Electronic supplementary information (ESI)

Constructing of novel phosphonate-based MOF/P-TiO2 Heterojunction

Photocatalysts: enhanced photocatalytic performance and mechanistic insight

Tianyu Zeng a, Dajun Shi b, Qingrong Cheng a, *, Guiying Liao c, Hong Zhou a and Zhiquan Pan a, *

a. Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan

Institute of Technology, Wuhan 430073, P. R. China.

b. Three Gorges Public Inspection and Testing Center, Yichang, P. R. China

c. Engineering Research Center of Nano-Geo Materials of Ministry of Education,

China University of Geosciences, Wuhan 430074, P. R. China

Corresponding author:

*(Z.P.) E-mail: zhiqpan@163.com

*(Q.C.) E-mail: chengqr383121@sina.com

Total number of pages: 23

Total number of Schemes: 2

Total number of Tables: 8

Total number of Figures: 20

Electronic Supplementary Material (ESI) for Environmental Science: Nano.This journal is © The Royal Society of Chemistry 2020

Analysis of Chromium (VI) reduction

Cr(VI) concentrations were measured using the 1,5-diphenylcarbazide(DPC)

colorimetric method by monitoring the purple complex at 540 nm on a UV-vis

spectrophotometer. The specific operation for measurement is as follows: In a 50 mL

volumetric flask, 2 mL of sample was mixed with 0.5 mL of H2SO4 solution (H2SO4:

H2O=1:1) and 0.5mL of H3PO4 solution (H3PO4: H2O=1:1). After adding water to the

constant volume, 2 mL of freshly prepared 0.25% (w/v) DPC in acetone was added to

the volumetric flask. The mixture was then shaken for about 15-30 s and allowed to

stand for 10-15 min (for full color development). The red-violet to purple color was

measured and the absorbance at 540 nm was denoted as Ai (i represents different

reaction time intervals.

Experiment of Photoluminescence Spectra (PL)

•OH radical reactions were performed as follows. 4.00 mg of the photocatalyst

was suspended in 40.0 mL aqueous solution containing 2.00×10-3 M NaOH and

5.00×10-4 M terephthalic acid. Before exposure to light, the suspension was stirred in

the dark for 1 h. And then 1.00 mL sample was removed every 10 min and

centrifuged for fluorescence spectroscopy measurements. A fluorescence

spectrophotometer was used to measure the fluorescence signal of the 2-hydroxy

terephthalic acid generated. The excitation light wavelength used in recording

fluorescence spectra was 320 nm and the emission wavelength appeared to be ~426

nm.

Radical Trapping Experiments

The radical trapping experiments just have one more additional procedure than

the RhB photocatalytic process: a certain amount of radical scavenger need to be

added to the system of photogradation before 500 W Xe lamp turned on at room

temperature. The specific dosage of t-BuOH, TEOA and NBT are 48.0 μL, 67.0 μL,

and 0.0408 g respectively. The RhB concentration changes were monitored by

measuring the absorption intensity at its maximum absorbance wavelength of λ = 554

nm using a UV-visible spectrophotometer.

First-principles calculation

In our work, the first-principles calculation has been employed to describe the

electronic properties with the projector augment wave method based on density

functional theory.[1] It was performed by the Vienna Ab-initio Simulation Package

(VASP). [2] In addition, the generalized gradient approximation of the PBE (Perdew-

Burke-Enzerhof) functional has been used to describe the exchange-correlation

function in our systems. [3-4] The cutoff energy is set as 520 eV, and structure

relaxation was performed until the convergence criteria of energy and force reached

10-6 eV and 0.05 eVÅ−1, respectively. The Brillouin zone has been sampled with

4×4×4 k-points for our structures. Finally, Our calculations are conducted for a TiO2

(2×4×1) supercells containing five P atoms in the substitutional site of Ti atoms to

investigate the P doping effect, corresponding the proportion of P:Ti=5:27.

Single-crystal structure analysis

PO3H2

H2O3P

Scheme S1. The structure of ligand H4L.

Table S1. Crystallographic data for the MOF.

Empirical fomula (Formula weight) C10H14CdO6P2 (404.55)

CCDC deposit no. 1946452

Temperature/K 173

Crystal size 0.21*0.03*0.02

Crystal system monoclinic

Space group p 21/c

a; b; c(Å) 4.6221(4); 19.4673(19); 14.7079(13)

α;β;γ/o 90; 98.627(4); 90

V/Å3 1308.4(2)

Z 4

D calc/g cm−3 2.054

μ /mm−1 1.932

F(000) 800

θrange(o) 2.802- 24.999

Limiting indices -5<=h<=4; -23<=k<=23; -16<=l<=17

Reflections total 2283

Reflections unique 1544

Goodness-of-fit on F2 1.015

Rint; R1; wR2 0.0936; 0.0500; 0.0904

Table S2. Selected bond distances (A°) and angles (º) for the MOF.

Bond lengths (A° ) Bond lengths (A° )

Cd1-O3 2.153(5) O2-Cd1 2.289(4)

Cd1-O4 2.192(5) Cd1-O2 2.289(5)

Cd1-O2 2.238(5) Cd1-O6 2.304(4)

Bond angles (º) Bond angles (º)

O2-Cd1-O2 78.79(18) O3-Cd1-O4 105.37(19)

O3-Cd1-O6 89.85(18) O3-Cd1-O2 121.3(2)

O4-Cd1-O6 87.25(17) O4-Cd1-O2 133.13(19)

O2-Cd1-O6 88.34(16) O3-Cd1-O2 88.94(17)

O2-Cd1-O6 164.12(18) O4-Cd1-O2 108.33(18)

Fig. S1 (a) Asymmetric unit of the complex; (b) The coordination environment of

Cd(II) in MOF; (c) the coordination mode of the ligand in MOF.

Fig.S2 (a) The 44-membered rings of the complex; (b) 3D pore structure unit along

the a-axis; (c) 3D stacking diagram viewed along b-axis and c-axis; (d) Topological

representation of the network of the complex.

Fig. S3 3D frame structure along the a-axis.

FT-IR analysis

Fig.S4 (a) FTIR spectra of TiO2 and P-TiO2; (b) FTIR spectra of MOF, MOF0.5/P-

TiO2 and P-TiO2.

XPS analysis

Fig. S5 Ti 2p XPS spectrum of pure TiO2 (a), P-TiO2 (b).

Morphologies of the prepared materials

Fig. S6 SEM images of TiO2 (a), P-TiO2 (b) and MOF1.5/ P-TiO2 (c and d).

Fig. S7 SEM mapping of P-TiO2.

BET analysis

Fig. S8 (a) N2 absorption isotherms of the MOF, P-TiO2 and MOF1.5/P-TiO2; (b) the

particle size distribution obtained by NLDFT kernel.

Table S3. Specific surface areas, pore volumes and mean pore diameters for the MOF,

P-TiO2 and MOF1.5/P-TiO2.

Sample MOF P-TiO2 MOF1.5/P-TiO2

Specific surface area (m2g-1) 3.812 74.16 50.60

Pore volume (cm3 g-1) 0.00349 0.3326 0.2011

Average pore diameter (nm) 1.9375 17.94 12.93

Photocatalytic activities

Fig. S9 The band gap energy MOF0.1/ P-TiO2 (a), MOF0.5/P-TiO2 (b) and MOF1.0/P-

TiO2 (c); the valence band positions of MOF0.1/ P-TiO2 (d), MOF0.5/P-TiO2 (e) and

MOF1.0/P-TiO2 (f).

Fig S10. The band gap energy, valence band positions and conduction band levels

(Ec = Ev - Eg) of selected photocatalysts.

Fig. S11 Time-dependent UV−vis absorption spectra of the RhB solution in the

presence of MOF1.5/ P-TiO2 sample.

Table S4 TOC analysis of the treated solution.

EntryInitial concentration

(mg L-1)

End-point concentration

(mg L-1)

1 9.747 (exclude) 2.054 (exclude)

2 9.328 1.714

3 9.432 1.628

Average concentration 9.380 1.671

TOC removal 82.20 %

LC-MS analysis

Fig. S12 Total ion chromatogram (TIC) of RhB degradation under different

illumination time over MOF1.5/ P-TiO2 sample.

Fig. S13 (a) Total ion chromatogram (TIC) of RhB; (b) the mass spectrogram of

tR=13.96 min.

Fig. S14 The TIC of RhB after 15 min of photocatalytic operation under Xe lamp

irradiation (a); the mass spectrogram of tR=13.29 min (b), tR=11.80 min (c), tR=10.71

min (d), tR=8.77 min (e), tR=15.88 min (f).

Fig. S15 The TIC of RhB after 25 min of photocatalytic operation under Xe lamp irradiation (a); the mass spectrogram of tR=0.73 min (b), tR=15.54-17.50 min (c), tR=0.21-21.92 min (d).

It is clearly observed that there is an intense prominent ion with m/z = 443 (Fig. 13b), which can be attributed to RhB. From the analysis of MS and the previous studies, the dye is N-de-ethylated in a stepwise manner accompanying a color change of the dispersion from initial red to colorless, and it is degraded by a series of successive deethylation reaction from N, N, N′, N′-tetraethylated rhodamine to rhodamine. In Fig. 14b, the major peak was located at m/z = 415 (tR = 13.29 min), owing to the loss of the ethyl group on the RhB dye structures. By the further removal of the residual ethyl groups on the RhB structures, the m/z values of 387 (tR = 11.80 min), 359 (tR = 10.71 min), 331 (tR = 8.77 min), 318 (tR = 15.88 min) were gradually detected in Fig. S14 (c-f). The m/z values of 274 (tR = 15.88 min) is assigned to the possible product which is attacked by •OH after removing of the ethylamino and carboxyl on the RhB dye structures (Fig. S14f and Fig. S15c). After 25 min of photocatalytic operation, the major peak was located at m/z = 116 (tR=0.21-21.92 min, Fig. S16d), might be assigned to the butenedioic acid (cis- or trans-). On the basis of

all the above experimental results, the possible pathways of degradation of RhB under Xe illumination are depicted in Scheme S2.

Scheme S2. Schematic illustration for reaction pathway of RhB degradation over

MOF1.5/ P-TiO2 sample.

Table S5 Comparison of the RhB degradation capacity of MOF1.5/P-TiO2 with other photocatalysts.

Catalyst/mg V (mL)/C0 (mg L−1) Light source Time (min) 1st cycle efficiency (%) Ref.

MOF1.5/P-TiO2/10 60/10 300 W Xe lamp 25 97.6 This work

m-TiO2-NTs/20 100/20 125 W Hg lamp 60 100a [5]

H3PW12O40/TiO2-g-C3N4 /100 100/20 300 W Xe lamp 70 99.3 [6]

Fe3O4/TiO2/g-C3N4/40 40/20 500 W Xe lamp 80 96.4 [7]

Ag-SrTiO3/TiO2/20 50/20 300 W Xe lamp 60 82.2 [8]

TSC NFM/50 100/8 15 W UV lamp 120 91.2b [9]

a m-TiO2-NTs = mesoporous TiO2 nanotubes;

b TSC NFM = a novel flexible and porous TiO2/SiO2/C nanofiber mat

Fig. S16 Cr 2p spectrum bound to MOF0.5/P-TiO2 hybrid materials after

photocatalytic Cr(VI) reduction.

Fig. S17 Time-dependent UV-vis absorption spectra of the Cr(VI) solution in the

presence of MOF0.5/P-TiO2 sample.

Table S6 Comparison of the Cr(VI) reduction capacity of MOF0.5/P-TiO2 with other photocatalysts.

Catalyst/mg V (mL)/ C0 (mg L−1)/pH Light source Time (min) 1st cycle efficiency (%) Ref.

MOF0.5/P-TiO2/20 60/10/2.0 300 W Xe lamp 60 96.1 This work

TiO2@MoSe2-30/20 40/20/5.0 300 W Xe lamp 60 100 [10]

BUC-21/TNTs/40 250/10/2.0 500 W Hg lamp 20 90.0a [11]

0.2CDs-TNs /50 50/10/3.0 500 W Xe lamp 120 99.2b [12]

GO/TiO2/500 1000/10/2.0 8 W Hg lamp 420 99.6c [13]g-C3N4/UiO-66/100 200/10/2.0 300 W Xe lamp 40 99.0 [14]

a BUC-21 = a chemically stable 2D MOF constructed from cis-1,3-dibenzyl-2-oxo-4,5-imidazolidinedicarboxylic acid as linker and Zn2+, TNTs

= titanate nanotubes;

b CDs-TNs = carbon dots-TiO2 nanosheets;

c GO/TiO2 = a graphene oxide@TiO2 composite.

Fig. S18 (a) Photocatalytic reduction of Cr(VI) over MOF0.5/ P-TiO2 with or without

electron scavenger of K2S2O8 (0.1 mmol); (b) Pseudo-first-order kinetics curves of

photocatalytic Cr(VI) reduction reactions shown in (a).

Reusability and stability of MOFx/P-TiO2

Table S7 The ICP result of centre metal ions concentration in RhB and Cr(VI)

aqueous

solution after photocatalysis

System ICP result removal rate

RhB and MOF1.0/ P-TiO2 0.760 ppm 1.09%a

Cr(VI) and MOF0.5/P-TiO2 0.380 ppm 1.36%b

a MOF1.0/ P-TiO2: 30 mg; RhB: 60 mL, 10 ppm.

b MOF0.5/ P-TiO2: 30 mg; Cr(VI): 100 mL, 10 ppm.

Fig. S19 PXRD patterns of MOF1.0/P-TiO2 (red and dark yellow) and MOF0.5/P-TiO2

(blue and magenta) before and after recycling 5 times; recycling performance of

MOFx/P-TiO2 in photocatalytic degradation of RhB and reduction of aqueous Cr(VI)

under xenon lamp irradiation (b).

Fig. S20 Photosensitization pathway for enhancing photodegradation efficiency of

RhB over MOFx/P-TiO2 under Xe lamp irradiation.

The process is described in detail below:

Table S8 Comparison of calculation results and experimental results of the VBM and

CBM of materials.

P-TiO2 VBM vs. NHE (eV) CBM vs. NHE (eV)

Calculation result 4.023 -0.190

Experimental result 4.09 -0.19

MOF

Calculation result 2.796 -0.513

Experimental result 2.85 -0.45

The absolute vacuum scale (EAVS) is taken as 0 eV and the normal hydrogen electrode

scale (Ee) as 4.6 eV.[15]

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