Semiconductor System in

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

On the Electron Transfer of the Metal/Semiconductor System in Photocatalysis Mohd Adnan Khan, Partha Maity, Maher Al-Oufi, Ibrahim Khalid Al-Howaish, and Hicham Idriss J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03741 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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The Journal of Physical Chemistry

1

On the Electron Transfer of the Metal/Semiconductor System in Photocatalysis M.A. Khan*1, P. Maity2, M. Al-Oufi1, I.K. Al-Howaish1 and H. Idriss*1 1 2

SABIC-CRD, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia. Solar and Photovoltaics Engineering Research Center, KAUST, Thuwal, Saudi Arabia

*

Electronic mail: [email protected]; [email protected]

Keywords Photo-catalysis; carbon nitride; transient absorption spectroscopy (TAS); hydrogen production; charge carrier lifetime.

Abstract Probing into the relationship between charge carriers’ dynamics and photo-catalytic reactions is central to the understanding and therefore the design of photocatalysts. We have studied the charge carriers’ lifetime of Pt/C3N4 and compared it to C3N4 alone using fs pump-probe transient absorption spectroscopy (TAS) in the 500-900 nm signal range. Parallel photocatalytic reactions were also conducted in order to link the extracted lifetime to corresponding rates of hydrogen production from water. The presence of Pt (with mean particle size of ca. 2.5 nm) on C3N4 decreased the lifetime of excited electrons in the conduction band. This has occurred in pure water as well as in the presence of the organic sacrificial agent used. These results suggest that Pt particles on this n-type semiconductor act as electron trap centers (either by pumping away conduction band electrons or by creating trap centers at the interface). The corresponding increase in reaction rates can be linked to this electron transfer.

1. Introduction Graphitic carbon nitride (g-C3N4), one of the most stable allotropes among various carbon nitrides, has received much attention because of its photocatalytic activity for a few years now.1 It is at present a benchmark of a polymer photocatalyst for photocatalytic water splitting1-6, CO2 reduction7-9 and dye degradation.10-13 While a combination of low cost, thermal and chemical stability, visible light absorption and adequate CB and VB edges [CB = -1.3 V, VB = 1.4 V vs. NHE, pH = 7] makes it suitable, its photocatalytic activity is still low.5,

14-18

Various strategies have been pursued to improve its activity and

these include different synthetic methods3,

19-21

, nano-structuring22-24, doping25-27 and composites10,

28-32

. In

addition, investigations of its fundamental properties related to photo-generated charge carriers is emerging, with the realization that understanding the fundamentals of charge transfer is needed for further progress in this field in general. 33-35

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Yu et. al.36 and Merschjann et. al.35 measured the fluorescence lifetime (in the nanosecond scale) of C3N4 prepared at different temperatures and indicated that its origin is singlet exciton with associated lifetime decreasing with increasing pyrolysis temperature. They concluded that the surface area of C3N4 rather than changes in charge carrier dynamics correlates with its photocatalytic activity. Also, Merschjann et. al.37 suggested that singlet excitons rapidly (within 200 fs) dissociate into singlet polaron pairs across neighboring carbon nitride sheets. Although g-C3N4 emerges as sheet-like crystallites, similar to graphitic structures, their electronic transport are predominantly perpendicular to the sheets leading to low electron mobility; 10-6 to 10-4 cm2V−1s−1.37 Ye et. al.38 carried out Transient Absorption (TA) experiments on protonated C3N4 and noticed slower decay kinetics when compared to pristine C3N4, with signal remaining up to few µs. Godin et. al.34 observed that free charge carriers in g-C3N4 are trapped in ns timescale. Using a quantitative analysis of free energy loss resulting from charge trapping the authors calculated an energy loss of > 1 eV, consistent with deeply trapped electrons. Also, the same authors have also found an inverse relationship between the population of trapped electrons and the photocatalytic activity of C3N4 annealed at different temperatures. Corp and Sclenker33 studied a composite system of graphitic and exfoliated C3N4 by TA experiments. The charge carriers’ lifetime of the composite was found to be shortest for the best performing materials for H2 generation from water. With increasing studies using time resolved spectroscopy techniques, a better understanding of the photo-physics of g-C3N4 is emerging yet the effect of the charge carrier dynamics on the H2 production remains unclear. Urea is one of the ideal precursors to synthesize g-C3N4 for photocatalysis.20,

39-40

The quantum

efficiency and H2 production rates from urea derived C3N4 has been reported to be an order of magnitude higher than that made from other precursors.40 In general, it has been reported that increasing the pyrolysis temperature and time of urea increases the photocatalytic activity of resulting g-C3N4.7, 20, 40 While the effect of synthesis temperature on the charge carrier dynamics has been studied using TA and tr-PL34-36, the effect of pyrolysis time on carrier dynamics is yet to be studied. In this work, we have studied the role of (i) pyrolysis time of urea, (ii) metal nanoparticles (Pt) and a (iii) sacrificial agent (Triethanolamine (TEOA)) on charge carrier dynamics in order to correlate these to photocatalytic performances of g-C3N4.

2. Experimental Catalysts preparation In a typical synthesis, urea powder was put into an alumina crucible with a cover and then heated to 675 C in a muffle furnace for different times (2, 3 and 5 hours) at a heating rate of 1

C/min. The resulted g-

C3N4 powder was collected for use without further treatment. The samples were impregnated with the


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required amount of H2PtCl6 (2.4 mM in H2O) to get a sample with 0.5 wt.% loading of Pt. The mixture was dried by evaporating excess water under constant stirring with slow heating at 80 °C. The dried catalysts were calcined at 350 °C for 3 hours. Catalysts characterization UV–VIS absorbance spectra were collected over the wavelength range of 250–900 nm with a Thermo Fisher Scientific spectrophotometer equipped with praying mantis diffuse reflectance accessory. Absorbance (A) and reflectance (% R) of the samples were recorded. The reflectance (% R) data was used to calculate the band gap of the samples using the Tauc plot (Kubelka–Munk function). The Kubelka-Munk method is commonly used for the analysis of diffuse reflectance spectra obtained from weakly absorbing samples. It provides a correlation between reflectance and concentration. The concentration of an absorbing species can ² be determined using the Kubelka Munk formula: F (R) = (1–R) /2R = k/s = Ac/s , where R is the reflectance, k is the absorption coefficient, s is the scattering coefficient, c is the concentration of absorbing species and A is the absorbance. The optical band gap of semiconductors can be determined by plotting (F(R) × E) 1/r against the radiation energy in (eV), using r = 2 for indirect band gap allowed transitions of charge carriers or r = ½ for direct band gap allowed transition. The resulting plot has a distinct linear regime, which denotes the onset of absorption. Thus, extrapolating this linear region to the abscissa yields the energy of the optical band gap of the material. BET surface areas of the catalysts were measured using Quantachrome Autosorb analyzer by N2 adsorption. The surface areas were found to be 80, 85, 35 and 14 m2/gCatal. for the samples prepared at 675oC for 2, 3, 5 and 7 hours, respectively (error bars are in the range of 10%). The addition of 0.5 wt. % of Pt did not change the BET surface areas of the catalysts within measurements reproducibility. XRD spectra were recorded using a Bruker D8 Advance X-ray diffractometer. A 2θ interval between 20° and 90° was used with a step size of 0.010° and a step time of 0.2 sec/step. Transmission electron microscopy analysis was performed with a Titan 80-300 ST microscope from FEI Company (Hillsboro, OR). The microscope was set to the operating voltage of 300 kV and scanning TEM (STEM) mode during the analysis. Furthermore, STEM signal was collected with a high-angle annular darkfield (HAADF) detector in the range of 75 mrad to 200 mrad to enhance atomic number (Z) contrast. In this way, STEM images were acquired at various image magnifications. The PL spectra were measured using a Hamamatsu C9920-02G - Quantum Yield Measurement system at an excitation wavelength of 350 nm.



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Transient absorption (TA) measurements were performed on 0.1 ps to 3 ns timescales which is based on a regeneratively amplified Ti:sapphire laser system (produces 800 nm laser pulses of ~100 fs pulse width at 1 kHz repetition rate), and in conjunction with Excipro pump-probe spectrometers (CDP, Moscow). The pump pulses at 332 nm were generated after passing through a fraction of 800 nm beam into the spectrally tunable (240−2600 nm) optical parametric amplifier (TOPAS Prime, Spectra-Physics) and a frequency mixer (NirUVis, Light conversion). The fluence of the pump power was adjusted by using neutral density (ND) filters to avoid multiple exciton generation. To generate the probe pulses (UV visible and NIR wavelength continuum, white light) another fraction of the 800 nm amplified pulses was focused onto a 2-mm thick calcium fluoride (CaF2) crystal. To detect the transient species following excitation at different time scales, the 800 nm amplified pulses were passed through a motorized delay stage before white light generation. Depending on the path length of the probe beam the excited state life time of the species was measured up to 3 ns time scale with respect to the pump pulses. To achieve better signal to noise ratios the resulting white light was split into two channels named as probe and reference, respectively and focused on two different fiber optics. The pump pulses were overlapped on the sample with the probe pulses after passing through a synchronized chopper (500 Hz) which blocked an alternative pump pulses. Then, the change in absorption (∆A) of the excited state are calculated by subtracting absorption of the excited and unexcited sample. The spectra were averaged over a time period of 2 s for each time delay, yielding each time-resolved spectrum. Catalysts testing Photocatalytic reactions were evaluated in a 150 mL volume Pyrex glass reactor. 30 mL of 10 vol. % tri ethanol amine solution was used and the reactor was purged with N2 gas to remove any O2 prior to starting the reaction. Photoreactions were carried out under UV+Visible (300-620 nm) light using a Xenon lamp, respectively. The light flux was measured with a spectro-radiometer (Spectral Evolution SR-500). The total flux from the Xenon lamp was ~ 33 mW/cm2 (UV (300-400nm ~ 5 mW/cm2) and visible (400-620 nm ~ 28 mW/cm2)) as shown in Figure S4. Products monitoring was performed by gas chromatograph (GC) equipped with thermal conductivity detector (TCD) connected to Porapak Q packed column (2 m long, 1/8 in. external diameter) at 45 ◦C and N2 was used as a carrier gas (flow rate of 20mL/min) at 8 psi.

3. Results and Discussion Structural properties g-C3N4 has a 3D packing close to graphite with a 2-D structure made of tri-s-triazine rings (heptazine (C6N7)) cross-linked by trigonal N atoms.5, 14 The morphology of g-C3N4 samples exhibits a porous layered platelet-like morphology. The edges of the platelets tend to bend/roll in order to reduce surface energy as ACS Paragon Plus Environment

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seen in Figure 1(a) for g-C3N4-5 hr sample. Similar morphology for urea derived carbon nitride has been observed earlier.20, 41 Figure 1(b) shows a dark field TEM image of Pt/ g-C3N4-5 hr. The bright dots identified as Pt nanoparticles were 2-4 nm in size with a mean particle size of 2.5 nm. Particle size distribution is given in Figure 1(d). It is worth noting that most (if not all) Pt particles occurs at the edges of the crystallites making the particles (figure S3). One can also note in figure 1c the crystalline structures of C3N4 in line with what has been reported by others89. Figure 1(f) shows XRD patterns of g-C3N4 prepared at 675 different pyrolysis time. Two peaks occur at around 27.6 around 27.6

and 13.0

C with

for all samples. The strong peak at

is a characteristic inter-planar stacking of aromatic systems (triazine or heptazine rings),

corresponding to (002) interlayer distance of 0.322 nm. The broad peak located around 13.0°, assigned to the (100) plane, relates to an in-plane structural packing motif such as the hole-to-hole distance of the nitride pores in the crystal.2-3, 42. A narrowing of these two main diffraction peaks of g-C3N4 is observed when the pyrolysis time is increased. This indicates an increase in crystallinity; within the 2D layers as well as within the 3D packing of the polymer sheets. The crystallite size was 11.7 nm for 2 and 3-hour samples and 12.3 nm for the 5-hour samples. With increasing pyrolysis time, the (002) peak shifts toward a higher diffraction angle from 27.6

to 27.8 , namely the interplanar stacking distance decreases slightly from 0.322 to 0.320

nm. This would lead to a tighter packing of the polymer layers and promotes stronger interlayer electron coupling.36,

42

Also, the peak at 13.0⁰ becomes narrower and its intensity decreases, indicating higher

polymerization degree. g-C3N4 is an indirect band gap semiconductor with an energy gap of 2.7-2.8 eV.5, 15, 43 The UV-Vis DRS absorbance profiles of our g-C3N4 catalysts are shown in Figure 2(a). All samples have an intense absorbance region below 400 nm corresponding to π-π* transition normally seen in heterocyclic aromatics.34, 36

There is a weak absorption band in the 430–550 nm range, due to sub-band gap states corresponding to n-

π* transitions of the asymmetric and nonplanar conjugated heptazine rings i.e. involving lone pairs on the edge N atoms of the triazine/heptazine rings.34, 36

While the π - π* transitions correspond to the optical band gap of g-C3N4, the n-π* is forbidden for perfectly planar heptazine units and it becomes allowed when the heptazine units developed some distortions.44 The optical band gaps (Eg) calculated from the Tauc plots (r = 2 for indirect band gap) are shown in Figures S1, S2. Increasing pyrolysis time slightly increased the band gap form 2.9 to 3 eV. Similar observations were made by others for g-C3N4 samples prepared by increasing the pyrolysis temperature.20, 41 Increasing the pyrolysis time leads to a decrease in the degree of delocalization, in other words a decrease of the π-plane conjugation degree.45 As a result more n-π* transitions by breaking the symmetry are made. This



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6

is supported by XRD data as discussed in Figure 1(d). The relation between the decrease of conjugation degree and increase in optical band gap of C3N4 has also been given by Zhang and coworkers.45


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Figure 1. (a, b and c) Transmission Electron Microscope (TEM) images of g-C3N4-5 hr; (d) Dark field TEM of Pt/C3N4-5 hr (a magnified image is shown in figure S3); (e) Particle size distribution as calculated from TEM images of Pt- g-C3N4-675-5 hr; (f) X-ray diffraction pattern of g-C3N4 samples synthetized at 675 C at different times (2, 3 and 5 hours). Crystallinity is circulated in figure 1c. The arrows in figure 1d are guides for the eyes of the inter Pt particles distance.

The PL spectra of the carbon nitride powders recorded under 350 nm excitation are shown in Figure 2(b). Typically, a broad PL is emitted with a maximum near 450-460 nm. The PL spectra of carbon nitride have been explained in terms of multiple transitions involving π-conjugated states in the conduction band and n lone pairs in the valence band. The states consisting of the sp3 C−N σ band, the sp2 C−N π band, and the lone pair (LP) state of the bridging nitride atom are responsible for the luminescence from g-C3N4.46 The high energy PL near 450-460 nm, lies close to the bandgap, and is attributed to emission from the band edges (π-π* transitions).34,

44, 46

Lower energy PL near 500-510 nm, is attributed to intra bandgap states (n-π*

transitions), possibly involving tertiary N atoms.34, 44, 46 The PL maxima for our samples lies around 490 nm. This has been observed by others for g-C3N4 samples prepared at high temperatures (> 600

C).44 The shift

to longer wavelengths indicates the presence of sub-gap defects in the material.34 The emission intensity decreases in the order of g-C3N4-2 hr > g-C3N4-3 hr > g-C3N4-5 hr > Pt-C3N4. To first approximation, the intensity of a photoluminescence signal is proportional to the recombination rate of photo-generated electron–hole pairs.47-48 We observe a significant attenuation of the photoluminescence signal of g-C3N4 upon Pt loading. This effect is reported for many semiconductors and it is understood that these noble metal particles act as electron sinks thereby reducing the e-h recombination process and correlates with higher H2 production rates. 47,48,53



8

1.2

(a)

(b)

2 hr 3 hr 5 hr

PL intensity

1.0

Absorbance (a.u.)

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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0.8 0.6

2 hr 3 hr 5 hr Pt/5 hr

0.4 0.2 0.0 300

400

500 600 700 Wavelength (nm)

800

900

400 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 2. (a) UV-Vis Diffuse Reflectance Spectra (DRS) and (b) Room temperature photoluminescence (PL) spectra of g-C3N4 samples synthetized at 675 C for different times: 2, 3 and 5 hours. (Excitation light wavelength: 350 nm) Photocatalytic activity The H2 production activity of the g-C3N4 samples was tested under constant light flux in the presence of a 10 vol. % TEOA solution. The measured light flux is shown in Figure S4A. It is important to point out that no H2 was detected using pure H2O. Figure 3 shows that the g-C3N4 heated for 2 hours has a H2 rate of ~ 1.8 x 10-6 mol g-1(cat) min-1, it increases to ~ 4.4 x 10-6 mol g-1(cat) min-1 for catalysts prepared at 5-hour heating time. The raw data from which the rates were extracted are shown in Figure S5. Upon deposition of Pt the activity increased by about an order of magnitude. The Pt/C3N4 catalysts follow a similar trend for increasing H2 production rates from 1.6 x 10-5 mol g-1(cat) min-1 to 2.3 x 10-5 mol g-1(cat) min-1 for catalysts heated for 2 hours to 5 hours. The photocatalytic activity of a semiconductor is a function of light absorption, crystallinity, surface area and availability of active sites among many other parameters. It is worth making two observations here. Firstly, with increasing pyrolysis time there is an increase in crystallinity, a decrease in the PL emissions and an increase in H2 production rates. Secondly, the addition of Pt led to a drastic decrease in the PL signal that was concomitant with a considerable increase in the H2 production rates.


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Hydrogen rate (mol.g-1.min-1) x 10-5

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


2.5 Pt/C3N4

2.0 1.5 1.0 C3N4

0.5 0.0

1

2 hr

2

3 hr

3

5 hr

4

2 hr

5

3 hr

6

5 hr

Figure 3. Rates of H2 production over g-C3N4 and Pt/g-C3N4 photocatalysts synthetized at 675 C for different times (2, 3 and 5 hours). Reaction conditions: Pyrex reactor, Catalyst amount = 5 mg, 10 vol. % TEOA aqueous solution (20 mL) and UV-Vis flux (300-620 nm) of 33 mW/cm2; UV (300-400 nm: 5 mW/cm2); Visible (400-620 nm: 28 mW/cm2)

Transient absorption spectroscopy Our knowledge of the photo physics and charge trapping of carbon nitrides is still limited as there have been few studies performed using Transient Absorption Spectroscopy (TAS).33-34, 36 Figure 4(a) shows the TAS spectra of g-C3N4-5 hr dispersed in H2O at various delay times. The catalyst concentration was kept constant at 0.2 mg/mL and all measurements were performed with a 332 nm pump excitation. TAS spectra revealed a small bleaching signal below 500 nm and positive absorption features above 500 nm. The negative absorption in the region below 500 nm coincides with the ground state absorption band of C3N4 as seen in the UV-Vis spectra of Figure 2(a). A comparable bleach has also been attributed to stimulated emission in previous TAS investigations on g-C3N4.37 The broad positive absorption features that extend from visible to near IR region (500 – 900 nm) has been attributed to photo-generated holes, electrons or electron/hole pairs (excitons) by others.34, 49 To further probe into the nature of this absorption we have used a hole scavenger TEOA and measured the TAS of the charge carriers. Figure 4(b) shows TAS of g-C3N4-5 hr sample dispersed in pure H2O and 10 vol.% TEOA with identical concentration of 0.2 mg/mL. At different delay times, the addition of TEOA shows much higher ∆A, suggesting that the visible and NIR regions (500 – 900 nm) region is



10

primarily due to photo-generated electrons in C3N4. This is also in line with previous studies on TAS of gC3N4.33-34 The normalized decay kinetics of g-C3N4-5 hr dispersions in H2O and TEOA probed at 700 nm are plotted in Figure 4(c). The use of TEOA had a drastic effect on the charge carrier dynamics with a much slower decay, consistent with hole scavenging by the TEOA, upto 3 ns. The signal was best fitted with a triexponential decay, table 1 (the time constant τi of an event, in the table, is equivalent to the reciprocal of the rate constant ki).

A fast component with a characteristic time constant of less than 10 ps, a second

component with a time constant of a few tens of picoseconds and a third slow component with a time constant of hundreds of picoseconds. The amplitude weighted average lifetime can be calculated using ( =

∑

∑

) where Ai is amplitude of ith time constant and τi is the respective time constant value.

20

(b)

30 (a) 15 5.2 ps

m∆A

m∆A

TEOA-5.2 ps

1.1 ps

20

20 ps 103 ps

10

10

H2O-5.2 ps

508 ps

TEOA-103 ps

1010 ps

5

-1.5 ps

0

H2O-103 ps

0 600

700 850 900 Wavelength (nm)

950

550 600 650 700 750 850 Wavelength (nm) 1.4 0.5

1.2

(c) 1.0

5hr-Water 5hr-TEOA

1.2 1.0

0.4

∆A (Norm)

0.8 0.6

(d)

0.8

-0.2 -2 0 2 4 6 8 1000 2000 3000 Time (ps)

200 160

0.3

120 0.2

80 2

3

4

5

Pyrolysis time (hours)

0.4

0.0

0.0

950

240 0.4

0.6

0.2

0.2

900

Avg lifetime (ps)

500

H2 (mol/g/min) x 10-5

-10

∆A (Norm)

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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-0.2

2hr-TEOA 3hr-TEOA 5hr-TEOA

-2 0 2 4 6 8 1000 2000 3000 Time (ps)

Figure 4. Pump probe TAS of g-C3N4 annealed for 5 hours at 670oC and excited by 332 nm pulsed excitation (0.1 mJ/cm2, width ∼120 fs). (a) fs-TAS of the sample, dispersed in H2O, at different delay time. (b) fs-TAS ACS Paragon Plus Environment

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of the sample dispersed in H2O and 10 vol.% TEOA solution at different delay times. (c) Normalized decay traces of the 700 nm signal in (a) and (b). Solid lines are the fitting of respective data. (d) Normalized decay traces of the 700 nm signal for g-C3N4 annealed at different times; Samples were dispersed in 10 vol.% TEOA solution; Solid lines are the fitting of respective data. The inset in (d) is a plot of the H2 production rates and average lifetime as a function of pyrolysis time.

Figure 4(d) shows the normalized TAS decay kinetics of g-C3N4 samples prepared at different pyrolysis times (2, 3 and 5 hours). The fitted time constants and average lifetime is also listed in Table 1. With increasing pyrolysis time, we see slower decay kinetics, indicating slower charge carrier recombination rates. A plot of the H2 rates and average lifetime as a function of pyrolysis time is shown in the inset of Figure 4(d). Increasing the pyrolysis time leads to higher crystallinity in these samples as seen from XRD data, which leads to slower charge carrier recombination and higher photocatalytic activity. The increased crystallinity of the samples affects the lifetime up to 3 ns. This is confirmed by an increase in all three time constants components as seen in table 1 and plotted in Figure S6. The fs-TAS decay kinetics of these catalysts dispersed in pure water are shown in Figure S7.

Table 1. Summary of different time constants at 700 nm fitted using a three exponential decay function:

= + + + . Note T stands for 10 vol. % TEOA solution and W stands for pure water. In the equation x is for time and τi is the time constant (inverse of the rate constant) System

τ1 (ps)

τ2 (ps)

τ3 (ps)

τavg (ps)

C3N4-2Hr-T

1.47 ± 0.13 (57%)

17.61 ± 1.24 (23%)

385 ± 37 (20%)

81.89 ± 7.75

C3N4-3Hr-T

1.85 ± 0.18 (56%)

23.76 ± 2.2 (22%)

631.4 ± 59 (22%)

145.08 ± 13.56

C3N4-5Hr-T

2.25 ± 0.22 (46%)

24.11 ± 2.3 (22%)

777.3 ± 70 (32%)

254.98 ± 23.01

C3N4-2Hr-W

1.55 ± 0.13 (68%)

13.10 ± 1.6 (20%)

170.1 ± 18.4 (12%)

24.09 ± 2.61

C3N4-3Hr-W

1.70 ± 0.16 (66%)

15.20 ± 1.1 (21%)

190.5 ± 20.5 (13%)

29.08 ± 3.00

C3N4-5Hr-W

1.46 ± 0.12 (67%)

17.1 ± 1.2 (20%)

236.2 ± 24.7 (13%)

35.10 ± 3.53



12

1.2

0.6

(a)

5hr-TEOA Pt/5hr-TEOA

1.0

∆A (Norm)

∆A (Norm)

(b)


0.5

0.8 0.6 0.4

0.4 0.3 0.2

0.2 0.1

0.0 0

500

1000 1500 2000 2500 3000 Time (ps)

0.0 20

30

40

50 60 70 Time (ps)

80

90 100

1.2

1.0

(c)

(d)


5hr-Water Pt/5hr-Water

1.0 0.8

0.8

∆A (Norm)

∆A (Norm)

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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τ1 = 1.25 ps; τ2 = 15.2 ps

0.6 0.4

0.6 0.4 0.2 0.0

τ1 = 1.05 ps; τ2 = 6.4 ps

0.2 0

5

10 Time (ps)

15

20

-0.2

0

500

1000 1500 2000 2500 3000 Time (ps)

Figure 5. (a) Normalized decay kinetics at 700 nm of g-C3N4-5 hr and 0.5 wt. % Pt/g-C3N4-5 hr in 10 vol.% TEOA solution. (b) Data in (a) plotted from 20 to 100 ps. (c) Data in (a) plotted from 0 to 20 ps and fitted using a two exponential decay function. (d) Normalized decay kinetics of g-C3N4-5 hr and Pt-g-C3N4-5 hr in pure H2O dispersion.

Next, we present results related to charge carrier dynamics of Pt/g-C3N4. Loading of noble metal nanoparticles such as Pt, Pd and Rh on various semiconductors has been shown to drastically improve their photocatalytic efficiency.50-52 While most interpretations explain the role of the metal as an electron sink thereby reducing recombination rates, others argue that the role of metal is for atomic hydrogen recombination and is not involved in capturing of excited electrons.53-54 The electron transfer kinetics in M/TiO2 and M/CdS photocatalysts has been investigated in detail,55-57 but not yet on a polymeric photocatalyst such as g-C3N4. The normalized TAS decay kinetics of our g-C3N4-5 hr and Pt/g-C3N4-5 hr samples dispersed in 10 vol.% TEOA solutions are plotted in Figure 5. A clear difference is observed at time delays of few picoseconds where the decay is much faster for Pt loaded C3N4. The faster decay seen in Pt/gC3N4 samples represents the transfer of electrons from g-C3N4 to Pt. It is not possible to know if this fast ACS Paragon Plus Environment

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13

transfer from the CB of the semiconductor to Pt is enough to directly reduce hydrogen ions to molecular hydrogen or that the action of pumping electrons away from the CB, increasing the lifetime of both electrons and holes, enhance the charge transfer reaction rate of the catalyst. At longer time scales (> 100 ps) the signal decays to almost zero from Pt/g-C3N4 samples versus samples without Pt. These decay kinetics are similar to those reported for Pt/TiO2 systems, both at short time (0 to 10 ps) and longer time (10 ps to 3 ns) scales.56, 58 Similar experiments in pure water (Figure 5(d)) also showed a fast electron transfer from g-C3N4 to Pt. Based on TAS and catalytic results, we propose a simple model to understand charge transfer to the metal in Pt/g-C3N4. We observe charge carrier excitation within a 100 fs excitation pulse. With the use of 332 nm light, inter-band excitations will dominate exciting electrons from the valence band into the conduction band. This is followed by charge trapping (surface and bulk) in the fs-ps time scales. To drive redox reactions, excited charge carriers need to reach the catalyst surface and in the absence of an external field, charge carriers move by diffusion. After that, these charge carriers either recombine or transfer to respective acceptor (electron scavengers) or donor molecules (hole scavengers). Since the signal from Pt/g-C3N4 samples decays to almost zero after 100 ps, to accurately compare with samples without metal we have plotted the data in Figure 5(a) at different time delays. Figure 5(b) shows the spectra at time delays of 20 to 100 ps. At 20 ps or later, there is little difference observed between the signal decay kinetics of Pt/C3N4 and that of C3N4. In this time region, decay kinetics of electrons is not affected by the Pt loading. There is a clear difference observed, however, at the time delays smaller than a few picoseconds (< 20 ps) as shown in Figure 5(c). These data have been fitted using a two exponential decay function (Figure 5(c)). Considering that the signal at 700 nm is primarily due to photogenerated free electrons, then the first decay component can be attributed to charge trapping (bulk and surface) while the second decay component can be attributed to charge transfer to metal and/or recombination. From the fitting in Figure 5(c), we get similar τ1 ~ 1.05-1.25 ps for both Pt/g-C3N4 and g-C3N4 indicating that the trapping of electrons within the g-C3N4 particles is not affected by the presence of metal on the surface. The difference in the second decay component (τ2) indicates electron transfer to metal. This difference of ~ 8.8 ps for our Pt/C3N4 catalysts may represent the transfer of generated electrons from C3N4 to Pt. This is similar to previously observed on M/TiO2 catalysts where it was ~ 2.3 ps.56 Similarly, the decay signal for Pt/g-C3N4-2hr catalysts was fitted using a two exponential decay function as shown in Figure S7 of supplementary information. The fitted time constants are listed in Table 2. Based on τ we can calculate the charge diffusion constant D (cm2/s) of these g-C3N4 catalysts. In the absence of electric field, charge carriers move by diffusion and their range is defined by the mean free diffusion length L which depends on D (cm2/s) and the carrier lifetime τ (s).59-60 = √; for charges



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reaching the surface, LD = r0/π (particle radius). From TEM and XRD results C3N4 crystallites have dimensions of 15 (XRD) to 20 nm (transverse diameter from TEM). The TEM image in figure 1d shows that many Pt particles are deposited on each C3N4 particle (made of these crystallites) so it is probably more suitable to focus on the effect of the crystallite size rather than the agglomerated ones. Using τ1 ~ 1.25 ps, the calculated D is ~ 0.2 cm2/s. Furthermore D is related to the mobility, µ, via the Nernst–Einstein equation:59 =

!" ; #

Where kB is Boltzmann constant, e is charge of an electron. µ is calculated to be ~ 7

cm2/V/s at 298 K. Similarly, using τ2, D and µ are found to be equal to 0.015 cm2/s and 0.6 cm2/V/s, respectively. It is important to mention that these numbers are based on a constant LD (15 nm). Table 2, shows that the addition of Pt has resulted in decreasing τ1 and τ2. While the physical meaning of τ1 and τ2 may not be understood from these measurements, the main conclusion, also stated by others34, is that Pt competes with the process of e-h recombination. This may be due to its ability to pump electrons away from the CB but it can also be due to increasing the number of trapping centers at the interface metalsemiconductor.-

Table 2. Summary of different time constants (in the 0 to 20 ps range) at 700 nm fitted using a two

exponential decay function: = + + System

τ1 (ps)

τ2 (ps)

C3N4-5Hr-T

1.25 ± 0.15 (25%)

15.20 ± 1.4 (75%)

Pt/C3N4-5Hr-T

1.05 ± 0.12 (35%)

6.40 ± 0.5 (65%)

Pt/C3N4-2Hr-T

0.60 ± 0.10 (20%)

8.80 ± 0.6 (80%)

Table 3. Electron Mobility (µ) and charge diffusion constant (D) of some semiconductors;;LD stands for Diffusion length. . µ (cm2V-1s-1)

D (cm2s-1)

(electron)

(electron)

Si

< 1500 61-62

< 36 63

Ge

< 3900 64-65

< 100 66

Semiconductor


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GaAs

< 8500 61, 67-69

< 200 70-71

CdS

300-600 61, 69, 72

2.0-7.7 73-74

ZnO

0.5-150 75-76

1.12 × 10−3 77

TiO2 (Anatase)

1-20 78-80

10-4 - 10-2 81-83

WO3

10 84

2.5 × 10-3 85

BiVO4

12 86

0.3 86

Epitaxial graphene C60 fullerene g-C3N4 (this work) g-C3N4 (this work)

~ 70,000 87

~ 11,000 87

~ 0.022 88 0.2 (based on 1.25 ps and LD = 15nm)

~ 5.6 88 7

0.015 (based on 15 ps and LD = 15 nm)

0.6

While there are still no experimental reports on the mobility and diffusion constant of g-C3N4 derived from urea to compare with, these are found to be similar to those other wide band gap oxide semiconductors (such as TiO2, ZnO, WO3 and BiVO4) as indicated in Table 3. In other words, because of its stability in water, C3N4 has a potential for hydrogen production, in particular if its complex surface and bulk structural and electronic properties are better understood.

4. Conclusions We have probed into the relationship between charge carrier recombination rates and photo-catalytic activity of g-C3N4 and Pt/ g-C3N4 for water splitting to hydrogen using transient absorption spectroscopy and photocatalytic reactions. Three main conclusions from this study are extracted: (i)

Increasing pyrolysis time of urea leads to higher crystallinity which results in longer-lived charge separation and correlates with higher photocatalytic activity.

(ii)

The kinetics of tri-ethanolamine (TEOA) as a hole scavenger is extracted by fs pump probe transient absorption spectroscopy (TAS). In the presence of TEOA aqueous solutions photogenerated electrons had much longer lifetimes (upto ns time scales) when compared to similar experiments conducted in the absence of TEOA.

(iii)

TAS results indicate that Pt nanoparticles on top of g-C3N4 decrease the conduction band electrons lifetime. Electron transfer from the conduction band of g-C3N4 to Pt particles seem to be the most plausible interpretation.



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Acknowledgments The authors thank Shahid Bashir (SABIC-KAUST) for conducting the BET surface area measurements and Lutfan Sinatra (KAUST) for conducting further TEM images.

Supporting Information Tauc plots and UV-Vis DRS spectra of g-C3N4 and Pt-g-C3N4. Higher magnification STEM of Figure 1d. Flux measurement and spectral profile of the Xenon lamp. H2 production as a function of reaction time over g-C3N4 (a) and Pt/g-C3N4. Plots of different time constants as a function of pyrolysis time. fs-TAS kinetics of g-C3N4-675 oC in pure H2O and in 10 vol.% TEOA solution.


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Semiconductor System in

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