[phpBB Debug] PHP Notice: in file /viewtopic.php on line 988: date(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'Europe/Moscow' for 'MSK/3.0/no DST' instead[phpBB Debug] PHP Notice: in file /viewtopic.php on line 988: getdate(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'Europe/Moscow' for 'MSK/3.0/no DST' insteadSolaraGuy.com • View topic - In regards to the Facebook Solaraguy group

Your long-gone mod here. I've been busy as hell, but keeping tabs when I can. For those of you who have been off-put by the fact that there was no moderation on the original Facebook group for SolaraGuy, that has changed. Members such as shotgun and crankine3 have helped cleaned house, and I am also an administrator as well. For those who haven't checked it out, check it out here: https://www.facebook.com/groups/solaraguy/. If the hyperlink doesn't work, it's under the name "Solaraguy". And to those of you who jumped to the other group, mosey on back! Its always fun when we stick together!

-Brian

EDIT: The SG staff and I did NOT authorize this "transferring" of groups that was being orchestrated. The one and only Solaraguy FB group is the one with 495+ members at the moment and is simply titled "Solaraguy". This is not a matter of discussion, there are owners of this intellectual property and that is not an issue we take lightly

Didn't know there was a Facebook. Just sent a request. I need to be on here on a more regular basis. Seven years of commuter hell to campus and back every day, not to mention a little negligence on my part, and I've got a lot of work to do on my car.


import React from 'react';
import useStyles from '@airbnb/lunar/lib/hooks/useStyles';
import { footerMarkStyleSheet } from '../../styles';

export type MarkProps = {
  children?: React.ReactNode;
};

export default function Mark({ children }: MarkProps) {
  const [styles, cx] = useStyles(footerMarkStyleSheet);

  return <mark className={cx(styles.mark)}>{children}</mark>;
}


The Heart Is a Lonely Hunter

Carson McCullers was all of 23 when she published her first novel, The Heart Is a Lonely Hunter. She became an overnight literary sensation, and soon such authors as Tennessee Williams were calling her "the greatest prose writer that the South [has] produced." The Heart Is a Lonely Hunter tells an unforgettable tale of moral isolation in a small southern mill town in the 1930s.

The Ballad of the Sad Café

A classic work that has charmed generations of readers, this collection assembles Carson McCullers' best stories, including her beloved novella The Ballad of the Sad Cafe. A haunting tale of a human triangle that culminates in an astonishing brawl, the novella introduces readers to Miss Amelia, a formidable southern woman whose cafe serves as the town's gathering place. Among other fine works, the collection also includes "Wunderkind", McCullers' first published story, written when she was only 17, about a musical prodigy who suddenly realizes she will not go on to become a great pianist.

Thérèse Raquin

Once upon a time, a teenaged Kate Winslet (The Reader, Titanic, Revolutionary Road) received a gift that would leave a lasting impression: a copy of Emile Zola’s classic Thérèse Raquin. Six Academy Award nominations and one Best Actress award later, she steps behind the microphone to perform this haunting classic of passion and disaster.

Reflections in a Golden Eye

Set on a Southern army base in the 1930s, Reflections in a Golden Eye tells the story of Captain Penderton, a bisexual whose life is upset by the arrival of Major Langdon, a charming womanizer who has an affair with Penderton's tempestuous and flirtatious wife, Leonora. Upon the novel's publication in 1941, reviewers were unsure of what to make of its relatively scandalous subject matter.

The Sheltering Sky

The Sheltering Sky is a landmark of 20th-century literature, a novel of existential despair that examines the limits of humanity when it touches the unfathomable emptiness of the desert. Academy Award-winning actress Jennifer Connelly (A Beautiful Mind, Requiem for a Dream) gives masterful voice to this American classic.

The End of the Affair

Graham Greene’s evocative analysis of the love of self, the love of another, and the love of God is an English classic that has been translated for the stage, the screen, and even the opera house. Academy Award-winning actor Colin Firth (The King’s Speech, A Single Man) turns in an authentic and stirring performance for this distinguished audio release.

Being There

Academy Award winner Dustin Hoffman gives an understated and exemplary performance of this satiric look at the unreality of American media culture. Chance, the enigmatic gardener, becomes Chauncey Gardiner after getting hit by a limo belonging to a Wall Street tycoon. The whirlwind that follows brings Chance to his new status of political policy advisor and possible vice presidential candidate. His garden-variety political responses, inspired by television, become heralded as visionary, and he is soon a media icon.

Slouching Towards Bethlehem

Universally acclaimed from the time it was first published in 1968, Slouching Towards Bethlehem has been admired for decades as a stylistic masterpiece. Academy Award-winning actress Diane Keaton (Annie Hall, The Family Stone) performs these classic essays, including the title piece, which will transport the listener back to a unique time and place: the Haight-Ashbury district of San Francisco during the neighborhood’s heyday as a countercultural center.

The Human Comedy

In World War II-era California, Homer Macauley and his mother, sister, and three brothers live out struggles and dreams that reflect those of America’s second-generation immigrants. Homer, determined to become one of the fastest telegraph messengers in the West, finds himself caught between reality and illusion as delivering his messages of wartime brings him face-to-face with human emotion at its most naked and raw.

Collected Stories of William Faulkner

This magisterial collection of short works by Nobel Prize-winning author William Faulkner reminds listeners of his ability to compress his epic vision into narratives as hard and wounding as bullets. Among the 42 selections in this audiobook are such classics as "A Bear Hunt", "A Rose for Emily", "Two Soldiers", and "The Brooch".

Hourglass: Time, Memory, Marriage

Hourglass is an inquiry into how marriage is transformed by time - abraded, strengthened, shaped in miraculous and sometimes terrifying ways by accident and experience. With courage and relentless honesty, Dani Shapiro opens the door to her house, her marriage, and her heart, and invites us to witness her own marital reckoning - a reckoning in which she confronts both the life she dreamed of and the life she made, and struggles to reconcile the girl she was with the woman she has become.

Wise Blood

Flannery O’Connor’s astonishing and haunting first novel is a classic of 20th-century literature. It is the story of Hazel Motes, a 22-year-old caught in an unending struggle against his innate, desperate faith. He falls under the spell of a “blind” street preacher named Asa Hawks and his degenerate fifteen-year-old daughter. In an ironic, malicious gesture of his own non-faith, and to prove himself a greater cynic than Hawks, Hazel founds The Church of God Without Christ but is still thwarted in his efforts to lose God.

Mrs. Dalloway

It is a June day in London in 1923, and the lovely Clarissa Dalloway is having a party. Whom will she see? Her friend Peter, back from India, who has never really stopped loving her? What about Sally, with whom Clarissa had her life’s happiest moment? Meanwhile, the shell-shocked Septimus Smith is struggling with his life on the same London day.

Light in August

An Oprah's Book Club Selection regarded as one of Faulkner's greatest and most accessible novels, Light in August is a timeless and riveting story of determination, tragedy, and hope. In Faulkner's iconic Yoknapatawpha County, race, sex, and religion collide around three memorable characters searching desperately for human connection and their own identities.

To the Lighthouse

To the Lighthouse is Virginia Woolf’s arresting analysis of domestic family life, centering on the Ramseys and their visits to the Isle of Skye in Scotland in the early 1900s. Nicole Kidman (Moulin Rouge, Eyes Wide Shut), who won an Oscar for her portrayal of Woolf in the film adaptation of Michael Cunningham’s Pulitzer Prize-winning novel The Hours, brings the impressionistic prose of this classic to vibrant life.

The Turn of the Screw

Academy Award, Golden Globe, and Emmy winner Emma Thompson lends her immense talent and experienced voice to Henry James' Gothic ghost tale, The Turn of the Screw. When a governess is hired to care for two children at a British country estate, she begins to sense an otherworldly presence around the grounds. Are they really ghosts she's seeing? Or is something far more sinister at work?

This complete collection includes all of the published stories of Eudora Welty. There are 41 stories in all, including those in the earlier collections A Curtain of Green, The Wide Net, The Golden Apples, and The Bride of the Innisfallen, as well as previously uncollected stories.

The long-awaited first novel from the author of Tenth of December: a moving and original father-son story featuring none other than Abraham Lincoln, as well as an unforgettable cast of supporting characters, living and dead, historical and invented. February 1862. The Civil War is less than one year old. The fighting has begun in earnest, and the nation has begun to realize it is in for a long, bloody struggle. Meanwhile, President Lincoln's beloved eleven-year-old son, Willie, lies upstairs in the White House, gravely ill.

Across seven tales, Haruki Murakami brings his powers of observation to bear on the lives of men who, in their own ways, find themselves alone. Here are vanishing cats and smoky bars, lonely hearts and mysterious women, baseball and the Beatles, woven together to tell stories that speak to us all. Marked by the same wry humor that has defined his entire body of work, in this collection Murakami has crafted another contemporary classic.

After a violent coup in the United States overthrows the Constitution and ushers in a new government regime, the Republic of Gilead imposes subservient roles on all women. Offred, now a Handmaid tasked with the singular role of procreation in the childless household of the enigmatic Commander and his bitter wife, can remember a time when she lived with her husband and daughter and had a job, before she lost everything, even her own name.

The Sound and the Fury

First published in 1929, Faulkner created his "heart's darling", the beautiful and tragic Caddy Compson, whose story Faulkner told through separate monologues by her three brothers: the idiot Benjy, the neurotic suicidal Quentin, and the monstrous Jason.

O, Pioneers!

On the harsh and wild frontier of the American West, Alexandra Bergson struggles to fulfill her father's dying wish of establishing his family on the Nebraska table lands. Through hard times and abundant, through love and loss, through joy and suffering, Alexandra challenges both her family and the land in her quiet, honest way.

Sanctuary

A powerful novel examining the nature of evil, informed by the works of T. S. Eliot and Freud, mythology, local lore, and hard-boiled detective fiction, Sanctuary is the dark, at times brutal, story of the kidnapping of Mississippi debutante Temple Drake. She introduces her own form of venality into the Memphis underworld where she is being held.

In the Great Green Room: The Brilliant and Bold Life of Margaret Wise Brown

The extraordinary life of the woman behind the beloved children's classics Goodnight Moon and The Runaway Bunny comes alive in this fascinating biography of Margaret Wise Brown. Margaret's books have sold millions of copies all over the world, but few people know that she was at the center of a children's book publishing revolution.

Publisher's Summary

Gangly, outspoken 12-year-old Frankie Addams yearns to belong to the “we of me,” and in a Southern kitchen, pours her heart out to the family cook, Berenice. One of the most beautiful plays ever written about loneliness and love.

...by Ruby Dee and a young Jena Malone. This was my second time listening, but it won't be my last. Highly, highly recommended for all mature audiences. Darker themes of violence, racial injustice, and tragedy striking both young and old join with more general themes of love, loss and loneliness to make this an unsuitable listen for kids.


1. Introduction {#sec1-nanomaterials-10-00378}
===============

Developing sustainable energy is one of the most urgent tasks of humankinds. With the development of science and technology, the explosive growth of energy demand is making limited traditional fossil energy stretched thin. Among various types of sustainable energy, solar energy is the most abundant renewable energy source on earth. However, it is dilute, unequally distributed, and intermittent. Hence, to harness solar energy facilely, efficiently and economically remains a challenge \[[@B1-nanomaterials-10-00378]\]. Solar driven photocatalysis based on particulate photocatalysts is a potentially scale-up industrial and economically practical technology to convert solar energy into chemical energy. Since 1972, when Fujishima and Honda first reported the phenomenon of hydrogen generation from the decomposition of water by TiO~2~ photocatalysts, photocatalysis has gradually attracted the attention of academia and industry \[[@B2-nanomaterials-10-00378]\]. In 2001, Zou et al. \[[@B3-nanomaterials-10-00378]\] first discovered In~1-*x*~Ni*~x~*TaO~4~ photocatalyst with visible light activity for photolytic water splitting. The past two decades, various kinds of semiconductor photocatalysts have been widely studied, such as oxides \[[@B4-nanomaterials-10-00378],[@B5-nanomaterials-10-00378],[@B6-nanomaterials-10-00378],[@B7-nanomaterials-10-00378],[@B8-nanomaterials-10-00378],[@B9-nanomaterials-10-00378]\], sulfides \[[@B10-nanomaterials-10-00378],[@B11-nanomaterials-10-00378],[@B12-nanomaterials-10-00378],[@B13-nanomaterials-10-00378],[@B14-nanomaterials-10-00378]\], nitrides \[[@B15-nanomaterials-10-00378],[@B16-nanomaterials-10-00378],[@B17-nanomaterials-10-00378],[@B18-nanomaterials-10-00378]\], carbides \[[@B15-nanomaterials-10-00378],[@B19-nanomaterials-10-00378],[@B20-nanomaterials-10-00378],[@B21-nanomaterials-10-00378]\], etc. Domen's recent studies based on particulate photocatalyst sheets provide a possible way for the large-scale and practical photocatalytic water splitting \[[@B22-nanomaterials-10-00378],[@B23-nanomaterials-10-00378],[@B24-nanomaterials-10-00378]\].

A typical photocatalytic process mainly consists of three steps: (i) semiconductors absorb light of corresponding wavelength with appropriate bandgap; (ii) transition of photo induced carriers from valence band to conduction band; (iii) carrier migration to material surface for redox reaction with reactants \[[@B25-nanomaterials-10-00378]\]. However, the band gaps of most metal oxides are too large to avail adequate absorption of the majority of photons in the solar spectrum \[[@B26-nanomaterials-10-00378]\]. There are limitations in most semiconductor photocatalysts like fast charge carriers recombination, surface carrier recombination, as well as surface state pinning which prevent photogenerated carriers from participating in redox reaction \[[@B25-nanomaterials-10-00378]\]. In the study of particle photocatalysts, however, photocatalytic quantum efficiency of the bulk photocatalysts is limited by carrier migration, trapping, and recombination inside the bulks which account for most of the volume \[[@B27-nanomaterials-10-00378]\]. The nano form photocatalysts, with dimensions reaching tens of nanometers in scale, or even less than their corresponding Bohr radii, ought to possess enhanced photocatalytic quantum efficiency \[[@B28-nanomaterials-10-00378]\]. On account of the higher specific surface area and the smaller grain size, more surface catalytic sites and shorter distances from interior to surface of the material enable photo induced carriers to participate in surface redox reaction instead of loss in bulks. Similarly, designing and constructing nanostructures is the trend of enhancing sensing performance of semiconductor-based gas sensors.

To this end, researchers have synthesized nanomaterials by various methods and improved the efficiency of solar absorption and conversion efficiency of the photocatalysts by doping, constructing heterostructures and surface modification. Among them, carbon sphere template derived hollow nanostructure is one of the most promising methods to prepare high-efficiency photocatalysts and gas sensors due to some unique characteristics. First of all, the raw materials (any of the saccharides and some biomass) and products of template (carbon sphere) are environmentally friendly. Then, the surface functional groups adsorb metal ions uniformly and non-selectively, so we can realize doping, solid-solution, heterostructure, and surface modification easily by simply adjust the adsorption or annealing conditions. In view of these facts, we introduce a synthetic method that synthesizes a variety of multiple metal compound hollow spheres by using ions adsorption and templating approaches. This strategy could avoid the bulk form of the multiple metal compound in traditional synthesis process and tune the composition of different metals precisely. These multiple metal compound hollow spheres have shells in nanoscale and high specific surface areas, which bound to be helpful for photocatalysis, gas sensing, and other related reactions.

In this review, we will first give an overview on the synthesis of carbon sphere template and then summarized various synthetic routines toward carbon sphere template derived hollow nanostructures, including their advantages in synthesizing different types of nanostructure for photocatalysis (doping, solid-solution, heterostructure, surface modification). Secondarily, their valuable applications in gas sensing using the same idea as photocatalyst modification will be summarized and discussed. At the end of this review, challenges and outlooks in the field of carbon sphere template derived hollow nanostructure are highlighted.

2. Carbon Sphere Template---Overview of Synthesis {#sec2-nanomaterials-10-00378}
=================================================

Carbon spheres have been widely researched and applied to various fields---including batteries, photocatalysis, capacitors, and fuel cells---due to their easily available sources and facile synthesis. The last few decades have witnessed a rapid growth in sustainable energy field and this has led to the research of various application and approaches of carbon spheres, including carbonization routes \[[@B29-nanomaterials-10-00378]\], chemical vapor deposition (CVD) \[[@B30-nanomaterials-10-00378]\], ultrasonic-spray \[[@B31-nanomaterials-10-00378]\], and hydrothermal carbonization (HTC) \[[@B6-nanomaterials-10-00378],[@B32-nanomaterials-10-00378],[@B33-nanomaterials-10-00378]\]. Among these methods, hydrothermal carbonization is the one that aroused the greatest interest due to its facile procedure and mild experimental condition. Besides, carbon spheres obtained through HTC method have tunable diameters, great monodispersity, and excellent surface activities compared to carbon spheres synthesized by other procedures. Accordingly, HTC method is considered a perfect way to obtain carbon spheres as templates and assist the subsequent modification.

Wang et al. \[[@B34-nanomaterials-10-00378]\] successfully synthesized a novel monodispersed hard carbon spheres which is perfectly round with good surface condition and evenly distributed nanopores. In this experiment, carbon spheres were facilely prepared by two steps, including dehydration at low temperatures and carbonization at high temperatures of glucose. Sun et al. \[[@B32-nanomaterials-10-00378]\] put forward a simplified hydrothermal procedure to synthesize micro- or nanospheres. Carbon nano- and microspheres were synthesized directly using aqueous glucose solution as precursor by the method of hydrothermal synthesis at 160--180 °C. The growth model of carbon spheres is shown schematically in [Figure 1](#nanomaterials-10-00378-f001){ref-type="fig"}. The synthesis process is facile, and the as-prepared hard carbon spheres were tunable in diameters, crystallinity, chemical composition, with reactive surface and inherited functional groups of the precursor. Titirici et al. \[[@B35-nanomaterials-10-00378]\] reported a one-pot synthesis to obtain hollow spherical metal oxides such as Fe~2~O~3~, Ni~2~O~3~, Co~3~O~4~, CeO~2~, MgO, and CuO via a hydrothermal approach by directly adding glucose or sucrose as precursor and metal oxide precursors. It is to be observed that, during the synthesis procedure, the carbon spheres were also formed and serve as templates and this method is more simplified but it has no harmful effects for metal oxide hollow spheres. Although the hydrothermal carbonization (HTC) method for carbon spheres is mature and widely used, there exist some limitations that restrict the formation of relatively small size of carbon sphere. Tan et al. \[[@B36-nanomaterials-10-00378]\] finally overcome this barrier by varying parameters of hydrothermal carbonization process---including glucose concentration, pH value, and temperature---and synthesize small-sized carbon spheres (40 nm) for the first time.

A typical hydrothermal synthesis for carbon sphere template consists of 5g glucose dissolving in 30 ml water, sealing in a 50 mL autoclave and maintaining at 180 ℃ for 6 h. The as-prepared templates were isolated by several rinsing-centrifugation cycles until the solution is clear and oven-dried. Among some details of the experiment, we found the dispersion of the template can be controlled by trace amount of surfactant, solubility of glucose and quenching of the autoclave.

Besides, surface property of carbon spheres and precursors of carbon spheres have been widely studied in recent years. The surface of carbon spheres, especially those synthesized from hydrothermal reaction, possess high density of hydrophilic groups such as --OH and C=O groups, so that can absorb various of metal ions easily and uniformly without any further modification \[[@B37-nanomaterials-10-00378]\]. This is the critical superiority when carbon spheres are applied as templates. Moreover, many researchers are focusing on a wide range of source to obtain carbon spheres by hydrothermal reaction, including general monosaccharide (xylose \[[@B38-nanomaterials-10-00378]\], glucose \[[@B38-nanomaterials-10-00378]\]), disaccharides (sucrose \[[@B6-nanomaterials-10-00378],[@B38-nanomaterials-10-00378]\], lactose \[[@B39-nanomaterials-10-00378]\]), polysaccharides (starch \[[@B40-nanomaterials-10-00378],[@B41-nanomaterials-10-00378]\], cellulose \[[@B42-nanomaterials-10-00378]\]), and some biomasses \[[@B39-nanomaterials-10-00378]\]. The possible formation procedure is shown schematically in [Figure 2](#nanomaterials-10-00378-f002){ref-type="fig"}. These studies showed that all these sources could be applied to synthesize sphere-like carbon sphere-like structures which contained amounts of hydrophilic reactive functional group on the surface, but the qualities (such as morphology and dispersivity) of carbon spheres are better when saccharides are chosen as precursors due to the structural complexity of biomasses \[[@B39-nanomaterials-10-00378]\]. Despite all of this, using biomasses to synthesize carbon spheres will continue to be a hot spot because of its abundant resources, environmental friendliness, and consistency of sustainable development.

In addition, there are also some reports about sphere-like carbon with some changes of morphology, including mesoporous carbon, hollow carbon spheres, which can also be applied as templates \[[@B43-nanomaterials-10-00378],[@B44-nanomaterials-10-00378],[@B45-nanomaterials-10-00378]\]. Mesoporous and hollow carbon spheres are synthesized by a numerous methods, such as carbonization and a template procedure. Mesoporous hollow carbon spheres could be obtained through carbonization of polystyrene (PS) \[[@B43-nanomaterials-10-00378]\]. Carbon hollow spheres with tunable diameter and surface properties have studied and prepared with the use of templates such as silica spheres, some polymer spheres, metallic templates \[[@B44-nanomaterials-10-00378]\], and some other core templates \[[@B45-nanomaterials-10-00378]\]. There is still great necessity to deeply research the preparation of various carbon spheres (both morphology and property) to realize a synthesis method that starts with environmental-friendly materials and involves a facile synthesis process.

3. Carbon Sphere Template Derived Hollow Nanostructure: Advantages in Synthesizing Different Types of Nanostructures for Photocatalysis {#sec3-nanomaterials-10-00378}
=======================================================================================================================================

From recent studies, the nanoscale morphology could conducive to enhance the charge carriers' separation and transfer efficiency as well as raise the density of catalytic sites and it is one of the keys to improve photocatalytic performances \[[@B27-nanomaterials-10-00378],[@B28-nanomaterials-10-00378],[@B37-nanomaterials-10-00378]\]. However, for these traditional methods such as sol--gel and sintering methods, a precipitation or solid-state reaction process is essential to mix the different metal ions on molecular level. Resultantly, the large particle size and the aggregation phenomena are inevitable in multiple metal compounds \[[@B46-nanomaterials-10-00378]\]. Thus a general method for the synthesis of multiple metal compounds consists of doping, heterostructure, and surface modification strategies in nanoscale is essential in photocatalysis. Carbon sphere template derived hollow nanostructure is such a way to achieve the above strategies. Even controllable multi-layer hollow structures can be realized by different post-processing methods \[[@B47-nanomaterials-10-00378]\]. As shown in [Figure 3](#nanomaterials-10-00378-f003){ref-type="fig"}, it is the illustration of carbon sphere template derived hollow nanoheterostructure and controllable multilayer structure.

3.1. Doping {#sec3dot1-nanomaterials-10-00378}
-----------

Most metal oxide semiconductors with wide band gap could hardly respond to the visible light region \[[@B48-nanomaterials-10-00378]\], which limit their application in photocatalysis. Hence it is urgent to design semiconductor-based photocatalysts active both in UV and visible light region. Doping metal oxides (TiO~2~, ZnO, WO~3~, etc.), polymetallic oxides and nitrides \[[@B49-nanomaterials-10-00378]\] with non-metals (C, O, S, N, P, etc.) \[[@B48-nanomaterials-10-00378]\] or metals (Na, Mg, Fe, Lanthanide, etc.) \[[@B50-nanomaterials-10-00378]\] expand the effective absorption to visible light region to promote light harvesting and photocatalytic performance. One of many valid methods to design efficient photocatalysts which also can absorb visible light is creating impurity levels in band gap by doping \[[@B51-nanomaterials-10-00378]\]. We can achieve doping of many kinds of desired metals into oxides taking the characteristics of good dispersion of carbon sphere template and its uniformly and non-selectively adsorption of metal ions. By further regulating the experimental process, the doping of non-metallic elements can also be realized.

For instance, Song et al. \[[@B52-nanomaterials-10-00378]\] demonstrated that Fe doped contents can be precisely regulated by using this simple templating method, so that they acquired broader light absorption region. Fe doping introduced impurity levels in the forbidden band confirmed by DFT (density functional theory) calculation, which narrowed the band gap of WO~3~. Meanwhile, WO~3~ nanostructured photocatalysts with 5 at % Fe showed the best photocatalytic performance due to the low recombination and high transformation of the photo-generated carriers. Besides, Na-doped ZnO nanostructures have been firstly fabricated by ion adsorption and template method utilizing carbon spheres as templates \[[@B53-nanomaterials-10-00378]\]. Na and Zn elements cannot be precipitated simultaneously because their chemical activities in solution differ from each other. Instead of co-precipitation method, they gather the Zn and Na cations uniformly and simultaneously on the surface of carbon spheres ([Figure 4](#nanomaterials-10-00378-f004){ref-type="fig"}a). Engineering the Zn-doped contents can tune the work function of Na-doped ZnO from 4.48 eV to 6.19 eV, and fermi level get closer to the level of valence band by the increasing of Na doping concentration, which presented the success of a *p*-type doping effect ([Figure 4](#nanomaterials-10-00378-f004){ref-type="fig"}b). They have got Na-ZnO/Pt with 0.7 at % which presented the highest photocatalytic H~2~ generation rate and it was obviously superior to pure ZnO ([Figure 4](#nanomaterials-10-00378-f004){ref-type="fig"}c).

Rare earth metals also play a vital role in metal doped semiconductor-based photocatalysts. According to Chao et al. \[[@B54-nanomaterials-10-00378]\], Nd^3+^ led to defects in the lattice which were acted as electron traps, and also could band with non-bridging O^2−^ around the network structure. Nd^3+^ doping could make a red shift of absorption spectrum for TiO~2~. In another study \[[@B55-nanomaterials-10-00378]\], they used cerium instead of neodymium to dope into titania hollow spheres. Ce^4+^ ions could trap the excited electrons and then electrons would be transferred to adsorbed oxygen on the surface like the reaction

With the increase of Ce^4+^ doping content, the space charge layer would be thinner, which efficiently separates electron--hole pairs.

Among all the non-metals used as dopant materials by using carbon spheres templating, carbon doping is relatively common. Carbon spheres are used as hard template and also provide a source of carbon doping. Shi et al. reported C-doped titania nanostructures and its photocatalytic application in 2012 \[[@B56-nanomaterials-10-00378]\]. They presented the preparation of carbon doped TiO~2~ nanostructures with a hierarchical macroporous channel structure ([Figure 5](#nanomaterials-10-00378-f005){ref-type="fig"}a), which showed high potocatalytic degradation performance. In another work \[[@B57-nanomaterials-10-00378]\], they deduced the formation mechanism of hollow 3D network structure ([Figure 5](#nanomaterials-10-00378-f005){ref-type="fig"}b), because the surface of carbon spheres was hydrophilic. Carbon doping narrowed the band gap of titania, and hollow 3D structure offered a pathway for guest molecules and enhanced the light harvesting. Recently, Shi's group \[[@B58-nanomaterials-10-00378]\] realized C~3~N~4~ decorated carbon-doped TiO~2~ by in situ growth ([Figure 5](#nanomaterials-10-00378-f005){ref-type="fig"}c). C doping influenced the energy band structure of TiO~2~ to enhance photocatalytic activity for application in hydrogen generation under simulated sunlight. Except carbon doping, there are a few works on non-metal doped semicondutor-based photocatalysis in hollow nanostructure, such as Br-doped TiO~2~ \[[@B59-nanomaterials-10-00378]\].

In addition to being visible-light absorption centers, dopant also could reduce recombination losses in photocatalyst by trapping photo-generated carriers. Reciprocally, it enhanced the carriers separation to obtain the superior photocatalytic performance. The non-selectively adsorption of carbon sphere template provides a good condition for various doping. For instance, Li et al. \[[@B60-nanomaterials-10-00378]\] described that Fe-doped LaTiO~2~N nanostructures exhibited high efficiency, because doping can match the level of conduction band between cocatalysts and LaTiO~2~N hollow nanostructures by creating empty mid-gap states in Fe doped LaTiO~2~N. As shown in [Figure 6](#nanomaterials-10-00378-f006){ref-type="fig"}a, Fe dopants created a defect level lower than the level of conduction band both of LaTiO~2~N and CoO*~x~*, so that the photo-generated electrons could be gathered to CoO*~x~* nanoparticles. As a result, the sample with 1.5% Fe doping exhibited quantum efficiency as high as 55% for O~2~ evolution which was fairly higher than other LaTiO~2~N system ([Figure 6](#nanomaterials-10-00378-f006){ref-type="fig"}b). Besides, dopant also can reduce the defect sites, where are electron--hole recombination's sites. Gao et al. \[[@B61-nanomaterials-10-00378]\] found that Mg dopants could not create new trapping states in TiO~2~ because it does not have d orbit, but the intrinsic defect states like oxygen vacancy (*V~o~*) could be suppressed by Mg doping ([Figure 7](#nanomaterials-10-00378-f007){ref-type="fig"}a). Moreover, the 2p orbit of Mg dopant apart from the level of conduction and valence band of TiO~2~ could change the level of defect states by hybridizing the oxygen vacancy in titania ([Figure 7](#nanomaterials-10-00378-f007){ref-type="fig"}b). Besides, as a narrow band-gap semiconductor, Ta~3~N~5~ also has numerous photo-generated carrier recombination centers, which seriously impairs its photocatalytic activity. Recently, Xiao et al. \[[@B62-nanomaterials-10-00378]\] prepared Mg-doped Ta~3~N~5~ hollow spheres using carbon spheres templating. Mg ions doped into Ta~3~N~5~ could not only increase the charge mobility but also tune the electronic structure to promote the charge transportation. From the result of Mott--Schottky measurement ([Figure 7](#nanomaterials-10-00378-f007){ref-type="fig"}c), Mg-Ta~3~N~5~ had a higher electron density. Theoretically, Mg ions acted as an electron acceptor in Ta~3~N~5~ photocatalysts. Mg doping was relatively efficient strategy, its photocatalytic hydrogen production is about 7.6-times that of prinstine Ta~3~N~5~ ([Figure 7](#nanomaterials-10-00378-f007){ref-type="fig"}d).

Researchers have successfully achieved the doping of different elements in various materials by using carbon sphere template method. According to the results, the distribution of doping elements is uniform and there is no phase separation. For some doping systems that cannot be achieved by other preparation methods, we can try to achieve it by the non-selective adsorption of carbon sphere templates. This also provides ideas for other systems such as single atomic catalysis. Different kinds of salts can be used to make the expected elements adsorb and anchor on the surface of the template, and metal particles with good dispersion may be obtained through subsequent treatment.

3.2. Solid Solution {#sec3dot2-nanomaterials-10-00378}
-------------------

The bandgap engineering of a semiconductor is very important for its application in various aspects. It not only determines the optical absorption edge and emission wavelength of the material, but also closely related to the electrical conductivity \[[@B63-nanomaterials-10-00378]\]. However, it is difficult to find some semiconductors in nature whose bandgaps fit the requirements of various applications precisely, and that makes bandgap engineering an indispensable study. In order to tailor the bandgap and optoelectronic property of semiconductors, scientists have come up with two main solutions, including doping and forming solid solutions ([Figure 8](#nanomaterials-10-00378-f008){ref-type="fig"}) \[[@B64-nanomaterials-10-00378]\]. The doping method has been described above. Forming solid solutions is to synthesize two or more different semiconductors (with similar crystal or electronic structure) into one material, and the bandgap-controllable solid solutions can be obtained by fine-tuning the content of different components. When applied to photocatalysis, the band gap is expected to be quite narrow while satisfying the potential of the catalytic reaction. Similar to the strategy of doping in previous section, carbon sphere template's uniform and non-selective adsorption of metal ions also enables the synthesis of solid solution hollow nanostructure. By precisely controlling the precursor concentration, a broad solid solubility can be achieved.

(Ga~1-*x*~Zn*~x~*) (N~1-*x*~O*~x~*) is formed by GaN and ZnO. Unlike a solid solution synthesized by a wide and a narrow band semiconductor, GaN and ZnO both have wide bandgaps, while their solid solution has a narrow bandgap about 2.58 eV when the content of ZnO is 13.3% \[[@B65-nanomaterials-10-00378]\]. It is calculated that the VBM (valence band maximum) is composed of Zn 3d orbitals and *N* 2p orbitals, and the repulsion of electrons of Zn 3d and *N* 2p result in the narrowing of the band gap \[[@B66-nanomaterials-10-00378]\], which makes the material more suitable for photocatalysis than sole GaN and ZnO. The water-splitting quantum yield of the bulk (Ga~1-*x*~Zn*~x~*)(N~1-*x*~O*~x~*) is limited by carrier migration, trapping and recombination. Maeda et al. \[[@B67-nanomaterials-10-00378]\] enhanced the quantum efficiency to 5.9% at 420--440 nm by surface modification, optimizing annealing temperature and composition *x* of bulk (Ga~1−*x*~Zn*~x~*)(N~1−*x*~O*~x~*). Many scientists have improved the overall conversion efficiency of the system by preparing nanomorphological materials. Reinert et al. \[[@B68-nanomaterials-10-00378]\] obtained nanorods arrays with diameter of about 100 nm. Their Ga~2~O~3~(ZnO)~16~ precursor was heat-treated and cooled under NH~3~ flow. By reducing the synthesis temperature to 850 °C, the volatilization of Zn was also greatly reduced, thus increasing the *x* value from \<0.42 to 0.55 \[[@B65-nanomaterials-10-00378]\], and reducing the band gap to 2.53 eV. Li et al. \[[@B69-nanomaterials-10-00378]\] synthesized (Ga~1−*x*~Zn*~x~*)(N~1−*x*~O*~x~*) nano-hollow spheres by ion adsorption and templating. They used carbon spheres as templates, DMF as solvent, supplemented with the subsequent nitridation, and obtained various composition of (Ga~1−*x*~Zn*~x~*)(N~1−*x*~O*~x~*) by adjusting the ratio of the Zn source to the Ga source, tuning the composition of ZnO to the full range from 0.18 to 0.95. The magnified TEM image ([Figure 9](#nanomaterials-10-00378-f009){ref-type="fig"}a) shows the nano-hollow-spheres, whose diameter are approximately 100--120 nm and the shell of the sphere is about 7 nm thick. [Figure 9](#nanomaterials-10-00378-f009){ref-type="fig"}c indicates that using this new synth etic method, the narrowing of the bandgap is related to the mutual approachment of both VBM and CBM (conduction band minimum). The nitridation temperature was only 600 °C attributing to the the nanoscale hollow structures, much lower than that of bulk form which is approximately 900 °C \[[@B70-nanomaterials-10-00378]\], thus it reduces the volatilization of Zn. This nanoscale structure also decreases the carrier diffusion distances and increases the number of reaction sites, enhancing the quantum efficiency to 17.3% (400 nm), which was about 3 times higher than bulk (Ga~0.82~Zn~0.18~)(N~0.82~O~0.18~) \[[@B67-nanomaterials-10-00378]\].

A similar situation exists in some other solid solution systems. Zn*~x~*Cd~1-*x*~S is a solid solution composed of CdS and ZnS, which not only has tunable band gap, but also has better photocorrosion resistance and higher photocatalytic activity than pure CdS \[[@B71-nanomaterials-10-00378],[@B72-nanomaterials-10-00378]\]. Gholipour et al. \[[@B73-nanomaterials-10-00378]\] synthesized Zn*~x~*Cd~1-*x*~S hollow spheres by a solvothermal method, using glycerin to obtain carbon microspheres \[[@B74-nanomaterials-10-00378]\], which contains large amount of OH radicals that can adsorb Zn^2+^ and Cd^2+^. After centrifugated, calcinated and calcinated in H~2~S/Ar flow, the Zn*~x~*Cd~1-*x*~S hollow spheres can finally be obtained. The value of x can be tuned by adjusting the ratio of zinc source to cadmium source.

The nanostructure of Zn*~x~*Cd~1-*x*~S is hollow sphere with an average diameter of 500 nm ([Figure 10](#nanomaterials-10-00378-f010){ref-type="fig"}a,b). With the increase of Cd content, the absorption edge moves towards longer wavelength ([Figure 10](#nanomaterials-10-00378-f010){ref-type="fig"}c), indicating the decrease of bandgap. At the same time, the XRD (X-ray diffraction) peak of the samples gradually shifted to lower angle ([Figure 10](#nanomaterials-10-00378-f010){ref-type="fig"}d), accounting for the transformation from hexagonal ZnS to hexagonal wurtzite CdS. The hydrogen evolution they received is as high as 12 mmol h^−1^ g^−1^ under solar irradiation using MoS~2~ as cocatalyst. The calculated quantum efficiencies (QE) reached to 46.6% at 400 nm, which was among the highest QE in this system.

So far, there are few solid solution systems realized by carbon sphere template method. By adjusting the precursor and subsequent treatment, this method may be able to achieve more solid solution hollow nanostructures.

3.3. Heterostructure {#sec3dot3-nanomaterials-10-00378}
--------------------

It is hardly for individual semiconductor materials to satisfy fundamental requirements of highly efficient photocatalysts, such as suitable bandgap for effective sun light absorption, proper charge separation, high carrier mobility and fast kinetics for surface reaction \[[@B75-nanomaterials-10-00378],[@B76-nanomaterials-10-00378]\]. Introducing heterojunction comes to be a potential strategy to meet these critical requirements simultaneously by combining the advantages of two or more semiconductors. A conventional heterojunction is established by coupling two semiconductors having suitable structures of energy band directly. Heterostructured hollow photocatalysts have received growing research attention by cause of their unusual advantages such as ultrahigh sunlight absorption, extremely large surface area and short carrier transfer distance in recent year \[[@B76-nanomaterials-10-00378],[@B77-nanomaterials-10-00378]\]. So far, heterostructured hollow photocatalysts have demonstrated great application potential in varied photocatalytic fields \[[@B49-nanomaterials-10-00378],[@B78-nanomaterials-10-00378]\]. Due to the good adsorption capacity of carbon sphere template, we are able to obtain hollow nano heterostructures by multiple adsorption and different annealing conditions.

Semiconductor-based photocatalysts applied in water splitting are identified as the most economic and environmentally friendly material systems for hydrogen production \[[@B79-nanomaterials-10-00378]\]. In recent years, a great deal of previous research spotlighted on water splitting based on advanced heterostructured hollow photocatalysts.

For instance, Song et al. \[[@B80-nanomaterials-10-00378]\] demonstrated the superior photocatalytic H~2~ evolution activity of ZnFe~2~O~4~/ZnO hollow nanoheterostuctures by a facile hard-templating approach. The nanoheterostuctures of ZnFe~2~O~4~/ZnO, which are about 15 nm in size, are homogeneously distributed in the hollow sphere shell. The champion sample without the help of co-catalysts exhibits an outstanding H~2~ evolution rate of 2.15 mmol h^−1^ g^−1^ when irradiated by visible light. The corresponding quantum efficiency reaching 1.61%, which is 7.6 times higher than that of bare hollow ZnFe~2~O~4~ photocatalysts. ZnO forms a type-II heterojunction with ZnFe~2~O~4~ in the sphere shell ([Figure 11](#nanomaterials-10-00378-f011){ref-type="fig"}) which confirmed by the combination investigation of XPS (X-ray photoelectron spectroscopy) technic as well as UV--vis diffuse reflectance spectroscopy. The photogenerated electrons and holes will be separated into the conduction band of ZnO and valence band of ZnFe~2~O~4~ by the built-in electric field, respectively, thereby minimizing the recombination and boosting the photocatalytic H~2~ generation performance of samples. Meanwhile, Li et al. \[[@B81-nanomaterials-10-00378]\] have reported a high-crystallinity g-C~3~N~4~/TiO~2~ hollowsphere nanoheterojunction photocatalyst with large surface area through a carbon sphere templating method. By carefully tuning the ratio of precursors, the optimum g-C~3~N~4~/TiO~2~ photocatalyst obtain a 5.5-fold enhancement in visible-light-driven hydrogen generation rate, reaching almost 470 μmol g^−1^ h^−1^. The improvement in photocatalytic performance primarily owes to the establishment of heterojunctions which accelerating carrier's separation efficiently confirmed by PL spectrum and electrochemical independence spectroscopy. While conventional heterojunctions are generally in form of crystalline/crystalline contact, Lou et al. \[[@B82-nanomaterials-10-00378]\] have investigated a novel type of crystalline/amorphous heterostructure. A crystalline Cu~2~O/amorphous Ta~2~O~5~ heterostructure fabricated by a two-step ion absorption approach based on carbon spheres template. Such heterostructures were found to possess the capacity for splitting water into H~2~ and O~2~ driven by visible light without using sacrificial agent. Theoretical calculation shows the existence of hole trap states in the crystalline/crystalline interface cause by lattice and electron mismatch, which does not exist at the crystal/amorphous interface ([Figure 12](#nanomaterials-10-00378-f012){ref-type="fig"}). Moreover, the type-II heterojunction of crystalline Cu~2~O/amorphous Ta~2~O~5~ greatly promotes the separation of photo-induced charges to perform effective overall water splitting.

In addition, other research groups also have reported hollow heterojunction photocatalysts with high performance. Waqas et al. \[[@B83-nanomaterials-10-00378]\] prepared hollow sphere TiO~2~/Fe~2~TiO~5~ heterojunction photocatalyst through a simple two-step templating method for high efficient solar water oxidation. The oxygen evolution reaction rate of the champion sample with 35% Fe reached 375 μmol h^−1^ g^−1^ benefited from the desirable energy band alignment of TiO~2~/Fe~2~TiO~5~ heterojunction, which improved carrier separation in these photocatalysts. In another work, heterostructured ZnFe~2~O~4~/ZnSe hollow sphere photocatalysts which were synthesized through two-step carbon templating approach used in visible-light-driven water splitting \[[@B84-nanomaterials-10-00378]\]. Heterostructured ZnSe photocatalysts with suitable shell thickness generated a large amount of H~2~ of 16.9 mmol g^−1^ in 6 h, owing to the effective charge separation and accelerated carrier transfer.

Hollow nanoheterostructured photocatalysts possess extremely large surface area, high catalytic activity and carrier separation efficiency, thus being suitable for application of pollutant degradation \[[@B49-nanomaterials-10-00378],[@B75-nanomaterials-10-00378],[@B77-nanomaterials-10-00378],[@B85-nanomaterials-10-00378]\].

In 2014, Li et al. \[[@B86-nanomaterials-10-00378]\] have pioneered a simple and universal method for fabricating a hybrid hollow nanoheterostructured photocatalyst with ultrathin shell. Taking the TiO~2~/SnO~2~ hollow sphere photocatalyst as a typical example, they mixed 0.5 g of carbon spheres and a proper amount of C~16~H~36~O~4~Ti and SnCl~2~·2H~2~O together and stirred for 24 h, followed by rinsing and drying in an oven at 80 °C for 6 h. The TiO~2~/SnO~2~ hybrid nanostructured hollow spheres finally obtained by calcination have 3--10 nm small grain size and ultrahigh surface area of 300 m^2^ g^−1^. Compared to single TiO~2~ and SnO~2~ hollow nanostructured photocatalysts, the TiO~2~/SnO~2~ hybrid hollow nanostructured photocatalyst shows a superior RhB dye photodegradation activity, which is profited from the built-in electric field induced by TiO~2~/SnO~2~ heterojunction as well as the short carrier diffusion length in TiO~2~/SnO~2~ nanocrystal ([Figure 13](#nanomaterials-10-00378-f013){ref-type="fig"}). Adopting a similar strategy, Lou et al. \[[@B87-nanomaterials-10-00378]\] have also successfully prepared a hollow Fe~2~TiO~5~/TiO~2~ nanoheterostructure to enhance the photocatalytic performance of TiO~2~-based photocatalysts driven by visible light. The favorable energy band alignment of Fe~2~TiO~5~/TiO~2~ heterojunction suppressed the carrier recombination significantly. As a result, the optimum nanoheterostructured photocatalyst exhibited a 3-times higher visible-light photocatalytic-degradation activity towards RhB compared to that of commercial P25 photocatalyst.

Diversification of structural morphology makes hollow sphere photocatalysts more attractive. Ang et al. \[[@B85-nanomaterials-10-00378]\] have reported multi-dimensional hierarchical hollow heterojunctioned CuO/TiO~2~ spheres with TiO~2~ thorns grown on the surface of CuO hollow spheres by a carbon spheres assisted template method ([Figure 14](#nanomaterials-10-00378-f014){ref-type="fig"}a,b). The hierarchical TiO~2~ thorns acted as water channels generating a mass of reactive oxygen species for pollutant degradation. The multi-dimensional CuO/TiO~2~ spheres show an excellent photodegradation activity in comparison to P25 thanks to the effective carrier separation and migration origin from the establishment of CuO/TiO~2~ heterojunction. Liu et al. \[[@B88-nanomaterials-10-00378]\] have successfully used a microwave-assistant carbon sphere templating method to synthesize hollow nest-like *γ*-Fe~2~O~3~/ZnO photocatalysts, which ZnO nanosheets were loaded on the shell of g-Fe~2~O~3~ hollow spheres ([Figure 14](#nanomaterials-10-00378-f014){ref-type="fig"}c,d). The large surface area and effective restraint of carrier recombination of g-Fe~2~O~3~/ZnO result in an enhanced photocatalytic activity compared with bare ZnO photocatalysts.

Other type-II hollow heterostructured photocatalysts derived from carbon sphere template such as CeO~2~/TiO~2~ \[[@B89-nanomaterials-10-00378]\], WO~3~/TiO~2~ \[[@B90-nanomaterials-10-00378]\], ZnO/CuO \[[@B91-nanomaterials-10-00378]\], ZnO/SnO~2~ \[[@B92-nanomaterials-10-00378]\], and ZnO/ZnS \[[@B93-nanomaterials-10-00378]\] also showed enhanced photocatalytic pollutant degradation performance by the promotion of heterojunction.

Nanoheterostructure is one of the most facilely realized structures by carbon sphere template method. One-step adsorption by the carbon sphere template can realize heterostructure on the same spherical shell, and multi-step adsorption can realize hollow spherical nanostructures with core--shell distribution or multilayer structure. Notably, as powder photocatalysts, recycling is a very important issue. Generally, we build a heterostructure with magnetic materials to achieve magnetic recycling. However, magnetic materials generally have low photocatalytic activity and light blocking, which will reduce the efficiency of light utilization. By carbon sphere template method, the magnetic materials may be confined in the hollow nanospheres of photocatalyst by the method of multi-step adsorption, so as to realize the light utilization efficiency and recyclability.

3.4. Surface Modification {#sec3dot4-nanomaterials-10-00378}
-------------------------

For photocatalysis, the more electron--hole pairs are photogenerated and come into play, the higher the efficiency will be. However, the recombination is proven to harmfully impede the process of water-splitting or degradation resulting in a low efficiency \[[@B27-nanomaterials-10-00378],[@B37-nanomaterials-10-00378],[@B82-nanomaterials-10-00378]\]. Hollow structures can shorten the transport distance of carries from bulk to their proper catalytic sites on the surface, which suppress bulk recombination tremendously \[[@B37-nanomaterials-10-00378],[@B77-nanomaterials-10-00378]\]. However, a majority of surface defects remain there and behave as another kind of recombination centers \[[@B94-nanomaterials-10-00378]\]. Meanwhile the extra electronic and optical states may have an influence on electronic structure or band structure \[[@B95-nanomaterials-10-00378]\]. Surface modification is the most effective measure to suppress surface recombination and increase active sites. Carbon sphere templates can absorb precious metals uniformly in solution, and also can be used as good skeleton support in subsequent treatment. The carbon sphere template derived ultra-thin hollow nanostructure also provides favorable conditions for the subsequent surface modification. Furthermore, combining surface modification and hollow structure will endow photocatalysts with both higher activity and better stability.

The introduction of disorder at the surface of photocatalysts is a common method of surface modification. It has been revealed that lattice disorder in semiconductors could yield band tail states. It would dominate the optical excitation and relaxtion, and provide intended trapping sites for photo-generated carriers to avoid rapid recombination \[[@B96-nanomaterials-10-00378],[@B97-nanomaterials-10-00378]\]. Chen et al. \[[@B96-nanomaterials-10-00378]\] prepared TiO~2~ nanocrystals with disordered surface layers via hydrogenation and found that it could help harvest infrared optical photons and contribute to organic molecules photo-oxidation and H~2~ evolution. Li et al. \[[@B95-nanomaterials-10-00378]\] reported a surface-reconstruced hollow LaTiO~2~N nanostructure with significantly improved photocatalytic activity ([Figure 15](#nanomaterials-10-00378-f015){ref-type="fig"}). The ultrathin hollow LaTiO~2~N spheres were firstly synthesized by utilizing carbon spheres as hard templates. Then H~2~ treatment was carried not only to load the cocatalyst Pt but also to modify the surface of LaTiO~2~N spheres. Pt/H-LTON achieved a remarkably improved H~2~ evolution rate (up to 192 μmol h^−1^, compared to 91 μmol h^−1^ of Pt/LTON) and stability. Interestingly, the contrast sample with extra N~2~ annealing, which maintained the presence of Ti^3+^ ions but removed the disordered surface even exhibited worse photocatalytic performance than Pt/LTON. It indicated that the excellent photocatalytic capacity for water splitting was a consequence of the disordered surface rather than the induced Ti^3+^ ions. Introducing H atom can passivate surface defects, thus avoiding a majority of surface recombination of photo-induced carriers \[[@B94-nanomaterials-10-00378],[@B95-nanomaterials-10-00378]\]. Therefore, surface disordering functions as a complementary strategy for improving photocatalytic activity combined with hollow structures.

Oxygen vacancies are also proved to strongly affect photocatalytic activity of metal oxides-based photocatalysts \[[@B98-nanomaterials-10-00378]\]. Black titanium oxide is a typical example, in which electrons transition occurs between levels of valence band of the photocatalyst and the introduced oxygen vacancy states under visible radiation \[[@B98-nanomaterials-10-00378],[@B99-nanomaterials-10-00378]\]. Recently, Song et al. \[[@B100-nanomaterials-10-00378]\] successfully prepared black titanium oxide hollow nanospheres with ultrathin shells for more effective solar-driven water splitting ([Figure 16](#nanomaterials-10-00378-f016){ref-type="fig"}). The high quality TiO~2~ possessed a high specific surface area of 168.8 m^2^ g^−1^ and was obtained by a conventional carbon sphere templating method combined with subsequent Al reduction. This black TiO~2~ nanostructure exhibited a much higher photocatalytic H~2~ evolution rate (56.7 mmol h^−1^ g^−1^), which was well above that of the as-prepared one (22.9 mmol h^−1^ g^−1^) when using xenon lamp as irradiation source. It was found that Al reduction introduced masses of oxygen vacancies into TiO~2~ which can act as donors and accelerate the separation and transportation of the photogenerated electron--hole pairs.

Besides, cocatalysts can help lower energy barriers, enhance photo-generated charge separation and provide reaction sites \[[@B101-nanomaterials-10-00378]\]. Different cocatalysts are designed to facilitate the oxidation or reduction process on the surface respectively \[[@B102-nanomaterials-10-00378]\]. Template method makes it easy to load related cocatalysts to the specified spots of photocatalysts. For example, Li et al. \[[@B102-nanomaterials-10-00378]\] designed Pt\@TiO~2~\@MnOx hollow spheres to improve the photo-oxidation of water and benzyl alcohol. Xiao et al. \[[@B62-nanomaterials-10-00378]\] reported that the Ta~3~N~5~ hollow spheres with Pt/CoOx dual cocatalysts exhibited better photocatalytic ability of water splitting compared with those only with Pt cocatalyst. Hollow TiO~2~ modified with spatially isolated Au and CoO dual cocatalysts for CO~2~ photoreduction was designed by Zhu et al. ([Figure 17](#nanomaterials-10-00378-f017){ref-type="fig"}) \[[@B103-nanomaterials-10-00378]\]. CoO was formed on the inner surface of carbon spheres by ions absorption, and then Au was decorated on the external surface by Au ions reduction. The optimal formation rate of CH~4~ from CO~2~ reduction of Au~2.0~\@THS\@CoO was nearly 60 times as high as that of TiO~2~ hollow nanospheres (THS), while the activity of THS with Au and CoO nanoparticles randomly deposited was not so good. There is convincing evidence that Au and CoO are the suitable co-catalysts for photo-generated electrons and holes respectively so that carriers can be separated effectively on the inner or outer surfaces of the hollow nanostructure. Carbon templating is a convenient way of designing cocatalyst loading.

Furthermore, plasmonic metal nanocrystals have been intensively investigated recently on account of its unique optical and electronic properties. Gold (Au) and silver (Ag) can work as cocatalysts as well as light sensitizers in aspect of photocatalysis. Wang et al. \[[@B104-nanomaterials-10-00378]\] synthesized yolk--shell Au\@TiO~2~ nanostructure by employing carbon spheres as templates ([Figure 18](#nanomaterials-10-00378-f018){ref-type="fig"}). Core--shell Au\@C was fabricated through hydrolysis and then it worked as a template to form yolk--shell Au\@TiO~2~ (Au\@TiO~2~-YS). The diameter of Au\@TiO~2~-YS was about 350 nm with the Au core 70--90 nm or so. Evidently, this morphology brought a much larger BET (Brunauer--Emmett--Teller) surface area reaching 128.9 m^2^/g and a higher content of mesoporous particles than core--shell Au\@TiO~2~. Owing to plasmonic effects, Au\@TiO~2~-YS had broader light absorption extended to about 570 nm. It was found to be able to degrade the gaseous toluene more efficiently than the core--shell structure, almost 1.63 times higher. Obviously, the large surface area and abundant mesoporous channels induced by the yolk--shell structure make it easy to absorb more VOCs (Volatile Organic Compounds) and proceed the degradation. In addition, thin shells are also beneficial for photo-generated carriers to separate and migrate and unique cavities increase light reflection and refraction, too, thus enhancing the LSPR (Localized Surface Plasmon Resonance) effect of Au.

In addition, passivation layers can also modify the surface of hollow structure. Xiao et al. \[[@B62-nanomaterials-10-00378]\] demonstrated that MgO nano-layer as passivation layer seemed to be an effective strategy for Mg-doped Ta~3~N~5~ hollow spheres ([Figure 19](#nanomaterials-10-00378-f019){ref-type="fig"}). The Mg-doped Ta~2~O~5~ precursor fabricated with the aid of carbon spheres templating, and subsequently it was well wrapped in-situ by a thin layer of MgO even less than 5 nm. The particle size of Pt further dercreased from 2.04 nm to 1.33 nm along with the deposition of MgO. The increased ratio of hydrogen generation for MgO modification is 17.3 compared to the bulk Ta~3~N~5~, mainly due to the efficient charge transfer and passivation of surface defects. It should be noted that excessive MgO may adversely affect the charge transfer between Ta~3~N~5~ and cocatalysts instead since MgO is known as a insulator. Therefore, it is necessary to manage the thickness of the passivation layer by controlling the parameters of ion absorption.

Surface modification is not unique to the carbon sphere template method, but the adsorption characteristics of carbon sphere can well realize the delocalization of different co-catalysts. The carbon in the template can also acts as reducing agent. Under different post-treated atmosphere, it can play a role not only as a template. The hollow nanostructures with controllable shell thickness prepared by carbon sphere template method also provide convenience for many subsequent processing. In photocatalysis, the carrier migration distance is a very important factor. The thin shell prepared by carbon sphere template with subsequent surface treatment can greatly improve the efficiency of photocatalyst carrier utilization.

A summary of the recent carbon sphere template derived hollow nanostructure photocatalysts is presented in [Table 1](#nanomaterials-10-00378-t001){ref-type="table"}.

4. Carbon Sphere Template Derived Hollow Nanostructure for Gas Sensing {#sec4-nanomaterials-10-00378}
======================================================================

Hollow nanostructure also has widely applied in gas sensing since it possesses ultrahigh specific area which can provide rich active sites for gas adsorption and diffusion, further promoting gas-sensing performance. Moreover, the porous structures resulting in high surface permeability also allows more gas exposure and makes gas sensing easier \[[@B105-nanomaterials-10-00378],[@B106-nanomaterials-10-00378],[@B107-nanomaterials-10-00378]\]. Inspired by carbon sphere templating methods in photocatalysis, researchers have successfully implemented the modification strategies into gas sensing in view of carbon spheres templated hollow nanostructure with adjustable composition, thickness, surface, and interface \[[@B108-nanomaterials-10-00378],[@B109-nanomaterials-10-00378],[@B110-nanomaterials-10-00378]\].

For instance, Zhang et al. \[[@B111-nanomaterials-10-00378]\] firstly designed porous Co~3~O~4~/Ta~2~O~5~ heterojunction hollow nanospheres. They prepared ultrathin Ta~2~O~5~ hollow shell (shell thickness \~5 nm) attached with numerous Co~3~O~4~ nanoparticles by adsorbing Co ions onto Ta^5+^/C template spheres ([Figure 20](#nanomaterials-10-00378-f020){ref-type="fig"}a--d). [Figure 20](#nanomaterials-10-00378-f020){ref-type="fig"}e displayed that the gas sensor owned a good linear relationship of current vs. voltage between the sensing material and the electrode, ensuring that measured electrical signals were due to the gas-sensing performance rather than the contact resistance of the sensing material and the electrode. The gas sensor revealed splendid selectivity and reliable long stability to ethanol at room temperature ([Figure 20](#nanomaterials-10-00378-f020){ref-type="fig"}h--k), which was attributed to two reasons: (1) the supporting effect of Ta~2~O~5~ hollow spheres hindered the agglomeration of Co~3~O~4~ particles and guaranteed larger BET area; (2) the formation of heterojunction resulted to widen the electronic depletion layer.

Furthermore, Zhang et al. \[[@B112-nanomaterials-10-00378]\] also synthesized WO~3~ mesoporous hollow nanospheres doped with Fe with a series of concentrations (0--5.2% by molar ratio) through a carbon colloidal nanosphere templating method, electrostatic adsorption of metal ions, as well as post-annealing treatment. This hollow structure showed tiny size and ultrahigh BET area. Gas sensor based on hollow nanostructured semiconductors finally realized outstanding performance applied for ppb-level NO~2~ detection at lower working temperature (100--140 °C) as exhibited in [Figure 21](#nanomaterials-10-00378-f021){ref-type="fig"}. According to the results of XPS, the enhancement of NO~2~ detecting performance ascribed to rich oxygen vacancies and surface chemical states altered by Fe dopants. What is worth mentioning is that the excellent detection limitation can meet the needs of atmospheric monitoring and exclusive detecting at the same time.

Zhang et al. \[[@B113-nanomaterials-10-00378]\] studied hollow carbon spheres of which Co~3~O~4~ nanowires assembled on the surface (Co~3~O~4~ NWs-HCSs) by hydrothermal method together with subsequent heat treatment for acetone gas sensing ([Figure 22](#nanomaterials-10-00378-f022){ref-type="fig"}a). Obviously, Co~3~O~4~ NWs-HCSs behaved talented selectivity for acetone, and whose response increased to 23 at 150 °C, even possessed high response at lower temperature of 75 °C, approaching room temperature.

Most researchers tend to study low temperature (even room temperature) and low concentration gas sensing taking the advantages of hollow structure, however they lacked research on humidity sensitivity. Notably, Mutuma et al. \[[@B114-nanomaterials-10-00378]\] reported that pristine hollow carbon spheres (HCSs), HCS/PVP (polyvinyl pyrrolidone) composite and annealed HCSs as ammonia sensors in damp conditions. The authors used a surfactant dispersion method to fabricate the HCSs and HCS/PVP. HCSs sensor should have responded in a high relative humidity (10--97% RH) owing to the presence of amorphous domains and oxygenated groups while high concentration of water molecules inhibited ammonia adsorption sites ([Figure 23](#nanomaterials-10-00378-f023){ref-type="fig"}a). After adding PVP, HCSs still behaved obvious conductivity attenuation under high humidity resulting from polymer swelling and plasticization ([Figure 23](#nanomaterials-10-00378-f023){ref-type="fig"}b). After subsequent annealing, the annealed HCSs obtained a great response to 74--295 ppm NH~3~ over a broad RH range ([Figure 23](#nanomaterials-10-00378-f023){ref-type="fig"}c,h,i), decreasing the dependence on RH and raising surface area. This study provided insights into a step forward to modify the HCSs sensor in high humidity.

The above research presents several promising strategies to enhance gas-sensing performance through carbon sphere derived hollow nanostructure. Based on these studies, after further optimization on structure and process conditions, as well as material systems innovation, carbon sphere template derived hollow nanostructures would be promising in low temperature especially room temperature and ppb-level gas detection in high relative humidity environment, making it possible to apply in exhaled breath detecting in the future. A summary of the recent carbon sphere template derived hollow nanostructure gas sensors is presented in [Table 2](#nanomaterials-10-00378-t002){ref-type="table"}. Furthermore, hollow structures also have plenty of applications in the fields in energy storage \[[@B115-nanomaterials-10-00378],[@B116-nanomaterials-10-00378]\].

5. Conclusion and Outlook {#sec5-nanomaterials-10-00378}
=========================

In summary, carbon sphere template derived hollow nanostructure has attracted considerable attention and achieved great progress in the fields of photocatalysis, gas sensing, etc. Among template methods, the use of highly hazardous, complex, inefficient, and costly fluoride-containing or strong acids and bases to etch the sacrificial templates limits their large-scale production and practical application. For these reasons, developing green, facile, and efficient strategies especially carbon sphere template derived hollow nanostructures with better photocatalysis and gas sensing performance is highly desirable and meaningful.

After tremendous efforts, the family of carbon sphere template derived hollow nanostructures have been broadened from monometal oxides to multimetal compounds. More importantly, doping, solid solution, heterostructure, and surface modification strategies have been successfully realized in carbon sphere template derived hollow nanostructures. These successful attempts are due to the unique characteristics of carbon sphere templates like the whole synthesis process is totally green and the surface radicals adsorb metal ions uniformly and non-selectively. This strategy is of great help to the three steps of photocatalytic process which enhancing light absorption, promoting carrier separation and benefiting surface redox reaction. This review summarized the recent development of carbon sphere template derived hollow nanostructures in photocatalysis. We summarize various synthetic routines of carbon spheres and carbon sphere template derived hollow nanostructures and highlight the mechanism for photocatalytic performance enhancement. The strategy has been also successfully applied in the field of gas sensing thus we present a summary of the recent progress in the development of carbon sphere template derived hollow nanostructures for gas sensing and related applications as well.

Despite significant progress in synthesis and effective property tuning of carbon sphere template derived hollow nanostructures, there are still some challenges and opportunities. For example, there are still many opportunities for research in the preparation of other types of fully adjustable solid solutions and tailoring bandgaps in most material systems. More systematic studies are needed to study the distribution of the minority atoms on the surface, such as whether the atomic dispersion can be achieved and applied to other reactions like photocatalytic CO~2~ reduction or nitrogen fixation. It is important to realize synthesis of configurationally disordered and entropy-stabilized mixed metal oxides, especially consist of earth-abundant elements, using this uniformly and non-selectively adsorption method. Any progress in the above urgent problems, will make great contributions to their corresponding application fields. We hope this review can give an overview of the research progress in carbon sphere template derived hollow nanostructures, so that researchers can have a more in-depth understanding of the process and application of carbon sphere templating method, and we have given a brief description on the outlook of future development, so as to promote photocatalysis and gas sensing to practicality and benefit the development of scientific researches in these fields.

Conceptualization, Z.L. and L.Z.; Investigation, Y.W. (Yichen Wang), Y.Y., Y.W. (Yanwen Wang), C.Q., R.L., and X.C.; Writing---original draft preparation, Y.W. (Yichen Wang); Writing---review and editing, Z.L.; Supervision, Z.Y. and L.Z.; Funding acquisition, Z.Y and L.Z. All authors have read and agreed to the published version of the manuscript.

This research was funded by National Natural Science Foundation of China (51572239 and 91333203), Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), the National Science and Technology Support Program (2012BAC08B08), and the Major Project of Zhejiang Natural Science Foundation of China (LD18E020002).

The authors declare no conflict of interest.

![Schematics of the possible synthesis procedure of carbon spheres. Reproduced with permission from \[[@B32-nanomaterials-10-00378]\]. Copyright WILEY, 2004.](nanomaterials-10-00378-g001){#nanomaterials-10-00378-f001}

![Proposed formation mechanism of sphere-like carbon synthesized from saccharides and biomasses. Reproduced with permission from \[[@B39-nanomaterials-10-00378]\]. Copyright American Chemical Society, 2012.](nanomaterials-10-00378-g002){#nanomaterials-10-00378-f002}

![General process for the formation of carbon sphere template derived (**a**) hollow nanoheterostructure and (**b**) controllable multilayer structure. Reproduced with permission from \[[@B47-nanomaterials-10-00378]\]. Copyright WILEY, 2016.](nanomaterials-10-00378-g003){#nanomaterials-10-00378-f003}

![(**a**) Schematic route for preparing Na-ZnO hollow nanostructures, (**b**) valence band measure of Na-ZnO with various Na doping concentrations, (**c**) corrensponding H~2~ generation for Na-ZnO/Pt hollow spheres with 0, 0.2, 0.7, 2, and 4 at % Na. Reproduced with permission from \[[@B53-nanomaterials-10-00378]\]. Copyright Royal Society of Chemistry, 2016.](nanomaterials-10-00378-g004){#nanomaterials-10-00378-f004}

![(**a**) SEM (scan electron microscope) image of broken TiO~2~ nanospheres with a macroporous channel structure by crushing THS. Reproduced with permission from \[[@B56-nanomaterials-10-00378]\]. Copyright WILEY, 2012. (**b**) Formation mechanism of the porous channels in the hollow 3D network structure. Reproduced with permission from \[[@B57-nanomaterials-10-00378]\]. Copyright Elsevier, 2012. (**c**) Schematic route for preparing C-TiO~2~\@g-C~3~N~4~. Reproduced with permission from \[[@B58-nanomaterials-10-00378]\]. Copyright Elsevier, 2017.](nanomaterials-10-00378-g005){#nanomaterials-10-00378-f005}

![(**a**) Band energy diagram of CoO*~x~*/LaTiO~2~N with1.5% Fe, and (**b**) oxygen generation rates for samples with various Fe doping concentration under visible light illumination. Reproduced with permission from \[[@B60-nanomaterials-10-00378]\]. Copyright Elsevier, 2016.](nanomaterials-10-00378-g006){#nanomaterials-10-00378-f006}

![(**a**) Schematic illustration of the band structure of anatase TiO~2~ with and without Mg doping for water splitting. (**b**) The density of states of Mg 2p in different samples. Reproduced with permission from \[[@B61-nanomaterials-10-00378]\]. Copyright Elsevier, 2017. (**c**) Mott--Schottky plot of Ta~3~N~5~ with and without Mg dopant in dark, and (**d**) normalization of the contribution of individual strategy to the photocatalytic activity. Reproduced with permission from \[[@B62-nanomaterials-10-00378]\]. Copyright Elsevier, 2019.](nanomaterials-10-00378-g007){#nanomaterials-10-00378-f007}

![Band engineering through doping and forming solid-solution. (**a**,**b**) Formation of a new VBM by doping a foreign element to create a donor level, and (**c**) forming solid solution to obtain a controllable bandgap. Reproduced with permission from \[[@B64-nanomaterials-10-00378]\]. Copyright American Chemical Society, 2004.](nanomaterials-10-00378-g008){#nanomaterials-10-00378-f008}

![(**a**) TEM image shows an extremely thin shell (about 7 nm) of (Ga~0.82~Zn~0.18~)(N~0.82~O~0.18~) hollow spheres. (**b**) The (100) peak shift between GaN (100) and ZnO (100) with different *x*. (**c**) The band structure of the (Ga~1-*x*~Zn*~x~*)(N~1-*x*~O*~x~*) nanostructures with different x. Reproduced with permission from \[[@B69-nanomaterials-10-00378]\]. Copyright WILEY, 2015.](nanomaterials-10-00378-g009){#nanomaterials-10-00378-f009}

![(**a**,**b**) TEM images of Zn*~x~*Cd~1-*x*~S hollow sphere. (**c**) UV--vis spectra of: Zn~0.5~Cd~0.5~O, Zn~0.5~Cd~0.5~S, Zn~0.7~Cd~0.3~S, Zn~0.3~Cd~0.7~S, ZnS, and CdS (**d**) XRD patterns of solid solutions of Zn*~x~*Cd~1-*x*~S after H~2~S treatment. Reproduced with permission from \[[@B73-nanomaterials-10-00378]\]. Copyright Elsevier, 2018.](nanomaterials-10-00378-g010){#nanomaterials-10-00378-f010}

![(**a**) Schematic diagram of preparation process of ZnFe~2~O~4~ nanostructures and ZnFe~2~O~4~/ZnO nanoheterostructures. (**b**) TEM images of ZnFe~2~O~4~/ZnO spheres. (**c**) H~2~ production over time by 50 mg photocatalysts of bare ZnFe~2~O~4~ and ZnFe~2~O~4~/ZnO (ZF2) sample driven by visible light (\>420 nm). (**d**) Schematic illustration of ZnFe~2~O~4~/ZnO hetrtojunction energy band position and predicted reaction mechanism under irradiation. Reproduced with permission from \[[@B80-nanomaterials-10-00378]\]. Copyright Royal Society of Chemistry, 2015.](nanomaterials-10-00378-g011){#nanomaterials-10-00378-f011}

![(**a**) Band bending diagram of Cu~2~O/crystalline Ta~2~O~5~. (**b**) Band bending diagram of Cu~2~O/amorphous Ta~2~O~5~ nanoheterojunctions. (**c**) HRTEM image of Cu~2~O/amorphous Ta~2~O~5~heterostructures. (**d**) Amount of evolved gases under full light for different samples. Reproduced with permission from \[[@B82-nanomaterials-10-00378]\]. Copyright Royal Society of Chemistry, 2017.](nanomaterials-10-00378-g012){#nanomaterials-10-00378-f012}

![(**a**) Schematic illustration of the formation path of nanoheterostructured photocatalysts. (**b**) TEM image of the TiO~2~/SnO~2~ hybrid nanostructure. (**c**) HRTEM images of the TiO~2~/SnO~2~ nanoheterojunctions. (**d**) Schematic illustration of band structure of TiO~2~/SnO~2~ heterojuction. (**e**) The PL spectra of three different hollow spheres, red line for SnO~2~, black line for TiO~2~, and blue line for TiO~2~/SnO~2~. (**f**) The photocatalytic activity measurement of different structures towards RhB under full light irradiation. Reproduced with permission from \[[@B86-nanomaterials-10-00378]\]. Copyright Royal Society of Chemistry, 2014.](nanomaterials-10-00378-g013){#nanomaterials-10-00378-f013}

![(**a**) Schematic diagram of the fabrication process of hollow hierarchical CuO/TiO~2~ structure. (**b**) SEM image of the CuO/TiO~2~ spheres. Reproduced with permission from \[[@B85-nanomaterials-10-00378]\]. Copyright 2017, RSC. (**c**) Schematic diagram of the preparation process of nest-like *γ*-Fe~2~O~3~/ZnO hollow nanostructures. (**d**) SEM images of *g*-Fe~2~O~3~/ZnO nest-like hollow nanostructures. Reproduced with permission from \[[@B88-nanomaterials-10-00378]\]. Copyright Royal Society of Chemistry, 2012.](nanomaterials-10-00378-g014){#nanomaterials-10-00378-f014}

![(**a**) The simple illustration of the synthesizing process of LaTiO~2~N nanostructures. (**b**) TEM image of LaTiO~2~N hollow nanostructures. (**c**) HRTEM image of the H~2~ treated LaTiO~2~N. (**d**,**e**) Hydrogen generation versus time on Pt/LTON and Pt/H-LTON irradiated by visible light (*λ* ≥ 420 nm) respectively. (**f**) Possible hydrogen generation principles proposed of the Pt/B-LTON, Pt/LTON and Pt/H-LTON respectively. Reproduced with permission from \[[@B95-nanomaterials-10-00378]\]. Copyright Elsevier, 2015.](nanomaterials-10-00378-g015){#nanomaterials-10-00378-f015}

![(**a**) The synthesis process of the as-prepared TiO~2~ nanostructures and black ones. (**b**) SEM image of black TiO~2~. (**c**) Time course of photocatalytic H~2~ evolution of TiO~2~ nanostructures. Reaction conditions: 20 mg photocatalysts, 1 wt % Pt loaded, under 300W Xe lamp irradiation. (**d**) EPR spectra of the as-prepared TiO~2~ hollow spheres and black ones. Reproduced with permission from \[[@B100-nanomaterials-10-00378]\]. Copyright American Chemical Society, 2017.](nanomaterials-10-00378-g016){#nanomaterials-10-00378-f016}

![(**a**) Diagrams of preparation of Au~x~\@THS\@CoO. (**b**) Production rates of CH~4~ for THS, THS\@CoO and Au~2.0~\@THS\@CoO. Reproduced with permission from \[[@B103-nanomaterials-10-00378]\]. Copyright Elsevier, 2019.](nanomaterials-10-00378-g017){#nanomaterials-10-00378-f017}

![Schematic diagram of synthesis of (**a**) core--shell, (**b**) yolk--shell Au\@TiO~2~. (**c**) UV--vis absorbance spectra for different kinds of TiO~2~ spheres. (**d**) The BET curves of yolk--shell and core--shell Au\@TiO~2~ respectively. (**e**) The possible photocatalysis principle of Au\@TiO~2~-YS. Reproduced with permission from \[[@B104-nanomaterials-10-00378]\]. Copyright Elsevier, 2019.](nanomaterials-10-00378-g018){#nanomaterials-10-00378-f018}

![(**a**) Schematic diagram of how these three proposed approaches solve the recombination problems synergistically during the photocatalytic process of Ta~3~N~5~. Red, purple, green, and yellow objects represent Ta~3~N~5~, MgO, Pt, and CoO*~x~* respectively. TEM images of (**b**) Mg-Ta~3~N~5~, (**c**) MgO/Mg-Ta~3~N~5~ and (**d**) Pt/CoO*~x~*/MgO/Mg-Ta~3~N~5~ hollow spheres. (**e**) Hydrogen generation rates of Pt/Ta~3~N~5~, Pt/Mg-Ta~3~N~5~, Pt/MgO/Ta~3~N~5~, and Pt/CoO*~x~*/MgO/Mg-Ta~3~N~5~. (**f**) Comparison between the contributions of each approach to improve the photocatalytic ability of Pt/CoO*~x~*/MgO/Mg-Ta~3~N~5~ photocatalyst irradiated under simulated sunlight. Reproduced with permission from \[[@B62-nanomaterials-10-00378]\]. Copyright Elsevier, 2019.](nanomaterials-10-00378-g019){#nanomaterials-10-00378-f019}

![(**a**) Synthetic schematic of Co~3~O~4~/Ta~2~O~5~ heterostructure hollow nanospheres, (**b**) FESEM images, (**c**) TEM images, (**d**) HRTEM images. (**e**) I--V characteristics of the Co~3~O~4~/Ta~2~O~5~ sensor. Proposed energy band diagram: (**f**) before contact, (**g**) after contact. (**h**) Response and recovery curves of the Co~3~O~4~/Ta~2~O~5~ sensor at room temperature. (**i**) Sensitivity curves between the heterostructure and pure Co~3~O~4~ sensor with 10--100 ppm. (**j**) Selective test to several reducing gases with concentration of 10--100 ppm. (**k**) Long-term stability test during two months. Reproduced with permission from \[[@B111-nanomaterials-10-00378]\]. Copyright Elsevier, 2017.](nanomaterials-10-00378-g020){#nanomaterials-10-00378-f020}

![(**a**) XRD patterns. (**b**) N~2~ adsorption and desorption isotherms and pore diameter distribution curves. (**c**) Response of all samples with different Fe doping contents from 100 to 140 °C to 1000 ppb NO~2~. The gas-sensing performance of the representative S5: (**d**) dynamic sensing curve to different NO~2~ concentrations, (**e**) dynamic response curves to 10 ppb NO~2~ at 120 °C, and (**f**) selective test to 1 ppm NO~2~ versus several 10 ppm interfering gases. Reproduced with permission from \[[@B112-nanomaterials-10-00378]\]. Copyright Elsevier, 2018.](nanomaterials-10-00378-g021){#nanomaterials-10-00378-f021}

![(**a**) Synthetic Schematic of Co~3~O~4~ NWs-HCSs. The gas-sensing performance of Co~3~O~4~ NWs-HCSs. (**b**) Sensing selectivity for several 200 ppm gases at 150 °C. (**c**) Sensing response versus operating temperature to 200 ppm acetone. (**d**,**e**) Dynamic sensing response to acetone within different concentration ranges at the operating temperature of 150 °C. Reproduced with permission from \[[@B113-nanomaterials-10-00378]\]. Copyright Elsevier, 2019.](nanomaterials-10-00378-g022){#nanomaterials-10-00378-f022}

![RH-dependent conductivity curves of (**a**) pure HCS, (**b**) HCS/PVP composite, and (**c**) annealed HCS based sensors at 40 °C. Ammonia concentration-dependent conductance and sensitivity of (**d**,**e**) pristine HCS, (**f**,**g**) HCS/PVP, and (**h**,**i**) annealed HCS based sensors at 10% RH at 40 °C. A schematic illustration of the sensing mechanism to NH~3~ of the HCSs at low RH (**j**) and high RH (**k**). Reproduced with permission from \[[@B114-nanomaterials-10-00378]\]. Copyright Royal Society of Chemistry, 2017.](nanomaterials-10-00378-g023){#nanomaterials-10-00378-f023}

nanomaterials-10-00378-t001_Table 1

######

Summary of carbon sphere template derived hollow nanostructure for photocatalysis.

  -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Strategy               Material                         Reaction                   Efficiency                       Reference
  ---------------------- -------------------------------- -------------------------- -------------------------------- -----------------------------------------------------------------------------------------------
  Doping                 WO~3~: Fe                        RhB degradation            *k*~RhB~ = 0.02 min^−1^          \[[@B52-nanomaterials-10-00378]\]

                         ZnO: Na                          H~2~ evolution             1380 μmol g^−1^ h^−1^\           \[[@B53-nanomaterials-10-00378]\]
                                                                                (AQY = 13.5% @ 350 nm)

                         TiO~2~: Nd                       *X*-3B degradation         *k~X~*~-3B~ = 0.0087 min^−1^     \[[@B54-nanomaterials-10-00378]\]

                         TiO~2~: Ce                       *X*-3B degradation         *k~X~*~-3B~ = 0.029 min^−1^      \[[@B55-nanomaterials-10-00378]\]

                         TiO~2~: C                        H~2~ evolution             35.6 μmol g^−1^ h^−1^            \[[@B56-nanomaterials-10-00378],[@B57-nanomaterials-10-00378],[@B58-nanomaterials-10-00378]\]

                         TiO~2~: Br                       RhB, MO, MB degradation    90% removal in 40 min            \[[@B59-nanomaterials-10-00378]\]

                         LaTiO~2~N: Fe                    O~2~ evolution             1.59 mmol h^−1^\                 \[[@B60-nanomaterials-10-00378]\]
                                                                                (AQY = 55% @ 450 nm)

                         TiO~2~: Mg                       Overall water splitting    H~2~: 850μmol g^−1^ h^−1^\       \[[@B61-nanomaterials-10-00378]\]
                                                                                O~2~: 425μmol g^−1^ h^−1^\
                                                                                (AQY = 19.4% @ 350 nm)

                         Ta~3~N~5~: Mg                    H~2~ evolution             11 μmol h^−1^                    \[[@B62-nanomaterials-10-00378]\]

  Solid-solution         (Ga~1−x~Zn~x~)(N~1−x~O~x~)       Overall water splitting    H~2~: 254.2μmol h^−1^\           \[[@B69-nanomaterials-10-00378]\]
                                                                                O~2~: 127.1μmol h^−1^\
                                                                                (AQY = 14.3% @ 400 nm)

                         Zn~x~Cd~1-x~S                    H~2~ evolution             12 mmol h^−1^ g^−1^\             \[[@B73-nanomaterials-10-00378]\]
                                                                                (AQY = 46.6% @ 400 nm)

  Heterostructure        ZnFe~2~O~4~/ZnO                  H~2~ evolution             2.15 mmol h^−1^ g^−1^\           \[[@B80-nanomaterials-10-00378]\]
                                                                                (AQY = 1.61% @ 440 nm)

                         g-C~3~N~4~/TiO~2~                H~2~ evolution             470 μmol g^−1^ h^−1^             \[[@B81-nanomaterials-10-00378]\]

                         Cu~2~O/a-Ta~2~O~5~               Overall water splitting    H~2~: 202 μmol h^−1^\            \[[@B82-nanomaterials-10-00378]\]
                                                                                O~2~: 98 μmol h^−1^\
                                                                                (AQY = 3% @ 480 nm)

                         TiO~2~/Fe~2~TiO~5~               O~2~ evolution             375 μmol g^−1^ h^−1^             \[[@B83-nanomaterials-10-00378]\]

                         ZnFe~2~O~4~/ZnSe                 H~2~ evolution             2.8 mmol g^−1^ h^−1^             \[[@B84-nanomaterials-10-00378]\]

                         TiO~2~/SnO~2~                    RhB degradation            *k*~RhB~ = 0.22 min^−1^          \[[@B86-nanomaterials-10-00378]\]

                         Fe~2~TiO~5~/TiO~2~               RhB degradation            *k*~RhB~ = 0.09 min^−1^          \[[@B87-nanomaterials-10-00378]\]

                         CuO/TiO~2~                       AO7 degradation            *k*~AO7~ = 0.057 min^−1^         \[[@B85-nanomaterials-10-00378]\]

                         γ-Fe~2~O~3~/ZnO                  RhB, MO, MB degradation    90% RhB, MB removal in 50 min\   \[[@B88-nanomaterials-10-00378]\]
                                                                                80% MO removal in 80 min

                         CeO~2~/TiO~2~                    RhB degradation            75% removal in 180 min           \[[@B89-nanomaterials-10-00378]\]

                         WO~3~/TiO~2~                     MB degradation             78% removal in 80 min            \[[@B90-nanomaterials-10-00378]\]

                         ZnO/CuO                          MB degradation             98% removal in 60 min            \[[@B91-nanomaterials-10-00378]\]

                         ZnO/SnO~2~                       TC degradation             81% removal in 180 min           \[[@B92-nanomaterials-10-00378]\]

                         ZnO/ZnS                          RhB degradation            99% removal in 40 min            \[[@B93-nanomaterials-10-00378]\]

  Surface modification   Surface-reconstruced LaTiO~2~N   H~2~ evolution             192 μmol h^−1^\                  \[[@B95-nanomaterials-10-00378]\]
                                                                                (AQY = 3.4% @ 440 nm)

                         Black TiO~2~                     H~2~ evolution             56.7 mmol h^−1^ g^−1^\           \[[@B100-nanomaterials-10-00378]\]
                                                                                (AQY = 90.6% @ 365 nm)

                         Pt\@TiO~2~\@MnO~x~               Benzyl alcohol oxidation   31.22 μmol in 14 h\              \[[@B102-nanomaterials-10-00378]\]
                                                                                (AQY = 63.14% @ 254 nm)

                         Pt/CoO~x~\@MgO~2~\@Ta~3~N~5~     H~2~ evolution             56.3 μmol h^−1^\                 \[[@B62-nanomaterials-10-00378]\]
                                                                                (AQY = 0.31% @ 400 nm)

                         Au~2.0~\@TiO~2~\@CoO             CO~2~ reduction to CH~4~   13.3μmolh^−1^g^−1^               \[[@B103-nanomaterials-10-00378]\]

                         Yolk--shell Au\@TiO~2~           Toluene degradation        57% removal in 180 min           \[[@B104-nanomaterials-10-00378]\]
  -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

nanomaterials-10-00378-t002_Table 2

######

Summary of carbon sphere template derived hollow nanostructure for gas sensing.

  Strategy               Material              Gas       Response                Reference
  ---------------------- --------------------- --------- ----------------------- ------------------------------------
  Doping                 WO~3~: C              acetone   5.1 @ 0.9 ppm, 300 °C   \[[@B109-nanomaterials-10-00378]\]
                         WO~3~: Fe             NO~2~     1.3 @ 10 ppb, 120 °C    \[[@B112-nanomaterials-10-00378]\]
  Heterostructure        Pd\@In~2~O~3~         ethanol   159 @ 5 ppm, 350 °C     \[[@B108-nanomaterials-10-00378]\]
                         Co~3~O~4~/Ta~2~O~5~   ethanol   1.8 @ 100 ppm, 25 °C    \[[@B111-nanomaterials-10-00378]\]
                         Co~3~O~4~\@C          acetone   23 @ 200 ppm, 150 °C    \[[@B113-nanomaterials-10-00378]\]
  Surface modification   Annealed C            ammonia   66 @ 74 ppm, 40 °C      \[[@B114-nanomaterials-10-00378]\]


(* WebAssembly-compatible type conversions to f64 implementation *)

let promote_f32 x =
  let xf = F32.to_float x in
  if xf = xf then F64.of_float xf else
  let nan32bits = I64_convert.extend_u_i32 (F32.to_bits x) in
  let sign_field = Int64.(shift_left (shift_right_logical nan32bits 31) 63) in
  let significand_field = Int64.(shift_right_logical (shift_left nan32bits 41) 12) in
  let fields = Int64.logor sign_field significand_field in
  let nan64bits = Int64.logor 0x7ff8000000000000L fields in
  F64.of_bits nan64bits

let convert_s_i32 x =
  F64.of_float (Int32.to_float x)

(*
 * Unlike the other convert_u functions, the high half of the i32 range is
 * within the range where f32 can represent odd numbers, so we can't do the
 * shift. Instead, we can use int64 signed arithmetic.
 *)
let convert_u_i32 x =
  F64.of_float Int64.(to_float (logand (of_int32 x) 0x00000000ffffffffL))

let convert_s_i64 x =
  F64.of_float (Int64.to_float x)

(*
 * Values in the low half of the int64 range can be converted with a signed
 * conversion. The high half is beyond the range where f64 can represent odd
 * numbers, so we can shift the value right, adjust the least significant
 * bit to round correctly, do a conversion, and then scale it back up.
 *)
let convert_u_i64 x =
  F64.of_float
    Int64.(if x >= zero then to_float x else
           to_float (logor (shift_right_logical x 1) (logand x 1L)) *. 2.0)

let reinterpret_i64 = F64.of_bits