Tunable Photocatalytic HER Activity of Single-Layered TiO2

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Tunable Photocatalytic HER Activity of Single Layered TiO2 Nanosheet with Transition Metal Doping and Biaxial Strain Jian Yuan, Chao Wang, Yanyu Liu, Ping Wu, and Wei Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09848 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018

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The Journal of Physical Chemistry

3

Tunable Photocatalytic HER Activity of Single Layered TiO2 Nanosheet with Transition Metal Doping and Biaxial Strain

4

Jian Yuana, Chao Wanga, Yanyu Liub, Ping Wua, and Wei Zhoua*

5

a

6

and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, P. R. China.

7

b

8

Abstract: The hydrogen evolution reaction and electronic properties of single-layered TiO 2

9

nanosheet effectively modulated by transition metal doping and biaxial strain were investigated in

10

this work. Surprisingly, the hydrogen adsorption free energy of Cr-doped LNS-TiO 2 nanosheet

11

was reduced to almost zero eV, which is comparable to the well-known highly efficient catalyst of

12

Pt metal. The tunable hydrogen adsorption ability was correlated with the O-2p z band center level.

13

Moreover, by up-shifting the p z band center level, both the tensile and compressive strain could

14

decrease the hydrogen adsorption free energy of V-doped LNS-TiO 2 nanosheet, even close to zero

15

eV with 7% tensile strain. These insights open a new avenue to realizing high HER activity in

16

oxide nanosheets.

1 2

Department of Applied Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics

School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China

17 18 19 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1. Introduction

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Recently, solar energy is widely accepted as a free, abundant and endlessly renewable source

3

of clean energy. Meanwhile, H 2 is an excellent energy carrier for the development of low-carbon

4

emission economy. Thus, storage of solar energy in form of H 2 is proposed to be one of the most

5

ideal routes for clean and sustainable energy in the future.1 However, efficient and stable catalysts

6

are desirable in photocatalytic hydrogen evolution reaction (HER, 2H+ + 2e− ＝H 2 ), such as

7

platinum (Pt) which exists high photocatalytic activity except for its scarcity and valuableness.2 It

8

is well known that generating active electronic states responding to catalysis is fundamental to

9

design of high-activity catalysts.3, 4 Transition metals (Co, Ni, Fe and Ru) as additives can adjust

10

electronic property and remarkably improve HER activity.5-8 The catalytic activity of inert 2D

11

MoS 2 surface can be efficiently triggered via single metal atom doping.9, 10 Gong et al. synthesized

12

W/N−TiO 2 existing high-performance photocatalysis.11 Besides, nonmetal dopants also have

13

better photocatalytic performance.12 Expect for doping, strain,13, 14 adsorption15-17 and substrate18

14

are usually applied to regulate active electronic states. Generally, the d-band center model has been

15

considered as an important parameter in determining the ability of the surface binding to a number

16

of adsorbates.19 Recently, many amendatory band center model have been reported to be applicable

17

to a broad class of systems.20-24

18

In the past few years, plenty of ultrathin two-dimensional (2D) materials have been prepared

19

beyond graphene. Owing to their exotic electronic properties and excellent charge carrier mobility,

20

as well as a superior surface-volume ratio, 2D materials, such as MoS 2 and boron nanosheets,25 2


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could be an advantage for a photocatalyst. It’s mentioning that the HER activity of 2D materials

2

mainly arises from the edge active sites. However, thermodynamics favors the presence of the

3

relatively inactive basal plane and impedes the formation of highly energetic and atomically

4

undercoordinated edge sites, limiting the number of active sites at the surface.26 As a most

5

promising candidate in photocatalyst, Titanium dioxide nanosheets has been applied into water

6

splitting because of a large band gap and proper band alignment relative to the water redox

7

potential.27 In 1996, Sasaki et al. had successfully exfoliated signal layered titanium dioxide

8

nanosheet (LNS-TiO 2 ) by means of soft-chemical procedures.28,

9

thickness for electron transferring and large surface area, LNS-TiO 2 nanosheets possess high

10

photocatalytic degradation efficiency.30 Surprisingly, the band gap of LNS-TiO 2 nanosheet is 3.8

11

eV, and it’s band edge suggests that photogenerated electron−hole pairs have stronger reduction

12

and oxidation power.31 Recently, Zhou et al. reported the strong anisotropic exciton and high

13

carrier mobility in LNS-TiO 2 nanosheet is beneficial to photochemical application.32 And Rh

14

dopant in LNS-TiO 2 nanosheet can effectively act as co-catalysts for the water splitting reaction.33

15

Besides, LNS-TiO 2 nanosheet had been investigated experimentally34, 35 and theoretically27, 36 in

16

photocatalyst.

29

Because it’s the ultra-thin

17

Despite these advantages, the H 2 production rate for the undoped nanosheet was very low.33

18

Thus, in this work, we simulated the HER performance of LNS−TiO 2 nanosheet doped with

19

transition metal (TM, TM＝V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Nb) by employing first-principle

20

calculations and explored the mechanism of tuning the photocatalytic hydrogen evolution activity. 3



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1

More interesting, the in-plane biaxial strain exhibits the ability to modulate the HER catalytic

2

activity by engineering the electronic structure of doped LNS−TiO 2 nanosheet. The corresponding

3

physical origin for this appearance was also analyzed and discusses in details.

4

2. Methods

5

The first-principle calculations were performed by using the Vienna ab initio simulation

6

package (VASP).37,

7

Ernzerhof (PBE) form and a cutoff energy of 400 eV for plane-wave basis set were adopted. The

8

convergence threshold was 10-5 eV and 0.01 eV/Å for energy and force, respectively. A vacuum

9

space at least 18 Å was used to avoid the interaction between two periodic units. The Brillouin

10

zone was sampled by Monkhorst-Pack k-point mesh of 4×3×1 for the 3×3×1 supercell (54 atoms).

11

The effect of van der Waals (vdW) interactions was included using the correction scheme of

12

Grimme, DFT-D2.39

13 14

17 18 19

The generalized gradient approximation (GGA) in the Perdew-Burke-

The assessment of HER catalytic activity is based on the free energy of hydrogen adsorption (△G H* ),40 which was calculated by: △G H* ＝△E H* + △E ZPE − T△S H

15 16

38

(1)

where * represents an adsorption site on the catalyst surface, and △E H* is the hydrogen adsorption energy

△E H* ＝E(host+H) − E(host) −E(H 2 )/2

(2)

in which E(host+H) and E(host) are the energies of the catalyst with and without H adsorption,

4


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2

respectively. △E ZPE represents the zero-point energy difference between the adsorbed state of the

3

is quite small. The E ZPE can be obtained via vibrational frequency calculation. Further ΔS H can be

1

H

2 system (E ZPE ) and gas-phase state (𝐸𝐸ZPE ). Considering that phonon contribution to the free energy

1

5

regarded as ∆𝑆𝑆H ≅ − 2 𝑆𝑆H0 2 , where 𝑆𝑆H0 2 is the entropy of H 2 in the gas phase at standard

6

with a and a 0 are strained and unstrained equilibrium lattice constants along a-direction,

4

condition.41 Besides, all strains are in-plane biaxial strain, where the strain is defined as 𝜀𝜀＝

7

respectively.

8

3. Result and discussion

9

3.1 HER catalytic activity and stability

𝑎𝑎−𝑎𝑎0 𝑎𝑎0

,

10

The detailed crystal structure of LNS−TiO 2 nanosheet adsorbed H atom is shown in figure 1.

11

LNS−TiO 2 nanosheet has two inequivalent O atoms which are 2-fold (surface) or 4-fold (inner)

12

coordinated respectively. And Ti atoms are fully coordinated (6-fold). For water dissociation, O

13

atoms at the surface is a more favorable H adsorption sites than those inside the nanosheet.33

14

Therefore, the ability of hydrogen adsorption at the surface O site was taken into account in this

15

work. As for the catalytic activity of HER at the surface of the host, the △G H* is considered as an

16

effective descriptor. Smaller △G H* will induce strong binding to the adsorbed H, whereas larger

18

△G H* will make the proton bonding to the catalytic surface difficult, both leading to the slow HER

19

~0), indicating the free energy of adsorbed H is close to that of both the reactant and product. This

17

kinetics. Hence, an optimal HER activity can be achieved at a value of △G H* close to zero (△G H*

5



1

general principle for HER activity estimation has been widely accepted in the previous

2

researches.40-42 Unfortunately, the value of hydrogen adsorption free energy is high (△G H* =1.05

3

eV) in undoped LNS−TiO 2 nanosheet, which results in too weak bonding between the proton and

4

the catalyst surface.

5

To improve the HER catalytic activity of LNS−TiO 2 , substitutional doping of the Ti site with

6

TM is adopted there since the doping strategy is considered as an effective way to modulate the

7

local electronic structure which determines the atomic bonding strength around the dopants. The

8

calculated △G H* of doped systems is shown in figure 2 in detail. Obviously, the △G H* of

9

LNS−TiO 2 nanosheet can be well adjusted in a relatively large energy range by transition metal

11

doping. More importantly, one can see that doping with Cr leads to a △G H* value very closer to

12

high HER catalytic activity will be achieved in Cr-doped LNS−TiO 2 nanosheet. In addition, the

13 14

calculated △G H* of Cr-doped LNS−TiO 2 nanosheet is also similar with other highly efficient 2D

15

catalysts, such as α and β 12 boron nanosheets (△G H* (α)=0.03 eV, △G H* (β 12 )=0.02 eV).25 Moreover, considering that the LNS-TiO 2 nanosheet is more stable than boron nanosheets in

16

experiment,28, 29, 43, 44 Cr-doped LNS−TiO 2 nanosheet could be a potential HER photocatalyst

17

candidate. On the other hand, as for Mn-doped LNS−TiO 2 nanosheet, although its negative

18

hydrogen adsorption free energy is beneficial for hydrogen adsorption, it is harder to form free

19

hydrogen molecular which leads to the decrease of HER efficiency.

10

zero (△G H* = 0.03 eV), and rather competitive to that of Pt (△G H* = -0.09 eV),40 indicating the

6


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1 2

Figure 1. The optimized structures of TM-doped and undoped LNS-TiO 2 nanosheet adsorbed H

3

atom. Color code: Ti: blue; O: red; H: white; TM: dark blue.

4 5

Figure 2. The calculated free energy diagram for HER of doped LNS-TiO 2 nanosheet.

6

In fact, when hydrogen atom adsorbs at the surface oxygen atom, the dominated interaction

7

is H 1s states with some mixture of O 2p z orbitals. This can be confirmed by the calculated partial

8

density of states (PDOS) of TM-doped LNS-TiO 2 nanosheet with H adsorption (see figure S1 of

9

Supporting Information). Hence, the binding energy of hydrogen adsorption reaction in the TM-

10

doped LNS-TiO 2 nanosheet is correlated to the center of O 2p z bands (𝜀𝜀𝑝𝑝z ) defined as: 7



𝜀𝜀𝑝𝑝z =

1

0

∫−∞ 𝐸𝐸⋅𝐷𝐷(𝐸𝐸) d𝐸𝐸 0

∫−∞ 𝐷𝐷(𝐸𝐸) d𝐸𝐸

Page 8 of 36

(3)

2

where D(E) is the DOS of O 2p z band at a given energy E.45, 46 The combination of H 1s orbital

3

and O 2p z orbital will form a fully filled bonding orbital (σ) and a partially filled anti-bonding

4

orbital (σ*). Moreover, the binding strength can be described by the bond order, which is equal to

5

half of the difference between the electron number of σ and σ* according to the molecular orbital

6

theory. Thus, the higher σ* occupancy will result in the weaker binding strength. Furthermore, the

7

electron filling of the anti-bonding states depends on O 2p z states relative to the Fermi level (E f ).

8

As shown in figure 3, it is clearly illustrated that a lower p z -band center location of active sites

9

makes the anti-bonding states down-shift and with higher occupancy, which weakens the

10

interaction between the adsorbate and surface.47 Based on the above discussion, the p z band center

11

of surface O atom around TM dopants are depicted in figure 4. It can be seen that the calculated

12

p z band level shifts up to the Fermi level from Nb-doped LNS-TiO 2 nanosheet to Zn-doped LNS-

13

TiO 2 nanosheet, which leads to a stronger binding strength between surface O atom and H atom.

14

This further lower the adsorption energy of hydrogen which is consistent with the evolution

15 16

tendency of calculated △G H* for doped LNS-TiO 2 nanosheets. Thus, p z band center model could

17

different transition metal dopants.

be employed to explain the change of hydrogen adsorption ability of LNS-TiO 2 nanosheet with

8


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1 2

Figure 3. Schematic illustration of bond formation between the surface and the adsorbate (Ads.)

3 4

Figure 4. Projected p z -orbital density of states of oxygen atom at reactive sites in TM-doped

5

LNS−TiO 2 . Light yellow area corresponds to the filled states up to the Fermi level. The red and

6

purple dash lines indicate 𝜀𝜀𝑝𝑝z and the Fermi level, respectively. The blue double arrow lines 9



1

Page 10 of 36

present the distance between 𝜀𝜀𝑝𝑝z and the Fermi level.

2

Next, the formation energy (E Formation ), which is reference to the undoped system, was

3

calculated with the equation (4) to estimate the relative stability of dopants in LNS-TiO 2

4

nanosheet:22, 48, 49

5

E Formation ＝E d − Ep + µTi − µdopant

6

where Ed and Ep are total energies of the supercell with and without defects, respectively. µTi and

7

µdopant is the chemical potential of Ti and dopant atom, respectively. It should be noticed that the

8

growth conditions including Ti-rich and O-rich conditions also have influence on the formation

9

energy of doped LNS−TiO2 nanosheet. Under Ti-rich condition, µTi is defined as µTi＝1/2E(Ti),

10

and under O-rich condition, µTi is defined as µTi＝E(TiO2) −2µO where µO is obtained by µO＝

11

1/2E(O2). According to these definitions, a small formation energy generally corresponds to a

12

thermodynamically stable system and a preferred doping scenario. From the calculated results in

13

table 1, formation energies are lower under Ti−rich conditions in comparison with those under

14

O−rich conditions for Nb and V doping. On the other hand, for other transition metal doped

15

LNS−TiO2 nanosheet, the formation energies under O−rich are relatively small, indicating they

16

are stable and easy to be synthesized from the energetic point of view. Furthermore, recently, Mn-,

17

Co- and Ni-doped LNS−TiO2 nanosheet have been successfully prepared in the experiment,50-52

18

which is consistent with our formation energy analysis. This confirms our estimation for the

19

stability of doped LNS-TiO2 nanosheets is reasonable. Besides, the structure stability of doped

20

and undoped LNS-TiO2 nanosheet models were also checked with the energy difference between 10


(4)

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1

their 3D bulk and 2D nanosheet structures. The calculated results are summarized in table S1. It

2

is obvious that the energy differences of doping systems are almost the same with that of undoped

3

LNS-TiO2 nanosheet. This further indicates the doped nanosheets are also stable and can be easily

4

exfoliated, since the undoped LNS-TiO2 nanosheet has been successfully fabricated and stable in

5

experimental condition.

6

Table 1. The formation energies EFormation (eV) of TM-doped LNS-TiO2 nanosheets under Ti−rich

7

and O−rich conditions.

EFormation (eV) TM-doped LNS-TiO2

8

Ti−rich

O−rich

Nb

1.83

3.64

V

2.74

3.12

Cr

5.21

-0.70

Mn

6.46

-1.52

Fe

7.40

0.43

Co

8.47

-2.45

Ni

8.38

-2.38

Cu

10.14

-0.43

Zn

9.34

0.57

3.2 The effect of biaxial strain on the HER catalytic activity

11



1

Recently, strain engineering has been proved to be a viable way to modify the electronic

2

properties of nano-materials.53, 54 That is to say, it can be expected that the hydrogen adsorption

3

free energy of low-active nanosheet will be effectively modulated to optimal value of 0 eV by

4

external strain. As shown in figure 2, the free energy of LNS-TiO2 nanosheet with Cr and Mn

5

dopants is already close to 0 eV enough compared with Pt. Thus, for other low-active cases, the

6

V-doped LNS-TiO2 nanosheet was chosen as an example for strain engineering, since a relatively

7

small strain amplitude, which is viable in experimental condition, is need for strain modulation

8

due to its adsorption free energy is not too far away from the optimal value of 0 eV. The calculated

9

hydrogen adsorption free energy and pz band center (𝜀𝜀𝑝𝑝z ) versus biaxial strain (ε) are presented

10

in figure 5. As a result, the hydrogen adsorption free energy reduces whether tensile or

11

compressive biaxial strain is applied. It also found that the value of hydrogen adsorption free

12

energy closer to zero at ε＝7%, which implies that the 7% tensile strain leads to the optimal HER

13

activity in V−doped LNS−TiO2. Furthermore, pz band center is also suitable for the strained

14 15

structures. Similar to the above discussion, the calculated 𝜀𝜀𝑝𝑝z shifts up in V−doped LNS-TiO2

16

energy.

under biaxial strain, which indicates the strain related reduction of hydrogen adsorption free

12


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1 2

Figure 5. Free energy for HER and pz band center versus biaxial strain from -8% to 8% for V-

3

doped LNS-TiO2 nanosheet.

4

In order to get more insight into the pz band center variation under biaxial strain, the

5

projected pz-orbital density of states of O atom at reaction sites and equation (3) are carefully

6

analyzed. First of all, it is found that the impurity states shift to the low energy part and cross the

7

Fermi level when compression is applied, whereas a high energy shift takes place in the V−doped

8

LNS-TiO2 nanosheet under biaxial tension (see figure S2). Hence, the contribution of impurity

9

states and valence band to hydrogen adsorption can be separately considered, i.e.,

𝜀𝜀𝑝𝑝z =

10

0

∫−∞ 𝐸𝐸∙𝐷𝐷(𝐸𝐸)d𝐸𝐸 0

∫−∞ 𝐷𝐷(𝐸𝐸)d𝐸𝐸

=

VBM

0

∫−∞ 𝐸𝐸∙𝐷𝐷(𝐸𝐸)d𝐸𝐸+ lim − ∫𝐸𝐸 𝐸𝐸∙𝐷𝐷(𝐸𝐸)d𝐸𝐸 0 𝐸𝐸0 →0 0


(5)

11

where E0 represents lower edge of the impurity states below the Fermi level. Relative to

12

∫−∞ 𝐷𝐷(𝐸𝐸)d𝐸𝐸 , lim− ∫𝐸𝐸 𝐸𝐸⋅𝐷𝐷(𝐸𝐸)d𝐸𝐸 is too small and close to zero. Therefore,

13

0

VBM

𝐸𝐸0 →0

0

0

0

∫−∞ 𝐸𝐸 ∙ 𝐷𝐷(𝐸𝐸) d𝐸𝐸 + lim− ∫𝐸𝐸 𝐸𝐸 ∙ 𝐷𝐷(𝐸𝐸) d𝐸𝐸 0

𝐸𝐸0 →0


0

=

VBM

∫−∞ 𝐸𝐸 ∙ 𝐷𝐷(𝐸𝐸) d𝐸𝐸 0


13



=

1

=

2

=

3

6

11 12 13

∙ �1 −

−

lim ∫

0

𝐷𝐷(𝐸𝐸)d𝐸𝐸

𝐸𝐸0 →0− 𝐸𝐸0 0 ∫−∞ 𝐷𝐷(𝐸𝐸)d𝐸𝐸

𝜀𝜀𝑝𝑝VBM 𝑧𝑧

∙

lim ∫

0

�

𝐷𝐷(𝐸𝐸)d𝐸𝐸

𝐸𝐸0 →0− 𝐸𝐸0 0 ∫−∞ 𝐷𝐷(𝐸𝐸)d𝐸𝐸

(6)

to valence band maximum,

and 𝜀𝜀𝑝𝑝∆𝑧𝑧 is represented by equation (8),

9

10


0


where 𝜀𝜀𝑝𝑝VBM represents pz valence band center, whose integral domain is from minimum energy 𝑧𝑧

7 8


∙

VBM


= 𝜀𝜀𝑝𝑝VBM + 𝜀𝜀𝑝𝑝∆𝑧𝑧 𝑧𝑧

4 5


Page 14 of 36

0 where lim− ∫𝐸𝐸 𝐷𝐷(𝐸𝐸)d𝐸𝐸 0 𝐸𝐸0 →0


𝜀𝜀𝑝𝑝∆𝑧𝑧 =

=

VBM

∫−∞

𝐸𝐸⋅𝐷𝐷(𝐸𝐸) d𝐸𝐸

(7)

VBM ∫−∞ 𝐷𝐷(𝐸𝐸) d𝐸𝐸

lim − ∫𝐸𝐸0 𝐷𝐷(𝐸𝐸) d𝐸𝐸 0 𝐸𝐸 →0 −𝜀𝜀𝑝𝑝VBM ⋅ 0 0 𝑧𝑧 ∫−∞ 𝐷𝐷(𝐸𝐸) d𝐸𝐸

is the number of electrons near the Fermi level. Then

(8) 0 lim ∫ 𝐷𝐷(𝐸𝐸)d𝐸𝐸 𝐸𝐸0 →0− 𝐸𝐸0 0 ∫−∞ 𝐷𝐷(𝐸𝐸)d𝐸𝐸

represents electron occupancy rate in the vicinity of the Fermi level. According to the equation (6), the variation of 𝜀𝜀𝑝𝑝z is determined by 𝜀𝜀𝑝𝑝VBM and 𝜀𝜀𝑝𝑝∆𝑧𝑧 . Thus, figure 6 displays the change of 𝑧𝑧 𝜀𝜀𝑝𝑝VBM and 𝜀𝜀𝑝𝑝∆𝑧𝑧 for biaxial strained V-doped LNS-TiO2 nanosheet and unstrained state. 𝑧𝑧

14


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1

Figure 6. 𝜀𝜀𝑝𝑝VBM and 𝜀𝜀𝑝𝑝∆𝑧𝑧 versus biaxial strain from -8% to 8% for V-doped LNS-TiO2. 𝑧𝑧

2

As shown in figure 6, 𝜀𝜀𝑝𝑝∆𝑧𝑧 augments as compressive strain increases, whereas 𝜀𝜀𝑝𝑝VBM 𝑧𝑧

3 4

slightly reduces. According to the equation (8), when 𝜀𝜀𝑝𝑝VBM slightly reduces, the variation of 𝜀𝜀𝑝𝑝∆𝑧𝑧 𝑧𝑧 0 lim ∫ 𝐷𝐷(𝐸𝐸)d𝐸𝐸 𝐸𝐸0 →0− 𝐸𝐸0 0 ∫−∞ 𝐷𝐷(𝐸𝐸)d𝐸𝐸

5

is ascribed to

6

the tensile strain case, 𝜀𝜀𝑝𝑝VBM continuously enhances, and 𝜀𝜀𝑝𝑝∆𝑧𝑧 remains zero, which is caused by 𝑧𝑧

, i.e. electron occupancy rate in the vicinity of the Fermi level. In

8

no electron occupancy near the Fermi level. Therefore 𝜀𝜀𝑝𝑝VBM and 𝜀𝜀𝑝𝑝∆𝑧𝑧 is responsible for 𝑧𝑧

9

discussion, the analyze of electron occupancy near the Fermi level is required for the study of

7

variation of 𝜀𝜀𝑝𝑝z in the system with tension and compression, respectively. Based on the above

11

𝜀𝜀𝑝𝑝∆𝑧𝑧 . In figure 7, it is clear that impurity states near the Fermi level are mainly dominated by the

12

the antibonding interaction between V 3dz2 and bonding state (ONN 2pz−Ti 3dxz). The above

13

scenario is also supported by the crystal orbital overlap population (COOP) analysis (see figure

14

S3).

10

V 3dz2, the pz-orbital of nearest neighbor O of vanadium (ONN) and the Ti 3dxz, and derived from

15



1 2

Figure 7. (a) The partial electron density isosurfaces (0.007 eÅ-3) corresponding to impurity

3

states. Color code: Ti: blue; O: red; V: dark blue. ONN indicates the oxygen atom connected to

4

vanadium. (b) Calculated partial density of impurity states for V-doped LNS−TiO2. The vertical

5

dash line indicates the Fermi level.

6

The corresponding bonding formation in the V-doped LNS−TiO2 nanosheet with and

7

without biaxial strain is reasonably deduced in figure 8(b). The interaction between ONN 2pz and

8

Ti 3dxz leads to a splitting of occupied bonding state Ⅰ and antibonding state Ⅰ. What’s more,

9

formed bonding state Ⅰ (ONN 2pz−Ti 3dxz) is hybridized with V 3dz2 to form occupied bonding

10

state Ⅱ (not shown) and antibonding state Ⅱ around Fermi level. When compressive strain is

11

applied in the V-doped LNS−TiO2 nanosheet, the splitting between ONN 2pz and Ti 3dxz is

12

strengthened with the variation of ONN−Ti distance (see figure 8(a)), which results in reduction

13

of bonding state Ⅰ. This behavior can weaken the hybridization between bonding state Ⅰ and V

14

3dz2. Meanwhile, under compressive strain, ONN−V bond can be elongated (see figure 8(a)) to

15

weaken the splitting between them. Hence, a synergetic effect arising from the reduction of

16

bonding state Ⅰ and ONN−V bond elongation is responsible for transferring antibonding state Ⅱ to

17

below the Fermi level. 16


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1

In the system with tensile strain, the change of the hybridization between ONN pz and Ti dxz

2

is similar to that under the compression. The bonding state Ⅰ shifts up to approach V 3dz2,

3

strengthening the hybridization between them and forming a large splitting in bonding state Ⅱ−

4

antibonding state Ⅱ. On the other hand, it can be seen that ONN−V bond elongates when tension

5

is applied, which is able to weaken the hybridization between bonding state Ⅰ and V 3dz2. Whereas,

6

ONN−V bond elongation could not be expected offset the impact caused by enhancement of

7

bonding state Ⅰ. Therefore, the antibonding state Ⅱ improves and keeps stay above the Fermi level.

8 9

Figure 8. (a) ONN−V and ONN−Ti bond length as a function of biaxial strain. (b) Schematic

10

illustration of bond formation among V, ONN and Ti. The horizontal black dash line indicates the

11

Fermi level. The blue, black and red indicates energy level of V−doped LNS−TiO2 under tension,

12

unstrain and compression, respectively. The blue and red arrows indicate tension and compression

13

is applied to doped system.

14

Then, the shift of 𝜀𝜀𝑝𝑝VBM is discussed through valence band movement. At the beginning, 𝑧𝑧

15

valence band of V−doped LNS−TiO2 is mainly located in the range from -8 eV to -2 eV. As shown

16

in figure 9, after applying tensile strain, the valence band edge was significantly enhanced, which 17



2

is responsible for the increasing of 𝜀𝜀𝑝𝑝VBM . When compression is applied, valence band edge has 𝑧𝑧

3

to valence band movement.

1

a slightly decline, corresponding to the reduction of 𝜀𝜀𝑝𝑝VBM . Thus, the shift of 𝜀𝜀𝑝𝑝VBM is attributed 𝑧𝑧 𝑧𝑧

4 5

Figure 9. The evolution of valence band edges under biaxial strain. The red and black symbols

6

indicate the variation of lower and upper valence band edge versus biaxial strain.

7

4. Conclusion

8

In this work, the first−principle calculations were performed to investigate the photocatalytic

9

HER activity of LNS−TiO2 nanosheet with transition metal doping and biaxial strain. The change

10

of hydrogen adsorption free energy of doped nanosheets is well-understood in light of the pz band

11

center model. Our results suggest that the HER activity of LNS−TiO2 nanosheet can be improved

12

by introducing optimal dopants. Especially, Cr−doped LNS−TiO2 nanosheet is considered to be

13

the highly efficient HER photocatalyst here since its hydrogen adsorption free energy is much

18


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1

close to 0 eV. Moreover, HER activity can be further modified by imposing external biaxial strain

2 3

as well. It is worth pointing out that △GH* of V−doped LNS−TiO2 nanosheet can be decreased

4

adsorption energy of about 0 eV implying the high HER activity. The up-shift of pz band center

5

level is responsible for the variation of △GH*. When compressive strain is applied, impurity states

6

with both the tensile or compressive biaxial strain. And the 7% tensile strain leads to the hydrogen

shift down to the low energy part and cross the Fermi level, which results in the reduction of

8

△GH*. Under tensile strain, the origin of improvement of HER activity is mainly attributed to the

9

of atomic thin TiO2-based nanosheet and provides a new potential candidate for the photocatalytic

7

up-shift of valence band. In short, our findings unveiled here will be beneficial for the application

10

HER of water splitting.

11

ASSOCIATED CONTENT

12

Supporting information

13

The calculated PDOS for TM-doped LNS−TiO2 (TM＝V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Nb)

14

with H adsorption (Figure S1). Calculated energy differences between single-layered nanosheet

15

and bulk structure of TM-doped LNS-TiO2 (Table S1). The projected pz-orbital density of states

16

of oxygen atom at reactive sites in V-doped LNS−TiO2 under biaxial strain (Figure S2). Crystal

17

orbital overlap population for V, ONN and Ti in V−doped LNS−TiO2 nanosheet (Figure S3).

18

Atomic position of TM-doped LNS−TiO2 nanosheet without and with H adsorption, as well as

19

V-doped LNS−TiO2 nanosheet with strain. 19



1

AUTHOR INFORMATION

2

Corresponding Author

3

* E−mail: [email protected]

4

Acknowledgements

5

This work was supported by the National Natural Science Foundation of China (11247224)

6

and (11804023), and the Natural Science Foundation of Tianjin (18JCQNJC02700).

7

References

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

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21

23



1

TOC Graphic

2

24


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Figure 1. The optimized structures of TM-doped and undoped LNS-TiO2 nanosheet adsorbed H atom. Color code: Ti: blue; O: red; H: white; TM: dark blue. 338x118mm (300 x 300 DPI)



Figure 2. The calculated free energy diagram for HER of doped LNS-TiO2 nanosheet. 226x168mm (300 x 300 DPI)


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Figure 3. Schematic illustration of bond formation between the surface and the adsorbate (Ads.) 226x194mm (300 x 300 DPI)



Figure 4. Projected pz-orbital density of states of oxygen atom at reactive sites in TM-doped LNS-TiO2. Light yellow area corresponds to the filled states up to the Fermi level. The red and purple dash lines indicate ε_(p_z ) and the Fermi level, respectively. The blue double arrow lines present the distance between ε_(p_z ) and the Fermi level. 107x90mm (300 x 300 DPI)


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Figure 5. Free energy for HER and pz band center versus biaxial strain from -8% to 8% for V-doped LNSTiO2. 259x179mm (300 x 300 DPI)



Figure 6. ε_(p_z)^VBM and ε_(p_z)^ versus biaxial strain from -8% to 8% for V-doped LNS-TiO2. 268x179mm (300 x 300 DPI)


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Figure 7. (a) The partial electron density isosurfaces (0.007 eÅ-3) corresponding to impurity states. Color code: Ti: blue; O: red; V: dark blue. ONN indicates the oxygen atom connected to vanadium. (b) Calculated partial density of impurity states for V-doped LNS-TiO2. The vertical dash line indicates the Fermi level. 158x48mm (300 x 300 DPI)



Figure 8. (a) ONN-V and ONN-Ti bond length as a function of biaxial strain. (b) Schematic illustration of bond formation among V, ONN and Ti. The horizontal black dash line indicates the Fermi level. The blue, black and red indicates energy level of V-doped LNS-TiO2 under tension, unstrain and compression, respectively. The blue and red arrows indicate tension and compression is applied to doped system. 345x115mm (300 x 300 DPI)


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Figure 9. The evolution of valence band edges under biaxial strain. The red and black symbols indicate the variation of lower and upper valence band edge versus biaxial strain. 226x184mm (300 x 300 DPI)



Figure S1. Calculated PDOS for TM-doped LNS-TiO2 (TM＝V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Nb) with H adsorption. The vertical dash line indicates the Fermi level. 119x232mm (300 x 300 DPI)


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Figure S2. Projected pz-orbital density of states of oxygen atom at reactive sites in V-doped LNS-TiO2 under biaxial strain. Light yellow area corresponds to the filled states up to the Fermi level. The red dash lines indicate ε_(p_z ). 124x237mm (300 x 300 DPI)



Figure S3. Crystal orbital overlap population for V, ONN and Ti in V-doped LNS-TiO2 nanosheet. 228x186mm (300 x 300 DPI)


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Tunable Photocatalytic HER Activity of Single-Layered TiO2

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