Nano Research - Towards full-spectrum photocatalysis ...Nano Res 1 Towards full-spectrum photocatalysis: Achieving Z scheme between Ag 2 S and TiO 2 by engineering energy band alignment

Nano Res

1

Towards full-spectrum photocatalysis: Achieving Z

scheme between Ag2S and TiO2 by engineering energy

band alignment with interfacial Ag

Yanrui Li, Leilei Li, Yunqi Gong, Song Bai, Huanxin Ju, Chengming Wang, Qian Xu, Junfa Zhu, Jun Jiang

(), and Yujie Xiong ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0862-3

http://www.thenanoresearch.com on July 16, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0862-3

TABLE OF CONTENTS (TOC)

Towards full-spectrum photocatalysis: achieving Z



Yanrui Li, Leilei Li, Yunqi Gong, Song Bai, Huanxin Ju,

Chengming Wang, Qian Xu, Junfa Zhu, Jun Jiang,* and

Yujie Xiong*

Hefei National Laboratory for Physical Sciences at the

Microscale, iChEM (Collaborative

Innovation Center of Chemistry for Energy Materials),

School of Chemistry and Materials Science, and National

Synchrotron Radiation Laboratory, University of Science

and Technology of China, Hefei, Anhui 230026, P. R.

China

A design has been proposed that interfacial Ag, which can engineer

semiconductor band alignment, enables constituting Z scheme between

wide-bandgap TiO2 and narrow-bandgap Ag2S. To fulfill the design,

a unique approach has been developed to synthesize Ag2S-Ag-TiO2

hybrid structures. The ternary hybrid structures exhibit dramatically

enhanced performance in photocatalytic hydrogen production under

full-spectrum light illumination.

Provide the authors’ webside if possible.

Yujie Xiong, http://staff.ustc.edu.cn/~yjxiong/






Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

photocatalysis • Z scheme

• semiconductor • band

structure • water splitting

ABSTRACT

Z scheme is a promising approach to achieve broad-spectrum photocatalysis.

Integration of TiO2 with another semiconductor at ~1.0 eV bandgap would be

ideal to harvest both ultraviolet and visible-near infrared light for

photocatalysis; however, most narrow-bandgap semiconductors have

straddling band structure alignments with TiO2, constituting the obstacle to

form the Z scheme for photocatalytic hydrogen production. In this

communication, we demonstrate by employing Ag2S as a model system that the

energy band upshift of the narrow-bandgap semiconductor by interfacing with

a metal can overcome this limitation. To fulfill the design, we have developed

a unique approach to synthesize Ag2S-Ag-TiO2 hybrid structures. The

obtained ternary hybrid structures exhibit dramatically enhanced performance

in photocatalytic hydrogen production under full-spectrum light illumination.

The activities are significantly higher than the sum of those by the

Ag2S-Ag-TiO2 structures under λ < 400 nm and λ > 400 nm irradiation as well as

those by their counterparts under any light illumination conditions.

1. Introduction

Light absorption is a key factor to the development of

a photocatalytic system.[1] To maximize the

number of photo-induced electron-hole pairs,

developing the catalysts that can harvest broad

spectrum of solar energy is the main pursuit for

researchers to optimize the charge-generation step of

photocatalysis. Given that most semiconductors

offering good photocatalysis performance primarily

absorb the ultraviolet (UV) photons which only

account for ca. 5% of solar spectrum, it is imperative

to implement various heterojunction designs such as

semiconductor-semiconductor junctions to extend

the spectral range for light absorption. Ideally the

two semiconductors are anticipated to

complementarily absorb UV and visible (vis)-near

infrared (NIR) light, respectively, thereby covering

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Address correspondence to Jun Jiang, [email protected]; Yujie Xiong, [email protected]

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2 Nano Res.

the broad solar spectrum.

To this end, Z scheme is a system that has been

widely used by mimicking the natural

photosynthesis system since 1997.[2-6] The

Z-scheme system is typically formed between two

semiconductors with staggered energy band

alignment (i.e., Type II semiconductor

heterojunction), in which the holes on reduction sites

(e.g., the H2 evolution in water splitting) and the

electrons on oxidation sites (e.g., the O2 evolution in

water splitting) should be consumed electron

acceptor/donor pairs.[6] As such, the electrons and

holes remained at the reduction and oxidation sites

can be supplied to H2 and O2 evolution, respectively.

In recent years, the all-solid-state Z scheme, where

the holes and electrons aforementioned can be

recombined and thus consumed through interfacial

Ohmic contact instead, has become the major

trend.[7-9] Alternatively, in a typical demonstration

of CdS-Au-TiO2 system,[10] the two semiconductors

have been synergized by adding an interfacial metal

layer for TiO2AuCdS electron transfer so that the

TiO2 and CdS are designated as oxidation and

reduction sites. The CdS is a semiconductor that

can absorb visible light, achieving complementary

light harvesting with UV-excited TiO2.

To further broaden the light absorption

spectrum towards NIR, it would be ideal to identify a

semiconductor with narrower bandgap (<1.5 eV) to

replace CdS in this design. Unfortunately, most

narrow-bandgap semiconductors have straddling

energy band alignments (i.e., Type I semiconductor

heterojunctions) with wide-bandgap semiconductors

such as TiO2,[11,12] constituting the obstacle to form

Z scheme. In this communication, we demonstrate

that the energy band upshift of narrow-bandgap

semiconductor by interfacing with a metal enables

the formation of Type II band alignment with TiO2,

thereby overcoming this limitation.

In this work, we use TiO2 and Ag2S as model

systems to demonstrate our concept. TiO2 is known

as a n-type semiconductor with 3.2-eV bandgap that

can offer high photocatalytic activities in UV region,

while the n-type semiconductor Ag2S can absorb both

visible and NIR light according to its 1.0-eV

bandgap.[11-14] However, the TiO2 and Ag2S have

a straddling alignment of band structures,[11]

constituting the obstacle to form the Z scheme. To

overcome this limitation, we have proposed a design

that interfacial Ag, which can upshift the energy

band of Ag2S, enables constituting Z scheme between

TiO2 and Ag2S. The ternary Ag2S-Ag-TiO2 hybrid

structures exhibit dramatically enhanced

performance in photocatalytic hydrogen production

under full-spectrum light illumination.

2. Experimental

2.1 Growth of TiO2 on Ag nanocubes: The

synthesis of Ag nanocubes followed our previously

established protocol.[15] The growth of TiO2

nanocrystal shells on the Ag nanocubes was achieved

through a hydrothermal process by modifying the

protocol in literature.[16] In a typical process,

1.6-mg Ag nanocubes were mixed with 8-mL TiF4

aqueous solution (0.02 mM, Sigma Aldrich,

333239-100g). The mixture was kept stirring for 30

min, diluted to 32 mL with deionized water, and

transferred into a 50-mL Teflon-lined stainless steel

autoclave that was subsequently maintained at

135 °C for 45 min. Finally, the product was cooled

to room temperature, centrifuged, and washed with

deionized water several times.

2.2 Sulfidation of Ag-TiO2 core-shell structures into

Ag2S-(Ag)-TiO2 hybrid structures: The sulfidation of

Ag cores into Ag2S was performed using Na2S

aqueous solution at room temperature. 5-mg

Ag-TiO2 core-shell structures were mixed with 20-mL

Na2S aqueous solution (10 M, Aladdin, S116078-25g)

under continuous magnetic stirring for 12 h at room

temperature. The product was centrifuged and

washed with deionized water several times.

2.3 Photoelectrochemical measurements. The

as-synthesized products containing 4.0-mg

Ag2S-(Ag)-TiO2 hybrid structures were dispersed in 1

mL of ethanol, which were then uniformly

spin-dropped onto a 2.5 cm × 2.5 cm indium tin oxide

(ITO)-coated glass by a spin coater (SC-1B, China).

In the measurements for bare Ag2S and bare TiO2,

their weights were kept the same as the Ag2S (0.554

mg) and TiO2 (3.446 mg) in the hybrid structures,

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

respectively. The weights of Ag2S and TiO2 were

determined by inductively-coupled plasma mass

spectrometry (ICP-MS) through the measurements of

Ag and Ti concentrations. Subsequently, the

ITO-coated glass was heated at 80 °C in a vacuum

oven for 1 h. The photocurrents were measured on

a CHI 660D electrochemical station (Shanghai

Chenhua, China) in ambient conditions under

irradiation of a 300-W Xe lamp (Solaredge 700, China)

in the presence or absence of 400-nm cutoff filter

(short-wave-pass, λ< 400 nm, or long-wave-pass, λ>

400 nm), respectively. The power densities were

measured to be 30 mW cm-2 and 100 mW cm-2 for λ<

400 nm and λ> 400 nm, respectively. Standard

three-electrode setup was used with the ITO-coated

glass as photoelectrode, a Pt foil as counter electrode,

and a Ag/AgCl electrode as reference electrode. The

three electrodes were inserted in a quartz cell filled

with 0.5-M Na2SO4 electrolyte. The Na2SO4

electrolyte was purged with Ar for 30 min prior to

the measurements. The photoresponse of the

prepared photoelectrodes (i.e., I-t) was operated by

measuring the photocurrent densities under chopped

light irradiation (light on/off cycles: 10 s) at a bias

potential of 0.6 V vs. Ag/AgCl for 400 s.

0.5-M Na2SO4 aqueous solution was used as

electrolyte for transient open-circuit voltage decay

(OCVD) (i.e., photovoltage-time) curves. The

average lifetimes of the photogenerated carriers (τn)

were obtained from the OCVD according to Equation

(1): 1

ocBn

dt

dV

q

Tk

(1)

where kB is the Boltzmann constant, T is the

temperature (in Kelvin), and q is the unsigned charge

of an electron. Mott-Schottky plots were taken with

an applied potential ranging from 1.0 to -0.5 V (vs.

Ag/AgCl) at frequency of 5000 Hz.

2.4 Photocatalytic hydrogen production

measurements. To investigate the photocatalytic

activities of Ag2S-(Ag)-TiO2 catalyst for hydrogen

generation, Na2S and Na2SO3 were used to sacrifice

the holes. 30 mg of photocatalysts were added to

25-mL aqueous solution of Na2S (0.25 M) and Na2SO3

(0.35 M). In the measurements for bare Ag2S and

bare TiO2, their weights were kept the same as the

Ag2S (4.155 mg) and TiO2 (25.845 mg) in the hybrid

structures, respectively. The samples were

sonicated to form uniform suspension, followed by

saturation with Ar to eliminate air. The

light-irradiation experiment was performed by using

a 300-W Xe lamp (Solaredge 700, China). The light

source and power intensity were the same as those

for photocurrent measurements. The photocatalytic

reaction was typically performed for 4 h. The

amount of H2 evolved was determined using gas

chromatography (GC, 7890A, TCD, Ar carrier,

Agilent). Three replicates were collected for each

sample with relative error < 10%.

3. Results and discussion

As indicated by the first-principles simulations [17]

(Figure 1a and S1, consistent with the literature

values [11]), the use of TiO2 and Ag2S in Z scheme is

mainly limited by two facts: (1) their band

structures are of a straddling alignment; (2) the

conduction band edge of Ag2S is close to the energy

level of H2 evolution, so it can barely offer the

capability of producing H2 given the reaction

overpotential.[12-14]

Figure 1 (a) Computed energy band structures of bulk TiO2 and

bulk Ag2S materials, in alignment with the oxidation and

reduction potential levels for water splitting reactions. (b)

Schematic illustration depicting the flow of electrons from Ag

with higher Fermi level (i.e., lower work function) to

semiconductor Ag2S with lower Fermi level (i.e., higher work

function) at the Ag-Ag2S interface, which causes energy band

bending and upshift in Ag2S. The energy levels are obtained

from the simulations in Figure S1-S5. (c) Computed differential

charge distribution at Ag-Ag2S interface, showing that Ag in each

supercell donates about 1.33 electrons to Ag2S. (d) Schematic

illustration depicting the formation of Z-scheme band structure


4 Nano Res.

between the well-coupled Ag-Ag2S and TiO2. The energy level

of H2 evolution is set to zero.

We have thus decided to adopt a different

approach to circumvent such an undesired situation

by integrating an interfacial metal layer between

TiO2 and Ag2S (Figure 1b). To examine the

interaction of metallic Ag with Ag2S, we have

simulated the partial projected density of states

(PDOS) diagrams (Figure S2), which show strong

hybridizations at the Ag-Ag2S interface (refer to

Supporting Information for their interface structure

optimization). In particular, the strongly coupled

electronic states inside the bandgap of bare Ag2S

indicate the high probability of charge transfer

between Ag and Ag2S. Figure S3 shows that the

work functions of Ag and Ag2S at their interface are

4.18 and 5.64 eV, respectively. According to the

potential alignments, free electrons in Ag may be

transferred to Ag2S when they are in contact. This

assumption is further validated by the computed

differential charge distribution (Figure 1c), which

shows that Ag in each supercell donates about 1.33

electrons to Ag2S. Consequently, the Ag-Ag2S

interfacing causes a ~0.47 eV upshift of Ag2S Fermi

level (and energy bands), as revealed by the

computed potential surfaces in Figure S4.

Depending on surface structures, the Ag-induced

energy band upshift varies from 0.29 to 0.47 eV

(Figure S5 and Table S1). On the other hand, the

electronic structure coupling of TiO2 with Ag is

relatively weak around the Ag Fermi level (Figure

S6). As a result, free electrons in Ag can hardly

migrate to the TiO2 in the absence of external field,

and thus the band structure of TiO2 cannot be

altered by the Ag-TiO2 interface. Taken together,

the Ag2S energy band upshift enables the formation

of Z scheme in the Ag2S-Ag-TiO2 ternary system

(Figure 1d). Under full-spectrum light

illumination, photoexcited electrons in TiO2 will

flow into Ag at lower energy level, and the electrons

at Ag migrate towards the Ag-Ag2S interface at

which they then relax and recombine with the

photo-induced hole carriers in Ag2S. This process

results in the accumulation of photo-induced

electrons and holes at the valence band of TiO2 and

the conduction band of Ag2S, respectively, which

are well separated in both the space and energy

domain. Meanwhile, the upshifted conduction band

of Ag2S ensures high potential energy for

photoexcited electrons to fulfill H2 evolution

reaction.

Figure 2 (a) Schematic illustration for the synthesis of

Ag2S-(Ag)-TiO2 hybrid structures. (b) TEM and (c) HRTEM

images of Ag2S-(Ag)-TiO2 hybrid structures. The inset of panel

c shows the lattice fringes of a TiO2 shell marked in the box. (d)

EDS mapping profiles of a single Ag2S-(Ag)-TiO2 hybrid

structure showing the elemental Ti, O, Ag and S.

To prove this concept, we have developed a

unique approach to synthesize the Ag2S-Ag-TiO2

hybrid structures (Figure 2a). According to the

simulations, one can recognize that the Ag layer in

the Ag2S-Ag-TiO2 hybrid structures should be thin

enough. Otherwise, the electrons transferred to Ag

will relax to too low energy level in the Ag layer and

can hardly reach the Ag-Ag2S interface to recombine

with the holes in Ag2S.[10] The deposition of such a

thin layer generally requires expensive atomic layer

deposition (ALD) technique; however, the ALD can


5 Nano Res.

barely achieve the formation of hybrid structures in

three dimensions at large scale. For this reason, we

have to develop a new strategy for the synthesis of

such hybrid structures. Our synthesis consists of

two steps: (1) growth of TiO2 on Ag nanocubes by

taking advantage of strong Ag-O bonding; (2)

sulfidation of Ag nanocubes into Ag2S. It has been

reported that phase separation of Ag from Ag2S may

take place in the sulfidation and redox syntheses of

Ag/Ag2S nanoparticles.[18,19] In our case, a trace

amount of Ag will be separated from Ag2S and

retained on the Ag2S surface owing to the strong

binding of Ag to the O atoms in TiO2, forming the

Ag2S-Ag-TiO2 interface.

We start the synthesis with 40-nm Ag nanocubes

enclosed by {100} facets,[15] and then encapsulate the

nanocubes with TiO2 nanocrystal shells by hydrolyze

TiF4.[16] As revealed by transmission electron

microscopy (TEM, Figure S7), the Ag nanocubes

become truncated after the hydrolysis of TiF4 while

the resulted TiO2 shells are composed of TiO2

nanocrystals via a flower-like pattern. Figure S8

shows an X-ray diffraction (XRD) pattern of the

Ag-TiO2 core-shell structures, in which all the

diffraction peaks can be assigned to face-centered

cubic (fcc) Ag (JCPDS 65-2871) and anatase TiO2

(JCPDS 83-2243).

Upon achieving the synthesis of core-shell

structures, we further perform a sulfidation on the

Ag cores using an aqueous Na2S solution. As

shown in Figure 2b, the resulted nanostructures

inherit the morphology and core-shell structures

from their Ag-TiO2 precursors. High-resolution

TEM (HRTEM) image (Figure 2c) shows that the

shells are composed of TiO2 nanocrystal branches

with lattice spacings of 0.19 nm corresponding to the

(200) facet of anatase TiO2. The sample is further

characterized by energy-dispersive spectroscopy

(EDS) mapping analyses (Figure 2d), confirming that

the shells and cores are of TiO2 and Ag2S, respectively.

In particular, we notice that Ag element is more

concentrated near the Ag2S-TiO2 interface as

compared with S, implying the possibility of

elemental Ag at the interface. Figure S8 shows an

EDS line mapping profile of the sample, which more

clearly demonstrates the enrichment of Ag at the

interface.

This feature is also observed on the XRD pattern

(Figure S9). The diffraction peaks can be indexed to

monoclinic Ag2S (JCPDS 14-0072) and anatase TiO2

(JCPDS 83-2243). It is worth noting that the

diffraction peak at 37.6-38.7 is asymmetric, which

can be split into two peaks. The split peak centered

at 37.8 can be assigned to TiO2(004) (37.81) and

Ag2S(-103) (37.72), while the one located at 38.1

corresponds to Ag(111) (38.12). It indicates that the

hybrid structures (namely, Ag2S-(Ag)-TiO2) may

contain a small portion of Ag.

To prove this possibility, we have collected X-ray

photoelectron spectra (XPS) on the sample. The Ag

3d3/2 and 3d5/2 binding energies at 373.1 eV and

367.1 eV in Figure S10a are characteristic values for

AgI, respectively, showing that the majority of Ag in

the sample is in the state of +1 corresponding to

Ag2S.[20,21] Meanwhile, the XPS also identifies the

Ag0 characteristics in the sample – the Ag 3d3/2 and

3d5/2 binding energies at 374.1 eV and 368.1 eV

with lower intensities, respectively. A more direct

evidence for the presence of elemental Ag is the Ag

M4VV kinetic energy at 358.2 eV, the characteristic

value for Ag0, which can be well separated from the

kinetic energy at 351.2 eV for AgI (see Figure

S10b).[20] It demonstrates that a trace amount of

elemental Ag is involved in the Ag2S-(Ag)-TiO2

hybrid structures indeed.

It is worth mentioning that TiO2 cannot be

uniformly grown on the Ag2S surface (Figure S11).

Thus our synthetic method, in which TiO2 is coated

on Ag nanocubes followed by Ag sulfidation into

Ag2S, is necessary to the formation of Ag2S-(Ag)-TiO2

hybrid structures. We believe that the strong

binding of Ag to the O atoms in TiO2 makes the trace

elemental Ag left at the interface. In the absence of

TiO2 coating, the sulfidation can completely take

place on Ag nanocubes.[22] Although a single Ag-O

layer may exist next to the TiO2 shell, the majority of

the interfacial material between the TiO2 and Ag2S is

the elemental Ag. As a matter of fact, the Ag-Ag2S

interface is indispensable for the formation of Z

scheme. According to the simulations (Figure S12),

the alignment of Ag2O and Ag2S Fermi levels is

unlikely to drive free electrons to flow from Ag2O to

Ag2S. For this reason, if the Ag2S in our hybrid

structures was directly interfaced with Ag2O in the

absence of elemental Ag, no energy band upshift

could occur. On the other hand, the single Ag-O


6 Nano Res.

layer between the TiO2 and Ag makes the

contribution to maintaining the band structure of

TiO2 in the hybrid structures as illustrated in Figure

S6: the Ag-O layer would reduce the coupling of TiO2

with Ag around the Ag Fermi level, and lower the

charge mobility from Ag metal to TiO2. As a result,

the band structure of TiO2 cannot be altered by the

interface with Ag.

Figure 3 (a) Photocatalytic average rates of hydrogen

production and (b) photocurrents vs. time (I-t) curves of

Ag2S-(Ag)-TiO2 hybrid structures under various light

illumination conditions (full spectrum, < 400 nm and > 400

nm). In the photocatalytic measurements, 30-mg photocatalysts

were added to the water containing 0.25-M Na2S and 0.35-M

Na2SO3. In the photocurrent measurements, 4-mg samples were

used to prepare the photoelectrodes, and photoresponses were

operated in a 0.5-M Na2SO4 electrolyte under chopped irradiation

at a bias potential of 0.6 V vs. Ag/AgCl.

The synthesized Ag2S-(Ag)-TiO2 hybrid structures

provide an ideal platform for photocatalysis owing to

their merits. First, the TiO2 shells are composed of

small nanocrystals so they have pores to allow

reactants to reach core surface and pick up electrons

for reduction reactions. The N2 sorption

measurements (Figure S13) have identified that the

TiO2 shells contain a few meso/macropores. Second,

the hybrid structures can absorb light in a very broad

spectral range. Figure S14 shows the UV-vis diffuse

reflectance spectra of Ag2S-(Ag)-TiO2 hybrid

structures in reference to Ag-TiO2 core-shell

structures, bare Ag2S and bare TiO2 (see their

morphologies and phases in Figure S15 and S16).

As the Ag2S-(Ag)-TiO2 hybrid structures contain both

TiO2 and Ag2S, their light absorption band covers UV,

visible and NIR regions, in which the extraordinarily

high absorbance between 360 and 550 nm may be

ascribed to the presence of trace Ag.[23]

We are now in a position to further investigate

their photocatalytic performance, using hydrogen

evolution from water as a model reaction (Figure 3a).

To better examine the photocatalytic production of

H2 that bare Ag2S originally is incapable of achieving

limited by its band structure, we use Na2S/Na2SO3

pairs to sacrifice the photoexcited holes by following

the mechanism:[24,25]

2S2- + 2h+ S22- (1)

SO32- + S2- + 2h+ S2O32- (2)

SO32- + S22- S2O32- + S2- (3)

The 30-mg Ag2S-(Ag)-TiO2 hybrid structures exhibit

H2 production rates at 1.310-7 and 1.910-8 mol/h

under < 400 nm and > 400 nm, respectively. To

better assess the photocatalytic activities, we have

used bare TiO2 and bare Ag2S as reference samples at

the same TiO2 or Ag2S weight as Ag2S-(Ag)-TiO2,

respectively (Figure S17). The comparison reveals

that the addition of Ag interfacing layer does not

affect the activity of TiO2 in H2 evolution at < 400

nm; however, bare Ag2S (in the absence of trace Ag,

see Figure S16b) cannot offer the capability of

generating H2 like the Ag2S-(Ag)-TiO2 hybrid

structures under > 400 nm light illumination,

despite their comparable efficiency of charge

generation (see Figure S18). This feature indicates

that the conduction band structure of bare Ag2S does

not meet the criteria for H2 evolution but can be

upshifted to the level for H2 production by adding

the Ag interfacing layer, which agrees with our

simulation findings in Figure 1b. This upshift of

Ag2S band structure by interfacial Ag has been

proven by flat band measurements (Figure S19).

Furthermore, we have identified that the

photocatalytic H2 production rate of Ag2S-(Ag)-TiO2

hybrid structures in full spectrum is significantly

higher than the sum of those under < 400 nm and

> 400 nm light irradiation (see Figure 3a). This

performance enhancement suggests that the charge


7 Nano Res.

generation and separation in full spectrum has been

substantially improved by forming a Z-scheme.

Note that from the TEM image (Figure 2b), few

particles contain two Ag nanocube cores although

most particles only have a single core. In the case of

double cores, there would be two Z-scheme

interfaces in a single particles, slightly reducing the

efficiency of charge separation. However, in the

assessment of photocatalytic activity, all the samples

have the same weights of TiO2 and/or Ag2S, so their

light absorption would not be much altered. To

better appreciate the niche of Z scheme, we have

measured the photocatalytic H2 production rates of

Ag-TiO2 core-shell structures under various light

conditions. As shown in Figure S20, the activity in

full spectrum is the same as that under < 400 nm,

while no activity under > 400 nm has been

observed. It indicates that the visible-light activity

originates from Ag2S and the Z scheme can promote

the full-spectrum activity.

Figure 4 (a) Transient open-circuit voltage decay (OCVD)

from the photoanodes made of Ag2S-(Ag)-TiO2 hybrid structures,

bare TiO2 and bare Ag2S following exposure to various light

illumination sources (full spectrum, < 400 nm and > 400 nm).

(b) Average lifetimes of the photogenerated carriers (τn) obtained

from the OCVD measurements.

The photocurrent measurements are believed to

offer an informative evaluation for the efficiency of

charge generation and separation. To this end, we

have collected photocurrents from Ag2S-(Ag)-TiO2

hybrid structures under various light illumination,

which gives similar findings to photocatalytic

activities as shown in Figure 3b. Note that TiO2 and

Ag2S components in the Ag2S-(Ag)-TiO2 hybrid

structures are mainly responsible for their activities

at < 400 nm and > 400 nm, respectively, according

to their comparisons with bare TiO2 and bare Ag2S

(Figure S18).

To decode the mechanism of Z scheme for

promoting charge separation, we collect transient

open-circuit voltage decay (OCVD) on the

Ag2S-(Ag)-TiO2 hybrid structures in reference to bare

TiO2 and bare Ag2S under various irradiation

conditions (Figure 4a). The OCVD has been

previously demonstrated as a tool to resolve charge

kinetics.[26-28] As revealed by the OCVD

characterizations, the lifetimes of the carriers that are

photogenerated in the hybrid structures under <

400 nm and > 400 nm are comparable to those of

bare TiO2 and bare Ag2S, respectively (Figure 4b),

well demonstrating the individual semiconductor

characteristics of each component. In stark contrast,

when the light illumination is switched to full

spectrum, the carrier lifetime in the Ag2S-(Ag)-TiO2 is

dramatically prolonged, indicating the improvement

of charge separation. The role of trace Ag in

forming the Z scheme (and thus facilitating charge

separation) has been further elucidated by a control

experiment. The control sample is obtained by

directly growing Ag2S nanoparticles on TiO2 (namely

TiO2-Ag2S). As shown in Figure S21, no

photocurrent enhancement can be observed for the

TiO2-Ag2S under full-spectrum irradiation, unlike the

case of Ag2S-(Ag)-TiO2 hybrid structures. This

result indicates that in the absence of Ag interfacing

layer, no effective charge transfer occurs between

TiO2 and Ag2S to promote each other when both are

photoexcited.

In summary, the interfacial Ag, which induces

the energy band bending and upshift of Ag2S, is the

key to constitute the Z scheme between

wide-bandgap TiO2 and narrow-bandgap Ag2S. The

demonstrated ternary hybrid structures have

exhibited dramatically enhanced performance in

photocatalytic hydrogen production under

full-spectrum light illumination against those under


8 Nano Res.

< 400 nm and > 400 nm irradiation. This work

provides a new approach to the band structure

engineering of semiconductors, and will absolutely

serve as a strategy for better implementing

narrow-bandgap semiconductors in the Z-scheme

design towards full-spectrum photocatalysis.

Acknowledgements

This work was financially supported by the 973

Program (No. 2014CB848900), NSFC (No. 21471141,

21473166), Recruitment Program of Global Experts,

CAS Hundred Talent Program, and Fundamental

Research Funds for the Central Universities (No.

WK2060190025, WK2310000035, WK2090050027).

Photoemission spectroscopy experiments were

performed at the Catalysis and Surface Physics

Endstation at the BL11U beamline in the National

Synchrotron Radiation Laboratory (NSRL) in Hefei,

China.

Electronic Supplementary Material: Supplementary

material is available in the online version of this

article at http://dx.doi.org/10.1007/s12274-***-****-* References

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Electronic Supplementary Material






Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

EXPERIMENTAL PROCEDURE:

Synthesis of Ag nanocubes. In a typical synthesis,[S1] ethylene glycol (50 mL, EG, Aladdin, 1095698-500 mL) was

added into a 250-mL round bottom flask and heated under magnetic stirring in an oil bath preset to 150 °C. NaSH (0.6

mL, 3 mM in EG, Sigma Aldrich, 02326AH) was quickly injected into the heated solution after its temperature had

reached 150 °C. After 2 min, a 3-mM HCl solution (5 mL) was injected into the reaction solution, followed by the

addition of poly(vinyl pyrrolidone) (PVP, 12.5 mL, 20 mg/mL in EG, M.W. ≈55,000, Sigma-Aldrich, 856568-100g).

The HCl solution was prepared by adding HCl (4 μL, 38% by weight) into EG (12 mL). After another 2 min, silver

trifluoroacetate (4 mL, 282 mM in EG, Sigma-Aldrich, 04514TH) was added into the mixture. The reaction mixture

was heated at 150 °C in air for 1 h. The samples were washed with acetone and then with ethanol and deionized water

several times to remove most of the EG and PVP by centrifugation.

Synthesis of bare TiO2 and bare Ag2S. The synthesis of bare TiO2 is similar to the protocol for Ag-TiO2 core-shell


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structures, except the absence of Ag nanocubes in the reaction solution. The synthesis of bare Ag2S is similar to the

sulfidation protocol for Ag2S-(Ag)-TiO2 hybrid structures, except the use of Ag nanocubes instead of Ag-TiO2 core-shell

structures.

Synthesis of TiO2-Ag2S reference sample. In a typical synthesis,[S2] the obtained bare TiO2 (20 mg) was mixed with

5-mg AgNO3 in 25-mL deionized water, which was then ultrasonicated for 30 min. 5-mL aqueous solution containing

3-mmol Na2S was prepared separately and added dropwise to the mixture solution. The resulted solution was stirred at

80 °C for 8 h, transferred into a 50-mL Teflon-lined stainless steel autoclave, and then heated at 140 °C for 10 h. The

product was centrifuged and washed with deionized water several times.

Sample Characterizations. Prior to electron microscopy characterizations, a drop of the aqueous suspension of

particles was placed on a piece of carbon-coated copper grid or Si wafer and dried under ambient conditions. TEM

images were taken on a JEOL JEM-2010 LaB6 high-resolution transmission electron microscope operated at 200 kV.

HRTEM/STEM images and EDS mapping profiles were taken on a JEOL JEM-2100F field-emission high-resolution

transmission electron microscope operated at 200 kV.

Powder X-ray powder diffraction (XRD) patterns were recorded by using a Philips X’Pert Pro Super X-ray

diffractometer with Cu-Kα radiation (λ = 1.5418 Å).

Photoemission spectroscopy (X-ray photoelectron spectroscopy, XPS) experiments were performed at the Catalysis and

Surface Physics Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei,


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China. This beamline is connected to an undulator and equipped with two gratings that offer soft X-rays from 20 to

600 eV with a typical photon flux of 5×1010 photons/s and a resolution (E/ΔE) better than 105 at 29 eV. This system is

comprised of four ultrahigh vacuum (UHV) chambers including analysis chamber, preparation chamber, molecular

beam epitaxy (MBE) chamber, and a radial distribution chamber. The base pressures are 7×10−11, 1×10−10, 5×10−10

and 2×10−11 mbar, respectively. A sample load-lock system is connected to the sample transfer chamber. The analysis

chamber is equipped with a VG Scienta R4000 analyzer, a monochromatic Al Ka X-ray source, a UV light source, low

energy electron diffraction (LEED), a flood electron gun, and a manipulator with high precision and

five-degree-of-freedom. The preparation chamber comprises an ion gun, a quartz crystal microbalance (QCM), a

residual gas analyzer, a manipulator with high precision and four-degree-of-freedom, and several evaporators. The

MBE chamber houses a QCM, several evaporators and a manipulator with two-degree-of-freedom. With this radial

distribution chamber, the time for each transfer process between two chambers is less than 1 minute. The expected

charging of samples was corrected by setting the C 1s binding energy of the adventitious carbon to 284.5 eV.

XPS Auger data were collected on an ESCALab 250 X-ray photoelectron spectrometer, using nonmonochromatized

Al-Kα X-ray as the excitation source.

The concentrations of metal elements were measured as follows: the samples were dissolved with a mixture of HCl and

HNO3 (3:1, volume ratio) which was then diluted with 1% HNO3. The concentrations of metals were then measured

with a Thermo Scientific PlasmaQuad 3 inductively-coupled plasma mass spectrometry (ICP-MS).

UV-vis diffuse reflectance data were recorded in the spectral region of 220-1500 nm with a Shimadzu SolidSpec-3700


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spectrophotometer.

Nitrogen sorption studies were performed in a Micromeritics ASAP 2020 adsorption apparatus at 77 K up to 1 bar.

Computational methods. We have employed the Vienna ab initio Simulation package (VASP) [S3] to simulate the

geometric, electronic and photocatalytic properties of various model systems. Calculations were performed at the

spin-polarized density functional theory (DFT) level using the frozen-core all-electron projector augmented wave (PAW)

model with the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) functions. An energy

cutoff of 500 eV was used for the plane-wave expansion of the electronic wave function. The force and energy

convergence criterion was set to 0.01 eV/Å and 10-5 eV, respectively. A system of 1 × 1 slab with six layers was

employed to model the surface structures of Ag and TiO2, with three bottom layers fixed to the bulk positions during the

relaxation simulations. The 11×11×1, 11×4×1 and 3×6×1 k-points test were performed for Ag(200), TiO2(200) and

Ag2S(103) for the first Brillouin zone using the gamma center scheme, respectively. As for the Ag(200)/Ag2S(103)

composite system, mainly due to the large lattice mismatch between their facets, a system of 4×2 Ag(200) facets with

three layers was chosen to match Ag2S(103). For the same reason, a system of 1×2 Ag(200) facets with three layers

was built to match TiO2(200) in the TiO2(200)/Ag(200) composite system. The 3×6×1 and 11×4×1 k-points tests were

performed for Ag(200)/Ag2S(103) and TiO2(200)/Ag(200) hybrid systems for the first Brillouin zone using the gamma

center scheme, respectively.


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Figure S1 Theoretically optimized atomic models for (a) bulk TiO2 and (b) bulk Ag2S. (c) Partial projected density

of states (PDOS) diagrams for bulk TiO2 and bulk Ag2S from first-principles simulations. The energy level of H2

evolution is set to zero.

The PDOS diagrams in Figure S1c reflect the bandgaps of bulk TiO2 and bulk Ag2S (as marked by sky blue shadows) as

well as their valence band and conduction band energy levels.


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Figure S2 (a) Theoretically optimized atomic model for the Ag2S at Ag(200)-Ag2S(103) interface with an interface

distance of 1.7 Å. (b) PDOS diagrams for the Ag2S at Ag(200)-Ag2S(103) interface from first-principles simulations,

in reference to bare Ag2S. The energy level of H2 evolution is set to zero. The selection of Ag(200) is based on the

surface facets of starting materials – Ag nanocubes. Upon selecting Ag(200), we have optimized the structure of Ag2S

on the Ag(200) substrate in the simulations, which turns out to be Ag2S(103) at the Ag-Ag2S interface.

The PDOS diagrams in Figure S2b show strong hybridizations at the Ag(200)-Ag2S(103) interface. In particular, the

strongly coupled electronic states inside the bandgap of bare Ag2S indicate the high probability of charge transfer

between Ag and Ag2S.


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Figure S3 Theoretically optimized atomic models for (a) Ag(200) and (b) Ag2S(103). Computed potential surfaces

with work function (WF) values for (c) Ag(200) and (d) Ag2S(103). The Fermi levels are set to zero.

Figure S3c and S3d show that the work functions of Ag(200) and Ag2S(103) are 4.18 and 5.64 eV, respectively.

According to the potential alignments, free electrons would be transferred from Ag(200) to Ag2S(103) when they are in

contact.


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Figure S4 Computed potential surfaces with work function (WF) values for the Ag2S at Ag(200)-Ag(103) interface in

reference to bare Ag2S, based on the theoretically optimized atomic model in Figure S2a. The Fermi levels are set to

zero.

The computed potential surfaces in Figure S4 show that the bare Ag2S(103) and the Ag2S at Ag(200)-Ag(103) have the

work functions of 5.64 eV and 5.17 eV, respectively. It implies a ~0.47 eV upshift of Ag2S Fermi level (and energy bands)

due to the Ag-Ag2S interfacing.


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Figure S5 Another 2 possible surface structures for Ag(200)-Ag(103) interface: (a, d) theoretically optimized atomic

models; (b, e) computed differential charge distributions; and (c, f) PDOS diagrams. The simulation results are obtained

based on the atomic model in the same column. The energy level of H2 evolution is set to zero.

Similarly to the theoretically optimized atomic model in Figure S2a, the simulations by these two models indicate the high

probability of electron transfer from Ag to Ag2S. The computed charges donated to the Ag2S part in a supercell, work

function (WF) values and estimated upshifts of WF are listed in Table S1.


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Figure S6 Simulations for TiO2(200)-Ag(200) interface: (a) theoretically optimized atomic models, (b) PDOS diagrams,

(c) computed differential charge distribution with an interface distance of 2.67 Å, and (d) band alignment diagram. The

energy level of H2 evolution is set to zero. The selection of TiO2(200) is based on the lattice fringes of TiO2 observed by

HRTEM.

The PDOS diagram in Figure S6b suggests that the electronic structure coupling of TiO2 with Ag is relatively weak around

the Ag Fermi level. As a result, free electrons in Ag can hardly migrate to the TiO2 in the absence of external field, as

indicated by the differential charge distribution (Figure S6c). Thus the band structure of TiO2 cannot be altered by the

Ag-TiO2 interface (Figure S6d).


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Figure S7 TEM images of (a) Ag nanocubes and (b) Ag-TiO2 core-shell structures.

Figure S7a shows that the used Ag nanocubes have an average edge length of 40 nm. After the growth of TiO2, the Ag

nanocubes become truncated while the resulted TiO2 shells are composed of TiO2 nanocrystals via a flower-like pattern

(Figure S7b).


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Figure S8 EDS line mapping profile of a Ag2S-(Ag)-TiO2 hybrid structure on STEM image.


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Figure S9 (a) XRD patterns of Ag-TiO2 core-shell structures and Ag2S-(Ag)-TiO2 hybrid structures. (b) XRD pattern

of Ag2S-(Ag)-TiO2 hybrid structures with 2θ at 36-40.

Figure S9a shows the XRD patterns of Ag-TiO2 core-shell structures and Ag2S-(Ag)-TiO2 hybrid structures. All the

diffraction peaks of Ag-TiO2 core-shell structures can be assigned to face-centered cubic (fcc) Ag (JCPDS 65-2871) and

anatase TiO2 (JCPDS 83-2243) (refer to the standard patterns below). As for the Ag2S-(Ag)-TiO2 hybrid structures, their

diffraction peaks can be indexed to monoclinic Ag2S (JCPDS 14-0072) and anatase TiO2 (JCPDS 83-2243) (refer to the

standard patterns below). Given the trace portion of Ag in the Ag2S-(Ag)-TiO2, it is difficult to well resolve the

diffraction peaks from Ag. However, it is worth noting that the diffraction peak at 37.6-38.7 is asymmetric, which can

be split into two peaks (Figure S9b). The split peak centered at 37.8 can be assigned to TiO2(004) (37.81) and

Ag2S(-103) (37.72), while the one located at 38.1 corresponds to Ag(111) (38.12). It indicates that the

Ag2S-(Ag)-TiO2 hybrid structures may contain a small portion of Ag.


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Figure S10 (a) Ag 3d3/2 and 3d5/2 XPS spectra of Ag2S-(Ag)-TiO2 hybrid structures collected with synchrotron radiation

X-ray as the excitation source. (b) Ag M4VV Auger XPS spectra of Ag2S-(Ag)-TiO2 hybrid structures collected with

nonmonochromatized Al-Kα X-ray as the excitation source.

The Ag 3d3/2 and 3d5/2 binding energies at 373.1 eV and 367.1 eV in Figure S10a are characteristic values for AgI,

respectively, showing that the majority of Ag in the sample is in the state of +1 corresponding to Ag2S.[S4,S5]

Meanwhile, the XPS also identifies the Ag0 characteristics in the sample – the Ag 3d3/2 and 3d5/2 binding energies at 374.1

eV and 368.1 eV with lower intensities, respectively.[S4,S5]

The Ag M4VV kinetic energies at 351.2 eV and 358.2 eV in Figure S10b are the characteristic values for AgI and Ag0,

respectively. It indicates that the Ag2S-(Ag)-TiO2 hybrid structures should contain a small portion of Ag.


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Figure S11 TEM image of the sample obtained by directly using Ag2S nanoparticles instead of Ag nanocubes in the

growth of TiO2. The Ag2S nanoparticles were prepared by completely sulfidizing Ag nanocubes (see the morphology in

Figure S15a). In other words, this sample was obtained by Ag sulfidation into Ag2S, followed by TiO2 growth.

Figure S11 shows that TiO2 cannot be uniformly grown on the Ag2S surface. It demonstrates the importance of our

synthetic method (i.e., growth of TiO2 on Ag nanocubes, followed by Ag sulfidation into Ag2S) to the formation of

Ag2S-(Ag)-TiO2 hybrid structures.


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Figure S12 (a) Computed potential surface with work function (WF) value for Ag2O(103). The Fermi levels are set to

zero. (b) Band structure alignment between Ag2S(103) and Ag2O(103).

Due to the small difference in work function (Figure S12a), the alignment of Ag2O and Ag2S Fermi levels (Figure S12b) is

unlikely to drive free electrons to flow from Ag2O to Ag2S. For this reason, if the Ag2S in our hybrid structures was

directly interfaced with Ag2O in the absence of elemental Ag, no energy band upshift could occur.


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Figure S13 N2 adsorption-desorption isotherms for Ag2S-(Ag)-TiO2 hybrid structures. The inset shows the

corresponding pore size distribution diagram.

The N2 sorption measurements reveal that the TiO2 shells contain a few meso/macropores.


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Figure S14 UV-vis diffuse reflectance spectra of Ag2S-(Ag)-TiO2 hybrid structures, Ag-TiO2 core-shell structures, bare

Ag2S and bare TiO2 at the same weight of Ag2S or TiO2.

Figure S14 shows that bare TiO2 and bare Ag2S have bandgaps of about 3.2 and 1.0 eV, respectively, in agreement with the

values in literature.[S6] In the spectrum of Ag-TiO2 core-shell structures, there is an absorption peak located at 460 nm

in addition to the light absorption of TiO2, which should be assigned to the surface plasmon of Ag nanoparticles.[S7] As

the Ag2S-(Ag)-TiO2 hybrid structures contain both TiO2 and Ag2S, their light absorption band covers UV, visible and near

IR regions, in which the extraordinarily high absorbance between 360 and 550 nm may be ascribed to the presence of trace

Ag.


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Figure S15 TEM images of (a) bare Ag2S and (b) bare TiO2.

The bare Ag2S (Figure S15a) was obtained through the same protocol as Ag2S-(Ag)-TiO2 hybrid structures, except the use

of Ag nanocubes instead of Ag-TiO2 core-shell structures.

The bare TiO2 (Figure S15b) was prepared using the same protocol as Ag-TiO2 core-shell structures but excluding the use

of Ag nanocubes. The morphology is analogous to that for Ag-TiO2 and Ag2S-(Ag)-TiO2 hybrid structures except the

absence of Ag and Ag2S cores.


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Figure S16 (a) XRD patterns of bare Ag2S and bare TiO2. (b) XRD pattern of bare Ag2S with 2θ at 36-40. The bare

Ag2S was obtained through the same protocol as Ag2S-(Ag)-TiO2 hybrid structures, except the use of Ag nanocubes

instead of Ag-TiO2 core-shell structures.

Figure S16a shows the XRD patterns of bare Ag2S and bare TiO2. All the diffraction peaks of bare TiO2 can be assigned

to anatase TiO2 (JCPDS 83-2243) (refer to the standard patterns below). As for the bare Ag2S, the diffraction peaks can

be indexed to monoclinic Ag2S (JCPDS 14-0072) (refer to the standard patterns below). It is worth noting that the

diffraction peak at 37.6-38.7 is quite symmetric (Figure S16b). The symmetric peak centered at 37.8 can be Ag2S(-103)

(37.72), while no distinct split peak corresponding to Ag(111) (38.12) can be found around 38.1. It indicates that the

Ag nanocubes can be completely sulfidized into Ag2S in the absence of TiO2 shells.


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Figure S17 Photocatalytic average rates of hydrogen production of (a) Ag2S-(Ag)-TiO2 hybrid structures and bare Ag2S

under > 400 nm light illumination and (b) Ag2S-(Ag)-TiO2 hybrid structures and bare TiO2 under < 400 nm light

illumination. In the photocatalytic measurements, 30-mg Ag2S-(Ag)-TiO2 hybrid structures were added to the water

containing 0.25-M Na2S and 0.35-M Na2SO3, while the weights of bare Ag2S and bare TiO2 are kept the same as those of

Ag2S and TiO2 in the Ag2S-(Ag)-TiO2 hybrid structures, respectively.

Figure S17a shows that bare Ag2S cannot offer the capability of generating H2 like Ag2S-(Ag)-TiO2 hybrid structures under

> 400 nm light illumination, although bare Ag2S exhibits slightly higher photoelectrochemical activity than

Ag2S-(Ag)-TiO2 hybrid structures under the same irradiation (Figure S18a). This feature verifies that the conduction

band structure of bare Ag2S does not meet the criteria for H2 evolution but can be upshifted to the level for H2 production

by adding the Ag interfacing layer.

Figure S17b shows that bare TiO2 offers comparable H2 production capability to Ag2S-(Ag)-TiO2 hybrid structures under

< 400 nm light illumination. It indicates that the addition of Ag interfacing layer does not affect the activity of TiO2 in

photocatalytic H2 evolution.


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Figure S18 Photocurrents vs. time (I-t) curves of (a) Ag2S-(Ag)-TiO2 hybrid structures and bare Ag2S under > 400 nm

light illumination and (b) Ag2S-(Ag)-TiO2 hybrid structures and bare TiO2 under < 400 nm light illumination. In the

photocurrent measurements, 4-mg Ag2S-(Ag)-TiO2 hybrid structures were used to prepare the photoelectrodes, and

photoresponses were operated in a 0.5-M Na2SO4 electrolyte under chopped irradiation at a bias potential of 0.6 V vs.

Ag/AgCl. The weights of bare Ag2S and bare TiO2 are kept the same as those of Ag2S and TiO2 in the Ag2S-(Ag)-TiO2

hybrid structures, respectively.

Figure S18a shows that bare Ag2S exhibits slightly higher photocurrents than Ag2S-(Ag)-TiO2 hybrid structures under >

400 nm light illumination. Figure S18b shows that bare TiO2 exhibits comparable photocurrents to Ag2S-(Ag)-TiO2

hybrid structures under < 400 nm light illumination. The photocurrent measurements are believed to offer an

informative evaluation for the efficiency of charge generation and separation. Thus the comparisons indicate that the

TiO2 and Ag2S components in the Ag2S-(Ag)-TiO2 hybrid structures are mainly responsible for their activities at < 400

nm and > 400 nm, respectively.


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Figure S19 Mott-Schottky plots of Ag2S-(Ag)-TiO2 hybrid structures, bare Ag2S and bare TiO2, taken with an applied

potential ranging from 1.0 to -0.5 V (vs. Ag/AgCl) at frequency of 5000 Hz.

The Mott-Schottky plots (Figure S19) show that the electrodes made of Ag2S-(Ag)-TiO2 hybrid structures and bare TiO2

have comparable flat band potentials (-0.5 V vs. Ag/AgCl) in terms of TiO2, suggesting that the conduction band structure

of TiO2 has not been altered in the hybrid structures. In comparison, the Ag2S-(Ag)-TiO2 hybrid structures show more

negative flat band potential (0 V vs. Ag/AgCl) than bare Ag2S (0.25 V vs. Ag/AgCl) in terms of Ag2S, indicating that the

conduction band bending and upshift indeed occur in Ag2S when interfaced with Ag. Note that the conduction band edge

not only depends on the flat band potential, but also is correlated with the concentration of carriers. Thus the values of

flat band potentials for different materials (e.g., TiO2 versus Ag2S) do not directly reflect their positions of conduction band

edges.

For n-type semiconductor, ln( )B DCB fb

C

k T NV U

q N , in which the VCB, Ufb, ND, NC, kB , T and q are conduction band edge,

flat band potential, number of donors (i.e., electrons), DOS in the conduction band, Boltzmann’s constant, absolute

temperature and unsigned charge of an electron, respectively. Both TiO2 and Ag2S are n-type semiconductors.


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Figure S20 Photocatalytic average rates of hydrogen production of Ag-TiO2 core-shell structures under various light

illumination conditions (full spectrum, < 400 nm and > 400 nm). In the photocatalytic measurements, Ag-TiO2

core-shell structures were added to the water containing 0.25-M Na2S and 0.35-M Na2SO3. The weight of TiO2 in the

core-shell structures is kept the same as that of TiO2 in the Ag2S-(Ag)-TiO2 hybrid structures.


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Figure S21 (a) TEM image of the sample obtained by growing Ag2S nanoparticles on TiO2 (namely TiO2-Ag2S). The

bare TiO2 (see the morphology in Figure S13b) was prepared using the same protocol as Ag-TiO2 core-shell structures but

excluding the use of Ag nanocubes. (b) Photocurrents vs. time (I-t) curves of TiO2-Ag2S hybrid structures under various

light illumination conditions (full spectrum, < 400 nm and > 400 nm). In the photocurrent measurements, 4-mg

samples were used to prepare the photoelectrodes, and photoresponses were operated in a 0.5-M Na2SO4 electrolyte under

chopped irradiation at a bias potential of 0.6 V vs. Ag/AgCl.

Figure S21b shows that the photocurrents of TiO2-Ag2S under full-spectrum irradiation are roughly comparable to the sum

of those under < 400 nm and > 400 nm light irradiation, unlike the case of Ag2S-(Ag)-TiO2 hybrid structures (Figure

3b). Given the bandgaps of TiO2 and Ag2S at 3.2 and 1.0 eV, respectively, this result indicates that in the absence of Ag

interfacing layer, no effective charge transfer occurs between TiO2 and Ag2S to promote each other when both are

photoexcited.


Nano Res.

Table S1 The computed charges donated to the Ag2S part in a supercell, work function (WF) values and estimated

upshifts of WF, based on the 3 surface models for Ag(200)-Ag2S(103) interface.

Ag-Ag2S models Charges donated to Ag2S (e) WF (eV) WF upshift (eV)

I (Figure S2a) 1.33 5.17 0.47

II (Figure S5a) 1.28 5.35 0.29

III (Figure S5d) 1.22 5.30 0.34


Nano Res.

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Address correspondence to Jun Jiang, [email protected]; Yujie Xiong, [email protected]

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Nano Research - Towards full-spectrum photocatalysis ...Nano Res 1 Towards full-spectrum photocatalysis: Achieving Z scheme between Ag 2 S and TiO 2 by engineering energy band alignment