Cu-Ag Bimetal Alloy Decorated SiO2&at;TiO2Hybrid Photocatalyst for Enhanced H2Evolution and Phenol Oxidation under Visible Light

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Cu−Ag Bimetal Alloy Decorated SiO @TiO Hybrid Photocatalyst for

Enhanced H Evolution and Phenol Oxidation under Visible Light

Pradeepta Babu and Brundabana Naik*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c01325

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sı Supporting Information

ABSTRACT: With a broader objective to replace visible light driven Pt-

based photoelectrochemical/catalytic hydrogen evolution, a series of cost-

eﬀective bimetallic nanoalloys of Cu−Ag have been deposited on core−shell

nanostructured SiO @TiO through a facile reduction route. The

physicochemical properties, i.e. crystal structure, morphology, chemical

environment, and optical properties of Cu−Ag bimetal alloy decorated

SiO @TiO hybrid photocatalyst, have been thoroughly investigated through

X-ray diﬀraction, high resolution transmission electron microscopy, ﬁeld

emission scanning electron microscopy, X-ray photoelectron spectroscopy,

UV−vis diﬀuse reﬂectance spectroscopy, and photoluminescence spectros-

copy, respectively. TEM study conﬁrms the coating of an ultrathin layer of

TiO shell on 100 nm sized SiO core, and about 4.5 nm of Ag−Cu nanoalloys are uniformly distributed on the core−shell

nanostructure. The higher light absorption throughout the visible range and better separation of charge carrier by Ag−Cu (1:3)

deposited SiO @TiO hybrid compared to other counterparts is conﬁrmed from UV−vis, diﬀuse reﬂectance spectroscopy,

photoluminescence, and electrochemical impedance studies. Eightfold higher photocurrent enhancements, threefold enhanced

photocatalytic hydrogen generation, and twofold higher phenol oxidation activities of Ag−Cu (1:3) deposited SiO @TiO hybrid

compared to those of the monometallic plasmonic catalyst may be attributed to the synergetic eﬀect of enriched light harvesting and

surface plasmon induced hot electron transfer from the nanoalloy to the TiO interface, resulting in eﬃcient charge transfer.

. INTRODUCTION

metals with high dielectric constant and electromagnetic

radiation, can extend the light absorbance to even near IR

radiation and shows high promise. Moreover, surface plasmon

induced hot electrons/hot hole injection and regeneration in

enhanced photocatalytic hydrogen and oxygen evolution is

Generation of clean and renewable energy such as H fuel

replacing conventional fossil fuel will not only provide a

sustainable energy future but also will secure a clean

−3

environment.

Therefore, utilization of solar energy and

27−31

currently a hot topic.

Hot electrons are the electrons

water through photocatalytic/photoelectrochemical hydrogen

evolution reaction has gained huge attention in the last several

decades.

from Pt/TiO photocatalyst by Fujshima and Honda in 1972

furnished a ray of hope for researchers. However, the practical

application of solar powered systems is still not achievable due

to low quantum eﬃciency, poor photostability, and high cost

of the catalyst for economic viability.

generated from exposure of energetic irradiation, electrons,

ions, or exothermic reaction with energy of 1−3 eV and most

importantly thermodynamic nonequilibrium with the system

they initiated. Hot electrons induced by surface plasmon

resonance can be transferred either through a metal−

semiconductor junction (Schottky junction) according to

barrier of the junction and electronegativity of the counter

parts or through a metal−insulator−metal (M−I−M)

−8

The pioneering discovery of hydrogen generation

0,11

A cutting edge hybrid

nanocatalyst having large solar light harvesting ability and cost-

eﬀective alternative to the Pt-based catalyst needs to be

developed to fulﬁll the aforementioned properties.

32,33

tunnel.

Clavero highlighted the signiﬁcant roles of hot

electrons/holes in photocatalytic and photoelectrochemical

Despite having outstanding oxidation power, low cost, and

hydrogen evolution reactions. We recently reported the

high abundance, TiO -based photocatalysts exhibit limited

practical application owing to their wide band gap that is

restricted up to the UV range only.

modiﬁcations such as doping,

heterojunctions, and construction of plasmonic components

on the TiO matrix

surface plasmon induced hot electron mediated Ag/Ag PO @

12−15

Therefore, several

photosensitization,

6,17

18,19

Received: May 5, 2020

21,22

have been reported to enrich the light

absorbance to the visible region. Particularly, the plasmonic

3−26

photocatalyst,

having surface plasmon resonance that is a

coherent oscillation of conduction band electrons of some

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

SiO photocatalyst for visible light induced water splitting.

puriﬁcation. Deionized water obtained from a Millipore system was

used throughout the reactions.

However, the high cost of plasmonic noble metal model

catalysts like Pt and Au creates a major challenge in

constructing such types of prototypes. Therefore, a cost-

eﬀective Pt-free alternative nanoalloy hybrid catalyst is the

need of the time. Bimetallic nanoalloy-based catalysts are

promising owing to their developed functional properties due

to synergism. Bimetallic nanoalloys of low cost plasmonic

metals like Ag, Cu, Al, and Bi may be designed to construct a

hybrid nanocatalyst to obtain higher eﬃciency in the entire

.2. Synthesis of SiO @TiO . Silica nanoparticles were prepared

2 2

by a modiﬁed Stober method, and the mechanism of formation was

described in our previous articles. Speciﬁcally, a solution of

480 mL of absolute ethanol, 7.75 mL of 25% NH OH, and 20 mL of

35,48−50

deionized water was magnetically stirred in a 1000 mL round

bottomed ﬂask for 30 min. Twenty-ﬁve milliliters of TEOS was added

to the above solution, and the reaction proceeded at room

temperature for 24 h. Thereafter, the colloidal particles were separated

and collected by centrifugation at 10 000 rpm and subsequently

washed with absolute ethanol three times to remove undesirable

^p^a^r^tⁱ^c^l^e^s^.^T^h^e^a^s^-^p^r^e^p^a^r^e^d^Sⁱ^O2 nanoparticles were thoroughly

dispersed in a solution containing 3:1 volume ratio of ethanol and

acetonitrile with 4 mL of ammonia solution and marked as A. One

milliliter of TTIP was added to another beaker marked as B

8,40

solar spectrum.

In this regard, Patra et al. highlighted a

Ag−Au bimetallic nanoalloy for the visible light induced

enhanced water splitting to produce hydrogen. The AgAu

nanoalloy exhibited 3.3% quantum eﬃciency with 718 μmol/h·

H evolution and overall water splitting eﬃciency of 0.04%.

The higher activity is attributed toward hot electrons and

electron integration for charge transfer to the TiO surface.

Boosting of hot electron ﬂux and its light induced hydrogen

−

containing 3:1 volume ratio of ethanol and acetonitrile. After proper

dispersion, solution B was added dropwise to solution A and stirred

for 12 h. The colloidal particles were separated by centrifugation and

dried at 110 °C followed by calcination for 6 h at 600 °C. The

oxidation by Co and Pt bimetallic nanoalloy through metal

preformation of SiO particles and negative zeta potential of hydroxyl

oxide semiconductor junction was elucidated by Lee et al.

ions present on surface of SiO nanoparticles attracts the TTIP to

They highlighted the role of the CoO/Pt Schotky junction that

transfers the hot electrons, contributing higher catalytic activity

for PtCo nanoalloy. Naetu et al. investigated the photocatalytic

H evolution and CO reduction by Cu−Au nanoalloy and

coat titania only on the surface rather than driving to bulk.

2.3. Synthesis of Ag Deposited SiO

@TiO . The as-synthesized

00 mg of SiO @TiO was dispersed in 200 mL of DI water to which

0 mM solution of AgNO was added and stirred in an ice bath for 1

−

−1

h. An excess amount of 0.1 M NaBH was added to it and aged for 2

generated methane of 2000 μmolh g with 97% selectivity.

h, followed by centrifugation and drying. The as-synthesized samples

were then calcined at 400 °C for 2 h and labeled as STS.

They emphasized the synergetic eﬀect of Au for higher visible

light absorption and Cu for binding CO and directing the

reduction selectivity. Multiple exciton generation was

evidenced by Li et al. by the plasmonic Ag−Cu nanoalloy on

.4. Synthesis of Cu Deposited SiO @TiO . The as-synthesized

2 2

00 mg of SiO @TiO was dispersed in 200 mL of DI water to which

2 2

0 mM solution of Cu(NO ) was added and stirred in an ice bath for

visible light irradiation where Cu atom is doped inside Au

1 h. An excess amount of 0.1 M NaBH was added to it and aged for 2

cluster. Ag−Co nanoalloy fabricated through an in situ

h, followed by centrifugation and drying. The as-synthesized samples

were then calcined at 400 °C for 2 h and labeled as STC.

photoreduction route from natural sunlight by Kumar et al.

.5. Synthesis of Ag−Cu Deposited SiO @TiO . The as-

generated 63 mmol/h/g of H . Ag−Co nanoalloy acted as an

electron sink for lower recombination of photoinduced charge

carrier generated by the anatase TiO nanoparticle. Cu−Ni

bimetal integrated TiO thin ﬁlm catalyst exhibited hydrogen

2 2

synthesized SiO @TiO was dispersed in 200 mL of DI water. A 20

mM solution of AgNO and Cu(NO ) was added to the above

3 2

dispersion in diﬀerent ratios (i.e., 1:1, 1:3, and 3:1) and stirred in an

ice bath for 1 h. An excess amount of 0.1 M NaBH was added to it

yield in solar light 24 times higher than that of the particulate

and aged for 2 h, followed by centrifugation and drying. The as-

synthesized samples were then calcined at 400 °C for 2 h. For

comparison, Ag−Cu (3:1) deposited SiO @TiO hybrid catalyst was

catalyst. Pellarin et al. studied the chemical structure of

bimetallic Cu−Ag alloy nanoclusters of 5 nm size prepared

through a laser vaporization route dispersed on an alumina

matrix and their resulting plasmonic spectroscopy through

high-resolution transmission electron microscopy (HRTEM)

and absorption spectra. Despite a number of studies, the

mechanism of hot electron transfer by bimetallic nanoalloy at

the interface of oxides for solar light driven water splitting for

H generation remains elusive due to the transformation of

structure, oxidation state, and lack of detailed mechanistic in-

depth studies during photocatalytic reaction.

Herein, we designed a plasmonic cost-eﬀective bimetal alloy

of Cu−Ag deposited on a SiO @TiO hybrid photocatalyst

through a simple chemical reduction route for photocatalytic

designated as STSC-3:1, Ag−Cu (1:1) deposited SiO @TiO hybrid

catalyst as STSC-1:1, and Ag−Cu (1:3) deposited SiO @TiO hybrid

catalyst as STSC-1:3. The detailed synthesis procedure has been

demonstrated in the Scheme 1.

.6. Photocatalytic Water Splitting and Phenol Oxidation

Reaction. Photocatalytic water splitting for hydrogen evolution

reaction was carried out in a 100 mL sealed quartz batch reactor using

a 150 W xenon lamp (≥400 nm) as the excitation source. Normally,

Scheme 1. A Detailed Scheme Showing the Synthetic

Procedure of STSC Catalyst

water splitting to generate H fuel and photo-oxidation of

phenol for environmental pollution abatement. The mecha-

nism of hot electrons generated by the combined surface

plasmon eﬀect of the nanoalloy and transfer to the interface of

ultrathin TiO₂on the hybrid catalyst for enhanced H₂

evolution and phenol photodegradation activity is investigated.

. EXPERIMENTAL PROCEDURES

The ﬁrst step involves the preparation of SiO @TiO nanoparticles.

.1. Materials. Tetraethyl orthosilicate (TEOS), acetonitrile,

In the second step, Ag NPs and Cu NPs are deposited by chemical

reduction method and calcined at 400 °C. EtOH and acetonitrile act

as the solvent, and ammonia acts as the reagent for basic hydrolysis of

TTIP.

titanium isopropoxide (TTIP), silver nitrate (AgNO ), copper nitrate

Cu(NO ) ), absolute ethanol (EtOH), and ammonia solution (25%)

of analytical grade were purchased from Merck and used without

(

3 2

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Figure 1. (a) Powder X-ray diﬀraction showing peaks of TiO , Cu NPs, and Ag NPs in STSC-1:3. (b) HRTEM images of STSC-1:3 showing

uniform distribution of Ag−Cu on core−shell SiO @TiO . (c) HRTEM image of a single SiO core−shell structure. (d) Core−shell structure

showing uniform distribution of Cu−Ag alloy. (e) Lattice fringe of Ag−Cu. (f) SAED pattern of STSC-1:3.

0 mg of each catalyst was spread in 30 mL of 10 vol %

counter and reference electrodes, respectively, and a 300 W xenon

lamp was used as the excitation source. The linear sweep voltammetry

(LSV) was recorded by sweeping the potential from −1.0 to 1.0 V.

Electrochemical impedance spectroscopy (EIS) was performed in the

triethanolamine (TEOA) solution. The photocatalyst was dispersed

in the solution with a magnetic stirrer and was evacuated by nitrogen

purging. The measurement was done at 20 °C controlled by a water

jacket connected with the photocatalytic reactor. The evolved gas was

collected by downward displacement of water and was examined

using a gas chromatograph. For the photocatalytic phenol oxidation

reaction, 20 mg of each catalyst was dispersed in 20 mL (20 ppm) of

phenol solution and stirred in the dark for establishment of

adsorption−desorption equilibrium. The solution was exposed to

direct sunlight for 90 min and then centrifuged to remove the catalyst.

The concentration of phenol solution was estimated using a

spectrophotometer centered at 270 nm.

−2

dark (at 0 V) where frequency was set from 10 −10 Hz. A 0.1 M

Na SO aqueous solution was used as the electrolyte during the

electrochemical studies.

RESULTS AND DISCUSSION

^■4

.1. Structural Investigation. Crystal structure, phase,

and crystallite size of the as-prepared samples are determined

from powder X-ray diﬀraction. XRD pattern of STSC

photocatalysts exhibit tetragonal structure of TiO with space

group I41/amd. The diﬀraction peaks in Figure 1a observed at

. CHARACTERIZATION

X-ray diﬀraction patterns were collected on an X-ray diﬀractometer

RIGAKU MINIFLEX) from 2θ from 10° to 80° with a Cu Kα

θ = 25°, 38°, 48°, 53°, and 62° are indexed to plane (101),

(

103), (200), (105), and (213) of tetragonal TiO , respectively

JCPDS No. 78-2486). The highest intense peak observed at

irradiation source (l = 1.54 Å). Kratos Axis 165 instrument with a

monochromatic Mg Kα source was used to obtain the X-ray

photoelectron spectra (XPS) and calibrated with C 1s (284.8 eV)

as reference. HRTEM (JEOL-JEM 2010, Japan) was carried out for

the analysis of the morphology, crystal pattern, selected area electron

diﬀraction, and elemental composition of the samples. The UV−vis

diﬀuse reﬂectance spectroscopy spectra were measured by JASCO-

5° indicates the anatase phase of TiO . The peak broadening

^o^b^s^e^r^v^e^d^u^p^t^o²¹^°ⁱ^s^d^u^e^t^o^t^h^e^a^m^o^r^p^h^o^u^sⁿ^a^t^u^r^e^o^f^Sⁱ^O2

sphere used as the substrate. The loading of Ag NPs reveals

distinct peaks at 38°, 44°, 64°, and 77° ascribed to (111),

(

200), (220), and (311) planes of cubic Ag with space group

50 UV−vis spectrophotometer where boric acid was used as a

Fm3m (JCPDS No. 87-0717). The introduction of Cu NPs

shows peaks at 43.5° and 74° ascribed to (111) and (220)

benchmark. The JASCO-FP-8300 spectrophotometer was used to

obtain the photoluminescence spectra with an excited wavelength of

planes of cubic Cu, respectively (JCPDS No. 01-1241, space

30 nm. An electrochemical workstation (IVIUMnSTAT instrument)

was used to characterize the photoelectrochemical properties using a

conventional three electrode system. The electrophoretic deposition

method was taken into consideration to prepare the working

electrode. For the fabrication process, 20 mg of each photocatalyst

and iodine were spread in 20 mL of acetone and sonicated for 20 min.

Two parallel FTO electrodes were immersed in the solution, and 60 V

bias potential was applied for 5 min. The FTO electrodes were

calcined at 200 °C for 2 h to obtain the working electrode for

electrochemical investigation. Pt and Ag/AgCl electrodes were the

group Fm3m). Another peak is observed at 35° which is

attributed to the 002 plane of CuO (JCPDS No. 41-0254),

which may arise due to open atmosphere calcination of the

samples. The PXRD data suggest the successful formation of

alloy in STSC. The crystallite sizes of the catalysts were

calculated using Scherer’s equation and were 14.8, 15.0, 14.4,

15.3, and 14.8 nm for STS, STC, STSC-1:1, STSC-3:1, and

STSC-1:3, respectively.

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Figure 2. XPS spectra of (a) C 1s, (b) Si 2p, (c) O 1s, (d) Ti 2p, (e) Ag 3d, and (f) Cu 2p present in the STSC-1:3 sample.

TEM analysis was performed to ascertain the morphology

and intergrowth of primary nanocrystals aggregated to form

bimetal alloy of Cu−Ag in hybrid STSC-1:3 catalysts. Figure

shown in Figure S1c and suggests a narrow distribution with

average size of 4.5 nm with a standard deviation of 1 nm. The

average particle size of SiO @TiO and the Cu−Ag alloy was

b represents the TEM image of spherical SiO nanoparticles

measured using several TEM and FESEM images with ImageJ

software. The homogeneous spread over of Ag−Cu alloy with

sharp and narrow size distribution of particle size on the

surface of SiO @TiO plays a pivotal role in enhancing

of 100 nm coated with an ultrathin layer of TiO where 5 nm

of Cu−Ag nanoalloys are uniformly distributed throughout the

SiO @TiO surface. A single nanoparticle of SiO of 100 nm

with an ultrathin 5 nm TiO coating is clearly visible in Figure

catalytic activity. An ultrathin anatase TiO shell not only

c. Cu−Ag particles of 4.5 nm are seen in the high resolution

increases photostability but also transfers hot electrons

generated by bimetal alloy by giving a platform for Schottky

images of a single SiO @TiO particle in Figure 1d.

Furthermore, ﬁ eld-emission scanning electron microscopy

FESEM) (Figure S1a ) shows the distribution of uniform

SiO @TiO nanoparticles in STSC-1:3. The particle size

junction for photocatalytic H evolution. Figure 1e shows the

(

HRTEM image of a Ag−Cu nanoalloy distributed on the

surface of the hybrid catalyst, which conﬁrms the lattice fringe

having d-space of 0.23 nm, indicating a (111) plane of the alloy

and correlation with the XRD results. We observed only one

set of d spacing fringes, indicating the good formation of the

distribution curve (Figure S1b) reveals that the average size of

SiO @TiO spheres is 104 nm with a standard deviation of 17

nm. The particle size distribution of the Ag−Cu nanoalloy is

Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) UV−vis spectra and color photograph of as-synthesized photocatalysts. (b) Photoluminescence spectra of as-synthesized materials.

c) Linear sweep voltammetry curve under light illumination showing the current density. (d) Electrochemical impedance spectroscopy plot of as-

(

synthesized catalysts.

nanoalloy. The selected area electron diﬀraction (SAED)

pattern (Figure 1f) of the bimetal alloy hybrid catalyst taken

focusing on nanoalloy indicates polycrystalline nature and

reveals (111), (200), and (220) planes of diﬀraction concentric

rings of the alloy. The lattice fringe with a d value of 0.25 nm

be seen from the Figure 2(c), the O 1s peak was deconvoluted

to give three peaks at 532.6, 531.0, and 529.5 eV. The peak at

532.6 eV is attributed to the oxygen present in SiO₂

nanoparticles, whereas the peaks at 531.0 and 529.5 eV are

ascribed to Ti−O bonding and oxygen vacancy, respectively.

(Figure S 1d ) suggests the presence of copper oxide (CuO) in

In the case of Ti 2p spectra, two bands obtained at 458.7 and

STC. Further, the HRTEM image of STC (Figure S1e) shows

the formation of larger particles that can be attributed to CuO

and fewer Cu nanoparticles, whereas in the case of STS, Ag

nanoparticles were dispersed homogeneously (Figure S1f ).

The lower number of CuO particles (less concentration of

CuO) in STSC-1:3 as compared to STC is again favoring the

formation of bimetal alloy in STSC 1:3 catalyst. Figure S2

shows the spatial distribution of the constituent elements in

the material (EDX), and the weight percentage of Ag−Cu alloy

was 3 wt %.

464.3 eV correspond to the 2p₃_/₂and 2p₁_/₂states of Ti ,

respectively, in Figure 2d with an energy gap of 5.6 eV between

the two spin states. In the Figure 2e, two distinct peaks of Ag

3d at 368.2 and 374.2 eV are ascribed to 3d₅_/₂and 3d₃_/₂spin

states, respectively, with an energy gap of 6.0 eV which

corresponds to the Ag(0) state of Ag nanoparticles. Figure 2f

shows two distinct peaks of Cu at 932.6 and 952.2 eV

corresponding to 2p₃_/₂and 2p₁_/₂spin states, which on further

deconvolution demonstrates four peaks. The peak at 932.4 eV

is due to Cu , whereas the peak at 934.2 eV is ascribed to the

.2. Electronic Environment and Chemical State. The

Cu oxidation state, which may arise due to the formation of

56−,58

detailed electronic environment and oxidation state of the

hybrid photocatalyst were explored through X-ray photo-

CuO during open atmosphere calcination.

A satellite

peak observed at 942.2 eV which has binding energy 8 eV

electron spectroscopy. The C peak at 284.8 eV observed in

Figure 2a was taken as reference for calibration XPS analysis.

The CASA XPS software package was used for deconvolution

of the peaks to know the chemical states of the elements. The

core level spectra of Si 2p (Figure 2b) were deconvoluted to

two peaks at 103.7 and 102.1 eV representing the Si−O

bonding in SiO₂and Si−O−Ti linkages and partially

higher and is attributed to the charge transfer in d

conﬁguration for the ground state of CuO.

4.3. Optical Properties. UV−vis diﬀuse absorbance

spectra were taken to investigate the light absorbance

properties of the photocatalysts. From Figure 3a, it is clear

that all of the photocatalysts absorb visible light, and the

absorbance maximum increases with incorporation of Ag−Cu

alloy. From the ﬁgure, it is clear that STS showed an

absorption edge at 420 nm, whereas the absorption edge

shifted to 590 nm in the case of STC. The shifting of the

unsaturated Si−O features, respectively. Because the energy

gap between the two spin states 2p₃and 2p₁_/₂for Si is too

low, distinct peaks of the spin states are not observed. As can

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Figure 4. (a) Bar diagram showing percent of phenol oxidation. (b) C/C with time.(c) First order kinetic plot. (d) Amount of hydrogen evolution

by all as-synthesized photocatalysts.

absorption range from 400 to 600 nm upon Cu loading on

nm is due to oxygen vacancy. The lower emission peak of

STSC-1:3 as compared to its other photocatalysts suggests

lesser electron−hole pair recombination rate and higher charge

transfer, which is the most important and essential step in

enhancing the eﬃciency. Again, the higher quenching in the

PL spectra signiﬁes the strong electronic interaction between

TiO enhanced its visible light absorption ability. When the

Ag−Cu alloy was loaded on TiO , the absorption band edge

was 600 nm for both STSC-1:1 and STSC-3:1, as portrayed

from the ﬁgure. Interestingly, the absorption tail was shifted to

10 nm in the case of STSC-1:3, indicating the better light

absorption ability by the catalyst. The maximum absorption in

the case of STSC-1:3 is due to the synergistic eﬀect of Ag and

Cu as well as the presence of a higher fraction of Cu on the

alloy. Again, the SPR peak of Ag and Cu is not noticeable

distinctly, which supplements the formation of the alloy.

Hence, maximum light absorption ability by the STSC-1:3

catalyst is accredited to the LSPR eﬀect of Ag (390 nm) and

Cu (650 nm) and synergistic eﬀect between them when

present in alloy form. The present of a small amount of CuO is

also supplemental to the higher visible light absorption ability

by the STSC-1:3.

the Ag−Cu alloy and TiO in the case of STSC-1:3. So, the

introduction of the nanoalloy not only enriched the light

absorption but also eﬀectively separated the charge carriers and

photostability.

4.4. Photoelectrochemical Study. A conventional three

electrode system was used for carrying out the photo-

electrochemical investigation, taking 0.1 M Na SO as the

electrolytic solution. Under light irradiation, the photocatalyst

STS and STC could generate −0.20 and −0.26 mA current.

STSC-1:3 catalyst shows −1.62 mA current, whereas −1.07

and −0.60 mA current generation was shown by STSC-1:1 and

STSC-3:1, respectively (Figure 3c). From the results, it is clear

that the STSC-1:3 could generate current around eightfold

higher as compared to the monometallic counterparts. Again, it

could generate current at an onset potential of −0.86 V, which

is lower compared to others. The outstanding enhancement in

the photocurrent by the best performing catalyst STSC-1:3 is

attributed to the low particle size and uniform distribution of

To observe the recombination and charge carrier properties

of photogenerated electron−hole pairs, photoluminescence

analysis was performed for all the catalysts. As shown in Figure

b, the PL peaks show a similar trend in all of the

photocatalysts. The PL spectra contain strong emission band

with maximum at 420 nm, which is due to the self-trapped

exciton of the TiO octahedra. Moreover, it consists of

several small peaks, suggesting that the emission mechanism

follows a multistep process involving participation of several

energy states within the band gap. The peaks at 410 and 440

nm are attributed to the oxygen vacancy located below the

Ag−Cu on the TiO surface. The Schottky junction could

channel the hot electrons to the semiconductor surface,

enhancing the photocatalytic activity. Electrochemical impe-

dance spectroscopy (EIS) was performed to investigate the

behavior of photoinduced electron−hole pair and charge

transfer resistance. The radius of curvature in the Nyquist

conduction band and surface states of TiO in that order.

The peak at 480 nm is due to charge carrier transition, and 493

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Inorganic Chemistry

Article

9,60

plot

depicts the reaction rate on the electrode surface. From Figure

d, it is clear that the order of arc value for the catalyst was

plotted between the imaginary part Z′ and real part Z″

Table 1. First Order Fitting Data, Half-Life Period, and

Percent Photo-Oxidation of All Catalysts

catalyst

STS

STC

STSC-3:1

STSC-1:1

STSC-1:3

k_a_p_p(min⁻¹)

t₁_/₂(min)

% of oxidation

R²

STS > STC > STSC-3:1 > STSC-1:1 > STSC-1:3. The

impedance values are 103.7, 100.8, 87.4, 85.4, and 76.0 for

STS, STC, STSC-3:1, STSC-1:1,and STSC-1:3, in that order.

The smallest arc of STSC-1:3 reﬂects lower resistance and

higher interfacial charge transfer (lower R in the inset Figure

d), which facilitates the hot electron transfer to the CB of

0.0072

0.0083

0.0107

0.0154

0.0245

96.2

83.5

64.7

45.0

28.2

0.994

0.995

0.961

0.998

0.996

TiO where the proton reduced to hydrogen.

From the table, it is clear that the rate constant for phenol oxidation

in the case of STSC-1:3 is 3.4 times more than that of STS. The

improved rate constant value justiﬁes the importance of the plasmonic

catalyst toward mitigation of pollutants.

.5. Photocatalytic Activity. To assess the photocatalytic

activity of STS, STC, STSC-3:1, STSC-1:1, and STSC-1:3,

phenol oxidation was performed under sunlight. The photo-

catalytic oxidation of phenol was done by dispersing 20 mg of

each catalyst in phenol solution (20 ppm, 20 mL), which was

exposed to sunlight for 90 min. No change in the

concentration of phenol solution was noticed in the absence

of catalyst. To understand the role of sunlight, the solutions

were stirred in the presence of catalyst in the dark for 20 min,

and concentration of phenol remained unchanged. Stirring the

solution in the dark is useful for establishment of the

absorption−desorption process. From this, it is clear that

both catalyst and sunlight play a vital role in the oxidation of

phenol. In the ﬁrst 18 min of exposure to sunlight, 13 and 14%

oxidation was noticed for STS and STC, where STSC-1:3

could oxidize 36% of phenol in the same time. Phenol

oxidation in the case of STSC-1:3 is due to the participation of

and h , respectively, in the photocatalytic phenol oxidation

process. In the experimental procedure, 5 mM solution of each

trapping agent was added in 20 mL phenol solution of

concentration 20 ppm and was exposed to sunlight. From the

scavenger test (Figure S3 a ), it was clear that the rate of photo-

oxidation process decreases considerably in the presence of

isopropyl alcohol and methanol suggesting the active

participation of hydroxyl radical and hole. The hydroxyl

radical can convert phenol into a polyhydroxy aromatic

compound, and holes can oxidize them into carbon dioxide

and water. After the ejection of hot electrons to the CB of

TiO , the hot holes left in the alloy are also responsible for the

oxidation of phenol. To elucidate the stability of the catalyst, a

reusability test was conducted, and the results suggest that the

catalyst is stable for up to four repeated cycles (Figure S3b).

The excellent stability of the photocatalyst is due to the core−

shell nanostructure of the material and uniform distribution of

hot holes of the Ag−Cu alloy and holes of TiO . Participation

of hot holes and formation of a Schottky barrier between TiO

and Ag−Cu could suppress the electron hole recombination

and enhance the photocatalytic activity. STSC-1:3 could

oxidize 89% phenol, whereas 75 and 62% phenol oxidation

the alloy on the surface of TiO .

(

Figure 4a) was noticed in the cases of STSC-1:1 and STSC-

:1 in 90 min. These results clearly indicate that the loading of

Photocatalytic hydrogen generation reactions were per-

formed for the as-synthesized photocatalyst by using a 10 vol%

TEOA solution (30 mL) and exposure to visible light (λ ≥ 420

nm). No hydrogen generation was noticed in the absence of

light and catalyst, which relates the pivotal role played by the

Ag−Cu could display superior catalytic activity as compared to

others in the series. Moreover the synergistic eﬀect between Ag

and Cu in the alloy is responsible for the excellent catalytic

activity by the alloy. C/C vs time graph portrayed (Figure 4b)

the photo-oxidation of phenol. The equation for the

calculation is:

catalyst and light for the water splitting reaction. SiO

@TiO

2 2

was not able to generate hydrogen in visible light due to its

−

1 −1

high band gap, whereas 520 and 550 μmolh g hydrogen

production was noticed in the case of STS and STC. The

hydrogen generation by these catalysts was attributed to the

SPR phenomena due to Ag and Cu. Interestingly, STSC-3:1,

STSC-1:1, and STSC-1:3 could produce 920, 1100, and 1560

C − C

C₀

of phenol oxidation =

× 100

where C and C represent the concentration of phenol solution

−1 −1

μmolh g of hydrogen (Figure 4d). The high value of

(

g/L) at time t = 0 min and at a particular time t min in that

hydrogen generation in the case of STSC-1:3 is attributed to

the synergistic eﬀect of Ag and Cu in the Ag−Cu alloy as well

as the SPR eﬀect, which tends to increase visible light

absorption and lead to better electron hole separation.

Moreover, the quantum eﬃciency of all of the catalysts was

estimated by using the following equation and found to be

order. Kinetics of the reaction was measured by using the

following equation:

C₀

−

⁼^kapp

.97, 1.02, 1.72, 2.05, and 2.91 for STS, STC, STSC-3:1,

where k_a_p_prepresents the apparent rate constant for the phenol

oxidation reaction and were calculated from the slope of −lnC/

C with exposure time (Figure 4c). The calculated k values

for all catalysts follow pseudo-ﬁrst-order kinetics based on the

Langmuir−Hinshelwood model. The k_a_p_p, half-life period of

phenol oxidation, and percent phenol oxidation are tabulated

in Table 1.

STSC-1:1, and STSC-1:3, respectively.

app

2 × number of H molecules

number of photons adsorbed

AQY(%) =

× 100

35,60

To know the role of active species

involvement, 5 mM

of each reagent (p-benzoquinone, isopropyl alcohol, dimethyl

sulfoxide, and methanol for O , OH, e , and h in that

order, respectively) was added to the photocatalytic reactor,

and experiments were performed. The result shows (Figure

S3c ) the active participation of electrons during the photo-

catalytic hydrogen reaction as the amount of hydrogen

•

−

•

−

To explore the role of reactive species and accomplish the

probable mechanism for oxidation of phenol, scavenger tests

35,60

were performed. The scavenger tests

were done by taking

reagents such as p-benzoquinone, isopropyl alcohol, dimethyl

•

−

•

−

sulfoxide, and methanol to elucidate the role of O , OH, e ,

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 2. (a) Plausible Mechanism for Photocatalytic Hydrogen Evolution and Phenol Oxidation by STSC-1:3 and (b)

Energy Band Diagram of STSC-1:3

E , E , χ, CB, VB, ϕ , and ϕ represent the Fermi energy level, vacuum energy level, electron aﬃnity, conduction band and valence band of

AgCu

vac

TiO , Schottky barrier height, and work function of Ag−Cu.

generation by STSC-1:3 remains almost equal in the presence

of other quenching agents. The reusability study was very

essential to understand the catalyst stability. Therefore, a

reusability test (Figure S3d) was performed four times for the

hydrogen generation reaction. The result suggests the excellent

stability of STSC-1:3 toward visible light driven hydrogen

production. Further the morphology and crystal structure

remain intact after reusability test, which is con ﬁ rmed from

PXRD (Figure S4a) and TEM images (Figure S4b) of used

STSC-1:3.

The excellent photocatalytic activity of STSC-1:3 toward

hydrogen generation and phenol oxidation is ascribed to the

following reasons. The catalyst could absorb more visible light

as compared to the rest of the as-synthesized materials,

revealed by UV−vis spectra and the lower photogenerated

electron−hole pair recombination rate, as suggested by PL

spectra. The lower arc in the Nyquist plot indicates the lower

interfacial charge transfer resistance in the catalyst, thereby

increasing the electron transfer process in the semiconductor−

electrolyte interface. The fast electron transfer process is again

supported by the excellent enhancement in the photocurrent

values in the case of STSC-1:3, which is eight times higher

compared to its monometallic counterparts. Photocurrent

generation at lower onset potential supplements further the

best catalytic property of the catalyst.

and STC, the Schottky barrier heights were estimated to be

.7 and 0.2 eV, respectively. The heights were calculated by the

diﬀerence between the work function of Ag (ϕ = 4.0 eV) and

Cu (ϕ = 4.0 eV) with the electron aﬃnity of TiO (χ = 4.7

eV). The larger value of the Schottky barrier height in the case

of STS suggests the lower photocatalytic activity by the

material as the hot electron requires more energy to pin into

the conduction band of TiO . Similarly, in the case of STC, the

height was 0.2 eV, suggesting ease of pinning of the electron to

the conduction band. The heights were borderline between Ag

and Cu in the case of the Ag−Cu alloy, suggesting better

pinning of the electron in the case of the alloy as compared to

the monometallic counterparts. The best photocatalytic

activity in the case of STSC-1:3 is attributed to the appropriate

Schottky barrier height. Upon light illumination, hot electrons

were generated by Ag−Cu and could pin into the conduction

band of TiO , leaving hot holes. The hot electrons were used

in the proton reduction reaction to produce hydrogen in the

conduction band of TiO . The hot holes and holes of TiO

could oxidize phenol at a faster rate due to the participation of

hot holes of the Ag−Cu alloy. In the alloy nanostructure,

electronegativity of each counterpart plays a vital role for the

reduction reaction. Because Cu is more electronegative than

Ag, electron density on Cu will be in the order STSC-1:3 >

STSC-1:1 > STSC-3:1. The presence of increased electron

density on Cu in the case of STSC-1:3 again supplements the

excellent hydrogen generation by the catalyst. The decrease in

the photocatalytic activity in the case of STSC-3:1 is due to the

presence of more Ag, which makes pinning of hot electrons to

the conduction band of TiO₂diﬃcult by increasing the

Schottky barrier height. The photocatalytic activity values

suggest that alloy containing an appropriate amount of Ag and

Cu mixed homogeneously can tune the activity to a remarkable

extent. The molecular mechanism of both oxidation and

reduction reaction on the surface of STSC-1:3 is as follows:

Therefore, a plausible mechanism can be envisaged for the

excellent photocatalytic activity shown by STSC-1:3 and

depicted in Scheme 2. Upon light illumination, the semi-

conductor absorbs light energy to generate photoexcited

electron−hole pairs. As the band structure of a semiconductor

is an important parameter to behave as photocatalyst, the

conduction band edge potential of TiO was calculated using

E_V_B= X − E + 0.5 E , E = E_V_B− E . E is the band gap

energy of TiO , X is the geometric mean of the electro-

negativity of Ti and O, and E is the energy of free electrons

(

4.5 eV). The conduction band edge potential was estimated

to be −0.7 eV, which relates the tendency of proton reduction

by TiO to produce hydrogen. Upon light illumination, the

valence electron of TiO is excited to the conduction band,

where the proton gets reduced to hydrogen. In the case of STS

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(3) Babu, P.; Mohanty, S.; Naik, B.; Parida, K. Synergistic Effects of

Article

ACKNOWLEDGMENTS

■

Both authors are thankful to Siksha ‘O’ Anusandhan,

Bhubaneswar. B.N. is grateful to SERB for Grant ECR/

016/000602. P.B. gratefully acknowledges CSIR (Council of

Scientiﬁc and Industrial Research) for awarding him SRF

(

Senior Research Fellowship).

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