The investigation of hydrogen evolution using Ca doped ZnO catalysts under visible light illumination

Materials Science in Semiconductor Processing 105 (2020) 104748

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The investigation of hydrogen evolution using Ca doped ZnO catalysts under visible light illumination Ahmad Irshada, *, Ahmed Ejaza, Mukhtar Ahmad a, Muhammad Shoaib Akhtar d, Muhammad Aqib Basharat b, Waheed Qamar Khan c, Muahmmad Irfan Ghauri a, Absar Ali a, Mian Faisal Manzoor a a

Department of Physics, Bahauddin Zakariya University, Multan, 60800, Pakistan Department of Physics, University of Agriculture Faisalabad, 38040, Pakistan Institute of Advanced Materials, Bahauddin Zakariya University, Multan, 6800, Pakistan d School of Computer and Communication, Lanzhou University of Technology, Lanzhou, 10731, China b c

A R T I C L E I N F O

A B S T R A C T

Keywords: ZnO DRS Sol gel Nanoparticles Hydrogen evolution

A series of Ca doped ZnO (CZO) nanoparticles have been prepared using sol gel self ignition method with an aim to increase the photocatalytic hydrogen evolution via water splitting under visible light illumination. The syn­ thesized catalysts were characterized by analytical tools such as XRD, SEM, BET, EDS, XPS, FTIR, UV–Vis DRS, PL, CV and EIS. The XRD analysis points towards substitution of Ca into ZnO as revealed by shift of plane towards smaller 2θ value, and reduced crystal size. UV–Vis DRS and EIS analysis evidence that Ca doped ZnO catalysts demonstrate enhanced optical absorption and reduced impedance. XPS study evidenced the substitution of Znþ2 by Caþ2 and enhanced Zn–O bond strengths in CZO catalyst. Textural properties of CZO nanoparticles were superior to that of pristine ZnO. Among series of prepared CZO nanoparticles, the 3 mol% Ca doped ZnO (CZO4) was found to be most efficient candidate for hydrogen evolution under visible light illumination, whose hydrogen evolution rate (819.75μmolh 1) is twice than that of previously reported optimal ZnO photocatalysts. Small particle size, enhanced optical absorption, reduced interfacial resistance, narrow band gap and high surface area were recognized as responsible factors towards increased photocatalytic hydrogen evolution activities of Ca doped ZnO nanoparticles.

1. Introduction Hydrogen evolution energy due to photocatalytic water splitting is a distinguished and attractive approach to generate clean energy [1,2]. So, photocatalytic water splitting represents important energy vector and achieved good attraction due to inexpensive, everlasting, eco friendly, acceptable and extensively efficient energy outcome [3–7]. Since last decade, photocatalytic water splitting technologies has attracted exceptional attention and extensive efforts have been made to synthesize different semiconductors photocatalyst like metal oxides (ZnO [8–10], TiO2 [11–13]) and sulphides photocatalyst [14,15]. The wide band gap TiO2 semiconductor photocatalyst was found to be most efficient photocatalyst due to larger stability, inexpensive and easily affordable aspects. But previous reports indicates that minimum of 1.23 eV photon energy is required for photocatalytic water splitting

using TiO2 photocatalyst. Further previously reported photocatalytic hydrogen energy generated from water methanol solution is 0.7 eV, much lower energy as compared to pure water [16,17]. So TiO2 pho­ tocatalyst had not been ideally recommended for commercial use. The researchers have now focused on TiO2 free catalyst, ZnO due to presence of intrinsic defects, higher electron mobility (200–300 cm2V 1s 1) and prolong life time (>10s) of photo induced charge carriers as compared to TiO2 (0.1–4.0 cm2V 1s 1) [18,19]. Important and inexpensive ZnO semiconductor photocatalyst have been considered useful due to its abundance, large catalytic activity, larger charge transfer rate, inex­ pensive, intrinsic stability and eco friendly aspects [20,21]. Recently, researchers had reported that quantum efficiency and photocatalytic activity of ZnO are higher than that of TiO2 [8,9]. In addition, facile synthesis of ZnO for all kinds of nanostructures and shapes like Nano­ tubes, nanoflowers, nanoparticles NPs [22], hollow spheres [23,24],

* Corresponding author. E-mail address: [email protected] (I. Ahmad). https://doi.org/10.1016/j.mssp.2019.104748 Received 25 July 2019; Received in revised form 17 September 2019; Accepted 20 September 2019 Available online 24 September 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.

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Materials Science in Semiconductor Processing 105 (2020) 104748

three dimensional branched ZnO nano wires [25,26] and nano needles [27], were also reported towards higher photocatalytic activity. Since photocatalytic efficiency heavily depends on crystallinity, morphology, energy band gap and high charge separation ability [28], therefore several novel methods have been attempted to synthesize highly effi­ cient photocatalysts with required aforementioned properties [29]. Also various reports reveal that lower surface catalyst dynamics greatly in­ fluence the photocatalytic activity of most semiconductors [30]. That is why; lower photocatalytic activity of pristine ZnO for hydrogen pro­ duction was strongly attributed to rapid charge recombination rate, backward reaction and inability to utilize visible light in photocatalytic mechanism [31,32]. Several documented reports evidence that photocatalytic activity of ZnO is lower under UV light illumination [33–36] which strongly dis­ allows the use of UV light for numerous applications since 5% of solar spectrum is in UV region while 43% is in visible range [37]. Extensive efforts had been made to trigger ZnO to harvest visible light with the help of metals doping process [33–35,38]. The researchers have attempted to eliminate this issue by doping of various elements in an attempt to enhance the surface defects [39]. The employed doping mechanism includes three main criteria: (1) ionic radius of doping ele­ ments are selected lower, equal or superior than Zn (2) optical energy band gap of the synthesized photocatalyst must be as low as possible (3) photocatalytic hydrogen evolution under visible light must be highest. Many reports were made for transition metal doped ZnO [M ¼ Ni (0.56 A� ), Ag (0.98 A� ), Mn (0.53 A� ), Cr (0.73 A� )] as catalyst [40,41]. In this paper, we reported inexpensive, simple and cost efficient sol gel method’s synthesize alkaline earth metal Ca doped ZnO photocatalyst for hydrogen evolution. To best of our knowledge, there has been no report for Ca-doped ZnO photocatalyst for hydrogen evolution. The Ca had been chosen for doping due to its ability for modulating the optical properties and improving the photocatalytic activity of ZnO. It has been already experimentally proved that optical band gap for Ca doped ZnO was red shifted [42], due to introduction of Ca into ZnO lattice shift optical absorption towards visible light and controls the exciton recombination [43]. Further an extra carrier could be produced due to substitution of Caþ2 to Znþ2 sites [44,45]. The photocatalytic water splitting results showed that optimal hydrogen evolution rate of nearly 819.25μmolh 1 was generated by 3-wt% Ca doped ZnO photocatalyst. The obtained result showed that rational enhance of Ca doping into ZnO improved the photocatalytic water splitting.

1.5, 3 and 5 mol% respectively). The overall reaction to form CZO nanoparticles in the presence of citric acid is as given below ZnðNO3Þ2:6H2O þ CaðNO3 Þ2

citric acid

→

Zn1 x Cax O þ N2 þ CO2 þ O vapors

3. Photocatalytic activity H2 evolution activity over ZnO and CZO nanoparticles was studied in an aqueous solution in the presence of ethanol as sacrificial substrate under visible light illumination. The presence of sacrificial substrate is crucial for photocatalytic hydrogen evolution, because it traps electrons to enhance charge separation. The key factor for any photocatalytic study is the irradiation source. The irradiation source consists of 300 W Xe lamp (perfect light, PLS-SXES 300) fitted with a UV cut-off filter (λ > 420 nm), irradiated light intensity was set at 0.1 W cm 2 for all activities. H2 evolution activity was tested on a closed gas circulation system. 0.5 g of photocatalyst powder was dispersed in 200 mL pure water and 200 mL methanol (50% Vol). The arrangement between Xe lamp and gas system was so adjusted that incident photon flux was maximized. The gas produced was evaluated with gas chromatograph (TCD, Agilent-6820) fitted with a molecular TDX-01 sieve-column with Ar carrier gas. Reactor system schematic diagram is shown in Fig. 1. 4. Characterization The crystal structure of synthesized ZnO and CZO photocatalysts were studied using a Rigaho dmax- IIIA X-ray diffractometer with Cu-Kα radiation (λ ¼ 1.5406 A� ) in 2θ range 20� -80� using step width of 0.02� . The particle size was estimated using Scherrer formula. A scanning electron microscope, Hitachi S-4800 coupled with EDS was used to study the morphology and compositional analysis of synthesized photo­ catalysts. Hitachi U-4100 UV–vis spectrometer was used to study the DRS spectra of synthesized photocatalysts in the range of 300–800 nm. Hitachi F-4500 fluorescence spectrophotometer was used to study the room temperature photoluminescence spectra of synthesized photo­ catalysts. The photocatalytic exciton was measured at 325 nm and emission was scanned between 350 and 600 nm . FTIR spectra and BET surface area were measured by JASCO-MFT 2000 and Gemini-2375, Shimadzu apparatus, respectively. Cyclic voltammetry (CV) and EIS measurements were carried out with PGSTAT204 using standard three electrode system. The photocatalysts were applied as working electrode, while platinum wire and Ag/AgCl were used as counter electrode and reference electrode respectively. 0.1 M of NaOH was used as electrolyte with pH value of 7.

2. Synthesis of photocatalyst Sol gel method was used to synthesize ZnO and calcium doped ZnO (CZO) photocatalysts. Samples of ZnO doped with Ca with nominal molar ratio CaxZn1-xO(x ¼ 0.5, 1, 1.5, 3 and 5 mol %) were synthesized by sol gel self-ignition method. All regents were bought from Merck, Pakistan with 99.9% purity. Zinc nitrate (Zn (NO3)3⋅6H2O) and citric acid (C6H8O7) were employed as initiating agent to synthesize pure ZnO, while calcium nitrate (Ca (NO3)2) and aforementioned materials were used as starting materials to synthesize nominal compositions. All re­ agents were dissolved in de-ionized water in desired compositions. Citric acid was employed as a catalyst to ignite the materials. The solution was made homogeneous with constant magnetic stirrer for 30 min on hot plate (~50 � C), then temperature was increased to 120 � C with constant stirring. The solution slowly transformed from viscous to highly viscous gel due to water evaporation. Further enhancement in temperature (~300 � C) led to ignition of the gel. The gel ignition led to momentarily lasted self controlled combustion reaction to yield voluminous and dry nano powders with large surface area. This combustion practice was observed for all samples and finally attained nano powders were annealed at 700 � C for 5 h to absorb any remained moisture and to develop the hexagonal wurtzite phase. The synthesized photocatalysts were named as CZO (calcium doped ZnO) and 1, 2, 3, 4 and 5 had been added at the end of their name for different Ca concentration (x ¼ 0.5, 1,

Fig. 1. Reactor system schematic diagram. 2

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5. Results and discussions

Table 1 Physical parameters of XRD results.

5.1. XRD analysis Fig. 2 shows the XRD patterns carried out to evaluate the crystal structure, peak intensity, and particle size of pure ZnO and CZO pho­ tocatalysts. The XRD patterns of CZO nanoparticles were quite parallel to those of pure ZnO nanoparticles and there was no evidence for spurious peaks due to Ca doping into ZnO lattice. The XRD pattern confirmed the presence of strong and sharp peaks for ZnO similar to hexagonal wurtzite crystal structure (JCPDS 36–1452). Three main diffraction peaks (100), (002) and (101) were observed at 2θ values of 31.72� , 34.6� and 36.28� for pure ZnO [46]. There was no evidence for Calcium oxide peak, which could be explained on two probable reasons. Firstly, Caþ2 ions were greatly dispersed in host ZnO matrix (introduc­ tion of Caþ2 ions into host ZnO lattice structure and substitution of Znþ2). Secondly, any formed CaO phase could be beyond the detectable capacity of XRD diffractometer. The inset of Fig. 2 clearly illustrates a small shift for (002) peak towards smaller 2θ value (~0.03� ) as well as reduction of peak intensity with increase of Ca doping into ZnO lattice, revealed Caþ2 substitution for Znþ2 into the lattice of host ZnO without modifying the hexagonal wurtzite phase [34]. The reduced crystallinity could be assigned to restrained nucleation and consequently retarded growth rate influenced by ionic radii difference of Caþ2 (99 nm) and Znþ2 (74 nm) ions. The particle size measured by Scherrer formula was reduced with enhancing Ca content, consistent with previous literature [47]. The introduction of Ca content to lattice of host probably creates barrier for shift of grain boundary and diminishes the grain growth. We consider the formation of Ca–O–Zn precursor is attained in the sol gel suspension. After annealing at 700 � C, Ca ions residing in the grain boundaries among nanoparticles could substitute Zn ions sites to reduce the interfacial energy of nanoparticles, which is combined to one single wurtzite-nano-crystal with more ZnO nano-particles in the crystalliza­ tion mechanism. This result in reduced particle size of Ca doped ZnO [48–51]. Table .1 shows the particle size values for pure ZnO and CZO photocatalysts. Thus it was concluded from XRD results that Caþ2 ions were successfully incorporated into host ZnO, intensity and particle size was decreased with increase in Ca content, consistent with previous studies [52,53]. The lattice parameters (Table 1) were also increased for CZO samples. This may be attributed to structural adjustment as the change in lattice parameter is dependent on the bond flex of cation and anion, modification of crystal structure and ionic radii difference of substituted ion. The observed c-axis elongation is quite possible for Ca doped ZnO as the ionic radius of Caþ2 (99 nm) is higher than that of Znþ2 (74 nm) [36,39].

Samples Name

ZnO

CZO1

CZO2

CZO2

CZO4

CZO5

Imax D (nm) a(nm) c(nm) c/a

6378 43.26 3.2 5.18 1.61

6282 42.61 3.225 5.205 1.61

4460 38.73 3.23 5.21 1.612

3986 36.97 3.235 5.22 1.62

3562 26.07 3.22 5.24 1.63

2564 43.26 3.225 5.23 1.62

5.2. SEM, BET and EDX analysis The surface morphology of representative photocatalysts was studied by using SEM analysis. The particles of pristine ZnO photocatalyst (Fig. 3) showed nearly spherical conformation, surrounded by nonuniform nano-particles with intermediate space occupied with some ZnO nanoparticles. Also, some nanoparticles are richly agglomerated and irregular in shape due to overlapping of smaller nanoparticles. However, the agglomeration degree was reduced to some extent for CZO4 photocatalyst due to Ca concentration (Fig. 3), which could be useful for photocatalytic activity because it would generate more active sites to trap charge carriers. The presence of Ca into ZnO also boosts the surface roughness, implying fair dispersion of Ca on the surface of the catalyst. Hence lower agglomeration of nanoparticles as well as good dispersion of Ca will consequently enhance the photo activity. The documented reports illustrate that maximum flux of light is absorbed when highly agglomeration of nanoparticles is avoided, consequently

Fig. 2. XRD pattern for ZnO and CZO photocatalysts.

Fig. 3. SEM images for ZnO and CZO4 photocatalysts. 3

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transfer more photo induced electrons to CB while rough surfaces are expected to perform better activity due to higher porosity and higher specific surface area [53]. Moreover particle size reduces with Ca doping, consistent with XRD results. Thus it is briefly concluded that doping and doping amount of Ca influence the particle size. Also higher doping triggered the morphological variation with decreased agglomeration. BET surface area of the photocatalyst is the most crucial factor influencing the photo-activity. The surface area for ZnO and CZO pho­ tocatalysts was measured by employing N2 gas absorption-adsorption technique and is shown in Table 2. The isotherm of Ca–ZnO (Fig. 4) demonstrates a kind II hysteresis loop. With increase in Ca doping amount, the steepness of the loop enhanced with end moved towards smaller relative pressure (P/P0), translating higher textural properties. Small particle size and enhanced Zn–O bond strength (later FTIR study) of Ca doped ZnO nanoparticles may be the significant factors improving the textural properties. Among all synthesized CZO photocatalysts, CZO4 showed largest specific surface area (98.26m2g-1). BET surface area of CZO4 catalyst is almost twice as compared to that of pure ZnO (Table .2). The enhancement in surface area could be due to decrease in crystallinity by growth inhibition due to introduction of Ca in the lattice of host ZnO. Thus it is obvious from above observations, CZO4 photo­ catalyst is most probable to show optimal hydrogen evolution rate due to availability of greater textural properties and narrow band gap [53]. The elemental composition of the prepared photocatalysts was investigated by using EDS analysis. The EDS analysis of ZnO and CZO4 photocatalysts are shown in Fig. 5. It was observed that there were only Zn and O elements presented in pure ZnO, while EDX spectra of CZO4 photocatalyst confirmed the presence of Ca in addition to Zn and O el­ ements. The peak observed at 0.53 keV is attributed to O while peaks at 1.02, 8.64 and 9.59 keV correspond to Zn. The peaks present at 0.77 and 5.01 keV belong to Ca. The atomic percentage of Zn, O and Ca are pre­ sented in Table 3. It is observed that O abundance is reduced with increasing Ca content attributed to electronegativity and ionic radius difference, identifies the substitution of Ca doping into lattice of ZnO in agreement with XRD results.

Fig. 4. N2 Adsorption-desorption isotherm for ZnO and CZO4 photocatalyst.

5.3. XPS analysis XPS analysis was employed to shed light on surface composition of synthesized catalysts. XPS spectra (Fig. 6) contain merely Zn, O and Ca for Ca doped ZnO samples, consistent with EDS result. XPS spectra indicate peaks for Zn2p1/2, O 1 S and Ca2p3/2 at binding energy values 1070.63, 531.04 and 341.3 eV, respectively. These binding energy values are consistent with documented literature [54,55]. The Zn2p1/2 peak indicates that valence state of Zn is not varied after introduction of Ca content into ZnO. Thus, zinc still exists in Znþ2 chemical states. O 1 S peak observed at 531.04 eV corresponds to chemisorbed oxygen of surface hydroxyl [56]. These surface hydroxyl acts as holes scavenger and enhance the charge separation under visible light illumination, simultaneously produce extremely reactive hydroxyl radicals [57], consequently assisting the photocatalytic process [58]. The peak posi­ tion of Ca2p3/2 shifts towards higher binding energy relative to standard binding energy of Ca, assigned to electrons migration from metal Ca to ZnO (formation of Caþ2). When Ca is introduced into ZnO, their corre­ sponding Fermi levels required to be matched. Thus electrons migra­ tions occur from Ca particles to CB of ZnO, reaching to higher valence of Ca [59]. A positive shift in binding energies for Zn, O and Ca can be assigned to replacement of Znþ2 by Caþ2 (consistent with XRD results)

Fig. 5. EDS spectra for ZnO and CZO4 photocatalysts. Table 3 Atomic % of ZnO and CZO nanoparticles.

ZnO

CZO1

CZO2

CZO3

CZO4

CZO5

Eg (eV) SBET(m2g 1)

3.25 55.74

3.20 63.21

3.15 71.13

3.11 85.17

3.05 98.26

3.09 88.39

Sample name

Atomic (%)

1

ZnO

2

CZO4

O ¼ 51.35 Zn ¼ 49.65 O ¼ .36.37 Zn ¼ 54.14 Ca ¼ 1.49

and strong interaction Zn–O–Ca added binding energy. This Zn–O bond strengthening was consistent with Zn–O bond stretching frequency of identified by next FTIR study. 5.4. FTIR analysis

Table 2 Band gap values and BET specific surface area for ZnO and CZO samples. Sample Name

S. no

FTIR analysis of pure ZnO and CZO4 is shown in Fig. 7. Different absorption peaks revealed in synthesized catalysts have been attributed to their relative functional groups. The peak observed at 456 Cm-1 cor­ responds to stretching vibration of Zn–O bond. The peak at 1636 cm 1 4

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Fig. 7. FTIR spectra of ZnO and CZO4 photocatalyst.

increase in Ca doping into ZnO and thus improving the abundance of hydroxyl group due to distortion of local ZnO lattice, consistent with above results, consequently increasing the photocatalytic process [60, 61]. It should be observed that Zn–O bond for Ca doped ZnO nano­ particles emerged at higher wave number compared to that for pure ZnO. This can be due to introduction of Caþ2 ions into lattice of host ZnO, consistent with XRD and XPS results. 5.5. UV–vis DRS analysis Diffuse reflectance spectroscopy analysis was employed to investi­ gate the energy band gap of the synthesized ZnO and CZO photo­ catalysts. To measure the absorbance, diffuse reflectance mode was changed in to absorbance by relation Absorbance ¼

log ð1=RÞ

(1)

The optical absorption spectrum of pure ZnO showed a strong ab­ sorption at 377 nm due to its wide band gap energy, indicated that pristine ZnO can be only triggered under UV light illumination, while Ca doped ZnO photocatalysts (Fig. 8) showed optical absorption in range of 387–406 nm due to incorporation influence of Ca into ZnO lattice. Further confirmation for enhanced optical absorption towards visible region was brought by threshold wavelength movement towards longer wavelength with enhancing Ca doping content, where CZO4 catalyst showed maximum absorption in visible region. The presence of Ca into ZnO may shift the light photon absorption ability of ZnO from UV to visible region, resulting in increased photo-activity [62]. The red shifted optical absorption could be explained on two probable reasons. Firstly, obvious optical absorption in visible region could be assigned to the absorption of metallic zinc core, usually noticed for metal oxides. Zeng et al. [63] and Ma et al. [64] had identified similar observations towards enhanced optical absorption for transition metal doped ZnO nano­ particles. Fu et al. [65] assigned red shifted optical absorption to the interaction between transition metal and surface oxides of zinc. Second probable reason for enhancement of optical absorption towards longer wavelength could be assigned to sp-d exchange interaction between CB electrons and localized d-electrons (Znþ2 ions substituted transition metal ions). Wu et al. [66,67] reported improved absorption abilities of transition-metal modified zinc oxide due to s-d and p-d strong exchange interactions. The optical band gap of ZnO and Ca doped ZnO photo­ catalysts were determined using Tauc’s relation

Fig. 6. XPS survey for ZnO and CZO photocatalyst.

was matched to bending-vibration of adsorbed H2O on the surface of the catalyst. The peak positioned at 2330 cm1 was assigned to atmospheric CO2. The peak originated at 2919 cm 1 due to bending vibration of C–H bond, while peak located at 3436 cm 1 was attributed to stretching vibrational of surface hydroxyl group of hexagonal ZnO and CZO4 catalyst. In this work, O–H stretching bands were enhanced with

αhν ¼ Aðhν 5

EgÞ1=n

(2)

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and CZO photocatalysts carried out at excitation wavelength of 325 nm to determine the excited states and defects. It can be observed that Ca doped ZnO samples demonstrate a three emission bands, observed at 387 nm (band edge emission), 423 nm (violet emission), 442, 453 and 470 nm (blue emission), 494 nm (blue green emission) and 531 and 542 nm (green emission). Many researchers credited the UV emission around 390 nm to free excitation recombination (donor acceptor recombination pair), originating from near band emission (NBE) tran­ sition from CB to VB of ZnO Vis an excitation mechanism [68,69]. The visible emission consisting of different colors is attributed to presence of several intrinsic defects Vis O vacancies (V0), Zn vacancies (VZn), O in­ terstitials (Oi), Zn interstitials (Zni) and O antisites (OZn). The docu­ mented reports suggest that violet emission is assigned to emissive electron migration from shallow donor level of Zn interstitial (Zni) to top level of VB [70–72]. Blue emission originates due to electron migration from shallow donor level of Zn interstitial (Zni) to an acceptor level of neutral Zn vacancy (VZn) [67]. Blue emission can be attributed to surface defects of ZnO nanoparticles or probably due to Zn vacancy (VZn), although the exact mechanism for emission 441 nm has not been iden­ tified. The peak observed at 527 nm is mainly due to oxygen vacancies, while peak at 542 nm is formed by CB to oxygen interstitial (Oi) tran­ sition. It is evident that intensity of defects peak slightly enhances with Ca doping, consistent with previous documented reports [73,74]. An enhance in the peak intensity (527 nm and 542 nm) with Ca doping content identifies the improvement in abundance of oxygen vacancies (Vo) and oxygen interstitials (Oi). In photocatalytic hydrogen evolution mechanism, the superior efficiency is related with excellent charge separation of photo-produced electron-hole, recombination can be diminished. Photo produced charge carriers (electrons-holes) can be scavenged by two impurity levels generated by Vo and Oi. Vo plays the role of electrons acceptor to capture electrons while Oi serves as shallow holes scavengers, both of these act to suppress the electron-hole recombination, thereby enhancing the photocatalytic efficiency. Petro­ nela et al. [75] and Kalita et al. [76] had reported enhanced photo­ catalytic activity for Co doped ZnO and Mn doped ZnO nanoparticles with similar PL emission response. A significant decrease in fluorescence emission intensity has been observed for CZO samples compared to that of pure one, assigned to the presence of Caþ2 in ZnO lattice that serves as non-radiative centers [77]. The observed quenching of PL emission in­ tensity has been observed with Ca incorporation up to CZO4 catalyst, however slight recovery of intensity is observed for CZO5 catalyst. The recovery of emission intensity may be assigned to possible

Fig. 8. UV–Vis DRS absorption spectra of all Ca–ZnO photocatalysts.

where A is constant, h is Planks constant, hѴ is incident light photon energy and Eg is band gap energy. The value of n is 1 here for direct band gap. To determine the band gap energy for pure ZnO and CZO photo­ catalysts, graph was plotted between (αhѴ) 2 and photon energy hѴ using above relation. The energy band gap determined from Tauc plot by extrapolation of the linear part of the curve to energy hѴ axis (αhѴ)2 ¼ 0, reduced from 3.25 eV to 3.05 eV as shown in Table 2. Thus, DRS analysis confirms enhanced optical absorption towards longer wavelength in visible region and reduced energy band due to Ca substitution into lat­ tice of host ZnO, consistent with above results. CZO4 photocatalyst is the most probable to demonstrate the optimal hydrogen evolution activity since it exhibited highest optical absorption and least energy band gap. It is evident from above observations that photocatalytic activities of Ca doped ZnO nanoparticles can be enlightened by their reduced optical band gap values and higher textural properties. Lattice constant is another significant factor influencing the optical band gap of Ca doped ZnO samples, a decrease in optical band gap was identified with increase in c/a ratio, attributed to difference in electro-negativity and ionic radius, replacement of Znþ2 by Caþ2 leads to c-axis elongation and consequently reduces band gap values [42]. It is evident from XRD, FTIR, XPS, BET and UV–Vis DRS observations that Ca doped ZnO nanoparticles leads towards narrowed band gap CZO photocatalysts consisting enhanced surface area. As enhanced in optical band gap values induces increased redox potential of the photo-produced charge carriers, which evidently improves the photocatalytic mechanism mainly occur on the photocatalyst surface, enhancing the surface area was one of the previously reported pathways to enhance the photo­ catalytic efficiency [59,64,67]. Though synthesized CZO catalysts possess the excellent textural properties and narrowed band gap to demonstrate efficient photocatalytic activities, the photocatalytic hydrogen evolution as found later would decrease with excess Ca doping amount. This identified the fact that higher textural properties and narrow band gap are not only parameters to fasten the photo mecha­ nisms of Ca doped ZnO nanoparticles. Poor textural characteristics originated by excessive Ca doping may be the crucial factor responsible for decreased hydrogen evolution rate of Ca doped ZnO sample (CZO4) [64]. 5.6. Photoluminescence analysis Fig. 9 depicts room temperature fluorescence spectra for pure ZnO

Fig. 9. PL spectra of ZnO and CZO photocatalysts. 6

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agglomeration of nanoparticles. Hence, presence of oxygen vacancies in our synthesized catalysts has a crucial role in photo activity mechanism because surface redox reactions are translated from oxygen vacancies. The abundance of oxygen vacancies and thus electrons concentration enhances with Ca doping content, originated due to electro-negativity and ionic radii difference of Zn and Ca as said above. Similar observa­ tion was reported by Etacheri et al. for Mg doped ZnO photocatalyst [74]. Furthermore, UV emission peak was red shifted for Ca doped ZnO nanoparticles, consistent with UV–Vis DRS identified enhanced optical absorption and reduced energy band gap. The significant variation in PL emission intensity verified a considerable modification in the electronic structure due introduction of Ca amount in ZnO, consistent binding energy shift of constitute atoms recognized by XPS analysis. Lower PL emission intensities originating from excellent charge separation were documented for ZnO doped with Al, Ni, Cu, C and Mn [56,60,65,75,76]. These metals doped ZnO revealed lower PL intensity with dopants to demonstrate enhanced photocatalytic activity. In summary any factor which can enhance charge separation can reduce fluorescence emission intensity and can induce efficient photocatalytic response.

gradually reduces with prolonging negative potential due to depleting electron mass transport barrier as a consequence of high mass transport barrier at the electrode. The potential shift and increased current may be attributed to excellent photocatalytic properties of Ca doped ZnO nanoparticles. The oxidation and reduction cycle were strongly depen­ dent on the electron transport and concentration of photoactive particles on the electrode surface. The peak current could be attributed to induce carrier concentration near CB of ZnO nanoparticles, which could be engaged in charge transport from ZnO to solution at semiconductor-solution junction [79]. To conclude CV measurements, CZO photocatalysts showed higher current compared to that of pure one, attributed to more charge transport ability due to introduction of Ca doping content. Fig. 11 shows EIS Nyquist plot of pristine ZnO and Ca doped ZnO catalysts under visible light illumination to study the charge migration impedance and separation-efficiency between photon produced electrons-holes as the increased charge separation between photon induced charge carriers is a crucial factor for efficient photocatalytic performance. The arc of EI-spectra translates the interface layer resis­ tance and charge migration resistance occurring at the surface of elec­ trode. The less arc radius reveals low interface layer resistance and high charge migration efficiency. The introduction of Ca into ZnO catalysts identifies the shortened semicircle of CZO electrodes than that of ZnO in plots, which indicated a decrease in interfacial resistance (impedance) and charge transfer barrier on the surface. Hence, introduction of Ca into ZnO nanoparticles will promote the electron trap and transfer efficiency in composite and could assist in inhibition of charge recombination in order to attain the higher photocatalytic efficiency. Among all synthe­ sized catalysts CZO4 exhibits the smallest semicircle arc which identifies the least interfacial resistance and electron transfer resistance at the surface of CZO4 catalyst to possess the highest possible hydrogen evo­ lution efficiency [80,81]. These observations are consistent with DRS and CV study. Also, EI-spectra supported the PL observation of various kinds of oxygen vacancies and defects, which remarkably support charge transfer and separation properties of photon induced charge carriers on the surface of Ca doped ZnO photocatalysts.

5.7. Cyclic voltammetry and EIS analysis CV measurements for pure ZnO and CZO nanoparticles (Fig. 10) were carried out in a redox reaction of 0.1 M NaOH electrolyte solution. The cyclic voltammograms were plotted from 0.7 V to þ0.7 V at scan rate of 50 mVs 1 to observe the presence of Ca ions into ZnO photocatalyst. The photocatalyst was mounted on GCE by droplet evaporation for 10 min, followed by dehydration in N2 atmosphere. When potential was positively scanned, the nearly spherical morphology of CZO nano­ particles revealed higher oxidation current compared to that of pure ZnO nanoparticles. The higher current signal as well as lower peak to peak potential clearly demonstrates the accelerated electrons transfer (oxidation current) for Ca doped ZnO samples. The CZO nanoparticles significantly generate a small mass transport barrier, leading to quick diffusion from bulk solution to modified electrode; consequently enhance the current (anodic peak current) at modified electrode. Firstly, the diffusion process generated barrier layer at the electrode due to electron mass transport continues to increase during positive scan, and then diffusion from bulk solution (catalyst) to modified electrode re­ duces with positive potential scan, consequently decreases the anodic current [78]. At switching potential the scan proceeds in reverse di­ rection to initiate the reverse electron mass transport process. The cathodic peak current was obtained due to maximum electron mass transport from electrode to catalyst suspension and then peak current

5.8. Photo activity Ethanol was used as sacrificial reagent due to its good documented record [82–84] and showed comparatively larger efficiency than other species [85]. Ethanol can be used as holes scavenger in water alcohol solution and as well as to show fast irreversible oxidation. This rapidly

Fig. 10. Cyclic voltammograms for ZnO and CZO photocatalyst at scan rate 50 mVs 1..

Fig. 11. EIS Nyquist plot for ZnO and CZO photocatalysts. 7

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took to oxidation through photo generated holes via direct reaction mechanism, instead of forming hydroxyl radicals [86,87]. Moreover ethanol possessed lower oxidation potential 0.02 V as compared to corresponding water potential 1.23 V. In this work, larger ethanol quantities were examined to compare previously reported results [88, 89]. Fig. 12 shows hydrogen evolution for pure ZnO and CZO photo­ catalysts in the presence of ethanol as holes scavenger. It can be observed from Fig. 12 that CZO4 photocatalyst showed a higher hydrogen evolution rate (819.75μmolh 1) than other tested photo­ catalysts. The reason behind this enhanced photocatalytic behavior of CZO4 is enlarged surface area with presence of defects and oxygen va­ cancies generated by Ca doping into ZnO lattice. The enhancement in the hydrogen evolution was 6 times higher than that reported for the unmodified commercial ZnO catalyst (94 μmolh 1) and almost 14 times higher than previously reported for Cu-doped ZnO photocatalyst [88]. The hydrogen evolution rate was in order of CZO4 > CZO5 > CZO3 > CZO 2 > CZO1 > ZnO There was a continuous decrease in hydrogen evolution rate from CZO4 to ZnO photocatalysts. The least hydrogen evolution rate (17.37μmolh 1) was observed for ZnO photocatalyst. The hydrogen evolution rate of 686.43μmolh 1, 313.69μmolh 1, 111.21μmolh 1 and 87.36μmolh 1 was observed for CZO5, CZO3, CZO2 and CZO1 photo­ catalysts respectively. Fig. 13 shows the hydrogen evolution rate for pure ZnO and CZO photocatalysts in the absence of ethanol. It can be clearly observed (Fig. 13) that hydrogen evolution rate was sufficiently decreased due to enhanced electron hole recombination rate. The hydrogen evolution rate was observed to be 6.37, 42.36, 61.21, 201.69, 559.75 and 426.43μmolh 1 for ZnO, CZO1, CZO2, CZO3, CZO4 and CZO5 photocatalysts, respectively. The photocatalytic hydrogen evolution for ZnO and CZO photo­ catalysts as a function of time is shown in Fig. 14. It is observed that photocatalytic activity of pure ZnO is very low (17 μmol for 7 h). However, it is observed that activity was increased with Ca doping in to ZnO. It was noted that the activity of CZO1 photocatalyst was about five times of that of pure ZnO low (87.36 μmol for 7 h). By further increasing the Ca concentration, the activity for CZO2 photocatalyst was even higher than CZO1 photocatalyst low (111.21 μmol for 7 h). Fig. 14 clearly shows that the activity of CZO3 photocatalyst is largely enhanced by Ca doping low (313.69 μmol for 7 h). The doping of Ca in to ZnO catalyst further greatly improved the performance remarkably low (819.75 μmol for 7 h). On further increasing the Ca content, the activity of CZO5 photocatalyst was reduced as compared to CZO4 photocatalyst

Fig. 13. Hydrogen evolution rate in the absence of ethanol for ZnO and CZO photocatalysts.

Fig. 14. Hydrogen evolution rate as a function of time for ZnO and CZO photocatalysts.

low (680.40 μmol for 7 h). The possible reason behind this could be the improvement of a charge transfer ability (electron-hole separation) and metallic-catalyst. Another possible reason, we suggest that the intro­ duced Ca into ZnO lattice is reduced to Ca0 by the photo generated electrons from the VB of ZnO ascribed to lower reduction potential of Caþ2 as compared to that of Zn. The capture of electrons by Ca atoms enhances the abundance of holes in the VB and reduces charge recom­ bination rate, consequently enhancing the photocatalytic water split­ ting. Over CZO4, the photocatalytic H2 production recues. The probable reason could be ascribable to (a) light filtration by the deposited metal, (b) partial obstruction of surface active site for ZnO in the oxidative branch during the photo process, (c) deterioration of catalytic activity of metal/ZnO nanoparticles at their enlargement, and (d) formation of recombination centers by excessive Ca metal clusters. 5.9. Photocatalytic stability and reusability To evaluate the photocatalytic stability of the CZO photocatalysts, the time course of photocatalytic H2 evolution over the CZO4

Fig. 12. Hydrogen evolution rate in the presence of ethanol for ZnO and CZO photocatalysts. 8

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Materials Science in Semiconductor Processing 105 (2020) 104748

photocatalyst were conducted for three consecutive runs. In Fig. 15, no noticeable degradation of H2 production was observed with prolonged light irradiation time, indicating that the CZO has good photocatalytic stability. After 12 h of photocatalytic reaction, H2 production rate dur­ ing the third cycle could maintain ca. 94% activity that of the first cycle, and the CZO4 photocatalyst was collected and characterized by XRD. As shown in Fig. 16, the CZO4 photocatalyst does not present any differ­ ence in XRD patterns, suggesting that the Ca doped ZnO photocatalyst has sufficient stability for the photocatalytic H2 evolution reaction. 6. Discussion Photocatalytic hydrogen evolution depends upon various factors like particle size, specific surface area, pore size, crystallinity etc. However, in this work hydrogen evolution activity showed a strong correlation with doping defects. The sol gel synthesized sample without doping, showed a small photocatalytic hydrogen evolution. Within CZO series, a distinctive increase in hydrogen generation was observed with Ca doping up to CZO4 photocatalyst. CZO4 photocatalyst showed the optimal hydrogen evolution rate 819.75 umolh-1. The high photo­ catalytic activity of Ca doped ZnO nanoparticles can be explained in terms of reduced particle size with increase in Ca doping content, since introduction of Ca would enhance the surface area of CZO catalysts. The improved textural properties would provide more active sites to trap charge carriers as identified by BET study. Furthermore, enhanced photocatalytic activity could be explained in terms of two factors. Firstly, introduction of Ca doping content into ZnO shifted optical ab­ sorption towards visible region; with increase of Ca content the threshold wavelength shifted towards longer wavelength thereby reduce the optical energy band gap due to Zn substitution by. Secondly, EIspectra confirmed that presence of Ca into ZnO lower the interfacial and charge transfer resistance, thereby accelerate the charge transfer to the surface and improve the charge separation to enhance the efficiency of Ca doped ZnO catalysts [90]. The hydrogen evolution activity of CZO photocatalysts was increased initially with increasing Ca doping and reached to maximum hydrogen evolution rate for CZO4 photocatalyst (mol-3% Ca–ZnO). This increased hydrogen evolution rate with Ca doping was attributed to enhanced charge transfer and separation rate to the catalyst surface and reduced energy band of CZO catalyst. This was in agreement with previous documented reports [91], which suggest that photocatalytic hydrogen evolution rate was enhanced with foreign

Fig. 16. XRD pattern of CZO4 before and after photocatalytic reaction.

doping. The higher hydrogen evolution rate of CZO photocatalysts in­ creases with Ca doping because presence of Ca facilitates more active sites on the surface of ZnO to trap charge carrier and thus control the electron hole pair rate. Previous reports [92,93] suggest that surface defects and oxygen vacancies of catalyst remarkably influence the photo-activity. In our work presence of oxygen vacancies due to electro-negativity and ionic radius difference between Ca and ZnO was confirmed by EDS study. Further ethnicity is brought by PL emission spectra to indicate peaks for oxygen vacancies and oxygen interstitials. Oxygen vacancies acts as trap electrons while oxygen interstitials acts as scavenge holes, thereby increasing the charge separation and promote the higher photocatalytic efficiency. These observations argue that surface defects are present in ZnO nanoparticles mainly due to oxygen vacancies and can strongly demonstrate the photocatalytic activity under visible light illumination. The higher photocatalytic hydrogen evolution of CZO4 under visible energy illumination is an indication of high photo-activity of CZO4, especially considering the wide band gap of ZnO. Upon further increase in Ca doping, hydrogen evolution rate was decreased. Hydrogen evolution rate for CZO5 photocatalyst was decreased probably due to light filtration by the presence of excess Ca on ZnO surface. The agglomeration of Ca may inhibit the creation of active sites on ZnO surface. Thus presence of excess Ca amount at higher doping deteriorated the photocatalytic properties of ZnO photocatalyst. This was also confirmed by UV–Vis DRS spectra that optical absorption was shifted towards smaller wavelength when compared to that of CZO4 catalyst. Thus introduction of Ca into CZO4 would facilitate more active sites to trap charge carriers, consequently enhancing charge separation and thus photocatalytic hydrogen evolution. 7. Conclusion A simple, safe, fast and environmental friendly hydrogen evolution activity by water splitting was developed with Ca doped ZnO nano­ particles using a sol gel method. The photocatalytic hydrogen evolution activity of Ca doped ZnO photocatalyst was determined by using ethanol as sacrificial agent under visible light illumination. The optimal hydrogen evolution rate was found to be 819.75umolh 1 for CZO4 photocatalyst. The mechanism of photocatalytic hydrogen evolution activity was investigated by XRD, SEM, BET, EDS, XPS, FTIR, UV–Vis optical band, PL emission spectra, CV oxidation reduction current peaks and EIS analysis. XRD study revealed successful substitution of Ca nanoparticles into ZnO. The BET surface area was enhanced with introduction of Ca into ZnO, due to growth inhibition by Ca doping. The

Fig. 15. Cyclic run for photocatalytic hydrogen evolution in the presence of CZO4. Measurement condition: 0.5 g of sample, 200 mL pure water and 200 mL methanol. 9

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Materials Science in Semiconductor Processing 105 (2020) 104748

optical band gap was reduced for Ca doped ZnO photocatalysts and showed stronger light absorption in visible light region as compared to pure ZnO photocatalyst. It was observed that presence of Ca supported the interfacial charge transport mechanism in such a way to use con­ duction band electron for increasing photocatalytic hydrogen evolution rate. PL analysis confirmed the presence of intrinsic defects acting as active sites to trap charge carrier. EIS analysis confirmed that low impedance and enhanced charge separation originate due to Ca doping content into ZnO. Also quenching of PL emission intensity with intro­ duction of Ca content was identified due to Caþ2 roles as non-radiative centers. The CV measurements showed that Ca doped ZnO photocatalyst experienced small mass transport barrier and showed increased redox reaction as compared to pure ZnO. The peak current could be attributed to induce carrier concentration near CB of ZnO nanoparticles. These observations identified that introduction of Ca content into ZnO pho­ tocatalysts reduces the optical band gap due to red shifted light ab­ sorption and promote the charge transfer to the surface of the catalyst due to small impedance, thereby improving the photocatalytic hydrogen evolution activity. Considering all studied, the highest photocatalytic hydrogen evolution (819.75μmolh 1) was attained with CZO4 nano­ particles under visible light illumination. The hydrogen evolution was two times higher than that previously reported for ZnO photocatalyst. This unexpected high hydrogen evolution could be assigned to small particle size, high surface area, enhanced optical absorption and charge transfer and oxygen vacancies as evidenced by above mentioned tools. Thus Ca doped ZnO photocatalysts showed higher hydrogen evolution activity and are potential candidate for industrial applications.

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