Supplementary information for

Sustained, photocatalytic CO2 reduction to CH4 in a continuous flow reactor

by earth-abundant materials: Reduced titania-Cu2O Z-Scheme

heterostructures

Shahzad Ali a,b, Junho Lee a, Hwapyong Kim a, Yunju Hwang a, Abdul Razzaq b, Jin-Woo Jung c, Chang-Hee Cho c and Su-Il In a,d,*

a Department of Energy Science & Engineering, DGIST, 333 Techno Jungang daero,

Hyeonpung-eup, Dalseong-gun, Daegu 42988, Republic of Korea

b Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, 1.5

KM Defense Road, Off Raiwind Road, Lahore 54000, Pakistan

c Department of Emerging Materials Science, DGIST, 333 Techno Jungang daero, Hyeonpung-

eup, Dalseong-gun, Daegu 42988, Republic of Korea

d Linde + Robinson Laboratories, California Institute of Technology, Pasadena, CA 91125, USA

*Email: insuil@dgist.ac.kr

1

Fig. S1 Schematic illustration for the synthesis of RT-Cux samples (x = 0.25, 0.50, 0.60, 0.75, 0.90 and 1.00 wt. % of Cu).

2

Fig. S2 XRD patterns for P25, RT and various RT-Cux samples.

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Fig. S3 (a) TEM image of overall scan for RT-Cu0.75 sample and elemental mapping of (b) Ti, (c) O, and (d) Cu within the sample.

4

Fig. S4 Raman spectra of P25 and RT, with inset showing raman shift.

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Fig. S5 (a) Tauc’s plot for Band gap estimation of P25, RT and RT-Cux samples, and VB-XPS of (b) P25, (c) RT and (d) Cu2O.

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Fig. S6 XPS survey scan for (a) RT-Cu0.75 and, (b) O 1s spectra for P25 and RT.

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Coumarin dye test:[1]

Coumarin dye test aimed to detect hydroxyl radicals (•OH) was performed by mixing 50 mg of

the photocatalyst with 50 ml (2 x 10-4 M) aqueous solution of coumarin dye (Sigma Aldrich,

95% ). The resultant mixture was stirred vigorously under dark for 30 min and then illuminated

under 1 sun conditions over a period of 1 h. After that the mixture was centrifuged and 4 ml of

supernatant was further analyzed by PL spectroscopy.

Fig. S7 PL spectra for RT, and RT-Cu0.75 originating from coumarin dye test

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Fig. S8 Different control tests for photocatalytic CO2 reaction.

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Table S1 Time resolved PL decay parameter for P25, RT and RT-Cu0.75.

τ1 (ns) A1 τ2 (ns) A2 τavg (ns)

P25 0.448 289.8 3.738 118.65 1.40

RT 0.3864 196.9 3.104 93.5 1.26

RT-Cu0.75 0.2742 110 2.586 45.99 0.96

I(t) = A 1 exp (‧ ‧ -tτ1

¿+A 2 exp (‧ ‧ -tτ 2

¿ (S1)

τ avg = A 1 ‧ τ1 + A 2 ‧τ 2 (S2) A⁖ 1 +A2 = 1

10

Calculations of Yield:

For a 100 ppm CH4 standard in He the yield can be calculated as,

CH 4 yield in ppm = Peak area of the CH 4 from tested samplePeak area of the standard CH 4

× 100 (S3)

CH 4 yield in µmole = [ CH4 yield in ppm ] × [ moles of the gaseous mixture containing CH4 ]

The yield in ppm can be written as,

CH 4 yield in ppm = µ mole of CH4 moles of gaseous mixture

moles of gaseous mixture = Volume of gaseous mixture ( L )Molar volume ( L mol‧ -1)

Assuming all molecules are ideal gases, at 298 K (25 ) temperature (T) and operating pressure℃

(P) of 1 atm,

Molar volume (L mol‧ -1) = R‧TP

= 0.08206 atm L mol‧ ‧ -1 K‧ -1 × 298 K1 atm

Molar volume (L mol‧ -1) = 24.45 L mol‧ -1

As one cycle is prolonged over 30 min and flow rate is set to 1.0 ml min‧ -1, thus over the course

of one cycle,

Volume of gaseous mixture ( L) = 0.030 L

Thus, in one cycle

moles of gaseous mixture = 0.030 L24.45 L mol‧ -1

= 0.0012 mol

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Thus, CH4 yield in ppm, as calculated in equation (S3), will be converted to µmole of CH4 if

multiplied by “0.0012 mol” and yield in ppm will be converted to nmole of CH4 if multiplied

with factor of “1.2”.

Calculations of AQY:

For RT-Cu0.75 [2]:

Primary product: CH4

Product yield = 77 nmol‧g-1‧h-1

Apparent Light input (H) = 1000 W‧m-2 (100 W xenon solar simulator (Oriel, LCS-100)

with an AM 1.5 filter)

Area of reactor under irradiation (A) = 0.00049 m2

Band gap (Eg) = 2.72 eV

The calculations to find the electrons, participated in photocatalytic reaction, are as follow

number of reacted electrons = [ Production rate of CH4 ] ×[electrons required per mole of CH 4 formation]× NA

As we know from balanced chemical equation, CO2 + 8H+ + 8e- → CH4 + 2H2O, 8 electrons are

consumed per mole of CH4 formed, therefore

number of reacted electrons = [77×10- 9 mol g‧ -1 h‧ -1 ]× [ 8 ] × 6.022× 1023 mol -1

number of reacted electrons = 3.71×1017 g-1 h‧ -1

Basis for calculations = 1 g

number of reacted electrons = 3.71×1017 h -1

The number of photons incident upon photocatalyst is calculated by

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number of incident photons = light absorbed by the photocatalystaverage energy of the photon

× t

Light absorbed by the photocatalyst and average energy of photon is calculated as

light absorbed by the photocatalyst = H × A = 1000 W m‧ -2 × 0.00049 m2 = 0.49 W

average energy of the photon = hcλ

Where, h is Planck’s constant (6.626x10-34 J s), c is speed of light (3x10‧ 8 m s‧ -1) and λ is the

average wavelength for the photocatalyst's absorption range.

For finding λ we calculated λmax from the band gap by the formula:

λmax = hcEg

= (6.626×10-34 J∙s ) × (3× 108 m s‧ -1 )2.72 eV

× 1 eV1.6× 10-19 J

× 109 nm m‧ -1 = 45 5 nm

Therefore, the average wavelength would be

λ = λmin + λmax

2 = 250 + 45 5

2 = 352.5 nm

The average photon energy is then computed to be

average photon energy = ( 6.626×10-34 ) × (3×108 )352.5× 10-9 m

= 5.63×10 -19 J

number of incident photons = 2.92 W5.63×10 -19 J

× 3600 s h‧ -1

number of incident photons = 3.13× 1021 h -1

AQY computation:

AQY (% ) = number of reacted electronsnumber of incident photons

× 100%

AQY (% ) = 3.71× 1017

3.13× 1021 × 100% = 0.012%

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Joule to joule computation:[3]

For calculating joule to joule efficiency, we computed the amount of the energy content

harvested after methane production,

Energy the methane possess = 810000 J mol‧ -1

Output energy as a methane formatio n = 77× 10-9 mol g‧ -1 h‧ -1 × 6 h × 0.04 g × 810000 J mol‧ -1

Output energy as a methane formation = 0.015 J

Input energy absorbed by photocatalyst = 0.49 J s‧ -1 × 21600 s = 10584 J

Joule to joule efficiency (%) = Output energy as a methane formationInput energy absorbed by photocatalyst

× 100%

Joule to joule efficiency (%) = 0.015 J10584 J

× 100% = 0.00014%

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Table S2 Photocatalytic CH4 yield from CO2 reduction employing P25, RT and RT-Cux samples

with corresponding A.Q.Y and joule to joule efficiency.

Sample Name CH4 Yield ( nmol g‧ −1 h‧ −1) Joule to Joule Efficiency AQY (%)

P25 0 - -

RT 7.3 0.13 x 10-4 0.001

RT-Cu0.25 12.6 0.23 x 10-4 0.002

RT-Cu0.50 36.9 0.68 x 10-4 0.006

RT-Cu0.60 52.4 0.96 x 10-4 0.008

RT-Cu0.75 77.0 1.41 x 10-4 0.012

RT-Cu0.90 52.9 0.97 x 10-4 0.008

RT-Cu1.00 28.1 0.52 x 10-4 0.004

RTs-Cu0.75 32.9 0.60 x 10-4 0.005

RTh-Cu0.75 18.9 0.35x 10-4 0.003

P25-Cu0.75 15.3 0.28 x 10-4 0.003

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Table S3 Comparison of the photocatalytic activities for the Cu based photocatalysts reported in

literature.

Photo catalyst

Composition of Photocatalyst and Synthesis Method

Light Source, Reaction Conditions

Product Yield

Cu/TiO2

[4]1% Cu/TiO2 Precipitation-calcination and then H2 pretreatment

Continuous mode, catalyst weight 50 mg, moist CO2 at flow of 2 ml min‧ -1 for 6.5 hLight Source: 150 W solar simulatorLight intensity: 90 mW cm‧ -2

Room temperatureStability not reported

CH4 4.0 μmol g‧ −1

CO 25 μmol g‧ −1

Cu2O/ TiO2

[5]0.03 wt.% Cu/TiO2

Sol–gelBatch mode in liquid water for1.5 hStability not reported

CH4 36 nmol g‧ −1

Cu/TiO2 [6] 1.2% Cu/TiO2

Thermal hydrolysisTiO2-coated optical fiberCO2 and H2O feed mixture with partial pressure of 1.19 and 0.03 bar, respectively, at 75 °C and wavelength of 365 nm

CH4 5.2-5.5 μmol g‧ −1

CuO/TiO2

Cu2O/TiO2

Hollow Microspheres [7]

One-pot template-free

Batch reactor10 mgHg UV lamp (40 W; 254 nm; lightintensity at the location of the catalyst: 20 mW cm‧ −2

CH4 2.1μmol g‧ −1 h‧ −1

CO 14.5 μmol g‧ −1 h‧ −1

H2 2.8 μmol g‧ −1 h‧ −1

Cu/TiO2 [8] 1% Cu/TiO2

Impregnation method

Batch reactor210 min450 W Xe lampUnder UV-vis light irradiationStability not reported

CH4 4.4 μmol g‧ −1

CO 25 μmol g‧ −1

Cu2O−TiO2

[9]5.0 wt.% of Cu pre-cursorSolvothermal process

Batch reactor, catalyst weight 30 mg, 20 °C and light source is Xe arc lamp λ ≥ 305 nm. Testing for 4 cycles (1 h each)

CO 2.11 μmol g‧ −1 h‧ −1

Silica supported

0.5% Cu/TiO2–SiO2

one-pot sol–gel Continuous mode with light source of Xe arc lamp

CH4 10 μmol g‧ −1 h‧ −1

CO 60 μmol g‧ −1 h‧ −1

16

Cu/TiO2 [10] method 2.4 mW cm‧ -2 for 250 nm < λ < 400 nmStability not reported

CuxO−TiO2

[11]Thermal decomposition and calcination

Batch reactorStability not reported

CH4 221.63 ppm g‧ −1

·h−1

Cu/TiO2 [12] 3.0 wt.% Cu/TiO2 Continuous mode at 100 °C with light source of 200 W Hg reflector lamp 150 mW‧cm-2, λ = 254 nm.Stability not reported

CH4 4.20 μmol g‧ −1 h‧ −1

CO 763 μmol g‧ −1 h‧ −1

CuO−TiO2

[13] -

300 W Xe lamp (AM 1.5 filter), 100  mW cm−2

100 mgStability not reported

CH4 26.2 ppm g−1 h‧ −1

Cu/mTiO2

(mesoporus)

[14]

solvothermal

Batch mode, catalyst weight 20 mg, moist pure CO2 at 25 °C for 24 h300 W Xe lamp (AM 1.5 filter), 0.8  mW cm−2

Stability for 5 cycles (10 h), also it decreased after 2nd

cycle

CH4 4.5 μmol g‧ −1 h‧ −1

CO 1.0 μmol g‧ −1 h‧ −1

H2 0.75 μmol g‧ −1 h‧ −1

RT/Cu2O

(This work)

0.75 wt.% of CuCombination of thermochemical reduction and photodeposition

CO2 (moist) 1000 ppm in He at flow rate of 1.2 ml‧min−1, 40 mg catalystAt ambient temperaturefor 6 h irradiation in continuous flow reactor with 100WXe solar simulator with an AM 1.5 filter;100 m W cm‧ -2

Stability 42 h

CH4 77 nmol g‧ −1 h‧ −1

17

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