − R)2 2R

(3)

(2)

where R is the reflectance and the value of F(R) is proportional to the 5

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Fig. 6. (a) The UV–Vis absorption spectra and (b) the DRS of the ZnO NRs and ZnO/Ag nanohybrids.

Fig. 7. The PL spectra of the ZnO NRs and ZnO/Ag nanohybrids.

Fig. 8. The MB degradation efficiency of the ZnO NRs and ZnO/Ag nanohybrids under UV irradiation.

light region are commonly called deep level emissions (DLE). The emission at 435 nm is related to the electron transition between the interstitial oxygen to the CB and VB. The emission centered at 466 nm is attributed to the presence of electronic transition between Zn vacancies VZn and Zn interstitials Zni. The emission bands in the green region (485–494 nm) are attributed to VZn and structural defects [16]. As shown in Fig. 7, the PL spectra of the ZnO/Ag nanohybrid demonstrates similar emission bands as the pure ZnO NRs, although the intensity is significantly reduced. This result indicates that there is a large reduction of optical recombinations that can be attributed to the existence of Ag NPs in the sample. The Ag NPs may act as an electron sink that traps the electrons from the ZnO. Due to the Fermi energy level, the conduction band of the ZnO is higher than the Fermi energy level of Ag and, thus, the photoexcited electrons can easily transfer from the CB of the ZnO to the Ag NPs and hinder the recombination of photogenerated excitons in the ZnO NRs [16,39,52].

photodegradation efficiency =

A0 − A(t ) × 100(%) A0

(4)

where A0 is the initial MB absorbance and A(t) is the MB absorbance after irradiation with UV and visible light for t seconds. As shown in Fig. 8, the degradation efficiency under UV light of the ZnO NRs and ZnO/Ag nanohybrids were 73% and 82%, respectively. Meanwhile, the degradation efficiency under visible light of the ZnO NRs and ZnO/Ag nanohybrids were 60% and 65%, respectively, as shown in Fig. 9. In general, the photodegradation efficiency for both pure ZnO and ZnO/ Ag in the UV light region is higher than in the visible light region. The photodegradation mechanism under UV light can be explained as follows [42]. The ZnO absorbs UV light with energy equal or greater than its band gap for the photoexcitation of electrons from the VB to the CB, which leaves a similar number of holes in the VB. In pure ZnO, free electrons react with the soluble O2 and the holes react with hydroxyl ions. Both produce the free radicals that can break MB into a less harmful degradation product. The enhanced photocatalytic activity under UV light by ZnO/Ag can also be explained. Following the photogeneration of the electrons and holes, the electrons flow from the ZnO to the Ag NPs. The Ag NPs on the surface of the ZnO NRs act as an electron sink and provide enhancement of the photogenerated electrons and holes with an efficient charge separation [16,39]. The excited electrons in the ZnO CB are transferred to the Ag metal, while the holes remain in the VB so that the electron–hole recombination process can be suppressed [53]. The electron structure of ZnO and Ag form the

Photocatalytic activity The photocatalytic activities of the ZnO NRs and ZnO/Ag nanohybrids were examined through the photodegradation of MB, as shown in Fig. 8; MB was used as a representative organic pollutant. The MB solution without a photocatalyst was used as a reference and was slightly degraded under UV and visible light irradiation. The photodegradation efficiency of the ZnO and ZnO/Ag photocatalysts was further evaluated using the following equation [14]: 6

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Fig. 9. The MB degradation efficiency of the ZnO NRs and ZnO/Ag nanohybrids under visible light irradiation.

Fig. 10. Recycling ability of ZnO NR and ZnO/Ag nanohybrid under UV irradiation.

Fig. 11. Recycling ability of ZnO NR and ZnO/Ag nanohybrid under visible light irradiation.

Schottky barrier at the interface where the Fermi level energy of Ag is lower than the ZnO CB; thus, the electrons will be transferred from the ZnO to the Ag [24,31,48]. The presence of Ag on the ZnO efficiently extended the lifetime of the photogenerated charge carriers and facilitated the facile charge migration [32]. This agrees with the decreasing Raman and photoluminescence intensities at all wavelength ranges and indicates that the electron transfer process to Ag NPs is a major cause of the increased photocatalytic activity. These separated electrons and holes then react with the oxygen and water to generate (·O2−) and (%OH) free radicals that can break the chemical bond of MB molecules [42]. Ag deposition also may be able to modify the electronic properties of ZnO, leading to the enhancement of the visible light absorption and the decrease band gap of ZnO from 3.25 eV to 3.22 eV.

This provides improvement of the photocatalytic activity in the UVlight region. The visible-light-driven photocatalytic activity can be attributed to the presence of the free electrons and holes that can react with the oxygen and water in the MB solution. This agrees with the relatively high absorbance in the visible light area as shown in Fig. 6(a) and the presence of wide emission in the visible light area in photoluminescence spectrum (Fig. 7). These free electrons and holes have energy levels lower than the main conduction band energy level of ZnO. The enhancement of visible-light-driven photocatalytic activity maybe due to of two things. First, the existence of Ag2O that has a narrow band gap. The formation of Ag2O may correlates with the

7

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between ZnO and the Ag NPs.

decrease Ag binding energy as shown in XPS spectrum (Fig. 4). Under the exposure of visible light, the parts of electrons in Ag2O are excited and transferred to the conduction band of ZnO and another part of electrons transfer to the Ag. Both could produce the reactive (·O2−) and (%OH) free radicals [15]. Second, under the visible light irradiation, the LSPR effect of Ag can enhance the visible light absorption as shown in Fig. 6(a) and the photoexcited electrons of Ag NPs are also transferred to the ZnO which will enhance the visible light photocatalysis [15,30,52]. It was difficult to compare the performance of the ZnO photocatalysts produced in this experiment with previous results because no similar experiments have been performed. Wu et al. [36] reported that the photodegradation efficiency increased as the Ag NPs were deposited on the surface of the ZnO NRs. In the experiment, the ZnO/Ag photocatalysts were used for the degradation of rhodamine B (RhB) irradiated with UV light (100 W). Interestingly, the ZnO/Ag heterostructures could degrade the RhB completely only for 30 min, which may have been due to the fact that the heterostructures were grown on both sides of the substrate and, thus, produced a large surface area. In a study by Sarma et al., the surface coverage of the ZnO also played a role in its photocatalytic performance of the decolorization of a methyl orange (MO) solution [39]. Sarma et al. reported that the high surface coverage of the ZnO microrods/Ag NPs deposited on PET led to better photocatalytic activity than those of the ZnO microrods. The distribution of the Ag NPs in the heterostructures is also an important parameter in the photocatalytic performance in visible light. A high amount of Ag could lead to a low charge separation efficiency of the electrons and holes, which would decrease the photocatalytic activity. The stability of the photocatalytic performance under UV and visible light irradiation, are shown in Figs. 10 and 11. After four cycles, under UV light irradiation, the photocatalytic efficiency of both ZnO and ZnO/Ag catalyst displays a 13% drop, but in the fifth cycle, the decline was quite sharp to 28%. Furthermore, the cycling performance both ZnO and ZnO/Ag photocatalyst under visible light irradiation, the photocatalytic performance shows a sharp decrease since the second cycle. After recycling for five times, the photocatalytic activity of both ZnO and ZnO/Ag photocatalyst drop to 40%. It clearly demonstrates that the ZnO/Ag nanohybrids has a better photocatalytic performance stability under UV light irradiation.

Acknowledgement This research was financially supported by Hibah Penelitian Dasar Unggulan Perguruan Tinggi No. 386/UN2.R3.1/HKP05.00/2018 from Ministry of Research, Technology and Higher Education Republic of Indonesia. References [1] Jia ZG, Peng KK, Li YH, Zhu RS. Preparation and photocatalytic performance of porous ZnO microrods loaded with Ag. Trans Nonferrous Met Soc China 2012;22:873–8. https://doi.org/10.1016/S1003-6326(11)61259-4. [2] Cao S, Liu T, Tsang Y, Chen C. Role of hydroxylation modification on the structure and property of reduced graphene oxide/TiO2 hybrids. Appl Surf Sci 2016;382:225–38. https://doi.org/10.1016/j.apsusc.2016.04.138. [3] Mei W, Lin M, Chen C, Yan Y, Lin L. Low-temperature synthesis and sunlight-catalytic performance of flower-like hierarchical graphene oxide/ZnO macrosphere. J Nanopart Res 2018;20:286. https://doi.org/10.1007/s11051-018-4392-2. [4] Wang S, Ren C, Tian H, Yu J, Sun M. 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Conclusions The ZnO/Ag nanohybrids grown directly on a glass substrate were successfully prepared as a photocatalyst for the photodegradation of MB. The deposition of the Ag NPs on the ZnO NRs performed using a hydrothermal method at 80 °C for 90 min did not change the morphological and microstructural properties of the ZnO NRs. The Ag NPs with a diameter that ranged from 5 to 30 nm spread uniformly on the surface of the ZnO NRs. The multiple peaks of the O1s XPS spectrum of the ZnO/Ag nanohybrids were slightly shifted from those of the ZnO spectrum, which could be attributed to the bound OH– and chemisorbed oxygen on the surface of the Ag in the form of the Ag(111) oxidized submonolayer. The binding energy of the Ag3d spectrum was also slightly shifted compared with the Ag bulk binding energy, which may be due to the electron transfer from Ag to ZnO. The photodegradation efficiency of MB by the ZnO/Ag photocatalyst under UV light irradiation was higher (82%) than the photodegradation efficiency by the pure ZnO NRs (73%) because the Ag NPs acted as an electron sink for the electrons in the ZnO CB. After the photogeneration of the electrons and holes, the electrons flow from the ZnO to the Ag, which provide enhancement of the photogenerated electrons and holes with an efficient charge separation so that the electron–hole recombination process can be suppressed. The enhancement of visible-light-driven photocatalytic activity maybe due to the formation of a narrow band gap Ag2O and the localized surface plasmon resonance effect of Ag NPs. Both facilitate an efficient charge separation 8

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