Author Contributions
Conceptualization, L.F.; methodology, L.F. and M.-A.M.; software, L.F. and M.-A.M.; validation, M.-A.M.; formal analysis, M.-A.M., É.M., M.J. and P.P.; investigation L.F. and M.-A.M.; resources, O.H.; data curation, M.-A.M., É.M., M.J., P.P. and L.F.; writing—original draft preparation, M.-A.M., L.F. and O.H.; writing—review and editing, O.H.; supervision, O.H. and L.F.; project administration, O.H.; funding acquisition, O.H. All authors have read and agreed to the published version of the manuscript.
Figure 1.
The rate of H
2 production as a function of time (
a) and sulfide concentration (
b). The red square and the blue circle symbols represent the rate for catalyst prepared in reverse and normal order, respectively (see in the text). The bars in
Figure 1a. shows the average rate calculated for the 200–400 min time region. The small filled squares gives the ratio of sulfide concentration to the total concentration of all sulfur containing anions.
Figure 1.
The rate of H
2 production as a function of time (
a) and sulfide concentration (
b). The red square and the blue circle symbols represent the rate for catalyst prepared in reverse and normal order, respectively (see in the text). The bars in
Figure 1a. shows the average rate calculated for the 200–400 min time region. The small filled squares gives the ratio of sulfide concentration to the total concentration of all sulfur containing anions.
Figure 2.
(a) Overall rate of H2 production for Cat-xN-(0H) before and after HTT (x: 0, 1, and 2) between 200 and 400 min, (b) overall rate of H2 production for Cat-1N before (-0H, empty marks) and after (filled marks) HTT as a function of time from a 0.13M Na2S/0.18 M Na2SO3 solution.
Figure 2.
(a) Overall rate of H2 production for Cat-xN-(0H) before and after HTT (x: 0, 1, and 2) between 200 and 400 min, (b) overall rate of H2 production for Cat-1N before (-0H, empty marks) and after (filled marks) HTT as a function of time from a 0.13M Na2S/0.18 M Na2SO3 solution.
Figure 3.
Kubelka-Munk functions of catalysts Cat-0N (solid blue), Cat-1N (solid red), Cat-2N (solid green), Cat-0N-0H (dashed blue), Cat-1N-0H (dashed red), and Cat-2N-0H (dashed green).
Figure 3.
Kubelka-Munk functions of catalysts Cat-0N (solid blue), Cat-1N (solid red), Cat-2N (solid green), Cat-0N-0H (dashed blue), Cat-1N-0H (dashed red), and Cat-2N-0H (dashed green).
Figure 4.
XRD patterns of photocatalysts: A: mixture of ZnS and CdS (in 3:1 molar ratio) synthesized and hydrothermally treated in the same way as other catalysts, B: Cat-0N, C: Cat-1N, D: Cat-2N, and E: Cat-2N-0H. The vertical lines designate the characteristic peaks of cubic sphalerite (gray) and hawleyite (red).
Figure 4.
XRD patterns of photocatalysts: A: mixture of ZnS and CdS (in 3:1 molar ratio) synthesized and hydrothermally treated in the same way as other catalysts, B: Cat-0N, C: Cat-1N, D: Cat-2N, and E: Cat-2N-0H. The vertical lines designate the characteristic peaks of cubic sphalerite (gray) and hawleyite (red).
Figure 5.
The overall rate of H2 production (red cross-hatch) and the crystallite size of ZnS (grey) and CdS (orange) for the Cat-xN-(0H) catalysts (x: 0, 1, and 2).
Figure 5.
The overall rate of H2 production (red cross-hatch) and the crystallite size of ZnS (grey) and CdS (orange) for the Cat-xN-(0H) catalysts (x: 0, 1, and 2).
Figure 6.
The calculated free Cd2+ (orange) and Zn2+ (gray) concentrations. The thin solid and dashed lines represent the values for 0.15 M total Zn2+ and 0.05 M total Cd2+ concentration for the excess (TNH3 = 3 M) and stoichiometric amount of ammonia (TNH3 = 1.5 M), respectively. The thick solid and dashed lines show the free equilibrium concentration at 0.22 M and 0.02 M total sulfide concentration, respectively. The thin dotted line shows the free Zn2+ concentration, taking only the formation of hydroxo complex into account. For the meaning of the vertical lines and the arrows, see the text.
Figure 6.
The calculated free Cd2+ (orange) and Zn2+ (gray) concentrations. The thin solid and dashed lines represent the values for 0.15 M total Zn2+ and 0.05 M total Cd2+ concentration for the excess (TNH3 = 3 M) and stoichiometric amount of ammonia (TNH3 = 1.5 M), respectively. The thick solid and dashed lines show the free equilibrium concentration at 0.22 M and 0.02 M total sulfide concentration, respectively. The thin dotted line shows the free Zn2+ concentration, taking only the formation of hydroxo complex into account. For the meaning of the vertical lines and the arrows, see the text.
Figure 7.
SEM images of Cat-0N-0H (a), Cat-0N (b), Cat-1N-0H (c), Cat-1N (d), Cat-2N-0H (e), and Cat-2N (f). The rectangles indicate the area analyzed by EDS. The “whole area” is not signed (by a green A) in the case of (c–f).
Figure 7.
SEM images of Cat-0N-0H (a), Cat-0N (b), Cat-1N-0H (c), Cat-1N (d), Cat-2N-0H (e), and Cat-2N (f). The rectangles indicate the area analyzed by EDS. The “whole area” is not signed (by a green A) in the case of (c–f).
Figure 8.
STEM images of Cat-0N (a), Cat-1N (b) and Cat-2N (c).
Figure 8.
STEM images of Cat-0N (a), Cat-1N (b) and Cat-2N (c).
Figure 9.
TEM/STEM results obtained from the Cat-1N-0H sample (left column, (A,B,E,F,I,J) images) and its heat-treated pair Cat-1N (right column, (C,D,G,H,K,L) images).
Figure 9.
TEM/STEM results obtained from the Cat-1N-0H sample (left column, (A,B,E,F,I,J) images) and its heat-treated pair Cat-1N (right column, (C,D,G,H,K,L) images).
Figure 10.
The rate of H2 production of Cat-2N-pHx (x: 10 (blue ☐), 11 (red ○), and 12 (green ♢) as functions of time from a 0.13 M Na2S/0.18 M Na2SO3 solution.
Figure 10.
The rate of H2 production of Cat-2N-pHx (x: 10 (blue ☐), 11 (red ○), and 12 (green ♢) as functions of time from a 0.13 M Na2S/0.18 M Na2SO3 solution.
Figure 11.
(a) The rate of H2 production of Cat-2N for 3 cycles as a function of time from a 0.13M Na2S/0.18 M Na2SO3 solution, (b) the rate of H2 production of Cat-1N for 3 cycles as a function of time from a 0.13M Na2S/0.18M Na2SO3 solution (inset: the overall rate of H2 production of Cat-1N and Cat-2N for 3 cycles between 200 and 400 min). The blue, red, and green colors designate the 1st, 2nd, and 3rd illumination.
Figure 11.
(a) The rate of H2 production of Cat-2N for 3 cycles as a function of time from a 0.13M Na2S/0.18 M Na2SO3 solution, (b) the rate of H2 production of Cat-1N for 3 cycles as a function of time from a 0.13M Na2S/0.18M Na2SO3 solution (inset: the overall rate of H2 production of Cat-1N and Cat-2N for 3 cycles between 200 and 400 min). The blue, red, and green colors designate the 1st, 2nd, and 3rd illumination.
Figure 12.
Normalized (
a) and cumulative (
b) intensity of the light sources applied for illumination (blue line: UV-LED, red: Hg-Xe arc lamp, green: vis LED). The emission spectra as functions of wavelength are also shown in the SM (
Figure S4). The solid black line represent the KM-function, while the gray dashed lines indicate the intermediate energy range where the absorption steeply increases from the band gap for the Cat-2N catalyst. The yellow background symbolizes the light absorbed by the photocatalyst.
Figure 12.
Normalized (
a) and cumulative (
b) intensity of the light sources applied for illumination (blue line: UV-LED, red: Hg-Xe arc lamp, green: vis LED). The emission spectra as functions of wavelength are also shown in the SM (
Figure S4). The solid black line represent the KM-function, while the gray dashed lines indicate the intermediate energy range where the absorption steeply increases from the band gap for the Cat-2N catalyst. The yellow background symbolizes the light absorbed by the photocatalyst.
Figure 13.
The overall rate of H2 production for Cat-2N, using different light sources (vis LED, Xenon lamp, UV LED).
Figure 13.
The overall rate of H2 production for Cat-2N, using different light sources (vis LED, Xenon lamp, UV LED).
Table 1.
The band-gap energies of different catalysts determined from Tauc-representation [
40] of KM-functions (
Figure S2).
Table 1.
The band-gap energies of different catalysts determined from Tauc-representation [
40] of KM-functions (
Figure S2).
| Band-Gap Energy/eV |
---|
Catalyst | Before HTT | After HTT |
---|
Cat-0N | 2.58 ± 0.03 | 2.50 ± 0.02 |
Cat-1N | 2.65 ± 0.03 | 2.60 ± 0.02 |
Cat-2N | 2.65 ± 0.03 | 2.57 ± 0.02 |
Table 2.
Overall stability constants used for the calculation of the initial free metal ion concentrations [
42,
43].
Table 2.
Overall stability constants used for the calculation of the initial free metal ion concentrations [
42,
43].
n | NH4+ | HAc | [Cd(NH3)n]2+ | [Zn(NH3)n]2+ | [Zn(OH)n]2−n |
---|
1 | 9.25 | 4.73 | 2.51 | 2.32 | 6.31 |
2 | - | - | 4.47 | 4.61 | 11.19 |
3 | - | - | 5.77 | 6.97 | 14.31 |
4 | - | - | 6.56 | 9.36 | 17.7 |
Table 3.
The initial pH of the solution of metal complexes before addition of Na2S reagent, and the initial supersaturation (c/c*) at the beginning of precipitation.
Table 3.
The initial pH of the solution of metal complexes before addition of Na2S reagent, and the initial supersaturation (c/c*) at the beginning of precipitation.
Catalyst | Cat-0N | Cat-1N | Cat-2N |
---|
pH | 6.0 | 9.6 | 10.5 |
lgSZnS | 8.9 | 5.5 | 3.9 |
lgSCdS | 11.7 | 10.9 | 9.4 |
Table 4.
The Zn:Cd atomic ratios of the investigated catalysts calculated from the EDS analyses of different areas: A: average values regarding the whole area shown in the SEM pictures, B: the average of some lighter spots, and C: the average of a darker area.
Table 4.
The Zn:Cd atomic ratios of the investigated catalysts calculated from the EDS analyses of different areas: A: average values regarding the whole area shown in the SEM pictures, B: the average of some lighter spots, and C: the average of a darker area.
Catalyst | Whole Average (A) | Lighter Spots (B) | Darker Area (C) |
---|
Cat-0N-0H | 3.09 | 2.0 | 4.3 |
Cat-0N | 3.14 | 2.5 | 3.4 |
Cat-1N-0H | 3.10 | 1.3 | 3.6 |
Cat-1N | 2.95 | 1.4 | 3.1 |
Cat-2N-0H | 3.09 | 0.9 | 3.4 |
Cat-2N | 3.02 | 1.3 | 3.1 |
Table 5.
Measured data for the calculation of IPCE and QY for the Cat-2N catalyst illuminated by different light sources, and the resulting IPCE and QY values.
Table 5.
Measured data for the calculation of IPCE and QY for the Cat-2N catalyst illuminated by different light sources, and the resulting IPCE and QY values.
| | Light Sources | |
---|
| UV-LED | Hg-Xe | Vis-LED |
---|
nphotons/t (mmol/h) | 12.06 | 11.25 | 27.75 |
RHP (mmol H2/h) | 0.551 | 0.384 | 0.261 |
RHP (mmol H2/g/h) | 30.6 | 21.3 | 14.5 |
IPCE | 9.14% | 6.82% | 1.88% |
Ratio of abs. photons | 99.8% | 85.0% | 47.3% |
QY | 9.16% | 8.02% | 3.98% |