1

Supplementary Information

Enhanced CO2 Photoreduction by Graphene-Porphyrin Metal-

Organic Framework under Visible Light Irradiation

Nasrin Sadeghia,b, Shahram Sharifniaa, Trong-On DOb,*

a Catalyst Research Center, Chemical Engineering Department, Razi University, Kermanshah, Iran.

b Department of Chemical Engineering, Laval University, Quebec, G1V0A8, Canada.

Email: Trong-On.Do@gch.ulaval.ca

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2018

2

1. Experimental Details

1.1. Materials

Tetrakis (4-carboxyphenyl) porphyrin (TCPP) was purchased from TCI AMERICA.

Graphene oxide (GO) was supplied by Graphenea. Aluminum chloride, ammonium

hydroxide, triethanolamine (TEOA) and ethylene glycol were purchased from Fluka,

Caledon, Alfa and Sigma-Aldrich, respectively. Aceton and dimethylformamide (DMF) were

supplied by Fisher Chemical. All chemicals were used as received without further

purification. The deionized water was used in catalyst preparation. The used CO2 gas was

ultra-high pure (99%).

1.2. Preparation of NH2-Graphene

100 mg of GO was added to 40 mL of ethylene glycol under ultrasonication. After a

while, 1 mL of ammonia water was added and the solution was transferred to a Teflon lined

autoclave for solvothermal reaction at 180 °C for 10 h. The resulting precipitate was filtered,

repeatedly washed with distilled water and dried at 60 °C for 24 h 1.

1.3. Preparation of Al-PMOF

Al-PMOF was synthesized according to the way provided in previous papers 2, 3. 400 mg

of TCPP and 130 mg AlCl3 were introduced into 40 mL deionized water. The solution was

stirred for 30 min at room temperature and then transferred to a Teflon lined autoclave and

heated at 180 °C for 24 h. The solution was allowed to cool down to room temperature. The

solid was recovered by centrifugation and washed with DMF and acetone several times in

order to remove the unreacted porphyrin. Finally, the obtained solid was dried at 60 °C.

3

1.4. Preparation of NH2-Graphene/Al-PMOF

400 mg of TCPP and 130 mg AlCl3 were introduced into 40 mL deionized water.

Varying amount of NH2-Graphene (5, 15, 25 wt %: based on the total mass of starting solid

materials) was added to the solution. The solution was stirred for 30 min at room temperature

and then transferred to a Teflon lined autoclave and heated at 180 °C for 24 h. The solution

was allowed to cool down to room temperature. The solid was recovered by centrifugation

and washed with DMF and acetone several times. Finally, the obtained solid was dried at 60

°C.

1.5. Catalyst characterization

Powder XRD patterns were recorded on Bruker SMART APEXII X-ray diffractometer

equipped with Cu Kα radiation source (λ = 1.5418 Å). N2 adsorption-desorption isotherms of

the samples were obtained at 77 K using a Quantachrome Autosorb-1MP analyzer. Prior to

the measurements, the samples were outgassed under vacuum for 6 h at 150 °C. Scanning

electron microscopy (SEM) images were obtained using a JEOL 6360 operated at 15 kV. X-

ray photoelectron spectroscopy (XPS) measurements were carried out in the ion-pumped

chamber (evacuated to 10-9 Torr) of a photoelectron spectrometer (Kratos Axis-Ultra)

equipped with a focused X-ray source (Al Kα, hυ = 1486.6 eV). UV-vis spectra were

recorded on a Cary 300 Bio UV-visible spectrophotometer. The Fourier transform infrared

spectroscopy (FTIR) was recorded with an ALPHA (Bruker, Germany) FTIR

spectrophotometer. Samples of 1–2 mg were mixed with 100 mg KBr and pressed into

translucent disks at room temperature. All spectra were taken in the range 400-4000 cm-1 at a

resolution of 4 cm-1. The photoluminescence (PL) spectra of samples were carried out with a

Varian Cary Eclipse fluorescence spectrophotometer (excitation wavelength: 420 nm). The

diffuse reflection spectroscopy (DRS) of the powders was obtained by using Avantes

4

spectrometer (Avaspec-2048-TEC) with a reflectance sphere in the range, 300-900 nm. In

order to perform electrochemical measurements, the working electrodes were prepared by

adding 15 µL of slurry to the surface of fluride-tin oxide (FTO) glass plates and covering

approximately 1 cm2. The slurry was prepared by mixing 0.02 g ample and 2 mL ethanol.

Electrochemical measurements were conducted with an Ivinum-Vertex Electrochemical

system in a conventional three-electrode cell, using a Pt plate as the counter electrode and

Hg/HgCl2 electrode (3 M KCl) as the reference electrode. The experiments were performed at

room temperature in 0.2 M Na2SO4 electrolyte deoxygenated using N2 stream.

1.6. Photoreactor system

Fig. S1 illustrates the schematic of the experimental setup for photocatalytic activity

measurement of synthesized photocatalysts. CO2 Photoreduction was carried out in an outer

irradiation-type quartz photoreactor designed by our laboratory. In order to control

temperature, the photoreactor was equipped with a condenser which was connected to the

cooling water recirculation system. The solution temperature was kept constant at 30 °C.

Herein, a set of three 125 W medium-pressure mercury lamp was used as the light source,

which are located externally around the photoreactor. To filter out UV light, a glass bulb with

special coating was applied (Osram GmbH, Germany). Typically, 50 mg of the photocatalyst

was dispersed in a solution containing MeCN and TEOA (60 ml, 5:1 v/v) by a magnetic

stirrer during the 6 h irradiation. Then, the photoreactor was sealed and prior to the

irradiation, the mixture was degassed by CO2 for about 30 min to ensure all impurities and

trapped air were completely removed. The products in the liquid phase were analyzed using a

HPLC (Agilent 1200) with Kromasil 5u C18 column. In order to detect alcohols, the reaction

solution has been analyzed with a gas chromatography system (Agilent) equipped with a DB-

5 column and a flame ionization detector (FID). No signal for alcohols can be detected.

5

Moreover, the gaseous products (CH4, CO) were examined by using an online commercial

gas chromatograph GC-CGCA-1 apparatus. These analyses were done isothermally at 50 °C

using a parallel setup of two packed columns (Molecular sieve and Propak Q). No signal can

also be observed for gaseous products.

Fig. S1. Schematic of experimental setup

6

Fig. S2. SEM images of (a) GO, (b) NH2-rGO, (c) Al-PMOF and (d) NH2-rGO (5 wt %)/Al-

PMOF, (e) NH2-rGO (15 wt %)/Al-PMOF, (f) NH2-rGO (25 wt %)/Al-PMOF.

The morphology of as prepared composites was analyzed by SEM. For comparison, the

SEM images of GO, NH2-rGO and Al-PMOF are also included. As illustrated in Fig. S2(b),

the NH2-rGO consist of randomly aggregated thin sheets that are closely associated and

forming a porous and disordered network. In the Al-PMOF sample (Fig. S2 (c)), the cubic

d

e

f

c

a

b

7

crystals of Al-PMOF are visible. Fig. S2 (d) demonstrates that the presence of the NH2-rGO

in the sample lead to change the cubic structures of Al-PMOF to platelets. As shown in Fig.

S2 (e and f), with increasing the content of NH2-rGO in the sample, the thin platelets more

agglomerated and decreased the crystallinity of the structure as shown in XRD. Notably,

graphene-based materials have extraordinary properties, such as large surface area, high

electrical conductivity, also, graphene acts as electron acceptor-transporter. Therefore, the

platelets structures increase the electrons generation, also, accessibility to the CO2 molecules,

consequently resulting in enhancement in CO2 photoreduction. Nevertheless, the overloading

of graphene can cause a negative effect on the photocatalytic reaction. With increasing the

content of NH2-rGO in the sample, the thin platelets more agglomerated and decrease the

crystallinity of the structure. In other words, when the amount of the graphene increased in

the structure, the Al-PMOF molecules were wrapped by the amine functionalized graphene

sheets and acts like a shield to prohibit the incident light adsorption by photocatalyst which

leads to reducing the generation rate of photoinduced electron-hole pairs 4, 5. All of these are

not favorable for CO2 photoreduction which also will show in photoreactor test, Fig. S9, the

CO2 photoreduction decreased with increasing the amount of the NH2-rGO in the structure.

8

Fig. S3. FTIR spectra of GO, NH2-rGO, NH2-rGO (5 wt %) / Al-PMOF and TCPP.

The FTIR spectra of pristine GO and NH2-rGO were shown in Fig. S3. In the spectrum

of GO, the absorption peaks at 1034, 1366 cm-1 are assigned to stretching vibrations of epoxy

C–O, C–OH bonds, also, 1720 and 1629 cm-1 can be ascribed to C=O in carboxylic acid and

carbonyl moieties. The broad absorption peak at 3000–3500 cm-1 is related to the O–H

stretching vibrations of hydroxyl groups on GO surface and adsorbed water molecules 6-9.

After amine functionalization, these peaks almost disappeared while new peaks arise at 1637,

1127–1529 and 3405 cm-1 that can be ascribed to the antisymmetric C–N stretching

vibrations coupled with out-of-plane NH2 and NH modes, also the N–H stretching vibrations

6, 10. Generally, the above results clearly demonstrate the successful linkage of nitrogen to

rGO. The FTIR spectrum of TCPP (Fig. S3) shows typical symmetric and asymmetric

400100016002200280034004000

Tra

nsm

itan

ce

Wavenumber (cm-1)

TCPP

NH2-rGO/Al-PMOF

NH2-rGO

GO

9

stretching bands of the pyrrole ring of ν (C–H), (C=C) and (C=N) of TCPP ligand in the

range 679-1700 cm-1. The N–H vibration of the pyrrole ring is seen at 3390 cm-1. The C=O

stretching vibration of carboxylate group in TCPP is observed at 1692 cm-1 11-14. The FTIR

spectrum of NH2-rGO (5 wt %)/ Al-PMOF (Fig. S3) more resembles that of TCPP. The

vibration band at 1652 cm-1 is assigned to the C=O stretch of carboxylate group. Also, the C-

N and C=N stretching vibration in NH2-rGO (5 wt %)/ Al-PMOF are observed at 1638 and

1580 cm-1.

10

Fig. S4. (a) Tauc plot of Al-PMOF (b) Mott-Schottky plots for Al-PMOF in 0.2 M Na2SO4

aqueous solution (pH=7). Inset: Energy diagram of the HOMO and LUMO levels of Al-PMOF.

Mott–Schottky plots of Al-PMOF were measured at frequencies of 500, 1000 and 1500 Hz

(Fig. S4(b)). According to the positive slope of the obtained C-2 values (vs the applied

potentials), suggesting that the Al-PMOF is n-type semiconductor. The intersection point is

independent of the frequency, and the flat band position determined from the intersection is ~

-1.12 V vs Ag/AgCl (i.e., -0.92 V vs NHE). Forasmuch as it is generally accepted that the

1,5 1,6 1,7 1,8 1,9 2,0 2,1 2,2 2,3

(αhʋ)2

(eV

)2

Eg (eV)

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

-1,5 -1,3 -1,1 -0,9 -0,7 -0,5 -0,3 -0,1 0,1 0,3 0,5 0,7 0,9

10

10C

-2(F

-2)

Potential (V) vs. Ag/AgCl

500 Hz

1000 Hz

1500 Hz

LUMO

HOMO

-0.92 V

-1.12 V

0.97 V

(a)

(b)

1.89 eV

11

bottom of the conduction band (LUMO) in the n-type semiconductors is almost equal to the

flat-band potential, the LUMO of Al-PMOF is -0.92 V vs NHE. With the bandgap energy of

Al-PMOF estimated to be 1.89 ev, from Tauc plot (Fig. S4 (a)), its valence band (HOMO) is

0.97 V vs NHE, (Fig. S4 (b)).

12

Fig. S5. Tauc plot of NH2-rGO(5 wt%)/ Al-PMOF

1,5 1,6 1,7 1,8 1,9 2,0 2,1 2,2 2,3

(αhʋ)2

(eV

)2

Eg (eV)

NH2-rGO (5 wt%)/Al-PMOF

13

Fig. S6. X-ray photoelectron spectroscopy curves for the elements of (a,b) C1s (c,d) N1s and (e)

Al2p in two samples of NH2-rGO and NH2-rGO (5 wt%)/Al-PMOF.

The scanned C 1s spectrum of NH2-rGO in Fig. S6 (a) can be deconvoluted into five peaks

of C–C / C=C (284.35 eV, sp2–hydridized carbon), C=O (287.04 eV), C–O (286.65 eV), O–

C=O (289.71 eV) and C–N (285.6) 15, 16. Notably, from NH2-rGO and NH2-rGO (5 wt %)/

395 397 399 401 403 405

(c) NH2-rGO

N1s

C=N 398.01

C-N 399.39

396 397 398 399 400 401 402

Binding Energy (eV)

N-Al porphyrin 397.48

C-N 399.57

N-Al bridging

400.5

C=N 398.32

NH2-rGO/Al-PMOF

N1s (d)

70 71 72 73 74 75 76

Binding Energy (eV)

NH2-rGO/Al-PMOF

Al 2p (e)

bridging Al 72.14

porphyrinic Al 72.69

280 282 284 286 288 290 292 294 296

C-C

284.35

C-O

286.65

C-N

285.6

C=O

287.04

(a)

O-C=O

289.71

NH2-rGO

C1s

282 284 286 288 290

C-C

284.5

C-N

285.72

C=O

287.89

O-C=O

289.52

NH2-rGO/Al-PMOF

C1s

C-O

286.08

(b)

14

Al-PMOF (Fig. S6 (a) and (b)), the intensity of sp2-hydridized carbon increases while those

of functional groups corresponding to the oxygenous part shows the opposite trends,

revealing partial restoration of the graphene structure and the effective amine

functionalization of the rGO 17, 18. This observation is consistent with the above FTIR results.

As shown in Fig. S6 (a) and (b), the presence of amine groups in the NH2-rGO and NH2-rGO

(5 % wt)/ Al-PMOF is confirmed by the peak at 285.6 and 285.72 eV for the C–N bond in the

primary amine 10, 17. The N 1s spectrum of NH2-rGO in Fig. 4 (c) can be fitted in two peaks

of pyridinic (398.01 eV) and primary amine (399.39 eV) 1, 19, 20. According to the obtained

peaks, it can be concluded that NH groups are functionalized on the structure of GO. It could

not be doped GO. In the doped cased, N will replace C atoms in GO. In current study case,

just functional groups (-OH, and -COOH or C-O-, etc.) on the surface react with NH3 to

produce C-N and C=N. Therefore, just two different species of N were obtained, which are

C-N and C=N. As shown in Fig. 4 (d), two new peaks emerged which are related to the Al

porphyrin (397.48 eV) and Al bridging (400.5 eV). The peak is assignable to the Al inside the

porphyrin core that connected to four electronegative nitrogen atoms from pyrrole groups

(397.48 eV) and the Al connected to the two electronegative oxygen groups of two adjacent

porphyrin sites (400.5 eV), respectively.

15

Fig. S7. Solid photoluminescence spectra of TCPP (black plot), Al-PMOF (blue plot) and NH2-

rGO (5 wt%)/ Al-PMOF (green plot) (excitation wavelength: 420 nm).

The PL properties of TCPP, Al-PMOF and NH2-rGO(5 wt%)/Al-PMOF were investigated in the

solid state at room temperature with an excitation wavelength 420 nm. The emission peaks of

photocatalysts are illustrated in Fig. S7. The emission intensity of Al-PMOF was diminished

obviously, indicating that lifetime of photogenerated charge carriers in Al-PMOF is longer than

TCPP. In other words, it implies that the photogenerated charge carriers can be separated efficiently

due to the electron transfer from excited TCPP to the conduction band of metal 12, 21, 22. When NH2-

rGO was added to Al-PMOF structure the emission intensity was quenched dramatically. Due to the

some graphene properties, e.g. Fermi level (0 V vs NHE), graphene is a good electron acceptor during

photocatalytic reaction. In other words, under light irradiation, the photoinduced electrons on

photocatalyst can be transferred to the graphene rapidly for reduction process, while photoinduced

holes can be remained on photocatalyst 4, 23. Overall, these results imply a high spatial separation of

the photoinduced electron-hole pairs on graphene-based photocatalysts which is favor for CO2

photoreduction. Also, this behavior can be validated by photoreactor tests (Fig. 3 (a)).

600 650 700 750 800

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

TCPP

Al-PMOF

NH2-rGO/Al-PMOF

16

Fig. S8. Nitrogen adsorption-desorption isotherms at 77 K for Al-PMOF and NH2-rGO(5 wt

%)/Al-PMOF.

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

V0

(m3

g-1

)

(P/Po)

NH2-rGO/Al-PMOF

Al-PMOF

17

Fig S9. Comparison of yield of HCOO- produced by different weight percentages of NH2-rGO

(5, 15, 25 wt %)/ Al-PMOF. Photocatalyst: 50 mg, MeCN/TEOA (5:1), solution volume: 60 mL

According to the results, it is apparent that increasing graphene has a negative effect on

photocatalytic activity. It may be explained by the fact that the high amount of graphene can

shield the photocatalyst and prevent the incident light adsorption by photocatalyst. In other

words, the generation rate of photoinduced electron-hole pairs will be reduced 4. Notably,

according to XRD and SEM results, the structures of NH2-rGO (15 wt%)/Al-PMOF and

NH2-rGO (25 wt%)/Al-PMOF are almost same, and also their photoactivities are nearly

similar.

685,6

479,8 476,4

0

100

200

300

400

500

600

700

800

5 wt% 15 wt% 25 wt%

nH

CO

O-/μ

mo

l g

cat-1

h -1

Weight percentage of NH2-rGO

18

Fig S10. Proposed mechanism of CO2 photoreduction over NH2-rGO/Al-PMOF.

TEOA

e-

e-

19

Table S1. Comparison of photocatalytic CO2 conversion performance.

Photocatalyst Sacrificial agent

Products HCOO ̶

(µmol/h. gcat.)

Ref.

NH2-rGO (5 wt%)/Al-PMOF

TEOA 685.6

This work

NH2-rGO (15 wt%)/Al-PMOF 479.8

NH2-rGO (25 wt%)/Al-PMOF 476.4

Al-PMOF 165.3

PCN-222 (Zr) TEOA 125 24

NH2-MIL-101 (Fe)

TEOA

445

25

MIL-101 (Fe) 147.5

NH2-MIL-53 (Fe) 116.25

MIL-53 (Fe) 74.25

NH2-MIL-83 (Fe) 75

MIL-83 (Fe) 22.5

UiO-66 (Zr) TEOA

0 26

NH2-UiO-66 (Zr) 26.4

MIL-125 (Ti) TEOA

0 27

NH2-MIL-125 (Ti) 16.28

MOF-525-Co TEOA CO: 200.6

28

CH4: 36.67

UiO-66/CNNS TEOA CO: 9.79 29

TiO2 0.1 M aqueous

NaOH

0

30 0.6 wt% ZnPc/TiO2 31.01

CdS (20 mg) TEOA

CO: 28.53 µmol.h-1 31

cocatalyst: Co-ZIF-9 (1 mg) H2: 12.93 µmol.h-1

Zn2GeO4/ZIF-8 0.1 M aqueous

Na2SO3

CH3OH: 0.22 32

1 wt% Pt-loaded Zn2GeO4/ZIF-8 CH3OH: 0.25

20

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