Away from TiO2: A critical minireview on the developing of new photocatalysts for degradation of contaminants in water

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Away from TiO2: A critical minireview on the developing of new photocatalysts for degradation of contaminants in water Nurit Shaham-Waldmann, Yaron Paz n Department of Chemical Engineering, Technion, Haifa, Israel

art ic l e i nf o

a b s t r a c t

Article history: Received 3 June 2015 Received in revised form 25 June 2015 Accepted 26 June 2015

During recent years there has been a growing tendency to study new photocatalysts for water and air treatment. To large extent this endeavor is part of a more general effort searching for efficient conversion of solar energy into electricity and into chemical energy stored in hydrogen. Yet, despite the so many man-years invested in this search titanium dioxide is still considered to be the photocatalyst of choice. This paper discusses the current status of research with non-TiO2 photocatalysts for water decontamination. It suggests that developing of new, highly photoactive photocatalysts for water and air treatment is hindered by a combination of reasons, reflecting the tendency of the scientific community to “search under the streetlight”. This includes the overemphasis on bandgap values while overlooking the importance of band positions, the use of dyes as model contaminants under visible light, the ignoring of the importance of transient phenomena, the under-emphasis of the role of surface area and the lack of implementation of theoretical tools in the developing of new photocatalysts. Stepping out of the comfort zone is not only possible but essential. & 2015 Published by Elsevier Ltd.

Keywords: Photocatalysis Review Non-TiO2 Sensitization Water treatment

1. Introduction The photocatalytic activity of titanium dioxide was realized already 90 years ago, however, there is no doubt that the famous Honda–Fujishima paper [1] marked a tremendous change with respect to the ability (and the will) to take advantage of this property. Over the years enormous number of manuscripts has been published on the application of photocatalysts for water and air decontamination, as well as for maintaining clean and superhydrophilic surfaces. As part of this scientific endeavor thousands of compounds have been tested [2], demonstrating the versatility of photocatalysis and its inherent non-preferential nature, which is closely connected to the radical mechanism involved in the photocatalytic degradation process. The general degradation scheme in case of light absorption by a photocatalyst (Fig. 1A) consists of separation of the photoinduced carriers, leading to the generation of active species on the surface of the photocatalyst. Generally speaking, the main active species under this mechanism are OH radicals formed by oxidation of water molecules by the photogenerated holes, hence the primary attack of the dye molecules is oxidative [3,4]. Evidence for direct oxidative attack by holes was also recorded. In parallel, reduction of di-oxygen molecules, forming superoxide anion radicals, takes n

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place, thus acting to minimize electron–hole recombination [5]. These superoxide anions may be utilized at secondary degradation steps [6]. Irreversible reductive degradation of contaminants, initiated by electrons or by superoxides was reported as well [7,8]. The bandgap of TiO2 is 3.2 eV, hence no more than 2.5–3.5% of the solar energy can be utilized by this photocatalyst. Taking into account the solar spectrum impinging on earth, these numbers may grow considerably upon developing materials or methods that can respond to wavelengths, longer than that of pristine TiO2. Indeed, the last fifteen years are marked by a growing effort towards the developing of photocatalysts that utilize not only UV light but also visible light. One approach is the manipulation of titanium dioxide by doping with non-metallic elements (in particular nitrogen [9,10], carbon [11], sulfur [12,13], fluorine [14], and their mixtures [15]), by doping with transition metals [16], and by coupling the photocatalyst with other semiconductors [17]. In parallel to the manipulation of titanium dioxide a zeal for developing new photocatalysts having bandgaps, narrower than that of TiO2, has been noticed. Apparently, this trend occurred (actually still occurs) in parallel with the developing of the Dye Sensitized Solar Cells (DSSC) and with the renewed interest in hydrogen production by photoinduced water splitting. The change in trend from TiO2 into non-TiO2 photocatalysts (defined hereby as NTP) is well reflected in Fig. 2, which presents the number of manuscripts retrieved by the SciFinder™ database upon using the terms “TiO2” and “Photocatalytic decontamination” according to the year of publication, and the number of

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No. of publications

Fig. 1. A schematic photocatalytic degradation of contaminant. (A) The mechanism of photocatalysis (B) the mechanism of sensitization.

1600 1400 1200 1000 800 600 400 200 0 1970

1980

1990 Year

2000

2010

Fig. 2. The annual number of manuscripts retrieved by the SciFinder™ database upon using the terms “TiO2” and “Photocatalytic decontamination” (empty circles), and the annual number of manuscripts retrieved upon using the terms “Bismuth oxides” and “Photocatalytic decontamination” (filled diamonds).

manuscripts retrieved upon using the terms “Bismuth oxides” and “Photocatalytic decontamination”. Here, “Bismuth oxides” were taken as a representative class for non-TiO2 photocatalysts. A clear lag time between the graphs is clearly observed. Generally speaking, the developing of most of the NTPs was driven by the enthusiasm for an efficient photoinduced water splitting process rather than by the will to find better ways for air and water decontamination. While photocatalytic decontamination shares a lot in common with water splitting, it differs by the number of transferred electrons (one versus four), by the minimal photon energy and by the fact that decontamination is basically exothermic whereas water splitting is endothermic. For a review on the similarities and differences between the two processes see Pasternak et al. [18]. Despite the differences between the two processes many of the non-TiO2 photocatalysts developed for water splitting were tested as photocatalysts for water and air decontamination. More on the different types of photocatalysts can be found in several reviews [19,20]. Tracing all the materials that were reported as having photocatalytic properties is very difficult, if not impossible. An incomplete literature search gave several hundred compounds, whose activities and properties were described in thousands of manuscripts. At large, these materials can be divided into oxides and non-oxides; the former comprises the larger group in terms of diversity and number of manuscripts describing their properties. Oxide photocatalysts differ from non-oxides photocatalysts by the sensitivity of the location of their valence bands and conduction

bands to pH, reflecting their tendency to adsorb H þ and OH  , whereas the effect of pH on the location of the energy bands of non-oxide photocatalysts is minute. This fact has a clear effect on the tendency of organic contaminants (weak acid/bases) to be adsorbed on the photocatalyst. Another difference has to do with the location of the valence band in oxide semiconductors. Since the valence band in oxide semiconductors originates from O2p orbitals [21], its location in the energy scale hardly deviates between different oxides (Fig. 3). From the point of view of oxidation, this location is sufficient for the formation of active OH radicals. On the other hand, it means that altering the metal cations, leading to smaller bandgaps, might be manifested by decreasing the ability to reduce oxygen, hence increasing the rates of recombination. This is in particular important in cases where direct oxidation by holes is possible due to strong adsorption of the contaminant [22]. In this case, a non-oxide photocatalyst, having a valence band which is slightly more positive than the redox level of the contaminant, may be sufficient to oxidize the contaminant (provided that the rate of recombination is slow enough). Many of the photocatalytic materials (but not all) are characterized by an internal electric field which assists in the separation of the charge carriers, hence reduces recombination rates. This field can often be found in layered- structured photocatalysts. One example is K4Nb6O17 which was found to be able to efficiently degrade phenol and 2,4-dichlorophenol under UV light [26]. Another example is BiOX, where X is an halogen [27]. Here, the layered structure consists of tetragonal [Bi2O2] slabs, “sandwiched” between two halide ions slabs to form a [Bi2O2X2] layer along the c-axis, thus forming an internal electric field between the [Bi2O2] positive slabs and the halide anionic slabs. A second structure found very active for water splitting as well as for photocatalytic decontamination comprises of materials that are composed of a network of corner-shared octahedral units of metal cations such as TaO6, NbO6 and TiO6, which act to increase the mobility of electrons and holes. These corner-shared octahedral can be found in a variety of crystalline structures [28] among which are orthorhombic weberite (La3TaO7 and La3NbO7), cubic pyrochlore (Y2Ti2O7, and Gd2Ti2O7) and monoclinic perovskite structure (La2Ti2O7). Composite photocatalysts, comprising of TiO2/NTP or NTP/NTP become more and more popular in recent years. The basic concept is to form a heterojunction between the two photocatalysts such that absorption of light by one of the photocatalysts is followed by charge transport to the other photocatalyst, thus improving charge

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Fig. 3. The bandgap of several photocatalysts and their location for non-oxides (left) and at pH ¼7 for oxides (right). The location of reduction potential of the various species follows Refs. [23–25].

separation and consequently increasing the photodegradation rate [29]. In that sense there is not much of a difference between NTP/ NTP and systems based on titanium dioxide connected to electron sinks such as noble metals or carbon nano tubes [30]. For nonoxide NTPs that may suffer from photocorrosion, such as CdS, coupling between the photocatalyst and a charge sink may assist in reducing the rate of photocorrosion [31,32]. Despite so many man-years invested in searching for replacement for TiO2, titanium dioxide is still considered to be the photocatalyst of choice. Moreover, P25 (previously manufactured by Degussa, currently by Evonik Industries) served and still serves as a reference in the process of assessing the photocatalytic activity of novel photocatalysts. While P25 is adequate to serve as a reference under UV light, the low activity of pristine P25 under visible light makes it unsuitable as a reference for the latter spectral region, for which most of the NTPs are designed. The low cost and the abundancy of titanium dioxide make the use of this material for UV photocatalysis a natural choice. Yet, when speaking about utilizing visible light, it is difficult to outline a single compound that may serve as a reference. Moreover, after so many years of research the community still lacks solid guidelines that may assist in searching for the right materials. The following paper discusses the current status of research with non-TiO2 photocatalysts and analyzes the reasons for the current situation. In light of the immense number of publications on NTPs and due to the need to restrict the volume of this manuscript we had no other choice but to limit the discussion to photocatalysts aimed at decontamination of water.

2.1. Band gaps and band positions The band gap of photocatalysts is regarded as one of the fundamental characteristics of these materials as it directly related to the range of photon energies that, potentially, can be utilized. The fact that a bandgap is a parameter that can be easily used to compare between different photocatalysts adds to its popularity. Whether for decontamination or for water splitting, almost all manuscripts that describe new materials use reflection spectroscopy as a major characteristic tool, as it is an easy-to-perform measurement that enables to estimate the bandgap of the new materials by plotting Tauc plots [33]. Since the calculated bandgap depends on the nature of the transition (i.e. direct or indirect) and since the type of transition is often unknown for new materials the calculated band gaps might be erroneous and should be taken with a grain of salt. Moreover, many of the reported new materials contain more than one phase, and this fact is quite often overlooked in manuscripts. Another source for lack of matching between calculated bandgaps and activity arises from lack of knowledge on the location of the valence and conduction bands relative to NHE. This has a crucial effect on the activity, since, as explained above, such mismatch might prevent the formation of critical species i.e. hydroxyl radicals and superoxides. There are several techniques claimed to estimate the location of the bands, among which are Kelvin Probe measurements [34], UPS and XPS [25] and electrochemistry [25]. For the same material, each of the methods might give a different value, reflecting small deviations in the conditions prevailing during the measurements. 2.2. The use of dyes as model systems

2. A critical view on the way by which we all work While finding highly efficient photocatalysts is a very challenging task, it is claimed hereby that at least part of the difficulties is an outcome from the way by which the scientific community, as a whole, tackles this mission. Moreover, it is claimed hereby that for a breakthrough progress to be made, the community has to reevaluate the various types of scientific tools used for this research, and, if needed, add some more tools into its toolbox. This conclusion is demonstrated by analyzing three aspects of the current scientific work: band gaps and band positions, the use of dyes to evaluate photoactivity, and the role of surface area versus that of crystallinity.

The use of dyes to demonstrate photocatalytic activity dates to the first days of photocatalysis, where un-doped titanium dioxide, operating under UV light was, practically, the only photocatalyst to be considered. At these times, the main motivation for using dyes as model contaminants was to study the optimal conditions for efficient removal of specific dyes that posed specific environmental hazard. This notion was reflected by the large number of manuscripts monitoring the kinetics of disappearance of the primary substrate and the various factors affecting the photocatalytic degradation rates such as dye concentration, solution's pH, light intensity, addition of oxidants such as H2O2 and S2O82  and the presence of co-existing ions. As part of this endeavor care was given to examine the toxicity of the intermediates, as well as that of the end-products.

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Table 1 An estimation regarding the number of publications on the photocatalytic degradation of dyes under UV light and under visible light, organized according to dye categories. The estimation is based on SciFinder™ data source, taking the words “photocatalysis” “visible light” (or “UV light”) and “a specific name of dye” as keywords. For comparison, data on the number of publications on the degradation of other contaminants is shown as well. Class

UV

Visible

Vis/UV ratio

Anthraquinones dyes Azo dyes Natural dyes Thiazine dyes Triarylmethane dyes Xanthene dyes

238 1285 187 7496 1439 5625

390 2006 303 13,471 2758 12,244

1.64 1.56 1.62 1.80 1.92 2.18

272 538 102 292 276

218 408 97 276 223

0.80 0.76 0.95 0.94 0.81

Herbicides Insecticides chlorobenzene NO2 TCE

The introduction of self-cleaning surfaces, i.e. self-cleaning glass [35,36] self-cleaning cementitious materials [37,38] and selfcleaning fabrics [39] marked a change in the role that dyes played in pohotocatalysis. For these applications, the degradation of dyes was no longer important per-se but, instead, began to serve as a tool for demonstrating the potential of photocatalysis as a means to protect esthetic assets. Two dyes appeared to be the most popular: rhodamine B and methylene blue. Indeed, the Japanese industrial standard JIS-R-1703-2:2007 utilizes methylene blue for evaluation of self-cleaning surfaces [40]. Intermediate products, mineralization and adsorption were no longer of importance. The only important characteristic was now the kinetics of de-coloring. In a similar manner, dyes were used as model contaminants, demonstrating specific properties of tailored photocatalysts, for example photocatalysts that were able to degrade specific contaminants [41]. While the above statements are based on impression from twenty years of experience in the field and occasional conversation with colleagues, they are also supported by statistics. Table 1 presents the main categories of dyes and an estimation regarding the number of publications on their photocatalytic degradation under UV light as well as under visible light. The estimation is based on the SciFinder™ data source, taking the words “photocatalysis” “visible light” (or “UV light”) and “a specific name of dye (in both its formal and its commercial name)” as keywords. A third column presents the ratio between the number of publications on degradation under visible light to the number of publications on degradation under UV light [42]. For comparison, more data, on the number of publications aimed at treating other contaminants is given as well. The table is based on data obtained for 250 different dyes. The most studied dyes are the thiazine dyes (with a dominance of methylene blue: 37% of all papers on thiazines), second to them are the xanthenes (with a dominance of rhodamine B: 30% of all manuscripts on xanthenes). From the table it is evident that for all dye categories the number of manuscripts on visible light photocatalysis of dyes is larger than that of the number of manuscripts on UV light photocatalysis. The higher ratio between the number of manuscripts on visible light photocatalysis and UV light photocatalysis is found for xanthenes (2.18) and thiazines (1.80). Azo dyes, despite their dominance in global production (50–70% of the market), hence their dominance in contributing to the environmental challenge, come only fourth. Moreover, their low Vis/ UV ratio, relative to the other dyes (1.56 versus 2.18 for the Xanthene dyes), is in correlation with our claim

regarding the historical shift in the role of dyes in photocatalysis. That dyes are used primarily for demonstrating activity under visible light can be deduced also from comparing between the Vis/ UV publications ratio of dye contaminants to that of non-dyes (Table 1). Here, we used herbicides, insecticides, chlorobenzene, NO2 and Trichloroethylene (TCE) as representative key words. For all five categories the Vis/UV ratio was smaller than one (0.76– 0.95), whereas for all categories of dyes this ratio was higher than 1.5 (1.56–2.18). It is claimed hereby that the use of dyes as model contaminants demonstrating the photocatalytic properties of new and tailored materials impedes the developing of efficient new, non-TiO2 photocatalysts. To understand the origin of this claim one has to look into the way by which dyes are degraded under visible light. Under visible light, a significant part of the impinging photons are absorbed by the dye molecules, opening a second avenue for degradation, self-sensitization, depicted in Fig. 1B. Here, charge transfer from the excited dye molecule to the conduction band of the semiconductor results in the formation of an unstable dye cation radical and in parallel an active specie on the semiconductor surface that attacks the destabilized dye molecule. One of the first demonstrations of this mechanism, published as early as 1977, described highly efficient N-deethylation of rhodamine B adsorbed on CdS [43]. This mechanism was claimed to be responsible for the fact that the de-coloring kinetics of methylene blue under solar light in the presence of (un-doped) TiO2 was faster than that of de-coloring kinetics under UV light [44]. As an example, one may look in depth into the photooxidation of alizarin red in TiO2 under visible light, where the main active species was found to be O−2  or OOH. A claim was made [45] that the electron transferred from the dye to the semiconductor is likely to arrive from the atom having the largest electron density in the ground state. This atom becomes later the site where the attack by the superoxide anions radicals takes place. Using any model contaminant as a probe for the effectiveness of a new photocatalyst requires that the degradation kinetics, whatever they are, will be, as much as possible, generic, in a sense that comparative studies performed on a set of photocatalysts will be relevant (at least partially) for the degradation of other contaminants. In practice, most manuscripts do not report that the degradation they measured under visible light could be due to photosensitization rather than photocatalysis. In many cases, even when sensitization is mentioned as the governing mechanism, the (ir)relevance for other contaminants is only rarely mentioned. One of the first studies pointing out the inadequacy of dyes as probe molecules for semiconductor photocatalysis (methylene blue in this case) was presented by Yan et. al. [46]. Here, the action spectra of S-doped TiO2 measured during the degradation of methylene blue revealed activity at 580–650 nm (in correlation with the absorption spectrum of MB), whereas no activity at this range of the spectrum was observed towards acetic acid. Recently, a clear recommendation not to use dye tests for activity assessment of visible light photocatalysts was presented, based on comparison between six visible-light photocatalysts that degraded five organic dyes [47]. Further details on the inadequacy of dyes as model contaminants for visible light photocatalysis are given in a recent review by us [42]. Basic ethics of integrity require that comparative studies, which are most likely relevant to one contaminant only, will be reported as such, and will not be publicized in a generalized manner that misleads the readers. Fig. 4 presents the annual ratio between the number of manuscripts obtained using the keywords “water treatment” and “sensitization” versus the number of manuscripts obtained upon using the keywords “water treatment” and “visible light”. The graph depicts a clear decrease in the ratio as a function of time, as of the beginning of the century. In other words, there is

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Fig. 4. The annual ratio between the number of manuscripts obtained using the keywords “water treatment” and “sensitization” versus the number of manuscripts obtained upon using the keywords “water treatment” and “visible light” (source: SciFinder™).

an increasing tendency not to assign water treatment under visible light to sensitization. One might interpret this trend as representing a situation where more and more photocatalysts acting under visible light not according to a sensitization mechanism but according to photocatalysis are developed. Unfortunately, the truth is different. Almost all the manuscripts on water treatment under visible light describe the degradation of dyes. Accordingly, the statistics depicted in Fig. 4 does not represent, at large, the finding of new photocatalysts operating under visible light but a growing tendency to disregard and to overlook the effect of sensitization. We leave it for the readers to decide whether this behavior is a matter of misinterpretation of data or reflects a humane response to the “Publish or Perish” pressure in the field. Regardless of the answer, it is quite clear that the way for developing NTPs operating under visible light goes through using model contaminants that are not dyes, or, if using dyes, through following not only the disappearance of color, but also the mineralization rates. At any case, non-compromised reporting on the origins of photocatalytic phenomena measured with dyes (i.e. sensitization versus true photocatalysis) should be the rule rather than the exception. 2.3. Transient measurements Regardless of the type of photocatalyst and contaminant, typical quantum efficiencies, defined hereby as the ratio between the number of degraded molecules to the number of absorbed photons (per the same time), are, in water, within one percent or even less. This means that the main obstacle for efficient photocatalysis is neither photon absorption, nor the adsorption of the contaminant on the surface of the photocatalyst, but the high rate of recombination and the slow process of transferring the photoinduced charge to the target molecules. Accordingly, one could have expected that large part of the scientific efforts will be dedicated to studies on the transient behavior of the photoinduced charge and in particular to understand the role that shallow traps play in reducing recombination rates. Unfortunately, this is not the situation and the number of manuscripts describing transient measurements is estimated by us (based on SciFinder™) as less of one percent of all published manuscripts on photocatalysis. Studying transient behavior of photocatalysts may be obtained by a limited number of techniques, the most popular of which are Electron Spin Resonance (ESR)/(EPR), Time Resolved Microwave Conductivity (TRMC) and Laser Flash Photolysis (LFP) coupled with UV–vis diffuse reflectance or transient photocurrent. EPR spectroscopy measures the transitions between spin levels of a given spin multiplet, split by a static magnetic induction. In combination with pulse EPR pulse light excitation can be used to

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study the time evolution of photo-generated species containing one unpaired electron. As such it can detect free radicals, point defect in solids, localized crystal imperfections, systems with conducting electrons and transition metal ions. To increase sensitivity, spin trapping of short – lived species by using nitrone or nitroso compounds is often used. The method is commonly used to elucidate the origin of the photoactivity of pristine and doped TiO2. In particular, it was used to probe trapped holes, trapped electrons, hydroxyl radicals, superoxide anions radicals, singlet dioxygen and many intermediate radicals. As such, it is a very important tool that may provide detailed information not only on the behavior of the photocatalyst but also on the distribution of short lived intermediate products, thus revealing the degradation mechanisms of specific molecules [48,49]. For reviews on the use of ESR for the study of TiO2 photocatalysis see Fittipaldi et al. [50] and Wang et al. [51]. Unfortunately, not much was done in the utilization of ESR to study photocatalytic processes in non-TiO2 photocatalysts. Nevertheless, the situation is much better than with LFP and TRMC. Examples include the investigation of unstable intermediates on CdS dispersed on CdO [52], studying the activity of polymermodified ZnO, using 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) for spin trapping, thus providing evidence for the role of OH radicals [53] and following the degradation of dyes on γ-Bi2MoO6 [54], SrTi1  xMnxO3 [55], hybrids of Al2O3 and γ-C3N4 [56] and ZnO/CuO nanocomposites [57]. Likewise, ESR was used to deduce the mechanism of degradation of antibiotics on Sr-doped β-Bi2O3 [58] and provided evidence for the role of superoxide radicals in the demineralization of sulfamethoxazole on Bi2O3/Bi2O2CO3/ Sr6Bi2O9 [59]. Time resolved microwave conductivity is a contactless technique, enabling to follow recombination, trapping, and interfacial charge transfer of photo-generated carriers [60]. TRMC is based on measuring the change in the microwave power reflected by a sample subjected to laser pulse illumination. This relative change in the reflected microwave power is caused by the variation of the sample's conductivity induced upon exposure to the laser pulse. The method was found to be very suitable for the study of photocatalytic phenomena. It is estimated that 25% of all manuscripts on TRMC measurements were performed on photocatalytic materials or are within the context of photocatalysis [61]. For TiO2, the method was able to successfully correlate between structural parameters, charge carrier dynamics [61], and the photodegradation of contaminants such as phenol [62–64]. The same method was used to study oxygen isotopic exchange in TiO2 photocatalysis [65], charge separation and trapping dynamics in N-doped TiO2 [66], in TiO2 over-coated with Au–Cu nanoparticles [67], and as a means to understand the differences between various types of TiO2 and platinized TiO2 [68,69]. This endeavor was, and still is, part of the attempt to provide a physically sound structure – activity relationship for TiO2 [70]. To date, TRMC measurements on non-TiO2 photocatalysts are very scarce. While reports on the dynamics of charge carriers in alumina, magnesium oxide, MoSe2 and MoS2 were published as early as the beginning of the nineties [60,71], not much was done on highly photoactive non-TiO2 materials. The only works that we could found were performed of TiO2-containing hybrid systems such as TiO2–SiO2–POM [72] and g-C3N4–Mn þ /CeO2–TiO2 [73]. This was accompanied by works involving photosensitization of oxides such as WO3 and (surprisingly) SiO2 [74] as well as SnO2 [75]. The only exception we found (an exception that hopefully is the first sign of a change) was a manuscript on the origin of slow carrier transport in thin films photoanodes made of BiVO4 [76]. Here, TRMC revealed that the poor carrier mobility of undoped bismuth vanadate (4  10  2 cm2 V  1 s  1) is compensated by an unexpectedly long carrier lifetime (40 ns).

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LFP is a mature technique used to study the oxidation of SCN  on TiO2 already in 1990 [77]. A variation of LFP is a two-color twolaser flash photolysis, used to study electron transfer from the conduction band of TiO2 to polyoxometalates en-route for efficient oxidation of aromatic sulfides [78]. The literature on LFP–UV/vis reflectance follows the same trend described for TRMC. Here also, the vast majority of works were performed on TiO2. During recent years a growing number of transient measurements on the behavior of organo-metallic complexes acting as photocatalysts were documented. Examples include 2Fe2S complexes [79], the report on water stable Zr-benzenedicarboxylate [80], and a donor–acceptor linked dyad comprising of 9-mesityl10-methylacridinium ion incorporated into nanosized mesoporous silica–alumina [81]. Likewise, LFP was used to study the direct action of organic sensitizers leading to the degradation of water contaminants. Here, one may mention the work of Diaz et. Al. on the role of riboflavin in the degradation of endocrine disrupting compounds in water [82]. Unlike the situation with TiO2, the LFP measurements in inorganic NTPs are quite rare. Large part of the few published manuscripts describes measurements that were performed in the context of studying water splitting (actually photoinduced hydrogen evolution). Examples include the use of the mixed-valence compound Mn(III)3Mn(IV)O3 that mimicked a biological oxygenevolving center [83], and the study of complexes based on Keggin structures, such as [Co4(H2O)2(a-PW9O34)2]10 [84]. The number of manuscripts using LFP of inorganic photocatalysts in the context of remediation of water and air is in particular small. One of these few works describes the finding of long-lived electrons in Ag–Ag2O nanocomposites, leading to fast degradation of 2-chlorophenol, 2,4-dichloroiphenol, and trichlorophenol [85]. Another work utilized LFP to study the photocatalytic properties of pillared interlayered clays [86]. The number of NTPs studied by LFP coupled with transient photocurrent measurements is also very limited. Here, we may outline the study of composites made of graphene – oxide and CdS [87]. The small number of given-above examples demonstrates the fact that the majority of materials that potentially can be used as photocatalysts for water treatment have never been studied by transient techniques. As a consequence it is currently very difficult to deduce, based on existing data, practical conclusions that may aid in designing new, highly efficient, photocatalysts. One reason for that is the very small number of research groups that are capable of performing these measurements. The small number of groups is in correlation with the higher complexity of these measurements in comparison with traditional kinetics measurements. We believe that that there is room for more groups to step in and perform transient measurements. It is possible that the reason for lack of emphasis on transient measurements stems not only from the complexity and cost of these techniques but also from a notion that the information that can be obtained from the mentioned-above techniques is inherently limited. There is definitively a need to develop means that will provide more information beyond measuring the life time of the carriers. It should be noted however, that there is a lot to be done even with current techniques, for example by studying the effect of faceting, the behavior under specific environment and the behavior as a function of impinging wavelength. Recent years have shown an increased tendency to implement “blind” approaches for the developing of new photocatalysts, such as combinatorial chemistry [88]. Yet, it is hard to believe that the holy-grail will be found based on combinatorial chemistry alone, taking into account the large number of parameters that may affect photoactivity, some of which are still, after so many years, unknown. Therefore, a deep understanding of transient phenomena will be needed, even when using the broad approach of

combinatorial chemistry. 2.4. The role of surface area Any discussion of the physical attributes of a given photocatalyst must include its surface area. Large surface area provides more active sites for the adsorption of water and contaminants as well as for the formation of hydroxyl radicals. It should be noted that the adsorption capacity for a given contaminant depends not only on the surface area but also on the type of functional groups on the surface, surface morphology, particle size and product tendency to desorb from the surface. Semiconductors photocatalysts can be prepared by several methods. High temperature methods such as the solid state reaction process result usually with materials having high crystallinity but with very low specific surface area. The fundamental problem in examining published data in order to decide the relative importance of surface area versus that of crystallinity stems from the different nature of these publications, reflecting preparation by different experimental routes and testing under different experimental conditions. There are many well-known methods for controlling surface area. For example, the use of surfactants and polymers as structure-directing agents [89,90]. High surface area can be obtained also in self-organized nanoporous photocatalysts [91] and nanotubes/nanocolumns structures [92]. Surface with minimum bulk defects (higher crystallinity) is obtainable by high temperature treatment. However, this, in turn, can induce the aggregation of small nanoparticles thereby decreasing the surface area. High crystallinity is well-accepted in solid-state science as improving charge-transport dynamics and therefore, this parameter should be crucial in dictating the photocatalytic activity of a given inorganic semiconductor. In general, the photocatalytic activity increases with increasing the specific surface area and increasing crystallinity. Optimizing these two properties simultaneously is quite challenging. Information on NTPs is very limited. It is thought that photocatalytic activity depends on the physical and structural properties of the photocatalyst and that there is a correlation between activity and each property. However, these properties are inter-related. Rajeshwar et al. [93] suggested that normalization of the activities according to the surface area should be done routinely, and demonstrated this attitude while studying the decomposition of methyl orange by AgBiW2O8 (synthesize by a low temperature process and by a solid state reaction). Among common known NTPs are iron oxides and bismuth oxides. The latter may be prepared by several methods. High temperature methods such as the solid state reaction process result usually with materials having high quality crystallites but with very low specific surface area. For example, Bi2MNbO7 (M ¼Al, Ga, In) compounds prepared by solid state reaction at 1100 °C had a specific surface area of no more than 0.5 m2g  1 [94]. Surveying the literature reveals a dominant preference towards high crystallinity, hence preparation by solid state methods. In this context it was shown for doped BiOCl that high activity can be obtained if the specific surface area is large enough as the large specific surface area more than compensates for less-than optimal crystallinity [95]. In contrast, when Iron(III) oxide (hematite) [96] with different degree of crystallinity was investigated under constant conditions (pH, concentrations of iron oxide and H2O2) it was found that the catalytic efficiency was determined by the crystallinity of the particles rather than by their surface area. 2.5. Theory and calculations Density Functional Theory (DFT) is probably the most widely

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used tool for the calculation of adsorption and reactions on solid surfaces [97]. Over the years a steady process of refining this method (the use of hybrid functionals (B3LYP) and the DFT(LDA/ GGA)þ U approach) has reduced the gap between theoretical predictions and experimental results. Nevertheless, when it comes to oxides, even the advanced methods such as PBE0 operating on relatively simplest oxides give an inaccuracy in the positions of Ev, Ec [98] which limits its usefulness for experimentalists. Precise knowledge of the ground and excited states of the photocatalysts, either naked or with adsorbed species, is very important for better understanding of the mechanism behind photocatalysis [99]. Despite the abundant use of theoretical tools in chemistry, their application in the field of photocatalysis is still very limited. Calculating the band edge positions of MnO, FeO, a-Fe2O3, Cu2O and NiO [100] and studies on the nature of the valence band and conduction bands in Bi2WO6 [101], AgTaO3 and AgNbO3 [102]. and bismuth molybdates, are a few examples [103]. For experimentalists, it is of upmost importance to be able to correlate between structure and charge transport phenomena within the photocatalyst, not to mention the existence and role of intra-bandgap levels associated with defects and dopants. In that respect, it seems that a lot of work is still to be done. Overall, it is not very common that the theory work is done in parallel (or even in collaboration) with experimental work. In fact, the two scientific communities, both working on photocatalysis, hardly meet each other. Moreover, cultural barriers (from different terminologies to lack of awareness to the challenges of each community) prevent efficient transfer and assimilation of existing knowledge. In light of the existing challenges a stronger collaboration seems to be essential.

3. Conclusions This paper aims at discussing the current status of research with non-TiO2 photocatalysts for water decontamination. It points to the fact that it is still difficult to outline a single compound (or even a group of compounds) that may replace (doped/tailored) TiO2 for water treatment. Furthermore, it claims that currently there are no solid guidelines for searching for the right materials. It is suggested that this situation originates from a combination of reasons, which reflects the tendency of the scientific community to “search under the streetlight”. This includes the overemphasis on bandgap values while overlooking the importance of band positions, the use of dyes as model systems under visible light, the ignoring of the importance of transient phenomena, the underemphasis of the role of surface area and the lack of implementation of theoretical tools in the developing of new photocatalysts. We believe that stepping out of the comfort zone is not only possible but essential.

Acknowledgments The authors are grateful to ICORE Program, to The Grand Technion Energy Program (GTEP) and to the Russell Berrie Nanotechnology Institute (RBNI) for their support. The assistance of Idan Cohen in the data mining is gratefully acknowledged.

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