Photocatalysts - Science topic

Dear all, the answer will depend whether the question is concerned by conventinal radical polymerization or by recent living/controlled polymerization ones. Among many factors that are detrimental: presence of oxygen (inhibition/retardation) and transfer reactions. My Regards

"As for the difference between S-scheme and Z-scheme photocatalyst, the S-scheme photocatalyst is usually composed of two n-type semiconductors, while the Z-scheme is usually composed of n-type and p-type semiconductors. In addition, the construction of the Z-scheme photocatalyst is inspired by the natural photosynthesis process and the S-scheme photocatalyst is named for its staggered band structure."

From

Yes, there are other photocatalyst concepts besides the Z-scheme that have been proposed for overall water splitting.

One such concept is the Type-II heterojunction, in which a semiconductor material with a wider bandgap is paired with a semiconductor material with a narrower bandgap. The two materials form a heterojunction, which allows for a more efficient separation of the electron-hole pairs generated by the absorption of photons. This concept has been proposed for use in overall water splitting by using semiconductors such as TiO2 and CdS as well as other combinations.

Another concept is the S-scheme, which involves the use of a single semiconductor material with a bandgap that is intermediate between the bandgaps of the materials used in the Z-scheme. This concept aims to improve the overall water splitting efficiency by reducing the recombination of the electron-hole pairs generated by the absorption of photons.

A good reference for these concepts is "Recent advances in photocatalytic water splitting" by D. Wang and Y. Wang, published in Journal of Materials Chemistry A (2013). This review article provides a detailed overview of the recent developments in photocatalytic water splitting and discusses the different photocatalyst concepts, including the Z-scheme, Type-II heterojunction, and S-scheme.

Synthesis of chemicals / API / Drugs by continuous flow process using Flow Engineering / Flow Chemistry / Flow reactors / Technology. All the best ...

Gentle Akinnuoye Likely formation of degradation products of chloramphenicol. You can expect some phenolic or quinonic products which will absorbe stronger in the UV-range.

For bandgap Calculation, you can follow this article...

To explore Scavenging effect, Follow this article

@ thanks a lot sir for your help

Dear Md Al-Amin, you can use electrochemical and ultrasound processes. My Regards

Try oxidizing agents: ozone, hydrogen peroxide (perhydrol), sodium or potassium hypochlorite. It is also possible with acids or when changing the pH with coagulants.

Filtration, through a sorbent.

a lot depends on what your photocalizer consists of. Maybe just ultraviolet.

With such a limited infomation it is impossible even guess. More details.

hydrothermal synthesis. you could try GO and something else. Or just prepare some MOFs/COFs.

No it isn't possible. Ostwald ripening effect will make your particles bigger with increasing temperature.

I'm not sure the size of your reactor. for the laboratory scale, the xenon lamp and mercury lamp are suitable because of their high light intensity. But in academic research, LED light sources may be a trend. I have compared conventional high-power light sources and LED in photocatalysis in my previous research, hope it works for you.

I am seeing this phenomena in a non-quartz vial.

Thank you for the reference!

Surely yes, but it depends much on the complexity of the system.

Dear Syahmi Basir, other option is the use of 'bimetallic NPs catalysts' (search with this keyword), they are by far more performing than nanocomposites. My Regards

https://link.springer.com/article/10.1007/s10562-014-1399-x#:~:text=Bimetallic%20nanoparticle%20catalysts%2C%20by%20alloying,catalyst%20%5B7%2C%208%5D.

That actually comes from some assumptions from electronic transitions in semiconductors, both direct and indirect bandgapped.

Starting with a direct bandgap in a 3D semiconductor, if you calculate the joint density of states for the absorption of a photon with energy bigger than the bandgap, you will find a result that is proportional to (hω-Eg)^1/2. α in this case is proportional to the transition rate divided by hω, which is proportional to the joint density of states. This is the simple one. You can find this explained very well in Mark Fox book Optical properties of solids.

For indirect bandgapped materials it is a little harder, but the electron-phonon interaction must come into play. I'm not going to go into detail though, and you can find it in Cardona's book Fundamentals of Semiconductors.

Hi Aarif Shah , in gas phase catalysis FTIR can be quite useful to probe the chemistry of the surface. (In water phase, may be not much will be seen because of water interference, you can perhaps do it, who knows)

If you have a pure metal oxide powder. The FTIR spectra (ATR is preferred) of the powder will already give a few important information about the surface. First, the water molecules that may have from the atmosphere. H2O can adsorb both as molecules, but also in the dissociated form: H and OH. H2O will also be present in the condensed form due to capillary condensation. Now, the dissociated H2O is interesting! Because it denotes a catalytic process. In TiO2, the active sites dissociates the water and then the H and OH are sitting on the active sites. On FTIR a small shoulder at 3596 cm-1 denotes dissociated water for TiO2. Same is observed for ZnO. Around 1600 cm-1 adsorption of molecular water is denoted by a peak. Further absorption bands from the metal oxide itself are difficult to interpret. But things can still be improved because data science is developing as well.

Now if you are investigating the photocatalysis with WO3 and TiO2, then you can study degradation of a surface organic molecule with FTIR. The C-H stretches of long chain organic molecules show strong absorption in the 2800 cm-1 to 3000 cm-1 region. This absorption peak you can monitor during the degradation. During degradation, CO2 peaks will also evolve. In this work, we exploited the FTIR analysis for studying TiO2, WO3, ZnO and CuO. You may find this interesting:

What is also interesting is DRIFT (Diffuse reflectance FTIR). So this is FTIR where you probe the diffusely reflected signal from the sample after IR incidence. For catalysis it is particularly interesting because it gives only the surface chemistry due to reflection based probing. DRIFT cells are accessories that can bought along with normal FTIR set-ups. An in-situ DRIFT cells where vacuum can be created or gas phase reaction can be carried out is an inexpensive but powerful set-up!

thank you very much

i experienced your comment a bout coating of FTO with thin layer of catalyst and also increasing annealing temperature until 470 degree C. and successfully was controlled cracking of coated film.

For that please follow these articles

The type-II heterojunction is the conventional mechanism which describes that, upon irradiation electrons of both the semiconductors (in the photocatalyst) get excited to the CB leaving holes in the VB. Then the lifetimes of these photogenerated electrons and holes are extended because of the shifting of electrons in the CB of the semiconductor with a more negative CB band edge to the CB of the semiconductor with less negative CB band edge and similarly holes will shift towards less positive VB. So, this prevents the recombination of electrons and holes.

In the Z-scheme mechanism, there are donor and acceptor terms involved.

Basically, in this mechanism, photogenerated electrons in the CB of the semiconductor with less CB band edge recombine with the holes in the VB of the semiconductor with less positive VB band edge. Thus, unwanted electrons and holes recombine and this enhances the lifetime of strong photogenerated electrons (in the CB of semiconductor with more negative VB band edge) and holes (in the VB of semiconductor with more positive VB band edge) which will subsequently take part in the photocatalysis.

This is the basic difference.

Yu et al. have explained these mechanisms in detail in their paper. (10.1016/j.chempr.2020.06.010)

@ Esther, the attached file may be useful to you.

For preparing g-C3N4, you can calcinate any specific amount of ammonia or urea in a Muffle furnace at 500-550 C° for 2-3 hours.

Dear Zakarya Zebiri

In order to explore heterojunction formation in multi-component photocatalysts, XPS analysis is a versatile instrument. Because in this technique, binding energy of elements is very sensitive to changes of electronic environment of elements. Hence, you can simply compare the binding energy of C, N, Co, and O elements in the composite relative to the pure components. For more information, please refer to the following valuable paper, pages 6-8:

-Liuyang Zhang, Jianjun Zhang, Huogen Yu, Jiaguo Yu, Emerging S-Scheme Photocatalyst, Adv. Mater. 2022, 2107668.

Regards

Aziz Habibi-Yangjeh

The most likely reason is due to the small amount of the solution (50 ml) and the high amount of eriochrome black (20,25,30,35, and 40 ppm). Try adding a small amount of eriochrome black (1 to 10 ppm).

Dear sir Francisco Manuel Marquez

thank you for your answer.

Dear sir, I have the bandgap and the potentials of the bands (CB and VB) for both semiconductors and I know throw the use of scavengers that the junction formed is type II. But I want to form a Z scheme junction to protect the high redox ability of the two semiconductors. What I'm asking is, are we able to control what junction that will be formed or that's something we couldn't know before the synthesis?

Matheus Mendes thaks for your valuable suggestion

Yes you can.

Also check please the following good link:

It can be calculated from UV data of photocatalyst as well as DRS data of photocatalyst. While using UV data Tauce's equation is used. While in other case kubelka munk equation is ised

Also check..

Hi Khaled Hassan

Phenolphthalein is a common acid-base indicator used to determine the endpoint in acid-base titrations. It is also the active ingredient in some laxatives. If excess base is present at the end of an acid-base titration, the pink phenolphthalein color fades if the solution is allowed to stand for a while.

The structure of the colorless form of phenolphthalein is C20H14O4. At a pH of 8 or lower, the structure will be abbreviated as H2P and in the pH range of 8-10 both acidic protons are rapidly removed from the colorless H2P by NaOH, (OH - ) to give the pink P2- ion according to the equation:

H2P+2OH-=P2-+2H2O(pink)

At a pH above 10 the pink color slowly fades as P2- reacts further with NaOH, (OH - ), to give the colorless POH3- ion:

P2-+OH-=POH3-(colorless)

For decolorization, pH is a major factor. I can say the above from my side.

Regards

By the introduction of different scavenging agents into the photocatalytic system.

Please refer to printbibliography output for "[author year] entrydata" (using biblatex-ext and its introcite option) if you are using a more recent TeX installation.

Do you need to create the bibliography or just cite the references in the body of the paper?

Manga Raju Imandi thanks for your valuable answer

Dear Bakhtyar K. Aziz, I think your repoted value of Eg is much higher than the most common ones. The synthesis techniques and protocols highly influence the Eg values. Please check the following RG free access documents. My Regards

Andrea Rogolino

You did not write the most important thing whether the supposed suspension is photocatalysis or not. To prove heterogeneity

1. Separate by long standing

2. Separate by centrifugation

3.Add an immiscible solvent and see the distribution of substances in each of them

4. Add larger nanoparticles and view by DLS.

5. Add coagulant chloride or aluminum sulfate and check if there is precipitation

6. Change the pH and see the change in precipitation

You must know the possible nanoparticles of the photocatalyst then you can discuss the problem more specifically.

Only on the basis of UV experiment one can not get the clear idea about why band gap increased. First, I would like to suggest to repeat the experiment again including synthesis. If possible then measure the standard sample using same UV measurement system to check whether instrument is working fine or not. If instrument is fine, then try to do other characterization such as temperature dependent (low temperature region) electrical resistivity from which you can also calculate the band gap. Another important point is to check the chemical composition as off stoichiometric sample can affect the electronic structure near chemical potential, which can lead to the shrink or broadening of the band gap depending upon the excess or deficiency of oxygen. Che king from different techniques and analysis with different model you can find the exact reason which is responsible for such unusual results. At the end I hope xrd will be single phase, just confirm it with crystal structure analysis method. Good luck.

In addition to scavengers the use of PL spectroscopy could be very useful. Comparison of the intensity (under adequate excitation wavelenght) with respect to the parent components would provide evidence of the type of interface you are analyzed.

Using g-C3N4 we measured the type of several interfaces.

Dear Andrea,

this is a very good and important technical question. As a synthetic inorganic chemist I'm absolutely not a specialist in this field. However, I just came across the following potentially useful review article which could help you in your analysis:

Unfortunately this review article has not been posted as public full text on RG. However, the first author has an RG profile (https://www.researchgate.net/profile/Yanyan-Sun-8). Thus you should be able to request the full text directly from this author via RG.

Good luck with your work!

Dear Serhat Sezer , have you tried to perform centrifugation on your solution with the catalyst? You can separate the powder catalyst from the solution, and also perform FTIR before and after the photocatalytic process.

Tq for the

Continuing the topic of possible dependencies of efficiency measures on, for example, photocatalyst concentration or light intensity, one should expect mainly empirical equations/functions.

Got it.

Brajesh Kumar Thanks a lot! Thanks again for your time.

Ridha Djellabi, Since you've linked your work, here's a quick comment. The most important thing is, of course, the possibility of complete removal of pollutants and the interpretation of the observed phenomena. The mathematical description of the kinetic data is of secondary importance. However, if you choose to do so, do not mislead the reader. There is no evidence in this paper that the processes under investigation are of the first-order. The mere compliance of the empirical exponential function with separate kinetic curves is not enough, and the results presented in Figs. 2 and 4 and in Table 1 even contradict it.

Hello Lea Bongat

There a lot of reasons that's why it is necessary to treat commercial TiO2:

1. In many of the cases, titanium dioxide have impurities that are necessary to remove because could affect catalytic activity.

2. Do not have a high surface area. Commercial TiO2 only have at least 34 m2/g, which could decrease considerably the reaction rate of your material in a specific reaction.

3. Commercial TiO2 have a very specific "shape" or "architecture" that could do not favor your reaction.

With kind regards,

Julián :)

Usually, certain amount of photocatalyst powder is placed in the dye/drug (organic contaminates) solution under UV/Vis light irradiation. I found some confusion about effect of reactive volume, sample volume, and solid/liquid ratio during photodegradation process. Thank you.

Kavya Bisaria

No - for other materials the value is different. For example - for sulfides it is around 0,033 eV.

Aarif Shah

"cellulose fibre" does not exist. By definition all organic materials contain C.

Read what you write and then respond to the titled question.

It depends on the photocatalyst, if you need more information, you can send me a message.

Azeez Abdullah Barzinjy Thomas Jüstel Luiz Fernando de Sousa Lima Thank you for your input. I most certainly agree with the idea that "Irrespective of the method of synthesis" (sol-gel method or photodeposition), the idea of Ag forming substitutional vacancies in TiO2 crystal is not rooted in theoretical observation. However, I do believe that Ti-O-Ag bonds are formed which somehow reduce the charge density and affect the lattice constants, thereby affecting the crystal strain however I am still working on proving this theory. The following paper has also reported the presence of Ti-O-Ag bonds. Perhaps oxygen vaccancies and its quantification is a good place to start.

Video link for determination of Point of zero charge (PZC)

No. It cant be. The role of photocatalyst is to initiate redox reaction only. The phenomena of excitation under UV or visible light can promote one electron to accept/donate from the catalyst, hence the process started. The utility of light (UV or vis) is no more.

For radiation catalyst, the role is completely different. X-ray is a source of much higher energy and hence, harder ionizing capability rather than visible light or UV. So, for much forceful phenomena is obtained by this type of catalysts

Aarif Shah

You're welcome.

In case of antipyretics - when it contains benzoic acid unit or something similar, probability of dimers formation is very high. Drugs can also form charge-transfer complexes with semiconductor (via hydroxyl or carboxyl groups) which can also affect the changes in concentration. It depends of course on the photocatalyst type, and most often it is included in adsorption-desorption equilibrium of the contaminant.

Nice discussion

Hi Joshua,

The best answer is that it depends. Primarily on the size and relative position of rGO w.r.t. your traditional p and and n-type photocatalysts. People have reported all different kind of systems including z-schemes and transistors using all kinds of charge mediators ranging from Au nanoparticles to graphene.

I haven't had a chance to go through recent literature on the topic, but I am guessing there might be some literature on that. Coming back to the question: even thin layer of rGO might be enough to disrupt the formation of a p-n junction as the semiconductors are not in contact. As to whether it is bad or not depends entirely on you application.

I would suggest you to read a review article that we published recently, which has a lot of examples of what such bi-phasic photocatalysts (with w/o a charge transfer interface) have been used for.

I hope it helps.

If you are interestes in the molecules in solution just use described procedure.

But this works only for organic dyes. If you have inorganic particles making the color you would have to do XRF and may be XRD with the sediment after centrifugation or at last SEM or TEM.

I wonder from where you get single TiO2 nano particles as well as color dyes from real life pollutants. Anyway I would strongly recomment TEM to clarify the real situation of your pollutant.

For Molecules adsorbed on TiO2 use microwafeextraction with some commonly used alcohol as solvent.

Photocatalytic processes are considered redox processes initiated by charge carriers (electrons and holes) separated after excitation of the photocatalyst. As it is known - the valence band and conduction band of photocatalyst lies around certain potential values. When the redox potential of the reaction is higher than conduction band edge potential - then the reaction is impossible due to the fact that electron wants to decrease its energy. The same happens when reaction redox potential lies under the potential of the edge of valence band. Pollutants can be also degraded by radicals which arise in described processes or in the case in which the organic molecule (most often dye - it is called a sensitizer) is excited and then the electron can be transferred from the LUMO orbital of this molecule to the conduction band of the semiconductor and then radicals can arise in reaction with the electron from conduction band. Of course the LUMO orbital of sensitizer must have greater energy than conduction band of the photocatalyst. This process in known in literature as photosensitization. In some cases the pollutant can be a sensitizer itself.

These phenomena, as well as possibility of adsorption of pollutants on photocatalyst surface or possibility of absorption of the light from certain range, or the rate of charge carrier recombination etc. can affect photocatalytic activity of the material.

Au and Ag nanoparticles can be formed to absorb visible spectrum efficiently that's why its commonly used photocatalysts.

Dear Joshua Nigel Fernandes thanks for your interesting question. Please see the following article entitled "Simple Pyrolysis of Urea into Graphitic Carbon Nitride with Recyclable Adsorption and Photocatalytic Activity". The paper is available as public full text on RG. In the experimental procedure it is stated: "After drying at 80 °C for 24 hours, the urea was put in a covered crucible and heated in an oven. The yellow colored g-C₃N₄ was obtained by heating at 550 °C for 3 hours." Thus the authors don't specify the heating rate at all. Perhaps the drying of the urea prior to the pyrolysis plays some role.

Please also find attached the Supplementary Information for this article. It does not contain any further details about the heating rate either. Apparently the heating rate is not crucial in this process.

See this link

If you are using TiO2 foam in a photoelectrochemical system as photoanode, the thickness of the electrode is very important. The thickness of the electrode (thin film) should be less than the electron diffusion length. Larger the thickness may lead to efficient recombination of photogenerated charge carriers.

Hi, Febiyanto,

May be, you can first do a bibliography on your synthetic materials to assess their stability with respect to pH, which I agree with you is an important parameter during the organic pollutants degradation such as dyes.

Good luck

Safia

F. Febiyanto

Dear F. Febiyanto ,

you know that the peak size is derived from the peak width such as the full width at half maximum (FWHM)

The maximum peak height come here into play because from the peaks with the largest heights this height(s) (and thus the half of the height(s)) can be determined with the highest confidence due to minimum noise, which statistically affects the individual height. Noise goes with sqrt(N) with N being the counts of the peak height. The relative noise N_r (as the noise relative to the peak height) is a measure of the measurement uncertainty of the peak height:

N_r =sqrt(N)/N= 1/sqrt(N)

So for the heighest N of the peaks we have the lowest uncertainty in peak height determination, which directly affects the (lowest) uncertainty of FWHM calculation.

Yes..I too agree with Prof Feroz, Graphene act as adsorbent.

In the right side p-n junction structure the field region in the transition region will direct the electrons to the n-side where they will be accumulate on the surface of the n-side. At the same time the photognerated holes will be directed to the p-side where they will accumulate on the surface of the p-type material. So, the p-side will be source of holes which is oxidizing. The electrons accumulating on the n-side will be reducing.

The structure in the the right side is composed of two intrinsic materials with the left side has an electron affinity greater than the right side. The two materials have almost an equal energy gap. Both materials are optically active where on absorbing photons they will both generate electron hole pairs.

The electrons will be accumulated on the left side while the holes will be accumulated in the right side.

So, the right side will be oxidizing while the left side will be reducing.

Both will have photovoltaic effect.

The second structure is simpler to produce than the first p-n junction structure.

Best wishes

https://www.amazon.com/EISCO-Premium-Quantitative-Spectroscope-Accuracy/dp/B00B84DGDA/ref=sr_1_2?dchild=1&keywords=spectroscope&qid=1601828755&sr=8-2

graphene being zero bandgap shows fast recombination of excitons, and has no photocatalytic activity. it is just adsorption of dye on the graphene surface. if you want to use graphene/carbon materials for photocatalysis, you can only use carbon or graphene quantum dots. no catalysis journal accepts the photocatalysis of graphene. if you have graphene, then graphene quantum dots synthesis is not that much difficult using the following method.

You can see these papers as a reference.

Aarif Shah

Not sure why there are issues here other than the formulae are not correct in the question or the attached paper. Chloroplatinic acid is usually formulated as H₂PtCl₆.xH₂O (x = nominally 6) and chloroauric acid as HAuCl₄.yH₂O. The x's and y's are important because both salts are hygroscopic and weighing out exact quantities is a problem. It's usual to measure the real metal loading afterwards by AAS or ICP - one usually finds that the actual loading is less than that aimed for because of wash out and the underestimation of 'x' and 'y'. The catalyst preparation isn't conventional either - normally one would just saturate the pores of the support (and this requires a knowledge or measurement of the pore volume. The calculations are not correct either: to make a 1 (wt)% loading of metal on support requires 1 g of metal and 99 g (not 100 g) of support. So, I suspect that chemists or catalyst engineers were not the authors of the paper.

Let's take one example. A desired loading of 1% Pt on 1 g of TiO₂ requires 0.01 g of Pt on 0.99 g of TiO₂. In order to get 0.01g of Pt (atomic weight 195.084) requires [517.90/195.084]*0.01 g of the chloroplatinic salt precursor (the MW of H₂PtCl₆.6H₂O is 517.90) or ~ 0.0265 g of the salt. It's freely soluble in water so you'd dissolve the approximate amount in water and use this solution with the 0.99 g of the TiO₂. Now the key here is the pore volume of the TiO₂ (and, of course, that x is not equal to 6 but practically greater). It wouldn't be normal in catalyst engineering to just dissolve 0.0265 g of the precursor in any amount of water. One would want to just saturate the pores. Imagine a TiO₂ with a known pore volume of 1 mL/g then the ideal situation would be to dissolve 0.0265 g of the salt precursor in 0.99*1 mL = 0.99 mL of water and add this to the dried TiO₂ support. This is not really practical and that's the reason why most people would make 25 or 100 g of final catalyst.

Good discussion!

Yes, of course. SEM image can also strictly identify the surface deterioration which meaning that catalyst surface and substrate may be inhibited by it.

You're welcome Muhammad Shuaib Khan.

No comments on FTIR, only on SEM. You do not have a "transparent layer". Electrons of 15KeV have very small depts of penetration, much, much less than your scale bar of 1 micron. You do not see surfaces covered with some layer (may be you have one, but you do not see it at least), you have just something (artifact?) on the bottom of one of the particles. For proper observation in secondary electrons you had too high voltage and as a result - very strong edge effect which obscure details. You may want to repeat your observations at lower accelerating voltages (1-5 kV) and higher magnifications.

If it is not the instrument error, then most probably the slight shifts of peak positions toward lower angles might be occurred due to the substitution of small radius ion by a larger one (Vegard’s law) which leads to an increase in the cell parameters that may cause a slight distortion in the lattice and a modification of the crystal structure.

In my work

You can get the best oxide catalyst to phenol stability at high temperatures

I have replied such a question a few months back. I repeat.

Higher photocatalytic activity of Cu-N_codoped TiO2 may be due to decrease in band gap of TiO2 as metal doping (Cu) added some new energy levels below conduction band resulting in lowering of conduction band. On the other hand, doping by a non-metal like N helps in raising the valence band by adding new energy levels above the valence band. As a consequence, the band gap decreases and in turn wavelength