doi.org/10.26434/chemrxiv.11105576.v1

Zn Doping Induced Band Gap Widening of Ag2O NanoparticlesArup Kumar De, Sourav Majumdar, Shaili Pal, Sunil Kumar, Indrajit Sinha

Submitted date: 26/11/2019 • Posted date: 02/12/2019Licence: CC BY-NC-ND 4.0Citation information: De, Arup Kumar; Majumdar, Sourav; Pal, Shaili; Kumar, Sunil; Sinha, Indrajit (2019): ZnDoping Induced Band Gap Widening of Ag2O Nanoparticles. ChemRxiv. Preprint.https://doi.org/10.26434/chemrxiv.11105576.v1

The present investigation widens the narrow bandgap of Ag2O to make it a semiconductor with moreattractive properties. A typical hydrothermal synthesis procedure was used to prepare Zn doped Ag2Onanoparticles. The X-ray diffraction analysis of the prepared powder samples showed that the latticeparameters of Ag2O increase with doping, indicating the occupation of interstitial positions by the dopantatoms. Density functional theory calculations also demonstrated the expansion of the Ag2O crystal lattice withthe dopant at an interstitial location. The bandgap of the Ag2O increases to 1.65 eV for 5-mole percent doping.The doped Ag2O nanoparticles photocatalytically degrade methyl orange under aerobic visible lightconditions. It appears that the rise in the percentage of higher valence Zn doping converts the originally p-typeto an n-type semiconductor.

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Zn doping induced band gap widening of Ag2O nanoparticles

Arup Kumar De, Sourav Majumdar, Shaili Pal, Sunil Kumar and Indrajit Sinha*

Department of Chemistry,

Indian Institute of Technology (Banaras Hindu University),

Varanasi 221005, India

*Corresponding author email: isinha.apc@iitbhu.ac.in

Abstract

The present investigation widens the narrow bandgap of Ag2O to make it a semiconductor with

more attractive properties. A typical hydrothermal synthesis procedure was used to prepare Zn

doped Ag2O nanoparticles. The X-ray diffraction analysis of the prepared powder samples

showed that the lattice parameters of Ag2O increase with doping, indicating the occupation of

interstitial positions by the dopant atoms. Density functional theory calculations also

demonstrated the expansion of the Ag2O crystal lattice with the dopant at an interstitial location.

The bandgap of the Ag2O increases to 1.65 eV for 5-mole percent doping. The doped Ag2O

nanoparticles photocatalytically degrade methyl orange under aerobic visible light conditions. It

appears that the rise in the percentage of higher valence Zn doping converts the originally p-type

to an n-type semiconductor.

Keywords: Hydrothermal synthesis, Doping, Bandgap widening, DFT calculations,

Photocatalysis

1. Introduction

One component photocatalysts suffer from several disadvantages. For instance, only

wideband semiconductors have conduction (CB) and valence band (VB) positions that offer

sufficient driving force for photo-oxidation or reduction reactions. However, such photocatalysts

suffer from recombination and photo-efficiency issues. They require UV radiation for photo-

excitation, which is only 4% of the solar spectrum [1]. Indeed nowadays, preference is for visible

light photocatalysts over those that need UV range radiation.

Doping of wideband semiconductors by metal or non-metal ions is one technique that has

been widely used in literature to tackle these issues. It introduces defects in the crystal lattice of

the pure phase semiconductor that may increase the catalytic activation of particular bonds in

substrate molecules. Alternatively, defects can trap the excitons and prevent their recombination.

Another significant effect is the introduction of new dopant bands that may narrow or widen the

bandgap of the photocatalyst. Such doping can result in the introduction of intermediate energy

levels that contract the original bandgap, making the initially wideband semiconductor photo-

responsive to visible light irradiation. Following this, quite a few researchers have reported

doping of ZnO, TiO2, and other wideband semiconductors by suitable metals such that this gives

rise to intermediate dopant bands [2]. For instance, reports show that the bandgap of ZnO can be

reduced to a value in the visible range by the addition of a small amount of Ag or Au [3].

The opposite approach is a less commonly followed strategy. In this case, a

semiconductor is doped with a metal ion to increase the bandgap of the native semiconductor. An

example of such bandgap widening is the Zn doping of CdS [4-5]. In these investigations, the

authors showed that Zn doping CdS modified the VB and CB positions of the resulting

semiconductor [5]. The change in the VB position was more than that in the CB energy.

Likewise, Ohtomo et al. [6] demonstrated that Mg doping of ZnO significantly widens the

bandgap to the solar-blind region. In another paper, Choopun et al. [7] reported ultra-wide

bandgaps above 5.0 eV in cubic phase MgxZn1-xO thin films. Recently, Xie et al. [8] investigated

the effect of lattice restraint on the widening of the ZnO bandgap due to the doping of Mg2+

cations along with Be ions as a co-dopant.

The present study follows this strategy and attempts to dope Zn2+ in the Ag2O lattice to

widen its bandgap. While silver oxide is a p-type semiconductor with a small bandgap, ranging

from 1.2 to 1.5 eV, ZnO is a semiconductor having a wide bandgap. It is essential to mention that

no attempt has been made to investigate this particular problem before. The main feature of Zn2+

ion doping in Ag2O is that the former ion is more than 50% smaller in size than Ag+ cations.

Given this size mismatch, the doping of only low percentages of Zn2+ ions in the Ag2O lattice is

possible. Also, the investigation of the position of Zn in the Ag2O lattice is a noteworthy issue.

The introduction of a dilute concentration of Zn salt under high temperature and pressure

conditions could lead to its inclusion in the Ag2O lattice. Generally, metal d-orbitals make the CB

and oxygen 2p orbitals usually compose the VB of a metal oxide semiconductor. In this case, the

3d orbitals of the dopant (Zn2+) would lie above the CB of Ag2O, which should lead to the

widening of the bandgap of the semiconductor.

Methods like vapor deposition, sol-gel, calcination, solvothermal, etc. have been

successfully used to fabricate doped metal oxides nanostructure [9-12]. The present study uses a

co-precipitation based hydrothermal technique to synthesize Zn doped Ag2O nanoparticles. For

the sake of comparison, the same protocol was utilized to prepare un-doped Ag2O nanoparticles

as well. X-ray diffraction (XRD) analysis and density functional theory (DFT) calculations were

employed to investigate the position of the dopant atoms in the Ag2O lattice. DFT studies were

used to calculate the formation energy due to the doping of Zn in substitutional and interstitial

sites of the Ag2O supercell. The doped Ag2O nanoparticle samples showed useful photocatalytic

properties for aerobic degradation of methyl orange (MO) and Rhodamine B (RhB) dyes in their

aqueous solutions. Active species responsible for such aerobic photocatalytic properties were

revealed by conducting the experiments in the presence of suitable scavenger molecules.

2. Experimental

2.1. Sample Preparation

A hydrothermal co-precipitation protocol was designed to fabricate Zn doped silver

oxides nanoparticles. The precursors were Zn(NO3)2.6H2O (SRLchem), AgNO3 (Merck), and

NaOH (Merck). All reagents were of analytical grade and used without further purification. All

synthesis procedures used double distilled water. Three different molar ratios of Zn(NO3)2.6H2O

and AgNO3 (corresponding to 1.25, 2.5, and 5% Zn salt mole percent in overall composition)

were dissolved in the proper volume of distilled water and mechanically stirred at 620

revolutions per minute. At the same time, 0.2 (M) NaOH solution was added in a drop-wise

manner until the pH of the mixture became nearly 11.24. Then the reaction mixture was

transferred to a stainless steel autoclave in an oven at 150oC. The heat treatment of the reaction

vessel at this temperature continued for 24 hours. Symbols A1, A2, and A3 respectively denote

the doped samples corresponding to 1.25, 2.5, and 5-mole percent Zn in the overall composition.

The same protocol was used to synthesize pure Ag2O (denoted by A0). In this instance, AgNO3

was the precursor.

2.2. Characterization

Rigaku SmartLab 9 kW powder type (RIGAKU Corporation) with Cu Kα irradiation

source ( λ = 1.54056 Å) at a scan rate of 5.09o 2θ min-1 with a step size of 0.01 recorded high-

resolution XRD patterns for identifying the crystalline phases formed in the synthesized

materials. The accelerating voltage and applied current was 40 kV and 200mA, respectively. The

Scherrer equation [13-14] (d = 0.9 λ/B cosθ) has been used to calculate the average crystallite

size (d) of the doped Ag2O. In this equation, d, λ, B and θ denote the crystallite size, Cu Kα

wavelength (0.154056 nm), full width at half maximum (FWHM) intensity in radians, and

Bragg’s diffraction angle, respectively. Transmission electron microscopy (TEM) and high-

resolution TEM (HRTEM) images were taken on Tecnai G2 20 TWIN (EDAX Inc.) using 200

kV accelerating voltage. Agilent Cary 60 UV-Vis spectrometer was used to record the absorption

spectrums in this research work. The HORIBA PTI QM-400 spectrofluorometer with xenon

lamp was used to record the photoluminescence spectrums of the samples.

2.3. Computational Details

All theoretical calculations utilized the plane-wave density functional theory (DFT) in

the Vienna ab-initio simulation package (VASP). The MedeA VASP software platform was used

to make the models investigated in the present research. The generalized gradient approximation

Perdew-Burke-Ernzerhoff (PBE) exchange-correlation functional was used to calculate the

defect formation energy, as in reference [15-16]. The Projected Augmented Wave (PAW)

pseudopotential modeled the interaction between valence electrons and core-shell electrons. All

calculations were performed using optimized parameters. The Kohn-Sham orbitals were

expanded using the plane-wave basis set to a cut-off value of 800 eV with a k-point

mesh size. The Ag2O unit cell was multiplied to construct a supercell, followed by its

geometry optimization and energy calculation. One model was built by substituting Zn for Ag at

a lattice position in the original Ag2O supercell (denoted by M1). In another model, Zn was

located in an interstitial site of the Ag2O supercell (represented by M2). The energies of the two

models were separately calculated after optimizing their geometry. Fig. 1a, 1b, and 1c displays

the optimized configurations of the Ag2O supercell, M1 and M2, respectively.

Fig. 1. Optimized supercells of a) Ag2O, b) Ag2O with Zn substituting an Ag, and c) Ag2O with

Zn placed in an interstitial position.

Equation (1) gave the formation energy for the model with the substitutional defect (M1), while

formula (2) was used to calculate the same for the interstitial defect system (M2).

(1)

(2)

Here is the defect formation energy, the energy of the M1,

and are the chemical potentials of Zn and Ag [17-18].

2.4. Photocatalytic Performance Measurements

The photocatalytic visible-light aerobic degradation of MO and RhB dyes was used to

assess the activity of the doped Ag2O samples. The reaction mixture of 2.4 ml aqueous solution

of MO (concentration 4.7 mg/L) and 0.23 mg photocatalyst were mixed in a standard quartz

cuvette of 1 cm path length. A few drops of 0.1 (M) HCl was added to reduce the pH of the

reaction mixture to ~3.0. In the case of RhB degradation 0.2 mg catalyst and 2.4 ml aqueous

solution of RhB (concentration 4.7 mg/L) was used. The cuvette was kept in a photocatalytic

chamber and irradiated with visible light from 14W Philips cool white LED bulb. The absorption

spectrum of the reaction mixture was measured after every two-minute interval. For the RhB

degradation, the same was done after every three minutes interval. The efficiency of all the

catalysts was compared estimating the turn over frequency (TOF).

3. Results and Discussion

Fig. 2a. Normalized HR-XRD patterns of pure Ag2O (A0) and different Zn doped Ag2O (A1, A2,

A3) samples.

Fig. 2b. Comparison of the Ag2O (111) peaks with the increase in Zn doping (from the HR-XRD

patterns).

3.1. Structural Properties

Fig. 2a shows high-resolution XRD spectra of undoped and Zn doped Ag2O nanoparticle

samples. All peaks in the XRD patterns of both doped and undoped Ag2O nanoparticle samples

match with the standard diffraction patterns of the FCC Ag2O (JCPDS Card No. 75-1532) phase.

These XRD patterns did not show any ZnO phase peaks. Fig. 2b compares the positions of peaks

corresponding to the (111) plane of various doped Ag2O samples. The diffraction peak for the

Ag2O (111) plane shifts slightly to the left, as Zn concentration increased from 1.25% to 5%. The

shift towards lower Bragg’s angle increases with the percentage of the Zn dopant in the Ag2O

sample. Thus, the change is maximum for 5% Zn doping and points towards the expansion of the

Ag2O lattice. When Zn goes into the Ag2O lattice, it can either go into an interstitial or a

substitutional position. Substitution of the larger ion by a smaller one will result in the

contraction of lattice, while the incorporation of a larger ion into a smaller interstitial void

expands the lattice [20]. Accordingly, the expansion of the lattice tells us that the dopant ions

occupy vacant interstitial positions in the Ag2O lattice.

Table 1 presents the change in the crystallite size and lattice parameters calculated from

the (111) plane XRD peak of doped and un-doped Ag2O powder. The average crystallite sizes of

the pure Ag2O and different Zn doped Ag2O samples were calculated using the Scherrer equation

from the broadening of Ag2O (111) facet diffraction peak. There are two unambiguous trends.

One that Ag2O lattice undergoes expansion with doping percentage. Fig. 3 displays the nature of

change in the lattice parameter with doping. If we consider only the doped samples (A1

onwards), then a linear relation following Vegard’s law may be invoked. Doping also results in a

decrease of the crystallite size. Reduction in crystallite size with doping is a frequently observed

phenomenon, due to an increase in lattice strain with defect concentration [8, 21].

Table 1 Crystallite sizes and lattice parameter values found from XRD patterns

Dopant concentrations(mol %)

Crystallite sizea (nm) Lattice parameter (Å)

0 86.8 4.7275

1.25 76.1 4.7278

2.5 73.4 4.7286

5 66.8 4.7305

Fig. 3. Variation in lattice parameter with Zn percentage.

3.2. DFT calculations

DFT calculations give additional evidence supporting the experimental findings. The

defect formation energy for substitutional and interstitial defect was found out to be 0.52 eV and

-0.96 eV, respectively, from the DFT calculations. Thus, interstitial doping is more favorable and

agrees with the experimental results. Table 2 presents the expansion in dimensions due to the

inclusion of a Zn in an interstitial position of the Ag2O supercell. The calculated parameters show

that the cubic Ag2O lattice expands in all three directions, but the increase is lesser in the ‘z’

direction.

Table 2 The change in supercell dimensions with Zn in an interstitial position,

Dimension Pure Ag2O After dopingX 9.620 9.678Y 9.620 9.678Z 9.620 9.647

3.3. TEM analysis

TEM images were used to analyze the morphology of the synthesized materials. The

representative images show the formation of approximately spherical nanoparticles in all

samples. TEM images of about 150 nanoparticles were sampled to calculate the particle size

distributions. Sample A1 displays the smallest average particle sizes (~14.9 nm). The average

particle size increases from 15.9 to 23.1 nm (Fig. 4d and 4f) with doping from 2.5 to 5%

(samples A2 and A3). Thus, the particles in sample A1 (with 1.25% doping) are much smaller

than those in A2 or A3.

Fig. 4. TEM images of Zn doped Ag2O samples a) A1, c) A2, and e) A3. Part b), d) and f) present

the particle size distributions corresponding to samples A1, A2, and A3, respectively.

The trend for change in particle sizes with doping is opposite to that found for crystallite

sizes, which decrease with doping. The mismatch between the average particle size obtained for

TEM images and XRD crystallite size is due to the following reason. The sample for TEM

imaging is prepared by re-dispersing the respective powder sample in an appropriate solvent by

sonication. Even after sonication, the larger nanoparticles in the sample tend to settle down. The

diluted supernatant of this sonicated suspension is taken to prepare the TEM sample on a carbon-

coated Cu grid. Since the liquid of the supernatant of the re-dispersed sample contains the

smaller nanoparticles, therefore, only these are seen in the TEM images.

3.4. Raman Analysis

Fig. 5. Comparison of the Raman spectrums of doped and un-doped Ag2O samples.

As mentioned earlier, doping increases the strain in the lattice that may disturb the crystal

symmetry. Such disruption also affects the vibrational modes of the crystal lattice. Thus, doping

may shift, broaden, and in some cases, may lead to the disappearance of some bands of the

Raman spectrum of the pure crystal. Fig. 5 presents the Raman spectra of the pure and Zn-doped

Ag2O samples. The Ag-lattice vibrational mode (i.e., phonons) causes the initial peak at 146 cm-

1. The next major peak is at 230 cm-1, which is due to Ag-O lattice vibration [22]. The peak shifts

increasingly to higher wavenumbers with dopant percentage, indicating that this is due to lattice

expansion as a consequence of Zn doping. Also, the shift to high wavenumber is characteristic of

p-type doping [23]. The Raman signals at 928 cm-1 and 971 cm-1 in the Ag2O samples are due to

chemisorbed atomic/molecular oxygen species. Other characteristic Ag-O stretching and bending

modes are at 1055 cm-1 and 1100 cm-1 [22]. The 1055 cm-1 peak also shifts to a higher

wavenumber value, but the change is more for sample A2. The peak movement is lesser for

sample A3. Moreover, the peaks at ~1100 cm-1 and higher wavenumbers disappear in Zn-doped

Ag2O samples. The presence of major Ag2O peaks and the absence of any new peak in all doped

samples indicates the incorporation of the dopant in the Ag2O crystal.

3.5. XPS analysis

XPS investigations were done to understand the chemical binding states in different

species in sample A0 and A3. Fig. 6 displays the high-resolution XPS spectra of Ag 3d and O1s

of Ag2O (A0) and Zn doped Ag2O (A3). In both doped and pure samples, spin-orbit coupling

splits the Ag 3d peak into Ag 3d5/2 and Ag 3d3/2 doublet. Fig. 6a shows that the deconvolution of

the Ag 3d5/2 region gives two peaks with binding energies (BE) 367.84 eV and 368.27 eV due to

Ag2O and Ag0, respectively. The Ag 3d3/2 region of the same plot exhibits peaks corresponding to

BE values 373.86 eV and 374.67 eV ascribed to Ag2O and Ag0, respectively.

Fig. 6. High resolution deconvoluted XPS spectra for (a,b) Ag 3d (samples A0 and A3) and (c,d)

O1s (samples A0 and A3).

The deconvolution of Ag 3d5/2 peak of the XPS of sample A3 displayed peaks at 367.30,

and 368.13 eV, while the Ag 3d3/2 gave peaks at 373.44 eV and 374.36 eV BE values. Compared

with pure Ag2O, the Ag 3d and O1s peaks of A3 shift toward lower BE values. It indicates an

increase in electron density on the surface of the material due to the incorporation of the dopant

in the semiconductor [25]. The shift towards lower BE values is characteristic of doping by an

element of lower electronegativity than Ag.

3.6. Bandgap analysis

Fig. 7. Tauc plots of prepared samples A1, A2, and A3.

Bandgap values are required to elucidate the mechanism of photocatalysis. Fig. 7 shows

the Tauc plots derived from corresponding UV-visible solid-state absorbance data of the doped

Ag2O nanomaterials. These plots were constructed using the following relation:

(4)

The intercept of the tangent to the linear part of the plot on the x-axis gives the bandgap energies

of these doped materials. Samples A1, A2, and A3 exhibit bandgap energies1.50, 1.64, and 1.65

eV, respectively. The bandgap increases with Zn doping and is significant compared to the

generally observed value of 1.2 (to 1.4) eV for pure Ag2O nanoparticles [26-27]. However, the

increase in bandgap from A2 to A3 is quite small.

3.7. Fluorescence spectra

Figure 8 displays the fluorescence spectra of the A0, A2, and A3 samples. The excitation

wavelength was 400 nm for all samples. The fluorescence intensities of doped materials (A2, A3)

were lower than pure Ag2O (A0). Moreover, the fluorescence intensity of A3 was lesser than A2.

Photoluminescence (PL) emission is directly associated with the recombination of photo-excited

holes and electrons. Hence, lower PL intensity demonstrates a delay in recombination and, thus,

better photocatalytic activity [24, 28-29]. It seems that doping Ag2O with Zn2+ ions causes the

formation of surface trapping sites due to the difference in metal ion charges. Such trap sites

selectively capture photo-excited electrons or holes and reduce photoluminescence intensity by

preventing recombination. Higher doping percentages induce more such trap sites that prevent

recombination.

Fig. 8. Photoluminescence spectra of samples A0, A2, and A3

3.8. Photocatalytic activity

The degradation of the typical water pollutants, methyl orange (MO) and Rhodamine-B

(RhB) under visible light irradiation was followed to evaluate the photocatalytic activity and

reusability of the prepared samples.

Fig. 9. a) Photocatalytic MO degradation of A3 b) Comparison of photocatalytic performance of

the different photocatalysts c) Recyclability of A3 d) Result of active species trapping

experiments for A3 catalyst in the presence of various scavenger molecules.

Fig. 9a displays the UV-visible absorbance spectra for aerobic MO degradation (in its

aqueous solution) and Fig. S2 (supporting information) shows the UV-visible absorption spectra

for aerobic RhB degradation in the presence of A3 photocatalyst. In the absence of catalysts,

there is no degradation of dyes. Supporting information in Fig. S1 indicates no degradation of

MO in the absence of catalysts. The photocatalyst degrades MO almost entirely in a short period

of ten minutes. Fig. 9b compares the photocatalytic performance of pure Ag2O and different

doped Ag2O photocatalysts. All considered samples degrade MO by zero-order kinetics. The rate

constant increases with doping, and photocatalyst A3 exhibits the best kinetics of degradation.

Surprisingly, MO degradation in the presence of pure Ag2O is faster than by samples A1 and A2.

Hence, the initial effect of doping is that it decreases their photocatalytic efficiency in

comparison to pure Ag2O. But on increasing the doping percentage to 5%, the photocatalytic

activity becomes better than pure Ag2O. The visible light degradation of RhB The trend is similar

for (Fig. S3 in supporting information). But in this case, the kinetics of RhB degradation is

nearly the same in the presence of A0 and A2, while it is again quickest for A3. Supporting

information in Table S1 and S2 gives details of the corresponding turnover frequency values and

rate constants for MO degradation and RhB degradation, respectively.

We carried out the active species trapping experiments using photocatalyst A3 to

understand the mechanism of photocatalytic reactions. Reactive species like OH., O2.- and h+ play

an essential role in the aerobic photocatalytic degradation of several organic pollutants. The

molecule p-benzoquinone (PBQ) was used to scavenge O2.- radical, potassium iodide (KI) was

the hole scavenger, and isopropyl alcohol (IPA) consumed the OH. radicals generated during

aerobic MO degradation reaction. Fig. 9d represents the results of the trapping experiments for

A3. The experimental results showed that the presence of KI inhibited the degradation of MO

more than other scavengers (Fig. 9d). It indicates that holes are the most generated reactive

species during the reaction. The superoxide scavenger, PBQ, also affects the photocatalytic

activity. Its effect is second only to KI. The order of activity of the reactive species for the

catalyst is h+ > O2.- > OH..

Visible light irradiation photo-excites the doped Ag2O, causing the generation of holes at

the VB and electrons at the CB. The main driving force for photocatalytic degradation of MO is

the production of holes (the reactive species). Thus, MO gets oxidized at the VB, resulting in the

transfer of its electrons to the photo-generated holes. Concurrently, the photo-excited electron

reduces the dissolved oxygen present in water to superoxide radicals, which then transform to

hydroxyl radicals (through well-known routes) to oxidize MO molecules. Since there is only

limited amount of oxygen in water, therefore, superoxide production does not dominate the MO

degradation process.

While the fluorescence spectrum intensity decreases with doping, the photocatalytic

activities for aerobic degradation of MO and RhB do not follow the same order. As mentioned

earlier, this implies that the surface trap sites increase with Zn2+ doping resulting in better charge

separation and, consequently, less intense fluorescence spectrums. Nevertheless, the

photocatalytic activity of undoped Ag2O (A0) is nearly the same as A2. Only A3 displays better

photocatalytic activity than A0. Photocatalytic activity is determined not only by charge

separation but also by the adsorption of the substrate on the catalyst surface. The present

photocatalytic reaction follows a photo-excited hole dominated substrate (MO or RhB) oxidation

mechanism. A vital step of such a mechanism is the adsorption of the substrate on to the surface

of the catalyst [30]. It seems that doping with Zn (up to 1.25-mole percent) weakens the surface

adsorption behavior of the catalyst towards these molecules. On further increase in doping, the

adsorption behavior becomes better than that of A0, and hence A3 displays the best aerobic

photocatalytic activity towards these dye molecules.

4. Conclusions

A hydrothermal procedure was used to dope Zn in Ag2O crystal lattice for widening the

bandgap of this semiconductor. The crystallite size of the samples decreased with an increase in

doping because of the disruption of lattice symmetry. XRD pattern analysis shows that the unit

cell parameters of Ag2O expand with Zn doping. DFT calculation results agree with this

conclusion. The formation energy of interstitially Zn-doped Ag2O is lesser than with Zn

substituting an Ag. DFT calculations also show Ag2O lattice expansion on interstitial Zn doping.

The bandgap of 5-mole percent Zn-doped Ag2O widens to 1.65 eV. Photoluminescence

spectroscopy points to increased charge separation with doping. The doped photocatalysts show

aerobic photocatalytic MO and RhB degradation. But, the photocatalytic activity exceeds that of

undoped Ag2O only after reaching the 5-mole percent doping level. The best-doped photocatalyst

also demonstrates excellent recyclability in photocatalytic efficiency. Reactive species trapping

experiments with individual scavenger molecules showed that the photocatalysis proceeds by a

photo-excited hole dominated mechanism. It appears that doping affects the substrate adsorption

behavior of the catalyst.

Acknowledgments

Arup Kumar De and Shaili Pal acknowledge financial support (SRF) received from IIT

(BHU). Authors thank Prof. Rajiv Prakash (School of Material Science and Technology) for the

measurement of solid-state UV-visible spectrum of the samples and Dr. Manoj Kumar

(Department of Chemical Engineering) for the fluorescence spectrum measurement of the

samples.

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Supporting Information

The rate constant is not a suitable parameter for comparing the photocatalytic activity of

two different materials when the order of the reaction is not the same. Also, the rate constant

calculation does not take into account the catalyst quantity. Turnover frequency (TOF) is better

for measuring the catalyst[44-46] activity. The following expression gives the calculation of TOF.

Turn Over Frequency ………………..(1)

Table S1. A comparison of turnover frequencies (TOF) of the visible light aerobic degradation of

MO on the prepared doped Ag2O photocatalysts.

Photocatalyst Light source The apparent rate

constant (min-1)

TOF (moles g-1min-1)

A0 (0 % dopant) Cool white LED

(0.1470 W/cm2,

visible range)

0.0763 2.54 x 10-5

A1 (1.25 % dopant) Same 0.0254 8.01 x 10-6

A2 (2.5 % dopant) Same 0.0488 1.60 x 10-5

A3 (5 % dopant) Same 0.1099 2.72 x 10-5

Table S2. A comparison of turnover frequencies (TOF) of the visible light aerobic degradation of

MO on the prepared doped Ag2O photocatalysts.

Photocatalyst Light source The apparent rate

constant (min-1)

TOF (moles g-1min-1)

A0 (0 % dopant) Cool white LED

(0.1470 W/cm2,

visible range)

0.0491 1.85 x 10-5

A1 (1.25 % dopant) Same 0.0244 5.68 x 10-6

A2 (2.5 % dopant) Same 0.0475 1.21 x 10-5

A3 (5 % dopant) Same 0.0635 1.99 x 10-5

Fig. S1 UV-visible of an aqueous solution of MO under cool white LED visible light irradiation

(without photocatalyst). In the absence of photocatalyst, there is no change in the intensity of the

MO UV-visible absorbance under light.

Fig. S2 Change in the UV-visible spectrum of RhB (showing its degradation) on visible light

irradiation with time in the presence of A3 photocatalyst sample

Fig. S3 RhB degradation kinetic plots for sample catalysts (A0, A1, A2, A3).

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